Steel sheet, steel tube, and method for producing steel sheet

Abstract

To provide a steel sheet having a yield strength of 300 MPa or more and exhibiting superior SSC resistance and HIC resistance.SOLUTION: In a steel sheet adopted herein, the metal structure at a depth of 0.1 mm from the surface contains, in terms of area ratio, one or both of polygonal ferrite and granular bainite in a total area ratio of 70-98%, with an average particle diameter of 30.0 μm or less; the metal structure at a depth of 0.5 mm from the surface contains, in terms of area ratio, one or both of polygonal ferrite and granular bainite in a total area ratio of 75-95%; when the thickness of the steel sheet is t, the metal structure at a depth of 2t / 5 from the surface contains one or both of polygonal ferrite and granular bainite in a total area ratio of 50-97%, with an average particle diameter of 20.0 μm or less; Hvmax, which is the maximum hardness in the surface layer portion up to a depth of 1.0 mm from the surface of the steel sheet, is 220 Hv or less; and the yield ratio is 70% or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to steel plates, steel pipes, and a method for manufacturing steel plates.

Background Art

[0002] In recent years, the drilling depth of oil wells and gas wells (hereinafter sometimes collectively referred to as "oil wells") has tended to become deeper and deeper. Along with this, higher strength is required for steel pipes for oil wells. In addition, steel pipes for oil wells are exposed to a severe acidic environment (high-pressure hydrogen sulfide environment) containing corrosive gas, so sulfide stress cracking (hereinafter sometimes referred to as "SSC") and hydrogen-induced cracking (hereinafter sometimes referred to as "HIC") become problems. Also, steel pipes used in pipelines for transporting oil and gas are also exposed to corrosive gas produced from oil wells. Therefore, steel pipes (line pipes) used in pipelines are also required to have sulfide stress cracking resistance (SSC resistance) and hydrogen-induced cracking resistance (HIC resistance). Hereinafter, sulfide stress cracking resistance (SSC resistance) and hydrogen-induced cracking resistance (HIC resistance) may sometimes be collectively referred to as "acid resistance".

[0003] SSC and HIC mainly occur in the vicinity of inclusions and welds. Therefore, conventionally, seamless steel pipes have mainly been used for applications that require high strength and acid resistance. Also, in a high-pressure hydrogen sulfide environment, stainless steel pipes and high-alloy steel pipes have been used instead of ordinary steel and low-alloy steel. However, recently, from the viewpoint of reducing manufacturing costs, inexpensive steel pipes that can be used in a high-pressure hydrogen sulfide environment have been demanded.

[0004] Also, for steel pipes for line pipes, higher strength is required for material savings due to thinning and weight reduction of the product. However, when the addition amount of alloying elements is increased for the purpose of increasing strength or the heat input amount is increased for high-efficiency welding, the low-temperature toughness of the heat-affected zone (HAZ) of the weld decreases.

[0005] As a method for enhancing sour resistance in a high-pressure hydrogen sulfide environment, for example, Non-Patent Document 1 proposes microstructure controls such as (a) finely dispersing non-metallic inclusions to prevent pitting corrosion, (b) utilizing nano-sized carbides to reduce dislocation density by high-temperature tempering, and (c) improving the morphology of grain boundary carbides by spheroidizing M3C and preventing the formation of coarse carbide M23C6 in 125 ksi grade high-strength oil well pipes. Non-Patent Document 1 discloses that the SSC resistance of steel developed by applying these methods was evaluated under different environmental conditions, and the developed steel has superior performance compared to conventional steel.

[0006] However, Non-Patent Document 1 relates to seamless steel pipes and has the problem that it cannot be applied to large-diameter welded steel pipes used in trunk lines and the like. In addition, the seamless steel pipes of Non-Patent Document 1 also have the problem that their application range is limited, such as inferior weldability, which is an important property for steel pipes used in pipelines.

[0007] In contrast, Patent Document 1 and Non-Patent Document 2 propose welded steel pipes with excellent sour resistance or steel plates for such steel pipes, in which the hardness of the base metal part and the welded part is specified to be 220 Hv or less based on the finding that hardness affects sour resistance.

[0008] Further, Patent Document 2 proposes a high-strength steel plate for sour-resistant line pipes, in which, in mass%, the CP value (= 4.46×[%C] + 2.37×[%Mn] / 6 + (1.74×[%Cu] + 1.7×[%Ni]) / 15 + (1.18×[%Cr] + 1.95×[%Mo] + 1.74×[%V]) / 5 + 22.36×[%P]), which is an index indicating the hardness of the center segregation part, is 1.0 or less, the steel structure is a bainite structure, the hardness variation ΔHV in the plate thickness direction is 30 or less, and the hardness variation (ΔHV) in the plate width direction is 30 or less.

[0009] In addition, Patent Document 3 proposes a high-strength steel sheet for sour line pipe with excellent material uniformity in the steel sheet, in which the metallographic structure is a bainite structure, the hardness variation in the plate thickness direction is 25 or less in terms of ΔHv10, the hardness variation in the plate width direction is 25 or less in terms of ΔHv10, and the maximum hardness of the steel sheet surface layer part is 220 or less in terms of Hv10.

[0010] In addition, Patent Document 4 proposes a quenched and tempered steel sheet with excellent hydrogen-induced cracking resistance, in which the metallographic structure in the range of 1 mm from the steel sheet surface in the plate thickness direction consists of one or two selected from tempered martensite and tempered bainite, and in the metallographic structure in the range of ±1 mm in the plate thickness direction from the plate thickness center part, the main phase consisting of one or two selected from tempered martensite and tempered bainite has an area ratio of 80% or more, and the remainder other than the main phase consists of one or more selected from ferrite, pearlite, cementite, and retained austenite. Furthermore, the hardness at a position 1 mm in the plate thickness direction from the steel sheet surface is 250 HV or less in terms of Vickers hardness, and the hardness difference between the position 1 mm from the steel sheet surface and the plate thickness center part is 60 HV or less in terms of Vickers hardness.

[0011] Regarding the sour resistance of the steel sheets of Patent Documents 1 to 4 and Non-Patent Document 2, it is improved in an environment with a hydrogen sulfide partial pressure of 0.1 MPa (1 bar) or less. However, recent oil well environments have become more severe, and the required level for the sour resistance of welded steel pipes used in pipelines and the like has become higher. Specifically, conventionally, sour resistance in an environment with a hydrogen sulfide partial pressure of 0.1 MPa (1 bar) or less has been required, but recently, materials that can withstand a high-pressure hydrogen sulfide environment exceeding 0.1 MPa have been required.

[0012] Furthermore, in the steel plate of Patent Document 5, as a material capable of withstanding the above high-pressure hydrogen sulfide environment, regarding the structure and hardness in the range of 1.0 mm from the surface of the steel pipe (surface layer part), the metal structure contains one or more selected from the group consisting of granular bainite, acicular ferrite, tempered bainite, and tempered martensite in a total area ratio of more than 80%, and the maximum hardness Hvmax in the surface layer part of the base material part is 250 Hv or less, and a non-quenched and tempered steel plate excellent in hydrogen-induced cracking resistance has been proposed.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0014]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0015] As described above, although there are several inventions regarding the surface structure and hardness of materials that can withstand a high-pressure hydrogen sulfide environment, for the surface structure and hardness of actual products, precise control of heating, rolling, and controlled cooling in the manufacturing process is required. Considering product variations, it is necessary to control the manufacturing conditions within a limited range to achieve this. For example, during the controlled cooling of a steel plate, it is necessary to start accelerated cooling at a temperature above the Ar3 point to obtain the above surface structure. However, if the starting temperature of accelerated cooling is too high, cooling will be insufficient and the desired metal structure cannot be obtained. In particular, during an online manufacturing process, due to different thermal histories at the leading end in the longitudinal direction of the steel plate that enters cooling first and the trailing end in the longitudinal direction of the steel plate that enters cooling later, various factors such as the cooling start temperature, cooling rate, and cooling stop temperature continuously change. Therefore, stably setting the surface hardness of the steel plate to Hvmax 220 or less or 200 or less is not easy considering variations in other manufacturing conditions.

[0016] Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a steel plate for use in the base material part of a steel pipe suitable for line pipes, which has a yield strength of 300 MPa or more and is excellent in SSC resistance and HIC resistance, a steel pipe using such a steel plate, and a method for manufacturing the steel plate.

Means for Solving the Problems

[0017] The present invention has been made in view of the above problems, and its gist is as follows.

[0018] [1] As a chemical composition, in mass %, C: 0.030 to 0.070%, Si: 0.005 to 0.500%, Mn: 0.80 to 1.65%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.070%, Ti: 0.004 to 0.018%, Nb: 0.005 to 0.050%, Ca: 0.0010 to 0.0050%, N: 0.0020 to 0.0070%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, B: 0 to 0.0030%, O: 0.0040% or less, The balance: a steel sheet composed of Fe and impurities, The said steel sheet, Ceq defined by the following formula (1) is within the range of 0.200 to 0.500, NPIP defined by the following formula (2) is 1.00 or less, and NPIM defined by the following formula (3) is 1.5×10 -5 or more, The metallographic structure at a position 0.1 mm deep from the surface contains, by area ratio, one or two of polygonal ferrite and granular bainite with a total area ratio of 70 to 98% and an average grain size of 30.0 μm or less, The metallographic structure at a position 0.5 mm deep from the surface contains, by area ratio, one or two of polygonal ferrite and granular bainite with a total area ratio of 75 to 95%, When the thickness of the said steel sheet is t, the metallographic structure at a position 2t / 5 deep from the surface of the said steel sheet contains, by area ratio, one or two of polygonal ferrite and granular bainite with a total area ratio of 50 to 97% and an average grain size of 20.0 μm or less, Hvmax, which is the maximum hardness in the surface layer part within the range from the surface of the said steel sheet to a depth of 1.0 mm, is 220 Hv or less, A steel sheet with a yield ratio of 70% or more. Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ···(1) NPIP = [Ti] / [N] × 0.29 ···(2) NPIM = [Ti] × [N] ···(3) Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] in the above formulas (1) to (3) respectively represent the contents in mass % of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel plate. [2] The steel plate according to [1], wherein the chemical composition satisfies the condition that ESSP defined by the following formula (4) is in the range of 1.0 or more and 15.0 or less. ESSP = [Ca] × (1 - 124 × [O]) / (1.25 × [S]) ···(4) Here, [Ca], [O], and [S] in the above formula (4) respectively represent the contents in mass % of Ca, O, and S in the steel plate. [3] The steel plate according to [1] or [2], wherein the metal structure at a position with a depth of 2t / 5 from the surface contains 50% or more of polygonal ferrite in terms of area ratio. [4] Among the chemical compositions, Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, The steel plate according to [1] or [2], which contains one or more of the following in the range of 0.0001 to 0.0100%: REM. [5] Among the chemical compositions, Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, The steel plate according to [3], which contains one or more of the following in the range of 0.0001 to 0.0100%: REM. [6] The steel plate according to [1] or [2], wherein the maximum hardness Hvmax in the surface layer portion within a range of 1.0 mm from the surface of the steel plate is 200 Hv or less. [7] The steel sheet according to [1] or [2], wherein the yield strength is 300 to 600 MPa. [8] The steel sheet according to [1] or [2], wherein the fracture toughness value after holding for 1000 hours in a high-pressure hydrogen environment with a hydrogen pressure of 200 atmospheres is 55 MPa√m or more. [9] The steel sheet according to [1] or [2], wherein the plate thickness is in the range of 10 to 40 mm.

