Stainless steel with excellent cold forging properties, corrosion resistance, and non-magnetic properties

Abstract

To provide a stainless steel having excellent cold forgeability, corrosion resistance and non-magnetism.SOLUTION: Provided is a stainless steel containing, 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%, and B: 0.0001-0.05%. The A value represented by formula (a) is -100 or less. The B grain boundary occupancy rate is 1% or more. A value=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to stainless steel, and more particularly to a non-magnetic stainless steel that is excellent in corrosion resistance and cold forgeability. [Background technology]

[0002] 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 to 50% martensite when a tensile true strain of 0.30 is applied to a single-phase austenite sample. The higher this value, the more unstable the material. Non-Patent Document 1 presents an equation for Md30 as a function of the chemical composition.

[0003] Conventionally, austenitic stainless steels such as SUS316 and SUS316L have been commonly used for non-magnetic parts. In contrast, Patent Documents 1 and 2 disclose a steel having a predetermined composition including C: 0.15 to 0.80%, Ni: 8.0 to 20.0%, Cr: 8.0 to 18.0%, Mo: 0.05 to 0.50%, V: 0.50 to 3.00%, and Al: 0.001 to 1.000%, in which the value of formula (3) obtained by modifying the Md30 formula described in Non-Patent Document 1 is set to −100 or less, and the number of V(C,N) precipitates of 50 nm or less is 3.5 × 10 -2 μm 2 The document discloses a high-hardness non-magnetic steel that is inexpensive and has excellent hydrogen embrittlement resistance, mechanical properties, and corrosion resistance, characterized by the presence of 50 or more dispersed particles in the steel.

[0004] Patent Document 3 discloses tilt rolling. In tilt rolling, three work rolls are arranged on roll axes that are twisted and tilted in the same direction around the material to be rolled. Each work roll revolves around the material to be rolled while rotating on its own axis. As a result, the material to be rolled is rolled in a spiral shape while moving forward. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-49036 [Patent Document 2] JP 2016-183372 A [Patent Document 3] Japanese Patent Application Publication No. 05-329510 [Non-patent literature]

[0006] [Non-Patent Document 1] Nohara et al., "Dependence of Deformation-Induced Martensitic Transformation on Composition and Grain Size in Metastable Austenitic Stainless Steels," Iron and Steel, 63rd Year (1977), No. 5, pp. 772-782 Summary of the Invention [Problem to be solved by the invention]

[0007] Non-magnetic steels with excellent mechanical strength have been achieved by using SUS316, SUS316L, or the austenitic stainless steels described in Patent Documents 1 and 2. However, it has been found that these conventional steels are difficult to simultaneously satisfy corrosion resistance, cold forgeability, and non-magnetic properties after cold working. In particular, conventional techniques suffer from sensitization due to high C content, resulting in poor corrosion resistance. Furthermore, the high material strength before cold forging shortens tool life and increases the forging load for large-diameter steel bars. These factors have been found to result in poor cold forgeability. Furthermore, it has been found that conventional steels reach their material processing limits (cracks) during high strain processing such as cold forging.

[0008] A third object of the present invention is to provide a stainless steel that can improve corrosion resistance, reduce tensile strength to improve cold forgeability, and further improve non-magnetic properties after cold working. [Means for solving the problem]

[0009] The invention of the original application resulted in 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. The invention of the present application, which is a divisional application of the original application, defines the following third invention. That is, the gist of the present invention is as follows.

[0010]

[14] <Third Invention> The chemical composition is, 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%, containing 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: 0-0.50%, S: 0-0.50%, P: 0-0.30%, and the balance: Fe and impurities; The A value represented by the following formula (a) is −100 or less, B Stainless steel with a grain boundary occupancy rate of 1% or more. A value=551-462(C+N)-9.2Si―8.1Mn―29(Ni+Cu)-13.7Cr―18.5Mo (a) However, the element symbol in formula (a) means the content (mass%) of the element in the steel. If the content of an element in formula (a) is 0%, the calculation is performed by substituting "0" for the corresponding symbol.

