WO2024209921A1

WO2024209921A1 - Steel material

Info

Publication number: WO2024209921A1
Application number: PCT/JP2024/010558
Authority: WO
Inventors: 裕紀神谷; 桂一近藤
Original assignee: 日本製鉄株式会社
Priority date: 2023-04-06
Filing date: 2024-03-18
Publication date: 2024-10-10

Abstract

Provided is a steel material having both high strength of at least 125 ksi and excellent low-temperature toughness. This steel material contains, in mass%, 0.15-0.45% of C, 0.05-1.00% of Si, 0.05-1.00% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.005-0.100% of Al, 0.30-1.50% of Cr, 0.20-2.00% of Mo, 0.002-0.030% of Ti, 0.002-0.100% of Nb, 0.0005-0.0040% of B, 0.0100% or less of N, and 0.0040% or less of O, with the remainder comprising Fe and impurities, and has a yield strength of 862-1034 MPa. In the steel material, the number density of Al oxides having a major axis of at least 5.0 μm is less than 30 counts/200 mm², and the number density of Si oxides having a major axis of at least 5.0 μm is 5 counts/200 mm² or less.

Description

Steel

This disclosure relates to steel materials, and more particularly to steel materials suitable for use in oil wells.

The deepening of oil and gas wells (hereinafter, oil and gas wells will be collectively referred to simply as "oil wells") has created a demand for higher strength oil well steel materials, such as oil well steel pipes. Specifically, 80 ksi grade (yield strength of 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and 95 ksi grade (yield strength of 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) oil well steel materials are widely used, and recently there has been an increasing demand for 110 ksi grade (yield strength of 758 to less than 862 MPa) oil well steel materials.

Oil wells may further contain corrosive hydrogen sulfide gas (H ₂ S) and carbon dioxide gas (CO ₂ ). Therefore, steel materials expected to be used as oil well steel materials are required to have not only high strength but also excellent corrosion resistance. In addition, steel materials for oil wells are subjected to stress during use. Therefore, sulfide stress cracking resistance (hereinafter referred to as SSC resistance) has been used as an index of excellent corrosion resistance of steel materials for oil wells.

Technologies for improving the strength and SSC resistance of steel materials have been proposed in JP 2006-28612 A (Patent Document 1), WO 2008/123422 A (Patent Document 2), and JP 2017-166060 A (Patent Document 3).

The steel material disclosed in Patent Document 1 is a steel for steel pipes, and consists of, by mass%, C: 0.2-0.7%, Si: 0.01-0.8%, Mn: 0.1-1.5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005-0.1%, Ti: 0.005-0.05%, Ca: 0.0004-0.005%, N: 0.007% or less, Cr: 0.1-1.5%, Mo: 0.2-1.0%, with the balance being Fe and impurities. This steel material further has a (Ca%)/(Al%) ratio of 0.55-1.72 and a (Ca%)/(Ti%) ratio of 0.7-19 in the non-metallic inclusions containing Ca, Al, Ti, N, O, and S. Patent Document 1 states that this steel has a high yield strength of more than 758 MPa and excellent SSC resistance.

The steel material disclosed in Patent Document 2 is a low alloy steel containing, by mass%, C: 0.10-0.20%, Si: 0.05-1.0%, Mn: 0.05-1.5%, Cr: 1.0-2.0%, Mo: 0.05-2.0%, Al: 0.10% or less, and Ti: 0.002-0.05%, with Ceq (=C+(Mn/6)+(Cr+Mo+V)/5) being 0.65 or more, the balance being Fe and impurities, with the impurities being P: 0.025% or less, S: 0.010% or less, N: 0.007% or less, and B: less than 0.0003%. This steel material further contains _0.1 _M23C6 type precipitates having a grain size of 1 μm or more per mm2 ^or less. Patent Document 2 describes that this steel has a yield strength of 654 to 793 MPa and has excellent SSC resistance even in a high-pressure hydrogen sulfide environment.

The steel material disclosed in Patent Document 3 is a material for high-strength steel pipes for oil wells, and consists, by mass%, of C: 0.20-0.45%, Si: 0.05-0.40%, Mn: 0.3-0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005-0.10%, N: 0.001-0.006%, Cr: 0.1-0.8%, Mo: 0.1-1.6%, V: 0.02-0.2%, Nb: 0.001-0.04%, B: 0.0003-0.0030%, O (oxygen): 0.0030% or less, and the balance being Fe and unavoidable impurities. This steel further has a Rockwell hardness HRC that satisfies the formula (15.6 x [%C] + 29.2 ≦ HRC < 60.5 x [%C] + 31.1). Patent Document 3 states that this steel can produce steel pipes with a yield strength of 758 to less than 862 MPa and excellent SSC resistance.

JP 2006-28612 A International Publication No. 2008/123422 JP 2017-166060 A

In recent years, there has also been active development of deep wells below sea level. Specifically, oil well steel materials intended for use in so-called deep sea oil fields at depths of 2000m or more are required to have high strength, for example, of 125ksi or more (yield strength of 125ksi or more, i.e. 862MPa or more). The water temperature in such deep sea oil fields is even lower. For this reason, low-temperature toughness is also required for oil well steel materials intended for use in such deep sea oil fields. However, there is concern that increasing the yield strength of the steel will result in a decrease in the low-temperature toughness of the steel.

In other words, oil well steel materials intended for use in such harsh environments are required to have both high strength of 125 ksi or more and excellent low-temperature toughness. However, the above Patent Documents 1 to 3 do not consider achieving both high strength of 125 ksi or more and excellent low-temperature toughness.

The objective of this disclosure is to provide a steel material that combines high strength of 125 ksi or more (862 MPa or more) with excellent low-temperature toughness.

The steel material according to the present disclosure is
In mass percent,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.50%,
Mo: 0.20-2.00%,
Ti: 0.002 to 0.030%,
Nb: 0.002-0.100%,
B: 0.0005-0.0040%,
N: 0.0100% or less,
O: 0.0040% or less,
V: 0 to 0.30%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0-0.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
The balance is Fe and impurities,
The yield strength is 862 to 1034 MPa;
In the steel material,
The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxides having a major axis of 5.0 μm or more is less than 30 pieces/200 ^mm2 , in terms of mass%,
In terms of mass%, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces/200 ^mm2 or less.

The steel material disclosed herein combines high strength of 125 ksi or more (862 MPa or more) with excellent low-temperature toughness.

FIG. 1 is a graph showing the relationship between the number density (pieces/200 ^mm2 ) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) and the fracture appearance transition temperature (°C), which is an index of low-temperature toughness, for examples having a yield strength of less than 945 MPa among the present examples. FIG. 2 is a graph showing the relationship between the number density (pieces/200 ^mm2 ) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) and the fracture appearance transition temperature (°C), which is an index of low-temperature toughness, for the examples having a yield strength of 945 MPa or more among the present examples.

First, the inventors focused on the chemical composition and investigated how to obtain a steel material that combines a yield strength of 125 ksi or more with excellent low-temperature toughness. As a result, the inventors discovered that the composition is, by mass%, C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, Cr: 0.30-1.50%, Mo: 0.20-2.00%, Ti: 0.002-0.030%, Nb: 0.002-0.100%, B: 0.0005-0.0040%, N: 0.01 00% or less, O: 0.0040% or less, V: 0-0.30%, Cu: 0-0.50%, Ni: 0-0.50%, W: 0-0.50%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, rare earth elements: 0-0.0100%, and the balance being Fe and impurities, it is believed that a steel material consisting of this may be able to achieve both a yield strength of 125 ksi or more and excellent low-temperature toughness.

Next, the inventors of the present invention investigated various methods for improving the low-temperature toughness of a steel material having the above-mentioned chemical composition and a yield strength of 125 ksi or more. Specifically, the inventors of the present invention considered that if coarse oxide-based inclusions could be reduced, it would be possible to improve the low-temperature toughness while maintaining the yield strength. Here, in a steel material having the above-mentioned chemical composition, Al _oxides mainly composed of _Al2O3 tend to coarsen. Therefore, the inventors of the present invention first focused on the coarse Al oxides.

