martensitic stainless steel

Abstract

Provided is a martensitic stainless steel material having both high yield strength and excellent SSC resistance. The martensitic stainless steel material of the present invention contains, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 7.50%, Mo: 1.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less, and the balance of Fe and impurities, and has a yield strength of 758 MPa or more. Within this range, the content of the elements and the yield strength satisfy formula (1) described in the specification.

Description

Technical Field

[0001] The present invention relates to a steel material, and more particularly to a martensitic stainless steel material. Background Art

[0002] The environments of some oil and gas wells (hereinafter collectively referred to as "oil wells") contain a high concentration of corrosive substances. Examples of these substances include corrosive gases such as hydrogen sulfide (H2S) and carbonic acid (CO2). Chromium (Cr) is known to be effective in improving the corrosion resistance of steel materials to carbonic acid gas. Therefore, in oil wells with high carbonic acid gas concentrations, martensitic stainless steels containing approximately 13% by mass of Cr, such as API L80 13Cr steel (conventional 13Cr steel) and super 13Cr steel with reduced carbon content, are used, depending on the partial pressure and temperature of the carbonic acid gas.

[0003] In recent years, as oil wells have deepened, there has been a demand for high-strength oil well steel. Specifically, 80 ksi grade (yield strength of 80 ksi or greater and less than 95 ksi, or 552 MPa or greater and less than 655 MPa) and 95 ksi grade (yield strength of 95 ksi or greater and less than 110 ksi, or 655 MPa or greater and less than 758 MPa) oil well steel have become widely used. In recent years, demand has also increased for oil well steel with a yield strength of 110 ksi or greater (yield strength of 758 MPa or greater).

[0004] In this specification, an environment containing hydrogen sulfide and carbon dioxide gas is referred to as a "sour environment." Oil well steel used in sour environments is required to have resistance to sulfide stress cracking (hereinafter referred to as SSC resistance). In other words, in recent years, there has been a demand for oil well steel that combines high strength with excellent SSC resistance.

[0005] Japanese Patent Application Laid-Open No. 2000-192196 (Patent Document 1), Japanese Patent Application Laid-Open No. 2012-136742 (Patent Document 2), and International Publication No. 2008 / 023702 (Patent Document 3) propose steel materials having high strength and excellent SSC resistance.

[0006] The steel disclosed in Patent Document 1 is a martensitic stainless steel for oil wells. It contains, by weight, 0.001-0.05% C, 0.05-1% Si, 0.05-2% Mn, 0.025% or less P, 0.01% or less S, 9-14% Cr, 3.1-7% Mo, 1-8% Ni, 0.5-7% Co, 0.001-0.1% Al, 0.05% or less N, 0.01% or less O (oxygen), 0-5% Cu, and 0-5% W, with the balance consisting of Fe and unavoidable impurities. The inclusion of Mo lowers the Ms point. Therefore, by incorporating Co along with Mo, this steel suppresses the decrease in the Ms point and achieves a single-phase martensitic microstructure. As a result, Patent Document 1 describes the steel's improved SSC resistance while maintaining a strength of 80 ksi or higher (552 MPa or higher).

[0007] The steel material disclosed in Patent Document 2 is a martensitic stainless steel seamless pipe containing, by mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1-2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0-15.5%, Ni: 5.5-7.0%, Mo: 2.0-3.5%, Cu: 0.3-3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, with the balance consisting of Fe and unavoidable impurities. This steel material has a yield strength of 655-862 MPa and a yield ratio of 0.90 or greater. Patent Document 2 describes that by setting the C content to 0.01% or less, adjusting the Cr, Ni, and Mo content to appropriate ranges, and further incorporating appropriate amounts of Cu and V or W, a strength of 655 MPa or greater and excellent SSC resistance can be achieved.

[0008] The steel material of Patent Document 3 is a martensitic stainless steel containing, by mass%, one or more of C: 0.010-0.030%, Mn: 0.30-0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00-15.00%, Ni: 2.50-8.00%, Mo: 1.00-5.00%, Ti: 0.050-0.250%, V: 0.25% or less, N: 0.07% or less, Si: 0.50% or less, and Al: 0.10% or less, with the balance consisting of Fe and impurities. The martensitic stainless steel satisfies the formula (6.0 ≤ Ti / C ≤ 10.1). Its yield strength is 758-862 MPa. The ratio of the Ti content to the C content (Ti / C) in the steel is correlated with the value obtained by subtracting the yield strength from the tensile strength. Furthermore, if the hardness variation within the steel is large, the SSC resistance of the steel decreases. Therefore, Patent Document 3 describes that, for this steel, the hardness variation is suppressed by adjusting the Ti / C ratio to an appropriate range, and the yield strength is adjusted to 758-862 MPa.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 2000-192196

[0012] Patent Document 2: Japanese Patent Application Laid-Open No. 2012-136742

[0013] Patent Document 3: International Publication No. 2008 / 023702 Summary of the Invention

[0014] Problems to be solved by the invention

[0015] Patent Documents 1 to 3 above propose techniques for increasing the yield strength and SSC resistance of steel. However, in addition to the techniques proposed in Patent Documents 1 to 3 above, other techniques can be used to obtain martensitic stainless steels that have both increased yield strength and excellent SSC resistance.

[0016] Furthermore, in recent years, the development of oil wells with higher hydrogen ion concentrations than before has become increasingly active. Generally, SSC is more likely to occur in environments with high hydrogen ion concentrations (i.e., low pH). Therefore, there is a need for a martensitic stainless steel material that exhibits excellent SSC resistance even in acidic environments with a pH of 3.0, where the hydrogen ion concentration is higher than before. However, Patent Documents 1 to 3 do not investigate the SSC resistance of steel materials in acidic environments with a pH of 3.0.

[0017] An object of the present invention is to provide a martensitic stainless steel material that can achieve both high yield strength and excellent SSC resistance in an acidic environment of pH 3.0.

[0018] Solutions for solving problems

[0019] The martensitic stainless steel material of the present invention is calculated as mass %.

[0020] C: 0.030% 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] Cu: 0.01~3.50%,

[0026] Cr: 10.00~14.00%,

[0027] Ni: 4.50-7.50%,

[0028] Mo: 1.00~4.00%,

[0029] Ti: 0.050~0.300%,

[0030] V: 0.01~1.00%,

[0031] Al: 0.001~0.100%,

[0032] Co: 0.010~0.500%,

[0033] Ca: 0.0005~0.0050%,

[0034] Sn: 0.0005~0.0500%,

[0035] N: 0.0010~0.0500%,

[0036] O: 0.050% or less,

[0037] W: 0~0.50%,

[0038] Nb: 0~0.500%,

[0039] As: 0~0.0100%,

[0040] Sb: 0 to 0.0100%, and

[0041] Balance: Fe and impurities,

[0042] The yield strength of the martensitic stainless steel is 758 MPa or more.

[0043] Within the ranges of the content of the element and the yield strength of the martensitic stainless steel material, the content of the element and the yield strength satisfy the formula (1).

[0044] 0.15≤(Sn+As+Sb) / {(Cu+Ni) / YS}≤1.00 (1)

[0045] In formula (1), the element symbol is replaced by the content of the corresponding element in mass %, and the yield strength in MPa is replaced by YS. It should be noted that when the corresponding element is not present, "0" is replaced by the element symbol.

[0046] Effects of the Invention

[0047] The martensitic stainless steel material of the present invention can achieve both high yield strength and excellent SSC resistance in an acidic environment of pH 3.0. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a graph showing the relationship between F1 (=(Sn+As+Sb) / {(Cu+Ni) / YS}) in this example and the number of pitting corrosions (pittings) which is an indicator of SSC resistance. DETAILED DESCRIPTION

[0049] First, the present inventors studied martensitic stainless steel materials that can achieve both high yield strength and excellent SSC resistance in an acidic environment of pH 3.0 from the perspective of chemical composition. As a result, the present inventors found that as long as the content is calculated by mass % of C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01-3.50%, Cr: 10.00-14.00%, Ni: 4.50-7.50%, Mo: 1.00-4.00%, Ti: 0.050-0.300%, V: 0.01-1.00 Martensitic stainless steel containing 0.1% Al: 0.001-0.100%, Co: 0.010-0.500%, Ca: 0.0005-0.0050%, N: 0.0010-0.0500%, O: 0.050% or less, W: 0-0.50%, and Nb: 0-0.500% may have both a yield strength of 758 MPa (110 ksi) or more and excellent SSC resistance in an acidic environment of pH 3.0.