[0011]

[15] The chemical composition further comprises, in mass %, as Group A, one or more selected from 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 to 2.5%, Sb: 0.01 to 2.5%, and Ta: 0.01 to 2.5%; as group B, one or more 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%; as Group C, one or more selected from 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%; The stainless steel according to

[14] , which contains one or more of Groups A to C.

[0012]

[16] The stainless steel according to

[14] or

[15] , having a pitting potential of 0.05 V or more.

[17] The stainless steel according to any one of

[14] to

[16] , having a tensile strength of 700 MPa or less.

[18] The stainless steel according to any one of

[14] to

[17] , which has a limiting 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. [Effects of the Invention]

[0013] The stainless steel of the third invention contains the specified components and has a B grain boundary occupancy rate of 1% or more, which makes it possible to satisfy corrosion resistance, cold forgeability, and non-magnetic properties after cold working. DETAILED DESCRIPTION OF THE INVENTION

[0014] The stainless steel of the present invention can be applied in either a bar shape or a plate shape. In particular, it is particularly suitable for use as a bar-shaped steel material. Bar-shaped steel materials include "steel bar," "wire rod," "steel wire," "deformed wire," "deformed steel bar," and the like. The stainless steel of the present invention is an austenitic stainless steel.

[0015] As mentioned above, the third invention has an object to provide stainless steel, particularly a steel bar, which can satisfy the requirements for corrosion resistance, cold forgeability, and non-magnetic properties after cold working.

[0016] Regarding cold forgeability, when an end-constrained compression test (processing temperature: RT (room temperature), strain rate: 10 / s) is conducted using a φ8 × 12 mm test piece, the maximum compression ratio at which no cracks occur on the side of the test piece after compression processing is defined as the limiting compression ratio, and the target limiting compression ratio is 60% or more.

[0017] For corrosion resistance, the center of the L-section of a 20mm x 30mm test piece (20mm wide x 30mm long x 1mm thick) was used as the evaluation surface. Passivation treatment was performed on the entire surface, including the evaluation surface, by immersing it in 15% nitric acid for 30 minutes. The evaluation surface was then subjected to a pitting potential test in accordance with JIS G 0577 (3.5% NaCl, 30°C, N=3 average, V vs. Ag / AgCl, saturated KCl) to measure the pitting potential. The goal of the third invention is a pitting potential of 0.05V or higher. In the examples, the pitting potential of the comparative material without passivation treatment was measured immediately after polishing.

[0018] Regarding the non-magnetic properties after cold working, first, a solution heat treatment is performed at 1100°C for 30 minutes (water cooling), and then a cold working sample with a cold working ratio (reduction of area) of 80% is prepared, and the relative permeability at 1000 Oe is measured. The goal of the third invention is to achieve a relative permeability of 1.10 or less.

[0019] The third aspect of the invention will now be described in detail.

[0020] <Third Invention> The present inventors conceived the idea of ​​controlling the B grain boundary occupancy rate of a steel material as a means of satisfying the corrosion resistance, cold forgeability, and non-magnetic properties after cold working in stainless steel, particularly in steel bars. The B grain boundary occupancy rate (%) is the ratio (B / A × 100) of the grain boundaries (B) where a finite amount of B exists to the total grain boundaries (A). The inventors conceived that a high B grain boundary occupancy rate promotes passivation of Cr-deficient regions due to grain boundary Cr precipitates, improving corrosion resistance, facilitating plastic deformation at the grain boundaries and improving cold forgeability, and suppressing local deformation at the grain boundaries, thereby suppressing the formation of strain-induced α' martensite in the magnetic phase and maintaining non-magnetic properties.

[0021] The B grain boundary occupancy rate was evaluated using EPMA analysis. The total length (A) of the grain boundaries in an arbitrary field of view was measured in the L cross section of the steel (a cross section including the center line in the case of a bar-shaped steel), and then an area analysis of the B concentration was performed in the same field of view. The grain boundaries with a higher B concentration than the matrix within the grains were defined as B grain boundary occupancy, and the length (B) of the B grain boundary occupancy was calculated, and the B grain boundary occupancy rate was calculated using the above formula.

