JP2003293076A - High strength steel pipe stock having low yield ratio after pipe making and production method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel pipe stock which has a low yield ratio after pipe making without deteriorating its toughness properties, and to provide a production method thereof. <P>SOLUTION: A steel slab having a composition containing, by mass, 0.04-0.08% C, and further containing one or more kinds of metals selected from Cu, Ni, Cr and Mo so as to satisfy the relation in the inequality of Mn/20+Cu/20+Ni/60+Cr/32+Mo/7≥0.11 is heated to 1,000-1,250°C. After that, the steel slab is subjected to hot rolling so as to satisfy a cumulative draft of ≥75% at ≤850°C and a rolling finishing temperature of 700-800°C. Subsequently, water cooling is performed from (the rolling finishing temperature to the rolling finishing temperature -50°C) to ≤200°C at a cooling rate of 10-30°C/sec, so that a bainite structure in which the ratio of a martensite-austenite coexistent structure having an aspect ratio of ≤4.0 is 5-20 vol.% of the whole structure is formed. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a large diameter welded steel pipe material used for pipelines or building structures,
In particular, the present invention relates to a material for low-yield ratio and high-strength steel pipes having strength exceeding API-5LX80 class and a manufacturing method thereof.

[0002]

2. Description of the Related Art As a steel pipe used for a pipeline for transporting crude oil or natural gas, a large diameter welded steel pipe formed by welding a thick steel plate by the UOE method or the roll bender method is mainly used. In recent years, the steel pipe has been strengthened to reduce the pipe thickness (material thickness), thereby reducing the material cost and thus the laying cost. Conventionally, as a material for the above-mentioned steel pipe, hot-rolled steel containing a large amount of alloying elements such as Mn, Cu, Ni, Cr, Mo, Nb, and V as disclosed in JP-A-08-35011 is used. After that, a thick steel plate that has been strengthened by subjecting it to accelerated cooling, and Japanese Patent Laid-Open No. 08-269544
No. as disclosed in Japanese, Ar _l and the so-called two-phase region rolling between transformation point to Ar ₃ transformation point after application of the machining strengthening of the ferrite, many more high strength steel plates and subjected to an accelerated cooling It is used.

However, as research on safety evaluation progresses, for example, from the viewpoint of preventing buckling of a steel pipe when an earthquake occurs, as a high-strength steel pipe used for a pipeline, a yield ratio (YS / TS) of the steel pipe is used. It has been found that a lower value is preferable. Recently, the number of cases in which steel pipes are used as column materials for high-rise buildings is increasing, and in this application as well, a low yield ratio is required in order to ensure plastic deformability during an earthquake. At present, the yield ratio (YS
/ TS) has a desirable value of 85% or less.

[0004]

However, it cannot be said that the high-strength steel pipe manufactured by the above-mentioned conventional technique can sufficiently satisfy the requirement of such a low yield ratio. That is, a thick steel sheet manufactured by adding a large amount of alloying elements and accelerating cooling has a high yield ratio due to martensitic structure during cooling. In addition, a thick steel sheet reinforced by ferrite by two-phase rolling has a particularly high YS, so that the yield ratio is remarkably high. Further, in the UOE method and the roll bending method, since the thick steel plate which is a raw material is bent, YS is further increased. Therefore, in order to obtain a low yield ratio at the steel pipe stage, the yield ratio must be made lower at the stage of the original steel sheet.

An object of the present invention is to propose a material for high-strength steel pipe having a low yield ratio after pipe making (≦ 85%) without deteriorating toughness characteristics and a method for producing the same.

[0006]

[Means for Solving the Problems] The inventors of the present invention have earnestly studied the enhancement of strength by controlling the microstructure. As a result, the microstructure of the material steel sheet is mainly composed of bainite, and low temperature transformation formation phase (Martensite) in which the hard martensite and austenite coexist in the bainite in a coexisting state.
Called ite-Austenite constituents. After that, "M-
It was found that TS is remarkably increased when the A) coexisting structure is abbreviated), particularly when the volume ratio of the MA coexisting structure is 5 vol% or more of the whole structure. M
The -A coexisting structure is generated when C in the steel is diffused and concentrated in untransformed austenite during the bainite transformation. Therefore, in order to secure the required volume ratio of the MA coexisting structure, it is necessary to secure the C content and optimize the addition amount of the alloying element for causing the bainite transformation and the cooling conditions.

