Steel material and method for producing steel material

Abstract

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

Description

Steel material and steel material manufacturing method

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

[0002] Existing energy infrastructure includes line pipes used to transport crude oil, natural gas, and the like. When line pipes manufactured using steel are used in an atmosphere containing hydrogen sulfide, they can experience hydrogen embrittlement, such as hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), which can pose safety issues. To prevent hydrogen embrittlement, such as hydrogen-induced cracking and sulfide stress corrosion cracking, various measures have been taken to prevent such hydrogen embrittlement, such as reducing the amount of MnS in the steel, which can cause crack initiation, suppressing the accumulation of Ti and Nb carbonitrides and oxides, or 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 or 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 as a clean energy source has been promoted 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 transported through natural gas line pipes. The transport pressure of 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 that reduces hydrogen accumulation within the steel.

[0004] Conventionally, austenitic stainless steels such as SUS316L, which exhibit hydrogen embrittlement resistance, have been used in steel structures used in high-pressure hydrogen gas environments. However, these have problems such as high material costs when manufacturing steel structures and low strength. On the other hand, when designing a steel structure using austenitic stainless steel to withstand high hydrogen pressures, it becomes necessary to increase the wall thickness, which results in a problem of extremely high prices for 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.

[0006] JP 2011-26695 A JP 2010-209461 A JP 2019-505675 A

[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.

[0009] The present inventors conducted technical studies to identify conditions that a steel material must satisfy to obtain a steel material with excellent hydrogen absorption resistance, with the aim of suppressing hydrogen absorption into the steel material, which is the root cause of hydrogen embrittlement in a hydrogen gas environment. As a result, they discovered that adjusting the steel material 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 a quarter of the wall thickness, improves the hydrogen absorption resistance of the steel material and provides excellent hydrogen embrittlement resistance in a hydrogen gas environment. Furthermore, achieving such a steel material structure requires strict control of the cooling conditions in the cooling process after the hot rolling process, and they have successfully identified the necessary 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 to 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 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0 to 0.10%, Zr: 0 to 0.050%, B: 0 to 0.0020%, A steel material having a chemical composition containing REM: 0-0.01%, Mg: 0-0.01%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and Co: 0-5.0%, with the balance being Fe and unavoidable impurities, and having a metallic structure at a 1 / 4 position of the wall thickness of the steel material, in which retained austenite is present in an area fraction of 0% or more and 3% or less, bainite is present in an area fraction of 90% or more, and island martensite (MA) is present in an area fraction of 0% or more and less than 5%, and in which the amount of saturated hydrogen that has penetrated 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 the hydrogen content C calculated from the following formula (1) when the tensile strength TS of the steel material is 500 MPa or more. Ｈ (ppm by mass), Ｈ (mass ppm)=5.4637e-0.004×TS(MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material. [2] The composition of the elements 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 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, The steel material according to [1], which contains one or more selected from Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%, and Co: 0.0001 to 5.0%. [3] The steel material according to [1] or [2], which is a welded steel pipe. [4] A hot rolling process in which a slab having the chemical composition according to [1] or [2] is heated at a heating temperature of 1000°C or more and 1250°C or less, and the slab heated in the heating process is hot-rolled to form a hot-rolled steel sheet. The hot-rolled steel sheet obtained in the hot-rolling process is subjected to a cooling start temperature: a surface temperature of the hot-rolled steel sheet. ３a cooling step of cooling the hot-rolled steel plate to a temperature above 750°C / s or above, and a 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 of 50°C or less, and a difference between the tip temperature and the tail temperature at the center of the thickness of the hot-rolled steel plate of 60°C or less, an average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel plate of 15°C / s or more and 50°C / s or less, a difference between the average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel plate of 20°C / s or less, and a cooling stop temperature of 250°C to 500°C at the temperature at the center of the thickness of the hot-rolled steel plate of 250°C to 500°C. [5] A method for producing a steel product according to [4], further comprising, after the cooling step, a pipe-making step of forming the steel product obtained after the cooling step into a cylindrical shape and butt-welding both circumferential ends of the cylindrical steel product to form a welded steel pipe.

