120-KG-GRADE ULTRAHIGH-STRENGTH GALVANIZED STEEL SHEET AND MANUFACTURING METHOD THEREFOR

Abstract

Provided in the present invention is a 120-kg-grade ultrahigh-strength galvanized steel sheet with a good resistance spot welding performance. The galvanized steel sheet comprises: 0.18-0.24% of C, 2.3-3.0% of Mn, 0.5-1.7% of Si, 0.02-1.0% of Al, 0.55<Si+Al1.75%, C+Si/30+Mn/200.395%, and at least one selected from Nb, Ti, B, Cr, Mo and REM, with the balance being Fe and inevitable impurities, wherein the thickness of the steel sheet is set as t, and the resistivity of the steel sheet is 0<R.sub.155 .Math.cm; in the direction starting from an interface between a plating and a steel sheet substrate and towards the steel sheet substrate, the resistivity of the steel sheet within the range of 0.025 t or more to no more than 0.05 t is 0<R.sub.215 .Math.cm, and the resistivity of the material within the range of 0.01 t or more to no more than 0.015 t is 0<R.sub.335 .Math.cm; and the resistivities satisfy: 1.5R.sub.1.sup.1/2-0.1R.sub.2-0.25R.sub.3>0.

Claims

1. A 120 Kg grade ultra-high strength galvanized steel plate, which comprises the following chemical elements: $C:0.18-0.24\%,$ $\mathrm{Mn}:2.3-3.\%,$ $\mathrm{Si}:0.5-1.7\%,$ $\mathrm{Al}:0.02-1.\%,$ $0.55<\mathrm{Si}+\mathrm{Al}1.75\%,$ $C+\mathrm{Si}/30+\mathrm{Mn}/200.395\%,$ at least one of Nb, Ti, B, Cr, Mo and REM, and a balance of Al and unavoidable impurities, wherein the thickness of the steel plate is set to t, the resistivity R.sub.1 of the steel plate is 0<R.sub.155 .Math.cm; in the direction from the interface between the plating layer and the steel plate matrix to the steel plate matrix, the resistivity R.sub.2 of the steel plate in the range of 0.025 t to 0.05 t is 0<R.sub.215 .Math.cm, and the resistivity R.sub.3 of the steel plate in the range of 0.01 t to 0.015 t is 0<R.sub.335 .Math.cm, and they satisfy 1.5R.sub.1.sup.1/2-0.1R.sub.2-0.25R.sub.3>0.

2. A 120 Kg grade ultra-high strength galvanized steel plate, which comprises the following chemical elements in addition to Fe and other unavoidable impurities: $C:0.18-0.24\%,$ $\mathrm{Mn}:2.3-3.\%,$ $\mathrm{Si}:0.5-1.7\%,$ $\mathrm{Al}:0.02-1.\%,$ $0.55<\mathrm{Si}+\mathrm{Al}1.75\%,$ $C+\mathrm{Si}/30+\mathrm{Mn}/200.395\%,$ at least one of Nb, Ti, B, Cr, Mo and REM, wherein, the thickness of the steel plate is set to t, the resistivity R.sub.1 of the steel plate is 0<R.sub.155 .Math.cm; in the direction from the interface between the plating layer and the steel plate matrix to the steel plate matrix, the resistivity R.sub.2 of the steel plate in the range of 0.025 t to 0.05 t is 0<R.sub.215 .Math.cm, and the resistivity R.sub.3 of the material in the range of 0.01 t to 0.015 t is 0<R.sub.335 .Math.cm, and they satisfy 1.5R.sub.1.sup.1/2-0.1R.sub.2-0.25R.sub.3>0.

3. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 1, wherein the contents of Nb, Ti, B, Cr, Mo and REM are as follows: $0\mathrm{Nb}0.1\%;$ $0\mathrm{Ti}0.1\%;$ $0B0.003\%;$ $0\mathrm{Cr}0.1\%;$ $0\mathrm{Mo}0.1\%;$ $0\mathrm{REM}<0.05\%.$

4. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 3, wherein, if present, the contents of Nb, Ti, B, Cr, Mo and REM are as follows: $\mathrm{Nb}:0.08-0.1\%;$ $\mathrm{Ti}:0.01-0.02\%;$ $B:0.0004-0.0023\%;$ $\mathrm{Cr}:0.05-0.1\%;$ $\mathrm{Mo}:0.02-0.1\%;$ $\mathrm{REM}:0.0035-0.05\%.$

5. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 1, wherein it comprises, by mass percentage, among other unavoidable impurities: $P<0.015\%,$ $S0.01\%,$ $N0.01\%.$

6. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 1, wherein when Type III specimens according to ISO 6892-1 standard are stretched at room temperature perpendicularly to the rolling direction, the steel plate has a tensile strength of 1180 MPa, a yield strength of 800 MPa, an elongation at break of 14%, and a hole expansion ratio of 30%.

7. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 1, wherein the thickness of the steel plate is set to t, the resistivity R.sub.1 of the steel plate is 41-55 .Math.cm; in the direction from the interface between the plating layer and the steel plate matrix to the steel plate matrix, the resistivity R.sub.2 of the steel plate in the range of 0.025 t to 0.05 t is 11-15 .Math.cm, and the resistivity R.sub.3 of the material in the range of 0.01 t to 0.015 t is 24-35 .Math.cm, and they satisfy 1.5R.sub.1.sup.1/2-0.1R.sub.2-0.25R.sub.3>0.

8. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 1, wherein when the welding current is 1.5*I.sub.splash, Type B or Type C LME cracks will not appear, and if Type D cracks appear, the length of Type D cracks is less than 10% of the thickness of the base metal; when the welding current is <I.sub.splash, Type B, Type C or Type D cracks will not appear, and if Type A cracks occur, the length is less than 5% of the thickness of the base metal, wherein I.sub.splash is the minimum current at which splashing occurs.

9. A manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 1, which comprises the following steps: (1) Smelting and continuous casting to obtain a cast billet that satisfies the composition of the steel plate according to claim 1; (2) Hot rolling: the cast billet obtained in step (1) is heated, subjected to final rolling, laminar flow cooling and coiling to obtain a hot-rolled coil; (3) Pickling and cold rolling: the hot-rolled coil obtained in step (2) is pickled, cold-rolled to obtain a cold rolled coil without annealing; (4) Continuous annealing: the cold rolled coil without annealing obtained in step (3) was subjected to a multi-stage heat treatment to obtain a strip steel; (5) Galvanizing: the strip steel obtained in step (4) enters the zinc pot at a temperature of (T.sub.ZP15 C.) for galvanizing to obtain a galvanized steel plate.

10. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 9, wherein the multi-stage heat treatment comprises the following (a)(d): (a) First stage annealing: the cold rolled coil without annealing is heated to a first stage temperature in the range of not less than 600 C.(A.sub.c1+40 C.) to obtain a steel coil, (b) Second stage annealing: the steel coil obtained in (a) is further heated to a second stage temperature in the range of (A.sub.c1+50 C.)(A.sub.c3+80 C.) or (A.sub.c1+50 C.)900 C., and held for 30300 s to obtain a strip steel, wherein the smaller value from (A.sub.c3+80 C.) and 900 C. is taken as the upper limit of the second stage temperature range, (c) Third stage annealing: the strip steel obtained in (b) is cooled to a third stage temperature in the range of M.sub.sM.sub.f at a cooling rate that is no less than a certain cooling rate V.sub.2-3 and held for 10120 s, (d) Fourth stage annealing: the strip steel obtained in (c) is heated again to a fourth stage temperature in the range of 350 C.T.sub.ZP and held for 1590 s, wherein A.sub.c1 is the temperature at which the pearlite transforms into austenite when heated, A.sub.c3 is the final temperature of transforming into austenite when heated, M.sub.s is the temperature at which martensite appears, and M.sub.f is the temperature of full martensitization, V.sub.2-3 represents the cooling rate, which is not less than 50 C./s

11. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 10, wherein in (a), the heating temperature is 680-720 C.; in (b), the heating temperature is 830-900 C., and the soaking time is 30-145 s; in (c), the third stage temperature is 220-310 C., and the holding time is 25-110 s; in (d), the heating temperature is 355-420 C., and the holding time is 20-86 s.

12. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 10, wherein, in step (4), the atmosphere in (a) contains 0.010.5% of O.sub.2 by volume, with a balance of N.sub.2 and unavoidable impurities; the atmosphere in (b) contains at least 1.5% by volume of H.sub.2, 0.2% or less by volume of water vapor, with a balance of N.sub.2 and unavoidable impurities, and a dew point of 2510 C.

13. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 9, wherein, in step (2), the heating temperature is in the range of 11501300 C., the temperature of the final rolling is in the range of A.sub.c31000 C., the holding temperature of laminar flow cooling is in the range of (A.sub.c145 C.), and the laminar flow cooling residence time is 530 s, and then the steel is cooled to 550650 C. and coiled, and the coiled steel coil is thermally insulated in the range of (a coiling temperature of T.sub.C30 C.) for 30300 min; and/or in step (5) when the galvanized steel plate is a hot-dip galvanized steel plate with a zinc plating layer, the steel plate with a zinc plating layer is cooled to room temperature after it is discharged from the zinc pot; when the galvanized steel plate is a hot-dip galvanized steel plate with a zinc iron alloy plating layer, the hot-dip galvanized steel plate with a zinc iron alloy plating layer is thermally insulated in the range of (zinc pot temperature T.sub.ZP20 C.)(zinc pot temperature T.sub.ZP+35 C.) for 560 s for alloying after it is discharged from the zinc pot, and then cooled to room temperature.

14. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 13, wherein, in step (2), the heating temperature is in the range of 11651270 C., the temperature of the final rolling is in the range of 885945 C., the holding temperature of laminar flow cooling is in the range of 680-720 C., and the laminar flow cooling residence time is 7-26 s; the coiling temperature is 550-645 C., the holding time after coiling is 45-270 min.

15. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 13, wherein, in step (5) the T.sub.ZP is 458-461 C.

16. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 2, wherein the contents of Nb, Ti, B, Cr, Mo and REM are as follows: 0Nb0.1%, 0Ti0.1%, 0B0.003%, 0Cr0.1%, 0Mo0.1%, and 0REM0.05%; or if present, the contents of Nb, Ti, B, Cr, Mo and REM are as follows: Nb: 0.08-0.1%, Ti: 0.01-0.02%, B: 0.0004-0.0023%, Cr: 0.05-0.1%, Mo: 0.02-0.1%, REM: 0.0035-0.05%; or the steel comprises, by mass percentage, among other unavoidable impurities: P0.015%, S0.010%, N0.010%.

17. The 120 Kg grade ultra-high strength galvanized steel plate according to claim 2, wherein: when Type III specimens according to ISO 6892-1 standard are stretched at room temperature perpendicularly to the rolling direction, the steel plate has a tensile strength of 1180 MPa, a yield strength of 800 MPa, an elongation at break of 14%, and a hole expansion ratio of 30%; and/or the thickness of the steel plate is set to t, the resistivity R.sub.1 of the steel plate is 41-55 .Math.cm; in the direction from the interface between the plating layer and the steel plate matrix to the steel plate matrix, the resistivity R.sub.2 of the steel plate in the range of 0.025 t to 0.05 t is 11-15 .Math.cm, and the resistivity R.sub.3 of the material in the range of 0.01 t to 0.015 t is 24-35 .Math.cm, and they satisfy 1.5R.sub.1.sup.1/2-0.1R.sub.2-0.25R.sub.3>0; and/or when the welding current is 1.5*I.sub.splash, Type B or Type C LME cracks will not appear, and if Type D cracks appear, the length of Type D cracks is less than 10% of the thickness of the base metal; when the welding current is <I.sub.splash, Type B, Type C or Type D cracks will not appear, and if Type A cracks occur, the length is less than 5% of the thickness of the base metal, wherein I.sub.splash is the minimum current at which splashing occurs.

18. A manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 2, which comprises the following steps: (1) Smelting and continuous casting to obtain a cast billet that satisfies the composition of the steel plate according to claim 2; (2) Hot rolling: the cast billet obtained in step (1) is heated, subjected to final rolling, laminar flow cooling and coiling to obtain a hot-rolled coil; (3) Pickling and cold rolling: the hot-rolled coil obtained in step (2) is pickled, cold-rolled to obtain a cold rolled coil without annealing; (4) Continuous annealing: the cold rolled coil without annealing obtained in step (3) was subjected to a multi-stage heat treatment to obtain a strip steel; (5) Galvanizing: the strip steel obtained in step (4) enters the zinc pot at a temperature of (T.sub.ZP15 C.) for galvanizing to obtain a galvanized steel plate.

19. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 18, wherein the multi-stage heat treatment comprises the following (a)(d): (a) First stage annealing: the cold rolled coil without annealing is heated to a first stage temperature in the range of not less than 600 C.(A.sub.c1+40 C.) to obtain a steel coil, (b) Second stage annealing: the steel coil obtained in (a) is further heated to a second stage temperature in the range of (A.sub.c1+50 C.)(A.sub.c3+80 C.) or (A.sub.c1+50 C.)900 C., and held for 30300 s to obtain a strip steel, wherein the smaller value from (A.sub.c3+80 C.) and 900 C. is taken as the upper limit of the second stage temperature range, (c) Third stage annealing: the strip steel obtained in (b) is cooled to a third stage temperature in the range of M.sub.sM.sub.f at a cooling rate that is no less than a certain cooling rate V.sub.2-3 and held for 10120 s, (d) Fourth stage annealing: the strip steel obtained in (c) is heated again to a fourth stage temperature in the range of 350 C.T.sub.ZP and held for 1590 s, wherein A.sub.c1 is the temperature at which the pearlite transforms into austenite when heated, A.sub.c3 is the final temperature of transforming into austenite when heated, M.sub.s is the temperature at which martensite appears, and M.sub.f is the temperature of full martensitization, V.sub.2-3 represents the cooling rate, which is not less than 50 C./s

20. The manufacturing method for the 120 Kg grade ultra-high strength galvanized steel plate according to claim 18, wherein: in step (4), the atmosphere in (a) contains 0.010.5% of O.sub.2 by volume, with a balance of N.sub.2 and unavoidable impurities; the atmosphere in (b) contains at least 1.5% by volume of H.sub.2, 0.2% or less by volume of water vapor, with a balance of N.sub.2 and unavoidable impurities, and a dew point of 2510 C.; and/or in step (2), the heating temperature is in the range of 11501300 C., the temperature of the final rolling is in the range of A.sub.c31000 C., and the holding temperature of laminar flow cooling is in the range of (A.sub.c145 C.), and the laminar flow cooling residence time is 530 s, and then the steel is cooled to 550650 C. and coiled, and the coiled steel coil is thermally insulated in the range of (a coiling temperature of T.sub.C30 C.) for 30300 min; and/or in step (5), when the galvanized steel plate is a hot-dip galvanized steel plate with a zinc plating layer, the steel plate with a zinc plating layer is cooled to room temperature after it is discharged from the zinc pot; when the galvanized steel plate is a hot-dip galvanized steel plate with a zinc iron alloy plating layer, the hot-dip galvanized steel plate with a zinc iron alloy plating layer is thermally insulated in the range of (zinc pot temperature T.sub.ZP20 C.)(zinc pot temperature T.sub.ZP+35 C.) for 560 s for alloying after it is discharged from the zinc pot, and then cooled to room temperature.

Description

DESCRIPTION OF THE DRAWINGS

[0070] FIG. 1 schematically shows the relative positions of the four kinds of resistance spot welding LME cracks of the present disclosure.

[0071] FIG. 2 illustratively shows the change of microstructure of the steel plate (not including the zinc layer) of the present disclosure in the thickness direction.

DETAILED DESCRIPTION

[0072] Specific embodiments will be further explained and illustrated below with reference to the specific examples. However, such explanation and illustration do not constitute any improper limitation on the technical solution of the present disclosure.

[0073] In the present disclosure, galvanizing refers to hot-dip zinc galvanizing or hot-dip zinc iron alloy galvanizing.

[0074] In the present disclosure, A.sub.c1 refers to the temperature at which the pearlite transforms into austenite when heated, and the unit is C.

[0075] In the present disclosure, A.sub.c3 refers to the final temperature of transforming into austenite when heated, and the unit is C.

[0076] In the present disclosure, T.sub.C refers to the coiling temperature in step (2), and the unit is C.

[0077] In the present disclosure, M.sub.s is the temperature at which martensite appears, and M.sub.f is the temperature of full martensitization, and the unit is C.

[0078] In the present disclosure, T.sub.ZP represents the zinc pot temperature, and the unit is C.

[0079] In the present disclosure, V.sub.2-3 represents the cooling rate.

