Steel material and method for producing steel material

Abstract

The purpose of the present invention is to provide: a steel material which is suitable as a steel structure used in a hydrogen gas environment, has excellent hydrogen absorption resistance in a hydrogen gas environment, and has excellent hydrogen embrittlement resistance; and a method for producing same.　The steel material has a specific component composition, wherein the amount of saturated hydrogen that has entered in a hydrogen gas environment at a hydrogen gas pressure of 40 MPa is less than a predetermined value. The predetermined value is 0.8 ppm by mass when the tensile strength TS of the steel material is less than 500 MPa, and is a hydrogen amount CH (ppm by mass) calculated from formula (1) when the tensile strength TS of the steel material is 500 MPa or more. (1): CH (ppm by mass) = 5.4637e-0.004 × TS (MPa) In formula (1), TS (MPa) is the tensile strength of the steel material.

Description

Steel material and manufacturing method thereof

[0001] The present invention relates to a steel material having excellent resistance to hydrogen absorption and a method for manufacturing the steel material.

[0002] Line pipes for natural gas transportation are an existing energy infrastructure. These line pipes have been required to suppress the occurrence of hydrogen-induced cracking in sour environments. Meanwhile, hydrogen has recently attracted significant attention worldwide as a clean energy source for building a decarbonized society. Therefore, with the aim of transporting large quantities of hydrogen gas, the construction of a hydrogen gas transportation network that pumps natural gas (natural gas partially mixed with hydrogen) and hydrogen gas (hydrogen-containing gas) is being considered. The expected pressure for transporting hydrogen gas to these line pipes is high, ranging from 1 to 40 MPa, and the line pipes will be placed in a high-pressure hydrogen gas environment. Line pipes used in such environments are susceptible to "hydrogen embrittlement," in which hydrogen penetrates the materials that make up the line pipe and deteriorates its properties. Therefore, line pipes used in hydrogen gas environments must have the hydrogen resistance required for such environments.

[0003] Austenitic stainless steels such as SUS316L, which are less susceptible to hydrogen embrittlement than low-alloy steels, have traditionally been used for steel structures used in high-pressure hydrogen gas environments. However, austenitic stainless steels such as SUS316L are expensive in material cost and have low strength, so when a structure is designed to withstand high hydrogen pressure, the wall thickness (plate thickness) becomes large, which increases the price of the structure itself. Therefore, there has been a strong demand for low-cost low-alloy steels that can withstand high-pressure hydrogen gas environments for use in hydrogen steel structures.

[0004] In response to such demands, for example, Patent Document 1 describes a steel for high-pressure hydrogen environments, which is a steel used in a high-pressure hydrogen environment, and by making the Ca / S ratio less than 1.5 or 11 or more, the diffusible hydrogen concentration ratio is reduced and embrittlement due to diffusible hydrogen is suppressed.

[0005] In Patent Document 2, by using a low-alloy high-strength steel adjusted to a specific component composition, it is said that in the air tensile strength range of 900 to 950 MPa, the reduction of area and elongation values ​​in a 45 MPa hydrogen atmosphere are greater than those of JIS G3128SHY685NS, and the steel has excellent resistance to embrittlement in a high-pressure hydrogen environment.

[0006] Furthermore, the low alloy steel described in Patent Document 3 is a Cr-Mo based high strength low alloy steel, which is tempered at a relatively high temperature of 560 to 580°C, and by adjusting the grain size after tempering to a grain size of 8.4 or more and the tensile strength to an extremely narrow range of 900 to 950 MPa, it is said to become a high strength low alloy steel that exhibits excellent elongation and drawing characteristics even in a 45 MPa hydrogen atmosphere and has excellent resistance to high pressure hydrogen environment embrittlement characteristics.

[0007] Furthermore, Patent Document 4 proposes a low-alloy steel for use in a high-pressure hydrogen gas environment. The low-alloy steel described in Patent Document 4 is said to have improved carbide morphology at grain boundaries and significantly improved resistance to hydrogen environment embrittlement by adding V, increasing the Mo content compared to existing steels, and increasing the tempering temperature to utilize V-Mo carbides.

[0008] Furthermore, Patent Document 5 proposes a steel for high-pressure hydrogen gas storage containers that has excellent hydrogen resistance. According to the technology described in Patent Document 5, when manufacturing a steel plate, long-term stress relief annealing is performed after normalizing treatment, whereby MC-based carbides (Mo, V)C are dispersed and precipitated finely and densely, improving the hydrogen resistance of the steel, such as hydrogen embrittlement resistance.

[0009] Furthermore, Patent Document 6 proposes a steel material for storing high-pressure hydrogen. The steel material described in Patent Document 6 is a steel material in which the metal structure is mainly composed of bainite with an area fraction of 90% or more, and cementite having an average grain size of 50 nm or less and an average aspect ratio of 3 or less is dispersed and precipitated in the bainite.

[0010] Non-Patent Document 1 describes the fatigue strength values ​​of low alloy steels.

[0011] JP 2005-2386 A JP 2009-46737 A JP 2009-275249 A JP 2009-74122 A JP 2010-37655 A JP 2012-107332 A

[0012] Matsunaga et al. , Int J Hydrogen Energy, Vol. 40 (2015), p. 5739-5748

[0013] The pressure inside a linepipe fluctuates due to operational conditions and periodic shutdowns, subjecting the linepipe to repeated stresses. Therefore, fatigue properties must be considered when designing steel structures such as linepipes. However, as shown in Non-Patent Document 1, fatigue life, one of the fatigue properties of materials, is known to be reduced in a hydrogen gas environment compared to that in air. In other words, if linepipe materials are designed based on conventional natural gas linepipes, their service life will be reduced. Furthermore, it is known that the amount of hydrogen absorption by the material in a hydrogen gas environment significantly affects fatigue properties in a hydrogen gas environment. Therefore, linepipe materials used in a hydrogen gas environment must have excellent hydrogen absorption resistance.

[0014] In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a steel material that has excellent resistance to hydrogen absorption in a hydrogen gas environment and excellent resistance to hydrogen embrittlement, and a method for manufacturing the same.

[0015] The present inventors conducted technical studies to determine the conditions that must be satisfied to obtain a steel material with excellent hydrogen absorption resistance, with the aim of suppressing hydrogen absorption into steel, which is the root cause of hydrogen embrittlement. As a result, they discovered that by controlling the ratio of the thickness of the slab before rolling to the thickness of the steel material after rolling in the hot rolling process and the conditions of the tempering process, the hydrogen absorption resistance of the steel material can be improved, resulting in excellent hydrogen embrittlement resistance, i.e., improved fatigue properties in a hydrogen gas environment. The present invention was made based on these findings. Specifically, the gist of the present invention is as follows. [1] In mass%, C: 0.10 to 0.45%, Si: 0.01 to 2.0%, Mn: 0.30 to 2.00%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.0050% or less, Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0%, Ti: 0 to 0.5%, Cu: 0 to 2.5%, Cr: 0 to 2.5%, Mo: 0 to 2.0%, W: 0 to 2.5%, V: 0 to 0.50%, Zr: 0 to 0.050%, B: 0 to 0.0050%, A steel material having a chemical composition containing REM: 0-0.05%, Mg: 0-0.05%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurities, wherein the amount of saturated hydrogen that has penetrated into the steel material in a hydrogen gas environment at a hydrogen gas pressure of 40 MPa is less than a predetermined value, where the predetermined value is 0.8 mass ppm when the tensile strength TS of the steel material is less than 500 MPa, or 0.8 mass ppm when the tensile strength TS of the steel material is 500 MPa or more, calculated from the following formula (1): Ｈ (ppm by mass), Ｈ(mass ppm) = 5.4637e - 0.004 × TS (MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material. [2] The steel material according to [1], wherein the metal structure at the center of the wall thickness of the steel material has an area fraction of retained austenite of 0 to 3%, and an area fraction of martensite of 90% or more. [3] The steel material according to [1] or [2], wherein the steel material is a seamless steel pipe. [4] In mass%, C: 0.10 to 0.45%, Si: 0.01 to 2.0%, Mn: 0.30 to 2.00%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.0050% or less, Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0%, Ti: 0 to 0.5%, Cu: 0 to 2.5%, Cr: 0 to 2.5%, Mo: 0 to 2.0%, W: 0 to 2.5%, V: 0 to 0.50%, Zr: 0 to 0.050%, B: 0 to 0.0050%, a casting step of casting molten steel having a composition containing REM: 0-0.05%, Mg: 0-0.05%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurities, at a casting speed of 0.1-1.8 m / min to obtain a slab; a heating step of heating the slab at a heating temperature of 1350°C or less; a hot rolling step of rolling the slab heated in the heating step under conditions where the rolling end temperature is 820°C or more and the ratio of the thickness of the slab to the thickness of the steel material after rolling is in the range of 5-35 to obtain a steel material; and ３ a holding step of holding the steel material at a holding temperature of 1000°C or higher and 800°C or lower; a first cooling step of cooling the steel material after the holding step to a cooling stop temperature of 50°C or lower under the conditions that a first average cooling rate from 800°C to 300°C is 10°C / s or higher at the center of the thickness of the steel material and a second average cooling rate from 300°C to 50°C is 5°C / s or lower at the center of the thickness of the steel material; １and a tempering step of performing tempering at a tempering temperature of not more than 10 minutes and less than 60 minutes after the first cooling step. [5] The method further includes a heat treatment step after the first cooling step and before the tempering step, wherein the heat treatment step comprises: ３ a second cooling step of cooling the steel material after the reheating step to a cooling stop temperature of 50°C or less under conditions of a first average cooling rate from 800°C to 300°C of 10°C / s or more at the center of the wall thickness of the steel material and a second average cooling rate from 300°C to 50°C of 5°C or less at the center of the wall thickness of the steel material. [6] A method of manufacturing a steel material according to [4] or [5], wherein the slab in the casting step is a billet and the steel material is a seamless steel pipe, and in the hot rolling step, the billet heated in the heating step is pierced and rolled to a seamless steel pipe under conditions of a rolling finish temperature of 820°C or more and a ratio of the billet radius to the wall thickness of the seamless steel pipe after rolling in a range of 5 to 35.

