980 MPA-GRADE ULTRA-LOW-CARBON MARTENSITE AND RETAINED AUSTENITE ULTRA-HIGH HOLE EXPANSION STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

A 980 MPa-grade ultra-low-carbon martensite and retained austenite ultra-high hole expansion steel and a manufacturing method therefor. The hole expansion steel comprises the following chemical components in percentage by weight: C 0.03%-0.06%, Si 0.8%-2.0%, Mn 1.0%-2.0%, P≤0.02%, S≤0.003%, Al 0.02%-0.08%, N≤0.004%, Mo 0.1%-0.5%, Ti 0.01%-0.05%, and O≤0.0030%. The high hole expansion steel of the present invention has the yield strength ≥800 MPa, the tensile strength ≥980 MPa, the elongation rate (horizontal A50≥10%), the cold bending property (d≤4a, 180°), and the hole expansion ratio ≥80%, and can be applied to a chassis part of a passenger vehicle such as a control arm, an auxiliary frame and other parts that require high-strength thinning.

Claims

1. A 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel, comprising the following chemical components in weight percentages: C 0.03%-0.06%, Si 0.8%-2.0%, Mn 1.0%-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1%-0.5%, Ti 0.01%-0.05%, O≤0.0030%, and a balance of Fe and other unavoidable impurities.

2. The 980 MPa-grade ultra-low-carbon martensitic retained austenitic ultra-high-hole-expandability steel according to claim 1, further comprising one or more elements selected from the group consisting of Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, and Ni≤0.5%.

3. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, comprising the following chemical components in weight percentages: C 0.03%-0.06%, Si 0.8%-2.0%, Mn 1.0%-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1%-0.5%, Ti 0.01%-0.05%, O≤0.0030%, Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5%, and comprising at least one of Cr, B, Ca, Nb, V, Cu and Ni, or comprising at least Cr and/or B, and a balance of Fe and other unavoidable impurities.

4. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein C: 0.04-0.055%.

5. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein Si: 1.2-1.6%.

6. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein Mn: 1.4-1.8%.

7. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein S is controlled to have a content of 0.0015% or lower, and/or N is controlled to have a content of 0.003% or lower.

8. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein Al: 0.02-0.05%.

9. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein Ti: 0.01-0.03%, and/or Mo: 0.15-0.35%.

10. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein the ultra-high-hole-expandability steel has a microstructure of bainite and a small amount of retained austenite.

11. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein the ultra-high-hole-expandability steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a transverse A.sub.50 of ≥10%, a hole expansion ratio of ≥80%, and has passed cold bending test (d≤4a, 180°).

12. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein the ultra-high-hole-expandability steel has an impact toughness at −40° C. of ≥140 J.

13. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, wherein the ultra-high-hole-expandability steel has a yield strength of ≥815 MPa, a tensile strength of ≥1000 MPa, a transverse A.sub.50 of ≥10%, a hole expansion ratio of ≥85%, an impact toughness at −40° C. of ≥150 J, and has passed cold bending test (d≤4a, 180°).

14. A method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 1, comprising the following steps: 1) Smelting, casting wherein the components according to claim 1 are subjected to smelting in a converter or electrical furnace, secondary refining in a vacuum furnace, and casting to form a cast blank or ingot; 2) Reheating of the cast blank or ingot, wherein a heating temperature is 1100-1200° C.; and a holding time is 1-2 hours; 3) Hot rolling wherein an initial rolling temperature is 950-1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of 950° C. or higher with an accumulated deformation rate of ≥50%, to obtain an intermediate blank; wherein the intermediate blank is held till 900-950° C., and then subjected to final 3-5 passes of rolling with an accumulated deformation rate of ≥70%, wherein a final rolling temperature is 800-920° C.; 4) Cooling wherein air cooling is performed for 0-10 s first, and then the strip steel is water cooled at a cooling rate of ≥50° C./s, to a martensite start temperature Ms or a lower temperature, and then coiled and cooled to room temperature; 5) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2%; wherein the strip steel is then subjected to rinsing, surface drying, and oiling.

15. The method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high hole-expandability steel according to claim 14, wherein in step 5), after the pickling, the rinsing is carried out at a temperature in a range of 35-50° C., and the surface of the strip steel is dried at 120-140° C., followed by oiling.

16. The 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to claim 2, wherein Cr has a content of 0.2-0.4%; Cu and Ni each have a content of ≤0.3%; Nb and V each have a content of ≤0.03%; B has a content of 0.0005-0.0015%; and Ca has a content of ≤0.002%.

