阅读说明：本技术 管线管用钢材及其制造方法以及管线管的制造方法 (Steel material for line pipe, method for producing same, and method for producing line pipe ) 是由 安田恭野 横田智之 谷泽彰彦 村冈隆二 长谷和邦 于 2019-01-22 设计创作，主要内容包括：本发明的目的在于,提供具有板厚为30mm以上的厚壁并具有应用于海底管线管所需的高强度、且低温韧性及DWTT性能优异的管线管用钢材及其制造方法以及管线管的制造方法。拉伸强度为570MPa以上、压缩强度为440MPa以上、板厚为30mm以上的管线管用钢材的制造方法,将具有特定成分组成的钢加热至1000～1200℃的温度,在实施未再结晶温度区域的累积压下率为60％以上且(轧制结束温度+20℃)以下的温度区域的累积压下率为50％以上、轧制结束温度为Ar<Sub>3</Sub>相变点以上且790℃以下的热轧后,从Ar<Sub>3</Sub>相变点以上的冷却开始温度起以10℃/s以上的冷却速度进行加速冷却直至钢板表面温度为300～500℃。(The purpose of the present invention is to provide a steel material for line pipes, which has a thick wall with a thickness of 30mm or more, has high strength required for application to submarine line pipes, and has excellent low-temperature toughness and DWTT performance, a method for producing the same, and a method for producing line pipes. A tensile strength of 570MPa or more, a compressive strength of 440MPa or more, and a sheet thickness of 30mm or moreThe method for producing a steel material for a line pipe according to (1), wherein a steel having a specific composition is heated to a temperature of 1000 to 1200 ℃, the cumulative reduction in a temperature region in which the cumulative reduction in the unrecrystallized temperature region is 60% or more and (rolling end temperature +20 ℃) or less is 50% or more, and the rolling end temperature is Ar 3 Hot rolling at a temperature of not lower than the transformation point and not higher than 790 ℃, and then rolling with Ar 3 Accelerated cooling is performed at a cooling rate of 10 ℃/s or more from a cooling start temperature of the transformation point or more until the surface temperature of the steel sheet is 300 to 500 ℃.)

1. A method for producing a steel material for line pipes, the steel material having a tensile strength of 570MPa or more, a compressive strength of 440MPa or more and a plate thickness of 30mm or more, wherein a steel having a composition of the following components is heated to a temperature of 1000 to 1200 ℃, the cumulative reduction in a temperature region in which the cumulative reduction in the unrecrystallized temperature region is 60% or more and (rolling end temperature +20 ℃) or less is 50% or more, and the rolling end temperature is Ar₃Hot rolling at a temperature of not lower than the transformation point and not higher than 790 ℃, and then rolling with Ar₃Accelerated cooling at a cooling start temperature of 10 ℃/s or more from the transformation point until the surface temperature of the steel sheet reaches 300 to 500 ℃,

the composition of the components comprises the following components in percentage by mass

C：0.030～0.10％、

Si：0.01～0.20％、

Mn：1.0～2.0％、

Nb：0.005～0.050％、

Ti：0.005～0.025％、

Al: the content of the active carbon is less than 0.08%,

the composition further contains, in mass%

Cu: less than 0.5 percent,

Ni: less than 1.0 percent,

Cr: less than 1.0 percent,

Mo: less than 0.5 percent,

V: 0.1% or less, Ceq value represented by formula (1) of 0.350 or more, Pcm value represented by formula (2) of 0.20 or less, and Ar represented by formula (3)₃A transformation point of 750 ℃ or below, and the balance of Fe and inevitable impurities,

Ceq＝C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)

Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)

Ar₃(℃)＝910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (3)

in the formulae (1) to (3), the element symbol indicates the mass% of the contained element, and is 0 when not contained.

2. A method for producing a line pipe having a tensile strength of 570MPa or more, a compressive strength of 440MPa or more, and a plate thickness of 30mm or more, wherein a steel material for line pipes produced by the method according to claim 1 is formed into a steel pipe shape by cold forming, and after butt seam welding, the steel pipe is expanded at an expansion ratio of 1.2% or less to produce a steel pipe.

