ES3038447T3 - Martensitic stainless steel material - Google Patents

Abstract

A martensitic stainless steel is provided that exhibits a high yield strength and excellent resistance to weld corrosion. This martensitic stainless steel contains, by mass percentage, 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 the remainder: Fe and impurities. Martensitic stainless steel has a yield strength of 758 MPa or higher. The amounts of the elements contained and the yield strength within these ranges satisfy formula (1) described in the description. (Automatic translation using Google Translate, not legally binding)

Description

DESCRIPTION

Martensitic stainless steel material

Technical field

This disclosure relates to a steel material and, more particularly, to a martensitic stainless steel material.

Background of the technique

Oil well and gas well environments (hereafter referred to collectively as "oil wells") include environments containing large quantities of corrosive substances. Corrosive substances include, for example, corrosive gases such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂). Chromium (Cr) is known to be effective in improving the carbonic acid corrosion resistance of steel materials. Therefore, in an oil well in an environment containing a large quantity of carbon dioxide gas, martensitic stainless steel materials containing approximately 13% Cr by mass, typified by API L80 13Cr steel (standard 13Cr steel) and Super 13Cr steel, in which the C content is reduced, are used depending on the partial pressure and temperature of the carbon dioxide gas.

In recent years, with deeper oil wells, there has been a demand for increased strength in oil well steel. Specifically, 80 ksi grade oil well steel (yield strength 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and 95 ksi grade oil well steel (yield strength 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) are in widespread use. Recently, demand has also grown for 110 ksi grade or higher (yield strength 758 MPa or higher) oil well steel.

In this document, an environment containing hydrogen sulfide gas and carbon dioxide gas is referred to as an "acid environment." Oil well steel materials intended for use in an acid environment must have sulfide stress cracking resistance (hereafter referred to as "SSC resistance"). In other words, in recent years, oil well steel materials have been required to have both high strength and excellent SSC resistance.

Japanese patent application no. 2000-192196 (Patent bibliography 1), Japanese patent application no. 2012-136742 (Patent bibliography 2) and international application no. WO2008/023702 (Patent bibliography 3) each propose a steel material having high strength and excellent resistance to SSC.

The steel material proposed in patent bibliography 1 is a martensitic stainless steel for oil wells consisting of, in wt%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.025% or less, S: 0.01% or less, Cr: 9 to 14%, Mo: 3.1 to 7%, Ni: 1 to 8%, Co: 0.5 to 7%, Al: 0.001 to 0.1%, N: 0.05% or less, O (oxygen): 0.01% or less, Cu: 0 to 5%, and W: 0 to 5%, with the remainder being Fe and unavoidable impurities. When a steel material contains Mo, the Ms point decreases. Therefore, since this steel material contains Co in addition to Mo, the Ms point decrease is suppressed, and the microstructure becomes a single-phase martensitic structure. Patent literature 1 describes that, as a result, the SSC resistance of this steel material can be increased while maintaining a tensile strength of 80 ksi or more (552 MPa or more).

The steel material proposed in patent bibliography 2 is a seamless martensitic stainless steel tube consisting of, by mass %, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less, the remainder being Fe and unavoidable impurities. The steel material has a yield strength of 655 to 862 MPa and a spring ratio of 0.90 or greater. It is described in patent bibliography 2 that by setting the C content at 0.01% or less, adjusting Cr, Ni, and Mo within a preferred range, and also containing adequate amounts of Cu and V or an adequate amount of W, excellent SSC resistance is obtained while also having a strength of 655 MPa or more.

The steel material proposed in patent bibliography 3 is a martensitic stainless steel consisting of, in % by 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 to 15.00%, Ni: 2.50 to 8.00%, Mo: 1.00 to 5.00%, Ti: 0.050 to 0.250%, V: 0.25% or less, N: 0.07% or less, and one or more types of elements between Si: 0.50% or less and Al: 0.10% or less, the remainder being Fe and impurities, and satisfying the formula (6.0 < Ti/C < 10.1). The yield strength is 758 to 862 MPa. There is a correlation between the Ti/C ratio (the ratio of Ti to C content in the steel) and a value obtained by subtracting the yield strength from the tensile strength. Furthermore, when there are large variations in hardness in a steel material, its SSC resistance decreases. Therefore, it is described in patent bibliography 3 that in this steel material, by adjusting Ti/C within a preferred range, hardness variations are suppressed, and the yield strength is achieved from 758 to 862 MPa. Patent bibliography 4 describes a high-strength, seamless stainless steel tube for oil well tubing that is excellent for hot workability, has high strength, suppresses strength dispersion, and has excellent carbon dioxide corrosion resistance. Seamless steel pipe is a high-strength, seamless stainless steel pipe for oil well tubular materials with a yield strength of 655 MPa or more, the seamless stainless steel pipe comprising a composition containing C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 12.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, and O: 0.010% or less in terms of % mass, with the remainder being Fe and unavoidable impurities, wherein Cr, Ni, Mo, Cu and C satisfy a specified expression, and Cr, Mo, Si, C, Mn, Ni, Cu, and N satisfy a specified expression.

List of appointments

Patent bibliography

Patent Bibliography 1: Japanese Patent Application No. 2000-192196 Patent Bibliography 2: Japanese Patent Application No. 2012-136742 Patent Bibliography 3: International Application No. WO2008/023702 Patent Bibliography 4: European Patent EP 3438305 A1 Summary of the Invention

Technical problem

The aforementioned patent bibliographies 1 to 3 propose techniques for increasing the yield strength and improving the SSC resistance of a steel material. However, a martensitic stainless steel material with excellent SSC resistance can be obtained while also increasing the yield strength using a technique different from those proposed in the aforementioned patent bibliographies 1 to 3. Additionally, in recent years, oil wells with higher hydrogen ion concentrations than in the past are being actively developed. In general, SSC can occur in an environment with a high hydrogen ion concentration (i.e., a low pH). Therefore, there is a need for a martensitic stainless steel material that has excellent SSC resistance even in an acidic environment with a pH of 3.0 where the hydrogen ion concentration is higher than in the past.

However, the aforementioned patent bibliographies 1 to 3 do not contain any consideration regarding the SSC resistance of steel materials in acidic environments with a pH of 3.0.

One objective of this disclosure is to provide a martensitic stainless steel material that can achieve both a high yield strength and excellent SSC resistance in an acidic environment with a pH of 3.0.

Solution

A martensitic stainless steel material according to the present disclosure consists of, in % by mass, C: 0.030% or less,

Yes: 1.00% or less,

Mn: 1.00% or less,

P: 0.030% or less,

S: 0.0050% or less,

Cu: from 0.01 to 3.50%,

Cr: from 10.00 to 14.00%

Ni: from 4.50 to 7.50%,

Mo: 1.00 to 4.00%,

Ti: from 0.050 to 0.300%,

V: from 0.01 to 1.00%

Al: from 0.001 to 0.100%,

Co: from 0.010 to 0.500%

Ca: from 0.0005 to 0.0050%

Sn: from 0.0005 to 0.0500%

N: from 0.0010 to 0.0500%

O: 0.050% or less,

W: from 0 to 0.50%

Nb: from 0 to 0.500%

As: from 0 to 0.0100%

Sb: from 0 to 0.0100%, and

The rest: Faith and impurities,

where:

An elastic limit is 758 MPa or more; and

Within a range of element content and yield strength of the martensitic stainless steel material, the element content and yield strength satisfy Formula (1);

0.15 < (Sn+As+Sb)/{(Cu+Ni)/YS} < 1.00 (1)

Where, in Formula (1), a corresponding element content in mass percent is replaced by each element symbol, and a yield strength in MPa is replaced by YS, and, if it does not contain the corresponding element, "0" is replaced by the symbol of the relevant element.

Advantageous effects of the invention

The martensitic stainless steel material according to this disclosure can achieve both a high yield strength and excellent SSC resistance in an acidic environment with a pH of 3.0.

Brief description of the drawing

[FIG. 1] FIG. 1 is a view illustrating the relationship between F1 (= (Sn+As+Sb)/{(Cu+Ni)/YS}) and the number of samples in which pitting occurred (samples) which is an index of resistance to SSC in the present Examples.