As a result of studies by the present inventors, it has become clear that in a steel material having the above-mentioned chemical composition and a yield strength of 125 ksi or more, if the number density of Al oxides having a major axis of 5.0 μm or more is less than 30 particles/200 ^mm2 , low-temperature toughness may be improved. Herein, particles having an Al content of 20% or more and an O content of 10% or more, in mass%, are also referred to as "Al oxides". Furthermore, in this specification, Al oxides having a major axis of 5.0 μm or more are also referred to as "coarse Al oxides".

Here, Al oxides are hard oxides and tend to reduce the toughness of the steel material. In particular, when the yield strength is increased to 125 ksi or more, the effect of coarse Al oxides tends to become apparent, and the low-temperature toughness tends to decrease significantly. Therefore, in the steel material according to this embodiment having the above-mentioned chemical composition and a yield strength of 125 ksi or more, the number density of coarse Al oxides is set to less than 30 particles/200 ^mm2 .

On the other hand, even if a steel material has the above-mentioned chemical composition and the number density of coarse Al oxides is less than 30/200 ^mm2 , when the steel material has a yield strength of 125 ksi or more, it may not be possible to stably obtain excellent low-temperature toughness. Therefore, the present inventors have studied various methods for stably obtaining excellent low-temperature toughness for a steel material having the above-mentioned chemical composition, a yield strength of 125 ksi or more, and a number density of coarse Al oxides of less than 30/200 ^mm2 . As a result of detailed studies by the present inventors, it has become clear that in a steel material having the above-mentioned chemical composition, a yield strength of 125 ksi or more, and a number density of coarse Al oxides of less than 30/200 ^mm2 , if not only the coarse Al oxides but also the coarse Si oxides in the steel material can be reduced, excellent low-temperature toughness may be stably obtained.

Here, in this specification, particles having an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more, in mass%, are also referred to as "Si oxides". In this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides". Hereinafter, the relationship between coarse Si oxides and low-temperature toughness will be specifically described with reference to the drawings for a steel material having the above-mentioned chemical composition, a yield strength of 125 ksi or more, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 .

Fig. 1 is a diagram showing the relationship between the number density (pieces/200 ^mm2 ) of coarse Si oxides (Si oxides with a major axis of 5.0 µm or more) and the fracture transition temperature (°C), which is an index of low-temperature toughness, for examples having a yield strength of less than 945 MPa among the present examples. Fig. 1 was created using the number density (pieces/200 mm2 ⁾ of coarse Si oxides determined by a method described later and the fracture transition temperature (°C) determined by a method described later for steel materials that satisfy the above-mentioned chemical composition, have a yield strength of less than 862 to 945 MPa, and have a number density of coarse Al oxides of less than 30 pieces/200 ^mm2 among the examples described later.

Referring to FIG. 1, in a steel material having the above-mentioned chemical composition, a yield strength of 862 to less than 945 MPa, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 , if the number density of coarse Si oxides is 5 particles/200 mm2 or ^less , the fracture appearance transition temperature is −50° C. or less, and excellent low-temperature toughness is exhibited.

Furthermore, Fig. 2 is a diagram showing the relationship between the number density (pieces/200 ^mm2 ) of coarse Si oxides (Si oxides with a major axis of 5.0 µm or more) and the fracture transition temperature (°C), which is an index of low temperature toughness, for examples having a yield strength of 945 MPa or more among the present examples. Fig. ² was created using the number density (pieces/200 mm2) of coarse Si oxides determined by a method described later and the fracture transition temperature (°C) determined by a method described later for steel materials that satisfy the above-mentioned chemical composition, have a yield strength of 945 to 1034 MPa, and have a number density of coarse Al oxides of less than 30 pieces/200 ^mm2 among the examples described later.

Referring to FIG. ² , in a steel material having the above-mentioned chemical composition and a yield strength of 945 to 1034 MPa, in which the number density of coarse Al oxides is less than 30 particles/200 mm2, if the number density of coarse Si oxides is 5 particles/200 mm2 or ^less , the fracture appearance transition temperature is −40° C. or less, and excellent low-temperature toughness is exhibited.

Therefore, in this embodiment, the steel has the above-mentioned chemical composition and a yield strength of 862 to 1034 MPa, the number density of coarse Al oxides is less ^than 30 particles/200 mm2, and the number density of coarse Si oxides is 5 particles/200 mm2 or ^less . As a result, the steel according to this embodiment can achieve both a yield strength of 125 ksi or more and excellent low-temperature toughness.

The reason why the low-temperature toughness of steel is improved by reducing the number density of coarse Si oxides has not been clarified in detail. However, the present inventors speculate as follows. When manufacturing a steel material having the above-mentioned chemical composition, deoxidation with aluminum (Al) is mainly performed in the steelmaking process. Therefore, in the steel material having the above-mentioned chemical composition, Al oxides represented by Al ₂ O ₃ have been studied, and Si oxides having a small number of oxides have not been paid attention to. However, when the yield strength is increased to 125 ksi or more, there is a possibility that the decrease in low-temperature toughness is likely to be evident not only for coarse Al oxides but also for coarse Si oxides having a small number of oxides. Therefore, the present inventors speculate that, by not only reducing the number density of coarse Al oxides to less than 30/200 mm ² but also reducing the number density of coarse Si oxides to 5/200 mm ² or less, excellent low-temperature toughness can be stably obtained even if the steel material has a yield strength of 125 ksi or more.

It is possible that the low-temperature toughness of the steel material is improved by a mechanism different from that presumed by the present inventors. However, it has been demonstrated by the examples described ^later that in a steel material having the above-mentioned chemical composition, a yield strength of 125 ksi or more, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 , excellent low-temperature toughness can be obtained by setting the number density of coarse Si oxides to 5 particles/200 mm2 or less.

The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows:

[1]
A steel material,
In mass percent,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.50%,
Mo: 0.20-2.00%,
Ti: 0.002 to 0.030%,
Nb: 0.002-0.100%,
B: 0.0005-0.0040%,
N: 0.0100% or less,
O: 0.0040% or less,
V: 0 to 0.30%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0-0.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
The balance is Fe and impurities,
The yield strength is 862 to 1034 MPa;
In the steel material,
The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxides having a major axis of 5.0 μm or more is less than 30 pieces/200 ^mm2 , in terms of mass%,
In terms of mass%, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces/200 ^mm2 or less.
Steel.

[2]
The steel material according to [1],
V: 0.01-0.30%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
W: 0.01-0.50%,
Ca: 0.0001-0.0100%,
Mg: 0.0001 to 0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth elements: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%;
Steel.

[3]
The steel material according to [1] or [2],
The steel material is a seamless steel pipe.
Steel.

The shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate. Note that round bar means a steel bar with a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[Chemical composition]
The chemical composition of the steel material according to this embodiment contains the following elements.

C: 0.15-0.45%
Carbon (C) improves the hardenability of steel and increases its strength. C also promotes the spheroidization of carbides during tempering in the manufacturing process, improving the low-temperature toughness of steel. If the C content is too low, If the C content is too high, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the ranges of this embodiment. Even if the amount of C is less than 100%, the amount of carbides becomes too large, and the low-temperature toughness of the steel material decreases. Therefore, the C content is 0.15 to 0.45%. The preferable lower limit of the C content is 0.18%. The upper limit of the C content is preferably 0.40%, more preferably 0.38%. %, more preferably 0.35%, and even more preferably 0.30%.

Si: 0.05-1.00%
Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. If the amount is too high, even if the contents of other elements are within the range of this embodiment, a large number of coarse Si oxides are generated, which may reduce the low-temperature toughness of the steel material. The lower limit of the Si content is preferably 0.10%, more preferably 0.15%, and even more preferably 0.20%. The upper limit is 0.85%, more preferably 0.75%, even more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.40%.

Mn: 0.05-1.00%
Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel. If the Mn content is too low, the above effects will not be achieved even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, coarse sulfide-based inclusions are generated, and the low-temperature toughness of the steel material is deteriorated. Therefore, the Mn content is 0.05 to 1.00%. The lower limit of the Mn content is preferably 0.06%, more preferably 0.08%, and even more preferably 0.10%. The upper limit of the Mn content is preferably 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and still more preferably 0.80%. is 0.50%, and more preferably 0.45%.