Examples 1-12 and Comparative Examples 1-2

[0080] Table 1 lists the mass percentages of the various chemical elements of the ultra-high-strength galvanized steel plates of Examples 1-12 and the galvanized steel plates of Comparative Examples 1-2.

TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and other unavoidable impurities except P, S and N) Steel Chemical elements grade C Si Mn Al Nb Ti Cr Mo B REM P S N Sb Sn A.sub.c1 A.sub.c3 A 0.180 0.88 2.92 0.85 0.0004 0.0009 0.0008 0.004 698 888 B 0.188 0.95 2.73 0.71 0.01 0.0011 0.0006 0.003 702 878 C 0.197 1.67 2.58 0.023 0.05 0.02 0.0014 0.0004 0.005 707 822 D 0.209 1.13 2.55 0.53 0.02 0.0010 0.0012 0.0007 0.004 708 863 E 0.222 0.75 2.48 0.60 0.0023 0.001 0.001 0.007 706 855 F 0.235 0.58 2.37 0.86 0.08 0.0035 0.0012 0.0009 0.003 710 881 CEx. 1 0.204 1.542 2.32 0.0017 0 0.018 0 0 0.0007 0 0.012 708 823 CEx. 2 0.184 1.121 2.234 0.0012 0.013 0.027 0.14 0.012 0.0005 0.014 0 704 815

[0081] The ultra-high-strength galvanized steel plates of Examples 1-12 were prepared by the following steps: [0082] (1) Smelting and continuous casting to obtain a cast billet that satisfies the composition of the present disclosure; [0083] (2) Hot rolling: the cast billet obtained in step (1) was heated in the range of 11501300 C., subjected to final rolling in the range of A.sub.c31000 C.; the strip steel after final rolling was cooled to the range of (A.sub.c145 C.), and the laminar flow cooling residence time was 530 s, then cooled to 550650 C. and coiled with a coiling temperature of T.sub.C, and the coiled steel coil was kept in the range of (T.sub.C30 C.) for 30300 min to obtain the hot-rolled coil; [0084] (3) Pickling and cold rolling: the hot-rolled coil obtained in step (1) was pickled, cold-rolled to obtain a cold rolled coil without annealing; [0085] (4) Continuous annealing: the cold rolled coil without annealing obtained in step (3) was subjected to a multi-stage heat treatment, [0086] (a) First stage annealing: the cold rolled coil without annealing was heated to a first stage temperature in the range of not less than 600 C.(A.sub.c1+40 C.) to obtain a steel coil; [0087] (b) Second stage annealing: the steel coil obtained in (a) was further heated to a second stage temperature in the range of (A.sub.c1+50 C.)(A.sub.c3+80 C.) or (A.sub.c1+50 C.)900 C., and held for 30300 s to obtain a strip steel, [0088] (c) Third stage annealing: the strip steel obtained in (b) was cooled to a third stage temperature in the range of M.sub.sM.sub.f at a cooling rate that is no less than a certain cooling rate V.sub.2-3 and held for 10200 s; [0089] (d) Fourth stage annealing: the strip steel obtained in (c) was heated again to a fourth stage temperature in the range of (350 C.T.sub.ZP) and held for 1590 s, [0090] wherein, in step (4), the atmosphere in (a) contained 0.010.5% of O.sub.2 by volume, with a balance of N.sub.2 and unavoidable impurities; the atmosphere in (b) contained at least 1.5% by volume of H.sub.2 and 0.2% or less by volume of water vapor, with a balance of N.sub.2 and unavoidable impurities, and a dew point of 2015 C.; the smaller value from (A.sub.c380 C.) and 900 C. was taken as the upper limit of the second stage temperature range; in (c), V.sub.2-3 represented the cooling rate, which was not less than 50 C./s; in (d), T.sub.ZP was the temperature of the zinc pot; [0091] (5) Galvanizing: the strip steel obtained after the fourth stage annealing (d) in step (4) entered the zinc pot at a temperature of (T.sub.ZP15 C.) for galvanizing, to obtain a galvanized steel plate, wherein, when the galvanized steel plate was hot-dip galvanized steel plate with a zinc plating layer, the steel plate with a zinc plating layer was cooled to room temperature after it was discharged from the zinc pot, and when the galvanized steel plate was hot-dip galvanized steel plate with a zinc iron alloy plating layer, the hot-dip galvanized steel plate with a zinc iron alloy plating layer was thermally insulated in the range of (T.sub.ZP20 C.)(T.sub.ZP+35 C.) for alloying for 560 s, and then cooled to room temperature.

