steel material

Abstract

Provided is a steel material having excellent SSC resistance. The steel material of the present invention has a chemical composition, in terms of mass%, of C: 0.035% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, sol. Al: 0.005 to 0.100%, N: 0.001 to 0.020%, Ni: 5.00 to 7.50%, Cr: 10.00 to 14.00%, Cu: 0.01% or more and less than 1.50%, Mo: 1.50 to 3.50%, V: 0.01 to 1.00%, Ti: 0.02 to 0.30%, Co: 0.01 to 0.50%, Ca: 0.0003 to 0.0030%, O: 0.0050% or less, W: 0 to 1.50%, Nb: 0 to 0.50%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements (REM): 0 to 0.020%, and the balance being Fe and impurities, and the total of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more is 0.50 pieces / mm 2 The following.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel material, and particularly to a steel material suitable for use in an acidic environment containing hydrogen sulfide and carbon dioxide. BACKGROUND

[0002] There is an environment containing a large amount of corrosive substances in an oil well or a gas well (hereinafter, the oil well and the gas well will be collectively referred to as "oil well"). The corrosive substances are, for example, hydrogen sulfide (H2S) gas and carbon dioxide (CO2) gas, and the like, which are corrosive gases. In the present specification, the environment containing hydrogen sulfide and carbon dioxide will be referred to as "acidic environment". The temperature of the acidic environment varies depending on the depth of the well, and is about normal temperature to 200°C. In the present specification, the normal temperature means 24±3°C.

[0003] It is known that chromium (Cr) is effective in improving the carbon dioxide corrosion resistance of a steel. Therefore, in an oil well in which a large amount of carbon dioxide is contained, a martensitic stainless steel material containing about 13 mass% of Cr, represented by API L8013Cr steel material (conventional 13Cr steel material), super 13Cr steel material in which the C content is reduced, and the like, is used depending on the partial pressure of carbon dioxide and the temperature.

[0004] Japanese Patent Application Laid-Open No. 10-503809 (Patent Literature 1), Japanese Patent Application Laid-Open No. 2000-192196 (Patent Literature 2), Japanese Patent Application Laid-Open No. 8-246107 (Patent Literature 3), and Japanese Patent Application Laid-Open No. 2012-136742 (Patent Literature 4) propose a steel material excellent in SSC resistance.

[0005] The steel material of Patent Literature 1 contains, in mass%, C: 0.005 to 0.05%, Si ≤ 0.50%, Mn: 0.1 to 1.0%, P ≤ 0.03%, S ≤ 0.005%, Mo: 1.0 to 3.0%, Cu: 1.0 to 4.0%, Ni: 5 to 8%, Al ≤ 0.06%, the balance consisting of Fe and impurities, satisfies Cr + 1.6Mo ≥ 13 and 40C + 34N + Ni + 0.3Cu - 1.1Cr - 1.8Mo ≥ -10.5. The microstructure of the martensitic stainless steel of this document is a tempered martensite structure. It is described in Patent Literature 1 that the SSC resistance can be improved by containing 1.0 to 3.0% of Mo.

[0006] The steel material of Patent Document 2 contains, in mass %, C: 0.001 to 0.05 %, Si: 0.05 to 1 %, Mn: 0.05 to 2 %, P: 0.025 % or less, S: 0.01 % or less, Cr: 9 to 14 %, Mo: 3.1 to 7 %, Ni: 1 to 8 %, Co: 0.5 to 7 %, sol. Al: 0.001 to 0.1 %, N: 0.05 % or less, O (oxygen): 0.01 % or less, Cu: 0 to 5 %, W: 0 to 5 %, and the balance consisting of Fe and inevitable impurities. When Mo is contained, the Ms point is lowered. To this end, by containing Co together with Mo, the lowering of the Ms point is suppressed, and the microstructure is made into a single-phase structure of martensite. It is described in Patent Document 2 that, by this, it is possible to improve the SSC resistance while maintaining the strength of 80 ksi or more (552 MPa or more).

[0007] The chemical composition of the martensitic stainless steel of Patent Document 3 contains, in mass %, C: 0.005 to 0.05 %, Si: 0.05 to 0.5 %, Mn: 0.1 to 1.0 %, P: 0.025 % or less, S: 0.015 % or less, Cr: 12 to 15 %, Ni: 4.5 to 9.0 %, Cu: 1 to 3 %, Mo: 2 to 3 %, W: 0.1 to 3 %, Al: 0.005 to 0.2 %, N: 0.005 to 0.1 %, and the balance consisting of Fe and inevitable impurities. The above chemical composition also satisfies 40C + 34N + Ni + 0.3Cu + Co - 1.1Cr - 1.8Mo - 0.9W ≥ -10. It is described in Patent Document 3 that, in a steel material having a Cr content of 12 to 15 %, by making the C content less than 0.05 %, the Ni content 4.5 % or more, the Cu content 1 to 3 %, Mo 2 to 3 %, and W 0.1 to 3 %, excellent SSC resistance can be obtained.

[0008] The martensitic stainless steel seamless pipe of Patent Document 4 contains, in mass %, C: 0.01 % or less, Si: 0.5 % or less, Mn: 0.1 to 2.0 %, P: 0.03 % or less, S: 0.005 % or less, Cr: 14.0 to 15.5 %, Ni: 5.5 to 7.0 %, Mo: 2.0 to 3.5 %, Cu: 0.3 to 3.5 %, V: 0.20 % or less, Al: 0.05 % or less, N: 0.06 % or less, and the balance consisting of Fe and inevitable impurities. The martensitic stainless steel seamless pipe of this document has a strength of yield strength: 655 to 862 MPa and a yield ratio: 0.90 or more. It is described in Patent Document 4 that, by making the C content 0.01 % or less, adjusting Cr, Ni, and Mo to appropriate ranges, and further containing an appropriate amount of Cu and V or an appropriate amount of W, it is possible to obtain excellent SSC resistance while having a strength of 655 MPa or more.

[0009] Prior art documents

[0010] Patent Literature

[0011] Patent Literature 1: Japanese Patent Application Laid-Open No. 10-503809

[0012] Patent Literature 2: Japanese Patent Application Laid-Open No. 2000-192196

[0013] Patent Literature 3: Japanese Patent Application Laid-Open No. 8-246107

[0014] Patent Literature 4: Japanese Patent Application Laid-Open No. 2012-136742 SUMMARY

[0015] PROBLEMS TO BE SOLVED BY THE INVENTION

[0016] The above Patent Literatures 1 to 4 all propose a method of improving the SSC resistance in an acid environment by adjusting the content of elements in the chemical composition. However, the SSC resistance of steel can be improved in an acid environment by other methods than those proposed in the above Patent Literatures.

[0017] An object of the present application is to provide a steel having excellent SSC resistance.

[0018] SOLUTION TO PROBLEM

[0019] The steel according to the present application has a chemical composition, in terms of mass%, of

[0020] C: 0.035% or less,

[0021] Si: 1.00% or less,

[0022] Mn: 1.00% or less,

[0023] P: 0.030% or less,

[0024] S: 0.0050% or less,

[0025] sol. Al: 0.005 to 0.100%,

[0026] N: 0.001 to 0.020%,

[0027] Ni: 5.00 to 7.50%,

[0028] Cr: 10.00 to 14.00%,

[0029] Cu: 0.01% or more and less than 1.50%,

[0030] Mo: 1.50 to 3.50%,

[0031] V: 0.01 to 1.00%,

[0032] Ti: 0.02 to 0.30%,

[0033] Co: 0.01 to 0.50%,

[0034] Ca: 0.0003 to 0.0030%,

[0035] O: 0.0050% or less,

[0036] W: 0 to 1.50%,

[0037] Nb: 0 to 0.50%,

[0038] B: 0 to 0.0050%,

[0039] Mg: 0 to 0.0050%,

[0040] rare earth elements (REM): 0 to 0.020%, and

[0041] the balance consisting of Fe and impurities,

[0042] Among the inclusions in the steel material, the sum of Mn sulfides having a Mn content of 10% or more, an S content of 10% or more, and a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a Ca content of 20% or more, an S content of 10% or more, a Mn content of less than 10%, and a circle equivalent diameter of 2.0 μm or more is 0.50 pieces / mm 2 The following.

[0043] Effects of the Invention

[0044] The steel material according to the present invention has excellent SSC resistance. DETAILED DESCRIPTION

[0045] The present inventors have studied a steel material having excellent SSC resistance in an acidic environment.

[0046] The inventors first studied the chemical composition of a steel material that can have excellent SSC resistance in an acidic environment. As a result, it was found that if a steel material having a chemical composition containing, in mass %, C: 0.035% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, sol. Al: 0.005 to 0.100%, N: 0.001 to 0.020%, Ni: 5.00 to 7.50%, Cr: 10.00 to 14.00%, Cu: 0.01% or more and less than 1.50%, Mo: 1.50 to 3.50%, V: 0.01 to 1.00%, Ti: 0.02 to 0.30%, Co: 0.01 to 0.50%, O: 0.0050% or less, W: 0 to 1.50%, Nb: 0 to 0.50%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, and rare earth elements (REM): 0 to 0.020%, with the balance being Fe and impurities, there is a possibility that excellent SSC resistance can be obtained in an acidic environment.

[0047] However, even if the steel material has the above-described range of each element content of the chemical composition, in the case of having a high yield strength of, for example, 110 ksi or more (758 MPa or more), there is a case where sufficient SSC resistance cannot be obtained. For this reason, the inventors studied the reason for the decrease in SSC resistance in a steel material having the above-described chemical composition. As a result, the inventors obtained the following insights.

[0048] It is known that in a low alloy steel material having a Cr content of 2.00% or less, in an acidic environment, inclusions (oxides, sulfides, nitrides, etc.) in the steel material become the starting point of cracks, and SSC easily occurs. In a high alloy steel material having a Cr content of 10.00% or more, because the Cr content is high, a strong passivation film is formed on the surface of the steel material compared to a low alloy steel material. Therefore, the prior art considers that SSC based on inclusions as the starting point does not easily occur.