[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.

[0011] 1 is a diagram showing a compact tension (CT) test piece used in a fatigue crack propagation test.

[0012] The present invention will be specifically described 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 component composition. The reasons for limiting the component composition of the steel material in the present invention will be described below. In the following description, all units shown in % are mass % unless otherwise specified.

[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 deteriorates. Furthermore, the hardness of the surface layer and central segregation of the steel increases during cooling, reducing hydrogen absorption resistance (increasing the amount of saturated hydrogen), resulting in reduced 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, more preferably 0.13% or less. If the C content exceeds 0.12%, toughness may decrease. Therefore, 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 is an element that acts as a deoxidizer. However, if the Si content is less than 0.01%, the deoxidizing effect is insufficient. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 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 is also reduced. 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. A P content exceeding 0.015% reduces weldability and increases the hardness of the central segregation region. 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 inclusion effect 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 its toughness decreases. Furthermore, Al-based inclusions act as hydrogen trapping sites, reducing hydrogen absorption resistance and 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 reduced toughness in hydrogen gas and reduced 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 can 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 in the present invention, the inclusion of these elements is not essential, 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 required 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 contained, 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 contained, 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 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 achieve 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 the steel 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 included, 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. On the other hand, since the inclusion of Cr is not essential, the lower limit of the Cr content may be 0%. However, to obtain the above effect, the Cr content is preferably 0.01% 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 hydrogen sulfide partial pressure. It 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. It also reduces weldability. 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. On the other hand, since the inclusion of Mo is not essential, the lower limit of the Mo content may be 0%. However, to achieve the above effects, the Mo content is preferably set to 0.01% or more, more preferably 0.10% 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 more 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 achieve 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 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 achieve the above effect, 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 contained, the Zr content is set to 0.050% or less. The Zr content is preferably set to 0.045% or less, more preferably 0.040% or less, even more preferably 0.030% or less, and most 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 achieve the above effects, the Zr content is preferably set to 0.0001% or more, more preferably 0.001% or more, even more preferably 0.005% or more, and most preferably 0.008% or more.

[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 achieve 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 increasing the toughness of steel by refining crystal grains and increasing the crack resistance of steel by controlling the properties of inclusions. These elements can be added at any amount depending on the desired properties. However, if the REM content and Mg content exceed 0.01%, the above effects saturate. Therefore, when REM and Mg are each contained, the REM content and Mg content should each be 0.01% or less. The REM content and Mg content should each be preferably 0.0090% or less, more preferably 0.0085% or less, even more preferably 0.0080% or less, and most preferably 0.0075% or less. However, since the inclusion of REM and Mg is not essential, the lower limits of the REM content and Mg content may each be 0%. However, in order 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 contained, the Hf content is set to 0.2% or less. The Hf content is preferably set to 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 achieve the above effects, the Hf content is preferably set to 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. Ta can be added at any amount depending on the required strength. However, a Ta content exceeding 0.2% can result in 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. Meanwhile, since the inclusion of Ta is not essential, the lower limit of the Ta content may be 0%. However, to achieve the above-mentioned 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 to 0.005% Re 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 Re content exceeds 0.005%, Re oxides increase and aggregate, resulting in a decrease in hydrogen absorption resistance. Therefore, when Re is contained, 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, to obtain the above effect, 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, resulting in increased costs. 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 each preferably set to 0.20% or less, even 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 each preferably set to 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.004% or more.