As a result of further research, the inventors have found that C
It was found that the content of 0.04 mass% or more can stably form the MA coexisting structure in bainite.
Furthermore, as a result of regression calculation of the experimental results, the alloying elements were adjusted to a composition range satisfying the following formula: Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11, and the cooling rate after rolling was adjusted.
It was found that by setting the temperature to 10 ° C./sec or more, bainite transformation can be caused without causing ferrite transformation, and further, if cooled to 200 ° C. or less, the MA coexisting structure can be generated following the bainite transformation. .

By the way, conventionally, the above-mentioned MA coexisting organization is
It has been considered that the toughness is lowered because it becomes a starting point of brittle fracture. As a result of investigating this point by paying attention to the morphology of the M-A coexisting structure, the inventors have found that the elongated M-A coexisting structure having an aspect ratio of more than 4 becomes a starting point of brittle fracture. That is, it was found that if the lumpy M-A coexisting structure having an aspect ratio of 4 or less has little adverse effect on the toughness in the range where the volume ratio of the M-A coexisting structure does not exceed 20 vol%. Then, in order to preferentially generate such a massive MA coexisting structure, a large deformation is applied in a temperature range of 850 ° C. or less at which austenite does not recrystallize, so that bainite is refined. It has also been found that a lumpy MA coexisting structure is easily generated.

The present invention completed on the basis of the above findings is C:
0.04 to 0.08 mass%, Si: 0.05 to 0.50 mass%, Mn: 1.5 to
2.5mass%, Al: 0.01 to 0.10mass%, Nb: 0.01 to 0.08mass
s%, Ti: 0.005-0.020mass%, and Cu, N
One or two or more selected from i, Cr and Mo are Cu: 0.2 to 0.7 mass%, Ni: 0.2 to 1.0 mass%, Cr: 0.
2 to 0.7mass%, Mo: 0.2 to 0.7mass%, and contains so as to satisfy the following formula; Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11 with the balance Fe and unavoidable impurities , Martensite with aspect ratio ≤ 4.0
Austenite coexisting structure has a microstructure mainly composed of bainite structure of 5 to 20% by volume of the whole structure, and the yield ratio after pipemaking is 85% or less, and the yield ratio after pipemaking is low. It is a high strength steel pipe material.

The steel pipe material of the present invention, in addition to the above-mentioned composition, further contains Ca: 0.001 to 0.003 mass% and REM: 0.
It is preferable to contain one or two selected from 005 to 0.020 mass%.

Further, according to the present invention, C: 0.04 to 0.08 mass
%, Si: 0.05 to 0.50 mass%, Mn: 1.5 to 2.5 mass%, Al:
0.01 to 0.10mass%, Nb: 0.01 to 0.08mass%, Ti: 0.005
To 0.020 mass%, and one or more selected from Cu, Ni, Cr and Mo, Cu: 0.2 to 0.7
mass%, Ni: 0.2 to 1.0 mass%, Cr: 0.2 to 0.7 mass%, M
o: 0.2 to 0.7 mass% and a steel slab containing the following formula; Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11 with the balance Fe and unavoidable impurities After heating to 1250 ° C,
Cumulative rolling reduction at 850 ° C or less is 75% or more, rolling end temperature is 7
Perform hot rolling to 00 ~ 800 ℃, then (rolling end temperature ~ rolling end temperature -50 ℃) to 200 ℃ or less, by water cooling at a cooling rate of 10 ~ 30 ℃ / aspect ratio ≤ The martensite-austenite coexistence structure of 4.0 has a microstructure mainly composed of bainite structure which is 5 to 20 vol% of the whole structure, and the yield ratio after pipe forming is 85% or less. We propose a manufacturing method of high strength steel pipe material with low yield ratio after pipe.

In the production method of the present invention, in addition to the above component composition, Ca: 0.001 to 0.003 mass%, REM: 0.
It is preferable to contain one or two selected from 005 to 0.020 mass%.

