Steel material and steel material manufacturing method

Abstract

An object of the present invention is to provide a steel material that is suitable for steel structures used in hydrogen gas environments, has excellent hydrogen absorption resistance in hydrogen gas environments, and has excellent hydrogen embrittlement resistance, and a method for manufacturing the same. A steel material having a specific chemical composition, in which the amount of saturated hydrogen that penetrates into the steel material in a hydrogen gas environment at a hydrogen gas pressure of 40 MPa 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 less than the amount of hydrogen C calculated by the following formula (1) when the tensile strength TS of the steel material is 500 MPa or more: H (ppm by mass), C H (mass ppm)=5.4637e -0.004×TS(MPa) ···(1) In equation (1), TS (MPa) is the tensile strength of the steel.

Description

[Technical Field]

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

[0002] Line pipes for transporting natural gas are an existing part of energy infrastructure. These line pipes have been required to suppress the occurrence of hydrogen-induced cracking in sour environments. Meanwhile, hydrogen has recently been attracting 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 can pump natural gas (a mixture of natural gas with some hydrogen) and hydrogen gas (hydrogen-containing gas) is being considered. The expected pressure for transporting hydrogen gas through 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 designing a structure to withstand high hydrogen pressures requires a thick wall (plate thickness), which increases the price of the structure itself. For this reason, there has been a strong demand for low-cost low-alloy steels for hydrogen steel structures that can withstand high-pressure hydrogen gas environments.

[0004] In response to such demands, for example, Patent Document 1 describes a steel for high-pressure hydrogen environments, which is used in high-pressure hydrogen environments, and claims that 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] Patent Document 2 states that by using a low-alloy high-strength steel adjusted to a specific component composition, the reduction of area and elongation values ​​in a 45 MPa hydrogen atmosphere are greater than those of JIS G3128SHY685NS in the air tensile strength range of 900 to 950 MPa, and the steel has excellent resistance to high-pressure hydrogen environment embrittlement.

[0006] Furthermore, the low alloy steel described in Patent Document 3 is a Cr-Mo high strength low alloy steel that is tempered at a relatively high temperature of 560 to 580°C, and by adjusting the grain size number after tempering to 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.

[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 its resistance to hydrogen embrittlement.

[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 whose metal structure is mainly composed of bainite with an area fraction of 90% or more, and in which cementite with 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. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-2386 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-46737 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-275249 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-74122 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-37655 [Patent Document 6] Japanese Patent Application Laid-Open No. 2012-107332 [Non-patent literature]

[0012] [Non-Patent Document 1] Matsunaga et al., Int J Hydrogen Energy, Vol.40(2015), p.5739-5748 Summary of the Invention [Problem to be solved by the invention]

[0013] The pressure inside a linepipe fluctuates due to operating 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 fatigue properties in a hydrogen gas environment are significantly affected by the amount of hydrogen absorbed by the material in the 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. [Means for solving the problem]