[0016] According to the present invention, it is possible to provide a steel material in which the amount of saturated hydrogen in a hydrogen gas environment is controlled, i.e., a steel material having excellent hydrogen absorption resistance and extremely improved hydrogen embrittlement resistance, and a method for manufacturing the same.

[0017] 1 is a diagram showing a compact tension (CT) test piece used in a fatigue crack propagation test.

[0018] 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.

[0019] First Embodiment A steel material in one embodiment of the present invention is a steel material having a specific component composition, and in which the amount of saturated hydrogen that penetrates in a hydrogen gas environment at a hydrogen gas pressure of 40 MPa is less than a predetermined value.

[0020] First, the steel material of the present invention will be described below. The reasons for limiting the composition of the steel material of the present invention will be described below. In the following description, all units shown in % are mass % unless otherwise specified.

[0021] Composition: C: 0.10-0.45%. C is an element necessary for increasing the strength of steel. Furthermore, solute C has the effect of suppressing hydrogen penetration into dislocations. Therefore, it is preferable to include C from the perspective of suppressing hydrogen absorption into steel. However, since the above effect is insufficient if the C content is less than 0.10%, the C content is set to 0.10% or more. The C content is preferably 0.15% or more, more preferably 0.20% or more, even more preferably 0.22% or more, and most preferably 0.25% or more. On the other hand, if the C content exceeds 0.45%, quench cracking may occur during quenching. Therefore, the C content is set to 0.45% or less. The C content is preferably 0.43% or less, more preferably 0.40% or less, even more preferably 0.38% or less, and most preferably 0.35% or less.

[0022] Si: 0.01 to 2.0% Si is known to retard the growth of carbides precipitated in steel and reduce the amount of hydrogen absorbed by the steel. However, if the Si content is less than 0.01%, the effect of suppressing carbide growth is insufficient. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.1% or more, more preferably 0.2% or more, even more preferably 0.3% or more, and most preferably 0.4% or more. On the other hand, since the above effect saturates even if the Si content exceeds 2.0%, the Si content is set to 2.0% or less. The Si content is preferably 1.0% or less, more preferably 0.8% or less, even more preferably 0.7% or less, and most preferably 0.6% or less.

[0023] Mn: 0.30 to 2.00% Mn is an element that effectively contributes to improving the strength and toughness of steel. However, if the Mn content is less than 0.30%, the above effects are poor, so the Mn content is set to 0.30% or more. The Mn content is preferably 0.50% or more, more preferably 0.60% or more, even more preferably 0.80% or more, and most preferably 1.00% or more. On the other hand, if the Mn content exceeds 2.00%, the hardness of the surface layer and central segregation part of the steel increases during the cooling process described below, increasing the amount of hydrogen absorption. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.80% or less, more preferably 1.50% or less, even more preferably 1.30% or less, and most preferably 1.20% or less.

[0024] P: 0.015% or less P is an element contained in steel as an unavoidable impurity. It reduces weldability, increases the hardness of the center segregation, and increases the amount of hydrogen absorption. Since the above tendency becomes more pronounced when the P content exceeds 0.015%, the P content is set to 0.015% or less. The P content is preferably 0.010% or less, more preferably 0.008% or less, even more preferably 0.005% or less, and most preferably 0.003% 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, excessive reduction of the P content increases refining costs, so the P content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0025] S: 0.0015% or less S is an element contained in steel as an unavoidable impurity, and in the steel, it forms MnS inclusions, increasing the amount of hydrogen absorption. Therefore, a low S content is preferable, but up to 0.0015% is acceptable. Therefore, the S content is set to 0.0015% or less. The S content is preferably 0.0013% or less, more preferably 0.0010% or less, even more preferably 0.0008% or less, and most preferably 0.0005% or less. On the other hand, since it is desirable to reduce S as much as possible, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction of the S content leads to an increase in refining costs, so the S content is preferably 0.0002% or more, more preferably 0.0003% or more, and even more preferably 0.0004% or more.

[0026] Al: 0.005 to 0.15% Al is an element contained as a deoxidizer. However, if the Al content is less than 0.005%, the effect of containing Al cannot be obtained. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.01% or more, more preferably 0.02% or more, even more preferably 0.03% 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. Therefore, the Al content is set to 0.15% or less. The Al content is preferably 0.12% or less, more preferably 0.10% or less, even more preferably 0.08% or less, and most preferably 0.05% or less.

[0027] O: 0.01% or less O causes the formation of oxide-based inclusions in steel, so the lower the content, the better. This effect is not a problem if the O content is 0.01% or less. Therefore, the O content is set to 0.01% or less. The O content is preferably 0.008% or less, more preferably 0.005% or less, and even more preferably less than 0.003%. On the other hand, since it is desirable to reduce O as much as possible, the lower limit of the O content is particularly limited and may be 0%. However, since excessive reduction of the O content increases refining costs, the O content is preferably 0.001% or more.

[0028] N: 0.0050% or less The effect of N on the fatigue properties of steel is small, and an N content of 0.0050% or less does not impair the effects of the present invention. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0045% or less, more preferably 0.0040% or less, even more preferably 0.0035% or less, even more preferably 0.0030% or less, and most preferably 0.0025% or less. On the other hand, from the viewpoint of improving toughness, it is desirable to reduce the N content as much as possible, so the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction of the N content increases the steelmaking cost, so the N content is preferably 0.0010% or more, more preferably 0.0015% or more.

[0029] 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 impurity elements.

[0030] Here, the term "unavoidable impurities" refers to impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing facilities, etc., and are allowed to be present to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include Zn, Pb, As, Bi, etc.

[0031] Furthermore, the component composition of the steel material in another embodiment of the present invention may further contain one or more of the elements listed below. Note that the inclusion of these elements is not essential in the present invention. Therefore, the lower limit of the content of each element may be 0%.

[0032] Nb: 0 to 0.10% Nb is an element that improves hardenability and precipitates as fine precipitates of Nb-based carbonitrides, pinning austenite grains during heating and suppressing grain coarsening. However, an Nb content exceeding 0.10% may result in the precipitation of coarse Nb-based carbonitrides, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.10% or less. The Nb content is preferably 0.09% or less, more preferably 0.08% or less, even more preferably 0.07% or less, and most preferably 0.06% or less. On the other hand, since the inclusion of Nb is not essential, the lower limit of the Nb content may be 0%. However, to obtain the above effects, the Nb content is preferably 0.005% or more, more preferably 0.01% or more, even more preferably 0.02% or more, and most preferably 0.03% or more.

[0033] Ca: 0 to 0.005% Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions. However, if the Ca content exceeds 0.005%, not only does the above effect saturate, but the cleanliness of the steel decreases, thereby reducing HIC resistance. Therefore, when Ca is contained, the Ca content is set to 0.005% or less. The Ca content is preferably 0.004% or less, more preferably 0.003% or less, even more preferably 0.002% or less, and most preferably 0.001% 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 0.0005% or more.

