980MPA GRADE COLD-ROLL STEEL SHEETS WITH HIGH HOLE EXPANSION RATE AND HIGHER PERCENTAGE ELONGATION AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed is a 980 MPa grade cold-roll steel sheets with high hole expansion rate and higher percentage elongation, and manufacturing method thereof. The mass percents of chemical components in the steel sheet are: C: 0.08%-0.12%, Si: 0.1%-1.0%, Mn: 1.9%-2.6%, Al: 0.01%-0.05%, Cr: 0.1-0.55%, Mo: 0.1-0.5%, Ti: 0.01-0.1%, the rest being Fe and other inevitable impurities. The steel plate has a yield strength >600 MPa, a tensile strength >980 MPa, a percentage elongation >11%, a hole expansion rate ≥45%, and a tensile strength up to 980 MPa grade; the microscopic structure is ferrite plus bainite plus martensite, with the volume fraction content of ferrite >10%, the volume fraction content of bainite >30%, and the volume fraction content of martensite >15%; the microscopic structure further comprises nanoscale precipitates in uniform dispersion distribution, the average size of precipitates being less than 20 nm.

Claims

1. A 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation, wherein the steel sheet has a chemical composition based on mass percentage of: C: 0.08%-0.12%, Si: 0.1%-1.0%, Mn: 1.9%-2.6%, Al: 0.01%-0.05%, Cr: 0.1-0.55%, Mo: 0.1-0.5%, Ti: 0.01-0.1%, and a balance of Fe and other unavoidable impurities, wherein the following relationships are satisfied: 1.8≥5×[C]+0.4×[Si]+0.1×([Mn]+[Cr]+[Mo]).sup.2≥1.3, [Mo]≥3×[Ti].

2. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the C content is 0.09%-0.11%.

3. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the Si content is 0.4%-0.8%.

4. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the Mn content is 2.1%-2.4%.

5. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the Al content is 0.015%-0.045%.

6. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the Cr content is 0.2%-0.4%.

7. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the Mo content is 0.2%-0.3%.

8. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the Ti content is 0.02%-0.05%.

9. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein 1.45≤5×[C]+0.4×[Si]+0.1×([Mn]+[Cr]+[Mo]).sup.2≤1.7.

10. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the cold-rolled steel sheet has a microstructure of ferrite+bainite+martensite, wherein ferrite has a volume fraction of greater than 10%; bainite has a volume fraction of greater than 30%; martensite has a volume fraction of greater than 15%; wherein the microstructure further comprises uniformly and dispersively distributed nano-scale precipitates having an average size of less than 20 nm.

11. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 10, wherein the microstructure of the cold-rolled steel sheet is ferrite+bainite+martensite, wherein ferrite has a volume fraction of greater than 10% to 30%; bainite has a volume fraction of 35-75%; and martensite has a volume fraction of greater than 15% to 35%.

12. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the cold-rolled steel sheet has a yield strength of greater than 600 MPa, a tensile strength of greater than 980 MPa, an elongation of greater than 11%, and a hole expansion rate of ≥45%.

13. A manufacturing method for the 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein the method comprises the following steps: 1) smelting and casting the composition in claim 1 into a blank; 2) first heating to 1150-1250° C., holding for 0.5 hours or more, hot-rolling at a temperature above Ar3, cooling rapidly at a rate of 30-100° C./s after rolling, and coiling at a temperature: 600-750° C.; 3) controlling a cold rolling reduction rate at 30-70%; 4) in an annealing process, soaking at a soaking temperature of 810-870° C. for a holding time of 50-100 s; then cooling at a rate of 3-10° C./s to a start temperature of rapid cooling which is 660-730° C.; and then cooling at a rate of 30-200° C./s to 200-460° C.; and 5) over-aging at an over-aging temperature of 320-460° C. for an over-aging time of 100-400 s.

