阅读说明：本技术 管线管用钢材及其制造方法以及管线管及其制造方法 (Steel material for line pipe, method for producing same, line pipe, and method for producing same ) 是由 安田恭野 嶋村纯二 村冈隆二 于 2020-03-19 设计创作，主要内容包括：目的在于提供厚壁、具有应用于海底管线所需的压缩强度及优异的低温韧性及DWTT性能且抗塌陷性能优异的管线管用钢材及其制造方法、以及具有需要的压缩强度、优异的低温韧性及DWTT性能且抗塌陷性能优异的管线管及其制造方法。管线管用钢材具有规定的成分组成,在距钢材表面为板厚1/8位置处的金属组织中,贝氏体的面积百分比为85％以上,多边形铁素体的面积百分比为10％以下,且岛状马氏体的面积百分比为5％以下,从钢材表面起到板厚1/8位置为止的轧制垂直方向上的0.23％压缩强度为340MPa以上,DWTT试验中的塑性断口率成为85％以上的温度为-10℃以下。(The purpose is to provide a thick steel material for a line pipe having a compressive strength required for application to an offshore line, excellent low-temperature toughness and DWTT performance, and excellent collapse resistance, a method for producing the same, a line pipe having a required compressive strength, excellent low-temperature toughness and DWTT performance, and excellent collapse resistance, and a method for producing the same. A steel material for line pipes having a predetermined composition, wherein in a microstructure at a position 1/8% in thickness from the surface of the steel material, the area percentage of bainite is 85% or more, the area percentage of polygonal ferrite is 10% or less, the area percentage of island-like martensite is 5% or less, the 0.23% compressive strength in a direction perpendicular to rolling from the surface of the steel material to a position 1/8 in thickness is 340MPa or more, and the temperature at which the plastic fracture rate in a DWTT test is 85% or more is-10 ℃ or less.)

1. A steel material for a line pipe, which has a composition of components containing, in mass%

C：0.030～0.10％、

Si：0.01～0.15％、

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, a Ceq value represented by formula (1) of 0.35 or more, a Pcm value represented by formula (2) of 0.20 or less, and the balance of Fe and unavoidable impurities,

in a metal structure at a position 1/8 DEG from the surface of the steel material in terms of the plate thickness, the area percentage of bainite is 85% or more, the area percentage of polygonal ferrite is 10% or less, and the area percentage of island-like martensite is 5% or less,

the steel material for line pipe has a 0.23% compressive strength in the direction perpendicular to rolling from the surface of the steel material to a position where the thickness is 1/8 MPa or more, a plastic fracture rate in DWTT test of 85% or more, and a temperature of-10 ℃ or less,

ceq value C + Mn/6+ (Cu + Ni)/15+ (Cr + Mo + V)/5. cndot. (1)

Pcm value of C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10. cndot. (2)

In the formulae (1) to (2), the symbol of the element represents the mass% of the element contained, and is 0 when not contained.

2. The steel product for line pipes according to claim 1, further comprising, in mass%, Ca: 0.0005 to 0.0035%.

3. A method for producing a steel material for line pipes, comprising heating a steel having the composition of claim 1 or 2 to a temperature of 1000 to 1200 ℃,

the rolling reduction rate is 60% or more in the non-recrystallization temperature range and the rolling finishing temperature is Ar₃Above the transformation point and (Ar)₃Hot rolling at a transformation point of +60 ℃ or lower,

from Ar₃Accelerated cooling at a cooling rate of 10 ℃/s or more from a temperature of not less than the transformation point to 200 to 450 ℃,

then, reheating is performed so that the steel sheet thickness is 350 ℃ or higher at the 1/8-point and 530 ℃ or lower at the steel surface,

the steel material for line pipe has a 0.23% compressive strength in the direction perpendicular to rolling from the surface of the steel material to a position where the thickness is 1/8 MPa or more, and a temperature at which the plastic fracture rate in DWTT test is 85% or more of-10 ℃ or less.

4. A line pipe having a component composition containing, in mass%

C：0.030～0.10％、

Si：0.01～0.15％、

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 line pipe 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, a Ceq value represented by formula (1) of 0.35 or more, a Pcm value represented by formula (2) of 0.20 or less, and the balance of Fe and unavoidable impurities,

in the metal structure at the position 1/8 tube thickness from the inner surface of the tube, the area percentage of bainite is more than 85%, the area percentage of polygonal ferrite is less than 10%, and the area percentage of island-shaped martensite is less than 5%,

the line pipe has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in the circumferential direction at a position of a long axis of the pipe from the inner surface of the pipe to a position of 1/8 mm thick, and has a temperature of-10 ℃ or less at which the plastic fracture rate in a DWTT test becomes 85% or more,

ceq value C + Mn/6+ (Cu + Ni)/15+ (Cr + Mo + V)/5. cndot. (1)

Pcm value of C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10. cndot. (2)

In the formulae (1) to (2), the symbol of the element represents the mass% of the element contained, and is 0 when not contained.

