WO2018117228A1

WO2018117228A1 - H-steel and method for manufacturing same

Info

Publication number: WO2018117228A1
Application number: PCT/JP2017/045965
Authority: WO
Inventors: 昌毅溝口; 市川　和利; 杉山　博一; 徹哉清家
Original assignee: 新日鐵住金株式会社
Priority date: 2016-12-21
Filing date: 2017-12-21
Publication date: 2018-06-28
Also published as: EP3533893A4; KR20190032625A; JP6468408B2; KR102021726B1; JPWO2018117228A1; EP3533893A1; US20190203309A1; PH12019500350A1; CN109715842B; CN109715842A

Abstract

Provided is an H-steel having a chemical composition containing C, Si, Mn, Nb, V, Ti, and N, and having a microstructure in which the area ratio of ferrite is at least 60% and less than 100%, the average grain size of the ferrite being 1-30 µm and the thickness of the ferrite being 20-140 mm, wherein the tensile yield stress is 385-530 MPa, and the Charpy absorbed energy at -20°C is at least 100 J.

Description

H-section steel and its manufacturing method

The present invention relates to a thick H-section steel excellent in strength and low-temperature toughness and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2016-248181 filed in Japan on December 21, 2016, the contents of which are incorporated herein by reference.

In recent years, buildings such as high-rise buildings are becoming larger and taller, and thick steel materials are used as strength members necessary for the structure. However, in general, as the thickness of a steel material increases, it becomes more difficult to ensure strength and toughness becomes more difficult.

With respect to such a problem, Patent Document 1 obtains a steel material that secures high strength by applying accelerated cooling while securing toughness by utilizing the refinement effect of prior austenite grains by Ca—Al-based oxides. Technology has been proposed.

Patent Document 2 proposes a technique for obtaining a steel material that secures high strength by applying accelerated cooling while securing toughness using the refinement effect of prior austenite grains due to Mg-S inclusions. Yes.

However, when manufacturing thick steel plates, if accelerated cooling is applied after hot rolling, the cooling rate is slower than the surface inside the steel plate, and there is a large difference in the temperature history during cooling between the surface and inside, Differences in mechanical properties such as strength, ductility, and toughness occur depending on the location of the steel material.

Also, the use of thick H-section steel is desired for large buildings, but this H-section has a unique shape. Universal rolling or the like is applied to form a steel slab into an H shape, but the rolling conditions (temperature, rolling reduction) are limited in universal rolling. Therefore, when manufacturing an H-section steel, especially when manufacturing a thick H-section steel having a flange thickness of 20 mm or more, the mechanical characteristics are controlled as compared with a general thick steel sheet (thick steel sheet). It is not easy.

For such problems, Patent Documents 3 and 4 propose a method of reducing the amount of C and hot-rolling a steel piece to which B is added and then allowing it to cool to ensure uniform mechanical properties. Yes.
Patent Documents 5 to 8 disclose thick H-shaped steels or methods for producing H-shaped steels for the purpose of high strength, high toughness and the like.

Japanese Patent No. 5655984 Japanese Patent No. 5867651 Japanese Laid-Open Patent Publication No. 2003-328070 Japanese Unexamined Patent Publication No. 2011-106006 Japanese Laid-Open Patent Publication No. 11-158543 Japanese Unexamined Patent Publication No. 11-335735 Japanese Unexamined Patent Publication No. 2016-84524 Japanese Unexamined Patent Publication No. 10-68016

Conventionally, it has been difficult to control the mechanical properties of thick H-section steels with a flange thickness of 20 mm or more, so such thick H-section steels have toughness at room temperature or at most 0 ° C. It was only required to satisfy. However, in recent years, in consideration of use in cold districts and the like, thick H-section steels are required to have excellent toughness at lower temperatures. In addition, considering the strength per unit weight as a structural material, it is also required that the yield stress (specifically, yield strength or 0.2% proof stress) be 385 MPa or more for thick H-section steel. Has been.

The present invention has been made in view of such a situation, and an object thereof is to provide a thick H-section steel excellent in strength and low-temperature toughness and a method for producing the same.

The gist of the present invention is as follows.
(1) In the H-section steel according to one aspect of the present invention, the steel is, as a chemical component, in mass%, C: 0.05 to 0.160%, Si: 0.01 to 0.60%, Mn: 0.80 to 1.70%, Nb: 0.005 to 0.050%, V: 0.05 to 0.120%, Ti: 0.001 to 0.025%, N: 0.0001 to 0. 0120%, Cr: 0 to 0.30%, Mo: 0 to 0.20%, Ni: 0 to 0.50%, Cu: 0 to 0.35%, W: 0 to 0.50%, Ca: 0 to 0.0050%, Zr: 0 to 0.0050%, Al: 0.10% or less, B: 0.0003% or less, the balance being Fe and impurities, Ceq = C + Mn / When 6+ (Cr + Mo + V) / 5 + (Ni + Cu) / 15, C, Mn, Cr, Mo, V, Ni, Cu in the chemical component are .30 ≦ Ceq ≦ 0.48, the steel has an area fraction of ferrite of 60 to less than 100% as a metal structure, and a mixed structure MA of martensite and austenite is 3.0% or less. The structure other than the ferrite and the MA is limited to 37% or less, the average grain size of the ferrite is 1 to 30 μm, and the shape is H when the steel is viewed in a cross section perpendicular to the rolling direction. And the thickness of the flange is 20 to 140 mm, and when the length in the width direction of the flange is F, the tensile yield stress is 385 at a position of (1/6) F from the end surface in the width direction of the flange. ˜530 MPa, the maximum tensile strength is 490 to 690 MPa, and the thickness of the flange is t ₂ , the position of (1/6) F and the outer surface in the thickness direction of the flange (1/4) ) At ₂ position, the absorbed energy of the Charpy test at -20 ° C. is at least 100 J.
(2) In the H-section steel described in (1) above, the steel may contain Nb: more than 0.02 to 0.050% by mass as the chemical component.
(3) In the H-section steel according to (1) or (2), the steel may contain N: more than 0.005 to 0.0120% by mass as the chemical component.
(4) In the H-section steel according to any one of (1) to (3), the steel may be limited to less than 0.03% by mass as the chemical component in terms of mass%. .
(5) In the H-section steel according to any one of the above (1) to (4), the steel may be limited to less than 0.003% Al by mass% as the chemical component. .
(6) In the H-section steel according to any one of (1) to (5) above, the thickness of the flange may be 25 to 140 mm.
(7) A method for producing an H-section steel according to an aspect of the present invention is the method for producing an H-section steel according to any one of (1) to (6) above, wherein the (1) to ( 5) A steel making process for obtaining molten steel having the chemical component according to any one of 5), a casting process for obtaining a steel slab by casting the molten steel after the steel making process, and 1100 for the steel slab after the casting process. From the end face in the width direction of the flange so that the shape when viewed in a cut surface perpendicular to the rolling direction is H-shaped with respect to the heating step of heating to ˜1350 ° C. and the steel slab after the heating step ( 1/6) Cumulative rolling reduction at the position of F is more than 20% at over 900 ° C. to 1100 ° C., cumulative rolling reduction at the above position is at least 15% at 730 to 900 ° C., rolling at 730 ° C. or more A hot rolling process in which rolling is performed under conditions to end the cooling, and cooling to cool the hot-rolled material after the hot rolling process Includes a degree, the.

