AUSTENITIC STAINLESS STEEL AND PRODUCTION METHOD THEREOF

Abstract

Provided are an austenitic stainless steel that has high strength and favorable shape retention properties after a heat treatment, and a production method thereof. One aspect of the present invention is the austenitic stainless steel wherein a component composition satisfies C: less than or equal to 0.12% by mass; Si: greater than or equal to 0.1% by mass and less than or equal to 1.0% by mass; Mn: greater than or equal to 0.1% by mass and less than or equal to 3.0% by mass; P: less than or equal to 0.05% by mass; S: less than or equal to 0.01% by mass; Cr: greater than or equal to 13.0% by mass and less than or equal to 22.0% by mass; Ni: greater than or equal to 4.0% by mass and less than or equal to 12.0% by mass; Cu: greater than or equal to 0.01% by mass and less than or equal to 0.50% by mass; Mo: less than or equal to 5.0% by mass; Al: less than or equal to 0.03% by mass; Nb: greater than or equal to 0.05% by mass and less than or equal to 0.30% by mass; N: greater than or equal to 0.10% by mass and less than or equal to 0.50% by mass; and a balance consisting of Fe and inevitable impurities, and a crystal grain size number is greater than or equal to 7.0.

Claims

1. An austenitic stainless steel, wherein a component composition satisfies C: less than or equal to 0.12% by mass; Si: greater than or equal to 0.1% by mass and less than or equal to 1.0% by mass; Mn: greater than or equal to 0.1% by mass and less than or equal to 2.7% by mass; P: less than or equal to 0.05% by mass; S: less than or equal to 0.01% by mass; Cr: greater than or equal to 13.0% by mass and less than or equal to 22.0% by mass; Ni: greater than or equal to 4.0% by mass and less than or equal to 12.0% by mass; Cu: greater than or equal to 0.01% by mass and less than or equal to 0.50% by mass; Mo: less than or equal to 5.0% by mass; Al: less than or equal to 0.03% by mass; Nb: greater than or equal to 0.05% by mass and less than or equal to 0.30% by mass; N: greater than or equal to 0.10% by mass and less than or equal to 0.50% by mass; and a balance consisting of Fe and inevitable impurities, and a crystal grain size number is greater than or equal to 7.0.

2. The austenitic stainless steel according to claim 1, wherein the component composition satisfies inequality (1):
2002090[% C]+12.8[% Cr]+320[% N]+42.3[% Nb]300(1) wherein, in the inequality (1), [% C], [% Cr], [% N], and [% Nb]represent a content (% by mass) of each component.

3. The austenitic stainless steel according to claim 1, wherein a maximum crystal grain diameter is less than or equal to 60 m.

4. The austenitic stainless steel according to claim 1, wherein the component composition satisfies inequality (2):
0.20[% C]+[% N]0.40(2) wherein, in the inequality (2), [% C] and [% N]represent a content in % by mass, of each component.

5. The austenitic stainless steel according to claim 1, wherein a maximum surface roughness height Ry is less than or equal to 10 m.

6. The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel is configured as a seamless steel tube.

7. An automobile fuel injection tube constituted of the stainless steel according to claim 1.

8. A method for producing the austenitic stainless steel according to claim 1, comprising: performing cold working on a steel material with a working rate per pass of greater than or equal to 20%; and performing a heat treatment on the steel material before and after performing the cold working, wherein a component composition of the steel material satisfies C: less than or equal to 0.12% by mass: Si: greater than or equal to 0.1% by mass and less than or equal to 1.0% by mass; Mn: greater than or equal to 0.1% by mass and less than or equal to 2.7% by mass; P: less than or equal to 0.05% by mass; S: less than or equal to 0.01% by mass; Cr: greater than or equal to 13.0% by mass and less than or equal to 22.0% Ni: greater than or equal to 4.0% by mass and less than or equal to 12.0% by mass; Cu: greater than or equal to 0.03% by mass and less than or equal to 0.50% by mass; Mo: less than or equal to 5.0% by mass; Al: less than or equal to 0.03% by mass; Nb: greater than or equal to 0.05% by mass and less than or equal to 0.30% by mass; N: greater than or equal to 0.10% by mass and less than or equal to 0.50% by mass; and a balance consisting of Fe and inevitable impurities, and wherein a heat treatment temperature T ( C.) in the heat treatment satisfies inequality (3):
1,000T2090[% C]+12.8[% Cr]+320[% N]+42.3[% Nb]+900(3) wherein, in the inequality (3), [% C], [% Cr], [% N], and [% Nb]represent a content (% by mass) of each component in the steel material.

