HIGH-STRENGTH STEEL COMPOSITE GALVANIZED SHEET RESISTANT TO LME CRACKING DURING SPOT WELDING, AND PREPARATION METHOD THEREFOR

Abstract

Disclosed in the present invention are a high-strength steel composite galvanized sheet resistant to LME cracking during spot welding, and a preparation method therefor. The high-strength steel composite galvanized sheet comprises a high-strength steel body (1), low-carbon steel composite layers (2) and a galvanized layer (3), wherein two low-carbon steel composite layers (2) are respectively compounded and rolled on two surfaces of the high-strength steel body (1), the galvanized layer (3) is formed on the surface of at least one low-carbon steel composite layer (2), and a high-strength steel composite galvanized sheet is thus formed. In the present invention, the low-carbon steel composite layers are compounded and rolled on the surfaces of the high-strength steel body, such that liquid metal is prevented from permeating into a base material along a grain boundary during spot welding; therefore, the sensitivity of the high-strength steel composite galvanized sheet to LME during spot welding is reduced, LME cracking during spot welding is effectively avoided, the problem of LME cracking during spot welding is mitigated, the resistance spot-welding performance of the high-strength steel body is improved, and the mechanical properties of a spot-welded joint are obviously improved.

Claims

1. A composite high-strength galvanized steel plate having resistance to spot-welding LME cracks, which comprises a high-strength steel matrix (1), low carbon steel composite layers (2), and a galvanized layer (3); wherein two low carbon steel composite layers (2) are affixed on two surfaces of the high-strength steel matrix (1) by composite rolling, the galvanized layer (3) is formed on the surface of at least one of the low carbon steel composite layers (2), thereby forming the composite high-strength galvanized steel plate.

2. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 1, wherein the high-strength steel matrix (1) is a high-strength steel that is sensitive to spot-welding LME having a tensile strength of 780 MPa, preferably 980 MPa, or 1180 MPa.

3. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 2, wherein the high strength steel comprises, by mass percentage, C0.1%, Mn1.0%, and Si0.07%.

4. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 2, wherein the high strength steel comprises, by mass percentage, C: 0.10.3%, Si: 0.42.50%, Mn: 1.011.0% and Al: 02.0%.

5. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 2, wherein the high strength steel is one or more selected from the group consisting of QP steel, TRIP steel, DH steel, 7Mn steel, 10 Mn steel and MS steel.

6. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 1, wherein the low carbon steel composite layers (2) is a low carbon steel that is not sensitive to spot-welding LME, wherein the low carbon steel comprises C and Mn, by mass percentage, the C content is 0.1%, the Mn content is 1.1% or Mn0.7%, the tensile strength is 590 MPa or is in a range of 150590 MPa or 150340 MPa.

7. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 6, wherein the low carbon steel comprises, by mass percentage, C: 0.0010.1%, Si: 0.0010.50%, Mn: 0.10.1.1%, Nb: 00.02%, Ti: 00.025%, Ni: 00.025%, Cr: 00.05%, P: 0.05%, with a balance of Fe and unavoidable impurities; or the low carbon steel comprises, by mass percentage, C: 0.0010.08%, Si: 0.0010.05%, Mn: 0.10.7%, Nb: 00.02%, Ti: 00.025%, Ni: 00.025%, Cr: 00.05%, P: 0.05%, with a balance of Fe and unavoidable impurities.

8. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 6, wherein the low carbon steel is one or more selected from the group consisting of IF steel, aluminum killed steel, cold-rolled carbon structural steel, phosphorus high-strength steel, bake-hardening steel and low-alloy steel.

9. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 1, wherein the original slab thickness of the high-strength steel matrix (1) and the original slab thickness of the low carbon steel composite layer (2) satisfy the following formula: LA+MB+NC=T; wherein, A is the percentage of the original slab thickness of one of the low-carbon steel composite layers (2) to the thickness of the total assembled slab, C is the percentage of the original slab thickness of the other of the low-carbon steel composite layers (2) to the thickness of the total assembled slab, and B is the percentage of the original slab of the high-strength steel matrix (1) to the thickness of the total assembled slab, and A+B+C=100%; wherein the total assembled slab is the total thickness of the original slab comprising the high-strength steel matrix (1) and the two low carbon steel composite layers (2); wherein L is the tensile strength of one of the low carbon steel composite layers (2) after annealing, N is the tensile strength of the other of the low carbon steel composite layers (2) after annealing, M is the tensile strength of the high-strength steel matrix (1) after annealing; wherein T is the target tensile strength of the composite high-strength galvanized steel plate; wherein, the unit of the tensile strength is MPa, and the unit of the thickness is micron.

10. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 9, wherein A:B and C:B are in the range of 1:35.5-1:5.

11. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 1, wherein: in the composite high-strength galvanized steel plate, each low carbon steel composite layer (2) has a thickness of 10-200 m; and/or the galvanized layer (3) has a thickness of 4-26 m.

12. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 1, wherein the composite high-strength galvanized steel plate has resistance to spot-welding LME cracks as follows: there are no LME cracks in the joint before the occurrence of welding spatter, and after the occurrence of welding spatter, the length of the Type A LME cracks in the joint does not exceed 10% of the plate thickness and the number is less than 6, the length of the Type D LME cracks does not exceed 3% of the plate thickness, and the number is less than 3, there are no Type B or Type C LME cracks.

13. A method for preparing the composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 1, which comprises the following steps: Step 1: taking an original slab of a high-strength steel matrix (1) and original slabs of two low-carbon steel composite layers (2), affixing the original slabs of the two low-carbon steel composite layers (2) to two surfaces of the original slab of the high-strength steel matrix (1), and welding the original slab edges of the two low-carbon steel composite layers (2) with the original slab edge of the high-strength steel matrix (1) to form a total assembled slab; Step 2: subjecting the total assembled slab to primary rolling after heating through blooming process of thick plate to reduce the thickness of the total assembled slab, and then hot rolling, pickling, cold rolling and annealing, so that the original slab of the high-strength steel matrix and the original slabs of the two low-carbon steel composite layers are metallurgically combined to form a billet having a tensile strength of T; Step 3: Plating a galvanized layer on at least one side of the billet to form the composite high-strength galvanized steel plate.

14. The preparing method according to claim 13, wherein in step 1, before the original slab of the two low-carbon steel composite layers (2) and the original slab of the high-strength steel matrix (1) are stacked, the affixing surfaces of the high-strength steel matrix (1) and the two low-carbon steel composite layers (2) need to be polished and cleaned.

15. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 2, wherein the tensile strength of the high-strength steel is 7801500 MPa or 11801500 MPa.

16. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 3, wherein the high strength steel comprises, by mass percentage, C0.14%, and/or Mn1.5%, and/or Si0.4%; or the high strength steel comprises, by mass percentage, C: 0.140.60%, Mn: 1.516%, Si: 0.072.0%; or the high strength steel comprises, by mass percentage, C: 0.140.30%, Mn: 1.53.5%, Si: 0.42.0%.

17. The composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to claim 6, wherein by mass percentage, the low carbon steel comprises C: 0.0010.1% and Mn: 0.11.1%.

18. The preparing method according to claim 13, wherein: the high-strength steel matrix (1) is a high-strength steel that is sensitive to spot-welding LME having a tensile strength of 780 MPa, or 980 MPa, or 1180 MPa; and/or the high strength steel comprises, by mass percentage, C0.1%, Mn1.0% and Si0.07%, or the high strength steel comprises, by mass percentage, C: 0.140.60%, Mn: 1.516% and Si: 0.072.0%, or the high strength steel comprises, by mass percentage, C: 0.140.30%, Mn: 1.53.5% and Si: 0.42.0%, or the high strength steel comprises, by mass percentage, C: 0.10.3%, Si: 0.42.50%, Mn: 1.011.0% and Al: 02.0%; or the high strength steel is one or more selected from the group consisting of QP steel, TRIP steel, DH steel, 7Mn steel, 10 Mn steel and MS steel.

19. The preparing method according to claim 13, wherein the low carbon steel composite layers (2) is a low carbon steel that is not sensitive to spot-welding LME, wherein the low carbon steel comprises C and Mn, by mass percentage, the C content is 0.1%, the Mn content is 1.1%, the tensile strength is 590 MPa; and/or the low carbon steel comprises, by mass percentage, C: 0.0010.1%, Si: 0.0010.50%, Mn: 0.10.1.1%, Nb: 00.02%, Ti: 00.025%, Ni: 00.025%, Cr: 00.05%, P: 0.05%, with a balance of Fe and unavoidable impurities; or the low carbon steel comprises, by mass percentage, C: 0.0010.08%, Si: 0.0010.05%, Mn: 0.10.7%, Nb: 00.02%, Ti: 00.025%, Ni: 00.025%, Cr: 00.05%, P: 0.05%, with a balance of Fe and unavoidable impurities; or the low carbon steel is one or more selected from the group consisting of IF steel, aluminum killed steel, cold-rolled carbon structural steel, phosphorus high-strength steel, bake-hardening steel and low-alloy steel.