[0034] Ni: 0 to 2.0% Ni is an element effective in improving the toughness and strength of steel. However, if the Ni content exceeds 2.0%, microcracks known as Fischer cracks are likely to occur in environments with low hydrogen sulfide partial pressures of less than 1 bar. Therefore, when Ni is contained, the Ni content is set to 2.0% or less. The Ni content is preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.8% or less, and most preferably 0.5% 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 0.0005% or more, more preferably 0.01% or more, even more preferably 0.05% or more, and most preferably 0.1% or more.

[0035] Ti: 0 to 0.5% Ti is an element that improves the hardenability of steel and precipitates as fine precipitates of Ti-based carbonitrides, pinning austenite grains and inhibiting their growth during heating. However, if the Ti content exceeds 0.5%, coarse, angular nitrides are more likely to form, reducing toughness. Therefore, when Ti is contained, the Ti content is limited to 0.5% or less. The Ti content is preferably 0.4% or less, more preferably 0.3% or less, and even more preferably 0.2% 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 0.01% or more, and even more preferably 0.05% or more.

[0036] Cu: 0 to 2.5% Cu is an element effective in improving the toughness and strength of steel. However, if the Cu content exceeds 2.5%, weldability decreases. Therefore, when Cu is contained, the Cu content is set to 2.5% or less. The Cu content is preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.2% or less, and most preferably 1.0% 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 effect, the Cu content is preferably 0.05% or more, more preferably 0.06% or more, even more preferably 0.07% or more, and most preferably 0.08% or more.

[0037] Cr: 0 to 2.5% Like Mn, Cr is an effective element for obtaining sufficient strength even when the C content is low. However, if the Cr content exceeds 2.5%, the hardenability becomes excessive, resulting in reduced SSCC resistance. Furthermore, weldability also deteriorates. Therefore, if Cr is contained, the Cr content is set to 2.5% or less. The Cr content is preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.2% or less, and most preferably 1.0% or less. On the other hand, since the inclusion of Cr is not essential, the lower limit of the Cr content may be 0%. However, to obtain the above effect, the Cr content is preferably 0.05% or more, more preferably 0.1% or more, even more preferably 0.3% or more, and most preferably 0.8% or more.

[0038] Mo: 0 to 2.0% Mo is an element that is effective in improving the toughness and strength of steel materials and in ameliorating the deterioration of fatigue properties due to hydrogen. However, if the Mo content exceeds 2.0%, the hardenability becomes excessive, resulting in a decrease in SSCC resistance. Furthermore, weldability also decreases. Therefore, when Mo is contained, the Mo content is set to 2.0% or less. The Mo content is preferably 1.8% or less, more preferably 1.5% or less, even more preferably 1.2% or less, and most preferably 1.0% 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 obtain the above effects, the Mo content is preferably 0.01% or more, more preferably 0.1% or more, even more preferably 0.5% or more, and most preferably 0.8% or more.

[0039] W: 0 to 2.5% W is an element that contributes to increasing the strength of steel. However, if the W content exceeds 2.5%, the above effect saturates, resulting in increased costs. Therefore, if W is contained, the W content is set to 2.5% or less. The W content is preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.0% or less, and most preferably 0.8% 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 0.0001% or more, more preferably 0.001% or more, even more preferably 0.01% or more, and most preferably 0.1% or more.

[0040] V: 0 to 0.50% V is an element that has the effect of increasing the strength and toughness of steel. However, if the V content exceeds 0.50%, the toughness of the weld decreases. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is preferably 0.40% or less, more preferably 0.30% or less, even more preferably 0.20% or less, and most preferably 0.10% 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 0.005% or more, more preferably 0.05% or more.

[0041] Zr: 0 to 0.050% Zr is an element that has the effect of increasing the toughness of steel materials through grain refinement and improving crack resistance through control of inclusion properties. However, if the Zr content exceeds 0.050%, the above effects saturate. Therefore, when Zr is contained, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% 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 obtain the above effects, the Zr content is preferably 0.005% or more, more preferably 0.010% or more.

[0042] B: 0 to 0.0050% 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 steel. However, if the B content exceeds 0.0050%, the above effects saturate, resulting in increased costs. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0040% or less, more preferably 0.0035% or less, even more preferably 0.0030% or less, and most preferably 0.0025% 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 more preferably 0.0001% or more, more preferably 0.0005% or more, and most preferably 0.0010% or more.

[0043] REM: 0-0.05%, Mg: 0-0.05% REM and Mg are elements that enhance the toughness of steel by refining crystal grains and improve crack resistance by controlling the properties of inclusions. However, if the REM and Mg content exceeds 0.05%, the above effects saturate. Therefore, when REM and Mg are contained, the REM content and Mg content are each set to 0.05% or less. The REM content and Mg content are each preferably 0.04% or less, more preferably 0.03% or less, even more preferably 0.02% or less, and most preferably 0.01% or less. On the other hand, since the inclusion of REM and Mg is not essential, the lower limit of the REM content and the lower limit of the Mg content may each be 0%. However, in order to obtain the above effects, the REM content and Mg content are each preferably 0.0005% or more, more preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.008% or more. Note that REM is an abbreviation for Rare Earth Metal, and refers to rare earth metals.

[0044] Hf: 0-0.2%, Ta: 0-0.2% Hf and Ta are elements that contribute to increasing the strength of steel. However, if the Hf content and Ta content exceed 0.2%, the above effects saturate, resulting in increased costs. Therefore, when Hf and Ta are contained, the Hf content and Ta content are each set to 0.2% or less. The Hf content and Ta content are each preferably set to 0.1% or less. On the other hand, since the inclusion of Hf and Ta is not essential, the lower limit of the Hf content and the lower limit of the Ta content may each be 0%. However, to obtain the above effects, the Hf content and Ta content are each preferably set to 0.001% or more.

[0045] Re: 0 to 0.005% Re is an element that contributes to increasing the strength of steel. However, if the Re content exceeds 0.005%, the amount of Re oxide increases, and if it aggregates, the hydrogen absorption resistance decreases. Therefore, when Re is contained, the Re content is set to 0.005% or less. The Re content is preferably 0.004% or less. On the other hand, since the inclusion of Re is not essential, the lower limit of the Re content may be 0%. However, to obtain the above effect, the Re content is preferably 0.0001% or more, and more preferably 0.001% or more.

[0046] Sn: 0-0.3%, Sb: 0-0.3% Sn and Sb are elements that contribute to increasing the strength and hardenability of steel. However, if the Sn content and Sb content exceed 0.3%, the above effects saturate, resulting in increased costs. Therefore, when Sn and Sb are contained, the Sn content and Sb content are each set to 0.3% or less. The Sn content and Sb content are each preferably 0.2% or less, and more preferably 0.1% or less. To reduce costs, the Sn content and Sb content are each more preferably 0.01% or less. On the other hand, since the inclusion of Sn and Sb is not essential, the lower limit of the Sn content and the lower limit of the Sb content may be 0%. However, to obtain the above effects, the Sn content and Sb content are each preferably 0.0001% or more, and more preferably 0.001% or more.

[0047] The metal structure of the steel material of the present invention will be described below.

[0048] The metallographic structure is not particularly limited as long as it achieves a saturated hydrogen content less than the value described below in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa and provides excellent hydrogen embrittlement resistance (excellent fatigue properties in a hydrogen gas environment). Here, fatigue cracks in steel in a hydrogen gas environment initiate from the surface of the steel, which is directly exposed to hydrogen gas, and propagate into the steel. In the case of a steel pipe, fatigue cracks initiate from the inner surface of the steel pipe and propagate into the steel pipe. Therefore, it is considered preferable to appropriately control the metallographic structure from the steel surface (inner surface of the steel pipe) to the steel interior. Meanwhile, in manufacturing steel, the interior of the steel, particularly the center of the steel wall thickness, is less likely to heat and cool than the surface. Therefore, it is considered possible to achieve the desired metallographic structure from the steel surface to the center of the wall thickness by achieving a desired metallographic structure in the center of the steel wall thickness. Based on the above considerations, the relationship between the metallographic structure in the center of the steel wall thickness and the saturated hydrogen content and hydrogen embrittlement resistance of the steel was evaluated. As a result, it was found that, as an example, the above-mentioned saturated hydrogen amount can be achieved and excellent hydrogen embrittlement resistance can be obtained by adjusting the metal structure at the center of the wall thickness to have an area fraction of retained austenite of 0 to 3% and an area fraction of martensite of 90% or more.

