Steel material and method for producing steel material

Abstract

The purpose of the present invention is to provide: a steel material having both high strength and excellent hydrogen embrittlement resistance in a hydrogen gas environment; and a method for producing a steel material.　The steel material has a specific component composition and a specific metal structure, wherein the relative reduction of area (RRA) represented by (reduction of area in hydrogen gas at hydrogen gas pressure of 40 MPa) / (reduction of area in atmosphere) is 0.90 or more in a tensile test with a strain rate of 7.0 × 10-4 mm-1.

Description

Steel material and steel material manufacturing method

[0001] The present invention relates to a steel material and a method for manufacturing the steel material.

[0002] In recent years, hydrogen has been attracting significant global attention as a clean energy source for building a decarbonized society. Furthermore, the construction of a hydrogen gas transportation network using pipelines is being considered for the purpose of transporting large quantities of hydrogen gas. The hydrogen gas transport pressure during pipeline operation is expected to be high, ranging from 1 to 40 MPa, and line pipes will be placed in a high-pressure hydrogen gas environment. Furthermore, when line pipes are manufactured using steel, there are concerns that the steel used in such an environment will suffer from "hydrogen embrittlement," in which hydrogen penetrates the steel and deteriorates its properties.

[0003] Conventional steel materials used in natural gas pipelines have been required to suppress the occurrence of hydrogen-induced cracking in sour environments. To this end, measures have been taken to suppress the formation of elongated MnS, suppress the accumulation of Ti and Nb carbonitrides and oxides, and suppress center segregation. Furthermore, Mn is an element that tends to segregate in the center of steel materials, and methods for suppressing Mn segregation have also been proposed.

[0004] Patent Document 1 proposes a steel sheet in which the ratio of the Mn content in segregated portions to the average Mn content in the steel is suppressed.

[0005] Furthermore, Patent Documents 2 and 3 propose high-strength line pipes in which the P concentration in the segregated portion is limited in addition to the size of the Mn segregation spot, and further Ca is utilized.

[0006] Furthermore, Patent Documents 4 and 5 propose methods for suppressing inclusions such as carbides and nitrides of Ti and Nb. Much research has been conducted to apply low-alloy steels, which have lower material costs, instead of austenitic stainless steels as materials for high-pressure hydrogen storage containers.

[0007] Patent Document 6 proposes a steel for high-pressure hydrogen environments that utilizes MnS, Ca-based inclusions, or VC as hydrogen trap sites in the steel to make the hydrogen non-diffusible, thereby suppressing embrittlement due to diffusible hydrogen.

[0008] JP 6-220577 JP 6-256894 JP 6-271974 JP 2006-63351 JP 2008-7841 JP 2005-2386

[0009] However, steel materials used in line pipes for transporting hydrogen gas must have not only the high strength required for conventional line pipes but also the hydrogen embrittlement resistance required in hydrogen gas environments. However, the technologies of Patent Documents 1 to 5 have room for improvement in terms of hydrogen embrittlement resistance in hydrogen gas environments. Furthermore, it has been found that the steel for high-pressure hydrogen environments proposed in Patent Document 6 contains large amounts of MnS and Ca-based inclusions, and therefore, although it can reduce diffusible hydrogen, there is still room for improvement in terms of hydrogen embrittlement resistance in hydrogen gas environments.

[0010] The present invention has been made in view of the above circumstances, and has as its object to provide a steel material that combines high strength with excellent resistance to hydrogen embrittlement in a hydrogen gas environment, and a method for manufacturing the steel material.

[0011] The present inventors have conducted extensive research into the conditions necessary to obtain a steel material that combines high strength with excellent resistance to hydrogen embrittlement in a hydrogen gas environment, and have invented a new steel material. The gist of the present invention is as follows. [1] In mass%, C: 0.02 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.0035% or less, N: 0.010% or less, Nb: 0.001 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0%, Ti: 0 to 0.1%, Cu: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0 to 0.10%, Zr: 0 to 0.050%, Ta: 0 to 0.050%, The steel sheet has a chemical composition containing B: 0 to 0.0020%, REM: 0 to 0.01%, Mg: 0 to 0.01%, Hf: 0 to 0.005%, Re: 0 to 0.005%, Sn: 0 to 0.3%, and Sb: 0 to 0.3%, with the balance being Fe and unavoidable impurities, and has a metal structure in which, at a quarter-wall thickness position, bainite accounts for 70% or more in area fraction, and the average grain size of the bainite is 15 μm or less, and a strain rate of 7.0×10 －４ mm －１In a tensile test, the steel has a relative reduction of area (RRA) of 0.90 or more, expressed as (reduction of area in hydrogen gas at a hydrogen gas pressure of 40 MPa) / (reduction of area in air). [2] The composition of the alloy is, in mass%, Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ni: 0.01 to 2.0%, Ti: 0.005 to 0.1%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, The steel material according to [1], containing one or more selected from Hf: 0.0001 to 0.005%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, and Sb: 0.0001 to 0.3%. [3] The steel material according to [1] or [2], wherein the steel material is a welded steel pipe. [4] A heating step of heating a slab having the chemical composition according to [1] or [2] to a heating temperature of 1000°C or more and 1250°C or less, and subjecting the slab heated in the heating step to a hot rolling start temperature of 1000°C or more and ... ３ a hot rolling step of hot rolling the hot rolled steel sheet at a temperature of 50°C or higher than the rolling point to obtain a hot rolled steel sheet; and a cooling start temperature of the hot rolled steel sheet obtained in the hot rolling step, wherein the cooling start temperature is Ar at the surface temperature of the hot rolled steel sheet. ３ point or higher and at the temperature of 1 / 4 of the thickness of the hot-rolled steel plate ３ a cooling step of cooling to obtain a steel material under the following conditions: a surface temperature of the hot-rolled steel plate is 750°C +20°C or more, an average cooling rate of 30°C / s or more from 750°C to 550°C at the temperature at 1 / 4 of the wall thickness position of the hot-rolled steel plate is 20°C / s or more from 750°C to 550°C at the temperature at 1 / 4 of the wall thickness position of the hot-rolled steel plate, and a cooling stop temperature of 550°C or more but 250°C or more at the temperature at 1 / 4 of the wall thickness position of the hot-rolled steel plate. [5] A method for producing a steel material according to [4], further comprising, after the cooling step, a pipe-making step of forming the steel material obtained after the cooling step into a cylindrical shape and butt-welding both circumferential ends of the cylindrical steel material to obtain a welded steel pipe.

[0012] According to the present invention, a steel material having both high strength and excellent resistance to hydrogen embrittlement in a hydrogen gas environment can be obtained.

[0013] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.

[0014] [Component Composition] The steel material of the present invention has the above-mentioned component composition. The reasons for limiting the component composition of the steel material in the present invention will be described below. In the following description, all units shown in % are mass % unless otherwise specified.

[0015] C: 0.02 to 0.50% C is an element that effectively contributes to improving the strength of steel. However, a C content of less than 0.02% does not ensure sufficient strength. Therefore, the C content is set to 0.02% or more. The C content is preferably set to 0.03% or more, more preferably set to 0.04% or more, and even more preferably set to 0.05% or more. On the other hand, if the C content exceeds 0.50%, the hardness of the surface layer and central segregation of the steel increases during cooling. This increases the amount of hydrogen in the steel in a hydrogen gas environment, and hydrogen embrittlement resistance deteriorates. This also significantly reduces weldability and leads to a decrease in the toughness of the base metal and weld heat-affected zone. Therefore, the C content is set to 0.50% or less. The C content is preferably set to 0.40% or less, more preferably set to 0.35% or less, even more preferably set to 0.25% or less, and most preferably set to 0.20% or less.

[0016] Si: 0.01 to 0.50% Si is an element that acts as a deoxidizer. However, if the Si content is less than 0.01%, the deoxidizing effect is insufficient. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.02% or more, more preferably 0.03% or more, even more preferably 0.08% or more, and most preferably 0.10% or more. On the other hand, if the Si content exceeds 0.50%, toughness and weldability decrease. Furthermore, an increase in Si-based inclusions increases the amount of hydrogen in the steel in a hydrogen gas environment, and hydrogen embrittlement resistance decreases. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.48% or less, more preferably 0.46% or less, even more preferably 0.40% or less, and most preferably 0.35% or less.