[0050] Next, the present inventors conducted detailed research on methods for improving SSC resistance while maintaining a yield strength of 758 MPa or higher in martensitic stainless steels containing the aforementioned elements. As a result, the present inventors discovered that tin (Sn), arsenic (As), and antimony (Sb), which had previously received little attention, can improve SSC resistance in martensitic stainless steels containing the aforementioned elements. Further detailed research by the present inventors revealed that, in martensitic stainless steels containing the aforementioned elements, Sn in particular significantly improves SSC resistance, and that As and Sb complement the SSC-improving effect of Sn.

[0051] To this end, the present inventors conducted detailed research on the contents of Sn, As, and Sb that can sufficiently improve the SSC resistance of martensitic stainless steel. The results showed that the martensitic stainless steel of the present embodiment can improve the SSC resistance of the steel by containing 0.0005% to 0.0500% Sn, 0% to 0.0100% As, and 0% to 0.0100% Sb in addition to the above-mentioned element contents. That is, as long as the following is calculated by mass %: C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01-3.50%, Cr: 10.00-14.00%, Ni: 4.50-7.50%, Mo: 1.00-4.00%, Ti: 0.050-0.300%, V: 0.01-1.00%, Al: 0.001-0.100%, Co: 0.010- Martensitic stainless steel with an element composition of 0.500%, Ca: 0.0005-0.0050%, Sn: 0.0005-0.0500%, N: 0.0010-0.0500%, O: 0.050% or less, W: 0-0.50%, Nb: 0-0.500%, As: 0-0.0100%, Sb: 0-0.0100%, and the balance: Fe and impurities, can achieve both a yield strength of more than 758 MPa and excellent SSC resistance in an acidic environment of pH 3.0.

[0052] Meanwhile, the present inventors have discovered that even martensitic stainless steels having the aforementioned chemical composition, while exhibiting a yield strength of 758 MPa or greater, sometimes fail to exhibit consistently improved SSC resistance in an acidic environment of pH 3.0. Therefore, the present inventors have conducted detailed research on methods for improving SSC resistance in an acidic environment of pH 3.0 while maintaining a yield strength of 758 MPa or greater for martensitic stainless steels having the aforementioned chemical composition. As a result, the present inventors have obtained the following findings.

[0053] The present inventors conducted detailed research and found that, in a martensitic stainless steel having the above-mentioned chemical composition and a yield strength of 758 MPa or more, if the element content and the yield strength satisfy formula (1), the SSC resistance of the steel in an acidic environment of pH 3.0 can be significantly improved.

[0054] 0.15≤(Sn+As+Sb) / {(Cu+Ni) / YS}≤1.00 (1)

[0055] In formula (1), the element symbol is replaced by the content of the corresponding element in mass %, and the yield strength in MPa is replaced by YS. It should be noted that when the corresponding element is not present, "0" is replaced by the element symbol.

[0056] Definition F1 = (Sn + As + Sb) / {(Cu + Ni) / YS}. As described above, As and Sb assist Sn in improving the SSC resistance of steel. In addition, by controlling the ratio of the Sn, As and Sb contents relative to the Cu and Ni contents within a certain range, the SSC resistance of the steel is significantly improved. On the other hand, the higher the yield strength of the steel, the more likely the SSC resistance of the steel is to decrease. For this reason, the denominator of F1 is set to the ratio of the Cu and Ni contents to the yield strength. Thus, the ratio of the Sn, As and Sb contents to the Cu and Ni contents adjusted according to the yield strength is defined as F1. That is, F1 is an indicator for improving the SSC resistance in an acidic environment of pH 3.0 by utilizing the synergistic effect of Sn, As and Sb adjusted according to the yield strength with Cu and Ni. The relationship between F1 and the SSC resistance in an acidic environment of pH 3.0 is described in detail using the accompanying drawings.

[0057] Figure 1 This is a graph showing the relationship between F1 and SSC resistance in this example. Figure 1 This test was conducted using F1 and the number of pitting corrosion (pitting corrosion) as an indicator of SSC resistance for the examples described below, which have the above chemical composition and a yield strength of 758 MPa or greater. The number of pitting corrosion was obtained through the SSC resistance evaluation test described below, which assumes an acidic environment of pH 3.0.

[0058] See also Figure 1 If F1 is too low, pitting will occur in more than one thread. Similarly, if F1 is too high, pitting will occur in more than one thread. On the other hand, if F1 is 0.15 to 1.00, no pitting will occur. In other words, see Figure 1 In the steel material having the above chemical composition and a yield strength of 758 MPa or more, if F1 is within the range of 0.15 to 1.00, excellent SSC resistance can be obtained in an acidic environment of pH 3.0.

[0059] It should be noted that the detailed mechanism by which the SSC resistance of steel having the above chemical composition and a yield strength of 758 MPa or more is improved in an acidic environment of pH 3.0 by adjusting F1 to 0.15 to 1.00 is not yet known. Figure 1 As shown in the examples, it has been confirmed that by adjusting F1 to 0.15 to 1.00, the SSC resistance of the martensitic stainless steel material having the above chemical composition and a yield strength of 758 MPa or more in an acidic environment at pH 3.0 is improved.

[0060] In summary, the martensitic stainless steel material of this embodiment has the above-described chemical composition, a yield strength of 758 MPa or greater, and within the ranges of element content and yield strength, the element content and yield strength satisfy equation (1). As a result, the martensitic stainless steel material of this embodiment can achieve both a high yield strength of 758 MPa or greater and excellent SSC resistance in an acidic environment of pH 3.0.

[0061] The main points of the martensitic stainless steel material of the present embodiment, which was completed based on the above findings, are as follows.

[0062] [1] A martensitic stainless steel material, which is calculated by mass %.

[0063] C: 0.030% or less,

[0064] Si: 1.00% or less,

[0065] Mn: 1.00% or less,

[0066] P: 0.030% or less,

[0067] S: 0.0050% or less,

[0068] Cu: 0.01~3.50%,

[0069] Cr: 10.00~14.00%,

[0070] Ni: 4.50-7.50%,

[0071] Mo: 1.00~4.00%,

[0072] Ti: 0.050~0.300%,

[0073] V: 0.01~1.00%,

[0074] Al: 0.001~0.100%,

[0075] Co: 0.010~0.500%,

[0076] Ca: 0.0005~0.0050%,

[0077] Sn: 0.0005~0.0500%,

[0078] N: 0.0010~0.0500%,

[0079] O: 0.050% or less,

[0080] W: 0~0.50%,

[0081] Nb: 0~0.500%,

[0082] As: 0~0.0100%,

[0083] Sb: 0 to 0.0100%, and

[0084] Balance: Fe and impurities,

[0085] The yield strength of the martensitic stainless steel is 758 MPa or more.

[0086] Within the ranges of the content of the element and the yield strength of the martensitic stainless steel material, the content of the element and the yield strength satisfy the formula (1).

[0087] 0.15≤(Sn+As+Sb) / {(Cu+Ni) / YS}≤1.00 (1)

[0088] Wherein, the content of the corresponding element is substituted in mass % at the element symbol in formula (1), and the yield strength is substituted in MPa at YS. It should be noted that when the corresponding element is not present, "0" is substituted at the element symbol.

