Non-magnetic austenitic stainless steel material and production method therefor

Abstract

To provide a non-magnetic austenitic stainless steel material having excellent strength and corrosion resistance, suitable for machine members to be used in a corrosive environment, and a production method therefor.SOLUTION: The present invention relates to a non-magnetic austenitic stainless steel material having a predetermined component composition, satisfying (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≤60, in which the content of an element M is represented by [M]% in terms of mass%, consisting of an austenite single phase structure, and having a critical pitting temperature of 50°C or higher, and a 0.2% proof stress of 970 MPa or more at a position at a depth of 1 inch from the surface (or, at a depth of T / 4 or D / 4 from the surface if the thickness T or the diameter D is less than 4 inches). A method for producing the steel material includes subjecting a steel ingot to hot working and then cooling treatment, and performing warm working at an area reduction rate of 15% to 50% in a temperature range of 800°C to 300°C during the cooling treatment.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a non-magnetic austenitic stainless steel material having excellent strength and corrosion resistance, and a method for producing the same. [Background technology]

[0002] Austenitic stainless steel materials, such as SUS304, are used for parts that require strength and corrosion resistance.

[0003] For example, Patent Document 1 describes that high-strength, highly corrosion-resistant stainless steel materials with large amounts of nitrogen added contain large amounts of expensive alloying elements such as Cr, Ni, and Mo, and are subjected to solution heat treatment at a high temperature of nearly 1200°C to dissolve nitrides that adversely affect cold workability, resulting in very high costs. Patent Document 1 then discloses an austenitic stainless steel material with excellent corrosion resistance whose strength has been increased by cold working. While cold working generally reduces corrosion resistance, the patent document states that Cu is added to the chemical composition to improve cold workability, and one or more of Nb, V, and W are added to improve strength, thereby improving both strength and corrosion resistance.

[0004] On the other hand, austenitic stainless steel is used for the steel used in accessories such as drill collars for drills used in offshore oil fields, as it is strong, corrosion-resistant, and non-magnetic so as not to affect the magnetic position control of the drill bit.

[0005] For example, Patent Document 2 discloses a method for manufacturing a forged product including a drill collar made of non-magnetic austenitic stainless steel. Since the steel used here has a chemical composition that makes it easy for precipitates such as carbides and nitrides to precipitate in the temperature range of 740 to 760°C, the steel ingot is warm worked at a surface temperature of 650 to 500°C to suppress the precipitation of precipitates such as carbides and nitrides at the grain boundaries and provide sufficient C and N in the austenite crystal grains, thereby achieving excellent strength and corrosion resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 8-269632 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-30139 Summary of the Invention [Problem to be solved by the invention]

[0007] Steel materials for accessories such as the drill collars of the above-mentioned earth-removing drills need to have not only strength and corrosion resistance, but also a more stable austenitic single-phase structure so as not to become magnetic even under harsher usage environments, i.e., so as not to cause deformation-induced martensitic transformation.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-magnetic austenitic stainless steel material having excellent strength and corrosion resistance and suitable for use as a machine component in a corrosive environment, particularly as a drill collar for a drilling drill, and a method for producing the same. [Means for solving the problem]

[0009] The non-magnetic austenitic stainless steel material according to the present invention has a composition, in mass percent, of C:<0.10%, Si:<0.3%, Mn: more than 4.5% and less than 10.0%, P:<0.05%, S:<0.0020%, Ni: 9.0-15.0%, Cr: 17.0-25.0%, Mo: 3.0-7.0%, N: 0.3-0.6%, the balance being Fe and unavoidable impurities, and when the content of element M is [M]% in mass percent, it satisfies (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≦60, and has an austenitic single-phase structure, and is prepared according to ASTM G48 Method It is characterized by a critical pitting temperature (CPT) of 50°C or higher according to the test method compliant with C, and a 0.2% yield strength of 970 MPa or higher at a depth of 1 inch from the surface (however, if the thickness T or diameter D is less than 4 inches, a depth of T / 4 or D / 4 from the surface).