[0049] However, the investigations and studies by the inventors revealed that even in a high alloy steel material, in the case of a high strength of a yield strength of 110 ksi or more (758 MPa or more), in an acidic environment, the steel material can have SSC based on the following mechanism. In the case where Mn sulfides exist in the surface layer of the steel material in an acidic environment, the Mn sulfides in the surface layer are dissolved by H2S gas in the acidic environment. As a trace of the dissolution of the Mn sulfides, a pit is formed on the surface of the steel material. The pit formed by the dissolution of a large-sized Mn sulfide easily becomes the starting point of SSC occurrence.

[0050] In low-alloy steels, all of the coarse inclusions (oxides, sulfides, nitrides, etc.) in the steel become the starting points of SSC. On the other hand, in high-alloy steels, SSC occurs by the dissolution of Mn sulfides, which are specific inclusions present on the surface layer of the steel, thereby forming pits on the surface of the steel. As described above, the present inventors have found that in high-alloy steels having a Cr content of 10.00% or more, SSC sometimes occurs based on a mechanism different from that in low-alloy steels.

[0051] Based on the above insight, the present inventors have found that in steels having the above chemical composition, by inhibiting the generation of large-sized Mn sulfides, the pits on the surface caused by the dissolution of Mn sulfides can be inhibited, and the SSC resistance of the steel can be improved. For this reason, the present inventors have found that if 0.0003 to 0.0030 mass% of Ca is further contained in the above chemical composition, the generation of large-sized Mn sulfides can be inhibited. That is, it has been found that if the steel has a chemical composition of, in mass%, C: 0.035% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, sol. Al: 0.005 to 0.100%, N: 0.001 to 0.020%, Ni: 5.00 to 7.50%, Cr: 10.00 to 14.00%, Cu: 0.01% or more and less than 1.50%, Mo: 1.50 to 3.50%, V: 0.01 to 1.00%, Ti: 0.02 to 0.30%, Co: 0.01 to 0.50%, Ca: 0.0003 to 0.0030%, O: 0.0050% or less, W: 0 to 1.50%, Nb: 0 to 0.50%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements (REM): 0 to 0.020%, and the balance being Fe and impurities, the generation of large-sized Mn sulfides can be inhibited. Specifically, by containing Ca, Ca combines with S to form Ca sulfides. By the generation of Ca sulfides, S combined with Mn is reduced. Thus, the generation of large-sized Mn sulfides is inhibited.

[0052] For this reason, steels having the above chemical composition containing Ca were manufactured, and the SSC resistance in an acidic environment was investigated. As a result, it was found that although the generation of large-sized Mn sulfides was inhibited, there were still cases where excellent SSC resistance could not be obtained. For this reason, the present inventors further investigated and researched the reasons for the low SSC resistance. As a result, it was found that when Ca is contained in the above amount, based on the following mechanism, excellent SSC resistance can not be obtained.

[0053] When Ca is contained in the chemical composition of the steel material in the above-mentioned content, Ca sulfides are formed, and the generation of large-sized Mn sulfides is inhibited. However, in an acidic environment, the Ca sulfides themselves are also as easily dissolved as the Mn sulfides. Therefore, if large-sized Ca sulfides exist in the surface layer of the steel material, the Ca sulfides, like the Mn sulfides, dissolve to form pits on the surface of the steel material. The pits on the surface caused by the Ca sulfides can cause the occurrence of SSC.

[0054] Based on the above-mentioned insights, the present inventors and others have found that if, in a steel material having the above-mentioned chemical composition, not only the generation of large-sized Mn sulfides but also the generation of large-sized Ca sulfides is inhibited, then even in the case of having a yield strength of 110 ksi or more (758 MPa or more), excellent SSC resistance in an acidic environment can be obtained. For this reason, the present inventors and others have further researched to what extent the total number of large-sized Mn sulfides and large-sized Ca sulfides per unit area should be inhibited in order to obtain excellent SSC resistance even in the case of having a yield strength of 110 ksi or more (758 MPa or more). As a result, it has been found that if, among the inclusions in the steel material, the sum of the Mn sulfides having a circle equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle equivalent diameter of 2.0 μm or more is 0.50 or less per mm 2 Therefore, even in the case of having a yield strength of 110 ksi or more (758 MPa or more), excellent SSC resistance in an acidic environment can be obtained.

[0055] As described above, the steel material according to the present embodiment is completed from the viewpoint of inhibiting pits on the surface of the steel material formed by Mn sulfides and Ca sulfides, which are inclusions dissolved in an acidic environment, in a steel material having a Cr content of 10.00% or more. The steel material according to the present embodiment has the following composition. [1]

[0057] A steel material having a chemical composition, in terms of mass%, of

[0058] C: 0.035% or less,

[0059] Si: 1.00% or less,

[0060] Mn: 1.00% or less,

[0061] P: 0.030% or less,

[0062] S: 0.0050% or less,

[0063] sol. Al: 0.005 to 0.100%,

[0064] N: 0.001 to 0.020%,

[0065] Ni: 5.00 to 7.50%,

[0066] Cr: 10.00 to 14.00%,

[0067] Cu: 0.01% or more and less than 1.50%,

[0068] Mo: 1.50 to 3.50%,

[0069] V: 0.01 to 1.00%,

[0070] Ti: 0.02 to 0.30%,

[0071] Co: 0.01 to 0.50%,

[0072] Ca: 0.0003 to 0.0030%,

[0073] O: 0.0050% or less,

[0074] W: 0 to 1.50%,

[0075] Nb: 0 to 0.50%,

[0076] B: 0 to 0.0050%,

[0077] Mg: 0 to 0.0050%,

[0078] a rare earth element (REM): 0 to 0.020%, and

[0079] the balance being Fe and impurities,

[0080] among the inclusions in the steel material, the sum of Mn sulfides having a Mn content of 10% or more, an S content of 10% or more, and a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a Ca content of 20% or more, an S content of 10% or more, a Mn content of less than 10%, and a circle equivalent diameter of 2.0 μm or more is 0.50 pieces / mm 2 or more. [2]

[0082] The steel material according to [1], wherein

[0083] the chemical composition contains

[0084] W: 0.01 to 1.50%. [3]

[0086] The steel material according to [1] or [2], wherein

[0087] the chemical composition contains

[0088] Nb: 0.01 to 0.50%. [4]

[0090] The steel material according to any one of [1] to [3], wherein

[0091] The chemical composition contains one or more selected from the group consisting of

[0092] B: 0.0001 to 0.0050%,

[0093] Mg: 0.0001 to 0.0050%, and

[0094] Rare earth element (REM): 0.001 to 0.020%. [5]

[0096] The steel material according to any one of [1] to [4], wherein

[0097] The steel material is a seamless steel pipe for oil well pipes.

[0098] Hereinafter, the steel material of the present embodiment will be described in detail. For the "% " concerning elements, unless otherwise specified, it means mass %.

[0099] [Chemical composition]

[0100] The chemical composition of the steel material of the present embodiment contains the following elements.

[0101] C: 0.035% or less

[0102] Carbon (C) is an element that is inevitably contained. That is, the C content is greater than 0%. C improves the hardenability of the steel material and thereby improves the strength of the steel material. However, if the C content is greater than 0.035%, even if the contents of the other elements are within the range of the present embodiment, the strength of the steel material becomes excessively high, and thereby the SSC resistance of the steel material decreases. Therefore, the C content is 0.035% or less. The C content is preferably as low as possible. However, if the C content is excessively reduced, the manufacturing cost becomes high. Therefore, considering industrial production, the lower limit of the C content is preferably 0.001%, further preferably 0.003%, further preferably 0.007%, further preferably 0.008%, further preferably 0.009%. The upper limit of the C content is 0.030%, further preferably 0.025%, further preferably 0.020%, further preferably 0.018%, further preferably 0.016%, further preferably 0.015%.

[0103] Si: 1.00% or less

[0104] Silicon (Si) is an element that is inevitably contained. That is, the Si content is greater than 0%. Si deoxidizes the steel. However, if the Si content is greater than 1.00%, even if the contents of the other elements are within the ranges of the present embodiment, the deoxidizing effect will be saturated, and the hot workability of the steel material will decrease. Therefore, the Si content is 1.00% or less. The lower limit of the Si content is preferably 0.01%, more preferably 0.05%, further more preferably 0.10%, further more preferably 0.15%, further more preferably 0.20%, further more preferably 0.25%. The upper limit of the Si content is preferably 0.70%, more preferably 0.60%, further more preferably 0.50%, further more preferably 0.45%.

[0105] Mn: 1.00% or less

[0106] Manganese (Mn) is an element that is inevitably contained. That is, the Mn content is greater than 0%. Mn improves the quenchability of the steel material, thereby improving the strength of the steel material. However, when the Mn content is too high, Mn will form a large amount of coarse Mn sulfides. In an acidic environment, coarse MnS present in the vicinity of the surface layer of the steel material sometimes dissolves. At this time, a pit that is a trace of the dissolved MnS is formed. Sometimes this pit becomes a starting point of SSC, causing SSC to occur. If the Mn content is greater than 1.00%, even if the contents of the other elements are within the ranges of the present embodiment, a pit that is a trace of the dissolved MnS will be generated, and the SSC resistance will decrease. Therefore, the Mn content is 1.00% or less. The lower limit of the Mn content is preferably 0.01%, more preferably 0.05%, further more preferably 0.10%, further more preferably 0.15%. The upper limit of the Mn content is preferably 0.80%, more preferably 0.70%, further more preferably 0.60%, further more preferably 0.50%.

[0107] P: 0.030% or less

[0108] Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is greater than 0%. P segregates at grain boundaries, causing SSC to easily occur. If the P content is greater than 0.030%, even if the contents of the other elements are within the ranges of the present embodiment, the SSC resistance of the steel material will significantly decrease. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, further more preferably 0.018%. The P content is preferably as low as possible. However, if the P content is excessively reduced, the manufacturing cost will increase. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, further more preferably 0.003%.

[0109] S: 0.0050% or less

[0110] Sulfur (S) is an unavoidable impurity. That is, the S content is greater than 0%. S also segregates at grain boundaries as P does, leading to easy occurrence of SSC. If the S content is greater than 0.0050%, the SSC resistance of the steel material is significantly reduced even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the S content is 0.0050% or less. The upper limit of the S content is preferably 0.0040%, further preferably 0.0030%, further preferably 0.0025%, further preferably 0.0020%, further preferably 0.0015%. The S content is preferably as low as possible. However, if the S content is excessively reduced, the manufacturing cost becomes high. Therefore, the lower limit of the S content is preferably 0.0001%, further preferably 0.0002%, further preferably 0.0003% in consideration of industrial production.