Description of the achievements

First, the present inventors carried out studies from the point of view of chemical composition with respect to a martensitic stainless steel material that can achieve both a high yield strength and excellent SSC resistance in an acidic environment with a pH of 3.0. As a result, the present inventors considered that if a martensitic stainless steel material contains, 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: from 0.01 to 3.50%, Cr: from 10.00 to 14.00%, Ni: from 4.50 to 7.50%, Mo: from 1.00 to 4.00%, Ti: from 0.050 to 0.300%, V: from 0.01 to 1.00%, Al: from 0.001 to 0.100%, Co: from 0.010 to 0.500%, Ca: from 0.0005 to 0.0050%, N: from 0.0010 to 0.0500%, O: 0.050% or less, W: from 0 to 0.50%, and Nb: from 0 to 0.500%, there is the possibility of obtaining both a yield strength of 758 MPa (110 ksi) or more and excellent resistance to SSC in an acidic environment with a pH of 3.0.

Subsequently, with respect to a martensitic stainless steel material containing the elements described above, the present inventors conducted detailed studies on means of increasing SSC resistance while maintaining a yield strength of 758 MPa or higher. As a result, the present inventors discovered that in a martensitic stainless steel material containing the aforementioned elements, tin (Sn), arsenic (As), and antimony (Sb), elements that had not previously received much attention, may increase SSC resistance. Further studies by the present inventors revealed that in a martensitic stainless steel material containing the aforementioned elements, Sn, in particular, may significantly increase SSC resistance, and that As and Sb may contribute to the SSC resistance-enhancing effect produced by Sn.

Therefore, the present inventors carried out detailed studies on the Sn, As, and Sb content that could sufficiently increase the SSC resistance of a martensitic stainless steel material. As a result, it has been clarified that, in addition to the content of the elements described above, the SSC resistance of the martensitic stainless steel material according to the present embodiment, which also contains Sn in an amount of 0.0005 to 0.0500%, As in an amount of 0 to 0.0100%, and Sb in an amount of 0 to 0.0100%, can be increased. That is, if a martensitic stainless steel material consists of, 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: from 0.01 to 3.50%, Cr: from 10.00 to 14.00%, Ni: from 4.50 to 7.50%, Mo: from 1.00 to 4.00%, Ti: from 0.050 to 0.300%, V: from 0.01 to 1.00%, Al: from 0.001 to 0.100%, Co: from 0.010 to 0.500%, Ca: from 0.0005 to 0.0050%, Sn: from 0.0005 to 0.0500%, N: from 0.0010 to 0.0500%, O: 0.050% or less, W: from 0 to 0.50%, Nb: from 0 to 0.500%, As: from 0 to 0.0100%, and Sb: from 0 to 0.0100%, with the remainder being Fe and impurities, it is possible to obtain both a yield strength of 758 MPa or more and excellent resistance to SSC in an acidic environment with a pH of 3.0.

On the other hand, the present inventors have discovered that, even in the case of a martensitic stainless steel material with the aforementioned chemical composition, when the martensitic stainless steel material has a yield strength of 758 MPa or higher, there are some instances where the SSC resistance does not increase stably in an acidic environment with a pH of 3.0. Therefore, with respect to a martensitic stainless steel material with the aforementioned chemical composition, the present inventors conducted detailed studies on means to increase the SSC resistance in an acidic environment with a pH of 3.0 while maintaining a yield strength of 758 MPa or higher. As a result, the present inventors obtained the following results.

It has been clarified, as a result of the detailed studies carried out by the present inventors, that in a martensitic stainless steel material having the aforementioned chemical composition and in which the yield strength is 758 MPa or more, if the element content and the yield strength satisfy Formula (1), the SSC resistance of the steel material increases markedly in an acidic environment with a pH of 3.0.

0.15 < (Sn+As+Sb)/{(Cu+Ni)/YS} < 1.00 (1)

Where, in Formula (1), each element symbol is replaced by the content of the corresponding element in mass percent, and YS is replaced by the yield strength in MPa. Note that, if it does not contain the corresponding element, "0" is replaced by the symbol of the relevant element.

Let's define F1 as F1 = (Sn+As+Sb)/{(Cu+Ni)/YS}. As described above, As and Sb contribute to increasing the steel material's SSC resistance due to Sn. Additionally, the steel material's SSC resistance increases significantly when the ratio of Sn, As, and Sb contents to Cu and Ni contents falls within a certain range. On the other hand, the higher the yield strength of the steel material, the more its SSC resistance tends to decrease. Therefore, the denominator of F1 is established as a ratio between Cu and Ni contents and the yield strength. Thus, a ratio of Sn, As, and Sb contents to Cu and Ni contents that adjusts according to the yield strength is defined as F1. In other words, F1 is an index of the increase in SSC resistance in an acidic environment with a pH of 3.0, obtained through a synergistic effect between Sn, As, and Sb, and Cu and Ni, which is adjusted according to the yield strength. The relationship between F1 and SSC resistance in an acidic medium with a pH of 3.0 is specifically described below using the accompanying diagram.

Figure 1 is a view illustrating the relationship between F1 and SSC resistance in the present examples.

1 was created using F1 and the number of samples in which pitting occurred (samples), which is an index of SSC resistance, with respect to Examples that had the mentioned chemical composition and in which the yield strength was 758 MPa or more among the Examples described below. Note that the number of samples in which pitting occurred was obtained by performing an SSC resistance evaluation test that assumed an acidic environment with a pH of 3.0, which will be described below.

With reference to Figure 1, when F1 was too low, pitting occurred in one or more samples. Similarly, when F1 was too high, pitting also occurred in one or more samples. On the other hand, when F1 was within the range of 0.15 to 1.00, no pitting occurred in any specimen. That is, with reference to Figure 1, in a steel material with the aforementioned chemical composition and a yield strength of 758 MPa or higher, when F1 is within the range of 0.15 to 1.00, excellent CSS resistance is obtained in an acidic environment with a pH of 3.0.

Note that, with respect to a steel material having the aforementioned chemical composition and a yield strength of 758 MPa or more, the detailed mechanism by which the SSC resistance of the steel material in an acidic environment with a pH of 3.0 is increased by adjusting F1 within the range of 0.15 to 1.00 has not been clarified. However, as also illustrated in FIG. 1, the fact that the CSS resistance in an acidic environment with a pH of 3.0 of a martensitic stainless steel material having the aforementioned chemical composition and a yield strength of 758 MPa or more is increased by adjusting F1 within the range of 0.15 to 1.00 has been demonstrated by the Examples.

As described above, the martensitic stainless steel material according to the present embodiment has the aforementioned chemical composition, a yield strength of 758 MPa or more, and, within the ranges of elemental content and yield strength, satisfies Formula (1). As a result, the martensitic stainless steel material according to the present embodiment can achieve both a high yield strength of 758 MPa or more and excellent SSC resistance in an acidic environment with a pH of 3.0.

The present invention is as follows.

[1]

Martensitic stainless steel material consisting of, in % by mass,

C: 0.030% or less,

Yes: 1.00% or less,

Mn: 1.00% or less,

P: 0.030% or less,

S: 0.0050% or less,

Cu: from 0.01 to 3.50%,

Cr: from 10.00 to 14.00%

Ni: from 4.50 to 7.50%,

Mo: 1.00 to 4.00%,

Ti: from 0.050 to 0.300%,

V: from 0.01 to 1.00%

Al: from 0.001 to 0.100%,

Co: from 0.010 to 0.500%

Ca: from 0.0005 to 0.0050%

Sn: from 0.0005 to 0.0500%

N: from 0.0010 to 0.0500%

O: 0.050% or less,

W: from 0 to 0.50%

Nb: from 0 to 0.500%

As: from 0 to 0.0100%

Sb: from 0 to 0.0100%, and

The rest: Faith and impurities,

where:

An elastic limit is 758 MPa or more; and

Within a range of element content and yield strength of the martensitic stainless steel material, the element content and yield strength satisfy Formula (1);

0.15 < (Sn+As+Sb)/{(Cu+Ni)/YS} < 1.00 (1)

Where, in Formula (1), a corresponding element content in mass percent is replaced by each element symbol, and a yield strength in MPa is replaced by YS, and, if it does not contain the corresponding element, "0" is replaced by the symbol of the relevant element.

