Nonmagnetic austenitic stainless steel material and method for manufacturing the same

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 excellent in strength and corrosion resistance and a method for producing the same.

Background Art

[0002] As a steel material for parts that require strength and corrosion resistance, austenitic stainless steel materials represented by SUS304 and the like are used.

[0003] For example, in Patent Document 1, in high-strength and highly corrosion-resistant stainless steel materials with a large amount of nitrogen added, all contain a large amount of expensive alloy elements such as Cr, Ni, and Mo, and nitride that has an adverse effect on cold workability is dissolved. After stating that it becomes very costly because of the solution heat treatment at a high temperature near 1200°C, an austenitic stainless steel material excellent in corrosion resistance with increased strength by cold working is disclosed. Generally, corrosion resistance decreases due to cold working. Here, for the purpose of improving cold workability, Cu is added to the component composition, and for the purpose of improving strength, any one or two or more of Nb, V, and W are added to improve both strength and corrosion resistance.

[0004] On the other hand, for steel materials for accessory parts such as drill collars for drills used in offshore oil fields, austenitic stainless steel materials that are non-magnetic are used so as not to affect the position control by the magnetism of the drill bit, along with strength and corrosion resistance.

[0005] For example, Patent Document 2 discloses a method for manufacturing a forged product including a drill collar made of a non-magnetic austenitic stainless steel material. In the steel material with the component composition used here, precipitates such as carbides or nitrides are likely to precipitate in the temperature range of 740 to 760°C. Therefore, the steel ingot is warm-worked at a temperature between 650 and 500°C on the surface temperature, suppressing the precipitation of precipitates such as carbides and nitrides at the grain boundaries and sufficiently supplying C and N into the austenite crystal particles to obtain excellent strength and corrosion resistance.

Prior Art Documents

[0006] [Patent Document 1] Japanese Patent Application Publication No. 8-269632 [Patent Document 2] Japanese Patent Publication No. 2009-30139 [Overview of the project] [Problems that the invention aims to solve]

[0007] For the steel materials used for the drill collars and other accessories of the drilling drills mentioned above, it is necessary to have a more stable austenite single-phase structure that not only provides strength and corrosion resistance, but also prevents magnetization even under harsher operating environments, i.e., prevents work-induced martensitic transformation.

[0008] The present invention has been made in view of the above circumstances, and its objective is to provide a non-magnetic austenitic stainless steel material with excellent strength and corrosion resistance, and a method for manufacturing the same, which is suitable for use in mechanical components used in corrosive environments, particularly for applications such as drill collars of drilling drills. [Means for solving the problem]

[0009] The nonmagnetic austenitic stainless steel material according to the present invention has a composition in mass percent of C:<0.10%, Si:<0.3%, Mn: greater than 4.5% to 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%, with the remainder being Fe and unavoidable impurities. Furthermore, if 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, according to the ASTM G48 Method. The test method conforming to C standards is characterized by a critical pitting temperature (CPT) of 50°C or higher, and a 0.2% proof stress 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, the depth is T / 4 or D / 4 from the surface).

[0010] These characteristics allow the material to maintain high strength and excellent corrosion resistance even in corrosive environments, while also possessing stable non-magnetic properties.

[0011] Furthermore, the method for producing non-magnetic austenitic stainless steel according to the present invention is a method for producing non-magnetic austenitic stainless steel having a component composition in mass percent of C:<0.10%, Si:<0.3%, Mn: greater than 4.5% to 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%, with the remainder being Fe and unavoidable impurities. The invention relates to a steel ingot having a predetermined component composition, which is subjected to a cooling treatment following hot working, and in the temperature range of 800 to 300°C during this cooling process, a hot working treatment is performed with a reduction in surface area of ​​15 to 50%, thereby obtaining the aforementioned component composition, and if the content of element M is [M]% by mass, the following conditions are met: (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≦60, the steel has an austenite single-phase structure, the critical pitting temperature (CPT) is 50°C or higher according to the measurement method in accordance with ASTM G48 Method C, and the 0.2% proof stress at a depth of 1 inch from the surface (however, if the thickness T or diameter D is less than 4 inches, the depth from the surface is T / 4 or D / 4) is 970 MPa or higher.