[0111] sol. Al: 0.005 to 0.100%

[0112] Aluminum (Al) deoxidizes the steel. If the sol. Al content is less than 0.005%, the above-mentioned effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the sol. Al content is greater than 0.100%, coarse oxides are generated even if the contents of the other elements are within the ranges of the present embodiment, and the toughness of the steel material is reduced. Therefore, the sol. Al content is 0.005 to 0.100%. The lower limit of the sol. Al content is preferably 0.010%, further preferably 0.013%, further preferably 0.015%, further preferably 0.018%. The upper limit of the sol. Al content is preferably 0.080%, further preferably 0.060%, further preferably 0.055%, further preferably 0.050%. The sol. Al content described in the present specification refers to the content of acid-soluble Al.

[0113] N: 0.001 to 0.020%

[0114] Nitrogen (N) combines with Ti to form fine Ti nitride. The fine TiN suppresses the coarsening of the crystal grains by pinning effect. As a result, the strength of the steel material is improved. If the N content is less than 0.001%, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the N content is more than 0.020%, coarse nitrides are formed even if the contents of the other elements are within the ranges of the present embodiment, and thus the toughness of the steel material is reduced. Therefore, the N content is 0.001 to 0.020%. The lower limit of the N content is preferably 0.002%, further preferably 0.003%, further preferably 0.004%, further preferably 0.005%. The upper limit of the N content is preferably 0.018%, further preferably 0.016%, further preferably 0.014%, further preferably 0.012%.

[0115] Ni: 5.00 to 7.50%

[0116] Nickel (Ni) is an austenite-forming element, and martensitizes the structure after quenching. As a result, the strength of the steel material is improved. Ni further forms sulfides on the passive film in an acidic environment. The Ni sulfides suppress the contact of chloride ions (CI - ) or hydrogen sulfide ions (HS - ) with the passive film, and suppress the destruction of the passive film by the chloride ions or the hydrogen sulfide ions. Therefore, the SSC resistance of the steel material is improved. If the Ni content is less than 5.00%, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the Ni content is more than 7.50%, the hydrogen diffusion coefficient in the steel material is reduced even if the contents of the other elements are within the ranges of the present embodiment. If the hydrogen diffusion coefficient in the steel material is reduced, the SSC resistance of the steel material is also reduced. Therefore, the Ni content is 5.00 to 7.50%. The lower limit of the Ni content is preferably 5.10%, further preferably 5.20%, further preferably 5.30%. The upper limit of the Ni content is preferably 7.40%, further preferably 7.30%, further preferably 7.20%.

[0117] Cr: 10.00 to 14.00%

[0118] Chromium (Cr) forms a passivation film on the surface of the steel material, and improves the SSC resistance of the steel material. If the Cr content is less than 10.00%, the above effect cannot be sufficiently obtained even if the contents of the other elements are within the range of the present embodiment. On the other hand, if the Cr content is more than 14.00%, delta (δ) ferrite is easily generated in the steel material even if the contents of the other elements are within the range of the present embodiment, and the toughness of the steel material is reduced. Therefore, the Cr content is 10.00 to 14.00%. The lower limit of the Cr content is preferably 10.50%, further preferably 11.00%, further preferably 11.50%, further preferably 12.00%, further preferably 12.20%. The upper limit of the Cr content is preferably 13.80%, further preferably 13.60%, further preferably 13.50%, further preferably 13.45%, further preferably 13.40%.

[0119] Cu: 0.01% or more and less than 1.50%

[0120] Copper (Cu) is an austenite-forming element like Ni, and martensitizes the structure after quenching. If the Cu content is less than 0.01%, the above effect cannot be sufficiently obtained. On the other hand, if the Cu content is 1.50% or more, the above effect is saturated, and the manufacturing cost is increased. Therefore, the Cu content is 0.01% or more and less than 1.50%. The lower limit of the Cu content is preferably 0.05%, further preferably 0.10%, further preferably 0.15%. The upper limit of the Cu content is preferably 1.40%, further preferably 1.25%, further preferably 1.00%.

[0121] Mo: 1.50 to 3.50%

[0122] Molybdenum (Mo) forms a sulfide on the passivation film in an acidic environment. The Mo sulfide suppresses contact of chloride ions (Cl - ) or hydrogen sulfide ions (HS - ) with the passivation film, and suppresses the passivation film from being damaged by the chloride ions or the hydrogen sulfide ions. Therefore, the SSC resistance of the steel material is improved. Mo is further solid-solved in the steel material to improve the strength of the steel material. If the Mo content is less than 1.50%, the above effect cannot be sufficiently obtained even if the contents of the other elements are within the range of the present embodiment. On the other hand, if the Mo content is more than 3.50%, the austenite is difficult to stabilize even if the contents of the other elements are within the range of the present embodiment. As a result, it is difficult to stably obtain a microstructure in which martensite is the main component. Therefore, the Mo content is 1.50 to 3.50%. The lower limit of the Mo content is preferably 1.60%, further preferably 1.70%, further preferably 1.80%. The upper limit of the Mo content is preferably 3.40%, further preferably 3.30%, further preferably 3.20%.

[0123] V: 0.01 to 1.00%

[0124] Vanadium (V) improves the hardenability of the steel material and increases the strength of the steel material. If the V content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the V content is more than 1.00%, the hardenability of the steel material becomes too high and the SSC resistance of the steel material decreases even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the V content is 0.01 to 1.00%. The lower limit of the V content is preferably 0.02%, further preferably 0.03%. The upper limit of the V content is preferably 0.70%, further preferably 0.50%, further preferably 0.30%, further preferably 0.20%, further preferably 0.15%, further preferably 0.10%.

[0125] Ti: 0.02 to 0.30%

[0126] Titanium (Ti) forms carbide or nitride in combination with C and / or N. At this time, the grain coarsening is suppressed by pinning effect and the strength of the steel material is increased. If the Ti content is less than 0.02%, the above effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the Ti content is more than 0.30%, δ ferrite is easily generated and the toughness of the steel material decreases even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the Ti content is 0.02 to 0.30%. The lower limit of the Ti content is preferably 0.05%, further preferably 0.07%. The upper limit of the Ti content is preferably 0.25%, further preferably 0.20%, further preferably 0.18%, further preferably 0.16%.

[0127] Co: 0.01 to 0.50%

[0128] Cobalt (Co) forms sulfide on the passive film in an acidic environment. The Co sulfide suppresses the generation of chloride ions (Cl - ) or hydrogen sulfide ions (HS -) with the passivation film, inhibiting the passivation film from being damaged by chloride ions or hydrogen sulfide ions. Thus, the SSC resistance of the steel material is improved. Co further improves the quenching property of the steel material, and particularly in industrial production, ensures stable high strength of the steel material. Specifically, Co inhibits the generation of residual austenite, and inhibits the strength unevenness of the steel material. If the Co content is less than 0.01%, even if the contents of the other elements are within the ranges of the present embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Co content is more than 0.50%, even if the contents of the other elements are within the ranges of the present embodiment, the toughness of the steel material is reduced. Thus, the Co content is 0.01 to 0.50%. The lower limit of the Co content is preferably 0.02%, more preferably 0.04%, further more preferably 0.08%, further more preferably 0.10%. The upper limit of the Co content is preferably 0.48%, more preferably 0.45%, further more preferably 0.40%, further more preferably 0.35%.

[0129] Ca: 0.0003 to 0.0030%

[0130] Calcium (Ca) combines with S in the steel material to generate Ca sulfide, and inhibits the generation of Mn sulfide. In the case where Mn sulfide having a circular equivalent diameter of 1.0 μm or more exists in the surface layer of the steel material, in an acidic environment, the Mn sulfide in the surface layer sometimes dissolves. At this time, as a trace of the dissolved Mn sulfide, a pit is formed. This pit formed on the surface of the steel material easily becomes a starting point of the occurrence of SSC. Ca inhibits the generation of Mn sulfide, and reduces the number density of Mn sulfide having a circular equivalent diameter of 1.0 μm or more. As a result, the SSC resistance of the steel material is improved. If the Ca content is less than 0.0003%, even if the contents of the other elements are within the ranges of the present embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Ca content is more than 0.0030%, even if the contents of the other elements are within the ranges of the present embodiment, Ca sulfide having a circular equivalent diameter of 2.0 μm or more is excessively generated. In the case where Ca sulfide having a circular equivalent diameter of 2.0 μm or more exists in the surface layer of the steel material, as with the above Mn sulfide, in an acidic environment, the Ca sulfide in the surface layer sometimes dissolves, and a pit is formed on the surface of the steel material. At this time, the SSC resistance of the steel material is reduced. Thus, the Ca content is 0.0003 to 0.0030%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0007%, further more preferably 0.0009%. The upper limit of the Ca content is preferably 0.0029%, more preferably 0.0028%, further more preferably 0.0027%, further more preferably 0.0026%.

[0131] O: 0.0050% or less

[0132] Oxygen (O) is an unavoidable impurity. That is, the O content is greater than 0%. O forms an oxide, and the toughness of the steel material decreases. If the O content is greater than 0.0050%, the toughness of the steel material decreases significantly even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the O content is 0.0050% or less. The upper limit of the O content is preferably 0.0045%, more preferably 0.0040%, further preferably 0.0035%, further preferably 0.0030%. The O content is preferably as low as possible. However, if the O content is excessively reduced, the manufacturing cost increases. Therefore, the lower limit of the O content is preferably 0.0001%, more preferably 0.0002% in consideration of industrial production.

[0133] The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the impurities refer to elements mixed from ores, waste materials, or the like as raw materials or from manufacturing environments and the like, and are not elements intentionally contained within a range that does not adversely affect the steel material of the present embodiment.

[0134] [About Optional Elements]

[0135] The chemical composition of the steel material according to the present embodiment can further contain W instead of a part of Fe.

