WO2022202913A1

WO2022202913A1 - Martensite stainless steel material

Info

Publication number: WO2022202913A1
Application number: PCT/JP2022/013603
Authority: WO
Inventors: 恭平神吉; 秀樹高部
Original assignee: 日本製鉄株式会社
Priority date: 2021-03-24
Filing date: 2022-03-23
Publication date: 2022-09-29
Also published as: JPWO2022202913A1; EP4286542A1; CN117043378A; JP7151945B1; BR112023014937A2

Abstract

Provided is a martensite stainless steel material which exhibits both high yield strength and excellent SSC resistance. This martensite stainless steel material comprises, in terms of 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; and remainder: Fe and impurities. The martensite stainless steel material has a yield strength of 758 MPa or more. The amounts of the elements contained and the yield strength within these ranges satisfy formula (1) set forth in the description.

Description

Martensitic stainless steel material

The present disclosure relates to steel materials, and more particularly to martensitic stainless steel materials.

Oil wells and gas wells (hereinafter collectively referred to as “oil wells”) have an environment containing a large amount of corrosive substances. Corrosive substances are, for example, corrosive gases such as hydrogen sulfide (H ₂ S) gas and carbonic acid (CO ₂ ) gas. Chromium (Cr) is known to be effective in improving the carbon dioxide gas corrosion resistance of steel. Therefore, in an oil well environment containing a large amount of carbon dioxide, depending on the partial pressure and temperature of carbon dioxide, API L80 13Cr steel (normal 13Cr steel), super 13Cr steel with reduced C content, etc. A martensitic stainless steel material containing about 13% by mass of Cr is used.

In recent years, due to the deepening of oil wells, there is a demand for higher strength steel materials for oil wells. Specifically, 80 ksi class (yield strength less than 80 to 95 ksi, that is, less than 552 to 655 MPa) and 95 ksi class (yield strength less than 95 to 110 ksi, that is, less than 655 to 758 MPa) oil well steel materials are widely used. It's been done. Recently, there is a growing demand for oil well steel materials with a yield strength of 110 ksi or more (yield strength of 758 MPa or more).

Here, in this specification, an environment containing hydrogen sulfide and carbon dioxide is referred to as a "sour environment". Steel materials for oil wells used in sour environments are required to have sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance). In other words, in recent years, there has been a demand for oil well steel materials that have both high strength and excellent SSC resistance.

JP 2000-192196 (Patent Document 1), JP 2012-136742 (Patent Document 2), and International Publication No. 2008/023702 (Patent Document 3), high strength and excellent SSC resistance We propose a steel material with

The steel material of Patent Document 1 is a martensitic stainless steel for oil wells, and in terms of weight %, 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: 9-14%, Mo: 3.1-7%, Ni: 1-8%, Co: 0.5-7%, 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 is Fe and inevitable impurities. When Mo is contained, the Ms point is lowered. Therefore, by containing Co together with Mo, this steel material suppresses a decrease in the Ms point and makes the microstructure a martensitic single-phase structure. As a result, Patent Document 1 describes that this steel material can improve SSC resistance while maintaining a strength of 80 ksi or more (552 MPa or more).

The steel material of Patent Document 2 is a martensitic stainless steel seamless steel pipe, and 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: 14.0-15.5%, Ni: 5.5-7.0%, Mo: 2.0-3.5%, Cu: 0 .3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, and the balance consists of Fe and unavoidable impurities. This steel material has a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or more. By setting the C content to 0.01% or less, adjusting Cr, Ni and Mo to appropriate ranges, and further containing appropriate amounts of Cu and V or appropriate amounts of W, it is possible to achieve a strength of 655 MPa or more. Patent Document 2 describes that , excellent SSC resistance can be obtained.

The steel material of Patent Document 3 is a martensitic stainless steel, and in mass %, C: 0.010 to 0.030%, Mn: 0.30 to 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, Al: 0.10% or less, and the balance is Fe and impurities and satisfies the formula (6.0≤Ti/C≤10.1). The yield strength is 758-862 MPa. There is a correlation between the ratio (Ti/C) of the Ti content to the C content in steel and the value obtained by subtracting the yield strength from the tensile strength. Moreover, when the hardness variation in the steel material is large, the SSC resistance of the steel material is lowered. Therefore, Patent Document 3 describes that this steel material has a yield strength of 758 to 862 MPa by adjusting Ti/C to an appropriate range to suppress variations in hardness.

Japanese Patent Application Laid-Open No. 2000-192196 JP 2012-136742 A WO2008/023702

Patent Documents 1 to 3 above propose techniques for increasing the yield strength of steel materials and improving SSC resistance. However, a martensitic stainless steel material having excellent SSC resistance while increasing the yield strength may be obtained by techniques other than the techniques proposed in Patent Documents 1 to 3 above.

Furthermore, in recent years, there has been active development of oil wells with higher hydrogen ion concentrations than before. Generally, SSC tends to occur in an environment with a high hydrogen ion concentration (that is, a low pH). Therefore, a martensitic stainless steel material having excellent SSC resistance even in a sour environment with a pH of 3.0, which has a higher hydrogen ion concentration than before, has been desired. However, Patent Documents 1 to 3 do not discuss the SSC resistance of steel materials in a sour environment with a pH of 3.0.

An object of the present disclosure is to provide a martensitic stainless steel material that can achieve both high yield strength and excellent SSC resistance in a sour environment of pH 3.0.

The martensitic stainless steel material according to the present disclosure is
in % 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 to 3.50%,
Cr: 10.00 to 14.00%,
Ni: 4.50-7.50%,
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,
W: 0 to 0.50%,
Nb: 0 to 0.500%,
As: 0 to 0.0100%,
Sb: 0 to 0.0100%, and
Balance: Fe and impurities,
Yield strength is 758 MPa or more,
Within the ranges of the element content and the yield strength of the martensitic stainless steel material, the element content and the yield strength satisfy formula (1).
0.15≦(Sn+As+Sb)/{(Cu+Ni)/YS}≦1.00 (1)
Here, the element symbol in the formula (1) is substituted with the content of the corresponding element in mass %, and YS is substituted with the yield strength in MPa. When the corresponding element is not contained, "0" is substituted for the element symbol.

The martensitic stainless steel material according to the present disclosure can achieve both high yield strength and excellent SSC resistance in a sour environment of pH 3.0.

FIG. 1 is a diagram showing the relationship between F1 (=(Sn+As+Sb)/{(Cu+Ni)/YS}) in this example and the number of pitting corrosion occurrences (lines), which is an index of SSC resistance.