[0092] Tables 2-1 and 2-2 list the specific process parameters for the ultra-high-strength galvanized steel plates of Examples 1-12.

TABLE-US-00002 TABLE 2-1 Laminar flow Laminar Final cooling cooling Holding Heating rolling insulation residence Coiling time after No. Ingredient temperature// C. temperature/ C. temperature/ C. time/s temperature/ C. coiling/min Ex. 1 A 1165 880 680 26 550 270 Ex. 2 A 1190 885 680 25 550 240 Ex. 3 B 1200 897 690 18 570 210 Ex. 4 B 1210 900 690 20 570 180 Ex. 5 C 1260 935 700 15 590 180 Ex. 6 C 1270 945 703 13 590 150 Ex. 7 D 1250 930 715 10 600 150 Ex. 8 D 1250 935 720 7 600 120 Ex. 9 E 1220 910 708 10 625 120 Ex. 10 E 1240 920 722 8 625 90 Ex. 11 F 1260 940 725 9 645 90 Ex. 12 F 1250 930 717 12 645 45

TABLE-US-00003 TABLE 2-2 Pre- heating/ Rapid Re- Temper- heating soaking cooling heating ature temper- temper- temper- Rapid temper- Re- for Zinc ature ature ature cooling ature heating entering pot Alloying Alloying (first (second Soaking dew (third holding (fourth holding zinc temper- temper- temper- Ingre- Plating stage)/ stage)/ time/ point/ stage)/ time/ stage)/ time/ pot/ ature/ ature/ ature/ No. dient layer C. C. s C. C. s C. s C. C. C. s Ex. 1 A GA 680 830 30 25 220 25 355 86 455 458 490 8 Ex. 2 A GI 690 840 50 20 230 40 375 73 460 457 Ex. 3 B GI 690 840 60 17 245 35 395 60 460 460 Ex. 4 B GA 690 850 45 13 250 55 410 45 455 461 470 25 Ex. 5 C GA 700 870 35 5 270 70 370 75 460 463 465 40 Ex. 6 C GI 690 850 95 9 260 45 410 55 465 461 Ex. 7 D GI 680 840 300 0 240 60 385 62 460 460 Ex. 8 D GA 700 860 120 3 260 85 405 45 455 459 475 35 Ex. 9 E GI 690 855 145 8 250 70 410 40 460 460 Ex. 10 E GA 700 875 100 5 270 90 420 20 455 461 455 55 Ex. 11 F GI 710 885 85 2 290 102 405 27 455 460 Ex. 12 F GA 715 895 100 3 305 113 415 35 460 458 480 30 GI: the plating layer is pure zinc layer, GA: the plating layer is zinc-iron alloy.

[0093] The ultra-high-strength galvanized steel plates of Examples 1-12 and the galvanized steel plates of Comparative Examples 1-2 were tested for mechanical properties and resistance spot welding performance, and the obtained test results are listed in Table 3.

[0094] The following methods were used to determine the mechanical properties, resistivity and resistance spot welding performance:

[0095] Mechanical properties: the test sample was stretched and processed in accordance with the requirements of Type III specimen in the ISO 6892-1 standard perpendicularly to the rolling direction of the steel plate, and the tensile strength TS, the yield strength YS and elongation at break EL were determined by stretching the sample at room temperature. It should be emphasized that, due to the difference in measurement methods and the difference in the geometric dimensions of the specimen, there is a difference between the elongation at break measured according to the ISO 6892-1 standard and the elongation at break measured according to the JIS Z2241 standard,

[0096] GB/T 228.1 standard, and the differences in the measured value of the elongation at break due to the difference between the reference standards all belong to the protection scope of the ultra-high-strength galvanized steel plate of the present disclosure.