[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: 0.10~0.45%, 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: 0-2.5% Cr: 0~2.5%, Mo: 0-2.0% W: 0-2.5%, V: 0~0.50%, Zr: 0 to 0.050%, B: 0~0.0050%, REM: 0~0.05%, Mg: 0 to 0.05% Hf: 0 to 0.2% Ta: 0 to 0.2%, Re:0~0.005%, Sn: 0 to 0.3%, and Sb: 0 to 0.3% The balance is Fe and unavoidable impurities, A steel material in which the amount of saturated hydrogen that penetrates in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa is less than a specified value. Here, the predetermined value is 0.8 mass ppm when the tensile strength TS of the steel material is less than 500 MPa, When the tensile strength TS of the steel material is 500 MPa or more, the hydrogen content C calculated from the following formula (1) H (mass ppm), is. C H (mass ppm)=5.4637e -0.004×TS(MPa) ···(1) In equation (1), TS (MPa) is the tensile strength of the steel. [2] The metal structure at the center of the steel wall thickness is The area fraction of retained austenite is 0 to 3%, The area fraction of martensite is 90% or more. [1] Steel material as described in [1]. [3] The steel material according to [1] or [2], wherein the steel material is a seamless steel pipe. [4] In mass %, C: 0.10~0.45%, 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: 0-2.5% Cr: 0~2.5%, Mo: 0-2.0% W: 0-2.5%, V: 0~0.50%, Zr: 0 to 0.050%, B: 0~0.0050%, REM: 0~0.05%, Mg: 0 to 0.05% Hf: 0 to 0.2% Ta: 0 to 0.2%, Re:0~0.005%, Sn: 0 to 0.3%, and Sb: 0 to 0.3% a casting step of casting molten steel having a composition with the remainder being Fe and unavoidable impurities at a casting speed of 0.1 to 1.8 m / min to form a cast slab; a heating step of heating the slab at a heating temperature of 1350°C or less; a hot rolling process in which the slab heated in the heating process is rolled to produce a steel material under conditions in which the rolling end temperature is 820°C or higher and the ratio of the thickness of the slab to the thickness of the steel material after rolling is in the range of 5 to 35; a holding step of holding the steel material obtained in the hot rolling step at a holding temperature of Ac3 point or higher and 1000°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 less under the conditions that a first average cooling rate from 800°C to 300°C is 10°C / s or more 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 less at the center of the thickness of the steel material; a tempering step in which the steel material obtained in the first cooling step is tempered at a tempering temperature of 400°C or higher and Ac1 point or lower at the center of the steel material thickness for a tempering time of 10 minutes or longer and shorter than 60 minutes. [5] Further, a heat treatment step is included after the first cooling step and before the tempering step, In the heat treatment step, a reheating step of reheating the steel material obtained in the first cooling step to a reheating temperature of not less than Ac3 point and not more than 1000°C; a second cooling step of cooling the steel material after the reheating step to a cooling stop temperature of 50°C or less under the conditions that a first average cooling rate from 800°C to 300°C is 10°C / s or more 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 less at the center of the thickness of the steel material; [4] A method for manufacturing a steel material according to [4]. [6] The cast piece in the casting process is a billet, and the steel material is a seamless steel pipe, A method for producing a steel material according to [4] or [5], wherein in the hot rolling step, the billet heated in the heating step is piercing-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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a drawing showing a compact tension (CT) test piece used in a fatigue crack growth test. DETAILED DESCRIPTION OF THE INVENTION

[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 according to one embodiment of the present invention has a specific component composition, and the amount of saturated hydrogen that penetrates into the steel material 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 as % are mass % unless otherwise specified.

[0021] Ingredient composition C: 0.10 to 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 viewpoint of suppressing hydrogen absorption into steel. However, if the C content is less than 0.10%, the above effect is insufficient, so 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% It is known that Si retards the growth of carbides that precipitate in steel and reduces 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, and the amount of hydrogen absorption increases. 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, and has the effects of reducing weldability, increasing the hardness of the center segregation, and increasing 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, which increase 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 increases 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 less O, 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 the effects of the present invention are not impaired if the N content is 0.0050% or less. 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, since excessive reduction of the N content increases the steelmaking cost, 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 permitted to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include Zn, Pb, As, and Bi.

[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 also precipitates as fine precipitates of Nb-based carbonitrides, pinning austenite grains during heating and inhibiting 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 is reduced, 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 called Fischer cracks are likely to occur 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 1.5% 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 obtain 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 materials and also 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 likely to be formed, reducing toughness. Therefore, when Ti is contained, the Ti content is set 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 obtain the above effects, 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, if Cu is contained, the Cu content should be 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 effects, 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 a decrease in SSCC resistance. Furthermore, weldability also decreases. 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 effects, 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: 0-2.0% Mo is an element that is effective in improving the toughness and strength of steel materials and in preventing 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, if 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 of increasing crack resistance through control of inclusion 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 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 inclusion properties. 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 be 0%. However, to obtain the above effects, the REM content and Mg content are each preferably 0.0005% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.008% or more. REM is an abbreviation for Rare Earth Metal, which refers to rare earth metals.

[0044] Hf: 0 to 0.2%, Ta: 0 to 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 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 be 0%. However, to obtain the above effects, the Hf content and Ta content are preferably set to 0.001% or more.

