ULTRA-HIGH-STRENGTH STEEL HAVING EXCELLENT PLASTICITY AND METHOD FOR MANUFACTURING SAME

Abstract

Disclosed is an ultra-high-strength steel having excellent plasticity, comprising in mass percent the chemical elements: C: 0.26-0.30 wt %; Si: 0.8-1.00 wt %; Mn: 2.80-3.30 wt %; Al: 0.04-0.08 wt %; with the balance being Fe and other inevitable impurities. Also disclosed is a manufacturing method for manufacturing the ultra-high-strength steel having excellent plasticity, comprising the following steps: (1) smelting and thin slab continuous casting; (2) heating; (3) hot rolling, wherein an oxide scale on the surface of a hot-rolled steel strip has a thickness of ≤6 μm, and (FeO+Fe.sub.3O.sub.4)≤40 wt % in the oxide scale on the surface of the hot-rolled strip steel; (4) acid pickling or acid pickling and cold rolling; and (5) continuous annealing: annealing at 800-920° C. and performing slow cooling at 3-10° C./s to 690-760° C.; performing fast cooling to 250-350° C. at 50-100° C.; and then heating to 360-460° C., maintaining the temperature for 100-400s and cooling to room temperature.

Claims

1. An ultra-high-strength steel having excellent plasticity, comprising chemical elements in mass percentages of: C: 0.26-0.30 wt %; Si: 0.8-1.00 wt %; Mn: 2.80-3.30 wt %; Al: 0.04-0.08 wt %; a balance of Fe and other unavoidable impurities.

2. The ultra-high strength steel having excellent plasticity according to claim 1, wherein the mass percentages of the chemical elements satisfy at least one of: C: 0.26-0.28 wt %; Si: 0.9-1.00 wt %; Mn: 2.9-3.1 wt %.

3. The ultra-high strength steel having excellent plasticity according to claim 1, further comprising at least one of the following chemical elements: 0<Cr≤0.05 wt %; 0<Mo≤0.05 wt %; 0<Nb≤0.03 wt %; 0<Ti≤0.05 wt %; 0<V≤0.03 wt %; 0<B≤0.001 wt %.

4. The ultra-high strength steel having excellent plasticity according to claim 3, wherein the mass percentages of the chemical elements satisfy at least one of: 0<Cr≤0.03 wt %; 0<Mo≤0.03 wt %; 0<Nb≤0.01 wt %; 0<Ti≤0.03 wt %; 0<V≤0.01 wt %.

5. The ultra-high strength steel having excellent plasticity according to claim 1, wherein among other unavoidable impurities: P≤0.01 wt %, S≤0.01 wt %, N≤0.006 wt %.

6. The ultra-high strength steel having excellent plasticity according to claim 1, wherein the ultra-high strength steel having excellent plasticity has a microstructure comprising 20 vol. %-40 vol. % ferrite+50 vol. %-70 vol. % martensite+retained austenite.

7. The ultra-high strength steel having excellent plasticity according to claim 6, wherein ferrite comprises 90% or more of grains of 10 μm or less, and 60% or more of grains of 5 μm or less.

8. The ultra-high strength steel having excellent plasticity according to claim 6, wherein an average grain size of retained austenite is ≤2 μm; and/or an average C content in retained austenite is ≥1.1 wt %.

9. The ultra-high strength steel having excellent plasticity according to claim 1, wherein the ultra-high strength steel having excellent plasticity has a yield strength of 850-1000 MPa, a tensile strength of 1180-1300 MPa, a uniform elongation of ≥11%; and an elongation at break of 15%-20%.