[10] A steel pipe having a base material portion made of the steel sheet according to [1] or [2] and a welded portion.

[11] In terms of mass%, the chemical composition is C: 0.030 to 0.070%, Si: 0.005 to 0.500%, Mn: 0.80 to 1.65%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.070%, Ti: 0.004 to 0.018%, Nb: 0.005 to 0.050%, Ca: 0.0010 to 0.0050%, N: 0.0020 to 0.0070%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, B: 0 to 0.0030%, O: 0.0040% or less, The balance consists of Fe and impurities, Ceq defined by the following formula (1) is in the range of 0.200 to 0.500, NPIP defined by the following formula (2) is 1.00 or less, and NPIM defined by the following formula (3) is 1.5×10 -5 A hot rolling step of heating the steel slab in the range of 1050 to 1210°C and performing hot rolling, A first cooling step, comprises a second cooling step, The hot rolling step includes a first rolling stage of rolling in a range of 930 °C or higher and a second rolling stage of rolling in a range of 900 °C or lower, In the first rolling stage, the total reduction ratio between 1130 and 930 °C is 30% or more, and the number of reduction passes with a reduction ratio of 10% or more per pass is 3 or more, In the second rolling stage, the total reduction ratio at 900 °C or lower is 30% or more, and the number of reduction passes with a reduction ratio of 10% or more per pass is 5 or more, In the first cooling step, the average cooling rate at a depth of 1.0 mm below the plate surface between the start of cooling and the water cooling stop temperature of 400 °C or lower is 100 °C / s or lower, and the cooling is such that the maximum temperature reached on the steel plate surface 15 seconds after the water cooling stop is 350 °C or higher and less than 650 °C, The second cooling step is a method for manufacturing a steel plate, which is performed after the first cooling step and after the steel plate surface temperature reaches the maximum temperature reached, and cools to 200 °C or lower at an average cooling rate of 5 °C / s or lower. Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ···(1) NPIP = [Ti] / [N] × 0.29 ···(2) NPIM = [Ti] × [N] ···(3) Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V] in the above formulas (1) to (3) respectively represent the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel plate.

[12] Further, a tempering step is provided in the range of a tempering temperature of 400 to 650 °C, and the holding time at the target temperature ±10 °C is 10 to 60 minutes. The method for manufacturing a steel plate according to

[11] .

Advantages of the Invention

[0019] According to the above aspect of the present invention, it is possible to provide a steel plate suitable for a line pipe having a yield strength of 300 MPa or more and excellent SSC resistance and HIC resistance, a steel pipe using such a steel plate, and a method for manufacturing a steel plate.

[0020] Specifically, it is possible to provide a steel sheet for use as a base material of a steel pipe excellent in SSC resistance (sulfide stress corrosion cracking resistance) and HIC resistance (hydrogen-induced cracking resistance). A steel pipe excellent in sour resistance (SSC resistance and HIC resistance) is suitable for use in a high-pressure hydrogen sulfide environment as a line pipe for transporting oil, natural gas, etc.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0022] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0023] <Regarding the steel sheet> The steel sheet according to the embodiment of the present invention has the following characteristics (i) to (v).

[0024] (i) The steel sheet has a predetermined chemical composition, and Ceq defined by the following formula (1) is in the range of 0.200 to 0.500, (ii) NPIP defined by the following formula (2) is 1.00 or less, and NPIM defined by the following formula (3) is 1.5 × 10 -5 or more, (iii) The metallographic structure at a depth of 0.1 mm from the surface contains, by area ratio, one or both of polygonal ferrite and granular bainite in a total area ratio of 70 to 98%, and the average grain size is 30.0 μm or less. The metallographic structure at a depth of 0.5 mm from the surface contains, by area ratio, one or both of polygonal ferrite and granular bainite in a total area ratio of 75 to 95%. When the plate thickness of the steel plate is t, the metallographic structure at a depth of 2t / 5 from the surface of the steel plate contains, by area ratio, one or both of polygonal ferrite and granular bainite in a total area ratio of 50 to 97%, and the average grain size is 20.0 μm or less. (iv) Hvmax, which is the maximum hardness in the surface layer portion within the range from the surface of the steel plate to a depth of 1.0 mm, is 220 Hv or less. (v) The yield ratio is 70% or more.

[0025] Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ···(1) NPIP = [Ti] / [N] × 0.29 ···(2) NPIM = [Ti] × [N] ···(3)

[0026] Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V] in the above formulas (1) to (3) respectively indicate the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel plate.

[0027] Also, the steel plate according to this embodiment preferably has an ESSP defined by the following formula (4) within the range of 1.0 or more and 15.0 or less.

[0028] ESSP = [Ca] × (1 - 124 × [O]) / (1.25 × [S]) ···(4)

[0029] Here, [Ca], [O], [S] in the above formula (4) respectively indicate the contents in mass% of Ca, O, and S in the steel plate.

[0030] Furthermore, the steel plate according to the present embodiment preferably has a plate thickness (more specifically, the plate thickness of the portion that becomes the base material when it becomes a steel pipe) of 10 to 40 mm.

[0031] Furthermore, the steel plate according to the present embodiment more preferably contains 50% or more of polygonal ferrite in terms of area ratio in the metal structure at a position 2t / 5 deep from the surface.

[0032] In addition, the steel plate according to the present embodiment is used for the base material part of a steel pipe suitable for line pipes, which is excellent in SSC resistance and HIC resistance. That is, by processing the steel plate according to the present embodiment into a cylindrical shape and welding the butted parts, a steel pipe for line pipes excellent in SSC resistance and HIC resistance can be obtained.

[0033] Hereinafter, the steel plate according to the present embodiment will be described in detail below.

[0034] (Regarding chemical composition) First, the chemical composition of the steel plate according to the present embodiment will be described in detail. The steel plate according to the present embodiment has the following chemical composition. In the present embodiment, unless otherwise specified, the % notation regarding the chemical composition means "mass %".

[0035] That is, the chemical composition of the steel plate according to the present embodiment is C: 0.030 to 0.070%, Si: 0.005 to 0.500%, Mn: 0.80 to 1.65%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.070%, Ti: 0.004 to 0.018%, Nb: 0.005 to 0.050%, Ca: 0.0010 to 0.0050%, N: 0.0020 to 0.0070%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, B: 0 to 0.0030%, O: 0.0040% or less, and the balance consists of Fe and impurities.

[0036] [C: 0.030 to 0.070%] C is an element necessary for improving the strength of steel. If the C content is less than 0.030%, the strength improvement effect cannot be sufficiently obtained. Therefore, the C content shall be 0.030% or more. The C content is preferably 0.040% or more.

[0037] On the other hand, when the C content exceeds 0.070%, the strength of the steel increases too much, and the hardness of the surface layer metal structure and the internal metal structure (especially the center segregation part) exceeds 248 Hv, resulting in a decrease in SSC resistance and HIC resistance. Therefore, the C content shall be 0.070% or less. Also, by setting the C content to 0.070% or less, an increase in the hardness of the steel can be suppressed, and the fracture toughness value in a high-pressure hydrogen environment can be ensured. In terms of suppressing a decrease in weldability, toughness, etc., the C content is preferably 0.060% or less. Note that the "surface layer part" means the range from the surface of the steel plate to a depth of 1.0 mm as described above.

[0038] [Si: 0.005~0.500%] Si is an element that functions as a deoxidizer during steelmaking. It is also an element that is inevitably mixed in during the steelmaking stage. If the Si content is less than 0.005%, the above effects cannot be sufficiently obtained. Therefore, the Si content shall be 0.005% or more. In order to sufficiently obtain the deoxidation effect, it is preferably 0.050% or more.

[0039] On the other hand, when the Si content exceeds 0.500%, the toughness of the heat-affected zone (HAZ) of the weld decreases. Therefore, the Si content shall be 0.500% or less. The Si content is preferably 0.350% or less. Also, if the Si content is 0.500% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured.

[0040] [Mn: 0.80~1.65%] Mn is an element that contributes to the improvement of the strength and toughness of steel. If the Mn content is less than 0.80%, the improvement effects on strength and toughness cannot be sufficiently obtained. Therefore, the Mn content shall be 0.80% or more. The Mn content is preferably 1.05% or more.

[0041] On the one hand, when the Mn content exceeds 1.65%, a large amount of MnS that deteriorates the HIC resistance is generated, and at the same time, the hardness of the internal metal structure, especially the center segregation part, exceeds 248 Hv, resulting in a decrease in the HIC resistance. Therefore, the Mn content should be 1.65% or less. The Mn content is preferably 1.50% or less.

[0042] [P: 0.015% or less] P is an impurity element, and the less its content, the more preferable it is. When the P content exceeds 0.015%, the HIC resistance significantly decreases. Therefore, the P content should be 0.015% or less. The P content is preferably 0.010% or less. Also, if the P content is 0.015% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured.

[0043] Since the lower the P content, the more preferable it is, the lower limit includes 0%. However, if the P content is reduced to less than 0.003%, the manufacturing cost will increase significantly. Therefore, on a practical steel plate, 0.003% or more is the substantial lower limit of the P content.

