Steel and method of manufacturing the same

Abstract

Steel has a chemical composition that contains 0.050% to 0.40% of C, 0.50% to 3.0% of Si, 3.0% to 8.0% of Mn, and 0.001% to 3.0% of sol. Al, by mass %, and has a metallographic structure that contains 10% to 40% of austenite in terms of % by volume. The average concentration of C in austenite is 0.30% by 0.60%, by mass %, structure uniformity, which is represented by a value obtained by subtracting the minimum value from the maximum value of Vickers hardness that is measured, in the metallographic structure is 30 Hv or less, and the tensile strength is 900 MPa to 1800 MPa.

Claims

1. A method of manufacturing a steel, comprising: performing a heat treatment with respect to a base steel having a metallographic structure in which an average grain size of a prior austenite is 20 m or less and which is composed of a martensite single phase, wherein the heat treatment includes: a retention process of retaining the base steel at a temperature that is equal to or higher than 670 C. and lower than 780 C., and is lower than an A.sub.c3 point for 5 seconds to 120 seconds; and a cooling process of cooling the base steel in such a manner that an average cooling rate from the temperature region to 150 C. is 5 C/second to 500 C./second after the retention process, and wherein the steel has a chemical composition comprising, by mass %: 0.050% to 0.40% of C, 0.50% to 3.0% of Si, 4.0% to 8.0% of Mn, 0.001% to 3.0% of sol. Al, 0.05% or less of P, 0,01% or less of S, 0.01% or less of N, 0% to 1.0% of Ti, 0% to 1.0% of Nb, 0% to 1.0% of V, 0% to 1.0% of Cr, 0% to 1.0% of Mo, 0% to 1.0% of Cu, 0% to 1.0% of Ni, 0% to 0.01% of Ca, 0% to 0.01% of Mg, 0% to 0.01% of REM, 0% to 0.01% of Zr, 0% to 0.01% of B, 0% to 0.01% of Bi, and the remainder including Fe and impurities.

Description

EXAMPLES

(1) Base steel having a chemical composition shown in Table 1 and a metallographic structure shown in Table 2 is used in a heat treatment under conditions shown in Table 3.

(2) The base steel, which was used, was prepared by subjecting slab that was obtained through melting in a laboratory to hot working. The base steel was cut into dimensions of 3 mm (thickness), 100 mm (width), and 200 mm (length), and was heated, retained, and cooled under conditions in Table 3. A thermocouple was attached to a surface of the steel to perform temperature measurement during a heat treatment. In Table 3, the average heating rate represents a value in a temperature region from room temperature to a heating temperature, a retention time represents time taken for retention at the heating temperature, and the average cooling rate represents a value in a temperature region from a retention temperature to 150 C. As described below, a metallographic structure of metal that was used in the heat treatment, and the metallographic structure and the mechanical properties of steel that was obtained through the heat treatment were investigated through metallographic structure observation, X-ray diffraction measurement, a tensile test, and a Charpy test. Test results are shown in Table 4.

(3) (Metallographic Structure of Steel (Base Steel) that is Subjected to Heat Treatment)

(4) A cross-section of steel, which was used in the heat treatment, was observed and photographed with an electron microscope, and a total region of 0.04 mm.sup.2 was analyzed to identify a metallographic structure and to measure an average grain size of prior austenite. The average grain size of prior austenite was obtained by measuring the average slice length in the observed image that was obtained, and by multiplying the length by 1.78.

(5) An observation position was set to a position that avoids the central segregation portion at a position (position of t) of approximately times the sheet thickness. The reason for avoiding the central segregation portion is as follows. The central segregation portion may have a metallographic structure that is locally different from a representative metallographic structure of steel. However, the central segregation portion is a minute region with respect to the entirety of the sheet thickness, and hardly has an effect on the characteristics of steel. That is, it cannot be said that the metallographic structure of the central segregation portion represents a metallographic structure of steel. According to this, it is preferable to avoid the central segregation portion in identification of the metallographic structure.

(6) (Volume Ratio of Austenite in Steel after Heat Treatment)

(7) A test specimen having a width of 25 mm and a length of 25 mm was cut out from the steel after the heat treatment, the test specimen was subjected to chemical polishing so as to reduce the thickness by 0.3 mm, and X-ray diffraction was performed three times with respect to a surface of the test specimen after the chemical polishing. Profiles, which were obtained, were analyzed, and were averaged to calculate the volume ratio of austenite.

(8) (Average Concentration of C in Austenite in Steel after Heat Treatment)

(9) The profiles, which were obtained in the X-ray diffraction, were analyzed to calculate a lattice constant (a: unit is A) of austenite, and the average concentration (c: unit is mass %) of C in austenite was determined on the basis of the following Expression (2).
c=(a3.572)/0.033(2)

(10) (Structure Uniformity) The hardness at five points under a load of 1 kg was measured by using a Vickers tester, and evaluation was made by setting a difference between the maximum value and the minimum value of the Vickers hardness as the structure uniformity.

