GPA-GRADE BAINITE STEEL HAVING ULTRA-HIGH YIELD RATIO AND MANUFACTURING METHOD FOR GPA-GRADE BAINITE STEEL

Abstract

GPa-grade bainite steel having an ultra-high yield ratio, containing, in addition to Fe, the following chemical elements in mass percentages: 0.12-0.24% of C; 0.2-0.5% of Si; 1.3-2.0% of Mn; 0.001-0.004% of B; 0.01-0.05% of Al; and at least one of Cr, Nb, Ti, and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, and Mo≤0.4%. Also disclosed are a manufacturing method and annealing process for the steel.

Claims

1. A GPa-grade bainite steel having an ultra-high yield ratio, comprising the following chemical elements in mass percentages in addition to Fe and unavoidable impurities: C: 0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%.

2. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, wherein the mass percentages of the chemical element are: C: 0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%; a balance of Fe and other unavoidable impurities.

3. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, wherein the mass percentages of the chemical elements satisfy at least one of: C: 0.15-0.20%, Mn: 1.6-2.0%.

4. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, wherein among the other unavoidable impurities: P≤0.015%; and/or S≤0.004%.

5. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, further comprising at least one of the following chemical elements: 0<Cu≤0.2%, 0<Ni≤0.2%, 0<V≤0.2%, 0<Ce≤0.2%.

6. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 5, wherein it satisfies 0.18≤M≤0.27, wherein M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, wherein Cr, V, Nb, Ti and Mo each represent a value in front of a percent sign in the mass percentage of each chemical element; and/or 0.20≤C.sub.b≤0.27, wherein an equivalent bainite carbon content C.sub.b=C−(Mo+Nb)/8−(Ti+V)/4−Cr/12+Ni/10+Mn/20+B×10, wherein each element in the above formula represents a value in front of a percent sign in the mass percentage of the element.

7. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, wherein its microstructure is mainly acicular lower bainite, and a phase proportion of the acicular lower bainite is ≥90%.

8. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 7, wherein its microstructure further comprises a nano-, submicron- or micron-scale granular carbide precipitate phase that is precipitated dispersively, and a total phase proportion of the granular carbide precipitate phase+acicular lower bainite is ≥99%; preferably, the granular carbide precipitate has a maximum diameter of ≤2 μm.

9. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, wherein it has a tensile strength of ≥980 MPa, a yield strength of ≥900 MPa, a yield ratio of ≥0.9, and a hole expansion rate of ≥55%; preferably a yield strength of ≥950 MPa, and a yield ratio of ≥0.95.

10. An annealing process for the GPa-grade bainite steel having an ultra-high yield ratio according to claim 1, comprising steps of: (a) Heating a strip steel to a soaking temperature Ts at a heating rate of ≤50° C./s at a heating stage, wherein Ts is 840-900° C.; (b) Holding the temperature Ts for 5 minutes or less at a soaking stage; (c) Cooling to (Ts-80) to (Ts-140) ° C. at a first cooling rate of ≤15° C./s at a slow cooling stage; (d) Cooling to (Ts-490) to (Ts-440) ° C. at a second cooling rate of ≥(130-Q)° C./s at a fast cooling stage; (e) Cooling at a third cooling rate for 10-40 s at a controlled cooling stage for self-temperature rise, wherein [(Q-80)/12]≤third cooling rate≤[(Q-80)/8]; (f) Finally, cooling the strip steel in air to room temperature at an air-cooling stage; wherein Q=C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000.

11. A manufacturing method for a GPa-grade bainite steel having an ultra-high yield ratio, comprising steps of: (1) Smelting and casting; (2) Hot rolling; (3) Post-rolling cooling and coiling; (4) Pickling and cold rolling. (5) The annealing process according to claim 10.

12. The manufacturing method according to claim 11, wherein in the step (2), a heating temperature is controlled at 1150-1260° C.; an initial rolling temperature of finishing rolling is controlled at 1100-1220° C.; and a final rolling temperature of finishing rolling is controlled at 900-950° C.