[0136] W: 0 to 1.50%

[0137] Tungsten (W) is an optional element, and can not be contained. That is, the W content can be 0%. In the case of being contained, W stabilizes a passivation film in an acidic environment, and inhibits the passivation film from being destroyed by chloride ions or hydrogen sulfide ions. Therefore, the SSC resistance of the steel material improves. As long as W is contained in a small amount, the above effects can be obtained to some extent. However, if the W content is greater than 1.50%, W combines with C, and forms a coarse carbide. At this time, the toughness of the steel material decreases even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the W content is 0 to 1.50%. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, further preferably 0.10%, further preferably 0.30%, further preferably 0.50%. The upper limit of the W content is preferably 1.45%, more preferably 1.40%, further preferably 1.37%.

[0138] The chemical composition of the steel material according to the present embodiment can further contain Nb instead of a part of Fe.

[0139] Nb: 0 to 0.50%

[0140] Niobium (Nb) is an optional element and can not be contained. That is, the content of Nb can be 0%. In the case of being contained, Nb forms Nb carbide, Nb carbonitride in combination with C and / or N. At this time, by pinning effect, the coarsening of the crystal grains is suppressed and the strength of the steel material is improved. As long as a small amount of Nb is contained, the above-mentioned effects can be obtained to some extent. However, if the content of Nb is more than 0.50%, even if the contents of the other elements are within the range of the present embodiment, Nb carbide and / or Nb carbonitride are excessively generated and the toughness of the steel material is reduced. Therefore, the content of Nb is 0 to 0.50%. The lower limit of the content of Nb is preferably 0.01%, further preferably 0.05%, further preferably 0.10%, further preferably 0.15%. The upper limit of the content of Nb is preferably 0.45%, further preferably 0.40%, further preferably 0.35%.

[0141] The chemical composition of the steel material according to the present embodiment can further contain B, Mg, and rare earth elements (REM) instead of a part of Fe.

[0142] B: 0 to 0.0050%

[0143] Boron (B) is an optional element and can not be contained. That is, the content of B can be 0%. In the case of being contained, B is solid-solved in the steel material and improves the hot workability of the steel material. As long as a small amount of B is contained, the above-mentioned effects can be obtained to some extent. However, if the content of B is more than 0.0050%, even if the contents of the other elements are within the range of the present embodiment, coarse B nitride is generated and the toughness of the steel material is reduced. Therefore, the content of B is 0 to 0.0050%. The lower limit of the content of B is preferably 0.0001%, further preferably 0.0002%, further preferably 0.0003%, further preferably 0.0004%. The upper limit of the content of B is preferably 0.0040%, further preferably 0.0030%, further preferably 0.0020%.

[0144] Mg: 0 to 0.0050%

[0145] Magnesium (Mg) is an optional element and can not be contained. That is, the content of Mg can be 0%. In the case of being contained, Mg controls the morphology of inclusions and improves the hot workability of the steel material. As long as Mg is contained in a small amount, the above effects can be obtained to some extent. However, if the content of Mg is greater than 0.0050%, coarse oxides are generated. At this time, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material is reduced. Therefore, the content of Mg is 0 to 0.0050%. The lower limit of the content of Mg is preferably 0.0001%, further preferably 0.0002%, and further preferably 0.0003%. The upper limit of the content of Mg is preferably 0.0040%, further preferably 0.0035%, further preferably 0.0030%, and further preferably 0.0025%.

[0146] Rare earth element (REM): 0 to 0.020%

[0147] The rare earth element (REM) is an optional element and can not be contained. That is, the content of REM can be 0%. In the case of being contained, REM controls the morphology of inclusions and improves the hot workability of the steel material, like Mg. As long as REM is contained in a small amount, the above effects can be obtained to some extent. However, if the content of REM is greater than 0.020%, coarse oxides are generated. At this time, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material is reduced. Therefore, the content of REM is 0 to 0.020%. The lower limit of the content of REM is preferably 0.001%, further preferably 0.003%, and further preferably 0.005%. The upper limit of the content of REM is preferably 0.019%, further preferably 0.018%, and further preferably 0.017%.

[0148] Note that REM in the present specification means one or more elements selected from the group consisting of scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium (Lu) having an atomic number of 71 as lanthanoid elements. In addition, the content of REM in the present specification means the total content of these elements.

[0149] As described later, the steel material preferably has a yield strength of 758 MPa or more (110 ksi or more), and further preferably 862 MPa or more (125 ksi or more). In the present embodiment, the preferred chemical composition based on the yield strength to be obtained is as follows.

[0150] Specifically, to obtain a yield strength of 758 MPa or more and less than 862 MPa, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the lower limit of the content of C is set to 0.002%, 0.005%, 0.007%, 0.008%, or 0.009%. To obtain a yield strength of 758 MPa or more and less than 862 MPa, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the lower limit of the content of Ni is set to 5.10%, 5.20%, or 5.30%. To obtain a yield strength of 758 MPa or more and less than 862 MPa, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the upper limit of the content of Ni is set to 7.00%, 6.80%, 6.50%, or less than 6.50%. To obtain a yield strength of 758 MPa or more and less than 862 MPa, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the lower limit of the content of Mo is set to 1.60%, 1.70%, or 1.80%. To obtain a yield strength of 758 MPa or more and less than 862 MPa, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the upper limit of the content of Mo is set to 3.20%, 3.00%, 2.80%, 2.50%, or less than 2.50%.

[0151] Specifically, to obtain a yield strength of 862 MPa or more, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the lower limit of the content of Ni is set to 5.50%, 6.00%, 6.30%, 6.50%, or more than 6.50%. To obtain a yield strength of 862 MPa or more, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the upper limit of the content of Ni is set to 7.40%, 7.30%, or 7.20%. To obtain a yield strength of 862 MPa or more, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the lower limit of the content of Mo is set to 1.80%, 2.10%, or 2.30%. To obtain a yield strength of 862 MPa or more, further, the chemical composition of the steel material preferably satisfies the above-described ranges of the contents of all the elements, and the upper limit of the content of Mo is set to 3.40%, 3.30%, or 3.20%.

[0152] [About Mn sulfides and Ca sulfides in the steel material]

[0153] In the steel material of the present embodiment, Mn sulfides and Ca sulfides in inclusions in the steel material are defined as follows.

[0154] Mn sulfide: an inclusion having a Mn content of 10% or more and an S content of 10% or more in terms of mass% when the mass% of the inclusion is taken as 100%

[0155] Ca sulfide: an inclusion having a Ca content of 20% or more, an S content of 10% or more, and a Mn content of less than 10% in terms of mass% when the mass% of the inclusion is taken as 100%

[0156] In the steel material of the present embodiment, among the inclusions in the steel material, the total number density (number / mm 2 ) of the Mn sulfide and the Ca sulfide, which are likely to dissolve in an acidic environment and form pits in the surface layer, is reduced. The Mn sulfide in the steel material exists extending in the length direction (rolling direction) of the steel material. On the other hand, the Ca sulfide in the steel material exists in a spherical shape. Therefore, among the Mn sulfide and the Ca sulfide, the sizes of the pits that are likely to become the starting points of SSC are different. The diameter when the area of the Mn sulfide and the Ca sulfide is converted into a circle is defined as the circle equivalent diameter. In the steel material in which the content of each element in the chemical composition is within the range of the present embodiment, the number per unit area of the Mn sulfide having a circle equivalent diameter of 1.0 μm or more and the Ca sulfide having a circle equivalent diameter of 2.0 μm or more is related to the SSC resistance in an acidic environment.

[0157] In the present specification, the total number of the Mn sulfide and the Ca sulfide per unit area (1 mm 2 ) is defined as the total number density (number / mm 2 ). Also, the total number density of the Mn sulfide having a circle equivalent diameter of 1.0 μm or more and the Ca sulfide having a circle equivalent diameter of 2.0 μm or more is defined as the total number density ND (Number Density). At this time, in the steel material of the present embodiment, the total number density ND of the Mn sulfide having a circle equivalent diameter of 1.0 μm or more and the Ca sulfide having a circle equivalent diameter of 2.0 μm or more is 0.50 number / mm 2 or more. That is, the total of the Mn sulfide having a circle equivalent diameter of 1.0 μm or more and the Ca sulfide having a circle equivalent diameter of 2.0 μm or more is 0.50 number / mm 2 or more.

[0158] The total number density ND of the Mn sulfide having a circle equivalent diameter of 1.0 μm or more and the Ca sulfide having a circle equivalent diameter of 2.0 μm or more is greater than 0.50 number / mm 2 , even if the content of each element in the chemical composition of the steel material is within the range of the present embodiment, the Mn sulfide and the Ca sulfide in the surface layer of the steel material are likely to dissolve in an acidic environment, and pits that become the starting points of SSC are likely to be generated on the surface of the steel material. Therefore, the SSC resistance of the steel material is reduced.

[0159] On the other hand, the total number density ND of the Mn sulfides having a circular equivalent diameter of 1.0 μm or more and the Ca sulfides having a circular equivalent diameter of 2.0 μm or more is preferably 0.50 or less per mm 2 Hereinafter, under the premise that the content of each element in the chemical composition of the steel material is within the range of the present embodiment, the number density of the Mn sulfides and the Ca sulfides of a size that is easily dissolved in an acidic environment is sufficiently low. Therefore, it is difficult to generate pits in the surface layer of the steel material even in an acidic environment. As a result, the SSC resistance of the steel material can be sufficiently improved.

[0160] The total number density ND of the Mn sulfides having a circular equivalent diameter of 1.0 μm or more and the Ca sulfides having a circular equivalent diameter of 2.0 μm or more is preferably 0.48 or less per mm 2 , further preferably 0.47 per mm 2 , further preferably 0.46 per mm 2 , further preferably 0.45 per mm 2 , further preferably 0.44 per mm 2 , further preferably 0.43 per mm 2 , further preferably 0.42 per mm 2 .

[0161] [Measurement method of total number density ND]

[0162] The total number density ND of the Mn sulfides having a circular equivalent diameter of 1.0 μm or more and the Ca sulfides having a circular equivalent diameter of 2.0 μm or more can be measured by the following method. Specifically, a test piece is collected from an arbitrary position of the steel material. When the steel material is a steel pipe, the test piece is collected from the central position of the wall thickness. When the steel material is a steel bar having a circular cross section, the test piece is collected from the R / 2 position. Note that, in the present specification, the R / 2 position refers to the central position of the radius R in a cross section perpendicular to the length direction of the steel bar. When the steel material is a steel plate, the test piece is collected from the central position of the plate thickness.