First, the present inventors studied a martensitic stainless steel material that can achieve both high yield strength and excellent SSC resistance in a sour environment of pH 3.0 from the viewpoint of chemical composition. As a result, the present inventors found that, 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 .050-0.300%, V: 0.01-1.00%, Al: 0.001-0.100%, Co: 0.010-0.500%, Ca: 0.0005-0.0050 %, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, and Nb: 0 to 0.500%. For example, it was considered possible to achieve both a yield strength of 758 MPa (110 ksi) or more and excellent SSC resistance in a sour environment of pH 3.0.

Next, the present inventors conducted a detailed study of means for enhancing the SSC resistance while maintaining the yield strength of 758 MPa or more for the martensitic stainless steel material containing the above elemental contents. As a result, tin (Sn), arsenic (As), and antimony (Sb), which have not received much attention so far, may improve the SSC resistance of the martensitic stainless steel material containing the above element contents. The inventors have found that. As a result of further detailed studies by the present inventors, in the martensitic stainless steel material containing the above-mentioned element contents, Sn in particular significantly increases the SSC resistance, and in As and Sb, Sn increases the SSC resistance. It may help the effect.

Therefore, the inventors conducted a detailed study on the Sn, As, and Sb contents that can sufficiently improve the SSC resistance of martensitic stainless steel materials. As a result, the martensitic stainless steel material according to the present embodiment contains 0.0005 to 0.0500% Sn, 0 to 0.0100% As, and 0 to 0 Sb in addition to the above element contents. It has been clarified that the SSC resistance of the steel material is enhanced by containing .0100%. That is, in terms of % 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: 0.005% or less. 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 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 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 the balance is Fe and impurities. may be compatible.

On the other hand, even if the martensitic stainless steel material has the chemical composition described above, if it has a yield strength of 758 MPa or more, it may not be possible to stably increase the SSC resistance in a sour environment of pH 3.0. The inventors have found out. Therefore, the inventors of the present invention conducted detailed studies on means for improving the SSC resistance in a sour environment of pH 3.0 while maintaining the yield strength of 758 MPa or more for the martensitic stainless steel material having the chemical composition described above. As a result, the present inventors obtained the following findings.

As a result of detailed studies by the present inventors, it was found that in a martensitic stainless steel material having the chemical composition described above and a yield strength of 758 MPa or more, if the element content and the yield strength satisfy the formula (1), , pH 3.0 sour environment, the SSC resistance of the steel can be significantly improved.
0.15≦(Sn+As+Sb)/{(Cu+Ni)/YS}≦1.00 (1)
Here, the element symbol in the formula (1) is substituted with the content of the corresponding element in mass %, and YS is substituted with the yield strength in MPa. When the corresponding element is not contained, "0" is substituted for the element symbol.

Define as F1=(Sn+As+Sb)/{(Cu+Ni)/YS}. As described above, As and Sb assist the effect that Sn enhances the SSC resistance of steel. Furthermore, the SSC resistance of the steel is remarkably enhanced by setting the ratio of the Sn, As and Sb contents to the Cu and Ni contents within a certain range. On the other hand, the higher the yield strength of the steel material, the more easily the SSC resistance of the steel material decreases. Therefore, the denominator of F1 is the ratio of the Cu and Ni contents to the yield strength. Thus, the ratio of Sn, As and Sb contents to Cu and Ni contents adjusted according to yield strength is defined as F1. That is, F1 is an index for enhancing the SSC resistance in a sour environment of pH 3.0 due to the synergistic effect of Sn, As and Sb, and Cu and Ni, which are adjusted according to the yield strength. The relationship between F1 and SSC resistance in a sour environment of pH 3.0 will be specifically described with reference to the drawings.

FIG. 1 is a diagram showing the relationship between F1 and SSC resistance in this example. FIG. 1 shows F1 and the number of pitting corrosion occurrences (numbers), which is an index of SSC resistance, for examples having the above-described chemical composition and a yield strength of 758 MPa or more among the examples described later. Created using The number of pitting corrosion occurrences was obtained by an SSC resistance evaluation test assuming a sour environment of pH 3.0, which will be described later.

With reference to Fig. 1, when F1 was too low, one or more pitting corrosion occurred. Similarly, when F1 was too high, one or more pitting corrosion occurred. On the other hand, when F1 was 0.15 to 1.00, no pitting corrosion occurred. That is, referring to FIG. 1, in a steel material having the above-described chemical composition and a yield strength of 758 MPa or more, if F1 is within the range of 0.15 to 1.00, a sour environment of pH 3.0 In, excellent SSC resistance is obtained.

For the steel material having the chemical composition described above and a yield strength of 758 MPa or more, by adjusting F1 to 0.15 to 1.00, the SSC resistance of the steel material in a sour environment of pH 3.0 is increased. The mechanism has not been elucidated. However, as shown in FIG. 1, by adjusting F1 to 0.15 to 1.00, the martensitic stainless steel material having the chemical composition described above and having a yield strength of 758 MPa or more has a pH of 3.0. The enhanced SSC resistance in a zero sour environment is demonstrated by the examples.

As described above, the martensitic stainless steel material according to the present embodiment has the chemical composition described above, the yield strength is 758 MPa or more, and the element content and the element content within the range of the yield strength are and the yield strength satisfy the formula (1). As a result, the martensitic stainless steel material according to this embodiment can achieve both a high yield strength of 758 MPa or more and excellent SSC resistance in a sour environment of pH 3.0.

The gist of the martensitic stainless steel material according to the present embodiment completed based on the above knowledge is as follows.

[1]
A martensitic stainless steel material,
in % 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 to 3.50%,
Cr: 10.00 to 14.00%,
Ni: 4.50-7.50%,
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,
W: 0 to 0.50%,
Nb: 0 to 0.500%,
As: 0 to 0.0100%,
Sb: 0 to 0.0100%, and
Balance: Fe and impurities,
Yield strength is 758 MPa or more,
Within the range of the element content and the yield strength of the martensitic stainless steel material, the element content and the yield strength satisfy the formula (1),
Martensitic stainless steel material.
0.15≦(Sn+As+Sb)/{(Cu+Ni)/YS}≦1.00 (1)
Here, the element symbol in the formula (1) is substituted with the content of the corresponding element in mass %, and YS is substituted with the yield strength in MPa. When the corresponding element is not contained, "0" is substituted for the element symbol.

[2]
The martensitic stainless steel material according to [1],
W: 0.01 to 0.50%,
Nb: 0.001 to 0.500%,
As: 0.0001 to 0.0100%, and
Sb: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%,
Martensitic stainless steel material.

The shape of the martensitic stainless steel material according to this embodiment is not particularly limited. The martensitic stainless steel material according to this embodiment may be a steel pipe, a round steel (solid material), or a steel plate. The round steel means a bar having a circular cross section perpendicular to the axial direction. Also, the steel pipe may be a seamless steel pipe or a welded steel pipe.

The martensitic stainless steel material according to this embodiment will be described in detail below. "%" for elements means % by weight unless otherwise specified. Further, in the following description, the martensitic stainless steel material is also simply referred to as "steel material".