[0089] [2] The martensitic stainless steel material according to [1], comprising a

[0090] W: 0.01~0.50%,

[0091] Nb: 0.001~0.500%,

[0092] As: 0.0001 to 0.0100%, and

[0093] Sb: One or more elements selected from the group consisting of 0.0001 to 0.0100%.

[0094] The shape of the martensitic stainless steel material of this embodiment is not particularly limited. The martensitic stainless steel material of this embodiment can be a steel pipe, round steel (solid material), or steel plate. It should be noted that round steel refers to a steel bar with a circular cross-section perpendicular to the axial direction. Furthermore, the steel pipe can be either seamless or welded.

[0095] The martensitic stainless steel material of this embodiment will be described in detail below. Unless otherwise specified, "%" for an element represents mass %. In the following description, the martensitic stainless steel material is also simply referred to as "steel material."

[0096] [Chemical composition]

[0097] The martensitic stainless steel material of the present embodiment contains the following elements.

[0098] C: 0.030% or less

[0099] It is inevitable to contain carbon (C). That is, the lower limit of the C content exceeds 0%. C improves the hardenability of the steel, thereby improving the strength of the steel. On the other hand, if the C content is too high, the strength of the steel will become too high even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel decreases. Therefore, the C content is 0.030% or less. The preferred upper limit of the C content is 0.028%, more preferably 0.025%, further preferably 0.020%, and further preferably 0.018%. The C content is preferably as low as possible. However, an extreme reduction in the C content will increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.003%, and further preferably 0.005%.

[0100] Si: 1.00% or less

[0101] Silicon (Si) is inevitably contained. That is, the lower limit of the Si content exceeds 0%. Si deoxidizes the steel. On the other hand, if the Si content is too high, the hot workability of the steel will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Si content is 1.00% or less. In order to effectively obtain the above-mentioned effects, the preferred lower limit of the Si content is 0.01%, more preferably 0.05%, further preferably 0.10%, and further preferably 0.15%. The preferred upper limit of the Si content is 0.80%, more preferably 0.60%, further preferably 0.50%, and further preferably 0.45%.

[0102] Mn: 1.00% or less

[0103] Manganese (Mn) is inevitably contained. That is, the lower limit of the Mn content exceeds 0%. Mn improves the hardenability of the steel, thereby increasing the strength of the steel. On the other hand, if the Mn content is too high, even if the contents of other elements are within the scope of this embodiment, Mn may segregate at the grain boundaries together with impurity elements such as P and S. In this case, the SSC resistance of the steel decreases. Therefore, the Mn content is 1.00% or less. In order to effectively obtain the above-mentioned effect, the preferred lower limit of the Mn content is 0.01%, more preferably 0.05%, further preferably 0.10%, and further preferably 0.15%. The preferred upper limit of the Mn content is 0.80%, more preferably 0.70%, further preferably 0.60%, and further preferably 0.50%.

[0104] P: 0.030% or less

[0105] Phosphorus (P) is an impurity that is inevitably contained. That is, the lower limit of the P content exceeds 0%. P segregates at the grain boundaries and easily causes SSC. Therefore, if the P content is too high, the SSC resistance of the steel will be significantly reduced even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%, more preferably 0.020%, and further preferably 0.018%. The P content is preferably as low as possible. However, an extreme reduction in the P content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and further preferably 0.003%.

[0106] S: 0.0050% or less

[0107] Sulfur (S) is an impurity that is inevitably contained. That is, the lower limit of the S content exceeds 0%. S, like P, segregates at the grain boundaries, making SSC more likely to occur. Therefore, if the S content is too high, the SSC resistance of the steel will be significantly reduced even if the contents of other elements are within the range of this embodiment. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further preferably 0.0020%. The S content is preferably as low as possible. However, an extreme reduction in the S content increases manufacturing costs. Therefore, considering industrial production, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%, and further preferably 0.0003%.

[0108] Cu: 0.01~3.50%

[0109] Copper (Cu) is an austenite-forming element that makes the microstructure after quenching become martensite. Cu also improves the SSC resistance of steel in an acidic environment of pH 3.0 through a synergistic effect with Sn, As and Sb. If the Cu content is too low, even if the contents of other elements are within the scope of the present embodiment, the above-mentioned effect cannot be fully obtained. On the other hand, if the Cu content is too high, even if the contents of other elements are within the scope of the present embodiment, the above-mentioned effect will be saturated, and the hot workability of the steel will be significantly reduced. In this case, the manufacturing cost will also increase. Therefore, the Cu content is 0.01 to 3.50%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.03%, and further preferably 0.05%. The preferred upper limit of the Cu content is 3.30%, more preferably 3.10%, and further preferably 2.90%.

[0110] Cr: 10.00~14.00%

[0111] Chromium (Cr) forms a passivation film on the surface of the steel, improving the SSC resistance of the steel. If the Cr content is too low, the above-mentioned effect cannot be fully achieved even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Cr content is too high, ferrite will be contained in the structure even if the contents of other elements are within the range of the present embodiment, making it difficult to ensure sufficient strength. Furthermore, if the Cr content is too high, intermetallic compounds and Cr carbonitrides will easily form in the steel even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel decreases. Therefore, the Cr content is 10.00-14.00%. The preferred lower limit of the Cr content is 10.30%, more preferably 10.50%, and even more preferably 11.00%. The preferred upper limit of the Cr content is 13.80%, more preferably 13.60%, more preferably 13.50%, more preferably 13.45%, more preferably 13.40%, and even more preferably 13.35%.

[0112] Ni: 4.50~7.50%

[0113] Nickel (Ni) is an austenite forming element, which makes the microstructure after quenching into martensite. Ni also forms sulfides on the passivation film in an acidic environment. Ni sulfides inhibit the chloride ion (Cl -), hydrogen sulfide ions (HS-) come into contact with the passivation film, inhibiting the passivation film from being destroyed by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel is improved. In addition, Ni improves the SSC resistance of the steel in an acidic environment of pH 3.0 through a synergistic effect with Sn, As and Sb. If the Ni content is too low, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ni content is too high, the hydrogen diffusion coefficient in the steel may decrease even if the contents of other elements are within the range of this embodiment. In this case, the SSC resistance of the steel decreases. Therefore, the Ni content is 4.50-7.50%. The preferred lower limit of the Ni content is 4.80%, more preferably 5.00%, and further preferably 5.50%. The preferred upper limit of the Ni content is 7.30%, more preferably 7.00%, and further preferably 6.50%.

[0114] Mo: 1.00~4.00%

[0115] Molybdenum (Mo) forms sulfide on the passivation film in an acidic environment. Mo sulfide inhibits the chloride ion (Cl - ), hydrogen sulfide ion (HS - ) contacts the passivation film, inhibiting the passivation film from being destroyed by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel is improved. In addition, Mo forms a solid solution in the steel, thereby increasing the strength of the steel. If the Mo content is too low, the above-mentioned effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, it is difficult to stabilize the austenite even if the contents of other elements are within the ranges of this embodiment. As a result, the microstructure after tempering may contain a large amount of ferrite. If the Mo content is too high, even if the contents of other elements are within the ranges of this embodiment, a large amount of Laves phase intermetallic compounds may be generated, and the yield strength of the steel may become too high. Therefore, the Mo content is 1.00-4.00%. The preferred lower limit of the Mo content is 1.30%, more preferably 1.50%, and even more preferably 1.80%. The preferred upper limit of the Mo content is 3.80%, more preferably 3.60%, and even more preferably 3.40%.

[0116] Ti: 0.050~0.300%

[0117] Titanium (Ti) combines with C and / or N to form carbides or nitrides. At this time, due to the pinning effect, the coarsening of the grains is suppressed and the yield strength of the steel is improved. If the Ti content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the Ti content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel will become too high and the SSC resistance of the steel will decrease. Therefore, the Ti content is 0.050 to 0.300%. The preferred lower limit of the Ti content is 0.060%, more preferably 0.080%. The preferred upper limit of the Ti content is 0.250%, more preferably 0.200%, and further preferably 0.180%.