13. The manufacturing method according to claim 11, wherein in step (3), a cooling rate is controlled at 30-150° C./s, and a coiling temperature is controlled at 450-580° C.

14. The manufacturing method according to claim 11, wherein in step (4), a cold rolling reduction rate is controlled at ≥50%.

15. The manufacturing method according to claim 11, wherein the GPa-grade bainite steel having an ultra-high yield ratio comprising the following chemical elements in mass percentages in addition to Fe and unavoidable impurities: C: 0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%.

16. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 2, wherein the mass percentages of the chemical elements satisfy at least one of C: 0.15-0.20%, and Mn: 1.6-2.0%; and/or among the other unavoidable impurities: P≤0.015%; and/or S≤0.004%; and/or the GPa-grade bainite steel having an ultra-high yield ratio further comprises at least one of the following chemical elements: 0<Cu≤0.2%, 0<Ni≤0.2%, 0<V≤0.2%, 0<Ce≤0.2%.

17. The GPa-grade bainite steel having an ultra-high yield ratio according to claim 16, wherein it satisfies 0.18≤M≤0.27, wherein M=Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7, wherein Cr, V, Nb, Ti and Mo each represent a value in front of a percent sign in the mass percentage of each chemical element; and/or 0.20≤C.sub.b≤0.27, wherein an equivalent bainite carbon content C.sub.b=C−(Mo+Nb)/8−(Ti+V)/4−Cr/12+Ni/10+Mn/20+B×10, wherein each element in the above formula represents a value in front of a percent sign in the mass percentage of the element.

18. The annealing process for the GPa-grade bainite steel having an ultra high yield ratio according to claim 10, wherein the mass percentages of the chemical element of the GPa-grade bainite steel having an ultra-high yield ratio are: C: 0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%; a balance of Fe and other unavoidable impurities.

19. The annealing process for the GPa-grade bainite steel having an ultra high yield ratio according to claim 18, wherein the mass percentages of the chemical elements satisfy at least one of C: 0.15-0.20%, and Mn: 1.6-2.0%; and/or among the other unavoidable impurities: P≤0.015%; and/or S≤0.004%; and/or the GPa-grade bainite steel having an ultra-high yield ratio further comprises at least one of the following chemical elements: 0<Cu≤0.2%, 0<Ni≤0.2%, 0<V≤0.2%, 0<Ce≤0.2%.

20. The manufacturing method according to claim 15, wherein the mass percentages of the chemical element of the GPa-grade bainite steel having an ultra high yield ratio are: C: 0.12-0.24%; Si: 0.2-0.5%; Mn: 1.3-2.0%; B: 0.001-0.004%; Al: 0.01-0.05%; at least one of Cr, Nb, Ti and Mo, wherein Cr≤0.4%, Nb≤0.06%, Ti≤0.1%, Mo≤0.4%; a balance of Fe and other unavoidable impurities.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] FIG. 1 is a photograph at 3000× magnification showing the microstructure of the GPa-grade bainite steel of Example 1.

[0092] FIG. 2 is a photograph at 3000× magnification showing the microstructure of the comparative steel in Comparative Example 7.

[0093] FIG. 3 is a photograph at 1000× magnification showing the microstructure of the comparative steel in Comparative Example 8.

DETAILED DESCRIPTION

[0094] The GPa-grade bainite steel having an ultra-high yield ratio according to the present disclosure and the manufacturing method for 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 1-14 and Comparative Examples 1-10

[0095] The GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1-14 was prepared using the following steps: [0096] (1) Subjecting the chemical composition shown in Table 1 to smelting and casting. [0097] (2) Hot rolling: the heating temperature was controlled at 1150-1260° C.; the initial temperature of the finishing rolling was controlled at 1100-1220° C.; and the final rolling temperature of the finishing rolling was controlled at 900-950° C. [0098] (3) Post-rolling cooling and coiling: the cooling rate was controlled at 30-150° C./s; and the coiling temperature was controlled at 450-580° C. [0099] (4) Pickling and cold rolling: the cold rolling reduction rate was controlled at ≥50%. [0100] (5) Annealing.