[Chemical composition]
The martensitic stainless steel material according to this embodiment contains the following elements.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is over 0%. C enhances the hardenability of the steel material and enhances the strength of the steel material. On the other hand, if the C content is too high, the strength of the steel material will be too high even if the contents of other elements are within the ranges of the present embodiment. As a result, the SSC resistance of the steel is lowered. Therefore, the C content is 0.030% or less. The preferred upper limit of the C content is 0.028%, more preferably 0.025%, still more preferably 0.020%, still more preferably 0.018%. The C content is preferably as low as possible. However, drastic reduction of C content increases manufacturing cost. Therefore, considering industrial production, the lower limit of the C content is preferably 0.001%, more preferably 0.003%, and still more preferably 0.005%.

Si: 1.00% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is over 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability of the steel deteriorates even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 1.00% or less. The preferred lower limit of the Si content for effectively obtaining the above effects is 0.01%, more preferably 0.05%, still more preferably 0.10%, and still more preferably 0.15%. be. A preferred upper limit of the Si content is 0.80%, more preferably 0.60%, still more preferably 0.50%, still more preferably 0.45%.

Mn: 1.00% or less Manganese (Mn) is inevitably contained. That is, the lower limit of the Mn content is over 0%. Mn enhances the hardenability of the steel material and enhances the strength of the steel material. On the other hand, if the Mn content is too high, Mn may segregate at grain boundaries together with impurity elements such as P and S even if the content of other elements is within the range of the present embodiment. In this case, the SSC resistance of the steel is lowered. Therefore, the Mn content is 1.00% or less. The preferred lower limit of the Mn content for effectively obtaining the above effect is 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.15%. be. A preferable upper limit of the Mn content is 0.80%, more preferably 0.70%, still more preferably 0.60%, still more preferably 0.50%.

P: 0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the lower limit of the P content is over 0%. P segregates at grain boundaries to facilitate the generation of SSC. Therefore, if the P content is too high, the SSC resistance of the steel is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.025%, more preferably 0.020%, and still more preferably 0.018%. The lower the P content is, the better. However, drastic reduction of P content increases manufacturing cost. Therefore, considering industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%.

S: 0.0050% or less Sulfur (S) is an unavoidable impurity. That is, the lower limit of the S content is over 0%. S, like P, segregates at grain boundaries and facilitates the generation of SSC. Therefore, if the S content is too high, the SSC resistance of the steel is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the S content is 0.0050% or less. The upper limit of the S content is preferably 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, still more preferably 0.0020%. It is preferable that the S content is as low as possible. However, drastic reduction of the S content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

Cu: 0.01-3.50%
Copper (Cu) is an austenite-forming element and makes the microstructure after quenching martensite. Cu further enhances the SSC resistance of steel materials in a sour environment of pH 3.0 due to a synergistic effect with Sn, As and Sb. If the Cu content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cu content is too high, even if the contents of the other elements are within the range of the present embodiment, the above effects will be saturated, and the hot workability of the steel material will be significantly reduced. In this case, the manufacturing costs are further increased. Therefore, the Cu content is 0.01-3.50%. A preferable lower limit of the Cu content is 0.02%, more preferably 0.03%, and still more preferably 0.05%. A preferable upper limit of the Cu content is 3.30%, more preferably 3.10%, and still more preferably 2.90%.

Cr: 10.00-14.00%
Chromium (Cr) forms a passivation film on the surface of the steel material and enhances the SSC resistance of the steel material. If the Cr content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content is too high, even if the content of other elements is within the range of the present embodiment, ferrite is included in the structure, making it difficult to ensure sufficient strength in some cases. If the Cr content is too high, intermetallic compounds and Cr carbonitrides are likely to form in the steel even if the content of other elements is within the range of the present embodiment. As a result, the SSC resistance of the steel is lowered. Therefore, the Cr content is 10.00-14.00%. A preferable lower limit of the Cr content is 10.30%, more preferably 10.50%, and still more preferably 11.00%. The preferred upper limit of the Cr content is 13.80%, more preferably 13.60%, still more preferably 13.50%, still more preferably 13.45%, still more preferably 13.40 %, more preferably 13.35%.

Ni: 4.50-7.50%
Nickel (Ni) is an austenite-forming element, and the microstructure after quenching becomes martensite. Ni also forms sulfides on the passive film in sour environments. Ni sulfide suppresses contact of chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) with the passive film, and suppresses destruction of the passive film by chloride ions and hydrogen sulfide ions. do. As a result, the SSC resistance of the steel is enhanced. Ni further enhances the SSC resistance of steel materials in a sour environment of pH 3.0 due to a synergistic effect with Sn, As and Sb. If the Ni content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ni content is too high, the hydrogen diffusion coefficient in the steel may decrease even if the content of other elements is within the range of the present embodiment. In this case, the SSC resistance of the steel is lowered. Therefore, the Ni content is 4.50-7.50%. A preferable lower limit of the Ni content is 4.80%, more preferably 5.00%, and still more preferably 5.50%. A preferable upper limit of the Ni content is 7.30%, more preferably 7.00%, and still more preferably 6.50%.

Mo: 1.00-4.00%
Molybdenum (Mo) forms sulfides on passive films in sour environments. Mo sulfide prevents chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) from coming into contact with the passive film, and prevents the passive film from being destroyed by chloride ions and hydrogen sulfide ions. do. As a result, the SSC resistance of the steel is enhanced. Mo also forms a solid solution in the steel material to increase the strength of the steel material. If the Mo content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content is too high, it becomes difficult to stabilize austenite even if the content of other elements is within the range of the present embodiment. As a result, a large amount of ferrite may be contained in the microstructure after tempering. If the Mo content is too high, a large amount of intermetallic compounds such as Laves phases may be generated even if the content of other elements is within the range of the present embodiment, and the yield strength of the steel material may become too high. Therefore, the Mo content is 1.00-4.00%. A preferable lower limit of the Mo content is 1.30%, more preferably 1.50%, and still more preferably 1.80%. A preferable upper limit of the Mo content is 3.80%, more preferably 3.60%, and still more preferably 3.40%.

Ti: 0.050-0.300%
Titanium (Ti) combines with C and/or N to form carbides or nitrides. In this case, the pinning effect suppresses grain coarsening and increases the yield strength of the steel material. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ti content is too high, the strength of the steel material becomes too high and the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Ti content is 0.050-0.300%. A preferable lower limit of the Ti content is 0.060%, more preferably 0.080%. A preferable upper limit of the Ti content is 0.250%, more preferably 0.200%, and still more preferably 0.180%.

