Steel material and manufacturing method thereof

Abstract

An object of the present invention is to provide a steel material having improved resistance to hydrogen absorption in a hydrogen gas environment and excellent resistance to hydrogen embrittlement, and a method for manufacturing the same. A steel material having a specific chemical composition and a specific structure, in which the amount of saturated hydrogen that penetrates into the steel in a hydrogen gas environment at a hydrogen gas pressure of 40 MPa is less than a specified value. When the tensile strength TS of the steel material is less than 500 MPa, the hydrogen content is 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) is 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 and a method for manufacturing the steel material. [Background technology]

[0002] Existing energy infrastructure includes line pipes used to transport crude oil, natural gas, and other products. When line pipes made of steel are used in environments containing hydrogen sulfide, they can experience hydrogen embrittlement, such as hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), posing safety risks. To prevent hydrogen embrittlement, such as hydrogen-induced cracking and sulfide stress corrosion cracking, various measures have been implemented for steel, including reducing the amount of MnS in the steel, which can cause crack initiation, suppressing the accumulation of Ti and Nb carbonitrides and oxides, and suppressing center segregation. Furthermore, from the perspective of suppressing the occurrence of crack initiation by improving the corrosion resistance of steel, the addition of Sn and Sb to the steel and controlling the segregation rates of Mn, Nb, and Ti have been proposed (e.g., Patent Documents 1 and 2).

[0003] In recent years, the use of hydrogen has been promoted as a clean energy source with the aim of building a decarbonized society. Therefore, with the aim of transporting large quantities of hydrogen gas, the construction of a hydrogen gas transportation network is being considered, in which natural gas mixed with a certain ratio of hydrogen and hydrogen gas (hydrogen-containing gas) are pressurized and sent through natural gas line pipes. The pressure for transporting hydrogen-containing gas through these line pipes is expected to be high, ranging from 1 to 40 MPa, and the line pipes will be exposed to a high-pressure hydrogen gas environment. Therefore, steel for line pipes used in hydrogen gas environments must have hydrogen absorption resistance properties that reduce hydrogen accumulation within the steel.

[0004] Conventionally, austenitic stainless steels such as SUS316L, which are resistant to hydrogen embrittlement, have been used in steel structures used in high-pressure hydrogen gas environments. However, these have problems with the high cost of materials used in manufacturing steel structures and their low strength. On the other hand, when designing steel structures using austenitic stainless steel to withstand high hydrogen pressures, the wall thickness must be increased, which increases the cost of the steel structure. Therefore, there is a demand for low-cost, high-strength steel materials that can withstand high-pressure hydrogen gas environments for hydrogen gas line pipes.

[0005] In order to solve the above problems, for example, Patent Document 3 proposes an austenitic steel material with a high Mn content. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-26695 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-209461 [Patent Document 3] Special Publication No. 2019-505675 Summary of the Invention [Problem to be solved by the invention]

[0007] The technology described in Patent Document 3 makes it possible to provide steel materials at lower costs than austenitic stainless steels. However, because it is an austenitic steel, it is more expensive than low-alloy steels, and there is room for improvement in terms of material costs. Furthermore, the hydrogen absorption resistance of austenitic steels is not taken into consideration, and there is room for improvement in this respect as well.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a steel material having improved hydrogen absorption resistance in a hydrogen gas environment and excellent hydrogen embrittlement resistance, and a method for manufacturing the same. [Means for solving the problem]

[0009] The present inventors conducted technical studies to determine the conditions that steel must satisfy to obtain steel with excellent hydrogen absorption resistance, with the aim of suppressing hydrogen absorption into steel, which is the root cause of hydrogen embrittlement in a hydrogen gas environment. As a result, they discovered that by adjusting the steel to a predetermined chemical composition and creating a metallographic structure with an area fraction of retained austenite of 3% or less, an area fraction of bainite of 90% or more, and an area fraction of island martensite (MA) of less than 5% from the surface to the quarter of the wall thickness, the hydrogen absorption resistance of the steel can be improved and excellent hydrogen embrittlement resistance can be obtained in a hydrogen gas environment. Furthermore, to achieve such a steel structure, it is necessary to strictly control the cooling conditions in the cooling process after the hot rolling process, and they have successfully identified the conditions. The present invention was made based on these findings. The gist of the present invention is as follows. [1] In mass %, C: 0.02~0.15%, Si: 0.01 to 2.00%, Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.0015% or less, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.001 to 0.10%, Ca: 0 to 0.005%, Ni: 0 to 2.0% Ti: 0 to 0.1%, Cu: 0-1.0% Cr: 0 to 1.0%, Mo: 0-0.60% W: 0-1.0%, V: 0~0.10%, Zr: 0 to 0.050%, B: 0~0.0020%, REM: 0~0.01%, Mg: 0 to 0.01%, Hf: 0 to 0.2% Ta: 0 to 0.2%, Re:0~0.005%, Sn: 0 to 0.3% Sb: 0 to 0.3%, and Co: 0 to 5.0% The balance is Fe and unavoidable impurities, At 1 / 4 of the steel wall thickness, Retained austenite is 0% or more and 3% or less in area fraction, Bainite is 90% or more in area fraction, and A metal structure having an area fraction of island martensite (MA) of 0% or more and less than 5%, 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 component composition is, in mass%, Ca: 0.0001 to 0.005%, Ni: 0.01 to 2.0% Ti: 0.005 to 0.1%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, B: 0.0001~0.0020%, REM: 0.0001~0.01%, Mg: 0.0001 to 0.01%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001~0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%, and The steel material according to [1], containing one or more selected from Co: 0.0001 to 5.0%. [3] The steel material according to [1] or [2], wherein the steel material is a welded steel pipe. [4] A heating step of heating a slab having the composition according to [1] or [2] at a heating temperature of 1000°C or more and 1250°C or less; a hot rolling step of hot rolling the slab heated in the heating step into a hot-rolled steel plate; The hot-rolled steel sheet obtained in the hot rolling step is Cooling start temperature: The surface temperature of the hot-rolled steel plate is Ar3 point or higher, and the difference between the surface temperature of the hot-rolled steel plate and the temperature at the center of the thickness of the hot-rolled steel plate is 50°C or less, and the difference between the tip temperature and the tail temperature at the center of the thickness of the hot-rolled steel plate is 60°C or less. Average cooling rate from 750°C to 500°C on the surface of the hot-rolled steel sheet: 15°C / sec or more and 50°C / sec or less; The difference between the average cooling rate from 750 ° C. to 500 ° C. on the surface of the hot-rolled steel sheet and the average cooling rate from 750 ° C. to 500 ° C. at the center of the thickness: 20 ° C. / sec or less; Cooling stop temperature: 250℃ to 500℃ at the center of the thickness of the hot-rolled steel plate a cooling step of cooling the steel material under the following conditions to obtain a steel material; A method for manufacturing a steel material having the above structure. [5] A method for manufacturing a steel material according to [4], further comprising, after the cooling step, a pipe-making step of forming the steel material obtained after the cooling step into a cylindrical shape, and butting and welding both circumferential ends of the cylindrical steel material to form a welded steel pipe. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a steel material having excellent hydrogen embrittlement resistance in a hydrogen gas environment and a method for producing the same. [Brief explanation of the drawings]

[0011] [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

[0012] 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.

[0013] [Component composition] The steel material of the present invention has the above-mentioned chemical composition. The reasons for limiting the chemical composition of the steel material in the present invention will be described below. In the following description, all units shown in % are mass % unless otherwise specified.