[0012] These characteristics make it possible to obtain a non-magnetic austenitic stainless steel material that has suitable corrosion resistance even in corrosive environments, maintains high strength, and possesses stable non-magnetic properties. [Brief explanation of the drawing]

[0013] [Figure 1] This is a table showing the component composition of the steel used in the mock exam. [Figure 2] This is a list of test results from the mock exam. [Figure 3] These are micrographs of (a) stepped tissue and (b) grooved tissue after sensitization testing. [Modes for carrying out the invention]

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

[0015] The austenitic stainless steel material in question consists of steel containing, by mass%, C:<0.10%, Si:<0.3%, Mn: greater than 4.5% but 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%, and N:0.3-0.6%. With such a composition, it can be made into a non-magnetic steel with an austenite single-phase structure.

[0016] In particular, the content of each component is adjusted to satisfy the following equation 1. (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≦60 … (Equation 1) The relationship shown (Equation 1) was discovered by the inventors of this invention as a condition for the component composition that can delay the onset of sensitization when a sensitization test (in accordance with 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 Ditch to Dual to Step, but from the viewpoint of corrosion resistance, it is preferable to exhibit a Dual or Step structure. Here, while hot working, as described later, generally causes precipitation of Cr-based carbides and nitrides at the grain boundaries, making sensitization more likely, the onset time of sensitization can be delayed by satisfying Equation 1. In other words, the resulting steel material can be made to have excellent corrosion resistance to harmful gases generated during seabed drilling and high-temperature, high-pressure seawater.

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

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

[0019] Among the elements of Group A above, except for W, the effect of grain refinement can be obtained, and for W, the effect of improving corrosion resistance can be obtained. In addition, the elements of Group B and Group C segregate at grain boundaries, thereby reducing the influence of grain boundary embrittlement elements such as P and S, and are effective for maintaining good workability in hot working processes and warm working processes. In particular, B is preferable in terms of easy adjustment of the content.

[0020] Moreover, as the above-described component composition, it is also preferable to be adjusted so as to further 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 such a relational expression (Formula 2), martensitic transformation due to processing induction can be suppressed, and a stable austenite single-phase structure effective for maintaining non-magnetism can be obtained.

[0021] As the above-described component composition, it is also preferable to be 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 such a relational expression (Formula 3), the above-described corrosion resistance of the obtained steel material can be further enhanced.

[0022] By the way, the manufacturing method for the non-magnetic austenitic stainless steel material described above is as follows. First, hot working such as hot forging or rolling is performed using a steel ingot having a predetermined component composition so that the above-mentioned component composition can be obtained. Furthermore, solution heat treatment is performed as necessary. In this embodiment, during the cooling process after these hot working processes, if solution heat treatment is performed, further warm working is performed in the temperature range of 800 to 300°C during the cooling process after the solution heat treatment. The cooling process is preferably rapid cooling from the viewpoint of suppressing the precipitation of precipitates such as carbides and nitrides at the grain boundaries, and can be done by blast cooling (air cooling), gas cooling, water cooling, oil cooling, etc. In the warm working process, a reduction in the cross-sectional area of ​​15 to 50% is applied. This works hardens the inside of the steel material, making it possible to achieve high strength even in thick-walled members with a thickness of 2 inches or more. Furthermore, if the thickness T or diameter D of the steel material is less than 4 inches, the 0.2% proof stress at a depth of T / 4 or D / 4 from the surface can be set to 970 MPa or higher; otherwise, the 0.2% proof stress at a depth of 1 inch from the surface can be set to 970 MPa or higher.

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

[0024] In particular, when hot forging is performed, the forging effect is higher in the outer circumference of steel materials with a relatively large diameter round bar shape compared to the core, resulting in a hot forged structure with a structural gradient from the core to the outer circumference. Therefore, the strength near the outer circumference is higher, but when using hollow cylindrical steel materials, such as steel for drill collars in drilling machines, the core is boring out to preserve the relatively strong outer circumference.