V: 0.01-1.00%
Vanadium (V) enhances the hardenability of the steel material and enhances the yield strength of the steel material. If the V content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content is too high, the strength of the steel material becomes too high and the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment. Therefore, the V content is 0.01-1.00%. A preferable lower limit of the V content is 0.02%, more preferably 0.03%. A preferable upper limit of the V content is 0.80%, more preferably 0.60%, and still more preferably 0.50%.

Al: 0.001-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content is too high, even if the contents of the other elements are within the ranges of the present embodiment, coarse oxides are formed and the SSC resistance of the steel is lowered. Therefore, the Al content is 0.001-0.100%. A preferable lower limit of the Al content is 0.005%, more preferably 0.010%, and still more preferably 0.015%. A preferable upper limit of the Al content is 0.080%, more preferably 0.060%, still more preferably 0.055%, still more preferably 0.050%. The Al content referred to in this specification is sol. It means the content of Al (acid-soluble Al).

Co: 0.010-0.500%
Cobalt (Co) forms sulfides on passivation films in sour environments. Co sulfide prevents chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) from coming into contact with the passive film, and prevents the passive film from being destroyed by chloride ions and hydrogen sulfide ions. do. As a result, the SSC resistance of the steel is enhanced. Co further enhances the hardenability of the steel material and ensures stable high strength of the steel material, especially during industrial production. Specifically, Co suppresses the formation of retained austenite and suppresses variations in the strength of the steel material. If the Co content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Co content is too high, the toughness of the steel material will decrease even if the content of other elements is within the range of the present embodiment. Therefore, the Co content is 0.010-0.500%. The lower limit of the Co content is preferably 0.015%, more preferably 0.020%, still more preferably 0.030%, still more preferably 0.050%, still more preferably 0.100 %. A preferable upper limit of the Co content is 0.450%, more preferably 0.400%, and still more preferably 0.350%.

Ca: 0.0005-0.0050%
Calcium (Ca) fixes S in the steel material as a sulfide to render it harmless and enhances the hot workability of the steel material. If the Ca content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ca content is too high, even if the contents of other elements are within the range of the present embodiment, coarse inclusions are formed in the steel material and the SSC resistance of the steel material is lowered. Therefore, the Ca content is 0.0005-0.0050%. The lower limit of the Ca content is preferably 0.0006%, more preferably 0.0008%, still more preferably 0.0010%. A preferable upper limit of the Ca content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0035%.

Sn: 0.0005-0.0500%
Tin (Sn) enhances the SSC resistance of steel in a sour environment of pH 3.0. If the Sn content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Sn content is too high, even if the contents of other elements are within the ranges of the present embodiment, Sn will segregate at the grain boundaries, and the SSC resistance of the steel material will rather deteriorate. Therefore, the Sn content is 0.0005-0.0500%. The preferred lower limit of the Sn content is 0.0008%, more preferably 0.0010%, still more preferably 0.0015%. The preferred upper limit of the Sn content is 0.0400%, more preferably 0.0300%, still more preferably 0.0200%, still more preferably 0.0100%, still more preferably 0.0080 %.

N: 0.0010-0.0500%
Nitrogen (N) combines with Ti to form fine Ti nitrides. Fine TiN suppresses coarsening of crystal grains due to the pinning effect. As a result, the yield strength of the steel is increased. If the N content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the N content is too high, even if the content of the other elements is within the range of the present embodiment, coarse nitrides are formed and the SSC resistance of the steel material is lowered. Therefore, the N content is 0.0010-0.0500%. The lower limit of the N content is preferably 0.0015%, more preferably 0.0020%, still more preferably 0.0030%, still more preferably 0.0040%. A preferred upper limit of the N content is 0.0450%, more preferably 0.0400%, still more preferably 0.0350%, still more preferably 0.0300%.

O: 0.050% or less Oxygen (O) is an unavoidable impurity. That is, the lower limit of the O content is over 0%. O forms oxides and lowers the SSC resistance of the steel material. Therefore, if the O content is too high, the SSC resistance of the steel is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the O content is 0.050% or less. A preferable upper limit of the O content is 0.040%, more preferably 0.030%, and still more preferably 0.020%. It is preferable that the O content is as low as possible. However, drastic reduction of O content increases manufacturing cost. Therefore, considering industrial production, the lower limit of the O content is preferably 0.0005%, more preferably 0.001%, and still more preferably 0.002%.

The balance of the martensitic stainless steel material according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when industrially producing steel materials, and are not intentionally included. It means that it is permissible within a range that does not adversely affect the martensitic stainless steel material due to

[Arbitrary element]
The martensitic stainless steel material according to this embodiment may further contain W instead of part of Fe.

W: 0-0.50%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When included, W stabilizes the passive film in a sour environment and inhibits destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel is enhanced. If even a small amount of W is contained, the above effect can be obtained to some extent. On the other hand, if the W content is too high, W will combine with C to form coarse carbides. In this case, even if the content of other elements is within the range of the present embodiment, the SSC resistance of the steel material is lowered. Therefore, the W content is 0-0.50%. A preferable lower limit of the W content is 0.01%, more preferably 0.03%, and still more preferably 0.05%. A preferable upper limit of the W content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

Furthermore, W remarkably increases the SSC resistance when the Cu content is high. 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%, more preferably 0.15%.

The martensitic stainless steel material according to this embodiment may further contain Nb instead of part of Fe.

Nb: 0-0.500%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When included, Nb combines with C and/or N to form Nb carbides, Nb carbonitrides. In this case, the pinning effect suppresses grain coarsening and increases the yield strength of the steel material. If even a small amount of Nb is contained, the above effect can be obtained to some extent. On the other hand, if the Nb content is too high, Nb carbides and/or Nb carbonitrides are excessively produced even if the other element contents are within the range of the present embodiment. As a result, the SSC resistance of the steel is lowered. Therefore, the Nb content is 0-0.500%. A preferable lower limit of the Nb content is 0.001%, more preferably 0.002%, and still more preferably 0.003%. A preferable upper limit of the Nb content is 0.450%, more preferably 0.400%, and still more preferably 0.350%.

The martensitic stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of As and Sb instead of part of Fe. All of these elements assist the effect of Sn that enhances the SSC resistance of the steel material.

As: 0-0.0100%
Arsenic (As) is an optional element and may not be contained. That is, the As content may be 0%. When contained, As assists the effect that Sn enhances the SSC resistance of the steel material. If even a small amount of As is contained, the above effect can be obtained to some extent. On the other hand, if the As content is too high, even if the content of other elements is within the range of the present embodiment, As will segregate at the grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the As content is 0-0.0100%. A preferable lower limit of the As content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the As content is 0.0090%, more preferably 0.0080%.

Sb: 0-0.0100%
Antimony (Sb) is an optional element and may not be contained. That is, the Sb content may be 0%. When contained, Sb assists the effect of Sn to improve the SSC resistance of the steel material. If even a small amount of Sb is contained, the above effect can be obtained to some extent. On the other hand, if the Sb content is too high, even if the contents of other elements are within the ranges of the present embodiment, Sb will segregate at the grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the Sb content is 0-0.0100%. A preferable lower limit of the Sb content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the Sb content is 0.0090%, more preferably 0.0080%.