[0014] C: 0.02 to 0.15% C is an element that effectively contributes to improving the strength of steel. However, if the C content is less than 0.02%, sufficient strength cannot be ensured. Therefore, the C content is set to 0.02% or more. The C content is preferably set to 0.03% or more, more preferably 0.04% or more, and even more preferably 0.05% or more. On the other hand, if the C content exceeds 0.15%, weldability decreases. Furthermore, the hardness of the surface layer and central segregation of the steel increases during cooling, and hydrogen absorption resistance decreases (the amount of saturated hydrogen increases), resulting in decreased hydrogen embrittlement resistance in a hydrogen gas environment. For this reason, the C content is set to 0.15% or less. The C content is preferably set to 0.14% or less, and more preferably 0.13% or less. If the C content exceeds 0.12%, toughness may decrease. For this reason, the C content is more preferably set to 0.12% or less, and most preferably 0.10% or less.

[0015] Si: 0.01 to 2.00% Si acts as a deoxidizer. However, if the Si content is less than 0.01%, the deoxidizing effect is insufficient. Therefore, the Si content is set to 0.01% or more. The Si content is preferably set to 0.02% or more, more preferably 0.04% or more, even more preferably 0.06% or more, and most preferably 0.08% or more. On the other hand, if the Si content exceeds 2.00%, grain boundary embrittlement occurs, resulting in a decrease in low-temperature toughness and hydrogen absorption resistance, and a decrease in hydrogen embrittlement resistance in a hydrogen gas environment. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.00% or less, more preferably 0.80% or less. Furthermore, if the Si content exceeds 0.60%, toughness and weldability may be reduced. Therefore, the Si content is more preferably set to 0.60% or less, and most preferably 0.40% or less.

[0016] Mn: 0.50 to 1.50% Mn is an element that effectively contributes to improving the strength and toughness of steel. However, if the Mn content is less than 0.50%, the effect of adding Mn is poor. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably set to 0.60% or more, more preferably 0.70% or more, even more preferably 0.80% or more, and most preferably 0.90% or more. On the other hand, if the Mn content exceeds 1.50%, the hardness of the surface layer and central segregation of the steel increases during cooling, reducing hydrogen absorption resistance and hydrogen embrittlement resistance in a hydrogen gas environment. Furthermore, weldability also deteriorates. Therefore, the Mn content is set to 1.50% or less. The Mn content is preferably set to 1.40% or less, more preferably 1.38% or less, even more preferably 1.36% or less, and most preferably 1.34% or less.

[0017] P:0.015% or less P is an element contained in steel as an unavoidable impurity. If the P content exceeds 0.015%, weldability decreases and the hardness of the center segregation increases. Furthermore, the segregation of P to grain boundaries promotes hydrogen trapping at the grain boundaries, reducing hydrogen absorption resistance and hydrogen embrittlement resistance in a hydrogen gas environment. Therefore, the P content is set to 0.015% or less. The P content is preferably set to 0.008% or less, more preferably 0.007% or less, even more preferably 0.006% or less, and most preferably 0.005% or less. Since it is desirable to reduce P as much as possible, the lower limit of the P content is not particularly limited and may be 0%. However, from the viewpoint of refining costs, the P content is preferably set to 0.0001% or more, more preferably 0.0002% or more.

[0018] S: 0.0015% or less S is an element contained in steel as an unavoidable impurity. In steel, S forms MnS inclusions that act as hydrogen trapping sites, reducing hydrogen absorption resistance and hydrogen embrittlement resistance in a hydrogen gas environment. Therefore, it is desirable to reduce S as much as possible, and an S content of up to 0.0015% is acceptable. Therefore, the S content is set to 0.0015% or less. The S content is preferably set to 0.0010% or less, more preferably 0.0008% or less, even more preferably 0.0006% or less, and most preferably 0.0004% 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, from the viewpoint of refining costs, the S content is preferably set to 0.0002% or more, more preferably 0.0003% or more.

[0019] Al: 0.005 to 0.15% Al is an element that functions as a deoxidizer. However, if the Al content is less than 0.005%, the Al content is ineffective. Therefore, the Al content is set to 0.005% or more. The Al content is preferably set to 0.006% or more, more preferably 0.007% or more, even more preferably 0.015% or more, and most preferably 0.020% or more. On the other hand, if the Al content exceeds 0.15%, the cleanliness of the steel decreases and the toughness decreases. Furthermore, Al-based inclusions act as hydrogen trapping sites, which reduces the hydrogen absorption resistance and the hydrogen embrittlement resistance in a hydrogen gas environment. Therefore, the Al content is set to 0.15% or less. The Al content is preferably set to 0.12% or less, more preferably 0.10% or less, even more preferably 0.08% or less, and most preferably 0.06% or less.

[0020] O: 0.01% or less O is an element contained in steel as an unavoidable impurity and causes the formation of oxide-based inclusions that act as hydrogen trapping sites. Therefore, the lower the O content, the better. However, if the O content is 0.01% or less, the above problem does not occur. Therefore, the O content is set to 0.01% or less. The O content is preferably set to 0.0080% or less, more preferably set to 0.0060% or less, and even more preferably set to less than 0.0030%. On the other hand, since it is desirable to reduce O as much as possible, the lower limit of the O content is not particularly limited and may be 0%. However, the O content is preferably set to 0.0005% or more, more preferably set to 0.0008% or more, and even more preferably set to 0.0010% or more.

[0021] N: 0.010% or less N is an element contained in steel as an unavoidable impurity and causes the formation of nitride-based inclusions that act as hydrogen trapping sites. However, if the N content is 0.010% or less, the nitride-based inclusions are hardly formed, and the effects of the present invention are not impaired. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.008% or less, more preferably set to 0.006% or less, and even more preferably set to 0.004% or less. On the other hand, from the viewpoint of improving toughness, it is desirable to reduce N as much as possible, so the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction increases the steelmaking cost. Therefore, the N content is preferably set to 0.0001% or more, more preferably set to 0.0005% or more, and even more preferably set to 0.001% or more.

[0022] Nb: 0.001 to 0.10% Nb is an element effective in increasing the strength and toughness of steel. However, if the Nb content is less than 0.001%, the above effects cannot be obtained. Therefore, the Nb content is set to 0.001% or more. The Nb content is preferably set to 0.006% or more, more preferably 0.015% or more, even more preferably 0.020% or more, and most preferably 0.025% or more. On the other hand, if the Nb content exceeds 0.10%, the amount of precipitates becomes excessive, which acts as hydrogen trapping sites, resulting in a decrease in toughness in hydrogen gas and a decrease in hydrogen absorption resistance. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably set to 0.095% or less, more preferably 0.08% or less, and even more preferably 0.07% or less.

[0023] A steel material according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities.

[0024] Here, the term "unavoidable impurities" refers to impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be included to the extent that the object of the present invention is not impaired. Examples of the raw materials include iron ore, reduced iron, and scrap. Examples of the impurities include H, Zn, Pb, As, and Bi.

[0025] Furthermore, the component composition of the steel material in another embodiment of the present invention may further optionally contain one or more elements selected from Ca, Ni, Ti, Cu, Cr, Mo, W, V, Zr, B, REM, Mg, Hf, Ta, Re, Sn, Sb, and Co. Note that the inclusion of these elements is not essential in the present invention, and therefore the lower limit of the content of each element may be 0%.

[0026] Ca: 0 to 0.005% Ca is an element effective in improving the HIC resistance of steel by controlling the morphology of sulfide-based inclusions, and can be added at any amount depending on the desired HIC resistance. However, if the Ca content exceeds 0.005%, not only does the effect saturate, but the HIC resistance also decreases due to a decrease in the cleanliness of the steel. Therefore, when Ca is added, the Ca content is set to 0.005% or less. The Ca content is preferably set to 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% 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 set to 0.0001% or more, more preferably 0.001% or more.