[0010] These characteristics allow the material to have suitable corrosion resistance and maintain high strength even in a corrosive environment, while also providing stable non-magnetic properties.

[0011] The present invention also provides a method for producing a non-magnetic austenitic stainless steel material having a chemical composition, in mass percent, of C: <0.10%, Si: <0.3%, Mn: more than 4.5% and less than 10.0%, P: <0.05%, S: <0.0020%, Ni: 9.0 to 15.0%, Cr: 17.0 to 25.0%, Mo: 3.0 to 7.0%, N: 0.3 to 0.6%, the balance being Fe and unavoidable impurities. The steel ingot has a predetermined chemical composition, and is subjected to a cooling treatment after hot working. During the cooling process, the ingot is warm worked at a temperature range of 800 to 300°C with a reduction in area of ​​15 to 50%. This results in a steel with the above-described chemical composition, where the content of element M, in mass %, satisfies (40[N] + 1.2[Cr] + 0.07exp(0.3[Ni] + 0.3[Cu])) × 1.5[Mo]^(-0.18) ≦ 60, with an austenitic single-phase structure, a critical pitting temperature (CPT) of 50°C or higher as measured in accordance with ASTM G48 Method C, and a 0.2% yield strength of 970 MPa or higher at a depth of 1 inch from the surface (however, if the thickness T or diameter D is less than 4 inches, a depth of T / 4 or D / 4 from the surface).

[0012] According to these characteristics, it is possible to obtain a non-magnetic austenitic stainless steel material that has suitable corrosion resistance and maintains high strength even in a corrosive environment, and also has stable non-magnetic properties. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a table showing the composition of steels used in the simulation test. [Figure 2] This is a list of test results from mock exams. [Figure 3] 1 shows micrographs of (a) stepped structure and (b) grooved structure after sensitization testing. DETAILED DESCRIPTION OF THE INVENTION

[0014] A non-magnetic austenitic stainless steel material and a method for manufacturing the same will be described as one embodiment of the present invention.

[0015] The target austenitic stainless steel material is steel containing, by mass%, C: <0.10%, Si: <0.3%, Mn: over 4.5% and less than 10.0%, P: <0.05%, S: <0.0020%, Ni: 9.0 to 15.0%, Cr: 17.0 to 25.0%, Mo: 3.0 to 7.0%, and N: 0.3 to 0.6%. With this composition, the austenitic single-phase structure can be a non-magnetic steel.

[0016] In particular, the content of each component is adjusted so as to satisfy the following formula 1. (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≦60 … (Equation 1) The inventors of the present application have discovered this relational expression (Equation 1) as a condition for the component composition that can delay the onset of sensitization when a sensitization test (based on ASTM A262 Practice A) is performed. In the sensitization test, as the amount of Cr-based nitrides at the grain boundaries decreases, the structure changes from a ditch structure to a dual structure to a step structure. From the viewpoint of corrosion resistance, it is preferable to have a dual structure or a step structure. Here, warm working, which will be described later, generally precipitates Cr-based carbides and nitrides at the grain boundaries, making sensitization more likely to occur. However, by satisfying Equation 1, the onset of sensitization can be delayed. In other words, the resulting steel has excellent corrosion resistance to hazardous gases generated during seabed drilling and high-temperature, high-pressure seawater.

[0017] The corrosion resistance of steel materials obtained from steels having such component compositions is such that the critical pitting temperature (CPT) measured in accordance with ASTM G48 Method C is 50°C or higher.