[Yield strength]
The yield strength of the martensitic stainless steel material according to this embodiment is 758 MPa (110 ksi) or more, more preferably 862 MPa (125 ksi) or more. Although the upper limit of the yield strength is not particularly limited, the upper limit of the yield strength of the steel material of this embodiment is, for example, 1034 MPa (150 ksi). A more preferable upper limit of the yield strength of the steel material is 1000 MPa (145 ksi). Yield strength as used herein means 0.2% offset yield strength (MPa) obtained by a tensile test at room temperature (24±3° C.) according to ASTM E8/E8M (2013).

Specifically, in this embodiment, the yield strength is obtained by the following method. First, a tensile test piece is produced from the martensitic stainless steel material according to this embodiment. When the steel material is a steel pipe, a tensile test piece is prepared from the thickness center position. When the steel material is a round bar, a tensile test piece is produced from the R/2 position. In this specification, the R/2 position of the round bar means the central position of the radius R in the cross section perpendicular to the axial direction of the round bar. When the steel material is a steel plate, a tensile test piece is prepared from the center 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 longitudinal direction of the parallel portion of the tensile test piece shall be parallel to the rolling direction and/or axial direction of the steel material. Using the prepared tensile test piece, a tensile test is performed at room temperature (24±3° C.) in accordance with ASTM E8/E8M (2013) to determine the 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength is defined as yield strength (MPa).

As described above, the yield strength of the martensitic stainless steel material according to this embodiment is 758 MPa or more, preferably 862 MPa or more. In this embodiment, when the Cu content is 1.00% or less and a yield strength of less than 758 to 862 MPa is to be obtained, the martensitic stainless steel material, 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: 4.50-6.50%, Mo: 1.00-3.00%, 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 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, and the balance is preferably composed of Fe and impurities.

Further, in the present embodiment, when the Cu content is 1.00% or less and a yield strength of 862 MPa or more is to be obtained, the martensitic stainless steel material 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: 0.01-1.00%, Cr: 10.00-14. 00%, Ni: 5.00-7.50%, Mo: 2.00-4.00%, 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 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, and the remainder It is preferably composed of Fe and impurities.

In the present embodiment, when the Cu content is 0.50% or more and a yield strength of 862 MPa or more is to be obtained, the martensitic stainless steel material, in terms of mass %, contains 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 to 3.50%, Cr: 10.00 to 14.00 %, Ni: 5.00-7.50%, Mo: 2.00-4.00%, Ti: 0.050-0.300%, V: 0.01-1.00%, Al: 0.00% 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: 0.10 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, and the balance is preferably composed of Fe and impurities.

[Regarding formula (1)]
In the martensitic stainless steel material according to the present embodiment, the element content and the yield strength satisfy formula (1) within the range of the above-described element content and yield strength of 758 MPa or more. As a result, the martensitic steel material according to this embodiment has excellent SSC resistance in a sour environment of pH 3.0 on condition that other configurations of this embodiment are satisfied.
0.15≦(Sn+As+Sb)/{(Cu+Ni)/YS}≦1.00 (1)
Here, the element symbol in the formula (1) is substituted with the content of the corresponding element in mass %, and YS is substituted with the yield strength in MPa. When the corresponding element is not contained, "0" is substituted for the element symbol.

F1 (=(Sn+As+Sb)/{(Cu+Ni)/YS}) is the synergistic effect of Sn, As and Sb and Cu and Ni, which are adjusted according to the yield strength, to withstand a sour environment at pH 3.0. It is an index for enhancing the SSC property. If F1 is too low, the steel does not exhibit excellent SSC resistance in pH 3.0 sour environments. Similarly, if F1 is too high, the steel will not exhibit good SSC resistance in pH 3.0 sour environments. On the other hand, when F1 is 0.15 to 1.00, the steel exhibits excellent SSC resistance in a sour environment of pH 3.0.

Therefore, the martensitic stainless steel material according to the present embodiment has an F1 of 0.15 to 1.00 while satisfying the above-described element content and yield strength of 758 MPa or more. A preferred lower limit for F1 is 0.16, more preferably 0.18. A preferred upper limit for F1 is 0.95, more preferably 0.90.

[Microstructure of steel]
The microstructure of the martensitic stainless steel material according to this embodiment is mainly composed of martensite. In this specification, "mainly composed of martensite" means that the microstructure is 0 to 5.0% by volume of retained austenite, 0 to 5.0% of ferrite, and the balance consists of martensite. means that As used herein, "consisting of retained austenite, ferrite and tempered martensite" means that phases other than retained austenite, ferrite and tempered martensite are negligibly small. For example, in the chemical composition of the martensitic stainless steel material according to this embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fractions of retained austenite, ferrite, and tempered martensite. That is, the microstructure of the martensitic stainless steel pipe according to the present embodiment may contain minute amounts of precipitates, inclusions, and the like in addition to retained austenite, ferrite, and tempered martensite.

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 according to this embodiment is 90.0%, more preferably 95.0%. More preferably, the microstructure of the steel material is martensite single phase.

In the microstructure, a small amount of retained austenite does not cause a significant decrease in strength and significantly increases the toughness of the steel material. However, if the volume fraction of retained austenite is too high, the strength of the steel significantly decreases. Therefore, in the microstructure of the steel material of this embodiment, the volume fraction of retained austenite is 0 to 5.0%. From the viewpoint of securing strength, the upper limit of the volume fraction of retained austenite is preferably 4.0%, more preferably 3.0%. The volume fraction of retained austenite may be 0%. On the other hand, when even a little retained austenite is contained, the volume fraction of retained austenite is more than 0 to 5.0%, more preferably more than 0 to 4.0%, more preferably more than 0 to 3.0%. is.

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

[Method for measuring volume fraction of martensite]
In this embodiment, the volume fraction (%) of martensite in the microstructure of the steel material is the volume fraction (%) of retained austenite determined by the method shown below and the volume fraction (%) of ferrite determined by the method shown below. %) is subtracted from 100%.

[Method for measuring volume fraction of retained austenite]
The volume fraction of retained austenite in the microstructure of the steel material is obtained by X-ray diffraction method. Specifically, a test piece for measuring the volume ratio of retained austenite is produced from the steel material according to the present embodiment. If the steel material is a steel pipe, take a test piece from the center of the wall thickness. When the steel material is a round bar, a test piece is taken from the R/2 position. When the steel material is a steel plate, a test piece is taken from the center position of the plate thickness. The size of the test piece is not particularly limited. The specimen is for example 15 mm x 15 mm x 2 mm thick. When the steel material is a steel pipe, the thickness direction of the test piece is the pipe radial direction. When the steel material is round steel, the thickness direction of the test piece is the radial direction. When the steel material is a steel plate, the thickness direction of the test piece is the plate thickness direction. Using the prepared test piece, 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 γ phase The X-ray diffraction intensity of each of the (200) plane of the γ phase and the (220) plane of the γ phase is measured, and the integrated intensity of each plane is calculated.