[0044] [S: 0.0015% or less] S is an element that forms MnS stretching in the rolling direction during hot rolling. This stretched MnS reduces the HIC resistance. When the S content exceeds 0.0015%, the HIC resistance significantly decreases. Therefore, the S content should be 0.0015% or less. The S content is preferably 0.0010% or less.

[0045] Since the lower the S content, the more preferable it is, the lower limit includes 0%. However, if the S content is reduced to less than 0.0001%, the manufacturing cost will increase significantly. Therefore, on a practical steel plate, 0.0001% or more is the substantial lower limit of the S content.

[0046] [Al: 0.010 - 0.070%] Al is an element added for deoxidation. When the Al content is less than 0.010%, the above effects cannot be fully obtained. Also, if the Al content is 0.070% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured. Therefore, the Al content should be 0.010% or more. The Al content is preferably 0.020% or more.

[0047] On the other hand, when the Al content exceeds 0.070%, Al oxides accumulate to form clusters, and the HIC resistance decreases. Therefore, the Al content should be 0.070% or less. The Al content is preferably 0.045% or less.

[0048] [Ti: 0.004~0.018%] Ti is one of the important elements in the present invention. Ti is an element that combines with N to form nitrides, and these nitrides contribute to the refinement of crystal grains. The grain refinement effect of γ (austenite) varies greatly depending on the Ti / N balance at this time and the solubility products of Ti and N. When the Ti content is less than 0.004%, the above effects cannot be fully obtained. Therefore, the Ti content should be 0.004% or more. The Ti content is preferably 0.005% or more.

[0049] On the other hand, when the Ti content exceeds 0.018%, coarse nitrides are formed and the HIC resistance decreases. Therefore, the Ti content should be 0.018% or less. The Ti content is preferably 0.016% or less. Also, if the Ti content is 0.004~0.018%, the fracture toughness value in a high-pressure hydrogen environment can be ensured.

[0050] [Nb: 0.005~0.050%] Nb is an element that expands the non-recrystallization temperature range to refine crystal grains and forms carbides and nitrides, contributing to the improvement of the strength of steel. When the Nb content is less than 0.005%, the above effects cannot be fully obtained. Therefore, the Nb content should be 0.005% or more. The Nb content is preferably 0.010% or more.

[0051] On the one hand, when the Nb content exceeds 0.050%, coarse carbides and nitrides are formed, and the HIC resistance decreases. Therefore, the Nb content should be 0.050% or less. The Nb content is preferably 0.040% or less. Also, if the Nb content is 0.005 - 0.050%, the fracture toughness value in a high-pressure hydrogen environment can be ensured.

[0052] [Ca: 0.0010 - 0.0050%] Ca is an element that contributes to the improvement of HIC resistance by combining with S to form CaS and suppressing the formation of MnS that elongates in the rolling direction. When the Ca content is less than 0.0010%, the above effects cannot be obtained sufficiently. Therefore, the Ca content should be 0.0010% or more. The Ca content is preferably 0.0020% or more.

[0053] On the other hand, when the Ca content exceeds 0.0050%, Ca oxides accumulate and the HIC resistance decreases. Also, if the Ca content is 0.0050% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured. Therefore, the Ca content should be 0.0050% or less. The Ca content is preferably 0.0040% or less.

[0054] [N: 0.0020 - 0.0070%] N is one of the important elements together with Ti in the present invention. N is an element that forms nitrides and contributes to suppressing the coarsening of austenite grains during heating. As described above, the grain refinement effect of γ (austenite) changes greatly depending on the Ti / N balance and the solubility product of Ti and N. When the N content is less than 0.0020%, the above effects cannot be obtained sufficiently. Therefore, the N content should be 0.0020% or more. The N content is preferably 0.0025% or more, and more preferably 0.0030% or more.

[0055] On the one hand, when the N content exceeds 0.0070%, coarse nitride-containing inclusions are generated, and the HIC resistance decreases. Therefore, the N content should be 0.0070% or less. The N content is preferably 0.0050% or less. Also, if the N content is 0.0020 - 0.0070%, the fracture toughness value in a high-pressure hydrogen environment can be ensured.

[0056] The chemical composition of the steel sheet according to this embodiment, in addition to the above elements, for the improvement of strength, toughness, and other properties, within the range that does not reduce the properties of the steel sheet according to this embodiment, instead of a part of Fe, Ni, Mo, Cr, Cu, V, Mg, REM, and B may be included within the ranges described later. These elements are optional elements and may not be contained. That is, the lower limit of the content of these elements is 0%.

[0057] [Ni: 0 - 0.50%] Ni is an element that contributes to the improvement of the toughness and strength of steel and the improvement of corrosion resistance. When obtaining these effects, the Ni content is preferably 0.05% or more. The N content is more preferably 0.10% or more.

[0058] On the other hand, when the Ni content exceeds 0.50%, the strength increases too much, the toughness decreases, and there is a possibility that the SSC resistance decreases due to intergranular selective corrosion on the surface. Therefore, even when including it, the Ni content is preferably 0.50% or less. The Ni content is more preferably 0.35% or less.

[0059] [Mo: 0 - 0.50%] Mo is an element that contributes to the improvement of the hardenability of steel. When obtaining this effect, the Mo content is preferably 0.05% or more. The Mo content is more preferably 0.10% or more.

[0060] On the one hand, if the Mo content exceeds 0.50%, the strength may increase too much and the toughness may decrease. Also, if the Mo content is 0.50% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured. Therefore, even when adding it, the Mo content is preferably 0.50% or less. The Mo content is more preferably 0.35% or less.

[0061] [Cr: 0 to 0.50%] Cr is an element that contributes to the improvement of the strength of steel. When obtaining this effect, the Cr content is preferably 0.05% or more. The Cr content is more preferably 0.10% or more.

[0062] On the one hand, if the Cr content exceeds 0.50%, the strength may increase too much and the toughness may decrease. Therefore, even when adding it, the Cr content is preferably 0.50% or less. The Cr content is more preferably 0.35% or less.

[0063] [Cu: 0 to 0.50%] Cu is an element that contributes to the improvement of the strength and corrosion resistance of steel. When obtaining these effects, the Cu content is preferably 0.05% or more. The Cu content is more preferably 0.10% or more.

[0064] On the one hand, if the Cu content exceeds 0.50%, the strength may increase too much and the toughness may decrease. Therefore, even when adding it, the Cu content is 0.50% or less. The Cu content is more preferably 0.35% or less.

[0065] [V: 0 to 0.100%] V is an element that contributes to the improvement of the strength of steel by forming carbides and / or nitrides. When obtaining this effect, the V content is preferably 0.010% or more. The V content is more preferably 0.030% or more.

[0066] On the one hand, if the V content exceeds 0.100%, the toughness may decrease. Also, if the V content is 0.100% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured. Therefore, when V is contained, the V content is preferably 0.100% or less. The V content is more preferably 0.080% or less.

[0067] [Mg: 0 to 0.0100%] Mg is an element that contributes to the improvement of the toughness of steel by forming fine oxides and suppressing the coarsening of crystal grains. When obtaining this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0010% or more.

[0068] On the other hand, if the Mg content exceeds 0.0100%, the oxides may aggregate and coarsen, and the HIC resistance and toughness may decrease. Therefore, even when contained, the Mg content is preferably 0.0100% or less. The Mg content is more preferably 0.0050% or less.

[0069] [REM: 0 to 0.0100%] REM is an element that contributes to the improvement of SSC resistance, HIC resistance, and toughness by controlling the morphology of sulfide inclusions. To obtain these effects, the REM content is preferably 0.0001% or more. The content of REM is more preferably 0.0010% or more.

[0070] On the other hand, if the REM content exceeds 0.0100%, oxides are generated and the cleanliness of the steel decreases, and as a result, the HIC resistance and toughness may decrease. Therefore, even when contained, the REM content is preferably 0.0100% or less. The content of REM is more preferably 0.0060% or less.

[0071] In this embodiment, REM means rare earth elements and is a general term for 16 elements of Sc and lanthanoids (La to Lu). The REM content in this embodiment indicates the total content of these 16 elements.

[0072] [B: 0 to 0.0030%] B is an element that significantly enhances hardenability even in small amounts and contributes to improving the strength of steel plates and steel pipes. Therefore, B may be contained, and its lower limit is 0% or more. However, it has the effect of increasing the surface hardness during controlled cooling, and if contained in a large amount, the SSC resistance decreases. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0015% or less.

[0073] [O: 0.0040% or less] O is an element that inevitably remains after deoxidation, and the smaller its content, the better. If the O content exceeds 0.0040%, a large amount of oxides are generated, and the HIC resistance is significantly reduced. Also, if the O content is 0.0040% or less, the fracture toughness value in a high-pressure hydrogen environment can be ensured. Therefore, the O content is limited to 0.0040% or less. The O content is preferably 0.0030% or less.

[0074] On the other hand, since the O content is preferably as small as possible, the lower limit includes 0%. However, if the O content is reduced to less than 0.0010%, the manufacturing cost increases significantly. Therefore, on practical steel plates, 0.0010% is the substantial lower limit of the O content.

[0075] As described above, the steel plate according to the present embodiment is based on a chemical composition containing the above essential elements and the balance consisting of Fe and impurities. However, it may also have a chemical composition containing the above essential elements, optionally containing the above optional elements, and the balance consisting of Fe and impurities. Here, impurities mean components that are mixed from raw materials such as ore or scrap during the industrial production of steel materials or from various environments in the manufacturing process and are allowed within a range that does not adversely affect the steel.

[0076] Among the above impurities, for W, Sb, Sn, Zr, Y, and Hf, considering their effects on the steel plate properties and steel pipe properties, for example, it is preferable that W, Sb, and Sn are each 0.50% or less, and Zr, Y, and Hf are each 0.01% or less.

[0077] [Ceq: 0.200 to 0.500] In the steel sheet according to this embodiment, at a position 0.1 mm from the surface, one or two of polygonal ferrite and granular bainite are included in a total area ratio of 70 to 98%, and at a position 0.5 mm from the surface, one or two of polygonal ferrite and granular bainite are included in a total area ratio of 75 to 95%. In order to obtain such a metallographic structure, it is necessary to appropriately control the hardenability of the steel. Specifically, the value of Ceq (carbon equivalent) defined by the following formula (1) needs to be in the range of 0.200 to 0.500. Preferably, it is in the range of 0.30 to 0.45.