[0022] It was also found that if the B grain boundary occupancy rate of the steel material is 1% or more, the above-mentioned target corrosion resistance, cold forgeability, and non-magnetic properties after cold working can be satisfied. It is more preferable for the B grain boundary occupancy rate to be 5% or more on average, even more preferably 15% or more, and even more preferably 20% or more.

[0023] <Composition of the stainless steel of the third invention> Next, the chemical composition of the stainless steel according to the third invention will be explained. In the chemical composition, % means mass %.

[0024] (C: 0.0010 to 0.15%) C suppresses the formation of deformation-induced martensite and improves non-magnetic properties, so the content is set to 0.0010% or more. Excessive addition of C reduces the B grain boundary occupancy rate, deteriorating corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the C content is set to 0.15%, preferably 0.12%, more preferably 0.05%, and even more preferably 0.02%. It is preferable to set the upper limit of C to less than 0.15%.

[0025] (Si: 0.01 to 2.00%) Si is added as a deoxidizing element and is set to 0.01% or more. Excessive addition of Si reduces the B grain boundary occupancy rate 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%, more preferably 0.6%, and even more preferably 0.5%.

[0026] (Mn: 0.01 to 10.00%) Mn suppresses the formation of strain-induced martensite and enhances non-magnetic properties, so its content is set to 0.01% or more. Excessive addition of Mn reduces the B grain boundary occupancy rate, deteriorating 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, and even more preferably 1.0% or less.

[0027] (Ni: 8.00 to 30.00%) Ni suppresses the formation of deformation-induced martensite and improves non-magnetic properties. 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 even more preferably 15.00% or more. Adding excessive Ni reduces the B grain boundary occupancy rate, deteriorating corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ni content is set to 30.00%, and preferably 25.00% or less.

[0028] (Cr: 9.0 to 21.0%) Cr suppresses the formation of deformation-induced martensite and improves non-magnetic properties. Furthermore, to improve corrosion resistance, the Cr content is set to 9.0% or more, preferably 10.5% or more. Adding excessive Cr reduces the B grain boundary occupancy rate, deteriorating 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, and more preferably 15.0% or less.

[0029] (Mo: 0.01 to 3.00%) Mo suppresses the formation of deformation-induced martensite and improves non-magnetic properties. Furthermore, in order to improve corrosion resistance and cold forgeability, the Mo content is set to 0.01% or more. Adding excessive Mo reduces the B grain boundary occupancy rate, deteriorating corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Mo content is set to 3.0%, preferably 2.8% or less, more preferably 2.5% or less, and even more preferably 1.0% or less.

[0030] (Cu: 0.01 to 5.00%) Cu suppresses the formation of deformation-induced martensite and improves non-magnetic properties. To improve cold forgeability, the Cu content is set to 0.01% or more. It is preferably set to 1.00% or more, and more preferably set to 2.00% or more. Excessive Cu addition reduces the B grain boundary occupancy rate, degrades corrosion resistance, cold forgeability, and non-magnetic properties, and also causes hot brittleness. Therefore, the upper limit of the Cu content is set to 5.00%, and preferably set to 3.50% or less.

[0031] (N:0.0010~0.10%) N suppresses the formation of deformation-induced martensite and improves non-magnetic properties, so the content is set to 0.0010% or more. Excessive addition of N reduces the B grain boundary occupancy rate, deteriorating 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, and even more preferably 0.03% or less.

[0032] (B: 0.0001 to 0.05%) B is a major element that increases the B grain boundary occupancy rate, and in order to improve corrosion resistance, cold forgeability, and non-magnetic properties, its content is set to 0.0001% or more, preferably 0.0005% or more. Excessive addition of B causes the formation of coarse B-based precipitates, which in turn 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.