14. The manufacturing method for the 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 13, further comprising step 6) flattening, wherein a flattening rate of 0.05-0.3% is used.

15. The manufacturing method for the 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 13, wherein in step 2), the holding time is 0.5-3 hours; in step 3), the cold rolling reduction rate is controlled at 50-70%; in step 4), an annealing temperature is 820-870° C.; the holding time of the soaking is 50-90 s; and the cooling is conducted at a cooling rate of 50-200° C./s to 320-460° C.

16. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 1, wherein: the C content of the 980 MPa grade cold-rolled steel sheet is 0.09%-0.11%; the Si content of the 980 MPa grade cold-rolled steel sheet is 0.4%-0.8%; the Mn content of the 980 MPa grade cold-rolled steel sheet is 2.1%-2.4%; the Al content of the 980 MPa grade cold-rolled steel sheet is 0.015%-0.045%; the Cr content of the 980 MPa grade cold-rolled steel sheet is 0.2%-0.4%; the Mo content of the 980 MPa grade cold-rolled steel sheet is 0.2%-0.3%; and the Ti content of the 980 MPa grade cold-rolled steel sheet is 0.02%-0.05%.

17. The 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 16, wherein: the cold-rolled steel sheet has a microstructure of ferrite+bainite+martensite, wherein ferrite has a volume fraction of greater than 10%; bainite has a volume fraction of greater than 30%; martensite has a volume fraction of greater than 15%; wherein the microstructure further comprises uniformly and dispersively distributed nano-scale precipitates having an average size of less than 20 nm; and the cold-rolled steel sheet has a yield strength of greater than 600 MPa, a tensile strength of greater than 980 MPa, an elongation of greater than 11%, and a hole expansion rate of ≥45%.

18. The manufacturing method for the 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 13, wherein: the C content of the 980 MPa grade cold-rolled steel sheet is 0.09%-0.11%; the Si content of the 980 MPa grade cold-rolled steel sheet is 0.4%-0.8%; the Mn content of the 980 MPa grade cold-rolled steel sheet is 2.1%-2.4%; the Al content of the 980 MPa grade cold-rolled steel sheet is 0.015%-0.045%; the Cr content of the 980 MPa grade cold-rolled steel sheet is 0.2%-0.4%; the Mo content of the 980 MPa grade cold-rolled steel sheet is 0.2%-0.3%; and/or the Ti content of the 980 MPa grade cold-rolled steel sheet is 0.02%-0.05%.

19. The manufacturing method for the 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 13, wherein the contents of C, Si, Mn, Cr and Mo of the 980 MPa grade cold-rolled steel shee satisfies: 1.45≤5×[C]+0.4×[Si]+0.1×([Mn]+[Cr]+[Mo]).sup.2≤1.7.

20. The manufacturing method for the 980 MPa grade cold-rolled steel sheet having a high hole expansion rate and a high elongation according to claim 13, wherein: the cold-rolled steel sheet has a microstructure of ferrite+bainite+martensite, wherein ferrite has a volume fraction of greater than 10%; bainite has a volume fraction of greater than 30%; martensite has a volume fraction of greater than 15%; wherein the microstructure further comprises uniformly and dispersively distributed nano-scale precipitates having an average size of less than 20 nm; and/or the cold-rolled steel sheet has a yield strength of greater than 600 MPa, a tensile strength of greater than 980 MPa, an elongation of greater than 11%, and a hole expansion rate of ≥45%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0048] The present disclosure will be further explained and illustrated with reference to the following specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the present disclosure.

[0049] The compositions of the steel examples of the present disclosure are shown in Table 1, and the balance of the compositions is Fe. Table 2 lists the process parameters of the steel sheets of the examples. The tensile test was performed in accordance with the standard ASTM A370-2017 method, and the hole expansion rate test was performed in accordance with the ISO/TS 16630-2017 method. Table 3 lists the relevant process parameters of the steel sheets of the examples.

