JP2019073789A - Stainless steel and stainless steel pipe - Google Patents

Abstract

To provide a stainless steel that has high strength only by heat treatment, and further has excellent strong acid resistance and carbon dioxide corrosiveness under a high temperature environment.SOLUTION: A stainless steel according to an embodiment has a chemical composition that comprises, in mass%, C: 0.040% or less, Si: 0.05-1.0%, Mn: 0.010-0.30%, Cr: more than 18.0% and 21.0% or less, Cu: 1.5-4.0%, Ni: 3.0-6.0%, sol. Al: 0.001-0.100%, with the balance being Fe and impurities, satisfying formula (1) and formula (2), and has a micro structure that comprises, by volume fraction, 20.0-60.0% of a ferrite phase and 1.0-10.0% of an austenite phase, with the balance being martensite. 15.0≤616-706(C+N)-22Si-24Mn-26 Ni-18Cr-8Cu-16Mo (1) and 20.0≤Cr+Cu≤24.5 (2).SELECTED DRAWING: Figure 2

Description

  The present invention relates to a stainless steel material and a stainless steel pipe, and more particularly to a stainless steel material and a stainless steel pipe for oil wells and geothermal wells.

Steel pipes are used for the extraction and extraction of oil and natural gas, and the extraction of steam for geothermal power generation. Because oil and natural gas exist deep underground, steel pipes used in oil wells are required to have high strength. Also, oil wells have completely different environments depending on the area and depth. In particular, the content of carbon dioxide gas (CO ₂ ) and the content of hydrogen sulfide gas (H ₂ S) greatly vary from one well to another. For example, in an oil well, both the content of carbon dioxide gas and the content of hydrogen sulfide gas are high. On the other hand, in another oil well, the content of hydrogen sulfide gas may be remarkably low although the content of carbon dioxide gas is high. The characteristics required for steel pipes differ depending on the environment of these oil wells.

  In recent years, in order to cope with the soaring price of crude oil and natural gas, the development of deeper oil wells and deeper natural gas wells has been promoted. Furthermore, with the background of rising awareness of renewable energy, the development of geothermal wells deeper than before has also been promoted in geothermal wells that collect steam for geothermal power generation.

Such deep oil wells and deep natural gas wells (hereinafter, both are collectively referred to as "deep oil wells") and deep geothermal wells are generally extremely deep. Furthermore, the atmosphere of deep oil wells and deep geothermal wells is high temperature and contains carbon dioxide gas (CO ₂ ), and additionally contains an acid such as chloride ion (Cl ⁻ ) or sulfuric acid (H ₂ SO ₄ ). It is a corrosive environment. Therefore, excellent carbon dioxide gas corrosion resistance is required for steel materials used as materials of steel pipes used in such deep oil wells and deep geothermal wells.

In corrosive environments such as deep oil wells and deep geothermal wells, martensite stainless steel (so-called "13% Cr steel"), which is excellent in carbon dioxide corrosion resistance, contains about 13% Cr by mass% The used steel pipe has been used. However, as the oil and geothermal wells become deeper, the environment becomes even higher temperature and pressure, and additionally, the CO ₂ partial pressure also becomes higher. Therefore, a steel pipe having sufficient carbon dioxide gas corrosion resistance even in a more severe carbon dioxide gas environment is required.

  It is known that carbon dioxide corrosion resistance at high temperatures can generally be improved by increasing the Cr content. Therefore, in recent years, based on martensitic stainless steel, a steel material is proposed in which the carbon dioxide gas corrosion resistance at high temperature is improved by increasing the amount of Cr more than ever before.

In JP 2005-336599 A (patent document 1), the contents of Cr, Ni, Mo, Cu and C are made appropriate, and the contents of C and N are adjusted to a certain level or less, thereby achieving 150 ° C. or higher A high strength stainless steel pipe for line pipe has been proposed which is excellent in CO ₂ corrosion resistance, shows excellent sulfide stress corrosion cracking resistance even in a hydrogen sulfide environment, and is also excellent in weldability.

In JP 2006-016637 A (patent document 2), based on 13% Cr steel, the contents of C, Ni, Si, Mn, V, Al, N and O are optimized, and further Mo, Cu, Nb High-strength stainless steel tubes for oil wells that are excellent in CO ₂ corrosion resistance of 170 ° C. or higher by adding Ti and Ti have been proposed.

In JP 2007-332431 A (patent document 3), after adding Ni, Mo and V, the contents of S, Si, Al and O are reduced to thereby reduce the CO ₂ corrosion resistance and the pipe expansion property. A stainless steel pipe for oil wells that is compatible with the above has been proposed.

The stainless steels proposed in Patent Literature 1 to Patent Literature 3 all exhibit good carbon dioxide gas corrosion resistance in a CO ₂ environment of about 200 ° C. However, in the case of deep oil wells and deep geothermal wells in which deep wells are further advanced than in the past, the environment becomes a higher temperature than assumed in Patent Documents 1 to 3. For deep oil wells and deep geothermal wells, for example, the high temperature environment exceeds 250 ° C. Furthermore, in such a high temperature environment, an environment containing not only CO ₂ but also chloride ions and sulfuric acid exists. Therefore, in a conventional steel pipe as disclosed in Patent Document 1 to Patent Document 3, for example, a deep oil well environment including CO ₂ and Cl ⁻ at about 250 ° C., or CO ₂ and sulfuric acid at about 250 ° C. In high-temperature, strong-acid CO ₂ environments such as deep-depth geothermal well environments, including stable water, the desired corrosion resistance (here, particularly, carbon dioxide corrosion resistance under strong acid environments containing chloride ions or sulfuric acid) is stabilized There was a problem of not showing.

In order to solve such a problem, in Japanese Patent Application Laid-Open No. 2008-297599 (patent document 4), a stainless steel containing Cr and Cu is immersed in a high pressure CO ₂ environment to form a film having an anticorrosive function on the surface. A stainless steel material which exhibits excellent corrosion resistance even in a high temperature CO ₂ environment of 250 ° C. by being formed has been proposed.

On the other hand, a duplex stainless steel composed of a ferrite phase and an austenite phase, in which the content of Cr is further increased, is known to have sufficient corrosion resistance under high temperature CO ₂ corrosion environment as a steel material.

JP 2005-336599 A JP, 2006-016637, A JP 2007-332431 A JP 2008-297599 A

However, stainless steel has been proposed in Patent Document 4, in order to exhibit excellent corrosion resistance, it is necessary anticorrosive film formation process in the high-pressure CO ₂ environment, there is a problem that manufacturing cost becomes high.