[0025] [Mock Exam] Next, the results of manufacturing test specimens simulating austenitic stainless steel using the manufacturing method described above will be explained with reference to Figures 1 and 2.

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

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

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

[0029] In this section, permeability measurements were performed in accordance with ASTM A342, with a permeability of 1.005 or less being considered good and marked as "A," while any other value was considered poor and marked as "C."

[0030] In the room temperature tensile test, the 0.2% proof stress, tensile strength, elongation, and reduction of area were measured. A "A" was indicated in each column of the "Tensile Properties" column if all of the following conditions were met: 0.2% proof stress of 970 MPa or higher, tensile strength of 1030 MPa or higher, elongation of 15% or higher, and reduction of area of ​​area of ​​50% or higher. A "C" was indicated for all other conditions.

[0031] Sensitization testing was performed in accordance with ASTM A262 Practice A. After immersion in the corrosive solution, microstructure observation was performed and the samples were classified into ditch structure (groove-like structure), step structure (step-like structure), and dual structure (mixed structure). Ditch structure was indicated as poor, and all other structures as good.

[0032] Figure 3 shows examples of stepped structures (see Figure (a)) and grooved structures (see Figure (b)). In the sensitization test, the degree of sensitization, which involves the deposition of precipitates such as chromium nitride at the grain boundaries by exposure to a high-temperature environment, is observed. As a result of etching, the more precipitates there are, the deeper the corrosion at the grain boundaries appears, resulting in a black appearance. In other words, compared to the stepped structure shown in Figure (a), as sensitization progresses, it becomes a grooved structure with black grain boundaries as shown in Figure (b).

[0033] The critical pitting temperature (CPT) was measured (corrosion resistance test) in accordance with ASTM G48 Method C. The CPT was measured, and good corrosion resistance was observed above 50°C.

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

[0035] On the other hand, in Comparative Example 1, the starting temperature for the warm working process was a high 830°C. As a result, a groove-like structure was observed in the sensitization test, and the CPT was low at 5°C, indicating poor corrosion resistance. When microstructure observation was performed on the specimen from Comparative Example 1, a large amount of Cr-based nitride was observed. In other words, setting a high starting temperature for warm working promotes the formation of Cr-based nitride, which is undesirable from the viewpoint of corrosion resistance.

[0036] In Comparative Example 2, the reduction in surface area during warm working was low at 12%. 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 content of each component was within the range described above, the component composition did not satisfy Equation 1. Reflecting this, the sensitization test showed a grooved structure and a low CPT of 10°C. In other words, the corrosion resistance was insufficient. This indicates that satisfying Equation 1 is effective in obtaining excellent corrosion resistance.

[0038] In Comparative Example 4, the N content was lower than in the other examples, resulting in a component composition that did not satisfy Equation 2. Reflecting this, the permeability and tensile properties were poor. In other words, a martensitic transformation induced by processing 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, resulting in a component composition that did not satisfy Equation 1. Reflecting this, the sensitization test showed a grooved structure, and the CPT was low at 20°C. In other words, sensitization progressed.

[0040] In Comparative Example 6, the Mn content was lower than in the other examples, resulting in a component composition that did not satisfy Equation 2. Reflecting this, the magnetic permeability was poor, and the CPT was also low at 40°C. In other words, a martensitic transformation induced by processing occurred, and the austenite single-phase structure could not be maintained.

[0041] In Comparative Example 7, the Cr content was lower compared to the other examples. As a result, the CPT was low at 45°C. In other words, a lower Cr content leads to lower corrosion resistance.

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

[0043] In Comparative Example 9, the Ni content was higher than in the other examples, resulting in a component composition that did not satisfy Equation 1. Reflecting this, the sensitization test showed a grooved structure, and the CPT was low at 30°C. In other words, sensitization progressed.

[0044] In Comparative Example 10, although the content of each component was within the range described above, the component composition did not satisfy Equation 2. Reflecting this, the magnetic permeability was poor. In other words, a martensitic transformation induced by processing occurred, and the austenite single-phase structure could not be maintained.