[0050] The method for manufacturing the steel examples of the present disclosure is as follows:

[0051] (1) Smelting and casting: the required alloy components were obtained, and the contents of S and P were minimized;

[0052] (2) Hot rolling: heating was conducted first to 1150-1250° C. which was held for 0.5 hours or more; then hot-rolling at a temperature above Ar3 was conducted; after the rolling, rapid cooling was conducted at a rate of 30-100° C./s; and coiling was conducted at a temperature of 600-750° C. in the hot rolling process;

[0053] (3) Cold rolling: the cold rolling reduction rate was controlled at 30-70%;

[0054] (4) Annealing: the soaking temperature in the annealing process was 810-870° C., preferably 830-860° C.; the holding time of the soaking was 50-100 s; then cooling was conducted at a rate of v1=3-10° C./s to a starting temperature of rapid cooling which was 660-730° C.; and then cooling was further conducted at a rate of v2=30-200° C./s to 200-460° C.;

[0055] (5) Over-aging: the over-aging temperature was 320-460° C., and the over-aging time was 100-400 s.

[0056] Optionally, the manufacturing method in each example further comprised step (6) flattening, wherein a flattening rate of 0.05-0.3% was employed.

[0057] Table 3 shows the mechanical properties of the cold-rolled steel sheets of Examples 1-12 obtained using the composition and process of the present disclosure: the yield strength is greater than 600 MPa; the tensile strength is greater than 980 MPa; the elongation is greater than 11%; and the hole expansion rate is ≥45%.

[0058] This demonstrates that the 980 MPa grade cold-rolled steel sheet of the present disclosure has a tensile strength greater than 980 MPa and has an excellent hole expansion rate.

TABLE-US-00001 TABLE 1 (unit: weight %) No. C Si Mn Al P S N Cr Mo Ti Ex. 1 0.107 0.52 2.19 0.024 0.012 0.0023 0.0025 0.33 0.22 0.026 Ex. 2 0.108 0.54 2.23 0.025 0.013 0.0022 0.0024 0.34 0.21 0.028 Ex. 3 0.108 0.53 2.22 0.022 0.012 0.0021 0.0025 0.31 0.25 0.027 Ex. 4 0.095 0.90 2.33 0.022 0.009 0.0021 0.0042 0.24 0.21 0.035 Ex. 5 0.097 0.91 2.34 0.025 0.008 0.0024 0.0041 0.21 0.20 0.039 Ex. 6 0.099 0.92 2.32 0.027 0.009 0.0024 0.0042 0.21 0.19 0.038 Ex. 7 0.113 0.86 2.25 0.035 0.012 0.0018 0.0021 0.13 0.31 0.019 Ex. 8 0.114 0.87 2.26 0.035 0.012 0.0014 0.0022 0.14 0.33 0.018 Ex. 9 0.111 0.88 2.23 0.037 0.009 0.0010 0.0022 0.14 0.32 0.017 Ex. 10 0.088 0.55 2.29 0.031 0.013 0.0015 0.0031 0.51 0.28 0.025 Ex. 11 0.089 0.55 2.28 0.029 0.014 0.0016 0.0030 0.49 0.31 0.026 Ex. 12 0.087 0.57 2.27 0.028 0.013 0.0017 0.0031 0.50 0.31 0.025 Ex. 13 0.098 0.46 2.03 0.022 0.013 0.0022 0.0024 0.34 0.21 0.068 Ex. 14 0.103 0.37 2.57 0.028 0.013 0.0017 0.0031 0.32 0.45 0.045 Ex. 15 0.118 0.66 1.92 0.025 0.012 0.0018 0.0021 0.13 0.47 0.011 Ex. 16 0.083 0.12 2.39 0.043 0.013 0.0015 0.0031 0.54 0.31 0.097 Ex. 17 0.095 0.97 2.23 0.048 0.009 0.0021 0.0042 0.27 0.41 0.048 Ex. 18 0.107 0.32 2.19 0.011 0.012 0.0023 0.0025 0.48 0.18 0.056