  In addition, duplex stainless steel consisting of a ferrite phase and austenite phase can achieve the strength of 125 ksi class (862 MPa or more) which is the strength required for steel pipes used for deep oil wells and deep geothermal wells as heat treated. Can not. Therefore, when using this duplex stainless steel for deep oil wells and deep geothermal wells, it is necessary to cold work, and there is a problem that the manufacturing cost is higher than steel materials based on martensitic stainless steel. .

Therefore, carbon dioxide corrosion resistance sufficient in the above-described high temperature / strong acid CO ₂ environment of about 250 ° C., particularly carbon dioxide corrosion resistance in a strong acid environment containing chloride ions or sulfuric acid (hereinafter referred to as strong acid / carbon dioxide gas) There is a demand for a steel material which is corrosive and has a strength of 125 ksi class (862 MPa or more) as it is heat treated.

  An object of the present invention is to provide a stainless steel material having high strength as it is heat treated and further having excellent strong acid and carbon dioxide gas corrosion resistance under high temperature environment.

The stainless steel material of the present embodiment is, by mass%, C: 0.040% or less, Si: 0.05 to 1.0%, Mn: 0.010 to 0.30%, Cr: 18.0% or more 21.0% or less, Cu: 1.5 to 4.0%, Ni: 3.0 to 6.0%, sol. Al: 0.001 to 0.100%, Mo: 0 to 0.60%, W: 0 to 2.0%, Co: 0 to 0.30%, Ti: 0 to 0.10%, V: 0 ~ 0.15%, Zr: 0 to 0.10%, Nb: 0 to 0.10%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.05% , B: 0 to 0.005%, and the balance is Fe and impurities, and among impurities, P, S, O, and N respectively are P: 0.050% or less, S: less than 0.0020%, O: not more than 0.020%, and not more than N: not more than 0.020%, and the chemical composition satisfying the formulas (1) and (2), and the ferrite phase of 20.0 to 60.0% in volume ratio , An austenite phase of 1.0 to 10.0%, and a microstructure in which the balance is martensite.
15.0 ≦ 616-706 (C + N) -22 Si-24 Mn-26 Ni-18 Cr-8 Cu-16 Mo (1)
20.0 ≦ Cr + Cu ≦ 24.5 (2)
Here, content (mass%) of each element is substituted to each element symbol in Formula (1) and Formula (2).

  The stainless steel pipe of the present embodiment is made of stainless steel having the above-described chemical composition.

  The stainless steel material and stainless steel pipe of the present embodiment have high strength as they are heat treated, and further have excellent strong acid and carbon dioxide gas corrosion resistance under high temperature environment.

FIG. 1 is a view showing the relationship between the Cr content of a stainless steel material, the total content of Ni and Cu, and the structure after quenching. FIG. 2 is a diagram in which the line segment L1 in FIG. 1 is moved to the line segment L2.

  Hereinafter, the present embodiment will be described in detail with reference to the drawings.

  The present inventors examined the strength of stainless steel materials and carbon dioxide gas corrosion resistance under high temperature environments, in particular, strong acid and carbon dioxide gas corrosion resistance in a strong acid environment containing chloride ions or sulfuric acid. As a result, the following findings were obtained.

  It is known that an increase in the Cr content is effective for enhancing the carbon dioxide gas corrosion resistance under a high temperature environment. If the Cr content is increased, the strong acid / carbon dioxide gas corrosion resistance in an environment containing chloride ions or sulfuric acid can also be enhanced. However, when the content of Cr, which is a ferrite stabilizing element, is simply increased, the microstructure becomes a two-phase structure consisting of ferrite and austenite or an austenite single-phase structure. In this case, it is difficult to obtain the strength of 125 ksi class (862 MPa or more) as it is heat treated. That is, in consideration of the relationship with strong acid and carbon dioxide gas corrosion resistance, the higher the Cr content, the better, but in view of the relationship with strength, the Cr content is limited.

  Therefore, the present inventors examined the chemical composition, the structure, and the strength of the stainless steel material.

  FIG. 1 is a view showing the relationship between the Cr content of a stainless steel material, the total content of Ni and Cu, and the structure after quenching. The horizontal axis in FIG. 1 represents the Cr content. The vertical axis in FIG. 1 represents the total content of Ni and Cu. Referring to FIG. 1, as the content of Cr, which is a ferrite stabilizing element, increases, the structure after quenching changes from a martensite single phase (region M in FIG. 1) to a two phase structure of martensite and ferrite (FIG. In FIG. 1, the region M + F) changes to a ferrite single phase (region F in FIG. 1). Line segment L1 in Drawing 1 is a line which shows a boundary of a chemical composition which produces martensitic transformation by hardening. The region above the line segment L1 does not undergo martensitic transformation by quenching. That is, when the contents of Ni and Cu, which are austenite stabilizing elements, are too high, quenching does not cause martensitic transformation (that is, it becomes austenite single phase) (region A in FIG. 1). The strength is low in the dual phase structure of ferrite and austenite (area F and area A in FIG. 1) or austenite single phase (area A in FIG. 1). On the other hand, if it is a dual phase structure of martensite and ferrite (region M + F in FIG. 1), a strength of 125 ksi class (862 MPa or more) can be obtained without heat treatment.

  Conventionally, in order to obtain a two phase structure of martensite and ferrite, the Cr content has been considered to be about 18.0% at the limit. If more Cr is contained, it becomes a ferrite and / or austenite-based microstructure and strength can not be obtained. Therefore, conventionally, the Cr content is about 18.0% at the maximum within the range of the chemical composition that can obtain a two phase structure of martensite and ferrite (region Y in FIG. 1), as in Patent Document 4 It has been considered difficult to further enhance the corrosion resistance other than enhancing the corrosion resistance by forming an anticorrosive film or the like.

  However, unlike the prior art, the present inventors found a method of obtaining high strength by adjusting the chemical composition other than Cr, even when the Cr content is further increased to more than 18.0%. The present inventors paid attention to the relationship between the chemical composition (line segment L1 in FIG. 1) causing the martensitic transformation and the Cr content, which has not been studied so far.

  As a result of conducting various studies, the present inventors have found that the position of the line segment L1 can be changed by adjusting the contents of C, Si, Mn, Ni, Cr, Cu, Mo and N. Based on this finding, the present inventors adjust the content of the above-mentioned element to move the line segment L1, and marten in the chemical composition with Cr content of 18.0% (region X in FIG. 1). It has been found that two-phase structure of site and ferrite can be obtained.