3. A steel material for a line pipe, which has a tensile strength of 570MPa or more, a compressive strength of 440MPa or more and a plate thickness of 30mm or more, and which has a composition containing, in mass%, (ii) a

C：0.030～0.10％、

Si：0.01～0.20％、

Mn：1.0～2.0％、

Nb：0.005～0.050％、

Ti：0.005～0.025％、

Al: the content of the active carbon is less than 0.08%,

the composition further contains, in mass%

Cu: less than 0.5 percent,

Ni: less than 1.0 percent,

Cr: less than 1.0 percent,

Mo: less than 0.5 percent,

V: 0.1% or less, Ceq value represented by formula (1) of 0.350 or more, Pcm value represented by formula (2) of 0.20 or less, and Ar represented by formula (3)₃A transformation point of 750 ℃ or below, and the balance of Fe and inevitable impurities,

the steel material for line pipes has a microstructure mainly composed of bainite, wherein the area fraction of polygonal ferrite is 10% or less and the area fraction of island-like martensite is 5% or less at a position 1/4 mm thick, the average grain size of bainite at a position 1/2 mm thick is 10 μm or less,

Ceq＝C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)

Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)

Ar₃(℃)＝910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (3)

in the formulae (1) to (3), the element symbol indicates the mass% of the contained element, and is 0 when not contained.

4. A method for producing a line pipe having a tensile strength of 570MPa or more, a compressive strength of 440MPa or more and a plate thickness of 30mm or more, which comprises cold forming the steel product for line pipe according to claim 3 into a steel pipe shape, seam welding the butt seams, and then expanding the steel pipe at an expansion ratio of 1.2% or less.

Technical Field

The present invention relates to a steel material for line pipes, a method for producing the same, and a method for producing line pipes. The present invention relates to a line pipe for transporting oil and natural gas, and more particularly to a steel material for a line pipe suitable for use in a submarine line pipe requiring high collapse resistance (collapse), a method for producing the same, and a method for producing a line pipe. The compressive strength of the present invention means 0.5% compressive yield strength, and is also referred to as compressive yield strength unless otherwise specified.

Background

With the recent increase in energy demand, development of oil and gas line pipes has been actively carried out, and many line pipes crossing the ocean have been developed because of the remote location of gas fields and oil fields and the diversification of transportation routes. In order to prevent a line pipe used for a submarine line pipe from collapsing (collapsing) due to water pressure, a line pipe having a larger pipe thickness than a land line pipe is used and is required to have high roundness. Further, as the characteristics of line pipes, high compressive strength is required in order to resist compressive stress generated in the pipe circumferential direction by external pressure.

UOE steel pipes have a pipe expansion process in the final pipe forming step, and are subjected to tensile deformation in the pipe circumferential direction and then to compression, and therefore, a decrease in compressive yield strength due to the bauschinger effect is problematic.

Many studies have been made on the improvement of the collapse resistance of UOE steel pipes, and patent document 1 discloses a method of heating a steel pipe by electric heating and expanding the pipe, and then maintaining the temperature for a certain time or more.

As a method of recovering from a decrease in compressive yield strength due to the bauschinger effect by similarly expanding a pipe and then heating, patent document 2 proposes the following method: in addition, patent document 3 proposes a work hardening method in which the bauschinger effect of a portion subjected to tensile deformation on the outer surface side is recovered and the compression on the inner surface side is maintained by heating the outer surface of the steel pipe to a temperature higher than that of the inner surface, and the method comprises: in the process of manufacturing steel sheet of Nb and Ti added steel, Ar is selected from₃Accelerated cooling from a temperature above the above point to 300 ℃ or below after hot rolling, forming a steel pipe by a UOE process, and then heating.

On the other hand, as a method for improving the compressive strength by a forming method of a steel pipe without heating after pipe expansion, patent document 4 discloses a method in which the compression ratio at the time of forming by O-press is made larger than the subsequent pipe expansion ratio.

Patent document 5 discloses a method for improving collapse resistance by setting the diameter of the steel pipe at a position 180 ° away from the welded portion in the vicinity of the welded portion where the compressive strength is low to the maximum diameter of the steel pipe.