P: 0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content is too high, even if the contents of other elements are within the range of this embodiment, P will segregate at the grain boundaries, and the low-temperature toughness of the steel material will decrease. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%, more preferably 0.020%, more preferably 0.015%, and even more preferably 0.010%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the range of this embodiment, S will segregate at the grain boundaries, and the low-temperature toughness of the steel material will decrease. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, more preferably 0.0020%, and even more preferably 0.0015%. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effects are not sufficiently obtained even if the contents of other elements are within the range of this embodiment, and the low temperature toughness of the steel material is deteriorated. On the other hand, if the Al content is too high, even if the contents of other elements are within the range of this embodiment, a large number of coarse Al oxides are generated, and the low-temperature toughness of the steel material is reduced. The Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.010%, more preferably 0.015%, and further preferably 0.020%. The upper limit of the content is preferably 0.080%, more preferably 0.060%, still more preferably 0.040%, and still more preferably 0.035%. The content of "acid-soluble Al" means the content of "sol. Al".

Cr: 0.30~1.50%
Chromium (Cr) improves the hardenability of steel. Cr also improves the tempering softening resistance of steel and enables high-temperature tempering. As a result, the low-temperature toughness of the steel is improved. If the Cr content is too low, other On the other hand, if the Cr content is too high, the above effect cannot be sufficiently obtained even if the other element contents are within the ranges of this embodiment. However, the low-temperature toughness of the steel material decreases. Therefore, the Cr content is 0.30 to 1.50%. The lower limit of the Cr content is preferably 0.35%, and more preferably 0.40%. The upper limit of the Cr content is preferably 1.40%, more preferably 1.30%, even more preferably 1.20%, and still more preferably 1. It is preferably 10%, and more preferably 1.05%.

Mo: 0.20~2.00%
Molybdenum (Mo) improves the hardenability of steel. Mo also improves the tempering softening resistance of steel and enables high-temperature tempering. As a result, the low-temperature toughness of steel is improved. If the Mo content is too low, other Even if the content of Mo is within the range of this embodiment, the above effect cannot be sufficiently obtained. On the other hand, if the Mo content is too high, the above effect saturates. Therefore, the Mo content is set to 0.20 The lower limit of the Mo content is preferably 0.22%, more preferably 0.25%, even more preferably 0.30%, and still more preferably 0.40%. The upper limit of the Mo content is preferably 1.80%, more preferably 1.50%, and even more preferably 0.60%. The Mo content is preferably 0.60%, more preferably 1.40%, and even more preferably 1.30%. When the yield strength is 945 MPa or more, the lower limit of the Mo content is 0.40%. preferable.

Ti: 0.002-0.030%
Titanium (Ti) combines with N to form nitrides, which refines the grains of steel through a pinning effect, thereby increasing the strength of the steel. If the Ti content is too low, the other element contents On the other hand, if the Ti content is too high, the Ti nitrided steel cannot be obtained sufficiently even if the contents of the other elements are within the range of the present embodiment. The Ti content is preferably 0.002 to 0.030%. The lower limit of the Ti content is preferably 0.003%, and more preferably 0. The upper limit of the Ti content is preferably 0.028%, more preferably 0.025%, still more preferably 0.023%, still more preferably 0.020%, and still more preferably 0.025%. The content is preferably 0.018%, more preferably 0.015%, more preferably 0.010%, and even more preferably 0.008%.

Nb: 0.002-0.100%
Niobium (Nb) combines with C and/or N to form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."). Carbonitrides, etc., have a pinning effect that causes the crystals of steel to be pinned. Nb also forms fine carbides during tempering, improving the tempering softening resistance of steel and increasing the strength of steel. If the Nb content is too low, it will be difficult to obtain the same results as other elements. Even if the content is within the range of this embodiment, the above-mentioned effects cannot be sufficiently obtained. On the other hand, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, Carbonitrides and the like are formed in excess, which reduces the low-temperature toughness of the steel material. Therefore, the Nb content is 0.002 to 0.100%. The preferred lower limit of the Nb content is 0.005%. The content is more preferably 0.010%, more preferably 0.015%, more preferably 0.020%, and even more preferably 0.025%. The upper limit of the Nb content is preferably 0.080%, more preferably 0.060%, further preferably 0.040%, and further preferably 0.035%.

B: 0.0005-0.0040%
Boron (B) dissolves in steel to improve the hardenability of the steel and to increase the strength of the steel. If the B content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned On the other hand, if the B content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are formed, and the low-temperature toughness of the steel material is reduced. Therefore, the B content is 0.0005 to 0.0040%. The lower limit of the B content is preferably 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The upper limit of the B content is preferably 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, and even more preferably It is 0.0015%.

N: 0.0100% or less Nitrogen (N) is inevitably contained. That is, the lower limit of the N content is more than 0%. N combines with Ti to form nitrides, and the grains of the steel are refined by the pinning effect. As a result, the strength of the steel is increased. However, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are formed, and the low-temperature toughness of the steel is reduced. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, more preferably 0.0050%, more preferably 0.0045%, and more preferably 0.0040%. In order to more effectively obtain the above-mentioned effects, the lower limit of the N content is preferably 0.0005%, more preferably 0.0010%, still more preferably 0.0015%, still more preferably 0.0020%, still more preferably 0.0025%, and still more preferably 0.0030%.

O: 0.0040% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is more than 0%. If the O content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxides are formed, and the low-temperature toughness of the steel material decreases. Therefore, the O content is 0.0040% or less. The preferred upper limit of the O content is 0.0035%, more preferably 0.0030%, more preferably 0.0025%, and even more preferably 0.0020%. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

The remainder of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during the industrial production of steel material, and are acceptable to the extent that they do not adversely affect the steel material according to this embodiment.

[Optional element]
The chemical composition of the above-mentioned steel material may further contain V instead of a portion of Fe.

V: 0 to 0.30%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms carbonitrides and the like. Carbonitrides and the like have a pinning effect to refine the grains of the steel material and increase the low-temperature toughness of the steel material. V also forms fine carbides during tempering to increase the tempering softening resistance of the steel material and increase the strength of the steel material. If even a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content is too high, even if the contents of other elements are within the range of this embodiment, excessive carbonitrides and the like are generated, and the low-temperature toughness of the steel material decreases. Therefore, the V content is 0 to 0.30%. The preferred lower limit of the V content is more than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.08%. The preferred upper limit of the V content is 0.25%, more preferably 0.20%, and more preferably 0.15%. When the yield strength is 945 MPa or more, the lower limit of the V content is preferably 0.01%.

The chemical composition of the above-mentioned steel material may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu and Ni. All of these elements are optional elements, and improve the hardenability of the steel material.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hardenability of the steel material and increases the strength of the steel material. If even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content is too high, the hardenability of the steel material is low even if the contents of other elements are within the range of this embodiment. The Cu content is preferably 0% to 0.50%. The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, and even more preferably 0.01%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.25%, and even more preferably 0.15%. %, more preferably 0.10%, and even more preferably 0.05%.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the hardenability of the steel material and increases the strength of the steel material. Ni also dissolves in steel and improves the low-temperature toughness of the steel. Even if even a small amount of Ni is contained, these effects can be obtained to a certain extent. However, if the Ni content is too high, the low-temperature toughness of the steel will be reduced. Even if the amount is within the range of this embodiment, local corrosion is promoted and the low-temperature toughness of the steel material is reduced. Therefore, the Ni content is 0 to 0.50%. The upper limit of the Ni content is preferably 0.30%, more preferably 0.20%. , more preferably 0.10%, and even more preferably 0.05%.

The chemical composition of the above-mentioned steel may further contain W instead of part of the Fe.

W: 0 to 0.50%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When W is contained, W forms a protective corrosion film in a sour environment and suppresses the penetration of hydrogen into the steel material. This increases the low-temperature toughness of the steel material. If even a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content is too high, even if the contents of other elements are within the range of this embodiment, coarse carbides are generated in the steel material, and the low-temperature toughness of the steel material decreases. Therefore, the W content is 0 to 0.50%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the W content is less than 0.50%, and more preferably 0.48%.

The chemical composition of the above-mentioned steel material may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements. All of these elements are optional elements, and render the S in the steel material harmless as sulfides. As a result, these elements increase the low-temperature toughness of the steel material.