[0041] Co: 0 to 5.0% Like Ni and Cu, Co is an element that improves hardenability. It contributes to increasing the strength of steel, suppresses the coarsening of prior austenite grains, and improves 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, resulting in 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 achieve 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] [Metallic Structure] The steel material of the present invention has a metallic structure at a quarter-thickness position of the steel material, in which the area fraction of retained austenite is 0% to 3%, the area fraction of bainite is 90% or more, and the area fraction of island martensite (MA) is 0% to less than 5%. Hydrogen absorption in steel material occurs from the surface that directly contacts hydrogen gas. Therefore, it is considered that it is sufficient to control the metallic structure at least from the surface of the steel material that directly contacts hydrogen gas to an appropriate metallic structure from the interior. Therefore, as a result of studies by the inventors, it was found that excellent hydrogen absorption resistance can be obtained by controlling the metallic structure at the quarter-thickness position based on the surface on the side that contacts hydrogen gas to the above metallic structure, and as a result, excellent hydrogen embrittlement resistance can be obtained. Here, it is sufficient for the steel material to have the above metallic structure at the quarter-thickness position based on at least one of the steel material surfaces, and the steel material surface from which the hydrogen absorption originated can be exposed to a hydrogen gas environment. For example, when the steel material is a welded steel pipe, the inner surface of the welded steel pipe comes into contact with hydrogen gas, so the metal structure at least at a position 1 / 4 of the wall thickness based on the inner surface of the welded steel pipe can be considered as the above metal structure.

[0043] Retained austenite area fraction: 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, more preferably 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 limitation on 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 of 90% or More: If the bainite area fraction 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, and strain concentrates 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 bainite area fraction is set to 90% or more. By setting the bainite area fraction to 90% or more, the strength and toughness required for steel pipes used in linepipes can be ensured. The bainite area fraction is preferably 92% or more, more preferably 93% or more, and even more preferably 95% or more. On the other hand, since it is desirable to increase the bainite area fraction as much as possible, the upper limit of the bainite area fraction is not particularly limited and may be 100%. The area fraction of bainite may be measured by 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) area fraction: 0% or more but 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 fraction 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 using the method described in the examples.

[0046] Other Structures: The metal structure in one embodiment of the present invention may consist of the above-described metal structure. Furthermore, the metal structure in other embodiments of the present invention may contain, in addition to the above-described metal structure, other metal structures 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 into the steel material in a hydrogen gas environment with a hydrogen gas pressure of 40 MPa that is less than a predetermined value. Here, the predetermined value is 0.8 mass ppm when the tensile strength TS of the steel material is less than 500 MPa, and is the amount of hydrogen C obtained by the following formula (1) when the tensile strength TS of the steel material is 500 MPa or more. Ｈ (mass ppm). Ｈ (mass ppm)=5.4637e-0.004×TS(MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material.

[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 SSCC (Sulfide Stress Corrosion Cracking), and stress concentration sources such as initial defects in the steel, promoting the propagation of cracks (fatigue cracks) generated by stress loads, leading to fracture. This results in a decrease in hydrogen embrittlement resistance in a hydrogen gas environment. Furthermore, generally, the higher the TS of a steel, the more likely it is to suffer from hydrogen embrittlement. In other words, whether 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 a relationship between the TS of a steel and its hydrogen tolerance (saturated hydrogen content). That is, it has been found that by making the saturated hydrogen content in the steel less than a predetermined value (improving hydrogen absorption resistance) according to 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 a 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. Ｈ (mass ppm) = 5.4637e - 0.004 × TS (MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel material. The saturated hydrogen amount 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 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 a pressure of 40 MPa, in accordance with ASTM E647, Standard test method for measurement of fatigue crack growth rate, under the following conditions: frequency: 1 Hz, repeated load waveform: sine wave, control method: load control, loading conditions: uniaxial tension, and stress ratio: R = 0.1. Here, for the fatigue crack growth test, a compact tension (CT) test piece used in a fatigue crack growth test in accordance with ASTM E647 may be 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 that the hydrogen embrittlement resistance is excellent. －３ If the thickness is less than 1 / 2 mm / cycle, it will be possible to design long-life steel structures for hydrogen use, such as line pipes, within the range of wall thickness that can be manufactured using steel materials, particularly welded steel pipes such as UOE.

