Cold-rolled high-strength steel having tensile strength of not less than 1500 MPA and excellent formability, and manufacturing method therefor

Abstract

Provided is a cold-rolled high-strength steel having a tensile strength of not less than 1500 MPa and an excellent formability, the chemical elements thereof having the following mass percent ratios: 0.25%-0.40% of C, 1.50%-2.50% of Si, 2.0%-3.0% of Mn, 0.03%-0.06% of Al, P≤0.02%, S≤0.01%, N≤0.01% and at least one of 0.1%-1.0% of Cr and 0.1%-0.5% of Mo, with the balance being Fe and other unavoidable impurities. The microstructure of the cold-rolled high-strength steel has 5%-20% of residual austenite and 70%-90% of martensite, and the carbon concentration ratio of the residual austenite to the martensite is greater than 3.5 and less than 15. The cold-rolled high-strength steel sheet has a high strength and an excellent formability through a rational ingredient design and microstructure control.

Claims

1. A cold-rolled high-strength steel comprising the following chemical elements in mass percentage: C: 0.25-0.40%, Si: 1.50-2.50%, Mn: 2.0-3.0%, Al: 0.03-0.06%, P≤0.02%, S≤0.01%, N≤0.01%, at least one of: 0.1-1.0% of Cr and 0.1-0.5% of Mo, and at least one of: 0.01-0.1% of Nb, 0.01-0.2% of V, and 0.01-0.05% of Ti, with the balance being Fe and other unavoidable impurities; wherein, the microstructure of the cold-rolled high-strength steel has 5-20% of residual austenite, 70-90% of martensite, and a ratio of the carbon concentration of the residual austenite to that of the martensite is more than 3.5 and less than 15, wherein the microstructure of the cold-rolled high-strength steel further comprises ferrite, and wherein the cold-rolled high-strength steel has a tensile strength of 1500 MPa or more.

2. The cold-rolled high-strength steel according to claim 1, wherein mass percentages of the chemical elements satisfy: Mn+Cr+Mo≤3.8%.

3. The cold-rolled high-strength steel according to claim 1, wherein mass percentages of the chemical elements satisfy:
C+Si/30+Mn/20+2P+4S≤0.56%.

4. The cold-rolled high-strength steel according to claim 1, wherein the steel has an elongation after fracture of 12% or more.

5. A manufacturing method for the cold-rolled high-strength steel according to claim 1, comprising the steps of: (1) smelting and casting; (2) hot rolling; (3) pickling; (4) cold rolling; (5) continuous annealing: heating a steel strip to a soaking temperature of 800-900° C. and holding for 60 s or more, then cooling the steel strip to 150-300° C. at a rate of 30-80° C./s, then reheating the steel strip to 350-440° C. and holding for 30-300 s, and finally cooling the steel strip to room temperature.

6. The manufacturing method according to claim 5, wherein in step (2), in a heating stage, a slab is heated to 1200-1300° C. and held for 0.5-4 h; and in a rolling stage, a final rolling temperature is controlled to 850° C. or more and a coiling temperature is controlled to 400-600° C.

7. The manufacturing method according to claim 5, wherein the pickling in step (3) is controlled at a speed of 80-120 m/min.

8. The manufacturing method according to claim 5, wherein a cold rolling reduction in step (4) is controlled to 40-60%.

Description

DETAILED DESCRIPTION

(1) The cold-rolled high-strength steel having a tensile strength of 1500 MPa or more and excellent formability and the manufacturing method thereof according to the present invention will be further explained and illustrated below with reference to specific Examples. However, the explanations and illustrations do not unduly limit the technical solutions of the present invention.

Examples 1-8 and Comparative Examples 1-4

(2) The cold-rolled high-strength steels of Examples 1-8 and the conventional steel sheets of Comparative Examples 1-4 were prepared by the following steps:

(3) (1) smelting and casting slabs according to the mass ratio of chemical elements listed in Table 1;

(4) (2) hot rolling: in the heating stage, the slab was heated to 1200˜1300° C. and held for 0.5˜4 h; in the rolling stage, the final rolling temperature was controlled to 850° C. or more and the coiling temperature was controlled to 400˜600° C.;

(5) (3) pickling: the pickling speed was controlled to 80˜120 m/min;

(6) (4) cold rolling: the cold rolling reduction was controlled to 40˜60%;

(7) (5) continuous annealing: the steel was heated to a soaking temperature of 800˜900° C. and held for 60 s or more, then cooled to 150˜300° C. at a rate of 30˜80° C./s, then reheated to 350˜440° C. and held for 30˜300 s, and finally cooled to room temperature.