[0118] V: 0.01~1.00%

[0119] Vanadium (V) improves the hardenability of steel and increases the yield strength of steel. If the V content is too low, even if the contents of other elements are within the scope of the present embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the V content is too high, even if the contents of other elements are within the scope of the present embodiment, the strength of the steel will become too high and the SSC resistance of the steel will decrease. Therefore, the V content is 0.01 to 1.00%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%. The preferred upper limit of the V content is 0.80%, more preferably 0.60%, and further preferably 0.50%.

[0120] Al: 0.001~0.100%

[0121] Aluminum (Al) deoxidizes steel. If the Al content is too low, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Al content is too high, coarse oxides will be generated even if the contents of other elements are within the range of the present embodiment, and the SSC resistance of the steel will decrease. Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.005%, more preferably 0.010%, and further preferably 0.015%. The preferred upper limit of the Al content is 0.080%, more preferably 0.060%, further preferably 0.055%, and further preferably 0.050%. The Al content referred to in this specification refers to the content of sol.Al (acid-soluble Al).

[0122] Co: 0.010~0.500%

[0123] Cobalt (Co) forms sulfides on the passivation film in an acidic environment. Co sulfides inhibit the chloride ion (Cl - ), hydrogen sulfide ion (HS -) with the passivation film, inhibiting the passivation film from being damaged by chloride ions, hydrogen sulfide ions. As a result, the SSC resistance of the steel material is improved. Further, Co improves the hardenability of the steel material, particularly, ensuring stable high strength of the steel material during industrial production. Specifically, Co suppresses the generation of retained austenite, suppressing uneven strength of the steel material. If the Co content is too low, even if the contents of the other elements are within the range of the present embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Co content is too high, even if the contents of the other elements are within the range of the present embodiment, the toughness of the steel material is reduced. Therefore, the Co content is 0.010 to 0.500%. The preferable lower limit of the Co content is 0.015%, more preferably 0.020%, further preferably 0.030%, further preferably 0.050%, further preferably 0.100%. The preferable upper limit of the Co content is 0.450%, more preferably 0.400%, further preferably 0.350%.

[0124] Ca: 0.0005 to 0.0050%

[0125] Calcium (Ca) makes S in the steel material harmless by fixing it as sulfide, improving the hot workability of the steel material. If the Ca content is too low, even if the contents of the other elements are within the range of the present embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Ca content is too high, even if the contents of the other elements are within the range of the present embodiment, coarse inclusions are generated in the steel material, and the SSC resistance of the steel material is reduced. Therefore, the Ca content is 0.0005 to 0.0050%. The preferable lower limit of the Ca content is 0.0006%, more preferably 0.0008%, further preferably 0.0010%. The preferable upper limit of the Ca content is 0.0045%, more preferably 0.0040%, further preferably 0.0035%.

[0126] Sn: 0.0005 to 0.0500%

[0127] Tin (Sn) improves the SSC resistance of the steel material in an acidic environment of pH 3.0. If the Sn content is too low, even if the contents of the other elements are within the range of the present embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Sn content is too high, even if the contents of the other elements are within the range of the present embodiment, Sn is segregated at grain boundaries, and the SSC resistance of the steel material is reduced. Therefore, the Sn content is 0.0005 to 0.0500%. The preferable lower limit of the Sn content is 0.0008%, more preferably 0.0010%, further preferably 0.0015%. The preferable upper limit of the Sn content is 0.0400%, more preferably 0.0300%, further preferably 0.0200%, further preferably 0.0100%, further preferably 0.0080%.

[0128] N: 0.0010~0.0500%

[0129] Nitrogen (N) combines with Ti to form fine Ti nitrides. Fine TiN suppresses the coarsening of grains through the pinning effect. As a result, the yield strength of the steel is improved. If the N content is too low, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the N content is too high, coarse nitrides will be generated even if the contents of other elements are within the range of this embodiment, thereby reducing the SSC resistance of the steel. Therefore, the N content is 0.0010 to 0.0500%. The preferred lower limit of the N content is 0.0015%, more preferably 0.0020%, further preferably 0.0030%, and further preferably 0.0040%. The preferred upper limit of the N content is 0.0450%, more preferably 0.0400%, further preferably 0.0350%, and further preferably 0.0300%.

[0130] O: 0.050% or less

[0131] Oxygen (O) is an impurity that is inevitably contained. That is, the lower limit of the O content exceeds 0%. O forms oxides, which reduce the SSC resistance of the steel. Therefore, if the O content is too high, the SSC resistance of the steel will be significantly reduced even if the contents of other elements are within the range of this embodiment. Therefore, the O content is 0.050% or less. The preferred upper limit of the O content is 0.040%, more preferably 0.030%, and further preferably 0.020%. The O content is preferably as low as possible. However, an extreme reduction in the O content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0005%, more preferably 0.001%, and further preferably 0.002%.

[0132] The balance of the martensitic stainless steel material of this embodiment consists of Fe and impurities. Impurities are substances that enter the martensitic stainless steel material during industrial steel production, such as from raw material ores and scrap, or from the manufacturing environment. These substances are not intentionally present and are permitted as long as they do not adversely affect the martensitic stainless steel material of this embodiment.

[0133] [Any element]

[0134] The martensitic stainless steel material of the present embodiment may further contain W in place of a portion of Fe.

[0135] W: 0~0.50%

[0136] Tungsten (W) is an arbitrary element and may not be contained. That is, the W content may be 0%. When contained, W stabilizes the passivation film in an acidic environment, thereby inhibiting the passivation film from being destroyed by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel is improved. Even if a small amount of W is contained, the above-mentioned effect can be obtained to a certain extent. On the other hand, if the W content is too high, W combines with C to form coarse carbides. In this case, even if the content of other elements is within the range of this embodiment, the SSC resistance of the steel will decrease. Therefore, the W content is 0 to 0.50%. The preferred lower limit of the W content is 0.01%, more preferably 0.03%, and further preferably 0.05%. The upper limit of the W content is preferably 0.45%, more preferably 0.40%, and further preferably 0.35%.

[0137] Furthermore, when the Cu content is high, W significantly improves SSC resistance. Specifically, when the Cu content is 0.50% or more, the W content is preferably 0.10% or more. When the Cu content is 0.50% or more, the lower limit of the W content is more preferably 0.12%, and even more preferably 0.15%.

[0138] The martensitic stainless steel material of the present embodiment may further contain Nb in place of a portion of Fe.

[0139] Nb: 0~0.500%

[0140] Niobium (Nb) is an arbitrary element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C and / or N to form Nb carbides and Nb carbonitrides. At this time, the coarsening of the grains is suppressed by the pinning effect, and the yield strength of the steel is improved. Even if a small amount of Nb is contained, the above-mentioned effect can be obtained to a certain extent. On the other hand, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, excessive Nb carbides and / or Nb carbonitrides will be produced. As a result, the SSC resistance of the steel decreases. Therefore, the Nb content is 0 to 0.500%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, and further preferably 0.003%. The preferred upper limit of the Nb content is 0.450%, more preferably 0.400%, and further preferably 0.350%.

[0141] The martensitic stainless steel material of this embodiment may further contain one or more elements selected from the group consisting of As and Sb in place of a portion of Fe. These elements assist the effect of Sn in improving the SSC resistance of the steel material.

[0142] As: 0~0.0100%

[0143] Arsenic (As) is an arbitrary element and may not be contained. That is, the As content may be 0%. When contained, As assists Sn in improving the SSC resistance of the steel. Even if a small amount of As is contained, the above-mentioned effect can be obtained to a certain extent. On the other hand, if the As content is too high, even if the contents of other elements are within the range of this embodiment, As will segregate at the grain boundaries, thereby reducing the SSC resistance of the steel. Therefore, the As content is 0 to 0.0100%. The preferred lower limit of the As content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the As content is 0.0090%, more preferably 0.0080%.