[0097] Hole expansion ratio: the sample was processed according to the ISO 16630 standard, and the hole expansion ratio was detected at room temperature according to the requirements of the standard to determine the hole expansion ratio HER. It should be emphasized that, due to the difference in measurement methods and the difference in the geometric size of the specimen, there is a difference between the hole expansion ratio measured according to the ISO 16630 standard and the hole expansion ratio measured according to the JFS T1001 standard, GB/T 15825.4 standard, and the differences in the measured value of the hole expansion ratio caused by the difference between the reference standards all belong to the protection range of the ultra-high strength galvanized steel plate of the present disclosure.

[0098] Resistivity: a steel plate having a certain area was taken and processed by processing methods including but not limited to grinding machine, wire-electrode cutting, milling machine, etc. The full thickness of the steel plate was taken as t, and the sample in the range of 0.010t to 0.035t and 0.01t to 0.015t was taken and detected for the resistivity R.sub.1, R.sub.2, R.sub.3 of the sample with a resistivity measuring equipment.

[0099] LME cracks: the welded joints were cut and polished, corroded and the cross sections of the welded joints were observed by a microscope (usually an optical microscope) at a certain magnification to determine the length of different types of cracks on the observed welded joints.

[0100] The ultra-high-strength galvanized steel plates of Examples 1-12 were tested for mechanical properties, resistivity and resistance spot welding, and the test results obtained are listed in Table 3, wherein I.sub.splash is the minimum current at which splashing occurs. The resistance spot welding performance tested for the galvanized steel plates of Comparative Examples 1-2 is shown in Table 4.

TABLE-US-00004 TABLE 3 Plating Thickness/ TS/ YS/ TEL/ HER/ R.sub.1/ R.sub.2/ R.sub.3/ Welding current I.sub.splash Type Type Type Type No. layer mm MPa MPa % % .Math. cm .Math. cm .Math. cm (kA) (kA) A/m B/m C/m D/m Ex. 1 GI 1.2 1205 893 18.5 33 41.2 14.4 24.5 I.sub.splash 10.5 68 N/D N/D 23 Ex. 2 GA 1.2 1230 932 17.1 33 41.5 14.5 24.8 0.95*I.sub.splash 10.3 N/D N/D N/D N/D Ex. 3 GA 1.2 1218 901 18.3 35 42.0 13.8 25.0 0.90*I.sub.splash 10.0 5.5 N/D N/D N/D Ex. 4 GI 1.2 1222 845 20.2 31 41.8 13.3 24.5 I.sub.splash + 9.8 69 N/D N/D 52 I.sub.splash*10% Ex. 5 GI 1.2 1238 992 16.8 38 47.1 13.8 28.5 0.80*I.sub.splash 9.5 57 N/D N/D N/D Ex. 6 GA 1.2 1197 967 17.5 42 47.5 14.1 28.8 I.sub.splash + 9.6 72 N/D N/D 111 I.sub.splash*15% Ex. 7 GA 1.2 1185 1003 16.5 47 45.2 12.3 27.1 I.sub.splash + 10.0 97 N/D N/D 35 I.sub.splash*5% Ex. 8 GI 1.2 1222 945 15.8 36 44.9 12.0 26.9 I.sub.splash + 10.0 108 N/D N/D 81 I.sub.splash*45% Ex. 9 GI 1.2 1210 1021 15.2 44 43.6 11.4 26.0 I.sub.splash + 10.7 41 N/D N/D N/D I.sub.splash*20% Ex. 10 GA 1.2 1254 1067 14.3 39 44.0 11.8 26.3 I.sub.splash + 11.2 N/D N/D N/D 44 I.sub.splash*35% Ex. 11 GA 1.2 1247 1030 15.5 37 44.2 11.5 26.2 I.sub.splash 11.4 N/D N/D N/D N/D Ex. 12 GI 1.2 1263 979 14.9 32 44.5 12.0 26.7 I.sub.splash 11.7 N/D N/D N/D N/D TS: tensile strength, YS: yield strength, TEL: elongation at break, HER: hole expansion ratio, N/D: Not detected.

[0101] The resistance spot welding performance test of the galvanized steel plate of Comparative Examples 1-2 was carried out, and the test results obtained are listed in Table 4.