20. The preparing method according to claim 13, wherein: the original slab thickness of the high-strength steel matrix (1) and the original slab thickness of the low carbon steel composite layer (2) satisfy the following formula: LA+MB+NC=T; wherein, A is the percentage of the original slab thickness of one of the low-carbon steel composite layers (2) to the thickness of the total assembled slab, C is the percentage of the original slab thickness of the other of the low-carbon steel composite layers (2) to the thickness of the total assembled slab, and B is the percentage of the original slab of the high-strength steel matrix (1) to the thickness of the total assembled slab, and A+B+C=100%; wherein the total assembled slab is the total thickness of the original slab comprising the high-strength steel matrix (1) and the two low carbon steel composite layers (2); wherein L is the tensile strength of one of the low carbon steel composite layers (2) after annealing, N is the tensile strength of the other of the low carbon steel composite layers (2) after annealing, M is the tensile strength of the high-strength steel matrix (1) after annealing; wherein T is the target tensile strength of the composite high-strength galvanized steel plate; wherein, the unit of the tensile strength is MPa, and the unit of the thickness is micron.

Description

DESCRIPTION OF THE DRAWINGS

[0053] FIG. 1 is a cross-sectional view of a composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to the present disclosure.

[0054] FIG. 2 is the spot-welding LME evaluation results (crack length) of the composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to the present disclosure in Example 1.

[0055] FIG. 3 is the spot-welding LME evaluation results (crack number) of the composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to the present disclosure in Example 1.

[0056] FIG. 4 is the spot-welding LME evaluation results (crack length) of the control example in Example 1.

[0057] FIG. 5 is the spot-welding joint hardness distribution diagram of the composite high-strength galvanized steel plate having resistance to spot-welding LME cracks according to the present disclosure in Example 1, wherein represents the spot-welding joint hardness distribution curve of the control example, represents the spot-welding joint hardness distribution curve of the composite high-strength galvanized steel plate of Example 1.

[0058] In the FIG. 1 is the high-strength steel matrix, 2 is the low carbon steel composite layer, and 3 is the galvanized layer.

DETAILED DESCRIPTION

[0059] The present disclosure will be further described in detail below in conjunction with the accompanying drawings and Examples. Unless otherwise specified, the composition of the steel involved herein conforms to the composition range specified in GB/T 13304.1-2008.

[0060] Referring to FIG. 1, a composite high-strength galvanized steel plate having resistance to spot-welding LME cracks comprises a high-strength steel matrix 1, a low-carbon steel composite layer 2 and a galvanized layer 3; wherein two low carbon steel composite layers are affixed on two surfaces of the high-strength steel matrix 1 by composite rolling, the galvanized layer 3 is formed on the surface of at least one of the low carbon steel composite layers 2, thereby consisting the composite high-strength galvanized steel plate. Different steel grades have different susceptibility to spot-welding LME, and spot-welding LME cracks occur on the surface of galvanized high-strength steel. The composite rolling of low carbon steel composite layer 2 makes the surface of high-strength steel matrix 1 not sensitive to spot-welding LME cracks, so that the occurrence of spot-welding LME cracks is avoided fundamentally.

[0061] The high-strength steel matrix 1 is a high strength steel that is sensitive to spot-welding LME. The high-strength steel matrix 1 comprises, by mass percentage, C0.14%, Mn1.5%, Si0.4%, with a balance of Fe, other alloy elements and impurity elements, and has a tensile strength of 780 MPa. The high-strength steel matrix 1 is any high strength steel that is sensitive to spot-welding LME, such as QP (Quenching and Partitioning steel), TRIP (transformation induced plasticity steel), TWIP (twinning induced plasticity steel), DH (dual-phase high ductility steel), 7Mn (Fe-7% Mn-0.3% C-2% Al), 10Mn (Fe-10% Mn-0.3% C-2% Al), DP (dual-phase steel), MS (Martensitic Steel), etc.

[0062] The low-carbon steel composite layer 2 is a low-carbon steel that is not sensitive to spot-welding LME. The low-carbon steel composite layer 2 comprises C and Mn. By mass percentage, the C content is 0.1%, the Mn content is 0.7%, and the tensile strength is 590 MPa grade. For example, IF (interstitial-free steel), aluminum killed steel, BH steel (Bake-Hardening steel), low alloy steel may be used.

[0063] The original slab thickness of the high-strength steel matrix 1 and the original slab thickness of the low carbon steel composite layer 2 should satisfy the following formula: LA+MB+NC=T, [0064] wherein, A is the percentage of the original slab thickness of one of the low-carbon steel composite layers 2 to the thickness of the total assembled slab, C is the percentage of the original slab thickness of the other of the low-carbon steel composite layers 2 to the thickness of the total assembled slab, and B is the percentage of the original slab of the high-strength steel matrix 1 to the thickness of the total assembled slab, and A+B+C=100%; wherein the total assembled slab is the total thickness of the original slab comprising the high-strength steel matrix 1 and the two low carbon steel composite layers 2; [0065] wherein L is the tensile strength of one of the low carbon steel composite layers 2 after annealing, N is the tensile strength of the other of the low carbon steel composite layers 2 after annealing, M is the tensile strength of the high-strength steel matrix 1 after annealing; [0066] wherein T is the target tensile strength of the composite high-strength galvanized steel plate.

