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

Abstract

In a steel sheet according to one embodiment of the present disclosure: in the metal structure at a depth of 0.1 mm from the surface, the total area ratio of polygonal ferrite and granular bainite is 70-100%, and the area ratio of polygonal ferrite is 70% or more; the average particle diameter of the metal structure at a depth of 0.1 mm from the surface is 30.0 μm or less; in the metal structure at a depth of 0.5 mm from the surface, the total area ratio of polygonal ferrite and granular bainite is 75-95%, and the area ratio of polygonal ferrite is 50% or more; in the metal structure at a depth of 2t / 5 from the surface of the steel sheet, the total area ratio of polygonal ferrite and granular bainite is 50-97%; the average particle diameter of the metal structure at a depth of 2t / 5 from the surface is 20.0 μm or less; Hvmax, which is the maximum hardness in the surface layer part, is 220 Hv or less; and the yield ratio is 70% or more.

Description

Steel plate, steel pipe, and method for manufacturing steel plate

[0001] This disclosure relates to a steel plate, a steel pipe, and a method for manufacturing the steel plate. This application claims priority to Japanese Patent Application No. 2024-004183, filed on January 15, 2024, the contents of which are incorporated herein by reference.

[0002] Steel pipes used in pipelines transporting oil and gas are exposed to corrosive gases produced from oil wells. Steel pipes exposed to a high-pressure hydrogen sulfide environment, which is a severe acidic environment containing corrosive gases, are susceptible to sulfide stress cracking (hereinafter sometimes referred to as "SSC") and hydrogen-induced cracking (hereinafter sometimes referred to as "HIC"). Therefore, line pipes, which are steel pipes used in pipelines, are required to have resistance to sulfide stress cracking (SSC resistance) and hydrogen-induced cracking (HIC resistance). Hereinafter, resistance to sulfide stress cracking (SSC resistance) and resistance to hydrogen-induced cracking (HIC resistance) may be collectively referred to as "sour resistance."

[0003] Furthermore, high strength is required for steel pipes for line pipes in order to conserve material by reducing the thickness and to reduce the weight of the product. However, if the amount of alloying elements added is increased or the heat input is increased for high-efficiency welding, the low-temperature toughness of the weld heat-affected zone (HAZ) will decrease.

[0004] Patent Document 1 and Non-Patent Document 1 propose a welded steel pipe or a steel plate for such a steel pipe that has excellent sour resistance, in which the hardness of the base metal and the weld is specified to be 220 Hv or less, based on the finding that sour resistance is affected by hardness.

[0005] Furthermore, Patent Document 2 proposes a high-strength steel plate for sour-resistant line pipes in which 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 showing the hardness of a center segregation portion, in mass%, is 1.0 or less, the steel structure is a bainite structure, the variation in hardness ΔHV in the plate thickness direction is 30 or less, and the variation in hardness (ΔHV) in the plate width direction is 30 or less.

[0006] Furthermore, Patent Document 3 proposes a high-strength steel plate for sour-resistant line pipes, which has a bainite metal structure, a hardness variation in the plate thickness direction of 25 or less at ΔHv10, a hardness variation in the plate width direction of 25 or less at ΔHv10, and a maximum hardness of the steel plate surface layer of 220 or less at Hv10, and which has excellent uniformity of material properties within the steel plate.

[0007] Patent Document 4 also proposes a tempered steel sheet with excellent resistance to hydrogen-induced cracking, in which the metal structure within a range of 1 mm from the steel sheet surface in the sheet thickness direction consists of one or two types selected from tempered martensite and tempered bainite, and in the metal structure within a range of ±1 mm from the sheet thickness center in the sheet thickness direction, the main phase consisting of one or two types selected from tempered martensite and tempered bainite accounts for 80% or more in area ratio, with the remainder other than the main phase consisting of one or more types selected from ferrite, pearlite, cementite and retained austenite, and further the hardness at a position 1 mm from the steel sheet surface in the sheet thickness direction is 250 HV or less in Vickers hardness and the difference in hardness between a position 1 mm from the steel sheet surface and the sheet thickness center is 60 HV or less in Vickers hardness.

[0008] The steel plates of Patent Documents 1 to 4 and Non-Patent Document 1 have improved sour resistance in an environment with a hydrogen sulfide partial pressure of 0.1 MPa (1 bar) or less. However, the required level of sour resistance for welded steel pipes used in pipelines and the like is becoming higher. Specifically, while sour resistance in an environment with a hydrogen sulfide partial pressure of 0.1 MPa (1 bar) or less was conventionally required, recently there has been a demand for materials that can withstand high-pressure hydrogen sulfide environments exceeding 0.1 MPa.

[0009] Furthermore, Patent Document 5 proposes a steel plate as a material that can withstand the above-mentioned high-pressure hydrogen sulfide environment. The steel plate has a metal structure in a range of 1.0 mm from the surface of the steel pipe (surface layer portion) that contains one or more types 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 has a maximum hardness in the surface layer portion of the base material, Hvmax, of 250 Hv or less, and is non-heat treated steel plate with excellent resistance to hydrogen-induced cracking.

[0010] JP 2011-017048 A JP 2012-077331 A JP 2013-139630 A JP 2014-218707 A International Publication No. 2019 / 058422

[0011] Nobuyuki ISHIKAWA, et al. “High performance UOE Linepipes”, JFE TECHNICAL REPORT, Japan, Jan. 2006, No.7, p20-p26

[0012] As mentioned above, there have been several disclosures regarding the surface structure and hardness of materials that can withstand a high-pressure hydrogen sulfide environment. However, the surface structure and hardness of actual products require precise control of heating, rolling, and controlled cooling in the manufacturing process. To achieve this, it is necessary to control the manufacturing conditions within a limited range. For example, in order to obtain the above-mentioned surface structure during controlled cooling of steel sheets, Ar ３ It is necessary to start accelerated cooling at a temperature equal to or higher than this point. On the other hand, if the start temperature of accelerated cooling is too high, cooling becomes insufficient and the desired metal structure cannot be obtained. Therefore, it is not easy to make the surface hardness of the steel sheet Hvmax 220 or less or 200 or less.

[0013] Therefore, the present disclosure has been made in consideration of the above-mentioned circumstances, and aims to provide a steel plate to be used as the base material of a steel pipe suitable for line pipe, which has 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 the steel plate.

[0014] The present disclosure has been made in view of the above problems, and the gist of the disclosure is as follows.

[0015] (1) A steel sheet according to an embodiment of the present disclosure has a chemical composition, in mass%, of 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%, W: 0 to 0.50%, O: 0.0040% or less, and the balance: Fe and impurities, wherein the steel sheet has a Ceq defined by the following formula (i) in the range of 0.200 to 0.500, an NPIP defined by the following formula (ii) of 1.00 or less, and an NPIM defined by the following formula (iii) of 1.5×10 －５or more, in the metal structure at a position 0.1 mm deep from the surface, the total area ratio of polygonal ferrite and granular bainite is 70 to 100%, and the area ratio of the polygonal ferrite is 70% or more, the average grain size of the metal structure at the position 0.1 mm deep from the surface is 30.0 μm or less, and in the metal structure at a position 0.5 mm deep from the surface, the total area ratio of polygonal ferrite and granular bainite is 75 to 95%, and The area ratio of polygonal ferrite is 50% or more, and when the thickness of the steel plate is t, in the metal structure at a position at a depth of 2t / 5 from the surface of the steel plate, the total area ratio of polygonal ferrite and granular bainite is 50 to 97%, the average grain size of the metal structure at the position at a depth of 2t / 5 from the surface is 20.0 μm or less, Hvmax which is the maximum hardness in a surface layer portion ranging from the surface to a depth of 1.0 mm of the steel plate is 220 Hv or less, and the yield ratio is 70% or more. Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 (i) NPIP = ([Ti] / [N]) × 0.29 (ii) NPIM = [Ti] × [N] (iii) In the above formulas (i) to (iii), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] respectively represent the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel sheet. (2) Preferably, in the steel sheet described in (1) above, the chemical composition has an ESSP defined by the following formula (iv) in the range of 1.0 to 15.0. ESSP=[Ca]×(1−124×[O]) / (1.25×[S]) (iv) Here, [Ca], [O], and [S] in the above formula (iv) respectively represent the contents in mass% of Ca, O, and S in the steel sheet. (3) Preferably, in the steel sheet according to (1) or (2) above, the metal structure at a depth of 2t / 5 from the surface contains polygonal ferrite in an area ratio of 50% or more.(4) Preferably, in the steel sheet according to any one of (1) to (3) above, the chemical composition contains one or more of 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%, and REM: 0.0001 to 0.0100%. (5) Preferably, in the steel sheet according to any one of (1) to (4) above, the chemical composition contains one or more of 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%, and REM: 0.0001 to 0.0100%. (6) Preferably, in the steel sheet according to any one of (1) to (5) above, Hvmax, which is the maximum hardness in a surface layer portion extending from the surface of the steel sheet to a depth of 1.0 mm, is 200 Hv or less. (7) Preferably, in the steel sheet according to any one of (1) to (6) above, the yield strength is 300 to 600 MPa. (8) Preferably, the steel sheet according to any one of (1) to (7) above has a sheet thickness in the range of 10 to 40 mm. (9) Preferably, the steel sheet according to any one of (1) to (8) above satisfies the following formula (v): PF. ０．１ / PF ０．５ ≧1.1 (v) Here, PF in the above formula (v) ０．１ is the area ratio of polygonal ferrite in the metal structure at a depth of 0.1 mm from the surface, and PF ０．５ is the area ratio of polygonal ferrite in the metal structure at a position 0.5 mm deep from the surface.

[0016] (10) A steel pipe according to another aspect of the present disclosure comprises a tubular base material portion made of the steel plate described in any one of (1) to (9) above, and a weld portion extending along the axial direction of the base material portion.

