Method for producing high elastic limit nonmagnetic steel material using an austenitic stainless steel sheet
10125404 ยท 2018-11-13
Assignee
Inventors
Cpc classification
C22C38/002
CHEMISTRY; METALLURGY
International classification
C21D8/04
CHEMISTRY; METALLURGY
Abstract
[Problem] To provide a raw material steel sheet for providing a high strength nonmagnetic austenitic stainless steel material that has a high elastic limit stress and excellent toughness. [Solution to Problem] An austenitic stainless steel sheet containing 0.12% or less of C, from 0.30 to 3.00% of Si, from 2.0 to 9.0% of Mn, from 7.0 to 15.0% of Ni, from 11.0 to 20.0% of Cr, and 0.30% or less of N, and further containing at least one kind of 3.0% or less of Mo, 1.0% or less of V, 1.0% or less of Nb, 1.0% or less of Ti, and 0.010% or less of B, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities, having a component composition having a Ni equivalent of 19.0 or more, having a value of d.sup.1/2 of 0.40 or more, wherein d (m) represents an average austenitic crystal grain diameter, and having a property that provides a magnetic permeability of 1.0100 or less after subjected to cold rolling with an equivalent strain of 0.50 or more.
Claims
1. A method for producing a high elastic limit nonmagnetic stainless steel material that is excellent in toughness, comprising: subjecting an austenitic stainless steel sheet consisting of 0.02 to 0.09% of C, from 0.30 to 3.00% of Si, from 2.0 to 9.0% of Mn, from 7.0 to 14.0% of Ni, from 16.0 to 20.0% of Cr, and from 0.02 to 0.30% of N, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities, having a component composition having a Ni equivalent defined by the following expression (1) of 19.0 or more to hot rolling, cold rolling, and annealing at a temperature of from 700 C. or more to 1000 C. or less to provide a value of d.sup.1/2 (m.sup.1/2) of 0.40 or more, wherein d (m) represents an average austenitic crystal grain diameter, then subjecting the stainless steel sheet to cold rolling at a rolling reduction ratio of 40% or more to provide a magnetic permeability of 1.0100 or less, and then subjecting the stainless steel sheet to an aging treatment at an aging temperature of from 300 to 600 C. under a condition that satisfies the following expression (4):
Ni equivalent=Ni+0.6Mn+9.69(C+N)+0.18Cr0.11Si.sup.2(1)
13,000<T(log t+20)<16,500(4) wherein T represents the aging temperature in K in terms of absolute temperature, and t represents the aging time in h.
2. A method for producing a high elastic limit nonmagnetic stainless steel material that is excellent in toughness, comprising: subjecting an austenitic stainless steel sheet consisting of 0.02 to 0.09% of C, from 0.30 to 3.00% of Si, from 2.0 to 9.0% of Mn, from 7.0 to 14.0% of Ni, from 16.0 to 20.0% of Cr, and from 0.02 to 0.30% of N, and further comprising at least one kind of 3.0% or less of Mo, 1.0% or less of Nb, 1.0% or less of Ti, and 0.010% or less of B, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities, having a component composition having a Ni equivalent defined by the following expression (3) of 19.0 or more to hot rolling, cold rolling, and annealing at a temperature of from 700 C. or more to 1000 C. or less to provide a value of d.sup.1/2 (m.sup.1/2) of 0.40 or more, wherein d (m) represents an average austenitic crystal grain diameter, then subjecting the austenitic stainless steel sheet to cold rolling at a rolling reduction ratio of 40% or more to provide a magnetic permeability of 1.0100 or less, and then subjecting the austenitic stainless steel sheet to an aging treatment at an aging temperature of from 300 to 600 C. under a condition that satisfies the following expression (4):
Ni equivalent=Ni+0.6Mn+9.69(C+N)+0.18Cr0.11Si.sup.2+0.6Mo+2.3(V+Nb+Ti)(3)
13,000<T(log t+20)<16,500(4) wherein T represents the aging temperature in K in terms of absolute temperature, and t represents the aging time in h.