[0049] Retained austenite area fraction: 0 to 3%. Retained austenite remains in the metal structure of the steel, acting as a hydrogen trap site and increasing the amount of saturated hydrogen in a hydrogen gas environment, i.e., significantly reducing the hydrogen absorption resistance of the steel. Furthermore, when the steel is used in a steel structure, the retained austenite transforms into martensite due to stress load during use, resulting in extremely hard fresh martensite with accumulated hydrogen. As a result, it may become a source or propagation path for fatigue cracks. Therefore, in the present invention, the retained austenite area fraction is preferably 3% or less, more preferably 2% or less, and even more preferably 1% or less. On the other hand, the retained austenite area fraction may be 0%. The area fraction of retained austenite may be measured using the method described in the examples.

[0050] Martensite area fraction: 90% or more To achieve high strength with a tensile strength of 520 MPa or more, the metal structure is preferably martensite. Furthermore, when a soft phase and a hard phase are mixed in a steel material, hydrogen that penetrates in a hydrogen gas environment is preferentially accumulated in the hard phase. As a result, fatigue cracks are more likely to initiate and propagate in the hard phase. Therefore, it is preferable that the metal structure be a single structure. That is, the area fraction of martensite is preferably 90% or more, more preferably 93% or more, and even more preferably 95% or more. On the other hand, the upper limit of the area fraction of martensite is not particularly limited, but is preferably 100% and may be 98% or less. Here, martensite includes tempered martensite. The area fraction of martensite may be measured by the method described in the examples.

[0051] Furthermore, tempering martensite can precipitate carbides such as cementite in the martensite. Fine precipitation of carbides in martensite inhibits the straightness of the fatigue crack propagation path in hydrogen, thereby reducing the fatigue crack growth rate. Therefore, the martensite is preferably tempered martensite. Furthermore, it is preferable to disperse and precipitate fine carbides in the martensite. Here, the average size of the carbides is preferably 1 μm or less, more preferably 0.5 μm or less, even more preferably 0.3 μm or less, and most preferably 0.1 μm or less. On the other hand, the lower limit of the carbide size is not particularly limited, but may be 0.01 μm or more on average. The carbide size X is determined by the following formula, assuming that the carbide has an elliptical shape, with the long side of the carbide as a and the short side as b:

[0052]

[0053] Other Structures The metal structure in one embodiment of the present invention may consist of the above-described metal structure. Furthermore, the metal structure in other embodiments of the present invention may contain, in addition to the above-described metal structure, another metal structure at an area fraction of 10% 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 bainite, ferrite, and pearlite. If the area fraction of the other metal structure exceeds 10%, it is not possible to ensure an area fraction of martensite of 90% or more, as described above, and the hydrogen absorption resistance deteriorates. Therefore, the area fraction of the other metal structure is set to 10% or less. The area fraction of the other metal structure is preferably set to 7% or less, more preferably 5% or less, and even more preferably 2% 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%.

[0054] Dislocation density Practical metals contain many dislocations. The amount of dislocations is expressed as 1m ３ Dislocation density (m ―２ Since dislocations can act as hydrogen traps, it is preferable to reduce the dislocation density in order to reduce the saturated hydrogen content of steel in a hydrogen gas environment. In particular, when the dislocation density of the steel is 5.0 × 10 １５ m －２ The hydrogen absorption resistance of the steel is further improved by the dislocation density being 5.0 × 10 or less. As a result, the fatigue properties in hydrogen are further improved. In other words, the hydrogen embrittlement resistance is further improved. １５ m －２ Preferably, it is 3.0 × 10 or less. １５ m －２ More preferably, it is 2.0 × 10 or less. １５ m －２ More preferably, it is 1.0 × 10 or less. １５ m －２ On the other hand, it is desirable that the lower limit of the dislocation density is low, and although there is no particular limitation, it is preferably 5.0 × 10 １３ m－２ or more, 5.0 × 10 １４ m －２ The dislocation density may be measured by the method described in the examples.

[0055] The steel material of the present invention has an amount of saturated hydrogen that penetrates into the steel material in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa, and the amount of saturated hydrogen that penetrates into the steel material is less than a predetermined value. Here, the predetermined value is 0.8 mass ppm when the tensile strength TS of the steel material is less than 500 MPa, and is the amount of hydrogen C obtained by the following formula (1) when the tensile strength TS of the steel material is 500 MPa or more. Ｈ (mass ppm). Ｈ (mass ppm)=5.4637e-0.004×TS(MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material.

[0056] The above-mentioned limitation of the saturated hydrogen content is the most important factor in the present invention. Hydrogen embrittlement of steel is fundamentally caused by the absorption of hydrogen by the steel. That is, hydrogen accumulates at hydrogen trap sites such as precipitates in the metal structure, corrosion pits caused by sulfide stress corrosion cracking (SSCC), and stress concentration sources such as initial defects in the steel, and promotes the propagation of cracks (fatigue cracks) generated by stress load, resulting in fracture. The inventors have conducted a detailed study on the relationship between the TS of steel and its fatigue crack growth rate in a hydrogen gas environment (hydrogen fatigue crack growth rate), and have found that the hydrogen fatigue crack growth rate can be reduced (i.e., hydrogen embrittlement resistance can be improved) by setting the saturated hydrogen content of the steel below a predetermined value. That is, the saturated hydrogen content of the steel material that can obtain excellent hydrogen embrittlement resistance is less than 0.8 ppm when the TS of the steel material is less than 500 MPa, and is less than 0.8 ppm when the TS of the steel material is 500 MPa or more, as expressed by the formula (1). ＨIt has been found that it is necessary that the saturated hydrogen content be less than (ppm by mass). Therefore, in the steel material of the present invention, the amount of saturated hydrogen that penetrates in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa is set to be less than the above-mentioned predetermined value. Preferably, when the tensile strength TS of the steel material is less than 500 MPa, it is preferably set to 0.7 ppm by mass or less, and more preferably set to 0.5 ppm by mass or less. Furthermore, when the tensile strength TS of the steel material is 500 MPa or more, it is necessary that the saturated hydrogen content be less than (ppm by mass). Ｈ -(C Ｈ × 3%)) ppm by mass or less, and Ｈ -(C Ｈ × 5%)) mass ppm or less. On the other hand, since a low saturated hydrogen content is desirable, the lower limit of the saturated hydrogen content is not particularly limited regardless of TS, and is preferably 0 ppm, and may be 0.001 ppm or more. The saturated hydrogen content may be measured by the method described in the Examples. The hydrogen absorption resistance of the present invention can be evaluated as excellent when the saturated hydrogen content is less than the predetermined value.

[0057] Tensile Strength (TS) The tensile strength TS (MPa) of the steel material is not particularly limited, but when steel structures such as line pipes used in a high-pressure hydrogen gas environment are assumed, it is preferably 460 MPa or more, more preferably 500 MPa or more, and even more preferably 520 MPa or more. Furthermore, although there is no particular upper limit for TS, excessively high strength tends to increase costs and reduce hydrogen embrittlement resistance. Therefore, TS is preferably 1500 MPa or less, more preferably 1300 MPa or less, and even more preferably 1000 MPa or less. The tensile strength may be evaluated by the method described in the examples.

[0058] Hydrogen Embrittlement Resistance In the present invention, hydrogen embrittlement resistance is evaluated using fatigue properties in hydrogen. The fatigue properties in hydrogen (fatigue crack growth properties in hydrogen) are evaluated by the following procedure. First, a fatigue crack growth test is performed in a hydrogen gas environment at room temperature (20±10°C) and a pressure of 40 MPa, in accordance with ASTM E647 Standard test method for measurement of fatigue crack growth rate, under the following conditions: frequency: 1 Hz, repeated load waveform: sine wave, control method: load control, loading conditions: uniaxial tension, and stress ratio: R = 0.1. Here, for the fatigue crack growth test, a compact tension (CT) test piece used in a fatigue crack growth test in accordance with ASTM E647 may be used. Next, the fatigue crack growth rate da / dN (m / cycle) in hydrogen in the stress intensity factor range ΔK = 30 MPa was calculated, and the obtained da / dN was 1.5 × 10 －６ When the da / dN is 1.5×10 m / cycle or less, the specimen is judged to have excellent fatigue crack growth characteristics in hydrogen, i.e., excellent resistance to hydrogen embrittlement. －６ If da / dN is 1.0 × 10 m / cycle or less, it becomes possible to design long-life steel structures for hydrogen use, such as line pipes, within the range of wall thickness that can be manufactured using the process for manufacturing steel materials, particularly seamless steel pipes. －６ m / cycle or less is preferable, and 9.0 × 10 －７ On the other hand, the lower the da / dN, the more preferable it is. The lower limit of da / dN is not particularly limited, but it is preferably 8.0 × 10 －８ It may be m / cycle or more.