10. A manufacturing method for the ultra-high-strength steel having excellent plasticity according to claim 1, comprising steps: (1) smelting and thin slab continuous casting: controlling a slab thickness at an exit end in the continuous casting at 55-60 mm; (2) heating; (3) hot rolling: thickness of an oxide scale on a surface of a hot-rolled steel strip: 6 μm; (FeO+Fe.sub.3O.sub.4) in the oxide scale on the surface of the hot-rolled steel strip: ≤40 wt %; (4) pickling or pickling+cold rolling; (5) continuous annealing: annealing at 800-920° C.; slowly cooling to 690-760° C. at a cooling rate of 3-10° C./s to obtain a certain proportion of ferrite; rapidly cooling to 250-350° C. at a cooling rate of 50-100° C./s to transform austenite partially to martensite; reheating to 360-460° C., holding for 100-400s, and finally cooling to room temperature.

11. The manufacturing method according to claim 10, wherein in the step (1), a drawing speed in the continuous casting is controlled at 2-5 m/min.

12. The manufacturing method according to claim 10, wherein in step (2), the slab is heated to 1200-1300° C.

13. The manufacturing method according to claim 10, wherein in the step (3), a finishing rolling temperature is controlled at 860-930° C., and a coiling temperature is 450-600° C.

14. The manufacturing method according to claim 10, wherein in step (4), when a step of pickling+cold rolling is employed, an amount of deformation is controlled at 40%-70%.

15. The manufacturing method according to claim 10, wherein in step (5), the continuous annealing process is controlled to satisfy at least one of: annealing temperature of 820-870° C.; slowly cooling to 700-730° C. at a cooling rate of 3-10° C./s; rapidly cooling to 270-330° C.; after rapid cooling, reheating to 400-430° C. and holding for 150-300 s; controlling a volume content of hydrogen in a reducing atmosphere in a continuous annealing furnace at 10-15%.

16. The ultra-high strength steel having excellent plasticity according to claim 2, further comprising at least one of the following chemical elements: 0<Cr≤0.05 wt %; 0<Mo≤0.05 wt %; 0<Nb≤0.03 wt %; 0<Ti≤0.05 wt %; 0<V≤0.03 wt %; 0<B≤0.001 wt %.

17. The manufacturing method according to claim 10, wherein the ultra-high-strength steel having excellent plasticity further comprises at least one of the following chemical elements: 0<Cr≤0.05 wt %; 0<Mo≤0.05 wt %; 0<Nb≤0.03 wt %; 0<Ti≤0.05 wt %; 0<V≤0.03 wt %; 0<B≤0.001 wt %.

18. The manufacturing method according to claim 10, wherein the ultra-high strength steel having excellent plasticity has a microstructure comprising 20 vol. %-40 vol. % ferrite+50 vol. %-70 vol. % martensite+retained austenite.

19. The manufacturing method according to claim 10, wherein ferrite comprises 90% or more of grains of 10 μm or less, and 60% or more of grains of 5 μm or less; and/or, wherein an average grain size of retained austenite is ≤2 μm; and/or an average C content in retained austenite is ≥1.1 wt %.

20. The manufacturing method according to claim 10, wherein the ultra-high strength steel having excellent plasticity has a yield strength of 850-1000 MPa, a tensile strength of 1180-1300 MPa, a uniform elongation of 11%; and an elongation at break of 15%-20%.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0089] FIG. 1 is an image showing the microstructure of the ultra-high-strength steel of Example 4.

[0090] FIG. 2 is an EBSD image showing the phase composition of the ultra-high-strength steel of Example 4.

DETAILED DESCRIPTIONS

[0091] The ultra-high-strength steel having excellent plasticity according to the present disclosure and the method for manufacturing the same will be further explained and illustrated with reference to the accompanying drawings of the specification and the specific Examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the present disclosure.