[0033] The stainless steel of the third invention contains the above-mentioned components, with the balance being Fe and impurities. It may further contain one or more components selected from the following components:

[0034] (Al: 0-2.0%) Al may be added to fix N, which reduces the B grain boundary occupancy rate. On the other hand, excessive addition of Al causes the formation of coarse Al-based precipitates, which deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Al content is set to 2.0%, preferably 1.0%, more preferably 0.5%, and even more preferably 0.05%. The preferred lower limit of Al is 0.001% or more.

[0035] (Ti: 0 to 2.00%) Ti may be added to fix C and N, which reduces the B grain boundary occupancy. On the other hand, excessive Ti addition causes the formation of coarse Ti-based precipitates, which deteriorates corrosion resistance, cold forgeability, and nonmagnetic properties. Therefore, the upper limit of the Ti content is set to 2.00%, preferably 1.0%, more preferably 0.7%, and even more preferably 0.5%. The preferred lower limit of Ti is 0.01%, more preferably 0.05%.

[0036] (Nb: 0 to 2.00%) Nb may be added to fix C and N, which reduces the B grain boundary occupancy. On the other hand, excessive addition of Nb results in the formation of coarse Nb-based precipitates, which deteriorates corrosion resistance, cold forgeability, and nonmagnetic properties. Therefore, the upper limit of the Nb content is set to 2.00%, preferably 1.0%, more preferably 0.7%, and even more preferably 0.5%. The preferred lower limit of Nb is 0.01%, more preferably 0.05%.

[0037] (Sn: 0 to 2.5%) Sn is an element effective in improving corrosion resistance, and may be contained. However, if excessive Sn is contained, the effect saturates, and there is a risk that corrosion resistance, cold forgeability, and non-magnetic properties may be deteriorated. Therefore, the upper limit of Sn content is set to 2.5%. More preferably, it is 1.0% or less, and even more preferably, it is 0.2% or less. To achieve the above effect, the Sn content is preferably 0.0001% or more, and even more preferably, it is 0.01% or more. More preferably, it is 0.05% or more.

[0038] (V:0~2.0%) V may be added to fix C and N, which reduces the B grain boundary occupancy. On the other hand, excessive V addition causes the formation of coarse V-based precipitates, which deteriorates corrosion resistance, cold forgeability, and nonmagnetic properties. Therefore, the upper limit of the V content is set to 2.0%, preferably 1.0%, more preferably 0.7%, and even more preferably 0.5%. The preferred lower limit of V is 0.001%.

[0039] (W:0~3.0%) W is an element effective in improving corrosion resistance, and may be contained. However, if W is contained in excess, the effect saturates and there is a risk that corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit of W content is set to 3.0%. More preferably, it is 2.0% or less, and even more preferably, it is 1.5% or less. To achieve the above effect, the W content is preferably 0.05% or more, and more preferably, it is 0.10% or more.

[0040] (Ga: 0 to 0.05%) Ga is an effective element for improving corrosion resistance, so it may be contained. However, if Ga is contained in excess, the effect saturates and there is a risk that the corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit of Ga content is set to 0.05%. To achieve the above effects, the Ga content is preferably 0.0004% or more.

[0041] (Co: 0-2.5%) Co may be added because it has the effect of improving corrosion resistance. However, if excessive Co is added, the effect saturates and, conversely, corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit of Co content is set to 2.5%. More preferably, it is 1.0% or less, and even more preferably, it is 0.8% or less. To achieve the above effect, the Co content is preferably 0.05% or more, and more preferably, 0.10% or more.

[0042] (Sb: 0-2.5%) Sb may be contained because it has the effect of improving corrosion resistance. However, if Sb is contained in excess, the effect saturates and, conversely, corrosion resistance, cold forgeability, and non-magnetic properties may deteriorate. Therefore, the upper limit of Sb content is set to 2.5%. More preferably, it is 1.0% or less, and even more preferably, it is 0.8% or less. To achieve the above effect, the Sb content is preferably 0.01% or more, and more preferably, 0.05% or more.

