WO2023105852A1 - Stainless steel having excellent cold forgeability, hydrogen embrittlement resistance properties or corrosion resistance and non-magnetism - Google Patents

Abstract

Provided is a stainless steel containing, in % by mass, 8.00 to 30.00% of Ni, 9.0 to 21.0% of Cr, 0.01 to 3.00% of Mo, 0.01 to 5.00% of Cu, and 0.0010 to 0.10% of N. Furthermore, the first invention contains at least one component selected from Al in an amount of 0.001 to 2.0% and B in an amount of 0.0001 to 0.05%, has a micro-strain in a region laying between a steel material surface layer to a depth of D/4 of 0.0040 or less, and has excellent cold forgeability and hydrogen embrittlement resistance properties. The second invention contains 0.0001 to 0.05% of B and 0.0001 to 0.50% of S and also contains at least one component selected from Al in an amount of 0.001 to 2.0% and Ca in an amount of 0.0001 to 0.05%, has a precipitated B amount of 0.0001% or more in terms of a boride, has an aspect ratio of a sulfide of 50 or less, and has excellent cold forgeability, cutting performance and hydrogen embrittlement resistance properties. The third invention contains 0.0001 to 0.05% of B, has a B grain boundary occupancy of 1% or more, has excellent corrosion resistance and cold forgeability, and is non-magnetic.

Description

Stainless steel with excellent cold forgeability, hydrogen embrittlement resistance, corrosion resistance, and non-magnetism

The present invention relates to stainless steel, and particularly to stainless steel that can satisfy both cold forgeability and resistance to hydrogen embrittlement after cold working. The present invention also relates to stainless steel, and more particularly to stainless steel that can satisfy all of cold forgeability, machinability, and resistance to hydrogen embrittlement after cold working. The present invention further relates to stainless steel, and more particularly to non-magnetic stainless steel that is excellent in corrosion resistance and cold forgeability.

Fuel cell vehicles and hydrogen stations that handle hydrogen fuel use many metal parts that come into contact with hydrogen in a high-pressure gas state. Metal parts that come into contact with high-pressure hydrogen gas are prone to hydrogen embrittlement due to penetration of hydrogen into the metal. Therefore, in a high-pressure hydrogen environment, it is required to have mechanical strength and corrosion resistance as well as hydrogen embrittlement resistance.

Conventionally, austenitic stainless steels such as SUS316 and SUS316L are generally used as stainless steels used for parts that come into contact with high-pressure hydrogen gas. SUS316 and SUS316L contain Mo. On the other hand, Patent Document 1 discloses austenitic stainless steel for high-pressure hydrogen that is excellent in mechanical strength and corrosion resistance, has low hydrogen embrittlement susceptibility even at a low temperature of -40 ° C., and is inexpensive in a composition system that does not contain Mo. is disclosed. In the examples, hydrogen embrittlement susceptibility is evaluated by SSRT (Slow Strain Rate Test) for test pieces that have been cold drawn at a cold working rate of 0 to 25%.

Patent Document 2 discloses a water-resistant steel made of austenitic stainless steel having a predetermined composition, subjected to a predetermined cold working, and having a face-centered cubic (fcc) lattice crystal structure after the working. A stainless steel wire for elemental springs is disclosed. In the examples, after the solid-solution heat treatment, cold wire drawing was performed with a final working ratio of 0 to 75%, and the test pieces were charged with hydrogen and then evaluated for bending stress and tensile stress.

　Non-Patent Document 1 presents Md30 as an evaluation index for the austenite stability of austenitic stainless steel. Md30 is the temperature (° C.) at which the structure transforms into 50% martensite phase when a tensile true strain of 0.30 is applied to an austenite single phase sample. The higher the value, the more unstable the material. Non-Patent Document 1 presents the formula for Md30 as a function of component composition.

In Patent Document 3, C: 0.15 to 0.80%, Ni: 8.0 to 20.0%, Cr: 8.0 to 18.0%, Mo: 0.05 to 0.50%, A steel having a predetermined composition containing V: 0.50 to 3.00% and Al: 0.001 to 1.000%, and is a modified formula (3) of the Md30 formula described in Non-Patent Document 1 is -100 or less, and 50 or more V (C, N) precipitates of 50 nm or less are present dispersedly in 3.5 × 10 ^-2 μm ² . A high hardness non-magnetic steel is disclosed that combines hydrogen embrittlement resistance, mechanical properties and corrosion resistance.

Patent Document 4 discloses tilt rolling. In the tilt rolling, three work rolls are twisted in the same direction around the material to be rolled and arranged on the roll axes tilted. Each work roll revolves around the material to be rolled while rotating. As a result, the material to be rolled is spirally rolled while moving forward.

Conventionally, austenitic stainless steels such as SUS316 and SUS316L are generally used as stainless steels used for non-magnetic parts. On the other hand, in Patent Documents 3 and 5, C: 0.15 to 0.80%, Ni: 8.0 to 20.0%, Cr: 8.0 to 18.0%, Mo: 0.05 0.50%, V: 0.50 to 3.00%, Al: 0.001 to 1.000%. The value of the modified formula (3) is set to −100 or less, and 50 or more V(C, N) precipitates of 50 nm or less are dispersed in 3.5×10 ⁻² μm ² . A high-hardness non-magnetic steel that is inexpensive and has excellent hydrogen embrittlement resistance, mechanical properties and corrosion resistance is disclosed.

JP 2014-114471 A JP 2009-84597 A JP 2019-49036 A JP-A-05-329510 JP 2016-183372 A

Nohara et al., "Composition and Grain Size Dependence of Deformation-Induced Martensitic Transformation in Metastable Austenitic Stainless Steel," Tetsu to Hagane, 63rd Year (1977), No. 5, pp. 772-782

By using SUS316, SUS316L, or the austenitic stainless steel described in Patent Documents 1 to 3, steel with excellent hydrogen embrittlement resistance, mechanical strength and corrosion resistance is realized. All of these steels have excellent resistance to hydrogen embrittlement after hot working or steels subjected to cold working followed by solid solution heat treatment. Even after cold working, the steel subjected to cold working at a cold working rate of 25% or less in Patent Document 1 and at a final working rate of 75% or less in Patent Document 2 has excellent hydrogen embrittlement resistance. It is shown.

However, it was found that it was difficult to satisfy both cold forgeability and resistance to hydrogen embrittlement after cold working with these conventionally known steels. In particular, in the conventional technology, the material strength before cold forging is high, so the tool life is short and the forging load increases with large diameter steel bars. Therefore, it was found that the cold forgeability deteriorated due to these factors. In addition, it has been found that the processing limit (cracking) of the material occurs in conventional steel in high-strain processing such as cold forging.

The first object of the present invention is to provide a stainless steel that can lower tensile strength, improve cold forgeability, and further improve hydrogen embrittlement resistance after cold working.

It has also been found that it is difficult for the conventionally known steels to satisfy all of cold forgeability, machinability, and resistance to hydrogen embrittlement after cold working. In particular, in the conventional technology, the material strength before cold forging is high, so the tool life is short and the forging load increases with large diameter steel bars. Therefore, it was found that cold forgeability and machinability deteriorated due to these factors. In addition, it has been found that the processing limit (cracking) of the material occurs in conventional steel in high-strain processing such as cold forging.

The second object of the present invention is to provide a stainless steel that can lower tensile strength, improve cold forgeability, improve machinability, and further improve hydrogen embrittlement resistance after cold working. aim.

By using SUS316, SUS316L, or the austenitic stainless steel described in Patent Documents 3 and 5, non-magnetic steel with excellent mechanical strength is realized. However, it has been found that it is difficult to simultaneously satisfy corrosion resistance, cold forgeability, and non-magnetism after cold working in these conventionally known steels. In particular, in the prior art, the corrosion resistance deteriorates due to sensitization due to the high C content. In addition, since the strength of the material before cold forging is high, the tool life is short and the forging load for large steel bars increases. Therefore, it was found that the cold forgeability deteriorated due to these factors. Furthermore, it has been found that the processing limit (cracking) of the material occurs in conventional steel in high-strain processing such as cold forging.

A third object of the present invention is to provide a stainless steel that can improve corrosion resistance, lower tensile strength, improve cold forgeability, and further improve non-magnetic properties after cold working.

In the present invention, the following three inventions, a first invention corresponding to the first object, a second invention corresponding to the second object, and a third invention corresponding to the third object, were achieved.
That is, the gist of the present invention is as follows.

[1] <First invention>
The chemical composition, in mass %,
C: 0.0010-0.15%, Si: 0.01-2.00%, Mn: 0.01-10.00%, Ni: 8.00-30.00%, Cr: 9.0- 21.0%, Mo: 0.01-3.00%, Cu: 0.01-5.00%, N: 0.0010-0.10%,
Ti: 0-2.00%, Nb: 0-2.00%, Sn: 0-2.5%, V: 0-2.0%, W: 0-3.0%, Ga: 0-0 .05%, Co: 0-2.5%, Sb: 0-2.5%, Ta: 0-2.5%, Ca: 0-0.05%, Mg: 0-0.012%, Zr : 0-0.012%, REM: 0-0.05%, Pb: 0-0.30%, Se: 0-0.80%, Te: 0-0.30%, Bi: 0-0. 50%, S: 0-0.50%, P: 0-0.30%, and
Al: 0.001 to 2.0%, containing one or more selected from B: 0.0001 to 0.05%, the balance being Fe and impurities,
The A value represented by the following formula (a) is −100 or less,
A stainless steel having an average microstrain of 0.0040 or less from the surface layer to D/4 of the steel material.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
However, the symbol of an element in the formula (a) means the content (% by mass) of the element in the steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol.
Here, D is the diameter or thickness of the steel material, and the micro strain means the lattice strain calculated from the half width obtained by X-ray diffraction.

[2] The chemical composition is further, in mass %,
As group A, Ti: 0.01 to 2.00%, Nb: 0.01 to 2.00%, Sn: 0.0001 to 2.5%, V: 0.001 to 2.0%, W: 0.05-3.0%, Ga: 0.0004-0.05%, Co: 0.05-2.5%, Sb: 0.01-2.5%, and Ta: 0.01-2 one or more selected from .5%,
Group B selected from Ca: 0.0002 to 0.05%, Mg: 0.0002 to 0.012%, Zr: 0.0002 to 0.012%, and REM: 0.0002 to 0.05% one or more
As group C, Pb: 0.0001 to 0.30%, Se: 0.0001 to 0.80%, Te: 0.0001 to 0.30%, Bi: 0.0001 to 0.50%, S: 0.0001 to 0.50%, P: one or more selected from 0.0001 to 0.30%,
The stainless steel according to [1], containing one or more of Groups A to C of

[3] The stainless steel according to [1] or [2], which has a tensile strength of 700 MPa or less.
[4] The stainless steel according to any one of [1] to [3], which has a critical compressibility of 60% or more.
[5] The stainless steel according to any one of [1] to [4], which has a relative tensile strength of 80% or more in high-pressure hydrogen after cold working.
[6] The stainless steel according to any one of [1] to [5], which has a relative reduction in area of 50% or more in high-pressure hydrogen after cold working.