[0050] Steel materials 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 assuming 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, the upper limit of the TS of the above steel material is not particularly limited, but excessively high strength leads to increased costs and reduced hydrogen absorption resistance. Therefore, 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 Material] The steel material of the present invention includes thin steel plates, thick steel plates, welded steel pipes, shaped steel, steel bars, and the like. The steel material has the above-described chemical composition and metallographic structure, and 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 manufacturing a steel material of the present invention will be described. The steel material can be manufactured by sequentially performing steps (a) to (c) on a slab (steel material) having the above-described chemical composition. (a) Heating step (b) Hot rolling step (c) Cooling step In the following description, unless otherwise specified, temperature refers to the temperature at the surface of the slab, hot-rolled steel plate, or steel material. The surface temperature of the slab, hot-rolled steel plate, or steel material can be measured using a radiation thermometer or the like. The center-thickness 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 Step In the heating step, the slab (steel material) having the above-described composition is heated at a heating temperature of 1000°C or higher and 1250°C or lower.

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

[0056] Heating temperature: 1000°C or higher and 1250°C or lower. If the heating temperature is lower than 1000°C, the diffusion of elements such as C, P, and S microsegregated in the slab will be insufficient, 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 Step 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: Ar ３ In the hot rolling process, the slab heated in the heating process is hot rolled to a desired thickness to produce a hot rolled steel sheet. ３ If the temperature is lower than this point, in the case of a process in which cooling is performed immediately after hot rolling, there is a risk of a decrease in strength due to the formation of soft ferrite. Therefore, the hot rolling finishing temperature is set to the ferrite formation temperature, Ar ３ It is preferable that the temperature is equal to or higher than Ar ３ It is more preferable to set the temperature at the temperature above 30°C. ３ It is more preferable that the temperature is set to be 50°C or higher than the temperature at which the temperature is reached. ３ It is most preferable to set the temperature at 70°C or higher than the temperature at which the Ar ３ It is most preferable that the hot rolling end temperature is the temperature above the rolling point +100°C. 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 1250°C or less, more preferably 1220°C or less, even more preferably 1180°C or less, and most preferably 950°C or less.

[0059] Ar ３Since the transformation temperature varies depending on the alloy composition of the steel material, it may be determined by measuring the transformation temperature of each steel material through an experiment. Alternatively, it may be determined from the composition of the steel material using the following formula (2): Ar ３ (°C) = 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 produce 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, ３ point or higher, 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. (2) Average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel plate: 15°C / sec or more and 50°C / sec or less. (3) 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 to 500°C at the center of the thickness of the hot-rolled steel plate.

[0061] Cooling start temperature The cooling start temperature of the steel material after the hot rolling process is the surface temperature of the hot-rolled steel sheet. ３ If the cooling temperature is lower than this point, ferrite will be generated before the start of cooling, resulting in a significant decrease in strength. Furthermore, the generation of ferrite will cause a decrease in hydrogen absorption resistance due to a difference in hardness, and the hydrogen embrittlement resistance in a hydrogen gas environment will decrease. For this reason, the cooling start temperature should be set to a value lower than the surface temperature of the hot-rolled steel sheet in Ar. ３ The cooling start temperature is Ar ３ It is preferable to set the temperature at the temperature above 20°C. ３ It is more preferable to set the temperature at the temperature above +40°C, and ３It is more preferable that the cooling start temperature is the temperature above the temperature point +60°C. On the other hand, if the cooling start temperature is too high, the grain size becomes too large and the toughness decreases. Therefore, the cooling start temperature is preferably less than 1250°C, more preferably 1100°C or less, even more preferably 1050°C or less, and most preferably 900°C or less.

[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. Furthermore, segregation zones that become hydrogen trapping sites will be formed, the amount of saturated hydrogen will increase, and hydrogen embrittlement resistance will be significantly reduced. 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. The lower limit of the temperature difference is not particularly limited, but is preferably set to 5°C or more, and more preferably 10°C or more.