[0043] (Ta: 0 to 2.5%) Ta may be added to fix C and N, which reduces the B grain boundary occupancy. On the other hand, excessive Ta addition causes the formation of coarse Ta-based precipitates, which deteriorates corrosion resistance, cold forgeability, and nonmagnetic properties. Therefore, the upper limit of the Ta content is set to 2.5%, preferably 1.0%, more preferably 0.7%, and even more preferably 0.5%. The preferred lower limit of Ta is 0.01%.

[0044] (Ca: 0-0.05%) Ca may be added as needed for deoxidation. However, excessive Ca addition can result in the formation of coarse Ca-based inclusions, which can deteriorate corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Ca content is set to 0.05%, preferably 0.010%, and more preferably 0.005%. The preferred lower limit of Ca is 0.0002%.

[0045] (Mg: 0 to 0.012%) Mg may be added as needed for deoxidation. However, excessive addition of Mg results in the formation of coarse Mg-based inclusions, which deteriorates cold forgeability and hydrogen embrittlement resistance. Therefore, the upper limit of the Mg content is set to 0.012%, preferably 0.010%, and more preferably 0.005%. The preferred lower limit of Mg is 0.0002%.

[0046] (Zr: 0 to 0.012%) Zr may be added as needed for deoxidation. However, excessive addition of Zr results in the formation of coarse Zr-based inclusions, which deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Zr content is set to 0.012%, preferably 0.010%, and more preferably 0.005%. The preferred lower limit of Zr is 0.0002%.

[0047] (REM: 0 to 0.05%) REM may be added as needed for deoxidation. However, excessive REM addition can result in the formation of coarse REM inclusions, which can degrade corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the REM content is set to 0.05%, preferably 0.010%, and more preferably 0.005%. The preferred lower limit of REM is 0.0002%.

[0048] (Pb: 0 to 0.30%) Pb is an element that improves machinability and may be added as needed. However, 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%, and more preferably 0.05%. The preferred lower limit of Pb is 0.0001%.

[0049] (Se:0~0.80%) Se is an element that improves machinability and may be added as needed. However, excessive Se addition deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Se content is set to 0.80%, preferably 0.1%, and more preferably 0.05%. The preferred lower limit of Se is 0.0001%.

[0050] (Te:0~0.30%) Te is an element that improves machinability and may be added as needed. However, 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. The preferred lower limit of Te is 0.0001%.

[0051] (Bi: 0 to 0.50%) Bi is an element that improves machinability and may be added as needed. However, excessive Bi addition deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the Bi content is set to 0.50%, preferably 0.1%, and more preferably 0.05%. The preferred lower limit of Bi is 0.0001%.

[0052] (S:0~0.50%) S is an element that improves machinability and may be contained as needed. However, excessive addition of S deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the S content is set to 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. The preferred lower limit of S is 0.0001%. Note that S is usually contained in steel as an impurity mixed in from the steelmaking raw materials.

[0053] (P: 0 to 0.30%) P is an element that improves machinability and may be added as needed. However, excessive P addition deteriorates corrosion resistance, cold forgeability, and non-magnetic properties. Therefore, the upper limit of the P content is set to 0.30%, preferably 0.1%, and more preferably 0.05%. The preferred lower limit of P is 0.0001%.

[0054] <Common to the first to third inventions> <<A value in 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 element symbols in formula (a) represent the content (mass%) of the corresponding element in the steel. When the content of an element in formula (a) is 0%, the calculation is performed by substituting "0" for the corresponding symbol. The above formula (a) corresponds to the formula for Md30 described in Non-Patent Document 1 with the Nb term deleted. The Nb term was deleted because the addition rate 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 deformation-induced martensite is suppressed, and work hardening is reduced, resulting in softening and suppressing cracking, thereby improving cold forgeability. Furthermore, the first invention has the effect of reducing microstrain and improving hydrogen embrittlement resistance. The second invention has the effect of softening the steel, thereby reducing cutting resistance and improving machinability. With regard to hydrogen embrittlement resistance, the amount of deformation-induced martensite at the fracture initiation point is reduced, improving hydrogen embrittlement resistance. The third invention has the effect of improving non-magnetic properties.