Examples 1-24 and Comparative Examples 1-3

[0092] The ultra-high-strength steel having excellent plasticity in each of Examples 1-24 was prepared by the following steps: [0093] (1) performing smelting and thin slab continuous casting on the chemical compositions shown in Table 1: the slab thickness at the exit end in the continuous casting was controlled at 55-60 mm, and the speed of drawing the continuously cast slab was controlled at 2-5 m/min. [0094] (2) heating: the slab was heated to 1200-1300° C. [0095] (3) hot rolling: the thickness of the oxide scale on the surface of the hot-rolled steel strip was ≤6 μm; (FeO+Fe.sub.3O.sub.4) in the oxide scale on the surface of the hot-rolled steel strip was ≤40 wt %; the finishing rolling temperature was controlled at 860-930° C.; and the coiling temperature was 450-600° C. [0096] (4) pickling or pickling+cold rolling: when the step of pickling+cold rolling was used, the amount of deformation was controlled at 40%-70%. [0097] (5) continuous annealing: the steel strip was annealed at 800-920° C.; slowly cooled to 690-760° C. at a cooling rate of 3-10° C./s to obtain a certain proportion of ferrite; rapidly cooled to 250-350° C. at a cooling rate of 50-100° C./s to transform austenite partially to martensite; reheated to 360-460° C., held for 100-400 s, and finally cooled to room temperature.

[0098] It should be noted that, in some preferred embodiments, in step (5), the parameters may be further controlled to satisfy at least one of the following: [0099] annealing temperature of 820-870° C.; [0100] slowly cooling to 700-730° C. at a cooling rate of 3-10° C./s; [0101] rapidly cooling to 270-330° C.; [0102] after rapid cooling, reheating to 400-430° C. and holding for 150-300 s; [0103] controlling a volume content of hydrogen in a reducing atmosphere in a continuous annealing furnace at 10-15%.

[0104] Comparative Examples 1-3 were obtained using the conventional process.

[0105] Table 1 lists the mass percentages of the chemical elements in the ultra-high-strength steel having excellent plasticity in each of Examples 1-24 and the comparative steel in each of Comparative Examples 1-3.

TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and other unavoidable impurities except for P, S and N) Steel No. grade C Si Mn Cr Nb Mo Ni V Ti B P S Al N Exs. 1-4 A 0.26   1 3.3 0.049 — 0.048 — — — — 0.010 0.007 0.040 0.0058 Exs. 5-8 B 0.27 0.9 2.8 — — — — — 0.049 0.0009 0.010 0.006 0.052 0.0051 Exs. 9-12 C 0.29 0.9 3.0 — — — — 0.029 0.025 — 0.005 0.008 0.058 0.0044 Exs. D 0.30 0.8 2.9 — 0.029 — — — 0.021 — 0.010 0.008 0.076 0.0048 13-16 Exs. E 0.30 0.9 3.0 — — — — — — — 0.008 0.008 0.068 0.0056 17-20 Exs. F 0.28 0.8 3.1 0.013 — 0.023 — 0.012 — 0.0005 0.010 0.007 0.069 0.0048 20-24 Comp. a 0.26 0.7 2.8 0.03  0.02  0.006 0.01 0.002 0.011 — 0.005 0.005 0.046 0.0042 Ex. 1 Comp. b 0.26 0.9 3.0 — — — — — — — 0.010 0.005 — 0.0031 Ex. 2 Comp. c 0.28 0.5 2.9 0.03  0.02  — — — 0.021 0.0010 0.010 0.001 0.060 0.0044 Ex. 3

[0106] Table 2-1 and Table 2-2 list the specific process parameters for the ultra-high-strength steel having excellent plasticity in each of Examples 1-24 and the comparative steel in each of Comparative Examples 1-3.