[0163] The collected test piece is resin-embedded. When the steel material is a steel pipe, a surface of the test piece including the pipe axis direction and the wall thickness direction is used as the observation surface. When the steel material is a steel bar, a surface of the test piece including the axial direction (length direction) and the radial direction is used as the observation surface. When the steel material is a steel plate, a surface including the length direction (rolling direction) and the plate thickness direction is used as the observation surface. The observation surface of the resin-embedded steel material is polished. Any 10 fields of view in the polished observation surface are observed. In each field of view, the number of inclusions is counted. The area of each field of view is set to 36 mm 2 (6 mm x 6 mm).

[0164] Specifically, elemental concentration analysis (EDS analysis) is performed on each inclusion in the field of view to determine the type of inclusion. In the EDS analysis, the acceleration voltage is set to 20 kV, and the elements of the analysis object are set to N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Cu, Zr, Nb.

[0165] Based on the results of the EDS analysis of each inclusion, it is determined whether the inclusion is a Mn sulfide or a Ca sulfide. When the Mn content is 10% or more and the S content is 10% or more in terms of mass%, it is determined that the inclusion is a "Mn sulfide". When the Ca content is 20% or more, the S content is 10% or more, and further, the Mn content is less than 10% in terms of mass%, it is determined that the inclusion is a "Ca sulfide".

[0166] The total number of Mn sulfides having a circular equivalent diameter of 1.0 μm or more is found among the Mn sulfides determined in the 10 fields of view. Further, the total number of Ca sulfides having a circular equivalent diameter of 2.0 μm or more is found among the Ca sulfides determined in the 10 fields of view. Based on the total number of Mn sulfides having a circular equivalent diameter of 1.0 μm or more, the total number of Ca sulfides having a circular equivalent diameter of 2.0 μm or more, and the total area of the 10 fields of view, the total number density ND (pieces / mm2) of Mn sulfides having a circular equivalent diameter of 1.0 μm or more and Ca sulfides having a circular equivalent diameter of 2.0 μm or more is found. 2

[0167] The determination of the total number density ND can be performed using a device (SEM-EDS device) equipped with a composition analysis function on a scanning electron microscope. As the SEM-EDS device, for example, an inclusion automatic analysis device manufactured by FEI (ASPEX) Co., Ltd. can be used, which is commercially available as Metals Quality Analyzer.

[0168] [Microstructure]

[0169] The microstructure of the steel material according to the present embodiment is mainly martensite. In the present specification, the martensite includes not only fresh martensite but also tempered martensite. In addition, in the present specification, the main body of the martensite means that the volume fraction of the martensite is 80% or more in the microstructure. The balance of the microstructure is retained austenite. That is, in the steel material of the present embodiment, the volume fraction of the retained austenite is 0 to 20%. The volume fraction of the retained austenite is preferably as low as possible. The lower limit of the volume fraction of the martensite in the microstructure of the steel material of the present embodiment is preferably 85%, and further preferably 90%. It is further preferable that the microstructure of the steel material is a single phase of the martensite.

[0170] ​In the microstructure, a small amount of residual austenite does not cause a significant decrease in strength, and significantly improves the toughness of the steel material. However, when the volume fraction of residual austenite is too high, the strength of the steel material significantly decreases. Therefore, as described above, in the microstructure of the steel material of the present embodiment, the volume fraction of residual austenite is 0 to 20%. From the viewpoint of securing strength, the upper limit of the volume fraction of residual austenite is preferably 15%, and further preferably 10%. As described above, the microstructure of the steel material of the present embodiment can be a single phase of martensite. Therefore, the volume fraction of residual austenite can be 0%. On the other hand, in the case where a small amount of residual austenite exists, the volume fraction of residual austenite is greater than 0% and 20% or less, and is further preferably greater than 0% and 15% or less, and further preferably greater than 0% and 10% or less.

[0171] [Method for measuring volume fraction of martensite]

[0172] The volume fraction (vol.%) of martensite in the microstructure of the steel material of the present embodiment is calculated by subtracting the volume fraction (vol.%) of residual austenite calculated by the method described below from 100%.

[0173] The volume fraction of residual austenite is calculated by X-ray diffraction method. Specifically, a test piece is collected from an arbitrary position of the steel material. When the steel material is a steel pipe, the test piece is collected from the central position of the wall thickness. When the steel material is a steel bar, the test piece is collected from the R / 2 position. When the steel material is a steel sheet, the test piece is collected from the central position of the sheet thickness. The size of the test piece is not particularly limited. The test piece is, for example, 15 mm x 15 mm x thickness 2 mm. At this time, when the steel material is a steel pipe, the thickness direction of the test piece is the pipe diameter direction. When the steel material is a steel bar, the thickness direction of the test piece is the radial direction. When the steel material is a steel sheet, the thickness direction of the test piece is the sheet thickness direction. Using the obtained test piece, the X-ray diffraction intensity of each of the (200) plane of the α phase, the (211) plane of the α phase, the (200) plane of the γ phase, the (220) plane of the γ phase, and the (311) plane of the γ phase is measured, and the integral intensity of each plane is calculated. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction device is set to Mo (Mo Kα ray), and the output power is set to 50 kV-40 mA. After the calculation, for each combination of the planes of the α phase and the planes of the γ phase (2 x 3 = 6 groups), the volume fraction Vγ (%) of residual austenite is calculated using Formula (I). Furthermore, the average value of the volume fractions Vγ of residual austenite of the 6 groups is defined as the volume fraction (%) of residual austenite.

[0174] Vγ = 100 / {1 + (Iα x Rγ) / (Iγ x Ra)} (I)

[0175] Here, Ia represents the integrated intensity of the a phase. Ra represents the crystallographically theoretically calculated value of the a phase. Ig represents the integrated intensity of the g phase. Rg represents the crystallographically theoretically calculated value of the g phase. Note that, in the present specification, Ra in the (200) plane of the a phase is set to 15.9, Ra in the (211) plane of the a phase is set to 29.2, Rg in the (200) plane of the g phase is set to 35.5, Rg in the (220) plane of the g phase is set to 20.8, and Rg in the (311) plane of the g phase is set to 21.8. Note that the volume fraction of retained austenite is rounded off to the first decimal place of the obtained value.

[0176] The volume fraction of the martensite of the microstructure of the steel material is calculated from the volume fraction (%) of the retained austenite obtained by the above-described X-ray diffraction method by the following equation.

[0177] Volume fraction of martensite = 100 - volume fraction (%) of retained austenite

[0178] [Yield strength]

[0179] The yield strength of the steel material according to the present embodiment is not particularly limited. The yield strength of the steel material is preferably 758 MPa or more (110 ksi or more), and further preferably 862 MPa or more (125 ksi or more). The upper limit of the yield strength is not particularly limited, but the upper limit of the yield strength of the steel material according to the present embodiment is, for example, less than 1069 MPa (less than 155 ksi). The further preferred upper limit of the yield strength of the steel material is 1000 MPa.

[0180] In the present specification, the yield strength refers to the 0.2% residual deformation stress (MPa) obtained by the tensile test at normal temperature (24 ± 3°C) based on ASTM E8 / E8M (2013). Specifically, the yield strength is calculated by the following method. A tensile test piece is collected from an arbitrary position of the steel material. When the steel material is a steel pipe, the tensile test piece is collected from the central position of the wall thickness. When the steel material is a steel bar, the tensile test piece is collected from the R / 2 position. When the steel material is a steel sheet, the tensile test piece is collected from the central position of the sheet thickness. The size of the tensile test piece is not particularly limited. For example, a round bar tensile test piece having a parallel portion diameter of 8.9 mm and a parallel portion length of 35.6 mm is used. The length direction of the parallel portion of the tensile test piece is parallel to the length direction (rolling direction) of the steel material. Using the tensile test piece, the tensile test is performed at normal temperature (24 ± 3°C) in accordance with ASTM E8 / E8M (2013), and the 0.2% residual deformation stress (MPa) is calculated. The calculated 0.2% residual deformation stress is defined as the yield strength (MPa).

[0181] [SSC resistance of steel material]

[0182] The steel material according to the present embodiment has excellent SSC resistance. The SSC resistance of the steel material according to the present embodiment can be evaluated by a SSC resistance evaluation test at normal temperature. The SSC resistance evaluation test is performed by a method based on NACE TM0177-2005 Method A.

[0183] Specifically, a round bar test piece is collected from the steel material according to the present embodiment. When the steel material is a steel pipe, the round bar test piece is collected from a wall thickness central position. When the steel material is a steel bar, the round bar test piece is collected from an R / 2 portion. When the steel material is a steel sheet, the round bar test piece is collected from a sheet thickness central position. The size of the round bar test piece is not particularly limited. For example, the diameter of a parallel portion of the round bar test piece is 6.35 mm, and the length of the parallel portion is 25.4 mm. Note that the axial direction of the round bar test piece is parallel to the length direction (rolling direction) of the steel material.

[0184] A test solution is a 0.17 mass% sodium chloride aqueous solution having a pH of 3.0. The test solution is an aqueous solution containing 0.17 mass% of sodium chloride and 0.41 g / L of sodium acetate, to which acetic acid is added to adjust the pH to 3.0. A stress equivalent to 90% of the actual yield stress is applied to the round bar test piece collected as described above. The test solution at 24°C is injected into a test vessel as a test bath in a manner that the round bar test piece to which the stress is applied is immersed. After degassing the test bath, 0.03 bar of H2S gas and 0.97 bar of CO2 gas are blown into the test bath to saturate the test bath with H2S gas. The test bath saturated with H2S gas is kept at 24°C for 720 hours. The surface of the parallel portion of the test piece after being kept for 720 hours is observed using a magnifying glass having a magnification of 10 times to confirm the presence or absence of a crack. In a case where a suspected crack is observed using the magnifying glass, the cross section of the suspected crack portion is observed using an optical microscope having a magnification of 100 times to confirm the presence or absence of a crack.

[0185] [Shape and use of the steel material]

[0186] The steel material according to the present embodiment is a steel pipe, a round bar (solid material), or a steel sheet. The steel pipe can be a seamless steel pipe or a welded steel pipe. The steel pipe is, for example, a steel pipe for oil well pipes. The steel pipe for oil well pipes refers to a steel pipe used for oil well pipes. The oil well pipe is, for example, a casing, a tubing, a drill pipe, or the like used in drilling an oil well or a gas well, or in the production of crude oil or natural gas. It is preferable that the steel material according to the present embodiment be a seamless steel pipe for oil well pipes.

