780 MPA-GRADE ULTRA-HIGH REAMING STEEL HAVING HIGH SURFACE QUALITY AND HIGH PERFORMANCE STABILITY, AND MANUFACTURING METHOD THEREFOR

Abstract

A 780 MPa-grade ultra-high reaming steel having high surface quality and high performance stability, and a manufacturing method therefor. The ultra-high reaming steel comprises the following components in percentage by weight: 0.03-0.08% of C, Si≤0.2%, 0.5-2.0% of Mn, P≤0.02%, S≤0.003%, 0.01-0.08% of Al, N≤0.004%, 0.05-0.20% of Ti, 0.1-0.5% of Mo, Mg≤0.005%, O≤0.0030%, and the remainder being Fe and other inevitable impurities. The ultra-high reaming steel of the present invention achieves matching between good structure homogeneity and performance homogeneity and excellent strength, plasticity, and ultra-high reaming rate; the ultra-high reaming steel has yield strength greater than or equal to 750 MPa, tensile strength greater than or equal to 780 MPa, an elongation A50 greater than or equal to 15%, and a reaming rate greater than or equal to 70%; moreover, appearance of red iron scales on the surface of a steel plate can be avoided, thereby improving the surface quality of pickled high-strength steel; the ultra-high reaming steel can satisfy user requirements well, and can be applied to parts of passenger vehicle chassis components such as a control arm and an auxiliary frame, which require high strength and thinning.

Claims

1. A 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability, comprising the following components in weight percentages: C: 0.03-0.08%, Si: ≤0.2%, Mn: 0.5-2.0%, P: ≤0.02% S: ≤0.003%, Al: 0.01-0.08%, N: 0.004%, Ti: 0.05-0.20%, Mo: 0.1-0.5%, Mg: ≤0.005%, O: ≤0.0030%, a balance of Fe and other unavoidable impurities.

2. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, further comprising any one or more of Cu, Ni, Cr, Nb, V, B, and Ca.

3. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein C: 0.04-0.07%.

4. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein Si≤0.15%, S≤0.0015%, and/or N≤0.003%.

5. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein Mn: 1.0-1.6%.

6. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein Al: 0.02-0.05%.

7. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein Ti: 0.07-0.10%.

8. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein Mo: 0.20-0.40%.

9. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein the ultra-high-hole-expandability steel has a microstructure of bainite+nano-scale carbide, wherein the nano-scale carbides are precipitated in bainitic ferrite.

10. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, wherein the ultra-high-hole-expandability steel has a yield strength of ≥750 MPa, a tensile strength of ≥780 MPa, an elongation A.sub.50 of ≥15%, and a hole expansion ratio of ≥70%.

11. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 10, wherein the ultra-high-hole-expandability steel has a yield strength of ≥760 MPa, a tensile strength of ≥810 MPa, an elongation A.sub.50 of 5%, and a hole expansion ratio of ≥80%.

12. A method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 1, comprising the following steps: 1) Smelting, casting wherein the components according to any one of claims 1-8 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 rate is ≥20 ° C./h; a heating temperature is ≥1230° C.; and a holding time is 1-2 hours; 3) Hot rolling wherein an initial rolling temperature is 1050-1150° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of 1050° C. or higher with an accumulated deformation rate of ≥50% to obtain an intermediate blank; wherein the intermediate blank is held till 950-1000° C., and then subjected to final 3-7 passes of rolling with an accumulated deformation rate of ≥70% to obtain a steel plate, wherein a final rolling temperature is 850-950° C.; wherein the steel plate is cooled to 300° C. or lower for coiling; 4) Annealing wherein bell type annealing is performed, wherein a heating rate is ≥20° C./h; a bell type annealing temperature is 500-650° C.; and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to 300° C. or lower at a cooling rate of ≥50° C./h, taken out and coiled; 5) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-140 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤3%; wherein the strip steel is then subjected to rinsing, surface drying, and oiling.

13. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, 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.

14. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, wherein the heating rate in step 2) is 20-40° C./h, and the heating temperature is 1230-1300° C.

15. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, wherein in step 4), the heating rate is 20-40° C./h, and the cooling rate is 15-50° C./h.

16. The 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 2, wherein Cu, Ni and Cr, each has a content of ≤0.3%; Nb and V, each has a content of ≤0.03%; B has a content of ≤0.0005%; and Ca has a content of ≤0.002%.

17. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, wherein the 780 MPa-grade steel further comprises any one or any two or more of Cu, Ni, Cr, Nb, V, B, and Ca.

18. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, wherein the 780 MPa-grade steel further comprises C: 0.04-0.07%, Si≤0.15%, S≤0.0015%, N≤0.003%, Mn: 1.0-1.6%, Al: 0.02-0.05%, Ti: 0.07-0.10%., and Mo: 0.20-0.40%.

19. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, wherein the 780 MPa-grade steel has a microstructure of bainite+nano-scale carbide, wherein the nano-scale carbides are precipitated in bainitic ferrite.

20. The method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to claim 12, wherein the 780 MPa-grade steel has a yield strength of ≥750 MPa, a tensile strength of ≥780 MPa, an elongation A.sub.50 of ≥15%, and a hole expansion ratio of ≥70%.

Description

DESCRIPTION OF THE DRAWINGS

[0080] FIG. 1 is a process flow chart of the method for manufacturing a 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to the present disclosure.

[0081] FIG. 2 is a schematic view showing the rolling and cooling processes in the method for manufacturing a 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to the present disclosure.

[0082] FIG. 3 is a schematic view showing the bell type annealing process in the method for manufacturing a 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to the present disclosure.

[0083] FIG. 4 is a typical metallographical photo of the ultra-high-hole-expandability steel of Example 1 according to the disclosure.

[0084] FIG. 5 is a typical metallographical photo of the ultra-high-hole-expandability steel of Example 3 according to the disclosure.

[0085] FIG. 6 is a typical metallographical photo of the ultra-high-hole-expandability steel of Example 7 according to the disclosure.

DETAILED DESCRIPTION

[0086] The disclosure will be further illustrated with reference to the following Examples and accompanying drawings.

[0087] Referring to FIG. 1-FIG. 3, the method for manufacturing the 780 MPa-grade steel having high surface quality, high performance stability and ultra-high hole expandability according to the present disclosure comprises the following steps: [0088] 1) Smelting, casting [0089] wherein the above composition is subjected to smelting in a converter or electrical furnace, secondary refining in a vacuum furnace, and casting to form a cast blank or ingot; [0090] 2) Reheating of the cast blank or ingot, wherein a heating rate is ≥20 ° C./h; a heating temperature is ≥1230° C.; and a holding time is 1-2 hours, as shown by FIG. 2; [0091] 3) Hot rolling [0092] wherein an initial rolling temperature is 1050-1150° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of 1050° C. or higher with an accumulated deformation rate of ≥50% to obtain an intermediate blank; wherein the intermediate blank is held till 950-1000° C., and then subjected to final 3-7 passes of rolling with an accumulated deformation rate of ≥70% to obtain a steel plate, wherein a final rolling temperature is 850-950° C., as shown by FIG. 3; [0093] 4) Annealing [0094] wherein bell type annealing is performed, wherein a heating rate is ≥20° C./h; a bell type annealing temperature is 500-650° C.; and a bell type annealing time is 12-48 h; wherein the steel plate is cooled to 300° C. or lower at a cooling rate of ≤50° C./h, taken out and coiled, as shown by FIG. 4; [0095] 5) Pickling [0096] wherein a moving speed of the strip steel is adjusted within a range of 30-140 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤3%; 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.

[0097] The compositions of the Examples of the ultra-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 IS06892-2-2018; and the hole expansion ratio was tested in accordance with International Standard IS016630-2017.

[0098] As it can be seen from Table 4, the yield strength of the steel coil is ≥750 MPa, while the tensile strength is ≥800 MPa, the elongation A.sub.50 is usually in the range of 16-18%, and the hole expansion ratio satisfies ≥70%. As it can be seen from the above Examples, the 780 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 complex parts such as wheels that need hole flanging. Therefore, it has broad application prospects.

[0099] FIG. 4, FIG. 5 and FIG. 6 show the typical metallographic photographs of Examples 1, 3 and 7 according to the present disclosure respectively when they were directly cooled on-line to low temperature for coiling. As it can be seen from these figures, for a system having a composition designed according to the present disclosure, the structure obtained during low-temperature coiling is uniform and fine low-carbon bainite. The precipitation of dispersed, fine and uniform nano-scale carbides in the bainitic ferrite lath in the subsequent bell type annealing process promotes the strength and plasticity, improves the structure uniformity, and also improves the hole expandability.