[0017] (11) A method for producing a steel sheet according to another embodiment of the present disclosure includes, 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%, W: 0 to 0.50%, O: 0.0040% or less, and the balance: Fe and impurities, wherein Ceq defined by the following formula (i) is in the range of 0.200 to 0.500, NPIP defined by the following formula (ii) is 1.00 or less, and NPIM defined by the following formula (iii) is 1.5×10 －５ The method includes a hot rolling step of heating the above-mentioned steel slab to a temperature in the range of 1050 to 1210°C and hot rolling it, a first cooling step, and a second cooling step, and the hot rolling step includes a first rolling stage of rolling the steel slab to a temperature in the range of 930°C or higher, and a second cooling step of rolling the steel slab to a temperature in the range of 930°C or higher. ３ and a second rolling stage in which the steel slab is rolled in a range of from 1130°C to 900°C, wherein the first rolling stage has a total reduction of 30% or more between 1130°C and 930°C, and includes three or more passes with a reduction rate of 10% or more per pass. The second rolling stage has a total reduction of 30% or more at 900°C or less, and includes five or more passes with a reduction rate of 10% or more per pass. The first cooling step is performed by cooling the surface temperature of the steel sheet at the start of water cooling with Ar. ３the surface temperature of the steel plate is set to 400°C or less when water cooling is stopped; the average cooling rate at a depth of 1.0 mm from the surface of the steel plate from the start of water cooling to the stop of water cooling is set to 100°C / s or less; and the maximum temperature of the surface of the steel plate 15 seconds after water cooling is stopped is set to 350°C or more and less than 650°C; the second cooling step is performed after the first cooling step and after the temperature of the surface of the steel plate has reached the maximum temperature, by cooling the steel plate until the temperature of the surface is 200°C or less, and the average cooling rate of the surface of the steel plate from the maximum temperature to 200°C is set to 5°C / s or less. Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (i) NPIP=([Ti] / [N])×0.29 (ii) NPIM=[Ti]×[N] (iii) In the above formulas (i) to (iii), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] respectively represent the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel slab. (12) Preferably, the method for producing a steel plate according to the above (11) further comprises a tempering step in which the tempering temperature is in the range of 400 to 650°C, and the holding time at the target temperature ±10°C is 10 to 60 minutes.

[0018] According to the above-described aspects of the present disclosure, it is possible to provide a steel plate suitable for line pipes 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 the steel plate.

[0019] Specifically, it is possible to provide a steel plate to be used as a base material for steel pipes that are excellent in SSC resistance (resistance to sulfide stress corrosion cracking) and HIC resistance (resistance to hydrogen-induced cracking). Steel pipes that are excellent in sour resistance (resistance to SSC and HIC) are suitable for use in high-pressure hydrogen sulfide environments as line pipes for transporting petroleum, natural gas, etc.

[0020] FIG. 1 is a diagram schematically showing one aspect of accelerated cooling after completion of hot rolling. FIG. 2 is a diagram showing an example of hardness distribution in the surface layer portion of an example steel. FIG. 3 is a diagram showing an example of hardness distribution in the surface layer portion of an example steel. FIG. 4 is a diagram showing an example of hardness distribution in the surface layer portion of a comparative steel. FIG. 5 is a diagram showing an example of hardness distribution in the surface layer portion of a comparative steel. FIG. 6 is a diagram showing a structure (imaged with a scanning electron microscope) 0.1 mm below the outermost surface of an example steel, and is a structural photograph showing an example of structural evaluation. FIG. 7 is a diagram showing a structure (imaged with a scanning electron microscope) 0.1 mm below the outermost surface of an example steel, and is a structural photograph showing an example of structural evaluation. FIG. 8 is a diagram showing a distribution of NPIP and maximum surface layer hardness of an example steel. FIG. 9 is a diagram showing a distribution of NPIM and maximum surface layer hardness of an example steel. FIG. 10 is a cross-sectional schematic diagram explaining the measurement positions of area fractions of polygonal ferrite and granular bainite. FIG. 11 is a schematic diagram explaining the extraction position of a full-thickness block test piece in a steel plate. FIG. 12 is a schematic diagram explaining the extraction position of a full-thickness block test piece in a steel pipe. FIG. 13 is a cross-sectional schematic diagram explaining the hardness measurement position in a full-thickness block test piece. 1 is a flowchart showing an example of a method for manufacturing a steel sheet.

[0021] Preferred embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0022] <Regarding Steel Sheet> The steel sheet according to an embodiment of the present disclosure has a chemical composition, in mass %, of 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%, W: 0 to 0.50%, O: 0.0040% or less, and the balance: Fe and impurities, wherein the steel sheet has a Ceq defined by the following formula (1) in the range of 0.200 to 0.500, an NPIP defined by the following formula (2) of 1.00 or less, and an NPIM defined by the following formula (3) of 1.5×10 －５or more, in the metal structure at a position 0.1 mm deep from the surface, the total area ratio of polygonal ferrite and granular bainite is 70 to 100%, and the area ratio of polygonal ferrite is 70% or more, the average grain size of the metal structure at a position 0.1 mm deep from the surface is 30.0 μm or less, and in the metal structure at a position 0.5 mm deep from the surface, the total area ratio of polygonal ferrite and granular bainite is 75 to 95%, and The area ratio of polygonal ferrite is 50% or more, and when the thickness of the steel plate is t, in the metal structure at a position at a depth of 2t / 5 from the surface of the steel plate, the total area ratio of polygonal ferrite and granular bainite is 50 to 97%, the average grain size of the metal structure at a position at a depth of 2t / 5 from the surface is 20.0 μm or less, Hvmax which is the maximum hardness in a surface layer portion ranging from the surface of the steel plate to a depth of 1.0 mm is 220 Hv or less, and the yield ratio is 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, in the above formulas (1) to (3), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] respectively represent the contents in mass% of C, Mn, Cu, Ni, Cr, Mo, V, Ti, and N in the steel plate.

[0023] Furthermore, the steel sheet according to this embodiment preferably has an ESSP defined by the following formula (4) in the range of 1.0 or more and 15.0 or less.

[0024] ESSP=[Ca]×(1-124×[O]) / (1.25×[S])...(4)

[0025] Here, [Ca], [O], and [S] in the above formula (4) represent the contents of Ca, O, and S in the steel sheet in mass %, respectively.

[0026] Furthermore, the steel plate according to this embodiment more preferably has a plate thickness of 10 to 40 mm. More specifically, the plate thickness is the plate thickness of the part of the steel plate that will become the base material when made into a steel pipe.

[0027] Furthermore, in the steel plate according to this embodiment, it is even more preferable that the metal structure at a depth of 2t / 5 from the surface contains polygonal ferrite in an area ratio of 50% or more.

[0028] Furthermore, the steel plate according to this embodiment is used as a base material for a steel pipe suitable for line pipes, which has excellent SSC resistance and HIC resistance. That is, by processing the steel plate according to this embodiment into a cylindrical shape and welding the butt joints, a steel pipe for line pipes, which has excellent SSC resistance and HIC resistance, can be obtained.

[0029] The steel sheet according to this embodiment will be described in detail below.

[0030] (Regarding Chemical Composition) First, the chemical composition of the steel sheet according to this embodiment will be described in detail. The steel sheet according to this embodiment has the chemical composition shown below. In this embodiment, unless otherwise specified, the notation of % regarding the chemical composition means "mass %." Furthermore, the preferred upper and lower limit values ​​of the contents of the elements described below can be selected individually and freely combined.

[0031] That is, the chemical composition of the steel sheet according to this 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 0.005%. 0%, 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%, W: 0 to 0.50%, O: 0.0040% or less, and the balance including Fe and impurities.

[0032] [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 is set to 0.030% or more. The C content is preferably 0.035% or more, 0.040% or more, or 0.045% or more.

[0033] On the other hand, if the C content exceeds 0.070%, the strength of the steel increases too much, and the hardness of the surface layer metallographic structure and the internal metallographic structure (particularly the metallographic structure of the center segregation portion) exceeds 248 Hv, resulting in a decrease in SSC resistance and HIC resistance. Therefore, the C content is set to 0.070% or less. In order to suppress a decrease in weldability, toughness, etc., the C content is preferably 0.065% or less, 0.060% or less, or 0.050% or less. As mentioned above, the "surface layer" means the range from the surface of the steel plate to a depth of 1.0 mm.

[0034] [Si: 0.005 to 0.500%] Si is an element that functions as a deoxidizer during steelmaking. Furthermore, Si is an element that is mixed into steel during the steelmaking stage. If the Si content is less than 0.005%, the above effect cannot be sufficiently obtained. Therefore, the Si content is set to 0.005% or more. In order to obtain a sufficient deoxidizing effect, the Si content is preferably set to 0.010% or more, 0.020% or more, or 0.050% or more.

[0035] On the other hand, if the Si content exceeds 0.500%, the toughness of the weld heat affected zone (HAZ) decreases. Therefore, the Si content is set to 0.500% or less. The Si content is preferably 0.400% or less, 0.350% or less, or 0.300% or less.

[0036] [Mn: 0.80 to 1.65%] Mn is an element that contributes to improving the strength and toughness of steel. If the Mn content is less than 0.80%, the effect of improving strength and toughness cannot be sufficiently obtained. Therefore, the Mn content is set to 0.80% or more. The Mn content is preferably 0.90% or more, 1.05% or more, or 1.20% or more.

[0037] On the other hand, if the Mn content exceeds 1.65%, a large amount of MnS, which deteriorates HIC resistance, is generated, and the hardness of the internal metal structure, particularly the center segregation portion, exceeds 248 Hv, thereby reducing HIC resistance. Therefore, the Mn content is set to 1.65% or less. The Mn content is preferably 1.50% or less, 1.50% or less, or 1.30% or less.

[0038] [P: 0.015% or less] P is an impurity element, and the lower its content, the better. If the P content exceeds 0.015%, HIC resistance will be significantly reduced. Therefore, the P content is set to 0.015% or less. The P content is preferably 0.012% or less, 0.010% or less, or 0.008% or less.

[0039] Since a lower P content is preferable, the lower limit includes 0%. However, reducing the P content to less than 0.003% significantly increases the production cost. Therefore, the P content may be set to 0.001% or more, 0.003% or more, or 0.005% or more.

[0040] [S: 0.0015% or less] S is an element that forms MnS that elongates in the rolling direction during hot rolling. This elongated MnS reduces HIC resistance. If the S content exceeds 0.0015%, HIC resistance significantly decreases. Therefore, the S content is set to 0.0015% or less. The S content is preferably 0.0012% or less, 0.0010% or less, or 0.0008% or less.

[0041] Since a lower S content is preferable, the lower limit includes 0%. However, if the S content is reduced to less than 0.0001%, the manufacturing cost will increase significantly. Therefore, the S content may be set to 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0042] [Al: 0.010 to 0.070%] Al is an element added for deoxidation. If the Al content is less than 0.010%, the above effect cannot be sufficiently obtained. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.015% or more, 0.020% or more, or 0.025% or more.

[0043] On the other hand, if the Al content exceeds 0.070%, Al oxides accumulate to form clusters, resulting in a decrease in HIC resistance. Therefore, the Al content is set to 0.070% or less. The Al content is preferably 0.060% or less, 0.050% or less, or 0.045% or less.

[0044] [Ti: 0.004 to 0.018%] Ti is one of the important elements in the steel sheet according to the present disclosure. Ti combines with N to form nitrides. These nitrides contribute to the refinement of crystal grains. The effect of refining γ (austenite) grains varies significantly depending on the Ti / N balance and the solubility product of Ti and N. If the Ti content is less than 0.004%, the above effect cannot be sufficiently obtained. Therefore, the Ti content is set to 0.004% or more. The Ti content is preferably 0.005% or more, 0.008% or more, or 0.010% or more.