TABLE-US-00005 TABLE 4 Plating Thickness/ Welding Type Type Type Type No layer mm current A/m B/m C/m D/m CEx. 1 GI 1.4 N/D 452 CEx. 2 GA 1.4 65 548

[0102] As can be seen from Table 1-3, the chemical elements of the steel grade used in Examples 1-12 of the present disclosure satisfy the requirement of the present disclosure. Therefore, the ultra-high-strength galvanized steel plate obtained in Examples 1-12 according to the method of the present disclosure has a tensile strength of 1180 MPa, a yield strength of 800 MPa, an elongation at break of 14%, a hole expansion ratio of 30%, and the resistivity satisfies the present disclosure. Therefore, when the steel plates of Examples 1-12 adopted the welding current of (I.sub.splash+I.sub.splash+50%), no Type B or Type C cracks were generated, and when the welding current was <I.sub.splash, Type A LME cracks did not appear, or when Type A LME cracks appeared, the length of Type A cracks was not greater than 5% of the thickness of the base metal plate. For example, in Examples 2 and 3, the lengths of Type A LME cracks were 0.5% of the thickness of the base metal, respectively. When the welding current was I.sub.splash, Type D LME cracks did not appear, or when Type D LME cracks appeared, the length of Type D cracks was not greater than 10% of the thickness of the base metal plate. For example, in Example 6, where the length of the Type D LME cracks was the longest in examples, the length of the Type D type cracks was 9.2% of the thickness of the base metal.

[0103] In contrast, Comparative Example 1 and Comparative Example 2 are existing steel plates, which were not produced by the manufacturing method of the steel plates of the present disclosure. Only the chemical elements Al and Sn in the steel grade of the steel plate of Comparative Example 1 did not satisfy the present disclosure. The chemical elements Mn, Al, Cr, Sb in the steel grade of Comparative Example 2 did not satisfy the present disclosure. When resistance spot welding was carried out, the Type C cracks that had a greater impact on the base metal was produced in the steel plate of Comparative Examples 1 and 2, and the Type C cracks that appeared in Comparative Examples 1 and 2 accounted for 32.3% and 39.1% of the thickness of the base metal respectively, and the cracks were larger. Compared with the steel in the examples of the present disclosure, the liquid metal embrittlement LME crack resistance and the resistance spot welding performance of the steel in the Comparative Example were obviously poor.

[0104] It can be seen that, the LME cracks of the ultra-high-strength galvanized steel plate obtained in Examples 1-12 has been suppressed by optimizing the chemical elements and controlling the manufacturing process, and the resistance spot welding performance is excellent.

[0105] Based on the above, it can be seen that through the reasonable design of chemical composition combined with the optimization process, the ultra-high-strength galvanized steel with excellent tensile strength, yield strength, elongation at break, and hole expansion ratio and having excellent resistance spot welding performance can be obtained in the present disclosure. In the welded joint combination in which one of at least two layers of steel plate is the steel plate according to the present disclosure, when the welding current is (I.sub.splash+I.sub.splash+50%), no Type B or Type C cracks will be generated, and when Type A cracks are generated, 1% or less of the total number of Type A cracks will appear when the welding current is <I.sub.splash, and the length of Type A cracks will not be greater than 5% of the thickness of the base metal plate; when Type D cracks are generated, 99.99% or more of the total number of Type D cracks will appear when the welding current is I.sub.splash, and the length of Type D cracks will not be greater than 10% of the thickness of the base metal plate. The 120 kg grade ultra-high-strength galvanized steel plate in the present disclosure has excellent resistance to liquid metal embrittlement LME cracks and resistance spot welding performance. It can realize stable production in batch and the comprehensive properties of the material make it able to meet the manufacture of body parts with complex shapes, and the material has excellent resistance spot welding performance and corrosion resistance, can be effectively applied to the manufacture of automobile body structure, meets the current development needs of automotive steel for automobile lightweight and safety, and has broad application prospects.

[0106] It should be noted that the prior art of the protection scope of the present disclosure is not limited to the examples given in the application documents, and all prior arts that do not conflict with the solution of the present disclosure, including but not limited to prior patent documents, prior publications, prior public use, etc., can be included in the protection scope of the present disclosure. In addition, combinations of the various technical features in this case are not limited to the combinations described in the claims of this case or the combinations described in the specific examples. All technical features recorded in this case can be combined freely or associated in any way unless a contradiction occurs.

[0107] It should also be noted that the examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above examples, and changes or modifications made thereto can be directly derived from the present disclosure or easily conceived of by those skilled in the art, all of which fall within the protection scope of the present disclosure.