According to the above aspect of the present invention, it is possible to provide a thick H-section steel excellent in strength and low temperature toughness and a method for producing the same.

It is a cross-sectional schematic diagram explaining the position which extract | collects the test piece of H-section steel which concerns on one Embodiment of this invention. It is a flowchart which shows the manufacturing method of the H-section steel which concerns on one Embodiment of this invention.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention. Moreover, a lower limit value and an upper limit value are included in the numerical limit range described below. Numerical values indicating “over” or “less than” are not included in the numerical range. “%” Regarding the content of each element means “mass%”.

As described above, until now, a thick H-section steel having a flange thickness of 20 mm or more has been required to have toughness at room temperature or at most 0 ° C. However, at present, considering the use in cold districts, etc., thick H-section steel is required to have excellent toughness at a lower temperature of about −20 ° C. In addition, considering the strength per unit weight as a structural material, it is also required that the yield stress (specifically, yield strength or 0.2% proof stress) be 385 MPa or more for thick H-section steel. Is done.

Therefore, the present inventors have investigated the steel composition that affects the strength and low temperature toughness with respect to thick H-section steel (hereinafter sometimes referred to as “steel material”), particularly with respect to the flange, which is an important part in the structure of H-section steel. We investigated the effects of the chemical composition of steel) and the steel structure (steel metal structure), and obtained the following knowledge. In this embodiment, the strength means the tensile yield stress and the maximum tensile strength, and the low temperature toughness means the absorbed energy of the Charpy test at −20 ° C.

First, an excessive increase in hardenability due to the addition of alloying elements promotes the formation of a martensite-austenite mixed structure (hereinafter referred to as MA) in the steel material, leading to a decrease in low-temperature toughness. In particular, B tends to promote the formation of MA among the alloy elements. Therefore, it is effective to limit B to an impurity level or less without positively adding B.

Also, Nb is effective to achieve high yield stress (yield strength or 0.2% yield strength) and at the same time improve the toughness at −20 ° C. Since Nb increases the strength of the steel material through precipitation strengthening, it is not necessary to excessively increase the hardenability, and the strength of the steel material can be increased without promoting the formation of MA. Nb also has the effect of suppressing recrystallization of austenite during hot rolling, accumulating strain in the steel material due to rolling, and reducing the ferrite grain size after transformation.

Further, in order to improve the toughness at −20 ° C., addition of V is effective. V precipitates as carbonitride (VC, VN, or a composite thereof) and functions as a nucleation site of ferrite, and has the effect of causing finer ferrite.

Also, the addition of Mn further improves strength and low temperature toughness. In addition, controlling the steel composition and controlling the ferrite area fraction, the MA area fraction, the average crystal grain size of ferrite, etc. as the steel structure can achieve both high strength and low temperature toughness. Is important.

In order to stably control the steel structure, it is necessary to give sufficient rolling strain in the recrystallization temperature range and the non-recrystallization temperature range of austenite when hot-rolling the steel slab with controlled steel composition. is necessary. Specifically, hot rolling with a cumulative reduction rate of 20% or more is performed in a temperature range from 900 ° C. to 1100 ° C., and further, a hot rolling with a cumulative reduction rate of 15% or more in a temperature range of 900 ° C. or less. I do. By rolling above 900 ° C., the austenite grains are refined and hardenability is lowered to reduce the amount of MA produced, etc. Increase the nucleation frequency to refine ferrite.

Also, in order to stably control the steel structure, it is preferable that the cooling rate difference between the surface and the inside of the steel material is small when cooling after hot rolling. When the steel material is allowed to cool without accelerated cooling after hot rolling, the cooling rate is reduced on the surface and inside of the steel material, and the difference is also reduced. For example, in an H-section steel having a flange thickness of 20 mm, when the steel material is allowed to cool after hot rolling, the average cooling rate on the surface and inside of the steel material from 800 ° C. to 500 ° C. is 1 ° C./second or less.

When the cooling rate after hot rolling is slow, it is generally not easy to ensure yield stress and low temperature toughness at the same time. However, it is possible to achieve both yield stress and low temperature toughness by optimally controlling the steel components and production conditions. For example, as a steel component, the C content is 0.05% to 0.160%, B is not added, it is limited to the impurity level or less, Nb and V are actively added, Mn, Ti, N, etc. The alloy element content is appropriately controlled, and the carbon equivalent Ceq is controlled within the range of 0.30 to 0.48. In addition, the manufacturing conditions are optimally controlled to create the ferrite area fraction, the MA area fraction, the average grain size of ferrite, and the like as the steel structure. As a result, it is possible to obtain a thick H-section steel having excellent strength and low temperature toughness.

Hereinafter, the H-section steel according to this embodiment will be described. First, the steel composition and the reasons for limitation will be described in detail.

The H-section steel according to the present embodiment includes a basic element as a chemical component, includes a selection element as necessary, and the balance is composed of Fe and impurities.

Among the chemical components of the H-section steel according to this embodiment, C, Si, Mn, Nb, V, Ti, and N are basic elements (main alloying elements).

(C: 0.05-0.160%)
C (carbon) is an element effective for strengthening steel. Therefore, the lower limit for the C content is 0.05%. Preferably, the lower limit of the C content is 0.060%, 0.070%, or 0.080%. On the other hand, when the C content exceeds 0.160%, the low temperature toughness is reduced. Therefore, the upper limit of C content is 0.160%. In order to further improve the low temperature toughness, the upper limit of the C content is preferably set to 0.140%, 0.130%, or 0.120%.