In the manufacturing method of the present invention, it is preferable that the steel sheet after hot rolling is tempered at a temperature of 450 ° C. or lower.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons why each component composition is limited to the above-mentioned claims in the present invention will be explained. C: 0.04 to 0.08 mass% C is 5% by volume or more of the entire structure in bainite.
In order to disperse the coexisting structure, it is necessary to contain 0.04 mass% or more. However, if added in excess of 0.08 mass%, martensitic transformation is likely to occur in the weld zone due to the large amount of alloying elements described below, and the hardness may become too high, impairing weldability. Therefore, the upper limit of C is
0.08 mass%

Si: 0.05 to 0.50 mass% Si needs to be added in an amount of 0.05 mass% or more as a deoxidizing element in steelmaking. In addition, the strength of steel increases with the addition amount.
Increased by solid solution strengthening. However, if added over 0.50%, the toughness is adversely affected, so the upper limit is 0.50 mass%.
And

Mn: 1.5 to 2.5 mass% Mn is an element that enhances the hardenability, and if added according to the formula described below, bainite transformation can be promoted. Also, since it is cheaper than other elements, the lower limit is 1.
By adding 5 mass% or more, cost increase can be suppressed and high strength can be achieved. However, if added in excess of 2.5 mass%, the toughness of the weld will deteriorate, so the upper limit is 2.5 mass.
%.

Al: 0.01 to 0.10 mass% Al must be added in an amount of 0.01 mass% or more as a deoxidizing agent in steelmaking. However, if it is added in excess of 0.10 mass%, the toughness of the base metal deteriorates with the increase of inclusions such as Al ₂ O ₃ and AlN, and the acicular ferrite formed especially from TiO by dilution into the weld metal. Therefore, since the toughness of the weld metal is significantly deteriorated in order to inhibit the formation of TiO in the weld metal achieving high toughness, the upper limit was made 0.10 mass%.

Nb: 0.01 to 0.08 mass% Nb must be added in an amount of 0.01 mass% or more in order to shift the recrystallization temperature of austenite to the high temperature side. Due to the effect of the addition of Nb, the bainite structure is remarkably refined by rolling at 850 ° C. or less, and accordingly, the formation of a massive MA coexisting structure having an aspect ratio of ≦ 4 is promoted. on the other hand,
If added in excess of 0.08 mass%, the toughness of the weld heat affected zone will be significantly deteriorated, so the upper limit was made 0.08 mass%.

Ti: 0.005 to 0.020 mass% Ti needs to be added in an amount of 0.005 mass% or more in order to fix the free N in steel inevitably present as TiN. Further, this TiN also contributes to suppressing the growth of austenite grains in the heat affected zone of welding. On the other hand, when added in excess of 0.020 mass%, excess Ti forms carbides, which causes an increase in YS of the steel and causes an increase in the yield ratio, so the upper limit is 0.020 m.
Assass%

Cu: 0.2 to 0.7 mass% Cu contributes to the promotion of bainite transformation by adding 0.2 mass% or more. However, if added in excess of 0.7 mass%, in particular, when tempered, precipitation occurs and precipitation strengthening occurs, and YS increases, leading to an increase in yield ratio.
0.7mass%

Ni: 0.2 to 1.0 mass% Ni contributes to the promotion of bainite transformation by adding 0.2 mass% or more. On the other hand, the effect is saturated even if added in excess of 1.0 mass%, so the upper limit is made 1.0 mass%.

Cr: 0.2 to 0.7 mass% Cr contributes to the promotion of bainite transformation by adding 0.2 mass% or more. On the other hand, if added in excess of 0.7 mass%, the toughness of the weld will be adversely affected.
s%

Mo: 0.2 to 0.7 mass% Mo contributes to the promotion of bainite transformation by adding 0.2 mass% or more. On the other hand, if added in excess of 0.7 mass%, Mo carbide precipitates and precipitation strengthening occurs, especially YS increases and the yield ratio is increased, so the upper limit is made 0.7 mass%.

Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11 Cu, Ni, Cr and Mo together with Mn satisfy the following formula: Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11 With such addition, when the cooling rate in water cooling after hot rolling is set to 10 ° C./sec or more, bainite transformation occurs without ferrite transformation, and contributes to the subsequent formation of the MA coexisting structure.

In the present invention, in addition to the above essential components,
The following components can be added as needed. Ca: 0.001 to 0.003 mass% Ca can be added as a morphology controlling element of the S compound. That is, when the non-metallic inclusion MnS inevitably formed in the steel becomes a problem in the toughness of the HAZ, etc.
Ca added at 001mass% or higher produces higher temperature Ca
It is possible to control the form of inclusions in S and eliminate its adverse effects. However, when added in excess of 0.003 mass%, CaS
Is generated in a cluster and adversely affects it, so the upper limit is made 0.003 mass%.