[0063] Furthermore, if the cooling start temperature exceeds 60°C as the difference (temperature difference) between the tip temperature and the tail temperature in the rolling direction at the center position of the thickness of the hot-rolled steel sheet, a temperature gradient occurs in the longitudinal direction (rolling direction) of the hot-rolled steel sheet at the center position of the thickness when cooling is stopped. This results in a significant temperature variation at the time of cooling stop, which reduces the area fraction of bainite at the quarter position of the thickness and increases the area fraction of island martensite, resulting in a deterioration of 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 the temperature variation at the end of cooling, this reduces 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 is formed, 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 lower. The cooling stop temperature is preferably set to 490°C or lower, more preferably 480°C or lower, and even more preferably 460°C or lower. On the other hand, if the cooling stop temperature is lower 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 higher. Furthermore, hydrogen present in the steel gradually escapes during cooling, but if the cooling stop temperature is lower 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 higher. 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 slowly cool the steel material 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 manufactured by sequentially performing the processes (a) to (d) on a cast slab having the above-mentioned composition. (a) Heating process (b) Hot rolling process (c) Cooling process (d) Pipe-making process However, the above processes (a) to (c) are as described above. Therefore, only the pipe-making process will be described 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 butted together and welded to form a welded steel pipe. The method for forming the steel material into a cylindrical shape is not particularly limited, but examples include bending and cold roll forming. Specifically, in the bending process, the end of the steel material is beveled, and the steel material is formed into a cylindrical shape using a C press, a U press, or an O press. Then, both circumferential ends of the obtained cylindrical steel material are butted together, and the butt joint is seam-welded by internal and external welding to produce a welded steel pipe. Alternatively, the welded steel pipe may be formed into a cylindrical shape by press bending, and then the butt joint is seam-welded to produce a welded steel pipe. If necessary, the welded steel pipe may be further manufactured through a pipe expansion process. Any welding method may be used as long as it provides sufficient joint strength and joint toughness. However, submerged arc welding is preferred from the viewpoints of excellent weld quality and production efficiency. An example of a welded steel pipe manufactured using the above-mentioned bending process is a UOE steel pipe. In cold roll forming, a steel material may be formed into a cylindrical shape by cold roll forming. Then, both circumferential ends of the obtained cylindrical steel material are butted together and electric resistance welded to manufacture 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 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) is applied to the inner surface of the electric resistance welded steel pipe material (internal pressure application process), thereby producing the electric resistance welded steel pipe. Note that the cylindrical shape refers to a "C"-shaped circumferential cross section of the steel pipe. 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. Furthermore, the diameter (mm) of the sizing roll is preferably 1800 mm or less. X < p ≤ X × 1.5 ... (4) Note that X = (Thickness of electric resistance welded steel pipe material (mm) / Radius of electric resistance welded steel pipe material (mm)) × Yield strength of electric resistance welded steel pipe material (MPa)

[0072] The above-mentioned internal pressure can be applied, for example, by sealing the pipe end of the electric resistance welded steel pipe material with a rubber packing and applying water pressure to the inside of the electric resistance welded steel pipe material. Furthermore, to stabilize the shape, a mold of the desired diameter can be used as an outer frame, if necessary. The wall thickness of the electric resistance welded steel pipe material is preferably 5 mm or more. It is also preferably 30 mm or less. There is no particular upper limit to the radius of the electric resistance welded steel pipe material, but as the radius of the electric resistance welded steel pipe material increases, the load on the equipment increases. Therefore, the radius of the electric resistance welded steel pipe material is preferably 600 mm or less, more preferably 500 mm or less. There is no particular lower limit to the radius of the electric resistance welded steel pipe material, but it is preferably 200 mm or more. 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. On the other hand, in order to avoid an increase in hydrogen embrittlement susceptibility, the yield strength of the electric resistance welded steel pipe material is preferably 560 MPa or less, and more preferably 550 MPa or less.

[0073] Sizing process: In the sizing process, bending deformation occurs in the pipe axis direction along the roll shape when the electric resistance welded steel pipe material passes through the rolls, generating residual stress in the pipe axis direction. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the pipe axis direction. The smaller the diameter of the sizing roll and the greater the thickness of the hot-rolled steel sheet, the greater the bending strain. If the diameter of the sizing roll does not satisfy the above formula (3), the residual stress in the electric resistance welded steel pipe will increase, which may induce the 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, i.e., to reduce the absolute value of the residual stress in the pipe axis direction, it is preferable to set the diameter of the sizing roll to satisfy the above formula (3).

