Stainless steel with excellent cold forging properties and hydrogen embrittlement resistance

Abstract

To provide a stainless steel which can satisfy all of cold forgeability, machinability and hydrogen embrittlement resistance after being cold worked.SOLUTION: A stainless steel is provided, containing, by mass%, 0.0010-0.15% of C, 0.01-2.00% of Si, 0.01-10.00% of Mn, 8.00-30.00% of Ni, 9.0-21.0% of Cr, 0.01-3.00% of Mo, 0.01-5.00% of Cu, 0.0010-0.10%, of N, 0.0001-0.05% of B, and 0.0001-0.50% of S, and further containing one or more selected from 0.001-2.0% of Al and 0.0001-0.05% of Ca, wherein an A value represented by formula (a) is -100 or less, an amount of precipitated B as a boride is 0.0001% or more, and an aspect ratio of a sulfide is 50 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to stainless steel, and more particularly to stainless steel that can satisfy all of the requirements for cold forgeability, machinability, and hydrogen embrittlement resistance after cold working. [Background technology]

[0002] Fuel cell vehicles and hydrogen stations that handle hydrogen fuel use many metal parts that come into contact with high-pressure hydrogen gas. Metal parts that come into contact with high-pressure hydrogen gas are prone to hydrogen embrittlement, which occurs when hydrogen penetrates the metal. Therefore, they are required to have mechanical strength and corrosion resistance as well as hydrogen embrittlement resistance in a high-pressure hydrogen environment.

[0003] Conventionally, austenitic stainless steels such as SUS316 and SUS316L have been commonly used as stainless steels in parts that come into contact with high-pressure hydrogen gas. SUS316 and SUS316L contain Mo. In contrast, Patent Document 1 discloses an inexpensive austenitic stainless steel for high-pressure hydrogen that does not contain Mo, has excellent mechanical strength and corrosion resistance, and exhibits low hydrogen embrittlement susceptibility even at temperatures as low as -40°C. In the examples, test pieces that were cold drawn at cold working rates of 0 to 25% were evaluated for hydrogen embrittlement susceptibility using the Slow Strain Rate Test (SSRT).

[0004] Patent Document 2 discloses a hydrogen-resistant stainless steel wire for springs that is made of austenitic stainless steel having a predetermined composition, that has been subjected to a predetermined cold working process, and that has a face-centered cubic (fcc) lattice crystal structure after the working process. In the examples, after solution heat treatment, cold drawing is performed with a final working ratio of 0 to 75%, and the test specimens are hydrogen-charged before evaluation of bending stress and tensile stress.

[0005] 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.

[0006] Patent Document 3 describes 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 above Md30 formula described in Non-Patent Document 1 is set to −100 or less, and V(C, N) precipitates of 50 nm or less are 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.

[0007] Patent Document 4 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.

[0008] Conventionally, austenitic stainless steels such as SUS316 and SUS316L have been commonly used for non-magnetic parts. In contrast, Patent Documents 3 and 5 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), which is a modified version of 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 2The 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. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-114471 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-84597 [Patent Document 3] Japanese Patent Application Publication No. 2019-49036 [Patent Document 4] Japanese Patent Application Publication No. 05-329510 [Patent Document 5] JP 2016-183372 A [Non-patent literature]

[0010] [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]

[0011] By using SUS316, SUS316L, or the austenitic stainless steels described in Patent Documents 1 to 3, steels that have excellent hydrogen embrittlement resistance as well as excellent mechanical strength and corrosion resistance have been realized. These steels all have excellent hydrogen embrittlement resistance after hot working or after cold working and subsequent solution heat treatment. Patent Document 1 shows that steels that have been cold worked to a cold working rate of 25% or less, and Patent Document 2 shows that steels that have been cold worked to a final working rate of 75% or less, have excellent hydrogen embrittlement resistance even after cold working.

