HIGH-STRENGTH STEEL WITH YIELD STRENGTH OF 800 MPA AND PRODUCTION METHOD THEREFOR

Abstract

A high-strength steel having a yield strength at a level of 800 MPa and a method of manufacturing the same, with the components and amounts thereof by weight percentage being: C:0.06-0.14%, Si: 0.1-0.30%, Mn: 0.8-1.60%, Cr: 0.2-0.70%, Mo: 0.1-0.40%, Ni: 0-0.30%, Nb: 0.01-0.030%, Ti: 0.01-0.030%, V: 0.01-0.05%, B: 0.0005-0.0030%, Al: 0.02-0.06%, Ca: 0.001-0.004%, N: 0.002-0.005%, P≦0.02%, S≦0.01%, O≦0.008%, the balance of Fe and unavoidable impurities; wherein the above elements meet the following relationships: 0.40%<Ceq<0.50%, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15; 0.7%≦Mo+0.8Ni+0.4Cr+6V≦1.1%; 3.7≦Ti/N≦7.0; 1.0≦Ca/S≦3.0.

Claims

1. A high-strength steel having a yield strength at a level of 800 MPa, consisting of the following components by weight percentage: C: 0.06-0.14%, Si: 0.10-0.30%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.10-0.40%, Ni: 0-0.30%, Nb: 0.010-0.030%, Ti: 0.010-0.030%, V: 0.010-0.050%, B: 0.0005-0.0030%, Al: 0.02-0.06%, Ca: 0.001-0.004%, N: 0.002-0.005%, P≦0.020%, S≦0.010%, O≦0.008%, and the balance of Fe and unavoidable impurities; wherein the above elements meet the following relationships: 0.40%<Ceq<0.50%, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15, 0.7%≦Mo+0.8Ni+0.4Cr+6V≦1.1%; 3.7≦Ti/N≦7.0; and 1.0≦Ca/S≦3.0.

2. The high-strength steel having a yield strength at a level of 800 MPa according to claim 1, wherein the high-strength steel has a microstructure of tempered martensite.

3. The high-strength steel having a yield strength at a level of 800 MPa according to claim 1, wherein the high-strength steel has a yield strength of 800-950 MPa, a tensile strength of 850-1000 MPa, an elongation >12%, and an impact energy at −40° C.>40 J.

4. A method of manufacturing the high-strength steel having a yield strength at a level of 800 MPa according to claim 1, comprising the following steps: 1) Smelting and casting smelting a composition as described in claim 1 in a converter or electrical furnace, refining, and casting to a cast blank; 2) Slab Heating heating the cast blank in a furnace at 1150-1270° C., wherein, when the temperature of the core of the cast blank arrives at the furnace temperature, the temperature is held, and the holding time is >1.5 h; 3) Rolling rolling the cast blank to a target thickness by single-stand reciprocating rolling or multi-stand hot continuous rolling, wherein a final rolling temperature is 820-920° C., and the final rolling temperature Tf meets: Ar.sub.3<Tf<Tnr, wherein Ar.sub.3 is a temperature at which hypo-eutectoid steel austenite begins to convert to ferrite: Ar.sub.3=901−325C−92Mn−126Cr−67Ni−149Mo; Tnr is non-recrystallization critical temperature: Tnr=887+464C+(6445Nb−644 sqrt(Nb))+(732V−230 sqrt(V))+890Ti+363Al−357Si; a rolling reduction rate at a final rolling path is >15%; 4) Quenching heat treatment process conducting on-line quenching to (Ms−150) ° C. or lower after the rolling by using a laminar cooling system to control a cooling speed V>e.sup.(5.3−2.53c−0.16Si−0.82Mn−0.95Cr−1.87Mo−160B)° C./s, so as to guarantee formation of full martensitic structure, wherein Ms is a temperature at which transformation of martensite begins, Ms=539−423C−30.4Mn−17.7Ni−12.1Cr−11.0Si−7.0Mo; 5) Annealing heat treatment process subjecting to annealing heat treatment at an annealing temperature of 400-550° C.; wherein when the temperature of the core of the steel plate arrives at the furnace temperature, the temperature is held, and the holding time is 20-180 min.

5. The method of manufacturing the high-strength steel having a yield strength at a level of 800 MPa according to claim 4, wherein the high-strength steel manufactured by the method has a microstructure of tempered martensite.

6. The method of manufacturing the high-strength steel having a yield strength at a level of 800 MPa according to claim 4, wherein the high-strength steel manufactured by the method has a yield strength of 800-950 MPa, a tensile strength of 850-1000 MPa, an elongation >12%, and an impact energy at −40° C.>40 J.

Description

DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 is an image of the metallographical structure of the steel of Example 1 according to the disclosure;

[0050] FIG. 2 is an image of the metallographical structure of the steel of Example 5 according to the disclosure;

[0051] FIG. 3 is an image of the metallographical structure of the steel of Example 8 according to the disclosure.