[7] <Second invention>
The chemical composition, in mass %,
C: 0.0010-0.15%, Si: 0.01-2.00%, Mn: 0.01-10.00%, Ni: 8.00-30.00%, Cr: 9.0- 21.0%, Mo: 0.01-3.00%, Cu: 0.01-5.00%, N: 0.0010-0.10%, B: 0.0001-0.05%, S : 0.0001 to 0.50%,
Ti: 0-2.00%, Nb: 0-2.00%, Sn: 0-2.5%, V: 0-2.0%, W: 0-3.0%, Ga: 0-0 .05%, Co: 0-2.5%, Sb: 0-2.5%, Ta: 0-2.5%, Mg: 0-0.012%, Zr: 0-0.012%, REM : 0-0.05%, Pb: 0-0.30%, Se: 0-0.80%, Te: 0-0.30%, Bi: 0-0.50%, P: 0-0. 30%,
and further
Al: 0.001 to 2.0%, Ca: 0.0001 to 0.05%, containing one or more selected from, the balance being Fe and impurities,
The A value represented by the following formula (a) is −100 or less,
A stainless steel characterized by having an amount of precipitated B as borides of 0.0001% or more and an aspect ratio of sulfides of 50 or less.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
However, the symbol of an element in the formula (a) means the content (% by mass) of the element in the steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol.

[8] The chemical composition is further, in mass %,
As group A, Ti: 0.01 to 2.00%, Nb: 0.01 to 2.00%, Sn: 0.0001 to 2.5%, V: 0.001 to 2.0%, W: 0.05-3.0%, Ga: 0.0004-0.05%, Co: 0.05-2.5%, Sb: 0.01-2.5%, and Ta: 0.01-2 .5%, one or more selected from
As group B, one or more selected from Mg: 0.0002 to 0.012%, Zr: 0.0002 to 0.012%, and REM: 0.0002 to 0.05%,
As group C, Pb: 0.0001 to 0.30%, Se: 0.0001 to 0.80%, Te: 0.0001 to 0.30%, Bi: 0.0001 to 0.50%, P: 0.0001 to 0.30%, one or more selected from
The stainless steel according to [7], containing one or more of Groups A to C of

[9] The stainless steel according to [7] or [8], which has a tensile strength of 700 MPa or less.
[10] The stainless steel according to any one of [7] to [9], which has a critical compressibility of 60% or more.
[11] The stainless steel according to any one of [7] to [10], which has a drilling life index VL-1000 of 1 m/min or more.
[12] The stainless steel according to any one of [7] to [11], which has a relative tensile strength of 80% or more in high-pressure hydrogen after cold working.
[13] The stainless steel according to any one of [7] to [12], which has a relative reduction in area of 50% or more in high-pressure hydrogen after cold working.

[14] <Third invention>
The chemical composition, in mass %,
C: 0.0010-0.15%, Si: 0.01-2.00%, Mn: 0.01-10.00%, Ni: 8.00-30.00%, Cr: 9.0- 21.0%, Mo: 0.01-3.00%, Cu: 0.01-5.00%, N: 0.0010-0.10%, B: 0.0001-0.05%,
Al: 0-2.0%, Ti: 0-2.00%, Nb: 0-2.00%, Sn: 0-2.5%, V: 0-2.0%, W: 0-3 .0%, Ga: 0-0.05%, Co: 0-2.5%, Sb: 0-2.5%, Ta: 0-2.5%, Ca: 0-0.05%, Mg : 0-0.012%, Zr: 0-0.012%, REM: 0-0.05%, Pb: 0-0.30%, Se: 0-0.80%, Te: 0-0. 30%, Bi: 0 to 0.50%, S: 0 to 0.50%, P: 0 to 0.30%, the balance is Fe and impurities,
The A value represented by the following formula (a) is −100 or less,
A stainless steel having a B grain boundary occupancy of 1% or more.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
However, the symbol of an element in the formula (a) means the content (% by mass) of the element in the steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol.

[15] The chemical composition is further, in mass %,
As group A, Al: 0.001 to 2.0%, Ti: 0.01 to 2.00%, Nb: 0.01 to 2.00%, Sn: 0.0001 to 2.5%, V: 0.001-2.0%, W: 0.05-3.0%, Ga: 0.0004-0.05%, Co: 0.05-2.5%, Sb: 0.01-2. 5%, and one or more selected from Ta: 0.01 to 2.5%,
As group B, Ca: 0.0002-0.05%, Mg: 0.0002-0.012%, Zr: 0.0002-0.012%, and REM: 0.0002-0.05%, from one or more selected
As group C, Pb: 0.0001 to 0.30%, Se: 0.0001 to 0.80%, Te: 0.0001 to 0.30%, Bi: 0.0001 to 0.50%, S: One or more selected from 0.0001 to 0.50%, P: 0.0001 to 0.30%,
The stainless steel according to [14], containing one or more of Groups A to C of

[16] The stainless steel according to [14] or [15], which has a pitting potential of 0.05 V or higher.
[17] The stainless steel according to any one of [14] to [16], which has a tensile strength of 700 MPa or less.
[18] The stainless steel according to any one of [14] to [17], which has a critical compressibility of 60% or more.
[19] The stainless steel according to any one of [14] to [18], which has a relative magnetic permeability of 1.10 or less after cold working.

The stainless steel of the first invention contains predetermined components, further contains one or both of Al and B, and has a micro strain of 0.0040 or less from the surface layer to D/4 of the steel material. and hydrogen embrittlement resistance after cold working.

The stainless steel of the second invention contains predetermined components, further contains one or both of Al and Ca, has an amount of B precipitated as borides of 0.0001% or more, and has a sulfide aspect ratio of 50. By having the following, it becomes possible to satisfy all of cold forgeability, machinability and resistance to hydrogen embrittlement after cold working.

The stainless steel of the third invention contains predetermined components and has a B grain boundary occupancy of 1% or more, so that it can satisfy corrosion resistance, cold forgeability, and non-magnetic properties after cold working. become.

The stainless steel of the present invention can be applied in either bar shape or plate shape. Above all, it can be used particularly suitably when used as a bar-shaped steel material. Steel bars include "steel bars", "wire rods", "steel wires", "deformed wires", "deformed steel bars" and the like. The stainless steel of the present invention is austenitic stainless steel.

As described above, the first invention aims to provide a stainless steel, particularly a bar steel, which can satisfy both cold forgeability and hydrogen embrittlement resistance after cold working.

The purpose of the second invention is to provide a stainless steel, particularly a bar steel, which can satisfy all of cold forgeability, machinability, and resistance to hydrogen embrittlement after cold working, as described above.

As described above, the third invention aims to provide stainless steel, particularly bar steel, which can satisfy corrosion resistance, cold forgeability, and non-magnetic properties after cold working.

For cold forgeability, a test piece of φ8 × 12 mm was used, and an end face restraint compression test (processing temperature: RT (room temperature), strain rate: 10 / s) was performed. The maximum compressibility at which cracks do not occur is defined as the limit compressibility, and the goal is to achieve the limit compressibility of 60% or more.

For the hydrogen embrittlement resistance after cold working, first, a sample is prepared by performing solution heat treatment and then cold working at a cold working rate of 80%. Two test pieces are prepared under the same processing conditions, one being a hydrogen test piece and the other an air test piece. A hydrogen test piece is subjected to a tensile test at a strain rate of 1×10 ⁻⁵ /s in a hydrogen atmosphere, −40° C., 70 MPa. For atmospheric specimens, tensile tests are performed at the same strain rate in atmospheric conditions. The strength and reduction of area are evaluated, respectively, and the percentage display of the value obtained by dividing the evaluation result of the hydrogen test piece by the evaluation result of the air test piece is defined as "relative strength" and "relative reduction of area", respectively. The goals of the first and second inventions are to achieve a relative strength of 80% or more and a relative reduction of area of 50% or more.

The machinability is evaluated by the drilling life index VL-1000 (maximum peripheral speed (m/min) that can be drilled to a cumulative hole depth of 1000 mm). A target of the second invention is to achieve VL-1000 of 1 m/min or more.

For corrosion resistance, the center of the L cross section (20 width x 30 length x 1 mm thickness) of a φ20 x 30 mm test piece was used as the evaluation surface, and the passivation treatment including the evaluation surface was immersed in 15% nitric acid for 30 minutes. It was carried out at After that, the evaluation surface was subjected to a pitting potential test according to JISG 0577 (3.5% NaCl, 30°C, N=3 average, V vs Ag/AgCl, saturated KCl) to measure the pitting potential. A target of the third invention is to achieve a pitting potential of 0.05 V or higher. In the examples, the pitting potential was measured immediately after polishing without the passivation treatment of the comparative material.

Regarding the non-magnetic properties after cold working, first, after performing heat treatment at 1100 ° C. for 30 minutes (water cooling) as solution heat treatment, cold working with a cold working rate (area reduction rate) of 80% is performed. A sample is prepared and the relative permeability at 1000 [Oe] is measured. The objective of the third invention is to achieve a relative permeability of 1.10 or less.

Details will be described below in the order of the first invention, the second invention, and the third invention.

<First Invention>
<<Micro strain of the stainless steel of the first invention>>
The present inventors conceived the idea of controlling the micro-strain near the surface of stainless steel, particularly bar steel, as a means of satisfying both cold forgeability and hydrogen embrittlement resistance after cold working. bottom. Microstrain is lattice strain calculated from the half width obtained by X-ray line profile analysis, and the magnitude of lattice strain is determined by lattice defects and solid solution elements in the steel material. If the micro strain is small, the crystal lattice will not be easily distorted, and the accumulation of strain in high strain processing such as cold forging will be small. In addition, we thought that cold forging of large-diameter bar steel would be possible due to the reduction of the forging load. In addition, it was conceived that by reducing the local strain concentration in the metal structure, it would lead to suppression of fracture during cold forging and suppression of fracture in a hydrogen atmosphere.

Evaluation of microstrain was measured by X-ray line profile analysis. In the L cross section of the steel material (the cross section including the center line in the case of a bar steel material), the steel material surface layer to the D/4 position are measured by X-ray diffraction using CuKα rays, and (111), (200), The half-value widths of (220) and (311) are measured, and the obtained half-value widths are substituted into the following formula (A) of the Direct-Fitting method. where D is the diameter or thickness of the steel.
ΔK=α+ε(K/ω) (A)
In formula (A), ε is microstrain and α is a value related to crystallite size. K, ΔK, and ω are as follows.
K=2 sin θ/λ,
ΔK=2β cos θ/λ
1/ω=(1/ω _h00 )−Γ{(1/ω _h00 )−1}/0.276
In the above formula, β, θ and λ are the half width (rad), Bragg reflection angle (rad) and X-ray wavelength (CuKα=0.15405 nm) of each diffraction line, respectively. Γ is a function of diffraction line indices h, k, and l, and is given below.
Γ=(h ² k ² +k ² l ² +l ² h ² )/(h ² +k ² +l ² ) ²
ω _h00 is a constant, and is determined so as to minimize the error when linearly approximating ΔK and (K/ω) in Equation ₍ A). After that, the slope of the linear approximation of the above ΔK and (K/ω) is calculated as the micro strain ε.