In the measurement of X-ray diffraction intensity, the target of the X-ray diffractometer is Co (CoKα rays) and the output is 30 kV-100 mA. The measurement angle (2θ) is set to 45 to 105°. After the calculation, the volume fraction Vγ (%) of retained austenite is calculated using the formula (I) for each combination (3×3=9 sets) of each α-phase plane and each γ-phase plane. Then, the average value of the volume fraction Vγ of retained austenite in the nine sets is defined as the volume fraction (%) of retained austenite.
Vγ=100/{1+(Iα×Rγ)/(Iγ×Rα)} (I)
where Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. As the values of Rα and Rγ on each surface, the values incorporated in the residual γ quantitative analysis system attached to RINT-TTR (trade name) manufactured by Rigaku Co., Ltd. can be used. For the volume fraction of retained austenite, the obtained numerical value is rounded off to the second decimal place.

[Method for measuring volume fraction of ferrite]
The volume fraction of ferrite in the microstructure of the steel material is determined by the point counting method. Specifically, a test piece for measuring the volume ratio of ferrite is produced from the steel material according to the present embodiment. If the steel material is a steel pipe, take a test piece from the center of the wall thickness. When the steel material is a round bar, a test piece is taken from the R/2 position. When the steel material is a steel plate, a test piece is taken from the center position of the plate thickness. The test piece is not particularly limited as long as it has a surface parallel to the rolling direction as an observation surface. For example, when the steel material is a steel pipe, the observation surface of the test piece is parallel to the pipe axis direction. After the observation surface is mechanically polished, the observation surface is electrolytically etched to expose the structure. Electrolytic etching is performed using an electrolytic solution of 30% sodium hydroxide aqueous solution, a current density of 1 A/cm ² and an electrolysis time of 1 minute.

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

Specifically, 20 vertical lines are drawn at equal intervals from the top end to the bottom end of the observation field of view. That is, the observation field of view is divided into 21 areas in the left-right direction by the 20 vertical lines. Further, 20 horizontal lines are drawn at regular intervals from the left end to the right end of the observation field of view. That is, the observation field of view is vertically divided into 21 areas by the 20 horizontal lines. At this time, the intersections of vertical lines and horizontal lines are called lattice points. That is, 400 lattice points are arranged at equal intervals in the observation field. In accordance with ASTM E562 (2019), the grid points overlapping ferrite are counted in the observation field. The number of lattice points overlapping ferrite obtained in 30 fields of view is divided by the total number of lattice points (400×30=12000) to define the ferrite area ratio. In the present embodiment, the ferrite area ratio obtained by the above method is defined as the ferrite volume ratio (%). For the volume fraction of ferrite, the obtained numerical value is rounded off to the second decimal place.

Using the volume fraction (%) of retained austenite obtained by the above-described X-ray diffraction method and the volume fraction (%) of ferrite obtained by the above-described point counting method, the volume fraction of martensite in the microstructure of the steel material (%) is calculated by the following formula.
Volume fraction of martensite (%) = 100.0 - {volume fraction of retained austenite (%) + volume fraction of ferrite (%)}

[SSC resistance of steel]
The martensitic stainless steel material according to this embodiment has excellent SSC resistance in a sour environment of pH 3.0 even though it has a high yield strength of 758 MPa or more. The SSC resistance of the martensitic stainless steel material according to this embodiment can be evaluated by an SSC resistance evaluation test at room temperature. The SSC resistance evaluation test is conducted according to NACE TM0177-2016 Method A.

Specifically, a round bar test piece is produced from the steel material according to this embodiment. When the steel material is a steel pipe, a round bar test piece is produced from the thickness center position. When the steel material is round steel, a round bar test piece is produced from the R/2 section. When the steel material is a steel plate, a round bar test piece is produced from the center position of the plate thickness. The size of the round bar test piece is not particularly limited. The round bar test piece, for example, has a parallel portion diameter of 6.35 mm and a parallel portion length of 25.4 mm. The axial direction of the round bar test piece is parallel to the rolling direction and/or axial direction of the steel material.

The test solution is a 0.17 mass % sodium chloride aqueous solution with a pH of 3.0. A test solution is prepared by adding acetic acid to an aqueous solution containing 0.17 mass % sodium chloride and 0.41 g/L sodium acetate 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 prepared as described above. The test solution at 24° C. is poured into the test container so that the stress-loaded round bar test piece is immersed to form a test bath. After degassing the test bath, 0.03 bar H ₂ S gas and 0.97 bar CO ₂ gas are blown into the test bath to saturate the test bath with H ₂ S gas. A test bath saturated with H ₂ S gas is held at 24° C. for 720 hours. After holding for 720 hours, the surface of the parallel portion of the test piece is observed with a magnifying glass having a magnification of 10 times to confirm the presence or absence of pitting corrosion. In the martensitic stainless steel material according to the present embodiment, pitting corrosion is not confirmed after 720 hours even in the SSC resistance evaluation test conducted by the method described above.

[Shapes and uses of steel]
As described above, the shape of the martensitic stainless steel material according to this embodiment is not particularly limited. Specifically, the martensitic stainless steel material according to this embodiment may be a steel pipe, a round steel (solid material), or a steel plate. The steel pipe may be a seamless steel pipe or a welded steel pipe. Steel pipes are, for example, steel pipes for oil country tubular goods. A steel pipe for oil country tubular goods means a steel pipe for oil country tubular goods. Oil country tubular goods are, for example, casings, tubings, drill pipes, etc. used for drilling oil wells or gas wells, extracting crude oil or natural gas, and the like. Preferably, the steel material of the present embodiment is a seamless steel pipe for oil country tubular goods.

As described above, the martensitic stainless steel material according to the present embodiment has a content of each element within the range of the present embodiment, a yield strength of 758 MPa or more, and the content of the above elements, , F1 satisfies 0.15 to 1.00 within the range of yield strength of 758 MPa or more. As a result, the steel material according to this embodiment achieves both high yield strength and excellent SSC resistance in a sour environment of pH 3.0.

[Production method]
An example of a method for producing a martensitic stainless steel material according to this embodiment will be described. The manufacturing method described below is merely an example, and the method for manufacturing a martensitic stainless steel material according to the present embodiment is not limited to the following description. That is, as long as the martensitic stainless steel material according to the present embodiment having the above-described structure can be manufactured, the manufacturing method is not limited to the method described below. However, the manufacturing method described below is a suitable manufacturing method for manufacturing the martensitic stainless steel material according to the present embodiment.