Further, patent document 6 proposes a steel sheet in which the yield stress reduction due to the bauschinger effect is small by reducing the hard phase 2 fraction at the surface layer portion of the steel sheet by reheating after accelerated cooling.

Further, patent document 7 proposes a method for producing a high-strength acid-resistant steel sheet for a line pipe having a sheet thickness of 30mm or more, in which a surface layer portion of the steel sheet is heated while suppressing a temperature increase in a center portion of the steel sheet, in a reheating treatment after accelerated cooling.

Disclosure of Invention

Problems to be solved by the invention

According to the method described in patent document 1, dislocations introduced by pipe expansion are recovered and the compressive strength is improved. However, since the energization heating is continued for 5 minutes or more after the tube expansion, the productivity is poor.

In the method described in patent document 2, it is necessary to separately manage the heating temperature and the heating time of the outer surface and the inner surface of the steel pipe. This is difficult in actual manufacturing and it is very difficult to perform quality control in a mass production process. In addition, the method described in patent document 3 requires that the accelerated cooling stop temperature in the production of the steel sheet be low, i.e., 300 ℃. Therefore, the strain of the steel sheet becomes large, and the roundness in the case of manufacturing the steel pipe by the UOE process is reduced, and further, in order to obtain the steel pipe from Ar₃Since accelerated cooling is performed at this point, rolling at a relatively high temperature is required, which causes a problem of deterioration in toughness.

According to the method described in patent document 4, since there is substantially no tensile pre-strain in the tube circumferential direction, the bauxitic effect is not exhibited, and high compressive strength can be obtained. However, if the expansion ratio is low, it becomes difficult to maintain the roundness of the steel pipe, and there is a possibility that the collapse resistance of the steel pipe deteriorates.

In actual pipeline laying, the collapse becomes a problem of a portion (sag-bend portion) where the pipe reaching the seabed is bent and deformed. For line pipes, circumferential welding is performed regardless of the position of the welded portion of the steel pipe, and the line pipes are laid on the seabed. Therefore, even if the steel pipe is manufactured by performing the pipe manufacturing process and welding so that the portion having the largest diameter in the cross section of the steel pipe becomes the seam-welded portion as described in patent document 5, the position of the seam-welded portion at the time of actual line pipe laying cannot be specified. Therefore, the technique of patent document 5 does not actually exert any effect.

The steel sheet described in patent document 6 is difficult to be applied to a thick-walled line pipe for deep sea because it is necessary to heat the steel sheet up to the center of the steel sheet during reheating, and the Drop Weight Tear Test (DWTT) performance may be degraded. Further, there is room for improvement from the viewpoint of thickening of the steel sheet.

According to the method described in patent document 7, the hard phase 2 fraction of the surface layer portion of the steel sheet is reduced while suppressing the reduction in DWTT (Drop Weight Tear Test) performance. Therefore, not only a steel sheet with a small hardness at the surface layer portion of the steel sheet and a small unevenness in material quality can be obtained, but also the bauschinger effect due to a small hardness by 2 nd order subtraction can be expected. However, it is difficult to stably obtain an intensity of X70 level or more while ensuring DWTT performance with the technique described in patent document 7.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel material for a line pipe, which has a thick wall of 30mm or more in thickness, has high strength required for application to a submarine line pipe, and is excellent in low-temperature toughness and DWTT performance, a method for producing the same, and a method for producing a line pipe.

Means for solving the problems

The inventors of the present application have conducted intensive studies in order to suppress a decrease in compressive strength due to the bauschinger effect and to obtain strength and toughness at the same time. As a result, the following findings were obtained.

(a) The reduction in compressive strength due to the bauschinger effect is caused by the occurrence of a stress (also referred to as back stress) due to the accumulation of dislocations at the heterogeneous interface and the hard phase 2, and in order to prevent this, it is effective to form a homogeneous structure in order to reduce the interface between the soft phase and the hard phase, which is the site where the dislocations are accumulated. Therefore, by making the metal structure a structure mainly composed of bainite in which the generation of soft polygonal ferrite and hard island-like martensite is suppressed, it is possible to suppress the reduction of the compressive strength due to the bauxitic effect.