(Si: 0.01-0.60%)
Si (silicon) is a deoxidizing element and is an element that contributes to an improvement in strength. Therefore, the lower limit for the Si content is 0.01%. Preferably, the lower limit of the Si content is 0.05%, 0.10%, or 0.15%. On the other hand, when the Si content exceeds 0.60%, the formation of MA is promoted and the low temperature toughness is reduced. Therefore, the upper limit of Si content is 0.60%. In order to further improve the low temperature toughness, the upper limit of the Si content is preferably set to 0.40% or 0.30%.

(Mn: 0.80 to 1.70%)
Mn (manganese) is an element that contributes to improvement in strength. Therefore, the lower limit of the Mn content is 0.80%. In order to increase the strength, the lower limit of the Mn content is preferably set to 1.0%, 1.1%, or 1.2%. On the other hand, if the Mn content exceeds 1.70%, the hardenability is excessively increased, the formation of MA is promoted, and the low temperature toughness is impaired. Therefore, the upper limit of the Mn content is 1.70%. Preferably, the upper limit of the Mn content is 1.60% or 1.50%.

(Nb: 0.005 to 0.050%)
Nb (niobium) is an element that suppresses recrystallization of austenite during hot rolling, contributes to finer ferrite by accumulating processing strain in the steel, and further contributes to improvement of strength by precipitation strengthening. is there. Therefore, the lower limit of the Nb content is set to 0.005%. Preferably, the lower limit of the Nb content is 0.010%, more than 0.020%, 0.025%, or 0.030%. However, if the Nb content exceeds 0.050%, a significant decrease in low temperature toughness may be caused. Therefore, the upper limit of Nb content is 0.050%. Preferably, the upper limit of the Nb content is 0.045%, 0.043%, or 0.040%. When Nb is not added intentionally, the Nb content contained as an impurity is less than 0.005%. In order to make the Nb content 0.005% or more, Nb is intentionally contained in the steel.

(V: 0.05-0.120%)
V (vanadium) is an element that has the effect of precipitating as a carbonitride in the austenite grains, acting as a transformation nucleus to ferrite, and refining the ferrite grains. Therefore, the lower limit of V content is 0.05%. Preferably, the lower limit of the V content is more than 0.05%, 0.06%, or 0.07%. However, if the V content exceeds 0.120%, the low temperature toughness may be impaired due to coarsening of precipitates. Therefore, the upper limit of V content is 0.120%. Preferably, the upper limit of the V content is 0.110% or 0.100%.

(Ti: 0.001 to 0.025%)
Ti (titanium) is an element that forms TiN and fixes N in steel. Therefore, the lower limit of the Ti content is set to 0.001%. In order to further refine austenite by the pinning effect of TiN, the lower limit of the Ti content is preferably set to 0.005%, 0.007%, or 0.010%. On the other hand, if the Ti content exceeds 0.025%, coarse TiN is generated and the low temperature toughness is impaired. Therefore, the upper limit of the Ti content is 0.025%. Preferably, the upper limit of the Ti content is 0.020%, 0.015%, or 0.012%.
Further, when Al is not positively added, Ti works as a deoxidizing element, so that N that does not bind to Ti is generated. However, this N is deposited as V carbonitrides using Ti oxide as a nucleus. That is, when Ti acts as a deoxidizing element and Ti oxide is precipitated, precipitation of V carbonitride is promoted, and low temperature toughness can be improved.

(N: 0.0001 to 0.0120%)
N (nitrogen) is an element that forms TiN and VN and contributes to finer structure and precipitation strengthening. Therefore, the lower limit of the N content is set to 0.0001%. Preferably, the lower limit of the N content is 0.0020%, 0.0035%, more than 0.0050%, or 0.0060%. However, when the N content exceeds 0.0120%, the low temperature toughness is lowered, which causes a material defect due to surface cracking during casting and strain aging of the manufactured steel. Therefore, the upper limit of N content is 0.0120%. Preferably, the upper limit of the N content is 0.0110%, 0.0100%, or 0.0090%.

The H-section steel according to the present embodiment contains impurities as chemical components. The “impurities” refer to those mixed from ore or scrap as a raw material or from a production environment when steel is industrially produced. For example, it means elements such as Al, B, P, S and O. Among these impurities, Al and B are preferably limited as follows in order to sufficiently exhibit the effects of the present embodiment. Moreover, since it is preferable that there is little content of an impurity, it is not necessary to restrict | limit a lower limit and the lower limit of an impurity may be 0%.

(Al: 0.10% or less)
Al (aluminum) is an element used as a deoxidizing element. However, if the Al content exceeds 0.10%, the oxide becomes coarse and becomes a starting point for brittle fracture, and low-temperature toughness decreases. Therefore, the upper limit of the Al content is limited to 0.10%. When Al is not actively used as a deoxidizing element, Ti works as a deoxidizing element, and Ti oxide is precipitated in the steel. This Ti oxide functions as a nucleation site for V carbonitrides, refines the ferrite grain size, and contributes to the improvement of low temperature toughness. Therefore, without using Al as a deoxidizing element, the upper limit of the Al content may be limited to less than 0.003%, 0.002%, or 0.001% using Al as an impurity. In general, in order to make the Al content 0.003% or more, Al is intentionally contained in the steel.

(B: 0.0003% or less)
B (boron) improves hardenability, promotes the formation of MA, and lowers low temperature toughness. For this reason, in the present embodiment, B is not actively added and is limited to the impurity level or less. The upper limit of B content is limited to 0.0003%. Preferably, the upper limit of the B content is limited to less than 0.0003%, 0.0002%, or 0.0001%. In general, in order to make the B content more than 0.0003%, B is intentionally contained in the steel.

(P: 0.03% or less, S: 0.02% or less, O: 0.005% or less)
P (phosphorus), S (sulfur), and O (oxygen) are impurities. P and S are segregated by solidification, promote weld cracking, and reduce low temperature toughness. Preferably, the upper limit of the P content is limited to 0.03%, 0.02%, or 0.01%. Preferably, the upper limit of the S content is limited to 0.02% or 0.01%. O dissolves in steel and lowers the low temperature toughness, and lowers the low temperature toughness by coarsening of oxide particles. Preferably, the upper limit of the O content is limited to 0.005%, 0.004%, or 0.003%.

The H-section steel according to the present embodiment may contain a selective element in addition to the basic elements and impurities described above. For example, instead of a part of Fe which is the above-described remaining part, Cr, Mo, Ni, Cu, W, Ca, Zr, Mg, and / or REM may be included as a selective element. These selective elements may be contained depending on the purpose. Therefore, it is not necessary to limit the lower limit values of these selected elements, and the lower limit value may be 0%. Moreover, even if these selective elements are contained as impurities, the above effects are not impaired.