[0017] Mn: 0.50 to 1.50% Mn is an element that effectively contributes to improving the strength and toughness of steel. However, if the Mn content is less than 0.50%, the effect of adding Mn is poor. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 0.55% or more, more preferably 0.60% or more, even more preferably 0.65% or more, and most preferably 0.70% or more. On the other hand, if the Mn content exceeds 1.50%, the hardness of the surface layer and central segregation of the steel increases during cooling. As a result, the amount of hydrogen in the steel increases in a hydrogen gas environment, and hydrogen embrittlement resistance deteriorates. Weldability also deteriorates. Therefore, the Mn content is set to 1.50% or less. The Mn content is preferably 1.40% or less, more preferably 1.30% or less, even more preferably 1.25% or less, and most preferably 1.20% or less.

[0018] P: 0.015% or less P is an element contained in steel as an unavoidable impurity. If the P content exceeds 0.015%, weldability decreases, and the hardness of the central segregation increases, increasing the amount of hydrogen in the steel in a hydrogen gas environment and reducing hydrogen embrittlement resistance. Therefore, the P content is set to 0.015% or less. The P content is preferably set to 0.013% or less, more preferably set to 0.010% or less, and even more preferably set to 0.008% or less. On the other hand, since it is desirable to reduce P as much as possible, the lower limit of the P content is not particularly limited and may be 0%. However, from the viewpoint of refining costs, the P content is preferably set to 0.001% or more, more preferably set to 0.002% or more.

[0019] S: 0.0015% or less S is an element contained in steel as an unavoidable impurity. S forms MnS inclusions in steel, which increases the amount of hydrogen in the steel in a hydrogen gas environment and reduces hydrogen embrittlement resistance. Therefore, it is desirable to reduce S as much as possible, and an S content of up to 0.0015% is permissible. Therefore, the S content is set to 0.0015% or less. The S content is preferably set to 0.0013% or less, more preferably set to 0.0012% or less, and even more preferably set to 0.0010% or less. On the other hand, since it is desirable to reduce S as much as possible, the lower limit of the S content may be 0%. However, from the viewpoint of refining costs, the S content is preferably set to 0.0002% or more, more preferably set to 0.0003% or more.

[0020] Al: 0.005 to 0.15% Al is an element that functions as a deoxidizer. However, if the Al content is less than 0.005%, the inclusion effect is ineffective. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.008% or more, more preferably 0.01% or more, even more preferably 0.02% or more, and most preferably 0.04% or more. On the other hand, if the Al content exceeds 0.15%, the cleanliness of the steel decreases and the toughness decreases. Furthermore, impurities act as hydrogen trapping sites, increasing the amount of hydrogen in the steel in a hydrogen gas environment and reducing hydrogen embrittlement resistance. Therefore, the Al content is set to 0.15% or less. The Al content is preferably 0.14% or less, more preferably 0.12% or less, even more preferably 0.11% or less, and most preferably 0.10% or less.

[0021] O: 0.0035% or less O is an element contained as an unavoidable impurity and causes the formation of oxide-based inclusions that act as hydrogen trapping sites. As a result, the amount of hydrogen in the steel increases in a hydrogen gas environment, resulting in a decrease in hydrogen embrittlement resistance. Therefore, the lower the O content, the better. However, if the O content is 0.0035% or less, the above problem does not occur. Therefore, the O content is set to 0.0035% or less. The O content is preferably set to 0.0030% or less, more preferably 0.0028% or less, even more preferably 0.0026% or less, and most preferably 0.0024% or less. On the other hand, since it is desirable to reduce O as much as possible, the lower limit of the O content is not particularly limited and may be 0%. However, the O content is preferably set to 0.0005% or more, more preferably 0.0008% or more.

[0022] N: 0.010% or less N is an element contained in steel as an unavoidable impurity and effectively contributes to improving the strength of steel. However, if the N content exceeds 0.010%, the hardness increases during cooling, thereby reducing toughness. Furthermore, it can cause the formation of nitride-based inclusions that act as hydrogen trapping sites. As a result, the amount of hydrogen in the steel increases in a hydrogen gas environment, reducing hydrogen embrittlement resistance. For this reason, the N content is set to 0.010% or less. The N content is preferably set to 0.008% or less, more preferably 0.007% or less, even more preferably 0.006% or less, and most preferably 0.005% or less. On the other hand, from the viewpoint of improving toughness, it is desirable to reduce N as much as possible. The lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction of N increases steelmaking costs. For this reason, the N content is preferably set to 0.00001% or more, more preferably 0.001% or more.

[0023] Nb: 0.001 to 0.10% Nb is an element that effectively increases the strength of steel and reduces the yield ratio. However, if the Nb content is less than 0.001%, the above effects cannot be obtained. Therefore, the Nb content is set to 0.001% or more. The Nb content is preferably set to 0.01% or more, more preferably set to 0.02% or more. On the other hand, if the Nb content exceeds 0.10%, the amount of precipitates becomes excessive, which acts as hydrogen trapping sites, increasing the amount of hydrogen in the steel in a hydrogen gas environment and reducing hydrogen embrittlement resistance. The yield ratio also increases. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably set to 0.095% or less, more preferably 0.08% or less, even more preferably 0.07% or less, and most preferably 0.06% or less.

[0024] A steel material according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities.

[0025] Here, the term "unavoidable impurities" refers to impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be included to the extent that the object of the present invention is not impaired. Examples of the raw materials include iron ore, reduced iron, and scrap. Examples of the impurities include H, Zn, Pb, As, Co, and Bi.

[0026] Furthermore, the component composition of the steel material in another embodiment of the present invention can further optionally contain one or more elements selected from Ca, Ni, Ti, Cu, Cr, Mo, W, V, Zr, Ta, B, REM, Mg, Hf, Re, Sn, and Sb. Note that the inclusion of these elements is not essential in the present invention, and therefore the lower limit of the content of each element may be 0%.

[0027] Ca: 0 to 0.005% Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, and can be added at any amount depending on the required HIC resistance. However, if the Ca content exceeds 0.005%, not only does the above effect saturate, but the cleanliness of the steel decreases, resulting in a decrease in HIC resistance. Therefore, if Ca is added, the Ca content should be 0.005% or less. The Ca content is preferably 0.004% or less, more preferably 0.003% or less. On the other hand, since the inclusion of Ca is not essential, the lower limit of the Ca content may be 0%. However, to obtain the above effect, the Ca content is preferably 0.0001% or more, more preferably 0.0008% or more.

[0028] Ni: 0 to 2.0% Ni is an element effective in improving the toughness and strength of steel materials and can be added at any amount depending on the desired properties. However, if the Ni content exceeds 2.0%, microcracks known as Fischer cracks are more likely to form in environments with low hydrogen sulfide partial pressures of less than 1 bar. Therefore, if Ni is contained, the Ni content should be 2.0% or less. The Ni content is preferably 1.8% or less, more preferably 1.6% or less, even more preferably 1.5% or less, and most preferably 1.4% or less. On the other hand, since the inclusion of Ni is not essential, the lower limit of the Ni content may be 0%. However, to achieve the above effects, the Ni content is preferably 0.01% or more, more preferably 0.1% or more, and even more preferably 0.2% or more.

[0029] Ti: 0 to 0.1% Ti is an effective element for increasing the strength and toughness of steel and can be added at any amount depending on the desired properties. However, if the Ti content exceeds 0.1%, the toughness of the weld decreases. Therefore, if Ti is added, the Ti content should be 0.1% or less. The Ti content is preferably 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less. On the other hand, since the inclusion of Ti is not essential, the lower limit of the Ti content may be 0%. However, to achieve the above effect, the Ti content is preferably 0.005% or more, more preferably 0.008% or more, and even more preferably 0.01% or more.