REM: 0.005 to 0.020 mass% REM, like Ca, is 0.00% for the purpose of controlling the morphology of MnS.
5 mass% or more can be added. But 0.020mass
%, The cleanliness of the steel deteriorates, so the upper limit was made 0.020 mass%.

Next, the microstructure of the steel pipe material according to the present invention will be described. In order to satisfy the strength of the large-diameter welded steel pipe exceeding the API-5LX80 class, which is the object of the present invention, at least the microstructure of the steel should be at least the microstructure of the steel in order to satisfy the strength of the API-5LX80 class which is the object of the present invention. Bainite needs to be the basis. Further, when forming a steel pipe by the UOE method or the roll bender method, even if the YS increases due to bending, in order to stably satisfy the yield ratio ≦ 85%, the yield ratio at the steel plate stage should be further reduced. Need to be kept.

At this time, when a hard MA coexisting structure as the second phase is dispersed in bainite, the TS of the steel sheet can be remarkably increased by dispersion strengthening, and as a result, high strength and low yield are achieved at the steel sheet stage. A ratio can be achieved.
Here, the hardness of the MA coexisting structure is about 2 to 3 of that of bainite.
Therefore, in order to achieve the target yield ratio of 85% or less, it is necessary to disperse at least 5 vol% of the MA coexisting structure. In order to prevent the M-A coexisting structure from becoming a starting point of brittle fracture and significantly lowering the toughness of the steel sheet, the form of the M-A coexisting structure is set to be an elongated form such as an aspect ratio> 4.0. Aspect ratio ≤
It is necessary to control the lumpy form of 4.0. However, even if the bulk morphology is controlled, if the volume ratio of the MA coexisting structure exceeds 20 vol%, the toughness deteriorates. Therefore, the upper limit of the volume ratio is 20 vol%. As described above, in order to achieve the low yield ratio while ensuring the toughness equivalent to that of the conventional material, the MA coexisting structure with the aspect ratio ≦ 4.0 is set to 5% of the whole structure.
~ 20vol% is required.

Next, the manufacturing conditions of the steel pipe material according to the present invention will be described. Slab heating temperature: 1000 to 1250 ° C. Since the slab heating temperature before hot rolling is 1000 ° C. or higher, a uniform austenite structure is obtained, so the lower limit temperature is set to 1000 ° C. On the other hand, when heated to 1250 ° C or higher, the austenite grains coarsen, and if hot-rolled as it is, deterioration of the toughness of the steel sheet is large.
℃ was made. The temperature range of 1050-1150 ° C is preferable.

Cumulative rolling reduction in the low temperature γ region of 850 ° C. or lower ≧ 75
% The heated slab immediately undergoes hot rolling to produce the MA coexisting structure, which is important for controlling the yield ratio. At this time, in order to control the morphology of the MA coexisting structure so as not to adversely affect the toughness of the base material, it is important to refine the bainite by performing strong working in a low temperature region where austenite does not recrystallize. Specifically, by setting the cumulative rolling reduction at 850 ° C. or lower to 75% or higher, the morphology of the MA coexisting structure mainly has an aspect ratio of ≦ 4.0, whereby a steel sheet with little deterioration in toughness can be obtained. .

Hot rolling end temperature: 700 to 800 ° C. In order not to recrystallize austenite, the lower the rolling temperature in hot rolling, the greater the effect, but when rolling at a temperature lower than 700 ° C., conversely, Ferrite is generated and YS and TS are reduced. Therefore, the hot rolling finish temperature is set to 700 ° C or higher. If the hot rolling temperature exceeds 800 ° C, the cumulative rolling reduction ratio of 7
Since it becomes difficult to add rolling of 5% or more, the upper limit of the rolling end temperature is 800 ° C.

Cooling start temperature ≧ (rolling end temperature to rolling end temperature−50 ° C.) In order to transform the hot-rolled sheet into bainite, it is necessary to start water cooling immediately. If the water cooling start temperature is low,
Ferrite transformation occurs by the start of water cooling, YS and TS
Therefore, the cooling start temperature is set to the rolling end temperature −50 ° C. or higher.