(Cr: 0 to 0.30%)
Cr (chromium) is an element that contributes to improving the strength. If necessary, the Cr content may be 0 to 0.30%. In order to further improve the strength, the lower limit of the Cr content is preferably set to 0.01%, 0.05%, or 0.10%. On the other hand, if the Cr content exceeds 0.30%, the formation of MA may be promoted and the low temperature toughness may be reduced. Therefore, preferably, the upper limit of the Cr content is set to 0.30%, 0.25%, or 0.20%.

(Mo: 0 to 0.20%)
Mo (molybdenum) is an element that contributes to improvement in strength by solid solution in steel. If necessary, the Mo content may be 0 to 0.20%. In order to further improve the strength, the lower limit of the Mo content is preferably set to 0.01%, 0.05%, or 0.10%. However, if the Mo content exceeds 0.20%, the formation of MA is promoted and the low temperature toughness may be lowered. Therefore, preferably, the upper limit of the Mo content is 0.20%, 0.17%, or 0.15%.

(Ni: 0 to 0.50%)
Ni (nickel) is an element that contributes to improvement in strength by solid solution in steel. If necessary, the Ni content may be 0 to 0.50%. In order to further improve the strength, the lower limit of the Ni content is preferably set to 0.01%, 0.05%, or 0.10%. However, if the Ni content exceeds 0.50%, the hardenability is increased, the formation of MA is promoted, and the low temperature toughness may be lowered. Therefore, preferably, the upper limit of the Ni content is 0.50%, 0.30%, or 0.20%.

(Cu: 0 to 0.35%)
Cu (copper) is an element contributing to the improvement of strength. If necessary, the Cu content may be 0 to 0.35%. However, the addition of Cu facilitates the formation of MA and may reduce the low temperature toughness. Therefore, preferably, even if the Cu content is limited to 0.30% or less, 0.20% or less, 0.10% or less, or less than 0.03% or less than 0.01%, which is an impurity level. Good.

(W: 0-0.50%)
W (tungsten) is an element that contributes to improvement in strength by solid solution in steel. If necessary, the W content may be 0 to 0.50%. Preferably, the lower limit of the W content is 0.001%, 0.01%, or 0.10%. However, if the W content exceeds 0.50%, the formation of MA may be promoted and the low temperature toughness may be reduced. Therefore, preferably, the upper limit of the W content is 0.50%, 0.40%, or 0.30%. In addition, when W is not added intentionally, W content contained as an impurity is less than 0.001%. In order to make the W content 0.001% or more, W is intentionally contained in the steel.

(Ca: 0 to 0.0050%)
Ca (calcium) is an element that is effective in controlling the form of sulfide, suppresses the formation of coarse MnS, and contributes to the improvement of low-temperature toughness. If necessary, the Ca content may be 0 to 0.0050%. Preferably, the lower limit of the Ca content is 0.0001%, 0.0005%, or 0.0010%. On the other hand, if the Ca content exceeds 0.0050%, the low temperature toughness may be lowered. Therefore, preferably, the upper limit of the Ca content is set to 0.0050%, 0.0040%, or 0.0030%.

(Zr: 0 to 0.0050%)
Zr (zirconium) is an element that precipitates as carbide, nitride, or a composite thereof and contributes to precipitation strengthening. If necessary, the Zr content may be 0 to 0.0050%. Preferably, the lower limit of the Zr content is 0.0001%, 0.0005%, or 0.0010%. On the other hand, when the Zr content exceeds 0.0050%, coarsening of Zr carbides and nitrides may be caused, and the low temperature toughness may be lowered. Therefore, preferably, the upper limit of the Zr content is 0.0050%, 0.0040%, or 0.0030%. When Zr is not intentionally added, the Zr content contained as an impurity is less than 0.0001%. In order to make the Zr content 0.0001% or more, Zr is intentionally contained in the steel.

(Mg: 0 to 0.0050%, REM: 0 to 0.0050%)
Mg (magnesium) and REM (rare earth elements) are elements that contribute to the improvement of the toughness of the base metal and the heat affected zone (HAZ). If necessary, the Mg content may be 0 to 0.0050% and the REM content may be 0 to 0.0050%. Preferably, the lower limit of the Mg content is 0.0005%, 0.0010%, or 0.0020%, and the lower limit of the REM content is 0.0005%, 0.0010%, or 0.0020%. To do. On the other hand, preferably, the upper limit of Mg content is 0.0040%, 0.0030%, or 0.0025%, and the upper limit of REM content is 0.0040%, 0.0030%, or 0.0025. %.

(Ceq: 0.30-0.48)
In the H-section steel according to the present embodiment, the carbon equivalent Ceq is controlled from the viewpoint of securing strength. Specifically, when Ceq is represented by the following formula 1, C, Mn, Cr, Mo, V, Ni, and Cu in the chemical components of the H-shaped steel are in mass%, and 0.30 ≦ Ceq ≦ 0. 48 is satisfied. If Ceq is less than 0.30, the strength is insufficient. Therefore, the lower limit of Ceq is set to 0.30. Preferably, the lower limit of Ceq is set to 0.32%, 0.34%, or 0.35%. On the other hand, when Ceq exceeds 0.48, low temperature toughness decreases. Therefore, the upper limit of Ceq is set to 0.48. Preferably, the upper limit of Ceq is 0.45%, 0.43%, or 0.40%. In addition, when calculating Ceq by the following formula 1, an element whose content in steel is equal to or lower than the detection limit may be calculated by substituting 0 into formula 1 as a value.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (Formula 1)

The above steel components may be measured by a general steel analysis method. For example, the steel component may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas melting-thermal conductivity method, and O may be measured using an inert gas melting-non-dispersive infrared absorption method.

Next, the steel structure of the H-section steel according to this embodiment and the reason for limitation will be described in detail.

In the H-section steel according to the present embodiment, the steel structure includes an area fraction of ferrite of less than 60 to 100%, the mixed structure MA of martensite and austenite is limited to 3.0% or less, and the ferrite and MA The organization other than is limited to 37% or less. Moreover, the average particle diameter of ferrite is 1 μm or more and 30 μm or less.

(Ferrite area fraction: 60 to less than 100%)
Ferrite is a main constituent phase in the steel structure of the H-section steel according to the present embodiment. When the area fraction of ferrite is less than 60%, the low temperature toughness decreases. Therefore, the lower limit of the ferrite fraction is set to 60%. Preferably, the lower limit of the ferrite fraction is 65%, 70%, or 75%. On the other hand, it is physically difficult to control the area fraction of ferrite to 100% because it involves generation of pearlite or bainite. Therefore, the upper limit of the ferrite fraction is set to less than 100%. In order to preferably control the strength and the low temperature toughness, the upper limit of the ferrite fraction is preferably 90%, 85%, or 80%.