[0045] On the other hand, if the Ti content exceeds 0.018%, coarse nitrides are formed and HIC resistance is reduced. Therefore, the Ti content is set to 0.018% or less. The Ti content is preferably 0.017% or less, 0.016% or less, or 0.014% or less.

[0046] [Nb: 0.005 to 0.050%] Nb is an element that expands the non-recrystallization temperature range, refines crystal grains, and forms carbides and nitrides, thereby contributing to improving the strength of steel. If the Nb content is less than 0.005%, the above effects cannot be fully achieved. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably 0.008% or more, 0.010% or more, or 0.020% or more.

[0047] On the other hand, if the Nb content exceeds 0.050%, coarse carbides and nitrides are formed, resulting in a decrease in HIC resistance. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.045% or less, 0.040% or less, or 0.030% or less.

[0048] [Ca: 0.0010 to 0.0050%] Ca is an element that combines with S to form CaS and suppresses the formation of MnS, which elongates in the rolling direction, thereby contributing to improving HIC resistance. If the Ca content is less than 0.0010%, the above effect cannot be sufficiently obtained. Therefore, the Ca content is set to 0.0010% or more. The Ca content is preferably 0.0015% or more, 0.0020% or more, or 0.0025% or more.

[0049] On the other hand, if the Ca content exceeds 0.0050%, Ca oxides accumulate and the HIC resistance decreases. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[0050] [N: 0.0020 to 0.0070%] N, along with Ti, is one of the important elements in the steel sheet according to the present disclosure. N is an element that forms nitrides and contributes to suppressing coarsening of austenite grains during heating. As described above, the effect of refining γ (austenite) grains varies significantly depending on the Ti / N balance and the solubility product of Ti and N. If the N content is less than 0.0020%, the above effect cannot be sufficiently obtained. Therefore, the N content is set to 0.0020% or more. The N content is preferably 0.0025% or more, more preferably 0.0030% or more, or 0.0035% or more.

[0051] On the other hand, if the N content exceeds 0.0070%, coarse nitride-containing inclusions are formed, resulting in a decrease in HIC resistance. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0060% or less, 0.0050% or less, or 0.0040% or less.

[0052] In addition to the above elements, the chemical composition of the steel sheet according to this embodiment may contain Ni, Mo, Cr, Cu, V, Mg, REM, and B in the ranges described below, in place of a portion of Fe, to improve strength, toughness, and other properties, as long as the properties of the steel sheet according to this embodiment are not impaired. These elements are optional elements and may not be contained. In other words, the lower limit of the content of these elements is 0%.

[0053] [Ni: 0 to 0.50%] Ni is an element that contributes to improving the toughness and strength of steel, as well as improving corrosion resistance. To obtain these effects, the Ni content is preferably 0.05% or more. The N content is more preferably 0.08% or more, 0.10% or more, or 0.15% or more.

[0054] On the other hand, if the Ni content exceeds 0.50%, the strength increases too much, resulting in a decrease in toughness, and the selective corrosion of the grain boundaries on the surface may cause a decrease in SSC resistance. Therefore, even if Ni is contained, the Ni content is preferably 0.50% or less. The Ni content is more preferably 0.40% or less, 0.35% or less, or 0.30% or less.

[0055] [Mo: 0 to 0.50%] Mo is an element that contributes to improving the hardenability of steel. To obtain this effect, the Mo content is preferably 0.05% or more. The Mo content is more preferably 0.08% or more, 0.10% or more, or 0.12% or more.

[0056] On the other hand, if the Mo content exceeds 0.50%, the strength increases too much and the toughness may decrease. Therefore, even if Mo is contained, the Mo content is preferably 0.50% or less. The Mo content is more preferably 0.35% or less, 0.30% or less, or 0.20% or less.

[0057] [Cr: 0 to 0.50%] Cr is an element that contributes to improving the strength of steel. To obtain this effect, the Cr content is preferably 0.05% or more. The Cr content is more preferably 0.08% or more, 0.10% or more, or 0.12% or more.

[0058] On the other hand, if the Cr content exceeds 0.50%, the strength may increase too much and the toughness may decrease. Therefore, even if Cr is contained, the Cr content is preferably 0.50% or less. The Cr content is more preferably 0.35% or less, 0.25% or less, or 0.15% or less.

[0059] [Cu: 0 to 0.50%] Cu is an element that contributes to improving the strength and corrosion resistance of steel. To obtain these effects, the Cu content is preferably 0.05% or more. The Cu content is more preferably 0.08% or more, 0.10% or more, or 0.12% or more.

[0060] On the other hand, if the Cu content exceeds 0.50%, the strength may increase too much and the toughness may decrease. Therefore, even if Cu is contained, the Cu content is set to 0.50% or less. The Cu content is more preferably 0.35% or less, 0.25% or less, or 0.15% or less.

[0061] [V: 0 to 0.100%] V is an element that contributes to improving the strength of steel by forming carbides and / or nitrides. To obtain this effect, the V content is preferably 0.010% or more. The V content is more preferably 0.015% or more, 0.020% or more, or 0.030% or more.

[0062] On the other hand, if the V content exceeds 0.100%, the toughness may decrease. Therefore, even if V is contained, the V content is preferably 0.100% or less. The V content is more preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0063] [Mg: 0 to 0.0100%] Mg is an element that contributes to improving the toughness of steel by forming fine oxides and suppressing the coarsening of crystal grains. To obtain this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0003% or more, 0.0006% or more, or 0.0010% or more.

[0064] On the other hand, if the Mg content exceeds 0.0100%, oxides may aggregate and coarsen, which may reduce HIC resistance and toughness. Therefore, even if Mg is contained, the Mg content is preferably 0.0100% or less. The Mg content is more preferably 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0065] [REM: 0 to 0.0100%] REM is an element that contributes to improving SSC resistance, HIC resistance, and toughness by controlling the morphology of sulfide-based inclusions. To achieve these effects, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0003% or more, 0.0005% or more, or 0.0010% or more.

[0066] On the other hand, if the REM content exceeds 0.0100%, oxides are generated, reducing the cleanliness of the steel, which may result in reduced HIC resistance and toughness. Therefore, even when REM is contained, the REM content is preferably 0.0100% or less. The REM content is more preferably 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0067] In this embodiment, REM refers to rare earth elements, which is a general term for 16 elements including Sc and lanthanides (La to Lu). The REM content in this embodiment refers to the total content of these 16 elements.

[0068] [B: 0 to 0.0030%] B is an element that significantly improves hardenability even in small amounts, and contributes to improving the strength of steel plates and steel pipes. Therefore, B may be contained, with the lower limit being 0% or more. The B content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more. However, B has the effect of increasing surface hardness during controlled cooling. If a steel plate contains a large amount of B, SSC resistance decreases. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0015% or less, 0.0013% or less, or 0.0010% or less.

[0069] [W: 0 to 0.50%] W is an element that contributes to improving the hardenability of steel. To obtain this effect, the W content is preferably 0.05% or more. The W content is more preferably 0.08% or more, 0.10% or more, or 0.12% or more.

[0070] On the other hand, if the W content exceeds 0.50%, the strength increases too much and the toughness may decrease. Therefore, even if W is contained, the W content is preferably 0.50% or less. The W content is more preferably 0.35% or less, 0.30% or less, or 0.20% or less.

[0071] [O: 0.0040% or less] O is an element that remains in the steel sheet after deoxidation, and the lower its content, the better. If the O content exceeds 0.0040%, a large amount of oxide is generated, significantly reducing HIC resistance. Therefore, the O content is limited to 0.0040% or less. The O content is preferably 0.0035% or less, 0.0030% or less, or 0.0025% or less.

[0072] On the other hand, since a lower O content is preferable, the lower limit includes 0%. However, reducing the O content to less than 0.0005% significantly increases the production cost. The O content may be 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more.

[0073] As described above, the steel sheet according to this embodiment basically has a chemical composition containing the above essential elements with the balance being Fe and impurities, but may also have a chemical composition containing the above essential elements and, as necessary, the above optional elements with the balance being Fe and impurities. Here, impurities refer to components that are mixed in from raw materials such as ore or scrap or from various environments in the manufacturing process when industrially producing steel, and are acceptable within a range that does not adversely affect the steel.

[0074] Of the above impurities, Sb, Sn, Zr, Y, and Hf are preferably contained in an amount of, for example, 0.50% or less each of W, Sb, and Sn, and 0.01% or less each of Zr, Y, and Hf, taking into consideration the influence on the steel sheet properties and the steel pipe properties.

[0075] [Ceq: 0.200 to 0.500] In the steel plate according to this embodiment, in order to obtain a metallographic structure containing one or both of polygonal ferrite and granular bainite in a total area ratio of 70 to 100% at a position 0.1 mm from the surface, and one or both of polygonal ferrite and granular bainite in a total area ratio of 75 to 95% at a position 0.5 mm from the surface, 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 within the range of 0.200 to 0.500, preferably within the range of 0.30 to 0.45.

[0076] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5...(1)

[0077] In the above formula (1), [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the contents of C, Mn, Cu, Ni, Cr, Mo, and V in mass %, respectively, and are set to 0 when not contained.

[0078] If the Ceq value is less than 0.200, ferrite and pearlite are generated in the metal structure, and the total of one or both of polygonal ferrite and granular bainite at the 0.1 mm position is less than 70%, or the total of one or both of polygonal ferrite and granular bainite at the 0.5 mm position is less than 75%. In this case, the steel sheet strength is reduced and sour resistance is also reduced. Therefore, the Ceq value is set to 0.200 or more. The Ceq value is preferably 0.300 or more, 0.320 or more, or 0.350 or more.

[0079] On the other hand, if the Ceq value of the steel plate exceeds 0.500, the surface hardness of the base metal and the portion that will become the weld when manufacturing a steel pipe from the steel plate becomes excessively high, and sour resistance decreases. Therefore, the Ceq value is set to 0.500 or less. The Ceq value is preferably 0.480 or less, 0.450 or less, or 0.430 or less.

[0080] In the steel sheet according to this embodiment, it is necessary to control the content of each element within the above-mentioned range, and then control the values ​​of NPIP and NPIM calculated from the content of the components within predetermined ranges as shown below.

[0081] [NPIP: 1.00 or less] In the steel sheet according to this embodiment, in order to achieve the above-described surface hardness of the steel sheet, the NPIP value 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), the 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, respectively, in mass %. The NPIP may be set to 0.90 or less, 0.80 or less, or 0.70 or less. Furthermore, from the above-described Ti content and N content, the lower limit of the NPIP is substantially about 0.12. The NPIP may be set to 0.30 or more, 0.50 or more, or 0.60 or more.