[0030] Cu: 0 to 1.0% Cu is an element effective in improving the toughness and strength of steel and can be added at any amount depending on the desired properties. However, if the Cu content exceeds 1.0%, weldability decreases. Therefore, when Cu is added, the Cu content is set to 1.0% or less. The Cu content is preferably set to 0.9% or less, more preferably 0.8% or less, even more preferably 0.7% or less, and most preferably 0.5% or less. On the other hand, since the inclusion of Cu is not essential, the lower limit of the Cu content may be 0%. However, to obtain the above effects, the Cu content is preferably set to 0.01% or more, more preferably 0.05% or more, and even more preferably 0.1% or more.

[0031] Cr: 0 to 1.0% Like Mn, Cr is an effective element for obtaining sufficient strength even with a low C content, and can be added at any amount depending on the required strength. However, if the Cr content exceeds 1.0%, the hardenability becomes excessive and SSCC resistance decreases. Weldability also decreases. Therefore, if Cr is added, the Cr content should be 1.0% or less. The Cr content is preferably 0.9% or less, more preferably 0.9% or less, even more preferably 0.8% or less, and most preferably 0.7% or less. Since the inclusion of Cr is not essential, the lower limit of the Cr content may be 0%. However, to obtain the above effects, the Cr content should preferably be 0.01% or more, more preferably 0.05% or more, and even more preferably 0.1% or more.

[0032] Mo: 0 to 0.60% Mo is an element effective in improving the toughness and strength of steel materials and in improving SSCC resistance regardless of hydrogen sulfide partial pressure, and can be added at any amount depending on the desired properties. However, if the Mo content exceeds 0.60%, the hardenability becomes excessive, the SSCC resistance decreases, and the weldability also decreases. Therefore, if Mo is contained, the Mo content is set to 0.60% or less. The Mo content is preferably set to 0.50% or less, more preferably 0.40% or less, even more preferably 0.35% or less, and most preferably 0.30% or less. On the other hand, since the inclusion of Mo is not essential, the lower limit of the Mo content may be 0%. However, to achieve the above effects, the Mo content is preferably set to 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, and most preferably 0.12% or more.

[0033] W: 0 to 1.0% W is an element that contributes to increasing the strength of steel and can be added at any amount depending on the required strength. However, if the W content exceeds 1.0%, the above effect saturates and costs increase. Therefore, if W is added, the W content is set to 1.0% or less. The W content is preferably set to 0.9% or less, more preferably 0.8% or less. From the perspective of cost reduction, the W content is further preferably set to 0.5% or less, and most preferably 0.4% or less. On the other hand, since the inclusion of W is not essential, the lower limit of the W content may be 0%. However, to obtain the above effect, the W content is preferably set to 0.01% or more, more preferably 0.08% or more, and even more preferably 0.2% or more.

[0034] V: 0 to 0.10% V is an element that contributes to improving the strength and toughness of steel and can be added at any amount depending on the desired properties. However, if the V content exceeds 0.10%, the toughness of the weld decreases. Therefore, when V is added, the V content is set to 0.10% or less. The V content is preferably set to 0.09% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. On the other hand, since the inclusion of V is not essential, the lower limit of the V content may be 0%. However, to obtain the above effect, the V content is preferably set to 0.01% or more, more preferably 0.02% or more.

[0035] Zr: 0 to 0.050% Zr is an effective element for improving toughness through grain refinement and crack resistance through control of inclusion properties, and can be added at any amount depending on the desired properties. However, if the Zr content exceeds 0.050%, the above effects saturate. Therefore, if Zr is contained, the Zr content is set to 0.050% or less. The Zr content is preferably set to 0.048% or less, more preferably 0.046% or less, and even more preferably 0.044% or less. On the other hand, since the inclusion of Zr is not essential, the lower limit of the Zr content may be 0%. However, to achieve the above effects, the Zr content is preferably set to 0.0001% or more, more preferably 0.001% or more, even more preferably 0.005% or more, and most preferably 0.008% or more.

[0036] Ta: 0 to 0.050% Ta is an element that forms carbides and nitrides and contributes to improving strength, and can be added at any amount depending on the required strength. However, a Ta content exceeding 0.050% can result in a decrease in toughness. Therefore, when Ta is added, the Ta content is set to 0.050% or less. The Ta content is preferably set to 0.045% or less, and more preferably set to 0.040% or less. On the other hand, since the inclusion of Ta is not essential, the lower limit of the Ta content may be 0%. However, to achieve the above effect, the Ta content is preferably set to 0.0001% or more. The Ta content is more preferably set to 0.0008% or more, even more preferably set to 0.001% or more, most preferably set to 0.004% or more, and even more preferably set to 0.012% or more.

[0037] B: 0 to 0.0020% B is an element that improves hardenability, contributes to increasing the strength of steel, and suppresses coarsening of prior austenite grains, thereby improving various properties of the steel. B can be added at any amount depending on the desired properties. However, if the B content exceeds 0.0020%, the above effects saturate and lead to increased costs. Therefore, if B is added, the B content is set to 0.0020% or less. The B content is preferably 0.0018% or less, more preferably 0.0016% or less. To reduce costs, it is even more preferably 0.0014% or less, and most preferably 0.0012% or less. On the other hand, since the inclusion of B is not essential, the lower limit of the B content may be 0%. However, to obtain the above effects, the B content is preferably 0.0001% or more, more preferably 0.0004% or more, and even more preferably 0.0008% or more.

[0038] REM: 0-0.01%, Mg: 0-0.01%. REM and Mg are elements that enhance toughness through grain refinement and crack resistance through inclusion property control, respectively. These elements can be added at any amount depending on the desired properties. However, if the REM content and Mg content exceed 0.01%, the above effects saturate. Therefore, when REM and Mg are each contained, the REM content and Mg content should each be 0.01% or less. The REM content and Mg content should each be preferably 0.009% or less, more preferably 0.008% or less, even more preferably 0.007% or less, and most preferably 0.006% or less. Since the inclusion of REM and Mg is not essential, the lower limits of the REM content and Mg content may each be 0%. However, to achieve the above effects, the REM content and Mg content should each be preferably 0.0001% or more, more preferably 0.0008% or more, and even more preferably 0.001% or more. It should be noted that REM is an abbreviation for Rare Earth Metal, which refers to rare earth metals.

[0039] Hf: 0 to 0.005%, Re: 0 to 0.005%. Hf and Re are elements that contribute to increasing the strength of steel and are optionally contained depending on the required strength. However, if the Hf content or Re content exceeds 0.005%, oxides increase and agglomerate, resulting in a decrease in hydrogen embrittlement resistance. Therefore, when Hf and Re are contained, the Hf content and Re content are each set to 0.005% or less. The Hf content and Re content are preferably set to 0.004% or less, more preferably 0.003% or less. On the other hand, since the inclusion of Hf and Re is not essential, the lower limits of the Hf content and Re content may be 0%. However, to obtain the above effects, the Hf content and Re content are each set to preferably 0.0001% or more, more preferably 0.0004% or more, and even more preferably 0.0006% or more.

[0040] Sn: 0-0.3%, Sb: 0-0.3%. Sn and Sb are elements that contribute to increasing the strength and hardenability of steel, respectively, and can be added at any amount depending on the desired properties. However, if the Sn content and Sb content exceed 0.3%, the above effects saturate and cost increases. Therefore, when Sn and Sb are each contained, the Sn content and Sb content are each set to 0.3% or less. The Sn content and Sb content are each preferably set to 0.28% or less, more preferably 0.26% or less, and even more preferably 0.24% or less to reduce costs. On the other hand, since the inclusion of Sn and Sb is not essential, the lower limits of the Sn content and Sb content may be 0%. However, to obtain the above effects, the Sn content and Sb content are each preferably set to 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.01% or more.

[0041] The steel material of the present invention has a metal structure in which bainite accounts for 70% or more in area fraction at the quarter-wall position and the average grain size of the bainite is 15 μm or less. The reason for this will be explained below.