5. The line pipe of claim 4, further comprising, in mass%, Ca: 0.0005 to 0.0035%.

6. The linepipe of claim 4 or 5 further having a coating.

7. A method for producing a line pipe, comprising cold-forming the steel material for a line pipe according to claim 1 or 2 into a pipe shape, seam-welding the butt seams, and then expanding the pipe to produce a pipe at an expansion ratio of 1.2% or less,

the line pipe has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in the circumferential direction at a position of a long axis of the pipe from the inner surface of the pipe to a position of 1/8 mm thick, and has a temperature of-10 ℃ or less at which the plastic fracture rate in a DWTT test becomes 85% or more.

8. A method for producing a line pipe, wherein a steel material for a line pipe produced by the method described in claim 3 is cold-formed into a pipe shape, butt seam-welded, and then expanded to a pipe expansion ratio of 1.2% or less to produce a pipe,

the line pipe has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in the circumferential direction at a position of a long axis of the pipe from the inner surface of the pipe to a position of 1/8 mm thick, and has a temperature of-10 ℃ or less at which the plastic fracture rate in a DWTT test becomes 85% or more.

9. The line pipe manufacturing method according to claim 7 or 8, wherein after the pipe expansion, a coating treatment including heating to a temperature of 200 ℃ or higher is performed on the pipe surface.

Technical Field

The present invention relates to a steel material for line pipes and a method for producing the same, and a line pipe and a method for producing the same. The present invention relates to a steel material for a line pipe suitable for a line pipe for transporting oil and natural gas, particularly a submarine line requiring high collapse resistance, a method for producing the same, a line pipe, and a method for producing the same. Unless otherwise specified, the compressive strength of the present invention means 0.23% compressive proof stress, and is also referred to as compressive yield strength.

Background

With the recent increase in energy demand, development of oil and gas pipelines has been pursued, and a large number of sea-crossing pipelines have been developed due to remote development of gas fields and oil fields and diversification of transportation routes. In order to prevent the pipeline pipes used for the submarine pipeline from collapsing (collapsing) due to water pressure, pipeline pipes having a pipe thickness greater than that of the land pipeline are used, and high roundness is required. Further, as a characteristic of the subsea line pipe, a high compressive strength is required in order to resist a compressive stress generated in the pipe circumferential direction by a water pressure from the outside.

The UOE steel pipe has a pipe expanding process in a final pipe manufacturing step, is laid on the seabed after being subjected to tensile deformation in the pipe circumferential direction, and is compressed in the pipe circumferential direction by external water pressure. Therefore, the decrease in compressive yield strength due to the bauschinger effect becomes a problem.

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 holding the pipe at a temperature for a certain time or more.

Further, as a method of recovering from a decrease in compressive yield strength due to the bauschinger effect by heating after expanding the pipe in the same manner, patent document 2 proposes a method of recovering from the bauschinger effect of a portion subjected to tensile deformation on the outer surface side by heating the outer surface of the steel pipe to a temperature higher than that of the inner surface, and maintaining work hardening of the compression on the inner surface side,patent document 3 proposes that in the steel sheet production process of Nb and Ti-containing steel, Ar is used as a starting material₃And a method comprising performing accelerated cooling after hot rolling at a temperature not lower than 300 ℃ and heating the steel pipe after forming the steel pipe by a UOE process.

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

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

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

Patent document 7 proposes a method for producing a high-strength sulfur-resistant line pipe steel sheet having a sheet thickness of 30mm or more by heating the surface layer portion of the steel sheet while suppressing the temperature rise in the center portion of the steel sheet in a reheating treatment after accelerated cooling.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 9-49025

Patent document 2: japanese patent laid-open publication No. 2003-342639

Patent document 3: japanese patent laid-open publication No. 2004-35925

Patent document 4: japanese patent laid-open publication No. 2002-102931

Patent document 5: japanese patent laid-open publication No. 2003-340519

Patent document 6: japanese patent laid-open No. 2008-56962

Patent document 7: japanese laid-open patent publication No. 2009-52137

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 needs to be 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 individually control the heating temperature and the heating time of the outer surface and the inner surface of the steel pipe, which is difficult in actual production and very difficult in quality control in a mass production process. In the method described in patent document 3, the accelerated cooling stop temperature in the production of the steel sheet must be set to a low temperature of 300 ℃. Therefore, the strain of the steel sheet increases, and the roundness when the steel pipe is manufactured by the UOE process decreases. In addition, in order to get from Ar₃Since the accelerated cooling is performed at this point, rolling at a relatively high temperature is required, and there is a problem that toughness deteriorates.

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

In actual pipeline laying, what becomes a problem of collapse resistance is a portion (sag-bend) of the seabed where the pipe bears bending deformation. When the pipeline is laid on the seabed, circumferential welding is not performed in consideration of the position of the welded portion of the steel pipe. Therefore, even if the pipe making process and welding are performed to manufacture the steel pipe so that the portion of the maximum diameter of 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 laying of the pipeline cannot be specified, and therefore the technique of patent document 5 does not exhibit any effect in practice.