[0078] Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ···(1)

[0079] Here, in the above formula (1), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] are the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, and V, respectively, and are 0 when not contained.

[0080] When the value of Ceq is less than 0.200, ferrite and pearlite are generated in the metallographic structure, and the total of one or two of polygonal ferrite and granular bainite at the 0.1 mm position is less than 70%, or the total of one or two of polygonal ferrite and granular bainite at the 0.5 mm position is less than 75%. In this case, the strength of the steel sheet decreases and the sour resistance decreases. Therefore, the value of Ceq should be 0.200 or more. The value of Ceq is preferably 0.300 or more or 0.320 or more.

[0081] On the other hand, when the value of Ceq exceeds 0.500, the surface hardness of the parts that become the base material part and the welded part when manufacturing the steel pipe becomes excessively high, and the sour resistance decreases. Therefore, the value of Ceq should be 0.500 or less. The value of Ceq is preferably 0.450 or less or 0.430 or less.

[0082] For the steel sheet according to this embodiment, after controlling the content of each element within the above-mentioned range, as shown below, it is necessary to control the values of NPIP and NPIM calculated from the component content within a predetermined range.

[0083] [NPIP: 1.00 or less] In the steel sheet according to this embodiment, in order to achieve the above-mentioned steel sheet surface hardness, the value of NPIP defined by the following formula (2) is set to 1.00 or less. Here, as is clear from the content of the following formula (2), NPIP defined by the following formula (2) is an index related to the Ti / N balance. In the following formula (2), [Ti] and [N] are the contents of Ti and N in mass%, respectively. Also, from the above Ti content and N content, the lower limit of NPIP is substantially about 0.12.

[0084] NPIP = [Ti] / [N] × 0.29 ···(2)

[0085] [NPIM: 1.5×10 -5 or more] In the steel sheet according to this embodiment, in order to achieve the above-mentioned steel sheet surface hardness, while satisfying the conditions for the above NPIP, the value of NPIM defined by the following formula (3) is 1.5×10 -5 or more. Here, as is clear from the content of the following formula (3), NPIM defined by the following formula (3) is an index related to the solubility product of Ti and N. In the following formula (3), [Ti] and [N] are the contents of Ti and N in mass%, respectively. Also, from the above Ti content and N content, the upper limit of NPIM is substantially 1.26×10 -4 or less.

[0086] NPIM = [Ti] × [N] ···(3)

[0087] To ensure SSC resistance, it is effective to reduce the surface hardness. However, it is often technically difficult to precisely control the cooling rate of the surface layer during water cooling of the steel plate. This is because the cooling rate varies depending on factors such as the uniformity of water temperature and water volume on the plate surface, and the soundness of each water cooling nozzle. To stably reduce the surface hardness under various cooling conditions, reducing the hardenability of the steel plate is an effective method. It is known that the hardenability of the steel plate decreases by refining the γ grain size before cooling. It is effective to refine the heating γ grain size before rolling, promote the recrystallization of γ grains by rough rolling, flatten the γ grains by controlled rolling, etc.

[0088] NPIP is one of the indices related to the existence state of Ti-containing carbonitrides on the heating γ grain size. Specifically, it is an index for fully exerting the γ grain pinning effect by Ti-containing carbonitrides by optimizing the balance between Ti and N. When the value of NPIP is greater than 1.00, it has been confirmed that the heating γ grain size on the steel plate surface becomes coarser. The cause is not clear, but when the amount of Ti is large relative to the amount of N, a large amount of solid solution Ti that does not contribute to the formation of Ti-containing carbonitrides remains, and a mechanism is considered in which the Ostwald growth of Ti-containing carbonitrides is promoted by these solid solution Ti. Also, when N is insufficient, the formation of Nb-containing carbonitrides becomes less likely to occur, and it is also considered that the corresponding pinning effect cannot be obtained. From the above, the range where the value of NPIP is 1.00 or less was set as the target component range.

[0089] Also, NPIM is, like the above NPIP, one of the indices related to the existence state of Ti-containing carbonitrides. Specifically, it is an index related to the total amount of Ti-containing carbonitrides that pin the growth of γ grains. When the value of NPIM is 1.5×10 -5 or more, it has been confirmed that the heating γ grain size on the steel plate surface becomes smaller. NPIM is an index related to the solubility product of Ti and N. When the value of NPIM is high, both the Ti concentration and the N concentration are in a high state, and an increase in the total amount of TiN precipitates is expected. That is, it is considered that the effect of refining the heating γ grain size can be obtained by increasing the number of Ti-containing carbonitrides per unit volume.

[0090] Furthermore, for the steel sheet according to the present embodiment, after controlling the content of each element within the above-mentioned range, it is preferable to control the value of ESSP calculated from the content of the components within a predetermined range as follows.

[0091] [ESSP: 1.0 to 15.0] In the steel sheet according to the present embodiment, in order to more reliably ensure HIC resistance equivalent to or higher than that of conventional steel, it is preferable that the value of ESSP defined by the following formula (4) is in the range of 1.0 or more and 15.0 or less. The value of ESSP may also be in the range of 3.0 or more and 10.0 or less. Here, in the following formula (4), [Ca], [O], and [S] are the contents of Ca, O, and S in mass%, respectively.

[0092] ESSP = [Ca] × (1 - 124 × [O]) / (1.25 × [S]) ···(4)

[0093] In order to more reliably ensure HIC resistance, it is effective to suppress the formation of MnS stretched in the rolling direction. Further, in order to more reliably suppress the formation of MnS stretched in the rolling direction, it is an effective method to reduce the S content and add Ca to form CaS to fix S. On the other hand, since Ca has a stronger oxygen affinity than S, reducing the O content is effective for forming the required amount of CaS.

[0094] ESSP defined by the above formula (4) is an index related to the remaining Ca (effective Ca) after subtracting Ca combined with oxygen. Specifically, it is an index indicating whether there is an amount of effective Ca required according to the S content on the premise of binding with S in an atomic weight ratio.

[0095] When the value of ESSP is less than 1.0, there is a possibility that the Ca content is insufficient with respect to the O content and the S content, and MnS may be generated. Since MnS stretched by rolling causes deterioration of HIC resistance, the value of ESSP is preferably 1.0 or more. The value of ESSP is more preferably 3.0 or more or 3.2 or more, and still more preferably 3.5 or more.

[0096] On the other hand, when the Ca content becomes excessive, mainly oxide inclusions are generated in large amounts, and there is concern that the HIC characteristics may deteriorate. Although the generation of inclusions can be suppressed by reducing the O content and S content, when the value of ESSP exceeds 15.0, there is concern that the manufacturing cost for reducing the O content and S content will increase significantly. Therefore, the value of ESSP is preferably 15.0 or less. The value of ESSP is more preferably 10.0 or less or 9.0 or less, and even more preferably 8.0 or less.

[0097] If the value of ESSP is within the range of 1.0 or more and 15.0 or less, the effective Ca amount is adjusted to be not less than the minimum amount required for controlling the morphology of MnS and not more than the critical amount at which cluster-like inclusions are not generated, so that more excellent HIC resistance can be obtained.

[0098] (Regarding the metal structure and hardness of the surface layer part) Next, the metal structure and hardness of the surface layer part (the part from the surface to a depth of 1.0 mm) of the steel sheet according to the present embodiment will be described in detail. In the present embodiment, the metal structure at a position 0.1 mm deep from the surface of the steel sheet contains, in terms of area ratio, one or two of polygonal ferrite and granular bainite in a total area ratio of 70 to 98% and has an average grain size of 30.0 μm or less. The metal structure at a position 0.5 mm deep from the surface of the steel sheet contains, in terms of area ratio, one or two of polygonal ferrite and granular bainite in a total area ratio of 75 to 95%. When the thickness of the steel sheet is t, the metal structure at a position 2t / 5 deep from the surface of the steel sheet needs to contain, in terms of area ratio, one or two of polygonal ferrite and granular bainite in a total area ratio of 50 to 97% and have an average grain size of 20.0 μm or less. More preferably, the metal structure at a position 2t / 5 deep from the surface contains 50% or more of polygonal ferrite in terms of area ratio.

[0099] In the steel sheet according to the present embodiment, in order to suppress the maximum hardness (Hvmax) of the surface layer portion to 220 Hv or less and ensure required strength and excellent sour resistance, in the surface layer portion, one or two selected from the group consisting of polygonal ferrite and granular bainite should be contained at 70 to 98% at a position 0.1 mm deep from the surface, 75 to 95% at a position 0.5 mm deep from the surface, and 50 to 97% at a position 2t / 5 deep from the surface. When the total area ratio of these structures at each depth is less than the lower limit value, the maximum hardness of the surface layer portion cannot be stably satisfied below 220 Hv, and sufficient strength and sour resistance cannot be obtained. Also, even if the total area ratio of these structures at each depth is higher than the upper limit value, there is no problem as a surface layer property, but it may be difficult to ensure the base metal strength and HIC resistance property. The maximum hardness Hvmax of the surface layer portion will be described again below.

[0100] In the metal structure, as structures other than polygonal ferrite and granular bainite (the remainder), one or more of tempered bainite, pearlite-like, tempered martensite, and austenite-martensite mixture may be included. If these structures are present in an amount exceeding 2% at a position 0.1 mm deep from the surface, exceeding 5% at a position 0.5 mm deep from the surface, and exceeding 3% at a position 2t / 5 deep from the surface, locally high-hardness regions are included in the structure, and the sour resistance deteriorates. Therefore, the total area ratio of tempered bainite, pearlite-like, tempered martensite, and austenite-martensite mixture is 2% or less at a position 0.1 mm deep from the surface, 5% or less at a position 0.5 mm deep from the surface, and 3% or less at a position 2t / 5 deep from the surface.

[0101] The measurement of the area ratio of each metal structure is obtained by grinding and polishing up to a position 0.1 mm deep from the surface, a position 0.5 mm deep from the surface, and a position 2t / 5 deep from the surface to expose the observation surfaces at each depth, and observing the nital-etched metal structure at a magnification of 1000 times using a scanning electron microscope. The observation field of view of the photograph used for measurement is 10000 μm 2 It is necessary to ensure the above.

