1500 MPA-grade steel with high product of strength and elongation for vehicles and manufacturing methods therefor

Abstract

Provided are a 1500 MPa-grade steel with a high product of strength and elongation for vehicles and a manufacturing method thereof. The mass percentages of the chemical elements thereof are: 0.1-0.3% of C, 0.1-2.0% of Si, 7.5-12% of Mn, 0.01-2.0% of Al, and the balance of iron and other inevitable impurities. The microstructure of the steel with a high product of strength and elongation for vehicles is austenite+martensite+ferrite or austenite+martensite. The steel for vehicles can reach a grade of 1500 MPa, and has a product of strength and elongation of no less than 30 GPa %.

Claims

1. An automotive steel with a high product of strength and elongation, with chemical elements in percentage by mass being: C: 0.1-0.3%, Si: 0.1-2.0%, Mn: 7.5-12%, Al: 0.01-2.0%, and a balance of iron and unavoidable impurities, wherein the automotive steel with a high product of strength and elongation comprises a microstructure of austenite+martensite+ferrite, wherein the phase of austenite has a proportion of 20%-40%, and the phase of the martensite has a proportion of 50%-70%, or the automotive steel with a high product of strength and elongation comprises a microstructure of austenite+martensite, wherein the phase of the austenite has a proportion of 20%-50%; wherein the tensile strength of the automotive steel with a product of strength and elongation is >1500 MPa, and its product of strength and elongation is ≥30 GPa %.

2. The automotive steel of claim 1, further comprising at least one chemical element of Nb: 0.01-0.07%, Ti: 0.02-0.15%, V: 0.05-0.20%, Cr: 0.15-0.50%, and Mo: 0.10-0.50%.

3. A method for manufacturing the automotive steel of claim 1, comprising the following steps in order: (1) Smelting and casting; (2) Hot rolling; (3) Bell furnace annealing, wherein an annealing temperature is 600-700° C., and an annealing time is 1-48 h; (4) Cold rolling; (5) First post-cold-rolling annealing: an annealing temperature is between Ac1 and Ac3 temperatures, and an annealing time is greater than 5 min; (6) Second post-cold-rolling annealing: an annealing temperature is 750-850° C., and an annealing time is 1-10 min; (7) Tempering: a tempering temperature is 200-300° C., and a tempering time is no less than 3 min.

4. The manufacturing method of claim 3, wherein in step (2), a cast blank is heated to 1100-1260° C., and then rolled under control, wherein a blooming temperature is 950-1150° C., a final rolling temperature is 750-900° C., and a coiling temperature is 500-850° C., wherein a pure martensitic structure is obtained after cooling to room temperature after the coiling.

5. The manufacturing method of claim 3, wherein in step (4), a cold rolling reduction is not less than 40%.

6. The manufacturing method of claim 3, wherein an acid pickling step exists between steps (3) and (4).

7. The method for manufacturing the automotive steel of claim 3, wherein the automotive steel further comprises at least one chemical element of Nb: 0.01-0.07%, Ti: 0.02-0.15%, V: 0.05-0.20%, Cr: 0.15-0.50%, and Mo: 0.10-0.50%.

8. The manufacturing method of claim 3, wherein the microstructure of the automotive steel is austenite+martensite+ferrite with a proportion of the austenite phase being 20%-40%, and a proportion of the martensite phase being 50%-70%.

9. The manufacturing method of claim 3, wherein the microstructure of the automotive steel is austenite+martensite with a proportion of the austenite phase being 20%-50%.

Description

DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic view showing a process curve of the manufacturing method for the 1500 MPa-grade automotive steel with a high product of strength and elongation according to the disclosure.

DETAILED DESCRIPTION

(2) The 1500 MPa-grade automotive steel with a high product of strength and elongation and the manufacturing method thereof according to the disclosure will be further explained and illustrated with reference to the accompanying drawing and the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

Examples 1-8 and Comparative Examples 1-4

(3) The 1500 MPa-grade automotive steel with a high product of strength and elongation in Examples 1-8 and the steel plates in Comparative Examples 1-4 were manufactured according to the following steps:

(4) (1) Smelting and casting: A converter was used for the smelting, and the mass percentages of the various chemical elements were controlled as shown by Table 1.

(5) (2) Hot rolling: A cast blank was heated to 1100-1260° C., and then rolled under control, wherein a blooming temperature was 950-1150° C., a final rolling temperature was 750-900° C., and a coiling temperature was 500-850° C. After coiling and after cooling to room temperature, a pure martensitic structure was obtained.