3. The method for producing a high elastic limit nonmagnetic stainless steel material according to claim 1, wherein the annealing temperature is from 700 C. or more to 900 C. or less.
4. The method for producing a high elastic limit nonmagnetic stainless steel material according to claim 2, wherein the annealing temperature is from 700 C. or more to 900 C. or less.
5. The method for producing a high elastic limit nonmagnetic stainless material according to claim 1, wherein the stainless steel has a property that provides an increment of elastic limit stress .sub.0.01 before and after an aging treatment of 150 N/mm.sup.2 or more.
6. The method for producing a high elastic limit nonmagnetic stainless material according to claim 2, wherein the stainless steel has a property that provides an increment of elastic limit stress .sub.0.01 before and after an aging treatment of 150 N/mm.sup.2 or more.
7. The method for producing a high elastic limit nonmagnetic stainless steel material according to claim 3, wherein the stainless steel has a property that provides an increment of elastic limit stress .sub.0.01 before and after an aging treatment of 150 N/mm.sup.2 or more.
8. The method for producing a high elastic limit nonmagnetic stainless steel material according to claim 4, wherein the stainless steel has a property that provides an increment of elastic limit stress .sub.0.01 before and after an aging treatment of 150 N/mm.sup.2 or more.
9. A method for producing a high elastic limit nonmagnetic stainless steel material that is excellent in toughness, comprising: subjecting an austenitic stainless steel sheet consisting of from 0.02 to 0.09% of C, from 0.30 to 3.00% of Si, from 2.0 to 9.0% of Mn, from 7.0 to 14.0% of Ni, from 16.0 to 20.0% of Cr, and from 0.02 to 0.30% of N, and 1.0% or less of V, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities, having a component composition having a Ni equivalent defined by the following expression (3) of 19.0 or more, to hot rolling, cold rolling, and annealing at an annealing temperature of from 700 C. or more to 900 C. or less to provide a value of d.sup.1/2 (m.sup.1/2) of 0.40 or more, wherein d in m represents an average austenitic crystal grain diameter, then subjecting the stainless steel sheet to cold rolling at a rolling reduction ratio of 40% or more to provide a magnetic permeability of 1.0100 or less, and then subjecting the stainless steel sheet to an aging treatment at an aging temperature of from 300 to 600 C. under a condition that satisfies the following expression (4):
Ni equivalent=Ni+0.6Mn+9.69(C+N)+0.18Cr0.11Si.sup.2+0.6Mo+2.3(V+Nb+Ti)(3)
13,000<T(log t+20)<16,500(4) wherein T represents the aging temperature in K in terms of absolute temperature, and t represents the aging time in h.
10. The method for producing a high elastic limit nonmagnetic stainless steel material according to claim 9, wherein the stainless steel has a property that provides an increment of elastic limit stress .sub.0.01 before and after an aging treatment of 150 N/mm.sup.2 or more.
Description
BRIEF DESCRIPTION OF DRAWINGS
(1)
(2)
(3)
DESCRIPTION OF EMBODIMENTS
(4) The value of d.sup.1/2 (i.e., reciprocal of square root of d), wherein d (m) represents the average austenitic crystal grain diameter, is hereinafter referred to as a crystal grain diameter d.sup.1/2. The present inventors have found that when the crystal grain diameter d.sup.1/2 is decreased to 0.40 or less, the austenitic crystal grains form a texture through rotation in a particular direction due to working deformation, but the elastic limit stress is enhanced through homogenization and refinement of the strain introduced.
(5) Using the A1 steel in Table 1 described later,
(6) In the invention, the steel species having such requirements that martensite is not induced even being subjected to working under severe condition, and the nonmagnetism is maintained under the use condition, is employed. As an index for securing the requirements, the Ni equivalent in PTL 1 proposed by the inventors is effective.