[0102] Further, the average grain size by EBSD measurement of the metallographic structure at a position 0.1 mm deep from the surface of the steel plate should be 30.0 μm or less, and the average grain size by EBSD measurement of the metallographic structure at a position 2t / 5 deep from the surface should be 20.0 μm or less. The reason is that in order to obtain the above-mentioned tissue fraction at each plate thickness position, it is necessary to refine and deform the γ grain size before transformation by recryrolling and non-recrystallized zone rolling to reduce hardenability. When recryrolling and non-recrystallized rolling are sufficiently performed, as a result, the structure after transformation is also refined, so the upper limit of the average grain size at each plate thickness position is defined. Also, by setting the upper limit of the average grain size in this way, it is possible to ensure the low-temperature toughness of the base material evaluated by Charpy test, DWTT test, etc. at a certain level according to the application. Since there are differences in the original casting structure and the effects of rolling, etc. between the vicinity of the surface and the inside of the plate thickness, the average grain size by EBSD measurement is 30.0 μm or less at a position 0.1 mm deep from the surface, and the average grain size by EBSD measurement is 20.0 μm or less at a position 2t / 5 deep from the surface.

[0103] In this embodiment, polygonal ferrite is a massive structure observed with a dark gray contrast, and contains little cementite or austenite-martensite mixture observed with a white contrast in the grains, and basically shows a uniform contrast. However, even in polygonal ferrite, there may be several dot-like cementites in the grains. Also, the prior austenite grain boundaries are unclear, and the ferrite grain boundaries show smooth curves with a clear white contrast. However, depending on the orientation difference between the crystal grains, the ferrite grain boundaries may be unclear.

[0104] Bainite is a structure in which a plurality of laths with a width of 0.5 to 3.0 μm are arranged almost parallel in the grains, and fine carbides and austenite-martensite mixtures are mixed in the laths and between the laths. Also, the prior austenite grain boundaries are clear. However, in this embodiment, bainite does not finally exist, and tempered bainite obtained by subsequent tempering exists.

[0105] In this embodiment, tempered bainite is a massive, acicular, or amorphous (mainly having curved grain boundaries and massive) structure in which lath boundaries observed with a distinct white contrast, cementite with a white contrast observed along the lath direction, and one or more of austenite-martensite mixtures are observed within grains of ferrite matrix observed with a dark gray contrast. The lath structure of tempered bainite has a lath width of 1.0 μm or more and a form in which a plurality of lath-like structures are arranged in parallel, but there are also forms in which the intervals and directions of the laths are not very regular. In addition, the cementite and austenite-martensite mixtures observed in tempered bainite may have sizes that can be individually identified even at a magnification of 1000 times, or may be observed as a shade of cloud-like contrast due to the aggregation of fine ones. The prior austenite grain boundaries are distinct, but the ferrite grain boundaries are not as distinct as those of polygonal ferrite, and may have a case where the contrast is unclear or a form on a saw blade with unevenness.

[0106] Granular bainite is formed at a transformation temperature intermediate between polygonal ferrite and bainite and has microstructural characteristics intermediate between polygonal ferrite and bainite. Specifically, it is a structure in which within the grains of the ferrite matrix observed with a dark gray contrast, one or more of the following can be observed: a white-contrast lath boundary, cementite with a white granular contrast, or an austenite-martensite mixture; it is a massive, acicular, or amorphous (mainly massive with curved grain boundaries) structure. The lath structure of granular bainite has a lath width of 1.0 μm or more, and the single or multiple lath-like structures are arranged generally in the same direction, and the lath spacing and direction are more likely to be disordered than in the tempered bainite structure. Also, compared with the tempered bainite structure, the size of the cementite and the austenite-martensite mixture is larger and the dispersion density is lower, so that even at a magnification of 1000 times, each cementite and austenite-martensite mixture can be identified. The prior austenite grain boundaries are unclear, and the ferrite grain boundaries are also less clear than those of the polygonal ferrite. In this embodiment, acicular ferrite is also included in the fraction of the granular bainite structure.

[0107] Tempered martensite is mainly a structure composed of fine laths with a clear white contrast. In this embodiment, it is a structure in which the lath width is less than 1.0 μm and a large number of cementites observed with a white contrast are dispersed within and at the boundaries of the laths. Since the cementite observed in the tempered martensite is fine and the precipitation density is high, it is difficult to identify each one at a magnification of 1000 times, and it is observed as the shade of a cloud-like contrast formed by innumerable white granular contrasts and fine particles gathered together.

[0108] Pseudo-pearlite structure is a form of a structure in which C discharged during ferrite formation is concentrated, and it is a structure in which cementite observed with a white contrast is dispersed at a high density in the ferrite matrix. The morphology of the cementite observed in the pseudo-pearlite is, at a magnification of 1000 times, each one is fine granular or short string-like, and in some cases, they are densely packed and observed as a white mass.

[0109] Austenite-martensite composite structures are a type of structure in which carbon, which was expelled during the formation of ferrite, is concentrated, and are observed as blocky structures with white to light gray contrast. At a magnification of 1000x, the contrast within the austenite-martensite composite structure is generally small and it is observed as a uniform structure. However, in some cases, the contrast of the martensite lath structure in the austenite-martensite composite is observed, and in some cases, granular or string-like white contrast of cementite is observed.

[0110] In determining the metal structure of the steel plate according to this embodiment, it is necessary to classify the structure into three types: the softest polygonal ferrite structure, the relatively soft granular bainite structure, and hard other structures, and the hard other structures include the above-mentioned tempered bainite, tempered martensite, pseudo-pearlite, and austenite-martensite mixtures. Note that cementite and austenite-martensite mixtures contained inside polygonal ferrite and granular bainite have little effect on the area fraction and may be included in any classification.

[0111] Fig. 4(a) shows the metal structure (imaged by a scanning electron microscope) at a depth of 0.1 mm from the surface of the steel plate according to this embodiment, and Fig. 4(b) shows the judgment result. In Fig. 4(b), polygonal ferrite and granular bainite are classified into hard other structures on the image. In Fig. 4(b), the light-colored areas are hard other structures.

[0112] The remaining metal structure of the above specifications is composed of bainite, tempered bainite, pseudo-pearlite, martensite, tempered martensite and austenite-martensite mixture.

[0113] (Maximum hardness of surface layer Hvmax: 220Hv or less) SSC is caused by minute flaws and microcracks on the surface of the steel plate. Therefore, in the steel plate and steel pipe manufactured from the steel plate, the metallographic structure and hardness of the surface layer, which are the sources of minute flaws and microcracks, are important. In particular, it is important to control the maximum hardness Hvmax not only at the position 1.0 mm deep from the surface but also in the entire range from the surface to a depth of 1.0 mm (i.e., the surface layer).

[0114] In the steel pipe with the steel plate according to this embodiment as the base material part, in order to ensure HIC resistance and excellent SSC resistance, after controlling the metallographic structure of the surface layer as described above, the maximum hardness Hvmax of the surface layer is set to 250 Hv or less. The maximum hardness Hvmax of the above surface layer is preferably 245 Hv or less, and more preferably 240 Hv or less. Considering these, the steel plate according to this embodiment sets the maximum hardness (more specifically, the maximum hardness of the surface layer in the steel plate as the material before pipe manufacturing) Hvmax of the surface layer to 220 Hv or less in consideration of pipe manufacturing strain and hardness increase during strain aging. The maximum hardness of the surface layer of the above steel plate is preferably 200 Hv or less.

[0115] On the other hand, although there is no particular regulation on the lower limit value of the maximum hardness Hvmax of the surface layer, from the viewpoint of ensuring the strength of the steel material, it is preferably 150 Hv or more.

[0116] The measurement of the maximum hardness of the surface layer from the steel plate surface to a depth of 1.0 mm is carried out as follows. First, from the positions of 1 / 4, 1 / 2, and 3 / 4 (in the case of steel pipes, the 3 o'clock, 6 o'clock, and 9 o'clock positions when the welded part is set as 0 o'clock) from the end in the width direction of the steel plate (corresponding to the butting part of the steel pipe), steel plates of 300 mm square (300 mm × 300 mm) are cut out by gas cutting. From the center of the cut-out steel plate, a full-thickness block test piece of 20 mm in length × 20 mm in width is collected by mechanical cutting, and the surface perpendicular to the rolling direction is polished by mechanical polishing. For one block test piece, with a Vickers hardness tester (load: 100 g), starting from 0.1 mm from the surface, 10 points are measured at 0.1 mm intervals in the plate thickness direction, and 10 points are measured at 1.0 mm intervals in the width direction at the same depth, for a total of 100 points. That is, a total of 300 points are measured with 3 block test pieces. From the obtained results, the maximum hardness of the surface layer metal structure can be obtained respectively.

[0117] In the steel plate according to the present embodiment, the maximum hardness of the 1 / 2t part (t: plate thickness), which is the position of 1 / 2 of the plate thickness from the surface, is not particularly defined. However, it is preferable that the maximum hardness when measuring 10 points or more in the longitudinal direction at a pitch of 1.0 mm near the 1 / 2t part is 300 Hv or less, because the HIC resistance can be further improved.

[0118] The steel plate according to the present embodiment does not generate sulfide stress cracking even when a stress of 90% or more of the yield strength is applied in an H2S-saturated solution environment containing 5% sodium chloride and acetic acid at 30 °C or lower. That is, the stress at which fracture occurs in the above solution environment is 90% or more of the yield strength.

[0119] Further, the steel plate according to the present embodiment can obtain excellent fracture toughness in a high-pressure hydrogen environment. Specifically, in the steel plate according to the present embodiment, the fracture toughness value K IH obtained by conducting the fracture toughness value evaluation test described later is 55 MPa√m or more.

[0120] The fracture toughness value K IH has a preferable lower limit of 56 MPa√m, and more preferably 57 MPa√m. The fracture toughness value K IH has no particular upper limit, but for example, it is 198 MPa√m or less.

[0121] [Fracture toughness value evaluation test under high-pressure hydrogen environment] The fracture toughness value K according to this embodiment IH can be obtained by the following method in accordance with the provisions of ASME B31.12:2019.