Then, if the diameter or thickness of the steel material is D, and the average microstrain of the steel material surface layer to D/4 is 0.0040 or less, the above target cold forgeability and resistance to hydrogen embrittlement after cold working It was found that both characteristics were satisfied. The average microstrain from the surface layer to D/4 of the steel material is more preferably 0.0020 or less, still more preferably 0.0010 or less, and even more preferably 0.0005 or less. The vicinity of the steel material surface layer tends to be a fracture starting point in cold forging, but by controlling the micro strain of the steel material surface layer to D/4 to be small, the deformability of the steel material is increased during cold forging. Strain control becomes important. The micro strain from the surface layer to D/4 of the steel material is the average value at the position from the surface layer to D/4 of the steel material.

<<Component composition of the stainless steel of the first invention>>
Next, the chemical composition of the stainless steel of the first invention will be explained. In the component composition, % means % by mass, including the second and third inventions.

(At least one selected from Al: 0.001 to 2.0%, B: 0.0001 to 0.05%)
The stainless steel of the first invention contains one or more selected from Al: 0.001 to 2.0% and B: 0.0001 to 0.05%. This reduces microstrain, improves cold forgeability, and enhances resistance to hydrogen embrittlement. Also, Al is effective as a deoxidizing element.
Excessive addition of Al results in the formation of coarse AlN and the like, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Al content is set to 2.0%, preferably 1.0% or less, more preferably 0.5% or less, further preferably 0.05% or less.
The reason for the lower limit of B is as described above, and in addition, it has the effect of forming B-based precipitates to improve machinability. Preferably it is 0.0005% or more. Excessive addition of B forms coarse B-based precipitates and the like, deteriorating cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the B content is set to 0.05%, preferably 0.02% or less, and more preferably 0.015% or less.
If neither Al nor B is contained, or if the lower limit is exceeded, the microstrain will be outside the range of the present invention, and the tensile strength, critical compressibility, relative tensile strength after cold working, and reduction of area will be poor.

(C: 0.0010 to 0.15%)
C is made 0.0010% or more in order to suppress the formation of deformation-induced martensite and improve hydrogen embrittlement resistance. Excessive addition of C increases microstrain and deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the C content is set to 0.15%, preferably 0.12% or less, more preferably 0.05% or less, still more preferably 0.02% or less. It is preferable to set the upper limit of C to less than 0.15%.

(Si: 0.01 to 2.00%)
Si is added as a deoxidizing element to be 0.01% or more. Excessive addition of Si increases microstrain and deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Si content is set to 2.0%, preferably 1.2% or less, more preferably 0.6% or less, still more preferably 0.5% or less.

(Mn: 0.01 to 10.00%)
Mn is made 0.01% or more in order to suppress the formation of deformation-induced martensite and improve hydrogen embrittlement resistance. Excessive addition of Mn increases microstrain and deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Mn content is set to 10.0%, preferably 2.5% or less, more preferably 1.5% or less, still more preferably 1.0% or less.

(Ni: 8.00-30.00%)
Ni suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. Also, in order to reduce microstrain and improve cold forgeability, the Ni content is made 8.00% or more. It is preferably 10.00% or more, more preferably 13.00% or more, and still more preferably 15.00% or more. Excessive addition of Ni, on the contrary, increases microstrain and deteriorates cold forgeability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Ni content is 30.00%, preferably 25.00% or less.

(Cr: 9.0 to 21.0%)
Cr suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. Moreover, in order to improve corrosion resistance, the Cr content is set to 9.0% or more. Preferably it is 10.5% or more. Excessive addition of Cr increases microstrain and degrades cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Cr content is set to 21.0%, preferably 19.5% or less, more preferably 15.0% or less.

(Mo: 0.01 to 3.00%)
Mo suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. In addition to improving corrosion resistance, the Mo content is set to 0.01% or more in order to reduce microstrain and improve cold forgeability. Excessive addition of Mo, on the contrary, increases microstrain and deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Mo content is 3.0%, preferably 2.8% or less, more preferably 2.5% or less, and even more preferably 1.0% or less.

(Cu: 0.01 to 5.00%)
Cu suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. Also, in order to reduce microstrain and improve cold forgeability, the Cu content is made 0.01% or more. It is preferably 1.00% or more, more preferably 2.00% or more. Excessive addition of Cu conversely increases microstrain, degrades cold forgeability and hydrogen embrittlement resistance, and causes hot shortness. Therefore, the upper limit of the Cu content is set to 5.00%, preferably 3.50% or less.

(N: 0.0010 to 0.10%)
N is made 0.0010% or more in order to suppress the formation of deformation-induced martensite and improve hydrogen embrittlement resistance. Excessive addition of N increases microstrain and deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the N content is set to 0.10%, preferably 0.08% or less, more preferably 0.05% or less, still more preferably 0.03% or less.

The stainless steel of the first invention contains the above components, and the balance is Fe and impurities. Furthermore, it may contain one or more selected from the following components.

(Ti: 0 to 2.00%)
Ti may be added for fixing C and N which enhance microstrain. On the other hand, if Ti is added excessively, coarse Ti-based precipitates and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Ti content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Ti is 0.03% or more, more preferably 0.05% or more.

(Nb: 0 to 2.00%)
Nb may be added for fixing C and N to enhance microstrain. On the other hand, if Nb is added excessively, coarse Nb-based precipitates and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Nb content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Nb is 0.03% or more, more preferably 0.05% or more.

(Sn: 0-2.5%)
Since Sn is an effective element for improving corrosion resistance, it may be contained. However, excessive Sn content saturates the effect, and conversely, there is a possibility that the cold forgeability and the hydrogen embrittlement resistance deteriorate. Therefore, the upper limit of the content of Sn is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.2% or less. In order to exhibit the above effects, the Sn content is preferably 0.0001% or more, more preferably 0.01% or more. More preferably, it is 0.05% or more.

(V: 0-2.0%)
V may be added for fixing C and N which enhances microstrain. On the other hand, excessive addition of V forms coarse V-based precipitates and the like, deteriorating cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the V content is set to 2.0%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of V is 0.001%.

(W: 0-3.0%)
Since W is an effective element for improving corrosion resistance, it may be contained. However, if W is contained excessively, the effect is saturated, and the cold forgeability and hydrogen embrittlement resistance may deteriorate. Therefore, when W is contained, the upper limit is set to 3.0%. More preferably, it is 2.0% or less, and still more preferably 1.5% or less. In order to exhibit the above effects, it is preferable to set the W amount to 0.05% or more. More preferably, it is 0.10% or more.

(Ga: 0 to 0.05%)
Ga is an effective element for improving corrosion resistance, so it may be contained. However, if Ga is contained excessively, the effect is saturated, and the cold forgeability and hydrogen embrittlement resistance may deteriorate. Therefore, the upper limit when Ga is contained is set to 0.05%. In order to exhibit the above effect, it is preferable to set the amount of Ga to 0.0004% or more.

(Co: 0-2.5%)
Co has the effect of improving the corrosion resistance, so it may be contained. However, if Co is contained excessively, the effect is saturated, and the cold forgeability and hydrogen embrittlement resistance may deteriorate. Therefore, the upper limit of the content of Co is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.8% or less. In order to exhibit the above effects, the Co content is preferably 0.05% or more, more preferably 0.10% or more.

(Sb: 0-2.5%)
Since Sb has an effect of improving corrosion resistance, it may be contained. However, if Sb is contained excessively, the effect is saturated, and the cold forgeability and hydrogen embrittlement resistance may deteriorate. Therefore, the upper limit when Sb is contained is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.8% or less. In order to exhibit the above effect, the Sb content is preferably 0.01% or more, more preferably 0.05% or more.

(Ta: 0-2.5%)
Ta may be added for fixing C and N to enhance microstrain. On the other hand, if Ta is added excessively, coarse Ta-based precipitates and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Ta content is set to 2.5%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Ta is 0.01%.

(Ca: 0-0.05%)
Ca may be contained as needed for deoxidation. On the other hand, if Ca is added excessively, coarse Ca-based inclusions and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Ca content is 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. A preferable lower limit of Ca is 0.0002%.

(Mg: 0-0.012%)
Mg may be contained as necessary for deoxidation. On the other hand, if Mg is excessively added, coarse Mg-based inclusions and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Mg content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. A preferred lower limit for Mg is 0.0002%.

(Zr: 0 to 0.012%)
Zr may be contained as necessary for deoxidation. On the other hand, if Zr is excessively added, coarse Zr-based inclusions and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Zr content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. A preferable lower limit of Zr is 0.0002%.

(REM: 0-0.05%)
REM may optionally be included for deacidification. On the other hand, if REM is excessively added, coarse REM inclusions and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the REM content is 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. A preferred lower limit for REM is 0.0002%.

(Pb: 0 to 0.30%)
Pb is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Pb deteriorates the cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Pb content is set to 0.30%, preferably 0.10% or less, and more preferably 0.05% or less. A preferable lower limit of Pb is 0.0001%.

(Se: 0 to 0.80%)
Se is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Se deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Se content is 0.80%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for Se is 0.0001%.

(Te: 0 to 0.30%)
Te is an element that enhances machinability and may be contained as necessary. On the other hand, if Te is added excessively, cold forgeability and hydrogen embrittlement resistance deteriorate. Therefore, the upper limit of the Te content is set to 0.30%, preferably 0.1% or less, and more preferably 0.05% or less. A preferable lower limit of Te is 0.0001%.

(Bi: 0 to 0.50%)
Bi is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Bi degrades cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Bi content is 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. A preferable lower limit of Bi is 0.0001%.

(S: 0-0.50%)
S is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of S degrades cold forgeability and resistance to hydrogen embrittlement. Therefore, the upper limit of the S content is 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for S is 0.0001%. Incidentally, S is usually contained in steel as an impurity mixed from steelmaking raw materials.

(P: 0-0.30%)
P is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of P degrades cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the P content is 0.30%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for P is 0.0001%.

<Second invention>
<<Amount of precipitated B as borides on the steel material of the second invention>>
The inventors of the present invention have found that as a means of satisfying all of the cold forgeability, machinability, and resistance to hydrogen embrittlement after cold working in stainless steel materials, the amount of B precipitated as borides in steel materials is controlled. was conceived. With regard to cold forgeability, the formation of borides reduces solute elements (such as N) and softens the steel. In addition, since borides are finely precipitated, they are less likely to initiate cracks, which also improves the cold forgeability. Regarding machinability, the lubricating action of borides prolongs the tool life during cutting. Regarding the resistance to hydrogen embrittlement, the formation of borides softens the material and increases the mobility of dislocations. In addition, the borides contribute as trap sites for hydrogen, thereby improving the resistance to hydrogen embrittlement. By setting the amount of precipitated B as a boride to 0.0001% by mass or more, together with the definition of the aspect ratio of the sulfide described below, cold forgeability, machinability, and hydrogen embrittlement resistance after cold working can satisfy all of

The amount of B precipitated as borides in the steel can be evaluated by subjecting the steel to electrolytic extraction residue, extracting the borides, and measuring the amount of B (Bpre) in the borides.