DETAILED DESCRIPTION

[0052] The disclosure will be further illustrated with reference to the following specific Examples.

[0053] A 50 kg vacuum electric furnace was used for smelting. The compositions of the steel according to the disclosure are shown in Table 1. Liquid steel smelted in the 50 kg vacuum electric furnace was cast into steel blanks having a thickness of 120 mm. The steel blanks were placed into an electric furnace for heating. The steel blanks were rolled to a target thickness of 10 mm in multiple paths. The final rolling temperature was 820-920° C. At the same time, the final rolling temperature Tf met: Ar.sub.3<Tf<Tnr. The reduction rate at the final path was set to 17%. On-line quenching was conducted after rolling, wherein the quenching cooling speed was V>e.sup.(5.3−2.53C−0.16S−0.82Mn−0.95Cr−1.87Mo−160B)° C./s. The final cooling temperature was (Ms˜150°) C. or less. In the tempering heat treatment process, the tempering temperature was 400-550° C., and the tempering time was 20-180 min after the core of the steel plate arrived at the tempering temperature. The specific process conditions are shown in Table 2.

[0054] The on-line quenched+tempered steel plate was subjected to longitudinal tensile testing and longitudinal impact testing. The properties of various sample plates are shown in Table 3. As can be seen from Table 3, a quenched and tempered high-strength steel having a yield strength of 8000 MPa or higher can be manufactured according to the disclosure, wherein the tensile strength is 850-1000 MPa, the elongation is >12%, and the impact energy at −40° C. is >40 J.

[0055] FIGS. 1-3 show the metallographical structure images of the test steels of Examples 1, 5 and 8. As can be seen, the metallographical structures of the final steel plates are homogeneous lath-shaped tempered martensite, and the structures are fine.

TABLE-US-00001 TABLE 1 Chemical compositions of the Examples according to the disclosure Unit: weight percentage No. C Si Mn Cr Mo V Ni Nb Ti B Al Ca P S N O Composition 1 0.06 0.15 1.59 0.7 0.12 0.05 0 0.016 0.01 0.0026 0.06 0.004 0.015 0.0015 0.0025 0.0056 Composition 2 0.14 0.12 0.82 0.41 0.28 0.012 0.3 0.008 0.03 0.001 0.034 0.004 0.01 0.0014 0.0044 0.0034 Composition 3 0.065 0.2 1.03 0.54 0.17 0.039 0.25 0.03 0.012 0.003 0.023 0.0026 0.007 0.0021 0.0027 0.0073 Composition 4 0.13 0.1 1.26 0.2 0.4 0.035 0.27 0.016 0.016 0.002 0.06 0.0025 0.013 0.0015 0.0038 0.0023 Composition 5 0.07 0.3 1.24 0.6 0.34 0.047 0.29 0.01 0.018 0.0005 0.05 0.0013 0.013 0.0011 0.0047 0.0056

TABLE-US-00002 TABLE 2 Rolling process conditions of the Examples according to the disclosure On-line Tempering Heating Final rolling quenching Final cooling heating Tempering Chemical Temperature, Holding time, temperature, cooling speed, temperature, temperature, holding time, Ex. composition ° C. min ° C. ° C./s ° C. ° C. min Ex. 1 Composition 1 1210 130 889 56 134 400 180 Ex. 2 Composition 1 1210 190 835 31 176 520 60 Ex. 3 Composition 2 1270 120 865 54 85 500 55 Ex. 4 Composition 2 1220 210 870 75 234 550 28 Ex. 5 Composition 3 1250 120 883 72 253 490 50 Ex. 6 Composition 3 1150 100 829 63 120 520 20 Ex. 7 Composition 4 1200 160 892 67 90 420 130 Ex. 8 Composition 4 1190 120 828 54 119 500 65 Ex. 9 Composition 5 1170 180 823 52 230 410 100 Ex. 10 Composition 5 1240 150 827 82 60 550 45

TABLE-US-00003 TABLE 3 Mechanical properties of the Examples according to the disclosure Yield Tensile strength strength Elongation Impact energy at −40° C. Ex. MPa MPa % (7.5 * 10 * 55 mm) J Ex. 1 917 958 12.8 46 52 58 Ex. 2 842 873 14.2 90 97 101 Ex. 3 888 909 13.8 53 58 63 Ex. 4 851 878 14.7 86 69 81 Ex. 5 862 904 14.2 93 79 82 Ex. 6 839 882 14.5 87 91 85 Ex. 7 894 929 13.6 46 54 49 Ex. 8 871 898 15.1 73 73 53 Ex. 9 902 933 13.1 89 76 83 Ex. 10 875 891 15.3 102 98 92