[2]

The martensitic stainless steel material according to [1], containing one or more elements selected from a group consisting of:

W: from 0.01 to 0.50%

Nb: from 0.001 to 0.500%

As: from 0.0001 to 0.0100%, and

Sb: from 0.0001 to 0.0100%.

The form of the martensitic stainless steel material according to the present embodiment is not particularly limited. The martensitic stainless steel material according to the present embodiment can be a tube, a round steel bar (solid material), or a steel sheet. Note that the expression "round steel bar" refers to a steel bar whose cross-section perpendicular to the axial direction is circular. Furthermore, the tube can be a seamless tube or a welded tube.

The martensitic stainless steel material according to the present embodiment is described in detail below. The symbol "%" in relation to an element means percentage by mass unless otherwise specified. Furthermore, in the following description, the martensitic stainless steel material is also referred to simply as "steel material."

[Chemical composition]

The martensitic stainless steel material according to the present embodiment contains the following elements. C: 0.030% or less

Carbon (C) is unavoidably present. That is, the lower limit of the C content is greater than 0%. C increases the hardenability of the steel and its strength. On the other hand, if the C content is too high, even if the content of other elements is within the range of the present embodiment, the strength of the steel will be too high. As a result, the SSC resistance of the steel will decrease. Therefore, the C content should be 0.030% or less. A preferred upper limit for the C content is 0.028%, more preferably 0.025%, preferably 0.020%, and preferably even lower than 0.018%. The C content is preferably as low as possible. However, drastically reducing the C content will increase production costs. Therefore, considering industrial production, a preferred lower limit of the C content is 0.001%, more preferably 0.003%, and preferably further 0.005%.

Yes: 1.00% or less

Silicon (Si) is unavoidably present. That is, the lower limit of the Si content is greater than 0%. Si deoxidizes the steel. On the other hand, if the Si content is too high, even if the content of other elements is within the range of the present embodiment, the hot workability of the steel material will decrease. Therefore, the Si content should be 1.00% or less. A preferred lower limit of the Si content to effectively obtain the aforementioned advantageous effect is 0.01%, more preferably 0.05%, preferably 0.10%, and preferably also 0.15%. A preferred upper limit of the Si content is 0.80%, more preferably 0.60%, preferably 0.50%, and preferably also 0.45%.

Mn: 1.00% or less

Manganese (Mn) is unavoidably present. That is, the lower limit of the Mn content is greater than 0%. Mn increases the hardenability of the steel and increases its strength. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of the present embodiment, in some cases the Mn will segregate at the grain boundaries along with impurity elements such as P and S. In such a case, the SSC resistance of the steel will decrease. Therefore, the Mn content should be 1.00% or less. A preferred lower limit of the Mn content to effectively obtain the aforementioned advantageous effect is 0.01%, more preferably 0.05%, preferably 0.10%, and preferably 0.15%. A preferred upper limit for the Mn content is 0.80%, more preferably 0.70%, preferably 0.60%, and preferably further 0.50%.

P: 0.030% or less

Phosphorus (P) is an unavoidable impurity. That is, the lower limit of the P content is above 0%. P segregates at the grain boundaries and facilitates the formation of SSC (Solid State Cracking). Therefore, if the P content is too high, even if the content of other elements is within the range of the present embodiment, the SSC resistance of the steel material will be significantly reduced. Therefore, the P content should be 0.030% or less. A preferred upper limit of the P content is 0.025%, more preferably 0.020%, and even more preferably 0.018%. The P content is preferably as low as possible. However, drastically reducing the P content will increase production costs. Consequently, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even 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 greater than 0%. Similar to phosphorus (P), S segregates at grain boundaries and facilitates the development of SSC (Solid State Cracking). Therefore, if the S content is too high, even if the content of other elements is within the range specified in this embodiment, the SSC resistance of the steel material will be significantly reduced. Therefore, the S content should be 0.0050% or less. A preferred upper limit for the S content is 0.0040%, more preferably 0.0030%, preferably 0.0025%, and preferably 0.0020%. The S content is preferably as low as possible. However, drastically reducing the S content will increase the production cost. Therefore, considering industrial production, a preferred lower limit for the S content is 0.0001%, more preferably 0.0002%, and preferably further 0.0003%.

Cu: from 0.01 to 3.50%

Copper (Cu) is an austenite-forming element and causes the microstructure after quenching to become martensitic. Cu also increases the steel's resistance to surface corrosion cracking (SCC) in an acidic environment with a pH of 3.0, through a synergistic effect with tin (Sn), arsenic (As), and antimony (Sb). If the Cu content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effects will not be sufficiently achieved. On the other hand, if the Cu content is too high, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effects will be saturated, and the hot workability of the steel will also decrease significantly. In this case, the production cost will also increase. Therefore, the Cu content will be from 0.01% to 3.50%. A preferred lower limit for the Cu content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit for Cu content is 3.30%, more preferably 3.10%, and preferably further 2.90%.

Cr: from 10.00 to 14.00%

Chromium (Cr) forms a passive film on the surface of the steel material, thereby increasing its SSC resistance. If the Cr content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effect will not be sufficiently achieved. On the other hand, if the Cr content is too high, even if the content of other elements is within the range specified in this embodiment, ferrite may be incorporated into the microstructure in some cases, making it difficult to ensure sufficient strength. Furthermore, if the Cr content is too high, even if the content of other elements is within the range specified in this embodiment, intermetallic compounds or Cr carbonitrides will readily form in the steel material. As a result, the SSC resistance of the steel material will decrease. Therefore, the Cr content should be between 10.00 and 14.00%. A preferred lower limit for the Cr content is 10.30%, more preferably 10.50%, and preferably also 11.00%. A preferred upper limit for the Cr content is 13.80%, more preferably 13.60%, preferably also 13.50%, preferably also 13.45%, preferably 13.40%, and preferably also 13.35%.

Ni: from 4.50 to 7.50%

Nickel (Ni) is an austenite-forming element and causes the microstructure after quenching to become martensitic. Ni also forms sulfides in the passive film in an acidic environment. Ni sulfides inhibit chloride (Cl-) and hydrogen sulfide (HS-) ions from contacting the passive film, thereby suppressing its destruction by chloride and hydrogen sulfide ions. As a result, the steel's resistance to systemic corrosion cracking (SSC) increases. Furthermore, Ni increases the SSC resistance of the steel in an acidic environment with a pH of 3.0 through a synergistic effect with tin (Sn), arsenic (As), and antimony (Sb). If the Ni content is too low, even if the content of other elements is within the range specified in the present embodiment, the aforementioned advantageous effects will not be sufficiently achieved. On the other hand, if the Ni content is too high, even if the content of other elements is within the range of the present embodiment, the hydrogen diffusion coefficient in the steel material may decrease. In such a case, the SSC resistance of the steel material will decrease. Therefore, the Ni content should be between 4.50% and 7.50%. A preferred lower limit for the Ni content is 4.80%, more preferably 5.00%, and preferably also 5.50%. A preferred upper limit for the Ni content is 7.30%, more preferably 7.00%, and preferably also 6.50%.

Mo: 1.00 to 4.00%

Molybdenum (Mo) forms sulfides in the passive film in an acidic environment. These Mo sulfides inhibit chloride (Cl-) and hydrogen sulfide (HS-) ions from contacting the passive film, thereby suppressing its destruction by these ions. As a result, the steel's SSC resistance increases. Mo also dissolves in the steel, further increasing its strength. If the Mo content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effects will not be sufficiently achieved. Conversely, if the Mo content is too high, even if the content of other elements is within the range specified in this embodiment, it will be difficult to stabilize the austenite. Consequently, in some cases, a significant amount of ferrite will be incorporated into the microstructure after tempering. If the Mo content is too high, even if the content of other elements is within the range of the present embodiment, a large amount of intermetallic compounds, such as Laves-phase-based intermetallic compounds, may form in some cases, and the yield strength of the steel material may become too high. Therefore, the Mo content shall be from 1.00 to 4.00%. A preferred lower limit for the Mo content is 1.30%, more preferably 1.50%, and preferably also 1.80%. A preferred upper limit for the Mo content is 3.80%, more preferably 3.60%, and preferably also 3.40%.