[0067] In the composite high-strength galvanized steel plate, each low carbon steel composite layer 2 has a thickness of 10-200 m. The two low carbon steel composite layers 2 on two sides of the high-strength steel matrix 1 may be the same or different.

[0068] In the composite high-strength galvanized steel plate, the thickness of the galvanized layer 3 is 4-26 m.

[0069] A method for preparing the composite high-strength galvanized steel plate having resistance to spot-welding LME cracks, which comprises the following steps: [0070] Step 1: an original slab of a high-strength steel matrix 1 and original slabs of two low-carbon steel composite layers 2 are taken and the original slabs of the two low-carbon steel composite layers 2 are affixed to two surfaces of the original slab of the high-strength steel matrix 1, and the original slab edges of the two low-carbon steel composite layers 2 and the original slab edge of the high-strength steel matrix 1 are welded to form a total assembled slab.

[0071] Before the original slab of the two low-carbon steel composite layers 2 and the original slab of the high-strength steel matrix 1 are stacked, the affixing surfaces of the high-strength steel matrix 1 and the two low-carbon steel composite layers 2 need to be polished and cleaned to ensure the smoothness. [0072] Step 2: the total assembled slab is subjected to primary rolling after heating through blooming process of thick plate to reduce the thickness of the total assembled slab, and then hot rolling, pickling, cold rolling and annealing, so that the original slab of the high-strength steel matrix 1 and the original slabs of the two low-carbon steel composite layers 2 are metallurgically combined to form a billet having a tensile strength of T. Primary rolling, hot rolling, pickling, cold rolling and annealing adopt the existing processes, which are not repeated here. [0073] Step 3: a galvanized layer 3 is plated on at least one side of the billet by hot dipping, electroplating or other ways to form the composite high-strength galvanized steel plate. The method of plating the galvanized layer 3 comprises hot dipping, electroplating or existing processes, which are not repeated here.

[0074] The composite high-strength galvanized steel plate of the present disclosure is evaluated for spot-welding LME sensitivity through Rapid LME Test Procedure for Coated Sheet Steels V2.0 standard issued by the North American Automotive/Steel Partnership Organization (A/SP). In this standard, cracks are divided into four categories: Type A, Type B, Type C and Type D according to the distribution and position of cracks, wherein Type A cracks are located in the contact area between the end of spot welding electrode and the steel plate on the upper and lower surfaces of the spot welding joint, and it is required in the standard that the length of Type A cracks should be 10% of the plate thickness before the occurrence of welding spatter, and there is no requirement in the standard for the length of Type A cracks after the occurrence of welding spatter; Type B cracks are located in the non-contact area between the spot welding electrode and the steel plate on the upper and lower surfaces of the spot welding joint, and it is required in the standard that the length of Type B cracks should be 5% of the plate thickness before and after the occurrence of welding spatter; Type C cracks are located in the contact area of the overlap of the upper and lower steel plates in the spot welding joint, and it is required in the standard that the length of Type C cracks should be 5% of the plate thickness before and after the occurrence of welding spatter; Type D cracks are located in the shoulder area of the indentation on the upper and lower surfaces of the spot welding joint, which has the largest deformation of the weld thickness, and it is required in the standard that the length of Type D cracks should be 5% of the plate thickness before the occurrence of welding spatter and the length of Type D cracks should be 10% of the plate thickness after the occurrence of welding spatter.

[0075] The composite high-strength galvanized steel plate of the present disclosure has good resistance to spot welding LME cracks: no LME cracks in the joint before occurrence of welding spatter, and after the occurrence of welding spatter, the length of Type A LME cracks in the joint does not exceed 10% of the plate thickness and the number does not exceed 6, and the length of the Type D LME crack does not exceed 3% of the plate thickness and the number does not exceed 3, and there are no Type B or Type C LME cracks. At the same time, the basic welding performance and the base metal mechanical properties of the composite high-strength galvanized steel plate of the present disclosure meet the application requirements of the related fields.

Example 1

[0076] Referring to FIG. 1, the original slab of QP1180 was taken as the high-strength steel matrix 1. The original slab of the high-strength steel matrix 1 comprised, by mass percentage, 0.18% of C, 1.67% of Si, 2.73% of Mn, 0.0089% of P, 0.0009% of S, 0.02% of Ti, 0.03% of Cr, with a balance of Fe and unavoidable impurity elements.

[0077] The original slab of IF steel DC04 was taken as the low carbon steel composite layer 2. The original slab of the low carbon steel composite layer 2 comprised, by mass percentage, 0.0015% of C, 0.002% of Si, 0.114% of Mn, 0.0119% of P, 0.0044% of S, with a balance of Fe and unavoidable impurity elements.