Ca: 0~0.0100%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca converts S in the steel into sulfides and renders the steel harmless. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, the steel material The oxides in the steel become coarse, and the low temperature toughness of the steel material decreases. Therefore, the Ca content is 0 to 0.0100%. The lower limit of the Ca content is preferably more than 0%, and more preferably less than 0. The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0025%, and even more preferably 0.0001%, 0.0003%, and even more preferably 0.0006%. The content is preferably 0.0020%, and more preferably 0.0015%.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg converts S in the steel into sulfides to be harmless, and the steel Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the low-temperature toughness of the steel material may be improved. The oxides in the steel become coarse, and the low-temperature toughness of the steel material decreases. Therefore, the Mg content is 0 to 0.0100%. The lower limit of the Mg content is preferably more than 0%, and more preferably 0. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0025%, and even more preferably 0.0001%, 0.0003%, and even more preferably 0.0006%. The content is preferably 0.0020%, and more preferably 0.0015%.

Zr: 0~0.0100%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr renders S in the steel material harmless as sulfides, and the steel material Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, the low-temperature toughness of the steel material may be improved. The oxides in the steel become coarse, and the low temperature toughness of the steel material decreases. Therefore, the Zr content is 0 to 0.0100%. The lower limit of the Zr content is preferably more than 0%, and more preferably 0. The upper limit of the Zr content is preferably 0.0040%, more preferably 0.0025%, and even more preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0006%. The content is preferably 0.0020%, more preferably 0.0015%, and even more preferably 0.0010%.

Rare earth elements (REM): 0 to 0.0100%
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM converts S in the steel into harmless sulfides, REM improves the low-temperature toughness of steel. REM also binds to P in the steel and suppresses the segregation of P at the grain boundaries. Therefore, the decrease in low-temperature toughness of steel caused by the segregation of P is suppressed. If even a small amount of REM is contained, the above effect can be obtained to some extent even if the contents of other elements are within the range of this embodiment. However, if the REM content is too high, the contents of other elements may be insufficient. Even within the range of the embodiment, the oxides in the steel material become coarse and the low-temperature toughness of the steel material decreases. Therefore, the REM content is 0 to 0.0100%. The preferable lower limit of the REM content is It is more than 0%, more preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the REM content is preferably 0.0040%, more preferably 0.0025%, still more preferably 0.0020%, and still more preferably 0.0015%.

In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[Yield strength]
The yield strength of the steel material according to this embodiment is 862 to 1034 MPa (125 to 150 ksi). The yield strength in this specification means the stress at 0.65% elongation (0.65% proof stress) obtained in a tensile test at room temperature (25 ° C.) in accordance with ASTM E8 / E8M (2021) when the yield strength is less than 862 to 945 MPa. The yield strength in this specification means the stress at 0.7% total elongation (0.7% total elongation proof stress) obtained in a tensile test at room temperature (25 ° C.) in accordance with ASTM E8 / E8M (2021) when the yield strength is 945 to 1034 MPa. The steel material according to this embodiment has the above-mentioned chemical composition and satisfies the number density of the coarse Al oxides and the number density of the coarse Si oxides described later, so that it has excellent low-temperature toughness even if the yield strength is 862 to 1034 MPa.

The yield strength of the steel material according to this embodiment is determined by the following method. First, a round bar test piece is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the round bar test piece is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the round bar test piece is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the steel pipe. If the steel material is a round bar, the round bar test piece is prepared from the R/2 position. In this specification, the R/2 position means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the round bar. The size of the round bar test piece is, for example, 8.9 mm in parallel part diameter and 35.6 mm in gauge length.

　Using the prepared round bar test specimen, a tensile test is carried out at room temperature (25°C) in air in accordance with a method conforming to ASTM E8/E8M (2021). If the obtained stress at 0.65% elongation (0.65% yield strength) is less than 862 to 945 MPa, the 0.65% yield strength is defined as the yield strength (MPa). If the obtained stress at 0.7% total elongation (0.7% total elongation yield strength) is 945 to 1034 MPa, the 0.7% total elongation yield strength is defined as the yield strength (MPa). Note that in this embodiment, the yield strength (MPa) is calculated by rounding the obtained value to the nearest tenth.

[Number density of coarse Al oxide particles]
The steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of 862 to 1034 MPa, and furthermore, the number density of the coarse Al oxides is less than 30 particles/200 ^mm2 . As described above, in this specification, particles having an Al content of 20% or more and an O content of 10% or more, in mass%, are also referred to as "Al oxides". As described above, in this specification, Al oxides having a major axis of 5.0 μm or more are also referred to as "coarse Al oxides". In other words, coarse Al oxides refer to particles having an Al content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, in mass%.

As described above, when manufacturing a steel material having the above-mentioned chemical composition, deoxidation is mainly performed with aluminum (Al) in the steelmaking process. Therefore, a large number of Al oxides are likely to be formed in the steel material having the above-mentioned chemical composition. Furthermore, Al oxides are hard oxides and tend to reduce the toughness of the steel material. In particular, when the steel material has a high yield strength of 125 ksi or more, the effect of coarse Al oxides tends to become apparent, and the low-temperature toughness tends to decrease significantly. Therefore, in the steel material according to this embodiment having the above-mentioned chemical composition and a yield strength of 862 to 1034 MPa, the number density of coarse Al oxides is set to less than 30 pieces/200 ^mm2 .

In this embodiment, the preferred upper limit of the number density of the coarse Al oxides is 28 pieces/200 mm ² , more preferably 26 pieces/200 mm ² , more preferably 25 pieces/200 mm ² , and even more preferably 22 pieces/200 mm ² . In this embodiment, the lower limit of the number density of the coarse Al oxides is not particularly limited, and may be 0 pieces/200 mm ² . The lower limit of the number density of the coarse Al oxides may be, for example, 5 pieces/200 mm ² , 7 pieces/200 mm ² , or 9 pieces/200 mm ² . A method for determining the number density of the coarse Al oxides will be described later.

[Number density of coarse Si oxide particles]
The steel material according to the present embodiment has the above-mentioned chemical composition and a yield strength of 862 to 1034 MPa, the number density of the coarse Al oxides is less than 30 pieces/200 ^mm2 , and the number density of the coarse Si oxides is 5 pieces/200 ^mm2 or less. As described above, in this specification, particles having an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more, in mass%, are also referred to as "Si oxides". As described above, in this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides". In other words, coarse Si oxides refer to particles having an Al content of less than 20%, a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, in mass%.

As described above, Si oxides have not been paid much attention to because of their small number. However, when the steel has a high yield strength of 125 ksi or more, not only the coarse Al oxides but also the coarse Si oxides with a small number may easily cause a decrease in low-temperature toughness. Therefore, by making the number density of the coarse Al oxides less than 30 pieces/200 mm ² and making the number density of the coarse Si oxides 5 pieces/200 mm ² or less, it is possible to stably obtain excellent low-temperature toughness even if the yield strength is increased to 125 ksi or more. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of 862 to 1034 MPa, and the number density of the coarse Al oxides in the steel material is less than 30 pieces/200 mm ² , and further, the number density of the coarse Si oxides is 5 pieces/200 mm ² or less.

In this embodiment, the preferred upper limit of the number density of the coarse Si oxides is 4 pieces/200 ^mm2 , and more preferably 3 pieces/200 ^mm2 . In this embodiment, the lower limit of the number density of the coarse Si oxides is not particularly limited, and may be 0 pieces/200 ^mm2 . The lower limit of the number density of the coarse Si oxides may be, for example, 1 piece/200 ^mm2 .

In this embodiment, the number density of coarse Al oxides and the number density of coarse Si oxides in the steel material can be found by the following method. First, a test piece is prepared from the steel material according to this embodiment, with the observation surface being a surface including the rolling direction and the reduction direction. Specifically, if the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a surface including the rolling direction and the plate thickness direction. If the steel material is a steel pipe, a test piece is prepared from the center of the plate thickness, with the observation surface being a surface including the pipe axial direction and the pipe radial direction. If the steel material is round bar, a test piece is prepared that includes the R/2 position in the center, with the observation surface being a surface including the axial direction and the radial direction.

The observation surface of the prepared test piece is polished to a mirror finish, and then the measurement is performed. The area of the observation surface is not limited, but is, for example, 300 mm ² (20 mm × 15 mm). The number of Al oxides with a major axis of 5.0 μm or more is determined on the observation surface. Furthermore, the number of Si oxides with a major axis of 5.0 μm or more is determined on the observation surface. Specifically, the particles on the observation surface are first identified from the contrast. An element concentration analysis (EDS analysis) is performed on each identified particle. In the EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb. Based on the EDS analysis results of each particle, if the Al content is 20% or more and the O content is 10% or more by mass%, the particle is identified as "Al oxide". Based on the EDS analysis results of each particle, if the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more, in mass%, the particle is identified as a "Si oxide".