[0082] NPIP=([Ti] / [N])×0.29...(2)

[0083] [NPIM: 1.5×10 －５ In the steel sheet according to this embodiment, in order to achieve the above-mentioned surface hardness of the steel sheet, the above-mentioned conditions for NPIP are satisfied and the value of NPIM defined by the following formula (3) is set to 1.5 × 10 －５ 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. When NPIM is 1.8 × 10 －５ That's it, 2.0 x 10 －５ or more, or 3.0 x 10 －５ Furthermore, from the Ti content and N content, the upper limit of NPIM is substantially 1.26 × 10 －４ The result is as follows: NPIM is 1.0 x 10 －４ Below, 8.0 x 10 －５ Below, 7.0 x 10 －５ or less, or 6.0 x 10 －５ The following may also be used.

[0084] NPIM=[Ti]×[N]...(3)

[0085] In order to ensure SSC resistance, it is effective to reduce the surface layer hardness. However, it is often technically difficult to precisely control the cooling rate of the surface layer during water cooling of a steel sheet. This is because the cooling rate varies depending on the uniformity of the water temperature and amount on the sheet surface, the soundness of each water cooling nozzle, and other factors. In order to stably reduce the surface layer hardness under various cooling conditions, reducing the hardenability of the steel sheet is an effective method. It is known that the hardenability of a steel sheet decreases by refining the γ grain size before cooling. Effective methods include refining the heated γ grain size before rolling, promoting the recrystallization of γ grains by rough rolling, and flattening the γ grains by controlled rolling.

[0086] NPIP is one of the indices relating to the state of Ti-containing carbonitrides that affects the heated gamma grain size. Specifically, it is an index for optimizing the balance between Ti and N to fully utilize the gamma grain pinning effect of Ti-containing carbonitrides. It has been confirmed that when the NPIP value is greater than 1.00, the heated gamma grain size in the steel sheet surface layer becomes coarse. Although the cause of this is unclear, a mechanism is thought to be that when the Ti content is high relative to the N content, a large amount of solute Ti that does not contribute to the formation of Ti-containing carbonitrides remains, and this solute Ti promotes Ostwald ripening of Ti-containing carbonitrides. Furthermore, if the N content is insufficient, the formation of Nb-containing carbonitrides is difficult, and the pinning effect may not be achieved. Based on the above, the target component range was determined to be a range in which the NPIP value is 1.00 or less.

[0087] Similarly to the above-mentioned NPIP, NPIM is an index relating to the state of existence of Ti-containing carbonitrides. Specifically, it is an index relating to the total amount of Ti-containing carbonitrides that pin γ grain growth. When the NPIM value is 1.5×10 －５ It has been confirmed that if the above value is met, the heated gamma grain size in the steel sheet surface layer becomes smaller. NPIM is an index related to the solubility product of Ti and N. When the NPIM value is high, both the Ti concentration and the N concentration are high, and an effect of increasing the total amount of TiN precipitates is expected. In other words, it is thought that the effect of refining the heated gamma grain size can be obtained by increasing the number of Ti-containing carbonitrides per unit volume.

[0088] Furthermore, in the steel sheet according to this embodiment, it is preferable to control the content of each element within the above-mentioned range, and then control the ESSP value calculated from the content of the components within a predetermined range as shown below.

[0089] [ESSP: 1.0 to 15.0] In the steel plate according to this embodiment, in order to more reliably ensure HIC resistance equal to or greater than that of conventional steel, the ESSP value defined by the following formula (4) is preferably set to a range of 1.0 to 15.0. The ESSP value may also be set to a range of 3.0 to 10.0. Here, in the following formula (4), [Ca], [O], and [S] are the contents of Ca, O, and S, respectively, in mass%.

[0090] ESSP=[Ca]×(1-124×[O]) / (1.25×[S])...(4)

[0091] In order to more reliably ensure HIC resistance, it is effective to suppress the formation of MnS that elongates in the rolling direction. Also, in order to more reliably suppress the formation of MnS that elongates in the rolling direction, it is an effective method to reduce the S content and add Ca to form CaS and fix the S. On the other hand, since Ca has a stronger oxygen affinity than S, reducing the O content is effective in forming the required amount of CaS.

[0092] The ESSP defined by the above formula (4) is an index of the remaining Ca (effective Ca) after subtracting the Ca bonded to oxygen. Specifically, it is an index showing whether or not there is a required amount of effective Ca depending on the S content, assuming that Ca bonds with S at the atomic weight ratio.

[0093] If the ESSP value is less than 1.0, the Ca content may be insufficient relative to the O and S contents, resulting in the formation of MnS. Since MnS elongated by rolling causes deterioration of HIC resistance, the ESSP value is preferably 1.0 or more. The ESSP value is more preferably 2.0, 3.0 or more, or 3.2 or more, and even more preferably 3.5 or more.

[0094] On the other hand, if the Ca content is excessive, a large amount of mainly oxide-based inclusions will be formed, which may cause a deterioration in HIC resistance. Reducing the O content and S content can suppress the formation of inclusions, but if the ESSP value exceeds 15.0, there is a concern that the manufacturing cost for reducing the O content and S content will increase significantly. Therefore, the ESSP value is preferably 15.0 or less. The ESSP value is more preferably 10.0 or less or 9.0 or less, and even more preferably 8.0 or less.

[0095] When the ESSP value is within the range of 1.0 or more and 15.0 or less, the effective Ca content is adjusted to be equal to or more than the minimum amount necessary for controlling the morphology of MnS and equal to or less than the critical amount at which cluster-like inclusions are not formed, thereby obtaining even better HIC resistance.

[0096] (Metal structure and hardness of surface layer portion) Next, the metal structure and hardness of the surface layer portion (region from the surface to a depth of 1.0 mm) of the steel sheet according to this embodiment will be described in detail. Both the metal structure and hardness are measured on the L cross section of the steel sheet, i.e., on a plane parallel to the rolling direction RD of the steel sheet and perpendicular to the sheet surface.

[0097] In this embodiment, the total area ratio of polygonal ferrite and granular bainite is 70 to 100% in the metal structure at position 12, which is 0.1 mm deep from the surface 11 of the steel sheet 1, as schematically shown in Figure 7. Furthermore, the area ratio of polygonal ferrite is 70% or more in the metal structure at position 12, which is 0.1 mm deep from the surface 11 of the steel sheet 1. Furthermore, the average grain size of the metal structure at position 12, which is 0.1 mm deep from the surface 11 of the steel sheet 1, is 30.0 µm or less.

[0098] 7, the total area ratio of polygonal ferrite and granular bainite is 75 to 95% in the metal structure at position 13, which is 0.5 mm deep from the surface 11 of the steel plate 1. Furthermore, the area ratio of polygonal ferrite in the metal structure at position 13, which is 0.5 mm deep from the surface 11 of the steel plate 1, is 50% or more.

[0099] In addition, when the thickness of the steel plate is t, the total area ratio of polygonal ferrite and granular bainite is 50 to 97% in the metallographic structure at position 14 at a depth of 2t / 5 from the surface 11 of the steel plate 1, as schematically shown in Figure 7. The average grain size of the metallographic structure at position 14 at a depth of 2t / 5 from the surface 11 of the steel plate 1 is 20.0 µm or less.

[0100] More preferably, the area ratio of polygonal ferrite in the metal structure at position 14 at a depth of 2t / 5 from the surface 11 of the steel plate 1 may be set to 50% or more.

[0101] In the steel sheet according to this embodiment, in order to suppress the maximum hardness (Hvmax) of the surface layer to 220 Hv or less and ensure the required strength and excellent sour resistance, the surface layer must contain one or two types selected from the group consisting of polygonal ferrite and granular bainite at 70 to 100% at a depth of 0.1 mm from the surface, 75 to 95% at a depth of 0.5 mm from the surface, and 50 to 97% at a depth of 2t / 5 from the surface. The starting point of SSC is often at a depth of 0.1 mm from the surface. Therefore, the area ratio and average grain size of polygonal ferrite at a depth of 0.1 mm from the surface are particularly important for improving SSC resistance.

[0102] If the total area ratio of polygonal ferrite and granular bainite in the metal structure at depths of 0.1 mm, 0.5 mm, and 2t / 5 from the surface of the steel plate is less than the lower limit, the maximum hardness of the surface layer cannot be consistently achieved at 220 Hv or less, and sufficient strength and sour resistance cannot be obtained. Also, even if the total area ratio of polygonal ferrite and granular bainite in the metal structure at depths of 0.1 mm, 0.5 mm, and 2t / 5 from the surface of the steel plate is higher than the upper limit, there is no problem with the properties of the surface layer, but it may be difficult to ensure the strength of the base material and HIC resistance.

[0103] The total area ratio of polygonal ferrite and granular bainite at a position 0.1 mm deep from the surface of the steel plate may be 72% or more, 75% or more, or 80% or more. The total area ratio of polygonal ferrite and granular bainite at a position 0.1 mm deep from the surface of the steel plate may be 98% or less, 95% or less, or 90% or less.

[0104] The total area ratio of polygonal ferrite and granular bainite at a position 0.5 mm deep from the surface of the steel plate may be 78% or more, 80% or more, or 82% or more. The total area ratio of polygonal ferrite and granular bainite at a position 0.5 mm deep from the surface of the steel plate may be 92% or less, 90% or less, or 88% or less.

[0105] The total area ratio of polygonal ferrite and granular bainite at a position at a depth of 2t / 5 mm from the surface of the steel plate may be 60% or more, 70% or more, or 75% or more. The total area ratio of polygonal ferrite and granular bainite at a position at a depth of 2t / 5 mm from the surface of the steel plate may be 90% or less, 85% or less, or 80% or less.

[0106] Furthermore, the metal structure at a depth of 0.1 mm from the surface must contain polygonal ferrite. The area ratio of polygonal ferrite in the metal structure at a depth of 0.1 mm from the surface is set to 70% or more. This stably reduces the hardness of the surface layer of the steel sheet. The area ratio of polygonal ferrite in the metal structure at a depth of 0.1 mm from the surface may be 75% or more, 80% or more, or 85% or more. The area ratio of polygonal ferrite in the metal structure at a depth of 0.1 mm from the surface may be 100% or less, 98% or less, 95% or less, or 90% or less. On the other hand, granular bainite is not essential in the metal structure at a depth of 0.1 mm from the surface. Therefore, the area ratio of granular bainite in the metal structure at a depth of 0.1 mm from the surface may be 0%.

[0107] The metal structure at a depth of 0.5 mm from the surface must also contain polygonal ferrite. The area ratio of polygonal ferrite in the metal structure at a depth of 0.5 mm from the surface is set to 50% or more. This results in a maximum hardness Hvmax in the surface layer ranging from the surface to a depth of 1.0 mm of the steel plate being 220 Hv or less. The area ratio of polygonal ferrite in the metal structure at a depth of 0.5 mm from the surface may be 60% or more, 70% or more, or 75% or more. The area ratio of polygonal ferrite in the metal structure at a depth of 0.5 mm from the surface may be 92% or less, 90% or less, or 88% or less. Note that, when the area ratio of polygonal ferrite in the metal structure at a depth of 0.5 mm from the surface is 75% or more, granular bainite is not essential, and its area ratio may be 0%. Note that the maximum hardness Hvmax in the surface layer will be described again below.