[0042] Bainite has an area fraction of 70% or more at the quarter-wall thickness position. First, the reason for limiting the metallographic structure at the quarter-wall thickness position will be explained. The inventors' studies have revealed that hydrogen absorption by steel occurs from the surface of the steel that directly contacts hydrogen gas, and therefore it is important to control the metallographic structure at least from the surface of the steel that directly contacts hydrogen gas to the quarter-wall thickness position (inside the steel), which is slightly inside the steel. Further studies have revealed that controlling the metallographic structure at the quarter-wall thickness position relative to the steel surface to the above metallographic structure can result in a steel with excellent hydrogen embrittlement resistance. Therefore, the metallographic structure at the quarter-wall thickness position is controlled. That is, it is sufficient for the steel to have the above metallographic structure at the quarter-wall thickness position relative to at least one of the front and back surfaces of the steel, and the steel surface having the above metallographic structure can be exposed to a hydrogen gas environment. For example, when the steel material is a welded steel pipe, the inner surface of the welded steel pipe comes into contact with hydrogen gas, so the metal structure at least at a position 1 / 4 of the way through the wall thickness based on the inner surface of the welded steel pipe should be the metal structure described above.

[0043] Next, the reason for setting the area fraction of bainite to 70% or more will be explained. If the area fraction of bainite at the 1 / 4 wall thickness position is less than 70%, the desired strength cannot be obtained. For this reason, the area fraction of bainite is set to 70% or more. The area fraction of bainite is preferably 80% or more, more preferably 85% or more. Furthermore, the presence of structures with different hardnesses generates strain at the interface of structures with different hardnesses due to the difference in hardness of the structures within the steel when stress is applied during use. This strain acts as a hydrogen trapping site, thereby degrading hydrogen embrittlement resistance. From the viewpoint of further improving hydrogen embrittlement resistance, the area fraction of bainite is more preferably 90% or more, and most preferably 94% or more. On the other hand, since a higher area fraction of bainite is preferable, the upper limit of the area fraction of bainite is not particularly limited and may be 100%.

[0044] Furthermore, the steel material of the present invention preferably contains lower bainite at the 1 / 4 position of the wall thickness, and further contains one or more of upper bainite and ferrite, the area fraction of lower bainite being 50% or more and 75% or less, and the total area fraction of upper bainite and ferrite being 25% or more and 50% or less.

[0045] Area fraction of lower bainite is 50% or more and 75% or less By setting the area fraction of lower bainite to 50% or more, the desired strength is more easily obtained. Therefore, the area fraction of lower bainite is preferably 50% or more, more preferably 54% or more, and even more preferably 58% or more. On the other hand, if the area fraction of lower bainite exceeds 75%, the yield ratio increases, and the desired yield ratio (93% or less) cannot be obtained. Therefore, the area fraction of lower bainite is preferably 75% or less, more preferably 72% or less, and even more preferably 70% or less. Here, the lower bainite is composed of one or more types selected from bainitic ferrite, granular bainite, and tempered bainite.

[0046] The sum of the area fractions of upper bainite and ferrite is 25% or more and 50% or less. If the sum of the area fractions of upper bainite and ferrite is less than 25%, the desired yield ratio cannot be obtained. Therefore, the sum of the area fractions of upper bainite and ferrite is preferably 25% or more, more preferably 27% or more. On the other hand, if the sum of the area fractions of upper bainite and ferrite exceeds 50%, it may be difficult to obtain the desired strength. Furthermore, during deformation of the steel material, a strain distribution occurs due to the difference in hardness between the lower bainite and the upper bainite and / or ferrite, which generates strain at the interface between the lower bainite and the upper bainite and / or ferrite. This strain may act as a hydrogen trapping site and reduce hydrogen embrittlement resistance. Therefore, the sum of the area fractions of upper bainite and ferrite is preferably 50% or less, more preferably 48% or less, and even more preferably 42% or less.

[0047] The sum of the above-mentioned lower bainite and upper bainite corresponds to the above-mentioned bainite, i.e., the area fraction of bainite is the sum of the area fractions of lower bainite and upper bainite. However, the area fraction of ferrite is set to 30% or less for the reasons described below.

[0048] Other Structures: The metal structure in one embodiment of the present invention may consist of the above-described metal structure. Furthermore, in other embodiments of the present invention, in addition to the above-described metal structure, the metal structure may further contain other metal structures at an area fraction of 30% or less. The other metal structure is not particularly limited and may be any metal structure. For example, the other metal structure may be at least one selected from martensite, ferrite, and pearlite. If the area fraction of the other metal structure exceeds 30%, the metal structure becomes nonuniform, resulting in a decrease in strength and inhibiting deformation above the yield strength, resulting in an increase in the yield ratio. Furthermore, the metal structure acts as a hydrogen trap, causing a decrease in hydrogen embrittlement resistance. Therefore, the area fraction of the other metal structure is set to 30% or less. The area fraction of the other metal structure is preferably set to 25% or less, more preferably 20% or less, and most preferably 15% or less. On the other hand, since it is desirable to reduce the area fraction of the other metal structure as much as possible, the lower limit of the area fraction of the other metal structure is not particularly limited and may be 0%.

[0049] Average grain size of bainite is 15 μm or less: In addition, when a steel is exposed to a hydrogen gas environment, hydrogen gas is adsorbed on the steel surface and then penetrates the steel as atomic hydrogen. The hydrogen that penetrates the steel accumulates at grain boundaries, causing hydrogen embrittlement. Specifically, when the same amount of hydrogen penetrates a steel, the hydrogen concentration accumulated at grain boundaries is higher when the average grain size of bainite is large than when the average grain size of bainite is small. As a result, hydrogen embrittlement is more likely to occur. As a result of research conducted by the inventors, it was found that by setting the average grain size of bainite to 15 μm or less, the grain boundary area is increased, the concentration of hydrogen acting at the grain boundaries is reduced, and excellent hydrogen embrittlement resistance is achieved. Therefore, the average grain size of bainite is set to 15 μm or less. The average grain size of bainite is preferably 14 μm or less, more preferably 13 μm or less, even more preferably 12 μm or less, and most preferably 11 μm or less. On the other hand, the lower limit of the average grain size of bainite is not particularly limited, but is preferably 4 μm or more, and more preferably 6 μm or more.

[0050] Average grain size of lower bainite is 15 μm or less: When bainite contains lower bainite, the average grain size of the lower bainite is preferably 15 μm or less for the same reasons as for bainite described above. The average grain size of the lower bainite is more preferably 13 μm or less, even more preferably 12 μm or less, and most preferably 11 μm or less. On the other hand, although there is no particular lower limit for the average grain size of the lower bainite, it is preferably 4 μm or more.

[0051] Average grain size of upper bainite is 15 μm or less: When bainite contains upper bainite, the average grain size of the upper bainite is preferably 15 μm or less for the same reasons as for bainite described above. The average grain size of the upper bainite is more preferably 14 μm or less, even more preferably 13 μm or less, and most preferably 12 μm or less. On the other hand, the lower limit of the average grain size of the upper bainite is not particularly limited, but is preferably 4 μm or more.

[0052] Average grain size of ferrite is 15 μm or less: When the metal structure contains ferrite, the average grain size of the ferrite is preferably 15 μm or less for the same reasons as for bainite described above. The average grain size of the ferrite is more preferably 14 μm or less, even more preferably 13 μm or less, and most preferably 12 μm or less. On the other hand, the lower limit of the average grain size of ferrite is not particularly limited, but is preferably 4 μm or more.

[0053] The area fraction and average grain size of each metal structure may be measured by the method described in the examples.