TABLE-US-00002 TABLE 2 Hot Rolling Cold Rolling Annealing Heating Holding Hot rolling Cooling Coiling Cold Rolling Annealing Soaking Temperature Time temperature Rate Temperature Reduction Temperature Time No. ° C. h ° C. ° C./s ° C. Rate % ° C. s Ex. 1 1150 0.8 880 50 630 50 830 90 Ex. 2 1150 0.8 880 50 630 50 850 90 Ex. 3 1150 0.8 880 50 630 50 870 90 Ex. 4 1200 1 890 60 660 60 830 80 Ex. 5 1200 1 890 60 660 60 850 80 Ex. 6 1200 1 890 60 660 60 870 80 Ex. 7 1230 1.2 900 70 690 55 830 70 Ex. 8 1230 1.2 900 70 690 55 850 70 Ex. 9 1230 1.2 900 70 690 55 870 70 Ex. 10 1250 1.5 910 50 720 60 830 60 Ex. 11 1250 1.5 910 50 720 60 850 60 Ex. 12 1250 1.5 910 50 720 60 870 60 Ex. 13 1150 0.8 880 30 600 30 850 90 Ex. 14 1250 1.5 910 50 720 60 870 60 Ex. 15 1230 1.2 900 70 690 55 830 70 Ex. 16 1250 1.5 910 50 750 70 810 50 Ex. 17 1200 1 890 60 660 100 830 80 Ex. 18 1150 0.8 880 100 630 50 830 100 Annealing Fast Cooling Fast Cooling Over-aging Cooling Start Fast Cooling Termination Over-aging Over-aging Flattening Rate v1 Temperature Rate v2 Temperature Temperature Time Flattening No. ° C./s (° C.) ° C./s ° C. ° C. s Rate % Ex. 1 3 670 60 370 370 180 0.05 Ex. 2 3 670 60 370 370 180 0.10 Ex. 3 3 670 60 370 370 180 0.15 Ex. 4 5 690 70 380 380 220 0.20 Ex. 5 5 690 70 380 380 220 0.25 Ex. 6 5 690 70 380 380 220 0.30 Ex. 7 6 700 80 390 390 250 0.08 Ex. 8 6 700 80 390 390 250 / Ex. 9 6 700 80 390 390 250 / Ex. 10 8 680 90 400 400 270 0.22 Ex. 11 8 680 90 400 400 270 0.17 Ex. 12 8 680 90 400 400 270 0.12 Ex. 13 3 660 30 200 460 180 0.08 Ex. 14 10 680 90 280 400 270 0.12 Ex. 15 6 700 80 320 320 400 0.25 Ex. 16 8 730 200 400 400 100 0.21 Ex. 17 5 690 70 460 460 220 0.18 Ex. 18 3 670 160 370 370 330 0.09 Note: “/” indicates not flattened.

TABLE-US-00003 TABLE 3 Yield Tensile Hole Strength Strength Elongation Expansion No. (MPa) (MPa) (%) Rate (%) Ex. 1 664 1040 12.3 47 Ex. 2 675 1018 11.8 51 Ex. 3 685 1027 11.9 50 Ex. 4 730 1096 12.2 52 Ex. 5 743 1108 12.1 54 Ex. 6 741 1087 12.3 54 Ex. 7 779 1024 11.4 60 Ex. 8 786 1024 11.6 58 Ex. 9 776 1019 11.5 61 Ex. 10 719 1038 12.4 47 Ex. 11 709 1028 12.1 51 Ex. 12 731 1017 11.8 52 Ex. 13 710 1029 13.4 49 Ex. 14 689 1019 12.9 50 Ex. 15 821 1045 11.9 56 Ex. 16 798 1098 11.4 67 Ex. 17 816 1087 12.1 67 Ex. 18 765 1076 11.9 49