[0018] The above-mentioned component composition preferably further contains one or more groups selected from the following groups A to C (where % is by mass). Group A - One or more selected from Nb: <0.2%, W: <1.0%, Al: <0.1%, Ti: <0.2%, V: <0.5%, and Ta: <0.2% Group B-B:≦0.0050% Group C: One or more selected from Ca: <0.0200%, Mg: <0.0200%, and Zr: <0.0200%

[0019] Of the elements in group A, all elements except W have the effect of refining crystal grains, and W has the effect of improving corrosion resistance. Furthermore, the elements in groups B and C segregate at grain boundaries to reduce the effects of grain boundary embrittlement elements such as P and S, and are effective in maintaining good workability in hot working and warm working processes. B is particularly preferred because its content can be easily adjusted.

[0020] Furthermore, it is also preferable that the above-mentioned component composition is further adjusted to satisfy the following formula 2. 756-555[C]-528[N]-10.3[Si]-12.5[Mn]-10.5[Cr]―24[Ni]-5.6[Mo]≦-110… (Formula 2) By satisfying this relational expression (Equation 2), it is possible to suppress the martensitic transformation induced by deformation, and to obtain a stable austenite single phase structure that is effective in maintaining non-magnetic properties.

[0021] The above-mentioned component composition is also preferably adjusted so as to further satisfy the following formula 3. 48≦[Cr]+1.27[Ni]+3.2[Mo]+5.45[Cu]… (Formula 3) By satisfying this relational expression (Equation 3), the corrosion resistance of the resulting steel material can be further improved.

[0022] The manufacturing method of the nonmagnetic austenitic stainless steel material described above is as follows. First, a steel ingot having a predetermined composition is subjected to hot working, such as hot forging or rolling, to obtain the above-described composition. Furthermore, solution heat treatment is performed as needed. In this embodiment, during the cooling treatment after these hot working steps, or if solution heat treatment is performed, further warm working is performed in the temperature range of 800 to 300°C during the cooling process. Rapid cooling is preferred for the cooling treatment to suppress the precipitation of precipitates such as carbides and nitrides at grain boundaries. Cooling treatment can be performed by air blast cooling (air cooling), gas cooling, water cooling, oil cooling, or the like. Warm working involves a reduction in cross-sectional area of ​​15 to 50%. This work-hardens the interior of the steel material, thereby enhancing the internal strength of thick-walled components, such as those with thicknesses of 2 inches or more. If the thickness T or diameter D of the steel material is less than 4 inches, the 0.2% yield strength at a depth of T / 4 or D / 4 from the surface can be 970 MPa or more, and in other cases, the 0.2% yield strength at a depth of 1 inch from the surface can be 970 MPa or more.

[0023] The 0.2% proof stress is measured by a tensile test. When manufacturing round bars as steel materials, for components with a diameter of less than 4 inches, the test specimen is taken so that a position half the radius deep from the surface is included in the parallel cross section of the tensile test specimen. For components with a diameter of 4 inches or more, the test specimen is taken so that a position 1 inch deep from the surface is included in the parallel cross section of the tensile test specimen.

[0024] In particular, when warm forging is performed on a steel material having a round bar shape with a relatively large diameter, the forging effect is higher in the outer periphery than in the core, and the warm forged structure has a structure gradient from the core to the outer periphery. Therefore, the strength near the outer periphery is higher, but when the steel material is made into a hollow cylindrical shape, such as a steel material for a drill collar for a drill, it is bored so as to leave the outer periphery, which has a relatively high strength.

[0025] [Mock exam] Next, the results of producing test pieces simulating austenitic stainless steel materials by the above-described production method will be described with reference to FIGS. 1 and 2. FIG.

[0026] As shown in Figure 1, steels with predetermined chemical compositions were prepared by air melting (arc furnace melting) and electroslag remelting, and 6 t ingots with the chemical compositions shown in Examples 1 to 17 and Comparative Examples 1 to 10 were obtained. These ingots were subjected to homogenization heat treatment at a predetermined temperature in the range of 1100 to 1250°C and hot forged into round bars with a diameter of 320 mm. Next, solution heat treatment was performed at a predetermined temperature in the range of 1050 to 1150°C, and the bars were air-cooled to 750°C.