[0078] The two low-carbon steel composite layers 2 had the same original slab thickness of 24 mm, and the original slab thickness of the high-strength steel matrix 1 was 182 mm. The thickness ratio of the original slab of the high-strength steel matrix 1 and the original slab (one piece) of the low-carbon steel composite layer 2 was 7.5:1. In this Example, A=C=10.5%, L=N=150 MPa, B=79%, M=1250 MPa, T=1019 MPa. Through the process of assembling-blooming of thick plate-hot rolling-pickling-cold rolling-annealing, the original slabs of two low-carbon steel composite layers 2 (with the same thickness) were affixed on two surfaces of the original slab of high-strength steel matrix 1 by composite rolling to obtain a billet, and the galvanized layer 3 was coated on two surfaces of the billet by hot-dip galvanizing to form a composite high-strength galvanized steel plate. The specific process was as follows: [0079] (1) hot rolling: the slab was heated to 1200 C. and held for 0.5 h, then hot rolled with a final rolling temperature of 850 C., followed by cooled to 400 C. at a rate of 30 and coiled; [0080] (2) cold rolling: the cold rolling reduction was 50%; [0081] (3) annealing: the steel was heated to a soaking temperature of 800 C. at a rate of 5 C./s and held for 50 sec; cooled to 650 C. at a rate of 3 C./s, then cooled to 280 C. at a rate of 20 C./s and held for 20 sec; and heated to 450 C. at a rate of 5 C./s and held for 200 sec; [0082] (4) hot-dip galvanizing: the galvanizing temperature was 450 C., and the galvanizing time was 20 seconds. After galvanizing, it was cooled to room temperature at a cooling rate of no less than 20 C./s.

[0083] The total thickness of the final composite high-strength galvanized steel plate was 1.5 mm. The thickness of the two low carbon steel composite layers 2 was about 160 m, and the thickness of the galvanized layer 3 was about 8.6 m. The composite high-strength galvanized steel plate had a yield strength of 696 MPa, a tensile strength of 1074 MPa, and an elongation at break of 15%, which met the requirements of QP980GI. The central layer was QP steel structure and contained martensite and residual austenite.

[0084] The composite high-strength galvanized steel plate obtained in this Example by composite rolling was evaluated for spot-welding LME sensitivity through Rapid LME Test Procedure for Coated Sheet Steels V2.0 standard issued by A/SP. Welding spatter occurred when the welding current is 11 kA. There were no LME cracks in the joint before the occurrence of welding spatter; and after the occurrence of welding spatter, the length of the Type A LME cracks in the joint did not exceed 10% of the plate thickness and the number did not exceed 6, the length of Type D LME cracks did not exceed 3% of the plate thickness, and the number did not exceed 3, and there were no Type B or Type C LME cracks, as shown in FIG. 2 and FIG. 3. According to the standard, the composite high-strength galvanized steel plate in the present Example belongs to a material that is low-sensitive to spot welding LME.

[0085] A QP980GI steel with a thickness of 1.5 mm was used as a control. The QP980GI steel had a composition, by mass percentage, comprising 0.18% of C, 1.8% of Si, 2.3% of Mn, 0.001% of S, 0.012% of P, 0.017% of Ti, with a balance of Fe and unavoidable impurities. The QP980GI steel had mechanical properties as follows: a yield strength of 687 MPa, a tensile strength of 1068 MPa, an elongation at break of 22%. The QP980GI steel was evaluated for spot-welding LME sensitivity through Rapid LME Test Procedure for Coated Sheet Steels V2.0 standard. Welding spatter occurred when the welding current was 10.5 kA. Since Type A cracks were acceptable after welding spatter, Type A cracks were not included in the evaluation results of QP980GI as a control. However, there were Type D cracks with a length of nearly 90% of the plate thickness and Type C cracks with a length of more than 20% of the plate thickness, as shown in FIG. 4. According to this standard, the material is a material sensitive to spot welding LME.

[0086] Using the spot-welding parameters listed in Table 1, the basic spot-welding performance experiments were carried out for the present Example and the control Example, and the hardness distribution of spot-welding joint was shown in FIG. 5.