<<Aspect ratio of sulfide in the steel material of the second invention>>
The present inventors conceived the idea of controlling the aspect ratio of sulfides in steel materials as a means of satisfying all of the cold forgeability, machinability, and resistance to hydrogen embrittlement after cold working in stainless steel. bottom. When the aspect ratio of sulfides in the steel material is small, it is difficult to act as a fracture starting point, so cold forgeability and hydrogen embrittlement resistance are improved. Also, when the aspect ratio of the sulfide is small, the lubricating effect increases and the tool life increases, thus improving the machinability. When the aspect ratio of the sulfide is 50 or less, together with the regulation of the amount of precipitated B as the boride, all of cold forgeability, machinability, and hydrogen embrittlement resistance after cold working are satisfied. be able to. The aspect ratio means a value calculated as L/W from the length (L) of the sulfide in the rolling direction and the length (W) of the sulfide in the direction perpendicular to the rolling direction.

Regarding the evaluation of the aspect ratio of sulfide, in the L cross section of the steel material (the cross section including the center line of the steel material), the surface layer, the central part, and the 1/4 depth position part existing between the surface layer and the central part In , one or more fields of view are measured in a field of view of 200 times, and the average value of the aspect ratio L/W of the sulfides in the same field of view is calculated from an optical microscope.

<<Component composition of the stainless steel of the second invention>>
Next, the chemical composition of the stainless steel of the second invention will be explained. In the component composition, % means % by mass.

(B: 0.0001 to 0.05%)
B is necessary in order to secure the amount of precipitated B as the boride. By containing 0.0001% or more of B, it is possible to ensure the amount of precipitated B as borides, coupled with the regulation of the manufacturing method described later. The B content is more preferably 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, if the B content exceeds 0.05%, coarse borides are formed in the steel, and the coarse borides become fracture starting points, so cold forgeability, machinability, and hydrogen embrittlement resistance deteriorate. Therefore, the upper limit is set to 0.05%. The B content is more preferably 0.02% or less. It is more preferably 0.015% or less.

(S: 0.0001 to 0.50%)
S is an element that forms sulfides in steel and improves machinability, and is contained in an amount of 0.0001% or more. On the other hand, if S is added excessively, cold forgeability, machinability, and resistance to hydrogen embrittlement deteriorate, so the upper limit is made 0.50%, preferably 0.1% or less, more preferably 0.5%. 05% or less. Incidentally, S is usually contained in steel as an impurity mixed from steelmaking raw materials.

(At least one selected from Al: 0.001 to 2.0% and Ca: 0.0001 to 0.05%)
The stainless steel of the second invention contains one or more selected from Al: 0.001 to 2.0% and Ca: 0.0001 to 0.05%. By containing one or more of Al and Ca in an amount equal to or higher than the above lower limit, an Al- or Ca-based oxide is formed, and together with the above-mentioned content of S and the provisions of the production method described later, it becomes the nucleus of sulfide and fine sulfidation. The aspect ratio of the sulfide after rolling can be 50 or less.
Excessive addition of Al forms coarse AlN and the like, deteriorating cold forgeability, machinability, and resistance to hydrogen embrittlement. Therefore, the upper limit of the Al content is set to 2.0%, preferably 1.0% or less, more preferably 0.5% or less, further preferably 0.05% or less.
When excessive Ca is added, coarse Ca-based inclusions and the like are formed, and the sulfides formed around them also become large. Forgeability, machinability and resistance to hydrogen embrittlement deteriorate. Therefore, the upper limit of Ca content is set to 0.05%. Ca is preferably 0.010% or less, more preferably 0.005% or less.
If neither Al nor Ca is contained, or if the lower limit is exceeded, the aspect ratio of the sulfide will be outside the range of the present invention, and the tensile strength, critical compressibility, machinability, relative tensile strength after cold working, and drawing will be poor. becomes.

(C: 0.0010 to 0.15%)
C is made 0.0010% or more in order to suppress the formation of deformation-induced martensite and improve hydrogen embrittlement resistance. Excessive addition of C degrades cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the C content is set to 0.15%, preferably 0.12% or less, more preferably 0.05% or less, still more preferably 0.02% or less. It is preferable to set the upper limit of C to less than 0.15%.

(Si: 0.01 to 2.00%)
Si is added as a deoxidizing element to be 0.01% or more. Excessive addition of Si deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Si content is set to 2.0%, preferably 1.2% or less, more preferably 0.6% or less, still more preferably 0.5% or less.

(Mn: 0.01 to 10.00%)
Mn is made 0.01% or more in order to suppress the formation of deformation-induced martensite and improve hydrogen embrittlement resistance. Excessive addition of Mn deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Mn content is set to 10.0%, preferably 2.5% or less, more preferably 1.5% or less, still more preferably 1.0% or less.

(Ni: 8.00-30.00%)
Ni suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. Moreover, in order to improve cold forgeability, the Ni content is set to 8.00% or more. It is preferably 10.00% or more, more preferably 13.00% or more, and still more preferably 15.00% or more. If Ni is added excessively, cold forgeability, machinability and resistance to hydrogen embrittlement deteriorate. Therefore, the upper limit of the Ni content is 30.00%, preferably 25.00% or less.

(Cr: 9.0 to 21.0%)
Cr suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. Moreover, in order to improve corrosion resistance, the Cr content is set to 9.0% or more. Preferably it is 10.5% or more. Excessive addition of Cr deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Cr content is set to 21.0%, preferably 19.5% or less, more preferably 15.0% or less.

(Mo: 0.01 to 3.00%)
Mo suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. In addition to improving corrosion resistance, the Mo content is set to 0.01% or more in order to improve cold forgeability. Excessive addition of Mo deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Mo content is 3.0%, preferably 2.8% or less, more preferably 2.5% or less, and even more preferably 1.0% or less.

(Cu: 0.01 to 5.00%)
Cu suppresses the formation of deformation-induced martensite and enhances hydrogen embrittlement resistance. Moreover, in order to improve the cold forgeability, the Cu content is made 0.01% or more. It is preferably 1.00% or more, more preferably 2.00% or more. Excessive addition of Cu deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement, and causes hot shortness. Therefore, the upper limit of the Cu content is set to 5.00%, preferably 3.50% or less.

(N: 0.0010 to 0.10%)
N is made 0.0010% or more in order to suppress the formation of deformation-induced martensite and improve hydrogen embrittlement resistance. Excessive addition of N degrades cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the N content is set to 0.10%, preferably 0.08% or less, more preferably 0.05% or less, still more preferably 0.03% or less.

The stainless steel of the second invention contains the above components, and the balance is Fe and impurities. Furthermore, it may contain one or more selected from the following components.

(Ti: 0 to 2.00%)
Ti may be added for fixing C and N which enhance microstrain. On the other hand, if Ti is added excessively, coarse Ti-based precipitates and the like are formed, degrading cold forgeability, machinability, and resistance to hydrogen embrittlement. Therefore, the upper limit of the Ti content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Ti is 0.01% or more, more preferably 0.05% or more.

(Nb: 0 to 2.00%)
Nb may be added for fixing C and N. On the other hand, if Nb is added excessively, coarse Nb-based precipitates and the like are formed, deteriorating cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Nb content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Nb is 0.01% or more, more preferably 0.05% or more.

(Sn: 0-2.5%)
Since Sn is an effective element for improving corrosion resistance, it may be contained. However, if Sn is contained excessively, the effect is saturated, and cold forgeability, machinability, and resistance to hydrogen embrittlement may deteriorate. Therefore, the upper limit of the content of Sn is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.2% or less. In order to exhibit the above effects, the Sn content is preferably 0.0001% or more, more preferably 0.01% or more. More preferably, it is 0.05% or more.

(V: 0-2.0%)
V may be added for fixing C and N. On the other hand, excessive addition of V forms coarse V-based precipitates and the like, deteriorating cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the V content is 2.0%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of V is 0.001%.

(W: 0-3.0%)
Since W is an effective element for improving corrosion resistance, it may be contained. However, if W is contained excessively, the effect is saturated, and cold forgeability, machinability, and resistance to hydrogen embrittlement may deteriorate. Therefore, when W is contained, the upper limit is set to 3.0%. More preferably, it is 2.0% or less, and still more preferably 1.5% or less. In order to exhibit the above effects, it is preferable to set the W amount to 0.05% or more. More preferably, it is 0.10% or more.

(Ga: 0 to 0.05%)
Ga is an effective element for improving corrosion resistance, so it may be contained. However, if Ga is contained excessively, the effect is saturated, and the cold forgeability, machinability, and resistance to hydrogen embrittlement may deteriorate. Therefore, the upper limit when Ga is contained is set to 0.05%. In order to exhibit the above effect, it is preferable to set the amount of Ga to 0.0004% or more.

(Co: 0-2.5%)
Co has the effect of improving the corrosion resistance, so it may be contained. However, if Co is contained excessively, the effect is saturated, and cold forgeability, machinability, and resistance to hydrogen embrittlement may deteriorate. Therefore, the upper limit of the content of Co is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.8% or less. In order to exhibit the above effects, the Co content is preferably 0.05% or more, more preferably 0.10% or more.

(Sb: 0-2.5%)
Since Sb has an effect of improving corrosion resistance, it may be contained. However, if Sb is contained excessively, the effect is saturated, and cold forgeability, machinability, and resistance to hydrogen embrittlement may deteriorate. Therefore, the upper limit when Sb is contained is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.8% or less. In order to exhibit the above effect, the Sb content is preferably 0.01% or more, more preferably 0.05% or more.

(Ta: 0-2.5%)
Ta may be added for fixing C and N. On the other hand, if Ta is added excessively, coarse Ta-based precipitates and the like are formed, degrading cold forgeability, machinability, and hydrogen embrittlement resistance. Therefore, the upper limit of the Ta content is set to 2.5%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Ta is 0.01%.

(Mg: 0-0.012%)
Mg may be contained as necessary for deoxidation. On the other hand, if Mg is excessively added, coarse Mg-based inclusions and the like are formed, deteriorating cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Mg content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. A preferred lower limit for Mg is 0.0002%.

(Zr: 0 to 0.012%)
Zr may be contained as necessary for deoxidation. On the other hand, if Zr is added excessively, coarse Zr-based inclusions and the like are formed, degrading cold forgeability, machinability, and resistance to hydrogen embrittlement. Therefore, the upper limit of the Zr content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. A preferable lower limit of Zr is 0.0002%.

(REM: 0-0.05%)
REM may optionally be included for deacidification. On the other hand, if REM is added excessively, coarse REM inclusions and the like are formed, deteriorating cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the REM content is 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. A preferred lower limit for REM is 0.0002%.

(Pb: 0 to 0.30%)
Pb is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Pb deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Pb content is set to 0.30%, preferably 0.10% or less, and more preferably 0.05% or less. A preferable lower limit of Pb is 0.0001%.

(Se: 0 to 0.80%)
Se is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Se deteriorates cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Se content is 0.80%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for Se is 0.0001%.

(Te: 0 to 0.30%)
Te is an element that enhances machinability and may be contained as necessary. On the other hand, if Te is added excessively, cold forgeability, machinability and resistance to hydrogen embrittlement deteriorate. Therefore, the upper limit of the Te content is set to 0.30%, preferably 0.1% or less, and more preferably 0.05% or less. A preferable lower limit of Te is 0.0001%.