Among the Al oxides identified on the observation surface, Al oxides with a major axis of 5.0 μm or more (coarse Al oxides) are identified, and the total number of coarse Al oxides is calculated. Furthermore, among the Si oxides identified on the observation surface, Si oxides with a major axis of 5.0 μm or more (coarse Si oxides) are identified, and the total number of coarse Si oxides is calculated. The major axes of Al oxides and Si oxides can be calculated by well-known methods. In this specification, the major axes of Al oxides and Si oxides refer to the longest line segment among the line segments connecting any two points on the periphery of Al oxides and Si oxides on the observation surface.

The number density of the coarse Al oxides (pieces/200 mm ² ) is calculated based on the total number of the coarse Al oxides and the total area of the observation surface. Furthermore, the number density of the coarse Si oxides (pieces/200 mm ² ) is calculated based on the total number of the coarse Si oxides and the total area of the observation surface. In this embodiment, the number density of the coarse Al oxides (pieces/200 mm ² ) and the number density of the coarse Si oxides (pieces/200 mm ² ) are both calculated by rounding off the obtained values to the first decimal place. Furthermore, the number density of the coarse Al oxides and the coarse Si oxides can be measured using a device (SEM-EDS device) in which a composition analysis function is provided to a scanning electron microscope. As the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX) under the product name: Metals Quality Analyzer can be used.

[Low temperature toughness]
The steel material according to the present embodiment has the above-mentioned chemical composition, a yield strength of 862 to 1034 MPa, a number density of coarse Al oxides of less than 30/200 ^mm2 , and a number density of coarse Si oxides of 5/200 ^mm2 or less. As a result, the steel material according to the present embodiment has excellent low-temperature toughness even when the yield strength is 125 ksi or more. In the present embodiment, having excellent low-temperature toughness is determined by a Charpy impact test in accordance with ASTM E23 (2018) described below.

First, a full-size or sub-size V-notch test piece is prepared from the steel material according to this embodiment in accordance with API 5CT (2019). Here, when the steel material is a steel plate, the rolling direction of the steel plate is defined as the "L direction" (Longitudinal), and the plate width direction of the steel plate is defined as the "T direction" (Transverse). When the steel material is a steel pipe, the pipe diameter direction of the steel pipe is defined as the "C direction", the pipe axial direction of the steel pipe is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction". When the steel material is a round bar, the cross-sectional radial direction of the round bar is defined as the "C direction", the axial direction of the round bar is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction".

　A Charpy impact test is performed on the prepared V-notch test specimen in accordance with ASTM E23 (2018). Specifically, the test temperature is set to eight levels (-120°C, -100°C, -80°C, -60°C, -40°C, -20°C, 0°C, and 20°C) that are changed by 20°C in the range of -120 to 20°C. The Charpy impact test is performed using two test specimens for each test temperature. Under the above conditions, the brittle fracture rate (%) of the test specimen after the test at each temperature is obtained. An approximation curve is obtained by plotting the test temperature (°C) and the obtained brittle fracture rate (%). From the obtained approximation curve, the temperature (°C) at which the brittle fracture rate becomes 50% is obtained and defined as the fracture transition temperature (°C). In this embodiment, the fracture transition temperature (°C) is obtained by rounding the obtained value to one decimal place.

In this embodiment, "excellent low-temperature toughness" is defined according to the range of yield strength. Specifically, if the yield strength is less than 862-945 MPa and the fracture transition temperature is -50°C or lower, it is determined to have excellent low-temperature toughness. Also, if the yield strength is 945-1034 MPa and the fracture transition temperature is -40°C or lower, it is determined to have excellent low-temperature toughness.

[Microstructure]
The microstructure of the steel material according to this embodiment has a total volume fraction of tempered martensite and tempered bainite of 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. If the microstructure of the steel material having the above-mentioned chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, it can achieve both a yield strength of 125 ksi or more and excellent low-temperature toughness, provided that other configurations of this embodiment are satisfied. That is, in this embodiment, if the steel material achieves both a yield strength of 125 ksi or more and excellent low-temperature toughness, it is determined that the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

When the volume fraction of tempered martensite and tempered bainite is determined by observation, it can be determined by the following method. First, a test piece having an observation surface is prepared from the steel material according to this embodiment. If the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a surface including the rolling direction and the plate thickness direction. If the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a surface including the pipe axial direction and the pipe radial direction. If the steel material is round bar, a test piece is prepared that includes the R/2 position in the center, with the observation surface being a surface including the axial direction and the radial direction.

The observation surface of the test piece is polished to a mirror finish, and then immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view as secondary electron images using a scanning electron microscope (SEM). The field area is, for example, 0.01 mm ² (magnification 1000 times). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area ratio of the identified tempered martensite and tempered bainite is obtained. The method for obtaining the area ratio is not particularly limited, and a well-known method may be used. For example, the area ratio of tempered martensite and tempered bainite can be obtained by image analysis. In this embodiment, the arithmetic average value of the area ratios of tempered martensite and tempered bainite obtained in all fields of view is defined as the volume ratio of tempered martensite and tempered bainite.

[Production method]
A method for manufacturing a steel material according to this embodiment will be described. Below, a method for manufacturing a seamless steel pipe will be described as an example of a steel material according to this embodiment. The method for manufacturing a seamless steel pipe includes a process for preparing a material (steelmaking process), a process for hot working the material to manufacture a mother pipe (hot working process), and a process for quenching and tempering the mother pipe to produce a seamless steel pipe (quenching process and tempering process). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each process will be described in detail below.

[Steelmaking process]
In the steelmaking process, first, molten pig iron produced by a known method is refined in a converter (primary refining). The molten steel produced by the primary refining is then subjected to secondary refining. In the secondary refining, alloy elements are added to adjust the composition, to produce molten steel that satisfies the above-mentioned chemical composition.

Secondary refining involves, for example, RH (Ruhrstahl-Hausen) vacuum degassing treatment. After that, final adjustment of the alloy components is made. In secondary refining, combined refining may also be performed. In this case, prior to the RH vacuum degassing treatment, for example, a refining treatment using LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed.

　Materials are manufactured using molten steel that has undergone secondary refinement. Specifically, cast pieces (slabs, blooms, or billets) are manufactured by continuous casting using molten steel that has undergone secondary refinement. In continuous casting, molten steel is first poured from a ladle into a tundish. At this time, packing sand is usually enclosed in the nozzle of the ladle to seal it. For this reason, packing sand may be mixed in with the molten steel from the ladle to the tundish. Also, when manufacturing materials having the above-mentioned chemical composition, silicon oxides may be used as packing sand. In this case, there is a concern that silicon oxides may be introduced into the manufactured material.

In this embodiment, therefore, in order to prevent the Si oxide sealed in the nozzle of the ladle from being introduced into the tundish, the molten steel and the Si oxide are separated. The method of separating the Si oxide is not particularly limited, but the following method can be used, for example. A tilted metal plate is placed below the nozzle of the ladle and above the opening of the tundish. When the nozzle of the ladle is opened, the Si oxide is discharged from the nozzle first, followed by the molten steel. Here, the Si oxide is lighter than the molten steel. Therefore, the Si oxide discharged from the nozzle is guided out of the opening of the tundish along the tilt of the metal plate. The tilt of the metal plate may be provided, for example, by placing a metal plate processed into a cone shape without a bottom so that its apex is directly below the nozzle of the ladle, or by other methods. Furthermore, a single metal plate may be used, or multiple metal plates may be used in a stack. Furthermore, the thickness of the metal plate is not particularly limited, but is, for example, about 1 to 10 mm.

After the Si oxides are discharged from the nozzle, the molten steel is discharged. At this time, the molten steel discharged from the nozzle is introduced into the tundish through the opening together with the metal plate. That is, in this embodiment, a part or all of the metal plate may be introduced into the tundish and mixed into the molten steel. Therefore, the metal plate in this embodiment is preferably a metal plate made of alloy elements contained in the molten steel. For example, an aluminum plate can be used as a metal plate made of alloy elements contained in the molten steel. In this specification, an aluminum plate means a metal plate made of aluminum and the remainder made of impurities.