[0059] Steel Materials The steel materials of the present invention include thin steel plates, thick steel plates, steel pipes (seamless steel pipes, welded steel pipes), shaped steel, steel bars, and the like. The steel materials have the above-described chemical composition and a saturated hydrogen content less than a predetermined value (excellent hydrogen absorption resistance), thereby improving hydrogen embrittlement resistance in hydrogen gas environments. Therefore, the steel materials can be suitably applied to steel structures used in hydrogen gas environments, particularly high-pressure hydrogen gas environments. In particular, when the steel materials are seamless steel pipes, they can be suitably applied to line pipes for hydrogen gas, so the steel materials are preferably seamless steel pipes. Here, the high-pressure hydrogen gas environment is a hydrogen gas environment containing hydrogen at a hydrogen partial pressure of 1 MPa or more.

[0060] Manufacturing Method Next, a method for manufacturing a steel material according to one embodiment of the present invention will be described. The steel material of the present invention can be manufactured by sequentially performing the following steps (1) to (6): (1) a casting step in which molten steel having a predetermined chemical composition is cast to form a cast slab; (2) a heating step in which the cast slab (which may be a billet or slab) obtained in the casting step is heated; (3) a hot rolling step in which the cast slab obtained in the heating step is rolled to form a steel material; (4) a holding step in which the steel material obtained in the hot rolling step is held; (5) a first cooling step in which the steel material obtained in the hot rolling step is cooled; and (6) a tempering step in which the steel material obtained in the cooling step is tempered. It is more preferable to perform a heat treatment step in which the steel is reheated and cooled after the first cooling step (5) and before the tempering step (6).

[0061] Each step will be described below. In the following description, the temperature refers to the temperature at the center of the thickness of the slab or steel material unless otherwise specified. The temperature at the center of the thickness of the steel material is a temperature estimated from the temperature of the steel material surface measured with a radiation thermometer using a heat transfer calculation or the like that takes into account the heat transfer coefficient of the steel material.

[0062] Casting Step In the casting step, the molten steel having the above-mentioned composition is cast at a casting speed of 0.1 to 1.8 m / min to produce a cast piece.

[0063] Molten steel: The molten steel having the above-mentioned composition may be produced by melting in a conventional manner, such as by using a converter, an electric furnace, or an induction furnace.

[0064] Casting speed: 0.1 m / min (min) or more and 1.8 m / min (min) or less. If the casting speed when casting molten steel is too fast, the number of inclusions in the slab increases, and the saturated hydrogen content (sometimes simply referred to as saturated hydrogen content) of the finally obtained steel in a hydrogen gas environment increases. Therefore, the casting speed is set to 1.8 m / min or less, and 1.5 m / min or less is preferable. On the other hand, a slower casting speed can reduce the number of inclusions in the slab, thereby reducing the saturated hydrogen content. To achieve this effect, the casting speed is more preferably set to 1.0 m / min or less. On the other hand, from the viewpoint of production efficiency, the lower limit of the casting speed is set to 0.1 m / min or more. The casting speed is preferably set to 0.2 m / min or more, and more preferably 0.3 m / min or more. The slab obtained in the casting process may be used as a steel slab.

[0065] The cast piece is not particularly limited, but may be, for example, a slab or billet obtained by a normal continuous casting method. From the viewpoint of productivity, the cast piece is preferably produced by a continuous casting method, but may also be produced by an ingot casting method.

[0066] Heating Step In the heating step, the slab is heated at a heating temperature of 1350°C or less.

[0067] Heating temperature: 1350°C or less If the heating temperature in the heating process exceeds 1350°C, the average grain size of prior austenite grains becomes excessively large, resulting in a deterioration in various properties of the final steel, such as hydrogen embrittlement resistance. Regarding hydrogen embrittlement resistance, when the saturated hydrogen content of the steel remains the same, as the average grain size of prior austenite grains increases, the hydrogen concentration accumulated at the grain boundaries increases. This increases the likelihood of fatigue cracks initiating at the grain boundaries. Furthermore, coarsening of prior austenite grains increases the likelihood of fatigue crack propagation. Therefore, the heating temperature is set to 1350°C or less. The heating temperature is preferably set to 1300°C or less, more preferably 1250°C or less, even more preferably 1230°C or less, and most preferably 1200°C or less. On the other hand, a lower heating temperature reduces the number of precipitates, thereby reducing the saturated hydrogen content of the steel. However, if the heating temperature is too low, the rolling temperature in the hot rolling process decreases, making rolling difficult. Therefore, the heating temperature is preferably 950°C or higher, more preferably 1000°C or higher, even more preferably 1050°C or higher, and most preferably 1100°C or higher. The heating time is not particularly limited, but if the heating time is too long, the number of precipitates in the final steel increases, increasing the risk of an increase in the saturated hydrogen content. Furthermore, the prior austenite grains become coarse, increasing the risk of a decrease in hydrogen embrittlement resistance. Therefore, the heating time is preferably 180 minutes or shorter, more preferably 150 minutes or shorter, even more preferably 135 minutes or shorter, and most preferably 120 minutes or shorter. On the other hand, the lower limit of the heating time is not particularly limited, but the heating time is preferably 30 minutes or longer, more preferably 45 minutes or longer, even more preferably 60 minutes or longer, and most preferably 75 minutes or longer.

[0068] Hot Rolling Step The slab heated in the heating step is rolled under the conditions described below to produce a steel material. A normal hot rolling mill can be used for rolling in the hot rolling step.

[0069] Rolling end temperature: 820°C or higher If the rolling end temperature is lower than 820°C, the rolling load becomes excessive, increasing the risk of rolling trouble. Therefore, the rolling end temperature is set to 820°C or higher. The rolling end temperature is preferably set to 850°C or higher, more preferably 870°C or higher, even more preferably 900°C or higher, and most preferably 920°C or higher. On the other hand, the upper limit of the rolling end temperature is not particularly limited. However, if the rolling end temperature is too high, the metal structure of the finally obtained steel material is likely to be non-uniform. Therefore, the rolling end temperature is preferably set to 1200°C or lower, more preferably 1100°C or lower, even more preferably 1000°C or lower, and most preferably 980°C or lower.

[0070] The ratio of the thickness of the slab to the thickness of the steel after rolling is 5 to 35. When rolling steel from a slab, excessive rolling increases strain in the steel, increasing dislocation density and causing an increase in the saturated hydrogen content of the steel. Therefore, it is necessary to control the ratio of the thickness of the slab to the thickness of the steel after rolling. If the ratio of the thickness of the slab to the thickness of the steel after rolling is less than 5, the steel will be too thick and difficult to manufacture. Therefore, the ratio is set to 5 or more. The ratio is preferably set to 7 or more, more preferably 9 or more, even more preferably 10 or more, and most preferably 12 or more. On the other hand, if the ratio exceeds 35, strain in the steel due to rolling increases, increasing the saturated hydrogen content of the steel. Therefore, the ratio is set to 35 or less. The ratio is preferably set to 25 or less, more preferably 20 or less, even more preferably 18 or less, and most preferably 15 or less. The ratio of the thickness of the slab to the thickness of the steel material after rolling can be calculated by the following formula: Ratio of the thickness of the slab to the thickness of the steel material after rolling = Thickness of the slab (mm) / Thickness of the steel material after rolling (mm)

[0071] Holding Step In the holding step, the steel material after the hot rolling step is held at a holding temperature of not less than the Ac3 point and not more than 1000°C.