[0144] Sb: 0~0.0100%

[0145] Antimony (Sb) is an arbitrary element and may not be contained. That is, the Sb content may be 0%. When contained, Sb assists Sn in improving the SSC resistance of the steel. Even if a small amount of Sb is contained, the above-mentioned effect can be obtained to a certain extent. On the other hand, if the Sb content is too high, even if the contents of other elements are within the range of this embodiment, Sb will segregate at the grain boundaries, thereby reducing the SSC resistance of the steel. Therefore, the Sb content is 0 to 0.0100%. The preferred lower limit of the Sb content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Sb content is 0.0090%, more preferably 0.0080%.

[0146] [Yield Strength]

[0147] The yield strength of the martensitic stainless steel material of the present embodiment is 758 MPa (110 ksi) or more, more preferably 862 MPa (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 of the present embodiment is, for example, 1034 MPa (150 ksi). The more preferred upper limit of the yield strength of the steel material is 1000 MPa (145 ksi). In this specification, the yield strength refers to the 0.2% residual strain stress (MPa) obtained by performing a tensile test at room temperature (24 ± 3 ° C) according to ASTM E8 / E8M (2013).

[0148] Specifically, in the present embodiment, the yield strength is obtained by the following method. First, a tensile test piece is prepared from the martensitic stainless steel material of the present embodiment. When the steel material is a steel pipe, the tensile test piece is prepared from the central position of the wall thickness. When the steel material is a round bar, the tensile test piece is prepared from the R / 2 position. Note that, in the present specification, the R / 2 position of a round bar refers to the central position of the radius R in a cross section perpendicular to the axial direction of the round bar. When the steel material is a steel plate, the tensile test piece is prepared from the central position of the plate thickness. The size of the tensile test piece is not particularly limited. The tensile test piece is, for example, a round bar tensile test piece having a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm. The length direction of the parallel portion of the tensile test piece is parallel to the rolling direction and / or the axial direction of the steel material. Using the prepared tensile test piece, a tensile test is performed according to ASTM E8 / E8M (2013) at room temperature (24 ± 3°C) to obtain the 0.2% residual strain stress (MPa). The obtained 0.2% residual strain stress is defined as the yield strength (MPa).

[0149] As described above, the yield strength of the martensitic stainless steel material of the present embodiment is 758 MPa or more, and preferably 862 MPa or more. In the present embodiment, the Cu content is 1.00% or less, and to obtain a yield strength of 758 MPa or more and less than 862 MPa, it is preferable that the martensitic stainless steel material be C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 1.00%, Cr: 10.00 to 14.00%, Ni: 4.50 to 6.50%, Mo: 1.00 to 3.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, and the balance: Fe and impurities, in mass%.

[0150] In addition, in the present embodiment, when the Cu content is 1.00% or less and a yield strength of 862 MPa or more is desired, the martensitic stainless steel material preferably comprises, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01-1.00%, Cr: 10.00-14.00%, Ni: 5.00-7.50%, Mo: 2.00-4.00%, and Ti: 0. 050~0.300%, V: 0.01~1.00%, Al: 0.001~0.100%, Co: 0.010~0.500%, Ca: 0.0005~0.0050%, Sn: 0.0005~0.0500%, N: 0.0010~0.0500%, O: less than 0.050%, W: 0~0.50%, Nb: 0~0.500%, As: 0~0.0100%, Sb: 0~0.0100%, and the balance: Fe and impurities.

[0151] In this embodiment, when the Cu content is 0.50% or more and a yield strength of 862 MPa or more is desired, the martensitic stainless steel preferably comprises, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.50-3.50%, Cr: 10.00-14.00%, Ni: 5.00-7.50%, Mo: 2.00-4.00%, and Ti: 0.050-1.00%. ~0.300%, V: 0.01~1.00%, Al: 0.001~0.100%, Co: 0.010~0.500%, Ca: 0.0005~0.0050%, Sn: 0.0005~0.0500%, N: 0.0010~0.0500%, O: less than 0.050%, W: 0.10~0.50%, Nb: 0~0.500%, As: 0~0.0100%, Sb: 0~0.0100%, and the balance: Fe and impurities.

[0152] [About formula (1)]

[0153] In the martensitic stainless steel material of this embodiment, within the range of the content of the above-mentioned elements and the yield strength of 758 MPa or more, the content of the elements and the yield strength satisfy the formula (1). As a result, the martensitic stainless steel material of this embodiment has excellent SSC resistance in an acidic environment of pH 3.0 under the conditions that satisfy the other components of this embodiment.

[0154] 0.15≤(Sn+As+Sb) / {(Cu+Ni) / YS}≤1.00 (1)

[0155] In formula (1), the element symbol is replaced by the content of the corresponding element in mass %, and the yield strength in MPa is replaced by YS. It should be noted that when the corresponding element is not present, "0" is replaced by the element symbol.

[0156] F1 (=(Sn+As+Sb) / {(Cu+Ni) / YS}) is an indicator of improved SSC resistance in an acidic environment at pH 3.0, utilizing the synergistic effect of Sn, As, and Sb, along with Cu and Ni, adjusted for yield strength. If F1 is too low, the steel will not exhibit excellent SSC resistance in an acidic environment at pH 3.0. Similarly, if F1 is too high, the steel will not exhibit excellent SSC resistance in an acidic environment at pH 3.0. On the other hand, if F1 is between 0.15 and 1.00, the steel will exhibit excellent SSC resistance in an acidic environment at pH 3.0.

[0157] Therefore, the martensitic stainless steel material of this embodiment satisfies the aforementioned element contents and a yield strength of 758 MPa or higher, and has F1 set to 0.15 to 1.00. The preferred lower limit of F1 is 0.16, more preferably 0.18. The preferred upper limit of F1 is 0.95, more preferably 0.90.

[0158] [Microstructure of Steel]

[0159] The microstructure of the martensitic stainless steel material of the present embodiment is mainly composed of martensite. In this specification, "mainly composed of martensite" means that the microstructure is composed of 0-5.0% retained austenite, 0-5.0% ferrite, and the balance martensite by volume. In this specification, "composed of retained austenite, ferrite, and tempered martensite" means that the phases other than retained austenite, ferrite, and tempered martensite are so small that they can be ignored. For example, in the chemical composition of the martensitic stainless steel material of the present embodiment, the volume fraction of precipitates and inclusions is so small as to be ignored compared to the volume fraction of retained austenite, ferrite, and tempered martensite. That is, the microstructure of the martensitic stainless steel material of the present embodiment may contain trace amounts of precipitates, inclusions, etc. in addition to retained austenite, ferrite, and tempered martensite.

[0160] In this specification, martensite includes not only fresh martensite but also tempered martensite. The lower limit of the volume fraction of martensite in the microstructure of the martensitic stainless steel material of this embodiment is 90.0%, more preferably 95.0%. More preferably, the microstructure of the steel material is a single martensite phase.

[0161] In the microstructure, a small amount of retained austenite does not cause a significant decrease in strength and significantly improves the toughness of the steel. However, if the volume fraction of retained austenite is too high, the strength of the steel will be significantly reduced. Therefore, in the microstructure of the steel of this embodiment, the volume fraction of retained austenite is 0 to 5.0%. From the perspective of ensuring strength, the preferred upper limit of the volume fraction of retained austenite is 4.0%, more preferably 3.0%. The volume fraction of retained austenite can be 0%. On the other hand, when a small amount of retained austenite is contained, the volume fraction of retained austenite is greater than 0 and less than 5.0%, more preferably greater than 0 and less than 4.0%, and further preferably greater than 0 and less than 3.0%.

[0162] A small amount of ferrite may be contained in the microstructure. However, if the volume fraction of ferrite is too high, the toughness of the steel material will be significantly reduced. Therefore, in the microstructure of the steel material of this embodiment, the volume fraction of ferrite is 0 to 5.0%. The preferred upper limit of the volume fraction of ferrite is 3.0%, more preferably 2.0%, and even more preferably 1.0%. The volume fraction of ferrite may be 0%. On the other hand, when a small amount of ferrite is contained, the volume fraction of ferrite is greater than 0 and less than 5.0%, more preferably greater than 0 and less than 3.0%, even more preferably greater than 0 and less than 2.0%, and even more preferably greater than 0 and less than 1.0%.