[0012] It has also been found that the previously known steels described above are difficult to satisfy in all of the requirements for cold forgeability, machinability, and hydrogen embrittlement resistance after cold forging. In particular, with conventional techniques, the material strength before cold forging is high, resulting in short tool life and increased forging loads for large-diameter steel bars. These factors have been found to result in poor cold forgeability and poor machinability. It has also been found that high-strain processing, such as cold forging, can lead to material processing limits (cracks) in conventional steels.

[0013] A second object of the present invention is to provide a stainless steel that can reduce tensile strength, increase cold forgeability, improve machinability, and further increase hydrogen embrittlement resistance after cold working. [Means for solving the problem]

[0014] 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 second invention. That is, the gist of the present invention is as follows.

[0015] [7] <Second 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%, S:0.0001~0.50%, 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 comprising containing one or more selected from Al: 0.001 to 2.0%, Ca: 0.0001 to 0.05%, and the balance being Fe and impurities; The A value represented by the following formula (a) is −100 or less, A stainless steel characterized in that the amount of B precipitated as borides is 0.0001% or more and the aspect ratio of sulfides is 50 or less. 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.

[0016] [8] The chemical composition further comprises, in mass %, as Group A, one or more selected from 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 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%, and P: 0.0001 to 0.30%; The stainless steel according to [7], which contains one or more of Groups A to C.

[0017] [9] The stainless steel according to [7] or [8], having a tensile strength of 700 MPa or less.

[10] The stainless steel according to any one of [7] to [9], which has a limiting compressibility of 60% or more.

[11] The stainless steel according to any one of [7] to

[10] , having 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 in high-pressure hydrogen after cold working of 80% or more.

[13] The stainless steel according to any one of [7] to

[12] , which has a relative reduction in area in high-pressure hydrogen after cold working of 50% or more. [Effects of the Invention]

[0018] The stainless steel of the second invention contains specified components, and further contains one or both of Al and Ca, with the amount of B precipitated as borides being 0.0001% or more, and the aspect ratio of sulfides being 50 or less, thereby making it possible to satisfy all of the requirements for cold forgeability, machinability, and hydrogen embrittlement resistance after cold working. DETAILED DESCRIPTION OF THE INVENTION

[0019] 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.

[0020] As mentioned above, the second invention has an object to provide stainless steel, particularly a steel bar, which can satisfy all of the requirements for cold forgeability, machinability, and hydrogen embrittlement resistance after cold working.

[0021] 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.

[0022] To evaluate the hydrogen embrittlement resistance after cold working, a sample was first prepared by performing solution heat treatment and then cold working with a cold working ratio of 80%. Two test pieces were prepared under the same processing conditions, one as a hydrogen test piece and the other as an air test piece. The hydrogen test piece was subjected to strain rate of 1×10 in a hydrogen atmosphere at -40°C and 70 MPa. -5 Tensile tests are conducted at a strain rate of 1 / s. For air test pieces, tensile tests are conducted in an air atmosphere at the same strain rate. Strength and reduction of area are evaluated for each, and the evaluation result for the hydrogen test piece is divided by the evaluation result for the air test piece, and the values ​​expressed as percentages are the "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.

[0023] Machinability is evaluated using the drill processing life index VL-1000 (the maximum peripheral speed (m / min) that can drill a cumulative hole depth of 1000 mm). The goal of the second invention is to achieve a VL-1000 of 1 m / min or more.

[0024] The second aspect of the invention will now be described in detail.