(7) Specifically, a magnetic permeability of 1.0100 or less in a magnetic field of 1 kOe (79.58 kA/m) is demanded for the application to a part used in various types of equipment and devices functioning by utilizing nonmagnetism. For such a magnetic permeability, the value of the Ni equivalent defined by the following expression (1) or (3) is necessarily 19.0 or more. The expression (3) is applied to a steel that contains at least one kind of Mo, V, Nb, Ti and B, and the expression (1) is applied to the other cases. In the expressions, the element symbols each represent the content of the corresponding element in terms of percentage by mass. In the case where the expression (3) is applied, the element symbol among Mo, V, Nb, Ti and B that is not added represents 0.
Ni equivalent=Ni+0.6Mn+9.69(C+N)+0.18Cr0.11Si.sup.2(1)
Ni equivalent=Ni+0.6Mn+9.69(C+N)+0.18Cr0.11Si.sup.2+0.6Mo+2.3(V+Nb+Ti)(3)
(8)
(9) For increasing the Ni equivalent, increase of the amounts of Ni and Mn is effective, but the work hardening capability of the steel may be lowered when the contents of these elements are too large, and thus the Ni equivalent is preferably in a range of from 19.0 to 21.0.
(10) A steel that has the component composition defined above is formed into a cold rolled steel sheet through ordinary hot rolling and cold rolling, and then annealed to provide the steel sheet of the invention. In this case, it is important to perform the annealing under the condition that provides a crystal grain diameter d.sup.1/2 of 0.40 or more. For achieving the crystal grain diameter, the annealing temperature is preferably in a range of 700 C. or more and 1,000 C. or less, and more preferably in a range of 700 C. or more and 860 C. or less. In consideration of the cold rolling reduction ratio before the annealing, the annealing condition is selected which provides a crystal grain diameter d.sup.1/2 of 0.40 or more. The annealing condition may be obtained in advance by a preliminary experiment corresponding to the production line. The crystal grain diameter d.sup.1/2 is preferably 0.45 or more, and more preferably 0.50 or more. However, the austenitic crystal grains are necessarily constituted by recrystallized grains.
(11) The steel sheet according to the invention having an austenitic crystal grain diameter d.sup.1/2 that is regulated as shown above may be formed into a shape of a part by being subjected to punching and then cold working, such as bending, and then may be imparted with high elasticity by the aging treatment. The nonmagnetism of the steel sheet may be maintained even though the steel sheet is subjected to the cold working under severe conditions resulting in an equivalent strain of 0.50 or more. In the case where an austenitic stainless steel sheet having a high elastic limit is provided as a raw material of a steel sheet, temper rolling may be performed to regulate the thickness and to enhance the strength, and then the steel sheet may be subj ected to the aging treatment. In this case, the annealing is performed before the temper rolling, and thus the annealing may be referred to as annealing before temper rolling in some cases. The nonmagnetism may be maintained even when the temper rolling is performed at a rolling reduction ratio providing an equivalent strain of 0.5 or more. The temper rolling reduction ratio may be more advantageously 40% or more (corresponding to an equivalent strain of 0.59 or more according to the expression (6)) for enhancing the strength. The upper limit of the temper rolling reduction may not be particularly determined, however, since excessive work hardening may result in difficulty in working of parts thereafter the temper rolling is preferably performed at a rolling reduction ratio of 80% or less (corresponding to an equivalent strain of 1.86 or less according to the expression (6)). The amount of cold working may be managed to a range that provides an equivalent strain of 1.5 or less.
(12) The austenitic stainless steel sheet thus having a refined crystal grain diameter may provide a texture having a homogeneous distribution of working strain when subjected to temper rolling. Accordingly, the elastic limit stress .sub.0.01 as an index of the elastic limit may be considerably increased by subjecting the steel sheet to the aging treatment thereafter. The condition for the aging treatment is preferably an aging temperature of from 300 to 600 C. and a condition that satisfies the following expression (4):
13,000<T(log t+20)<16,500(4)
wherein T represents the aging temperature (K) in terms of absolute temperature, and t represents the aging time (h).