[0054] Characteristics Hydrogen embrittlement resistance: Hydrogen embrittlement resistance is measured at a strain rate of 7.0 x 10 －４ mm －１In a tensile test (slow strain rate tensile test (SSRT)) in which the reduction of area is 40 MPa, the reduction of area is evaluated by the relative reduction of area (RRA) expressed as (reduction of area in hydrogen gas at a hydrogen gas pressure of 40 MPa) / (reduction of area in air), and a steel is judged to be excellent when the RRA value is 0.90 or more. Generally, when a tensile test of a steel material is performed in hydrogen gas, the elongation of the steel material decreases compared to when it is performed in air. Therefore, the steel material tends to break before it is reduced in area. As a result, the area of ​​the fracture surface in hydrogen gas tends to be larger than the area of ​​the fracture surface in air. Therefore, the reduction of area φ obtained in a tensile test in hydrogen gas Ｈ is the reduction of area φ obtained in the tensile test in air ａｉｒ The relative aperture ratio (RRA) is expressed as follows: RRA = φ Ｈ / φ ａｉｒ , and the larger the RRA, the better the hydrogen embrittlement resistance. －４ mm －１ In a tensile test, the reduction of area in hydrogen gas at a hydrogen gas pressure of 40 MPa was Ｈ ) / (Aperture value in air: φ ａｉｒ The greater the relative reduction of area (RRA), expressed as 0.96 or less, the better the hydrogen embrittlement resistance. The steel material of the present invention has a relative reduction of area (RRA) of 0.90 or more in a 40 MPa hydrogen gas environment for the safe operation of steel structures in a hydrogen gas environment. The relative reduction of area is preferably 0.91 or more. On the other hand, the upper limit of the relative reduction of area is not particularly limited, but the relative reduction of area may be 1.00. The RRA may more preferably be 0.96 or less, even more preferably be 0.94 or less, and most preferably be 0.92 or less.

[0055] Since it is desirable to have sufficient ductility in a hydrogen gas environment, i.e., deformation characteristics equivalent to those in air, it is important to judge hydrogen embrittlement resistance by the relative reduction of area at a low strain rate. Furthermore, hydrogen embrittlement is caused by hydrogen accumulation at stress concentration points such as notch tips and crack tips. Therefore, when evaluating the hydrogen embrittlement resistance of steel, it is necessary to reduce the strain rate in the tensile test and allow a sufficient amount of hydrogen to accumulate at stress concentration points. For this reason, the strain rate is set to 7.0 x 10 －４ mm －１ The hydrogen embrittlement resistance may be evaluated by the method described in the examples.

[0056] Tensile Strength (TS) The tensile strength (TS) of the steel material of the present invention is not particularly limited, but when assuming a steel material constituting a steel structure, the TS of the steel material is preferably 520 MPa or more, more preferably 530 MPa or more, and even more preferably 540 MPa or more. On the other hand, although there is no particular upper limit for the TS of the steel material, excessively high strength tends to increase costs and reduce hydrogen embrittlement resistance. Therefore, TS is preferably 700 MPa or less, more preferably 670 MPa or less, and even more preferably 640 MPa or less. The tensile strength may be evaluated by the method described in the examples.

[0057] Yield Ratio (YR) When the steel material of the present invention is used, for example, as a steel pipe for a line pipe, the steel material is required to have excellent deformation characteristics, i.e., a low yield ratio, so that hydrogen gas does not leak from the line pipe even if the line pipe is significantly deformed by an earthquake or the like. The yield ratio (YR) of the steel material of the present invention is preferably 93% or less, assuming large strain loads due to seismic motion expected in buried pipelines. The yield ratio is more preferably 92% or less, even more preferably 91% or less, and most preferably 90% or less. While there is no particular restriction on the lower limit of the yield ratio of the steel material, excessively lowering the yield ratio increases the risk of brittle fracture under large strain loads. Therefore, the yield ratio is preferably 75% or more. The yield ratio is more preferably 78% or more, even more preferably 80% or more, and most preferably 82% or more. The yield ratio may be evaluated using the method described in the Examples.

[0058] [Steel Material] The steel material of the present invention includes thin steel plates, thick steel plates, welded steel pipes, shaped steel, steel bars, and the like. The steel material has the above-described chemical composition and metallographic structure, and a relative reduction of area (RRA) of 0.90 or more, thereby providing the strength required for steel structures and excellent hydrogen absorption resistance in hydrogen gas environments. Therefore, the steel material can be suitably applied to steel structures used in hydrogen gas environments, particularly high-pressure hydrogen gas environments. In particular, when the steel material is a welded steel pipe, it can be suitably applied to hydrogen line pipes. Therefore, the steel material is preferably a welded steel pipe. Here, examples of high-pressure hydrogen gas environments include hydrogen gas at a pressure of 1 MPa or more, and a natural gas (mainly composed of hydrocarbons such as methane and ethane) mixed atmosphere containing hydrogen at a hydrogen partial pressure of 1 MPa or more.

[0059] [Manufacturing Method of Steel Material] Next, a manufacturing method of the steel material of the present invention will be described. The steel material can be manufactured by sequentially performing the steps (a) to (c) on a slab (steel material) having the above-described composition. (a) Heating step (b) Hot rolling step (c) Cooling step In the following description, unless otherwise specified, temperature refers to the temperature at the surface of the slab, hot-rolled steel plate, or steel material. The surface temperature of the slab, hot-rolled steel plate, or steel material can be measured using a radiation thermometer or the like. The temperature at the 1 / 4 wall thickness position may be measured using a thermocouple or the like, or may be predicted from the measured surface temperature using a finite element method or the like.

[0060] Heating Step In the heating step, the slab (steel material) having the above-described composition is heated at a heating temperature of 1000° C. to 1250° C. The heating temperature is the surface temperature of the slab.

[0061] Slabs Any form of material can be used as the slab (also referred to as steel material). The slab may be, for example, a billet or a steel slab. The method for producing the slab is not particularly limited, and the slab can be produced, for example, by melting steel having the above-mentioned composition and casting it. The melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, from the viewpoint of productivity, the casting is preferably carried out by a continuous casting method, but can also be carried out by an ingot casting-breakdown rolling method.

[0062] Heating temperature: 1000°C or higher and 1250°C or lower. If the heating temperature is lower than 1000°C, the diffusion of elements such as C, P, and S microsegregated in the slab will be insufficient, resulting in a non-uniform material. Therefore, the heating temperature is set to 1000°C or higher. The heating temperature is preferably set to 1025°C or higher, more preferably 1050°C or higher, even more preferably 1075°C or higher, and most preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains will become coarse, resulting in a decrease in strength and hydrogen embrittlement resistance in a hydrogen gas environment. Furthermore, the formation of ferrite will further decrease strength. Therefore, the heating temperature is set to 1250°C or lower. The heating temperature is preferably set to 1230°C or lower, more preferably 1210°C or lower, even more preferably 1200°C or lower, and most preferably 1190°C or lower.

[0063] In the hot rolling process, the slab heated in the heating process is rolled at a temperature of 10 ... ３ The steel sheet is then hot-rolled at a temperature of 50°C or higher to obtain a hot-rolled steel sheet.

[0064] Hot rolling start temperature: Ar ３ Point + 50℃ or more Hot rolling start temperature is Ar ３ If the hot rolling temperature is less than the recrystallization temperature +50°C, the temperature falls far below the lower limit of the recrystallization temperature, and no refinement by rolling occurs, resulting in large grain sizes of upper bainite and ferrite. These coarse upper bainite and coarse ferrite cause brittle fracture such as cleavage and grain boundary cracking in hydrogen gas, so depending on the area fraction of the coarse upper bainite and coarse ferrite, the hydrogen embrittlement properties in a hydrogen gas environment may be reduced. Therefore, the hot rolling start temperature is set to Ar ３ The hot rolling start temperature is Ar ３ It is preferable to set the temperature at the temperature above 60°C. ３ It is more preferable to set the temperature at the temperature above 80°C. ３ On the other hand, when the hot rolling start temperature is Ar ３ If the rolling temperature is higher than the rolling point +200°C, recrystallization due to rolling will proceed excessively, and the area fraction of lower bainite will increase. As a result, the yield ratio will increase due to a decrease in deformability. ３ The hot rolling start temperature is preferably set to 200°C or less than the rolling point. ３ It is more preferable to set the temperature at or below +180°C. ３ It is more preferable that the temperature is set to 160°C or lower than the temperature at which the Ar ３ It is most preferable that the hot rolling start temperature is not higher than the temperature at which the hot rolling is started +150° C. Here, the hot rolling start temperature is the surface temperature of the slab.