[0072] Holding temperature: Ac ３ Point or more and 1000 ° C or less Holding temperature is Ac ３If the holding temperature is less than this point, ferrite will remain in the steel material after the first cooling step described below, and the strength and fatigue properties of the steel material in hydrogen will decrease. ３ The holding temperature is Ac ３ It is preferable to set the temperature at or above the temperature point +30°C, ３ It is more preferable to set the temperature at the temperature above +50°C, ３ It is more preferable to set the holding temperature at 80°C or higher than the temperature at which the steel is heated. On the other hand, if the holding temperature is higher than 1000°C, the austenite grains of the steel material will become coarse, which may cause a decrease in the impact absorption energy value and toughness of the steel material after the tempering process described below. Furthermore, if the austenite grains become coarse when the saturated hydrogen content of the steel material is the same, the hydrogen concentration accumulated at the grain boundaries will increase, making fatigue cracks more likely to occur. Furthermore, crack propagation will also be more likely to progress. Therefore, the holding temperature is set to 1000°C or lower. The holding temperature is preferably set to 980°C or lower, more preferably 950°C or lower, and even more preferably 930°C or lower. However, Ac ３ For steel materials with a temperature above 930°C, the holding temperature is ３ It is preferred that the holding temperature be equal to or higher than this point. In this holding step, if the temperature of the steel material after the hot rolling process satisfies the holding temperature, the steel material may be held at the holding temperature without being heated, and then cooled under the cooling conditions described below. Alternatively, the steel material may be reheated to the holding temperature after the hot rolling process, held at the holding temperature, and then cooled under the cooling conditions described below. The holding time at the holding temperature is not particularly limited, but is preferably held at the holding temperature for 10 minutes or more, more preferably 15 minutes or more, and even more preferably 20 minutes or more. Furthermore, the upper limit of the holding time is not particularly limited, but is preferably 60 minutes or less, more preferably 50 minutes or less, and even more preferably 40 minutes or less.

[0073] In the present invention, Ac ３ The score is calculated using the following formula: ３ (℃)=910-203[C] １／２-30[Mn] + 44.7[Si] + 700[P] + 100[Al] + 31.5[Mo] - 11[Cr] - 15.2[Ni] - 20[Cu] + 104[V] where [M] represents the content (mass%) of element M.

[0074] First Cooling Step: The steel material after the holding step is cooled under the following cooling conditions (first average cooling rate, second average cooling rate, cooling stop temperature).

[0075] First average cooling rate: 10°C / s or more The steel material after the holding step is cooled at a first average cooling rate. The first average cooling rate is the average cooling rate from 800°C to 300°C at the center of the steel material's thickness, and is 10°C / s or more. If the first average cooling rate is less than 10°C / s, the precipitated carbides will coarsen, resulting in a deterioration in fatigue properties. As a result, fatigue properties in hydrogen will also deteriorate. Furthermore, if the first average cooling rate is less than 10°C / s, it will be difficult to obtain a martensite area fraction of 90% or more. For this reason, the first average cooling rate is 10°C / s or more. From the viewpoint of suppressing variations in the metal structure, the first average cooling rate is preferably 15°C / s or more, more preferably 17°C / s or more, even more preferably 20°C / s or more, and most preferably 22°C / s or more. On the other hand, the upper limit of the first average cooling rate is not particularly limited, but the first average cooling rate is preferably 50°C / s or less, more preferably 45°C / s or less, and even more preferably 40°C / s or less.

[0076] Second average cooling rate: 5°C / s or less The steel material cooled at the first average cooling rate is then cooled at the second average cooling rate. The second average cooling rate is the average cooling rate from 300°C to 50°C at the center of the steel material's thickness, and is 5°C / s or less. By setting the second average cooling rate to 5°C / s or less, it is possible to promote the diffusion of elements in the metal structure and reduce inclusions by not performing cooling more than necessary. As a result, the saturated hydrogen content of the steel material can be reduced. For this reason, the second average cooling rate is set to 5°C / s or less. The second average cooling rate is preferably 3°C / s or less, more preferably 2°C / s or less, and even more preferably 1°C / s or less. There is no particular lower limit to the second average cooling rate, but the second average cooling rate is preferably 0.1°C / s or more, more preferably 0.3°C / s or more, and even more preferably 0.5°C / s or more.

[0077] Cooling stop temperature: 50°C or less If the cooling stop temperature exceeds 50°C, the precipitated carbides will coarsen, resulting in a decrease in fatigue properties and a decrease in fatigue properties in hydrogen. Furthermore, the transformation will not be completed, making it impossible to obtain the desired metal structure after tempering. For this reason, the cooling stop temperature is set to 50°C or less. The cooling stop temperature is preferably set to 45°C or less, and more preferably to 40°C or less. The lower limit of the cooling stop temperature is not particularly limited, but may be room temperature or higher. Here, room temperature refers to 20±10°C. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination. For example, it is preferable to use water cooling or oil cooling from 800°C to 300°C, and air cooling from 300°C to 50°C.

[0078] Tempering Step In the tempering step, the steel material obtained in the cooling step is tempered under the following conditions (tempering temperature, tempering time).

[0079] Tempering temperature: 400°C or higher Ac １The tempering temperature is set to 400°C or higher. By setting the tempering temperature to 400°C or higher, it is possible to alleviate the strain in the steel material that occurs during cooling in the cooling step, and the dislocation density can be significantly reduced. As a result, the saturated hydrogen content of the steel material can be reduced. The tempering temperature is preferably set to 450°C or higher, more preferably set to 500°C or higher, even more preferably set to 550°C or higher, and most preferably set to 600°C or higher. On the other hand, when the tempering temperature is set to Ac １ If the tempering temperature exceeds this point, the amount of austenite increases and the amount of saturated hydrogen in the steel may increase. １ The tempering temperature is Ac １ It is preferable that the temperature is set to -10°C or lower. １ It is more preferable that the temperature is -30°C or lower. １ It is more preferable that the temperature is -50°C or lower. １ The most preferable temperature is -80°C or lower.

[0080] Tempering time: 10 minutes or more and less than 60 minutes after reaching the tempering temperature The tempering time is 10 minutes or more and less than 60 minutes after reaching the tempering temperature. If the tempering time is too short, the dislocation density will not be sufficiently reduced, and the saturated hydrogen content of the steel will not be reduced. Therefore, the tempering time is 10 minutes or more. The tempering time is preferably 15 minutes or more, more preferably 20 minutes or more, even more preferably 25 minutes or more, and most preferably 30 minutes or more. On the other hand, if the tempering time is too long, the precipitates will become coarse, so the tempering time is less than 60 minutes. The tempering time is preferably 55 minutes or less, more preferably 50 minutes or less, even more preferably 45 minutes or less, and most preferably 40 minutes or less.

[0081] In the present invention, Ac １ There is no particular rule about how to find the points, but for example, １ (°C) = 723 - 14Mn + 22Si - 14.4Ni + 23.3Cr. In the above formula, each element symbol represents the content (mass%) of each element in the steel material, and elements that are not contained are represented as 0.

[0082] In another embodiment of the present invention, in order to improve the toughness of the steel material and further improve its hydrogen embrittlement resistance, a heat treatment process is carried out after the first cooling process and before the tempering process. The heat treatment process includes a reheating process in which the steel material obtained in the first cooling process is reheated, and a second cooling process.

[0083] Reheating step: In the reheating step, the steel material after the first cooling step is cooled by Ac ３ The material is reheated to a reheating temperature of 1000°C or higher.

[0084] Reheating temperature: Ac ３ In the heat treatment process, the steel material after the first cooling process is cooled to 1000°C or less. ３ By reheating to the Ac point or higher, it is possible to reduce the amount of untransformed austenite remaining in the steel material after the first cooling step. ３ The reheating temperature is Ac ３ It is preferable to set the temperature at or above the temperature point +10°C, ３ It is more preferable to set the temperature at the temperature above +30°C, ３ It is more preferable that the temperature is set to 50°C or higher than the temperature at which the temperature is reached. ３ The reheating temperature is most preferably 80°C or higher than the initial austenite grain size. On the other hand, when a heat treatment step is performed to prevent the initial austenite grains from becoming coarse and to improve production efficiency, the reheating temperature is 1000°C or lower. The reheating temperature is preferably 980°C or lower, more preferably 950°C or lower, and even more preferably 930°C or lower. However, Ac ３ In the case of steel materials where the temperature difference +30°C is over 1000°C, the reheating temperature should be set to 1000°C or less. ３ Even in the case of steel materials where the temperature at the point +50°C exceeds 1000°C, it is preferred to set the reheating temperature to 1000°C or less.

[0085] In the heat treatment process, by subjecting the steel material after the reheating process to a second cooling process, it is possible to refine the prior austenite grains of the finally obtained steel material and to precipitate fine and dispersed carbides. As a result, further improvement in hydrogen embrittlement resistance can be achieved. The first average cooling rate, second average cooling rate, and cooling stop temperature in the second cooling process are the same as those in the first cooling process described above.