[0027] Ni: 0 to 2.0% Ni is an element effective in improving the toughness and strength of steel and can be added at any amount depending on the desired properties. However, if the Ni content exceeds 2.0%, microcracks known as Fischer cracks are more likely to form in environments with low hydrogen sulfide partial pressures of less than 1 bar. Therefore, if Ni is added, the Ni content should be 2.0% or less. The Ni content is preferably 1.8% or less, more preferably 1.4% or less, even more preferably 1.2% or less, and most preferably 1.0% or less. On the other hand, since the inclusion of Ni is not essential, the lower limit of the Ni content may be 0%. However, to achieve the above effects, the Ni content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.08% or more, and most preferably 0.12% or more.

[0028] Ti: 0 to 0.1% Ti is an element effective in increasing the strength of steel materials and can be added at any amount depending on the required strength. However, if the Ti content exceeds 0.1%, the above effect saturates and costs increase. Therefore, if Ti is added, the Ti content is set to 0.1% or less. To reduce costs, the Ti content is preferably set to 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less. On the other hand, since the inclusion of Ti is not essential, the lower limit of the Ti content may be 0%. However, to obtain the above effect, the Ti content is preferably set to 0.005% or more, more preferably 0.006% or more, even more preferably 0.008% or more, and most preferably 0.010% or more.

[0029] Cu: 0 to 1.0% Cu is an element effective in improving the toughness and strength of steel, and can be added at any amount depending on the desired properties. However, if the Cu content exceeds 1.0%, weldability decreases. Therefore, if Cu is added, the Cu content should be 1.0% or less. The Cu content is preferably 0.95% or less, more preferably 0.80% or less, even more preferably 0.70% or less, and most preferably 0.60% 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 achieve the above effects, the Cu content is preferably 0.01% or more, more preferably 0.04% or more, even more preferably 0.06% or more, and most preferably 0.08% or more.

[0030] Cr: 0 to 1.0% Like Mn, Cr is an effective element for obtaining sufficient strength even when the C content of a steel material is low, and can be added at any amount depending on the required strength. However, if the Cr content is too high, the hardenability becomes excessive, resulting in reduced SSCC resistance. It also reduces weldability. Therefore, if Cr is added, the Cr content should be 1.0% or less. The Cr content is preferably 0.95% or less, more preferably 0.90% or less, even more preferably 0.85% or less, and most preferably 0.80% or less. Since the inclusion of Cr is not essential, the lower limit of the Cr content may be 0%. However, to obtain the above effects, the Cr content 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.

[0031] Mo: 0 to 0.60% Mo is an element effective in improving the toughness and strength of steel, as well as in improving SSCC resistance regardless of the hydrogen sulfide partial pressure. Therefore, Mo can be added at any amount depending on the desired properties. However, if the Mo content exceeds 0.60%, the hardenability becomes excessive, resulting in a decrease in SSCC resistance. Furthermore, weldability also decreases. Therefore, when Mo is added, the Mo content is set to 0.60% or less. The Mo content is preferably set to 0.55% or less, more preferably 0.50% or less, even more preferably 0.45% or less, and most preferably 0.40% or less. However, since the inclusion of Mo is not essential, the lower limit of the Mo content may be 0%. However, to achieve the above effects, the Mo content is preferably set to 0.01% or more, more preferably 0.10% or more, even more preferably 0.12% or more, and most preferably 0.14% or more.

[0032] W: 0 to 1.0% W is an element that contributes to increasing the strength of steel and can be added at any amount depending on the required strength. However, if the W content exceeds 1.0%, the above effect saturates and costs increase. Therefore, if W is added, the W content is set to 1.0% or less. The W content is preferably set to 0.8% or less, and more preferably set to 0.7% or less. To reduce costs, the W content is further preferably set to 0.5% or less, and most preferably set to 0.4% or less. On the other hand, since the inclusion of W is not essential, the lower limit of the W content may be 0%. However, to obtain the above effect, the W content is preferably set to 0.01% or more, more preferably set to 0.02% or more, even more preferably set to 0.03% or more, and most preferably set to 0.04% or more.

[0033] V: 0 to 0.10% V is an element effective in increasing the strength and toughness of steel materials and can be added at any amount depending on the desired properties. However, if the V content exceeds 0.10%, the toughness of the weld decreases. Therefore, when V is added, the V content is set to 0.10% or less. The V content is preferably set to 0.09% or less, more preferably 0.08% or less, even more preferably 0.07% or less, and most preferably 0.06% 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 effects, the V content is preferably set to 0.01% or more, more preferably 0.02% or more, and most preferably 0.03% or more.

[0034] Zr: 0 to 0.050% Zr is an effective element for improving the toughness of steel materials by refining crystal grains and for improving the crack resistance of steel materials by controlling the properties of inclusions. Zr can be added at any amount depending on the desired properties. However, if the Zr content exceeds 0.050%, the above effects saturate. Therefore, if Zr is added, the Zr content should be 0.050% or less. The Zr content is preferably 0.045% or less, more preferably 0.040% or less, even more preferably 0.030% or less, and most preferably 0.020% or less. Since the inclusion of Zr is not essential, the lower limit of the Zr content may be 0%. However, to achieve the above effects, the Zr content is preferably 0.0001% or more, more preferably 0.001% or more, even more preferably 0.005% or more, and most preferably 0.008% or more.

[0035] B: 0 to 0.0020% B is an element that improves hardenability, contributes to increasing the strength of steel, and suppresses coarsening of prior austenite grains, thereby improving various properties of steel. B can be added at any amount depending on the desired properties. However, if the B content exceeds 0.0020%, the above effects saturate, resulting in increased costs. Therefore, when B is added, the B content is set to 0.0020% or less. The B content is preferably set to 0.0018% or less, and more preferably set to 0.0016% or less. To reduce costs, the B content is further preferably set to 0.0010% or less. On the other hand, since the inclusion of B is not essential, the lower limit of the B content may be 0%. However, to obtain the above effects, the B content is preferably set to 0.0001% or more, more preferably set to 0.0005% or more, and even more preferably set to 0.0008% or more.

[0036] REM: 0-0.01%, Mg: 0-0.01% REM and Mg are elements that have the effect of improving the toughness of steel by refining crystal grains and improving the crack resistance of steel by controlling the properties of inclusions, and can be added at any amount depending on the desired properties. However, if the REM content and Mg content exceed 0.01%, the above effects saturate. Therefore, when REM and Mg are each contained, the REM content and Mg content should each be 0.01% or less. The REM content and Mg content should each be preferably 0.0090% or less, more preferably 0.0085% or less, even more preferably 0.0080% or less, and most preferably 0.0075% or less. On the other hand, since the inclusion of REM and Mg is not essential, the lower limit of the REM content and Mg content may each be 0%. However, to obtain the above effects, the REM content and Mg content are each preferably 0.0001% or more, more preferably 0.0010% or more, even more preferably 0.0020% or more, and most preferably 0.0040% or more. REM is an abbreviation for rare earth metal.

[0037] Hf: 0 to 0.2% Hf is an element that contributes to increasing the strength of steel and can be added at any amount depending on the required strength. However, if the Hf content exceeds 0.2%, the amount of Hf oxide increases and agglomerates, resulting in a decrease in hydrogen absorption resistance. Therefore, if Hf is added, the Hf content should be 0.2% or less. The Hf content is preferably 0.18% or less, more preferably 0.16% or less, even more preferably 0.14% or less, and most preferably 0.12% or less. On the other hand, since the inclusion of Hf is not essential, the lower limit of the Hf content may be 0%. However, to obtain the above effects, the Hf content is preferably 0.0001% or more, more preferably 0.001% or more, even more preferably 0.002% or more, and most preferably 0.02% or more.