(13) By subjecting the steel sheet according to the invention to the aging treatment under the aforementioned condition, the increment of elastic limit stress .sub.0.01 before and after the aging treatment shown by the following expression (2) may be 150 N/mm.sup.2 or more:
.sub.0.01=.sub.0.01[1].sub.0.01[0](2)
wherein .sub.0.01[0] represents the elastic limit stress .sub.0.01 (N/mm.sup.2) in the rolling direction of the steel sheet before the aging treatment, and .sub.0.01[1] represents the elastic limit stress .sub.0.01 (N/mm.sup.2) in the rolling direction of the steel sheet after the aging treatment.
(14) The content ranges of the alloy components will be described below. The percentages for the contents of the alloy components mean percentages by mass unless otherwise indicated.
(15) C: 0.12% or Less
(16) C is an element that strongly stabilizes the austenitic phase and is effective for enhancing the strength through working. It is more effective to ensure the C content of 0.02% or more. The increase of the C content may be a factor resulting in deterioration of corrosion resistance and the like, and thus the C content is restricted to 0.12% or less, and is more preferably 0.09% or less.
(17) Si: 0.30 to 3.00%
(18) Si is an element that is effective for enhancing the strength, and a Si content of 0.30 or more is ensured. However, the increase of the Si content may sharply increase the magnetic permeability after the cold working to fail to maintain the nonmagnetism. As a result of various investigations, the Si content is restricted to 3.00% or less.
(19) Mn: 2.0 to 9.0%
(20) Mn is an element that stabilizes austenite as similar to Ni, and suppresses the increase of the magnetic permeability due to cold working. Mn is also an element that enhances the solid solubility of N. For exhibiting these functions, a Mn content of 2.0% or more is ensured. A large amount of Mn contained may be a factor of deteriorating the low temperature toughness, and thus the Mn content is in a range of 9.0% or less.
(21) Cr: 11.0 to 20.0%
(22) Cr is a basic component of a stainless steel and is necessarily contained in an amount of 11.0% or more for providing corrosion resistance. Cr is more effectively contained in an amount of 16.0% or more for enhancing the corrosion resistance. When the Cr content is increased, the amount of ferrite formed may be increased to inhibit the maintenance of the nonmagnetism. The Cr content is restricted to 20.0% or less.
(23) Ni: 7.0 to 15.0%
(24) Ni is an element that is essential for stabilizing the austenitic phase. A Ni content of 7.0% is necessary for ensuring the nonmagnetism after cold working. A large amount of Ni contained may be a factor of lowering the strength enhancement effect of cold rolling, and thus the Ni content is restricted to 15.0% or less, and is more preferably 14.0% or less.
(25) N: 0.30% or Less
(26) N is an element that is effective for enhancing the strength and stabilizing the austenitic phase. It is more effective to ensure an N content of 0.02% or more. When the N content is increased, however, a casted slab in good condition may not be obtained in some cases. In the invention, the N content is restricted to 0.30% or less.
(27) Mo: 3.0% or Less
(28) Mo has a useful function including enhancement of the corrosion resistance and enhancement of the work hardening capability, and thus may be added depending on necessity. In the case where Mo is added, the content thereof is more effectively 0.2% or more. However, a large amount thereof added may increase the amount of ferrite formed, which is disadvantageous for maintaining the nonmagnetism. In the case where Mo is added, the content thereof is in a range of 3.0% or less, and more preferably 2.5% or less.
(29) V: 1.0% or Less, Nb: 1.0% or Less, Ti: 1.0% or Less
(30) V, Nb and Ti all have a function of enhancing the work hardening capability, and thus at least one kind thereof may be added depending on necessity. In the case where these elements are added, the contents thereof are more effectively 0.1% or more for V, 0.1% or more for Nb, and 0.1% or more for Ti. However, large amounts of the elements added may cause deterioration of the hot workability and formation of ferrite. In the case where at least one kind of these elements is added, the amounts thereof added each are necessarily 1.0% or less.