(Bi: 0 to 0.50%)
Bi is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Bi degrades cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the Bi content is 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. A preferable lower limit of Bi is 0.0001%.

(P: 0-0.30%)
P is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of P degrades cold forgeability, machinability and resistance to hydrogen embrittlement. Therefore, the upper limit of the P content is 0.30%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for P is 0.0001%.

<Third invention>
<<B Grain Boundary Occupancy of the Stainless Steel of the Third Invention>>
The present inventors came up with the idea of controlling the B grain boundary occupancy of stainless steel, particularly bar steel, as a means of satisfying corrosion resistance, cold forgeability, and non-magnetic properties after cold working. The B grain boundary occupation ratio (%) is the ratio (B/A×100) occupied by the grain boundaries (B) in which a finite amount of B exists with respect to all the grain boundaries (A). When the B grain boundary occupancy is large, passivation is promoted in the Cr-deficient region due to grain boundary Cr-based precipitates, improving corrosion resistance, facilitating plastic deformation at grain boundaries, and improving cold forgeability. It was conceived that the local deformation in the field is suppressed, suppressing the formation of deformation-induced α'-martensite in the magnetic phase, and maintaining the non-magnetism.

The evaluation of the B grain boundary occupation rate was measured by EPMA analysis. In the L cross section of the steel material (the cross section including the center line if it is a bar steel material), the total length (A) of the grain boundary in an arbitrary field of view is measured. A grain boundary having a higher B concentration than the parent phase was defined as a B grain boundary occupancy, the B grain boundary occupancy length (B) was calculated, and the B grain boundary occupancy was calculated from the above formula.

It was also found that if the B grain boundary occupation ratio of the steel material is 1% or more, the above-mentioned targets of corrosion resistance, cold forgeability, and non-magnetic properties after cold working can be satisfied. The average B grain boundary occupation rate is more preferably 5% or more, more preferably 15% or more, and even more preferably 20% or more.

<<Component composition of the stainless steel of the third invention>>
Next, the chemical composition of the stainless steel of the third invention will be explained. In the component composition, % means % by mass.

(C: 0.0010 to 0.15%)
C is made 0.0010% or more in order to suppress the formation of deformation-induced martensite and improve non-magnetic properties. Excessive addition of C lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the C content is set to 0.15%, preferably 0.12% or less, more preferably 0.05% or less, still more preferably 0.02% or less. It is preferable to set the upper limit of C to less than 0.15%.

(Si: 0.01 to 2.00%)
Si is added as a deoxidizing element to be 0.01% or more. Excessive addition of Si lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Si content is set to 2.0%, preferably 1.2% or less, more preferably 0.6% or less, still more preferably 0.5% or less.

(Mn: 0.01 to 10.00%)
Mn is made 0.01% or more in order to suppress the formation of deformation-induced martensite and improve non-magnetic properties. Excessive addition of Mn lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Mn content is set to 10.0%, preferably 2.5% or less, more preferably 1.5% or less, still more preferably 1.0% or less.

(Ni: 8.00-30.00%)
Ni suppresses the formation of deformation-induced martensite and enhances non-magnetic properties. Moreover, in order to improve cold forgeability, the Ni content is set to 8.00% or more. It is preferably 10.00% or more, more preferably 13.00% or more, and still more preferably 15.00% or more. Excessive addition of Ni lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ni content is 30.00%, preferably 25.00% or less.

(Cr: 9.0 to 21.0%)
Cr suppresses the formation of deformation-induced martensite and enhances non-magnetic properties. Moreover, in order to improve corrosion resistance, the Cr content is set to 9.0% or more. Preferably it is 10.5% or more. Excessive addition of Cr lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Cr content is set to 21.0%, preferably 19.5% or less, more preferably 15.0% or less.

(Mo: 0.01 to 3.00%)
Mo suppresses the formation of deformation-induced martensite and enhances non-magnetic properties. In addition to improving corrosion resistance, the Mo content is set to 0.01% or more in order to improve cold forgeability. Excessive addition of Mo lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Mo content is 3.0%, preferably 2.8% or less, more preferably 2.5% or less, and even more preferably 1.0% or less.

(Cu: 0.01 to 5.00%)
Cu suppresses the formation of deformation-induced martensite and enhances non-magnetic properties. Moreover, in order to improve the cold forgeability, the Cu content is made 0.01% or more. It is preferably 1.00% or more, more preferably 2.00% or more. Excessive addition of Cu lowers the B grain boundary occupancy, degrades corrosion resistance, cold forgeability, and non-magnetic properties, and causes hot shortness. Therefore, the upper limit of the Cu content is set to 5.00%, preferably 3.50% or less.

(N: 0.0010 to 0.10%)
N is made 0.0010% or more in order to suppress the formation of deformation-induced martensite and improve non-magnetic properties. Excessive addition of N lowers the B grain boundary occupancy and deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the N content is set to 0.10%, preferably 0.08% or less, more preferably 0.05% or less, still more preferably 0.03% or less.

(B: 0.0001 to 0.05%)
B is the main element that increases the B grain boundary occupation ratio, and is made 0.0001% or more in order to improve corrosion resistance, cold forgeability, and non-magnetic properties. Preferably it is 0.0005% or more. Excessive addition of B causes the formation of coarse B-based precipitates and conversely deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the B content is set to 0.05%, preferably 0.02% or less, and more preferably 0.015% or less.

The stainless steel of the third invention contains the above components, and the balance is Fe and impurities. Furthermore, it may contain one or more selected from the following components.

(Al: 0-2.0%)
Al may be added for fixing N which lowers the B grain boundary occupancy. On the other hand, if Al is added excessively, coarse Al-based precipitates are formed, and corrosion resistance, cold forgeability, and non-magnetic properties deteriorate. Therefore, the upper limit of the Al content is set to 2.0%, preferably 1.0% or less, more preferably 0.5% or less, further preferably 0.05% or less. A preferable lower limit of Al is 0.001% or more.

(Ti: 0 to 2.00%)
Ti may be added for fixing C and N which lowers the B grain boundary occupancy. On the other hand, if Ti is added excessively, coarse Ti-based precipitates and the like are formed, degrading corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ti content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Ti is 0.01% or more, more preferably 0.05% or more.

(Nb: 0 to 2.00%)
Nb may be added for fixing C and N which lowers the B grain boundary occupancy. On the other hand, if Ti is added excessively, coarse Nb-based precipitates and the like are formed, degrading corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Nb content is set to 2.00%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Nb is 0.01% or more, more preferably 0.05% or more.

(Sn: 0-2.5%)
Since Sn is an effective element for improving corrosion resistance, it may be contained. However, excessive Sn content saturates the effect, and conversely, the corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit of the content of Sn is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.2% or less. In order to exhibit the above effects, the Sn content is preferably 0.0001% or more, more preferably 0.01% or more. More preferably, it is 0.05% or more.

(V: 0-2.0%)
V may be added for fixing C and N which lower the B grain boundary occupation rate. On the other hand, excessive addition of V forms coarse V-based precipitates and the like, deteriorating corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the V content is set to 2.0%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of V is 0.001%.

(W: 0-3.0%)
Since W is an effective element for improving corrosion resistance, it may be contained. However, if W is contained excessively, the effect is saturated, and the corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, when W is contained, the upper limit is set to 3.0%. More preferably, it is 2.0% or less, and still more preferably 1.5% or less. In order to exhibit the above effects, it is preferable to set the W amount to 0.05% or more. More preferably, it is 0.10% or more.

(Ga: 0 to 0.05%)
Ga is an effective element for improving corrosion resistance, so it may be contained. However, if Ga is contained excessively, the effect is saturated, and the corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit when Ga is contained is set to 0.05%. In order to exhibit the above effect, it is preferable to set the amount of Ga to 0.0004% or more.

(Co: 0-2.5%)
Co has the effect of improving the corrosion resistance, so it may be contained. However, if Co is contained excessively, the effect is saturated, and the corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit of the content of Co is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.8% or less. In order to exhibit the above effects, the Co content is preferably 0.05% or more, more preferably 0.10% or more.

(Sb: 0-2.5%)
Since Sb has an effect of improving corrosion resistance, it may be contained. However, if Sb is contained excessively, the effect is saturated, and the corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit when Sb is contained is set to 2.5%. More preferably, it is 1.0% or less, and still more preferably 0.8% or less. In order to exhibit the above effect, the Sb content is preferably 0.01% or more, more preferably 0.05% or more.

(Ta: 0-2.5%)
Ta may be added for fixing C and N which lowers the B grain boundary occupancy. On the other hand, if Ta is added excessively, coarse Ta-based precipitates and the like are formed, degrading corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ta content is set to 2.5%, preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less. A preferable lower limit of Ta is 0.01%.

(Ca: 0-0.05%)
Ca may be contained as needed for deoxidation. On the other hand, if Ca is added excessively, coarse Ca-based inclusions and the like are formed, degrading corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ca content is 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. A preferable lower limit of Ca is 0.0002%.

(Mg: 0-0.012%)
Mg may be contained as necessary for deoxidation. On the other hand, if Mg is excessively added, coarse Mg-based inclusions and the like are formed, degrading cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Mg content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. A preferred lower limit for Mg is 0.0002%.

(Zr: 0 to 0.012%)
Zr may be contained as necessary for deoxidation. On the other hand, if Zr is added excessively, coarse Zr-based inclusions and the like are formed, degrading corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Zr content is 0.012%, preferably 0.010% or less, and more preferably 0.005% or less. A preferable lower limit of Zr is 0.0002%.

(REM: 0-0.05%)
REM may optionally be included for deacidification. On the other hand, if REM is added excessively, coarse REM-based inclusions are formed, and corrosion resistance, cold forgeability, and non-magnetic properties deteriorate. Therefore, the upper limit of the REM content is 0.05%, preferably 0.010% or less, and more preferably 0.005% or less. A preferred lower limit for REM is 0.0002%.

(Pb: 0 to 0.30%)
Pb is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Pb deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Pb content is set to 0.30%, preferably 0.10% or less, and more preferably 0.05% or less. A preferable lower limit of Pb is 0.0001%.

(Se: 0 to 0.80%)
Se is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Se deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Se content is 0.80%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for Se is 0.0001%.

(Te: 0 to 0.30%)
Te is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Te deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Te content is set to 0.30%, preferably 0.1% or less, and more preferably 0.05% or less. A preferable lower limit of Te is 0.0001%.

(Bi: 0 to 0.50%)
Bi is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of Bi degrades corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Bi content is 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. A preferable lower limit of Bi is 0.0001%.

(S: 0-0.50%)
S is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of S degrades corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the S content is 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for S is 0.0001%. Incidentally, S is usually contained in steel as an impurity mixed from steelmaking raw materials.

(P: 0-0.30%)
P is an element that enhances machinability and may be contained as necessary. On the other hand, excessive addition of P degrades corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the P content is 0.30%, preferably 0.1% or less, and more preferably 0.05% or less. A preferred lower limit for P is 0.0001%.