[0045] Re:0~0.005% Re is an element that contributes to increasing the strength of steel. However, if the Re content exceeds 0.005%, Re oxides increase, and if they aggregate, 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 or Sb content exceeds 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 a targeted metallographic structure from the steel surface to the center of the wall thickness by achieving a targeted metallographic structure in the center of the wall thickness. Based on the above considerations, the relationship between the metallographic structure in the center of the wall thickness of a steel, the saturated hydrogen content, and hydrogen embrittlement resistance was evaluated. As a result, it was discovered that, as an example, the above-mentioned saturated hydrogen amount can be achieved and excellent hydrogen embrittlement resistance can be obtained by making the metal structure at the center of the thickness 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, remaining in the metal structure of steel, acts as a hydrogen trapping site, 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 very 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 area fraction of retained austenite is preferably 3% or less, more preferably 2% or less, and even more preferably 1% or less. On the other hand, the area fraction of retained austenite may be 0%. The area fraction of retained austenite may be measured by the method described in the Examples.

[0050] Martensite area fraction is 90% or more To achieve a high strength of 520 MPa or more in tensile strength, 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, the 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 within the martensite. Fine carbide precipitation within 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 have fine carbides dispersed and precipitated within the martensite. The average carbide size 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. While the lower limit of the carbide size is not particularly limited, it 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 a and the short side b:

[0052]

number

[0053] Other organizations The metal structure in one embodiment of the present invention may consist of the above-described metal structure. Furthermore, the metal structure in another embodiment 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 will be impossible to ensure an area fraction of martensite of 90% or more, as described above, and hydrogen absorption resistance will deteriorate. 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 ) are listed. Dislocations can act as hydrogen traps, so 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 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 x 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 × 1013 m -2 may be greater than or equal to 5.0 x 10 14 m -2 The dislocation density may be measured by the method described in the examples.

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

[0056] The above-mentioned limitation of the saturated hydrogen content is the most important element in the present invention. Hydrogen embrittlement of steel is fundamentally caused by the steel's absorption of hydrogen. That is, hydrogen accumulates at hydrogen trap sites such as precipitates in the metal structure, corrosion pits caused by SSCC (Sulfide Stress Corrosion Cracking), and stress concentration sources such as initial defects in the steel, and promotes the propagation of cracks (fatigue cracks) generated by stress loading, resulting in destruction. The inventors have conducted a detailed study of 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 (hydrogen fatigue properties, 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 steel that provides excellent hydrogen embrittlement resistance is less than 0.8 ppm when the TS of the steel is less than 500 MPa, and is less than C expressed by Equation (1) when the TS of the steel is 500 MPa or more. 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 set to be 0.7 ppm by mass or less, and more preferably 0.5 ppm by mass or less. Furthermore, when the tensile strength TS of the steel material is 500 MPa or more, it is set to be less than (C H -(C H × 3%)) ppm by mass or less, H -(C H × 5%) ppm by mass or less is more preferable. On the other hand, since it is desirable that the saturated hydrogen content is low, 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 amount 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 considering steel structures such as line pipes used in a high-pressure hydrogen gas environment, 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 using the method described in the examples.

[0058] Hydrogen embrittlement resistance In the present invention, the hydrogen embrittlement resistance is evaluated using the fatigue property in hydrogen, which is evaluated by the following procedure. First, a fatigue crack growth test is conducted in a hydrogen gas environment at room temperature (20±10°C) and 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: sinusoidal wave, control method: load control, loading conditions: uniaxial tension, stress ratio: R = 0.1. For the fatigue crack growth test, compact tension (CT) test specimens used in fatigue crack growth tests in accordance with ASTM E647 can be used. Next, the fatigue crack growth rate in hydrogen, da / dN (m / cycle), is calculated in the stress intensity factor range ΔK = 30 MPa, and the obtained da / dN is 1.5 × 10 -6 m / cycle or less, the fatigue crack growth characteristics in hydrogen, i.e., the hydrogen embrittlement resistance, are judged to be excellent. -6 If the da / dN is less than 1.0×10 m / cycle, it will be possible to design long-life steel structures for hydrogen use, such as line pipes, within the range of thicknesses that can be manufactured using steel materials, especially 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. The lower limit of da / dN is not particularly limited, but it is preferably 8.0 × 10 -8 m / cycle or more.