  FIG. 2 is a diagram in which the line segment L1 is moved to the line segment L2 by adjusting the content of each element described above. Referring to FIG. 2, by adjusting the contents of C, Si, Mn, Ni, Cr, Cu, Mo and N described above, martensite is obtained in a chemical composition in which martensite has not been obtained so far. Be able to (In FIG. 1, area X). As a result, it is possible to realize the coexistence of a Cr content of more than 18.0% and a two-phase structure of martensite and ferrite, which have been considered to be difficult. That is, both the high strength of stainless steel and the strong acid and carbon dioxide gas corrosion resistance can be achieved.

By satisfying the equation (1) which defines the relationship of the elements that affect the position of the line segment L1 as the chemical composition, even if the Cr content is more than 18.0%, the structure sufficiently contains martensite be able to.
15.0 ≦ 616-706 (C + N) -22 Si-24 Mn-26 Ni-18 Cr-8 Cu-16 Mo (1)
Here, the content (mass%) of each element is substituted into each element symbol in the formula (1).

On the other hand, it is effective to contain Cu in addition to Cr in order to enhance the resistance to strong acid and carbon dioxide gas corrosion. Therefore, when the chemical composition satisfies the formula (2) which defines the total content of Cr and Cu, the resistance to strong acid and carbon dioxide gas corrosion containing chloride ion or sulfuric acid is enhanced.
20.0 ≦ Cr + Cu ≦ 24.5 (2)
Here, the content (mass%) of each element is substituted into each element symbol in the formula (2).

  On the other hand, many conventional oil well stainless steel materials often contain a large amount of Mo in order to improve resistance to sulfide stress cracking. However, for stainless steel materials used in oil well environments where the content of hydrogen sulfide is extremely low, Mo was a factor in cost increase. The stainless steel material of the present embodiment is intended to enhance resistance to strong acid and carbon dioxide gas corrosion, rather than resistance to sulfide stress cracking. Therefore, the Mo content may be lower than that of stainless steel intended mainly to improve the resistance to sulfide stress cracking.

  As described above, by adjusting the content of elements affecting the position of the line segment L1 according to the formula (1), it is possible to achieve a balance between Cr content of more than 18.0% and a structure sufficiently containing martensite. Become. Thereby, it becomes possible to reconcile the strength of 125 ksi class (862 MPa or more) and the resistance to strong acid and carbon dioxide gas corrosion in a high temperature environment exceeding 250 ° C. only by heat treatment. Further, by including Cu in addition to Cr according to the formula (2), the resistance to strong acid and carbon dioxide gas corrosion is enhanced. Therefore, it is possible to provide a stainless steel material suitable for deep oil wells or deep geothermal wells.

The stainless steel material of the present embodiment completed based on the above findings is, by mass%, C: 0.040% or less, Si: 0.05 to 1.0%, Mn: 0.010 to 0.30%, Cr: more than 18.0% and not more than 21.0%, Cu: 1.5 to 4.0%, Ni: 3.0 to 6.0%, sol. Al: 0.001 to 0.100%, Mo: 0 to 0.60%, W: 0 to 2.0%, Co: 0 to 0.30%, Ti: 0 to 0.10%, V: 0 ~ 0.15%, Zr: 0 to 0.10%, Nb: 0 to 0.10%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.05% , B: 0 to 0.005%, and the balance is Fe and impurities, and among impurities, P, S, O, and N respectively are P: 0.050% or less, S: less than 0.0020%, O: not more than 0.020%, and not more than N: not more than 0.020%, and the chemical composition satisfying the formulas (1) and (2), and the ferrite phase of 20.0 to 60.0% in volume ratio , An austenite phase of 1.0 to 10.0%, and a microstructure in which the balance is martensite.
15.0 ≦ 616-706 (C + N) -22 Si-24 Mn-26 Ni-18 Cr-8 Cu-16 Mo (1)
20.0 ≦ Cr + Cu ≦ 24.5 (2)
Here, content (mass%) of each element is substituted to each element symbol in Formula (1) and Formula (2).

  The stainless steel material of the present embodiment has high strength as it is heat treated, and further has excellent strong acid / carbon dioxide gas corrosion resistance under high temperature environment.

  Preferably, the chemical composition is selected from the group consisting of Mo: 0.02 to 0.60%, W: 0.01 to 2.0%, and Co: 0.01 to 0.30%. Contains a species or two or more species.

  In this case, the corrosion resistance of the stainless steel material is further enhanced.

  Preferably, the chemical composition is Ti: 0.005 to 0.10%, V: 0.005 to 0.15%, Zr: 0.005 to 0.10%, and Nb: 0.005 to 0 .1 or 2 or more selected from the group consisting of 10%.

  In this case, the strength of the stainless steel material is further enhanced.

  Preferably, the chemical composition is such that Ca: 0.0005 to 0.010%, Mg: 0.0005 to 0.010%, REM: 0.0005 to 0.05%, and B: 0.0005% to It contains one or more selected from the group consisting of 0.005%.

  In this case, the hot workability of the stainless steel material is enhanced.

  The stainless steel pipe of the present embodiment is made of a stainless steel material having any of the above chemical compositions.

  Hereinafter, the stainless steel material of the present embodiment will be described in detail.

[Chemical composition]
The chemical composition of the stainless steel material of the present embodiment contains the following elements. Unless otherwise stated,% with respect to an element means mass%.

C: 0.040% or less Carbon (C) is unavoidably contained. C precipitates as carbides during tempering. In this case, the carbon dioxide gas corrosion resistance of the stainless steel material at high temperatures is reduced. Furthermore, if the C content is too high, the amount of retained austenite produced during quenching increases. In this case, in order to reduce the formation amount of retained austenite, the Cu and Ni content effective for strength and toughness must be reduced. Therefore, the C content is 0.040% or less. The upper limit of the C content is preferably 0.030%, more preferably 0.020%, still more preferably 0.015%, and most preferably 0.010%. The lower the C content, the better. However, in consideration of the cost of decarburizing treatment in the steel making process, the lower limit of the C content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.004%. And most preferably 0.005%.

Si: 0.05 to 1.0%
Silicon (Si) deoxidizes stainless steel. If the Si content is too low, this effect can not be obtained. On the other hand, if the Si content is too high, the hot workability of the stainless steel material is reduced. Therefore, the Si content is 0.05 to 1.0%. The upper limit of the Si content is preferably 0.9%, more preferably 0.7%, still more preferably 0.6%, and most preferably 0.5%. The lower limit of the Si content is preferably 0.06%, more preferably 0.08%, still more preferably 0.10%, and most preferably 0.12%.