[0122] Collect a bolt load test piece compliant with ASTM E1681:2020 from the steel plate. The thickness of the bolt load test piece shall be 85% or more of the thickness of the steel plate.

[0123] Among the surfaces of the collected bolt load test piece, form a machining notch in the atmosphere at the central position of one surface parallel to the width direction of the bolt load test piece, and further introduce a fatigue pre-crack. Define the thickness B (m), width W (m), and initial crack length a0 (m) of the bolt load test piece. The ratio of the width W to the thickness B of the bolt load test piece shall be W:B = 2:1. The ratio of the width W of the bolt load test piece to the initial crack length a0 shall be a0 / W = 0.5. In this case, the machining notch before fatigue pre-crack introduction shall be a0 (m) - 0.003 (m), and the depth (length in the width direction) of the fatigue pre-crack shall be 3 mm. That is, the initial crack length a0 (m) is the sum of the machining notch and the depth of the fatigue pre-crack.

[0124] Perform a fracture toughness test compliant with ASME B31.12:2019 and ASME BPVC.VIII.3:2021 using the bolt load test piece with a fatigue pre-crack introduced. Specifically, sufficiently degas the inside of the glove box by nitrogen gas substitution to reduce the oxygen concentration inside the glove box to less than 5 ppm. Rotate the bolt of the bolt load test piece inside the glove box to open the fatigue pre-crack and give an initial K value (KIAPP) of 110 - 130 MPa√m. By giving the initial K value, a micro-crack propagates at the tip of the fatigue pre-crack, and a fresh surface without a surface oxide film is obtained. Since hydrogen penetrates from the obtained fresh surface, it is necessary to suppress the oxidation of the crack tip opened when moving the bolt load test piece from the glove box to the autoclave.

[0125] The bolt-loaded test piece is transferred to an autoclave that has been purged with nitrogen gas or evacuated to prevent oxidation of the crack tip of the opened crack. Then, a high-pressure hydrogen environment is created inside the autoclave. Specifically, the inside of the autoclave is purged with hydrogen gas, and the hydrogen gas pressure is further increased to 200 atmospheres. Then, the bolt-loaded test piece with the initial K value is exposed to the normal-temperature hydrogen gas at 200 atmospheres for 1000 hours. When the steel is hydrogen embrittled by the exposure to hydrogen in the high-pressure hydrogen environment, cracks will propagate in the bolt-loaded test piece. On the other hand, when the fracture toughness value of the steel in the high-pressure hydrogen environment is extremely high, cracks are less likely to propagate.

[0126] After 1000 hours of exposure, the crack length a (m) is measured, and the bolt-loaded test piece is heated to 300 - 400 °C and held at 300 - 400 °C for 30 minutes to 1 hour. This colors the part where the crack in the bolt-loaded test piece has propagated. The measurement of the crack length a (m) may be performed after coloring. After coloring the crack, the fracture surface is forced to open. The method of forcing it to open may be fatigue fracture or brittle fracture after immersion in liquid nitrogen for cooling. The crack that has propagated by high-pressure hydrogen treatment is observed using a scanning electron microscope (SEM), and it is determined whether a crack exceeding 0.25 mm has propagated.

[0127] When the crack propagation length measured by SEM observation is 0.25 mm or more, based on the crack length and the shape of the bolt-loaded test piece, in accordance with ASME BPVC.VIII.3 KD-10 and ASTM E1681:2020, the fracture toughness value K IH (MPa√m) is determined. Specifically, the opening displacement of the clip gauge is V m (m), Young's modulus E (MPa), crack length a (m), and the width W (m) of the bolt-loaded test piece are substituted into the following formula to determine the fracture toughness value K IH (MPa√m).

[0128] K IH ={V m ×(E / W 1 / 2 )}×f(a / W) f(a / W)=(1 - a / W)1 / 2 × {0.654 - 1.88×(a / W) + 2.66×(a / W) 2 - 1.233×(a / W) 3}

[0129] When the length of crack propagation measured by SEM observation does not exceed 0.25 mm, the fracture toughness value K IH shall be set to 0.5 times the initial K value (KIAPP).

[0130] Next, the mechanical properties and the like of the steel plate according to the present embodiment will be described. The steel plate according to the present embodiment preferably has a strength that satisfies X52 to X70 defined in API 5L in consideration of its application to a line pipe.

[0131] (Yield ratio: 70% or more) The yield ratio of the steel plate according to the present embodiment is 70% or more. If the yield ratio is too low, the tensile strength relative to the required yield strength becomes too high, so the yield ratio is set to 70% or more. Note that the upper limit value of the yield ratio is not particularly defined, and the higher it is, the more preferable it is.

[0132] (Yield strength: 300 to 600 MPa) The yield strength is preferably in the range of 300 to 600 MPa, and may be in the range of 350 to 550 MPa.

[0133] (Tensile strength: 400 to 700 MPa) The tensile strength is preferably in the range of 400 to 700 MPa.

[0134] Here, the yield strength, tensile strength, and yield ratio are obtained by taking an API full-thickness tensile test piece perpendicular to the longitudinal direction of the steel plate (C direction), flattening it, and then performing a tensile test.

[0135] (Plate thickness t: 10 to 40 mm) When the steel plate according to the present embodiment is used as a steel pipe for drilling or transportation of oil, natural gas, etc., the plate thickness of the steel plate is preferably in the range of 10 to 40 mm.

[0136] <Method for manufacturing steel plate> Next, a preferred method for manufacturing the steel plate according to the present embodiment will be described. The steel plate according to the present embodiment can obtain its effects as long as it has the above-described configuration regardless of the manufacturing method. However, for example, according to the following manufacturing method, it is preferable because the steel plate according to the present embodiment can be stably obtained.

[0137] The steel plate according to the present embodiment can be obtained, for example, by a manufacturing method including the following steps (I) to (IV).

[0138] (I) In order to control to a predetermined chemical composition, various steelmaking processes and alloy additions are performed under atmospheric or reduced pressure conditions, and a steel slab with a thickness of 200 to 650 mm is obtained by continuous casting (steelmaking process). (II) A step of heating a steel slab having a predetermined chemical composition to 1050 to 1210 °C and performing hot rolling (hot rolling process). (III) After the hot rolling process, the steel plate is water-cooled at an average cooling rate of 100 °C / s or less so that the water-cooling stop temperature becomes 400 °C or less, and accelerated cooling is performed so that the maximum temperature reached by reheating becomes 350 °C or more after stopping the water-cooling (first cooling process). (IV) After the first cooling process, a step of cooling to 200 °C or less at an average cooling rate of 5 °C / s or less (second cooling process).

[0139] Hereinafter, preferred conditions for each of the above steps will be described in detail.

[0140] (Steelmaking process) In order to adjust the molten steel to have the same chemical composition as the steel plate according to this embodiment (that is, the chemical composition in which each component is within the above range and satisfies the above formulas (1) to (3) (preferably, formulas (1) to (4))), various steelmaking processes and alloy additions are performed under atmospheric or reduced pressure conditions, and a steel slab with a thickness of 200 to 650 mm is obtained by continuous casting. In this embodiment, since the N content and the Ti content are important control factors, it is important to control the N content and the Ti content within the target ranges by steelmaking processes and alloy additions. Note that the control of the N content may be adjusted by the content in the hot metal or intentionally added from the outside.

[0141] (Hot rolling process) A steel slab produced by casting molten steel having the same chemical composition as the base material of the steel pipe according to this embodiment is heated within the range of 1050 to 1210 °C and subjected to hot rolling. Considering the toughness of the base material, the rolling is carried out at a temperature of 930 °C or higher and at a temperature of 900 °C or lower and Ar3 or higher, respectively, to ensure a total reduction ratio of 30% or more. Ar3 (°C) is as follows.

[0142] Ar3 (°C) = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo + 0.35(t - 8) …(A)

[0143] In the above formula (A), C, Mn, Cu, Cr, Ni, and Mo are the contents (mass %) of each element in the steel, and t is the plate thickness (mm) of the steel plate after hot rolling.

[0144] When rolling the steel slab, if the heating temperature is less than 1050 °C, the solid solution of Nb-containing carbides is not sufficient and the HIC resistance cannot be ensured. The heating temperature of the steel slab is preferably 1100 °C or higher. On the other hand, if the heating temperature exceeds 1210 °C, the crystal grains of the steel slab become coarser and the toughness decreases. Also, if the heating temperature of the steel slab is 1210 °C or lower, the fracture toughness value in a high-pressure hydrogen environment can be ensured. Therefore, the heating temperature is set to 1210 °C or lower. The heating temperature of the steel slab is preferably 1190 °C or lower.

[0145] In addition, although there are no specific regulations regarding the total reduction ratio during rolling at 930°C or higher and the upper limit of the total reduction ratio during rolling at 900°C or lower and above Ar3, from the perspectives of porosity bonding and segregation element homogenization, it is preferably 95% or less respectively.

[0146] For the rough rolling (the first rolling stage) after heating, from the perspective of grain refinement, it is necessary to have a total reduction ratio of 30% or more between 1130 and 930°C. The rough rolling after heating is preferably carried out with a total reduction ratio of 40% or more between 1130 and 930°C, and it is also necessary to ensure at least 3 passes with a reduction ratio of 10% or more per pass.

[0147] During the finish rolling (the second rolling stage) after rough rolling, hot rolling is terminated at Ar3 or higher. At this time, during rolling at 930°C or higher, even if Nb is added, it will not be rolling in the non-recrystallized temperature range, so the effect of refining the base metal grains is insufficient and it is difficult to ensure the toughness of the base metal. On the other hand, during rolling below Ar3, polygonal ferrite is generated on the surface layer of the plate before the start of controlled cooling, and a hard structure with C enrichment is formed along with the generation of polygonal ferrite, so stress corrosion cracking may occur. From the above perspectives, the finish rolling needs to be carried out at Ar3 or higher and 900°C or lower with a total reduction ratio of 30% or more. The finish rolling is preferably carried out at Ar3 or higher and 900°C or lower with a total reduction ratio of 70% or more, and it is also necessary to ensure at least 5 passes with a reduction ratio of 10% or more per pass. Also, by performing the finish rolling (the second rolling stage) under the above conditions, the fracture toughness value in a high-pressure hydrogen environment can be ensured.