[0059] steel material The steel material of the present invention includes thin steel plates, thick steel plates, steel pipes (seamless steel pipes, welded steel pipes), shaped steel, steel bars, etc. The steel material has the above-mentioned chemical composition and a saturated hydrogen content less than a predetermined value (has excellent hydrogen absorption resistance), thereby improving hydrogen embrittlement resistance in a hydrogen gas environment. Therefore, the steel material can be suitably applied to steel structures used in a hydrogen gas environment, particularly a high-pressure hydrogen gas environment. In particular, when the steel material is a seamless steel pipe, it can be suitably applied to line pipes for hydrogen gas, so the steel material is preferably a seamless steel pipe. 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 producing a steel material according to one embodiment of the present invention will be described. The steel material according to the present invention can be produced by sequentially carrying out the following steps (1) to (6). (1) A casting process in which molten steel having a predetermined composition is cast into slabs. (2) a heating step for heating the slab (which may be a billet or a slab) obtained in the casting step; (3) a hot rolling process in which the slab obtained in the heating process is rolled into a steel material; (4) a holding process for holding the steel material obtained in the hot rolling process; (5) a first cooling step for cooling the steel material obtained in the hot rolling step; (6) A tempering process in which the steel obtained in the cooling process is tempered. It is more preferable to carry out a heat treatment step of reheating and cooling 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 process In the casting step, 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 slab.

[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) to 1.8 m / min (min) 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 final 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, the slower the casting speed, the fewer the inclusions in the slab, and therefore the lower 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 to 0.3 m / min or more. The slab obtained in the casting process may be used as a steel slab.

[0065] Castings: 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 process In the heating step, the slab is heated at a heating temperature of 1350°C or less.

[0067] Heating temperature: 1350℃ or less If the heating temperature in the heating process exceeds 1350°C, the average grain size of the prior austenite grains becomes too 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, a larger average grain size of the prior austenite grains increases the hydrogen concentration accumulated at the grain boundaries. This increases the likelihood of fatigue crack initiation at the grain boundaries. Furthermore, coarsening of the 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 increased 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 less, more preferably 150 minutes or less, even more preferably 135 minutes or less, and most preferably 120 minutes or less. On the other hand, the lower limit of the heating time is not particularly limited, but the heating time is preferably 30 minutes or more, more preferably 45 minutes or more, even more preferably 60 minutes or more, and most preferably 75 minutes or more.

[0068] Hot rolling process 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 less than 820°C, the rolling load becomes excessive, increasing the risk of rolling problems. 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, there is no particular upper limit to the rolling end temperature. However, if the rolling end temperature is too high, the metal structure of the steel material obtained as a result 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 material after rolling is 5 to 35. When rolling a slab to obtain a steel product, excessive rolling increases strain in the steel, increasing dislocation density and resulting in 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 product after rolling. If the ratio of the thickness of the slab to the thickness of the steel product after rolling is less than 5, the steel product will be too thick, making manufacturing 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 steel product due to rolling increases, increasing the saturated hydrogen content of the steel product. 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 billet thickness to post-rolling steel thickness = billet thickness (mm) / post-rolling steel thickness (mm)

[0071] Holding process 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 points or more and 1000℃ or less If the holding temperature is lower than the Ac3 point, ferrite remains in the steel after the first cooling step described below, reducing the strength and fatigue properties of the steel in hydrogen. Therefore, the holding temperature is set to the Ac3 point or higher. The holding temperature is preferably set to the Ac3 point +30°C or higher, more preferably set to the Ac3 point +50°C or higher, and even more preferably set to the Ac3 point +80°C or higher. On the other hand, if the holding temperature is higher than 1000°C, the austenite grains in the steel may coarsen, which may cause a decrease in the impact absorption energy value and toughness of the steel after the tempering step described below. Furthermore, if the austenite grains coarsen when the saturated hydrogen content of the steel is the same, the hydrogen concentration accumulated at the grain boundaries increases, making fatigue cracks more likely to occur. Furthermore, crack propagation also becomes more likely. Therefore, the holding temperature is set to 1000°C or lower. The holding temperature is preferably set to 980°C or lower, more preferably set to 950°C or lower, and even more preferably set to 930°C or lower. However, in the case of steel materials with an Ac3 point exceeding 930°C, priority is given to maintaining the holding temperature at or above the Ac3 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, the Ac3 point is calculated by the following formula. Ac3(℃)=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] In the formula, [M] represents the content (mass %) of element M.