[0086] Second Embodiment Hereinafter, a specific description will be given of the case where the steel material of the present invention is a seamless steel pipe. The chemical composition, metal structure, saturated hydrogen content, and properties of the seamless steel pipe are the same as those described for the steel material. Meanwhile, in the method for manufacturing a seamless steel pipe, the processes other than the hot rolling process (casting process, heating process, holding process, cooling process, tempering process, and heat treatment process) are carried out in the same manner as those described for the steel material. However, when manufacturing a seamless steel pipe, a billet is used as the cast piece. Furthermore, the temperature at the center of the wall thickness of the seamless steel pipe is a temperature estimated from the temperature of the outer surface of the seamless steel pipe measured with a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the seamless steel pipe.

[0087] A method for manufacturing a seamless steel pipe will be described below. First, a billet, which is one of the cast pieces, is manufactured under the same conditions as in the casting process of the steel material described in the first embodiment. Next, the billet obtained in the casting process is heated under the same conditions as in the heating process of the steel material described in the first embodiment, and is subjected to the hot rolling process described below for manufacturing a seamless steel pipe, thereby manufacturing the seamless steel pipe. The obtained seamless steel pipe is subjected to a holding process, a first cooling process, and a tempering process under the same conditions as in the steel material described in the first embodiment, to obtain a final seamless steel pipe. Here, between the first cooling process and the tempering process, a heat treatment process under the same conditions as in the steel material described in the first embodiment may be performed.

[0088] In the hot rolling process, the billet heated in the heating process is pierced and rolled under the conditions described below to form a seamless steel pipe. Piercing and rolling in the hot rolling process can be performed by a conventional Mannesmann plug mill or Mannesmann mandrel mill.

[0089] Rolling end temperature: 820°C or higher If the rolling end temperature is lower than 820°C, the rolling load becomes excessive, increasing the risk of rolling trouble. Therefore, the rolling end temperature is set to 820°C or higher. The rolling end temperature is preferably set to 850°C or higher, more preferably 870°C or higher, even more preferably 900°C or higher, and most preferably 920°C or higher. On the other hand, the upper limit of the rolling end temperature is not particularly limited. However, if the rolling end temperature is too high, the metal structure of the seamless steel pipe obtained as a result tends to be non-uniform. Therefore, the rolling end temperature is preferably set to 1200°C or lower, more preferably 1100°C or lower, even more preferably 1000°C or lower, and most preferably 980°C or lower.

[0090] The ratio of the billet radius to the wall thickness of the rolled seamless steel pipe is 5 to 35. When rolling (forming) a seamless steel pipe from a billet, excessive forming increases strain in the seamless steel pipe, which increases dislocation density and causes an increase in the saturated hydrogen content of the seamless steel pipe. Therefore, it is necessary to control the ratio of the billet radius to the wall thickness of the rolled seamless steel pipe. If the ratio of the billet radius to the wall thickness of the rolled seamless steel pipe is less than 5, the seamless steel pipe will be too thick, making production difficult. Therefore, the ratio is set to 5 or more. The ratio is preferably set to 7 or more, more preferably 9 or more, even more preferably 10 or more, and most preferably 12 or more. On the other hand, if the ratio exceeds 35, strain in the seamless steel pipe due to rolling increases, and the saturated hydrogen content of the seamless steel pipe increases. Therefore, the ratio is set to 35 or less. The ratio is preferably 30 or less, more preferably 25 or less, even more preferably 20 or less, and most preferably 15 or less. The ratio of the billet radius to the wall thickness of the rolled seamless steel pipe can be calculated by the following formula: Ratio of the billet radius to the wall thickness of the rolled seamless steel pipe = Billet radius (mm) / Wall thickness of the formed seamless steel pipe (mm)

[0091] The seamless steel pipe after piercing and rolling may be expanded. Expanding the pipe using an expanding mill can improve the roundness of the seamless steel pipe. The expansion rate may be set appropriately depending on the target wall thickness (pipe thickness), outer diameter, strength, and target roundness of the seamless steel pipe.

[0092] Examples in which the effects of the present invention were verified will be described below. These examples show preferred examples of the present invention, and the present invention is not limited to these examples. In the following examples, seamless steel pipes were used as the steel material for evaluation.

[0093] Billets (slabs) having the chemical compositions shown in Tables 1-1, 1-2, and 1-3 were produced at a casting speed of 1.0 m / min (casting process), heated at the heating temperatures shown in Tables 2-1, 2-2, and 2-3 (heating process), and then piercing-rolled, expanding, and plug mill rolling were performed at the ratios of billet radius to wall thickness of the seamless steel pipe after rolling shown in Tables 2-1, 2-2, and 2-3 to obtain seamless steel pipes (hot rolling process). The seamless steel pipes were produced under conditions in which expansion was completed at 820°C or higher (rolling end temperature). The obtained seamless steel pipes were water-cooled under the conditions shown in Tables 2-1, 2-2, and 2-3 (first cooling process), and then tempered at 650°C (tempering temperature) for 30 minutes (tempering time) (tempering process). Steel pipes Nos. 28, 34, 97, and 101 to 104 were subjected to heat treatment under the conditions shown in Tables 2-1 and 2-3 after the first cooling step and before the tempering step (heat treatment step).

[0094] The obtained seamless steel pipes (sometimes simply referred to as steel pipes) were subjected to measurements of the area fraction of retained austenite, the area fraction of martensite, the amount of saturated hydrogen in a hydrogen gas environment, and evaluation of hydrogen embrittlement resistance.

[0095] Measurement of the Area Fraction of Retained Austenite Test specimens were taken from the center of the rolling direction (also referred to as the L direction, axial direction, or longitudinal direction) of the obtained steel pipe, so that the cross section parallel to the axial direction and circumferential (C) direction of the pipe served as the evaluation surface, allowing for observation of the metallographic structure at the center of the wall thickness. First, the evaluation surface of the test specimen was buffed, and then the surface layer of the evaluation surface was chemically polished by etching with picric acid to remove it. The area fraction of retained austenite was measured using X-ray diffraction (XRD). Specifically, a Co-Kα radiation source was used for the incident X-rays, and the volume fraction of retained austenite was calculated from the peak intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. The volume fraction of retained austenite was then taken as the area fraction of retained austenite, assuming that the retained austenite was three-dimensionally homogeneous.

[0096] Measurement of Area Fraction of Martensite Test specimens for metallographic observation were taken from the center of the rolling direction of the obtained steel pipe, so that the observation surface would be the same as that of the test specimen for evaluating retained austenite. First, the observation surface of the test specimen was etched using a 3 vol% nital solution. Then, a microstructure image of the observation surface was obtained using a scanning electron microscope at an appropriate magnification between 1000 and 5000 times, and martensite (including tempered martensite), ferrite, bainite, pearlite, etc. were observed. The metallographic structure was judged visually by comparing it with the structure photograph in Reference 1, and based on the judgment, the structure image was divided into regions for each metallographic structure, and the fraction of each metallographic structure was determined by image analysis (for example, when calculating the fraction of martensite, the martensite and other regions are binarized to determine the fraction of martensite), and the obtained fraction was used as the area fraction of each metallographic phase. [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 the Material, 2004

[0097] Measurement of Dislocation Density Dislocation density was measured using an XRD (X-ray diffraction) device. Test pieces for measuring dislocation density were taken in a 10 mm square at the center of the rolling direction of the steel pipe so that the center of the wall thickness of the steel pipe could be measured, with the same surface as the evaluation surface of the test piece for evaluating retained austenite being the measurement surface. First, the measurement surface was mechanically polished. Next, the measurement surface was subjected to electrolytic polishing so that strain due to mechanical polishing would not affect the measurement of dislocation density. Electrolytic polishing was performed using A-2 solution (HClO ４ The measurement was carried out using a mixture of 78 ml of ammonium hydroxide, 700 ml of ethanol, and 120 ml of distilled water at a temperature of 23°C, a current of 6 A, and a voltage of 27 V. Subsequently, XRD measurements were carried out on the measurement surface based on Reference 2. The measured diffraction planes were BCC-Fe (110), (200), (211), (220), (310), and (222), and the measurement 2θ range was 35 to 154°. A CuK X-ray source was used, and the tube voltage and tube current were 45 kV and 200 mA, respectively. The dislocation density was calculated from the obtained XRD peak intensity using the modified Williamson-Hall method (mWH / WA method). [Reference 2] T. Ungar et al., The effect of dislocation contrast on X-ray line broadening: A new approach to line profile analysis, Applied Physics Letters, Vol. 69 No. 21

[0098] Measurement of saturated hydrogen content The saturated hydrogen content of the obtained steel pipe was determined as the saturated hydrogen content that had penetrated into the steel pipe by subjecting the steel pipe to the hydrogen absorption test described below. The saturated hydrogen content of the steel pipe was measured using the thermal desorption analysis method described below. The steel pipe of the present invention was determined to have excellent hydrogen absorption resistance when the saturated hydrogen content that penetrated into the steel pipe in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa was less than a predetermined value. Here, the predetermined value is 0.8 ppm when the tensile strength (TS) of the steel pipe obtained in the tensile test described below is less than 500 MPa, and is C expressed by formula (1) when the TS of the steel pipe is 500 MPa or more. Ｈ (ppm by mass). Ｈ(mass ppm)=5.4637e-0.004×TS(MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material.