(6) (3) Bell furnace annealing, wherein an annealing temperature was 600-700° C., and an annealing time was 1-48 h.

(7) (4) Cold rolling: A cold rolling reduction was not less than 40%.

(8) (5) First post-cold-rolling annealing: an annealing temperature was between Ac1 and Ac3 temperatures, and an annealing time was greater than 5 min.

(9) (6) Second post-cold-rolling annealing: an annealing temperature was 750-850° C., and an annealing time was 1-10 min. It should be noted that, in order to demonstrate the influence of the process parameters of the second post-cold-rolling annealing defined by this disclosure on the implementing effects of this disclosure, the annealing temperatures used in Comparative Examples 1-3 were outside of the scope defined by this disclosure, wherein the annealing temperature of the second post-cold-rolling annealing in Comparative Example 1 was 720° C., the annealing time of the second post-cold-rolling annealing in Comparative Example 2 was 15 min, and the annealing temperature of the second post-cold-rolling annealing in Comparative Example 3 was 760° C.

(10) (7) Tempering: a tempering temperature was 200-300° C., and a tempering time was no less than 3 min.

(11) In addition, it should be noted that the thickness of the hot-rolled steel plate in step (2) was not greater than 8 mm. The thickness of the cold-rolled steel plate in step (4) was not greater than 2.5 mm.

(12) In addition, it should be noted that, in other embodiments, an electric furnace or an induction furnace may be utilized for the smelting in step (1).

(13) In addition, it should be noted that, in other embodiments, preferably, an acid pickling step may further exist between steps (3) and (4).

(14) Table 1 lists the mass percentages of the various chemical elements in Examples 1-8 and Comparative Examples 1-4.

(15) TABLE-US-00001 TABLE 1 (wt %, the balance being Fe and impurity elements other than impurity elements S, P and N) Composition Number C Si Mn Al P N S Other Elements A 0.25 1.86 8.19 0.038 0.010 0.004 0.007 Cr = 0.41% B 0.29 0.68 7.91 0.042 0.014 0.003 0.004 V = 0.19% C 0.14 0.18 9.88 1.56 0.015 0.005 0.009 — D 0.12 0.25 8.46 0.045 0.010 0.005 0.005 Nb = 0.06%   Ti = 0.12% E 0.19 0.64 11.27  1.82 0.011 0.004 0.004 Mo = 0.18% F 0.16 0.25 6.57 0.031 0.009 0.004 0.005 —

(16) Table 2 lists the specific process parameters of the manufacturing method in Examples 1-8 and Comparative Examples 1-4.

(17) TABLE-US-00002 TABLE 2 Step (2) Final Step (3) Heating Blooming Rolling Coiling Annealing Annealing Composition Temperature Temperature Temperature Temperature Temperature Time number (° C.) (° C.) (° C.) (° C.) (° C.) (h) Ex. 1 A 1170 1100 850 700 600 12 Ex. 2 B 1230 1070 830 650 630 12 Ex. 3 C 1180 1080 890 730 630 12 Ex. 4 C 1190 1110 870 500 620 24 Ex. 5 C 1230 1100 880 840 650 48 Ex. 6 C 1230 1130 890 560 600 1 Ex. 7 D 1220 1100 860 640 640 24 Ex. 8 E 1200 1120 870 600 650 12 Comp. B 1230 1105 865 600 650 12 Ex. 1 Comp. C 1200 1140 830 650 700 12 Ex. 2 Comp. D 1250 1120 890 650 650 1 Ex. 3 Comp. F 1220 1090 845 650 660 48 Ex. 4 Step (4) Step (5) Step (6) Step (7) Cold Rolling Annealing Annealing Annealing Annealing Tempering Tempering Reduction Temperature Time Temperature Time Temperature Time (%) (° C.) (min) (° C.) (min) (° C.) (min) Ex. 1 40 620 720 750 1 200 5 Ex. 2 50 640 30 770 3 240 3 Ex. 3 70 650 60 820 3 300 3 Ex. 4 60 620 10 810 5 260 5 Ex. 5 60 650 5 820 2 220 3 Ex. 6 60 650 360 830 5 200 3 Ex. 7 60 680 60 790 10  260 5 Ex. 8 55 600 120 790 1 220 5 Comp. 60 690 120 720 1 200 3 Ex. 1 Comp. 70 620 360 820 15  240 3 Ex. 2 Comp. 65 640 720 860 6 220 5 Ex. 3 Comp. 60 650 30 800 5 210 5 Ex. 4

(18) It should be noted that the composition numbers for the Examples and Comparative Examples in Table 2 refer to the corresponding composition numbers in Table 1.