(8) Table 1 lists the mass ratios of chemical elements in the cold-rolled high-strength steels of Examples and the conventional steel sheets of Comparative Examples.

(9) TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and other inevitable impurity elements other than S, P, N) Chemical Mn + C + Si/30 + component Cr + Mn/20 + 2 NO. C Si Mn Cr Mo P S Al N Ti Nb V Mo P + 4S A 0.28 1.8 2.7 0.5 — 0.015 0.002 0.037 0.0042 — — 0.05 3.2 0.51 B 0.31 1.7 2.5 — 0.3  0.01 0.004 0.053 0.0035 0.03 0.03 — 2.8 0.53 C 0.33 2   2.1 0.2 0.15 0.009 0.005 0.04 0.0028 — 0.03 — 2.45 0.54 D 0.36 1.6 2.4 0.1 — 0.007 0.003 0.032 0.0043 0.02 — — 2.5 0.56 E 0.23 1.8 2.5 — 0.25 0.009 0.01 0.046 0.0032 — 0.05 — 2.75 0.47 F 0.3  1.2 2.7 — — 0.013 0.008 0.031 0.0029 — — — 2.7 0.53

(10) Table 2 lists the specific process parameters of the manufacturing method of the Examples and Comparative Examples.

(11) TABLE-US-00002 TABLE 2 step (1) step (2) step step (5) Chem- Finish step (4) Rapid ical Heating Hold- rolling Coiling (3) Cold Rapid cooling Reheating Re- compo- temper- ing temper- temper- Pickling rolling Soaking Soaking cooling termination temper- heating nent ature time ature ature speed reduction temperature time rate temperature ature time NO. (° C.) (h) (° C.) (° C.) (m/min) (%) (° C.) (min) (° C./s) (° C.) (° C.) (s) Example 1 A 1212 1.3 851 547 81 50 835 2.8 45 227 411 235 Example 2 A 1271 1.8 890 469 101 55 802 1.3 34 256 364 267 Example 3 B 1257 2.0 901 424 117 52 871 1.6 33 192 363 47 Example 4 B 1249 3.8 896 540 118 43 821 2.3 68 232 416 202 Example 5 C 1264 0.9 861 440 99 49 892 1.7 62 259 390 31 Example 6 C 1263 2.5 853 491 84 50 886 1.8 35 178 391 272 Example 7 D 1277 2.3 864 549 90 60 853 3 54 262 357 113 Example 8 D 1243 3.6 870 458 110 50 880 2.9 40 265 404 84 Comparative B 1223 2 861 511 100 50 930 1.8 45 195 373 265 Example 1 Comparative C 1206 2 919 531 100 50 829 2 45 320 404 83 Example 2 Comparative E 1212 2 857 407 100 50 808 1.4 45 174 389 44 Example 3 Comparative F 1259 2 894 481 100 50 841 2.9 45 167 421 198 Example 4 Note: The rapid cooling rate in Table 2 refers to the cooling rate when the steel is cooled to 150-300° C. at a speed of 30-80° C./s as defined in the claims. The chemical component numbers in Table 2 have the mass ratios of chemical elements of the corresponding chemical component numbers listed in Table 1, for example, Example 1 uses the mass ratio of chemical component A in Table 1.

(12) The cold-rolled high-strength steels of the Examples and the conventional steel sheets of the Comparative Examples were subjected to various performance tests, and the obtained results of the tests were listed in Table 3. The yield strength and tensile strength tests are routine ones, methods of other tests are as follows:

(13) Tensile test method: JIS No. 5 tensile test specimen was used, the stretching direction was parallel to the rolling direction.

(14) Test method for the ratio of residual austenite phase (V.sub.γ): Samples having a size of 15×15 mm were cut out from the steel sheets, and ground and polished to perform XRD quantitative test.