Mn: 0.010 to 0.30%
Manganese (Mn) deoxidizes stainless steel. If the Mn content is too low, this effect can not be obtained. On the other hand, if the Mn content is too high, austenite tends to remain excessively after quenching and tempering, and the volume fraction of retained austenite may exceed 10%. In this case, the strength of the stainless steel material after tempering is reduced. Therefore, the Mn content is 0.010 to 0.30%. The upper limit of the Mn content is preferably 0.27%, more preferably 0.25%, still more preferably 0.22%, and most preferably less than 0.20%. The lower limit of the Mn content is preferably 0.015%, more preferably 0.030%, still more preferably 0.035%, and most preferably 0.050%.

Cr: more than 18.0% and 21.0% or less Chromium (Cr) is essential for securing strong acid and carbon dioxide corrosion resistance in an environment containing CO ₂ and chloride ion or sulfuric acid at about 250 ° C. It is an element. If the Cr content is too low, this effect can not be obtained. On the other hand, since Cr is a ferrite forming element, if the content of Cr is too high, the amount of ferrite in the stainless steel material becomes excessively large, and the strength of the stainless steel material is lowered. Therefore, the Cr content is more than 18.0% and not more than 21.0%. The upper limit of the Cr content is preferably 20.8%, more preferably 20.5%, still more preferably 20.3%, and most preferably 20.2%. The lower limit of the Cr content is preferably 18.3%, more preferably 18.5%, still more preferably 18.7%, and most preferably 18.8%.

Cu: 1.5 to 4.0%
Copper (Cu) precipitates as fine Cu particles during tempering to enhance the strength of the stainless steel material. Furthermore, Cu improves the resistance to strong acid and carbon dioxide gas corrosion in high temperature environments such as containing chloride ions or sulfuric acid. These effects can not be obtained if the Cu content is too low. On the other hand, if the Cu content is too high, martensitic transformation does not proceed sufficiently during quenching, and the amount of retained austenite becomes excessively large. In this case, the strength of the stainless steel material is reduced. Therefore, the Cu content is 1.5 to 4.0%. The upper limit of the Cu content is preferably 3.8%, more preferably 3.5%, still more preferably 3.2%, and most preferably 3.0%. The lower limit of the Cu content is preferably 1.6%, more preferably 1.7%, still more preferably 1.8%, and most preferably 1.9%.

Ni: 3.0 to 6.0%
Nickel (Ni) is an austenite-forming element and stabilizes austenite at high temperatures. Therefore, Ni increases the amount of martensite at normal temperature and enhances the strength of the stainless steel material. Ni further enhances strong acid and carbon dioxide corrosion resistance in high temperature environments such as those containing chloride ions or sulfuric acid. If the amount of Ni is too low, these effects can not be obtained. On the other hand, if the Ni content is too high, stable ferrite and martensite dual phase structures can not be obtained. That is, austenite tends to remain excessively after quenching and tempering, and the strength of the stainless steel material after tempering is reduced. Therefore, the Ni content is 3.0 to 6.0%. The upper limit of the Ni content is preferably 5.8%, more preferably 5.5%, still more preferably 5.2%, and most preferably 5.0%. The lower limit of the Ni content is preferably 3.2%, more preferably 3.4%, still more preferably 3.6%, and most preferably 3.8%.

sol. Al: 0.001 to 0.100%
Aluminum (Al) deoxidizes the stainless steel material. sol. If the Al content is too low, this effect can not be obtained. Meanwhile, sol. If the Al content is too high, the effect is saturated. sol. If the Al content is too high, inclusions are further formed in excess, and the resistance to strong acid / carbon dioxide gas corrosion and toughness in a high temperature environment containing chloride ions or sulfuric acid decreases. Therefore, sol. The Al content is 0.001 to 0.100%. sol. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%, still more preferably 0.055%, and most preferably 0.050%. sol. The lower limit of the Al content is preferably 0.005%, more preferably 0.008%, still more preferably 0.010%, and most preferably 0.015%.

  The balance of the chemical composition of the stainless steel material according to the present embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are mixed from ore as a raw material, scrap, or manufacturing environment when industrially manufacturing stainless steel materials, and the stainless steel materials of the present embodiment are adversely affected. It means what is permitted without giving it.

[About any element]
The stainless steel material of the present embodiment may contain the following optional elements.

Mo: 0 to 0.60%
Molybdenum (Mo) is an optional element and may not be contained. Mo improves the resistance to strong acid and carbon dioxide in a high temperature environment containing chloride ions or sulfuric acid. The above effect can be obtained if Mo is contained in any amount. On the other hand, since Mo is a ferrite forming element, if the content of Mo is too high, ferrite is excessively formed in the stainless steel material. In this case, the strength of the stainless steel material is reduced. Therefore, the Mo content is 0 to 0.60%. The upper limit of the Mo content is preferably 0.58%, more preferably 0.56%, still more preferably 0.54%, and most preferably 0.52%. The lower limit of the Mo content is preferably 0.02%, more preferably 0.04%, still more preferably 0.05%, and most preferably 0.06%.

W: 0 to 2.0%
Tungsten (W) is an optional element and may not be contained. W, like Mo, increases resistance to strong acid and carbon dioxide gas corrosion in high temperature environments containing chloride ions or sulfuric acid. The above effect can be obtained as long as W is contained. On the other hand, since W is a ferrite forming element, if the W content is too high, ferrite is excessively formed in the stainless steel material, and the strength of the stainless steel material is lowered. Therefore, the W content is 0 to 2.0%. The upper limit of the W content is preferably 1.8%, more preferably 1.5%, still more preferably 1.3%, and most preferably 1.0%. The lower limit of the W content is preferably 0.01%, more preferably 0.02%.

Co: 0 to 0.30%
Cobalt (Co) is an optional element and may not be contained. Co, like Mo and W, increases the resistance to strong acid and carbon dioxide gas corrosion in high temperature environments containing chloride ions or sulfuric acid. However, if the Co content is too high, the manufacturability is lowered and the manufacturing cost is also increased. Therefore, the Co content is 0 to 0.30%. The upper limit of the Co content is preferably 0.25%, more preferably 0.23%, still more preferably 0.20%, and most preferably 0.18%. The lower limit of the Co content is preferably 0.01%, more preferably 0.02%.