9. A method of producing the austenitic stainless steel according to claim 1, comprising: performing cold working on a steel material with a working rate per pass of greater than or equal to 20%; and performing a heat treatment on the steel material before and after performing the cold working; wherein a component composition of the steel material satisfies C: less than or equal to 0.12% by mass; Si: greater than or equal to 0.1% by mass and less than or equal to 1.0% by mass; Mn: greater than or equal to 0.1% by mass and less than or equal to 2.7% by mass; P: less than or equal to 0.05% by mass; S: less than or equal to 0.01% by mass; Cr: greater than or equal to 13.0% by mass and less than or equal to 22.0% by mass; Ni; greater than or equal to 4.0% by mass and less than or equal to 12.0% by mass; Cu: greater than or equal to 0.01% by mass and less than or equal to 0.50% by mass; Mo: less than or equal to 5.0% by mass; Al: less than or equal to 0.03% by mass; Nb: greater than or equal to 0.05% by mass and less than or equal to 0.30% by mass; N: greater than or equal to 0.10% by mass and less than or equal to 0.50% by mass; and a balance consisting of Fe and inevitable impurities, and wherein a heat treatment temperature T ( C.) in the heat treatment is greater than or equal to 1,000 C. and less than or equal to 1,200 C.

10. The method of method of producing the austenitic stainless steel according to claim 8, wherein a final heat treatment after performing the cold working comprises bright annealing.

11. The method of method of producing the austenitic stainless steel according to claim 9, wherein a final heat treatment after performing the cold working comprises bright annealing.

Description

EXAMPLES

[0092] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

Examples 1 to 7 and Comparative Examples 1 to 3: Preparation of Steel Plate (Plate Material)

[0093] Using a vacuum induction melting furnace (VIF), an ingot weighing 20 kg and being cylindrical, and having the component composition described in Table 1 (the balance being Fe and inevitable impurities) was prepared. The ingot was heat-treated at greater than or equal to 1,250 C. for 24 hours, and hot-forged at a temperature in a range of greater than or equal to 1,200 C. to less than or equal to 1,000 C. to prepare a plate material of W60 mmL250 mmt17 mm. The plate material was heat-treated at a pre-cold working heat treatment temperature (Tc) as described in Table 1. Then, the plate material was cold-rolled at a working rate of 30%. Thereafter, as the final heat treatment, the plate material was heat-treated in a bright annealing furnace at a post-cold working heat treatment temperature (TO described in Table 1 to give test samples for Examples 1 to 7 and Comparative Examples 1 to 3. It is to be noted that Examples 1 to 7 and Comparative Examples 1 to 2 are austenitic stainless steels, and Comparative Example 3 is a duplex stainless steel.

Example 8: Preparation of Steel Tube

[0094] Using a vacuum induction melting furnace (VIF), an ingot weighing 150 kg and being cylindrical, and having the component composition (the balance being Fe and inevitable impurities) described in Table 1 was prepared. The ingot was heat-treated for 24 hours at greater than or equal to 1,250 C., and hot-forged at a temperature in a range of greater than or equal to 1,200 C. to less than or equal to 1,000 C. to prepare a bloom of 150 mm. A billet of 146 mm330 mm was prepared from the bloom, and a steel tube was obtained by the Ugine-Sejournet hot extrusion method. After being subjected to the heat treatment and the cold working multiple times, the steel tube was heat-treated at the pre-cold working heat treatment temperature (Tc) described in Table 1. Then, the steel tube was shaped by cold working with a working rate of 35%. Subsequently, as the final heat treatment, the steel was heat-treated in a bright annealing furnace at the post-cold working heat treatment temperature (TO described in Table 1 to give the test sample (austenitic stainless steel) of Example 8.