TABLE-US-00002 TABLE 2-1 Step (1) Drawing Step (3) speed in Step (2) (FeO + Finishing Average Slab continuous Slab heating Oxide scale Fe.sub.3O.sub.4) rolling coiling Steel thickness casting temperature thickness content temperature temperature No. grade (mm) (m/min) (° C.) (μm) (%) (° C.) (° C.) Ex. 1 A 55 2.0 1220 3.1 26.7 903 476 Ex. 2 A 55 2.0 1235 3.5 30.3 910 490 Ex. 3 A 55 2.0 1260 4.5 33.5 908 500 Ex. 4 A 55 2.0 1280 5.8 28.6 925 470 Ex. 5 B 58 3.0 1230 4.8 32.7 930 530 Ex. 6 B 58 3.0 1290 5.7 28.6 922 497 Ex. 7 B 58 3.0 1210 3.8 31.7 917 450 Ex. 8 B 58 3.0 1260 4.3 33.3 920 562 Ex. 9 C 58 4.0 1230 3.2 28.6 860 455 Ex. 10 C 58 4.0 1245 3.3 25.2 920 500 Ex. 11 C 58 4.0 1210 3.5 29.3 924 525 Ex. 12 C 58 4.0 1290 5.8 32.9 919 560 Ex. 13 D 60 5.0 1245 4.2 34.4 875 580 Ex. 14 D 60 5.0 1230 3.6 35.8 886 525 Ex. 15 D 60 5.0 1210 3.1 28.9 885 495 Ex. 16 D 60 5.0 1290 5.9 38.1 913 600 Ex. 17 E 56 2.5 1240 4.6 33.7 930 530 Ex. 18 E 56 2.5 1290 5.8 38.6 922 497 Ex. 19 E 56 2.5 1220 3.8 28.7 880 470 Ex. 20 E 56 2.5 1260 4.1 33.3 920 562 Ex. 21 F 59 4.5 1250 4.3 34.4 875 580 Ex. 22 F 59 4.5 1230 3.6 32.8 870 525 Ex. 23 F 59 4.5 1210 3.3 28.9 885 475 Ex. 24 F 59 4.5 1290 5.9 38.1 913 590 Comp. Ex. a 200 1.0 1350 8.0 60.0 950 620 1 Comp. Ex. b 200 2.5 1250 4.3 50.0 870 580 2 Comp. Ex. c 60 3.0 1230 4.5 34.5 900 500 3

TABLE-US-00003 TABLE 2-2 Step (4) Step (5) Cold rolling Slow Initial temperature Final temperature Rapid deformation Annealing cooling of rapid of rapid cooling Reheating Reheating amount temperature rate cooling cooling rate temperature hold No. (%) (° C.) (° C./s) (° C.) (° C.) (° C./s) (° C.) time (s) Ex. 1 40 850 9 700 290 90 410 200 Ex. 2 40 845 7 715 280 70 420 160 Ex. 3 40 830 4 690 250 91 360 320 Ex. 4 40 835 8 700 285 57 405 220 Ex. 5 50 870 4 710 290 80 410 150 Ex. 6 50 840 9 730 270 90 460 110 Ex. 7 50 845 5 710 310 95 415 280 Ex. 8 50 825 4 690 260 90 420 220 Ex. 9 60 920 8 750 350 85 430 200 Ex. 10 60 900 9 730 260 62 420 240 Ex. 11 60 860 6 700 270 66 410 280 Ex. 12 60 840 8 725 280 86 430 150 Ex. 13 70 810 6 730 270 55 390 250 Ex. 14 70 820 5 710 290 76 410 220 Ex. 15 70 840 10 700 270 100 430 140 Ex. 16 70 880 10 720 260 70 420 200 Ex. 17 55 870 4 700 290 80 410 150 Ex. 18 55 840 9 730 270 90 450 110 Ex. 19 55 850 5 710 330 95 415 280 Ex. 20 55 860 4 690 260 90 420 240 Ex. 21 65 850 6 730 280 55 390 250 Ex. 22 65 830 5 710 300 80 400 220 Ex. 23 65 840 10 690 270 100 420 150 Ex. 24 65 880 10 730 260 70 430 200 Comp. Ex. 1 50 840 8 700 440 50 440 300 Comp. Ex. 2 60 900 4 770 200 13 390 200 Comp. Ex. 3 30 820 20 650 470 80 450 300

[0107] Table 3 lists the test results of the mechanical properties of the ultra-high-strength steel having excellent plasticity in each of Examples 1-24 and the comparative steel in each of Comparative Examples 1-3. ISO 6892:1998 (Room Temperature Tensile Test Method For Metallic Materials) and P14 (A.sub.50) Tensile Specimen Standard were used for testing.