The steel sheet described in patent document 6 is difficult to be applied to thick-walled line pipes for deep sea because it is necessary to heat the steel sheet up to the center of the steel sheet during reheating, which may result in a reduction in DWTT (Drop Weight Tear Test) performance. Further, there is room for improvement from the viewpoint of thickening of the steel sheet. Furthermore, the collapse resistance of the steel pipe is related to the compressive flow stress at the surface layer inside the pipe close to the elastic limit. In patent document 6, the collapse resistance is evaluated at the position of the plate thickness 1/4. However, even if high compressive strength is obtained at the 1/4 th site, the effect of the ultimate collapse pressure on the actual steel pipe is small.

According to the method described in patent document 7, the percentage of hard phase 2 in the surface layer portion of the steel sheet can be reduced while suppressing the reduction in DWTT (Drop Weight Tear Test) performance. Therefore, not only can the hardness of the surface layer portion of the steel sheet be reduced and the steel sheet with small unevenness in material quality be obtained, but also the reduction of the bauschinger effect due to the small hardness number 2 minus can be expected. However, in the technique described in patent document 7, since the surface of the steel sheet is heated to 550 ℃ or higher, the compressive strength at the surface layer is reduced, and there is a possibility that the collapse resistance is deteriorated.

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 is thick and has a compressive strength required for application to a submarine line, excellent low-temperature toughness and DWTT performance, and excellent collapse resistance, a method for producing the same, a line pipe which has a required compressive strength, excellent low-temperature toughness and DWTT performance, and excellent collapse resistance, and a method for producing the same.

In the present invention, the excellent collapse resistance means that the 0.23% compressive strength in the vertical direction of rolling from the surface of the steel material to the 1/8 th position of the sheet thickness is 340MPa or more in the case of a steel material for line pipe, and means that the 0.23% compressive strength in the circumferential direction from the inner surface of the pipe to the 1/8 th position of the pipe thickness and at the axial position of the long axis of the pipe is 340MPa or more and the collapse pressure is 35MPa or more in the case of line pipe.

Means for solving the problems

The inventors of the present application have conducted intensive studies to improve the collapse resistance, and as a result, have obtained the following findings.

(a) The reason why the compressive strength is reduced by the bauschinger effect is that the adverse stress (also referred to as back stress) is caused by the dislocation deposition at the hetero-phase interface and the hard phase 2, and in order to prevent this, it is effective to form a homogeneous structure by reducing the interface between the soft phase and the hard phase which becomes the dislocation deposition portion. Therefore, the microstructure is a microstructure mainly composed of bainite in which the generation of soft polygonal ferrite or hard island martensite is suppressed, whereby the reduction of the compressive strength due to the bauxitic effect can be suppressed.

(b) High-strength steel produced by accelerated cooling, particularly thick-walled steel plate used for marine pipelines, has high hardenability because it contains many alloying elements in order to obtain a desired strength, and it is difficult to completely suppress the generation of island Martensite (Martensite-Austenite constituent, hereinafter also referred to as MA). However, by suppressing MA formation by component control, accelerating reheating after cooling, and the like, the MA is decomposed into cementite, and thereby a decrease in compressive strength due to the bauschinger effect can be suppressed. On the other hand, excessive reheating may reduce the compressive strength, but the reheating temperature at the surface layer is controlled to obtain a desired compressive strength.

(c) The compressive strength was evaluated generally at 0.5% compressive strength, and the collapse resistance was correlated with the 0.23% compressive strength at the surface layer in the pipe near the elastic limit, and by increasing the 0.23% compressive strength from the inner surface of the pipe to the position 1/8 of the pipe thickness, excellent collapse resistance was obtained.

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 steel material for line pipes, which has a composition of components containing, in mass%, C: 0.030 to 0.10%, Si: 0.01-0.15%, Mn: 1.0-2.0%, Nb: 0.005-0.050%, Ti: 0.005-0.025%, Al: 0.08% or less, the composition 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, a Ceq value represented by formula (1) of 0.35 or more, a Pcm value represented by formula (2) of 0.20 or less, and the balance of Fe and unavoidable impurities,

in a metal structure at a position 1/8 DEG from the surface of the steel material in terms of the plate thickness, the area percentage of bainite is 85% or more, the area percentage of polygonal ferrite is 10% or less, and the area percentage of island-like martensite is 5% or less,

the steel material for line pipe has a 0.23% compressive strength in the direction perpendicular to rolling from the surface of the steel material to a position where the thickness is 1/8 MPa or more, and a temperature at which the plastic fracture rate in DWTT test is 85% or more of-10 ℃ or less.

Ceq value C + Mn/6+ (Cu + Ni)/15+ (Cr + Mo + V)/5. cndot. (1)

Pcm value of C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10. cndot. (2)

In the formulae (1) to (2), the symbol of the element represents the mass% of the element contained, and is 0 when not contained.

[2] The steel material for line pipes according to [1], further comprising, in mass%, Ca: 0.0005 to 0.0035%.