(11) (Tensile Test)

(12) A tensile test specimen of No. JIS 5 having a thickness of 2.0 mm was collected from steel after the heat treatment, and a tensile test was performed in conformity to JIS Z2241 to measure TS (tensile strength) and EL (total elongation). In addition, TSEL was calculated from TS and EL.

(13) (Impact Characteristics)

(14) Front and rear surfaces of the steel after the heat treatment were grinded to have a thickness of 1.2 mm, and a V-notched test specimen was prepared. Four sheets of the test specimen were laminated and were fixed with a screw, and the resultant laminated sheets were provided to a Charpy impact test in conformity to JIS Z2242. With regard to impact characteristics, a case where an impact value at 0 C. became 20 J/cm.sup.2 or greater was regarded as Good, and a case where an impact value at 0 C. was less than 20 J/cm.sup.2 was regarded as Poor.

(15) TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %), remainder: Fe and impurities Ac.sub.3 Symbol C Si Mn P S sol. Al N Other ( C.) A 0.23 1.68 3.31 0.012 0.0013 0.035 0.0042 811 B 0.074 1.76 5.25 0.012 0.0013 0.029 0.0043 Ca: 0.0013 796 C 0.14 1.73 4.21 0.010 0.0011 0.034 0.0035 REM: 0.0021 806 D 0.035 1.56 6.98 0.012 0.0011 0.032 0.0051 754 E 0.11 1.96 4.92 0.010 0.021 0.031 0.0039 802 F 0.095 1.87 3.64 0.012 0.0014 0.035 0.0042 Ni: 0.87 831 G 0.092 2.05 4.95 0.012 0.0013 0.028 0.0041 Mg: 0.0014 811 Bi: 0.0016 H 0.10 3.25 6.31 0.012 0.0013 0.028 0.0042 821 I 0.098 1.43 4.26 0.009 0.0012 0.028 0.0046 Cu: 0.32 787 Ni: 0.45 Zr: 0.0012 J 0.10 2.02 4.84 0.011 0.0011 0.029 0.0048 V: 0.024 813 B: 0.0007 K 0.097 0.24 3.35 0.009 0.0009 0.030 0.0044 775 L 0.52 1.26 3.13 0.011 0.0011 0.028 0.0045 745 M 0.15 1.89 4.64 0.012 0.0014 0.031 0.0045 Ti: 0.015 793 Nb: 0.022 Cr: 0.43 N 0.10 1.98 4.97 0.010 0.0011 0.028 0.0041 803 O 0.23 1.43 1.02 0.012 0.0012 0.037 0.0041 869 P 0.11 1.52 4.42 0.011 0.0009 0.23 0.0042 Mo: 0.12 881 Q 0.12 0.75 4.63 0.013 0.0012 0.032 0.0042 756 R 0.25 1.12 2.52 0.016 0.0012 0.031 0.0039 807 S 0.32 2.03 4.89 0.011 0.0009 0.034 0.0047 761 T 0.11 1.34 5.01 0.013 0.0007 0.55 0.0033 981 U 0.10 2.42 7.82 0.011 0.0008 0.042 0.0036 808 (Remark) an underline represents that a value is not in a range of the invention

(16) TABLE-US-00002 TABLE 2 Average Sample Steel grain size of prior No. symbol Metallographic structure austenite (m) 1 A Martensite single phase 11 2 A Austenite and bainite plural phases 12 3 B Martensite single phase 15 4 C Martensite single phase 13 5 C Martensite single phase 25 6 D Martensite single phase 14 7 E Martensite single phase 11 8 F Martensite single phase 12 9 F Martensite single phase 15 10 G Martensite single phase 13 11 H Martensite single phase 15 12 I Martensite single phase 12 13 I Martensite single phase 14 14 J Martensite single phase 13 15 J Martensite single phase 12 16 K Martensite single phase 11 17 L Martensite single phase 12 18 M Martensite single phase 13 19 M Martensite single phase 12 20 N Martensite single phase 14 21 N Martensite single phase 15 22 O Martensite single phase 11 23 P Martensite single phase 12 24 Q Martensite single phase 13 25 R Martensite single phase 11 26 S Martensite single phase 12 27 T Martensite single phase 11 28 U Martensite single phase 13 (Remark) an underline represents that a value is not in a range of the invention

(17) TABLE-US-00003 TABLE 3 Retention Sample Average heating temperature Retention time Cooling rate No. rate ( C./s) ( C.) (second) ( C./s) 1 10 700 30 50 2 10 700 30 50 3 10 710 30 50 4 10 720 30 50 5 10 680 15 50 6 10 680 30 50 7 10 700 30 50 8 10 720 30 50 9 10 680 30 3 10 10 700 30 50 11 10 700 30 50 12 10 700 30 50 13 10 800 30 50 14 10 700 30 50 15 10 790 30 50 16 10 690 30 50 17 10 700 30 50 18 10 700 30 30 19 10 660 30 30 20 10 700 30 50 21 10 700 1500 50 22 10 730 30 50 23 10 700 30 50 24 10 700 30 50 25 10 740 30 50 26 10 680 95 50 27 10 760 10 50 28 10 700 30 50 (Remark) an underline represents that a value is not in a range of the invention