[0074] 1st 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] 1st average cooling rate: 10℃ / 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 set to 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 achieve an area fraction of martensite of 90% or more. For this reason, the first average cooling rate is set to 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] 2nd average cooling rate: 5℃ / s or less The steel material cooled at the first average cooling rate is then cooled at a 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 prevent excessive cooling, thereby promoting the diffusion of elements in the metal structure and reducing inclusions. 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℃ or less If the cooling stop temperature exceeds 50°C, the precipitated carbides will become coarse, resulting in a deterioration in fatigue properties, and the fatigue properties in hydrogen will also deteriorate. 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, air cooling, etc. 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 process 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℃ or more, Ac1 point or less The 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 occurred during cooling in the cooling step, and to significantly reduce the dislocation density. 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 500°C or higher, even more preferably 550°C or higher, and most preferably 600°C or higher. On the other hand, if the tempering temperature exceeds the Ac1 point, austenite may increase, and the saturated hydrogen content in the steel material may increase. For this reason, the tempering temperature is set to the Ac1 point or lower. The tempering temperature is preferably set to the Ac1 point -10°C or lower, more preferably the Ac1 point -30°C or lower, even more preferably the Ac1 point -50°C or lower, and most preferably the Ac1 point -80°C or lower.

[0080] Tempering time: 10 minutes or more but less than 60 minutes after reaching the tempering temperature The tempering time is set to 10 minutes or more and less than 60 minutes after the tempering temperature is reached. 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 set to 10 minutes or more. The tempering time is preferably set to 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 set to less than 60 minutes. The tempering time is preferably set to 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, the method for determining the Ac1 point is not particularly specified. For example, Ac1 (°C) can be calculated as follows: Ac1 (°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] Heat treatment process (optimal conditions) 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 performed 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 process: In the reheating step, the steel material after the first cooling step is reheated to a reheating temperature of not less than the Ac3 point and not more than 1000°C.

[0084] Reheating temperature: Ac 3 points or more and 1000℃ or less In the heat treatment step, by reheating the steel material after the first cooling step to the Ac3 point or higher, it is possible to reduce the amount of untransformed austenite remaining in the steel material after the first cooling step. Therefore, when the heat treatment step is performed, the reheating temperature is set to the Ac3 point or higher. The reheating temperature is preferably set to the Ac3 point + 10°C or higher, more preferably set to the Ac3 point + 30°C or higher, even more preferably set to the Ac3 point + 50°C or higher, and most preferably set to the Ac3 point + 80°C or higher. On the other hand, in order to suppress coarsening of the initial austenite grains and improve production efficiency, when the heat treatment step is performed, the reheating temperature is set to 1000°C or lower. The reheating temperature is preferably set to 980°C or lower, more preferably set to 950°C or lower, and even more preferably set to 930°C or lower. However, in the case of steel where the Ac3 point + 30°C exceeds 1000°C, priority is given to setting the reheating temperature to 1000°C or less, and even in the case of steel where the Ac3 point + 50°C exceeds 1000°C, priority is given to setting the reheating temperature to 1000°C or less.

[0085] In the heat treatment process, the steel material after the reheating process is subjected to a second cooling process, which refines the prior austenite grains in the final steel material and allows carbides to precipitate finely and dispersedly. 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 Below, 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 of manufacturing the 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 slab. 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 is described below. First, a billet, which is one of the cast pieces, is produced under the same conditions as in the casting process for 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 for 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 produce 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] Hot rolling process In the hot rolling step, the billet heated in the heating step is pierced and rolled under the conditions described below to form a seamless steel pipe. Piercing and rolling in the hot rolling step can be performed by a conventional Mannesmann plug mill method or a Mannesmann mandrel mill method.

[0089] Rolling end temperature: 820°C or higher If the rolling end temperature is less than 820°C, the rolling load becomes excessive, increasing the risk of rolling problems. 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, there is no particular upper limit to the rolling end temperature. However, if the rolling end temperature is too high, the metal structure of the seamless steel pipe obtained as a result 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.