(b) High-strength steel produced by accelerated cooling, particularly thick-walled steel sheet used for marine line pipes, contains many alloying elements to obtain a required strength, and therefore has high hardenability and it is difficult to completely suppress the generation of island Martensite (hereinafter, also referred to as "MA"). However, by decomposing MA into cementite by optimizing accelerated cooling, a decrease in compressive strength due to the bauschinger effect can be suppressed.

(c) In order to improve the low temperature toughness, it is effective to lower the rolling temperature at the time of hot rolling of the steel sheet and refine the structure. However, when the rolling temperature is too low, polygonal ferrite is generated, and the structure after accelerated cooling is a structure in which bainite and polygonal ferrite are mixed, and the bauschinger effect becomes large. On the other hand, by optimizing the composition, the generation of polygonal ferrite after rolling at low temperature can be suppressed, and low-temperature toughness and compressive strength can be obtained at the same time. Further, by controlling the reduction amount during hot rolling, it is possible to introduce many deformed zones which become nuclei of transformation and to refine the structure, and it is possible to obtain high low-temperature toughness even in a thick steel plate having a thickness of 30mm or more.

The present invention has been completed by further studies based on the above findings. The gist of the present invention is as follows.

[1]A method for producing a steel material for a line pipe, the steel material having a tensile strength of 570MPa or more, a compressive strength of 440MPa or more and a plate thickness of 30mm or more, wherein a steel having a composition of the following components is heated to a temperature of 1000 to 1200 ℃, the cumulative reduction ratio in a temperature region in which the cumulative reduction ratio in a non-recrystallization temperature region is 60% or more and (rolling end temperature +20 ℃) is 50% or more, and the rolling end temperature is Ar₃Hot rolling at a temperature of not lower than the transformation point and not higher than 790 ℃, and then rolling with Ar₃Accelerated cooling at a cooling rate of 10 ℃/s or more from a cooling start temperature of 10 ℃/s or moreThe surface temperature of the steel plate is 300-500 ℃,

the above composition contains, in mass%, C: 0.030 to 0.10%, Si: 0.01-0.20%, Mn: 1.0-2.0%, Nb: 0.005-0.050%, Ti: 0.005-0.025%, Al: 0.08% or less, and further containing Cu in mass%: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.1% or less, Ceq value represented by formula (1) of 0.350 or more, Pcm value represented by formula (2) of 0.20 or less, and Ar represented by formula (3)₃A transformation point of 750 ℃ or below, and the balance of Fe and inevitable impurities,

Ceq＝C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)

Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)

Ar₃(℃)＝910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (3)

in the formulae (1) to (3), the element symbol indicates the mass% of the contained element, and is 0 when not contained.

[2] A method for producing a line pipe, wherein the line pipe has a tensile strength of 570MPa or more, a compressive strength of 440MPa or more, and a plate thickness of 30mm or more, wherein a steel material for line pipes produced by the method described in [1] is formed into a steel pipe shape by cold forming, and after seam welding of a butt portion, the steel pipe is expanded at an expansion ratio of 1.2% or less to produce a steel pipe.

[3]A steel material for a line pipe, which has a tensile strength of 570MPa or more, a compressive strength of 440MPa or more, and a plate thickness of 30mm or more, and which has a composition comprising, in mass%, C: 0.030 to 0.10%, Si: 0.01-0.20%, Mn: 1.0-2.0%, Nb: 0.005-0.050%, Ti: 0.005-0.025%, Al: 0.08% or less, and further containing Cu in mass%: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.1% or less, Ceq value represented by formula (1) of 0.350 or more, Pcm value represented by formula (2) of 0.20 or less, and Ar represented by formula (3)₃The phase transition point is less than 750 ℃, the remainderThe amount of Fe and inevitable impurities,

the steel material for line pipes has a microstructure mainly comprising bainite, wherein at a position 1/4 in sheet thickness, the area fraction of polygonal ferrite is 10% or less, the area fraction of island-shaped martensite is 5% or less, the average grain size of bainite at a position 1/2 in sheet thickness is 10 μm or less,

Ceq＝C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)

Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)

Ar₃(℃)＝910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (3)

in the formulae (1) to (3), the element symbol indicates the mass% of the contained element, and is 0 when not contained.