TABLE-US-00001 TABLE 1 spot welding parameters Plate thickness/mm 1.5 Diameter of the electrode end surface/mm 6 Welding pressure/kN 3.6 First welding time/ms 120 First welding current/kA I First cooling time/ms 20 Second welding time/ms 120 Second welding current/kA I Second cooling time/ms 20 Third welding time/ms 120 Third welding current/kA I Holding time/ms 250

[0087] It can be seen from FIG. 5 that the weldable interval of the composite high-strength galvanized steel plate in the Example was 6.6 kA-9.0 kA, and the interval width was 2.4 kA. The weldable interval of QP980GI in the control example was 6.7 kA-9.1 kA, and the interval width was 2.4 kA. Their weldable intervals were comparable. The lower limit of the weldable interval corresponded to the welding current at which a melt nugget with a diameter of 5 mm was formed, and the upper limit of the weldable interval corresponded to the minimum welding current at the occurrence of welding spatter. Accordingly, when the diameter of the melt nugget was 5 mm for both cases, the joint strength TSS (i.e. tensile shear strength) and the joint strength CTS (i.e. cross tensile strength) were tested for two samples. For the inventive Example, the joint strength TSS was 13.89 kN (this TSS value was the average value of three samples tested in this Example) and the joint strength CTS was 6.22 kN (this CTS value was the average value of three samples tested in this Example). For the control Example, the joint strength TSS was 14.033 kN (this TSS value was the average value of three samples tested in the control Example) and the joint strength CTS was 4.27 kN (this CTS value was the average value of three samples tested in the control Example). It can be seen that the joint strength TSS of the present Example was comparable to that of the control Example. But the existence of the composite rolling layer of IF steel reduced the carbon equivalent in the melt nugget area, so that the hardness of the melt nugget was reduced and the plasticity was improved. Therefore, the joint strength CTS of the present Example is greatly improved compared with the control Example.

Example 2

[0088] Referring to FIG. 1, the original slab of QP1180 was taken as the high-strength steel matrix 1. The original slab of the high-strength steel matrix 1 comprised, by mass percentage, 0.18% of C, 1.67% of Si, 2.73% of Mn, 0.0089% of P, 0.0009% of S, 0.02% of Ti, 0.03% of Cr, with a balance of Fe and unavoidable impurity elements.

[0089] The original slab of IF steel DC04 was taken as the low carbon steel composite layer 2. The original slab of the low carbon steel composite layer 2 comprised, by mass percentage, 0.0015% of C, 0.002% of Si, 0.114% of Mn, 0.0119% of P, 0.0044% of S, with a balance of Fe and unavoidable impurity elements.

[0090] The original slab of the high-strength steel matrix 1 and the original slab of the low-carbon steel composite layer 2 (the original slab thicknesses of the low-carbon steel composite layers 2 on both sides were the same) were respectively assembled with a thickness ratio of 5.5:1, 7.5:1, 10.5:1, 16.5:1 or 35.5:1 (the ratio referred to the ratio of the original slab thickness of the high-strength steel matrix 1 to the original slab thickness of one low-carbon steel composite layer). The composite high-strength galvanized steel plates with different thickness of low-carbon steel composite layer 2 were obtained according to the preparing method of the present disclosure through the process of assembling-blooming of thick plate-hot rolling-pickling-coldrolling-annealing-hot-dip galvanizing. The details of each composite high-strength galvanized steel plate (1 #5 #) were shown in Table 2. The specific process was as follows: [0091] (1) hot rolling: the slab was heated to 1230 C. and held for 1 h, then hot rolled with a final rolling temperature of 920 C., followed by cooled to 550 C. at a rate of 50 C./s and coiled; [0092] (2) cold rolling: the cold rolling reduction was 60%; [0093] (3) annealing: the steel was heated to a soaking temperature of 810 C. at a rate of 10 C./s and held for 100 sec; cooled to 700 C. at a rate of 7 C./s, then cooled to 300 C. at a rate of 50 C./s and held for 80 sec; and heated to 470 C. at a rate of 15 C./s and held for 300 sec; [0094] (4) hot-dip galvanizing: the galvanizing temperature was 480 C., and the galvanizing time was 100 seconds. After galvanizing, it was cooled to room temperature at a cooling rate of no less than 20 C./s.

TABLE-US-00002 TABLE 2 the composite high-strength galvanized steel plates 1#~5# with different thickness of low-carbon steel composite layer 2 Thickness of Composite layer the composite Tensile strength thickness of Low carbon high-strength of the composite the composite High strength steel composite galvanized high-strength high-strength steel matrix 1 layer 2 Thickness steel galvanized galvanized No. (original slab) (original slab) ratio plate /mm steel plate /Mpa steel plate /m 1# QP1180 DC04 5.5:1 1.5 1020 200 2# 7.5:1 1074 160 3# 10.5:1 1141 120 4# 16.5:1 1165 80 5# 35.5:1 1201 40

[0095] The composite high-strength galvanized steel plates 1 #-5 # were evaluated for spot-welding LME sensitivity through Rapid LME Test Procedure for Coated Sheet Steels V2.0 standard issued by A/SP. The results showed that, for the composite high-strength galvanized steel plates 1 #-5 #, there were no LME cracks in the spot-welding joint before the occurrence of welding spatter; after the occurrence of welding spatter, there were no Type B or Type C LME cracks, the length of the Type A LME cracks did not exceed 10% of the plate thickness, and the number did not exceed 6; the length of the Type D LME cracks did not exceed 3% of the plate thickness, and the number did not exceed 3, as shown in Table 3.