TABLE-US-00001 TABLE 1 (unit: weight %) Ex. C Si Mn P S Al N Mo Ti O  1 0.06 0.09 1.25 0.014 0.0023 0.045 0.0034 0.2 0.1 0.001  2 0.032 0.18 1.98 0.01 0.0018 0.05 0.004 0.48 0.2 0.002  3 0.043 0.2 1.02 0.016 0.002 0.034 0.0028 0.38 0.18 0.0005  4 0.071 0.06 0.85 0.011 0.0015 0.079 0.0029 0.25 0.08 0.0021  5 0.031 0.12 1.86 0.02 0.0012 0.023 0.0023 0.1 0.19 0.0028  6 0.08 0.18 0.52 0.009 0.0009 0.062 0.0033 0.18 0.05 0.0017  7 0.058 0.15 1.43 0.012 0.0005 0.011 0.0027 0.15 0.15 0.0024  8 0.055 0.05 1.62 0.013 0.0028 0.038 0.0035 0.31 0.12 0.0029  9 0.047 0.08 1.25 0.016 0.001 0.049 0.0025 0.4 0.14 0.0026 10 0.066 0.1 1.77 0.015 0.0021 0.072 0.003 0.27 0.09 0.0023 Ex. Mg Cu Ni Cr Nb V B Ca  1 0.0015 / / / / / / 0.0015  2 / 0.2 0.2 / / / / /  3 0.0048 / 0.1 / / / 0.0008 0.001  4 0.0025 / / 0.5 / 0.05 / /  5 0.003 0.5 / / 0.06 / / /  6 0.0045 / / / 0.02 0.03 / 0.005  7 / / 0.5 0.1 / / 0.0004 /  8 0.004 0.1 / / 0.04 / / /  9 0.0022 / 0.3 0.3 / / / 0.002 10 / 0.3 / / / 0.01 0.0005 /

TABLE-US-00002 TABLE 2 Accu- Accu- mulated mulated defor- Inter- defor- Initial mation mediate mation Final Heating rolling rate during blank rate during rolling Water Steel Coiling Heating temper- Holding temper- rough temper- finishing temper- cooling plate temper- rate ature time ature rolling ature rolling ature rate thickness ature ° C./h ° C. h ° C. % ° C. % ° C. ° C./s mm ° C. Ex. 1 38 1270 1.5 1100 80 970 75 890 90 2 150 Ex. 2 25 1240 1.8 1060 50 965 90 860 50 6 300 Ex. 3 20 1290 1.1 1140 80 990 75 930 80 2 100 Ex. 4 28 1260 1.6 1080 55 960 91 880 30 5 250 Ex. 5 22 1230 2.0 1050 75 955 90 850 60 3 r.t. Ex. 6 30 1300 1.0 1150 60 1000 92 950 40 4 200 Ex. 7 40 1280 1.2 1120 80 980 75 910 100 2 50 Ex. 8 36 1250 1.7 1070 70 950 92 870 70 3 r.t. Ex. 9 32 1235 1.9 1065 67 975 93 920 35 2.6 180 Ex. 10 35 1275 1.4 1085 71 995 95 885 55 1.8 230

TABLE-US-00003 TABLE 3 Temper- ature of strip Moving Bell type steel speed annealing Bell type Cooling leaving of strip Pickling Tension Rinsing Drying Heating temper- annealing temper- annealing steel during temper- leveling temper- temper- rate ature time ature furnace pickling ature rate ature ature ° C./h ° C. h ° C./h ° C. m/min ° C. % ° C. ° C. Ex. 1 30 520 44 30 260 60 82 2.5 40 135 Ex. 2 23 610 24 40 200 45 76 1.0 35 120 Ex. 3 35 500 48 20 300 30 75 1.5 47 128 Ex. 4 20 590 30 45 r.t. 140 80 0.8 42 140 Ex. 5 32 650 12 20 250 40 77 1.2 50 133 Ex. 6 27 540 40 50 100 100 79 2.3 37 125 Ex. 7 25 570 36 25 150 70 81 0.6 41 134 Ex. 8 40 630 16 15 210 120 83 1.1 38 130 Ex. 9 37 580 32 35 170 80 78 3.0 44 122 Ex. 10 33 615 20 30 280 50 85 0.9 46 138

TABLE-US-00004 TABLE 4 Mechanical performances of steel plates Yield Tensile Hole expansion strength strength Elongation ratio MPa MPa % % Ex. 1 765 821 17 95 Ex. 2 771 820 16 89 Ex. 3 774 826 17 77 Ex. 4 780 815 18 101 Ex. 5 772 817 16 97 Ex. 6 775 819 17 92 Ex. 7 764 829 16 110 Ex. 8 770 823 17 86 Ex. 9 751 818 18 94 Ex. 10 756 821 19 99