[0038] Ta: 0 to 0.2% Ta is an element that forms carbides and nitrides and contributes to increasing the strength of steel, and can be added at any amount depending on the required strength. However, a Ta content exceeding 0.2% can lead to a decrease in the toughness of the steel. Therefore, when Ta is added, the Ta content is set to 0.2% or less. The Ta content is preferably set to 0.16% or less, more preferably 0.14% or less, even more preferably 0.12% or less, and most preferably 0.10% or less. On the other hand, the inclusion of Ta is not essential, so the lower limit of the Ta content may be 0%. However, to obtain the above effects, the Ta content is preferably set to 0.0001% or more, more preferably 0.02% or more, and even more preferably 0.04% or more.

[0039] Re:0~0.005% Re is an element that contributes to increasing the strength of steel materials and can be added at any amount depending on the required strength. However, if the Re content exceeds 0.005%, Re oxides increase and agglomerate, resulting in a decrease in hydrogen absorption resistance. Therefore, when Re is added, the Re content is set to 0.005% or less. The Re content is preferably set to 0.004% or less, and more preferably set to 0.002% 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, in order to obtain the above effects, the Re content is preferably set to 0.0001% or more, and more preferably set to 0.001% or more.

[0040] Sn: 0-0.3%, Sb: 0-0.3% Sn and Sb are elements that contribute to increasing the strength and hardenability of steel materials, and can be added at any amount depending on the desired properties. However, if the Sn and Sb contents exceed 0.3%, the above effects saturate and cost increases. Therefore, when Sn and Sb are each contained, the Sn and Sb contents are each set to 0.3% or less. To reduce costs, the Sn and Sb contents are preferably set to 0.20% or less, more preferably 0.15% or less, and most preferably 0.10% or less. On the other hand, since the inclusion of Sn and Sb is not essential, the lower limits of the Sn and Sb contents may be 0%. However, to obtain the above effects, the Sn and Sb contents are preferably set to 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.004% or more.

[0041] Co: 0-5.0% Co, like Ni and Cu, is an element that improves hardenability, contributes to increasing the strength of steel, suppresses coarsening of prior austenite grains, and has the effect of improving various properties of steel. Co can be added at any amount depending on the desired properties. However, if the Co content exceeds 5.0%, the above effects saturate and cause increased costs. Therefore, when Co is added, the Co content is set to 5.0% or less. To reduce costs, the Co content is preferably set to 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 Co is not essential, the lower limit of the Co content may be 0%. However, to obtain the above effects, the Co content is preferably set to 0.001% or more, more preferably 0.01% or more, even more preferably 0.05% or more, and most preferably 0.1% or more.

[0042] [Metal structure] The steel material of the present invention has a metal structure at a 1 / 4 position in the wall thickness of the steel material, in which the area fraction of retained austenite is 0% or more and 3% or less, the area fraction of bainite is 90% or more, and the area fraction of island martensite (MA) is 0% or more and less than 5%. Hydrogen absorption in steel occurs from the surface that directly contacts hydrogen gas. Therefore, it is considered appropriate to control the metallographic structure of at least the steel material from the surface that directly contacts hydrogen gas to the interior to an appropriate metallographic structure. Therefore, the inventors conducted research and found that excellent hydrogen absorption resistance can be achieved by controlling the metallographic structure at a quarter-thickness position relative to the surface on the side that contacts hydrogen gas to the above metallographic structure, and as a result, excellent hydrogen embrittlement resistance can be achieved. Here, in a steel material, it is sufficient that the metallographic structure at a quarter-thickness position relative to at least one of the steel material surfaces is the above metallographic structure, and the steel material surface serving as the starting point is exposed to a hydrogen gas environment. For example, in the case of a welded steel pipe, since the inner surface of the welded steel pipe comes into contact with hydrogen gas, the metallographic structure at at least a quarter-thickness position relative to the inner surface of the welded steel pipe should be the above metallographic structure.

[0043] Retained austenite area fraction is 0% to 3% Retained austenite, when remaining in the metal structure of a steel material, acts as a hydrogen trap site, increasing the amount of saturated hydrogen in the steel material in a hydrogen gas environment, i.e., significantly reducing the hydrogen absorption resistance of the steel material. Furthermore, when the steel material is used as a steel structure, austenite may transform into martensite due to stress load during use. The transformed martensite is very hard and may serve as a source or propagation path for fatigue cracks. Therefore, it is desirable to reduce the area fraction of retained austenite as much as possible, but an area fraction of retained austenite of 3% or less is acceptable. Therefore, the area fraction of retained austenite is set to 3% or less. The area fraction of retained austenite is preferably set to 2% or less, and more preferably set to 1% or less. On the other hand, since it is desirable to reduce the area fraction of retained austenite as much as possible, there is no particular limit to the lower limit of the area fraction of retained austenite, and it is set to 0%. The area fraction of retained austenite may be measured using the method described in the Examples.

[0044] Bainite area fraction is 90% or more If the area fraction of bainite is less than 90%, heterogeneous metal structures are mixed, resulting in reduced strength and toughness. Furthermore, the presence of metal structures with different hardnesses causes stress distribution within the steel material when stress is applied during use, resulting in strain concentration at the interface between the soft and hard phases. As a result, the interface acts as a hydrogen accumulation source, increasing the amount of saturated hydrogen in a hydrogen gas environment (reducing hydrogen absorption resistance) and reducing hydrogen embrittlement resistance. Therefore, the area fraction of bainite is set to 90% or more. By setting the area fraction of bainite to 90% or more, the strength and toughness required for steel pipes used in linepipes can be ensured. The area fraction of bainite is preferably 92% or more, more preferably 93% or more, and even more preferably 95% or more. Since it is desirable to increase the area fraction of bainite as much as possible, the upper limit of the area fraction of bainite is not particularly limited and may be 100%. The area fraction of bainite may be measured using the method described in the Examples. Here, bainite may be any one of bainitic ferrite, granular bainite, and tempered bainite, which are transformed during or after cooling and contribute to transformation strengthening, or may be composed of two or more of them.

[0045] Island martensite (MA) is an area fraction of 0% to less than 5% Island martensite is a metal structure that acts as a hydrogen trapping site and significantly reduces the hydrogen absorption resistance of steel. Therefore, it is desirable to reduce the area fraction of island martensite (MA) as much as possible, but a value of less than 5% is acceptable. Therefore, the area fraction of island martensite is set to less than 5%. The area fraction of island martensite is preferably 4% or less, more preferably 3% or less, and even more preferably 2% or less. On the other hand, since it is desirable to reduce the area fraction of island martensite as much as possible, the lower limit of the area fraction of island martensite is set to 0%. The area fraction of island martensite (MA) may be measured by the method described in the Examples.

[0046] 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 ferrite, martensite, and pearlite. If the area fraction of the other metal structure exceeds 10%, as described above, bainite cannot be ensured at an area fraction of 90% or more, resulting in reduced strength, toughness, and hydrogen absorption resistance. 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%.

[0047] [Amount of saturated hydrogen] 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.