17. The method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high hole-expandability according to claim 14, wherein: the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel further comprises one or more elements selected from Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V0.05%, Cu≤0.5%, and Ni≤0.5%; or the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel comprises the following chemical components in weight percentages: C 0.03%-0.06%, Si 0.8%-2.0%, Mn 1.0%-2.0%, P≤0.02%, S0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1%-0.5%, Ti 0.01%-0.05%, O≤0.0030%, Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V0.05%, Cu≤0.5%, Ni≤0.5%, and comprises at least one of Cr, B, Ca, Nb, V, Cu and Ni, or comprises at least Cr and/or B, and a balance of Fe and other unavoidable impurities.

18. The method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high hole-expandability according to claim 14, wherein the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel comprises: C: 0.04-0.055%, Si: 1.2-1.6%, Mn: 1.4-1.8%, S: 0.0015% or lower and/or N: 0.003% or lower, Al: 0.02-0.05%, Ti: 0.01-0.03% and/or Mo: 0.15-0.35%; or the 980 MPa-grade ultra-high-hole-expandability steel has a microstructure of bainite and a small amount of retained austenite.

19. The method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high hole-expandability according to claim 14, wherein the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel has a yield strength of ≥300 MPa, a tensile strength of ≥980 MPa, a transverse A.sub.50 of ≥10%, a hole expansion ratio of ≥80%, and has passed cold bending test (d≤4a, 180°).

20. The method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high hole-expandability according to claim 14, wherein: in step 3), 3-5 passes of heavy reduction rolling is performed at a temperature of 950° C. or higher with an accumulated deformation rate of ≥60% to obtain an intermediate blank; the intermediate blank is subjected to final 3-5 passes of rolling with an accumulated deformation rate of ≥85%; in step 4), the cooling rate is 50-85° C./s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 is a process flow chart of the method for manufacturing a 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to the present disclosure;

[0056] FIG. 2 is a schematic view showing the rolling process in the method for manufacturing a 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to the present disclosure;

[0057] FIG. 3 is a schematic view showing the cooling process in the method for manufacturing a 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to the present disclosure.

DETAILED DESCRIPTION

[0058] Referring to FIG. 1-FIG. 3, the method for manufacturing the 980 MPa-grade ultra-low-carbon martensitic-retained austenitic ultra-high-hole-expandability steel according to the present disclosure comprises the following steps: [0059] 1) Smelting, casting [0060] wherein the above components are subjected to smelting in a converter or electrical furnace, secondary refining in a vacuum furnace, and casting to form a cast blank or ingot; [0061] 2) Reheating of the cast blank or ingot, wherein a heating temperature is 1100-1200° C.; and a holding time is 1-2 hours; [0062] 3) Hot rolling [0063] wherein an initial rolling temperature is 950-1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of 950° C. or higher with an accumulated deformation rate of ≥50% to obtain an intermediate blank; wherein the intermediate blank is held till 900-950° C., and then subjected to final 3-5 passes of rolling with an accumulated deformation rate of ≥70%, wherein a final rolling temperature is 800-920° C.; [0064] 4) Cooling [0065] wherein air cooling is performed for 0-10 s first, and then the strip steel is water cooled at a cooling rate of ≥50° C./s to a martensite start temperature Ms or a lower temperature, and then coiled and cooled (cooling rate ≤20° C./h) to room temperature; [0066] 5) Pickling [0067] wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2%; wherein the strip steel is then subjected to rinsing at a temperature in a range of 35-50° C., surface drying at a temperature in a range of 120-140° C., and oiling.

[0068] The compositions of the Examples of the high-hole-expandability steel according to the present disclosure are shown in Table 1. The production process parameters for the Examples of the steel according to the present disclosure are listed in Table 2 and Table 3, wherein the thickness of the steel blank in the rolling process is 120 mm. The mechanical performances of the Examples of the steel plates according to the present disclosure are listed in Table 4. The tensile performances (yield strength, tensile strength, elongation) were tested in accordance with International Standard ISO6892-2-2018; the hole expansion ratio was tested in accordance with International Standard ISO16630-2017; and the impact toughness at −40° C. was tested in accordance with International Standard ISO14556-2015. The bending performance was tested in accordance with International Standard ISO7438-2005.

[0069] As it can be seen from Table 4, the yield strength of the steel coil is ≥800 MPa, while the tensile strength is ≥980 MPa, and the elongation is usually in the range of 10-13%. The impact energy is relatively stable. The low-temperature impact energy at −40° C. is stabilized in the range of 140-180 J. The content of retained austenite varies as a function of the coiling temperature, generally by 2-5%. The hole expansion ratio satisfies ≥80%.