[0095] Crystal Grain Size Number

[0096] A sample of 1 cm1 cm1.2 cm was cut out from each test sample (plate material) obtained in Examples 1 to 7 and Comparative Examples 1 to 3. Each sample was filled with a resin so that a width-thickness cross-section was visible, and a surface thereof was polished to a mirror finish; subsequently, each sample was subjected to a 65% nitric acid electrolytic etch to reveal a structure. Further, a sample was cut out from a test sample (steel tube) obtained in Example 8 so that a vertical surface in a lengthwise direction was visible. The sample was filled wi th a resin so that a width-thickness cross-section was visible, and a surface thereof was polished to a mirror finish; subsequently, the sample was subjected to a 65% nitric acid electrolytic etch to reveal a structure. For each sample, the structure was observed with an optical microscope at a magnification of 400 to measure crystal grain size numbers in five fields of view, and a median value was determined to be the crystal grain size number. The measurement results are shown in Table 1. It is to be noted that - in the table indicates that a measurement was not performed.

[0097] Maximum Crystal Grain Diameter

[0098] An average value of minor and major diameters of the largest crystal grain observed in the five fields of view in which the crystal grain size numbers were measured was determined to be the maximum crystal grain diameter. The measurement results are shown in Table 1. It is to be noted that - in the table indicates that a measurement was not performed.

[0099] Maximum Height Ry

[0100] The maximum height Ry was obtained in accordance with JIS B0601 (1994). A roughness meter was used to perform the measurement for 3 mm in an axial direction. It is to be noted that for the test sample (steel tube) in Example 8, the external surface was measured for 3 mm in a length direction. The measurement results are shown in Table 1.

[0101] Evaluations

[0102] Tensile Test: 0.2% Proof Stress and Tensile Strength

[0103] A tensile test specimen having a parallel section of 4L15 was prepared from each test sample (plate material) of Examples 1 to 7 and Comparative Examples 1 to 3, to be used for a tensile test. Further, for Example 8 (steel tube), a No. 11 test specimen in compliance with JIS Z 2241 was prepared for use in the tensile test. In the tensile test, the test specimen was pulled at a constant speed at an initial strain rate of 2.010.sup.3 s.sup.1. The 0.2% proof stress and the tensile strength were measured. For the 0.2% proof stress, greater than or equal to 400 MPa was evaluated as A, greater than or equal to 370 MPa and less than 400 MPa was evaluated as B, and less than 370 MPa was evaluated as C. For the tensile strength, greater than or equal to 800 MPa was evaluated as A, greater than or equal to 710 MPa and less than 800 MPa was evaluated as B, and less than 710 MPa was evaluated as C. The results are shown in Table 2.

[0104] Amount of Warp after Heat Treatment

[0105] The test samples were wire-cut into plate materials having a length of 200 mm and a thickness of 2.0 mm. Each of the plate materials was heat-treated at 1,100 C. for 5 minutes under an air-cooling condition while being supported at two points being 50 mm away from both ends of the plate material. To measure the amount of warp of the plate material after the heat treatment, image data was used to draw a perpendicular line from a line connecting both ends of the plate material, and a length at a time at which the perpendicular line was the longest was determined to be the amount of warp caused by the heat treatment. The amount of warp being less than or equal to 0.1 mm was evaluated as A, greater than 0.1 mm and less than or equal to 1 mm was evaluated as B, and greater than 1 mm was evaluated as C. The measurement results are shown in Table 2.