TABLE-US-00003 TABLE 3 test results of LME cracks in spot-welding joint after welding spatter for the composite high-strength galvanized steel plates 1#~5# LME cracks after welding spatter Type A Type B Type C Type D No. length number length number length number length number 1# 5% 2 0 0 0 0 2% 1 2# 10% 6 0 0 0 0 3% 3 3# 9.5% 4 0 0 0 0 2.8%.sup. 2 4# 10% 6 0 0 0 0 3% 3 5# 10% 6 0 0 0 0 3% 3

Example 3

[0096] Referring to FIG. 1, the original slab of QP1180 was taken as the high-strength steel matrix 1. The original slab of the high-strength steel matrix 1 comprised, by mass percentage, 0.18% of C, 1.67% of Si, 2.73% of Mn, 0.0089% of P, 0.0009% of S, 0.02% of Ti, 0.03% of Cr, with a balance of Fe and unavoidable impurity elements.

[0097] The original slabs of cold-rolled carbon structural steel St37-2G, phosphorus high-strength steel HC220P, high-strength IF steel HC180Y, bake-hardening steel HC180B, and low-alloy high-strength steel HC300LA were taken as the original slab of low carbon steel composite layer 2, respectively. The main chemical compositions of original slabs of these five low carbon steel composite layers 2 were listed in Table 4, with a balance of Fe and unavoidable impurity elements.

TABLE-US-00004 TABLE 4 the main chemical compositions of the original slabs of the five low carbon steel composite layers 2 No. Brand C Si Mn P Nb Ti Ni Cr 6# St37-2G 0.075 0.01 0.612 0.016 \ 0.002 0.006 0.026 7# HC220P 0.026 0.006 0.414 0.021 \ 0.001 0.009 0.027 8# HC180Y 0.002 0.045 0.277 0.043 0.012 0.018 0.011 0.042 9# HC180B 0.0018 0.007 0.233 0.023 \ \ 0.011 0.026 10# HC300LA 0.0665 0.016 0.593 0.020 0.012 \ 0.019 0.029

[0098] The original slab of the high-strength steel matrix 1 and the original slab (one piece) of the five low-carbon steel composite layers 2 were assembled with a thickness ratio of 7.5:1. The composite high-strength galvanized steel plates 6 #-10 # with different low-carbon steel composite layer 2 were obtained according to the preparing method of the present disclosure through the process of assembling-bloomingofthickplate-hotrolling-pickling-coldrolling-annealing-hot-dip galvanizing. The details of each composite high-strength galvanized steel plate (6 #-10 #) were shown in Table 5. The specific process was as follows: [0099] (1) hot rolling: the slab was heated to 1250 C. and held for 2 h, then hot rolled with a final rolling temperature of 980 C., followed by cooled to 630 C. at a rate of 90 C./s and coiled; [0100] (2) cold rolling: the cold rolling reduction was 70%; [0101] (3) annealing: the steel was heated to a soaking temperature of 830 C. at a rate of 20 C./s and held for 160 sec; cooled to 750 C. at a rate of 8 C./s, then cooled to 300 C. at a rate of 70 C./s and held for 100 sec; and heated to 470 C. at a rate of 15 C./s and held for 200 sec; [0102] (4) hot-dip galvanizing: the galvanizing temperature was 500 C., and the galvanizing time was 180 seconds. After galvanizing, it was cooled to room temperature at a cooling rate of no less than 20 C./s.

TABLE-US-00005 TABLE 5 the composite high-strength galvanized steel plates 6#~10# with different thickness of low-carbon steel composite layer 2 Composite layer Thickness of Tensile strength thickness of low-carbon the composite of the composite the composite High strength steel composite high-strength high-strength high-strength steel matrix 1 layer 2 Thickness galvanized galvanized steel galvanized No. (original slab) (original slab) ratio steel plate/mm plate /MPa steel plate /m 6# QP1180 St37-2G 7.5:1 1.5 1139 160 7# HC220P 1092 8# HC180Y 1101 9# HC180B 1084 10# HC300LA 1143

[0103] The composite high-strength galvanized steel plates 6 #-10 #obtained in this Example by composite rolling were evaluated for spot-welding LME sensitivity through Rapid LME Test Procedure for Coated Sheet Steels V2.0 standard issued by A/SP. The results showed that, for the composite high-strength galvanized steel plates 6 #-10 #, there were no LME cracks in the spot-welding joint before the occurrence of welding spatter; after the occurrence of welding spatter, there were no Type B or Type C LME cracks in the spot-welding joint of the composite high-strength galvanized steel plates 6 #-10 #, the length of the Type A LME cracks did not exceed 10% of the plate thickness, and the number did not exceed 6; the length of the Type D LME cracks did not exceed 3% of the plate thickness, and the number did not exceed 3.