Cooling rate: 10 to 30 ° C./sec By optimizing the amounts of Mn, Cu, Ni, Cr and Mo, the cooling rate by water cooling after rolling is 10 within the above range of hot rolling conditions. By securing at least C / sec, bainite transformation can be caused without ferrite transformation, and then a MA coexisting structure can be generated. On the other hand, when the cooling rate exceeds 30 ° C / sec, the martensitic transformation occurs without the bainite transformation, and the YS,
Although TS increases, the yield ratio also increases, so
The object of the present invention cannot be achieved. Therefore, the upper limit of the cooling rate is in the range of 10 to 30 ° C / sec.

Cooling stop temperature ≦ 200 ° C. During the cooling, the M-A coexisting structure is generated at about 400 ° C.
It is considered to be between ~ 200 ° C, and if water cooling is stopped at 200 ° C or higher, the required volume ratio of the MA coexisting structure cannot be secured. Therefore, the cooling stop temperature is set to 200 ° C or lower.

Tempering temperature ≦ 450 ° C. In the present invention, the steel sheet after hot rolling can be subjected to tempering treatment for the purpose of improving strain aging characteristics.
If the tempering temperature in this case exceeds 450 ° C., the MA coexisting structure formed in bainite is decomposed, and the yield ratio of the steel sheet after tempering becomes high. Therefore, the upper limit of the tempering temperature is 450 ° C.

The method for producing the slab to be subjected to hot rolling is not particularly limited, and after the composition of the steel is adjusted by the conventional converter method or electric furnace method, the continuous casting method or the ingot making method is used. Any of the above may be used for casting. As the method for forming the manufactured steel sheet into a steel pipe, either the UOE method or the roll bender method can achieve the high strength and the low yield ratio targeted by the present invention, and either method may be used.

Next, the characteristics of the steel pipe produced by using the material produced under the above conditions will be described. The yield ratio of the steel pipe after pipe making must be 85% or less. If this yield ratio exceeds 85%, in high strength steel pipes exceeding API-5LX80 class,
This is because it becomes difficult to secure plastic deformability when an earthquake occurs.

Further, the steel pipe material of the present invention is characterized in that the deterioration of the toughness characteristics is small despite the existence of the MA coexisting structure. For that purpose, the Charpy property at a temperature of -20 ° C after pipe forming is vE-20 ≥
It is preferable that vE-20 ≧ 150 (J) or more at 250 (J) and welded portion (HAZ).

[0039]

EXAMPLE Using a steel slab having the composition shown in Table 1, slab heating, hot rolling, cooling and tempering were performed under the conditions shown in Table 2 to produce a thick steel plate having a thickness of 15 to 30 mm. From this steel plate, the total thickness for microstructure observation × 20 mm width ×
A 10 mm long block was taken. After mirror-polishing the surface of the cross section parallel to the rolling direction of this sample, electrolytic corrosion was performed at a voltage of 3 V for 3 seconds in an electrolytic solution in which 5 g of ethylenediamine tetraacetic acid and 0.5 g of NaF were dissolved in 100 ml of distilled water. In an electrolytic solution prepared by dissolving 25 g of NaOH and 5 g of picric acid in 100 ml of distilled water, electrolytic corrosion is performed at a voltage of 6 V for 30 seconds, and M
-A A coexisting organization was revealed. Then, the corroded surface of this sample was randomly photographed with a scanning electron microscope at an appropriate magnification of 800 to 2000 times for 4 or more fields of view, and the morphology of the MA coexisting structure shown in each photograph was taken. And the area ratio was calculated by image analysis processing. In addition, since the MA coexisting structure in the steel sheet produced in the present invention is considered to be isotropic in three dimensions, the area ratio obtained by the scanning electron micrograph, which is a two-dimensional cross-sectional image, is defined as the volume ratio. Regarded

[0040]

[Table 1]

[0041]

[Table 2]

Next, using the steel sheet produced above, Table 3
After forming into a steel pipe by the UOE method or roll bender method as shown in, a tensile test piece (JIS No. 4 test piece) is taken in parallel to the longitudinal direction of the steel pipe from the position of 180 ° clockwise from the welded part investigated. In addition, a Charpy test piece (JIS No. 4 test piece, V notch) was sampled in the circumferential direction from the welded portion and a position 180 ° clockwise from the welded portion, and the Charpy property at a temperature of −20 ° C. was also evaluated. The Charpy test piece of the welded portion was sampled so that a notch was formed in the center of the heat-affected zone of the welding.