<Common to the first to third inventions>
<<A value of formula (a)>>
Based on the formula for Md30 described in Non-Patent Document 1, the following formula (a) was introduced.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
The symbol of an element in formula (a) means the content (% by mass) of the element in steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol. The above formula (a) corresponds to the Md30 formula described in Non-Patent Document 1 with the Nb term removed. The reason for deleting the Nb term is that the addition ratio of Nb is small and its contribution to Md30 is small.
In the first to third inventions, the A value represented by the above formula (a) is -100 or less. By setting the A value to −100 or less, the formation of strain-induced martensite is suppressed, and work hardening is reduced, which softens the steel and suppresses the occurrence of cracks, thereby improving the cold forgeability. Furthermore, the first invention can obtain the effects of reducing microstrain and improving hydrogen embrittlement resistance. In the second invention, softening reduces cutting resistance and improves machinability. As for the resistance to hydrogen embrittlement, since deformation-induced martensite at the starting point of fracture is reduced, the resistance to hydrogen embrittlement is improved. The third invention can obtain the effect of improving the non-magnetic properties.

<<Quality of the steel materials of the first to third inventions>>
The stainless steel of the present invention, particularly the bar steel, has the above-mentioned chemical composition, and the first invention has a micro strain of D/4 from the surface layer of the steel material. As a result of having the aspect ratio of , the third invention can realize the following qualities as a result of having the B grain boundary occupancy of the steel material.

<Common to the first to third inventions>
It can be stainless steel with a tensile strength of 700 MPa or less.
<Common to the first to third inventions>
A stainless steel having a critical compressibility of 60% or more can be used. Here, regarding the evaluation of the critical compressibility, the above-described method is used for all of the shape of the test piece, the contents of the compression test, and the definition of the critical compressibility.

<Common to the first and second inventions>
A stainless steel having a relative tensile strength of 80% or more in high-pressure hydrogen after cold working can be used.
<Common to the first and second inventions>
A stainless steel having a relative reduction in area of 50% or more in high-pressure hydrogen after cold working can be used.
Here, the cold working rate (area reduction rate) of the cold working is 80%. For the evaluation of tensile strength and reduction in area in high-pressure hydrogen, a tensile test is performed at a strain rate of 1×10 ⁻⁵ /s in a hydrogen atmosphere at −40° C. and 70 MPa. The % display of the value obtained by dividing the tensile strength and reduction of area in the hydrogen atmosphere obtained in this way by the tensile strength and reduction of area evaluated at the same strain rate in the air atmosphere is the relative tensile strength in high pressure hydrogen, and the value in high pressure hydrogen. is the relative aperture of

<Second invention>
Stainless steel having a drilling life index VL-1000 of 1 m/min or more can be used.

<Third invention>
Stainless steel having a pitting potential of 0.05 V or higher can be used.

<Third invention>
Stainless steel having a relative magnetic permeability of 1.10 or less after cold working can be used.
Here, the cold working rate (area reduction rate) of the cold working is 80%.

<<Manufacturing method of steel materials according to the first to third inventions>>
Hereinafter, the steel material manufacturing methods of the first to third inventions will be described in order.

<First Invention>
In producing the stainless steel of the first invention, particularly the bar-shaped steel, it is preferable to use tilt rolling as the hot working method and use induction heating to heat the raw material before tilt rolling.

A set temperature is provided in the induction heating device that heats the rolled steel material. After specifying this set temperature to be 1000 to 1400° C., the time for passing the material to be rolled through the induction heating device is set within the range of 10 to 300 seconds. It is more preferable that the set temperature is in the range of 1000 to 1300° C. and the passing time is in the range of 10 to 200 seconds. The set temperature is more preferably 1050 to 1300°C, more preferably 1100 to 1300°C. The passage time is more preferably 20 to 200 seconds, more preferably 20 to 150 seconds. As a result, the steel material temperature from the steel material surface layer to the D/4 position can be uniformly set to the set temperature.

If the set temperature of the induction heating device is less than 1000 ° C., the strain on the steel material accumulates during hot rolling, and in addition to the large micro strain of the steel material surface layer to D / 4, coarse undissolved precipitates are formed. It remains and deteriorates cold forgeability and hydrogen embrittlement resistance. When the temperature exceeds 1400°C, the undissolved precipitates are dissolved, and the increase in the dissolved elements increases the micro strain of the steel material. In addition, high-temperature heating causes a reduction in yield due to oxidation of the steel material, or creep deformation of the steel material during rolling, resulting in poor rolling. In addition, if the rolling material passes through the induction heating device for less than 10 seconds, the steel material temperature does not uniformly reach the set temperature in the steel material surface layer to D/4, and the strain in the steel material accumulates during hot rolling. In addition to the increase in micro strain in the steel material surface layer to D/4, coarse undissolved precipitates remain, resulting in deterioration of cold forgeability and hydrogen embrittlement resistance. When the induction heating device passing time exceeds 300 seconds, the steel material temperature in the steel material surface layer to D/4 is locally overheated non-uniformly with respect to the set temperature, and the undissolved precipitates dissolve into solid solution. An increase in the number of elements increases the micro strain of the steel. In addition, high-temperature heating causes a decrease in yield due to oxidation of the steel material, or creep deformation of the steel material during rolling, resulting in poor rolling. The set temperature of the induction heating device specifically means the output temperature in the induction heating device through which the steel material passes.

After heating the rolled material in this way, tilt rolling is performed. In tilt rolling, for example, as disclosed in Patent Document 4, three work rolls are arranged on roll axes that are tilted in the same direction around the material to be rolled, and each work roll rotates the material to be rolled. It revolves around itself while rotating. As a result, the material to be rolled is spirally rolled while moving forward. Regarding the temperature distribution of the material to be rolled when tilt rolling is performed, the temperature distribution is such that the steel material temperature from the steel material surface layer to the D/4 position uniformly coincides with the set temperature. As a result, the micro strain of the steel material surface layer to D/4 can be reduced to 0.0040 or less by the mechanism of reducing the strain accumulation by recrystallization in the steel material surface layer to D/4. In addition, the above process eliminates coarse undissolved precipitates and refines the precipitates, which also contributes to the enhancement of cold forgeability and hydrogen embrittlement resistance.

It is preferable to perform in-line heat treatment, hot rolling, heat treatment, pickling, etc. after tilt rolling. After that, the shape of the steel material may be adjusted by peeling or drawing of the steel material.

In the steel manufacturing method of the first invention, it is preferable that hot working is performed using tilt rolling as described above. Note that hot working is not limited to tilt rolling, and any method that follows a similar heat working history may be used. For example, blooming rolling (breakdown) can be used as long as a similar heat working history can be obtained. can.

<Second invention>
In producing the stainless steel of the second invention, particularly the bar steel, it is preferable to employ tilt rolling as the hot working method and use induction heating to heat the raw material before tilt rolling.

A set temperature is provided in an induction heating device that heats a rolled steel material. After specifying this set temperature to be 1000 to 1400° C., the speed at which the material to be rolled passes through the induction heating device is set within the range of 0.003 to 4.0 m/s. It is more preferable that the set temperature is in the range of 1000 to 1300° C. and the feed speed is in the range of 0.005 to 2.0 m/s. The set temperature is more preferably 1050 to 1300°C, more preferably 1100 to 1300°C. The threading speed is more preferably 0.01 to 2.0 m/s, more preferably 0.1 to 1.0 m/s.
By setting the set temperature of the induction heating device and the feeding speed within the above ranges, coupled with containing B in the steel, it is possible to make the amount of precipitated B as borides in the steel 0.0001% or more. can.
In addition, by setting the set temperature of the induction heating device and the feed speed within the above ranges, the aspect ratio of the sulfides is reduced to 50 or less, coupled with the inclusion of S in the steel and the inclusion of one or more types of Al and Ca. can be

If the set temperature of the induction heating device is less than 1000°C, the amount of precipitated B decreases, the aspect ratio of sulfides increases, and coarse undissolved precipitates remain, resulting in poor cold forgeability and machinability. Hydrogen embrittlement resistance deteriorates. If the temperature exceeds 1400° C., cold forgeability, machinability, and resistance to hydrogen embrittlement deteriorate due to a decrease in the amount of precipitated B and an increase in aspect ratio due to elongation of sulfides. In addition, high-temperature heating causes a decrease in yield due to oxidation of the steel material, or creep deformation of the steel material during rolling, resulting in poor rolling. In addition, when the speed of passing through the induction heating device of the rolled material is less than 0.003 m / s, the amount of precipitated B decreases and the sulfide is elongated, which causes the sulfide to have a high aspect ratio and cold forgeability. , the machinability and hydrogen embrittlement resistance deteriorate. If the induction heating device passing speed exceeds 4.0 m / s, the amount of precipitated B decreases, the aspect ratio of sulfide increases, and coarse undissolved precipitates remain, so cold forgeability and machinability properties and hydrogen embrittlement resistance deteriorate. In addition, high-temperature heating causes a decrease in yield due to oxidation of the steel material, or creep deformation of the steel material during rolling, resulting in poor rolling. The set temperature of the induction heating device specifically means the output temperature in the induction heating device through which the steel material passes.

After heating the rolled material in this way, tilt rolling is performed. In tilt rolling, for example, as disclosed in Patent Document 4, three work rolls are arranged on roll axes that are tilted in the same direction around the material to be rolled, and each work roll rotates the material to be rolled. It revolves around itself while rotating. As a result, the material to be rolled is spirally rolled while moving forward. Regarding the temperature distribution of the material to be rolled when tilt rolling is performed, the temperature distribution is such that the steel material temperature from the steel material surface layer to the D/4 position uniformly coincides with the set temperature. This reduces the amount of precipitated B and suppresses elongation of sulfides, thereby improving cold forgeability, machinability, and resistance to hydrogen embrittlement. In addition, the above process eliminates coarse undissolved precipitates and refines the precipitates, which also contributes to the enhancement of cold forgeability, machinability, and hydrogen embrittlement resistance.

It is preferable to perform in-line heat treatment, hot rolling, heat treatment, pickling, etc. after tilt rolling. After that, the shape of the steel material may be adjusted by peeling or drawing of the steel material.

In the steel manufacturing method of the second invention, it is preferable that hot working is performed using tilt rolling as described above. Note that hot working is not limited to tilt rolling, and any method that follows a similar heat working history may be used. For example, blooming rolling (breakdown) can be used as long as a similar heat working history can be obtained. can.

<Third invention>
In the production of the stainless steel of the third invention, particularly the steel bar, it is preferable to subject the raw material to heating, hot rolling (tilt rolling, BD, bar rolling, etc.), heat treatment, pickling, etc. In particular, rough rolling is preferred. It is preferable to control the entry-side temperature and the average time between rough rolling stands and perform the passivation treatment.