[0048] 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 hydrogen absorption 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, leading to fracture. This results in a decrease in hydrogen embrittlement resistance in a hydrogen gas environment. In addition, generally, the higher the TS of a steel, the more likely it is to suffer from hydrogen embrittlement. In other words, whether or not absorbed hydrogen causes hydrogen embrittlement can be said to depend on the TS of the steel. Based on these findings, the inventors conducted a detailed study on the relationship between the amount of hydrogen in steel at each TS and its hydrogen embrittlement resistance, and discovered the relationship between the TS of a steel and its hydrogen tolerance (saturated hydrogen content). That is, it was found that by making the saturated hydrogen content in the steel less than a predetermined value (improving hydrogen absorption resistance) depending on the TS level of the steel, the hydrogen fatigue crack growth rate can be reduced, i.e., the hydrogen fatigue crack growth characteristics can be improved, and excellent hydrogen embrittlement resistance can be obtained. Specifically, the saturated hydrogen content of steel that can obtain 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 formula (1) when the TS of the steel is 500 MPa or more. H (ppm by mass) or less. C H (mass ppm)=5.4637e -0.004×TS(MPa) ···(1) In equation (1), TS (MPa) is the tensile strength of the steel. 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.

[0049] Hydrogen embrittlement resistance In the present invention, the hydrogen embrittlement resistance is 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 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, stress ratio: R = 0.1. Here, for the fatigue crack growth test, a compact tension (CT) test specimen used in fatigue crack growth tests in accordance with ASTM E647 can be taken and used. Next, the fatigue crack growth rate da / dN (mm / cycle) in hydrogen in the stress intensity factor range ΔK = 25 MPa was calculated, and the obtained da / dN was 2.0 × 10 -3 When the da / dN is 2.0×10 mm / cycle or less, it is judged to have excellent hydrogen embrittlement resistance. -3 If the yield strength is less than mm / 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, particularly welded steel pipes such as UOE.

[0050] Steel with excellent hydrogen absorption resistance can suppress the occurrence of hydrogen embrittlement such as hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), and can be said to have excellent HIC and SSCC resistance.

[0051] Tensile strength (TS) The tensile strength (TS) of the steel material of the present invention is not particularly limited, but when it is assumed to be a steel material constituting a steel structure, it is preferably 470 MPa or more, more preferably 500 MPa or more, even more preferably 520 MPa or more, and most preferably 540 MPa or more. On the other hand, there is no particular upper limit for the TS of the above steel material, but excessively high strength leads to increased costs and reduced hydrogen absorption resistance. Therefore, the TS is preferably 700 MPa or less, more preferably 680 MPa or less. The tensile strength (TS) may be measured by the method described in the Examples.

[0052] [Steel] The steel material of the present invention includes thin steel plates, thick steel plates, welded steel pipes, shaped steel, steel bars, etc. The steel material has the above-mentioned chemical composition and metallographic structure, and has a saturated hydrogen content less than a predetermined value (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 welded steel pipe, it can be suitably applied to hydrogen line pipes, so the steel material is preferably a welded steel pipe. Here, the high-pressure hydrogen gas environment is assumed to be a high-pressure hydrogen gas with a hydrogen partial pressure of 1 MPa or more, or an environment containing 0.2% or more hydrogen gas.

[0053] [Manufacturing method] Next, a method for producing a steel material of the present invention will be described. The steel material can be produced by sequentially carrying out steps (a) to (c) on a cast slab (steel material) having the above-mentioned component composition. (a) Heating process (b) Hot rolling process (c) Cooling process In the following description, unless otherwise specified, the temperature refers to the temperature at the surface of a slab, a hot-rolled steel plate, or a steel material. The surface temperature of a slab, a hot-rolled steel plate, or a steel material can be measured using a radiation thermometer or the like. The thickness center temperature may be measured using a thermocouple or the like, or may be predicted from the measured surface temperature using the finite element method or the like.

[0054] heating process In the heating step, the slab (steel material) having the above-mentioned composition is heated at a heating temperature of 1000°C or higher and 1250°C or lower.

[0055] Cast slab Any form of material can be used as the slab (also referred to as steel raw material). The slab may be, for example, a billet or a steel slab. The method for producing the slab is not particularly limited, but it can be produced, for example, by melting steel having the above-mentioned composition and casting it. The melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, from the viewpoint of productivity, the casting is preferably carried out by a continuous casting method, but can also be carried out by an ingot casting-splitting rolling method.

[0056] Heating temperature: 1000℃ or higher and 1250℃ or lower If the heating temperature is less than 1000°C, the diffusion of elements such as C, P, and S that are microsegregated in the slab will be insufficient, and a homogeneous material will not be obtained. Therefore, the heating temperature is set to 1000°C or higher. The heating temperature is preferably set to 1020°C or higher, more preferably 1050°C or higher, even more preferably 1080°C or higher, and most preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains will become coarse and the toughness will decrease. Therefore, the heating temperature is set to 1250°C or lower. The heating temperature is preferably set to 1200°C or lower, more preferably 1190°C or lower, and even more preferably 1180°C or lower.

[0057] Hot rolling process In the hot rolling step, the slab heated in the heating step is hot rolled to form a hot-rolled steel sheet.

[0058] Hot rolling finish temperature: Ar3 point or higher (preferable condition) In the hot rolling process, the slab heated in the heating process is hot-rolled to the desired thickness to produce a hot-rolled steel sheet. However, if the hot-rolling end temperature is lower than the Ar3 point, the formation of soft ferrite may result in a decrease in strength if the process involves immediate cooling after hot rolling. Therefore, the hot-rolling end temperature is preferably equal to or higher than the ferrite formation temperature, Ar3 point +30°C or higher, more preferably Ar3 point +50°C or higher, most preferably Ar3 point +70°C or higher, and most preferably Ar3 point +100°C or higher. On the other hand, if the hot-rolling end temperature exceeds 1250°C, the crystal grains may become coarse and the toughness may decrease. Therefore, the hot-rolling end temperature is preferably equal to or lower than 1250°C, more preferably equal to or lower than 1220°C, even more preferably equal to or lower than 1180°C, and most preferably equal to or lower than 950°C.

[0059] Since the Ar3 point varies depending on the alloying composition of the steel, it may be determined by experimentally measuring the transformation temperature of each steel, or it may be determined from the composition of the steel using the following formula (2): Ar3(℃)=910-310C(%)-80Mn(%)-20Cu(%)-15Cr(%)-55Ni(%)-80Mo(%)...(2) The content of each element is expressed as a percentage by mass.

[0060] cooling process In the cooling process, the hot-rolled steel sheet obtained in the hot rolling process is cooled under the conditions (1) to (4) described below to obtain a steel material. Note that the temperatures in the cooling process decrease from the leading edge to the trailing edge in the rolling direction on the surface and each face in the thickness direction of the hot-rolled steel sheet. Therefore, unless otherwise specified, the temperatures are those at the trailing edge of each face. (1) Cooling start temperature: The surface temperature of the hot-rolled steel sheet is Ar3 point or higher, the difference between the surface temperature of the hot-rolled steel sheet and the temperature at the center of the thickness of the hot-rolled steel sheet is 50°C or less, and the difference between the tip temperature and the tail temperature at the center of the thickness of the hot-rolled steel sheet is 60°C or less. (2) Average cooling rate from 750°C to 500°C on the surface of the hot-rolled steel sheet: 15°C / sec or more and 50°C / sec or less. (3) The difference between the average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel plate and the average cooling rate from 750°C to 500°C at the center of the thickness: 20°C / sec or less. (4) Cooling stop temperature: 250°C or higher and 500°C or lower at the center of the thickness of the hot-rolled steel plate.