[0163] [Measurement method of volume fraction of martensite]

[0164] In this embodiment, the volume fraction (%) of martensite in the microstructure of the steel material is determined by subtracting the volume fraction (%) of retained austenite and the volume fraction (%) of ferrite determined by the following method from 100%.

[0165] [Method for measuring the volume fraction of retained austenite]

[0166] The volume fraction of retained austenite in the microstructure of the steel is determined by X-ray diffraction. Specifically, a test piece for measuring the volume fraction of retained austenite is made from the steel of this embodiment. When the steel is a steel pipe, the test piece is collected from the center of the wall thickness. When the steel is round steel, the test piece is collected from the R / 2 position. When the steel is a steel plate, the test piece is collected from the center of the plate thickness. There is no particular restriction on the size of the test piece. The test piece is, for example, 15 mm × 15 mm × 2 mm thick. When the steel is a steel pipe, the thickness direction of the test piece is the pipe diameter direction. When the steel is round steel, the thickness direction of the test piece is the radial direction. When the steel is a steel plate, the thickness direction of the test piece is the plate thickness direction. Using the prepared test pieces, the X-ray diffraction intensities of the (110) plane of the α phase (martensite), the (200) plane of the α phase, the (211) plane of the α phase, the (111) plane of the γ phase (retained austenite), the (200) plane of the γ phase, and the (220) plane of the γ phase were measured, and the integrated intensity of each plane was calculated.

[0167] For X-ray diffraction intensity measurements, the X-ray diffraction device's target was Co (CoKα radiation), and the output was set at 30 kV / 100 mA. The measurement angle (2θ) was set between 45° and 105°. After calculation, the volume fraction Vγ (%) of retained austenite was calculated for each combination of α-phase and γ-phase planes (3 × 3 = 9 pairs) using formula (I). The average of the retained austenite volume fractions Vγ for the 9 pairs was defined as the retained austenite volume fraction (%).

[0168] Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)}(I)

[0169] Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value for the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value for the γ phase. The values ​​of Rα and Rγ for each surface can be entered using the retained γ quantitative analysis system supplied with the trade name RINT-TTR, manufactured by Rigaku Corporation. It should be noted that the volume fraction of retained austenite is obtained by rounding off the obtained value to the second decimal place.

[0170] [Method for measuring the volume fraction of ferrite]

[0171] The volume fraction of ferrite in the microstructure of the steel is determined by the point counting method. Specifically, a test piece for measuring the volume fraction of ferrite is made from the steel of this embodiment. When the steel is a steel pipe, the test piece is collected from the center of the wall thickness. When the steel is a round steel, the test piece is collected from the R / 2 position. When the steel is a steel plate, the test piece is collected from the center of the plate thickness. The test piece is not particularly limited as long as it has a surface parallel to the rolling direction as the observation surface. For example, when the steel is a steel pipe, the observation surface of the test piece is parallel to the pipe axis direction. After mechanically grinding the observation surface, the observation surface is electrolytically etched to expose the structure. Electrolytic etching is performed with an electrolyte: 30% sodium hydroxide aqueous solution, a current density: 1A / cm 2 , Electrolysis time: 1 minute.

[0172] The electrolytically etched observation surface was observed in 30 fields of view using an optical microscope. The observation field was set to a rectangle of 250 μm × 250 μm. It should be noted that the observation magnification was 400 times. In each observation field, those skilled in the art can distinguish ferrite from other phases (retained austenite, tempered martensite) based on contrast. Therefore, the ferrite in each observation field is determined based on contrast. The area ratio of the determined ferrite is obtained by the point counting method based on ASTM E562 (2019).

[0173] Specifically, for the observation field, 20 vertical lines are drawn at equal intervals from the upper end to the lower end of the observation field. That is, the observation field is divided into 21 areas in the left and right directions by 20 vertical lines. Further, for the observation field, 20 horizontal lines are drawn at equal intervals from the left end to the right end of the observation field. That is, the observation field is divided into 21 areas in the up and down directions by 20 horizontal lines. At this time, the intersection of the vertical line and the horizontal line is called a lattice point. That is, 400 lattice points are arranged at equal intervals in the observation field. According to ASTM E562 (2019), the lattice points overlapping with ferrite are counted in the observation field. The number of lattice points overlapping with ferrite obtained in 30 fields of view divided by the total number of lattice points (400×30=12000) is defined as the ferrite area ratio. In this embodiment, the area ratio of ferrite obtained by the above method is regarded as the volume ratio (%) of ferrite. It should be noted that the volume fraction of ferrite is obtained by rounding off the value to the second decimal place.

[0174] The volume fraction (%) of martensite in the microstructure of the steel material is determined by the following formula using the volume fraction (%) of retained austenite obtained by the above X-ray diffraction method and the volume fraction (%) of ferrite obtained by the above point counting method.

[0175] Volume fraction of martensite (%) = 100.0 - {volume fraction of retained austenite (%) + volume fraction of ferrite (%)}

[0176] [SSC resistance of steel materials]

[0177] The martensitic stainless steel material of this embodiment has excellent SSC resistance even in an acidic environment of pH 3.0, despite having a high yield strength of 758 MPa or higher. The SSC resistance of the martensitic stainless steel material of this embodiment can be evaluated by an SSC resistance evaluation test at room temperature. The SSC resistance evaluation test is conducted in accordance with NACE TM0177-2016 Method A.

[0178] Specifically, a round rod test piece is made from the steel material of this embodiment. When the steel material is a steel pipe, the round rod test piece is collected from the center of the wall thickness. When the steel material is round steel, the round rod test piece is collected from the R / 2 position. When the steel material is a steel plate, the round rod test piece is collected from the center of the plate thickness. The size of the round rod test piece is not particularly limited. For example, the diameter of the parallel portion of the round rod test piece is 6.35 mm, and the length of the parallel portion is 25.4 mm. It should be noted that the axial direction of the round rod test piece is parallel to the rolling direction and / or axial direction of the steel material.

[0179] The test solution was set to a 0.17 mass% sodium chloride aqueous solution with a pH of 3.0. For the test solution, acetic acid was added to an aqueous solution containing 0.17 mass% sodium chloride and 0.41 g / L sodium acetate to adjust the pH to 3.0. The round bar test piece prepared as described above was loaded with a stress equivalent to 90% of the actual yield stress. The test solution at 24°C was injected into the test container in a manner such that the round bar test piece loaded with stress was immersed, thereby preparing 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 kept at 24°C for 720 hours. For the test piece kept for 720 hours, the surface of the parallel part of the test piece was observed with a magnifying glass with a magnification of 10 times to confirm the presence or absence of pitting. In the SSC resistance evaluation test implemented by the above method, no pitting was confirmed after 720 hours for the martensitic stainless steel material of this embodiment.

[0180] [Shapes and uses of steel]

[0181] As described above, the shape of the martensitic stainless steel material of this embodiment is not particularly limited. Specifically, the martensitic stainless steel material of this embodiment can be a steel pipe, round steel (solid material), or steel plate. The steel pipe can be a seamless steel pipe or a welded steel pipe. For example, the steel pipe is a steel pipe for oil well pipe. Oil well pipe refers to a steel pipe used for oil well pipe purposes. Oil well pipes include, for example, casing, tubing, drill pipe, etc. used in the excavation of oil or gas wells, the collection of crude oil or natural gas, etc. Preferably, the steel material of this embodiment is a seamless steel pipe for oil well pipe.