(31) B: 0.010% or Less
(32) B has a function of improving the hot workability, and thus may be added depending on necessity in a range of 0.010% or less. In the case where B is added, the amount thereof contained is more effectively 0.001% or more.
(33) In addition to the aforementioned elements, Ca and REM (rare earth elements) used as a deoxidizing agent and a desulfurizing agent are allowed to be incorporated in an amount of 0.01% or less in total. Al used as a deoxidizing agent is allowed to be incorporated in an amount of 0.10% or less.
EXAMPLE
(34) Steels having a chemical composition shown in Table 1 were produced with a vacuum melting furnace, subjected to hot rolling, then subjected to a solution treatment and cold rolling, subjected to intermediate annealing and cold rolling once or plural times, subjected to finishing annealing (corresponding to annealing before temper rolling), then subjected to temper rolling to make a thickness of 0.2 mm, and further subjected to an aging treatment. The condition for the aging treatment was 500 C.1 h. In this case, the value of T(log t+20) in the expression (4) is 15,460. The finishing annealing temperature and the temper rolling reduction ratio are shown in Table 2. The equivalent strain according to the expression (6) is 0.59 for the case of a rolling reduction of 40%, 1.06 for the case of a rolling reduction of 60%, and 1.39 for the case of a rolling reduction of 70%.
(35) The ND plane of the finishing annealed material was observed for the structure thereof, and the average crystal grain diameter d of the austenitic crystal grains was obtained as a circle equivalent diameter by image analysis. The average crystal grain diameter d and the crystal grain diameter d.sup.1/2 are shown in Table 2.
(36) The plate surface of the temper rolled material was measured for Vickers hardness. A JIS 13B test piece in parallel to the rolling direction was subjected to a tensile test at a strain rate of 1.6710.sup.3 (s.sup.1) to measure the elastic limit stress .sub.0.01, the 0.2% proof stress .sub.0.2, and the tensile strength .sub.B. The temper rolled material was measured for the magnetic permeability in a magnetic field of 1 kOe (79.58 kA/m) with a vibrating sample magnetometer (produced by Riken Denshi Co., Ltd.). The measurement results are shown in Table 2.
(37) The aging treated material was measured for hardness, 0.01, 0.2 and .sub.B in the same manner as the temper rolled material. The test piece after the tensile test was measured for the cross sectional contraction ratio (reduction) in the broken portion. The increment .sub.0.01 of elastic limit stress .sub.0.01 due to the aging treatment was obtained from the expression (2), and the effect of enhancement of the elastic limit was evaluated thereby. The values are shown in Table 2.