[0025] <Second Invention> <Amount of B precipitated as boride in the steel material of the second invention> The inventors came up with the idea of ​​controlling the amount of B precipitated as borides in a steel material as a means of satisfying all of the cold forgeability, machinability, and hydrogen embrittlement resistance properties after cold working in a stainless steel material. Regarding cold forgeability, the formation of borides reduces solid solution elements (such as N) and softens the material, resulting in an improved limiting compression ratio during cold forging and improved cold forgeability. Furthermore, the fine precipitation of borides makes them less likely to serve as crack initiation sites, further improving cold forgeability. Regarding machinability, the lubricating action of borides extends tool life during cutting. Regarding hydrogen embrittlement resistance, the formation of borides softens the material, increasing dislocation mobility, and the borides also serve as hydrogen trapping sites, improving hydrogen embrittlement resistance. Furthermore, by setting the amount of B precipitated as boride to 0.0001% by mass or more, in combination with the specification of the aspect ratio of sulfides below, it is possible to satisfy all of the cold forgeability, machinability, and hydrogen embrittlement resistance after cold working.

[0026] The amount of B precipitated as borides in steel can be evaluated by subjecting the steel to electrolytic extraction residue, extracting borides, and measuring the amount of B in the borides (Bpre).

[0027] <Aspect ratio of sulfides in the steel material of the second invention> The inventors conceived the idea of ​​controlling the aspect ratio of sulfides in a steel material as a means of satisfying all of the cold forgeability, machinability, and hydrogen embrittlement resistance properties after cold working in stainless steel. A small aspect ratio of sulfides in a steel material reduces the likelihood of fracture initiation, improving cold forgeability and hydrogen embrittlement resistance. A small aspect ratio of sulfides also enhances lubrication and extends tool life, improving machinability. Furthermore, a sulfide aspect ratio of 50 or less, combined with the above-mentioned specification for the amount of precipitated B as boride, can satisfy all of the cold forgeability, machinability, and hydrogen embrittlement resistance properties after cold working. The aspect ratio refers to the value calculated as L / W, where L is the length of the sulfide in the rolling direction and W is the length of the sulfide in the direction perpendicular to the rolling direction.

[0028] The aspect ratio of sulfides can be evaluated by measuring one or more fields of view at a magnification of 200x in the surface layer, the center, and the 1 / 4 depth position between the surface layer and the center in the L cross section of the steel material (cross section including the center line of the steel material), and calculating the average value of the aspect ratios L / W of sulfides in the same fields of view using an optical microscope.

[0029] <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 chemical composition, % means mass %.

[0030] (B: 0.0001 to 0.05%) B is necessary to ensure the amount of B precipitated as the above-mentioned borides. By including 0.0001% or more of B, in combination with the regulations of the manufacturing method described below, it is possible to ensure the amount of B precipitated as borides. The B content is more preferably 0.0005% or more, and even more preferably 0.0020% or more. On the other hand, if the B content exceeds 0.05%, coarse borides are formed in the steel, and these coarse borides become the origin of fracture, thereby deteriorating cold forgeability, machinability, and hydrogen embrittlement resistance, so the upper limit is set to 0.05%. The B content is more preferably 0.02% or less, and even more preferably 0.015% or less.

[0031] (S:0.0001~0.50%) S is an element that forms sulfides in steel and improves machinability, and should be contained in an amount of 0.0001% or more. However, excessive addition of S deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance, so the upper limit is set to 0.50%, preferably 0.1% or less, and more preferably 0.05% or less. Note that S is usually contained in steel as an impurity mixed in from the steelmaking raw materials.

[0032] (Al: 0.001 to 2.0%, 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 at least one of Al and Ca in an amount equal to or greater than the lower limit described above, Al- or Ca-based oxides are formed, which, in combination with the above-mentioned S content and the regulations of the manufacturing method described below, act as nuclei for sulfides, forming fine sulfides, and making it possible to set the aspect ratio of the sulfides after rolling to 50 or less. Excessive addition of Al causes the formation of coarse AlN, etc., which deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%. Excessive addition of Ca results in the formation of coarse Ca-based inclusions, which in turn result in larger sulfides formed around them. The sulfides are elongated during rolling, increasing their aspect ratios and deteriorating cold forgeability, machinability, and hydrogen embrittlement resistance. Therefore, the upper limit of the Ca content is set to 0.05%. Ca is preferably 0.010% or less, and more preferably 0.005% or less. If neither Al nor Ca is contained or if the content is outside the lower limit, the aspect ratio of the sulfides will deviate from the range of the present invention, resulting in poor tensile strength, limiting compressibility, machinability, relative tensile strength after cold working, and reduction of area.