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

[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] Billets (cast pieces) having the chemical compositions shown in Tables 1-1 and 1-2 were prepared, and the billets were heated (heating process), hot-rolled to form hot-rolled steel sheets (hot-rolling process), and the hot-rolled steel sheets were cooled (cooling process) to obtain steel materials. 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 sheets after the hot-rolling process was 20 mm. In the above-mentioned pipe-making process, the steel materials obtained in the cooling process were cold-rolled and then butt-welded to form welded steel pipes (electric-resistance welded steel pipes). The metallographic structure and properties of the resulting steel plates and welded steel pipes (sometimes simply referred to as steel pipes) were evaluated by the following methods.

[0077] Evaluation of Metallographic Structure Measurement of Area Fraction of Retained Austenite Test pieces for X-ray diffraction (XRD) 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 of the way in the wall thickness direction (plate thickness) with respect to the surface of the steel plate, and at a position 1 / 4 of the way in the wall thickness direction with respect to the inner surface of the steel pipe. Furthermore, in the case of steel plate, a cross section parallel to the rolling direction and width direction (C direction) served as the evaluation surface, and in the case of steel pipe, a cross section parallel to the pipe axis direction and pipe circumferential direction (C direction) served as the evaluation surface. The evaluation surface of the obtained XRD test piece 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 radiation 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 Island Martensite

[0046] 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 thickness (plate thickness) direction relative to the surface of the steel plate for the steel plate, and at a position 1 / 4 of the way through the thickness direction relative to the inner surface of the steel pipe for the steel pipe. The observation surface of the obtained MA observation specimen was subjected to two-stage etching to reveal the island martensite. Ten fields of view were then 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 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 island martensite from cementite, electrolytic etching is performed in an electrolyte of EDTA (ethylenediaminetetraacetic acid) and NaF, and then electrolytic etching is performed in sodium picrate to remove the cementite.

[0079] Measurement of Bainite Area Fraction: Bainite 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 quarter position in the wall thickness direction (plate thickness direction) relative to the surface of the steel plate for the steel plate, and at a quarter position in the wall thickness direction relative to the inner surface of the steel pipe for the steel pipe. The observation surfaces of the obtained bainite observation specimens were buffed and etched with 3 vol% nital. Three fields of view of the observation surface were then observed at 100x magnification using an optical microscope to obtain microstructural images. Bainite was visually determined by comparing with the microstructural photographs in Reference 1, and the microstructural images were binarized into bainite and other regions based on the above determination. The binarized microstructural images were then used for image analysis to determine the area fraction of bainite. The average value of the area fractions of bainite in the three fields of view was taken as the area fraction of bainite. [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 content of the steel plate and steel pipe obtained as described above was determined by subjecting the steel plate and steel pipe to the hydrogen absorption test described below, and the saturated hydrogen content was determined as the amount of saturated hydrogen that had penetrated into the steel plate and steel pipe. The saturated hydrogen content of the steel plate and steel pipe was measured using the thermal desorption analysis method described below. The steel material of the present invention was determined to have excellent hydrogen absorption resistance when the amount of saturated hydrogen that penetrated 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 plate or steel pipe obtained in the tensile test described below is less than 500 MPa, and is 0.8 ppm when the TS of the steel plate or steel pipe is 500 MPa or more, as expressed by formula (1). Ｈ (ppm by mass). Ｈ (mass ppm)=5.4637e-0.004×TS(MPa) (1) In formula (1), TS (MPa) is the tensile strength of the steel plate or steel pipe.

[0081] The hydrogen absorption test was performed as follows. For the steel plates, test specimens for the hydrogen absorption test were cut from a quarter-way position in the wall thickness direction (plate thickness) 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 a quarter-way position 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 to #1000 using a wet rotary polisher with SiC paper, followed by water and acetone cleaning. Furthermore, to eliminate the effect of an oxide film formed on all surfaces of the test specimens that inhibits hydrogen absorption, all surfaces of the test specimens were Pd-plated (Pd plating layer thickness: 3-4 μm). The test specimens were then placed in the chamber of a high-pressure gas exposure test device, and the chamber was filled with hydrogen gas. The test temperature was 25° C., the hydrogen gas pressure was 40 MPa, and the hydrogen gas concentration was 99.9999%. The hydrogen absorption test was 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] The thermal desorption analysis was carried out as follows. The saturated hydrogen amount of the test piece after the hydrogen absorption test was measured using thermal desorption analysis. A low-temperature temperature-programmed hydrogen analyzer (gas chromatograph type) (JTF-20AL) was used for the measurement. The thermal desorption analysis was carried out in the temperature range from room temperature to 400°C at an average heating rate of 200°C / h from room temperature to 400°C, and the sum of the obtained hydrogen amounts was taken as the saturated hydrogen amount.