[0061] Cooling start temperature If the cooling start temperature of the steel material after the hot rolling process is less than the Ar3 point on the surface of the hot-rolled steel sheet, ferrite will form before the start of cooling, resulting in a significant decrease in strength. Furthermore, the formation of ferrite causes a decrease in hydrogen absorption resistance due to differences in hardness, resulting in a decrease in hydrogen embrittlement resistance in a hydrogen gas environment. Therefore, the cooling start temperature is set to the Ar3 point or higher on the surface of the hot-rolled steel sheet. The cooling start temperature is preferably set to the Ar3 point + 20°C or higher, more preferably the Ar3 point + 40°C or higher, and even more preferably the Ar3 point + 60°C or higher. On the other hand, if the cooling start temperature is too high, the grain size will become too large, resulting in a decrease in toughness. Therefore, the cooling start temperature is preferably set to less than 1250°C, more preferably 1100°C or lower, even more preferably 1050°C or lower, and most preferably 900°C or lower.

[0062] Furthermore, if the cooling start temperature (temperature difference) between the surface temperature and the center temperature of the hot-rolled steel sheet exceeds 50°C, the temperature gradient in the thickness direction will cause the metal structure from the steel surface to the 1 / 4 position to become non-uniform, resulting in a decrease in the strength and toughness of the steel. In addition, segregation zones that act as hydrogen trapping sites will form, increasing the amount of saturated hydrogen and significantly reducing hydrogen embrittlement resistance. Therefore, the temperature difference is set to 50°C or less. The temperature difference is preferably set to 48°C or less, more preferably 46°C or less, even more preferably 44°C or less, and most preferably 40°C or less. There is no particular lower limit to the temperature difference, but it is preferably set to 5°C or more, and more preferably 10°C or more.

[0063] Furthermore, if the cooling start temperature (temperature difference) between the tip and tail temperatures in the rolling direction at the center of the thickness of the hot-rolled steel sheet exceeds 60°C, a temperature gradient will occur in the longitudinal direction (rolling direction) of the hot-rolled steel sheet at the center of the thickness when cooling is stopped. This will result in significant temperature variation at the time of cooling stop, leading to a decrease in the area fraction of bainite at the quarter-thickness position and an increase in the area fraction of island martensite, resulting in a decrease in hydrogen embrittlement resistance. For this reason, the temperature difference is set to 60°C or less. The temperature difference is preferably set to 45°C or less, more preferably 42°C or less, even more preferably 40°C or less, and most preferably 38°C or less. While shortening the length of the hot-rolled steel sheet can reduce the temperature difference at the start of cooling and suppress temperature variation at the end of cooling, this will degrade manufacturability. Therefore, it is preferable to shorten the time until cooling starts by increasing the transport speed of the hot-rolled steel sheet and thereby suppress the temperature difference. On the other hand, the lower limit of the temperature difference is not particularly limited, but since there is a risk of productivity decreasing due to an increase in equipment load, the temperature difference is preferably 10°C or more, more preferably 12°C or more, and even more preferably 14°C or more.

[0064] average cooling rate If the average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel sheet is less than 15°C / sec, the average cooling rate at the center of the thickness is insufficient, making it more likely to produce untransformed austenite. As a result, the area fraction of retained austenite increases, island martensite forms, and hydrogen absorption resistance deteriorates. Therefore, the average cooling rate is set to 15°C / sec or more. From the viewpoint of suppressing bainite variation, the average cooling rate is preferably set to 17°C / sec or more, more preferably 20°C / sec or more, even more preferably 25°C / sec or more, and most preferably 28°C / sec or more. On the other hand, if the average cooling rate exceeds 50°C / sec, the area fraction of martensite, which acts as a hydrogen trapping site, increases and the area fraction of bainite decreases. As a result, hydrogen absorption resistance deteriorates, and hydrogen embrittlement resistance in a hydrogen gas environment deteriorates. Therefore, the average cooling rate is set to 50°C / sec or less. The average cooling rate is preferably 45° C. / sec or less, more preferably 42° C. / sec or less, even more preferably 40° C. / sec or less, and most preferably 38° C. / sec or less.

[0065] Furthermore, if the difference between the average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel sheet and the average cooling rate from 750°C to 500°C at the center of the thickness (average cooling rate difference) exceeds 20°C / sec, cooling near the 1 / 4 position in the thickness direction is not performed at a sufficient rate, island martensite forms, and a segregation zone is formed, resulting in a decrease in hydrogen absorption resistance. Therefore, the difference in average cooling rates is set to 20°C / sec or less. The difference in average cooling rates is preferably set to 18°C / sec or less, more preferably 16°C / sec or less, even more preferably 14°C / sec or less, and most preferably 12°C / sec or less. The lower limit of the difference in average cooling rates is preferably set to 5°C / sec or more, more preferably 8°C / sec or more, and even more preferably 10°C / sec or more.

[0066] Cooling stop temperature If the cooling stop temperature exceeds 500°C at the center of thickness of the hot-rolled steel sheet, the area fraction of ferrite and the average grain size of bainite increase, resulting in a significant decrease in strength. Furthermore, island martensite is more likely to form. For this reason, the cooling stop temperature is set to 500°C or less. The cooling stop temperature is preferably set to 490°C or less, more preferably 480°C or less, and even more preferably 460°C or less. On the other hand, if the cooling stop temperature is less than 250°C, quench cracking is more likely to occur during cooling. Furthermore, the remaining austenite phase increases the area fraction of retained austenite and decreases the area fraction of bainite, so the cooling stop temperature is set to 250°C or more. Furthermore, hydrogen present in the steel gradually escapes during cooling, but if the cooling stop temperature is less than 250°C, overcooling occurs, tending to increase the amount of hydrogen remaining in the steel. Furthermore, retained austenite, which absorbs large amounts of hydrogen, is more likely to form, resulting in a decrease in hydrogen absorption resistance. For this reason, the cooling stop temperature is set to 250°C or more. The cooling stop temperature is more preferably 280° C. or higher, even more preferably 320° C. or higher, and most preferably 350° C. or higher. After cooling is stopped (in a temperature range lower than the cooling stop temperature), the steel material may be allowed to cool naturally, but in order to promote the formation of bainite, it is more preferable to cool slowly at least in a temperature range from the cooling stop temperature to 80° C.

[0067] The steel material after the cooling step may be wound into a coil. When winding into a coil, the winding temperature is preferably 500°C or less. Furthermore, the winding temperature is preferably 250°C or more. In particular, when the steel material is an electric resistance welded steel pipe and an electric resistance welded steel pipe is to be manufactured, the steel material wound into a coil may be used to carry out the pipe-making step described below.

[0068] In another embodiment of the present invention, a pipe-making process may be provided after the cooling process. That is, in the above embodiment, a welded steel pipe, which is one type of steel material, can be produced by sequentially performing the steps (a) to (d) on a cast slab having the above-mentioned chemical composition. (a) Heating process (b) Hot rolling process (c) Cooling process (d) Pipe making process However, the steps (a) to (c) are as explained above, so only the pipe-making step will be explained below.

[0069] Pipe making process In the pipe-making process, after the cooling process, the steel material obtained after the cooling process is further formed into a cylindrical shape, and both circumferential ends of the cylindrical steel material are butt-welded to form a welded steel pipe. The method for forming the steel material into a cylindrical shape is not particularly limited, but examples thereof include bending and cold roll forming. Specifically, in the bending process, the end of the steel material is beveled and the steel material is formed into a cylindrical shape using a C press, a U press, or an O press. Then, both circumferential ends of the obtained cylindrical steel material are butted together, and the butt joints are seam-welded by internal welding and external welding to produce a welded steel pipe. Alternatively, a welded steel pipe may be produced by press-bending into a cylindrical shape and then seam-welding the butt joint. If necessary, the welded steel pipe may be produced by further undergoing a pipe expansion process. Any welding method may be used as long as it provides sufficient joint strength and joint toughness, but submerged arc welding is preferred from the viewpoints of excellent weld quality and manufacturing efficiency. An example of a welded steel pipe manufactured using the above-mentioned bending process is a UOE steel pipe. In cold roll forming, a steel material is formed into a cylindrical shape by cold roll forming, and then both circumferential ends of the resulting cylindrical steel material are butted together and electric resistance welded to produce a welded steel pipe.