[0099] The hydrogen absorption test was performed as follows. Test specimens for the hydrogen absorption test were cut from the center of the wall thickness of the steel pipe, measuring 10 mm in the circumferential direction, 10 mm in the wall thickness direction, and 20 mm in the rolling direction. All surfaces of the obtained test specimens were polished to #1000 using SiC paper with a wet rotary polisher, and then washed with water and acetone. Furthermore, to eliminate the effect of the oxide film formed on all surfaces of the test specimens, which inhibits hydrogen absorption, all surfaces of the test specimens were Pd-plated (Pd plating layer thickness: 3-4 μm). The test specimens were then placed in the chamber of a high-pressure gas exposure test device, and the chamber was filled with hydrogen gas. The test temperature was 25°C, the hydrogen gas pressure was 40 MPa, and the hydrogen gas concentration was 99.9999%. The hydrogen absorption test was performed for 24 hours, which is longer than the time required for hydrogen penetration into the steel pipe to become saturated.

[0100] The thermal desorption analysis was carried out as follows. The saturated hydrogen amount of the test piece after the hydrogen absorption test was measured using thermal desorption analysis. A low-temperature temperature-programmed hydrogen analyzer (gas chromatograph type) (JTF-20AL) was used for the measurement. The thermal desorption analysis was carried out in the temperature range from room temperature to 400°C at an average heating rate of 200°C / h from room temperature to 400°C, and the sum of the obtained hydrogen amounts was taken as the saturated hydrogen amount.

[0101] Measurement of Tensile Strength Tensile strength (TS) was measured by conducting a tensile test. The tensile test specimens used in the tensile test were taken in the circumferential direction of the steel pipe so that the center of the wall thickness was the center, and processed to obtain 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, and the TS (MPa) of the steel pipe was determined by dividing the maximum load by the initial cross-sectional area of ​​the tensile test specimen.

[0102] Evaluation of Hydrogen Embrittlement Resistance The hydrogen embrittlement resistance was evaluated using the fatigue properties in hydrogen (fatigue crack growth properties in hydrogen). The fatigue crack growth properties in hydrogen were evaluated by performing a fatigue crack growth test according to the following procedure, calculating the fatigue crack growth rate in hydrogen da / dN (m / cycle) in the stress intensity factor range ΔK = 30 MPa, and using the obtained da / dN (m / cycle). In the present invention, the da / dN was 1.5 × 10 －６ A fatigue crack growth test was performed in a hydrogen gas environment at room temperature (20±10°C) and a pressure of 40 MPa, in accordance with ASTM E 647, Standard test method for measurement of fatigue crack growth rate, under the following conditions: frequency: 1 Hz, repeated load waveform: sine wave, control method: load control, loading conditions: uniaxial tension, stress ratio R: 0.1. For the fatigue crack growth test, a compact tension (CT) test piece 2 used for the fatigue crack growth test shown in Figure 1 was taken from the center of the wall thickness of the steel pipe in accordance with ASTM E 647, so that the loading direction was parallel to the direction 3 perpendicular to the rolling (C direction, pipe circumferential direction). The thickness of the test piece 2 was 8 mm in the wall thickness direction of the steel pipe. The front and back surfaces of test specimen 2 were mirror-polished to avoid variations in the fatigue crack growth rate in hydrogen due to surface finishing. Fatigue pre-cracks were introduced into the test specimen in an air environment. Note that reference numeral 1 indicates the rolling direction (L direction, tube axis direction).

[0103] The steel pipes of the present invention had saturated hydrogen contents below a predetermined value in a hydrogen gas environment, and exhibited excellent hydrogen absorption resistance. Furthermore, the fatigue crack growth rate in hydrogen of steel pipes with saturated hydrogen contents below a predetermined value was 1.5 × 10 －６ m / cycle or less, showing excellent hydrogen embrittlement resistance.

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[0110] 1. Rolling direction (L direction) 2. Compact tension (CT) test piece (test piece) used for fatigue crack propagation test 3. Direction perpendicular to rolling (C direction)

Claims

1. In mass%, C: 0.10 to 0.45%, Si: 0.01 to 2.0%, Mn: 0.30 to 2.00%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.0050% or less, Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0%, Ti: 0 to 0.5%, Cu: 0 to 2.5%, Cr: 0 to 2.5%, Mo: 0 to 2.0%, W: 0 to 2.5%, V: 0 to 0.50%, Zr: 0 to 0.050%, B: 0 to 0.0050%, A steel material having a chemical composition containing REM: 0-0.05%, Mg: 0-0.05%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurities, wherein the amount of saturated hydrogen that has penetrated into the steel material in a hydrogen gas environment at a hydrogen gas pressure of 40 MPa is less than a predetermined value, where the predetermined value is 0.8 mass ppm when the tensile strength TS of the steel material is less than 500 MPa, or 0.8 mass ppm when the tensile strength TS of the steel material is 500 MPa or more, calculated from the following formula (1): Ｈ (ppm by mass), C Ｈ (mass ppm)=5.4637e-0.004×TS(MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material.

2. The steel material according to claim 1, wherein the metal structure at the center of the thickness of the steel material has an area fraction of retained austenite of 0 to 3% and an area fraction of martensite of 90% or more.

3. The steel material according to claim 1 or 2, which is a seamless steel pipe.

4. In mass%, C: 0.10 to 0.45%, Si: 0.01 to 2.0%, Mn: 0.30 to 2.00%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.0050% or less, Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0%, Ti: 0 to 0.5%, Cu: 0 to 2.5%, Cr: 0 to 2.5%, Mo: 0 to 2.0%, W: 0 to 2.5%, V: 0 to 0.50%, Zr: 0 to 0.050%, B: 0 to 0.0050%, a casting step of casting molten steel having a composition containing REM: 0-0.05%, Mg: 0-0.05%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurities, at a casting speed of 0.1-1.8 m / min to obtain a slab; a heating step of heating the slab at a heating temperature of 1350°C or less; a hot rolling step of rolling the slab heated in the heating step under conditions where the rolling end temperature is 820°C or more and the ratio of the thickness of the slab to the thickness of the steel material after rolling is in the range of 5-35 to obtain a steel material; and ３ a holding step of holding the steel material at a holding temperature of 1000°C or higher and 800°C or lower; a first cooling step of cooling the steel material after the holding step to a cooling stop temperature of 50°C or lower under the conditions that a first average cooling rate from 800°C to 300°C is 10°C / s or higher at the center of the thickness of the steel material and a second average cooling rate from 300°C to 50°C is 5°C / s or lower at the center of the thickness of the steel material; １ and a tempering step of performing tempering at a tempering temperature of not more than 10 minutes and for a tempering time of not less than 60 minutes.

5. The method further includes a heat treatment step after the first cooling step and before the tempering step, wherein the heat treatment step comprises: ３ 5. The method for manufacturing a steel material according to claim 4, comprising: a reheating step of reheating the steel material to a reheating temperature of from 800°C to 1000°C inclusive; and a second cooling step of cooling the steel material after the reheating step to a cooling stop temperature of 50°C or less under conditions where a first average cooling rate from 800°C to 300°C is 10°C / s or more at a center of thickness of the steel material and a second average cooling rate from 300°C to 50°C is 5°C / s or less at a center of thickness of the steel material.

6. A method for producing a steel material according to claim 4 or 5, wherein the slab in the casting step is a billet, the steel material is a seamless steel pipe, and in the hot rolling step, the billet heated in the heating step is pierced and rolled to form a seamless steel pipe under conditions where the rolling end temperature is 820°C or higher and the ratio of the billet radius to the wall thickness of the seamless steel pipe after rolling is in the range of 5 to 35.

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