[0083] Measurement of Tensile Strength The tensile strength (TS) of the steel plate and steel pipe 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 maximum load divided by the initial cross-sectional area of ​​the tensile test specimen was taken as the TS (MPa) of the steel plate and steel pipe, respectively. The test specimens 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 maximum load divided by the initial cross-sectional area of ​​the tensile test specimen was taken as the TS (MPa) of the steel plate and steel pipe, respectively.

[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 was evaluated as 2.0 × 10 －３The fatigue crack growth test was performed in a hydrogen gas environment at room temperature (20±10°C) and a pressure of 40 MPa, in accordance with ASTM 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. For the fatigue crack propagation test, compact tension (CT) test specimens 2 shown in Figure 1 were taken in accordance with ASTM E 647. The test specimens were taken from a quarter-way position in the wall thickness direction (plate thickness direction) relative to the surface of the steel plate, and from a quarter-way position in the wall thickness direction relative to the inner surface of the steel pipe, so that the load direction was parallel to the direction transverse to the rolling direction 3 (C direction, also referred to as the width direction for steel plates and the circumferential direction for steel pipes). The thickness of the test specimens 2 was 10 mm in the wall thickness direction of the steel plate and steel pipe. The front and back surfaces of the test specimens 2 were mirror-polished to avoid variations in the fatigue crack propagation rate in hydrogen due to surface finishing. Fatigue pre-cracks were introduced into the test specimens in an air environment. Reference symbol 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 predetermined 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, and showed excellent hydrogen embrittlement resistance.

[0086]

[0087]

[0088]

[0089]

[0090] 1. Rolling direction (L direction) 2. Compact tension (CT) test piece (test piece) used for fatigue crack propagation test 3. Direction perpendicular to rolling (C direction)

Claims

1. In mass%, C: 0.02 to 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 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0 to 0.10%, Zr: 0 to 0.050%, B: 0 to 0.0020%, A steel material having a chemical composition containing REM: 0-0.01%, Mg: 0-0.01%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and Co: 0-5.0%, with the balance being Fe and unavoidable impurities, and having a metallic structure at a 1 / 4 position of the wall thickness of the steel material, in which retained austenite is present in an area fraction of 0% or more and 3% or less, bainite is present in an area fraction of 90% or more, and island martensite (MA) is present in an area fraction of 0% or more and less than 5%, and in which the amount of saturated hydrogen that has penetrated 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 the hydrogen content C calculated from 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 formula (1), TS (MPa) is the tensile strength of the steel material.

2. The chemical 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 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, The steel material according to claim 1, comprising one or more selected from Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%, and Co: 0.0001 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 chemical composition according to claim 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 to form a hot-rolled steel sheet; and a cooling start temperature: the surface temperature of the hot-rolled steel sheet, at which the hot-rolled steel sheet obtained in the hot-rolling step is cooled with Ar. ３ a cooling step of cooling the hot-rolled steel plate to a temperature above the target point, and a difference between the surface temperature of the hot-rolled steel plate and the temperature at the thickness center of the hot-rolled steel plate of 50°C or less, and a difference between the tip temperature and the tail temperature at the thickness center of the hot-rolled steel plate of 60°C or less; an average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel plate of 15°C / sec or more and 50°C / sec or less; a difference between the average cooling rate from 750°C to 500°C at the surface of the hot-rolled steel plate of 20°C / sec or less; and a cooling stop temperature of 250°C to 500°C at the thickness center of the hot-rolled steel plate of 250°C to 500°C.

5. A method for manufacturing steel material as described in 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.

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