Table 3 shows the examination results of the mechanical properties of the above steel pipes. As is clear from this table, all the steel pipes manufactured from the materials satisfying the conditions of the present invention have a bainite-based structure and a MA coexisting structure is 5 vol% or more of the total structure of the bainite structure. The yield ratio is 85% or less,
The Charpy impact properties show good values of vE-20: 250J or more for the base metal and vE-20: 150J or more for the weld. on the other hand,
Steel pipes manufactured from materials that deviate from the composition of ingredients or manufacturing conditions of the present invention have yield ratios of more than 85%, or have poor Charpy properties of either the base metal or the weld. Absent.

[0044]

[Table 3]

[0045]

As described above, according to the present invention,
By optimizing the chemical composition and cooling after hot rolling, and by making the microstructure of the steel pipe material a structure mainly composed of bainite in which the MA coexisting structure with an aspect ratio ≤ 4.0 is 5 to 20 vol% of the total structure. A high-strength steel pipe material having a yield ratio after pipe production of 85% or less can be obtained without deteriorating the toughness characteristics of the steel pipe.

Claims (5)

[Claims] 1. C: 0.04 to 0.08 mass%, Si: 0.05 to 0.50 ma
ss%, Mn: 1.5 to 2.5 mass%, Al: 0.01 to 0.10 mass%, N
b: 0.01 to 0.08 mass%, Ti: 0.005 to 0.020 mass%, and one or more selected from Cu, Ni, Cr and Mo, Cu: 0.2 to 0.7 mass%, Ni: 0.2 ~ 1.0
mass%, Cr: 0.2 to 0.7 mass%, Mo: 0.2 to 0.7 mass%,
In addition, the following formula: Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11 is contained so that the balance is Fe and unavoidable impurities, and the aspect ratio is ≦ 4.0 martensite −
Austenite coexisting structure has a microstructure mainly composed of bainite structure of 5 to 20% by volume of the whole structure, and the yield ratio after pipemaking is 85% or less, and the yield ratio after pipemaking is low. High strength steel pipe material. 2. The high component according to claim 1, further comprising, in addition to the above component composition, one or two selected from Ca: 0.001 to 0.003 mass% and REM: 0.005 to 0.020 mass%. High strength steel pipe material. 3. C: 0.04 to 0.08 mass%, Si: 0.05 to 0.50 ma
ss%, Mn: 1.5 to 2.5 mass%, Al: 0.01 to 0.10 mass%, N
b: 0.01 to 0.08 mass%, Ti: 0.005 to 0.020 mass%, and one or more selected from Cu, Ni, Cr and Mo, Cu: 0.2 to 0.7 mass%, Ni: 0.2 ~ 1.0
mass%, Cr: 0.2 to 0.7 mass%, Mo: 0.2 to 0.7 mass%,
Also, after heating a steel slab containing the following formula: Mn / 20 + Cu / 20 + Ni / 60 + Cr / 32 + Mo / 7 ≧ 0.11 and the balance Fe and unavoidable impurities to 1000 to 1250 ° C.,
Cumulative rolling reduction at 850 ° C or less is 75% or more, rolling end temperature is 7
Perform hot rolling to 00 ~ 800 ℃, then (rolling end temperature ~ rolling end temperature -50 ℃) to 200 ℃ or less, by water cooling at a cooling rate of 10 ~ 30 ℃ / aspect ratio ≤ The martensite-austenite coexistence structure of 4.0 has a microstructure mainly composed of bainite structure which is 5 to 20 vol% of the whole structure, and the yield ratio after pipe forming is 85% or less. A method for producing a high-strength steel pipe material having a low yield ratio after the pipe. 4. The composition according to claim 3, further comprising, in addition to the above composition, one or two selected from Ca: 0.001 to 0.003 mass% and REM: 0.005 to 0.020 mass%. A method for producing the high-strength steel pipe material described. 5. The method for producing a high-strength steel pipe material according to claim 3, wherein the steel sheet after hot rolling is tempered at a temperature of 450 ° C. or lower in the production method.