[0065] In the cooling step, the hot-rolled steel sheet obtained in the hot rolling step is cooled under the following conditions to obtain a steel material. (1) Cooling start temperature: The surface temperature of the hot-rolled steel sheet is cooled under the following conditions. ３ point or higher and at the temperature of 1 / 4 of the thickness of the hot-rolled steel plate ３(2) Average cooling rate from 750°C to 550°C at the surface temperature of the hot-rolled steel plate: 30°C / s or more. (3) Average cooling rate from 750°C to 550°C at the temperature at 1 / 4 of the thickness of the hot-rolled steel plate: 20°C / s or more. (4) Cooling stop temperature: 250°C or more, but not more than 550°C at the temperature at 1 / 4 of the thickness of the hot-rolled steel plate.

[0066] Cooling start temperature By controlling both the temperature at the surface of the hot-rolled steel sheet (surface temperature of the hot-rolled steel sheet) and the temperature at a quarter position in the thickness direction based on the surface of the hot-rolled steel sheet (temperature at the quarter position of the thickness of the hot-rolled steel sheet), it is possible to control the metal structure at the quarter position of the thickness to a desired metal structure, and it is possible to achieve both the desired strength and hydrogen embrittlement resistance. ３ If the cooling temperature is below this point, coarse ferrite is generated before cooling, and the grain size of bainite increases. This results in a decrease in strength. Among bainite, the grain size of lower bainite tends to increase. While the area fraction of lower bainite tends to decrease, coarse ferrite causes brittle fracture such as cleavage and intergranular cracking in hydrogen gas. Furthermore, coarse lower bainite causes intergranular cracking in hydrogen gas. Therefore, depending on the area fraction of coarse ferrite and coarse lower bainite, hydrogen embrittlement resistance in a hydrogen gas environment decreases. For this reason, the cooling start temperature is set to be the surface temperature of the hot-rolled steel sheet at a temperature of Ar. ３ The cooling start temperature is the surface temperature of the hot-rolled steel sheet, and is preferably Ar. ３ point +5°C or higher, more preferably Ar ３ point +10°C or higher, more preferably Ar ３ The cooling start temperature is the temperature at the 1 / 4 position of the thickness of the hot-rolled steel plate. ３ If the cooling temperature is less than the temperature at the point +20°C, ferrite is generated, and the strength tends to decrease. Furthermore, the grain size of the lower bainite tends to increase. Furthermore, since the coarse lower bainite causes grain boundary cracking in hydrogen gas, the hydrogen embrittlement resistance decreases depending on the area fraction of the coarse lower bainite. For this reason, the cooling start temperature is set to the temperature at the 1 / 4 position of the thickness of the hot-rolled steel sheet in Ar. ３The cooling start temperature is the temperature at the 1 / 4 position of the thickness of the hot-rolled steel sheet, preferably Ar. ３ Point +22°C or higher, more preferably Ar ３ Point +25°C or higher, more preferably Ar ３ On the other hand, the upper limit of the cooling start temperature is not particularly limited, but it is preferable that the surface temperature of the hot-rolled steel sheet is higher than the Ar temperature. ３ The temperature is preferably 100°C or lower than the temperature at which the temperature reaches the Ar ３ More preferably, the temperature is 90°C or lower. ３ The temperature is more preferably 80°C or less than the temperature at the 1 / 4 position of the thickness of the hot-rolled steel sheet. ３ It is preferable to set the temperature at or below +120°C. ３ More preferably, the temperature is 100°C or lower. ３ It is more preferable that the temperature is not higher than the point +80° C. The temperature at the quarter thickness position is a temperature at a position that is one-quarter of the thickness from the surface in the thickness direction, with the surface of the hot-rolled steel sheet as the reference.

[0067] In the present invention, the Ar3 point is calculated by the following formula: Ar3 (°C) = 910 - 310 [C] - 80 [Mn] - 20 [Cu] - 15 [Cr] - 55 [Ni] - 80 [Mo], where [M] represents the content (mass%) of element M.

[0068] Average Cooling Rate By controlling both the average cooling rate at the surface temperature of the hot-rolled steel plate from 750°C to 550°C and the average cooling rate at the temperature at 1 / 4 of the wall thickness of the hot-rolled steel plate from 750°C to 550°C, it becomes possible to control the metal structure at the 1 / 4 of the wall thickness position to a desired metal structure, thereby achieving both the desired strength and hydrogen embrittlement resistance.

[0069] Average cooling rate from 750°C to 550°C at the surface temperature of the hot-rolled steel sheet: 30°C / s or more. If the average cooling rate from 750°C to 550°C at the surface temperature of the hot-rolled steel sheet is less than 30°C / s, ferrite will form, the desired area fraction of bainite will not be obtained, and the ferrite and bainite will coarsen, preventing the desired strength from being obtained. Furthermore, coarse upper bainite and coarse ferrite will cause brittle fracture such as cleavage and intergranular cracking in hydrogen gas. Furthermore, coarse lower bainite will cause intergranular cracking in hydrogen gas. The area fractions of coarse upper bainite, coarse ferrite, and coarse lower bainite tend to increase, and depending on these area fractions, hydrogen embrittlement resistance in a hydrogen gas environment will deteriorate. Therefore, the average cooling rate from 750°C to 550°C at the surface temperature of the hot-rolled steel sheet is set to 30°C / s or more. The average cooling rate is preferably 32° C. / s or more, more preferably 34° C. / s or more, even more preferably 36° C. / s or more, and most preferably 38° C. / s or more. On the other hand, although there is no particular upper limit to the average cooling rate, the average cooling rate is preferably 50° C. / s or less, more preferably 48° C. / s or less, even more preferably 46° C. / s or less, and most preferably 44° C. / s or less.

[0070] Average cooling rate from 750°C to 550°C at the 1 / 4 wall thickness position of the hot-rolled steel sheet: 20°C / s or more. If the average cooling rate from 750°C to 550°C at the 1 / 4 wall thickness position of the hot-rolled steel sheet is less than 20°C / s, the bainite, particularly the lower bainite, at the 1 / 4 wall thickness position will coarsen, the area fraction of ferrite will increase, and the ferrite will also coarsen, resulting in failure to obtain the desired strength. Furthermore, a distribution of grain sizes among the lower bainite, upper bainite, and ferrite will occur in the wall thickness direction. The distribution of grains with different grain sizes can cause localized strain concentration within the steel material, which acts as a hydrogen trap, resulting in reduced hydrogen embrittlement resistance in a hydrogen gas environment. In particular, because the grain size distribution tends to coarsen from the surface to the interior of the steel material, it is important to control the average cooling rate at the 1 / 4 wall thickness position of the hot-rolled steel sheet. Therefore, the average cooling rate from 750°C to 550°C at the wall thickness 1 / 4 position temperature of the hot-rolled steel sheet is set to 20°C / s or more. The average cooling rate is preferably 21°C / s or more, more preferably 22°C / s or more, even more preferably 23°C / s or more, and most preferably 24°C / s or more. On the other hand, although there is no particular upper limit, the average cooling rate is preferably 40°C / s or less, more preferably 39°C / s or less, even more preferably 38°C / s or less, and most preferably 37°C / s or less.

[0071] Cooling stop temperature: 550°C or less, 250°C or more at the 1 / 4 wall thickness position of the hot-rolled steel sheet. If the cooling stop temperature exceeds 550°C, coarse bainite, especially coarse upper bainite, is likely to form, and the area fraction of coarse ferrite increases, preventing the desired area fraction of bainite from being obtained. As a result, the desired strength cannot be obtained. Furthermore, coarse upper bainite and coarse ferrite can cause brittle fracture, such as cleavage and intergranular cracking, in hydrogen gas. Therefore, depending on the area fraction of coarse upper bainite and coarse ferrite, hydrogen embrittlement resistance in a hydrogen gas environment decreases. Therefore, the cooling stop temperature is set to 550°C or less. The cooling stop temperature is preferably set to 540°C or less, more preferably 530°C or less, even more preferably 525°C or less, and most preferably 520°C or less. On the other hand, if the cooling stop temperature is less than 250°C, the area fraction of martensite increases, and martensite acts as a hydrogen trapping site, resulting in decreased hydrogen embrittlement resistance. For this reason, the cooling stop temperature is set to 250° C. or higher, preferably 300° C. or higher, and more preferably 350° C. or higher.