(15) Test method for the ratio of martensite phase (Vα): Samples having a size of 15×15 mm were cut out from the steel sheets, and ground and polished to perform EBSD quantitative analysis.

(16) Test method for carbon concentration of residual austenite (x.sub.C.sup.γ): Assuming that the Mn and Al concentrations in each constituent phase in the steel sheet structure did not change, the lattice constant a.sub.γ was obtained from the diffraction peak data of the residual austenite in XRD, and the carbon concentration was calculated by using the following empirical formula: α.sub.γ=0.3556+0.00453x.sub.C.sup.γ+0.000095x.sub.Mn.sup.γ+0.00056x.sub.Al.sup.γ, wherein x.sub.C.sup.γ, x.sub.Mn.sup.γ and x.sub.Al.sup.γ represent carbon concentration, manganese concentration and aluminium concentration of the residual austenite, respectively.

(17) Test method for carbon concentration of martensite (x.sub.C.sup.α): the carbon concentration was calculated by the formula: x.sub.C.sup.α=(x.sub.C−x.sub.C.sup.γ×V.sub.γ)/V.sub.α, using the obtained V.sub.γ, V.sub.α, x.sub.C.sup.γ and the carbon concentration of the design composition (x.sub.C).

(18) Table 3 lists the test results of the cold-rolled high-strength steels of Examples and the conventional steel sheets of Comparative Examples.

(19) TABLE-US-00003 TABLE 3 Carbon Yield Tensile Elongation Ratio of Carbon Carbon concentration ratio strength strength after residual Ratio of concentration concentration of residual Rp0.2 Rm fracture EL austenite martensite of residual of martensite austenite to (MPa) (MPa) (%) phase (%) phase (%) austenite (%) (%) martensite (%/%) Example 1 1190 1541 12.8  7 75 1.06 0.27 3.9 Example 2 1045 1522 13.2  8 70 1.26 0.26 4.9 Example 3 1145 1594 15.2 10 82 1.45 0.21 6.8 Example 4 1089 1556 19.3 15 80 1.31 0.15 8.5 Example 5 1212 1589 17.7 11 85 1.37 0.23 5.8 Example 6 1299 1615 16.1  8 87 1.20 0.29 4.1 Example 7 964 1529 15.1 11 74 1.35 0.34 4.0 Example 8 866 1571 17.4 11 78 1.03 0.37 2.8 Comparative 816 1675 10.5  4 90 1.01 0.31 3.3 Example 1 Comparative 892 1574  9.2  7 80 0.90 0.36 2.5 Example 2 Comparative 1025 1282 14.1 12 72 1.16 0.13 9.2 Example 3 Comparative 1132 1512 11.3 10 80 1.12 0.24 4.7 Example 4

(20) It can be seen from Table 3 that all Examples of the present application have a tensile strength of 1500 MPa or more and an elongation after fracture of 12% or more, indicating that the cold-rolled high-strength steel of Examples of the present application has high strength and good formability.

(21) As can be seen from Table 1 and Table 3, Comparative Example 3 used the chemical elements ratio of E, wherein the mass percentage of carbon is less than 0.25%, resulting in a strength of Comparative Example 3 being less than 1500 MPa; and Comparative Example 4 used the chemical elements ratio of F, wherein the mass percentage of silicon is less than 1.5%, resulting in a lower elongation after fracture of Comparative Example 4.

(22) As can be seen from Table 2 and Table 3, the soaking temperature of Comparative Example 1 is higher than 900° C., so that the steel sheet undergoes a complete austenite transformation after soaking, resulting in austenite content of less than 5%. Therefore, although Comparative Example 1 has a tensile strength of more than 1500 MPa, the elongation after fracture is insufficient and the formability is poor. The rapid cooling termination temperature of Comparative Example 2 is higher than 300° C., so that the phase transformation of martensite is insufficient, resulting in a low content of residual austenite during the subsequent reheating, which ultimately leads to insufficient elongation after fracture of Comparative Example 2.

(23) It should be noted that the above are merely specific examples of the invention. It is obvious that the present invention is not limited to the above Examples, but has many similar variations. All modifications that can be directly derived or associated by those skilled in the art are intended to be within the scope of the present invention.