[0070] An example of the welded steel pipe manufactured by the above-mentioned manufacturing method is an electric resistance welded steel pipe.

[0071] When the welded steel pipe is an electric resistance welded steel pipe, the welded steel pipe after the above pipe-making process may be further formed into an electric resistance welded steel pipe material using a sizing roll that satisfies the following formula (3) (sizing process), and an internal pressure p (MPa) that satisfies the following formula (4) may be applied to the inner surface of the electric resistance welded steel pipe material (internal pressure application process), thereby producing an electric resistance welded steel pipe. The cylindrical shape means that the circumferential cross section of the steel pipe is "C" shaped. Diameter of sizing roll (mm) ≥ Thickness of hot-rolled steel sheet (mm) / 0.020 (3) The thickness of the hot-rolled steel sheet refers to the thickness of the hot-rolled steel sheet (steel material after the cooling process) before the pipe-making process. The diameter (mm) of the sizing roll is preferably 1800 mm or less. X <p≦X×1.5 ···(4) X = (thickness of electric resistance welded steel pipe material (mm) / radius of electric resistance welded steel pipe material (mm)) x yield strength of electric resistance welded steel pipe material (MPa)

[0072] The application of the internal pressure can be achieved, for example, by sealing the end of the electric resistance welded steel pipe material with a rubber packing and applying water pressure to the inside of the electric resistance welded steel pipe material. In addition, in order to stabilize the shape, a mold of the desired diameter can be used as an outer frame, if necessary. The wall thickness of the electric resistance welded steel pipe material is preferably 5 mm or more, and 30 mm or less. Although there is no particular upper limit to the radius of the electric resistance welded steel pipe material, a larger radius increases the load on the equipment. Therefore, the radius of the electric resistance welded steel pipe material is preferably 600 mm or less, and more preferably 500 mm or less. Furthermore, there is no particular lower limit to the radius of the electric resistance welded steel pipe material, but a radius of 200 mm or more is preferred. The yield strength of the electric resistance welded steel pipe material is preferably 480 MPa or more, more preferably 500 MPa or more, in order to withstand the gas pressure in pipeline operation, while the yield strength of the electric resistance welded steel pipe material is preferably 560 MPa or less, more preferably 550 MPa or less, in order to avoid an increase in hydrogen embrittlement susceptibility.

[0073] Sizing process In the sizing process, when the electric resistance welded steel pipe material passes through the rolls, it undergoes bending deformation in the pipe axis direction along the roll shape, generating residual stress in the pipe axis direction. The larger the bending strain in the bending deformation, the larger the absolute value of the residual stress in the pipe axis direction. The bending strain increases as the diameter of the sizing rolls decreases and as the thickness of the hot-rolled steel sheet increases. If the diameter of the sizing roll does not satisfy the above formula (3), the residual stress in the electric resistance welded steel pipe will increase, which may induce accumulation of hydrogen that has penetrated the electric resistance welded steel pipe and cause an increase in the rate of fatigue crack propagation in hydrogen.In order to reduce the residual stress in the electric resistance welded steel pipe, that is, to reduce the absolute value of the residual stress in the pipe axis direction, it is preferable to set the diameter of the sizing roll to satisfy the above formula (3).

[0074] Internal pressure loading process In the internal pressure application process, the electric resistance welded steel pipe material is expanded to generate tensile stress in the circumferential direction of the pipe, thereby reducing the absolute value of residual stress in the circumferential direction of the pipe. The greater the internal pressure p (MPa) in the internal pressure loading step, the smaller the absolute value of the residual stress in the circumferential direction of the pipe. The tensile stress generated in the circumferential direction of the pipe becomes higher as the radius of the electric resistance welded steel pipe increases and the wall thickness of the steel pipe decreases. The left side (X) of the above equation (4) corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the pipe is equal to the yield stress of the electric resistance welded steel pipe material. In the present invention, in order to reduce the absolute value of the residual stress in the pipe axis direction, it is preferable to set the internal pressure p to a value greater than (X) on the left side of equation (4) and expand the electric resistance welded steel pipe material to the plastic region. On the other hand, if the internal pressure p exceeds (X × 1.5) on the right side of equation (4), the absolute value of the residual stress in the pipe circumferential direction will be small, but the amount of work hardening due to pipe expansion will be too large, increasing the dislocation density on the steel pipe surface and potentially reducing hydrogen embrittlement resistance. [Example]

[0075] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to the following examples in any way.

[0076] A billet (slab) having the chemical composition shown in Tables 1-1 and 1-2 was prepared, and the billet was heated (heating process), hot-rolled to form a hot-rolled steel plate (hot-rolling process), and the hot-rolled steel plate was cooled (cooling process) to obtain a steel material. Steel materials Nos. 1 and 50 in Table 2-2 are the steel materials obtained after the cooling process, i.e., steel plates. Other than Nos. 1 and 50, Nos. 2 to 49 and 51 to 59 were formed into pipes after the cooling process (pipe-making process) to obtain welded steel pipes (electric-resistance welded steel pipes). The manufacturing conditions are shown in Tables 2-1 and 2-2. Here, the wall thickness (plate thickness) of the hot-rolled steel plate after the hot-rolling process was 20 mm. In the above-mentioned pipe-making process, the steel material obtained in the cooling process was cold-rolled and then butt-welded to form a welded steel pipe (electric-resistance welded steel pipe). The metallographic structure and properties of the obtained steel plates and welded steel pipes (sometimes simply referred to as steel pipes) were evaluated by the following methods.

[0077] Metal structure evaluation Measurement of the area fraction of retained austenite X-ray diffraction (XRD) specimens were taken from the center of the rolling direction (L direction, or in the case of steel pipes, also referred to as the longitudinal direction or pipe axis direction) of the steel plate and steel pipe obtained as described above, so that the metallographic structure could be evaluated at a position 1 / 4 in the wall thickness direction (plate thickness) relative to the surface of the steel plate, and at a position 1 / 4 in the wall thickness direction relative to the inner surface of the steel pipe. Furthermore, for steel plate, cross sections parallel to the rolling direction and width direction (C direction) and for steel pipe, cross sections parallel to the pipe axis direction and pipe circumferential direction (C direction) were used as the evaluation surface. The evaluation surface of the obtained XRD specimens was buffed, and then the surface layer was removed by chemical polishing using picric acid etching, and XRD measurement was performed. Specifically, a Co-Kα source was used for the incident X-rays, and the volume fraction of retained austenite was calculated from the 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.

[0078] Measurement of the area fraction of martensite islands Island martensite (MA) observation specimens were taken from the rolling direction center of the steel plate and steel pipe obtained as described above, so that the metallographic structure could be observed at a position 1 / 4 of the way through the wall thickness (plate thickness) relative to the surface of the steel plate for the steel plate, and at a position 1 / 4 of the way through the wall thickness (plate thickness) relative to the inner surface of the steel pipe for the steel pipe. The observation surface of the obtained MA observation specimens was subjected to two-stage etching to reveal the island martensite, and 10 fields of view were photographed at approximately 3000x magnification using a scanning electron microscope. Image analysis was performed using ImageJ, and the area fraction of island martensite in each field of view was calculated. The average of the area fractions of island martensite in the 10 fields of view was defined as the area fraction of island martensite. The two-stage etching is a method in which, in order to distinguish between island martensite and cementite, electrolytic etching is performed in an electrolyte of EDTA (ethylenediaminetetraacetic acid) and NaF, followed by further electrolytic etching in sodium picrate to remove the cementite.