[0101] It should be noted that in step (5), the annealing process comprises the following steps: [0102] (a) Heating to a soaking temperature Ts at a heating rate of ≤50° C./s at a heating stage, wherein Ts was 840-900° C.; [0103] (b) Holding the temperature Ts for 5 minutes or less at a soaking stage; [0104] (c) Cooling to (Ts-80) to (Ts-140) ° C. at a first cooling rate of ≤15° C./s at a slow cooling stage; [0105] (d) Cooling to (Ts-490) to (Ts-440) ° C. at a second cooling rate of ≥(130-Q)° C./s at a fast cooling stage, wherein Q=C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000; [0106] (e) Cooling at a third cooling rate for 10-40 s at a controlled cooling stage for self-temperature rise, wherein [(Q-80)/12]≤third cooling rate≤[(Q-80)/8]; [0107] (f) Finally, cooling the strip steel in air to room temperature at an air-cooling stage.

[0108] In addition, it should be noted that the GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1-14 according to the present disclosure was prepared using the above steps. The chemical compositions and related process parameters in these Examples all met the control requirements of the design specification according to the present disclosure.

[0109] The comparative steel in each of Comparative Examples 1-10 was also made by the process comprising smelting and casting, hot rolling, post-rolling cooling and coiling, pickling and cold rolling, and annealing. However, the chemical composition and the relevant process parameters in each of Comparative Examples 1-6 included parameters that failed to meet the requirements of the design according to the present disclosure. Although the chemical composition in each of Comparative Examples 7-10 met the requirements of the design according to the present disclosure, these Comparative Examples all included parameters that failed to meet the requirements of the design according to the present disclosure.

[0110] Among the Examples according to the present disclosure and the Comparative Examples, Comparative Example 7 and Example 1 had the same composition of chemical elements; Comparative Example 8 and Example 2 had the same composition of chemical elements; Comparative Example 9 and Example 6 had the same composition of chemical elements; and Comparative Example 10 and Example 11 had the same composition of chemical elements.

[0111] Table 1 lists the mass percentages (%) of the chemical elements in the GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1-14 and the mass percentages (%) of the chemical elements in the comparative steel in each of Comparative Examples 1-10.