[0108] The steel sheet according to this embodiment more preferably satisfies the following formula (5): PF ０．１ / PF ０．５ ≧1.1 (5) Here, PF in the above formula (5) ０．１ is the area ratio of polygonal ferrite in the metal structure at a depth of 0.1 mm from the surface, and PF ０．５ is the area ratio of polygonal ferrite in the metal structure at a position 0.5 mm deep from the surface. In a steel plate that satisfies formula (5), the area ratio of polygonal ferrite at a position 0.1 mm deep from the surface is higher than that at a position 0.5 mm deep from the surface. In such a steel plate, SSC resistance and HIC resistance are even more excellent.

[0109] The metal structure may contain one or more of tempered bainite, pseudo-pearlite, tempered martensite, and austenite-martensite mixtures as the remainder of the structure other than polygonal ferrite and granular bainite. If these structures are present in an amount of more than 2% at a depth of 0.1 mm from the surface, more than 5% at a depth of 0.5 mm from the surface, or more than 3% at a depth of 2t / 5 from the surface, localized areas of high hardness will be included in the structure, resulting in a decrease in sour resistance. Therefore, the total area ratio of tempered bainite, pseudo-pearlite, tempered martensite, and austenite-martensite mixtures is set to 2% or less at a depth of 0.1 mm from the surface, 5% or less at a depth of 0.5 mm from the surface, and 3% or less at a depth of 2t / 5 from the surface.

[0110] As shown in the cross-sectional schematic diagram of FIG. 7 , the area ratio of each metal structure is measured on the L-section of the steel sheet. First, a position 12 at a depth of 0.1 mm from the surface 11, a position 13 at a depth of 0.5 mm from the surface 11, and a position 14 at a depth of 2t / 5 from the surface 11 are identified. Next, an observation field 121, an observation field 131, and an observation field 141 are set. The observation field 121 is set so that its center is at the position 12 at a depth of 0.1 mm from the surface. The observation field 131 is set so that its center is at the position 13 at a depth of 0.1 mm from the surface. The observation field 141 is set so that its center is at the position 14 at a depth of 0.1 mm from the surface. Each of the observation fields 121, 131, and 141 has, for example, a dimension of 120 μm along the surface of the steel sheet and a dimension of 80 μm along the thickness direction of the steel sheet, and an area of ​​9600 μm. ２ The area of ​​the observation field is 9600 μm ２ That is all.

[0111] The metallographic structures in these observation fields 121, 131, and 141 are observed at a magnification of 1000 times using a scanning electron microscope. The metallographic structures are revealed by nital etching. The area ratio of the metallographic structures in the observation fields is obtained by the structure observation.

[0112] Furthermore, the average grain size measured by EBSD in the metallographic structure at a depth of 0.1 mm from the surface of the steel plate must be 30.0 μm or less, and the average grain size measured by EBSD in the metallographic structure at a depth of 2t / 5 from the surface must be 20.0 μm or less. The reason for this is that in order to obtain the above-mentioned structure fraction at each thickness position, it is necessary to refine and deform the γ grain size before transformation by recrystallization rolling and non-recrystallization rolling, thereby reducing hardenability. Since sufficient recrystallization rolling and non-recrystallization rolling will also result in the refinement of the structure after transformation, it was decided to specify an upper limit for the average grain size at each thickness position. Furthermore, by specifying an upper limit for the average grain size in this way, it is possible to ensure a certain level of low-temperature toughness of the base metal, as evaluated by Charpy tests, DWTT tests, etc., depending on the application. Since there are differences in the original cast structure and the effect of rolling between the surface and the interior of the plate, the average grain size measured by EBSD at a depth of 0.1 mm from the surface is set to 30.0 μm or less, and the average grain size measured by EBSD at a depth of 2t / 5 from the surface is set to 20.0 μm or less.

[0113] The average grain size of the metal structure at a depth of 0.1 mm from the surface may be 28.0 μm or less, 25.0 μm or less, or 20.0 μm or less. The average grain size of the metal structure at a depth of 0.1 mm from the surface may be 5.0 μm or more, 8.0 μm or more, or 10.0 μm or more.

[0114] The average grain size of the metal structure at a depth of 2t / 5 from the surface may be 19.0 μm or less, 18.0 μm or less, or 17.0 μm or less. The average grain size of the metal structure at a depth of 2t / 5 from the surface may be 5.0 μm or more, 8.0 μm or more, or 10.0 μm or more.

[0115] Average grain size d of the metal structure at a depth of 0.1 mm from the surface ａｖｅ is the area-weighted average value of crystal grains at a depth of 0.1 mm from the surface, measured by crystal orientation analysis using EBSD. The average grain size d of the metal structure at a depth of 2t / 5 from the surface ａｖｅ is the area-weighted average value of the grains at a depth of 2t / 5 from the surface, measured by crystal envelopment analysis using EBSD. ａｖｅThe calculation formula for is shown below. In the EBSD measurement, a "crystal grain" is defined as a region surrounded by a 15° tilt grain boundary. The pixel shape is a hexagon. a ｋ is the area of ​​the kth grain in the observation field. ｋ is the number of pixels in the kth grain. S is the step size. The step size is the measurement interval when the field of view is analyzed by EBSD, and corresponds to the pixel interval described above. ｋ is the observation field ｋ where N is the number of crystal grains included in the observation field, and A is the area of ​​the observation field.

[0116] When displaying a particle size chart using the JEOL EBSD analysis software (OIM analysis), the area fraction is automatically calculated and displayed based on the above-mentioned d ａｖｅ is equivalent to

[0117] The secondary electron emission from ferrite is low. Therefore, ferrite generally appears dark gray in secondary electron images. On the other hand, the secondary electron emission from grain boundaries and cementite is high. Therefore, grain boundaries and cementite generally appear white in secondary electron images. In this embodiment, polygonal ferrite is a massive structure observed with a dark gray contrast. The grains do not contain cementite or austenite-martensite mixtures, which are observed with a white contrast, and exhibit a basically uniform contrast. However, even in polygonal ferrite, one to three dot-like cementite particles may be present within the grains. Furthermore, prior austenite grain boundaries are not visible. Polygonal ferrite is surrounded by ferrite grain boundaries, which appear as smooth curves with a clear white contrast. However, depending on the orientation difference between crystal grains, the ferrite grain boundaries may become unclear and may become discontinuous.

[0118] Bainite has a distinct lath-like structure in which multiple laths with a width of 0.5 to 3.0 μm are arranged almost parallel to each other within the grains, and fine carbides and austenite-martensite mixtures are present within and between the laths. Furthermore, prior austenite grain boundaries are clearly defined. However, in this embodiment, bainite does not ultimately exist, and tempered bainite, which has been tempered by subsequent reheating, exists.

[0119] In this embodiment, tempered bainite is a massive, acicular, or amorphous (mainly massive with curved grain boundaries) structure in which lath boundaries are observed with clear white contrast within ferrite matrix grains, and one or more types of cementite and austenite-martensite composites are observed with white contrast along the lath direction. The lath structure of tempered bainite has a lath width of 1.0 μm or more, with multiple lath structures arranged in parallel, although some have less uniform lath spacing or orientation. Furthermore, the cementite and austenite-martensite composites observed in tempered bainite can be individually distinguishable even at 1000x magnification, while others may be observed as a cloud-like contrast of varying shades due to the accumulation of fine particles. While prior austenite grain boundaries are clear, ferrite grain boundaries are less clear than polygonal ferrite, resulting in unclear contrast or a saw-edge-like irregularity.

[0120] Granular bainite is formed at a transformation temperature intermediate between polygonal ferrite and bainite and has structural characteristics intermediate between polygonal ferrite and bainite. Specifically, it is a massive, acicular, or amorphous (mainly massive with curved grain boundaries) structure in which lath boundaries, cementite, and / or austenite-martensite composites, each of which are observed as white granular contrast, are observed within ferrite matrix grains, which are observed as dark gray contrast. The lath structure of granular bainite has a lath width of 1.0 μm or more, with one or more lath structures aligned in roughly the same direction. The lath spacing and orientation tend to be more irregular than in the tempered bainite structure. Furthermore, the cementite and austenite-martensite composites are larger in size and less dispersed than in the tempered bainite structure, making it possible to distinguish individual cementite and austenite-martensite composites even at a magnification of 1000x. 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, the acicular ferrite is also included in the fraction of the granular bainite structure.

[0121] Tempered martensite is a structure mainly composed of fine laths with a clear white contrast. In this embodiment, the lath width is less than 1.0 μm, and a large amount of cementite observed with a white contrast is dispersed within the laths and at the lath boundaries. Because the cementite observed in tempered martensite is fine and has a high precipitation density, it is difficult to distinguish each one at an observation magnification of 1000 times, and it is observed as countless white granular contrasts or cloud-like contrasts of fine particles.

[0122] The pseudo-pearlite structure is a form of structure in which C, which was released during ferrite formation, is concentrated, and is a structure in which cementite, which is observed with a white contrast, is dispersed at a high density in a ferrite matrix. At a magnification of 1000x, the cementite observed in the pseudo-pearlite is in the form of fine grains or short strings, and in some cases these are observed to be densely packed together as white lumps.

[0123] The austenite-martensite composite structure is a form of structure in which carbon, which was released during ferrite formation, is concentrated, and is observed as a massive structure with a white to light gray contrast. At a magnification of 1000x, the austenite-martensite composite structure is generally observed as a uniform structure with little contrast within it. However, in some cases, the contrast of the martensite lath structure within the austenite-martensite composite is observed, and in other cases, granular or string-like white contrast of cementite is partially observed.

[0124] In determining the metallographic 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 within polygonal ferrite and granular bainite have little effect on the area fraction and may be included in any classification.

[0125] Fig. 4A shows the metallographic structure (imaged with a scanning electron microscope) at a depth of 0.1 mm from the surface of the steel plate according to this embodiment. Fig. 4B shows the results of the determination. Fig. 4B is a colored version of the photograph of Fig. 4A. Therefore, Fig. 4A and Fig. 4B are photographs of the same field of view. In Fig. 4B, polygonal ferrite, granular bainite, and other hard structures are classified on the image. In Fig. 4B, the dark gray area that occupies the majority of the image is polygonal ferrite, and the light-colored areas are other hard structures. The distinction between polygonal ferrite, granular bainite, and other hard structures was performed based on the means described above.

[0126] The remaining metal structure of the above-mentioned specifications is composed of bainite, tempered bainite, pseudo-pearlite, martensite, tempered martensite, and austenite-martensite mixture.