Preferably, after the Si oxide is discharged from the nozzle and before the molten steel is discharged, the metal plate is removed from below the nozzle. In this case, it is possible to prevent the Si oxide adhering to the metal plate from being mixed into the molten steel. The method for removing the metal plate from below the nozzle is not particularly limited, but for example, a hole may be formed in part of the metal plate and the metal plate may be removed using a rod with a hook at its tip. In this case, the hook at the tip of the rod can be hooked into the hole in the metal plate and the rod can be pulled to remove the metal plate. By the above method, the Si oxide can be separated from the molten steel and the molten steel can be introduced into the tundish. The method for separating the Si oxide from the molten steel is not limited to the above method.

Then, the prepared molten steel is cast to produce the material. The casting method is not particularly limited, but may be, for example, a continuous casting method. When producing the material by continuous casting, it is preferable to carry out the following method.

The casting speed in the continuous casting machine is preferably 1.0 to 3.0 m/min. If the casting speed is too slow, an accumulation zone of Al oxides may form in the material. In this case, the produced steel material will contain a large amount of coarse Al oxides, and the low-temperature toughness of the steel material will decrease. On the other hand, if the casting speed is too fast, the Al oxides may not float to the surface, and a large amount of Al oxide may remain in the material. In this case, the produced steel material will contain a large amount of coarse Al oxides, and the low-temperature toughness of the steel material will decrease. Therefore, the casting speed in the continuous casting machine is preferably 1.0 to 3.0 m/min.

When manufacturing a material by continuous casting, it is preferable to further electromagnetically stir the molten steel in the mold. Specifically, by carrying out electromagnetic stirring in the mold at a current value of 330 to 450 A, it becomes difficult for an accumulation band of Al oxide to form in the material. If the current value of the electromagnetic stirring in the mold is too low, the stirring of the molten steel is insufficient, and an accumulation band of Al oxide may form in the material. In this case, the manufactured steel material contains a large amount of coarse Al oxides, and the low-temperature toughness of the steel material decreases. On the other hand, if the current value of the electromagnetic stirring in the mold is too high, the manufacturing equipment may be overloaded. Therefore, in this embodiment, it is preferable to set the current value of the electromagnetic stirring in the mold to 330 to 450 A. Molten steel is cast by the above method to manufacture the material.

[Hot processing process]
In the hot working process, the prepared material is hot worked to produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The method of hot working is not particularly limited, and may be a well-known method.

For example, the Mannesmann process may be carried out as hot working to manufacture a blank tube. In this case, a round billet is pierced and rolled using a piercing machine. When piercing and rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced and rolled round billet is further hot rolled using a mandrel mill, reducer, sizing mill, etc. to produce a blank tube. The cumulative reduction in area during the hot working process is, for example, 20 to 70%.

Other hot working methods may be used to produce a blank pipe from the billet. For example, in the case of short, thick-walled steel material such as a coupling, the blank pipe may be produced by forging using the Erhardt method or the like. The blank pipe is produced by the above process. There are no particular limitations on the thickness of the blank pipe, but it is, for example, 9 to 60 mm.

When the steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot processing to produce intermediate steel material with a circular cross section perpendicular to the axial direction. The hot processing is, for example, blooming using a blooming mill, or hot rolling using a continuous rolling mill. A continuous rolling mill has an alternating arrangement of horizontal stands having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands having a pair of grooved rolls arranged side by side in the horizontal direction.

When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot-rolled using a blooming mill and a continuous rolling mill to produce intermediate steel material in the shape of a steel plate.

　The blank pipe produced by hot working may be air-cooled (as-rolled). The blank pipe produced by hot working may be quenched directly after hot working without being cooled to room temperature, or it may be quenched after being reheated after hot working.

When quenching is performed directly after hot working, or after quenching with additional heating, cooling may be stopped midway through quenching, or slow cooling may be performed. In this case, the occurrence of quench cracks in the blank tube can be suppressed. When quenching is performed directly after hot working, or after quenching with additional heating, stress relief annealing (SR) may also be performed after quenching and before the next heat treatment step. In this case, residual stress in the blank tube is removed.

As described above, in the hot working process, the prepared material is hot worked to produce intermediate steel. The quenching process is described in detail below.

[Quenching process]
In the quenching process, quenching is performed on the prepared intermediate steel material (blank pipe). In this specification, "quenching" means rapidly cooling the intermediate steel material at the _A3 point or higher. The preferred quenching temperature is 800 to 1000°C. If the quenching temperature is too high, the prior γ grains may become coarse, and the low-temperature toughness of the steel material may decrease. Therefore, the quenching temperature is preferably 800 to 1000°C.

In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed at the outlet of the equipment that performs the final hot processing, when quenching is performed directly after hot processing. Furthermore, when quenching is performed after supplementary heating or reheating after hot processing, the quenching temperature corresponds to the temperature of the furnace in which supplementary heating or reheating is performed.

The quenching method, for example, involves continuously cooling the intermediate steel material (bare pipe) from the quenching start temperature, and continuously lowering the surface temperature of the raw pipe. The method of continuous cooling is not particularly limited, and any well-known method may be used. For example, the method of continuous cooling is a method of cooling the raw pipe by immersing it in a water tank, or a method of accelerating cooling the raw pipe by shower water cooling or mist cooling.

If the cooling rate during quenching is too slow, the microstructure will not be mainly martensite and bainite, and the mechanical properties specified in this embodiment (yield strength of 862 to 1034 MPa) will not be obtained. In this case, excellent low-temperature toughness will also not be obtained.

Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel is quenched during quenching. Specifically, in the quenching process, the average cooling rate in the range of the surface temperature of the intermediate steel (blank tube) during quenching from 800 to 500°C is defined as the cooling rate during quenching CR _800-500 . More specifically, the cooling rate during quenching CR _800-500 is determined from the temperature measured at the location that is cooled the slowest in the cross section of the intermediate steel to be quenched (for example, the center of the thickness of the intermediate steel when both surfaces are forcibly cooled).

The cooling rate CR _800-500 during quenching is preferably 300° C./min or more. The lower limit of the cooling rate CR _800-500 during quenching is more preferably 450° C./min, and even more preferably 600° C./min. The upper limit of the cooling rate CR _800-500 during quenching is not particularly specified, but is, for example, 60,000° C./min.

Preferably, the blank tube is heated in the austenite region multiple times and then quenched. In this case, the austenite grains before quenching are refined, thereby improving the low-temperature toughness of the steel. By quenching multiple times, heating in the austenite region may be repeated multiple times, or by normalizing and quenching, heating in the austenite region may be repeated multiple times. Quenching and tempering, which will be described later, may also be combined and performed multiple times. In other words, quenching and tempering may be performed multiple times. In this case, the low-temperature toughness of the steel is further improved. The tempering process is described in detail below.

[Tempering process]
In the tempering process, the blank pipe that has been quenched is tempered. In this specification, "tempering" means reheating the quenched intermediate steel material at a temperature lower than the A _c1 point and holding the material at that temperature. The tempering temperature corresponds to the furnace temperature when the quenched intermediate steel material is heated and held at that temperature. The tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature to when the intermediate steel material is extracted from the heat treatment furnace.

The tempering temperature is adjusted appropriately depending on the chemical composition of the seamless steel pipe and the yield strength to be obtained. In other words, for a blank pipe having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the seamless steel pipe to 862 to 1034 MPa. It is of course possible for a person skilled in the art to adjust the yield strength of the seamless steel pipe to 862 to 1034 MPa by adjusting the tempering temperature. Specifically, in the tempering process according to this embodiment, the preferred tempering temperature is 580 to 690°C.

If the tempering time is too short, a microstructure mainly composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effects saturate. Therefore, in the tempering process of this embodiment, the tempering time is preferably 10 to 90 minutes. A more preferable lower limit of the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 80 minutes.

The steel material according to this embodiment can be manufactured by the above manufacturing method. Note that in the above manufacturing method, a method for manufacturing a seamless steel pipe has been described as one example. However, the steel material according to this embodiment may be a steel plate or other shape. Similar to the above manufacturing method, a manufacturing method for a steel plate or other shape also includes, for example, a preparation step, a quenching step, and a tempering step. Furthermore, the above manufacturing method is one example, and the steel material may be manufactured by other manufacturing methods.