[0106] Crystal Structure after Heat Treatment at 1,100 C. for 5 Minutes

[0107] Each test sample was heat-treated at 1,100 C. for 5 minutes under a water-cooling condition. Subsequently, each test sample (plate material) of Examples 1 to 7 and Comparative Examples 1 to 3 was cut so that a width-thickness cross-section was visible and filled with a resin, and a surface thereof was polished to a mirror finish; subsequently, each test sample was subjected to a 65% nitric acid electrolytic etch to reveal a structure. The test sample (steel tube) of Example 8 was cut so that a surface perpendicular to a length direction was visible, then filled with a resin, and a surface thereof was polished to a mirror finish; subsequently, the test sample was subjected to a 65% nitric acid electrolytic etch to reveal a structure. Each structure was observed with an optical microscope at a magnification of 400 to measure crystal grain size numbers in 5 fields of view. Each median value was determined to be the crystal grain size number. The crystal grain size number being greater than or equal to 9.0 was evaluated as A, as the coarsening of crystal grains had been inhibited even after the heat treatment, and the crystal grain size number being less than 9.0 was evaluated as B. The results are shown in Table 2. It is to be noted that - in the table indicates that a measurement was not performed.

[0108] As an evaluation of the presence/absence of the mixed grain sizes: a case in which less than or equal to 5% of crystal grains in one field of view had crystal grain size numbers that differed from each other by greater than or equal to 2 was evaluated as A; a case in which greater than 5% and less than or equal to 20% of crystal grains in one field of view had crystal grain size numbers that differed from each other by greater than or equal to 2 was evaluated as B; and a case in which greater than 20% of crystal grains in one field of view had crystal grain size numbers that differed from each other by greater than or equal to 2 was evaluated as C. The results are shown in Table 2. It is to be noted that - in the table indicates that a measurement was not performed.

TABLE-US-00001 TABLE 1 Component Composition (% by mass) C Si Mn P S Cr Ni Cu Mo Example 1 0.050 0.66 1.75 0.027 0.002 18.12 8.05 0.21 0.28 Example 2 0.040 0.57 0.75 0.026 0.001 18.54 8.25 0.22 0.19 Example 3 0.049 0.51 2.21 0.018 0.001 19.19 9.55 0.30 0.32 Example 4 0.020 0.71 1.25 0.013 0.002 19.20 8.16 0.21 0.26 Example 5 0.036 0.51 2.14 0.021 0.001 18.62 8.13 0.13 0.12 Example 6 0.057 0.52 2.20 0.019 0.001 18.09 9.15 0.30 0.25 Example 7 0.120 0.54 2.21 0.024 0.001 18.54 8.25 0.23 0.43 Example 8 0.040 0.49 0.75 0.009 0.001 19.61 7.91 0.20 0.21 Comparative 0.031 0.52 1.33 0.020 0.002 18.54 8.06 0.29 0.17 Example 1 Comparative 0.040 0.62 2.15 0.022 0.002 18.55 8.29 0.23 0.32 Example 2 Comparative 0.029 0.51 1.65 0.023 0.001 20.42 4.36 0.15 0.21 Example 3 Heat Treatment Temperature Pre-Cold Post-Cold Component Composition (% by mass) Working Tc Working Tf Al Nb N X .sup.(*.sup.1) Y .sup.(*.sup.2) ( C.) ( C.) Example 1 0.013 0.08 0.23 204.4 0.280 1070 1050 Example 2 0.017 0.07 0.25 236.7 0.290 1080 1080 Example 3 0.015 0.10 0.22 217.9 0.269 1080 1080 Example 4 0.016 0.11 0.28 298.2 0.300 1100 1100 Example 5 0.013 0.12 0.15 216.2 0.186 1080 1080 Example 6 0.012 0.08 0.21 183.0 0.267 1080 1080 Example 7 0.013 0.12 0.22 62.0 0.340 1080 1080 Example 8 0.010 0.12 0.22 242.9 0.260 1080 1080 Comparative 0.014 0.13 0.21 245.2 0.241 1220 1080 Example 1 Comparative 0.014 0.01 0.23 227.9 0.270 1080 1080 Example 2 Comparative 0.011 0.12 0.18 263.4 0.209 1100 1100 Example 3 Crystal Structure Maximum Crystal Crystal Grain Steel Final Heat Grain Size Diameter Ry Type Treatment Number (m) (m) (*3) Example 1 Bright 9.5 38.6 5.2 I annealing Example 2 Bright 10.0 34.2 3.9 I annealing Example 3 Bright 10.5 23.0 3.2 I annealing Example 4 Bright 11.5 15.3 5.9 I annealing Example 5 Bright 10.5 38.4 4.8 I annealing Example 6 Bright 10.0 115.2 4.4 I annealing Example 7 Bright 9.0 122.2 5.2 I annealing Example 8 aright 12.0 20.0 6.0 I annealing Comparative Bright 5.5 120.9 3.1 I Example 1 annealing Comparative Bright 7.0 145.9 4.8 I Example 2 annealing Comparative Bright 13.0 II Example 3 annealing .sup.(*.sup.1) X = 2090 [% C] + 12.8 [% Cr] + 320 [% N] + 42.3[% Nb] .sup.(*.sup.2) Y = [% C] + [% N] (*3) I: Austenitic Stainless Steel, II: Duplex Stainless Steel