[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, leading to an increase in dislocation density and 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 seamless steel pipe after rolling. If the ratio of the billet radius to the wall thickness of the seamless steel pipe after rolling is less than 5, the wall thickness of 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, leading to an increase in the saturated hydrogen content of the seamless steel pipe. Therefore, the ratio is set to 35 or less. The ratio is preferably set to 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 billet radius to wall thickness of rolled seamless steel pipe = Billet radius (mm) / Wall thickness of 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. [Example]

[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 to obtain seamless steel pipes (hot rolling process) with the billet radius to wall thickness ratios shown in Tables 2-1, 2-2, and 2-3. The seamless steel pipes were manufactured under conditions in which expansion was completed at 820°C or higher (rolling end temperature). The resulting 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 called the L direction, axial direction, or longitudinal direction) of the resulting steel pipe, with the cross section parallel to the axial and circumferential (C) directions serving as the evaluation surface. The evaluation surface of the test specimen was first buffed, and then the surface layer was chemically polished by etching with picric acid to remove the surface. The area fraction of retained austenite was measured using X-ray diffraction (XRD). Specifically, a Co-Kα source was used as the incident X-ray source, 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 martensite area fraction A test specimen for metallographic observation was 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 microstructural image of the observation surface was taken using a scanning electron microscope at an appropriate magnification of 1000 to 5000x, and martensite (including tempered martensite), ferrite, bainite, pearlite, etc. were observed. The metallographic structure was visually determined by comparing it with the microstructural photographs in Reference 1. Based on this determination, the microstructural image was divided into regions for each metallographic structure. Image analysis was then performed to determine the fraction of each metallographic structure (for example, to calculate the fraction of martensite, the martensite and other regions were binarized to determine the fraction of martensite). The obtained fraction was used as the area fraction of each metallographic phase. [Reference 1] Japan Heat Treatment Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Make the Most of Materials, 2004

[0097] Dislocation density measurement Dislocation density was measured using an XRD (X-ray diffraction) device. The test specimens for dislocation density measurement were 10 mm square, taken from the center of the steel pipe in the rolling direction, so that the measurement surface would be the same as the evaluation surface of the test specimen for retained austenite evaluation, allowing measurements to be made at the center of the wall thickness of the steel pipe. First, the measurement surface was mechanically polished. Next, the measurement surface was electropolished to prevent distortion due to mechanical polishing from affecting the dislocation density measurement. Electropolishing was performed using A-2 solution (478 ml of HCl, 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. XRD measurements were then performed on the measurement surfaces based on Reference 2. The diffraction planes measured were BCC-Fe (110), (200), (211), (220), (310), and (222), with a 2θ range of 35 to 154°. A CuK X-ray source was used, with a tube voltage and current of 45 kV and 200 mA, respectively. The dislocation density was calculated from the obtained XRD peak intensities 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 by subjecting the steel pipe to a hydrogen absorption test as described below, and the saturated hydrogen content of the steel pipe was determined as the amount of saturated hydrogen that had penetrated into the steel pipe. The saturated hydrogen content of the steel pipe was measured using the thermal desorption analysis method as described below. The steel pipe of the present invention was determined to have excellent hydrogen absorption resistance when the amount of saturated hydrogen 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 later is less than 500 MPa, and when the TS of the steel pipe is 500 MPa or more, the predetermined value is 0.8 ppm when the TS of the steel pipe is 500 MPa or more, and the predetermined value is 0.8 ppm when the TS of the steel pipe is less than 500 MPa. H(ppm by mass). C H (mass ppm)=5.4637e -0.004×TS(MPa) ···(1) In equation (1), TS (MPa) is the tensile strength of the steel.

[0099] The hydrogen absorption test was performed as follows. Test specimens for the hydrogen absorption test were cut from the center of the steel pipe wall, 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 a wet rotary polisher with SiC paper and then rinsed 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, which was then 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 was longer than the time required for hydrogen penetration into the steel pipe to become saturated.

[0100] 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] Tensile strength measurement 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 form 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 calculated 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 conducting a fatigue crack growth test according to the following procedure, calculating the fatigue crack growth rate da / dN (m / cycle) in hydrogen in the stress intensity factor range ΔK = 30 MPa, and using the obtained da / dN (m / cycle). In the present invention, the fatigue crack growth rate was evaluated using the da / dN (m / cycle) calculated in the stress intensity factor range ΔK = 30 MPa. -6 m / cycle or less was judged to be excellent. Fatigue crack growth tests were conducted 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, cyclic load waveform: sinusoidal, load control method: load control, loading conditions: uniaxial tension, and stress ratio R: 0.1. For the fatigue crack growth tests, compact tension (CT) specimens 2 (shown in Figure 1) were 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 transverse direction 3 (C direction, pipe circumferential direction). The thickness of specimen 2 was 8 mm in the wall thickness direction of the steel pipe. The front and back surfaces of specimen 2 were mirror-polished to minimize variations in the fatigue crack growth rate in hydrogen. A fatigue pre-crack was introduced into the specimen in an air environment. Note that reference numeral 1 indicates the rolling direction (L direction, pipe axial direction).