[0187] As described above, in the steel material according to the present embodiment, each element in the chemical composition is within the range of the present embodiment, and the sum of the Mn sulfides having a circle equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle equivalent diameter of 2.0 μm or more is 0.50 or less per mm 2The following. As a result, the steel material according to the present embodiment has excellent SSC resistance.

[0188] [Manufacturing method]

[0189] An example of a manufacturing method of the steel material of the present embodiment will be described. Note that the manufacturing method described below is one example, and the manufacturing method of the steel material of the present embodiment is not limited to this. That is, as long as the steel material of the present embodiment having the above-described configuration can be manufactured, the manufacturing method described below is not limited thereto. However, the manufacturing method described below is a preferred manufacturing method of the steel material of the present embodiment.

[0190] An example of the manufacturing method of the steel material of the present embodiment includes a step of manufacturing a billet (steel manufacturing step), a step of manufacturing an intermediate steel material by subjecting the billet to hot working (hot working step), and a step of subjecting the intermediate steel material to quenching and tempering (heat treatment step). Hereinafter, each step will be described.

[0191] [Steel manufacturing step]

[0192] The steel manufacturing step includes a step of manufacturing molten steel (refining step), and a step of manufacturing a billet by casting using the molten steel (billet manufacturing step).

[0193] [Refining step]

[0194] In the refining step, first, the molten steel containing Cr is stored in a ladle, and decarburization treatment is performed on the molten steel in the ladle under atmospheric pressure (coarse decarburization refining step). By the decarburization treatment in the coarse decarburization refining step, molten slag is generated. The molten slag generated by the decarburization treatment floats on the liquid surface of the molten steel after the coarse decarburization refining step. In the coarse decarburization refining step, Cr in the molten steel is oxidized to generate Cr203. The Cr203 is absorbed into the molten slag. For this reason, a deoxidizer is added to the ladle, the Cr203 in the molten slag is reduced, and Cr is recovered into the molten steel (Cr reduction treatment step). The coarse decarburization refining step and the Cr reduction treatment step are performed, for example, by an electric furnace method, a converter method, or an AOD (Argon Oxygen Decarburization) method. After the Cr reduction treatment step, the molten slag is removed from the molten steel (deslagging treatment step).

[0195] The Cr-containing steel has a reduced C activity due to the presence of Cr, and thus, the decarburization reaction is inhibited. For this reason, the molten steel after the deslagging treatment step is further subjected to final decarburization treatment (final decarburization refining step). In the final decarburization refining step, the decarburization treatment is performed under reduced pressure. If the decarburization treatment is performed under reduced pressure, the CO gas partial pressure (P CO) will be reduced, oxidation of Cr in the molten steel will be suppressed. Therefore, if the decarburization treatment is performed under reduced pressure, oxidation of Cr can be suppressed while further reducing the C concentration in the molten steel. After the final decarburization refining process, a deoxidizer is added to the molten steel, and a Cr reduction treatment (Cr reduction treatment process) of reducing Cr203in the molten slag is performed again. The final decarburization refining process and the Cr reduction treatment process after the final decarburization refining process can be performed, for example, by a VOD (Vacuum Oxygen Decarburization) method or by an RH (Ruhrstahl-Heraeus) method.

[0196] After the Cr reduction treatment process, a final composition adjustment and temperature adjustment of the molten steel in the ladle before the billet manufacturing process (composition adjustment process) is performed. The composition adjustment process is performed, for example, by an LT (Ladle Treatment). In the latter half of the composition adjustment process, Ca is added to the molten steel. Here, the time from the addition of Ca until Ca is uniformly dispersed in the molten steel is defined as the "uniform mixing time" τ. The uniform mixing time τ can be found by the following equation (A).

[0197] τ = 800 x ε -0.4 (A)

[0198] Here, ε represents the stirring power density of the molten steel in the LT and is defined by equation (B).

[0199] ε = 28.5 (Q / W) x T x log (1 + H / 1.48) (B)

[0200] Here, Q represents the top gas flow rate (Nm 3 / min). W represents the mass of the molten steel (t). T represents the temperature of the molten steel (K). H represents the depth of the molten steel in the ladle (bath depth) (m).

[0201] In the composition adjustment process, the temperature of the molten steel in the ladle is maintained at 1500 to 1700°C. Further, the holding time after the uniform mixing time is defined as the "holding time t" (seconds). At this time, in the present embodiment, the holding time t after the uniform mixing time is set to 60 seconds or more.

[0202] When the holding time t is less than 60 seconds, the Ca added to the molten steel cannot sufficiently modify the Mn sulfides in the molten steel. In this case, large-sized Mn sulfides remain in the steel. Therefore, the number of Mn sulfides having a circle-equivalent diameter of 1.0 μm or more per unit area in the steel becomes too large. As a result, the total number density ND (number / mm 2 ) of the Mn sulfides having a circle-equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle-equivalent diameter of 2.0 μm or more becomes larger than 0.50 number / mm 2 . Alternatively, although the Mn sulfides are modified by the reaction with the Ca, the number of the Mn sulfides having a circle-equivalent diameter of 1.0 μm or more per unit area becomes small, but the Ca sulfides generated by the combination with S are not sufficiently absorbed by the slag and remain in the molten steel. As a result, the total number density ND (number / mm 2 ) of the Mn sulfides having a circle-equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle-equivalent diameter of 2.0 μm or more becomes larger than 0.50 number / mm 2 .

[0203] On the other hand, when the holding time t is 60 seconds or more, the Ca added to the molten steel sufficiently modifies the Mn sulfides in the molten steel, and the large-sized Mn sulfides are reduced. Therefore, the number of the Mn sulfides having a circle-equivalent diameter of 1.0 μm or more per unit area is sufficiently reduced. Further, the large-sized Ca sulfides generated by the combination with S are sufficient to secure the time for floating up from the molten steel to be absorbed by the slag. Therefore, the number of the Ca sulfides having a circle-equivalent diameter of 2.0 μm or more per unit area is also sufficiently reduced. As a result, the total number density ND (number / mm 2 ) of the Mn sulfides having a circle-equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle-equivalent diameter of 2.0 μm or more is 0.50 number / mm 2 or less.

[0204] As described above, in the component adjustment step of the present embodiment, it is preferable that the holding time t after the homogenizing time be 60 seconds or more. Note that the upper limit of the holding time t after the homogenizing time in the component adjustment step of the present embodiment is not particularly limited, and is, for example, 3600 seconds.

[0205] [Blank manufacturing step]

[0206] Using the molten steel produced by the above refining step, a blank (a cast blank or a cast ingot) is produced. Specifically, using the molten steel, a cast blank is produced by a continuous casting method. The cast blank can be a slab, a bloom, or a billet. Alternatively, using the molten steel, a cast ingot can be produced by an ingot casting method. The cast blank or the cast ingot can be further subjected to breakdown rolling or the like, thereby producing a billet. Through the above steps, a blank is produced.

[0207] [hot working step]

[0208] In the hot working step, the intermediate steel material is manufactured by hot working the billet. When the steel material is a steel pipe, the intermediate steel material corresponds to a pipe blank. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1100 to 1300°C. The billet taken out of the heating furnace is subjected to hot working, and a pipe blank (seamless steel pipe) is manufactured as the intermediate steel material. The method of hot working is not particularly limited, and a publicly known method can be employed. For example, as the hot working, Mannesmann method can be performed, and thereby a pipe blank is manufactured. At this time, the round billet is subjected to piercing rolling by a piercing mill. The piercing reduction is not particularly limited, and is, for example, 1.0 to 4.0. The round billet after the piercing rolling is further subjected to hot rolling by a mandrel mill, a reducing mill, a sizing mill, or the like, and thereby a pipe blank is manufactured. The cumulative cross-sectional reduction in the hot working step is, for example, 20 to 70%.

[0209] The pipe blank can also be manufactured from the billet by other hot working methods. For example, short and thick steel materials such as a coupling can also be manufactured into a pipe blank by forging based on the Elkhart pipe method or the like. The pipe blank can be manufactured by the above-described process.

[0210] When the steel material is a steel bar, first, the billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1100 to 1300°C. The billet taken out of the heating furnace is subjected to hot working, and a steel bar is manufactured as the intermediate steel material. The hot working is, for example, blooming by a blooming mill, or hot rolling by a continuous rolling mill. The continuous rolling mill has alternately arranged horizontal stands having a pair of hole-type rollers arranged side by side in the vertical direction, and vertical stands having a pair of hole-type rollers arranged side by side in the horizontal direction.

[0211] When the steel material is a steel plate, first, the billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1100 to 1300°C. The billet taken out of the heating furnace is subjected to hot rolling by a blooming mill and a continuous rolling mill, and a steel plate is manufactured as the intermediate steel material.

[0212] The intermediate steel material manufactured by the hot working can also be subjected to air cooling (As-Rolled). The intermediate steel material manufactured by the hot working can also not be cooled to room temperature, but can be directly subjected to quenching after the hot working, or can be subjected to quenching after re-heating after the hot working.

[0213] When the quenching is directly performed after the hot working, or the quenching is performed after the re-heating after the hot working, stress relief annealing (SR treatment) can also be performed before the heat treatment step (quenching and tempering) of the next step for the purpose of removing residual stress.

[0214] [heat treatment step]

[0215] The heat treatment process includes a quenching process and a tempering process.

[0216] [Quenching process]

[0217] In the heat treatment process, first, the intermediate steel material manufactured through the hot working process is subjected to quenching (quenching process). The quenching is performed by a publicly known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace and held at a quenching temperature. The quenching temperature is A C3 900 to 1000°C. After the intermediate steel material is held at the quenching temperature, rapid cooling (quenching) is performed. The holding time at the quenching temperature is not particularly limited and is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. The quenching method is not particularly limited. When the intermediate steel material is a pipe blank, for example, the pipe blank can be rapidly cooled by being immersed in a water tank or an oil tank, or cooling water can be injected or sprayed to the outer surface and / or the inner surface of the pipe blank by spray cooling or mist cooling.