The present invention will be explained in more detail below with reference to examples.

In Example 1, steel materials with a yield strength of 862 to less than 945 MPa were investigated. Specifically, molten steel was produced having the chemical composition shown in Tables 1-1 and 1-2. Note that "-" in Table 1-2 means that the content of each element is at the impurity level. Specifically, the V content, Cu content, Ni content, and W content of Steel A were rounded to the nearest two decimal places and were 0%. Furthermore, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel A were rounded to the nearest five decimal places and were 0%.

The above molten steel was used to manufacture a round billet by continuous casting. In the continuous casting process, when molten steel was poured from the ladle into the tundish, a metal plate processed into a bottomless cone shape was placed above the opening of the tundish so that its apex was directly below the nozzle of the ladle. Table 2 shows whether or not a metal plate of the above shape was placed above the opening of the tundish. Specifically, when a metal plate of the above shape was placed above the opening of the tundish, "A" is indicated in the "Metal Plate" column of Table 2. When a metal plate of the above shape was not placed above the opening of the tundish, "B" is indicated in the "Metal Plate" column of Table 2. The metal plate of the above shape placed above the opening of the tundish was an aluminum plate. Specifically, three aluminum plates with a thickness of 2 mm were used in a stack. When a metal plate was placed, after the silicon oxide was discharged from the nozzle and before the molten steel was discharged, the metal plate was removed from below the nozzle using a rod with a hook formed at its tip. Furthermore, the molten steel was cast into a round billet at the casting speed shown in Table 2. At this time, electromagnetic stirring was performed in the mold at the current value shown in Table 2.

The manufactured round billets of test numbers 1 to 20 were held at 1250°C for 1 hour, and then hot rolling was performed by the Mannesmann-mandrel method to manufacture mother pipes (seamless steel pipes) of test numbers 1 to 20. Furthermore, the obtained mother pipes of test numbers 1 to 20 were quenched. Specifically, the mother pipes of test numbers 1 to 20 were held at the temperature (°C) for the time (minutes) shown in the "Quenching process" column in Table 2, and then quenched by shower water cooling. In addition, the cooling rate during quenching CR _800-500 in test numbers 1 to 20 was in the range of 480 to 30000°C/min. Here, the quenching temperature (°C) shown in Table 2 was the temperature (°C) of the heat treatment furnace in which the mother pipe was heated. Furthermore, the quenching time (minutes) shown in Table 2 was the time (minutes) during which the mother pipe was held at the quenching temperature.

The obtained blank pipes of test numbers 1 to 20 were tempered. Specifically, blank pipes of test numbers 1 to 20 were tempered by holding them at the temperature (°C) for the time (minutes) listed in the "Tempering process" column of Table 2. Here, the tempering temperature (°C) listed in Table 2 is the temperature (°C) of the tempering furnace in which the blank pipes were heated. Furthermore, the tempering time (minutes) listed in Table 2 is the time (minutes) the blank pipes were held at the tempering temperature. Through the above manufacturing process, seamless steel pipes of test numbers 1 to 20 were obtained.

[Evaluation test]
The seamless steel pipes of test numbers 1 to 20 after tempering were subjected to a tensile test, a test for measuring the number density of coarse Al oxides and coarse Si oxides, and a Charpy impact test, which will be described below.

[Tensile test]
A tensile test was performed on the seamless steel pipes of test numbers 1 to 20 to determine the yield strength. The tensile test was performed in accordance with ASTM E8/E8M (2021). Round bar test pieces with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the center of the wall thickness of the seamless steel pipes of test numbers 1 to 20. The axial direction of the round bar test pieces was parallel to the axial direction of the seamless steel pipes. Using the prepared round bar test pieces, a tensile test was performed at room temperature (25 ° C.) in air to obtain the yield strength (MPa) of the seamless steel pipes of test numbers 1 to 20. In this example, the stress at 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. The obtained yield strength (MPa) is shown in Table 3 as "YS (MPa)".

[Measurement test of number density of coarse Al oxides and coarse Si oxides]
A number density measurement test of coarse Al oxides and coarse Si oxides was carried out on the seamless steel pipes of test numbers 1 to 20 to determine the number density of Al oxides (coarse Al oxides) having a major axis of 5.0 μm or more and Si oxides (coarse Si oxides) having a major axis of 5.0 μm or more. The number densities of coarse Al oxides and coarse Si oxides were determined by the above-mentioned method using test pieces prepared from the central part of the wall thickness of the seamless steel pipes of test numbers 1 to 20. The number densities (pieces/200 mm ² ) of the coarse Al oxides thus obtained are shown in the "coarse Al oxides (pieces/200 mm ² )" column of Table 3. The number densities (pieces/200 mm ² ) of the coarse Si oxides thus obtained are shown in the "coarse Si oxides (pieces/200 mm ² )" column of Table 3.

[Charpy impact test]
A Charpy impact test was performed on the seamless steel pipes of test numbers 1 to 20 to evaluate the low temperature toughness. A full-size V-notch test piece was prepared from the center of the wall thickness of the seamless steel pipes of test numbers 1 to 20. The longitudinal direction of the test piece was parallel to the circumferential direction of the pipe. The circumferential direction of the pipe means a direction perpendicular to both the axial direction and the radial direction of the seamless steel pipe. The notch surface of the test piece was perpendicular to the axial direction of the seamless steel pipe. A Charpy impact test was performed under the above conditions in accordance with ASTM E23 (2018) to determine the brittle fracture rate (%) of test numbers 1 to 20. The temperature (°C) at which the brittle fracture rate becomes 50% was determined from the approximation curve of the plot of the test temperature (°C) and the brittle fracture rate (%), and was taken as the fracture transition temperature (°C). The obtained fracture transition temperatures (°C) are shown in Table 3.

[Evaluation Results]
With reference to Tables 1-1, 1-2, 2, and 3, the seamless steel pipes of test numbers 1 to 12 had appropriate chemical compositions, and the manufacturing methods also satisfied the above-mentioned preferred conditions. As a result, the seamless steel pipes had a yield strength of 862 to less than 945 MPa, a number density of coarse Al oxides of less than 30 pieces/200 ^mm2 , and a number density of coarse Si oxides of 5 pieces/200 mm2 or less. As a result, the seamless steel pipes had a fracture transition temperature of -50°C or ^less in the Charpy impact test. That is, the seamless steel pipes of test numbers 1 to 12 had a yield strength of 862 to less than 945 MPa and excellent low-temperature toughness. It was determined that the sum of the volume fractions of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was 90% or more.

On the other hand, the casting speed in the steelmaking process of the seamless steel pipes of test numbers 13 and 14 was too fast. As a result, the number density of coarse Al oxides in these seamless steel pipes was 30/200 mm2 or ^more . As a result, the fracture transition temperature of these seamless steel pipes exceeded -50°C in the Charpy impact test, and they did not have excellent low-temperature toughness.

For the seamless steel pipes of test numbers 15 to 17, no metal plate was used in the steelmaking process. As a result, the number density of coarse Si oxides in these seamless steel pipes exceeded 5 pieces/200 ^mm2 . As a result, the fracture appearance transition temperature of these seamless steel pipes exceeded -50°C in the Charpy impact test, and they did not have excellent low-temperature toughness.

The seamless steel pipe of test number 18 had too high an O content. As a result, the fracture transition temperature of this seamless steel pipe exceeded -50°C in the Charpy impact test, and it did not have excellent low-temperature toughness.

The seamless steel pipe of test number 19 had too low a Mo content. As a result, the fracture transition temperature of this seamless steel pipe exceeded -50°C in the Charpy impact test, and it did not have excellent low-temperature toughness.

The seamless steel pipe of test number 20 had too high a S content. As a result, the fracture transition temperature of this seamless steel pipe exceeded -50°C in the Charpy impact test, and it did not have excellent low-temperature toughness.

In Example 2, steel materials with yield strengths of 945 to 1034 MPa were investigated. Specifically, molten steels were produced having the chemical compositions shown in Tables 4-1 and 4-2. Note that "-" in Table 4-2 means that the content of each element is at the impurity level. Specifically, the Cu content, Ni content, and W content of Steel A were rounded to the nearest two decimal places and were 0%. Furthermore, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel A were rounded to the nearest five decimal places and were 0%.