[0033] (C: 0.0010 to 0.15%) C suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance, so the content is set to 0.0010% or more. Excessive addition of C deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0034] (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 deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0035] (Mn: 0.01 to 10.00%) Mn suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance, so its content is set to 0.01% or more. Excessive addition of Mn deteriorates cold forgeability, machinability, 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, and even more preferably 1.0% or less.

[0036] (Ni: 8.00 to 30.00%) Ni suppresses the formation of deformation-induced martensite and improves hydrogen embrittlement resistance. Furthermore, 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 adversely deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. Therefore, the upper limit of the Ni content is set to 30.00%, and preferably 25.00% or less.

[0037] (Cr: 9.0 to 21.0%) Cr suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance. Furthermore, to improve corrosion resistance, the Cr content is set to 9.0% or more, preferably 10.5% or more. Adding excessive Cr deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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.

[0038] (Mo: 0.01 to 3.00%) Mo suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance. Furthermore, in order to improve corrosion resistance and cold forgeability, the Mo content is set to 0.01% or more. Adding excessive Mo adversely deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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.

[0039] (Cu: 0.01 to 5.00%) Cu suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance. To improve cold forgeability, the Cu content is set to 0.01% or more, preferably 1.00% or more, and more preferably 2.00% or more. Adding excessive Cu adversely deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance, and also causes hot embrittlement. Therefore, the upper limit of the Cu content is set to 5.00%, and preferably 3.50% or less.

[0040] (N:0.0010~0.10%) N suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance, so the content is set to 0.0010% or more. Excessive addition of N deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. Therefore, the upper limit of the N content is set to 0.10%, preferably 0.08%, more preferably 0.05%, and even more preferably 0.03%.

[0041] The stainless steel of the second 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:

[0042] (Ti: 0 to 2.00%) Ti may be added to fix C and N, which increase microstrain. On the other hand, excessive Ti addition leads to the formation of coarse Ti-based precipitates, which deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0043] (Nb: 0 to 2.00%) Nb may be added to fix C and N. However, excessive addition of Nb results in the formation of coarse Nb-based precipitates, which deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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 lower limit of Nb is preferably 0.01%, more preferably 0.05%.

[0044] (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, conversely, cold forgeability, machinability, and hydrogen embrittlement resistance may deteriorate. 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 effects, 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.

[0045] (V:0~2.0%) V may be added to fix C and N. However, excessive V addition leads to the formation of coarse V-based precipitates, which deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0046] (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, conversely, there is a risk of deterioration in cold forgeability, machinability, and hydrogen embrittlement resistance. 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.

[0047] (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 cold forgeability, machinability, and hydrogen embrittlement resistance 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.

[0048] (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, cold forgeability, machinability, and hydrogen embrittlement resistance 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.

[0049] (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, cold forgeability, machinability, and hydrogen embrittlement resistance 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.

[0050] (Ta: 0 to 2.5%) Ta may be added to fix C and N. However, excessive Ta addition can result in the formation of coarse Ta-based precipitates, which can deteriorate cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0051] (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, machinability, 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%.

[0052] (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 cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0053] (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 deteriorate cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0054] (Pb: 0 to 0.30%) Pb is an element that improves machinability and may be added as needed. However, excessive addition of Pb deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0055] (Se:0~0.80%) Se is an element that improves machinability and may be contained as needed. However, excessive addition of Se deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. Therefore, the upper limit of the Se content is set to 0.80%, preferably 0.1% or less, and more preferably 0.05% or less. The preferred lower limit of Se is 0.0001%.

