High-formability and super-strength hot galvanizing steel plate and manufacturing method thereof

Abstract

A high-formability, super-high-strength, hot-dip galvanized steel plate, the chemical composition of which comprises, based on weight percentage, C: 0.15-0.25 wt %, Si: 1.00-2.00 wt %, Mn: 1.50-3.00 wt %, P0.015 wt %, S0.012 wt %, Al: 0.03-0.06 wt %, N0.008 wt %, and the balance of iron and unavoidable impurities. The room temperature structure of the steel plate comprises 10-30% ferrite, 60-80% martensite and 5-15% residual austenite. The steel plate has a yield strength of 600-900 MPa, a tensile strength of 980-1200 MPa, and an elongation of 15-22%. Through an appropriate composition design, a super-high-strength, cold rolled, hot-dip galvanized steel plate is manufactured by continuous annealing, wherein no expensive alloy elements are added; instead, remarkable increase of strength along with good plasticity can be realized just by appropriate augment of Si, Mn contents in combination with suitable processes of annealing and furnace atmosphere control. In addition, the steel plate possesses good galvanization quality that meets the requirement of a super-high-strength, cold rolled, hot-dip galvanized steel plate for automobiles.

Claims

1. A high-formability, ultra-high-strength, hot-dip galvanized steel plate, consisting of: a) 0.150.25 wt % carbon (C) b) 1.002.00 wt % silicon (Si) c) 1.503.00 wt % manganese (Mn) d) <0.015 wt % phosphorus (P) e) <0.012 wt % sulfur (S) f) 0.030.06 wt % aluminum (Al) g) <0.008 wt% nitrogen (N), and h) a balance of iron (Fe) and unavoidable impurities; wherein the steel plate structure at room temperature consists of 1030% ferrite, 6080% martensite, and 515% residual austenite; and wherein the steel plate exhibits a yield strength of 600900 MPa, a tensile strength of 9801200 MPa, and an elongation of 1522%.

2. The high-formability, ultra-high-strength, hot-dip galvanized steel plate of claim 1, wherein carbon is present in an amount ranging from 0.180.22 wt %.

3. The high-formability, ultra-high-strength, hot-dip galvanized steel plate of claim 1, wherein silicon is present in an amount ranging from 1.41.8 wt %.

4. The high-formability, ultra-high-strength, hot-dip galvanized steel plate of claim 1, wherein manganese is present in an amount ranging from 1.82.3 wt %.

5. The high-formability, ultra-high-strength, hot-dip galvanized steel plate of claim 1, wherein phosphorus is present in an amount <0.012 wt % and sulfur is present in an amount <0.008 wt %.

6. A method for manufacturing the high-formability, ultra-high-strength, hot-dip galvanized steel plate of claim 1, comprising the following steps: a) smelting the raw materials according to the composition of the high-formability, ultra-high-strength, hot-dip galvanized steel plate of claim 1; b) casting the raw materials of step a) into a plate blank; c) heating the plate blank of step b) to 11701230 C.; d) hot rolling the plate blank of step c) at an end rolling temperature of 88030 C., and a coiling temperature of 550650 C.; e) acid pickling the steel of step d); f) cold rolling the acid pickled steel of step e) to a reduction rate of 40-60%, wherein a steel strip is formed; g) annealing the steel strip of step f) by (1) to (5), wherein the annealing is performed in a continuous mode using a two-stage heating procedure comprising a direct flame heating in an oxidative atmosphere and an irradiation heating in a reducing atmosphere, (1) direct flame heating the steel strip to 680750 C. in an oxidative atmosphere, wherein the dew point in the continuous annealing furnace is controlled at a value higher than 35 C., and the heating time is 10-30 s; (2) heating the steel strip to 840920 C. by irradiation in a reducing atmosphere and holding at this temperature for 48-80 s while the H content in the continuous annealing furnace being controlled at 815%; (3) cooling the steel strip at a cooling rate of 310 C/s to 720800 C. so that a proportion of ferrite is generated in the material; (4) cooling the steel strip to 260360 C. at a cooling rate >50 C/s so that part of austenite is converted into martensite; (5) reheating the steel strip to 460470 C. and holding at this temperature for 60-120 s; h) feeding the steel strip of step g) into a zinc pot to complete hot-dip galvanization, wherein carbon is distributed from martensite into austenite to make austenite rich in carbon and stabilized during the above course of reheating, holding and galvanization; and i) cooling the steel strip to room temperature, wherein, at room temperature, the structure of the steel plate consists of 10-30% ferrite, 60-80% martensite, and 5-15% residual austenite and the steel plate exhibits a yield strength of 600900 MPa, a tensile strength of 9801200 MPa, and an elongation of 1522%.

7. The method of claim 6, wherein the plate blank of step c) is heated to 11701200 C.

8. The method of claim 6, wherein the coiling temperature of step d) is 550600 C.

9. The method of claim 6, wherein the steel strip is heated to 680720 C. by direct flame in oxidative atmosphere in (1).

10. The method of claim 6, wherein in (1), the dew point in the furnace is controlled at 30 to 20 C. during the direct flame heating in oxidative atmosphere.

11. The method of claim 6, wherein the steel strip is further heated to 860890 C. by irradiation in reducing atmosphere in (2).

12. The method of claim 6, wherein in (2), the hydrogen (H) content in the continuous annealing furnace is controlled at 1015% during the irradiation heating in reducing atmosphere.

13. The method of claim 6, wherein in (4), the steel strip is cooled to 280320 C.

14. The method of claim 6, wherein after cooling in (4), the steel strip is reheated to 460465 C. and held for 80110 s in (5).

15. The method of claim 6, wherein the steel strip is cooled to 730760 C. in (3).

Description

DESCRIPTION OF DRAWING

(1) FIG. 1 is a photo showing the exemplary steel according to the invention.