[0055] <Quality of the steel materials of the first to third inventions> The stainless steel of the present invention, particularly the bar-shaped steel material, can achieve the following qualities as a result of having the above-mentioned chemical composition, and further, as a result of having a microstrain of D / 4 in the steel surface layer in the first invention, as a result of having an amount of B precipitated as borides and an aspect ratio of sulfides in the second invention, and as a result of having a B grain boundary occupancy rate in the steel in the third invention.

[0056] <Common to the first to third inventions> The stainless steel may have a tensile strength of 700 MPa or less. <Common to the first to third inventions> The stainless steel may have a limiting compression ratio of 60% or more. Here, the evaluation of the limiting compression ratio shall be carried out in the same manner as above, with respect to the shape of the test piece, the content of the compression test, and the definition of the limiting compression ratio.

[0057] <Third Invention> The stainless steel may have a pitting potential of 0.05V or more.

[0058] <Third Invention> The stainless steel may have a relative magnetic permeability of 1.10 or less after cold working. Here, the cold working rate (area reduction rate) of the cold working is 80%.

[0059] <<Method for manufacturing steel material according to the third invention>> The method for producing the steel material according to the third invention will be described below.

[0060] <Third Invention> In producing the stainless steel of the third invention, particularly a bar-shaped steel material, it is preferable to heat the material, hot roll (tilt rolling, BD, bar rolling, etc.), heat treat, pickle, etc., but it is particularly preferable to control the rough rolling entry temperature and the average time between rough rolling stands and to perform a passivation treatment.

[0061] The rough rolling entry temperature of the steel material is specified to be 1000 to 1400°C, and the rough rolling inter-stand average time of the rolling material is set to be within the range of 0.01 to 30 seconds. It is more preferable that the rough rolling entry temperature is within the range of 1000 to 1300°C and the rough rolling inter-stand average time is within the range of 0.03 to 10 seconds. The rough rolling entry temperature is more preferably 1050 to 1300°C, and even more preferably 1100 to 1300°C. The rough rolling inter-stand average time is more preferably 0.05 to 5 seconds, and even more preferably 0.1 to 2 seconds. If the rough rolling entry temperature is less than 1000°C, strain accumulates in the steel material during hot rolling, B-based precipitates are formed within the grains, and the B grain boundary occupancy rate decreases, resulting in deterioration of corrosion resistance, cold forgeability, and non-magnetic properties. If the rough rolling entry temperature exceeds 1400°C, B present at the grain boundaries diffuses into the grain interior and forms intragranular B precipitates during rolling, reducing the B grain boundary occupancy rate. Furthermore, high-temperature heating can reduce yields due to oxidation of the steel, or the steel can creep deform during passage, resulting in poor rolling. Furthermore, if the average time between rough rolling stands is less than 0.01 seconds, strain accumulates in the steel during hot rolling, forming intragranular B precipitates and reducing the B grain boundary occupancy rate, resulting in poor corrosion resistance, cold forgeability, and nonmagnetic properties. If the average time between rough rolling stands exceeds 30 seconds, B present at the grain boundaries diffuses into the grain interior and forms intragranular B precipitates during rolling, reducing the B grain boundary occupancy rate. Furthermore, high-temperature heating can reduce yields due to oxidation of the steel, or the steel can creep deform during passage, resulting in poor rolling.

[0062] When the rolled material controlled under the above conditions is heat-treated and the surface scale is removed, passivation treatment is performed to promote passivation of Cr-depleted regions due to grain boundary Cr precipitates, improving corrosion resistance. Furthermore, in the rolled and heat-treated material under the above conditions, plastic deformation at the grain boundaries is facilitated, improving cold forgeability, and localized deformation at the grain boundaries is suppressed, suppressing the formation of strain-induced α' martensite in the magnetic phase and maintaining non-magnetic properties. Here, passivation treatment involves immersing the material in a solution such as nitric acid, and can be a single treatment or one of several acidic processes. This treatment is effective when applied to stainless steel (especially bar-shaped steel), and also when applied to products obtained by secondary processing (drawing, forging, cutting, etc.) from the bar-shaped steel. [Example]

[0063] <Third Invention> Example 3-1 The steel was produced using AOD melting, an inexpensive stainless steel melting process, in a 100 kg vacuum melting furnace, and then cast into a 180 mm diameter slab. Subsequently, stainless steel bars with a diameter of 20.0 mm were produced under the following production conditions, with the chemical compositions shown in Tables 1 and 2. In Tables 1 to 5, items outside the scope of the third invention and items outside the preferred production conditions of the third invention are underlined.