TABLE-US-00001 TABLE 1 (the balance is Fe and other unavoidable impurities except for P and S) Steel Chemical elements No. Grade C Si Mn Cr B Mo Nb Ti P S Al Cu Ni IV Ce C.sub.b M Ex. 1 and A 0.16 0.45 1.75 0.3 0.002 0.19 0 0 0.010 0.002 0.02 0 0 0 0 0.22 0.23 Comp. Ex. 7 Ex. 2 and B 0.17 0.35 1.82 0.2 0.004 0.23 0 0 0.012 0.003 0.03 0 0 0 0 0.26 0.22 Comp. Ex. 8 Ex. 3 C 0.13 0.5 1.68 0.25 0.004 0.16 0.03 0.03 0.013 0.001 0.01 0 0 0 0 0.20 0.24 Ex. 4 D 0.19 0.23 2 0.23 0.003 0.13 0.04 0.02 0.011 0.002 0.03 0 0 0 0 0.27 0.21 Ex. 5 E 0.14 0.27 1.88 0.22 0.004 0.15 0 0 0.010 0.002 0.02 0.2 0 0 0 0.24 0.18 Ex. 6 and F 0.17 0.22 1.71 0.3 0.004 0.25 0 0 0.008 0.001 0.03 0 0 0 0 0.24 0.27 Comp. Ex. 9 Ex. 7 G 0.16 0.43 1.56 0.21 0.002 0.15 0.02 0.08 0.009 0.001 0.04 0 0 0 0 0.20 10.26 Ex. 8 H 0.19 0.41 1.53 0.35 0.002 0.12 0.06 0.01 0.010 0.001 0.05 0 0 0 0 0.23 0.26 Ex. 9 I 0.18 0.31 1.61 0.38 10.002 0 0 0.1 0.015 0.003 0.04 0 0 0 0 0.22 0.25 Ex. 10 J 0.2 0.37 1.94 0.25 0.001 0.20 0 0.01 0.010 0.002 0.02 0 0 0.2 0 0.21 0.27 Ex. 11 K 0.15 0.48 1.58 0 0.003 0.37 0 0 0.008 0.002 0.02 0 0 0 0 0.21 0.22 and Comp. Ex. 10 Ex. 12 L 0.22 0.46 1.35 0.16 0.004 0.31 0.04 0 0.009 0.001 0.03 0 0 0 0.2 0.27 0.27 Ex. 13 M 0.12 0.4 1.93 0.11 0.003 0.20 0 0.05 0.008 0.004 0.03 0 0.2 0 0 0.22 0.21 Ex. 14 N 0.24 0.25 1.3 0.17 0.001 0.27 0.01 0.03 0.010 0.001 0.02 0 0 0 0 0.26 0.26 Comp. 0 0.25 0.35 2.1 0.35 0.001 0.2 0 0 0.012 0.001 0.02 0 0 0 0 0.31 0.26 Ex. 1 Comp. P 0.10 0.4 1.7 0.3 0.002 0.25 0 0 0.013 0.003 0.02 0 0 0 0 0.15 0.27 Ex. 2 Comp. Q 0.18 0.3 1.68 0.28 0.004 0.2 0.02 0.05 0.011 0.004 0.02 0 0 0 0 0.24 0.29 Ex. 3 Comp. R 0.17 0.22 1.96 0.2 0.001 0.13 0 0 0.014 0.002 0.02 0 0 0 0 0.25 0.16 Ex. 4 Comp. S 0.15 0.22 1.51 0.22 0.001 0.3 0 0 0.015 0.002 0.02 0.2 0 0 0 0.18 0.26 Ex. 5 Comp. T 0.19 0.22 1.86 0.3 0.004 0.16 0 0.01 0.012 0.003 0.02 0 0 0 0 0.28 0.22 Ex. 6 Note: in the above table, Cb = C − (Mo + Nb)/−(Ti + V)/4 − Cr/12 + Ni/10 + Mn/20 + B × 10, wherein each element in the formula represents the value in front of the percent sign in the mass percentage of the element; M = Cr/2.5 + Ti + V/5 + Nb/1.7 + Mo/1.7, wherein Cr, V, Nb, Ti and Mo each represent the value in front of the percent sign in the mass percentage of the chemical element.

[0112] Table 2-1 and Table 2-2 list the specific process parameters for the GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1-14 and the comparative steel in each of Comparative Examples 1-10.

TABLE-US-00002 TABLE 2-1 Step (2) Initial rolling Final rolling Step (3) Step (4) Heating temperature of temperature of Coiling Cold rolling Steel temperature finishing finishing Cooling rate temperature reduction rate No. grade (° C.) rolling (° C.) rolling (° C.) (° C./s) (° C.) (%) Ex. 1 A 1180 1140 920 70 500 50 Ex. 2 B 1205 1165 945 100 525 55 Ex. 3 C 1210 1170 950 50 570 80 Ex. 4 D 1190 1150 930 90 480 65 Ex. 5 E 1155 1115 900 100 450 70 Ex. 6 F 1190 1150 930 110 510 60 Ex. 7 G 1240 1200 905 40 535 75 Ex. 8 H 1230 1190 910 30 580 55 Ex. 9 I 1255 1215 915 150 470 60 Ex. 10 J 1195 1155 935 120 515 65 Ex. 11 K 1205 1165 900 100 525 55 Ex. 12 L 1175 1135 915 140 495 50 Ex. 13 M 1230 1190 905 130 550 60 Ex. 14 N 1240 1200 950 150 460 65 Comp. Ex. 1 O 1170 1130 910 50 490 50 Comp. Ex. 2 P 1225 1185 915 100 545 70 Comp. Ex. 3 Q 1235 1195 925 120 495 65 Comp. Ex. 4 R 1215 1175 920 100 535 60 Comp. Ex. 5 S 1235 1195 900 80 505 55 Comp. Ex. 6 T 1230 1190 910 90 550 50 Comp. Ex. 7 A 1180 1140 920 70 500 70 Comp. Ex. 8 B 1205 1165 945 110 525 55 Comp. Ex. 9 F 1190 1150 930 90 510 60 Comp. Ex. K 1205 1165 900 100 525 60 10