Ti: from 0.050 to 0.300%

Titanium (Ti) combines with carbon (C) and/or nitrogen (N) to form carbides or nitrides. In such cases, the anchoring effect suppresses grain coarsening and increases the yield strength of the steel. If the Ti content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effect will not be sufficiently achieved. Conversely, if the Ti content is too high, even if the content of other elements is within the range specified in this embodiment, the strength of the steel will be too high, and its SSC resistance will decrease. Therefore, the Ti content should be between 0.050% and 0.300%. A preferred lower limit for the Ti content is 0.060%, and more preferably 0.080%. A preferred upper limit for the Ti content is 0.250%, more preferably 0.200%, and preferably 0.180%.

V: from 0.01 to 1.00%

Vanadium (V) increases the hardenability of steel and raises its yield strength. If the V content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effect will not be sufficiently achieved. Conversely, if the V content is too high, even if the content of other elements is within the range specified in this embodiment, the strength of the steel will be excessively high, and its SSC resistance will decrease. Therefore, the V content will be from 0.01% to 1.00%. A preferred lower limit for the V content is 0.02%, and more preferably 0.03%. A preferred upper limit for the V content is 0.80%, more preferably 0.60%, and even more preferably 0.50%.

Al: from 0.001 to 0.100%

Aluminum (Al) deoxidizes steel. If the Al content is too low, even if the content of other elements is within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently achieved. On the other hand, if the Al content is too high, even if the content of other elements is within the range of the present embodiment, coarse oxides will form, and the SSC resistance of the steel material will decrease. Therefore, the Al content shall be from 0.001 to 0.100%. A preferred lower limit for the Al content is 0.005%, more preferably 0.010%, and preferably also 0.015%. A preferred upper limit for the Al content is 0.080%, more preferably 0.060%, preferably 0.055%, and preferably also 0.050%. As used herein, the expression "Al content" means the content of Al sol (acid-soluble Al).

Co: from 0.010 to 0.500%

Cobalt (Co) forms sulfides in the passive film in an acidic environment. These Co sulfides inhibit chloride (Cl-) and hydrogen sulfide (HS-) ions from contacting the passive film, thereby suppressing its destruction by these ions. As a result, the steel's resistance to surface corrosion cracking (SSC) increases. Co also enhances the hardenability of the steel and, particularly during industrial production, ensures consistently high strength. Specifically, Co suppresses the formation of retained austenite and prevents variations in the steel's strength. If the Co content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effects will not be sufficiently achieved. Conversely, if the Co content is too high, even if the content of other elements is within the range specified in this embodiment, the steel's toughness will decrease. Therefore, the Co content will be from 0.010 to 0.500%. A preferred lower limit for the Co content is 0.015%, more preferably 0.020%, more preferably 0.030%, more preferably 0.050%, and more preferably 0.100%. A preferred upper limit for the Co content is 0.450%, more preferably 0.400%, and more preferably 0.350%.

Ca: from 0.0005 to 0.0050%

Calcium (Ca) immobilizes sulfur (S) in the steel material as sulfide, rendering it harmless and thereby improving the hot workability of the steel. If the Ca content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effect will not be sufficiently achieved. Conversely, if the Ca content is too high, even if the content of other elements is within the range specified in this embodiment, coarse inclusions will form in the steel material, reducing its SSC resistance. Therefore, the Ca content should be from 0.0005% to 0.0050%. A preferred lower limit for the Ca content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. A preferred upper limit for Ca content is 0.0045%, more preferably 0.0040%, and preferably further 0.0035%.

Sn: from 0.0005 to 0.0500%

Tin (Sn) increases the SSC resistance of steel in an acidic environment with a pH of 3.0. If the Sn content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effect will not be sufficiently achieved. Conversely, if the Sn content is too high, even if the content of other elements is within the range specified in this embodiment, the Sn will segregate at the grain boundaries, and the SSC resistance of the steel will decrease. Therefore, the Sn content should be between 0.0005% and 0.0500%. A preferred lower limit for the Sn content is 0.0008%, more preferably 0.0010%, and even more preferably 0.0015%. A preferred upper limit for the Sn content is 0.0400%, more preferably 0.0300%, preferably further 0.0200%, preferably 0.0100%, and preferably further 0.0080%.

N: from 0.0010 to 0.0500%

Nitrogen (N) combines with Ti to form fine Ti nitrides. The fine TiN suppresses grain coarsening through an anchoring effect. As a result, the yield strength of the steel material increases. If the N content is too low, even if the content of other elements is within the range specified in this embodiment, the aforementioned advantageous effect will not be sufficiently achieved. On the other hand, if the N content is too high, even if the content of other elements is within the range specified in this embodiment, coarse nitrides will form, and the steel material's SSC resistance will decrease. Therefore, the N content should be from 0.0010 to 0.0500%. A preferred lower limit for the N content is 0.0015%, more preferably 0.0020%, preferably 0.0030%, and preferably 0.0040%. A preferred upper limit for the N content is 0.0450%, more preferably 0.0400%, preferably 0.0350%, and preferably further 0.0300%.

O: 0.050% or less

Oxygen (O) is an unavoidable impurity. That is, the lower limit of the O content is greater than 0%. O forms oxides and reduces the steel material's resistance to shock absorption and corrosion (SSC). Consequently, if the O content is too high, even if the content of other elements is within the range of the present embodiment, the steel material's resistance to SSC will be significantly reduced. Therefore, the O content should be 0.050% or less. A preferred upper limit for the O content is 0.040%, more preferably 0.030%, and even more preferably 0.020%. The O content is preferably as low as possible. However, drastically reducing the O content will increase production costs. Therefore, considering industrial production, a preferred lower limit for the O content is 0.0005%, more preferably 0.001%, and even more preferably 0.002%.

The remainder in the martensitic stainless steel material according to the present embodiment is iron and impurities. In this document, the term "impurities" refers to elements that, during the industrial production of the steel material, are mixed in from ore or scrap used as raw material or from the production environment or similar sources, and which are not intentionally contained but are permitted within a range that does not adversely affect the martensitic stainless steel material according to the present embodiment.

[Optional elements]

The martensitic stainless steel material according to the present embodiment may also contain W instead of a portion of Fe.

W: from 0 to 0.50%

Tungsten (W) is an optional element and its presence is not required. That is, the W content can be 0%. When present, W stabilizes the passive film in an acidic environment and suppresses its destruction by chloride and hydrogen sulfide ions. As a result, the steel material's SSC resistance increases. Even a small amount of W will produce this effect to some extent. On the other hand, if the W content is too high, W will combine with carbon and form coarse carbides. In such a case, the steel material's SSC resistance will decrease even if the content of other elements is within the range specified in this embodiment. Therefore, the W content will be from 0 to 0.50%. A preferred lower limit for the W content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit for the W content is 0.45%, more preferably 0.40%, and preferably further 0.35%.

Additionally, W significantly increases SSC resistance when the Cu content is high. Specifically, when the Cu content is 0.50% or higher, it is preferable to set the W content at 0.10% or higher. When the Cu content is 0.50% or higher, a more preferred lower limit for the W content is 0.12%, and preferably also 0.15%.

The martensitic stainless steel material according to the present embodiment may also contain Nb instead of a portion of Fe.

Nb: from 0 to 0.500%

Niobium (Nb) is an optional element and its presence is not required. That is, the Nb content can be 0%. When present, Nb combines with carbon and/or nitrogen to form Nb carbides and Nb carbonitrides. In such cases, the anchoring effect suppresses grain coarsening and increases the yield strength of the steel. Even a small amount of Nb will produce this effect to some extent. On the other hand, if the Nb content is too high, excessive Nb carbides and/or Nb carbonitrides will form, even if the content of other elements is within the range specified in this embodiment. As a result, the steel's SSC resistance will decrease. Therefore, the Nb content should be between 0 and 0.500%. A preferred lower limit for Nb content is 0.001%, more preferably 0.002%, and preferably further 0.003%. A preferred upper limit for Nb content is 0.450%, more preferably 0.400%, and preferably further 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 a portion of Fe. Each of these elements contributes to the effect of increasing the SSC resistance of the steel material produced by Sn.