[0182] As described above, the martensitic stainless steel material of this embodiment has a yield strength of 758 MPa or greater within the ranges of the present embodiment, and within the ranges of the aforementioned element contents and a yield strength of 758 MPa or greater, F1 satisfies a value of 0.15 to 1.00. As a result, the steel material of this embodiment achieves both high yield strength and excellent SSC resistance in an acidic environment of pH 3.0.

[0183] [Manufacturing method]

[0184] An example of a method for producing the martensitic stainless steel material of the present embodiment will be described. It should be noted that the production method described below is an example, and the production method of the martensitic stainless steel material of the present embodiment is not limited to the following description. In other words, as long as the martensitic stainless steel material of the present embodiment having the above-described structure can be produced, the production method is not limited to the following description. However, the production method described below is a suitable method for producing the martensitic stainless steel material of the present embodiment.

[0185] An example of a method for producing a martensitic stainless steel material according to the present embodiment includes a step of preparing an intermediate steel material (preparation step) and a step of quenching and tempering the intermediate steel material (heat treatment step). Detailed descriptions of each step will be given below.

[0186] [Preparation process]

[0187] In the preparatory step, an intermediate steel material having the aforementioned chemical composition is prepared. The method for producing the intermediate steel material is not particularly limited, as long as the intermediate steel material has the aforementioned chemical composition. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate or welded steel pipe, and a pipe blank when the final product is a seamless steel pipe.

[0188] The preparation process may include: a process of preparing a billet (bill preparation process); and a process of hot working the billet to produce an intermediate steel material (hot working process). The case including the billet preparation process and the hot working process will be described in detail below.

[0189] [Blank preparation process]

[0190] In the billet preparation step, billets are produced using molten steel having the above-described chemical composition. The billet production method is not particularly limited and can be a known method. Specifically, molten steel is used to produce billets (slabs, blooms, or billets) through continuous casting. Alternatively, the molten steel can be used to produce ingots through ingot casting. Alternatively, billets can be produced by performing bloom rolling on the slabs, blooms, or ingots as needed. The billets (slabs, blooms, or billets) are produced through the above steps.

[0191] [Hot working step]

[0192] In the hot working step, the prepared billet is subjected to hot working to thereby produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to a pipe blank. First, the bloom is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1100 to 1300°C. The bloom taken out of the heating furnace is subjected to hot working to thereby produce a pipe blank (seamless steel pipe). The method of hot working is not particularly limited, and can be a publicly known method.

[0193] For example, as the hot working, Mannesmann method can be performed to produce a pipe blank. At this time, the round billet is subjected to piercing rolling by a piercing mill. In the case of the piercing rolling, the piercing ratio is not particularly limited, and is, for example, 1.0 to 4.0. The round billet subjected to the piercing rolling is subjected to hot rolling by a mandrel mill, a reducing mill, a sizing mill, or the like to thereby produce a pipe blank. The cumulative cross-sectional reduction ratio in the hot working step is, for example, 20 to 70%.

[0194] Other hot working methods can be used to produce a pipe blank from a bloom. For example, when the steel material is a short-thick-walled steel pipe such as a coupling, the pipe blank can be produced by forging using Elshoff pipe forging method or the like. The pipe blank is produced by the above steps. The wall thickness of the pipe blank is not particularly limited, and is, for example, 9 to 60 mm.

[0195] When the steel material is a round steel, 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 to thereby produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, cogging using a cogging mill or hot rolling using a continuous rolling mill. In the continuous rolling mill, a horizontal stand having a pair of roll passes arranged in the vertical direction and a vertical stand having a pair of roll passes arranged in the horizontal direction are alternately arranged.

[0196] When the steel material is a steel sheet, 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 using a cogging mill and a continuous rolling mill to thereby produce an intermediate steel material having a steel sheet shape.

[0197] The pipe blank produced by the hot working can be subjected to air cooling (As-Rolled). The pipe blank produced by the hot working can be subjected to direct quenching after the hot working without being cooled to normal temperature, or can be subjected to quenching after being reheated after the hot working.

[0198] When quenching is performed directly after hot working, or when quenching is performed after supplemental heating, cooling can be stopped during the quenching process, or slow cooling can be performed. This can suppress the occurrence of quench cracks in the tube. When quenching is performed after hot working, or when quenching is performed after supplemental heating, stress relief annealing (SR) can be performed after quenching and before the next heat treatment step. This eliminates residual stress in the tube.

[0199] As described above, an intermediate steel material is prepared in the preparation step. The intermediate steel material can be produced using the preferred steps described above, or it can be produced by a third party, or by a factory or business other than the factory that performs the quenching and tempering steps described below. The heat treatment step is described in detail below.

[0200] [Heat treatment process]

[0201] The heat treatment process includes quenching and tempering.

[0202] [Quenching process]

[0203] In the heat treatment process, first, the intermediate steel material produced by the hot working process is quenched (quenching process). Quenching is carried out by a known method. Specifically, the intermediate steel material after the hot working process is placed in a heat treatment furnace and maintained at a quenching temperature. The quenching temperature is A C3 The intermediate steel is held at a quenching temperature and then rapidly cooled (quenched). 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. For example, when the intermediate steel is a billet, the billet can be rapidly cooled by immersing it in a water tank or an oil tank; spray cooling or mist cooling can also be used to pour or spray cooling water onto the outer surface and / or inner surface of the billet to rapidly cool the billet.

[0204] It should be noted that, as described above, after the hot working process, quenching (direct quenching) can be performed immediately after the hot working without cooling the intermediate steel to room temperature; or the hot-worked tube blank can be loaded into a supplementary heating furnace before the temperature drops, thereby maintaining the quenching temperature and then performing quenching.

[0205] [Tempering process]

[0206] The intermediate steel material after quenching is further subjected to a tempering process. During the tempering process, the yield strength of the steel material is adjusted. In this embodiment, the tempering temperature is set to 540-620°C. The holding time at the tempering temperature is not particularly limited, but is, for example, 10-180 minutes. Those skilled in the art are well aware that the yield strength of the steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. To this end, the tempering conditions are adjusted so that the yield strength of the steel material is at least 758 MPa.

[0207] The above process can produce the martensitic stainless steel material of the present embodiment. It should be noted that, as mentioned above, the martensitic stainless steel material of the present embodiment is not limited to the above-mentioned manufacturing method. Specifically, as long as it is possible to produce a martensitic stainless steel material having a chemical composition with each element content within the range of the present embodiment, a microstructure composed of 0-5.0% retained austenite, 0-5.0% ferrite and the remainder tempered martensite in volume %, a yield strength of 758 MPa or more, and an F1 of 0.15-1.00, the manufacturing method of the present embodiment is not limited to the above-mentioned manufacturing method. The martensitic stainless steel material of the present embodiment is described in more detail below through examples.

[0208] Example

[0209] Molten steel having the chemical composition shown in Table 1 was produced. It should be noted that "-" in Table 1 indicates that the content of the element is an impurity level. For example, the W content in Test No. 1 is rounded to the third decimal place and is 0%. The Nb content in Test No. 1 is rounded to the fourth decimal place and is 0%. The As and Sb contents in Test No. 1 are rounded to the fifth decimal place and are 0%.

[0210] [Table 1]

[0211]

[0212] The molten steel was melted in an 180kg vacuum furnace and cast into ingots. The ingots were heated at 1250°C for 3 hours. The heated ingots were hot forged to form blanks. The hot-forged blanks were heated at 1230°C for 3 hours and then hot rolled. This produced steel products (steel plates) with a thickness of 13mm.

[0213] The steel materials of each test number were quenched. Specifically, the steel plates of each test number were heated to the quenching temperature (°C) listed in Table 2. The steel plates of each test number were held at the quenching temperature for 15 minutes and then water-cooled. After quenching, the steel materials of each test number were tempered by holding them at the tempering temperature (°C) listed in Table 2 for 30 minutes.

[0214] [Table 2]

[0215] Table 2

[0216]

[0217] Steel plates of respective test numbers were manufactured through the above-described manufacturing steps.