(MA area fraction: 3.0% or less)
When the formation of MA is promoted, low temperature toughness decreases. In the H-section steel according to this embodiment, the strength of the steel material is increased without promoting the generation of MA. Therefore, the MA fraction is limited to 3.0% or less. Preferably, the upper limit of the MA fraction is 2.5%, 2.0%, or 1.5%. Since the MA fraction is preferably as small as possible, the lower limit of the MA fraction may be 0%.

(Area fraction of structures other than ferrite and MA: 37% or less)
The steel structure of the H-section steel according to the present embodiment includes bainite, pearlite, and the like as structures other than the above-described ferrite and MA. If the structure other than ferrite and MA is excessively contained, the low temperature toughness is lowered. Therefore, the area fraction of the structure other than ferrite and MA (the above-mentioned ferrite and the remainder of MA) is limited to 37% or less. Preferably, the fraction of the structure other than ferrite and MA is 35% or less, 30% or less, or 25% or less. Since the fraction of the structure other than ferrite and MA is preferably as small as possible, this lower limit may be 0%.

(Average ferrite particle size: 1-30μm)
The average particle diameter of the ferrite is preferably fine. When the ferrite particle size exceeds 30 μm, the low temperature toughness is lowered. Therefore, the upper limit of the ferrite grain size is set to 30 μm. Preferably, the upper limit of the ferrite grain size is 25 μm, 22 μm, or 18 μm. On the other hand, it is industrially difficult to control the ferrite grain size to less than 1 μm. Therefore, the lower limit of the ferrite particle size is 1 μm. Preferably, the lower limit of the ferrite grain size is 3 μm, 5 μm, or 10 μm.

The above steel structure may be determined by observation with an optical microscope. For example, FIG. 1 is a schematic cross-sectional view orthogonal to the rolling direction of H-section steel, but the steel structure is observed using the vicinity of the evaluation site 7 shown in FIG. 1 as an observation surface. Specifically, in FIG. 1, the evaluation part is located at a position (1/6) F from the flange width direction end surface 5 a and a position (1/4) t ₂ from the outer surface 5 b in the thickness direction of the flange. The steel structure is observed using the vicinity of 7 as the observation surface. This observation surface is a surface parallel to the flange end surface 5a in the width direction.

研磨 Grind and corrode the above observation surface to observe the steel structure. Polishing is performed until the observation surface becomes a mirror surface, and corrosion is performed using a corrosive liquid suitable for identification of the constituent phases. For example, when the observation surface finished to a mirror surface is corroded with a nital solution to reveal a steel structure, pearlite and bainite are colored, so that ferrite, martensite, and austenite can be identified. Further, when the steel structure is revealed by corroding the mirror-finished observation surface with a repeller solution, the constituent phase other than martensite and austenite is colored black, so the mixed structure MA of martensite and austenite is identified. be able to.

In the H-section steel according to the present embodiment, the fraction of ferrite and MA is obtained from the observation surface that has undergone nital corrosion, the remainder is the fraction of the structure of pearlite and bainite, and the MA fraction is obtained from the observation surface that has undergone repeller corrosion. Specifically, measurement points are arranged in a lattice shape with a side of 25 μm on a 200 × optical micrograph (if necessary, multiple fields of view) taken on the observation surface that has been corroded at night, and at least 1000 measurement points Whether it is ferrite or MA is determined, and the value obtained by dividing the number of measurement points determined to be ferrite or MA by the number of all measurement points is defined as the ferrite or MA fraction.

Similarly, measuring points are arranged in a lattice shape with a side of 25 μm on a 200 × optical micrograph (if necessary, multiple fields of view) taken on an observation surface that has undergone repeller corrosion. A value obtained by dividing the number of measurement points determined to be MA by the number of all measurement points is defined as an MA fraction. The ferrite fraction is obtained by subtracting the total fraction of pearlite, bainite, and MA fraction obtained above from 100%.

Moreover, in the H-section steel according to the present embodiment, the average particle diameter of the ferrite is calculated from the cutting method in accordance with JIS G0551 (2013) using a 200-fold optical micrograph taken on the above-mentioned observation surface subjected to the nital corrosion. Ask.

Next, the mechanical characteristics of the H-section steel according to this embodiment will be described in detail.

In the H-section steel according to this embodiment, a test piece is taken from a region including the evaluation portion 7 shown in FIG. 1 as a position where average mechanical properties (strength and low temperature toughness) are obtained, and mechanical properties are evaluated.

First, the evaluation part 7 in FIG. 1 will be described. FIG. 1 is a schematic cross-sectional view orthogonal to the rolling direction of H-section steel. In FIG. 1, the X-axis direction is defined as the flange width direction, the Y-axis is defined as the flange thickness direction, and the Z-axis direction is defined as the rolling direction.

As shown in FIG. 1, the center of the evaluation part 7 is (1/6) F from the width direction end face of the flange, where F is the length in the width direction of the flange and t ₂ is the thickness of the flange. The position is (1/4) t ₂ from the outer surface in the thickness direction of the flange. The surface on the outer side in the thickness direction of the flange is one surface in the thickness direction of the flange and is the surface not in contact with the web 6, and is the surface 5b shown in FIG. Further, the end face in the width direction of the flange is the end face 5a shown in FIG.

A test piece for evaluating low temperature toughness by the Charpy test is collected from the position of the evaluation site 7 so that the longitudinal direction of the test piece is parallel to the rolling direction. In addition, the surface on which the notch is formed in the test piece is any surface parallel to the end surface 5a in the width direction of the flange. Further, the test piece is taken from any position as long as it is a position (1/6) F from the flange width direction end surface 5a and a position (1/4) t ₂ from the outer surface 5b in the thickness direction of the flange. May be.

A test piece for evaluating the yield stress (yield strength or 0.2% proof stress) and the tensile strength (maximum tensile strength) by a tensile test is (1/6) F from the width direction end face 5a of the flange in FIG. Samples are taken so that the position is the center of the specimen in the thickness direction. The test piece may be formed such that the longitudinal direction of the test piece is parallel to the rolling direction and the entire thickness direction of the flange is cut out. The test piece may be collected from any position as long as the position is (1/6) F from the end face 5a in the width direction of the flange.