[0072] Furthermore, by setting the cooling stop temperature to 400°C or higher, the area fraction of lower bainite can be reduced. As a result, a desired yield ratio can be obtained. For this reason, the cooling stop temperature is more preferably 400°C or higher. The cooling stop temperature is most preferably 420°C or higher. Here, the cooling stop temperature is the temperature at the 1 / 4 wall thickness position of the hot-rolled steel sheet. After cooling is stopped, it is possible to allow the steel sheet to cool naturally, but this will promote the formation of lower bainite and excessively increase the area fraction of lower bainite. As a result, the deformability will decrease and the yield ratio will increase. For this reason, it is more preferable to perform slow cooling from the cooling stop temperature to room temperature (20±10°C).

[0073] The steel material after the cooling step may be wound into a coil. When winding into a coil, the winding temperature is preferably 500°C or less, more preferably 490°C or less. The lower limit of the coiling temperature is not particularly limited, but is preferably 400°C or more, more preferably 425°C or more. In particular, when the steel material is an electric resistance welded steel pipe and an electric resistance welded steel pipe is to be manufactured, the steel material wound into a coil after the cooling step may be used to carry out the pipe making step described below.

[0074] In another embodiment of the present invention, a pipe-making process may be provided after the cooling process. That is, in the above embodiment, a welded steel pipe, which is one type of steel material, can be manufactured by sequentially performing the processes (a) to (d) on a cast slab having the above-mentioned composition. (a) Heating process (b) Hot rolling process (c) Cooling process (d) Pipe-making process However, the above processes (a) to (c) are as described above. Therefore, only the pipe-making process (d) will be described below.

[0075] Pipe-Making Process In the pipe-making process, the steel material after the cooling process is formed into a cylindrical shape, and both circumferential ends of the cylindrical steel material are butted together and welded to form a welded steel pipe. The method for forming the steel material into a cylindrical shape is not particularly limited, but examples include bending and cold roll forming. Specifically, in the bending process, the end of the steel material is beveled, and the steel material is formed into a cylindrical shape using a C press, a U press, or an O press. Then, both circumferential ends of the resulting cylindrical steel material are butted together, and the butt joint is seam-welded by internal and external welding to produce a welded steel pipe. Alternatively, the steel material may be formed into a cylindrical shape by press bending, and then the butt joint is seam-welded to produce a welded steel pipe. If necessary, a pipe expansion process may be further performed to produce a welded steel pipe. Any welding method may be used as long as it provides sufficient joint strength and joint toughness. However, submerged arc welding is preferred from the viewpoints of excellent weld quality and production efficiency. An example of a welded steel pipe manufactured using the above-mentioned bending process is a UOE steel pipe.

[0076] In cold roll forming, a steel material may be formed into a cylindrical shape by cold roll forming. Then, both circumferential ends of the obtained cylindrical steel material are butted together and electric resistance welded to produce a welded steel pipe. An example of a welded steel pipe manufactured by the above-mentioned manufacturing method is an electric resistance welded steel pipe. When the welded steel pipe is an electric resistance welded steel pipe, the welded steel pipe after the pipe-making process may be further formed into an electric resistance welded steel pipe material using a sizing roll that satisfies the following formula (1) (sizing process), and an internal pressure p (MPa) that satisfies the following formula (2) is applied to the inner surface of the electric resistance welded steel pipe material (internal pressure application process), thereby producing an electric resistance welded steel pipe. The cylindrical shape refers to a steel pipe with a "C"-shaped circumferential cross section. Diameter (mm) of the sizing roll ≧ Thickness (mm) of the hot-rolled steel sheet / 0.020 (1) The thickness of the hot-rolled steel sheet refers to the thickness of the hot-rolled steel sheet (steel sheet after the cooling process) before the sizing process. X<p≦X×1.5 (2) where X=(wall thickness of electric resistance welded steel pipe material (mm) / radius of electric resistance welded steel pipe material (mm))×yield strength of electric resistance welded steel pipe material (MPa).

[0077] The above-mentioned internal pressure can be applied, for example, by sealing the end of the electric resistance welded steel pipe material with a rubber packing and applying water pressure to the inside of the electric resistance welded steel pipe material. Furthermore, to stabilize the shape, a mold of the desired diameter can be used as an outer frame, if necessary. The wall thickness of the electric resistance welded steel pipe material is preferably 5 mm or more. Furthermore, the wall thickness of the electric resistance welded steel pipe material is preferably 30 mm or less. While there is no particular upper limit on the radius of the electric resistance welded steel pipe material, a larger radius of the electric resistance welded steel pipe material increases the load on the equipment. Therefore, the radius of the electric resistance welded steel pipe material is preferably 400 mm or less. Furthermore, the yield strength of the electric resistance welded steel pipe material is preferably 480 MPa or more, more preferably 490 MPa or more, and even more preferably 500 MPa or more, in order to withstand the gas pressure in pipeline operation. On the other hand, in order to avoid an increase in hydrogen embrittlement susceptibility, the yield strength of the electric resistance welded steel pipe material is preferably 560 MPa or less, more preferably 550 MPa or less, and even more preferably 540 MPa or less.

[0078] Sizing process: In the sizing process, the electric resistance welded steel pipe material undergoes bending deformation in the pipe axis direction along the roll shape as it passes through the rolls, generating residual stress in the pipe axis direction. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the pipe axis direction. The smaller the diameter of the sizing roll and the greater the thickness of the hot-rolled steel sheet, the greater the bending strain. If the diameter of the sizing roll does not satisfy the above formula (1), the residual stress in the electric resistance welded steel pipe will increase, which may induce accumulation of hydrogen that has penetrated the electric resistance welded steel pipe and cause hydrogen embrittlement. In order to reduce the residual stress in the electric resistance welded steel pipe, i.e., to reduce the absolute value of the residual stress in the pipe axis direction, it is preferable to set the diameter of the sizing roll to satisfy the above formula (1). Furthermore, the diameter of the sizing roll is not particularly limited, but it is preferable that it be 1800 mm or less.

[0079] Internal Pressure Loading Process In the internal pressure loading process, the electric resistance welded steel pipe material is expanded to generate tensile stress in the circumferential direction of the pipe, thereby reducing the absolute value of the residual stress in the circumferential direction. The greater the internal pressure p (MPa) in this internal pressure loading process, the smaller the absolute value of the residual stress in the circumferential direction. The tensile stress generated in the circumferential direction increases as the radius of the electric resistance welded steel pipe increases and as the wall thickness of the electric resistance welded steel pipe decreases. The left side (X) of equation (2) corresponds to the internal pressure p when the tensile stress generated in the circumferential direction is equal to the yield stress of the electric resistance welded steel pipe material. In the present invention, in order to reduce the absolute value of the residual stress in the axial direction of the pipe, it is preferable to set the internal pressure p to a value greater than the left side (X) of equation (2) and expand the electric resistance welded steel pipe material to the plastic region. On the other hand, if the internal pressure p exceeds the right-hand side of equation (2) (X × 1.5), the absolute value of the residual stress in the circumferential direction of the pipe will decrease, but the amount of work hardening due to pipe expansion will become too large, increasing the dislocation density on the pipe surface and potentially reducing hydrogen embrittlement resistance.

[0080] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to the following examples in any way.

