METHOD FOR FORMING ZINC-PLATED STEEL PLATE OR STEEL BELT HAVING GOOD CORROSION RESISTANCE

Abstract

A method for forming a zinc-plated steel plate or steel belt having good corrosion resistance, comprising the following steps: 1) conveying the steel plate or steel belt into a heating furnace higher than Ac.sub.3, austenitizing the steel plate or steel belt, and keeping the temperature for 1-7 minutes; 2) before the steel plate or steel belt leaves the heating furnace and enters a forming mold, applying different cooling manners according to different plated thicknesses, wherein when the single-sided weight of the plating is less than or equal to 50 g/m.sup.2, only air cooling is required before transferring to the forming mold, when the single-sided weight of the plating is greater than 50 g/m.sup.2, aerial fog or water mist precooling is performed before transferring to the forming mold, and the steel plate is cooled to be below 700° C. at a cooling sped greater than 30° C./s; and 3) rapidly transferring the steel plate or steel belt to the mold for hot stamping forming, wherein a transferring duration does not exceed 1 minute, and the temperature of the hot stamping forming is controlled between 400-800° C. The tensile strength of the steel plate or steel belt after hot stamping is greater than 1,450 MPa, and substrate cracks caused by local press and LME in an element are avoided.

Claims

1. A method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance, comprising the following steps: 1) Delivering a zinc-based plated steel plate or steel belt to a heating furnace to completely austenitize the zinc-based plated steel plate or steel belt at a temperature that is higher than Ac.sub.3 and held for 1 to 7 minutes; wherein the zinc-based plated steel plate or steel belt comprises a substrate and a zinc-based coating on at least one surface thereof; 2) Cooling the zinc-based plated steel plate or steel belt after it exits the heating furnace but before it enters a forming mold, wherein it is cooled in a cooling process selected from the following cooling processes depending on coating thickness: for a thin-coating steel plate or steel belt having a one-side coating weight ≤50 g/m.sup.2, cooling it only in air to 800° C. or less, before it is transferred to the forming mold; for a thick-coating steel plate or steel belt having a one-side coating weight >50 g/m.sup.2, pre-cooling it with gas spray or water spray, and cooling the steel plate or steel belt to 700° C. or less at a cooling rate of greater than 30° C./s before it is transferred to the forming mold; 3) After the cooling, transferring the zinc-based plated steel plate or steel belt to the forming mold for hot stamping forming in a period of time of no more than 1 minute, wherein the hot stamping forming is performed at a temperature controlled at 400-800° C.

2. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein the substrate of the zinc-based plated steel plate or steel belt has a composition based on mass of: C: 0.1-0.8%, Si: 0.05-2.0%, Mn: 0.5-3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, and a balance of Fe and unavoidable impurities.

3. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 2, wherein the substrate of the zinc-based plated steel plate or steel belt further comprises, based on mass, at least one of Cr: 0.01-1.0%, Mo: 0.01-1.0%, Ti: 0.2% or less, Nb: 0.01-0.08%, and V: 0.01-1.0%, B: 0.005-0.08%.

4. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein the substrate of the zinc-based plated steel plate or steel belt is an ultra-high-strength steel 22MnB5.

5. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein the zinc-based coating has a chemical composition based on mass of Al 0.15%-6.5%, Mg 0.2-7%, RE<0.2%, Sr 0.002-0.2%, and a balance of Zn and unavoidable impurities, wherein Mg/Al in the chemical composition of the coating is greater than 1.0.

6. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 5, wherein the zinc-based coating further comprises at least one or two of Mn and Cr, the total amount of which is greater than 0.1% and less than 5%.

7. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein before being transferred to the forming mold, the thin-coating steel plate or steel belt is cooled in air at a cooling rate of not more than 20° C./s to 650-800° C. to start the forming.

8. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein before being transferred to the forming mold, the thick-coating steel plate or steel belt is cooled with gas spray or water spray at a cooling rate of greater than 30° C./s to 400-650° C. to start the forming.

9. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein a pressure holding time of the hot stamping forming is 3-15 seconds, and a stamping force is 300-1000 tons.

10. The method for forming a zinc-based plated steel plate or steel belt having good corrosion resistance according to claim 1, wherein the method further comprises cooling the steel plate or steel belt in the mold to 200° C. or less after the hot stamping forming is completed, removing it from the mold, and then cooling it to room temperature.

Description

BRIEFLY DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a cross-sectional metallography (free of microcracks) of a zinc-based plated hot-formed steel plate in Example 3 according to the present disclosure;

[0029] FIG. 2 is a cross-sectional metallography (with cracks) of the Comparative Example.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present disclosure will be further illustrated with reference to the following Examples and accompanying drawings.

[0031] See Table 1 for the embodiments of the present disclosure. The substrates of the zinc-based plated steel plates of the Examples and Comparative Examples are all ultra-high-strength steel 22MnB5, and the coatings contain Al, Mg, Sr, Zn and unavoidable impurities, wherein the Al content ranges from 0.15% to 6.5%, the Mg content ranges from 0.2% to 7%, the Sr content ranges from 0.002% to 0.2%, the balance is Zn and unavoidable impurities, and Mg/Al is greater than 1.0.

[0032] From the results of the Examples and Comparative Examples in Table 1, it can be seen that when the coating is thin, as in Examples 1 to 3, normal air cooling before hardening in the mold does not lead to generation of microcracks. As the coating thickness increases, under higher heating temperature and longer temperature holding time, an additional pre-cooling process can help to control the generation of microcracks, as in Examples 6 and 10. Therefore, as the coating thickness increases, in order to obtain qualified parts without microcracks, it is necessary to: 1) increase the soaking temperature in the heating furnace; 2) increase the temperature holding time for austenization in the heating furnace; and 3) add a pre-cooling process.

[0033] According to the method of the present disclosure, different cooling processes are utilized for different coating thicknesses before the steel plates or belts enter the mold for forming.

[0034] Macroscopic liquid metal embrittlement (LME) cracks or microcracks are always generated in zinc-based plated hot stamped steel obtained by conventional hot stamping (see FIG. 2).

[0035] The hot stamping process of the present disclosure can avoid generation of cracks in the substrate of the zinc-based plated hot stamped steel plate during stamping forming (see FIG. 1), and the steel plate has better corrosion resistance. This process is of great significance to the development of zinc-based plated steel for hot stamping.

TABLE-US-00001 TABLE 1 Hot Stamping Forming Process and Evaluation Forming Conditions Zn Content Heating Process Average in Coating Coating Heating Holding Cooling Rate Forming Start Cooling End After Evaluation Test Thickness Temperature Time Cooling Before Forming Temperature Temperature Forming For- Micro- Tensile No. (g/m.sup.2) (° C.) (min) Process (° C./s) (° C.) (° C.) (wt %) mality cracks Strength Ex. 1 35 850 4 Air about 8 664 178 32 ∘ ∘ 1477 Ex. 2 35 880 3 Air about 8 682 185 30 ∘ ∘ 1499 Ex. 3 35 900 2 Air about 8 685 183 25 ∘ ∘ 1512 Comp. 60 880 4 Air about 8 785 185 35 ∘ x 1495 Ex. 4 Comp. 60 880 4 Water about 50 582 156 35 ∘ x 1509 Ex. 5 Spray Comp. 60 930 4 Water about 50 612 167 30 ∘ ∘ 1521 Ex. 6 Spray Comp. 105 880 4 Air about 8 796 185 41 ∘ x 1483 Ex. 7 Comp. 105 880 4 Water about 50 778 156 43 ∘ x 1489 Ex. 8 Spray Comp. 105 930 4 Water about 50 810 167 37 ∘ x 1502 Ex. 9 Spray Ex. 10 105 930 6 Water about 50 521 165 33 ∘ ∘ 1514 Spray