[0148] (The first cooling process) In the first cooling process, the steel plate after the hot rolling process is water-cooled from Ar3 or higher so that the water-cooling stop temperature is 400°C or lower, and accelerated cooling is carried out so that the maximum temperature reached by reheating exceeds 350°C after the water-cooling is stopped.

[0149] Here, when the water cooling stop temperature exceeds 400°C, cooling inside the plate thickness is insufficient and the strength becomes insufficient, which is not preferable. The water cooling stop temperature is preferably 350°C or lower. Since it is difficult to actually measure the water cooling stop temperature, the management of the water cooling stop temperature may be an estimated value by cooling simulation.

[0150] More specifically, for the steel plate after hot rolling, accelerated cooling is started to ensure the strength of the steel material. Since the surface hardness needs to be reduced as described above, it is preferable that the average cooling rate at a depth of 1.0 mm below the plate surface during the period from the start of cooling to a water cooling stop temperature of 400°C or lower is slow cooling at 100°C / s or lower. More preferably, the average cooling rate at a depth of 1.0 mm from the plate surface is slow cooling at 80°C / s or lower. Note that the cooling end temperature is the temperature at which the management of the cooling rate ends.

[0151] Also, aiming at optimizing the mechanical properties due to the tempering effect of the base material, the maximum temperature reached by reheating 15 seconds after the end of cooling needs to be 350°C or higher and less than 650°C. Such a maximum temperature reached can be more reliably realized by setting the average cooling rate in the temperature range of 500°C or higher from the start of cooling under the above conditions.

[0152] Figure 1 is a diagram schematically showing an aspect of accelerated cooling when water cooling is performed such that the maximum temperature reached by reheating is 500°C to 600°C 15 seconds after the end of water cooling after hot rolling.

[0153] When the maximum temperature reached by reheating is less than 350°C, the hardness of the steel plate, particularly the maximum hardness of the surface layer portion from the surface to a depth of 1.0 mm, cannot be stably set to 220 Hv or lower.

[0154] Also, when the maximum temperature reached by reheating is 650°C or higher, transformation stagnates inside the steel plate, and thus the base material strength and toughness cannot be satisfied. Therefore, the maximum temperature reached by reheating is set to less than 650°C.

[0155] (Second cooling step) After the first cooling (after water cooling and reheating are completed), it is cooled to 200°C or lower at an average cooling rate of 5°C / s or lower. If the average cooling rate to 200°C or lower is greater than 5°C / s, the effect of self-tempering is insufficient and the surface hardness becomes high, which is not preferable. Also, if the cooling end temperature is higher than 200°C, the effect of self-tempering becomes insufficient, which is not preferable. The average cooling rate is preferably 4°C / s or lower, and the cooling end temperature is preferably 100°C or lower. Note that the cooling end temperature is the temperature at which the control of the cooling rate ends, meaning that the cooling rate from the cooling end temperature to room temperature is arbitrary.

[0156] By going through the steps as described above, the metallographic structure of the surface layer portion in the obtained steel sheet contains one or two of polygonal ferrite and granular bainite in a total area ratio of 70 to 95%, the maximum hardness Hvmax of the surface layer portion becomes 220 Hv or lower, and further, the yield ratio becomes 70% or higher.

[0157] (Tempering treatment) Basically, it is possible to manufacture a steel sheet having the target structure and properties by the above-described steps. However, depending on the required size, properties, and their combinations, stable production may be difficult and productivity and product yield may decrease. In that case, in order to stably impart sour resistance while optimizing the production process and cost, tempering treatment (tempering) may be performed. The tempering treatment can be set under arbitrary conditions in the range of a target temperature of 400 to 650°C and a holding time of 10 to 60 minutes at the target temperature ±10°C after the second cooling step. Cooling after tempering may be air cooling.

[0158] Also, regarding the metallographic structure at the 1 / 4 thickness position of the plate, one or two of polygonal ferrite and granular bainite, as described above, form a metallographic structure containing them at a predetermined total area ratio.

[0159] As a result, the steel sheet obtained through the above steps has a yield strength of 300 MPa or higher, preferably 350 MPa or higher, and is excellent in SSC resistance and HIC resistance, and becomes a steel sheet suitable for use as the base material part of a steel pipe for line pipes.

[0160] Note that since the base material part of the steel pipe using the steel sheet according to this embodiment is not heat-treated, the metal structure of the steel pipe base material part is the same as the metal structure of the steel sheet according to this embodiment. The steel pipe using the steel sheet according to this embodiment has excellent SSC resistance in addition to HIC resistance equal to or higher than that of conventional steel in both the base material part and the welded part.

[0161] Figs. 2 to 3 show an example of the hardness distribution in the surface layer part of the steel sheet. Figs. 2(a) and 2(b) show the hardness distribution in the depth direction of the plate thickness of the example of the present invention that satisfies the above-described conditions, and Figs. 3(a) and 3(b) show the hardness distribution in the depth direction of the plate thickness of the comparative example with a high surface layer maximum hardness. Figs. 2(a) and 2(b) are the results measured at different positions. Similarly, Figs. 3(a) and 3(b) are the results measured at different positions. In the figures, the surface layer maximum hardness is the highest hardness Hvmax in the surface layer part from the surface of the steel sheet to a depth of 1.0 mm. It can be seen that in the comparative example, the surface layer maximum hardness exceeds 220 Hv.

[0162] <Regarding the steel pipe> As described above, the steel pipe according to this embodiment can be obtained by processing the steel sheet according to this embodiment into a cylindrical shape, butting both ends of the cylindrical steel sheet, and welding the butting part. The obtained steel pipe has a base material part made of the steel sheet according to this embodiment and a welded part. Such a welded part is usually continuously provided from one end to the other end in the longitudinal direction of the steel sheet.

[0163] Generally, in steel pipe welding, the welded part is constructed to be thicker than the base material part. Also, the weld metal has a higher alloy content than the base material and higher corrosion resistance. Therefore, it is almost impossible for the welded part to be the starting point of SSC. Therefore, the welded part of the steel pipe according to this embodiment is not particularly limited as long as it is obtained under normal conditions by SAW welding or the like.

[0164] The steel plates and steel pipes of the present embodiment have a yield strength of 300 MPa or more and are excellent in SSC resistance and HIC resistance. Further, the steel plates and steel pipes of the present embodiment have a fracture toughness of 55 MPa√m or more in a high-pressure hydrogen environment, and high fracture toughness can be ensured.

Example

[0165] Next, examples of the present invention will be described. The conditions in the examples are one set of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one set of conditions. The present invention can adopt various conditions without departing from the gist of the present invention as long as the object of the present invention is achieved.

[0166] Molten steel having the component compositions shown in Tables 1A to 2B was continuously cast to produce steel slabs with a thickness of 240 mm or 300 mm. Using this slab, steel plates shown in Tables 5 to 10 were produced under the manufacturing conditions shown in Tables 3A to 4B. Nos. 1 to 25 are invention steel plates, and Nos. 26 to 51 are comparative steel plates. Also, Ar3 (°C) is shown in Tables 1A to 2B.

[0167]

Table 1A

[0168]

Table 1B

[0169]

Table 2A

[0170]

Table 2B

[0171]

Table 3A

[0172]

Table 3B

[0173]

Table 4A

[0174]

Table 4B

[0175] JIS No. 5 tensile test pieces were prepared from the obtained steel plates, and the tensile tests specified in JIS Z 2241:2011 were carried out to measure the yield strength and the tensile strength. When the yield strength was less than 300 MPa, it was regarded as unqualified. In addition, the hardness of the surface layer portion from the surface to a depth of 1.0 mm was measured, and the metallographic structure of the surface layer portion was observed with a scanning electron microscope. Also, the HIC resistance property and the SSC resistance property were evaluated.

[0176] (Maximum hardness of the surface layer portion) At two arbitrary locations on the steel plate, steel plates with a size of 300 mm square (300 mm × 300 mm) were cut out by gas cutting from the positions of 1 / 4, 1 / 2, and 3 / 4 (in the case of steel pipes, corresponding to the butt joint part, these are the positions of 3 o'clock, 6 o'clock, and 9 o'clock when the welded part is set as 0 o'clock) from the ends in the width direction of the steel plate. From the center of the cut-out steel plate, block test pieces with a size of 20 mm in length × 20 mm in width were collected by mechanical cutting and polished by mechanical polishing. For one block test piece, with a Vickers hardness tester (load: 100 g), starting from 0.1 mm from the surface, 10 points were measured at intervals of 0.1 mm in the plate thickness direction, 10 points were measured at intervals of 1.0 mm in the width direction at the same depth, for a total of 100 points. That is, a total of 300 points were measured with three block test pieces. From these results, the maximum hardness of the surface layer metallographic structure at two locations in the longitudinal direction of the steel plate was obtained respectively. For the inventive steel plate, the relationship between the obtained maximum hardness of the surface layer portion, NPIP, and NPIM was graphed in FIGS. 5 and 6 respectively.

[0177] (Microstructure Observation) For the metal microstructure of the surface layer, the polished test piece was immersed in a mixed solution of 3% nitric acid and 97% ethanol for several seconds to several tens of seconds for etching to reveal the metal microstructure. Using a scanning electron microscope at a magnification of 1000 times, observations were made at positions 0.1 mm, 0.2 mm, and 0.5 mm from the surface. Using the tissue photographs taken of two consecutive fields of view, the fractions (area fractions) of polygonal ferrite, granular bainite, and the remaining part were calculated. The area fraction of each tissue was taken as the average of these fields of view. An example of the obtained microstructure observation results is shown in Figure 4.

[0178] Also, for the metal microstructure at the 1 / 4 plate thickness position, the polished test piece was observed in the same manner, and using the obtained tissue photographs, the fractions (area fractions) of polygonal ferrite, granular bainite, and the remaining part were calculated. The area fraction of each tissue was taken as the average of these fields of view.

[0179] (Evaluation of HIC Resistance) From the steel plate, a full-thickness test piece with a length of 100 mm × a width of 20 mm was taken, and a test was conducted in accordance with NACE (National Association of Corrosion and Engineer)'s TM0284 to observe the occurrence of HIC (hydrogen-induced cracking). If the HIC area fraction is 5% or less, it is evaluated as having excellent HIC resistance, and if it exceeds 5%, it is evaluated as having poor HIC characteristics.

