980 MPA-GRADE BAINITE HIGH HOLE EXPANSION STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed are a 980 MPa-grade bainite high hole expansion steel and a manufacturing method therefor. The steel contains the following chemical components in percentages by weight: 0.05-0.10% of C, 0.5-2.0% of Si, 1.0-2.0% of Mn, P≤0.02%, S≤0.003%, 0.02-0.08% of Al, N≤0.004%, Mo≥0.1%, 0.01-0.05% of Ti, Cr≤0.5%, B≤0.002%, O≤0.0030%, and the balance of Fe and other inevitable impurities. The high hole expansion steel of the present invention has a yield strength of ≥800 MPa and a tensile strength of ≥980 MPa, has a good elongation rate (the transverse A.sub.50 being ≥11%) and hole expansion performance (the hole expansion ratio being ≥40%), and can be applied to a position on a chassis part of a passenger car, such as a control arm and a vice frame, where high strength and thinning are required.

Claims

1. A 980 MPa grade bainite high hole expansion steel having a chemical composition based on weight percentage of: C 0.05-0.10%, Si 0.5-2.0%, Mn 1.0%-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo≥0.1%, Ti 0.01-0.05%, Cr≤0.5%, B≤0.002%, O≤0.0030%, and a balance of Fe and other unavoidable impurities, wherein the high hole expansion steel has a microstructure of bainite+residual austenite.

2. The 980 MPa grade bainite high hole expansion steel according to claim 1 further comprising one or more elements of Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5% and C≤a0.005%.

3. The 980 MPa grade bainite high hole expansion steel according to claim 1 having a chemical composition based on weight percentage of: C 0.05-0.10%, Si 0.5-2.0%, Mn 1.0%-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo≥0.1%, Ti 0.01-0.05%, Cr≤0.5%, B≤0.002%, O≤0.0030%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5%, Ca≤0.005%, and a balance of Fe and other unavoidable impurities, wherein the 980 MPa grade bainite high hole expansion steel comprises at least one of Nb, V, Cu, Ni and Ca.

4. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of C is 0.06-0.08%.

5. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of Si is 0.8-1.6%.

6. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of Mn is 1.4-1.8%.

7. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of S is controlled at 0.0015% or less, and/or the content of N is controlled at 0.003% or less.

8. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of Al is 0.02-0.05%.

9. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of Ti is 0.01-0.03%.

10. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of Mo is ≥0.15%.

11. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the content of Cr is 0.2-0.4%, and/or the content of B is 0.0005-0.0015%, and/or the content of Mo is 0.1-0.55%.

12. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the high hole expansion steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a traverse elongation A.sub.50≥10%, and a hole expansion ratio of ≥40%.

13. The 980 MPa grade bainite high hole expansion steel according to claim 1, wherein the high hole expansion steel has yield strength of ≥850 MPa, a tensile strength of ≥1000 MPa, a traverse elongation A.sub.50≥11%, and a hole expansion ratio of ≥50%.

14. A manufacture method of the 980 MPa grade bainite high hole expansion steel according to claim 1, which comprises the following steps: 1) Smelting and casting: Smelting the above composition according to claim 1 by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting it into a blank or ingot; 2 ) Re-heating the blank or ingot at the heating temperature of 1100-1200° C., holding for 1-2 hours; 3) Hot rolling: the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C. and has a cumulative deformation of ≥50%, after 3-5 passes of heavy reduction rolling at ≥950° C.; the intermediate blanket is then held till 920-950° C., then subjected to final 3-7 passes of rolling with cumulative deformation of ≥70% the final rolling temperature is 800-920° C.; 4) Cooling: first, air-cooling for 0-10 s is carried out, and then the strip steel is water cooled to 400-550° C. at a cooling speed of ≥0° C./s, for coiling, and naturally cooled to room temperature after coiling; 5) Pickling the running speed of pickling of the strip steel is adjusted in the range of 30˜100m/min, the pickling temperature is controlled at 75˜85° C., and the tensile levelling rate is controlled at ≥2%, and the strip steel is rinsed, and the strip steel surface is dried and oiled at the temperature of 120-140° C.

15. The manufacture method of the 980 MPa grade bainite high hole expansion steel according to claim 14, wherein after step 5) of pickling, the strip steel is rinsed at a temperature of 35-50° C., and the strip steel surface is dried and oiled at 120-140° C.

16. The 980 MPa grade bainite high hole expansion steel according to claim 2, wherein the content of Nb, V is ≤0.03%, respectively; the content of Cu, Ni is ≤0.3%, respectively; and/or the content of Ca is ≤0.002%.