[0045] As described above, in Examples 1 to 17, good results were obtained in all aspects, including permeability, tensile properties, sensitization test, and CPT. In other words, we were able to obtain a non-magnetic austenitic stainless steel material with excellent strength and corrosion resistance, suitable for mechanical components used in corrosive environments.

[0046] Incidentally, the composition range of steel that can provide mechanical properties and other characteristics nearly equivalent to those of non-magnetic austenitic stainless steel materials with excellent strength and corrosion resistance, including the examples described above, is defined as follows.

[0047] While carbon (C) refines the crystal grains, it can form compounds with Cr and Mo, potentially degrading corrosion resistance. Considering these factors, the carbon content is within the range of less than 0.10% by mass, preferably less than 0.05%.

[0048] Although silicon (Si) is a deoxidizing element, excessive addition reduces hot workability and promotes the formation of δ-ferrite, a ferromagnetic phase. Considering these factors, the Si content is kept within the range of less than 0.3% by mass.

[0049] As the Mn content increases, the amount of N that can be added also increases, and the effect of improved corrosion resistance due to the inclusion of N can be obtained. On the other hand, excessive addition degrades corrosion resistance and promotes segregation. Taking these factors into consideration, the Mn content is in the range of more than 4.5% to less than 10.0% by mass, preferably more than 4.5% to less than 8.0%.

[0050] Since phosphorus (P) segregates at grain boundaries and inhibits processability in hot and warm working processes, it is preferable to reduce its content. Therefore, the P content is within the range of less than 0.05% by mass.

[0051] Since sulfur (S) segregates at grain boundaries and inhibits processability in hot and warm working processes, it is preferable to reduce its content. Therefore, the S content is within the range of less than 0.0020% by mass.

[0052] Cu is an impurity that is inevitably present due to contamination from raw material scraps, etc., and it segregates at grain boundaries, reducing hot workability, so it is preferable to reduce its content. For this reason, the Cu content is in the range of less than 1.0% by mass. On the other hand, excessive reduction would increase steelmaking costs, so the lower limit of the content can be set at 0.005% or more.

[0053] Ni is actively added to improve corrosion resistance, as well as to enhance non-magnetic properties and hydrogen embrittlement resistance. However, excessive addition can increase costs, reduce work hardening properties, and increase sensitization susceptibility. Considering these factors, the Ni content is within the range of 9.0% to 15.0% by mass.

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

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

[0056] Co is an impurity that is inevitably present due to contamination from raw material scraps, etc. If present in excess, it can promote processing-induced transformation and become magnetic, so it is preferable to reduce the amount of Co. For this reason, the Co content should be in the range of less than 1.0% by mass, preferably less than 0.1%. On the other hand, excessive reduction would increase steelmaking costs, so the lower limit of the Co content can be set to 0.005% or more.

[0057] B can be added to segregate at grain boundaries and suppress the reduction in workability during hot and warm working processes caused by grain boundary embrittlement elements such as P and S. On the other hand, excessive addition can cause 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] Nitrogen (N) is actively added to improve corrosion resistance through solid solution and to significantly enhance work hardening during warm working. However, excessive addition can promote the formation of chromium-based nitrides and increase sensitivity. Considering these factors, the N content is within the range of 0.3% to 0.6% by mass.

[0059] While Al is effective as a deoxidizing element, excessive amounts can promote the formation of δ-ferrite, a ferromagnetic phase, and can also form nitrides, reducing the amount of dissolved nitrogen and impairing mechanical strength and corrosion resistance. Therefore, it is preferable to reduce the Al content. For this reason, the Al content is in the range of less than 0.1% by mass, preferably less than 0.01%. On the other hand, excessive reduction will increase steelmaking costs, so the lower limit of the Al content may be set at 0.005% or more.

[0060] Nb, Ti, V, and Ta can be added to form carbides and nitrides by bonding with C and N, respectively, and contribute to grain refinement. On the other hand, excessive addition may reduce the amount of solid-solution N, impairing mechanical strength and corrosion resistance. Therefore, the Nb content should be less than 0.2% by mass, preferably less than 0.1%. The Ti content should be less than 0.2% by mass. The V content should be less than 0.5% by mass. The Ta content should also be less than 0.2% by mass.