(19) The 1500 MPa-grade automotive steel with a high product of strength and elongation in Examples 1-8 and the steel plates in Comparative Examples 1-4 were sampled for testing of various properties. The relevant property parameters obtained by the testing are listed in Table 3.

(20) Table 3 lists the property parameters of the 1500 MPa-grade automotive steel with a high product of strength and elongation in Examples 1-8 and the steel plates in Comparative Examples 1-4. The product of strength and elongation is a product of tensile strength and elongation rate.

(21) TABLE-US-00003 TABLE 3 Yield Tensile Elongation Product of Strength Proportion of Proportion of Strength ReL Strength Rm Rate A50 and Elongation Austenitic Phase Martensitic Phase (MPa) (MPa) (%) (GPa %) (%) (%) Ex. 1 908 1623 19.8 32.1 23 65 Ex. 2 895 1668 18.1 30.2 29 67 Ex. 3 856 1559 25.6 39.9 35 65 Ex. 4 837 1546 23.8 36.8 40 60 Ex. 5 769 1601 20.6 33.0 28 72 Ex. 6 953 1643 18.7 30.7 22 78 Ex. 7 821 1512 26.8 40.5 31 69 Ex. 8 789 1587 22.2 35.2 43 57 Comp. 668 1132 30.8 34.9 28 41 Ex. 1 Comp. 901 1591 16.5 26.3 16 84 Ex. 2 Comp. 1001 1783 12.4 22.1 7 93 Ex. 3 Comp. 1048 1653 15.6 25.8 13 87 Ex. 4

(22) As shown by Table 3, the 1500 MPa-grade automotive steel with a high product of strength and elongation in the inventive Examples had a tensile strength >1500 MPa, and a product of strength and elongation >30 GPa %, which demonstrates that the automotive steel in the Examples possessed high strength and good tensile ductility.

(23) As shown by Tables 1 and 3 in combination, the mass percentage of manganese in Comparative Example 4 was less than 7.5%. Its product of strength and elongation failed to arrive at 30 GPa %, and its elongation rate was low. The reason for this is that the mass percentage of manganese in Comparative Example 4 was low, so that the proportion of the austenitic phase generated in the second post-cold-rolling annealing was not high enough and the austenitic phase was not sufficiently stable, leading to a low elongation rate, and thus a low product of strength and elongation.

(24) As shown by Tables 2 and 3 in combination, the annealing temperature of the second post-cold-rolling annealing in Comparative Example 1 was lower than 750° C. As a result, less ferrite transformed to austenite in the second post-cold-rolling annealing, and a large amount of ferrite still existed after cooling to room temperature. Thus, the elongation rate of the steel plate in Comparative Example 1 was greater than 30%, the product of strength and elongation was greater than 30 GPa %, but its tensile strength was lower than 1500 MPa.

(25) Again, as shown by Tables 2 and 3 in combination, the annealing time of the second post-cold-rolling annealing in Comparative Example 2 was longer than 10 min, and the annealing temperature of the second post-cold-rolling annealing in Comparative Example 3 was higher than 850° C. As a result, the austenite became less stable, and the proportion of the austenitic phase at room temperature was low. The products of strength and elongation of the steel plates in Comparative Examples 2 and 3 were both less than 30 GPa %.

(26) FIG. 1 is a schematic view showing a process curve of the manufacturing method for the 1500 MPa-grade automotive steel with a high product of strength and elongation in Example 1 according to the disclosure.

(27) As shown by FIG. 1, the manufacturing process in the technical solution according to the disclosure includes a first annealing after hot rolling 1, i.e. bell furnace annealing 2; cold rolling 3; a second annealing after the cold rolling, i.e. a first post-cold-rolling annealing 4; then a third annealing, i.e. a second post-cold-rolling annealing 5; and finally tempering 6. The horizontal axis in FIG. 1 represents time, and the vertical axis represents temperature. Hence, the curve in FIG. 1 schematically shows temperature as a function of time. As shown by FIG. 1, the bell furnace annealing 2 and the first post-cold-rolling annealing 4 employ common ART annealing, while the second post-cold-rolling annealing 5 employs a higher annealing temperature and a shorter annealing time as compared with the common ART annealing. Consequently, a microstructure desired by the present disclosure is obtained, i.e. a combination of a large quantity of martensitic structure and a relatively large amount of austenitic structure.

(28) It is to be noted that there are listed above only specific examples of the invention. Obviously, the invention is not limited to the above examples. Instead, there exist many similar variations. All variations derived or envisioned directly from the disclosure of the invention by those skilled in the art should be all included in the protection scope of the invention.