[0127] (Maximum hardness Hvmax of surface layer: 220 Hv or less) SSC occurs due to microdefects and microcracks on the surface of the steel sheet. Therefore, in steel sheets and steel pipes manufactured from steel sheets, the metal structure and hardness of the surface layer, which is the source of microdefects and microcracks, are important. In particular, it is important to control the maximum hardness Hvmax not only at a position 1.0 mm deep from the surface but also over the entire range from the surface to a depth of 1.0 mm (i.e., the surface layer).

[0128] In a steel pipe using the steel plate according to this embodiment as a base material, in order to ensure HIC resistance and excellent SSC resistance, the metal structure of the surface layer is controlled as described above, and the maximum hardness of the surface layer (more specifically, the maximum hardness of the surface layer of the steel plate as a raw material before pipe making) Hvmax is set to 220 Hv or less, taking into consideration the increase in hardness due to pipe making strain and strain aging. The maximum hardness of the surface layer of the steel plate is preferably 200 Hv or less.

[0129] On the other hand, the lower limit of the maximum hardness Hvmax of the surface layer is not particularly specified, but from the viewpoint of ensuring the strength of the steel material, it is preferably 150 Hv or more, 160 Hv or more, or 170 Hv or more.

[0130] The maximum hardness of the surface layer of a steel plate or steel pipe from the surface to a depth of 1.0 mm is measured as follows.

[0131] First, as shown schematically in Fig. 8, 300 mm square (300 mm x 300 mm) steel plates are cut out by gas cutting from the widthwise end 161 of the steel plate 1 at positions W / 4 162, W / 2 163, and 3W / 4 164. W is the width of the steel plate. Alternatively, as shown schematically in Fig. 9, when the welded portion 21 of the steel pipe 2 is set to 0 o'clock, 300 mm square (300 mm x 300 mm) steel plates having curved surfaces are cut out using a saw from positions 221, 222, and 223, respectively, at 3 o'clock, 6 o'clock, and 9 o'clock.

[0132] A full-thickness block test piece 3 measuring 20 mm in length and 20 mm in width is mechanically cut from the center of a sample cut from the steel plate 1 or steel pipe 2. The thickness of the full-thickness block test piece 3 is equal to the thickness of the steel plate 1 or steel pipe 2. The cross section of the full-thickness block test piece 3 is then mechanically polished. The mechanical polishing is finished by alumina polishing. The cross section is formed perpendicular to the rolling direction of the steel plate 1 or steel pipe 2. As shown schematically in Figure 10, a Vickers hardness tester (load: 100 g) is used to measure the cross section of one block test piece 3 at 10 points at 0.1 mm intervals in the thickness direction, starting from 0.1 mm from the surface, and 10 points at 1.0 mm intervals in the width direction at the same depth, for a total of 100 points. In other words, a total of 300 points are measured for three block test pieces 3. From the obtained results, the maximum hardness of the metal structure of the surface layer 31 can be determined.

[0133] The symbol X in Fig. 9 indicates a measurement point. The scales in the vertical and horizontal directions of Fig. 9 are different for the sake of convenience. In actual measurements, the thickness of the surface layer 31 and the distance between the measurement points X in the horizontal direction on the paper are the same.

[0134] In the steel sheet according to this embodiment, the maximum hardness at the t / 2 part (t: sheet thickness), which is the position from the surface to half the sheet thickness, is not particularly specified. However, it is preferable that the maximum hardness measured at 10 or more points in the longitudinal direction at 1.0 mm intervals near the t / 2 part is 300 Hv or less, since this can further improve HIC resistance.

[0135] The steel sheet according to this embodiment does not suffer from sulfide stress cracking even when immersed in the solution described below and subjected to the stress described below. Solution temperature range: 24±3°C Solution components: NACE TM0177 Solution A (5 wt% NaCl and 0.5 wt% CH ３ H containing COOH ２S-saturated solution) Stress: 90% of the actual yield stress (AYS) of the steel sheet. In other words, in the steel sheet according to this embodiment, the stress that causes cracks in the above solution environment is 90% or more of the allowable yield strength. Sulfide stress cracking is confirmed by observing the steel sheet with an optical microscope at a magnification of 50x. A steel sheet in which no cracks are found as a result of observation under these conditions is determined to be a steel sheet in which sulfide stress cracking has not occurred.

[0136] Next, the mechanical properties of the steel plate according to this embodiment will be described. Taking into consideration its application to line pipes, the steel plate according to this embodiment preferably has a strength that satisfies X52 to X70 specified in API 5L.

[0137] (Yield ratio: 70% or more) The yield ratio of the steel plate according to this 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. Preferably, the yield ratio is 72% or more, 75% or more, or 78% or more. There is no particular upper limit for the yield ratio, and the higher the yield ratio, the better. The yield ratio may be 95% or less, 92% or less, or 90% or less.

[0138] (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. The yield strength may be 320 MPa or more, 350 MPa or more, or 380 MPa or more. The yield strength may be 580 MPa or less, 550 MPa or less, or 520 MPa or less.

[0139] (Tensile strength: 400 to 700 MPa) The tensile strength is preferably in the range of 400 to 700 MPa. The tensile strength of the steel plate may be 450 MPa or more, 480 MPa or more, or 500 MPa or more. The tensile strength of the steel plate may be 680 MPa or less, 650 MPa or less, or 600 MPa or less.

[0140] Here, the yield strength, tensile strength, and yield ratio are obtained by taking a tensile test specimen perpendicular to the longitudinal direction of the steel plate (direction C) and conducting a tensile test. When the test specimen is taken from a steel pipe, the steel plate is flattened before the test specimen is taken.

[0141] (Thickness t: 10 to 40 mm) When the steel plate according to this embodiment is used as a steel pipe for drilling or transporting oil, natural gas, etc., the thickness of the steel plate is preferably within a range of 10 to 40 mm. The thickness of the steel plate may be 12 mm or more, 15 mm or more, or 18 mm or more. The thickness of the steel plate may be 38 mm or less, 35 mm or less, or 30 mm or less.

[0142] <Regarding the manufacturing method of the steel sheet> Next, a preferred manufacturing method of the steel sheet according to this embodiment will be described. The steel sheet according to this embodiment can obtain the effects as long as it has the above-mentioned configuration regardless of the manufacturing method. However, for example, the following manufacturing method is preferable because it allows the steel sheet according to this embodiment to be stably obtained.

[0143] As shown in FIG. 11, the steel plate according to this embodiment includes a process of hot-rolling a steel slab having a predetermined chemical composition by heating it to a temperature in the range of 1050 to 1210°C, a first cooling process, and a second cooling process. The hot-rolling process includes a first rolling stage in which the steel slab is rolled at a temperature in the range of 930°C or higher, and a second cooling stage in which the steel slab is rolled with Ar. ３ and a second rolling stage in which the steel slab is rolled in a temperature range of from 1130°C to 900°C, wherein the first rolling stage has a total reduction of 30% or more between 1130°C and 930°C, and includes three or more passes with a reduction rate of 10% or more per pass; the second rolling stage has a total reduction of 30% or more at 900°C or less, and includes five or more passes with a reduction rate of 10% or more per pass; the first cooling step is performed at an average cooling rate of 100°C / s or less to a depth of 1.0 mm from the surface of the steel plate during a period from the start of cooling of the steel plate to a water cooling stop temperature of 400°C or less, and is cooling such that the maximum temperature of the surface of the steel plate 15 seconds after water cooling is stopped is 350°C or more and less than 650°C; and the second cooling step is performed after the first cooling step, when the surface temperature of the steel plate has reached the maximum temperature, at an average cooling rate of 5°C / s or less to 200°C or less.

[0144] Preferred conditions for each of the above steps will be explained in detail below.

[0145] (Steelmaking Process) In order to prepare molten steel having the same chemical composition as the steel plate according to this embodiment (i.e., a chemical composition in which each component falls within the above-mentioned range and satisfies the above formulas (1) to (3) (preferably formulas (1) to (4))), various smelting processes and alloy additions are carried out under atmospheric or reduced pressure conditions, and a steel slab having a thickness of 200 to 650 mm is produced by continuous casting. In this embodiment, the N content and the Ti content are important control factors, so it is important to control the N content and the Ti content within the target ranges by the smelting process and alloy addition. The N content may be controlled by adjusting the content of the molten iron, or may be intentionally added from the outside.

[0146] (Hot Rolling Step) A slab produced by casting molten steel having the same chemical composition as the base material of the steel plate according to this embodiment is heated to a temperature range of 1050 to 1210°C and subjected to hot rolling. The hot rolling is performed in two stages: a first rolling stage in which the slab temperature is 930°C or higher, taking into consideration the toughness of the base material; and a second rolling stage in which the slab temperature is 900°C or lower and the slab temperature is 1000°C or lower. ３ The method has a second rolling stage carried out within the above range. The total rolling reduction in each of the first rolling stage and the second rolling stage is ensured to be 30% or more. ３ (°C) are as follows:

[0147] Ar ３ (℃)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35(t-8)...(A)

[0148] In the above formula (A), C, Mn, Cu, Cr, Ni, and Mo are the contents (mass%) of each element in the steel, t is the thickness (mm) of the steel plate after hot rolling, and the slab temperature means the surface temperature of the slab.

[0149] If the heating temperature of the steel slab before rolling is less than 1050°C, the solid solution of Nb-containing carbides is insufficient, and 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 of the steel slab exceeds 1210°C, the crystal grains of the steel slab become coarse, and toughness decreases. Therefore, the heating temperature of the steel slab is set to 1210°C or lower. The heating temperature of the steel slab is preferably 1190°C or lower.

[0150] In addition, the upper limit of the total reduction rate in the first rolling stage, which is carried out at a steel slab temperature of 930°C or higher, and the upper limit of the total reduction rate in the first rolling stage, which is carried out at a steel slab temperature of 900°C or lower and Ar ３ The upper limit of the total reduction in the second rolling stage carried out within the above range is not particularly specified. The total reduction in each of the first rolling stage and the second rolling stage is usually 95% or less.

[0151] In the first rolling stage, which is rough rolling after heating, from the viewpoint of grain refinement, it is necessary to set the total reduction rate to 30% or more between 1130 and 930° C. In the rough rolling after heating, the total reduction rate is preferably 40% or more between 1130 and 930° C. Furthermore, in the rough rolling after heating, preferably, three or more rolling passes are ensured, with a reduction rate of 10% or more per pass.

[0152] In the second rolling stage, which is the finishing rolling after the rough rolling, Ar ３ At this time, when rolling is performed at 930°C or higher, the rolling does not reach the non-recrystallization temperature range even if Nb is added, so the grain refinement effect of the base material is insufficient, and it is difficult to ensure the toughness of the base material. ３ In the rolling below the rolling point, polygonal ferrite is generated in the surface layer of the sheet before the start of controlled cooling, and a hard structure with concentrated C is formed with the generation of polygonal ferrite, which may cause stress corrosion cracking. ３ The rolling temperature must be above 900°C and the total reduction must be 30% or more. ３ The rolling temperature is from 100°C to 900°C, the total rolling reduction is 70% or more, and five or more rolling passes are performed with a rolling reduction of 10% or more per pass.