After specifying the rough rolling entrance temperature of the steel material to be 1000 to 1400°C, the average time between the rough rolling stands of the rolled material is set within the range of 0.01 to 30 seconds. More preferably, the rough rolling entrance temperature is in the range of 1000 to 1300° C., and the average time between rough rolling stands is in the range of 0.03 to 10 seconds. The entry temperature for rough rolling is more preferably 1050 to 1300°C, more preferably 1100 to 1300°C. The average time between rough rolling stands is more preferably 0.05 to 5 seconds, more preferably 0.1 to 2 seconds. If the rough rolling inlet temperature is less than 1000°C, the strain on the steel material accumulates during hot rolling, B-based precipitates are formed in the grains, and the B grain boundary occupancy decreases, so corrosion resistance and cold rolling are improved. Forgeability and non-magnetic properties deteriorate. When the entry temperature of rough rolling exceeds 1400° C., B present at grain boundaries diffuses into grains and forms intragranular B precipitates during rolling, reducing the B grain boundary occupancy. In addition, high-temperature heating causes a decrease in yield due to oxidation of the steel material, or creep deformation of the steel material during rolling, resulting in poor rolling. In addition, if the average time between rough rolling stands is less than 0.01 seconds, the strain on the steel material accumulates during hot rolling, B-based precipitates are formed in grains, and the B grain boundary occupancy decreases. Therefore, corrosion resistance, cold forgeability, and non-magnetic properties deteriorate. When the average time between rough rolling stands exceeds 30 seconds, B present at grain boundaries diffuses into grains and forms intragranular B precipitates during rolling, reducing the B grain boundary occupancy. In addition, high-temperature heating causes a decrease in yield due to oxidation of the steel material, or creep deformation of the steel material during rolling, resulting in poor rolling.

When the rolled material controlled under the above conditions is heat-treated, surface scale is removed, and then passivation treatment is performed, passivation is promoted in the Cr-deficient region due to intergranular Cr-based precipitates, and corrosion resistance is improved. In addition, in the rolled and heat-treated material under the above conditions, plastic deformation at grain boundaries is facilitated, cold forgeability is improved, local deformation at grain boundaries is suppressed, and the formation of deformation-induced α' martensite in the magnetic phase is suppressed. and remain non-magnetic. Here, the passivation treatment is a treatment of immersing the material in a solution such as nitric acid, and may be a single treatment or an acid treatment. This is effective in the treatment of stainless steel (especially bar steel), and also in the treatment of products obtained by secondary processing (drawing, forging, cutting, etc.) of the bar steel.

<First Invention>
(Example 1-1)
When the steel was smelted, assuming AOD smelting, which is a low-cost stainless steel smelting process, the steel was melted in a 100 kg vacuum melting furnace and cast into a slab with a diameter of 180 mm. Thereafter, stainless steel rods having a diameter of 20.0 mm and chemical compositions shown in Tables 1 to 3 were manufactured under the following manufacturing conditions. In Tables 1 to 6, items outside the scope of the present invention and items outside the preferred manufacturing conditions of the present invention are underlined.

The cast slab is heated in a heating furnace at 1130 ° C., then induction heating is used to heat the rolling material before tilt rolling, the set temperature of the induction heating device is 1210 ° C., and the induction heating device passing time is 110 s. After heating, tilt rolling, in-line heat treatment, bar and wire rolling, offline heat treatment at 1100° C. for 30 minutes (water cooling) and pickling, a bar steel with a diameter of 20.0 mm was produced.

The methods described above were used for measuring the micro strain of bar steel, measuring the critical compressibility, and evaluating the relative tensile strength and relative reduction of area after cold working.

Regarding the microstrain, AA is 0.0005 or less, A is more than 0.0005 and 0.0020 or less, B is more than 0.0020 and 0.0040 or less, and C is more than 0.0040.
Regarding the tensile strength, AA is 500 MPa or less, A is more than 500 MPa and 620 MPa or less, B is more than 620 MPa and 700 MPa or less, and C is more than 700 MPa.
Regarding the limit compression rate, AA is 80% or more, A is 70% or more and less than 80%, B is 60% or more and less than 70%, and C is less than 60%.
Regarding the relative tensile strength in high-pressure hydrogen after cold working, AA indicates 95% or more, A indicates 90% or more and less than 95%, B indicates 80% or more and less than 90%, and C indicates less than 80%.
Regarding the relative reduction of area in high-pressure hydrogen after cold working, AA indicates 70% or more, A indicates 60% or more and less than 70%, B indicates 50% or more and less than 60%, and C indicates less than 50%.
Evaluation results are shown in Tables 4 and 5.

Inventive Example No. 1 to 39, the steel bars have the chemical composition and microstrain specified in the first invention, and all of the tensile strength, critical compressibility, relative tensile strength and reduction of area after cold working. , AA, A, or B, and was good.

On the other hand, Comparative Example No. For 40 to 54, one of the components is outside the first invention range, and the microstrain is outside the first invention range, resulting in tensile strength, critical compressibility, relative tensile strength after cold working, and Aperture was C in both cases.

(Example 1-2)
Using steel type P in Table 1 as the chemical composition, induction heating conditions before tilt rolling were set to the conditions shown in Table 6, and other manufacturing conditions were the same as in Example 1-1 above, to produce bar steels.

As shown in Table 6, invention example No. 55 to 64 have the manufacturing method under the preferred conditions of the first invention, have the composition and microstrain specified in the first invention, and have tensile strength, critical compressibility, and relative tensile strength after cold working. All of AA, A, and B were good in both of AA, A, and B.

On the other hand, Comparative Example No. For 65 to 70, one of the manufacturing conditions is outside the preferred range of the first invention, and the microstrain is outside the range of the first invention, resulting in tensile strength, critical compressibility, relative after cold working Both tensile strength and area of drawing were C.

<Second invention>
(Example 2-1)
When the steel was smelted, assuming AOD smelting, which is a low-cost stainless steel smelting process, the steel was melted in a 100 kg vacuum melting furnace and cast into a slab with a diameter of 180 mm. Thereafter, stainless steel rods having a diameter of 20.0 mm and chemical compositions shown in Tables 7 to 9 were manufactured under the following manufacturing conditions. In Tables 7 to 12, items outside the scope of the second invention and items outside the preferred manufacturing conditions of the second invention are underlined.

The cast slab is heated at 1130 ° C. in a heating furnace, and then induction heating is used to heat the rolling material before tilt rolling. After heating at 3 m/s, tilt rolling, in-line heat treatment, bar and wire rolling, offline heat treatment at 1100°C for 30 minutes (water cooling), pickling, and bar steel with a diameter of 20.0 mm. was made.

Amount of precipitated B as borides in bar steel, method for measuring aspect ratio of sulfides, method for measuring critical compressibility, method for evaluating VL-1000 drilling life index, relative tensile strength and relative drawing after cold working As for the evaluation method, the method as described above was used.

Regarding the amount of precipitated B as borides, in mass%, 0.0010% or more is AA, 0.0005% or more and less than 0.0010% is A, 0.0001% or more and less than 0.0005% is B, and 0.0001%. % less than C. When coarse borides were formed due to excessive B content, it was rated as CC.
Regarding the aspect ratio of the sulfide, AA is 5 or less, A is 5 to 30, B is 30 to 50, and C is more than 50.
Regarding the tensile strength, AA is 500 MPa or less, A is more than 500 MPa and 620 MPa or less, B is more than 620 MPa and 700 MPa or less, and C is more than 700 MPa.
Regarding the limit compression rate, AA is 80% or more, A is 70% or more and less than 80%, B is 60% or more and less than 70%, and C is less than 60%.
For the VL-1000 drilling life index, 20 m/min or more is AA, 10 m/min or more and less than 20 m/min is A, 1 m/min or more and less than 10 m/min is B, and less than 1 m/min is C.
Regarding the relative tensile strength in high-pressure hydrogen after cold working, AA indicates 95% or more, A indicates 90% or more and less than 95%, B indicates 80% or more and less than 90%, and C indicates less than 80%.
Regarding the relative reduction of area in high-pressure hydrogen after cold working, AA indicates 70% or more, A indicates 60% or more and less than 70%, B indicates 50% or more and less than 60%, and C indicates less than 50%.
Evaluation results are shown in Tables 10 and 11.

Inventive Example No. 1 to 39 have the chemical composition, the amount of precipitated B as borides, and the aspect ratio of sulfides specified in the second invention, and have tensile strength, critical compressibility, and VL-1000. , the relative tensile strength and reduction of area after cold working were either AA, A, or B, and were good.

On the other hand, Comparative Example No. For 40 to 50 and 52 to 56, one of the components is outside the second invention range, the amount of precipitated B as borides and the aspect ratio of sulfides are outside the second invention range, and as a result, the tensile strength It was C in all of the strength, critical compressibility, VL-1000, relative tensile strength and reduction of area after cold working. In addition, comparative example No. In 51, coarse borides are formed due to excessive B content, and the coarse borides become the starting point of fracture, and as a result, tensile strength, critical compressibility, VL-1000, relative after cold working Both tensile strength and area of drawing were C.

(Example 2-2)
Using steel type P in Table 7 as the chemical composition, the induction heating conditions before tilt rolling were set to the conditions shown in Table 12, and other manufacturing conditions were the same as in Example 2-1 above to produce a bar steel.

As shown in Table 12, invention example No. 55 to 64 have the manufacturing method under the suitable conditions of the second invention, have the chemical composition specified in the second invention, the amount of precipitated B as borides, and the aspect ratio of sulfides, and have tensile strength and limit All of compressibility, relative tensile strength and reduction of area after cold working were AA, A, or B, and were good.

On the other hand, Comparative Example No. For 65 to 70, one of the production conditions is out of the preferred range of the second invention, the amount of precipitated B as borides and the aspect ratio of sulfides are out of the scope of the second invention, and as a result, the tensile strength , critical compressibility, VL-1000, relative tensile strength and reduction of area after cold working were all C.

<Third invention>
(Example 3-1)
When the steel was smelted, assuming AOD smelting, which is a low-cost stainless steel smelting process, the steel was melted in a 100 kg vacuum melting furnace and cast into a slab with a diameter of 180 mm. Thereafter, stainless steel rods having a diameter of 20.0 mm and chemical compositions shown in Tables 13 and 14 were manufactured under the following manufacturing conditions. In Tables 13 to 17, items outside the scope of the third invention and items outside the preferred manufacturing conditions of the third invention are underlined.

The cast slab is subjected to heating, tilt rolling, and in-line heat treatment, the temperature at the entry side of rough rolling is adjusted to 1130 ° C., rough rolling is performed, and the average time between stands for rough rolling is 1.8 s. After bar rolling, heat treatment at 1100° C. for 30 minutes (water cooling) was performed as a solution treatment, followed by pickling to produce a steel bar with a diameter of 20.0 mm. From this bar steel material (φ20 mm), an L cross section of φ20×30 mm was taken for corrosion resistance evaluation, and a test piece of φ8×12 mm was taken for the end face restraint compression test from the D (diameter) / 4 part position of the steel material C cross section 12 mm in the L direction. Taken long.

Regarding the method of measuring the B grain boundary occupation ratio of the bar steel, the L cross section of the solution-treated bar (φ20 mm) was used, and the method described above was used. For corrosion resistance, a test piece with a diameter of φ20×30 mm was used, and the method as described above was used. Tensile strength was evaluated by an ordinary method using solution-treated bar steel (φ20 mm). Regarding the cold forgeability, a test piece of φ8×12 mm was used for the limit compressibility measurement method, and the method as described above was used. Regarding the evaluation method of the relative magnetic permeability after cold working, the above-mentioned method is used using a bar steel material of φ9 mm obtained by cold drawing the bar steel material subjected to the above solution heat treatment at a cross-sectional reduction rate of 80%. board.