[4] A method for producing a line pipe, wherein the line pipe has a tensile strength of 570MPa or more, a compressive strength of 440MPa or more, and a plate thickness of 30mm or more, wherein the steel product for line pipe as defined in [3] is cold-formed into a steel pipe shape, and after seam welding of a butt portion, a steel pipe is produced by pipe expansion at an expansion ratio of 1.2% or less.

Effects of the invention

According to the present invention, a steel material for line pipe having high strength and excellent low-temperature toughness and DWTT performance can be obtained. The invention is suitable for submarine pipeline pipes.

Further, according to the present invention, it is possible to provide a thick-walled line pipe which does not require special molding conditions in steel pipe molding, does not require heat treatment after pipe fabrication, and has excellent low-temperature toughness and high compressive strength.

Detailed Description

Hereinafter, embodiments of the present invention will be described. Unless otherwise specified, "%" indicating the content of the component elements means "% by mass".

1. With respect to chemical composition

C：0.030～0.10％

C is the most effective element for improving the strength of a steel sheet produced by accelerated cooling. However, if it is less than 0.030%, sufficient strength cannot be secured, while if it exceeds 0.10%, not only toughness deteriorates but also generation of MA is promoted, resulting in a decrease in compression strength. Therefore, the C content is set to 0.030 to 0.10%. Preferably 0.040% or more and preferably 0.098% or less.

Si：0.01～0.20％

Si is contained for deoxidation. However, if it is less than 0.01%, the deoxidation effect is insufficient, while if it exceeds 0.20%, not only toughness deteriorates but also MA generation is promoted, thereby resulting in a decrease in compressive strength. Therefore, the Si content is defined to be 0.01 to 0.20%. Preferably 0.03% or more and preferably 0.15% or less.

Mn：1.0～2.0％

Setting the ratio of Mn: 1.0 to 2.0 percent. Mn is contained to improve strength and toughness. However, if the content is less than 1.0%, the effect is insufficient, while if the content exceeds 2.0%, the toughness is deteriorated. Therefore, the Mn content is defined to be 1.0 to 2.0%. Preferably 1.5% or more and preferably 1.95% or less.

Nb：0.005～0.050％

Nb improves toughness by refining the structure, and also forms carbide, contributing to improvement in strength. However, if it is less than 0.005%, the effect is insufficient, while if it exceeds 0.050%, the toughness of the welding heat-affected zone is deteriorated. Therefore, the Nb content is set to 0.005 to 0.050%. Preferably 0.010% or more and preferably 0.040% or less.

Ti：0.005～0.025％

Ti suppresses coarsening of austenite during slab heating by the pinning effect of TiN, and improves toughness. However, if it is less than 0.005%, the effect is insufficient, while if it exceeds 0.025%, the toughness is deteriorated. Therefore, the Ti content is set to 0.005-0.025%. Preferably 0.008% or more and preferably 0.023% or less.

Al: less than 0.08%

Al is contained as a deoxidizer. However, if it exceeds 0.08%, the cleanliness of the steel is lowered, resulting in deterioration of toughness. Therefore, the Al content is defined to be 0.08% or less. Preferably 0.05% or less.

Further, the present invention contains Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.1% or less of 1 or more.

Cu: less than 0.5%

Cu is an element effective for improving toughness and strength. However, if it exceeds 0.5%, the HAZ toughness of the weld portion deteriorates. Therefore, when Cu is contained, 0.5% or less is set. On the other hand, the lower limit is not particularly limited, and the content in the case of containing Cu is preferably 0.01% or more.

Ni: 1.0% or less

Ni is an element effective for toughness improvement and strength improvement. However, if it exceeds 1.0%, there is a possibility that HAZ toughness of the weld portion may deteriorate. Therefore, when Ni is contained, it is 1.0% or less. On the other hand, the lower limit is not particularly limited, and the content in the case of containing Ni is preferably 0.01% or more.