TABLE-US-00002 TABLE 2 Crystal after Heat Treatment at 1,100 C. 0.2% Amount of Warp for 5 minutes Proof Stress Tensile Strength after Heat Treatment Crystal Grain Size Mixed Grain Shape (MPa) (MPa) mm (Number) Sizes Example 1 Steel plate 462 A 806 A 0.02 A 9.0 A B Example 2 Steel plate 445 A 809 A 0.04 A 9.5 A B Example 3 Steel plate 499 A 840 A 0.04 A 10.5 A A Example 4 Steel plate 550 A 845 A 0.06 A 11.5 A A Example 5 Steel plate 370 B 735 B 0.04 A 9.5 A A Example 6 Steel plate 377 B 720 B 0.03 A 8.5 B C Example 7 Steel plate 466 A 839 A 0.06 A 8.0 B C Example 8 Steel tube 530 A 846 A 0.09 A 11.0 A A Comparative Steel plate 359 C 703 C 0.03 A 5.5 B C Example 1 Comparative Steel plate 354 C 709 C 0.02 A 6.5 B B Example 2 Comparative Steel plate 480 A 711 B 2.3 C Example 3

[0109] As shown in Table 2, all of Examples 1 to 8 received an evaluation of A or B for the 0.2% proof stress and the tensile strength, revealing high strength, and an evaluation of A for the amount of warp after the heat treatment, revealing favorable shape retention properties after the heat treatment. Further, among the Examples, in the evaluations pertaining to crystal structure after the heat treatment at 1,100 C. for 5 minutes, Examples 1 to 5 and 8, in which X (=2090 [% C]+12.8 [% Cr]+320 [% N]+42.3 [% Nb]) was greater than or equal to 200 and less than or equal to 300; and the maximum crystal grain diameter was less than or equal to 60 m, received an evaluation of A for the crystal grain size number, and an evaluation of A or B for the mixed grain sizes. These evaluations indicate that the coarsening of crystal grains after the heat treatment was inhibited in Examples 1 to 5 and Example 8. In other words, it is concluded that, in Examples 1 to 5 and Example 8, the steel had high strength, and the high strength was maintained even after the heat treatment. Further, among Examples 1 to 5 and 8, Examples 1 to 4 and 8, in which Y (=[% C]+[% N]) was greater than or equal to 0.20 and less than or equal to 0.40, received an evaluation of A for the 0.2% proof stress and the tensile strength, indicating particularly high strength.

[0110] It is to be noted that in Comparative Example 3, Ry was greater than 10 m even though the final heat treatment was performed through bright annealing. The Ry is considered to have increased because, in Comparative Example 8, the stainless steel had a structure of an / duplex stainless steel rather than that of a single-phase austenitic stainless steel, and the -phase and the -phase each had different strengths and/or deformation behaviors.

INDUSTRIAL APPLICABILITY

[0111] The austenitic stainless steel according to the present invention can be suitably used for automobile fuel injection tubes and the like.