Ti: 0 to 0.10%
Titanium (Ti) is an optional element and may not be contained. When it is contained, Ti precipitates as fine precipitates during tempering to increase the strength of the stainless steel material. The above effects can be obtained as long as Ti is contained. On the other hand, if the Ti content is too high, the resistance to strong acid and carbon dioxide gas corrosion and toughness in a high temperature environment containing chloride ions or sulfuric acid decreases. Therefore, the Ti content is 0 to 0.10%. The upper limit of the Ti content is preferably 0.08%, more preferably 0.05%. The lower limit of the Ti content is preferably 0.005%, more preferably 0.010%.

V: 0 to 0.15%
Vanadium (V) is an optional element and may not be contained. When it is contained, V precipitates as fine precipitates during tempering to increase the strength of the stainless steel material. The above effect can be obtained as long as V is contained. On the other hand, if the V content is too high, the resistance to strong acid and carbon dioxide gas corrosion and toughness in a high temperature environment containing chloride ions or sulfuric acid decreases. Therefore, the V content is 0 to 0.15%. The upper limit of the V content is preferably 0.13%, more preferably 0.12%. The lower limit of the V content is preferably 0.005%, more preferably 0.010%, and still more preferably 0.020%.

Zr: 0 to 0.10%
Zirconium (Zr) is an optional element and may not be contained. When it is contained, similarly to Ti and V, Zr precipitates as fine precipitates during tempering and enhances the strength of the stainless steel material. The above effect can be obtained if any of Zr is contained. On the other hand, if the Zr content is too high, the toughness of the stainless steel material decreases. Therefore, the Zr content is 0 to 0.10%. The upper limit of the Zr content is preferably 0.08%, more preferably 0.05%. The lower limit of the Zr content is preferably 0.005%, more preferably 0.010%.

Nb: 0 to 0.10%
Niobium (Nb) is an optional element and may not be contained. When it is contained, Nb precipitates as fine precipitates during tempering to increase the strength of the stainless steel material. The above effect can be obtained if Nb is contained even in small amounts. On the other hand, if the Nb content is too high, resistance to strong acid and carbon dioxide gas corrosion and toughness in a high temperature environment containing chloride ions or sulfuric acid decreases. Therefore, the Nb content is 0 to 0.10%. The upper limit of the Nb content is preferably 0.08%, more preferably 0.05%. The lower limit of the Nb content is preferably 0.005%, more preferably 0.010%.

Ca: 0 to 0.010%
Calcium (Ca) is an optional element and may not be contained. When it is contained, Ca improves the hot workability of the stainless steel material. The above effect can be obtained if any amount of Ca is contained. On the other hand, if the Ca content is too high, inclusions will be generated in excess. In this case, the toughness of the stainless steel and the strong acid corrosion resistance in a high temperature environment containing sulfuric acid are reduced. Therefore, the Ca content is 0 to 0.010%. The upper limit of the Ca content is preferably 0.008%, more preferably 0.005%. The lower limit of the Ca content is preferably 0.0005%.

Mg: 0 to 0.010%
Magnesium (Mg) is an optional element and may not be contained. When it is contained, Mg enhances the hot workability of the stainless steel material. The above effects can be obtained as long as Mg is contained. On the other hand, if the Mg content is too high, inclusions are generated in excess. In this case, the toughness of the stainless steel and the strong acid corrosion resistance in a high temperature environment containing sulfuric acid are reduced. Therefore, the Mg content is 0 to 0.010%. The upper limit of the Mg content is preferably 0.008%, more preferably 0.005%. The lower limit of the Mg content is preferably 0.0005%.

REM: 0 to 0.05%
The rare earth element (REM) is an optional element and may not be contained. REM, like Ca and Mg, enhances the hot workability of stainless steel materials. On the other hand, if the REM content is too high, inclusions of oxides and sulfides are excessively formed, and the toughness of the stainless steel material and the strong acid corrosion resistance in a high temperature environment containing sulfuric acid are lowered. Therefore, the REM content is 0 to 0.05%. The upper limit of the REM content is preferably 0.03%, more preferably 0.02%. The lower limit of the REM content is preferably 0.0005%. Here, REM means 17 elements obtained by adding yttrium (Y) and scandium (Sc) to elements from lanthanum (La) of element number 57 to lutetium (Lu) of element number 71 in the periodic table. The REM content means the total content of these elements.

B: 0 to 0.005%
Boron (B) is an optional element and may not be contained. B is added to enhance the hot workability of the stainless steel like Ca, Mg and REM. On the other hand, if the B content is too high, Cr carbides precipitate at grain boundaries and the toughness of the stainless steel decreases. Therefore, the B content is 0 to 0.005%. The upper limit of the B content is preferably 0.004%, more preferably 0.003%. The lower limit of the B content is preferably 0.0005%.

[About impurity elements]
The following elements may be contained as impurities in the chemical composition of the stainless steel material of the present embodiment. The content of these elements is limited for the following reasons.

P: 0.050% or less Phosphorus (P) is an impurity. P segregates at grain boundaries to reduce the strong acid and carbon dioxide gas corrosion resistance of stainless steel materials. Therefore, the P content is 0.050% or less. The upper limit of the P content is preferably 0.045%, more preferably 0.040%, still more preferably 0.035%, and most preferably 0.030%. The P content is preferably as low as possible. The lower limit of the P content is not particularly limited, and is, for example, 0.001%.

S: less than 0.0020% Sulfur (S) is an impurity. S segregates at grain boundaries to reduce the strong acid and carbon dioxide gas corrosion resistance of stainless steel materials. S further reduces the hot workability of the stainless steel material. Therefore, the S content is less than 0.0020%. The upper limit of the S content is preferably 0.0017%, more preferably 0.0015%. The S content is preferably as low as possible. The lower limit of the S content is not particularly limited, and is, for example, 0.0003%.

O: 0.020% or less Oxygen (O) is an impurity. O forms a coarse oxide to lower the toughness of stainless steel and the resistance to strong acid corrosion in a high temperature environment containing sulfuric acid. Therefore, the O content is 0.020% or less. The upper limit of the O content is preferably 0.015%, more preferably 0.010%. The O content is preferably as low as possible. The lower limit of the O content is not particularly limited, and is, for example, 0.001%.

N: 0.020% or less Nitrogen (N) is an impurity. N forms a coarse nitride. Coarse nitrides become a starting point of pitting corrosion and lower the strong acid corrosion resistance in a high temperature environment such as that containing sulfuric acid of stainless steel. Therefore, the N content is 0.020% or less. The upper limit of the N content is preferably 0.018%, more preferably 0.015%. It is preferable that the N content be as low as possible. The lower limit of the N content is not particularly limited, and is, for example, 0.001%.