[0079] Measurement of bainite area fraction Bainite observation specimens were prepared from the central portion of the rolling direction of the steel plate and steel pipe obtained as described above, so that the observation surface could be the same as that of the XRD specimen and the MA observation specimen. The observation surface of the obtained bainite observation specimen was buffed and then etched with 3 vol% nital. The observation surface was then observed at 100x magnification in three fields using an optical microscope, and microstructure images were obtained. Bainite was visually determined by comparing with the microstructure photographs in Reference 1. Based on this determination, the microstructure images were binarized into bainite and other regions. The binarized microstructure images were then used to determine the area fraction of bainite. The average of the area fractions of bainite in the three fields was then used as the bainite area fraction. [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

[0080] Measurement of saturated hydrogen content The saturated hydrogen amounts of the steel plates and steel pipes obtained as described above were determined by subjecting the steel plates and steel pipes to the hydrogen absorption test described below, and the saturated hydrogen amounts that had penetrated into the steel plates and steel pipes were measured using the thermal desorption analysis method described below. The steel materials of the present invention were determined to have excellent hydrogen absorption resistance when the amount of saturated hydrogen that penetrated into the steel plates and steel pipes in a hydrogen gas environment at 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 plate or steel pipe obtained in the tensile test described later is less than 500 MPa, and when the TS of the steel plate or steel pipe is 500 MPa or more, the predetermined value is 0.8 ppm when the TS of the steel plate or steel pipe is 500 MPa or more, and the predetermined value is 0.8 ppm when the TS of the steel plate or 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 plate or steel pipe.

[0081] The hydrogen absorption test was conducted as follows. For the steel plates, test specimens were cut from the quarter-way point in the wall thickness direction, based on the surface of the steel plate, measuring 10 mm in the width direction, 10 mm in the wall thickness direction, and 20 mm in the rolling direction. For the steel pipes, test specimens were cut from the quarter-way point in the wall thickness direction, based on the inner surface 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 with SiC paper to #1000 using a wet rotary polisher, then rinsed with water and acetone. In addition, 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 thickness: 3–4 μm). The test specimens were then placed in the chamber of a high-pressure gas exposure test apparatus, 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 carried out for 24 hours, which is longer than the time required for hydrogen to penetrate into the steel plate and steel pipe to become saturated.

[0082] 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.

[0083] Tensile strength measurement The tensile strength (TS) of the steel plates and steel pipes obtained as described above was measured by conducting a tensile test. The tensile test specimens used in the tensile test were taken and processed into bar-shaped test specimens 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 plate and steel pipe was calculated by dividing the maximum load by the initial cross-sectional area of ​​the tensile test specimen.

[0084] Evaluation of hydrogen embrittlement resistance The hydrogen embrittlement resistance of the steel plate and steel pipe obtained as described above was evaluated using the fatigue crack growth characteristics in hydrogen. The fatigue crack growth characteristics in hydrogen were evaluated by conducting a fatigue crack growth test according to the following procedure, calculating the fatigue crack growth rate da / dN (mm / cycle) in hydrogen in the stress intensity factor range ΔK = 25 MPa, and using the obtained da / dN (mm / cycle). In the present invention, da / dN is 2.0 × 10 -3A fatigue crack growth rate of less than 1 / 2 mm / cycle was considered 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 E647, Standard Test Method for Measurement of Fatigue Crack Growth Rate, under the following conditions: frequency: 1 Hz, repeated 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 cut in accordance with ASTM E647, from a quarter-way position in the thickness direction (plate thickness) relative to the surface of the steel plate, and from a quarter-way position in the thickness direction (pipe inner surface) relative to the inner surface of the steel pipe, so that the loading direction was parallel to the transverse direction 3 (C direction, also known as the width direction for steel plate and the circumferential direction for steel pipe). The thickness of the specimens 2 in the thickness direction of the steel plate and steel pipe was 10 mm. The front and back surfaces of specimen 2 were mirror-polished to prevent variations in the fatigue crack growth rate in hydrogen due to surface finishing. A fatigue pre-crack was introduced into the specimen in an air environment. Note that reference number 1 indicates the rolling direction (L direction).

[0085] The steel plates and steel pipes shown as examples of the present invention satisfy the chemical composition, metal structure, and saturated hydrogen content less than the specified value as shown in Tables 1-1, 1-2, 2-1, and 2-2. As a result, the fatigue crack growth rate in hydrogen gas (fatigue crack growth rate in hydrogen) is 2.0 × 10 -3 mm / cycle or less, demonstrating excellent hydrogen embrittlement resistance.

[0086] [Table 1-1]

[0087] [Table 1-2]

[0088] [Table 2-1]

[0089] [Table 2-2] [Explanation of symbols]

[0090] 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: 0.02-0.15%, Si: 0.01-2.00%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.0015% or less, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.001 to 0.10%, Ca: 0-0.005%, Ni: 0-2.0%, Ti: 0 to 0.1%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0-0.10%, Zr: 0 to 0.050%, B: 0 to 0.0020%, REM: 0-0.01%, Mg: 0 to 0.01%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: 0 to 0.3%, and Co: 0 to 5.0%, The balance is Fe and unavoidable impurities, At 1 / 4 of the thickness of the steel material, The area fraction of retained austenite is 0% or more and 3% or less, Bainite has an area fraction of 90% or more, and The steel has a metal structure in which island martensite (MA) has an area fraction of 0% or more and less than 5%; A steel material 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 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 component composition is, in mass%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Ti: 0.005-0.1%, Cu: 0.01 to 1.0%, Cr: 0.01-1.0%, Mo: 0.01 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, REM: 0.0001-0.01%, Mg: 0.0001-0.01%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001-0.3%, Sb: 0.0001 to 0.3%, and The steel material according to claim 1, comprising one or more selected from Co: 0.01 to 5.0%.

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

4. a heating step of heating a slab having the component composition according to claim 1 or 2 at a heating temperature of 1000°C or higher and 1250°C or lower; a hot rolling step of hot rolling the slab heated in the heating step into a hot-rolled steel plate; The hot-rolled steel sheet obtained in the hot rolling step is Cooling start temperature: Surface temperature of hot-rolled steel sheet ３ point or higher, and the difference between the surface temperature of the hot-rolled steel plate and the temperature at the center of the thickness of the hot-rolled steel plate is 50°C or less, and the difference between the tip temperature and the tail temperature at the center of the thickness of the hot-rolled steel plate is 60°C or less, Average cooling rate from 750 ° C. to 500 ° C. on the surface of the hot-rolled steel plate: 15 ° C. / sec or more and 50 ° C. / sec or less; The difference between the average cooling rate from 750 ° C. to 500 ° C. at the surface of the hot-rolled steel plate and the average cooling rate from 750 ° C. to 500 ° C. at the center of the thickness: 20 ° C. / sec or less; Cooling stop temperature: 250°C or more and 500°C or less at the center of the thickness of the hot-rolled steel plate; a cooling step of cooling the steel material under the following conditions to obtain a steel material; A method for manufacturing a steel material having the above structure.

5. 5. The method for producing a steel material according to claim 4, further comprising, after the cooling step, a pipe-making step of forming the steel material obtained after the cooling step into a cylindrical shape and butt-welding both circumferential ends of the cylindrical steel material to form a welded steel pipe.