[0064] [Table 1]

[0065] [Table 2]

[0066] The cast slab was heated, tilt-rolled, and inline heat treated. The rough rolling entry temperature was adjusted to 1130°C, and rough rolling was performed. The average interstand time for rough rolling was 1.8 s. Subsequently, bar rolling was performed, followed by solution treatment at 1100°C for 30 minutes (water cooling), followed by pickling to produce 20.0 mm diameter steel bars. From this 20 mm diameter bar, a 20 x 30 mm L cross section was taken for corrosion resistance evaluation, and a 8 x 12 mm test piece was taken from the D (diameter) / 4 position of the C cross section of the steel, with a length of 12 mm in the L direction, for end-constrained compression testing.

[0067] The method for measuring the B-grain boundary occupancy rate of the bar-shaped steel material was the same as described above, using the L-section of a solution-treated bar-shaped steel material (φ20 mm). For corrosion resistance, a test piece with a diameter of φ20 × 30 mm was used, and the same method as described above was used. For tensile strength, a solution-treated bar-shaped steel material (φ20 mm) was used and evaluated using a conventional method. For cold forgeability, a test piece with a diameter of φ8 × 12 mm was used to measure the limiting compression ratio, and the same method as described above was used. For evaluation of the relative permeability after cold working, the above solution-treated bar-shaped steel material was cold-drawn to a cross-sectional area reduction rate of 80% to form a φ9 mm bar-shaped steel material, and the same method as described above was used.

[0068] The B grain boundary occupancy rate was rated as AA when it was 15% or more, A when it was 5% or more and less than 15%, B when it was 1% or more and less than 5%, and C when it was less than 1%. Regarding corrosion resistance, 0.20V or more was rated AA, 0.10V or more but less than 0.20V was rated A, 0.05V or more but less than 0.10V was rated B, and less than 0.05V was rated C. Regarding tensile strength, AA was given for 500 MPa or less, A for over 500 MPa and less than 620 MPa, B for over 620 MPa and less than 700 MPa, and C for over 700 MPa. The limiting compression rate was rated as AA for 80% or more, A for 70% or more but less than 80%, B for 60% or more but less than 70%, and C for less than 60%. The relative permeability after cold working was rated as AA for 1.03 or less, A for over 1.03 and 1.05 or less, B for over 1.05 and 1.10 or less, and C for over 1.10. The evaluation results are shown in Tables 3 and 4.

[0069] [Table 3]

[0070] [Table 4]

[0071] The bar-shaped steel materials described in Inventive Examples Nos. 1 to 39 had the chemical composition and B grain boundary occupancy rate specified in the third invention, and were excellent in corrosion resistance, tensile strength, limiting compression ratio, and relative permeability after cold working, being either AA, A, or B.

[0072] On the other hand, in Comparative Examples 40 to 54, any of the components was outside the range of the third invention, and the B grain boundary occupancy rate was outside the range of the third invention. As a result, the corrosion resistance, tensile strength, limit compression ratio, and relative permeability after cold working were all C.

[0073] (Example 3-2) Steel type P in Table 1 was used as the chemical composition, and the conditions shown in Table 4 were set by varying the rough rolling entry temperature, the average time between rough rolling stands, and whether or not the test specimens were passivated during corrosion resistance evaluation (if no passivation treatment was performed, they were left as polished).The other manufacturing conditions were the same as in Example 3-1 above, and steel bars were manufactured and test specimens were prepared.