TABLE-US-00004 TABLE 3 YS TS UEL TEL No. (MPa) (MPa) (%) (%) Ex. 1 886 1211 11.22 16.88 Ex. 2 892 1201 12.23 17.21 Ex. 3 912 1189 11.67 15.38 Ex. 4 890 1192 12.11 16.79 Ex. 5 892 1242 12.12 17.24 Ex. 6 865 1182 13.16 18.21 Ex. 7 858 1203 12.34 16.12 Ex. 8 921 1197 13.23 18.91 Ex. 9 945 1300 11.13 15.71 Ex. 10 991 1276 12.14 16.22 Ex. 11 916 1221 12.63 17.22 Ex. 12 892 1194 12.34 16.96 Ex. 13 876 1184 12.76 18.21 Ex. 14 863 1197 13.01 19.21 Ex. 15 890 1212 12.67 17.89 Ex. 16 943 1266 11.78 16.23 Ex. 17 942 1258 12.32 17.64 Ex. 18 887 1203 13.36 18.41 Ex. 19 901 1243 11.84 15.89 Ex. 20 911 1198 13.21 18.76 Ex. 21 880 1194 12.66 18.56 Ex. 22 868 1202 13.23 19.35 Ex. 23 896 1222 12.67 17.67 Ex. 24 955 1286 11.12 15.34 Comp. Ex. 1 670 1080 10.4 14.3 Comp. Ex. 2 630 1035 11.2 13.5 Comp. Ex. 3 660 1198 9.2 11.3

[0108] As it can be seen from Table 3, the ultra-high-strength steel having excellent plasticity in each of Examples 1-24 according to the present disclosure exhibits excellent ductility while the strength is guaranteed. The yield strength YS is 850-1000 MPa; the tensile strength TS is 1180-1300 MPa; the uniform elongation UEL is ≥11%; and the elongation at break TEL is 15%-20%.

[0109] Table 4. Microstructure observations of the ultra-high-strength steel having excellent plasticity in each of Examples 1-24.

TABLE-US-00005 TABLE 4 Retained Fraction of ≤ Fraction Average size C content in Ferrite Martensite austenite 10 μm ferrite of ≤ 5 μm of retained retained fraction fraction fraction grains ferrite austenite austenite No. (%) (%) (%) (%) grains (%) (μm) (%) Ex. 1 28.25 59.56 12.19 91.77 72.35 0.6 1.17 Ex. 2 29.98 58.55 11.47 95.64 64.53 0.7 1.23 Ex. 3 28.76 60.12 11.12 90.97 74.24 0.8 1.25 Ex. 4 31.32 56.34 12.34 95.98 69.62 1.1 1.15 Ex. 5 24.08 63.15 12.77 96.06 64.35 1.0 1.27 Ex. 6 27.81 58.78 13.41 92.19 77.60 1.3 1.24 Ex. 7 29.78 58.33 11.89 93.24 62.88 0.9 1.18 Ex. 8 25.70 61.45 12.85 94.26 64.45 1.4 1.21 Ex. 9 20.54 69.34 10.12 97.24 73.50 0.7 1.31 Ex. 10 21.55 66.67 11.78 98.66 72.49 1.2 1.24 Ex. 11 26.70 61.34 11.96 93.84 73.96 0.9 1.13 Ex. 12 29.11 58.12 12.77 91.29 76.51 0.6 1.17 Ex. 13 38.89 50.23 10.88 97.12 79.01 1.6 1.26 Ex. 14 30.34 57.96 11.70 93.22 75.06 1.1 1.24 Ex. 15 29.67 58.45 11.88 93.67 74.51 1.4 1.15 Ex. 16 21.07 67.56 11.37 95.74 70.73 1.2 1.22 Ex. 17 25.08 61.15 13.77 95.06 64.35 1.0 1.24 Ex. 18 30.81 57.78 11.41 93.19 75.60 1.2 1.26 Ex. 19 29.78 58.33 11.89 93.54 65.88 0.9 1.15 Ex. 20 26.70 60.45 12.85 95.44 64.65 1.6 1.28 Ex. 21 39.89 49.23 10.88 94.12 69.01 1.5 1.16 Ex. 22 28.34 59.96 11.70 93.54 73.06 1.2 1.44 Ex. 23 31.67 56.45 11.88 94.67 74.51 1.4 1.16 Ex. 24 22.07 66.56 11.37 96.74 72.73 1.5 1.25