[0056] (Te:0~0.30%) Te is an element that improves machinability and may be added as needed. However, excessive addition of Te deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0057] (Bi: 0 to 0.50%) Bi is an element that improves machinability and may be added as needed. However, excessive addition of Bi deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0058] (P: 0 to 0.30%) P is an element that improves machinability and may be added as needed. However, excessive P addition deteriorates cold forgeability, machinability, and hydrogen embrittlement resistance. 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%.

[0059] <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.

[0060] <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.

[0061] <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.

[0062] <Common to the first and second inventions> The stainless steel may have a relative tensile strength in high-pressure hydrogen of 80% or more after cold working. <Common to the first and second inventions> The stainless steel may have a relative reduction of area in high-pressure hydrogen after cold working of 50% or more. Here, the cold working rate (area reduction rate) of the above cold working is 80%. The evaluation of the tensile strength and reduction of area in high-pressure hydrogen was carried out in a hydrogen atmosphere, -40°C, 70 MPa environment, at a strain rate of 1 × 10 -5 The tensile strength and reduction of area in a hydrogen atmosphere thus obtained are divided by the tensile strength and reduction of area evaluated at the same strain rate in an air atmosphere, and the percentage values ​​are the relative tensile strength and relative reduction of area in high-pressure hydrogen.

[0063] <Second Invention> The stainless steel may have a drilling life index VL-1000 of 1 m / min or more.

[0064] <<Method for manufacturing steel material according to the second invention>> The method for producing the steel material according to the second invention will be described below.

[0065] <Second Invention> In producing the stainless steel of the second invention, particularly a steel bar, it is preferable to employ tilt rolling as the hot working method and use induction heating to heat the material before tilt rolling.

[0066] A set temperature is set in an induction heating device that heats the steel material to be rolled. This set temperature is specified to 1000 to 1400°C, and the speed at which the rolling material passes through the induction heating device is set to a 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 speed at which the rolling material passes is in the range of 0.005 to 2.0 m / s. The set temperature is more preferably 1050 to 1300°C, and even more preferably 1100 to 1300°C. The speed at which the rolling material passes is more preferably 0.01 to 2.0 m / s, and even more preferably 0.1 to 1.0 m / s. By setting the induction heating device temperature and material passing speed within the above ranges, in combination with the inclusion of B in the steel, the amount of B precipitated as borides in the steel can be made 0.0001% or more. Furthermore, by setting the temperature of the induction heating device and the material passing speed within the above ranges, the aspect ratio of sulfides can be set to 50 or less, in combination with the inclusion of S in the steel and one or more of Al and Ca.

[0067] If the induction heating device is set at a temperature below 1000°C, the amount of precipitated B decreases, the aspect ratio of sulfides increases, and coarse, undissolved precipitates remain, resulting in poor cold forgeability, machinability, and hydrogen embrittlement resistance. If the temperature exceeds 1400°C, the amount of precipitated B decreases and the aspect ratio increases due to the elongation of sulfides, resulting in poor cold forgeability, machinability, and hydrogen embrittlement resistance. Furthermore, high-temperature heating can cause oxidation of the steel, resulting in a low yield, or the steel can creep during passage, resulting in poor rolling. Furthermore, if the speed at which the rolling material passes through the induction heating device is below 0.003 m / s, the amount of precipitated B decreases and the elongation of sulfides occurs, resulting in a high aspect ratio of the sulfides, resulting in poor cold forgeability, machinability, and hydrogen embrittlement resistance. When the material passing speed through the induction heating device exceeds 4.0 m / s, the amount of precipitated B decreases, the aspect ratio of sulfides increases, and coarse undissolved precipitates remain, resulting in deterioration of cold forgeability, machinability, and hydrogen embrittlement resistance. Furthermore, high-temperature heating can cause oxidation of the steel, resulting in a decrease in yield, or the steel can creep and deform during passage, resulting in poor rolling. The set temperature of the induction heating device specifically refers to the output temperature within the induction heating device through which the steel passes.