[0070] As it can be seen from the above Examples, the 980 MPa high-strength steel according to the present disclosure exhibits good matching of strength, plasticity, toughness and hole expandability. It is especially suitable for parts that require high strength, reduced thickness, hole expansion and flanging forming, such as a control arm in an automobile chassis structure. It can also be used for parts such as wheels that need hole-flanging. Therefore, it has broad application prospects.

TABLE-US-00001 TABLE 1 (unit: weight %) Ex. C Si Mn P S Al N Mo Ti Cr B Ca Nb V Cu Ni O 1 0.060 1.98 1.76 0.0011 0.0028 0.066 0.0030 0.10 0.029 0.23 0.0011 / 0.030 / / 0.40 0.0027 2 0.030 1.77 1.59 0.0013 0.0029 0.047 0.0027 0.17 0.025 0.35 / 0.001 / 0.015 / / 0.0020 3 0.048 0.80 1.68 0.0016 0.0010 0.020 0.0028 0.49 0.050 0.50 0.0009 / 0.015 / / 0.50 0.0025 4 0.055 1.26 1.99 0.0014 0.0024 0.039 0.0029 0.29 0.014 0.30 0.0010 0.002 / 0.50 / 0.0029 5 0.042 1.18 1.88 0.0010 0.0028 0.065 0.0038 0.38 0.028 0.28 0.0012 / 0.060 0.010 / / 0.0024 6 0.057 1.89 1.02 0.0015 0.0022 0.054 0.0033 0.33 0.015 0.10 0.0013 0.005 / 0.030 / 0.10 0.0028 7 0.053 1.53 1.96 0.0014 0.0017 0.079 0.0035 0.43 0.010 0.33 0.0020 / / / 0.15 / 0.0021 8 0.041 1.64 1.73 0.0012 0.0025 0.036 0.0022 0.18 0.020 / 0.0019 0.003 / 0.050 0.30 0.30 0.0030

TABLE-US-00002 TABLE 2 Accumulated Accumulated Initial deformation Intermediate deformation Final Water Steel Heating Holding rolling rate during blank rate during rolling Air cooling plate Coiling temperature time temperature rough rolling temperature finishing temperature cooling rate thickness temperature ° C. h ° C. % ° C. rolling % ° C. times ° C./s mm ° C. Ex. 1 1170 1.3 1040 70 950 89 880 5 60 6 390 Ex. 2 1160 1.4 1090 50 900 92 820 9 50 5 125 Ex. 3 1190 1.0 1100 65 930 90 900 4 55 3 280 Ex. 4 1130 1.7 960 55 910 94 820 7 70 4 250 Ex. 5 1150 1.5 1020 60 940 88 850 10 65 4 r.t. Ex. 6 1120 2.0 1000 75 920 93 810 6 80 2 180 Ex. 7 1140 1.8 980 80 930 90 920 0 75 3 230 Ex. 8 1180 1.2 1050 70 925 91 830 8 85 2 150

TABLE-US-00003 TABLE 3 Moving speed of strip steel during Pickling Tension Rinsing Drying pickling temperature leveling temperature temperature m/min ° C. rate % ° C. ° C. Ex. 1 90 76 1.2 40 128 Ex. 2 45 83 2.0 35 140 Ex. 3 60 75 0.6 48 133 Ex. 4 100 79 1.4 36 130 Ex. 5 40 82 1.8 45 125 Ex. 6 50 85 0.8 50 136 Ex. 7 30 78 1.6 42 120 Ex. 8 80 80 0.4 38 132

TABLE-US-00004 TABLE 4 Mechanical performances of steel plates Hole −40° C. Retained Yield Tensile expansion impact austenite strength strength Elongation ratio energy content MPa MPa % % J % Ex. 1 809 1002 11.5 106.5 168 1.86 Ex. 2 821 1034 12.5 90.9 170 2.49 Ex. 3 806 1104 11.0 95.1 154 4.58 Ex. 4 850 1011 10.5 82.8 144 2.64 Ex. 5 819 1033 11.0 86.9 158 0.66 Ex. 6 820 1032 12.0 87.2 182 4.33 Ex. 7 883 1039 11.0 83.5 162 4.02 Ex. 8 866 1050 10.5 100.7 180 3.56 Note: The impact energy is obtained by converting the measured impact energy of a sample having an actual thickness into the impact energy of a standard sample of 10*10*55 mm in proportion based on equivalent effect.