[0218] In addition, as described above, after the hot working process, the intermediate steel material can not be cooled to normal temperature, but quenching can be performed immediately after the hot working (direct quenching), or the pipe blank after the hot working can be charged into a heat supplement furnace before the temperature of the pipe blank decreases, held at the quenching temperature, and then subjected to quenching.

[0219] [Quenching process]

[0220] The intermediate steel material after the quenching is further subjected to a tempering process. In the tempering process, the yield strength of the steel material is adjusted. In the present embodiment, the tempering temperature is set to 540 to 620°C. The holding time at the tempering temperature is not particularly limited and is, for example, 10 to 180 minutes. It is publicly known to those skilled in the art that the yield strength of the steel material can be adjusted by appropriately adjusting the tempering temperature depending on the chemical composition. It is preferable to adjust the tempering conditions in such a manner that the yield strength of the steel material reaches 758 MPa or more (110 ksi or more).

[0221] The steel material of the present embodiment can be manufactured by the above-described process. The manufacturing method of the steel material of the present embodiment is not limited to the above-described manufacturing method. As long as the content of each element in the chemical composition is within the range of the present embodiment, the total number density ND (pieces / mm 2 ) of the Mn sulfides having a circle equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle equivalent diameter of 2.0 μm or more in the steel material is 0.50 pieces / mm 2 The following steel material is not limited to the manufacturing method of the steel material of the present embodiment.

[0222] Example 1

[0223] In Example 1, the SSC resistance in a steel material having a yield strength of 125 ksi or more (a yield strength of 862 MPa or more) was investigated. Specifically, a molten steel having the chemical composition shown in Table 1 was manufactured.

[0224] [Table 1]

[0225]

[0226] "-" in Table 1 indicates that the content of the corresponding element is less than the detection limit value. For example, the content of W in Test No. 1 indicates 0% rounded off to the third decimal place. The content of Nb in Test No. 1 indicates 0% rounded off to the third decimal place. The content of B in Test No. 1 indicates 0% rounded off to the fifth decimal place. The content of Mg in Test No. 1 indicates 0% rounded off to the fifth decimal place. The content of REM in Test No. 1 indicates 0% rounded off to the fourth decimal place.

[0227] The molten steel of Test Nos. 1 to 23 was manufactured as follows. The molten steel containing Cr was stored in a ladle, and a publicly known rough decarburization refining process and Cr reduction process were performed by an AOD method. After the Cr reduction process, a deslagging process of removing slag from the molten steel was performed. Further, a publicly known final decarburization refining process and Cr reduction process were performed by a VOD method.

[0228] After the Cr reduction process was performed by the VOD method, the final composition adjustment of the molten steel in the ladle and the temperature adjustment of the molten steel before the billet manufacturing process were performed by LT. The molten steel temperature was 1500 to 1700°C. Further, Ca was added to the molten steel. As shown in Table 2, the holding time t (seconds) after the uniform mixing time of the added Ca was adjusted. Through the above processes, the molten steel having the chemical composition shown in Table 1 was manufactured.

[0229] [Table 2]

[0230] Table 2

[0231]

[0232] Using the molten steel of Test Nos. 1 to 23, a small square billet having an outer diameter of 310 mm was manufactured. The manufactured small square billet was heated to 1250°C, and hot-rolled by a Mannesmann method to manufacture a pipe billet (seamless steel pipe) having an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.

[0233] The pipe blanks of Test Nos. 1 to 23 were subjected to quenching and tempering. For the pipe blanks of Test Nos. 1 to 23, the quenching temperature was set to 920°C, and the holding time at the quenching temperature was set to 10 minutes. The pipe blanks of Test Nos. 1 to 23 after quenching were subjected to tempering. The tempering temperature was adjusted in the range of 540 to 580°C for each Test No. in such a manner that the yield strength of the steel material (seamless steel pipe) after tempering becomes 862 MPa or more. The holding time at the tempering temperature was set to 30 minutes for all Test Nos.

[0234] The steel materials (seamless steel pipes) of Test Nos. 1 to 23 were manufactured by the above manufacturing process.

[0235] [Evaluation Test]

[0236] The steel materials of Test Nos. 1 to 23 after the above tempering were subjected to a microstructure observation test, a total number density ND measurement test, a tensile test, and an SSC resistance evaluation test.

[0237] [Measurement Test of Martensite Volume Fraction in Microstructure]

[0238] The martensite volume fraction in the microstructure of the steel materials (seamless steel pipes) of Test Nos. 1 to 23 was found by the following method. First, the volume fraction of retained austenite in the microstructure of the steel material of each Test No. was found by an X-ray diffraction method. Specifically, a test piece was collected from the central position of the wall thickness of the steel material (seamless steel pipe) of each Test No. The test piece had a size of 15 mm x 15 mm x thickness 2 mm. The thickness direction of the test piece was set to the pipe diameter direction. Using the obtained test piece, the X-ray diffraction intensity of each of the (200) plane of the α phase, the (211) plane of the α phase, the (200) plane of the γ phase, the (220) plane of the γ phase, and the (311) plane of the γ phase was measured, and the integral intensity of each plane was calculated. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction device was set to Mo (Mo Kα ray), and the output power was set to 50 kV-40 mA. After the calculation, the volume fraction Vγ (%) of the retained austenite was calculated for each combination of the planes of the α phase and the planes of the γ phase (2 x 3 = 6 groups) using Formula (I). Also, the average value of the volume fractions Vγ of the retained austenite of the 6 groups was defined as the volume fraction (%) of the retained austenite.

[0239] Vγ = 100 / {1 + (Iα x Rγ) / (Iγ x Ra)} (I)

[0240] Here, Ia represents the integrated intensity of the a phase. Ra represents the crystallographically theoretically calculated value of the a phase. Ig represents the integrated intensity of the γ phase. Rγ represents the crystallographically theoretically calculated value of the γ phase. Note that, in the present specification, Ra in the (200) plane of the a phase is set to 15.9, Ra in the (211) plane of the a phase is set to 29.2, Rγ in the (200) plane of the γ phase is set to 35.5, Rγ in the (220) plane of the γ phase is set to 20.8, and Rγ in the (311) plane of the γ phase is set to 21.8. Note that the volume fraction of retained austenite is rounded off to the first decimal place of the obtained value.

[0241] The volume fraction of the martensite of the steel materials of Test Nos. 1 to 23 was calculated from the volume fraction of the retained austenite (%) obtained by the above-described X-ray diffraction method by the following equation.

[0242] Volume fraction of martensite = 100 - volume fraction of retained austenite (%)

[0243] The obtained volume fraction of the martensite of Test Nos. 1 to 23 is shown in the column of "Volume fraction of martensite (%)" of Table 2.

[0244] [Measurement test of total number density ND]

[0245] The total number density ND of the Mn sulfides having a circle equivalent diameter of 1.0 μm or more and the Ca sulfides having a circle equivalent diameter of 2.0 μm or more in the steel materials (seamless steel pipes) of Test Nos. 1 to 23 was measured by the following method. A test piece was collected from the center position of the wall thickness of the steel material of each test number. The collected test piece was resin-embedded. The surface of the test piece including the tube axis direction and the wall thickness direction was used as an observation surface. The observation surface of the resin-embedded steel material was polished. Any 10 fields of view in the polished observation surface were observed. In each field of view, the number of inclusions was calculated. The area of each field of view was set to 36 mm 2 (6 mm x 6 mm).

[0246] Elemental concentration analysis (EDS analysis) was performed on each inclusion in the field of view to determine the type of the inclusion. In the EDS analysis, the acceleration voltage was set to 20 kV, and the elements to be analyzed were set to N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb.

[0247] Based on the results of the element analysis of the specific inclusion, it is determined whether the inclusion is a Mn sulfide or a Ca sulfide. Specifically, when the Mn content is 10% or more and the S content is 10% or more in mass%, it is determined that the inclusion thereof is "Mn sulfide". When the Ca content is 20% or more, the S content is 10% or more, and further, the Mn content is less than 10% in mass%, it is determined that the inclusion thereof is "Ca sulfide".

[0248] The total number of Mn sulfides having a circle equivalent diameter of 1.0 μm or more among the Mn sulfides determined in each field of view is found. Further, the total number of Ca sulfides having a circle equivalent diameter of 2.0 μm or more among the Ca sulfides determined in each field of view is found. Based on the total number of Mn sulfides having a circle equivalent diameter of 1.0 μm or more, the total number of Ca sulfides having a circle equivalent diameter of 2.0 μm or more, and the total area of 10 fields of view, the total number density ND (pieces / mm2) of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more is found. 2 Note that, at the time of measurement, an inclusion automatic analysis device manufactured by FEI (ASPEX) Co., Ltd. was used, and the trade name thereof was Metals Quality Analyzer. The total number density ND found for Test Nos. 1 to 23 is shown in the column of "Total number density ND (pieces / mm2)" in Table 2. 2

[0249] [Tensile test]

[0250] A tensile test was performed on the steel materials (seamless steel pipes) of Test Nos. 1 to 23 in accordance with ASTM E8 / E8M (2013). Specifically, a round bar tensile test piece was collected from the wall thickness central position of the steel material of each test number. The diameter of the parallel portion of the round bar tensile test piece was 8.9 mm, and the length of the parallel portion was 35.6 mm. The length direction of the round bar tensile test piece was parallel to the length direction (rolling direction) of the steel material. Using the round bar tensile test pieces of Test Nos. 1 to 23, a tensile test was performed in the atmosphere at normal temperature (25°C), and the 0.2% residual strain stress (MPa) was found. The found 0.2% residual strain stress was defined as the yield strength (MPa). The yield strengths of Test Nos. 1 to 23 obtained are shown in the column of "YS (MPa)" in Table 2.

[0251] [SSC resistance evaluation test]

[0252] ​The SSC resistance evaluation test of the steel materials (seamless steel pipes) of Test Nos. 1 to 23 was performed by the following method. A round bar test piece was taken from the center of the wall thickness of the steel material of each test number. The diameter of the parallel portion of the round bar test piece was 6.35 mm, and the length of the parallel portion was 25.4 mm. The length direction of the round bar test piece was parallel to the length direction (pipe axis direction) of the steel material.