As: from 0 to 0.0100%

Arsenic (As) is an optional element and its presence is not required. That is, the As content can be 0%. When present, As contributes to the increased SSC resistance of the steel material produced by Sn. Even a small amount of As will produce this effect to some extent. However, if the As content is too high, even if the content of other elements is within the range specified in this embodiment, As will segregate at the grain boundaries, and the SSC resistance of the steel material will decrease. Therefore, the As content should be between 0 and 0.0100%. A preferred lower limit for the As content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. A preferred upper limit for the As content is 0.0090%, and even more preferably 0.0080%.

Sb: from 0 to 0.0100%

Antimony (Sb) is an optional element and its presence is not required. That is, the Sb content can be 0%. When present, Sb contributes to the increased SSC resistance of the steel material produced by Sn. Even a small amount of Sb will produce this effect to some extent. On the other hand, if the Sb content is too high, even if the content of other elements is within the range of the present embodiment, the Sb will segregate at the grain boundaries, and the SSC resistance of the steel material will decrease. Therefore, the Sb content should be between 0 and 0.0100%. A preferred lower limit for the Sb content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. A preferred upper limit for the Sb content is 0.0090%, and even more preferably 0.0080%.

[Elastic limit]

According to the present embodiment, the yield strength of the martensitic stainless steel material is 758 MPa (110 ksi) or higher, and more preferably 862 MPa (125 ksi) or higher. Although the upper limit of the yield strength is not particularly restricted, the upper limit of the yield strength of the steel material of the present embodiment is, for example, 1034 MPa (150 ksi). A more preferred upper limit of the yield strength of the steel material is 1000 MPa (145 ksi). As used herein, the term "yield strength" means the yield strength shifted to 0.2% (MPa) obtained by a tensile test at normal temperature (24 ± 3 °C) carried out in accordance with ASTM E8/E8M (2013).

Specifically, in the present embodiment, the yield strength is determined by the following method. First, a tensile test specimen is prepared from the martensitic stainless steel material according to the present embodiment. If the steel material is a tube, the tensile test specimen is prepared from a position centered on the wall thickness. If the steel material is a round steel bar, the tensile test specimen is prepared from a position R/2. Note that, in the present description, the expression "position R/2" of a round steel bar means the position centered on a radius R in a cross-section perpendicular to the axial direction of the round steel bar. If the material is a steel sheet, the tensile test specimen is prepared from a position centered on the thickness. The size of the tensile test specimen is not particularly limited. For example, a round bar with a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm is used as a tensile test specimen. The longitudinal direction of the parallel portion of the tensile test specimen must be parallel to the rolling direction and/or the axial direction of the steel material. A tensile test is performed at normal temperature (24 ± 3 °C) according to ASTM E8/E8M (2013) using the prepared tensile test specimen, and the 0.2% shifted yield strength (MPa) is determined. The determined 0.2% shifted yield strength is defined as the yield strength (MPa).

As described above, the yield strength of the martensitic stainless steel material according to the present embodiment is 758 MPa or more, and preferably 862 MPa or more. In the present embodiment, when the Cu content is 1.00% or less and a yield strength of 758 to less than 862 MPa is desired, the martensitic stainless steel material preferably consists of, in mass percent, 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: from 0.0005 to 0.0050%, Sn: from 0.0005 to 0.0500%, N: from 0.0010 to 0.0500%, O: 0.050% or less, W: from 0 to 0.50%, Nb: from 0 to 0.500%, As: from 0 to 0.0100%, and Sb: from 0 to 0.0100%, with the remainder being Fe and impurities.

Additionally, 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 consists of, 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 1.00%, Cr: 10.00 to 14.00%, Ni: 5.00 to 7.50%, Mo: 2.00 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%, and Sb: 0 to 0.0100%, with the remainder being 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 desired, the martensitic stainless steel material preferably consists of, 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: 0.50 to 3.50%, Cr: 10.00 to 14.00%, Ni: 5.00 to 7.50%, Mo: 2.00 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: from 0.0005 to 0.0050%, Sn: from 0.0005 to 0.0500%, N: from 0.0010 to 0.0500%, O: 0.050% or less, W: from 0.10 to 0.50%, Nb: from 0 to 0.500%, As: from 0 to 0.0100%, and Sb: from 0 to 0.0100%, with the remainder being Fe and impurities.

[Regarding Formula (1)]

In the martensitic stainless steel material according to the present embodiment, within the range of the elemental content described above and a yield strength of 758 MPa or more, the elemental content and yield strength satisfy Formula (1). As a result, provided that the other requirements of the present embodiment are met, the martensitic stainless steel material according to the present embodiment has excellent resistance to SSC in an acidic environment with a pH of 3.0.

0.15 < (Sn+As+Sb)/{(Cu+Ni)/YS} < 1.00 (1)

Where, in Formula (1), each element symbol is replaced by the content of the corresponding element in mass percent, and YS is replaced by the yield strength in MPa. Note that, if it does not contain the corresponding element, "0" is replaced by the symbol of the relevant element.

F1 (= (Sn+As+Sb)/{(Cu+Ni)/YS}) is an index of increased resistance to SSC in an acidic environment with a pH of 3.0, obtained by the synergistic effect of Sn, As, and Sb, and Cu and Ni, adjusted according to the yield strength. When F1 is too low, the steel material will not exhibit excellent resistance to SSC in an acidic environment with a pH of 3.0. Similarly, when F1 is too high, the steel material will not exhibit excellent resistance to SSC in an acidic environment with a pH of 3.0. On the other hand, if F1 is in the range of 0.15 to 1.00, the steel material will exhibit excellent resistance to SSC in an acidic environment with a pH of 3.0.

Therefore, in the martensitic stainless steel material according to the present embodiment, in addition to meeting the requirements relating to the content of elements described above and the yield strength of 758 MPa or more, F1 must be within a range of 0.15 to 1.00. A preferred lower limit of F1 is 0.16, and more preferably 0.18. A preferred upper limit of F1 is 0.95, and more preferably 0.90.

[Microstructure of the steel material]

The microstructure of the martensitic stainless steel material according to the present embodiment consists mainly of martensite. As used herein, the expression "consisting mainly of martensite" means that the microstructure is composed, by volume ratio, of 0 to 5.0% retained austenite and 0 to 5.0% ferrite, with the remainder being martensite. In the present description, the expression "composed of retained austenite, ferrite, and tempered martensite" means that the amount of any phase other than retained austenite, ferrite, and tempered martensite is negligible. For example, in the chemical composition of the martensitic stainless steel material according to the present embodiment, the volume ratios of precipitates and inclusions are negligible compared to the volume ratios of retained austenite, ferrite, and tempered martensite. That is, the microstructure of the martensitic stainless steel material according to the present embodiment may contain minute quantities of precipitates, inclusions and the like, in addition to retained austenite, ferrite and tempered martensite.

In this description, the term "martensite" includes not only fresh martensite but also tempered martensite. The lower limit of the martensite volume ratio in the microstructure of the martensitic stainless steel material according to the present embodiment is 90.0%, and more preferably 95.0%. Furthermore, the microstructure of the steel material preferably consists of single-phase martensite.

In the microstructure, a small amount of retained austenite does not significantly decrease strength and significantly increases the toughness of the steel material. However, if the volume ratio of retained austenite is too high, the strength of the steel material will decrease considerably. Therefore, in the microstructure of the steel material of the present embodiment, the volume ratio of retained austenite should be from 0 to 5.0%. From the standpoint of ensuring strength, a preferred upper limit for the volume ratio of retained austenite is 4.0%, and more preferably 3.0%. The volume ratio of retained austenite may be 0%. On the other hand, if even a small amount of retained austenite is present, the volume ratio of retained austenite should be more than 0 to 5.0%, more preferably more than 0 to 4.0%, and preferably also more than 0 to 3.0%.