[3] A method for producing a steel material for a line pipe, comprising heating a steel having the composition as defined in [1] or [2] to a temperature of 1000 to 1200 ℃,

the rolling reduction rate is 60% or more in the non-recrystallization temperature range and the rolling finishing temperature is Ar₃Above the transformation point and (Ar)₃Hot rolling at a transformation point of +60 ℃ or lower,

from Ar₃Accelerated cooling at a cooling rate of 10 ℃/s or more from a temperature of not less than the transformation point to 200 to 450 ℃,

then, reheating is performed so that the steel sheet thickness is 350 ℃ or higher at the 1/8-point and 530 ℃ or lower at the steel surface,

the steel material for line pipe has a 0.23% compressive strength in the direction perpendicular to rolling from the surface of the steel material to a position where the thickness is 1/8 MPa or more, and a temperature at which the plastic fracture rate in DWTT test is 85% or more of-10 ℃ or less.

[4] A line pipe having a component composition containing, in mass%, C: 0.030 to 0.10%, Si: 0.01-0.15%, Mn: 1.0-2.0%, Nb: 0.005-0.050%, Ti: 0.005-0.025%, Al: 0.08% or less, the line pipe 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, a Ceq value represented by formula (1) of 0.35 or more, a Pcm value represented by formula (2) of 0.20 or less, and the balance of Fe and unavoidable impurities,

in the metal structure at the position 1/8 tube thickness from the inner surface of the tube, the area percentage of bainite is more than 85%, the area percentage of polygonal ferrite is less than 10%, and the area percentage of island-shaped martensite is less than 5%,

the line pipe has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in the circumferential direction at a position of a long axis of the pipe from the inner surface of the pipe to a position of 1/8 mm thick, and has a temperature of-10 ℃ or less at which the plastic fracture rate in a DWTT test becomes 85% or more.

Ceq value C + Mn/6+ (Cu + Ni)/15+ (Cr + Mo + V)/5. cndot. (1)

Pcm value of C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10. cndot. (2)

In the formulae (1) to (2), the symbol of the element represents the mass% of the element contained, and is 0 when not contained.

[5] The line pipe according to [4], further containing Ca: 0.0005 to 0.0035%.

[6] The line pipe according to [4] or [5], further having a coating layer.

[7] A method for producing a line pipe, wherein the steel material for a line pipe according to [1] or [2] is cold-formed into a pipe shape, and after butt seam welding, the pipe is expanded to a pipe expansion ratio of 1.2% or less to produce a pipe, and wherein the line pipe has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in a circumferential direction at a pipe longitudinal axis position from a pipe inner surface to a pipe thickness 1/8 position, and a temperature of-10 ℃ or less at which a plastic fracture ratio in a DWTT test becomes 85% or more.

[8] A method for producing a line pipe, wherein a steel material for a line pipe produced by the method described in [3] is cold-formed into a pipe shape, and after butt seam welding, the pipe is expanded at an expansion rate of 1.2% or less to produce a pipe, and wherein the line pipe has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in a circumferential direction at a pipe longitudinal axis position from a pipe inner surface to a pipe thickness 1/8 position, and a temperature of-10 ℃ or less at which a plastic fracture rate in a DWTT test becomes 85% or more.

[9] The process for producing a line pipe according to [7] or [8], wherein after the pipe expansion, a coating treatment including heating to a temperature of 200 ℃ or higher is performed on the pipe surface.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a steel material for a line pipe excellent in collapse resistance can be obtained. The invention is suitable for deep sea pipelines.

Further, according to the present invention, a thick line pipe having excellent low-temperature toughness and high compressive strength can be provided without requiring special molding conditions in steel pipe molding or heat treatment after pipe production.

Detailed Description

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

1. Chemical composition of steel material for line pipe or line pipe

C：0.030～0.10％

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

Si：0.01～0.15％

Si is contained for deoxidation. However, if the content is less than 0.01%, the deoxidation effect is insufficient, and therefore the Si content is 0.01% or more. Preferably 0.03% or more. On the other hand, if it exceeds 0.15%, not only toughness deteriorates but also MA generation is promoted to lower compressive strength, so that the Si content is 0.15% or less. Preferably 0.10% or less.

Mn：1.0～2.0％

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, and therefore the Mn content is 1.0% or more. Preferably 1.5% or more. On the other hand, if it exceeds 2.0%, the toughness deteriorates, so the Mn content is 2.0% or less. Preferably 1.95% or less.

Nb：0.005～0.050％

Nb improves toughness by refining the structure, and contributes to strength improvement by forming carbide. However, if less than 0.005%, the effect is insufficient, so the Nb content is 0.005% or more. Preferably 0.010% or more. On the other hand, if it exceeds 0.050%, the weld heat affects the deterioration of toughness, so the Nb content is set to 0.050% or less. Preferably 0.040% or less.

Ti：0.005～0.025％

Ti suppresses coarsening of austenite during billet heating by the pinning effect of TiN, and improves toughness. However, if the content is less than 0.005%, the effect is insufficient, and therefore the Ti content is set to 0.005% or more. Preferably 0.008% or more. On the other hand, if it exceeds 0.025%, the toughness deteriorates, so the Ti content is set to 0.025% or less. Preferably 0.023% or less.