[0027] Next, warm working was started at the warm working starting temperature shown in Figure 2. During warm working, the bars were forged to a predetermined reduction in cross-sectional area within the range of 15 to 50% (see the same figure). Tensile test specimens were taken from each bar so that a position 1 inch deep from the surface was included in the parallel cross section of the test specimen.

[0028] The figure shows the test results for each of the obtained steel materials, as well as the values ​​on the left side of the above (Equation 1) and (Equation 2) and the value on the right side of (Equation 3).

[0029] Here, the magnetic permeability measurement was carried out in accordance with ASTM A342, and a magnetic permeability of 1.005 or less was regarded as good and given a grade of "A," while anything other than that was regarded as bad and given a grade of "C."

[0030] In the room temperature tensile test, 0.2% yield strength, tensile strength, elongation, and reduction of area were measured. If the 0.2% yield strength was 970 MPa or more, the tensile strength was 1030 MPa or more, the elongation was 15% or more, and the reduction of area was 50% or more, all of these were met and the "tensile properties" column was marked with an "A," while anything else was deemed poor and marked with a "C."

[0031] The sensitization test was conducted in accordance with ASTM A262 Practice A. After immersion in the corrosive solution, the structure was observed and classified into ditch structure (groove-shaped structure), step structure (step-shaped structure), and dual structure (mixed structure), with ditch structure being rated as poor and all other structures being rated as good.

[0032] Figure 3 shows an example of a stepped structure (see Figure 3(a)) and an example of a grooved structure (see Figure 3(b)). In sensitization tests, the degree of sensitization is observed, whereby precipitates such as chromium nitride are formed at the grain boundaries due to exposure to a high-temperature environment. As a result of etching, the more precipitates there are, the deeper the grain boundaries are corroded and appear black. In other words, as sensitization progresses, a stepped structure like Figure 3(a) becomes a grooved structure where the grain boundaries appear black, as shown in Figure 3(b).

[0033] Measurement of the critical pitting temperature (CPT) (corrosion resistance test) was carried out in accordance with ASTM G48 Method C. When this CPT was measured, the corrosion resistance was good at 50°C or higher.

[0034] For Examples 1 to 17, the starting temperature of the warm working was all within the range of 800 to 300°C (more specifically, 650 to 600°C), the area reduction rate was within the range of 15 to 50%, and all of the formulas 1 to 3 were satisfied. As a result, good results were obtained in all of the magnetic permeability, tensile properties, sensitization tests, and CPT.

[0035] On the other hand, in Comparative Example 1, the starting temperature of warm working was as high as 830°C. As a result, a grooved structure was observed in the sensitization test, and the CPT was also low at 5°C, resulting in poor corrosion resistance. When the test piece of Comparative Example 1 was further observed, a large amount of Cr-based nitrides was observed. In other words, setting the starting temperature of warm working to a high temperature promotes the formation of Cr-based nitrides, which is undesirable from the viewpoint of corrosion resistance.

[0036] In Comparative Example 2, the area reduction rate of the warm working was low at 12%, and as a result, the tensile properties were judged to be poor. This is thought to be because the work hardening inside the steel material was insufficient.

[0037] In Comparative Example 3, although the contents of the individual components were within the above-mentioned ranges, the component composition did not satisfy Formula 1. Reflecting this, the sensitization test showed a grooved structure and the CPT was as low as 10°C. In other words, the corrosion resistance was insufficient. This shows that satisfying Formula 1 is effective in obtaining excellent corrosion resistance.

[0038] In Comparative Example 4, the N content was lower than in the other Examples, and the composition did not satisfy Formula 2. As a result, the magnetic permeability and tensile properties were poor. In other words, deformation-induced martensitic transformation occurred, and the austenite single-phase structure could not be maintained.

[0039] In Comparative Example 5, the Mo content was lower than in the other Examples, and the composition did not satisfy Formula 1. Reflecting this, a grooved structure was observed in the sensitization test, and the CPT was low at 20° C. In other words, sensitization had progressed.