A small amount of ferrite may be included in the microstructure. However, if the ferrite volume ratio is too high, the toughness of the steel material will be significantly reduced. Therefore, in the microstructure of the steel material of the present embodiment, the ferrite volume ratio should be from 0 to 5.0%. A preferred upper limit for the ferrite volume ratio is 3.0%, more preferably 2.0%, and preferably also 1.0%. The ferrite volume ratio may be 0%. On the other hand, if even a small amount of ferrite is present, the ferrite volume ratio should be more than 0 to 5.0%, more preferably more than 0 to 3.0%, more preferably more than 0 to 2.0%, and more preferably more than 0 to 1.0%.

[Method for measuring the volume ratio of martensite]

In the present embodiment, the volume ratio (%) of martensite in the microstructure of the steel material is determined by subtracting from 100% a volume ratio (%) of retained austenite determined by a method described below and a volume ratio (%) of ferrite determined by a method described below.

[Method for measuring the retained austenite volume ratio]

The volume ratio of retained austenite in the microstructure of the steel material is determined by an X-ray diffraction method. Specifically, a test specimen is prepared to measure the volume ratio of retained austenite from the steel material according to the present embodiment. If the steel material is a tube, the test specimen is taken from a position centered on the wall thickness. If the steel material is a round steel bar, the test specimen is taken from a position R/2. If the material is a steel sheet, the test specimen is taken from a position centered on the thickness. The size of the test specimen is not particularly limited. The test specimen is, for example, 15 mm x 15 mm x 2 mm thick. If the steel material is a tube, the direction of the test specimen thickness is the direction of the tube diameter. If the steel material is a round steel bar, the thickness direction of the test specimen is the radial direction. If the material is a steel plate, the thickness direction of the test specimen is the plate thickness direction. Using the prepared test specimen, the X-ray diffraction intensity is measured for each of the (110) phase plane α (martensite), the (200) phase plane α, the (211) phase plane α, the (111) phase plane <y> (retained austenite), the (200) phase plane <y>, and the (220) phase plane <y>, and an integrated intensity for the respective planes is calculated.

In the X-ray diffraction intensity measurement, the target of the X-ray diffraction apparatus is Co (Co Ka radiation), and the output is set to 30 kV - 100 mA. The measurement angle (20) is set from 45 to 105°. After calculation, the retained austenite volume ratio (Vy) (%) is calculated using Formula (I) for combinations (3 x 3 = 9 pairs) of each phase plane a and each phase plane <y>. An average value of the retained austenite volume ratios Vy of the nine pairs is then defined as the retained austenite volume ratio (%)

V<y>= 100/{1+(Ia x R<y>)/(I<y>x Ra)} (I)

Where, Ia is the integrated intensity of a phase. Ra is a crystallographic theoretical calculation value of phase a. Iy is the integrated intensity of phase y. Ry is a crystallographic theoretical calculation value of phase y. The values incorporated in a quantitative and retained analysis system belonging to RINT-TTR (product name) manufactured by Rigaku Corporation can be used for the Ra and Ry values of each plane. Note that the retained austenite volume ratio is adopted as a value obtained by rounding the numerical value to the second decimal place.

[Method for measuring the ferrite volume ratio]

The ferrite volume ratio in the microstructure of steel material is determined by a point-counting method. Specifically, a test sample is prepared from the steel material according to the present embodiment to measure the ferrite volume ratio. If the steel material is a tube, the test sample is taken from a central position within the wall thickness. If the steel material is a round steel bar, the test sample is taken from a position R/2. If the material is a steel sheet, the test sample is taken from a central position within the thickness. The test sample is not particularly restricted, provided that it has a face parallel to the rolling direction as the observation surface. For example, if the steel material is a tube, the observation surface of the test sample is parallel to the axial direction of the tube. After the observation surface is mechanically polished, it is subjected to electrolytic etching to reveal the microstructure. Electrolytic etching is performed using a 30% aqueous solution of sodium hydroxide as the electrolyte, at a current density of 1 A/cm2, for an electrolysis time of 1 minute.

On the surface subjected to electrolytic etching, 30 fields of view are observed using an optical microscope. Each field of view is defined as a 250 µm x 250 µm rectangle. Note that the magnification is 400x. Skilled practitioners can distinguish ferrite from other phases (retained austenite or tempered martensite) based on the contrast of each field of view. Therefore, ferrite in each field of view is identified based on its contrast. The fraction of the identified ferrite area is determined using a point-counting method according to ASTM E562 (2019).

Specifically, for each field of view, 20 vertical lines are drawn at regular intervals from the top to the bottom of the field of view. That is, the field of view is divided into 21 regions in the left-right direction by the 20 vertical lines. Additionally, for each field of view, 20 horizontal lines are also drawn at regular intervals from the left to the right of the field of view. That is, the field of view is divided into 21 regions in the vertical direction by the 20 horizontal lines. The intersections of the vertical and horizontal lines are called grid points. Therefore, in each field of view, 400 grid points are arranged at regular intervals. According to ASTM E562 (2019), the grid points that overlap with the ferrite in the field of view are counted. The number of overlapping ferrite grid points obtained in the 30 visual fields is divided by the total number of grid points (400 x 30 = 12,000), and the resulting value is defined as the ferrite area fraction. In the present embodiment, the ferrite area fraction determined by the above method is defined as the ferrite volume ratio (%). Note that the ferrite volume ratio is adopted as a value obtained by rounding the second decimal place of the numerical value obtained.

Using the volume ratio (%) of retained austenite obtained by the aforementioned X-ray diffraction method, and the volume ratio (%) of ferrite obtained by the aforementioned dot counting method, the volume ratio (%) of martensite in the microstructure of the steel material is determined by the following formula.

Martensite volume ratio (%) = 100.0 - {retained austenite volume ratio (%) ferrite volume ratio (%)}

[SSC resistance of the steel material]

Although the martensitic stainless steel material according to the present embodiment has a high yield strength of 758 MPa or more, it exhibits excellent SSC resistance in an acidic environment with a pH of 3.0. The SSC resistance of the martensitic stainless steel material according to the present embodiment can be evaluated by an SSC resistance assessment test at normal temperature. The SSC resistance assessment test is performed using a method conforming to NACE TM0177-2016 Method A.

Specifically, a round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a tube, the round bar specimen is prepared from a position centered on the wall thickness. If the steel material is a round steel bar, the round bar specimen is prepared from a portion R/2. If the material is a steel sheet, the round bar specimen is prepared from a position centered on the thickness. The size of the round bar specimen is not particularly limited. The round bar specimen, for example, is a specimen in which the diameter of the parallel portion is 6.35 mm and the length of the parallel portion is 25.4 mm. Note that the axial direction of the specimen will be parallel to the rolling direction and/or the axial direction of the steel material.

An aqueous solution containing 0.17% sodium chloride by mass and having a pH of 3.0 is used as the test solution. The test solution is prepared by adding acetic acid to an aqueous solution containing 0.17% sodium chloride by mass and 0.41 g/L sodium acetate to adjust the pH to 3.0. A stress equivalent to 90% of the actual yield strength is applied to the round bar specimen prepared as described above. The test solution at 24 °C is poured into a test vessel so that the stressed round bar specimen is immersed within it to form a test bath. After degassing the test bath, H₂S gas at 0.003 MPa (0.03 bar) and CO₂ gas at 0.097 MPa (0.97 bar) are blown into the test bath to saturate it with H₂S gas. The test bath, saturated with H₂S gas, is maintained at 24 °C for 720 hours. After the test sample has been maintained for 720 hours, the surface of the parallel portion of the test sample is observed with a magnifying glass at 10x magnification to confirm the presence or absence of pitting. In the martensitic stainless steel material according to the present embodiment, the presence of pitting is not confirmed after 720 hours in a SSC resistance evaluation test performed by the method described above.

[Form and uses of steel material]

As mentioned above, the form of the martensitic stainless steel material according to the present embodiment is not particularly limited. Specifically, the martensitic stainless steel material according to the present embodiment can be a tube, a round steel bar (solid material), or a steel sheet. The tube can be seamless or welded. The tube is, for example, a tube for oil well tubing. The expression "tube for oil well tubing" means a tube that is to be used in oil well tubing. Oil well tubing includes, for example, casing, pipes, and drill pipe used to drill an oil or gas well, collect crude oil or natural gas, and the like. Preferably, the steel material of the present embodiment is a seamless tube for oil well tubing.