In the H-shaped steel according to the present embodiment, as mechanical properties, the yield stress at room temperature is 385 MPa or more, the tensile strength is 490 MPa or more, and the Charpy absorbed energy at −20 ° C. is 100 J or more. Since the low temperature toughness may be impaired when the strength is too high, the upper limit of the yield stress is preferably 530 MPa and the upper limit of the tensile strength is preferably 690 MPa. Further, since it is industrially difficult to make the Charpy absorbed energy at −20 ° C. over 500 J, the upper limit of the Charpy absorbed energy at −20 ° C. may be set to 500 J. In addition, normal temperature refers to 20 degreeC.

When evaluating the mechanical properties of the H-section steel according to the present embodiment, the tensile test is performed according to JIS Z2241 (2011), and the Charpy test is performed according to JIS Z2242 (2005). When a yield phenomenon is found in the stress-strain curve obtained from the tensile test, the yield strength is obtained as the yield stress, and when no yield phenomenon is found in the stress-strain curve, the 0.2% yield strength is obtained as the yield stress.

Next, the shape of the H-section steel according to this embodiment will be described in detail.

The H-shaped steel according to the present embodiment, the flange thickness _{t 2} and 20 ~ 140 mm. For example, in a high-rise building structure, thick H-section steel is required as a strength member. Therefore, the lower limit of the flange thickness is 20 mm. Preferably, the lower limit of the flange thickness is 25 mm, 40 mm, or 56 mm. On the other hand, if the thickness t ₂ of the flange is greater than 140 mm, it is difficult achieve both the hot working volume during processing is insufficient strength and low temperature toughness. Therefore, the upper limit of the flange thickness is 140 mm. Preferably, the upper limit of the flange thickness is set to 125 mm, 89 mm, or 77 mm. For example, the flange thickness t ₂ is preferably 25 to 140 mm. The thickness t ₁ of the H-shaped steel web is not particularly specified, but is preferably 20 to 140 mm, and more preferably 25 to 140 mm.

When the H-section steel is manufactured by hot rolling, the flange thickness / web thickness ratio (t ₂ / t ₁ ) is preferably 0.5 to 2.0. When the flange thickness / web thickness ratio (t ₂ / t ₁ ) exceeds 2.0, the web may be deformed into a wavy shape. On the other hand, when the flange thickness / web thickness ratio (t ₂ / t ₁ ) is less than 0.5, the flange may be deformed into a wavy shape.

In the prior art, it was difficult to achieve both strength and toughness with a thick H-shaped steel having a flange thickness of 20 mm or more. However, in the H-section steel according to the present embodiment, the steel composition and the steel structure are optimally controlled in spite of the thick H-section steel having a flange thickness of 20 mm or more, so that both strength and low temperature toughness can be achieved. It becomes possible.

Next, a preferred method for manufacturing the H-section steel according to this embodiment will be described in detail.

The manufacturing method of the H-section steel according to the present embodiment includes a steel making process, a casting process, a heating process, a hot rolling process, and a cooling process.

In the steelmaking process, the chemical composition of the molten steel is adjusted so that the above steel composition is obtained. In the steel making process, molten steel produced by converter refining or secondary refining may be used, or molten steel melted in an electric furnace may be used as a raw material. In the steel making process, deoxidation treatment or vacuum degassing treatment may be performed as necessary.

In the casting process, the molten steel after the steel making process is cast to obtain a steel piece. Casting is performed by a continuous casting method, an ingot method, or the like. From the viewpoint of productivity, continuous casting is preferable. The shape of the billet is preferably a beam blank having a shape close to the H-shaped steel to be manufactured, but is not particularly limited. In addition, the thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation, homogeneity of the heating temperature before hot rolling, and the like.

In the heating process, the steel slab after the casting process is heated to 1100 to 1350 ° C. When the heating temperature of the steel slab is less than 1100 ° C., deformation resistance during finish rolling increases. Therefore, the lower limit of the heating temperature is 1100 ° C. Preferably, the lower limit of the heating temperature is set to 1150 ° C. in order to sufficiently dissolve elements that form carbides or nitrides such as Nb. On the other hand, if the heating temperature exceeds 1350 ° C., the scale on the surface of the steel slab becomes liquefied, which hinders production. Therefore, the upper limit of the heating temperature is 1350 ° C. In the heating process, a steel piece that has not been cooled to room temperature after the casting process may be used.

In the hot rolling process, rough rolling, intermediate rolling, and finish rolling are performed on the steel pieces after the heating process. In rough rolling, forming is performed such that the shape when viewed on a cut surface perpendicular to the rolling direction is substantially H-shaped. This nearly H-shaped steel slab is hot-rolled with a cumulative rolling reduction of 20% or more in a temperature range where the steel surface temperature is over 900 ° C to 1100 ° C, and the steel surface temperature is 730 ° C to Hot rolling is performed in a temperature range of 900 ° C. with a cumulative rolling reduction of 15% or more. In this hot rolling, forming is performed so that the shape when viewed on the cut surface is finally H-shaped.

In the temperature range above 900 ° C to 1100 ° C, the cumulative reduction ratio is set to 20% or more in order to reduce the amount of bainite and MA produced by refining austenite grains. Preferably, the lower limit of the cumulative rolling reduction in the temperature range of more than 900 ° C. to 1100 ° C. is 25%, 30%, or 35%. If necessary, the upper limit of the cumulative rolling reduction in the temperature range from over 900 ° C. to 1100 ° C. may be set to 60%.

In the temperature range of 730 ° C to 900 ° C, the cumulative rolling reduction is set to 15% or more due to finer ferrite. Preferably, the lower limit of the cumulative rolling reduction in the temperature range of 730 ° C. to 900 ° C. is 20%, 25%, or 30%. If necessary, the upper limit of the cumulative rolling reduction in the temperature range of 730 ° C. to 900 ° C. may be set to 50%.

In addition, when rolling is performed at a temperature lower than 730 ° C., low temperature toughness may be reduced. Therefore, the rolling end temperature (rolling finishing temperature) is 730 ° C. or higher at the surface temperature of the steel. Preferably, the upper limit of the rolling finishing temperature is 750 ° C.

In the hot rolling process, rough rolling, intermediate rolling, and finish rolling are performed. For example, rolling in a temperature range of over 900 ° C. to 1100 ° C. may be performed by rough rolling, intermediate rolling, or finish rolling. . Similarly, rolling in the temperature range of 730 ° C. to 900 ° C. may be performed by any of rough rolling, intermediate rolling, or finish rolling. In the method for manufacturing the H-section steel according to the present embodiment, the cumulative rolling reduction in the above temperature range may be controlled.