[0103] The steel pipes of the present invention had saturated hydrogen content below the specified 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 content below the specified value was 1.5 × 10 -6 m / cycle or less, demonstrating excellent hydrogen embrittlement resistance.

[0104] [Table 1-1]

[0105] [Table 1-2]

[0106] [Table 1-3]

[0107] [Table 2-1]

[0108] [Table 2-2]

[0109] [Table 2-3] [Explanation of symbols]

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

Claims

1. In mass%, C: 0.10-0.45%, Si: 0.01-2.0%, Mn: 0.30-2.00%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005-0.15%, O: 0.01% or less, N: 0.0050% or less, Nb: 0 to 0.10%, Ca: 0-0.005%, Ni: 0-2.0%, Ti: 0 to 0.5%, Cu: 0 to 2.5%, Cr: 0-2.5%, Mo: 0-2.0%, W: 0 to 2.5%, V: 0-0.50%, Zr: 0 to 0.050%, B: 0 to 0.0050%, REM: 0-0.05%, Mg: 0-0.05%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0 to 0.3%, and Sb: 0 to 0.3%; The balance is Fe and unavoidable impurities, A steel material in which the amount of saturated hydrogen that penetrates in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa is less than a specified value. Here, the predetermined value is When the tensile strength TS of the steel material is less than 500 MPa, 0.8 mass ppm, When the tensile strength TS of the steel material is 500 MPa or more, the hydrogen content C calculated from the following formula (1) Ｈ (mass ppm), is. 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 metal structure at the center of the steel thickness is The area fraction of retained austenite is 0 to 3%, The area fraction of martensite is 90% or more. The steel material according to claim 1.

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

4. In mass%, C: 0.10-0.45%, Si: 0.01-2.0%, Mn: 0.30-2.00%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005-0.15%, O: 0.01% or less, N: 0.0050% or less, Nb: 0 to 0.10%, Ca: 0-0.005%, Ni: 0-2.0%, Ti: 0 to 0.5%, Cu: 0 to 2.5%, Cr: 0-2.5%, Mo: 0-2.0%, W: 0 to 2.5%, V: 0-0.50%, Zr: 0 to 0.050%, B: 0 to 0.0050%, REM: 0-0.05%, Mg: 0-0.05%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0 to 0.3%, and Sb: 0 to 0.3%; a casting step of casting molten steel having a composition with the remainder being Fe and unavoidable impurities at a casting speed of 0.1 to 1.8 m / min to form a cast 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 into a steel material under conditions where the rolling end temperature is 820°C or higher and the ratio of the thickness of the slab to the thickness of the steel material after rolling is in the range of 5 to 35; The steel material obtained in the hot rolling process is ３ a holding step of holding the temperature at a temperature equal to or higher than the temperature point and equal to or lower than 1000°C; a first cooling step of cooling the steel material after the holding step to a cooling stop temperature of 50°C or less under the conditions that a first average cooling rate from 800°C to 300°C is 10°C / s or more 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 less at the center of the thickness of the steel material; The steel material obtained in the first cooling step is cooled to 400°C or more 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. Further, a heat treatment step is included after the first cooling step and before the tempering step, In the heat treatment step, The steel material obtained in the first cooling step is cooled by Ac ３ a reheating step of reheating the material to a reheating temperature of 1000°C or higher; a second cooling step of cooling the steel material after the reheating step to a cooling stop temperature of 50°C or less under the conditions that a first average cooling rate from 800°C to 300°C is 10°C / s or more 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 less at the center of the thickness of the steel material; The method for producing a steel material according to claim 4, comprising:

6. The cast piece in the casting process is a billet, and the steel material is a seamless steel pipe, 6. The method for producing a steel material according to claim 4 or 5, wherein in the hot rolling step, the billet heated in the heating step is piercing-rolled under conditions in which the rolling finish 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 to form a seamless steel pipe.

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