Cr: 1.0% or less

Cr is an element effective for improving strength by improving hardenability. However, if it exceeds 1.0%, the HAZ toughness of the weld portion deteriorates. Therefore, when Cr is contained, it is 1.0% or less. On the other hand, the lower limit is not particularly limited, and the content when Cr is contained is preferably 0.01% or more.

Mo: less than 0.5%

Mo is an element effective for toughness improvement and strength improvement. However, if it exceeds 0.5%, there is a possibility that HAZ toughness of the weld portion deteriorates. Therefore, when Mo is contained, the content is 0.5% or less. On the other hand, the lower limit is not particularly limited, and the content in the case of containing Mo is preferably 0.01% or more.

V: less than 0.1%

V is an element that forms a composite carbide and is very effective for improving the strength by precipitation strengthening, similarly to Nb and Ti. However, if it exceeds 0.1%, there is a possibility that HAZ toughness of the weld portion may deteriorate. Therefore, when V is contained, it is set to 0.1% or less. On the other hand, the lower limit is not particularly limited, and the content in the case of containing V is preferably 0.01% or more.

The present invention is characterized in that the Ceq value represented by formula (1) is 0.350 or more, the Pcm value represented by formula (2) is 0.20 or less, and Ar represented by formula (3)₃The transformation point is 750 ℃ or lower.

Ceq value: above 0.350

The Ceq value is set to 0.350 or more. The Ceq value is represented by the following formula (1). The Ceq value is related to the base metal strength and used as an index of strength. If the Ceq value is less than 0.350, a high tensile strength of 570MPa or more cannot be obtained. Therefore, the Ceq value is defined to be 0.350 or more. The Ceq value is preferably 0.360 or more.

Ceq＝C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)

In the formula (1), the symbol of the element represents the mass% of the element contained, and is 0 when not contained.

Pcm value: 0.20 or less

The Pcm value is 0.20 or less. The Pcm value is expressed by the following formula (2). The Pcm value is used as an index of weldability, and the higher the Pcm value is, the worse the toughness of the welded HAZ portion is. Particularly, in thick-walled high-strength steel, since the influence thereof becomes remarkable, Pcm needs to be strictly limited. Therefore, Pcm value is defined to be 0.20 or less. The Pcm value is preferably 0.19 or less.

Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)

In the formula (2), the symbol of the element represents the mass% of the element contained, and is 0 when not contained.

Ar₃Phase transformation point: below 750 deg.C

Ar₃The transformation point is 750 ℃ or lower. The following formula (3) is Ar₃The formula of the phase transformation point. Ar (Ar)₃The higher the transformation point is, the more difficult it is to obtain the microstructure of the present invention because ferrite is generated at a high temperature, and it is difficult to obtain both the compressive strength and the toughness. Therefore, the composition of the components is controlled so that Ar₃The transformation point is 750 ℃ or lower.

Ar₃(℃)＝910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (3)

In the formula (3), the symbol of the element indicates that the element is contained by mass%, and is 0 when not contained.

The balance other than the above components is Fe and inevitable impurities. However, other elements than the above elements may be contained within a range not affecting the operation and effect of the present invention.

2. About the metal structure

Mainly comprising bainite

The microstructure of the present invention is mainly composed of bainite from the viewpoint of suppressing a decrease in compressive strength due to the bauschinger effect. The microstructure of the present invention mainly includes bainite, and means that the area fraction of bainite with respect to the entire microstructure is 85% or more. In order to suppress the decrease in compressive strength due to the bauschinger effect, the microstructure of a bainite single phase is preferable because the accumulation of dislocations at the heterogeneous interface and in the hard phase 2 is avoided. The remaining structure other than bainite may be allowed to be 15% or less. The area fraction of bainite is a value at a position 1/4 in the sheet thickness.

At the position 1/4, the area fraction of polygonal ferrite is 10% or less and the area fraction of island-shaped martensite is 5% or less

In order to suppress the bauschinger effect and obtain high compressive strength, it is desirable to form a uniform structure without a soft polygonal ferrite phase and hard island-like martensite, and to suppress local dislocation aggregation generated inside the structure during deformation. Therefore, as described above, a structure mainly composed of bainite is formed, and the area fraction of polygonal ferrite is 10% or less and the area fraction of island-shaped martensite is 5% or less at the position 1/4 in the sheet thickness. The area fraction of polygonal ferrite and island-like martensite may be 0%.