[About the formula]
The chemical composition of the stainless steel material of the present embodiment has an appropriate content of each element described above, and satisfies the following formulas (1) and (2). This makes it possible to simultaneously achieve high strength only by heat treatment and excellent strong acid / carbon dioxide gas corrosion resistance under a high temperature environment.

[About formula (1)]
The chemical composition satisfies the formula (1).
15.0 ≦ 616-706 (C + N) -22 Si-24 Mn-26 Ni-18 Cr-8 Cu-16 Mo (1)
Here, the content (mass%) of each element is substituted into each element symbol in the formula (1).

  It defines as Fn1 = 616-706 (C + N) -22Si-24Mn-26Ni-18Cr-8Cu-16Mo. Fn1 is an index indicating the chemical composition that can obtain martensite. If the content of each of the above elements is appropriate and Fn1 is appropriate, a two-phase structure of martensite and ferrite described later can be obtained even if the Cr content exceeds 18.0%. This structure has a sufficiently high volume fraction of martensite phase (hereinafter referred to as martensite fraction). This increases the strength of the stainless steel material. That is, only by adjusting Fn1, it is possible to achieve both strong acid and carbon dioxide gas corrosion resistance in a high temperature environment containing chloride ions or sulfuric acid and strength of 125 ksi class (862 MPa or more) only by heat treatment. If Fn1 is less than 15.0, the martensite fraction decreases and the strength of the stainless steel material decreases. Therefore, 15.0 ≦ Fn1. The lower limit of Fn1 is preferably 40.0, more preferably 50.0, still more preferably 60.0, and most preferably 70.0. The upper limit of Fn1 is not particularly limited, but is preferably 300, more preferably 250, still more preferably 200, and most preferably 150, in consideration of the production cost.

[About formula (2)]
The chemical composition satisfies the formula (2).
20.0 ≦ Cr + Cu ≦ 24.5 (2)
Here, the content (mass%) of each element is substituted into each element symbol in the formula (2).

  It defines as Fn2 = Cr + Cu. Fn2 is an index showing strong acid and carbon dioxide gas corrosion resistance in a high temperature environment containing chloride ion or sulfuric acid. If the content of each of the above elements is appropriate and if Fn 2 is appropriate, strong acid and carbon dioxide gas corrosion resistance is enhanced. If Fn2 is less than 20.0, the strong acid / carbon dioxide gas corrosion resistance of the stainless steel material is lowered. On the other hand, if Fn2 exceeds 24.5, the ferrite fraction or the retained austenite fraction becomes excessively high, and the strength of the stainless steel material decreases. Therefore, 20.0 ≦ Fn2 ≦ 24.5. The upper limit of Fn2 is preferably 24.2, more preferably 24.0, still more preferably 23.8, and most preferably 23.5. The lower limit of Fn2 is preferably 20.3.

[Formula (3)]
Preferably, the chemical composition further satisfies the formula (3).
0.35 ≦ Cu / Ni ≦ 1.0 (3)
Here, the content (mass%) of each element is substituted for each element symbol in the formula (3).

  It defines as Fn3 = Cu / Ni. Fn3 is an index showing the strength and hot workability of stainless steel materials. If the content of each element is appropriate and Fn 3 is appropriate, the strength and hot workability of the stainless steel material are further enhanced. If Fn3 is 1.0 or less, the ratio of the Ni content to the Cu content is sufficiently high, and the hot workability of the stainless steel material is enhanced. On the other hand, if Fn3 is 0.35 or more, the ratio of the Cu content to the Ni content is sufficiently high, the effect of precipitation strengthening of Cu at tempering is enhanced, and the strength of the stainless steel material is further enhanced. Therefore, it is preferable that 0.35 ≦ Fn3 ≦ 1.0. The upper limit of Fn3 is more preferably 0.9, still more preferably 0.8, and most preferably 0.7. The lower limit of Fn3 is more preferably 0.40, still more preferably 0.45, and most preferably 0.50.

[About Microstructure]
The stainless steel material of the present embodiment is a two-phase stainless steel material of ferrite and martensite. Specifically, the microstructure of the stainless steel material contains 20.0 to 60.0% of a ferrite phase and 1.0 to 10.0% of an austenite phase in volume ratio, and the balance is made of martensite.

  The ferrite phase in the microstructure enhances the strong acid and carbon dioxide corrosion resistance in high temperature environments such as those containing chloride ions or sulfuric acid. If the volume fraction of the ferrite phase (hereinafter referred to as ferrite fraction) is too low, the above effect can not be obtained. On the other hand, if the ferrite fraction is too high, the strength and toughness of the stainless steel material decrease. Therefore, the ferrite fraction is 20.0 to 60.0%. The upper limit of the ferrite fraction is preferably 58.0%, more preferably 55.0%, still more preferably 52.0%. The lower limit of the ferrite fraction is preferably 20.5%, more preferably 21.0%.

  The retained austenite after tempering improves the toughness of the stainless steel material. If the volume fraction of retained austenite (the volume fraction of the austenite phase, hereinafter also referred to as the retained austenite fraction) is too low, the above effect can not be obtained. On the other hand, if the retained austenite fraction is too high, the yield strength of the stainless steel material decreases, and a yield strength of 862 MPa or more can not be stably obtained. Therefore, the retained austenite fraction is 1.0 to 10.0%. The upper limit of the retained austenite fraction is preferably 9.5%, more preferably 9.0%. The lower limit of the retained austenite fraction is preferably 1.5%, more preferably 2.0%.

[Method of measuring volume fraction of each phase in microstructure]
The ferrite fraction (volume%), the retained austenite fraction (volume%) and the martensite fraction (volume%) in the microstructure of the stainless steel material in the present embodiment are measured by the following method.

[Measuring method of ferrite fraction]
First, a specimen for microstructure observation is collected from a stainless steel material. If the stainless steel material is a steel plate, a cross section (hereinafter referred to as an observation surface) perpendicular to the plate width direction of the steel plate is polished among the surfaces of the test pieces. If the stainless steel material is a steel pipe, a bar steel or a wire rod, a cross section (observation surface) perpendicular to the axial direction of the stainless steel material is polished among the surfaces of the test pieces. Next, the observation surface after polishing is etched using a mixed solution of aqua regia and glycerin. The ferrite is identified in the etched observation surface. And the area ratio of the specified ferrite is measured by the point calculation method based on JISG0555 (2003). The area fraction measured is defined as the ferrite fraction (volume%), assuming that it is equal to the volume fraction.