[0040] In Comparative Example 6, the Mn content was lower than in the other Examples, and the composition did not satisfy Formula 2. Reflecting this, the magnetic permeability was poor and the CPT was low at 40° C. In other words, deformation-induced martensitic transformation occurred, and the austenite single-phase structure could not be maintained.

[0041] In Comparative Example 7, the Cr content was lower than in the other Examples. As a result, the CPT was low at 45° C. In other words, when the Cr content is low, the corrosion resistance is reduced.

[0042] In Comparative Example 8, the Cr content was higher than in the other Examples, and the composition did not satisfy Formula 1. Reflecting this, the sensitization test showed a grooved structure and the CPT was low at 25°C. In other words, sensitization had progressed.

[0043] In Comparative Example 9, the Ni content was higher than in the other Examples, and the composition did not satisfy Formula 1. Reflecting this, a grooved structure was observed in the sensitization test, and the CPT was low at 30° C. In other words, sensitization had progressed.

[0044] In Comparative Example 10, although the contents of the individual components were within the above-mentioned ranges, the component composition did not satisfy Formula 2. As a result, the magnetic permeability was poor. In other words, deformation-induced martensitic transformation occurred, and the austenite single-phase structure could not be maintained.

[0045] As described above, good results were obtained in all of the magnetic permeability, tensile properties, sensitization test, and CPT in Examples 1 to 17. In other words, non-magnetic austenitic stainless steel materials with excellent strength and corrosion resistance suitable for machine parts used in corrosive environments were obtained.

[0046] The composition range of steel that can provide mechanical properties and the like substantially equivalent to those of the non-magnetic austenitic stainless steel material having excellent strength and corrosion resistance, including the above-mentioned examples, is determined as follows.

[0047] C refines crystal grains, but may deteriorate corrosion resistance by forming compounds with Cr and Mo. Taking these into consideration, the C content is set to less than 0.10%, preferably less than 0.05%, by mass%.

[0048] Although Si is a deoxidizing element, excessive addition not only reduces hot workability but also promotes the formation of δ-ferrite, a ferromagnetic phase. Taking these factors into consideration, the Si content is set to less than 0.3% by mass.

[0049] As the Mn content increases, the amount of N that can be added increases, and the inclusion of N can have the effect of improving corrosion resistance. On the other hand, excessive addition of Mn not only deteriorates corrosion resistance but also promotes segregation. Taking these factors into consideration, the Mn content is in the range of more than 4.5% and less than 10.0%, preferably more than 4.5% and less than 8.0%, by mass%.

[0050] Since P segregates at grain boundaries and impairs workability in hot working and warm working processes, the lower its content, the better. Therefore, the P content is in the range of less than 0.05% by mass.

[0051] Since S segregates at grain boundaries and impairs workability in hot working and warm working processes, the lower the S content, the better. Therefore, the S content is in the range of less than 0.0020% by mass.

[0052] Cu is an unavoidable impurity that is mixed in from raw material scrap, etc., and segregates at grain boundaries, reducing hot workability. Therefore, the lower the Cu content, the better. Therefore, the Cu content is less than 1.0% by mass. However, excessive reduction increases steelmaking costs, so the lower limit of the Cu content can be set to 0.005% or more.

[0053] Ni is actively added because it not only improves corrosion resistance but also contributes to non-magnetic properties and improved hydrogen embrittlement resistance. However, excessive addition increases costs and may also reduce work hardenability and increase sensitization susceptibility. Taking these factors into consideration, the Ni content is set within the range of 9.0% to 15.0% by mass.

[0054] Cr contributes to improving corrosion resistance. However, excessive addition of Cr may promote the formation of δ-ferrite, a ferromagnetic phase, and may increase sensitization susceptibility. Taking these factors into consideration, the Cr content is set within the range of 17.0% to 25.0% by mass.