The B grain boundary occupancy rate was AA when 15% or more, A when 5% or more and less than 15%, B when 1% or more and less than 5%, and C when less than 1%.
Corrosion resistance was rated AA when 0.20V or more, A when 0.10V or more and less than 0.20V, B when 0.05V or more and less than 0.10V, and C when less than 0.05V.
Regarding the tensile strength, AA is 500 MPa or less, A is more than 500 MPa and 620 MPa or less, B is more than 620 MPa and 700 MPa or less, and C is more than 700 MPa.
Regarding the limit compression rate, AA is 80% or more, A is 70% or more and less than 80%, B is 60% or more and less than 70%, and C is less than 60%.
Regarding the relative magnetic permeability after cold working, AA indicates 1.03 or less, A indicates more than 1.03 and 1.05 or less, B indicates more than 1.05 and 1.10 or less, and C indicates more than 1.10.
Evaluation results are shown in Tables 15 and 16.

Inventive Example No. 1 to 39 have the chemical composition and B grain boundary occupancy specified in the third invention, corrosion resistance, tensile strength, critical compressibility, relative magnetic permeability after cold working, were either AA, A, or B, and were good.

On the other hand, Comparative Example No. For 40 to 54, one of the components is outside the third invention range, and the B grain boundary occupation ratio is outside the third invention range, resulting in corrosion resistance, tensile strength, critical compressibility, and after cold working Both of the relative magnetic permeability of

(Example 3-2)
Using steel type P in Table 13 as the chemical composition, the temperature at the entry side of rough rolling, the average time between rough rolling stands, and the presence or absence of passivation treatment of test pieces during corrosion resistance evaluation (if not, as polished) are changed. 16, and the other manufacturing conditions were the same as in Example 3-1, steel rods were manufactured and test pieces were prepared.

As shown in Table 17, invention example No. 55 to 64 have the manufacturing method under the preferred conditions of the third invention, have the chemical composition and the B grain boundary occupation ratio specified in the third invention, and have corrosion resistance, tensile strength, critical compressibility, and cold working. All of the subsequent relative magnetic permeability were either AA, A, or B, and were good.

On the other hand, Comparative Example No. For 65 to 68 and 70, one of the manufacturing conditions is out of the preferred range of the third invention, and the B grain boundary occupation rate is out of the range of the third invention, resulting in corrosion resistance, tensile strength, and critical compressibility. , the relative magnetic permeability after cold working, were both C. Comparative example no. For No. 69, passivation treatment was not performed, passivation in the Cr-deficient region was not promoted, and the B grain boundary occupation ratio was outside the scope of the third invention, resulting in poor corrosion resistance, tensile strength, Both the critical compressibility and the relative magnetic permeability after cold working were C.

Claims (19)

The chemical composition, in mass %,
C: 0.0010 to 0.15%,
Si: 0.01 to 2.00%,
Mn: 0.01 to 10.00%,
Ni: 8.00 to 30.00%,
Cr: 9.0 to 21.0%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 5.00%,
N: 0.0010 to 0.10%,
Ti: 0 to 2.00%,
Nb: 0 to 2.00%,
Sn: 0-2.5%,
V: 0 to 2.0%,
W: 0 to 3.0%,
Ga: 0-0.05%,
Co: 0-2.5%,
Sb: 0-2.5%,
Ta: 0-2.5%,
Ca: 0-0.05%,
Mg: 0-0.012%,
Zr: 0 to 0.012%,
REM: 0-0.05%,
Pb: 0 to 0.30%,
Se: 0 to 0.80%,
Te: 0 to 0.30%,
Bi: 0 to 0.50%,
S: 0 to 0.50%,
P: 0 to 0.30%,
and further
Al: 0.001 to 2.0%,
B: 0.0001 to 0.05%,
containing one or more selected from, the balance being Fe and impurities,
The A value represented by the following formula (a) is −100 or less,
A stainless steel having an average microstrain of 0.0040 or less from the surface layer to D/4 of the steel material.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
However, the symbol of an element in the formula (a) means the content (% by mass) of the element in the steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol.
Here, D is the diameter or thickness of the steel material, and the micro strain means the lattice strain calculated from the half width obtained by X-ray line profile analysis. Further, the chemical composition is, in mass %,
As group A,
Ti: 0.01 to 2.00%,
Nb: 0.01 to 2.00%,
Sn: 0.0001 to 2.5%,
V: 0.001 to 2.0%,
W: 0.05 to 3.0%,
Ga: 0.0004 to 0.05%,
Co: 0.05-2.5%,
Sb: 0.01-2.5%, and Ta: 0.01-2.5%,
one or more selected from
As group B,
Ca: 0.0002-0.05%,
Mg: 0.0002-0.012%,
Zr: 0.0002-0.012%, and REM: 0.0002-0.05%,
one or more selected from
As group C,
Pb: 0.0001 to 0.30%,
Se: 0.0001 to 0.80%,
Te: 0.0001 to 0.30%,
Bi: 0.0001 to 0.50%,
S: 0.0001 to 0.50%, and P: 0.0001 to 0.30%,
one or more selected from
containing one or more groups of Groups A to C of
The stainless steel according to claim 1. The stainless steel according to claim 1 or claim 2, which has a tensile strength of 700 MPa or less. The stainless steel according to claim 1 or claim 2, which has a critical compressibility of 60% or more. The stainless steel according to claim 1 or claim 2, wherein the relative tensile strength in high-pressure hydrogen after cold working is 80% or more. The stainless steel according to claim 1 or claim 2, which has a relative reduction in area of 50% or more in high-pressure hydrogen after cold working. The chemical composition, in mass %,
C: 0.0010 to 0.15%,
Si: 0.01 to 2.00%,
Mn: 0.01 to 10.00%,
Ni: 8.00 to 30.00%,
Cr: 9.0 to 21.0%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 5.00%,
N: 0.0010 to 0.10%,
B: 0.0001 to 0.05%,
S: 0.0001 to 0.50%,
Ti: 0 to 2.00%,
Nb: 0 to 2.00%,
Sn: 0-2.5%,
V: 0 to 2.0%,
W: 0 to 3.0%,
Ga: 0-0.05%,
Co: 0-2.5%,
Sb: 0-2.5%,
Ta: 0-2.5%,
Mg: 0-0.012%,
Zr: 0 to 0.012%,
REM: 0-0.05%,
Pb: 0 to 0.30%,
Se: 0 to 0.80%,
Te: 0 to 0.30%,
Bi: 0 to 0.50%,
P: 0 to 0.30%,
and further
Al: 0.001 to 2.0%,
Ca: 0.0001 to 0.05%,
contains one or more selected from
balance: Fe and impurities,
The A value represented by the following formula (a) is −100 or less,
A stainless steel characterized by having an amount of precipitated B as borides of 0.0001% or more and an aspect ratio of sulfides of 50 or less.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
However, the symbol of an element in the formula (a) means the content (% by mass) of the element in the steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol. Further, the chemical composition is, in mass %,
As group A,
Ti: 0.01 to 2.00%,
Nb: 0.01 to 2.00%,
Sn: 0.0001 to 2.5%,
V: 0.001 to 2.0%,
W: 0.05 to 3.0%,
Ga: 0.0004 to 0.05%,
Co: 0.05-2.5%,
Sb: 0.01-2.5%, and Ta: 0.01-2.5%,
one or more selected from
As group B,
Mg: 0.0002-0.012%,
Zr: 0.0002-0.012%, and REM: 0.0002-0.05%,
one or more selected from
As group C,
Pb: 0.0001 to 0.30%,
Se: 0.0001 to 0.80%,
Te: 0.0001 to 0.30%,
Bi: 0.0001 to 0.50%, and P: 0.0001 to 0.30%,
one or more selected from
containing one or more groups of Groups A to C of
The stainless steel according to claim 7. The stainless steel according to claim 7 or claim 8, which has a tensile strength of 700 MPa or less. The stainless steel according to claim 7 or claim 8, which has a critical compressibility of 60% or more. The stainless steel according to claim 7 or claim 8, wherein VL-1000 of the drilling life index is 1 m/min or more. The stainless steel according to claim 7 or claim 8, which has a relative tensile strength of 80% or more in high-pressure hydrogen after cold working. The stainless steel according to claim 7 or claim 8, which has a relative reduction in area of 50% or more in high-pressure hydrogen after cold working. The chemical composition, in mass %,
C: 0.0010 to 0.15%,
Si: 0.01 to 2.00%,
Mn: 0.01 to 10.00%,
Ni: 8.00 to 30.00%,
Cr: 9.0 to 21.0%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 5.00%,
N: 0.0010 to 0.10%,
B: 0.0001 to 0.05%,
Al: 0 to 2.0%,
Ti: 0 to 2.00%,
Nb: 0 to 2.00%,
Sn: 0-2.5%,
V: 0 to 2.0%,
W: 0 to 3.0%,
Ga: 0-0.05%,
Co: 0-2.5%,
Sb: 0-2.5%,
Ta: 0-2.5%,
Ca: 0-0.05%,
Mg: 0-0.012%,
Zr: 0 to 0.012%,
REM: 0-0.05%,
Pb: 0 to 0.30%,
Se: 0 to 0.80%,
Te: 0 to 0.30%,
Bi: 0 to 0.50%,
S: 0 to 0.50%,
P: 0 to 0.30%,
and the balance is Fe and impurities,
The A value represented by the following formula (a) is −100 or less,
A stainless steel having a B grain boundary occupancy of 1% or more.
A value = 551-462 (C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a)
However, the symbol of an element in the formula (a) means the content (% by mass) of the element in the steel. Further, when the content of the element in the formula (a) is 0%, the calculation is performed by substituting "0" in the corresponding symbol. Further, the chemical composition is, in mass %,
As group A,
Al: 0.001 to 2.0%,
Ti: 0.01 to 2.00%,
Nb: 0.01 to 2.00%,
Sn: 0.0001 to 2.5%,
V: 0.001 to 2.0%,
W: 0.05 to 3.0%,
Ga: 0.0004 to 0.05%,
Co: 0.05-2.5%,
Sb: 0.01-2.5%, and Ta: 0.01-2.5%,
one or more selected from
As group B,
Ca: 0.0002-0.05%,
Mg: 0.0002-0.012%,
Zr: 0.0002-0.012%, and REM: 0.0002-0.05%,
one or more selected from
As group C,
Pb: 0.0001 to 0.30%,
Se: 0.0001 to 0.80%,
Te: 0.0001 to 0.30%,
Bi: 0.0001 to 0.50%,
S: 0.0001 to 0.50%, and P: 0.0001 to 0.30%,
one or more selected from
containing one or more groups of Groups A to C of
15. The stainless steel of claim 14. The stainless steel according to claim 14 or 15, which has a pitting potential of 0.05 V or more. The stainless steel according to claim 14 or 15, which has a tensile strength of 700 MPa or less. The stainless steel according to claim 14 or 15, which has a critical compressibility of 60% or more. The stainless steel according to claim 14 or 15, wherein the relative magnetic permeability after cold working is 1.10 or less.