17. The manufacture method of the 980 MPa grade bainite high hole expansion steel according to claim 14, wherein the 980 MPa grade bainite high hole expansion steel has a chemical composition based on weight percentage of: C 0.05-0.10%, Si 0.5-2.0%, Mn 1.0%-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo≥0.1%, Ti 0.01-0.05%, Cr≤0.5%, B≤0.002%, CO≤0.0030%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5%, Ca≤0.005%, and a balance of Fe and other unavoidable impurities, wherein the 980 MPa grade bainite high hole expansion steel comprises at least one of Nb, V, Cu, Ni and Ca.

18. The manufacture method of the 980 MPa grade bainite high hole expansion steel according to claim 14, wherein in the composition of the 980 MPa grade bainite high hole expansion steel: the content of C is 0.06-0.08%; the content of Si is 0.8-1.6%; the content of Mn is 1.4-1.8%; the content of S is controlled at 0.0015% or less, and/or the content of N is controlled at 0.003% or less; the content of Al is 0.02-0.05%; the content of Ti is 0.01-0.03%; the content of Mo is ≥0.15%; and/or the content of Cr is 0.2-0.4%, and/or the content of B is 0.0005-0.0015%, and/or the content of Mo is 0.1-0.55%.

19. The manufacture method of the 980 MPa grade bainite high hole expansion steel according to claim 14, wherein the high hole expansion steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a traverse elongation A.sub.50≥10%, and a hole expansion ratio of ≥40%.

20. The manufacture method of the 980 MPa grade bainite high hole expansion steel according to claim 14, wherein: in step 3), the blank or ingot is hot rolled at an initial rolling temperature of 950-1100° C. and has a cumulative deformation of ≥60%; the intermediate blanket is subjected to final 3-7 passes of rolling with cumulative deformation of ≥85%; in step 4), the cooling speed is ≥30° C./s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 is a process flow diagram of the manufacturing method of 980 MPa grade bainite high hole expansion steel described in the present disclosure.

[0056] FIG. 2 is a schematic diagram of the rolling process in the manufacturing method of 980 MPa grade bainite high hole expansion steel described in the present disclosure.

[0057] FIG. 3 is a schematic diagram of the cooling process in the manufacturing method of 980 MPa grade bainite high hole expansion steel described in the present disclosure.

[0058] FIG. 4 is a typical metallographic photo of the 980 MPa grade bainite high hole expansion steel of Example 2 according to the present disclosure.

[0059] FIG. 5 is a typical metallographic photo of the 980 MPa grade bainite high hole expansion steel of Example 4 according to the present disclosure.

[0060] FIG. 6 is a typical metallographic photo of the 980 MPa grade bainite high hole expansion steel of Example 6 according to the present disclosure.

[0061] FIG. 7 is a typical metallographic photo of the 980 MPa grade bainite high hole expansion steel of Example 8 according to the present disclosure.

DETAILED DESCRIPTION

[0062] Referring to FIG. 1 to 3, the manufacturing method of 980 MPa grade bainite high hole expansion steel according to the present disclosure comprises the following steps: [0063] 1) Smelting and casting: [0064] Smelting the above composition by a converter or an electric furnace, secondary refining by a vacuum furnace, and then casting it into a blank or ingot; [0065] 2) Re-heating the blank or ingot at the heating temperature of 1100-1200° C., holding for 1-2 hours; [0066] 3) Hot rolling: [0067] the blank or ingot is hot rolled at an initial rolling temperature of 950˜1100° C. Its cumulative deformation after 3-5 passes of heavy reduction rolling at ≥950° C. is ≥50%; the intermediate blanket is then held till 920-950° C., then subjected to final 3-7 passes of rolling with cumulative deformation of ≥70%; the final rolling temperature is 800-920° C.; [0068] 4) Cooling: [0069] first, air-cooling for 0-10s is carried out for dynamic recovery to make the deformed austenite more uniform, and then the strip steel is water cooled to 400-550° C. at a cooling speed of ≥10° C./s for coiling, and naturally cooled to room temperature after coiling; [0070] 5) Pickling [0071] the running speed of pickling of the strip steel is adjusted in the range of 30˜100 m/min, the pickling temperature is controlled at 75˜85° C., and the tensile levelling rate is controlled at ≤2%, and the strip steel is rinsed at the temperature of 35-50° C., and the surface is dried and oiled at the temperature of 120-140° C.

[0072] The composition of the high hole expansion steel in examples of the present disclosure is described in Table 1. Table 2-3 show the production process parameters of the steel in the examples of the present disclosure, wherein the thickness of the blank in the rolling process is 120 mm Table 4 shows the mechanical properties of the steel plate in the examples of the present disclosure. In the examples, the tensile properties (yield strength, tensile strength, elongation) were tested in accordance with the International Standard ISO6892-2-2018; the hole expansion ratio was tested in accordance with the International Standard ISO16630-2017.