(38) TABLE-US-00001 TABLE 1 Chemical composition (% by mass) Steel C Si Mn P S Ni Cr N Mo V Nb Ti B Ni equivalent A1 0.052 0.62 2.80 0.023 0.006 12.90 18.20 0.090 19.19 A2 0.073 0.60 3.53 0.021 0.004 12.70 17.60 0.120 19.82 A3 0.024 1.70 4.24 0.020 0.007 12.88 19.88 0.190 20.76 A4 0.050 2.81 3.90 0.018 0.006 12.46 18.70 0.154 19.27 A5 0.060 1.70 3.31 0.025 0.010 12.44 18.00 0.132 2.00 20.41 A6 0.052 1.64 3.10 0.030 0.009 12.42 17.98 0.141 0.34 19.87 A7 0.060 1.50 3.40 0.025 0.009 12.40 18.20 0.140 0.35 20.21 A8 0.064 1.63 3.00 0.028 0.011 12.60 18.12 0.189 0.45 20.86 A9 0.090 0.50 8.80 0.025 0.011 7.50 20.00 0.290 20.03 A10 0.120 0.59 3.50 0.021 0.009 13.98 17.00 0.100 21.23 A11 0.119 0.78 6.60 0.019 0.013 14.90 11.80 0.080 22.85 A12 0.050 0.59 3.10 0.022 0.007 13.00 17.98 0.088 0.0055 19.40 A13 0.050 0.58 1.10 0.031 0.011 8.30 18.22 0.019 12.87 A14 0.015 0.55 1.13 0.033 0.012 9.99 18.60 0.015 14.27 A15 0.059 0.49 1.54 0.034 0.008 9.80 18.41 0.148 16.02 underlined value: outside the scope of the invention
(39) TABLE-US-00002 TABLE 2 Finishing annealed material Temper rolled material Average. Temper Annealing crystal grain Crystal grain rolling Ni temperature diameter diameter reduction Hardness .sub.0.01 .sub.0.2 Class No. Steel equivalent ( C.) d (m) d.sup.1/2 (%) (HV) (N/mm.sup.2) (N/mm.sup.2) Invention 1 A1 19.19 800 0.5 1.41 40 421 804 1202 2 850 2.8 0.60 392 762 1142 3 900 5.0 0.45 373 700 1037 4 A2 19.82 850 3.0 0.58 381 753 1132 5 A3 20.76 3.2 0.56 393 762 1144 6 A4 19.27 2.8 0.60 40 380 754 1135 7 60 448 903 1355 8 70 459 918 1377 9 A5 20.41 3.0 0.58 40 381 768 1154 10 A6 19.87 2.7 0.61 381 754 1133 11 A7 20.21 3.3 0.55 377 766 1150 12 A8 20.86 2.3 0.66 379 755 1130 13 A9 20.03 3.0 0.58 380 760 1151 14 A10 21.23 1.8 0.75 400 900 1366 15 A11 22.85 2.0 0.71 381 881 1278 16 A12 19.40 1.5 0.82 390 760 1140 Comparison 17 A1 19.19 1050 25.0 0.20 40 360 740 1111 18 A2 19.82 28.0 0.19 374 731 1103 19 A3 20.76 27.0 0.19 371 724 1086 20 A4 19.27 20.0 0.22 70 402 804 1206 21 A13 12.87 850 3.1 0.57 40 370 603 1000 22 A14 14.27 2.8 0.60 375 610 1021 23 A15 16.02 3.3 0.55 380 630 1050 Temper rolled material Aging treated material .sub.B Magnetic .sub.0.01 .sub.0.2 .sub.B .sub.0.01 Reduction Hardness Class No. (N/mm.sup.2) permeability (N/mm.sup.2) (N/mm.sup.2) (N/mm.sup.2) (N/mm.sup.2) (%) (HV) Invention 1 1262 1.0091 1050 1419 1470 246 31 470 2 1202 1.0090 962 1381 1400 200 33 442 3 1097 1.0087 900 1302 1350 200 36 420 4 1192 1.0048 948 1348 1400 195 34 431 5 1204 1.0043 955 1380 1400 193 35 443 6 1195 1.0075 1000 1370 1380 246 34 431 7 1420 1.0080 1098 1439 1460 195 33 501 8 1440 1.0090 1154 1455 1465 236 30 509 9 1214 1.0045 955 1387 1405 187 32 431 10 1193 1.0050 966 1384 1403 212 31 433 11 1210 1.0062 941 1383 1398 175 33 427 12 1190 1.0058 950 1378 1399 195 32 434 13 1211 1.0040 964 1382 1407 204 30 433 14 1446 1.0038 1100 1412 1472 200 30 467 15 1321 1.0054 1099 1398 1467 218 30 470 16 1200 1.0061 960 1380 1399 200 34 471 Comparison 17 1170 1.0090 800 1198 1240 60 44 399 18 1160 1.0048 780 1178 1198 49 42 397 19 1146 1.0043 790 1193 1203 66 40 404 20 1266 1.0075 890 1270 1289 86 10 410 21 1060 6.4560 750 1204 1255 147 42 363 22 1081 5.5611 760 1190 1212 150 45 370 23 1110 2.5222 780 1190 1248 150 40 380 underlined value: outside the scope of the invention
(40)