[0068] After heating the rolled material in this manner, tilt rolling is performed. As disclosed in Patent Document 4, for example, tilt rolling involves arranging three work rolls on roll axes that are twisted and tilted in the same direction around the rolled material, and each work roll revolves around the rolled material while rotating on its axis. This causes the rolled material to be spirally rolled while moving forward. The temperature distribution of the rolled material during tilt rolling is adjusted so that the steel temperature from the surface layer to the D / 4 position uniformly matches the set temperature. This reduces the amount of precipitated B and suppresses the elongation of sulfides, improving cold forgeability, machinability, and hydrogen embrittlement resistance. Furthermore, the above process eliminates coarse undissolved precipitates and refines the precipitates, contributing to improved cold forgeability, machinability, and hydrogen embrittlement resistance.

[0069] After tilt rolling, it is preferable to carry out in-line heat treatment, hot rolling, heat treatment, pickling, etc. After that, the shape of the steel material may be adjusted by peeling, drawing, etc.

[0070] In the method for producing a steel material according to the second aspect of the present invention, it is preferable to carry out hot working using tilt rolling as described above. However, the hot working is not limited to tilt rolling, and any method that follows a similar thermal working history may be used. For example, blooming (breakdown) may also be used as long as it follows a similar thermal working history. [Example]

[0071] <Second Invention> Example 2-1 The steel was produced using AOD melting, an inexpensive stainless steel melting process, in a 100 kg vacuum melting furnace, and 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 to 3. In Tables 1 to 6, items outside the scope of the second invention and items outside the preferred production conditions of the second invention are underlined.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] The cast slab was heated to 1130°C in a heating furnace, and then induction heating was used to heat the rolling material before tilt rolling. The induction heating device was set to a temperature of 1210°C and the material passing speed through the induction heating device was 0.3 m / s, and tilt rolling was performed. After inline heat treatment and bar wire rolling, offline heat treatment was performed at 1100°C for 30 minutes (water cooling), and the material was pickled to produce bar-shaped steel material with a diameter of 20.0 mm.

[0076] The methods used for measuring the amount of precipitated B as borides in the steel bar, the aspect ratio of sulfides, the limiting compression ratio, the VL-1000 drilling life index evaluation method, and the relative tensile strength and relative reduction of area after cold working were as described above.

[0077] The amount of B precipitated as borides was rated in mass percent as AA for 0.0010% or more, A for 0.0005% or more but less than 0.0010%, B for 0.0001% or more but less than 0.0005%, and C for less than 0.0001%. Note that when coarse borides were formed due to an excessive B content, they were rated as CC. The aspect ratio of sulfides was rated as AA for 5 or less, A for more than 5 and 30 or less, B for more than 30 and 50 or less, and C for more than 50. 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%. For the VL-1000 drilling life index, 20 m / min or more was rated AA, 10 m / min or more but less than 20 m / min was rated A, 1 m / min or more but less than 10 m / min was rated B, and less than 1 m / min was rated C. The relative tensile strength in high-pressure hydrogen after cold working was rated as AA for 95% or more, A for 90% or more but less than 95%, B for 80% or more but less than 90%, and C for less than 80%. Regarding the relative reduction in area in high pressure hydrogen after cold working, 70% or more was rated as AA, 60% or more but less than 70% as A, 50% or more but less than 60% as B, and less than 50% as C. The evaluation results are shown in Tables 4 and 5.

[0078] [Table 4]

[0079] [Table 5]

[0080] The steel bars described in Invention Examples Nos. 1 to 39 had the chemical composition, the amount of precipitated B as borides, and the aspect ratio of sulfides specified in the second invention, and were good in tensile strength, limiting compression ratio, VL-1000, relative tensile strength after cold working, and reduction of area, all of which were AA, A, or B.