[0055] Mo contributes to improving corrosion resistance. However, excessive addition may promote the formation of δ-ferrite, a ferromagnetic phase. Taking these factors into consideration, the Mo content is set within the range of 3.0% to 7% by mass, preferably 4.0% to 7.0%, and more preferably 4.5% to 7.0%.

[0056] Co is an unavoidable impurity that is mixed in from raw material scrap, etc., and excessive Co content can promote deformation-induced transformation and cause magnetism, so the lower the Co content, the better. Therefore, the Co content is less than 1.0%, preferably less than 0.1%, by mass. On the other hand, excessive Co content increases steelmaking costs, so the lower limit of the Co content can be set to 0.005% or more.

[0057] B segregates at grain boundaries and can be added to suppress the deterioration of workability in hot working and warm working processes caused by grain boundary embrittlement elements such as P and S. On the other hand, excessive addition causes embrittlement across all temperature ranges from cold to hot. Taking these factors into consideration, B can be added in a range of less than 0.0050% by mass.

[0058] N is actively added because it dissolves in solid solution to improve corrosion resistance and significantly improves work hardening during warm working. However, excessive addition can promote the formation of Cr-based nitrides and increase sensitization susceptibility. Taking these factors into consideration, the N content is set within the range of 0.3% to 0.6% by mass.

[0059] While Al is effective as a deoxidizing element, excessive Al content promotes the formation of δ-ferrite, a ferromagnetic phase, and may also form nitrides, reducing the amount of solute N and impairing mechanical strength and corrosion resistance. Therefore, the lower the Al content, the better. Therefore, the Al content is less than 0.1%, preferably less than 0.01%, by mass. On the other hand, excessive reduction increases steelmaking costs, so the lower limit of the Al content may be set to 0.005% or more.

[0060] Nb, Ti, V, and Ta can be added because they bond with C and N to form carbides and nitrides, respectively, and contribute to grain refinement. However, excessive addition may reduce the amount of dissolved N, impairing mechanical strength and corrosion resistance. Therefore, the Nb content is less than 0.2%, preferably less than 0.1%, by mass. The Ti content is less than 0.2%, by mass. The V content is less than 0.5%, by mass. The Ta content is less than 0.2%, by mass.

[0061] Although W contributes to improving corrosion resistance, it increases the manufacturing cost due to the cost of raw materials and may promote the formation of ferromagnetic δ-ferrite. Therefore, the W content is set to less than 1.0% by mass, preferably less than 0.1% by mass.

[0062] Ca, Mg, and Zr can be added to suppress the deterioration of workability in hot working and warm working processes due to grain boundary embrittlement elements such as P and S. However, excessive addition of these elements causes embrittlement across the entire temperature range from cold to hot. Taking these factors into consideration, Ca may be added in a range of less than 0.0200% by mass. Mg may be added in a range of less than 0.0200% by mass. Furthermore, Zr may be added in a range of less than 0.0200% by mass.

[0063] Although typical embodiments of the present invention have been described above, the present invention is not necessarily limited to these, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

Claims

1. In mass%, Mn: more than 4.5% to less than 10.0%; Ni: 9.0 to 15.0%, Cr: 17.0-25.0%, Mo: 3.0 to 7.0%, N: 0.3 to 0.6%; C: <0.10%, Si: <0.3%, P: <0.05%, S:<0.0020%, The balance is Fe and unavoidable impurities, When the content of element M is [M]% by mass, (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≦60 Fulfilling A non-magnetic austenitic stainless steel material characterized by having an austenitic single-phase structure, a critical pitting temperature (CPT) of 50°C or higher as determined by a test method in accordance with ASTM G48 Method C, and a 0.2% yield strength of 970 MPa or higher at a depth of 1 inch from the surface (however, in the case of thickness T or diameter D being less than 4 inches, at a depth of T / 4 or D / 4 from the surface).