[Method of measuring retained austenite fraction]
The retained austenite fraction is determined using X-ray diffraction. First, a test piece of 15 mm × 15 mm × 2 mm is collected from a stainless steel material. Next, using the collected test pieces, each of the (200) plane and the (211) plane of the ferrite (α phase), the (200) plane, the (220) plane and the (311) plane of the austenite (γ phase) The X-ray diffraction profile of is measured, and the integrated intensity of each surface is calculated. After calculation, the residual austenite fraction Vγ is determined using the following equation for each combination (total 6 sets) of each face of the α phase and each face of the γ phase.
Vγ = 100 / (1+ (Iα × Rγ) / (Iγ × Rα))
Here, “Iα” in the formula is the integrated intensity of the α phase, and “Iγ” is the integrated intensity of the γ phase. “Rα” is a crystallographic theoretical calculation value of the α phase, and “Rγ” is a crystallographic theoretical calculation value of the γ phase. An average value of the volume fraction Vγ of each surface is defined as a retained austenite fraction (volume%).

[Measuring method of martensite fraction]
Of the microstructure of the stainless steel material of the present embodiment, the balance other than ferrite and retained austenite is made of martensite. The martensite fraction is determined by the following equation.
Martensite fraction = 100-(ferrite fraction + retained austenite fraction)

[Stainless steel]
The shape of the stainless steel material of the present embodiment is not particularly limited. The stainless steel material may be, for example, a steel pipe, a steel plate, a steel bar, or a wire rod.

[Production method]
A method of producing a seamless steel pipe will be described as an example of the method of producing a stainless steel material of the present embodiment. A material having a chemical composition that satisfies the content of each of the above-described elements and the formulas (1) and (2) is prepared. The material may be a slab produced by a continuous casting method (including round CC) or a steel slab produced from a slab. Moreover, it may be a billet manufactured by hot working an ingot manufactured by the ingot method.

  The prepared material is charged into a heating furnace or a soaking furnace and heated to, for example, 1100 to 1300 ° C. Subsequently, the heated material is hot-worked to produce a raw pipe. For example, the Mannesmann method is implemented as hot working. Specifically, the material is pierced and rolled by a piercing machine to form a raw pipe. Subsequently, the blank is further rolled by a mandrel mill or a sizing mill. Hot extrusion may be performed as hot working, or hot forging may be performed. The temperature for hot working is, for example, 1000 to 1300 ° C.

  The core pipe after hot working is quenched and tempered to adjust the strength so that the yield strength is 862 MPa or more (125 ksi class). The quenching temperature is 800 to 1050 ° C. The preferred tempering temperature is 650 ° C. or less.

  The stainless steel material manufactured by the above steps has high strength as it is heat treated, and preferably has a yield strength of 862 MPa or more. Stainless steel materials are further excellent in strong acid and carbon dioxide gas corrosion resistance in a high temperature environment containing chloride ions or sulfuric acid.

  In the method of manufacturing stainless steel, the method of manufacturing a seamless steel pipe has been described as an example. However, the method of manufacturing the stainless steel material of the present embodiment is not limited to the above manufacturing method. When the stainless steel material is not a seamless steel pipe, the stainless steel material of the present embodiment can be manufactured by hot working, quenching and tempering treatment in the same temperature range as the above-mentioned manufacturing method. In addition, the stainless steel material of the present embodiment is not cold-worked, and high strength can be obtained even with heat treatment. However, cold working, heat treatment or the like may be performed as necessary.

  A molten steel having the chemical composition shown in Table 1 was melted in a vacuum high frequency melting furnace to produce a 50 kg ingot. Among the chemical compositions shown in Table 1, Test Nos. 1 to 20 are inventive examples, and Test Nos. 21 to 26 are comparative examples outside the scope of the present invention.

  Each ingot was heated at 1200 ° C. for 3 hours. The heated ingot was subjected to hot forging to obtain a steel material having a thickness of 45 mm and a width of 60 mm. The steel was heated at 1230 ° C. for 1 hour. The steel material after heating was hot-rolled to produce a steel plate having a thickness of 13 mm. Quenching and tempering was performed on the steel sheet after hot rolling. In quenching, the steel plate was heated at 950 ° C. for 15 minutes and then water quenched. The steel sheet after quenching was tempered at 550 ° C. for 30 minutes, and then air-cooled.

A microstructure observation test, a tensile test, and a corrosion test in a high temperature strong acid CO ₂ environment were performed using the steel plate of each test number after quenching and tempering.

[Microstructure observation test (measurement of volume fraction of each phase)]
The test piece for microstructure observation was extract | collected from the steel plate of each test number. Among the surfaces of the collected test pieces, a cross section (viewing surface) perpendicular to the plate width direction of the steel plate was polished. The observation surface after polishing was etched using a mixture of aqua regia and glycerin. The ferrite fraction (volume%) was determined by the above-mentioned measurement method using the etched observation surface. The results are shown in Table 2.

  A test piece of 15 mm × 15 mm × 2 mm was collected from the steel plate of each test number. The retained austenite fraction (volume%) was determined by the measurement method described above using the collected test pieces. The results are shown in Table 2.

  Based on the obtained ferrite fraction and retained austenite fraction, the martensite fraction (volume%) was determined by the above method. The results are shown in Table 2.

[Tension test]
Round bar tensile test specimens were collected from the center of thickness of the steel plate of each test number. The longitudinal direction of the round bar tensile test piece was a direction (L direction) parallel to the rolling direction of the steel plate. The diameter of the parallel part of the round bar tensile test piece was 6 mm, and the distance between marks was 40 mm. The tensile test was carried out at room temperature on the collected round bar tensile test pieces to determine the yield strength (0.2% proof stress). The results are shown in Table 2.

[Corrosion test in high temperature strong acid CO ₂ environment]
First, corrosion test specimens were taken from the steel plate of each test number. The test piece had a length of 40 mm, a width of 10 mm, and a thickness of 3 mm, and had a hole with a diameter of 3 mm for hanging on a jig. Next, the weight of this test piece was measured. Subsequently, the test piece was immersed in the test solution shown in Test Condition A or Test Condition B, and the test piece was housed in an autoclave while being immersed in the test solution. The inside of the autoclave was adjusted to the test gas atmosphere and the test temperature shown in Test Condition A or Test Condition B, respectively. The test time was 336 hours.