Further, the cumulative reduction ratio in the above temperature range is obtained based on the flange thickness at the position corresponding to (1/6) F from the width direction end face 5a of the flange shown in FIG. For example, the cumulative rolling reduction in the temperature range above 900 ° C. to 1100 ° C. is the rolling reduction calculated from the difference between the flange thickness when the surface temperature of the steel is 1100 ° C. and the flange thickness just before reaching 900 ° C. To do. Similarly, the cumulative rolling reduction in the temperature range of 730 ° C. to 900 ° C. is a rolling reduction calculated from the difference between the flange thickness at the time when the surface temperature of the steel is 900 ° C. and the flange thickness at the time of 730 ° C.

The method of rough rolling, intermediate rolling, and finish rolling in the hot rolling process is not particularly limited. For example, breakdown rolling is performed as rough rolling, universal rolling or edging rolling is performed as intermediate rolling, and universal rolling is performed as finishing rolling, so that the shape when viewed in a cross section perpendicular to the rolling direction becomes H-shaped. What is necessary is just to shape | mold.

In the hot rolling process, water cooling may be performed between rolling passes. Water cooling between rolling passes is cooling performed for the purpose of temperature control in a temperature range higher than the temperature at which austenite undergoes phase transformation. Bainite and MA are not generated in the steel by water cooling between rolling passes.

In the hot rolling process, two-heat rolling may be performed. The two-heat rolling is a rolling method in which the steel slab is cooled to 500 ° C. or lower after the primary rolling, and then the steel slab is heated again to 1100 to 1350 ° C. to perform secondary rolling. In the two-heat rolling, the amount of plastic deformation in the hot rolling is small and the decrease in temperature in the rolling process is small, so that the second heating temperature can be lowered.

In the cooling process, the hot rolled material after the hot rolling process is cooled. In the manufacturing method of the H-section steel according to the present embodiment, the hot-rolled material is allowed to cool in the air as it is after the hot rolling is finished. When the hot-rolled material is allowed to cool in the air, the average cooling rate on the surface and inside of the steel material from 800 ° C to 500 ° C is 1 ° C / second or less. By allowing the hot-rolled material to cool in the air, the cooling rate on the surface and inside of the steel material becomes uniform, so that variations in mechanical properties due to the portion of the steel material are suppressed. In addition, in the manufacturing method of the H-section steel according to the present embodiment, the cooling is performed in the atmosphere without performing forced cooling from immediately after hot rolling until the steel material temperature becomes 400 ° C. or lower. means.

In the prior art, hot-rolled material was accelerated and cooled in order to achieve both strength and toughness, resulting in variations in mechanical properties on the surface and inside of the steel material. On the other hand, in the manufacturing method of the H-section steel according to the present embodiment, the steel composition and the steel structure are optimally controlled even though the hot-rolled material is allowed to cool in the atmosphere. It is possible to achieve both strength and low temperature toughness without causing variations in mechanical properties.

Since the manufacturing method of the H-section steel according to the present embodiment does not require advanced steelmaking technology or accelerated cooling, it is possible to reduce the manufacturing load and the work period. Therefore, the H-section steel according to the present embodiment can improve the reliability of a large building without impairing the economy.

Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. However, the conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steels having chemical components shown in Tables 1 to 3 were melted, and steel pieces having a thickness of 240 to 300 mm were manufactured by continuous casting. The steel was melted in a converter, subjected to primary deoxidation, alloy elements were added to adjust the components, and vacuum degassing was performed as necessary. The obtained steel slab was heated and subjected to hot rolling to produce an H-shaped steel. Ingredient No. The steel components shown as 1 to 48 were obtained by chemical analysis of samples collected from each H-shaped steel after production. Although not shown in the table, in all Examples, P was 0.03% or less, S was 0.02% or less, and O was 0.005% or less. In addition, the blank of the chemical component in a table | surface represents that it was not actively added to steel or content was below the detection limit.

The manufacturing process of H-section steel is shown in FIG. In the hot rolling, the steel slab heated in the heating furnace 1 was subjected to a universal rolling apparatus row including a rough rolling mill 2a, an intermediate rolling mill 2b, and a finishing rolling mill 2c. The hot-rolled material was allowed to cool to 400 ° C. or less as it was after the hot rolling. The average cooling rate on the surface and inside of the hot rolled material from the hot rolling end temperature to 500 ° C. was 1 ° C./second or less. When water cooling was performed between hot rolling passes, the outer surface of the flange was spray cooled using water cooling devices 3 provided before and after the intermediate universal rolling mill (intermediate rolling mill) 2b. At this time, reverse rolling was performed.

Tables 4 to 6 show manufacturing conditions and manufacturing results. The rolling reduction during hot rolling shown in Tables 4 to 6 is the cumulative rolling reduction in each temperature region at a position corresponding to (1/6) F from the widthwise end face 5a of the flange shown in FIG.

As described above, the manufactured H-shaped steel was subjected to a Charpy test at −20 ° C. using a test piece taken from the evaluation site 7 shown in FIG. In addition, a tensile test was performed at normal temperature (20 ° C.) using a test piece having a position (1/6) F from the flange width direction end surface 5a at the center in the thickness direction, and tensile properties were evaluated. Further, the structure was observed using a sample having an observation surface in the vicinity of the evaluation site 7 shown in FIG. 1 to evaluate the steel structure.

The tensile test was performed according to JIS Z2241 (2005). The yield stress was taken as the yield point when the stress-strain curve of the tensile test showed yield behavior, and the yield stress was taken as 0.2% proof stress when no yield behavior was shown. The Charpy impact test was performed according to JIS Z2242 (2005). The Charpy impact test was conducted at -20 ° C.

In the structure observation, the ferrite fraction, the MA fraction, and the fraction of the structure other than ferrite and MA were measured by the above-described method using an optical micrograph. The structure other than ferrite and MA is bainite or pearlite. Moreover, the average particle diameter of the ferrite was calculated | required by the cutting method based on JISG0551 (2013) using the optical microscope photograph.

As a tensile property, a steel material having a yield stress (YS) at room temperature of 385 MPa or more and a tensile strength (TS) of 490 MPa or more was judged to be acceptable. Further, as a low temperature toughness, a steel material having Charpy absorbed energy (vE-20) at −20 ° C. of 100 J or more was judged to be acceptable.

As shown in Tables 1 to 6, the production numbers of the present invention examples. 1-8, Production No. 11 to 18 and production no. Nos. 34 to 43 all satisfy the scope of the present invention in terms of steel composition, steel structure, and mechanical properties.