[0153] (First Cooling Step) In the first cooling step, the steel sheet after the hot rolling step is water-cooled. The surface temperature of the steel sheet at the start of water-cooling, i.e., the water-cooling start temperature, is set to Ar. ３ The surface temperature of the steel plate when water cooling is stopped, i.e., the water cooling stop temperature, is set to 400°C or lower. Furthermore, the maximum temperature reached on the surface of the steel plate by reheating after water cooling is stopped is set to exceed 350°C. In addition, the average cooling rate at a depth of 1.0 mm from the surface of the steel plate from the start of water cooling to the stop of water cooling is set to 100°C / s or lower. That is, the value CR1 obtained by the following formula is set to 100°C / s or lower: CR1 = (T1 - T2) ÷ time T1 is the temperature at a depth of 1.0 mm from the surface of the steel plate when water cooling is started. T2 is the temperature at a depth of 1.0 mm from the surface of the steel plate when water cooling is stopped. Time is the water cooling time. The temperature at a depth of 1.0 mm from the surface of the steel plate is calculated by simulation. Hereinafter, the position 1.0 mm below the surface of the steel plate may be simply referred to as the interior of the steel plate.

[0154] Here, if the water cooling stop temperature exceeds 400°C, cooling inside the plate thickness will be insufficient, resulting in insufficient strength, which is not preferable. The water cooling stop temperature is preferably 350°C or less. Note that, since it is difficult to actually measure the water cooling stop temperature, the water cooling stop temperature may be managed using an estimated value obtained through a cooling simulation.

[0155] More specifically, after hot rolling, accelerated water cooling is initiated on the steel sheet to ensure the strength of the steel material. Since the cooling is required to reduce the surface hardness as described above, it is preferable to perform slow cooling at an average cooling rate of 100°C / s or less at a depth of 1.0 mm from the sheet surface between the start of cooling and the water cooling stop temperature of 400°C or less. More preferably, the average cooling rate inside the steel sheet at a depth of 1.0 mm from the sheet surface is slow cooling at 80°C / s or less. The cooling stop temperature is the temperature at which cooling rate control ends. Specifically, the cooling stop temperature is the surface temperature of the steel sheet at the time when the spraying of the refrigerant onto the steel sheet is completed. For example, in a manufacturing site where the steel sheet is cooled by passing through a cooling zone equipped with water-cooled nozzles, the surface temperature of the steel sheet at the time when the steel sheet leaves the cooling zone is the cooling stop temperature.

[0156] Furthermore, in order to optimize the mechanical properties by the tempering effect of the base material, it is necessary to set the maximum temperature reached on the steel sheet surface by reheating 15 seconds after the end of cooling to 350°C or higher and lower than 650°C. Such a maximum temperature can be more reliably achieved by setting the average cooling rate inside the steel sheet in the temperature range of 500°C or higher from the start of cooling to the above conditions. The maximum temperature reached on the steel sheet surface is measured using a radiation thermometer.

[0157] FIG. 1 is a diagram schematically showing one embodiment of accelerated cooling in which water cooling is performed after the end of hot rolling so that the maximum temperature of the steel sheet surface reaches 500°C to 600°C due to reheating 15 seconds after the end of water cooling. The vertical axis of the graph in FIG. 1 represents the surface temperature of the steel sheet (°C). The horizontal axis of the graph in FIG. 1 represents the elapsed time (s) from the start of water cooling. In FIG. 1, water cooling was performed for approximately 20 seconds.

[0158] During water cooling, the temperature of the surface of the steel plate drops quickly, while the temperature inside the steel plate drops slowly. At the end of water cooling, the temperature inside the steel plate is higher than the temperature on the surface of the steel plate. After water cooling is completed, heat transfers from the inside of the steel plate to the surface of the steel plate, causing the surface temperature of the steel plate to rise. This phenomenon is called recuperation.

[0159] If the maximum temperature reached by reheating on the steel sheet surface is less than 350°C, the hardness of the steel sheet, particularly the maximum hardness of the surface layer portion from the surface to a depth of 1.0 mm, cannot be stably made 220 Hv or less.

[0160] Furthermore, if the maximum temperature reached on the steel sheet surface by reheating is 650°C or higher, transformation will stagnate inside the steel sheet, making it impossible to satisfy the strength and toughness of the base material. Therefore, the maximum temperature reached by reheating is set to less than 650°C.

[0161] (Second Cooling Step) After the first cooling (i.e., after completion of water cooling and reheating), the steel plate is cooled at an average cooling rate of the surface of the steel plate of 5°C / s or less until the surface temperature of the steel plate reaches 200°C or less. For example, the steel plate after the first cooling may be placed on a cooling bed until its surface temperature reaches 200°C or less. As long as the above-mentioned average cooling rate is satisfied, the steel plate may be cooled by blowing air. On the other hand, in order to slow down the cooling rate of the steel plate, the steel plate may be left in a stacked state.

[0162] If the average cooling rate to 200°C or less is greater than 5°C / s, the effect of self-tempering will be insufficient and the surface hardness will increase, which is not preferable. Furthermore, if the cooling stop temperature is higher than 200°C, the effect of self-tempering will also be insufficient, which is also not preferable. The average cooling rate is preferably 4°C / s or less, and the cooling stop temperature is preferably 100°C or less. The cooling stop temperature is the temperature at which the management of the cooling rate is terminated. Specifically, the cooling stop temperature is the temperature of the surface of the steel sheet at the time when the spraying of the refrigerant onto the steel sheet is completed. For example, in a manufacturing site where the steel sheet is cooled by placing it on a cooling bed, the surface temperature of the steel sheet at the time when it is removed from the cooling bed is the cooling stop temperature. The cooling rate from the cooling stop temperature to room temperature is arbitrary.

[0163] By undergoing the steps described above, the metal structure of the surface layer portion of the obtained steel plate will contain one or both of polygonal ferrite and granular bainite in a total area ratio of 70 to 95%, the maximum hardness Hvmax of the surface layer portion will be 220 Hv or less, and further, the yield ratio will be 70% or more.

[0164] (Tempering process) Basically, it is possible to manufacture a steel sheet having the target structure and properties through the above-mentioned process. However, there are cases where the steel sheet is subjected to tempering treatment (tempering) after completion of the second cooling process. The tempering treatment may be performed under any conditions, such as a tempering temperature in the range of 400 to 650°C after the second cooling process, with a holding time of 10 to 60 minutes at the target temperature ±10°C. Cooling after tempering may be air cooling.

[0165] Furthermore, the metal structure at the 1 / 4 position in the plate thickness direction also becomes a metal structure containing one or both of polygonal ferrite and granular bainite at a predetermined total area ratio, as explained above.

[0166] As a result, the steel plate obtained through the above steps has a yield strength of 300 MPa or more, preferably 350 MPa or more, and is excellent in SSC resistance and HIC resistance, making it suitable for use as the base material of steel pipe for line pipe.

[0167] It should be noted that the base material of the steel pipe made using the steel plate according to this embodiment is not subjected to heat treatment. The steel pipe may be painted and then heated to approximately 200°C to dry, but this heating does not change the metal structure of the steel pipe. Therefore, the metal structure of the base material of the steel pipe is the same as that of the steel plate according to this embodiment. The steel pipe made using the steel plate according to this embodiment has excellent SSC resistance in addition to HIC resistance equal to or greater than that of conventional steel, both in the base material and the welded portion.

[0168] Figures 2A, 2B, 3A, and 3B show examples of hardness distribution in the surface layer of a steel plate. Figures 2A and 2B show the hardness distribution in the depth direction of the plate thickness of an example steel plate that satisfies the above-mentioned conditions. Figures 3A and 3B show the hardness distribution in the depth direction of the plate thickness of a comparative example in which the maximum surface layer hardness is high. Figures 2A and 2B show the results of measurements at different positions. Similarly, Figures 3A and 3B show the results of measurements at different positions. In the figures, the maximum surface layer hardness is the maximum hardness Hvmax of the surface layer from the surface of the steel plate to a depth of 1.0 mm. It can be seen that in the comparative example, the maximum surface layer hardness exceeds 220 Hv.

[0169] <Regarding the Steel Pipe> A steel pipe 2 as illustrated in FIG. 9 can be obtained by processing the steel plate according to this embodiment as described above into a cylindrical shape, butting both ends of the cylindrical steel plate together, and welding the butt joint. The obtained steel pipe 2 has a base metal portion 22 made of the steel plate according to this embodiment, and a welded portion 21. Such a welded portion 21 is usually provided continuously from one end of the steel plate to the other end in the longitudinal direction. Therefore, in the steel pipe 2, the welded portion 21 extends from one end to the other end of the steel pipe 2 along the axial direction of the cylindrical base metal portion 22. The welded portion 21 is a so-called seam welded portion.

[0170] Generally, in steel pipe welding, the weld is made thicker than the base metal. Furthermore, the weld metal is a higher alloy than the base metal and has higher corrosion resistance. Therefore, the weld rarely becomes the starting point for SSC. Therefore, the weld 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.

[0171] Next, examples of the present disclosure will be described. The conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0172] Molten steel having the chemical compositions shown in Tables 1A to 2B was continuously cast to produce steel slabs with a thickness of 240 mm or 300 mm. These slabs were used to produce the steel plates shown in Tables 5A to 10 under the production conditions shown in Tables 3A to 4B. Nos. 1 to 25 are example steel plates, and Nos. 26 to 51 are comparative steel plates. Tables 1A to 2B also show the results of the calculation of the Ar content. ３ (°C) is shown.

[0173]

[0174]

[0175]

[0176]

[0177]

[0178]

[0179]

[0180]

[0181] JIS No. 5 tensile test specimens were prepared from the obtained steel plates, and a tensile test specified in JIS Z 2241:2011 was performed to measure the yield strength and tensile strength. The longitudinal direction of the tensile test specimens was aligned with the C-direction of the steel plate. The C-direction of the steel plate is the direction perpendicular to the rolling direction of the steel plate. A yield strength of less than 300 MPa was deemed to be unacceptable. In addition, the hardness of the surface layer from the surface to a depth of 1.0 mm was measured, and the metal structure of the surface layer was observed with a scanning electron microscope. HIC resistance and SSC resistance were also evaluated.