(2) FIG. 2 is a photo showing the steel of the comparative example.

DETAILED DESCRIPTION OF THE INVENTION

(3) The invention will be further illustrated with reference to the following examples and the accompanying drawings.

(4) Table 1 lists the chemical compositions of the examples of the steel according to the invention.

(5) After smelting, hot rolling, cold rolling, annealing and galvanization, there were obtained steel products of the invention, the annealing process for and the mechanical properties of which are shown in Table 2. As indicated by Table 2, a super-high-strength, cold-rolled, hot-dip galvanized steel plate having a yield strength of 600900 MPa, a tensile strength of 9801200 MPa, and an elongation of 1522% has been obtained according to the invention by suitable coordination of process.

(6) C: 0.150.25 wt %, Si: 1.002.00 wt %, Mn: 1.503.00 wt %, P0.015 wt %, S0.012 wt %, Al: 0.030.06 wt %, N0.008 wt %;

(7) C content: 0.180.22%, Si content: 1.41.8%, Mn content: 1.82.3%, P0.012%, S0.008%.

(8) TABLE-US-00001 TABLE 1 Chemical composition of the inventive steel, wt % C Si Mn Cr Mo Nb Ti V P S Al N Example 1 0.22 1.8 2.0 0.006 0.010 0.040 0.0043 Example 2 0.16 2.0 1.8 0.012 0.007 0.050 0.0053 Example 3 0.20 1.4 3.3 0.009 0.006 0.030 0.0058 Example 4 0.18 1.6 2.3 0.008 0.008 0.050 0.0072 Example 5 0.25 1.0 1.5 0.010 0.007 0.060 0.0064 Example 6 0.15 1.2 3.0 0.015 0.012 0.050 0.0068 Comp. Ex. 1 0.084 1.51 1.41 0.009 0.0014 0.031 0.0031 Comp. Ex. 2 0.120 0.05 1.90 0.4 0.3 0.030 0.016 0.010 0.004 0.020 0.004 Comp. Ex. 3 0.095 0.33 2.56 0.112 0.123 0.019 0.002 0.063 0.0049

(9) TABLE-US-00002 TABLE 2 Process and mechanical properties of the Examples Annealing process Heating Initial Dew H temperature Heating Heating temperature point at content at time at temperature Heating Slow for direct at direct direct at time at cooling rapid Process flame irradiation flame flame irradiation irradiation speed cooling number stage C. stage % stage C. stage s stage C. stage s C./s C. Example 1 i 30 8 739 30 842 50 4 712 ii 27 8 692 25 886 50 6 735 iii 31 9 681 20 890 60 8 757 Example 2 i 30 10 745 20 857 70 3 733 ii 33 10 742 20 902 70 6 738 iii 31 9 698 20 868 80 9 732 Example 3 i 35 7 685 10 865 40 10 786 ii 32 8 712 20 920 50 8 732 iii 25 9 734 20 890 60 6 720 Example 4 i 30 9 721 20 869 60 8 765 ii 42 11 744 10 882 40 10 753 iii 31 10 725 30 867 80 8 738 Example 5 i 35 13 706 20 878 60 5 733 ii 36 15 748 20 904 60 5 754 iii 32 10 741 20 916 60 5 761 Example 6 i 31 13 718 15 878 50 6 793 ii 20 15 729 20 889 50 6 754 iii 23 10 738 25 860 50 7 800 Comp. Ex. 1 850 Comp. Ex. 2 820 Comp. Ex. 3 838 Annealing process End temperature for Mechanical rapid Galvanization Galvanization properties Zinc Process cooling temperature holding YS TS TEL layer number C. C. time s (MPa) (MPa) (%) adhesion Example 1 i 351 469 110 679 968 21.1 OK ii 343 463 70 710 985 19.3 OK iii 336 467 70 772 1058 18.1 OK Example 2 i 323 465 120 615 952 23.5 OK ii 306 468 60 676 967 20.2 OK iii 287 464 70 715 1065 18.3 OK Example 3 i 283 458 80 812 1143 16.9 NG ii 286 452 100 783 1195 17.2 OK iii 331 454 90 705 1157 17.0 OK Example 4 i 280 456 70 822 1150 16.8 OK ii 289 462 80 825 1157 17.3 NG iii 260 466 100 776 1101 18.0 OK Example 5 i 360 470 60 701 988 20.3 OK ii 354 468 90 805 1012 18.1 OK iii 316 456 80 887 1098 18.6 OK Example 6 i 305 460 60 845 1112 18.1 OK ii 320 457 100 683 983 23.3 OK iii 306 462 90 887 1158 17.1 OK Comp. Ex. 1 520 490 50 635 34.9 OK Comp. Ex. 2 460 10 598 1022 9.5 OK Comp. Ex. 3 520 463 43 659 1001 18.1 OK Note: Tension test method: JIS5 tension samples were used, and the tension direction was perpendicular to the rolling direction. Test method for zinc layer adhesion: A sample plate sized 300 70 mm was cut from a steel plate, and cold bended to 180 on a bending machine with the bend diameter three times the plate thickness. Then, a transparent tape was adhered to the outside of the cleaned bend angle, and the tape was torn off to observe if any peeling was transferred onto the tape. If no peeling was found, the zinc layer adhesion was given a passing grade (OK); otherwise, a non-passing grade (NG) would be given. Turn to FIGS. 1 and 2 which compare the galvanization effect between the inventive steel (using the furnace atmosphere control process according to the invention) and the Comparative Example (not using the furnace atmosphere control process according to the invention). It is demonstrated that the high Si composition of the invention results in good galvanization quality when the furnace atmosphere control process is used.