As described above, in the martensitic stainless steel material according to the present embodiment, the content of each element is within the range of the present embodiment, the yield strength is 758 MPa or more, and within the range of the element contents described above and the yield strength of 758 MPa or more, the condition that F1 is from 0.15 to 1.00 is satisfied. As a result, the steel material according to the present embodiment has both a high yield strength and excellent resistance to SSC in an acidic environment with a pH of 3.0.

[Production Method]

An example of a method for producing martensitic stainless steel according to the present embodiment is described below. Note that the production method described below is an example, and a method for producing martensitic stainless steel according to the present embodiment is not limited to the production method described below. That is, provided that martensitic stainless steel can be produced according to the present embodiment, composed as described above, a method for producing martensitic stainless steel is not limited to the production method described below. However, the production method described below is a favorable method for producing martensitic stainless steel according to the present embodiment.

An example of a method for producing martensitic stainless steel according to the present embodiment includes a process for preparing an intermediate steel material (preparation process), and a process for quenching and tempering the intermediate steel material (heat treatment process). Each of these processes is described in detail below.

[Preparation process]

In the preparation process, an intermediate steel material is prepared that has the aforementioned chemical composition. The method for preparing the intermediate steel material is not particularly limited, provided that the intermediate steel material has the aforementioned chemical composition. As used herein, the term "intermediate steel material" refers to a steel material in the form of a plate when the final product is a welded steel sheet or tube, and to a hollow shell when the final product is a seamless tube.

The preparation process may include a process for preparing a starting material (starting material preparation process) and a process of subjecting the starting material to hot working to produce an intermediate steel material (hot working process). A case in which the preparation process includes both a starting material preparation process and a hot working process is described in detail later.

[Preparation process of the starting material]

In the process of preparing the starting material, a starting material is produced using molten steel with the aforementioned chemical composition. The method for producing the starting material is not particularly limited; any known method is sufficient. Specifically, a casting (a slab, a billet, or a stock) can be produced by a continuous casting process using the molten steel. An ingot can also be produced by an ingot-making process using the molten steel. As needed, the slab, billet, or stock can be roughed to produce a billet. A starting material (a slab, a billet, or a stock) is produced by the process described above.

[Hot working process]

In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel product. If the starting steel product is a seamless tube, the intermediate steel product corresponds to a hollow shell. First, a billet is heated in a reheating furnace. Although not particularly restricted, the heating temperature is, for example, 1100 to 1300 °C. After removing the billet from the reheating furnace, it is subjected to hot working to produce a hollow shell (seamless tube). The hot working method is not particularly restricted, and it is sufficient to use a known method.

For example, the Mannesmann process can be performed as a hot working process to produce a hollow shell. In this case, a round billet is punched and rolled using a punching machine. While not particularly limited during the punching and rolling process, the punching ratio is, for example, 1.0 to 4.0. The punched and rolled round billet is then hot rolled using a mandrel mill, reducer, finishing mill, or similar equipment to produce a hollow shell. The cumulative area reduction in the hot working process is, for example, 20 to 70%.

A hollow shell can be produced from a billet using another hot working method. For example, if the steel material is a short, thick-walled tube such as a coupling, a hollow shell can be produced by forging using the Ehrhardt process or similar methods. The hollow shell is produced using the process described above. Although not particularly limited, the wall thickness of the hollow shell is, for example, 9 to 60 mm.

If the steel material is a round steel bar, the starting material is first heated in a reheating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300 °C. The starting material removed from the reheating furnace is then hot-worked to produce an intermediate steel material in which a cross-section perpendicular to the axial direction is circular. Hot working includes, for example, roughing in a roughing mill or hot rolling in a continuous rolling mill. In a continuous rolling mill, a horizontal stand with a pair of grooved rolls arranged side-by-side in the vertical direction and a vertical stand with a pair of grooved rolls arranged side-by-side in the horizontal direction are arranged alternately.

If the material is a steel sheet, the starting material is first heated in a reheating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300 °C. The starting material removed from the reheating furnace is then hot-rolled using a roughing mill and a continuous mill to produce an intermediate steel material in the form of a steel sheet.

The hollow shell produced by hot working can be air-cooled (rolling rough). The hollow shell produced by hot working can be quenched directly after hot working without cooling to normal temperature, or it can be quenched after undergoing additional heating (reheating) after hot working.

If direct hardening is performed after hot working, or after supplementary heating, cooling can be stopped midway through the hardening process or a slow cooling process can be implemented. This can prevent hardening cracking in the hollow shell. Additionally, if direct hardening is performed after hot working, or after supplementary heating, stress-relief annealing (SR) can be carried out after hardening and before the subsequent heat treatment process. This eliminates residual stress in the hollow shell.

As described above, an intermediate steel material is prepared in the preparation process. This intermediate steel material may be produced by the previously mentioned preferred process, or it may be an intermediate steel material produced by a third party, or it may be an intermediate steel material produced in a different factory than the one where the quenching and tempering processes described later are carried out, or at a different worksite. The heat treatment process is described in detail below.

[Heat treatment process]

The heat treatment process includes a quenching process and an annealing process.

[Tempering process]

In the heat treatment process, the intermediate steel material produced in the hot working process is first subjected to quenching (quenching process). Quenching is performed using a well-established method. Specifically, the intermediate steel material, after the hot working process, is loaded into a heat treatment furnace and held at a quenching temperature. The quenching temperature is equal to or higher than the Ac3 transformation point and, for example, is 900 to 1000 °C. After being held at the quenching temperature, the intermediate steel material is rapidly cooled (quenched). Although not particularly limited, the holding time at the quenching temperature is, for example, 10 to 60 minutes. The quenching method is, for example, water quenching. The quenching method is not particularly restricted. In a case where the intermediate steel material is a hollow shell, for example, the hollow shell can be rapidly cooled by immersing the hollow shell in a water bath or oil bath, or the hollow shell can be rapidly cooled by pouring or spraying a jet of cold water onto the outer and/or inner surface of the hollow shell by means of a cooling shower or mist cooling.

Note that, as mentioned above, after the hot working process, quenching (direct quenching) can be carried out immediately after hot working, without cooling the intermediate steel material to normal temperature, or quenching can be carried out after the intermediate steel material has been held at the quenching temperature after being loaded into a holding furnace before the hollow shell temperature decreased after hot working.

[Anning process]

After quenching, the intermediate steel is also subjected to annealing. During the annealing process, the yield strength of the steel material is adjusted. In the present embodiment, the annealing temperature is set in the range of 540 to 620 °C. Although not particularly restricted, the holding time at the annealing temperature is, for example, from 10 to 180 minutes. It is well known to those skilled in the art that the yield strength of a steel material can be adjusted by appropriately regulating the annealing temperature according to its chemical composition. Therefore, the annealing conditions are adjusted so that the yield strength of the steel material is 758 MPa or higher.

The martensitic stainless steel material according to the present embodiment can be produced by the process described above. Note that, as mentioned above, a method for producing the martensitic stainless steel material according to the present embodiment is not limited to the above production method. Specifically, while a martensitic stainless steel material in which the content of each element in the chemical composition is within the range of the present embodiment, and which has a microstructure composed, in vol.%, of 0 to 5.0% retained austenite and 0 to 5.0% ferrite, with the remainder being tempered martensite, has a yield strength of 758 MPa or more, and in which, additionally, F1 is from 0.15 to 1.00, can be produced, the production method of the present embodiment is not limited to the above production method. The martensitic stainless steel material according to the present embodiment is described more specifically below by way of example.

Examples

Cast steels were produced that have the chemical compositions indicated in Table 1. Note that the symbol "-" in Table 1 means that the content of the corresponding element was at an impurity level. For example, it means that the W content of Test No. 1 was 0% when rounded to two decimal places. It means that the Nb content of Test No. 1 was 0% when rounded to three decimal places. It means that the As and Sb content of Test No. 1 were each 0% when rounded to four decimal places.

The respective molten steels described above were melted in a 180 kg vacuum furnace, and the ingots were produced using an ingot-making process. The ingots were heated for three hours at 1250 °C. After heating, the ingot was hot-forged to produce a block. Following hot forging, the block was heated for three hours at 1230 °C and then hot-rolled. This process produced a steel material (a steel sheet) with a thickness of 13 mm.