[0182] (Maximum hardness of surface layer) At any two locations on the steel plate, 300 mm square (300 mm × 300 mm) steel plates were cut out by gas cutting from positions 1 / 4, 1 / 2, and 3 / 4 from the end of the steel plate in the width direction (corresponding to the butt joint in the case of a steel pipe) (3 o'clock, 6 o'clock, and 9 o'clock positions in the case of a steel pipe, respectively, when the weld is considered to be 0 o'clock). A block test piece 20 mm long × 20 mm wide was taken from the center of the cut steel plate by mechanical cutting and polished by mechanical polishing. For each block test piece, a Vickers hardness tester (load: 100 g) was used to measure 10 points at 0.1 mm intervals in the thickness direction, starting from 0.1 mm from the surface, and 10 points 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 were measured on three block test pieces. From these results, the maximum hardness of the surface layer metal structure at two locations in the longitudinal direction of the steel sheet was calculated. The relationship between the obtained maximum hardness of the surface layer and NPIP and NPIM for the example steel sheets is graphed in Figures 5 and 6, respectively.

[0183] (Structural Observation) The metallographic structure of the surface layer was etched by immersing the polished test specimen in a mixed solution of 3% nitric acid and 97% ethanol for several seconds to several tens of seconds to reveal the metallographic structure. The metallographic structure was then observed at positions 0.1 mm, 0.2 mm, and 0.5 mm from the surface using a scanning electron microscope at 1000x magnification. The fractions (area fractions) of polygonal ferrite, granular bainite, and the remainder were calculated using structural photographs of two consecutive fields of view. The dimensions of the measurement field of view were approximately 80 μm in the thickness direction and approximately 120 μm in the surface direction. The area fractions of each structure were calculated as the average of these fields of view. An example of the obtained structural observation results is shown in FIGS. 4A and 4B .

[0184] Furthermore, the polished test specimens were similarly observed for their metal structures at a quarter-thickness position, and the fractions (area fractions) of polygonal ferrite, granular bainite, and the remainder were calculated using the obtained microstructural photographs. The area fractions of each structure were calculated as the average of these fields of view.

[0185] (Evaluation of HIC Resistance) A full-thickness test piece measuring 100 mm in length and 20 mm in width was taken from the steel plate. The longitudinal direction of the full-thickness test piece was aligned with the C-direction of the steel plate. A test was conducted in accordance with TM0284 of the National Association of Corrosion and Engineers (NACE) to observe the occurrence of HIC (hydrogen-induced cracking). The HIC area ratio was also measured. The HIC area ratio was measured using Ultrasonic Testing (UST). If the HIC area ratio (CAR, Crack Area Ratio) was 5% or less, the HIC resistance was evaluated as excellent, and if it was more than 5%, the HIC resistance was evaluated as poor.

[0186] The NACE test involves immersing a steel sheet in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, at 24±3°C, saturated with 1 atm of hydrogen sulfide gas, and observing whether cracks occur after 96 hours.

[0187] (Evaluation of SSC Resistance) A full-thickness test piece measuring 15 mm wide x 115 mm long was taken from the steel plate in the width direction, and the SSC resistance was evaluated by a four-point bending test in accordance with NACE TM0284m and ASTM (American Society for Testing and Materials) G39. During the test, scale attached to the surface of the steel plate was not removed. However, if only black scale was formed on the surface of the steel plate, the scale may be removed before the test.

[0188] In a four-point bending test, a stress equivalent to 90% of the 0.2% proof stress derived from the tensile test was applied to a test piece, which was then immersed for 720 hours in an aqueous solution of 5% sodium chloride + 0.5% acetic acid, pH 2.7, saturated with 1 atm of hydrogen sulfide gas, at room temperature (24°C), and the surface of the test piece was observed at 10x magnification to determine whether SSC had occurred.

[0189] Those in which no SSC occurred were rated as pass (No Crack), and those in which SSC occurred were rated as fail (Crack).

[0190]

[0191]

[0192]

[0193]

[0194]

[0195]

[0196]

[0197]

[0198]

[0199]

[0200] As described above, it is clear that the example steel plates (Test Nos. 1 to 25) have excellent sulfide stress cracking resistance (SSC resistance) equal to or greater than that of conventional steels.

[0201] In Comparative Examples 26 to 31, one or more of the Ti content, N content, NPIP, and NPIM were outside the ranges of the present disclosure, and the steel sheet surface layer hardness showed a high value, and the SSC resistance properties were reduced.

[0202] Comparative Examples 32 to 35 had Ceq. outside the range of the present disclosure, and either had a yield strength of less than 300 MPa or had reduced SSC resistance.

[0203] Comparative Example No. 36 had a C content outside the range of the present disclosure, and the steel sheet surface layer hardness showed a high value, resulting in reduced SSC resistance.

[0204] Comparative Examples 37 to 43 had Mn, P, S, Nb, Ca, N, and O contents outside the ranges of the present disclosure, and therefore had reduced HIC resistance.

[0205] In Comparative Examples Nos. 44 and 45, the slab heating temperature before rolling was outside the range of the present disclosure, and No. 44 exhibited reduced HIC resistance, and No. 45 exhibited reduced SSC resistance.

[0206] Comparative Examples 46 and 47 had a low number of rolling passes in rough rolling or finish rolling, so the effect of refining γ grains before transformation was small, the steel sheet surface layer hardness was high, and the SSC resistance was reduced.

[0207] In Comparative Example No. 48, the cooling rate in the first cooling step was at a high level, and the steel sheet surface layer hardness exhibited a high value, resulting in a decrease in SSC resistance.

[0208] In Comparative Examples 49 and 50, the reheating temperature after the first cooling step was outside the range of the present disclosure, and No. 49 exhibited a high value of steel sheet surface layer hardness, resulting in decreased SSC resistance and decreased HIC resistance. In No. 50, the steel structure was outside the disclosed range, and the steel sheet surface layer hardness tended to be high, resulting in decreased HIC resistance.

[0209] In Comparative Example No. 51, the cooling rate in the second cooling step was high, and the tempering effect was not sufficient, so the steel sheet surface layer hardness showed a high value and the SSC resistance properties were reduced.

[0210] Although the preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings, the present disclosure is not limited to such examples. It is clear that a person skilled in the art to which the present disclosure pertains can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present disclosure.

[0211] As described above, according to the present disclosure, it is possible to provide a steel plate suitable for steel pipes having a yield strength of 300 MPa or more, sufficient HIC resistance, and excellent SSC resistance over a wide range of the inner surface. Therefore, the present disclosure has high industrial applicability.

[0212] 1 Steel plate 11 Surface 12 Position at a depth of 0.1 mm from the surface 121 Observation field at a depth of 0.1 mm from the surface 13 Position at a depth of 0.5 mm from the surface 131 Observation field at a depth of 0.5 mm from the surface 14 Position at a depth of 2t / 5 from the surface 141 Observation field at a depth of 2t / 5 from the surface t Plate thickness 161 End 162 Position at W / 4 from the end 163 Position at W / 2 from the end 164 Position at 3W / 4 from the end W Width 2 Steel pipe 21 Welded portion 22 Base material portion 221 Position at 3 o'clock in the base material portion 222 Position at 6 o'clock in the base material portion 223 Position at 9 o'clock in the base material portion 3 Full thickness block test piece 31 Surface layer X Measurement point

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%, W: 0 to 0.50%, O: 0.0040% or less, balance: Fe and impurities, and the steel sheet is such that 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 The above, in the metallographic structure at a position 0.1 mm deep from the surface, the total area ratio of polygonal ferrite and granular bainite is 70 to 100%, and the area ratio of the polygonal ferrite is 70% or more. The average grain size of the metallographic structure at the position 0.1 mm deep from the surface is 30.0 μm or less. In the metallographic structure at a position 0.5 mm deep from the surface, the total area ratio of polygonal ferrite and granular bainite is 75 to 95%, and the area ratio of the polygonal ferrite is 50% or more. When the plate thickness of the steel plate is t, in the metallographic structure at a position 2t / 5 deep from the surface of the steel plate, the total area ratio of polygonal ferrite and granular bainite is 50 to 97%. The average grain size of the metallographic structure at the position 2t / 5 deep from the surface is 20.0 μm or less. The maximum hardness Hvmax in the surface layer portion in the range from the surface of the steel plate to a depth of 1.0 mm is 220 Hv or less. The yield ratio is 70% or more. Steel plate. 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 plate.

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

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

4. Among the chemical composition, the steel sheet according to claim 1 or 2 contains one or more of the following: 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%, REM: 0.0001 to 0.0100%.

5. Among the chemical composition, the steel sheet according to claim 3 contains one or more of the following: 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%, REM: 0.0001 to 0.0100%.

6. The steel sheet according to claim 1 or 2, wherein 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 200 Hv or less.

7. The steel sheet according to claim 1 or 2, wherein the yield strength is 300 to 600 MPa.

8. The steel sheet according to claim 1 or 2, wherein the plate thickness is in the range of 10 to 40 mm.

9. The steel sheet according to claim 1 or claim 2, characterized by satisfying the following formula (5). PF ０．１ / PF ０．５ ≧1.1... (5) Here, PF ０．１ in the above formula (5) is the area ratio of polygonal ferrite in the metal structure at a position 0.1 mm deep from the surface, and PF ０．５ is the area ratio of polygonal ferrite in the metal structure at a position 0.5 mm deep from the surface.

10. A steel pipe comprising a cylindrical base material portion made of the steel sheet according to claim 1 or 2, and a welded portion extending along the axial direction of the base material portion.

11. In terms of 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%, W: 0 to 0.50%, O: 0.0040% or less, balance: Fe and impurities, and Ceq defined by the following formula (1) is in the range of 0.200 to 0.500, and 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 The hot rolling process of heating the steel sheet in the range of 1050 to 1210°C and performing hot rolling, the first cooling process, and the second cooling process are provided. The hot rolling process includes a first rolling stage of rolling the steel sheet in the range of 930°C or higher and a second rolling stage of rolling the steel sheet in the range of above Ar ３ point to 900°C or lower. The first rolling stage has a total reduction rate of 30% or more between 1130 and 930°C and 3 or more reduction passes with a reduction rate of 10% or more per pass. The second rolling stage has a total reduction rate of 30% or more at 900°C or lower and 5 or more reduction passes with a reduction rate of 10% or more per pass. The first cooling process has the surface temperature of the steel plate at the start of water cooling at Ar ３ Set it to be above a certain point, set the surface temperature of the steel plate at the time of water cooling stop to 400 °C or lower, set the average cooling rate at a depth of 1.0 mm below the surface of the steel plate during the period from the start of water cooling to the stop of water cooling to 100 °C / s or lower, and set the cooling such that the maximum temperature reached on the surface of the steel plate 15 seconds after the stop of water cooling is 350 °C or higher and less than 650 °C. The second cooling step is after the first cooling step and after the temperature of the surface of the steel plate reaches the maximum temperature reached, the steel plate is cooled until the temperature of the surface becomes 200 °C or lower, and the average cooling rate of the surface of the steel plate from the maximum temperature reached to 200 °C is 5 °C / s or lower. A method for manufacturing a steel plate. 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 sheet.

12. The method for manufacturing a steel sheet according to claim 11, further comprising an annealing process in the range of an annealing 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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