[0180] Note that the NACE test is a test in which a 5% NaCl solution + 0.5% acetic acid solution with a pH of 2.7 is saturated with hydrogen sulfide gas at 1 atm, the steel plate is immersed in the solution, and whether cracks occur after 96 hours is observed.

[0181] (Evaluation of SSC Resistance) From the steel plate, a full-thickness test piece with a width of 15 mm × a length of 115 mm was taken from the width direction, and the SSC resistance was evaluated by a four-point bending test in accordance with NACE's TM0284m and ASTM (American Society for Testing and Materials)' G39.

[0182] In the four-point bending test, specimens to which a stress corresponding to 90% of the 0.2% proof stress derived from the tensile test was applied were immersed in an aqueous solution of 5% sodium chloride + 0.5% acetic acid at room temperature (24°C) saturated with hydrogen sulfide gas at 1 atm and having a pH of 2.7 for 720 hours. To determine the presence or absence of SSC, the surface of the specimens was observed at a magnification of 10 times.

[0183] Those in which SSC did not occur were judged as qualified (No Crack), and those in which it occurred were judged as unqualified (Crack).

[0184] (Fracture toughness in a high-pressure hydrogen environment) For the steel plates No. 1 to 25 which are examples of the present invention, a fracture toughness value evaluation test was carried out based on the method described in the above [Fracture toughness value evaluation test in a high-pressure hydrogen environment]. The crack length a was measured, and it was determined whether the length of crack propagation measured by SEM exceeded 0.25 mm. According to the determination result, the fracture toughness value K IH (MPa√m) was obtained. The obtained fracture toughness value K IH (MPa√m) is shown in the column of "Fracture toughness value K IH (MPa√m)" in Table 11.

[0185]

Table 5

[0186]

Table 6

[0187]

Table 7

[0188]

Table 8

[0189]

Table 9

[0190]

Table 10

[0191]

Table 11

[0192] As described above, it can be seen that the steel plates (Test Nos. 1 to 25) of the present invention example are excellent in sulfide stress cracking resistance (SSC resistance) equal to or higher than that of conventional steels.

[0193] Also, referring to Table 11, in the steel plates (Test Nos. 1 to 25) of the present invention example, when the fracture toughness value evaluation test was further carried out in a high-pressure hydrogen environment, the fracture toughness value K IH was sufficiently high.

[0194] In Comparative Examples Nos. 26 to 31, one or more of the Ti content, N content, NPIP, and NPIM are outside the scope of the present invention, the surface hardness of the steel plate shows a high value, and the SSC resistance characteristics are deteriorated.

[0195] In Comparative Examples Nos. 32 to 35, Ceq. is outside the scope of the present invention, the yield strength is less than 300 MPa, or the SSC resistance characteristics are deteriorated.

[0196] In Comparative Example No. 36, the C content is outside the scope of the present invention, the surface hardness of the steel plate shows a high value, and the SSC resistance characteristics are deteriorated.

[0197] In Comparative Examples Nos. 37 to 43, the contents of Mn, P, S, Nb, Ca, N, and O are outside the scope of the present invention, and the HIC resistance characteristics are deteriorated.

[0198] In Comparative Examples Nos. 44 and 45, the slab heating temperature before rolling is outside the scope of the present invention. In No. 44, the HIC resistance characteristics are deteriorated, and in No. 45, the SSC resistance characteristics are deteriorated.

[0199] In Comparative Examples Nos. 46 and 47, the number of rolling passes in rough rolling or finish rolling was at a low level, the effect of refining γ grains before transformation was small, the surface hardness of the steel plate showed a high value, and the SSC resistance property decreased.

[0200] In Comparative Example No. 48, the cooling rate in the first cooling step was at a high level, the surface hardness of the steel plate showed a high value, and the SSC resistance property decreased.

[0201] In Comparative Examples Nos. 49 and 50, the reheating temperature after the first cooling step was outside the scope of the present invention. In No. 49, the surface hardness of the steel plate showed a high value, the SSC resistance property decreased, and the HIC resistance property also decreased. In No. 50, the structure of the steel deviated from the scope of the invention, the surface hardness of the steel plate tended to be high, and the HIC resistance property decreased.

[0202] In Comparative Example No. 51, the cooling rate in the second cooling step was at a high level, the tempering effect could not be sufficiently obtained, the surface hardness of the steel plate showed a high value, and the SSC resistance property decreased.

[0203] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that those having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modification examples or correction examples within the scope of the technical idea described in the claims, and it is naturally understood that these also belong to the technical scope of the present invention.

Industrial Applicability

[0204] As described above, according to the present invention, it is possible to provide a steel plate suitable for a steel pipe having a yield strength of 300 MPa or more, sufficient HIC resistance, and excellent SSC resistance over a wide range of the inner surface. Furthermore, according to the present invention, it is possible to provide a steel plate suitable for a steel pipe having excellent fracture toughness in a high-pressure hydrogen environment, and it is also applicable to a line pipe for high-pressure hydrogen gas. Therefore, the present invention has high industrial applicability.

Claims

1. As a chemical composition, by mass%, C: 0.030 to 0.070%, Si: 0.005 to 0.500%, Mn: 0.80 to 1.65%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.070%, Ti: 0.004 to 0.018%, Nb: 0.005 to 0.050%, Ca: 0.0010 to 0.0050%, N: 0.0020 to 0.0070%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, B: 0 to 0.0030%, O: 0.0040% or less, The balance: Fe and impurities, and is a steel sheet consisting of The steel sheet Ceq defined by the following formula (1) is in the range of 0.200 to 0.500, NPIP defined by the following formula (2) is 1.00 or less, and The NPIM defined by the following formula (3) is 1.5×10 －５ or more, The metallographic structure at a position 0.1 mm deep from the surface is, by area ratio, one or both of polygonal ferrite and granular bainite, with a total area ratio of 70 to 98% and an average grain size of 30.0 μm or less, The metallographic structure at a position 0.5 mm deep from the surface is, by area ratio, one or both of polygonal ferrite and granular bainite, with a total area ratio of 75 to 95%, When the thickness of the steel sheet is t, the metallographic structure at a position 2t / 5 deep from the surface of the steel sheet contains one or both of polygonal ferrite and granular bainite in a total area ratio of 50 to 97% and has an average grain size of 20.0 μm or less, Hvmax, which is the maximum hardness in the surface layer portion in the range from the surface of the steel sheet to a depth of 1.0 mm, is 220 Hv or less, A steel sheet having a yield ratio of 70% or more. Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5... (1) NPIP = [Ti] / [N] × 0.29... (2) NPIM = [Ti] × [N]... (3) Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V] in the above formulas (1) to (3) respectively indicate the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, N in the steel sheet.

2. The steel sheet according to Claim 1, wherein the chemical composition is such that ESSP defined by the following formula (4) is in the range of 1.0 or more and 15.0 or less. ESSP = [Ca] × (1 - 124 × [O]) / (1.25 × [S])... (4) Here, [Ca], [O], and [S] in the above formula (4) respectively represent the contents of Ca, O, and S in the steel sheet in mass%.

3. The steel sheet according to claim 1 or claim 2, wherein the metal structure at a position with a depth of 2t / 5 from the surface contains 50% or more of polygonal ferrite by area ratio.

4. Among the chemical compositions, Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, The steel sheet according to claim 1 or claim 2, containing one or more of REM: 0.0001 to 0.0100%.

5. Among the chemical compositions, Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, The steel sheet according to claim 3, containing one or more of REM: 0.0001 to 0.0100%.

6. The steel sheet according to claim 1 or claim 2, wherein Hvmax, which is the maximum hardness in the surface layer portion within a range of 1.0 mm from the surface of the steel sheet, is 200 Hv or less.

7. The steel sheet according to claim 1 or claim 2, having a yield strength of 300 to 600 MPa.

8. The steel sheet according to claim 1 or claim 2, having a fracture toughness value of 55 MPa√m or more after being held for 1000 hours in a high-pressure hydrogen environment with a hydrogen pressure of 200 atmospheres.

9. The steel sheet according to claim 1 or claim 2, having a plate thickness within the range of 10 to 40 mm.

10. A steel pipe having a base material portion made of the steel sheet according to claim 1 or claim 2 and a welded portion.

11. As the chemical composition, in mass%, C: 0.030 to 0.070%, Si: 0.005 to 0.500%, Mn: 0.80 to 1.65%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.070%, Ti: 0.004 to 0.018%, Nb: 0.005 to 0.050%, Ca: 0.0010 to 0.0050%, N: 0.0020 to 0.0070%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, B: 0 to 0.0030%, O: 0.0040% or less, Balance: Consisting of Fe and impurities, Ceq defined by the following formula (1) is in the range of 0.200 to 0.500, NPIP defined by the following formula (2) is 1.00 or less, and The steel sheet in which NPIM defined by the following formula (3) is 1.5×10 －５ or more is heated in the range of 1050 to 1210°C and hot-rolled in a hot-rolling step, a first cooling step and a second cooling step, the hot rolling step includes a first rolling stage of rolling in the range of 930°C or higher and a second rolling stage of rolling in the range of 900°C or lower, in the first rolling stage, the total reduction ratio between 1130 and 930°C is 30% or more, and the reduction passes with a reduction ratio of 10% or more per pass are 3 or more times, in the second rolling stage, the total reduction ratio at 900°C or lower is 30% or more, and the reduction passes with a reduction ratio of 10% or more per pass are 5 or more times, in the first cooling step, the average cooling rate at a depth of 1.0 mm below the plate surface between the start of cooling and the water cooling stop temperature of 400°C or lower is 100°C / s or lower, and the cooling is such that the maximum temperature reached on the steel plate surface 15 seconds after the water cooling stop is 350°C or higher and less than 650°C, the second cooling step is a method for manufacturing a steel plate, which, after the first cooling step and after the steel plate surface temperature reaches the maximum temperature reached, cools to 200°C or lower at an average cooling rate of 5°C / s or lower. Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5... (1) NPIP = [Ti] / [N] × 0.29... (2) NPIM = [Ti] × [N]... (3) Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V] in the above formulas (1) to (3) respectively indicate the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel plate.

12. Furthermore, the method for manufacturing a steel plate according to claim 11, further comprising a tempering step in the range of a tempering temperature of 400 to 650°C, with a holding time of 10 to 60 minutes at a target temperature ±10°C.

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