2. The component composition further includes: In mass%, Group A—Nb: <0.2%, W: <1.0%, Al: <0.1%, Ti: <0.2%, V: <0.5%, and Ta: <0.2%; Group B-B: ≦0.0050%, Group C—Ca: <0.0200%, Mg: <0.0200%, and Zr: <0.0200%; 2. The non-magnetic austenitic stainless steel material according to claim 1, characterized in that it contains at least one of the groups listed above.

3. 3. The non-magnetic austenitic stainless steel material according to claim 1, wherein the non-magnetic austenitic stainless steel material has a round bar shape.

4. 4. The non-magnetic austenitic stainless steel material according to claim 3, which has a warm forged structure with a structure gradient from the core to the outer periphery.

5. 2. The non-magnetic austenitic stainless steel material according to claim 1, which is a steel material for a cylindrical drill collar that has been bored.

6. The component composition is as follows: The non-magnetic austenitic stainless steel material according to claim 1 or 2, characterized in that it satisfies 756-555[C]-528[N]-10.3[Si]-12.5[Mn]-10.5[Cr]-24[Ni]-5.6[Mo]≦-110.

7. The component composition is as follows, where the content of element M is [M]% by mass:

3. The non-magnetic austenitic stainless steel material according to claim 1, wherein the following relationship is satisfied: 48≦[Cr]+1.27[Ni]+3.2[Mo]+5.45[Cu].

8. In mass percent, C: <0.10%, Si: <0.3%, Mn: more than 4.5% to less than 10.0%; P: <0.05%, S:<0.0020%, Ni: 9.0 to 15.0%, Cr: 17.0-25.0%, Mo: 3.0 to 7.0%, N: 0.3 to 0.6%, A method for producing a non-magnetic austenitic stainless steel material having a component composition including the balance being Fe and unavoidable impurities, A steel ingot having a predetermined chemical composition is subjected to hot working followed by cooling treatment, and during this cooling process, warm working is performed at a temperature range of 800 to 300°C with an area reduction rate of 15 to 50%, When the composition is as described above and the content of element M is [M]% by mass, A method for producing a non-magnetic austenitic stainless steel material, characterized in that the material satisfies (40[N] + 1.2[Cr] + 0.07exp(0.3[Ni] + 0.3[Cu])) x 1.5[Mo]^(-0.18) ≦ 60, has an austenitic single-phase structure, has a critical pitting temperature (CPT) of 50°C or higher as measured by a method conforming to ASTM G48 Method C, and has a 0.2% yield strength of 970 MPa or higher at a depth of 1 inch from the surface (however, in the case of thickness T or diameter D being less than 4 inches, at a depth of T / 4 or D / 4 from the surface).

9. The component composition further includes: In mass%, Group A—Nb: <0.2%, W: <1.0%, Al: <0.1%, Ti: <0.2%, V: <0.5%, and Ta: <0.2%; Group B-B: ≦0.0050%, Group C—Ca: <0.0200%, Mg: <0.0200%, and Zr: <0.0200%; 9. The method for producing a non-magnetic austenitic stainless steel material according to claim 8, wherein the method includes at least one group selected from the group consisting of the above.

10. 10. The method for producing a non-magnetic austenitic stainless steel material according to claim 8, wherein the material is formed into a round bar shape.

11. 11. The method for producing a non-magnetic austenitic stainless steel material according to claim 10, wherein the steel material is bored to form a cylindrical steel material for drill collars.

12. The component composition is as follows, where the content of element M is [M]% by mass: The method for producing a non-magnetic austenitic stainless steel material according to claim 8 or 9, characterized in that 756-555[C]-528[N]-10.3[Si]-12.5[Mn]-10.5[Cr]-24[Ni]-5.6[Mo]≦-110 is satisfied.

13. The component composition is as follows, where the content of element M is [M]% by mass:

10. The method for producing a non-magnetic austenitic stainless steel material according to claim 8, wherein the relationship satisfies 48≦[Cr]+1.27[Ni]+3.2[Mo]+5.45[Cu].

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