The average grain diameter of bainite at 1/2 th position of plate thickness is 10 μm or less

In the case of a thick material, a fine structure is effective particularly in order to obtain sufficient toughness of the base material at the position 1/2 of the thickness. This effect can be obtained by setting the bainite grain diameter at the 1/2 th position of the plate thickness to 10 μm or less. Therefore, the average grain size of bainite at the 1/2 th site of the plate thickness is defined to be 10 μm or less.

The microstructure of the present invention may contain other phases than bainite, polygonal ferrite, and island martensite as long as the microstructure has the above-described configuration. As the other phase, pearlite, cementite, martensite, or the like is present, for example. The smaller the amount of these other phases, the better, the area ratio at the position 1/4 of the sheet thickness is preferably 5% or less.

3. Method for producing steel material for line pipe

In the method for producing a steel material for a line pipe according to the present invention, a slab containing the above chemical components is heated and hot-rolled, and then accelerated cooling is performed. The reason for limiting the production conditions will be described below. In the following description, unless otherwise specified, the temperature is an average temperature of a steel sheet (steel material) in a thickness direction thereof. The average temperature of the steel sheet (steel material) in the thickness direction is determined by simulation calculation or the like based on the thickness, surface temperature, cooling conditions, and the like. For example, the average temperature of a steel sheet (steel material) in the thickness direction is obtained by calculating the temperature distribution in the thickness direction by the difference method.

Heating temperature of steel billet: 1000 to 1200 DEG C

If the billet heating temperature is less than 1000 ℃, the NbC is insufficiently dissolved and the subsequent strengthening by precipitation cannot be obtained. On the other hand, if it exceeds 1200 ℃, the low temperature toughness deteriorates. Therefore, the billet heating temperature is set to 1000 to 1200 ℃. Preferably 1000 ℃ or higher and preferably 1150 ℃ or lower.

Cumulative reduction in unrecrystallized temperature region: a cumulative reduction ratio in a temperature range of 60% or more and (rolling end temperature +20 ℃) or less: over 50 percent

In order to obtain high base material toughness, it is necessary to perform sufficient rolling in a non-recrystallization temperature region in a hot rolling step. However, if the cumulative reduction ratio in the non-recrystallization temperature region is less than 60% or the cumulative reduction amount in the temperature region of not more than (rolling end temperature +20 ℃) is less than 50%, the effect of refining crystal grains is insufficient. Therefore, the cumulative reduction ratio in the non-recrystallization temperature region is 60% or more, and the cumulative reduction amount in the temperature region of (rolling end temperature +20 ℃) or less is 50% or more. The cumulative reduction ratio in the non-recrystallization temperature range is preferably 65% or more. The cumulative reduction ratio in a temperature range of (rolling end temperature +20 ℃) or less is preferably 55% or more.

Rolling finishing temperature: ar (Ar)₃Above the transformation point and below 790 DEG C

In order to suppress the decrease in strength due to the bauxige effect, it is necessary to suppress the generation of a soft structure such as polygonal ferrite by mainly forming a metal structure into a bainite structure. Therefore, hot rolling needs to be performed in a temperature region where polygonal ferrite is not generated, i.e., Ar₃In a temperature region above the phase transition point. Therefore, the rolling end temperature is defined as Ar₃Above the transformation point. In addition, in order to obtain high toughness of the base material, Ar is used₃In the temperature range of the transformation point or more, rolling needs to be performed in the low temperature range, so the upper limit of the rolling completion temperature is 790 ℃. The rolling completion temperature is preferably 780 ℃ or lower.

Cooling start temperature: ar (Ar)₃Above the phase transition point

If the cooling initiation temperature is lower than Ar₃At the transformation point, the area fraction of polygonal ferrite at the position 1/4 where the plate thickness is greater than 10%, and a sufficient compressive strength cannot be secured due to the bauschinger effect. Therefore, the cooling start temperature is defined as Ar₃Above the transformation point. Is preferably (Ar)₃The phase transition point +10 ℃ C. or higher.