Al: less than 0.08%

Al is contained as a deoxidizer. In order to exert this effect, the Al content is preferably 0.01% or more. However, if it exceeds 0.08%, the cleanliness of the steel is lowered, resulting in deterioration of toughness. Therefore, the Al content is set to 0.08% or less. Preferably 0.05% or less.

In the present invention, 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, the content of Cu is 0.5% or less. On the other hand, the lower limit is not particularly limited, and when Cu is contained, the content 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, the content is 1.0% or less. On the other hand, the lower limit is not particularly limited, and when Ni is contained, the content is preferably 0.01% or more.

Cr: 1.0% or less

Cr is an element effective for improving strength by 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 when Cr is contained, the content thereof 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 when Mo is contained, the content is preferably 0.01% or more.

V: less than 0.1%

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

The present invention is characterized in that the Ceq value represented by formula (1) is 0.35 or more, and the Pcm value represented by formula (2) is 0.20 or less.

Ceq value: 0.35 or more

Ceq value: above 0.35. The Ceq value is represented by the following formula (1). The Ceq value is related to the base metal strength and is used as an index of the strength. If the Ceq value is less than 0.35, a high tensile strength of 570MPa or more cannot be obtained. Therefore, the Ceq value is set to 0.35 or more. Preferably 0.36 or more.

Ceq value C + Mn/6+ (Cu + Ni)/15+ (Cr + Mo + V)/5. cndot. (1)

Wherein the symbol of the element in the formula (1) represents the mass% of the element contained, and is 0 in the case of not containing the element.

Pcm value: 0.20 or less

Pcm value: 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. In particular, thick-walled high-strength steel has a significant influence, and therefore the Pcm value needs to be strictly limited. Therefore, Pcm value is set to 0.20 or less. Preferably 0.19 or less.

Pcm value of C + Si/30+ (Mn + Cu + Cr)/20+ Ni/60+ Mo/15+ V/10. cndot. (2)

Wherein the symbol of the element in the formula (2) represents the mass% of the element contained, and is 0 in the case of not containing the element.

In the present invention, the following elements may be contained as necessary.

Ca：0.0005～0.0035％

Ca is an element effective for controlling the morphology of sulfide-based inclusions and improving ductility. Since this effect is exhibited when the Ca content is 0.0005% or more, it is preferable to set the content to 0.0005% or more when Ca is contained. Even if Ca is contained in an amount exceeding 0.0035%, the effect is saturated, and conversely, the cleanliness may be reduced and the toughness may be deteriorated. Therefore, when Ca is contained, the content is preferably 0.0035% or less.

The balance of Fe and unavoidable impurities other than the above components. Elements other than the above elements may be contained as long as the operation and effect of the present invention are not affected.

2. Steel material for line pipe or metal structure of line pipe

In the present invention, the metal structure is defined at a position 1/8 in thickness from the steel material surface or 1/8 in thickness from the inner surface of the pipe. In the present invention, the compressive strength can be improved by controlling the metal structure at the position 1/8 the thickness of which is from the surface of the steel material, and a steel material for a line pipe or a line pipe having excellent collapse resistance can be obtained.

The area percentage of bainite is more than 85 percent

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 percentage of bainite with respect to the entire microstructure is 85% or more. In order to suppress the decrease in compressive strength due to the bauxiger effect and avoid dislocation deposition at the heterogeneous interface and the hard phase 2, it is preferable that the bainite single-phase metal structure is obtained, but a remaining structure other than bainite is allowable if it is 15% or less.

The area percentage of polygonal ferrite is 10% or less, and the area percentage of island-like martensite is 5% or less

In order to suppress the bauschinger effect and obtain high compressive strength, it is preferable to use a uniform structure having no soft polygonal ferrite phase or hard island-like martensite, and to suppress local dislocation deposition occurring inside the structure during deformation. Therefore, as described above, a structure mainly composed of bainite is adopted, and the area percentage of polygonal ferrite is defined to be 10% or less and the area percentage of island-like martensite is defined to be 5% or less. The percentage of the island-shaped martensite may be 0%. In addition, the area percentage of polygonal ferrite may be 0%.

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. Examples of the other phase include pearlite, cementite, and martensite. The other phases are preferably small, and the area percentage of the position 1/8 away from the steel material surface is preferably 5% or less.

In the present invention, the metal structure is not particularly limited in the portion closer to the plate thickness center than the position of 1/8% from the surface of the steel material or the portion closer to the tube thickness center than the position of 1/8% from the tube inner surface, and bainite is preferably 70% or more, more preferably 75% or more, from the viewpoint of balance of strength, toughness, and the like. As the remaining structure, ferrite, pearlite, martensite, island Martensite (MA), and the like are allowable if the total is 30% or less, more preferably 25% or less.