[0081] Molten steel having the chemical compositions shown in Tables 1-1 and 1-2 was cast into slabs (steel materials) by continuous casting, which were then heated (heating process), hot-rolled to produce hot-rolled steel plates with a thickness of 20 mm (hot-rolling process), cooled (cooling process), and formed into pipes (pipe-making process) to obtain welded steel pipes (electric resistance welded steel pipes). The manufacturing conditions for the electric resistance welded steel pipes (sometimes simply referred to as steel pipes) are shown in Tables 2-1 and 2-2. The steel pipes in Tables 2-1 and 2-2 use the same steel types as in Tables 1-1 and 1-2. The metal structures and properties of the obtained steel pipes were evaluated. The evaluation methods are as follows:

[0082] Evaluation of Metallographic Structure Test specimens were taken from the center of the rolling direction (also referred to as the L direction, longitudinal direction, or pipe axis direction) of the steel pipe obtained as described above, so that the metallographic structure at a position 1 / 4 of the wall thickness relative to the inner surface of the steel pipe could be evaluated, with the cross section parallel to the pipe axis direction and the pipe circumferential direction (C direction) serving as the evaluation surface. Each metallographic structure (bainite, lower bainite, upper bainite, and ferrite) was observed in detail using a scanning electron microscope (SEM) as described below, and the area fraction of each metallographic structure was measured. The average crystal grain size of each metallographic structure was also determined by EBSD measurement as described below.

[0083] Measurement of Area Fractions of Bainite, Lower Bainite, Upper Bainite, and Ferrite The evaluation surface of the test specimen was buffed and etched using a 3 vol% nital solution. Using a scanning electron microscope at an appropriate magnification between 1000 and 5000 times, microstructure images of three fields of view were obtained for the evaluation surface under conditions of an acceleration voltage of 10 kV, and each metal structure was extracted. Here, each metal structure was visually judged by comparing it with the microstructure photograph in Reference 1. The area fraction of each metal structure was then determined and calculated for each microstructure image by image analysis based on the above judgment. For example, the area fraction of lower bainite was determined by binarizing the lower bainite and other structure regions in the above microstructure image, determining the area fraction by image analysis, and the average of the obtained area fractions for the three fields of view was taken as the area fraction of lower bainite. The area fraction of upper bainite and the area fraction of ferrite were also calculated using the same procedure. The area fraction of bainite was calculated by adding the area fraction of lower bainite and the area fraction of upper bainite calculated above. [Reference 1] Japan Society for Heat Treatment Technology (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Make the Most of Materials, 2004

[0084]

[0043] First, the evaluation surface of the test specimen was mirror-polished and then etched with colloidal silica. Crystal data was collected in a 1 mm x 1 mm field of view by EBSD (acceleration voltage: 15 kV, Electron Backscatter Diffraction) (measurement step: 0.8 μm). The crystal data was collected in three fields of view, the crystal grain size of each metallographic structure was calculated, and the average value of the crystal grain sizes in the three fields of view was taken as the average crystal grain size of each metallographic structure. The crystal grain size was defined as the area grain size (a weighted average when a boundary with an orientation difference of 15° or more is defined as a grain boundary).

[0085] Tensile Test: Tensile test specimens were taken from the steel pipes obtained as described above, with the center at 1 / 4 of the wall thickness, based on the inner surface, and with the longitudinal direction (tensile direction) of the tensile test specimen aligned circumferentially. These specimens were then processed into bar-shaped test specimens as specified in JIS Z 2201 "Tensile Test Specimens for Metallic Materials." The tensile test was performed using the method specified in JIS Z 2241. The maximum load was divided by the initial cross-sectional area of ​​the tensile test specimen to determine the tensile strength TS (MPa) of the steel pipe. The yield ratio was calculated from the ratio of yield stress to tensile stress (yield stress / tensile stress) in the stress-strain curve obtained in the tensile test. If an upper yield point and a lower yield point were observed in the stress-strain curve, the upper yield point was used for the calculation. If no yield point was observed, the 0.2% proof stress was used as the yield stress.

[0086] Evaluation of Hydrogen Embrittlement Resistance Round bar tensile test specimens (parallel diameter 7 mm, gauge length 35 mm) shaped according to ASTM G 142 were taken from the steel pipes obtained as described above, and slow strain rate tensile tests (SSRT) were performed in air and in hydrogen gas at a hydrogen gas pressure of 40 MPa to determine the relative reduction of area (RRA), which is (reduction of area in hydrogen gas at a hydrogen gas pressure of 40 MPa) / (reduction of area in air). The round bar tensile test specimens were centered at a quarter-wall position relative to the inner surface of the steel pipe, and the longitudinal direction (tensile direction) of the round bar tensile test specimens was aligned with the circumferential direction of the pipe. The strain rate in the slow strain rate tensile test was 7.0 x 10 －４ mm －１ The slow strain rate tensile test in hydrogen at a hydrogen gas pressure of 40 MPa was carried out by introducing hydrogen gas into a high-pressure chamber after placing the round bar tensile test specimen therein, and immediately starting the slow strain rate tensile test after the hydrogen gas pressure reached the above-mentioned value. The test was carried out while maintaining a constant hydrogen gas pressure throughout the test.

[0087] Tables 2-1 and 2-2 show the manufacturing conditions, metal structure, tensile strength, yield ratio, and hydrogen embrittlement resistance (relative reduction of area (RRA)). The inventive examples of the present invention satisfied the conditions of a relative reduction of area (RRA) of 0.90 or more and a tensile strength of 520 MPa or more, and were found to have high strength and excellent hydrogen embrittlement resistance.

[0088]

[0089]

[0090]

[0091]

Claims

1. In mass%, C: 0.02 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.0035% or less, N: 0.010% or less, Nb: 0.001 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0%, Ti: 0 to 0.1%, Cu: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0 to 0.10%, Zr: 0 to 0.050%, Ta: 0 to 0.050%, The steel sheet has a chemical composition containing B: 0 to 0.0020%, REM: 0 to 0.01%, Mg: 0 to 0.01%, Hf: 0 to 0.005%, Re: 0 to 0.005%, Sn: 0 to 0.3%, and Sb: 0 to 0.3%, with the balance being Fe and unavoidable impurities, and has a metal structure in which, at a quarter-wall thickness position, bainite accounts for 70% or more in area fraction, and the average grain size of the bainite is 15 μm or less, and a strain rate of 7.0×10 －４ mm －１ In a tensile test, the steel has a relative reduction of area (RRA) of 0.90 or more, expressed as (reduction of area in hydrogen gas at a hydrogen gas pressure of 40 MPa) / (reduction of area in air).

2. The chemical composition is, in mass%, Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ni: 0.01 to 2.0%, Ti: 0.005 to 0.1%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, The steel material according to claim 1, comprising one or more selected from Hf: 0.0001 to 0.005%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, and Sb: 0.0001 to 0.3%.

3. The steel material according to claim 1 or 2, wherein the steel material is a welded steel pipe.

4. A heating step of heating a slab having the chemical composition according to claim 1 or 2 to a heating temperature of 1000°C or more and 1250°C or less, and a hot rolling start temperature of the slab in Ar at the surface temperature of the slab. ３ a hot rolling step of hot rolling the hot rolled steel sheet at a temperature of 50°C or higher than the rolling point to obtain a hot rolled steel sheet; and a cooling start temperature of the hot rolled steel sheet obtained in the hot rolling step, wherein the cooling start temperature is Ar at the surface temperature of the hot rolled steel sheet. ３ point or higher and at the temperature of 1 / 4 of the thickness of the hot-rolled steel plate ３ a cooling step of cooling the hot-rolled steel plate to a temperature of 750°C +20°C or more at the surface temperature, an average cooling rate of 30°C / s or more from 750°C to 550°C at the temperature at 1 / 4 of the wall thickness of the hot-rolled steel plate, an average cooling rate of 20°C / s or more from 750°C to 550°C at the temperature at 1 / 4 of the wall thickness of the hot-rolled steel plate, and a cooling stop temperature of 250°C or more but not more than 550°C at the temperature at 1 / 4 of the wall thickness of the hot-rolled steel plate to obtain a steel material.

5. A method for manufacturing steel material as described in claim 4, further comprising, after the cooling step, forming the steel material obtained after the cooling step into a cylindrical shape, butting and welding both circumferential ends of the cylindrical steel material together to form a welded steel pipe.

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