[0074] [Table 5]

[0075] As shown in Table 5, invention examples Nos. 55 to 64 were manufactured using the preferred conditions of the third invention, had the component composition and B grain boundary occupancy rate specified in the third invention, and were excellent in corrosion resistance, tensile strength, limiting compression ratio, and relative permeability after cold working, all of which were AA, A, or B.

[0076] On the other hand, for Comparative Examples 65 to 68 and 70, one of the manufacturing conditions was outside the preferred range of the third invention, and the B grain boundary occupancy rate was outside the range of the third invention. As a result, the corrosion resistance, tensile strength, critical compression ratio, and relative permeability after cold working were all rated C. For Comparative Example 69, no passivation treatment was performed, and passivation in the Cr-deficient region was not promoted, and the B grain boundary occupancy rate was outside the range of the third invention. As a result, the corrosion resistance, tensile strength, critical compression ratio, and relative permeability after cold working were all rated C.

Claims

1. The chemical composition, in mass%, is C: 0.0010 to 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 to 0.10%, B: 0.0001 to 0.05%, Al: 0-2.0%, Ti: 0-2.00%, Nb: 0 to 2.00%, Sn: 0 to 2.5%, V: 0 to 2.0%, W: 0 to 3.0%, Ga: 0-0.05%, Co: 0 to 2.5%, Sb: 0 to 2.5%, Ta: 0 to 2.5%, Ca: 0-0.05%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0-0.05%, Pb: 0 to 0.30%, Se: 0-0.80%, Te: 0 to 0.30%, Bi: 0 to 0.50%, S: 0-0.50%, P: 0-0.30%, and the balance being Fe and impurities, The A value represented by the following formula (a) is −100 or less, Stainless steel with a grain boundary occupancy rate of 1% or more. A value=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (a) In the formula (a), the element symbol represents the content (mass%) of the element in the steel. When the content of an element in the formula (a) is 0%, "0" is substituted for the corresponding symbol in the calculation.

2. The chemical composition further comprises, in mass %, As group A, Al: 0.001-2.0%, Ti: 0.01-2.00%, Nb: 0.01-2.00%, 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 to 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 to 0.012%, and REM: 0.0002-0.05%, One or more selected from As group C, Pb: 0.0001 to 0.30%, Se: 0.0001-0.80%, Te: 0.0001 to 0.30%, Bi: 0.0001 to 0.50%, S: 0.0001 to 0.50%, and P: 0.0001-0.30%, One or more selected from Contains one or more of Groups A to C of The stainless steel of claim 1.

3. 3. The stainless steel according to claim 1, wherein the pitting potential is 0.05 V or more.

4. 3. The stainless steel according to claim 1, having a tensile strength of 700 MPa or less.

5. 3. The stainless steel according to claim 1, wherein the limiting compressibility is 60% or more.

6. 3. The stainless steel according to claim 1, wherein the relative magnetic permeability after cold working is 1.10 or less.

7. 4. The stainless steel according to claim 3, having a tensile strength of 700 MPa or less.

8. 4. The stainless steel according to claim 3, having a limiting compressibility of 60% or more.

9. 5. The stainless steel according to claim 4, having a limiting compressibility of 60% or more.

10. The stainless steel according to claim 7, having a limiting compressibility of 60% or more.

11. 4. The stainless steel according to claim 3, having a relative magnetic permeability of 1.10 or less after cold working.

12. 5. The stainless steel according to claim 4, which has a relative magnetic permeability of 1.10 or less after cold working.

13. 6. The stainless steel according to claim 5, having a relative magnetic permeability of 1.10 or less after cold working.

14. 8. The stainless steel according to claim 7, having a relative magnetic permeability of 1.10 or less after cold working.

15. 9. The stainless steel according to claim 8, having a relative magnetic permeability of 1.10 or less after cold working.

16. 10. The stainless steel according to claim 9, having a relative magnetic permeability of 1.10 or less after cold working.

17. 11. The stainless steel according to claim 10, having a relative magnetic permeability of 1.10 or less after cold working.

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