In the present invention, when the metal structure at the position 1/8 away from the steel material surface is the above-described structure, the compressive strength from the steel material surface to the position 1/8 of the plate thickness and the compressive strength from the inner surface of the pipe to the position 1/8 of the pipe thickness can be increased, and as a result, excellent collapse resistance can be obtained.

3. Method for producing steel material for line pipe

The method for producing a steel material for a line pipe according to the present invention is a method for producing a steel material for a line pipe, which comprises heating and hot rolling a steel material containing the above chemical components, then subjecting the steel material to accelerated cooling, and then tempering (reheating). 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 the steel material (steel sheet) in the thickness direction. The average temperature of the steel sheet 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 the steel sheet in the thickness direction is obtained by calculating the temperature distribution in the thickness direction using a difference method.

Heating temperature of steel blank: 1000 to 1200 DEG C

If the heating temperature of the steel billet is less than 1000 ℃, the NbC is insufficiently dissolved, so that the subsequent strengthening by precipitation cannot be obtained, and the HIC resistance is deteriorated due to coarse undissolved carbides. On the other hand, when it exceeds 1200 ℃, the DWTT characteristics deteriorate. Therefore, the heating temperature of the steel billet is set to 1000 to 1200 ℃. Preferably 1000 ℃ or higher, preferably 1150 ℃ or lower.

Cumulative reduction in unrecrystallized temperature range: over 60 percent

In the step of hot rolling the heated steel slab, rolling in the non-recrystallization temperature range is performed after rolling in the recrystallization temperature range. The rolling conditions in the recrystallization temperature range are not particularly limited. In order to obtain high base material toughness, it is necessary to perform sufficient rolling in a non-recrystallization temperature range in the hot rolling step. However, if the cumulative reduction ratio in the non-recrystallization temperature range is less than 60%, the effect of refining crystal grains is insufficient, and thus sufficient DWTT performance cannot be obtained. Therefore, the cumulative reduction in the non-recrystallization temperature range is 60% or more. The cumulative reduction ratio in the non-recrystallization temperature range is preferably 63% or more.

Rolling finishing temperature: ar (Ar)₃Above the transformation point and (Ar)₃Phase transition point +60 ℃ or lower

In order to suppress the decrease in strength due to the bauxige effect, it is necessary to suppress the formation of a soft structure such as polygonal ferrite by using a microstructure mainly composed of bainite as a metal structure. Therefore, hot rolling needs to be performed in a temperature range in which polygonal ferrite is not generated, that is, in Ar₃A temperature range above the phase transition point. Therefore, the rolling end temperature is defined as Ar₃At least the transformation point, preferably (Ar)₃The phase transition point +10 ℃ C. or higher. In addition, in order to obtain high base material toughness, Ar is required₃Since rolling is performed in a low temperature range within a temperature range of not less than the transformation point, the upper limit of the rolling end temperature is (Ar)₃Phase transition point +60 deg.C). Preferably, the rolling end temperature is (Ar)₃Transformation point +50 ℃ or lower.

In addition, Ar is₃The transformation point can be determined by the following formula (3).

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

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

If the cooling initiation temperature is lower than Ar₃When the area percentage of polygonal ferrite at the transformation point exceeds 10%, the strength is lowered by the bauschinger effect, and thus sufficient compressive strength cannot be secured. Therefore, the cooling start temperature is defined as Ar₃Above the transformation point. Is preferably (Ar)₃The phase transition point +10 ℃ C. or higher.

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. If the cooling rate is less than 10 ℃/s, not only is sufficient strength not obtained, but also C diffusion occurs, so that C is enriched into the non-transformed austenite, and the amount of MA produced increases. As described above, the presence of the hard phase 2 such as MA promotes the bauschinger effect, resulting in a decrease in compressive strength. However, when the cooling rate is 10 ℃/s or more, C diffusion during cooling is small, and MA generation 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. If the cooling rate is too high, a hard structure such as martensite is formed, and the toughness and the compressive strength are reduced by the acceleration of the bauxitic effect, and therefore the cooling rate is preferably 200 ℃/s or less.

Cooling stop temperature: 200 to 450 DEG C

The bainite phase can be generated by quenching to 200-450 ℃ with accelerated cooling after rolling is finished, and a uniform structure can be obtained. However, if the cooling stop temperature is less than 200 ℃, island Martensite (MA) is excessively generated, which leads to a decrease in compressive strength and deterioration in toughness due to the bauschinger effect. On the other hand, if the cooling stop temperature exceeds 450 ℃, 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 200 to 450 ℃. Preferably 250 ℃ or higher, and preferably 430 ℃ or lower.