[0081] On the other hand, in Comparative Examples 40 to 50 and 52 to 56, one or more of the components was outside the range of the second invention, and the amount of B precipitated as borides and the aspect ratio of sulfides were outside the range of the second invention, resulting in tensile strength, critical compression ratio, VL-1000, relative tensile strength and reduction of area after cold working all being C. In Comparative Example 51, coarse borides were formed due to the excessive B content, and the coarse borides became the origin of fracture, resulting in tensile strength, critical compression ratio, VL-1000, relative tensile strength and reduction of area after cold working all being C.

[0082] (Example 2-2) Steel bar materials were produced using steel type P in Table 1 as the chemical composition, under the induction heating conditions shown in Table 6 before tilt rolling, and under the other production conditions similar to those of Example 2-1.

[0083] [Table 6]

[0084] As shown in Table 6, Invention Examples Nos. 55 to 64 were manufactured under the preferred conditions of the second invention, and had the chemical composition, amount of precipitated B as boride, and aspect ratio of sulfide specified in the second invention.The tensile strength, limiting compression ratio, relative tensile strength and reduction of area after cold working were all either AA, A, or B, which were good.

[0085] On the other hand, for Comparative Examples 65 to 70, any of the manufacturing conditions was outside the preferred range of the second invention, and the amount of precipitated B as borides and the aspect ratio of sulfides were outside the range of the second invention. As a result, the tensile strength, limiting compression ratio, VL-1000, relative tensile strength and reduction of area after cold working were all 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%, S: 0.0001-0.50%, 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%, 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: 0-0.50%, P: 0-0.30%, and further comprising Al: 0.001-2.0%, Ca: 0.0001-0.05%, Contains one or more selected from The balance is Fe and impurities. The A value represented by the following formula (a) is −100 or less, A stainless steel characterized in that the amount of B precipitated as borides is 0.0001% or more and the aspect ratio of sulfides is 50 or less. 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, 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, 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 to 0.80%, Te: 0.0001 to 0.30%, Bi: 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, having a tensile strength of 700 MPa or less.

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

5. The stainless steel according to claim 1 or 2, wherein the drilling life index VL-1000 is 1 m / min or more.

6. 3. The stainless steel according to claim 1, wherein the relative tensile strength in high-pressure hydrogen after cold working is 80% or more.

7. 3. The stainless steel according to claim 1, wherein the relative reduction of area in high-pressure hydrogen after cold working is 50% or more.

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

9. The stainless steel according to claim 3, having a drilling life index VL-1000 of 1 m / min or more.

10. The stainless steel according to claim 4, having a drilling life index VL-1000 of 1 m / min or more.

11. The stainless steel according to claim 8, having a drilling life index VL-1000 of 1 m / min or more.

12. 4. The stainless steel according to claim 3, which has a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

13. 5. The stainless steel according to claim 4, which has a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

14. 6. The stainless steel according to claim 5, which has a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

15. 9. The stainless steel according to claim 8, which has a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

16. 10. The stainless steel according to claim 9, which has a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

17. 11. The stainless steel according to claim 10, having a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

18. 12. The stainless steel according to claim 11, having a relative tensile strength in high-pressure hydrogen after cold working of 80% or more.

19. 4. The stainless steel according to claim 3, which has a relative reduction in area in high-pressure hydrogen after cold working of 50% or more.

20. 5. The stainless steel according to claim 4, which has a relative reduction in area in high-pressure hydrogen after cold working of 50% or more.

21. 6. The stainless steel according to claim 5, which has a relative reduction in area in high-pressure hydrogen after cold working of 50% or more.

22. 9. The stainless steel according to claim 8, which has a relative reduction in area in high-pressure hydrogen after cold working of 50% or more.

23. 10. The stainless steel according to claim 9, which has a relative reduction in area in high-pressure hydrogen after cold working of 50% or more.

24. 11. The stainless steel according to claim 10, having a relative reduction in area in high-pressure hydrogen after cold working of 50% or more.

25. 12. The stainless steel according to claim 11, having a relative reduction of area in high-pressure hydrogen after cold working of 50% or more.

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