TABLE-US-00003 TABLE 2-2 Step (5) Step (e) Step (c) Cooling Cooling Step (d) time at Step (b) at Cooling controlled Step (a) soaking temperature temperature cooling stage Heating Soaking First slow Second at fast Third Self- for self- rate at heating Soaking time at cooling cooling cooling cooling cooling temperature temperature stage temperature stage rate stage rate stage rate rise rise No. (° C./s) (° C.) (min) (° C./s) (° C.) (° C./s) Q (° C.) (° C./s) (° C.) (s) Ex. 1 5 850 2.0 5 740 55 97 365 2 108 10 Ex. 2 15 860 3.0 8 730 50 103 382 2 119 34 Ex. 3 45 900 4.5 10 775 45 93 410 1 65 40 Ex. 4 10 840 1.5 7 755 55 108 390 2.5 120 22 Ex. 5 20 865 1.5 12 737 50 98 385 2 100 23 Ex. 6 5 855 3.5 11 741 50 100 382 2 112 22 Ex. 7 20 875 2.5 4 762 60 89 397 1 58 30 Ex. 8 35 893 4.0 15 755 35 95 405 1.5 65 28 Ex. 9 30 862 1.5 13 725 45 94 400 1.5 51 14 Ex. 10 10 846 2.0 10 735 45 107 405 2.5 115 30 Ex. 11 15 868 1.0 8 746 50 91 380 1 80 20 Ex. 12 40 887 2.5 7 750 40 100 400 2 98 25 Ex. 13 25 858 3.0 7 725 45 104 379 2.5 98 12 Ex. 14 20 880 2.5 5 745 55 93 393 1.5 92 36 Comp. 30 860 2.5 5 740 25 122 391 4.5 123 12 Ex. 1 Comp. 25 863 3.0 8 745 45 85 386 1 64 20 Ex. 2 Comp. 10 858 2.0 7 735 40 101 396 2.5 111 18 Ex. 3 Comp. 15 866 1.5 10 733 42 99 382 3.5 83 10 Ex. 4 Comp. 5 854 2.0 15 720 50 84 378 71 30 Ex. 5 Comp. 20 857 2.5 10 725 30 107 393 3 111 25 Ex. 6 Comp. 5 852 4.0 7 740 20 97 405 2 48 10 Ex. 7 Comp. 35 860 3.5 8 730 30 103 450 2.5 45 10 Ex. 8 Comp. 40 855 3.0 5 741 35 100 382 1 112 10 Ex. 9 Comp. 30 868 1.5 10 746 45 91 380 3 80 20 Ex. 10 Note: in the above table, Q = C × 180 + Si × 10 + Mn × 30 + Ni × 50 + Cr × 15 + Mo × 15 + B × 2000, whereineach element in the formula represents the value in front of the percent sign in the mass percentage of the element.

[0113] Relevant mechanical performance tests were performed on the GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1-14 and the comparative steel in each of Comparative Example 1-10. The mechanical performance test results of the Examples and Comparative Examples are listed in Table 3. The relevant performance test methods are described as follows.

[0114] The resulting GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1 14 and the comparative steel in each of Comparative Example 1-10 were sampled respectively. A transverse JIS 5 #tensile sample was used to determine the yield strength and tensile strength of the steel, and the middle area of the sheet was used to determine the hole expansion rate and bending performance of the steel.