Test condition A
Test solution: 25 mass% NaCl aqueous solution Test gas: 30 bar CO ₂
Test temperature: 250 ° C

Test condition B
Test solution: 0.01 mol / L H ₂ SO ₄ aqueous solution Test gas: 30 bar CO ₂
Test temperature: 250 ° C

  The weight of the test piece after the test time was measured, and the corrosion loss of each test piece was determined based on the amount of change in weight before and after the test. The annual amount of corrosion (mm / y) of the steel plate of each test number was calculated from the obtained corrosion loss. The results are shown in Table 2.

  Furthermore, under corrosion condition A, with respect to each test piece after the test, the presence or absence of pitting corrosion on the surface of the test piece was confirmed using a loupe with a magnification of 10 times. The results are shown in Table 2.

[Evaluation results]
Under test condition A, when the annual corrosion amount was less than 0.050 (mm / y), it was determined that strong acid and carbon dioxide gas corrosion resistance was good. Under test condition B, when the annual corrosion amount was less than 0.100 (mm / y), it was determined that strong acid and carbon dioxide gas corrosion resistance was good.

With reference to Table 1 and Table 2, the steel plates of Test Nos. 1 to 20 had chemical compositions in which the content of each element was appropriate, and the formulas (1) and (2) were satisfied. Therefore, the steel plates of test numbers 1 to 20 had high strengths with a yield strength of 862 MPa or more. Further, the steel plates of test numbers 1 to 20 had a low corrosion rate and no pitting corrosion under any of the test conditions A and the test conditions B assuming the environments of deep oil wells and deep geothermal wells. Therefore, the steel plates of test numbers 1 to 20 have high strength as they are heat-treated, and further have excellent acid and carbon dioxide corrosion resistance in an environment containing CO ₂ and chloride ions or sulfuric acid at 250 ° C. Indicated.

  Furthermore, the steel plate of the test numbers 1 to 19 having the chemical composition satisfying the equation (3) is compared with the steel plate of the test number 20 having the chemical composition (of Fn3 = 0.32) not satisfying the equation (3), The yield strength (YS) was even higher.

  On the other hand, in Test No. 21, the Cr content was too low. In Test No. 21, Fn2 was as low as 18.9, and did not satisfy Formula (2). Therefore, in the corrosion test, the annual corrosion amount of the test condition A was 0.120 mm / y in the corrosion test, and pitting corrosion further occurred. Furthermore, the annual corrosion amount of test condition B became 0.280 mm / y of the steel plate of test number 21. In Test No. 21, resistance to strong acid and carbon dioxide gas was inferior.

  In Test No. 22, the Cu content was too low. In Test No. 22, Fn2 was as low as 19.9, and did not satisfy Formula (2). Therefore, the steel plate of test number 22 had a low yield strength of 351.3 MPa. Further, in the corrosion test, the steel plate of Test No. 22 had an annual corrosion amount of 0.160 mm / y under the test condition B, and the strong acid and carbon dioxide gas corrosion resistance was inferior.

  In Test No. 23, the Ni content was too high. In Test No. 23, Fn1 was as low as 14.3 and did not satisfy Formula (1). Therefore, the residual austenite fraction of the steel plate of Test No. 23 was as high as 32.5% by volume, and the yield strength was as low as 405.4 MPa.

  Although the chemical composition of the steel plate of the test number 24 had Fn1 as low as 7.5, although content of each element was appropriate, it did not satisfy Formula (1). Therefore, the steel plate of test No. 24 had a high retained austenite fraction of 47.2% by volume and a low yield strength of 386.7 MPa.

  Although the content of each element was appropriate, the chemical composition of the steel plate of the test number 25 had Fn2 as high as 24.7, and did not satisfy Formula (2). Therefore, the steel plate of test No. 25 had a high retained austenite fraction of 11.7% by volume and a low yield strength of 537.9 MPa.

  Although the content of each element was appropriate, the chemical composition of the steel plate of the test number 26 had Fn2 as low as 19.8, and did not satisfy Formula (2). Therefore, in the corrosion test, the annual corrosion amount of the test condition A was 0.142 mm / y in the corrosion test, and pitting corrosion further occurred. Furthermore, the annual corrosion amount of test condition B became 0.268 mm / y of the steel plate of test number 26. In Test No. 26, resistance to strong acid and carbon dioxide gas was inferior.

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

M Martensite phase M + F Dual phase structure of martensite phase and ferrite phase F ferrite phase A austenite phase

Claims (5)

In mass%,
C: 0.040% or less,
Si: 0.05 to 1.0%,
Mn: 0.010 to 0.30%,
Cr: over 18.0% and under 21.0%,
Cu: 1.5 to 4.0%,
Ni: 3.0 to 6.0%,
sol. Al: 0.001 to 0.100%,
Mo: 0 to 0.60%,
W: 0 to 2.0%,
Co: 0 to 0.30%,
Ti: 0 to 0.10%,
V: 0 to 0.15%,
Zr: 0 to 0.10%,
Nb: 0 to 0.10%,
Ca: 0 to 0.010%
Mg: 0 to 0.010%
REM: 0 to 0.05%,
B: 0 to 0.005%, and
The balance consists of Fe and impurities,
Among the impurities, P, S, O and N are each
P: 0.050% or less,
S: less than 0.0020%,
O: 0.020% or less, and
N: 0.020% or less,
A chemical composition satisfying the formula (1) and the formula (2),
In volume ratio,
A stainless steel material having 20.0 to 60.0% of a ferrite phase, 1.0 to 10.0% of an austenite phase, and a microstructure having a balance of martensite.
15.0 ≦ 616-706 (C + N) -22 Si-24 Mn-26 Ni-18 Cr-8 Cu-16 Mo (1)
20.0 ≦ Cr + Cu ≦ 24.5 (2)
Here, content (mass%) of each element is substituted to each element symbol in Formula (1) and Formula (2). The stainless steel material according to claim 1, wherein
The chemical composition is
Mo: 0.02 to 0.60%,
W: 0.01 to 2.0%, and
Co: 0.01 to 0.30%
A stainless steel material containing one or more selected from the group consisting of It is the stainless steel material according to claim 1 or 2,
The chemical composition is
Ti: 0.005 to 0.10%,
V: 0.005 to 0.15%,
Zr: 0.005 to 0.10%, and
Nb: 0.005 to 0.10%
A stainless steel material containing one or more selected from the group consisting of The stainless steel material according to any one of claims 1 to 3,
Ca: 0.0005 to 0.010%,
Mg: 0.0005 to 0.010%,
REM: 0.0005 to 0.05%, and
B: 0.0005% to 0.005%
A stainless steel material containing one or more selected from the group consisting of   The stainless steel pipe which consists of a stainless steel material of any one of Claims 1-4.

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