[0253] The test solution was a 0.17 mass% sodium chloride aqueous solution having a pH of 3.0. The test solution was prepared by adding acetic acid to a 0.17 mass% sodium chloride and 0.41 g / L sodium acetate aqueous solution, and adjusting the pH to 3.0. A stress equivalent to 90% of the actual yield stress was applied to the round bar test piece. The test vessel was filled with the test solution at 24°C by immersing the round bar test piece to which the stress was applied, as a test bath. After degassing the test bath, 0.03 bar of H2S gas and 0.97 bar of CO2 gas were blown into the test bath to saturate the test bath with H2S gas. The test bath saturated with H2S gas was maintained at 24°C for 720 hours. The surface of the parallel portion of the round bar test piece after the maintenance for 720 hours was observed using a magnifying glass with a magnification of 10 times, and the presence or absence of a crack was confirmed. In the case where the presence of a suspected crack was confirmed by the observation of the site of the suspected crack using the magnifying glass, the cross section of the site of the suspected crack was observed using an optical microscope with a magnification of 100 times, and the presence or absence of a crack was confirmed.

[0254] In the case where no crack was confirmed after the observation of the surface of the round bar test piece using the magnifying glass with a magnification of 10 times and the optical microscope with a magnification of 100 times, it was judged that excellent SSC resistance was obtained (marked as "E (Excellent)" in the "SSC resistance" column in Table 2). In the case where a crack was confirmed, it was judged that excellent SSC resistance was not obtained (marked as "B (Bad)" in the "SSC resistance" column in Table 2).

[0255] [Results of Evaluation]

[0256] As is apparent from Tables 1 and 2, the chemical compositions of the steel materials of Test Nos. 1 to 16 were appropriate. Further, the volume fraction of the martensite in the microstructure was 80% or more, and the yield strength was 862 MPa or more (125 ksi or more). Further, the total number density ND of the Mn sulfides having a circular equivalent diameter of 1.0 μm or more and the Ca sulfides having a circular equivalent diameter of 2.0 μm or more was 0.50 pieces / mm 2 As a result, the steel materials of Test Nos. 1 to 16 obtained excellent SSC resistance.

[0257] On the other hand, in Test Nos. 17 and 18, the S content in the chemical composition of the steel material was too high. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel material of Test No. 17 and 18 failed to obtain excellent SSC resistance.

[0258] In Test No. 19, the Ca content in the chemical composition of the steel material was too low. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel material of Test No. 19 failed to obtain excellent SSC resistance.

[0259] In Test No. 20, the Ca content in the chemical composition of the steel material was too high. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel material of Test No. 20 failed to obtain excellent SSC resistance.

[0260] In Test Nos. 21 to 23, the holding time t after the uniform mixing time in the manufacturing process of the steel material was too short. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel material of Test Nos. 21 to 23 failed to obtain excellent SSC resistance.

[0261] Example 2

[0262] In Example 2, the SSC resistance in a steel material having a yield strength of 110 ksi (758 MPa or more and less than 862 MPa) was investigated. Specifically, a molten steel having a chemical composition shown in Table 3 was manufactured. As in Table 1 described in Example 1, "-" in Table 3 indicates that the content of the corresponding element is less than the detection limit value.

[0263] [Table 3]

[0264]

[0265] The molten steel of test Nos. 24 to 46 was manufactured in the same manner as the molten steel of test Nos. 1 to 23 of Example 1. The molten steel containing Cr was stored in a ladle, and a publicly known rough decarburization refining process and Cr reduction process were performed by an AOD method. After the Cr reduction process, a deslagging process for removing slag from the molten steel was performed. Further, a publicly known final decarburization refining process and Cr reduction process were performed by a VOD method.

[0266] After the Cr reduction process by the VOD method, the final composition adjustment of the molten steel in the ladle and the temperature adjustment of the molten steel before the billet manufacturing process were performed by LT. The temperature of the molten steel was 1500 to 1700°C. Further, Ca was added to the molten steel. The holding time t (sec) after the uniform mixing time after the addition of Ca was adjusted as shown in Table 4. By the above process, the molten steel having the chemical composition shown in Table 3 was manufactured.

[0267] [Table 4]

[0268] Table 4

[0269]

[0270] Using the molten steel of test Nos. 24 to 46, a small square billet having an outer diameter of 310 mm was manufactured. After the manufactured small square billet was heated to 1250°C, hot rolling was performed by a Mannesmann method, and a pipe billet (seamless steel pipe) having an outer diameter of 244.48 mm and a wall thickness of 13.84 mm was manufactured.

[0271] Quenching and tempering were performed on the pipe billets of test Nos. 24 to 46. In the same manner as Example 1, the quenching temperature was set to 920°C and the holding time at the quenching temperature was set to 10 minutes for the pipe billets of test Nos. 24 to 46. The pipe billets of test Nos. 24 to 46 after quenching were tempered. The tempering temperature was adjusted in the range of 580 to 620°C for each test number in such a manner that the yield strength of the steel material (seamless steel pipe) after tempering reaches 758 MPa or more and less than 862 MPa (110 ksi grade). The holding time at the tempering temperature was set to 30 minutes for all test numbers.

[0272] By the above manufacturing process, the steel material (seamless steel pipe) of test Nos. 24 to 46 was manufactured.

[0273] [Evaluation Test]

[0274] In the same manner as the above Example 1, the steel material of test Nos. 24 to 46 after tempering was subjected to a microstructure observation test, a total number density ND measurement test, a tensile test, and an SSC resistance evaluation test.

[0275] [Measurement Test of Martensite Volume Fraction in Microstructure]

[0276] The volume fraction of martensite in the microstructure of the steel material (seamless steel pipe) of Test Nos. 24 to 46 was found by the same method as in Example 1. The volume fraction of martensite of Test Nos. 24 to 46 obtained is shown in the column of "Volume fraction of martensite (%) " in Table 4.

[0277] [Measurement test of total number density ND]

[0278] The total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more in the steel material (seamless steel pipe) of Test Nos. 24 to 46 was measured by the same method as in Example 1. The total number density ND of Test Nos. 24 to 46 found is shown in the column of "Total number density ND (number / mm 2 )" in Table 4.

[0279] [Tensile test]

[0280] The tensile test was performed on the steel material (seamless steel pipe) of Test Nos. 24 to 46 by the same method as in Example 1 in accordance with ASTM E8 / E8M (2013). The yield strength of Test Nos. 24 to 46 obtained is shown in the column of "YS (MPa)" in Table 4.

[0281] [SSC resistance evaluation test]

[0282] The SSC resistance evaluation test was performed on the steel material (seamless steel pipe) of Test Nos. 24 to 46 by the same method as in Example 1. In the case where no cracks were confirmed after the observation of the surface of the round bar test piece using a 10 times magnifying glass and a 100 times optical microscope, it was judged that excellent SSC resistance was obtained (marked as "E (Excellent)" in the column of "SSC resistance" in Table 4). In the case where cracks were confirmed, it was judged that excellent SSC resistance was not obtained (marked as "B (Bad)" in the column of "SSC resistance" in Table 4).

[0283] [Results of evaluation]

[0284] As is apparent from Table 3 and Table 4, the chemical composition of the steel material of Test Nos. 24 to 40 was appropriate. Further, the volume fraction of martensite in the microstructure was 80% or more, the yield strength was 758 MPa or more and less than 862 MPa (110 ksi grade), and the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was 0.50 number / mm 2 or less. As a result, the steel material of Test Nos. 24 to 40 obtained excellent SSC resistance.

[0285] On the other hand, in the steel materials of Test Nos. 41 and 42, the S content was too high in the chemical composition. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel materials of Test Nos. 41 and 42 failed to obtain excellent SSC resistance.

[0286] In the steel material of Test No. 43, the Ca content was too low in the chemical composition. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel material of Test No. 43 failed to obtain excellent SSC resistance.

[0287] In the steel material of Test No. 44, the Ca content was too high in the chemical composition. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel material of Test No. 44 failed to obtain excellent SSC resistance.

[0288] In Test Nos. 45 and 46, the holding time t after the uniform mixing time was too short in the manufacturing process of the steel material. As a result, the total number density ND of Mn sulfides having a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a circle equivalent diameter of 2.0 μm or more was greater than 0.50 pieces / mm 2 As a result, the steel materials of Test Nos. 45 and 46 failed to obtain excellent SSC resistance.

[0289] The above describes the embodiments of the present application. However, the above-described embodiments are merely examples for implementing the present application. Therefore, the present application is not limited to the above-described embodiments, and can be implemented by appropriately changing the above-described embodiments within the scope of the gist thereof.

Claims

1. A steel material, a chemical composition of which is, in terms of mass%, C: 0.035% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, sol. Al: 0.005 to 0.100%, Ni: 5.00 to 7.50%, Cu: 0.01% or more and less than 1.50%, Mo: 1.50 to 3.50%, Ti: 0.02 to 0.30%, Co: 0.01 to 0.50%, Ca: 0.0003 to 0.0030%, O: 0.0050% or less, Nb: 0 to 0.50%, Mg: 0 to 0.0050%, rare earth elements (REM): 0 to 0.020%, and the balance being Fe and impurities.

2. The steel material according to claim 1, wherein the chemical composition contains 3. The steel material according to claim 1 or claim 2, wherein the chemical composition contains Nb: 0.01 to 0.50%. the chemical composition contains one or more selected from the group consisting of Mg: 0.0001 to 0.0050%, and N：0.001～0.020％、 rare earth elements (REM): 0.001 to 0.020%. Cr：10.00～14.00％、 the steel material is a seamless steel pipe for an oil well pipe. ​ V：0.01～1.00％、 ​ ​ ​ ​ W：0～1.50％、 ​ B：0～0.0050％、 ​ ​ ​ The sum of Mn sulfides having a Mn content of 10% or more, an S content of 10% or more, and a circle equivalent diameter of 1.0 μm or more and Ca sulfides having a Ca content of 20% or more, an S content of 10% or more, a Mn content of less than 10%, and a circle equivalent diameter of 2.0 μm or more is 0.50 pieces / mm 2 The following. ​ ​ W：0.01～1.50％。 ​ ​ ​ 4. The steel material according to any one of claim 1 to claim 3, wherein, ​ B：0.0001～0.0050％、 ​ ​ 5. The steel material according to any one of claims 1 to 4, wherein, ​

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