[0115] The hole expansion rate of the steel was determined in a hole expanding test, wherein a test piece with a hole in the center was pressed into a die with a punch to expand the central hole of the test piece until the edge of the hole in the plate necked or through-plate cracks appeared. Since the manner for preparing the original hole in the center of the test piece and the quality of the corresponding edge of the original hole have a great influence on the test result of the hole expansion rate, the test and test method were implemented according to the test method of hole expansion rate specified in the ISO/DIS 16630 standard. The original hole in the center was in the form of a punched hole (corresponding to the processing method for an original hole having the worst edge quality). The 180° bending test was implemented using the method for determining bending performance in the GB/T232-2010 standard (bending diameter d=1a).

[0116] Table 3 lists the test results of the mechanical performances of the GPa-grade bainite steel having an ultra-high yield ratio in each of Examples 1-14 and the comparative steel in each of Comparative Examples 1-10.

TABLE-US-00004 TABLE 3 Mechanical performances Yield strength Tensile strength Yield Elongation Hole expansion rate 180º No. (MPa) (MPa) ratio (%) (%) bending Ex. 1 959 1003 0.96 9.3 68.2 d = 1a qualified Ex. 2 933 1022 0.91 9.7 62.2 d = 1a qualified Ex. 3 927 1010 0.92 9.8 62 d = 1a qualified Ex. 4 936 1038 0.90 9.6 56.7 d = 1a qualified Ex. 5 918 1019 0.90 10.5 58.4 d = 1a qualified Ex. 6 966 1031 0.94 9.5 65.4 d = 1a qualified Ex. 7 924 1016 0.91 9.6 61.3 d = 1a qualified Ex. 8 922 1026 0.90 10.5 61.8 d = 1a qualified Ex. 9 915 1015 0.90 11 55.5 d = 1a qualified Ex. 10 948 1020 0.93 9.7 61 d = 1a qualified Ex. 11 911 1008 0.90 10.7 55.9 d = 1a qualified Ex. 12 942 1037 0.91 10.3 59.3 d = 1a qualified Ex. 13 915 1008 0.91 10.8 57 d = 1a qualified Ex. 14 941 1005 0.94 9.4 66.2 d = 1a qualified Comp. 1075 1158 0.93 7.8 44.2 d = 1a cracked Ex. 1 Comp. 849 942 0.90 11.2 68.8 d = 1a Ex. 2 qualified Comp. 1008 1098 0.92 8.2 51.9 d = 1a cracked Ex. 3 Comp. 1031 1129 0.91 8 47.7 d = 1a cracked Ex. 4 Comp. 875 977 0.90 10.8 66.9 d = 1a Ex. 5 qualified Comp. 1020 1147 0.89 8.9 50.3 d = 1a cracked Ex. 6 Comp. 802 999 0.80 11.9 43.9 d = 1a cracked Ex. 7 Comp. 781 996 0.78 12.4 40.1 d = 1a cracked Ex. 8 Comp. 974 1035 0.94 8.8 52.2 d = 1a cracked Ex. 9 Comp. 887 1002 0.89 11.6 53.4 d = 1a cracked Ex. 10

[0117] As it can be seen from Table 3, as compared with the comparative steels in Comparative Examples 1-10, the mechanical performances of the GPa-grade bainite steels having an ultra-high yield ratio in Examples 1-14 according to the present disclosure are obviously better. The GPa-grade bainite steels having an ultra-high yield ratio in Examples 1-14 according to the present disclosure have an ultra-high yield ratio, an ultra-high strength and excellent hole-expanding and bending performances at the same time, with a tensile strength of ≥980 MPa, a yield strength of ≥900 MPa, a yield ratio of ≥0.9, and a hole expansion rate of ≥55%.

[0118] In some individual preferred embodiments, as in Example 1, the GPa-grade bainite steel having an ultra-high yield ratio in Example 1 has a yield strength of ≥950 MPa, and a yield ratio of ≥0.95. That is, it has an ultra-high yield ratio and an ultra-high yield strength.