Example 4

[0104] Referring to FIG. 1, the original slab of DH980 was taken as the high-strength steel matrix 1. The original slab of the high-strength steel matrix 1 comprised, by mass percentage, 0.18% of C, 0.48% of Si, 2.4% of Mn, 0.021% of Cr, 0.04% of Nb, 0.02% of Ti, 0.0004% of B, 0.009% of P, 0.001% of S, 0.23% of Al, with a balance of Fe and unavoidable impurity elements.

[0105] The original slabs of IF steel DC04, cold-rolled carbon structural steel St37-2G, phosphorus high-strength steel HC220P, high-strength IF steel HC180Y, and bake-hardening steel HC180B were taken as the original slab of low carbon steel composite layer 2, respectively. The main chemical compositions of original slabs of these five low carbon steel composite layers 2 were listed in Table 6, with a balance of Fe and unavoidable impurity elements.

TABLE-US-00006 TABLE 6 the main chemical compositions of the original slabs of the five low carbon steel composite layers 2 No. Brand C Si Mn P Nb Ti Ni Cr 11# DC04 0.0015 0.002 0.114 0.012 \ \ \ \ 12# St37-2G 0.075 0.01 0.612 0.016 \ 0.002 0.006 0.026 13# HC220P 0.026 0.006 0.414 0.021 \ 0.001 0.009 0.027 14# HC180Y 0.002 0.045 0.277 0.043 0.012 0.018 0.011 0.042 15# HC180B 0.0018 0.007 0.233 0.023 \ \ 0.011 0.026

[0106] The slab raw materials of each composite layer were rolled to a thickness of 7.5:1 (the ratio of the high-strength steel matrix thickness to the thickness of one piece of low-carbon steel slab) for later use. The adjacent interfaces of each composite layer were cleaned to remove impurities such as oxide scale. The composite layers were sealed at the contact boundary by welding and vacuumed to remove the oxygen between the composite layers, and then the slabs were assembled by composite rolling. The composite high-strength galvanized steel plates 11 #-15 # with different low-carbon steel composite layers 2 were obtained through the process of -the following hot rolling-pickling-coldrolling-annealing-hot-dip galvanizing. The details of each composite high-strength galvanized steel plate (11 #-15 #) were shown in Table 7. [0107] (1) hot rolling: the slab was heated to 1280 C. and held for 4 h, then hot rolled with a final rolling temperature of 950 C., followed by cooled to 600 C. at a rate of 80 C./s and coiled; [0108] (2) pickling; [0109] (3) cold rolling: the cold rolling reduction was 40%; [0110] (4) annealing: the steel was heated to a soaking temperature of 810 C. at a rate of 15 C./s with a holding time of 200 seconds and a dew point temperature controlled at 0 C., cooled to 670 C. at a rate of 10 C./s, then cooled to 320 C. at a rate of 60 C./s and held for 80 sec; [0111] (6) hot-dip galvanizing: the galvanizing temperature was 450 C., and the galvanizing time was 1600 seconds. After galvanizing, it was cooled to room temperature at a cooling rate of no less than 20 C./s.

TABLE-US-00007 TABLE 7 the composite high-strength galvanized steel plates 11#~15# with different thickness of low-carbon steel composite layer 2 Composite layer Thickness of Tensile strength thickness of low-carbon the composite of the composite the composite High strength steel composite high-strength high-strength high-strength steel matrix 1 layer 2 Thickness galvanized galvanized steel galvanized No. (original slab) (original slab) ratio steel plate/mm plate /Mpa steel plate /m 11# DH980 DC04 7.5:1 1.5 862 160 12# St37-2G 890 13# HC220P 881 14# HC180Y 877 15# HC180B 871

[0112] The composite high-strength galvanized steel plates 11 #-15 #obtained in this Example by composite rolling were evaluated for spot-welding LME sensitivity through Rapid LME Test Procedure for Coated Sheet Steels V2.0 standard issued by A/SP. The results showed that, for the composite high-strength galvanized steel plates 11 #-15 #, there were no LME cracks in the spot-welding joint before the occurrence of welding spatter; after the occurrence of welding spatter, there were no Type B or Type C LME cracks in the spot-welding joint of the composite high-strength galvanized steel plates 11 #-15 #, the length of the Type A LME cracks did not exceed 10% of the plate thickness, and the number did not exceed 6; the length of the Type D LME cracks did not exceed 3% of the plate thickness, and the number did not exceed 3.

[0113] The above content is only a preferable example of the present disclosure and is not intended to limit the protection scope of the present disclosure. Therefore, any modification, equivalent replacement, improvement, etc., within the spirit and principles of the present disclosure shall be included in the scope of the present disclosure.