In addition, Ar is₃The phase transformation point can be obtained by equation (3) as described above.

Ar₃(℃)＝910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (3)

In the formula (3), the symbol of the element indicates that the element is contained by mass%, and is 0 when not contained.

Cooling rate: 10 ℃/s or more

Accelerated cooling at a cooling rate of 10 ℃/s or more is an essential process for obtaining a high-strength and high-toughness steel sheet, and the strength-improving effect by phase transformation strengthening can be obtained by cooling at a high cooling rate. However, if the cooling rate is less than 10 ℃/s, not only is sufficient strength not obtained, but also C is diffused during cooling to enrich C in the non-transformed austenite, and the amount of MA produced increases. As mentioned above, the presence of a hard phase 2, such as MA, promotes the bauschinger effect and thus leads to a reduction in compressive strength. However, when the cooling rate is 10 ℃/s or more, the diffusion of C during cooling is small, and the generation of MA is also suppressed. Therefore, the cooling rate at the time of accelerated cooling is set to 10 ℃/s or more. Preferably 20 ℃/s or more.

Cooling stop temperature: the surface temperature of the steel plate is 300-500 DEG C

The steel sheet is quenched to a surface temperature of 300 to 500 ℃ by accelerated cooling after rolling, thereby suppressing the generation of MA and pearlite and obtaining a uniform structure mainly composed of bainite. However, when the cooling stop temperature is lower than 300 ℃, MA is generated, which causes a decrease in compressive strength and deterioration in toughness due to the bauschinger effect. When the cooling stop temperature of the steel sheet surface is 300 ℃ or higher, MA is decomposed by reheating, and thus a uniform structure can be obtained. On the other hand, if the cooling stop temperature exceeds 500 ℃, pearlite is produced, and not only is sufficient strength not obtained, but also the compressive strength is reduced due to the bauxitic effect. Therefore, the cooling stop temperature is set to 300 to 500 ℃ for the steel sheet surface temperature. Preferably 350 ℃ or higher and preferably 490 ℃ or lower.

4. Method for manufacturing line pipe

The present invention manufactures a steel pipe (line pipe) using the steel sheet (steel material) manufactured by the above method. Examples of a method of forming a steel material include a method of forming a steel pipe into a shape by cold forming using an UOE process, press bending (also referred to as bending press), or the like. In the UOE process, after the widthwise end portions of a steel sheet (steel material) as a material are beveled, the widthwise end portions of the steel sheet are bent using a C-shaped press machine, and then the steel sheet is formed into a cylindrical shape so that the widthwise end portions of the steel sheet face each other using U-shaped and O-shaped press machines. Next, the opposing widthwise ends of the steel sheets are butted and welded. This welding is called seam welding. In the seam welding, a method including the following two steps is preferable: a tack welding step of constraining the cylindrical steel plates, aligning the width-direction ends of the opposing steel plates with each other, and tack welding the aligned steel plates; and a main welding step of performing seam welding on the inner and outer surfaces of the butted portion of the steel sheets by a submerged arc welding method. After seam welding, pipe expansion is performed to eliminate welding residual stress and improve the roundness of the steel pipe. In the pipe expanding step, the pipe expansion ratio (the ratio of the amount of change in the outer diameter before and after pipe expansion to the outer diameter of the pipe before pipe expansion) is 1.2% or less. This is because, if the expansion ratio is too large, the reduction in compressive strength due to the bauschinger effect becomes large, and therefore the expansion ratio is preferably 1.0% or less. From the viewpoint of reducing the welding residual stress and improving the roundness of the steel pipe, the expansion ratio is preferably 0.4% or more, and more preferably 0.6% or more.

In the case of press bending, a steel pipe having a substantially circular cross-sectional shape is manufactured by repeating three-point bending of a steel plate and then successively forming the steel plate. Then, seam welding is performed in the same manner as in the UOE process. In the case of press bending, the pipe may be expanded after seam welding.