[0119] FIG. 1 is a photograph at 3000× magnification showing the microstructure of the GPa-grade bainite steel of Example 1.

[0120] As shown by FIG. 1, the microstructure of the matrix of the GPa-grade bainite steel in Example 1 is acicular lower bainite because it was cooled to the lower bainite phase region (the cooling temperature of the fast cooling met the requirement of the present disclosure) at a sufficiently fast cooling rate (the second cooling rate met the requirement of the present disclosure) at the fast cooling stage. In addition, because the cooling rate at the controlled cooling stage for self-temperature rise was suitable (the third cooling rate met the requirement of the present disclosure), the structure also contains a fine nano-, submicron- or micron-scale granular carbide precipitate phase that has been dispersively precipitated. The phase proportion of the acicular lower bainite is ≥90%; the total phase proportion of the granular carbide precipitate phase+the acicular lower bainite is ≥99%; and the maximum diameter of the granular carbide precipitate is ≤2 μm.

[0121] FIG. 2 is a photograph at 3000× magnification showing the microstructure of the comparative steel in Comparative Example 7.

[0122] As shown by FIG. 2, because the cooling rate was insufficient (the second cooling rate did not meet the requirement of the present disclosure) when the comparative steel in Comparative Example 7 was cooled at the fast cooling stage, bainite phase transformation occurred in the comparative steel at high temperature before it was cooled to the lower bainite phase region. Although it was still cooled to an appropriate temperature of the lower bainite phase region eventually, its microstructure is still dominated by massive equiaxed bainite, nearly free of acicular lower bainite, and the carbide precipitate is also not fine and uniform enough.

[0123] FIG. 3 is a photograph at 1000× magnification showing the microstructure of the comparative steel in Comparative Example 8.

[0124] As shown by FIG. 3, although the cooling rate was appropriate (the second cooling rate met the requirement of the present disclosure) when the comparative steel in Comparative Example 8 was cooled at the fast cooling stage, the cooling temperature of the fast cooling was too high (the cooling temperature at the fast cooling stage did not meet the requirement of the present disclosure). As a result, its microstructure almost consists of massive equiaxed bainite, nearly free of acicular lower bainite, and the carbide precipitate is also not fine and uniform enough.

[0125] To sum up, as it can be seen, by designing the chemical composition appropriately according to the present disclosure, a GPa-grade bainite steel having an ultra-high yield ratio that has a tensile strength of ≥980 MPa, a yield strength of ≥900 MPa, a yield ratio of ≥0.9, and a hole expansion rate of ≥55% can be obtained. The GPa-grade bainite steel has an ultra-high yield ratio, an ultra-high strength, and excellent hole-expanding and bending performances at the same time. It can be used to prepare automotive structural parts, and realize the new design concept of “green-safety” for automobiles. It has good popularization prospect and application value.

[0126] The annealing process according to the present disclosure plays a key role for the performances of the steel. The annealing process comprises a heating stage, a soaking stage, a slow cooling stage, a fast cooling stage, a controlled cooling stage for self-temperature rise, and an air cooling stage. By designing the process appropriately and controlling relevant process parameters, the GPa-grade bainite steel having an ultra-high yield ratio can be obtained.

[0127] Correspondingly, the manufacturing method according to the present disclosure employs a unique production process. Particularly, the above annealing process is utilized to guarantee the performances of the resulting GPa-grade bainite steel. The resulting GPa-grade bainite steel not only has ultra-high strength and yield ratio, but also has excellent hole-expanding and bending performances.

[0128] 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.

[0129] It should also be noted that the Examples set forth above are only specific examples according to the present disclosure. Obviously, the present disclosure is not limited to the above Examples. Similar variations or modifications made thereto can be directly derived or easily contemplated from the present disclosure by those skilled in the art. They all fall in the protection scope of the present disclosure.