[0061] While W contributes to improved corrosion resistance, it increases manufacturing costs due to its raw material cost and may promote the formation of δ-ferrite, a ferromagnetic phase. Therefore, the W content is within the range of less than 1.0% by mass, preferably less than 0.1%.

[0062] Ca, Mg, and Zr may be added to suppress the reduction in workability during hot and warm working processes caused by grain boundary embrittlement elements such as P and S. On the other hand, excessive addition can cause embrittlement across all temperature ranges 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. Zr may also be added in a range of less than 0.0200% by mass.

[0063] Although representative embodiments of the present invention have been described above, the present invention is not necessarily limited to these, and those 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 attached claims.

Claims

1. In mass percent, Mn: greater 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: 0.3-0.6%, including C: <0.10%, Si: <0.3%, P: <0.05%, S:<0.0020%, It has a component composition consisting of Fe and unavoidable impurities, If 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, and 756-555[C]-528[N]-10.3[Si]-12.5[Mn]-10.5[Cr]-24[Ni]-5.6[Mo]≦-110 Satisfying the conditions, 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 according to a test method compliant 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, the depth from the surface is T / 4 or D / 4).

2. The aforementioned component composition is further, In mass percent, 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%. The nonmagnetic austenitic stainless steel material according to claim 1, characterized in that it includes one or more of the groups.

3. A nonmagnetic austenitic stainless steel material according to claim 1 or 2, characterized by having a round bar shape.

4. The nonmagnetic austenitic stainless steel material according to claim 3, characterized in that it has a warm forging structure in which the structure is inclined from the core to the outer circumference.

5. The non-magnetic austenitic stainless steel material according to claim 1, characterized in that it is a steel material for drill collars with a bored cylindrical shape.

6. The above component composition is such that the content of element M is [M]% by mass, A nonmagnetic austenitic stainless steel material according to claim 1 or 2, characterized in that it satisfies 48 ≤ [Cr] + 1.27 [Ni] + 3.2 [Mo] + 5.45 [Cu].

7. In mass percentage, C: <0.10%, Si: <0.3%, Mn: greater 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: 0.3-0.6%, A method for producing a nonmagnetic austenitic stainless steel material having a component composition consisting of the remainder being Fe and unavoidable impurities, A steel ingot having a predetermined component composition is subjected to hot working followed by a cooling treatment, and during this cooling process, a hot working treatment is performed with a reduction in surface area of ​​15-50% in the temperature range of 800-300°C. Having the above component composition, and with the element M content being [M]% by mass, (40[N]+1.2[Cr]+0.07exp(0.3[Ni]+0.3[Cu]))×1.5[Mo]^(-0.18)≦60, and 756-555[C]-528[N]-10.3[Si]-12.5[Mn]-10.5[Cr]-24[Ni]-5.6[Mo]≦-110 A method for producing a nonmagnetic austenitic stainless steel material, characterized by satisfying the following conditions, having an austenitic single-phase structure, a critical pitting temperature (CPT) of 50°C or higher according to a measurement method compliant with ASTM G48 Method C, and a 0.2% proof stress of 970 MPa or higher at a depth of 1 inch from the surface (however, in the case of a thickness T or diameter D of less than 4 inches, the depth from the surface is T / 4 or D / 4).

8. The aforementioned component composition is further, In mass percent, 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%. A method for producing a nonmagnetic austenitic stainless steel material according to claim 7, characterized in that it includes one or more of the groups.

9. A method for producing a nonmagnetic austenitic stainless steel material according to claim 7 or 8, characterized by giving it a round bar shape.

10. A method for manufacturing a non-magnetic austenitic stainless steel material according to claim 9, characterized by boring the inside to form a cylindrical steel material for drill collars.

11. The above component composition is such that the content of element M is [M]% by mass, A method for producing a nonmagnetic austenitic stainless steel material according to one of 7 or 8, characterized in that 48 ≤ [Cr] + 1.27 [Ni] + 3.2 [Mo] + 5.45 [Cu] is satisfied.

Images