On the other hand, production No. which is a comparative example. 9-10, Production No. 19-33, and production no. In Nos. 44 to 50, any of the steel composition, the steel structure, and the mechanical properties did not satisfy the scope of the present invention.

Manufacturing No. No. 9 had a rolling reduction ratio of over 900 ° C. to 1100 ° C., the ferrite fraction in the steel structure was insufficient, and the fraction of the structure other than ferrite and MA became excessive, and at −20 ° C. This is an example where Charpy absorbed energy is insufficient.

Manufacturing No. No. 10 is an example in which since the rolling reduction at 730 ° C. to 900 ° C. was insufficient, the ferrite grain size became coarse and the Charpy absorbed energy at −20 ° C. was insufficient.

Manufacturing No. No. 19 had an insufficient rolling reduction at temperatures exceeding 900 ° C. to 1100 ° C., so that the ferrite fraction became insufficient, the MA fraction became excessive, the fraction of the structure other than ferrite and MA became excessive, and −20 This is an example in which Charpy absorbed energy at ℃ is insufficient.

Manufacturing No. No. 20 has a high C content. No. 25 has a high Nb content. No. 26 has a high V content, and production no. No. 28 has a high Al content. No. 29 has a high Ti content, and production No. No. 30 has a high N content. No. 31 is an example in which the Charpy absorption energy at −20 ° C. is insufficient because Ceq is excessive.

Manufacturing No. No. 21 has a low C content. No. 24 has a low Mn content, and production no. No. 32 has insufficient Ceq. No. 46 is an example in which YS and TS are insufficient because the Si content is low.

Manufacturing No. No. 22 has a high Si content, and production No. 22 No. 23 is an example in which the Charpy absorbed energy at −20 ° C. is insufficient because the Mn content is large and the MA fraction is excessive.

Manufacturing No. No. 27 is an example in which since the V content was small, the ferrite grain size became coarse and the Charpy absorbed energy at −20 ° C. was insufficient.

Manufacturing No. No. 33 has an excess of B content and Ceq. No. 49 is an example in which since the B content was large, the MA fraction was excessive and the Charpy absorbed energy at −20 ° C. was insufficient.

Manufacturing No. 44 and production no. No. 45 is an example in which since the V content was small, the ferrite grain size became coarse and the Charpy absorbed energy at −20 ° C. was insufficient.

Manufacturing No. No. 47 is an example where the Nb content was small, the ferrite grain size was coarse, YS and TS were insufficient, and Charpy absorbed energy at −20 ° C. was insufficient.

Manufacturing No. No. 48 is an example in which since the Ti content was small, the ferrite grain size became coarse and Charpy absorbed energy at −20 ° C. was insufficient.

Manufacturing No. No. 50 is an example in which the Charpy absorbed energy at −20 ° C. was insufficient because the rolling finishing temperature was low.

According to the above aspect of the present invention, it is possible to provide a thick H-section steel excellent in strength and low-temperature toughness and a method for producing the same, so that industrial applicability is high.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2a Rough rolling mill 2b Intermediate rolling mill 2c Finishing rolling mill 3 Water cooling device before and after the intermediate rolling mill 4 H-section steel 5 Flange 5a End face in the width direction of the flange 5b Outer face in the thickness direction of the flange 6 Web 7 Tensile property, Low temperature toughness and evaluation part of steel structure F Length of flange in width direction H Height t ₁ Web thickness t ₂ Flange thickness

Claims

Steel is a chemical component in mass%,
C: 0.05 to 0.160%,
Si: 0.01-0.60%,
Mn: 0.80 to 1.70%,
Nb: 0.005 to 0.050%,
V: 0.05 to 0.120%,
Ti: 0.001 to 0.025%,
N: 0.0001 to 0.0120%,
Cr: 0 to 0.30%,
Mo: 0 to 0.20%,
Ni: 0 to 0.50%,
Cu: 0 to 0.35%,
W: 0 to 0.50%,
Ca: 0 to 0.0050%,
Zr: 0 to 0.0050%
Containing
Al: 0.10% or less,
B: limited to 0.0003% or less,
The balance consists of Fe and impurities,
When Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, C, Mn, Cr, Mo, V, Ni, and Cu in the chemical component satisfy 0.30 ≦ Ceq ≦ 0.48. And
The steel, as a metal structure, in area fraction,
Containing less than 60-100% ferrite,
Limiting the mixed structure MA of martensite and austenite to 3.0% or less,
The structure other than the ferrite and the MA is limited to 37% or less,
The ferrite has an average particle size of 1 to 30 μm,
When the steel is viewed in a cross section perpendicular to the rolling direction, the shape is H-shaped, and the flange thickness is 20 to 140 mm.
When the length in the width direction of the flange is F, the tensile yield stress is 385 to 530 MPa and the maximum tensile strength is 490 to 690 MPa at a position of (1/6) F from the end surface in the width direction of the flange.
When the thickness of the flange is t 2 , Charpy at −20 ° C. at the position of (1/6) F and (1/4) t 2 from the outer surface in the thickness direction of the flange. H-section steel characterized in that the absorbed energy of the test is 100 J or more.
The steel, as the chemical component, in mass%,
Nb: more than 0.02 to 0.050%
The H-section steel according to claim 1, comprising:
The steel, as the chemical component, in mass%,
N: more than 0.005 to 0.0120%
The H-section steel according to claim 1, comprising:
The steel, as the chemical component, in mass%,
The H-section steel according to claim 1, characterized by being limited to Cu: less than 0.03%.
The steel, as the chemical component, in mass%,
The H-section steel according to claim 1, characterized by being limited to Al: less than 0.003%.
The H-section steel according to claim 1, wherein the thickness of the flange is 25 to 140 mm.
A method for producing the H-section steel according to any one of claims 1 to 6,
A steel making process for obtaining a molten steel having the chemical component according to any one of claims 1 to 5;
A casting step of casting the molten steel after the steel making step to obtain a steel piece;
A heating step of heating the steel slab after the casting step to 1100 to 1350 ° C .;
Accumulation at the position of (1/6) F from the end surface in the width direction of the flange so that the shape when viewed on the cut surface perpendicular to the rolling direction is H-shaped with respect to the steel slab after the heating step. The rolling reduction is 20% or more at over 900 ° C. to 1100 ° C., the cumulative reduction at the above position is 15% or more at 730 to 900 ° C. Rolling process;
And a cooling step of cooling the hot-rolled material after the hot rolling step.