An example of the method of manufacturing a martensitic stainless steel according to this embodiment includes a step of preparing an intermediate steel (preparation step) and a step of quenching and tempering the intermediate steel (heat treatment step). Each step will be described in detail below.

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

The preparation process may include a process of preparing the material (material preparation process) and a process of hot working the material to manufacture the intermediate steel material (hot working process). Hereinafter, the case where the material preparation process and the hot working process are included will be described in detail.

[Material preparation process]
In the material preparation step, the material is manufactured using molten steel having the chemical composition described above. The method of manufacturing the raw material is not particularly limited, and a known method may be used. Specifically, a slab (slab, bloom, or billet) may be produced by continuous casting using molten steel. You may manufacture an ingot by an ingot casting method using molten steel. If desired, the slab, bloom or ingot may be bloomed to produce a billet. A raw material (slab, bloom, or billet) is manufactured by the above steps.

[Hot working process]
In the hot working process, the prepared material is hot worked to produce an intermediate steel material. If the steel material is a seamless steel pipe, the intermediate steel material corresponds to the base pipe. First, the billet is heated in a heating furnace. Although the heating temperature is not particularly limited, it is, for example, 1100 to 1300.degree. A billet extracted from a heating furnace is subjected to hot working to produce a blank pipe (seamless steel pipe). The method of hot working is not particularly limited, and a known method may be used.

For example, the Mannesmann method may be carried out as hot working to produce a mother tube. In this case, the round billet is pierced and rolled by a piercing machine. In the case of piercing-rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced-rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill, or the like to form a mother tube. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%.

A blank tube may be manufactured from a billet by implementing other hot working methods. For example, when the steel material is a short thick steel pipe such as a coupling, the blank pipe may be manufactured by forging such as the Ehrhardt method. A blank pipe is manufactured by the above steps. Although the wall thickness of the blank tube is not particularly limited, it is, for example, 9 to 60 mm.

When the steel material is round steel, the material is first heated in a heating furnace. Although the heating temperature is not particularly limited, it is, for example, 1100 to 1300.degree. The raw material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. Hot working is, for example, blooming by a blooming mill or hot rolling by a continuous rolling mill. In the continuous rolling mill, a horizontal stand having a pair of grooved rolls arranged vertically and a vertical stand having a pair of grooved rolls arranged horizontally are arranged alternately.

When the steel material is a steel plate, the material is first heated in a heating furnace. Although the heating temperature is not particularly limited, it is, for example, 1100 to 1300.degree. The raw material extracted from the heating furnace is subjected to hot rolling using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the form of a steel plate.

The blank tube manufactured by hot working may be air-cooled (As-Rolled). A mother tube manufactured by hot working may be quenched directly after hot working without cooling to room temperature, or may be quenched after supplementary heating (reheating) after hot working. good.

When quenching is performed directly after hot working, or when quenching is performed after supplementary heat, cooling may be stopped or slow cooling may be performed during quenching. In this case, it is possible to suppress the occurrence of quench cracks in the mother pipe. When quenching is performed directly after hot working or quenching is performed after reheating, stress relief annealing (SR) may be performed after quenching and before heat treatment in the next step. In this case, the residual stress of the blank pipe is removed.

As described above, intermediate steel materials are prepared in the preparation process. The intermediate steel material may be manufactured by the above-mentioned preferable process, or the intermediate steel material manufactured by a third party, or by a factory other than the factory where the quenching process and the tempering process described below are performed, or at another place of business. You may prepare an intermediate steel material manufactured by The heat treatment process will be described in detail below.

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

[Quenching process]
In the heat treatment process, first, the intermediate steel material produced in the hot working process is quenched (quenching process). Quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace and held at the quenching temperature. The quenching temperature is above the AC3 transformation point, eg, 900-1000° _C . After holding the intermediate steel material at the quenching temperature, it is rapidly cooled (quenched). Although the holding time at the quenching temperature is not particularly limited, it 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 blank pipe, for example, the blank pipe may be rapidly cooled by being immersed in a water tank or an oil bath, or by shower cooling or mist cooling, cooling water may be poured onto the outer surface and/or the inner surface of the blank pipe. , or the like, to rapidly cool the tube.

As described above, after the hot working step, quenching (direct quenching) may be performed immediately after hot working without cooling the intermediate steel material to room temperature, or the temperature of the mother tube after hot working may be Quenching may be performed after the steel is charged into a reheating furnace and held at the quenching temperature before the temperature drops.

[Tempering process]
After quenching, the intermediate steel material is further subjected to a tempering process. The tempering process adjusts the yield strength of the steel material. In this embodiment, the tempering temperature is 540-620.degree. Although the holding time at the tempering temperature is not particularly limited, it is, for example, 10 to 180 minutes. It is well known to those skilled in the art that the yield strength of steel materials can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Therefore, the tempering conditions are adjusted so that the yield strength of the steel material is 758 MPa or more.

The martensitic stainless steel material according to this embodiment can be manufactured by the above steps. As described above, the martensitic stainless steel material according to the present embodiment is not limited to the manufacturing method described above. Specifically, the content of each element in the chemical composition is within the range of the present embodiment, and the volume % is 0 to 5.0% retained austenite, 0 to 5.0% ferrite, and the balance is If a martensitic stainless steel material having a tempered martensite microstructure, a yield strength of 758 MPa or more, and an F1 of 0.15 to 1.00 can be produced, the production method of the present embodiment can be applied as described above. is not limited to the manufacturing method of Hereinafter, the martensitic stainless steel material according to the present embodiment will be described more specifically by way of examples.

A molten steel having the chemical composition shown in Table 1 was produced. "-" in Table 1 means that the content of the corresponding element was at the impurity level. For example, the W content of Test No. 1 was rounded to the third decimal place, meaning that it was 0%. It means that the Nb content of Test No. 1 was 0%, rounded to the fourth decimal place. The As content and Sb content of Test No. 1 were rounded off to the fifth decimal place, meaning that they were 0%.

　The above molten steel was melted in a 180 kg vacuum furnace, and an ingot was produced by an ingot casting method. The ingot was heated at 1250° C. for 3 hours. A block was manufactured by hot forging the ingot after heating. After hot forging, the block was heated at 1230° C. for 3 hours, and then hot rolled. Thus, a steel material (steel plate) having a thickness of 13 mm was produced.

Quenching was performed on the steel material of each test number. Specifically, the steel sheets of each test number were heated to the quenching temperature (°C) shown in Table 2. The steel sheets of each test number were held at the quenching temperature for 15 minutes and then water-cooled. The steel material of each test number after quenching was tempered at the tempering temperature (° C.) shown in Table 2 for 30 minutes.

A steel plate with each test number was manufactured through the above manufacturing process.