Similar to Example 1, a round billet was produced by continuous casting using the above molten steel. In the continuous casting method, when pouring molten steel from the ladle into the tundish, a metal plate processed into a bottomless cone shape was placed above the opening of the tundish so that its apex was directly below the nozzle of the ladle. Table 5 shows whether or not a metal plate of the above shape was placed above the opening of the tundish. Specifically, when a metal plate of the above shape was placed above the opening of the tundish, "A" is indicated in the "Metal Plate" column of Table 5. When a metal plate of the above shape was not placed above the opening of the tundish, "B" is indicated in the "Metal Plate" column of Table 5. The metal plate of the above shape placed above the opening of the tundish was an aluminum plate. Specifically, three aluminum plates with a thickness of 2 mm were used in a stack. When a metal plate was placed, after the Si oxide was discharged from the nozzle and before the molten steel was discharged, the metal plate was removed from below the nozzle using a rod with a hook formed at its tip. Furthermore, the molten steel was cast into a round billet at the casting speed shown in Table 5. At this time, electromagnetic stirring was performed in the mold at the current value shown in Table 5.

As in Example 1, the manufactured round billets of test numbers 21 to 38 were held at 1250°C for 1 hour, and then hot rolling was performed by the Mannesmann-mandrel method to manufacture mother pipes (seamless steel pipes) of test numbers 21 to 38. Furthermore, the obtained mother pipes of test numbers 21 to 38 were quenched. Specifically, the mother pipes of test numbers 21 to 38 were held at the temperature (°C) for the time (minutes) shown in the "Quenching process" column in Table 5, and then quenched by shower water cooling. In addition, the cooling rate during quenching CR _800-500 in test numbers 21 to 38 was in the range of 480 to 30,000°C/min. Here, the quenching temperature (°C) shown in Table 5 was the temperature (°C) of the heat treatment furnace in which the mother pipe was heated. Furthermore, the quenching time (minutes) shown in Table 5 was the time (minutes) during which the mother pipe was held at the quenching temperature.

The obtained blank pipes of test numbers 21 to 38 were tempered in the same manner as in Example 1. Specifically, blank pipes of test numbers 21 to 38 were tempered by holding them at the temperature (°C) for the time (minutes) listed in the "Tempering process" column of Table 5. Here, the tempering temperature (°C) listed in Table 5 is the temperature (°C) of the tempering furnace in which the blank pipes were heated. Furthermore, the tempering time (minutes) listed in Table 5 is the time (minutes) during which the blank pipes were held at the tempering temperature. Through the above manufacturing process, seamless steel pipes of test numbers 21 to 38 were obtained.

[Evaluation test]
The seamless steel pipes of test numbers 21 to 38 after tempering were subjected to a tensile test, a test for measuring the number density of coarse Al oxides and coarse Si oxides, and a Charpy impact test, which will be described below.

[Tensile test]
A tensile test was performed on the seamless steel pipes of test numbers 21 to 38 to determine the yield strength. The tensile test was performed in accordance with ASTM E8/E8M (2021). Round bar test pieces with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the center of the wall thickness of the seamless steel pipes of test numbers 21 to 38. The axial direction of the round bar test pieces was parallel to the pipe axial direction of the seamless steel pipes. Using the prepared round bar test pieces, a tensile test was performed at room temperature (25 ° C.) in air to obtain the yield strength (MPa) of the seamless steel pipes of test numbers 21 to 38. In this example, the stress at 0.7% total elongation (0.7% total elongation yield strength) obtained in the tensile test was defined as the yield strength. The obtained yield strength (MPa) is shown in Table 6 as "YS (MPa)".

[Measurement test of number density of coarse Al oxides and coarse Si oxides]
A number density measurement test of coarse Al oxides and coarse Si oxides was carried out on the seamless steel pipes of test numbers 21 to 38 to determine the number density of Al oxides (coarse Al oxides) with a major axis of 5.0 μm or more and Si oxides (coarse Si oxides) with a major axis of 5.0 μm or more. The number densities of coarse Al oxides and coarse Si oxides were determined by the above-mentioned method using test pieces prepared from the central part of the wall thickness of the seamless steel pipes of test numbers 21 to 38. The number densities (pieces/200 mm ² ) of the coarse Al oxides thus obtained are shown in the "Coarse Al oxides (pieces/200 mm ² )" column of Table 6. The number densities (pieces/200 mm ² ) of the coarse Si oxides thus obtained are shown in the "Coarse Si oxides (pieces/200 mm ² )" column of Table 6.

[Charpy impact test]
A Charpy impact test was performed on the seamless steel pipes of test numbers 21 to 38 to evaluate their low-temperature toughness. Full-size V-notch test pieces were prepared from the center of the wall thickness of the seamless steel pipes of test numbers 21 to 38. The longitudinal direction of the test piece was parallel to the circumferential direction of the pipe. The circumferential direction of the pipe means a direction perpendicular to both the axial direction and the radial direction of the seamless steel pipe. The notch surface of the test piece was perpendicular to the axial direction of the seamless steel pipe. A Charpy impact test was performed under the above conditions in accordance with ASTM E23 (2018) to determine the brittle fracture rate (%) of test numbers 21 to 38. The temperature (°C) at which the brittle fracture rate becomes 50% was determined from the approximation curve of the plot of the test temperature (°C) and the brittle fracture rate (%), and was taken as the fracture transition temperature (°C). The obtained fracture transition temperatures (°C) are shown in Table 6.

[Evaluation Results]
With reference to Tables 4-1, 4-2, 5, and 6, the seamless steel pipes of test numbers 21 to 32 had appropriate chemical compositions, and the manufacturing methods also satisfied the above-mentioned preferred conditions. As a result, these seamless steel pipes had a yield strength of 945 to 1034 MPa, a number density of coarse Al oxides of less than 30 pieces/200 ^mm2 , and a number density of coarse Si oxides of 5 pieces/200 mm2 or ^less . As a result, these seamless steel pipes had a fracture transition temperature of -40°C or less in a Charpy impact test. That is, the seamless steel pipes of test numbers 21 to 32 had a yield strength of 945 to 1034 MPa and excellent low-temperature toughness. It was determined that the sum of the volume fractions of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was 90% or more.

On the other hand, the casting speed in the steelmaking process of the seamless steel pipes of test numbers 33 and 34 was too fast. As a result, the number density of coarse Al oxides in these seamless steel pipes was 30/200 mm2 or ^more . As a result, the fracture transition temperature of these seamless steel pipes exceeded -40°C in the Charpy impact test, and they did not have excellent low-temperature toughness.

For the seamless steel pipes of test numbers 35 and 36, no metal plate was used in the steelmaking process. As a result, the number density of coarse Si oxides in these seamless steel pipes exceeded 5 pieces/200 ^mm2 . As a result, the fracture appearance transition temperature of these seamless steel pipes exceeded -40°C in the Charpy impact test, and they did not have excellent low-temperature toughness.

The seamless steel pipe of test number 37 had too high an O content. As a result, the fracture transition temperature of this seamless steel pipe exceeded -40°C in the Charpy impact test, and it did not have excellent low-temperature toughness.

The seamless steel pipe of test number 38 had too high a S content. As a result, the fracture transition temperature of this seamless steel pipe exceeded -40°C in the Charpy impact test, and it did not have excellent low-temperature toughness.

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims

A steel material,
In mass percent,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.50%,
Mo: 0.20-2.00%,
Ti: 0.002 to 0.030%,
Nb: 0.002 to 0.100%,
B: 0.0005-0.0040%,
N: 0.0100% or less,
O: 0.0040% or less,
V: 0-0.30%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0-0.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
The balance is Fe and impurities,
The yield strength is 862 to 1034 MPa;
In the steel material,
The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxides having a major axis of 5.0 μm or more is less than 30 pieces/200 mm2 , in terms of mass%,
In terms of mass%, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces/200 mm2 or less.
Steel.
The steel material according to claim 1,
V: 0.01-0.30%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
W: 0.01-0.50%,
Ca: 0.0001-0.0100%,
Mg: 0.0001 to 0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth elements: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%;
Steel.
The steel material according to claim 1 or 2,
The steel material is a seamless steel pipe.
Steel.