(18) TABLE-US-00004 TABLE 4 Average Volume concentration ratio of of C in Structure Sample Steel austenite austenite uniformity TS EL TS EL Impact No. symbol (%) (% mass %) (Hv) (MPa) (%) (MPa .Math. %) characteristics Remark 1 A 16 0.56 29 987 25 24675 Good Invention Example 2 A 8 0.52 21 954 19 18126 Good Comparative Example 3 B 18 0.37 27 953 28 26684 Good Invention Example 4 C 12 0.56 26 1045 24 25080 Good Invention Example 5 C 13 0.66 28 958 26 24908 Poor Comparative Example 6 D 4 0.37 25 768 20 15360 Good Comparative Example 7 E 18 0.41 23 1035 24 24840 Poor Comparative Example 8 F 12 0.39 25 994 25 24850 Good Invention Example 9 F 13 0.56 24 894 30 26820 Good Comparative Example 10 G 21 0.43 26 1083 25 27075 Good Invention Example 11 H 13 0.41 22 1102 25 27550 Poor Comparative Example 12 I 21 0.45 24 1108 25 27700 Good Invention Example 13 I 5 0.52 28 1206 6 7236 Good Comparative Example 14 J 15 0.39 26 1153 21 24213 Good Invention Example 15 J 7 0.41 22 1242 10 12420 Good Comparative Example 16 K 9 0.56 24 975 21 20475 Good Comparative Example 17 L 25 0.73 25 1345 21 28245 Poor Comparative Example 18 M 19 0.43 23 1225 20 24500 Good Invention Example 19 M 16 0.64 24 895 29 25955 Poor Comparative Example 20 N 20 0.52 27 1073 26 27898 Good Invention Example 21 N 18 0.28 24 1042 24 25008 Poor Comparative Example 22 O 8 0.51 33 804 20 16080 Good Comparative Example 23 P 18 0.43 22 1105 25 27625 Good Invention Example 24 Q 15 0.45 25 1013 24 24312 Good Invention Example 25 R 6 0.48 36 872 25 21800 Good Comparative Example 26 S 27 0.55 28 1289 23 29647 Good Invention Example 27 T 18 0.48 27 1003 25 25075 Good Invention Example 28 U 25 0.51 23 1175 21 24675 Good Invention Example (Remark) art underline represents that a value is not in a range of the invention

(19) As shown in Table 4, sample Nos. 1, 3, 4, 8, 10, 12, 14, 18, 20, 23, 24, 26, 27, and 28 according to the present invention had a tensile strength of 900 MPa or greater, and the value of the product of the tensile strength and the total elongation (TSEL) was 24000 MPa.Math.% or greater. According to this, it could be seen that the ductility was excellent. In addition, an impact value in the Charpy test at 0 C. was 20 J/cm.sup.2 or greater, and thus it could be seen that impact characteristics were also good. Particularly, in Sample Nos. 4, 10, 12, 14, 18, 20, 23, 24, 26, 27, and 28, the amount of C and the amount of Mn were in a preferable range, and the tensile strength was very high as 1000 MPa or greater.

(20) Furthermore, a structure other than austenite was composed of martensite.

(21) On the other hand, in sample No. 2, the metallographic structure of steel, which was used in the heat treatment, was not appropriate, and thus the volume ratio of austenite was low and the ductility was low after the heat treatment. In sample No. 5, the grain size of prior austenite of the steel (base steel), which was used in the heat treatment, was not appropriate, and thus the average concentration of C in austenite in the steel after the heat treatment was high, and the impact characteristics were poor. In Sample Nos. 6, 22, and 25, the chemical composition was not appropriate, and thus the ductility was poor. Accordingly, a target tensile strength was not obtained. In addition, in Sample Nos. 22 and 25, the structure uniformity did not satisfy a target value. In Sample Nos. 7, 11, and 17, the chemical composition was not appropriate, and thus the impact characteristics were poor. In Sample No. 9, the cooling rate after the heat treatment was too slow, and thus a required tensile strength was not obtained. In Sample Nos. 13 and 15, the retention temperature during the heat treatment was too high, and thus a desired structure was not obtained. Accordingly, the ductility was inferior. In Sample No. 16, the chemical composition was not appropriate, and thus the ductility was inferior. In Sample No. 19, the retention temperature during the heat treatment was too low, and thus a desired structure was not obtained. Accordingly, the impact characteristics were poor, and a required tensile strength was not obtained. In Sample No. 21, the retention time during the heat treatment was too long, and thus a desired structure was not obtained. Accordingly, the impact characteristics were poor.

INDUSTRIAL APPLICABILITY

(22) According to the present invention, it is possible to manufacture ultrahigh-strength steel excellent in ductility and impact characteristics while having a high strength such as a tensile strength of 900 MPa or greater. For example, the ultrahigh-strength steel according to the present invention can be widely used in a vehicle field, an energy field, and a building field, and thus an industrial use value thereof is high.