[0218] [Evaluation test]

[0219] The manufactured steel plates of each test number were subjected to a microstructure observation test, a tensile test, and an SSC resistance evaluation test.

[0220] [Microstructure observation test]

[0221] A microstructure observation test was performed on the steel plates of each test number. First, the volume fraction (%) of retained austenite was determined for the steel plates of each test number using the above-mentioned X-ray diffraction method. It should be noted that in the measurement of the X-ray diffraction intensity, a product name RINT-TTR manufactured by Rigaku Corporation was used as an X-ray diffraction apparatus. The radiation source was set to CoKα, the output was set to 30kV-100mA, and the measurement angle (2θ) was set to 45° to 105° for measurement. The volume fraction (%) of retained austenite in the steel plates of each test number obtained is shown in the "Retained γ (%)" column of Table 2.

[0222] In addition, the volume fraction (%) of ferrite was calculated using the above-mentioned point method. Specifically, a test piece was made from the center of the thickness of the steel plate of each test number. The observation surface of the test piece was set to a surface parallel to the rolling direction. It should be noted that in this embodiment, the area ratio of ferrite calculated by the method according to the above-mentioned ASTM E562 (2019) was set to the volume fraction (%) of ferrite. The volume fraction of ferrite in the steel plate of each test number obtained is shown in "Ferrite (%)" in Table 2.

[0223] Furthermore, for the steel plate of each test number, the volume fraction (%) of martensite was determined by the following formula using the volume fraction (%) of retained austenite and the volume fraction (%) of ferrite.

[0224] Volume fraction of martensite (%) = 100 - {volume fraction of retained austenite (%) + volume fraction of ferrite (%)}

[0225] The volume ratio (%) of martensite obtained for each test number is shown in the "Martensite (%)" column of Table 2.

[0226] [Tensile test]

[0227] According to ASTM E8 / E8M (2013), a tensile test was performed on the steel plates of each test number. Specifically, a round bar tensile test piece with a diameter of 8.9 mm and a gauge length of 35.6 mm in the parallel portion was made from the center position of the plate thickness of the steel plates of each test number. The length direction of the round bar tensile test piece is parallel to the rolling direction of the steel plate. Using the round bar tensile test piece of each test number, a tensile test was performed at room temperature (24±3°C) and in the atmosphere to determine the 0.2% residual deformation stress (MPa). The obtained 0.2% residual deformation stress is defined as the yield strength (MPa). The yield strength of each test number obtained is shown in the "YS (MPa)" column of Table 2. Furthermore, for the steel plates of each test number, F1 was calculated using the chemical composition, yield strength and formula (1). The F1 value of each test number obtained is shown in the "F1" column of Table 2.

[0228] [SSC resistance evaluation test]

[0229] The steel plates with each test number were subjected to an SSC resistance evaluation test. Specifically, a round bar test piece with a diameter of 6.35 mm and a parallel portion length of 25.4 mm was prepared from the center of the thickness of the steel plate with each test number. Three of the prepared round bar test pieces were subjected to an SSC resistance evaluation test according to NACE TM0177-2016 Method A. The axial direction of the round bar test piece was parallel to the rolling direction.

[0230] The test solution was set to a 0.17 mass% sodium chloride aqueous solution with a pH of 3.0. For the test solution, acetic acid was added to an aqueous solution containing 0.17 mass% sodium chloride and 0.41 g / L sodium acetate to adjust the pH to 3.0. The round rod test piece of each test number was loaded with a stress equivalent to 90% of the actual yield stress. The test solution at 24°C was injected into the test container in a manner such that the round rod test piece loaded with stress was immersed to prepare 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 kept at 24°C for 720 hours.

[0231] The surfaces of the parallel portions of the round bar test pieces after 720 hours were observed with a magnifying glass at a magnification of 10 times to confirm the presence of pitting. The number of pits confirmed in the three round bar test pieces is shown in Table 2 as "Number of Pitting Occurrence (Pitting)".

[0232] [Evaluation results]

[0233] Referring to Tables 1 and 2, the chemical compositions of the steel plates from Test Nos. 1 to 22 were suitable, with microstructures consisting of 0-5.0% by volume of retained austenite, 0-5.0% by volume of ferrite, and the balance of martensite. Furthermore, these steel plates exhibited high strength, with yield strengths exceeding 758 MPa. Furthermore, the F1 values ​​for these steel plates were within the range of 0.15 to 1.00. As a result, these steel plates exhibited zero pitting corrosion in an acidic environment of pH 3.0, demonstrating excellent SSC resistance.

[0234] On the other hand, the F1 of the steel plates of test numbers 23 to 26 was too low. As a result, these steel plates suffered pitting corrosion in at least one of the steel plates in an acidic environment of pH 3.0, and did not have excellent SSC resistance.

[0235] The F1 of the steel plates of test numbers 27 to 30 was too high. As a result, three of these steel plates developed pitting corrosion in an acidic environment of pH 3.0, indicating that they did not have excellent SSC resistance.

[0236] The steel sheet of test number 31 did not contain Sn. As a result, pitting corrosion occurred in one steel sheet under an acidic environment of pH 3.0, indicating that the steel sheet did not have excellent SSC resistance.

[0237] The steel sheets of test numbers 32 to 34 contained no Sn. Furthermore, the F1 values ​​of these steel sheets were too low. Consequently, at least one of these steel sheets experienced pitting corrosion in an acidic environment of pH 3.0, indicating that they did not exhibit excellent SSC resistance.

[0238] The steel plate of test number 35 contained no Sn. Furthermore, the F1 of this steel plate was too high. Consequently, three of the steel plates exhibited pitting corrosion in an acidic environment of pH 3.0, indicating that this steel plate did not exhibit excellent SSC resistance.

[0239] The Co content of the steel plate of test number 36 was too low. As a result, pitting corrosion occurred in one steel plate in an acidic environment of pH 3.0, indicating that the steel plate did not have excellent SSC resistance.

[0240] The Co content of the steel plate of test number 37 was too low. Furthermore, the F1 of this steel plate was too low. As a result, three of the steel plates exhibited pitting corrosion in an acidic environment of pH 3.0, indicating that this steel plate did not have excellent SSC resistance.

[0241] The embodiments of the present invention have been described above. However, the above embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and the above embodiments may be appropriately modified and implemented within the scope of the present invention.

Claims

1. A martensitic stainless steel material comprising, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01~3.50%, Cr：10.00～14.00％、 Ni: 4.50-7.50%, Mo: 1.00~4.00%, Ti: 0.050~0.300%, V：0.01～1.00％、 Al：0.001～0.100％、 Co: 0.010~0.500%, Ca: 0.0005~0.0050%, Sn: 0.0005~0.0500%, N：0.0010～0.0500％、 O: 0.050% or less, W：0～0.50％、 Nb: 0~0.500%, As: 0~0.0100%, Sb: 0 to 0.0100%, and Balance: Fe and impurities, The yield strength of the martensitic stainless steel is 758 MPa or more. Within the range of the content of the element of the martensitic stainless steel material and the yield strength, the content of the element and the yield strength satisfy the formula (1), 0.15≤(Sn+As+Sb) / {(Cu+Ni) / YS}≤1.00 (1) in, In formula (1), the content of the corresponding element is substituted in mass % at the element symbol, and the yield strength is substituted in MPa at YS. It should be noted that when the corresponding element is not present, "0" is substituted at the element symbol.

2. The martensitic stainless steel material according to claim 1, comprising W：0.01～0.50％、 Nb: 0.001~0.500%, As: 0.0001 to 0.0100%, and Sb: One or more elements selected from the group consisting of 0.0001 to 0.0100%.

3. The martensitic stainless steel material according to claim 1, comprising 10.00 to 13.40% Cr. The martensitic stainless steel material according to claim 1 , wherein the microstructure of the martensitic stainless steel material is composed of 0 to 5.0% by volume of retained austenite, 0 to 5.0% by volume of ferrite, and the balance of martensite.

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