[0110] As it can be seen from Table 3 and Table 4 in combination, the microstructure of the ultra-high-strength steel having excellent plasticity in Examples 1-24 according to the present disclosure comprises 20%-40% ferrite+50%-70% martensite+retained austenite. Ferrite comprises 90% or more grains of 10 μm or smaller, and 60% or more grains of 5 μm or smaller; the average grain size of retained austenite is ≤2 μm; and/or the average C content in retained austenite is ≥1.1 wt %. It is thus demonstrated that the ultra-high-strength steel having excellent plasticity in each of the Examples according to the present disclosure has a certain amount of fine-grained ferrite and good structure uniformity, so the high-strength steel in each of the Examples has excellent plasticity while having high strength.

[0111] FIG. 1 is an image showing the microstructure of the ultra-high-strength steel of Example 4.

[0112] FIG. 2 is an EBSD image showing the phase composition of the ultra-high-strength steel of Example 4.

[0113] As it can be seen from FIG. 1 and FIG. 4 in combination, the microstructure of the ultra-high-strength steel in Example 4 comprises 20%-40% ferrite+50%-70% martensite+retained austenite. Ferrite comprises 90% or more grains of 10 μm or smaller, and 60% or more grains of 5 μm or smaller; the average grain size of retained austenite is ≤2 μm; and/or the average C content in retained austenite is ≥1.1 wt %.

[0114] In summary, it can be seen that the ultra-high-strength steel according to the present disclosure is based on a carbon-silicon-manganese steel with no addition of any expensive alloying element. By optimizing the fractions of carbon, silicon and manganese, and using the thin slab continuous casting technology, it has inherent advantages in terms of structure uniformity and segregation control.

[0115] The manufacturing method according to the present disclosure is simple in production process. The ultra-high-strength steel thus obtained has a significantly increased elongation when the strength is the same. It is promising for use in automobile safety structural parts, especially suitable for manufacture of vehicle structural parts and safety parts having complex shapes and high formability requirements, such as A/B pillars, door crash bars, side members, bumpers, etc.

[0116] It's to be noted that the prior art portions in the protection scope of the present disclosure are not limited to the examples set forth in the present application file. All the prior art contents not contradictory to the technical solution of the present disclosure, including but not limited to prior patent literature, prior publications, prior public uses and the like, may all be incorporated into the protection scope of the present disclosure.

[0117] In addition, the ways in which the various technical features of the present disclosure are combined are not limited to the ways recited in the claims of the present disclosure or the ways described in the specific examples. All the technical features recited in the present disclosure may be combined or integrated freely in any manner, unless contradictions are resulted.

[0118] It should also be noted that the Examples listed above are only specific examples according to the present disclosure. Obviously, the present disclosure is not limited to the above Examples. Similar changes or modifications made accordingly can be directly derived from the present disclosure or are readily conceivable to those skilled in the art, and they all fall in the protection scope of the present disclosure.