The steel material produced from each test number was quenched. Specifically, the steel sheet from each test number was heated to the quenching temperature (°C) described in Table 2. The steel sheet from each test number was held at the quenching temperature for 15 minutes and then quenched with water. After quenching, the steel material from each test number was tempered at the tempering temperature (°C) described in Table 2 for 30 minutes.

[Table 2]

TABLE 2

The steel plate for each test number was manufactured using the above production process.

[Evaluation trials]

The steel sheet produced from each trial number was subjected to a microstructure observation test, a tensile test, and a CSS resistance assessment test.

[Microstructure observation essay]

The steel plate from each test number was subjected to a microstructure observation test. First, the volume ratio (%) of retained austenite in the steel plate from each test number was determined using the aforementioned X-ray diffraction method. Note that, in measuring the X-ray diffraction intensity, an X-ray diffraction apparatus with the product name "RINT-TTR" manufactured by Rigaku Corporation was used. The measurement was performed using Co Ka radiation as the radiation source, with the output set at 30 kV-100 mA, and the measurement angle (20) set from 45 to 105°. The resulting volume ratio (%) of retained austenite in the steel plate from each test number is shown in the "Retained (%)" column of Table 2.

Furthermore, the ferrite volume ratio (%) was determined using the previously described point-counting method. Specifically, a test specimen was prepared from a central position within the thickness of the steel plate for each test number. For each test specimen, a surface parallel to the rolling direction was used as the observation surface. Note that in these Examples, a ferrite area fraction determined by a method conforming to ASTM E562 (2019) described above was defined as the ferrite volume ratio (%). The ferrite volume ratio obtained from the steel plate for each test number is shown in the "Ferrite (%)" column of Table 2.

Additionally, for the steel plate of each test number, the volume ratio (%) of martensite was determined using the following formula using the volume ratio (%) of retained austenite and the volume ratio (%) of ferrite.

Martensite volume ratio (%) = 100 - {retained austenite volume ratio (%) ferrite volume ratio (%)}

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

[Tensile test]

The steel plate from each test number was subjected to a tensile test according to ASTM E8/E8M (2013). Specifically, from a central position within the thickness of the steel plate for each test number, a round bar tensile test specimen was prepared where the diameter of the parallel portion was 8.9 mm and the gauge length was 35.6 mm. The longitudinal direction of the round bar tensile test specimen 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 the round bar tensile test specimen from each test number, and the 0.2% shifted yield strength (MPa) was determined. The determined 0.2% shifted yield strength was defined as the yield strength (MPa). The yield strength obtained from each test number is shown in the "YS (MPa)" column of Table 2. Additionally, for the steel plate of each test number, F1 was determined using the chemical composition, the yield strength, and Formula (1). The value obtained for F1 for each test number is shown in the "F1" column of Table 2.

[SSC resistance assessment test]

The steel plate from each test number was subjected to a steel shrinkage cracking (SSC) strength evaluation test. Specifically, round bar samples with a diameter of 6.35 mm were prepared, in which the length of the parallel portion was 25.4 mm from a central position within the thickness of the steel plate for each test number. Three of the prepared round bar samples were subjected to an SSC strength evaluation test performed in accordance with Method A of NACE TM0177-2016. Note that the axial direction of each round bar specimen was parallel to the rolling direction.

An aqueous solution containing 0.17% sodium chloride by mass and having a pH of 3.0 was used as the test solution. The test solution was prepared by adding acetic acid to an aqueous solution containing 0.17% sodium chloride by mass and 0.41 g/L sodium acetate to adjust the pH to 3.0. A stress equivalent to 90% of the actual yield strength was applied to the round bar specimen for each test number. The test solution at 24 °C was poured into a test vessel so that the stressed round bar specimen was immersed within it to form a test bath. After degassing the test bath, H2S gas at 0.003 MPa (0.03 bar) and CO2 gas at 0.097 MPa (0.97 bar) were blown into the test bath to saturate it with H2S gas. The H2S-saturated test bath was maintained at 24 °C for 720 hours.

The surface of the parallel portion of each round bar specimen, after being held for 720 hours, was observed with a magnifying glass at 10x magnification, and the presence or absence of pitting was confirmed. The number of samples in which pitting was confirmed among the three round bar samples is shown in Table 2 as "Number of samples in which pitting occurred (samples)."

[Evaluation Results]

See Table 1 and Table 2. In the steel plates from Tests 1 through 22, the chemical composition was adequate, and each plate had a microstructure consisting of 0 to 5.0% by volume of retained austenite and 0 to 5.0% by volume of ferrite, with the remainder being martensite. Additionally, each plate had high strength, specifically a yield strength of 758 MPa or higher. Furthermore, the condition that F1 was within the range of 0.15 to 1.00 was met in these plates. Consequently, in the acidic environment with a pH of 3.0, pitting occurred in zero samples, indicating excellent resistance to steel shock.

On the other hand, in the steel plates from Tests No. 23 to 26, F1 was too low. As a result, pitting occurred in at least one of the samples of each of these steel plates in the acidic environment with a pH of 3.0, indicating that these steel plates did not have excellent resistance to SSC.

In the steel plates from Tests No. 27 to 30, F1 was too high. As a result, pitting occurred in three samples of each of these steel plates in the acidic environment with a pH of 3.0, indicating that these steel plates did not have excellent resistance to SSC.

The steel plate from Test No. 31 did not contain Sn. As a result, with respect to this steel plate, in the acidic environment with a pH of 3.0, pitting occurred on one specimen, so the steel plate did not have excellent resistance to SSC.

The steel plates from Tests No. 32 to 34 did not contain Sn. Additionally, F1 was too low in these steel plates. As a result, pitting occurred in at least one of the samples of each of these steel plates in the acidic environment with a pH of 3.0, indicating that these steel plates did not have excellent resistance to SSC.

The steel plate from Test No. 35 did not contain Sn. Additionally, F1 was too high in this steel plate. As a result, pitting occurred in three samples of this steel plate in the acidic environment with a pH of 3.0, indicating that the steel plate did not have excellent resistance to SSC.

In the steel plate from Test No. 36, the Co content was too low. As a result, pitting occurred on one specimen of this steel plate in the acidic environment with a pH of 3.0, indicating that the steel plate did not exhibit excellent SSC resistance.

In the steel plate from Test No. 37, the Co content was too low. Additionally, F1 was too low in this steel plate. As a result, pitting occurred in three samples of this steel plate in the acidic environment with a pH of 3.0, indicating that the steel plate did not have excellent resistance to SSC.

An embodiment of this disclosure has been described above. However, the embodiment described above is merely an example of how to carry out this disclosure. Therefore, this disclosure is not limited to the embodiment described above and may be implemented by appropriately modifying the embodiment described above within the scope of the appended claims.

Claims (2)

1. Martensitic stainless steel material consisting of, in % by mass, C: 0.030% or less, Yes: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: from 0.01 to 3.50%, Cr: from 10.00 to 14.00% Ni: from 4.50 to 7.50%, Mo: 1.00 to 4.00%, Ti: from 0.050 to 0.300%, V: from 0.01 to 1.00% Al: from 0.001 to 0.100%, Co: from 0.010 to 0.500% Ca: from 0.0005 to 0.0050% Sn: from 0.0005 to 0.0500% N: from 0.0010 to 0.0500% O: 0.050% or less, W: from 0 to 0.50% Nb: from 0 to 0.500% As: from 0 to 0.0100% Sb: from 0 to 0.0100%, and The rest: Faith and impurities, where: A yield strength is 758 MPa or more, measured according to the method described in the description; and Within a range of element content and yield strength of the martensitic stainless steel material, the element content and yield strength satisfy Formula (1); 0.15 < (Sn+As+Sb)/{(Cu+Ni)/YS} < 1.00 (1) Where, in Formula (1), a corresponding element content in mass percent is replaced by each element symbol, and a yield strength in MPa is replaced by YS, and, if it does not contain the corresponding element, "0" is replaced by the symbol of the relevant element. 2. The martensitic stainless steel material according to claim 1, containing one or more elements selected from a group consisting of: W: from 0.01 to 0.50%