[Evaluation test]
A microstructure observation test, a tensile test, and an SSC resistance evaluation test were performed on the manufactured steel sheets of each test number.

[Microstructure Observation Test]
A microstructure observation test was performed on the steel sheets of each test number. First, the volume fraction (%) of retained austenite was obtained for the steel sheets of each test number by the above-described X-ray diffraction method. In the measurement of the X-ray diffraction intensity, RINT-TTR manufactured by Rigaku Corporation was used as an X-ray diffraction device. The radiation source was CoKα rays, the output was 30 kV-100 mA, and the measurement angle (2θ) was 45 to 105°. The volume fraction (%) of retained austenite in the obtained steel sheets of each test number is shown in the column of "retained γ (%)" in Table 2.

In addition, the volume fraction (%) of ferrite was obtained by the above-mentioned point counting method. Specifically, a test piece was produced from the thickness center position of the steel plate of each test number. The observation surface of the test piece was parallel to the rolling direction. In this example, the area ratio of ferrite determined by the method based on ASTM E562 (2019) was used as the volume ratio (%) of ferrite. The volume fraction of ferrite in the obtained steel sheets of each test number is shown in "Ferrite (%)" in Table 2.

Furthermore, for the steel sheets 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.
Volume fraction of martensite (%) = 100 - {volume fraction of retained austenite (%) + volume fraction of ferrite (%)}
The obtained volume fraction (%) of martensite for each test number is shown in Table 2 in the “Martensite (%)” column.

[Tensile test]
A tensile test was performed on the steel sheets of each test number in accordance with ASTM E8/E8M (2013). Specifically, a round-bar tensile test piece having a diameter of a parallel portion of 8.9 mm and a gauge length of 35.6 mm was prepared from the plate thickness center position of the steel plate of each test number. The longitudinal direction of the round bar tensile test piece was parallel to the rolling direction of the steel plate. A tensile test was performed at normal temperature (24±3° C.) in the atmosphere using a round bar tensile test piece of each test number to obtain a 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength was defined as the yield strength (MPa). The obtained yield strength of each test number is shown in Table 2, "YS (MPa)" column. Furthermore, F1 was obtained for the steel sheet of each test number using the chemical composition, yield strength, and formula (1). The obtained F1 value for each test number is shown in Table 2, column “F1”.

[SSC resistance evaluation test]
An SSC resistance evaluation test was performed on the steel sheets of each test number. Specifically, a round bar test piece having a diameter of 6.35 mm and a parallel portion length of 25.4 mm was prepared from the plate thickness center position of the steel plate of each test number. An SSC resistance evaluation test based on NACE TM0177-2016 Method A was performed on three of the prepared round bar test pieces. The axial direction of the round bar test piece was parallel to the rolling direction.

The test solution was a 0.17 mass % sodium chloride aqueous solution with a pH of 3.0. A test solution was prepared by adding acetic acid to an aqueous solution containing 0.17 mass % sodium chloride and 0.41 g/L sodium acetate to adjust the pH to 3.0. A stress equivalent to 90% of the actual yield stress was applied to the round bar test piece of each test number. The test solution at 24° C. was poured into the test container so that the stress-loaded round-bar test piece was immersed therein to form a test bath. After degassing the test bath, 0.03 bar H ₂ S gas and 0.97 bar CO ₂ gas were blown into the test bath to saturate the test bath with H ₂ S gas. A test bath saturated with H ₂ S gas was held at 24° C. for 720 hours.

The surface of the parallel portion of the round bar test piece after holding for 720 hours was observed with a loupe with a magnification of 10 times to confirm the presence or absence of pitting corrosion. Table 2 shows the number of test pieces for which pitting corrosion was confirmed among the three round bar test pieces as "the number of pitting corrosion occurrences (pieces)".

[Evaluation results]
With reference to Tables 1 and 2, the steel sheets of test numbers 1 to 22 have appropriate chemical compositions, 0 to 5.0% by volume of retained austenite, 0 to 5.0% by volume of ferrite, and the balance had a microstructure consisting of martensite. These steel sheets also had a high yield strength of 758 MPa or more. These steel sheets further satisfied F1 of 0.15 to 1.00. As a result, none of these steel sheets had pitting corrosion in a sour environment of pH 3.0, and had excellent SSC resistance.

On the other hand, the steel sheets with test numbers 23 to 26 had too low F1. As a result, these steel sheets had at least one pitting corrosion in a sour environment of pH 3.0 and did not have excellent SSC resistance.

The steel sheets with test numbers 27 to 30 had too high F1. As a result, three of these steel sheets suffered from pitting corrosion in a sour environment of pH 3.0, and did not have excellent SSC resistance.

The steel plate of test number 31 did not contain Sn. As a result, one of the steel sheets suffered from pitting corrosion in a sour environment of pH 3.0, and did not have excellent SSC resistance.

The steel sheets of test numbers 32 to 34 did not contain Sn. These steel plates also had too low F1. As a result, these steel sheets had at least one pitting corrosion in a sour environment of pH 3.0 and did not have excellent SSC resistance.

The steel plate of test number 35 did not contain Sn. This steel plate also had a too high F1. As a result, this steel sheet had pitting corrosion in three in a sour environment of pH 3.0, and did not have excellent SSC resistance.

The Co content of the steel sheet of test number 36 was too low. As a result, one of the steel sheets suffered from pitting corrosion in a sour environment of pH 3.0, and did not have excellent SSC resistance.

The Co content of the steel sheet of test number 37 was too low. This steel plate also had a too low F1. As a result, this steel sheet had pitting corrosion in three in a sour environment of pH 3.0, and did not have excellent SSC resistance.

The embodiment of the present disclosure has been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be modified as appropriate without departing from the scope of the present disclosure.

Claims

A martensitic stainless steel material,
in % 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 to 3.50%,
Cr: 10.00 to 14.00%,
Ni: 4.50-7.50%,
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,
W: 0 to 0.50%,
Nb: 0 to 0.500%,
As: 0 to 0.0100%,
Sb: 0 to 0.0100%, and
Balance: Fe and impurities,
Yield strength is 758 MPa or more,
Within the range of the element content and the yield strength of the martensitic stainless steel material, the element content and the yield strength satisfy the formula (1),
Martensitic stainless steel material.
0.15≦(Sn+As+Sb)/{(Cu+Ni)/YS}≦1.00 (1)
Here, the element symbol in the formula (1) is substituted with the content of the corresponding element in mass %, and YS is substituted with the yield strength in MPa. When the corresponding element is not contained, "0" is substituted for the element symbol.
The martensitic stainless steel material according to claim 1,
W: 0.01 to 0.50%,
Nb: 0.001 to 0.500%,
As: 0.0001 to 0.0100%, and
Sb: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%,
Martensitic stainless steel material.