Reheating temperature: 350 ℃ or higher at the 1/8-position of the sheet thickness and 530 ℃ or lower at the surface of the steel material

Reheating is performed after the above-mentioned accelerated cooling. In the accelerated cooling of a thick steel plate, the cooling rate of the surface layer portion of the steel plate is high, and the surface layer portion of the steel plate is cooled to a lower temperature than the inside of the steel plate. Therefore, island-like Martensite (MA) is easily generated in the surface layer portion of the steel sheet. Since the hard phase such as MA promotes the bauschinger effect, the degradation of the compressive strength due to the bauschinger effect can be suppressed by heating the surface layer portion of the steel sheet after accelerated cooling to decompose MA. However, when the temperature is lower than 350 ℃ at the 1/8 th steel sheet thickness, the decomposition of MA is insufficient, and when the temperature exceeds 530 ℃ at the steel sheet surface, the strength is lowered, so that it is difficult to obtain a predetermined strength. Further, the collapse resistance is related to the compressive strength from the steel material surface to the position of 1/8 mm thick, and the strength can be ensured while decomposing MA by controlling the reheating temperature from the steel material surface to the position of 1/8 mm thick. Therefore, the steel sheet is defined to have a thickness of 350 ℃ or more at the 1/8-point and 530 ℃ or less at the steel surface. Preferably, the steel sheet has a thickness of 1/8 ℃ or more at the position of 370 ℃ or less and a surface temperature of 520 ℃ or less.

The reheating means after accelerated cooling is not particularly limited, and for example, atmospheric furnace heating, gas combustion, induction heating, or the like can be used. In consideration of economy, controllability, and the like, induction heating is preferable.

4. Method for manufacturing line pipe

Steel pipes (line pipes) can be produced using the steel sheet (steel material) of the present invention or the steel sheet (steel material) produced by the above-described method. Examples of a method of forming a steel material include a UOE process, and a method of forming a steel pipe into a shape by cold forming such as bending (bending press). In the UOE process, after the widthwise end portions of a steel sheet (steel material) to be 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. Then, the opposing widthwise ends of the steel sheets are butted and welded. This welding is called seam welding. In the seam welding, it is preferable to have a two-stage process of a tack welding process in which a cylindrical steel plate is restrained and widthwise end portions of the opposing steel plates are aligned and tack welded; in the main welding step, seam welding is performed on the inner and outer surfaces of the butt joint portion of the steel plates by submerged arc welding. After seam welding, pipe expansion is performed to remove 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 of the pipe before and after expansion with respect to the outer diameter of the pipe before expansion) is 1.2% or less. This is because, when the expansion ratio is too large, the reduction in compressive strength becomes large due to the bauschinger effect, and 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.

After the pipe expansion, a coating treatment can be performed for corrosion prevention. As the coating treatment, for example, after heating the expanded steel pipe (pipe) to a temperature range of 200 ℃ or higher, a known resin may be applied to the outer surface or the inner surface of the steel pipe.

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 gradually forming. Thereafter, seam welding is performed in the same manner as in the UOE process described above. In the case of press bending, the pipe may be expanded after seam welding.

5. Steel material for pipeline

The steel material for line pipes of the present invention has the above-described composition and metal structure, and has a 0.23% compressive strength in the direction perpendicular to rolling from the surface of the steel material to a position where the thickness is 1/8 MPa or more and a temperature at which the plastic fracture rate in DWTT test is 85% or more of-10 ℃ or less. The steel material for a line pipe of the present invention has excellent collapse resistance by having a 0.23% compressive strength in the direction perpendicular to rolling from the surface of the steel material to a position where the plate thickness is 1/8 MPa or more. The 0.23% compressive strength can be measured by the method described in examples.

6. Pipeline pipe

The line pipe of the present invention has the above-described composition and metal structure, and has a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in the circumferential direction at a position of the long axis of the pipe from the inner surface of the pipe to a position of 1/8 mm in thickness, and has a temperature of-10 ℃ or less at which the plastic fracture rate in the DWTT test becomes 85% or more. The line pipe of the present invention has excellent collapse resistance by having a 0.23% compressive strength of 340MPa or more and a collapse pressure of 35MPa or more in the circumferential direction at a position along the long axis of the pipe from the inner surface of the pipe to a position where the pipe is 1/8 thick. The line pipe of the present invention having the above-described composition and metal structure and having a coating layer formed by coating treatment has a 0.23% compressive strength of 390MPa or more and a collapse pressure of 40MPa or more in the circumferential direction at the position of the long axis of the pipe from the inner surface of the pipe to the position of 1/8 mm in thickness, and is excellent in collapse resistance. Here, the tube longitudinal axis position is a position rotated by 90 degrees from the position of the minimum radius of the tube in consideration of the position in the circumferential direction of the tube. The 0.23% compressive strength can be measured by the method described in examples.

Examples

Steels (steel types a to J) having chemical compositions shown in table 1 were formed into ingots by a continuous casting method. Immediately after rolling the heated slab by hot rolling, the slab was heated and cooled by a water-cooling type cooling facility and reheated by an induction heating furnace or a gas combustion furnace to produce thick steel plates (nos. 1 to 23) having a thickness of 40 mm. The heating temperature, the rolling completion temperature, the cooling start temperature, and the cooling stop temperature are average temperatures of the steel sheet, and the reheating temperature is a temperature at the surface and a position 1/8 in the sheet thickness. The average temperature and the temperature at the 1/8 th position were calculated from parameters such as the sheet thickness and the thermal conductivity based on the surface temperature of the material or the steel sheet.