[0073] As can be seen from Table 4, the yield strength of the steel coil is ≥800 MPa, the tensile strength is ≥980 MPa, the elongation is between 10-13%, and the hole expansion ratio is ≥40%.

[0074] Typical metallographic photographs of Examples 2, 4, 6 and 8 are shown in FIGS. 4-7, respectively. It can be seen that the typical microstructure is bainite and contains a small amount of residual austenite.

[0075] As can be seen from the above Examples, the 980 MPa high hole expansion steel of the present disclosure has excellent matching of strength, plasticity and hole expansion performance, especially suitable for automotive chassis structure and other parts that require high strength and thinning, and hole expansion and flange forming, such as control arms, etc., and can also be used for wheels and other parts that need hole flanging. It has broad application prospects.

TABLE-US-00001 TABLE 1 (unit: weight %) Example C Si Mn P S Al N Mo Ti Cr B Ca Nb V Cu Ni O 1 0.077 0.95 1.75 0.009 0.0026 0.043 0.0038 0.11 0.019 0.42 0.0008 / 0.030 / / / 0.0025 2 0.084 1.89 1.13 0.011 0.0020 0.035 0.0028 0.22 0.050 0.11 / 0.002 / 0.025 / / 0.0024 3 0.099 0.50 1.04 0.013 0.0012 0.079 0.0032 0.55 0.015 0.28 0.0015 / 0.033 / / 0.12 0.0028 4 0.061 1.98 1.98 0.009 0.0028 0.022 0.0035 0.18 0.033 / 0.0010 0.003 0.025 / 0.20 0.21 0.0025 5 0.080 1.60 1.85 0.008 0.0011 0.065 0.0029 0.24 0.011 / / 0.005 / 0.033 / 0.50 0.0023 6 0.065 1.77 1.40 0.015 0.0023 0.058 0.0034 0.42 0.023 0.36 0.0018 / / 0.048 0.25 0.43 0.0020 7 0.090 1.24 1.94 0.013 0.0005 0.028 0.0029 0.31 0.018 0.31 0.0005 0.001 0.059 / / / 0.0027 8 0.051 1.40 1.80 0.012 0.0024 0.071 0.0040 0.37 0.029 0.50 0.0011 / / / 0.50 0.30 0.0029

TABLE-US-00002 TABLE 2 Rolling process (thickness of steel blank is 120 mm) Initial Rough Intermediate Finish Final Cooling Heating rolling rolling blank rolling rolling Air Water Steel Coiling rate temper- Holding temper- cumulative temper- cumulative temper- cooling cooling plate temper- after ature time ature deformation ature deformation ature time rate thickness ature coiling ° C. h ° C. % ° C. % ° C. s ° C./s mm ° C. ° C./h Ex. 1 1170 1.3 1040 70 920 89 870 8 50 2 430 10 Ex. 2 1180 1.2 1080 50 930 92 920 5 40 3 550 20 Ex. 3 1130 2.0 1100 65 935 90 840 3 45 5 470 13 Ex. 4 1190 1.0 950 55 925 94 820 2 60 4 400 8 Ex. 5 1150 1.7 1020 60 940 88 830 5 35 6 510 18 Ex. 6 1150 1.5 1000 75 950 93 800 7 50 4 480 15 Ex. 7 1130 1.9 980 80 920 90 850 0 45 2 450 12 Ex. 8 1160 1.4 1060 70 945 91 860 10 30 3 500 16

TABLE-US-00003 TABLE 3 Strip steel Pickling Tensile Rinsing Drying pickling temper- levelling temper- temper- running rate ature rate ature ature m/min ° C. % ° C. ° C. Ex. 1 100 82 1.8 40 135 Ex. 2 60 76 1.1 35 120 Ex. 3 70 75 1.6 47 128 Ex. 4 80 80 0.8 42 140 Ex. 5 30 77 2.0 50 133 Ex. 6 55 79 1.2 37 125 Ex. 7 45 81 0.5 41 134 Ex. 8 90 83 1.4 38 130

TABLE-US-00004 TABLE 4 mechanical performance of the steel plate Yield Tensile Hole expansion Residual strength strength Elongation ratio austenite Example MPa MPa % % content 1 809 1015 13.0 44 4.24 2 888 1057 13.0 49 2.33 3 868 1020 11.5 61 4.39 4 812 1039 12.5 43 1.85 5 877 1056 12.0 55 3.57 6 804 1024 12.5 45 4.92 7 834 1008 13.0 77 4.88 8 846 1031 11.0 40 3.86