Further, a pipe having a pipe thickness of 39mm and an outer diameter of 813mm was produced by a UOE process using these steel sheets. Seam welding was performed by 4-electrode submerged arc welding in 1 pass for each of the inner and outer surfaces, and the input heat during welding was set to a range of 100kJ/cm depending on the thickness of the steel sheet. And (3) expanding the welded pipe with the pipe expansion rate of 0.6-1.5%. In addition, the expanded pipe was subjected to a coating treatment at 230 ℃. The steel sheet production conditions and steel pipe production conditions (expansion ratio) are shown in table 2.

[ Table 1]

[ Table 2]

The compression properties of the steel sheets produced as described above were evaluated by collecting compression test pieces from the surface of the steel sheet to a position where the sheet thickness was 1/8. Specifically, a rectangular test piece having a parallel portion of 2.5mm in thickness, 2.5mm in width and 4.0mm in length was obtained by subjecting a small piece of steel sheet for collecting a compression test piece having a direction perpendicular to rolling of the steel sheet to cutting or grinding from the other surface of the steel sheet to reduce the thickness of the small piece of steel sheet to 1/8. For this test piece, in order to simulate tube formation, a compressive strain of 2.5% was applied, and then a tensile strain of 1.0% was applied. A compression test in which a load was applied in the compression direction was performed using a test piece simulated by tube fabrication, and the stress at which the compressive strain on the obtained compressive stress-compressive strain curve was 0.23% was evaluated as 0.23% compressive strength.

With respect to the tensile properties of the pipe produced in the above manner, a tensile test was performed using a full thickness test piece in the pipe circumferential direction of API 5L as a test piece, and the tensile strength was evaluated. The compression characteristics of the tube were evaluated using test pieces taken from the circumferential direction of the tube on the inner surface side at the position of the longitudinal axis of the tube. Specifically, the tube sheet for collecting the compression test piece having the tube circumferential direction as the longitudinal direction was subjected to cutting or grinding from the outer surface side of the tube to thin a small piece of steel plate to a plate thickness of 1/8, and then a rectangular test piece having a parallel portion of 2.5mm in thickness, 2.5mm in width and 4.0mm in length was collected. A compression test in which a load was applied in the compression direction was performed on the test piece, and the stress at which the strain on the obtained stress-strain curve was 0.23% was evaluated as 0.23% compressive strength. The collapse resistance was evaluated by gradually applying water pressure to a 7 m-cut pipe in a pressure vessel and then evaluating the pressure at which the water pressure started to decrease as a collapse pressure. The compressive properties and the collapse resistance were measured after pipe expansion (in a pipe-forming state) (still in a pipe-forming state)) and after coating treatment at 230 ℃ (after heating at 230 ℃).

Further, using a DWTT test piece taken from the circumferential direction of the pipe, the temperature at which the plastic fracture rate became 85% was determined as 85% SATT.

For the HAZ toughness of the joint, the temperature at which the plastic fracture ratio became 50% was determined as vTrs. The notch position was set so that the charpy test piece had a weld line at the center of the notch bottom, and the weld metal and the base metal (including the weld heat-affected zone) were 1: 1, position.

The metal structure was sampled from a position spaced from the inner surface of the tube by a thickness of 1/8 a, a cross section parallel to the longitudinal direction of the tube was polished, and then etched with nital, and observed with an optical microscope. Then, the area percentages of bainite and polygonal ferrite were determined by image analysis using 3 photographs taken at 200 times. For the observation of MA, using a sample in which the area percentages of bainite and polygonal ferrite were measured, electrolytic etching (2-stage etching) was performed after nital etching, and then observation was performed using a Scanning Electron Microscope (SEM). Then, the area percentage of MA was obtained from 3 photographs taken at 1000-fold magnification by image analysis.

In the examples, the microstructure in the pipe was determined, and the result was treated as the microstructure of the steel sheet.

The results of the metal structure and the mechanical properties are shown in table 3.

[ Table 3]

In Table 3, Nos. 1 to 9 each had a tensile strength of 570MPa or more, a 0.23% compressive strength of 340MPa or more in a still-in-steel-plate state, 340MPa or more in a pipe-forming state and 390MPa or more after heating at 230 ℃, a collapse pressure of 35MPa or more and 40MPa or more after heating at 230 ℃ in a still-in-pipe state, 85% SATT in DWTT properties of-10 ℃ or less, and HAZ toughness of-20 ℃ or less, and all had good evaluation results.

On the other hand, the compositions of Nos. 10 to 19 are within the range of the present invention, but the production method is outside the range of the present invention, and therefore, the desired metal structure cannot be obtained. As a result, some of the tensile strength, 0.23% compressive strength, or DWTT characteristics are deteriorated. No.20 to 23 are chemical components other than those of the present invention, and therefore some of tensile strength, compressive strength, DWTT characteristics or HAZ toughness are deteriorated.

According to the present invention, a pipe having high strength and excellent low-temperature toughness and having an API-X70 rating or higher can be obtained, and the pipe can be applied to a deep-sea line pipe requiring high collapse resistance.