Method for the manufacturing of liquid metal embrittlement resistant galvannealed steel sheet

Abstract

The present invention relates to a method for the manufacture of a galvannealed steel sheet including the steps of A.) coating of the steel sheet with a first coating consisting of nickel and having a thickness between 150 nm and 650 nm, the steel sheet having the following composition in weight percentage 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<3.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0%, and on a purely optional basis, one or more elements such as Nb≤0.5%, B≤0.010%, Cr≤1.0%, Mo≤0.50%, Ni≤1.0%, Ti≤0.5%, the remainder of the composition is made up of iron and inevitable impurities resulting from the elaboration, B.) annealing of the coated steel sheet being annealed at a temperature between 600 to 1200° C., C.) coating of the steel sheet obtained in step B.) with a second coating based on zinc and D.) an alloying heat treatment to form a galvannealed steel sheet.

Claims

1. A method for the manufacture of a galvannealed steel sheet comprising the following successive steps: A) coating a steel sheet with a first coating consisting of nickel and having a thickness between 200 nm and 500 nm, said steel sheet having a composition including in weight percentage: 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<3.0%, 0≤Nb≤0.5%, 0≤B≤0.010%, 0≤Cr≤1.0%, 0≤Mo≤0.50%, 0≤Ni≤1.0%, 0≤Ti≤0.5%, a remainder of the composition being iron and inevitable impurities resulting from the processing; B) annealing the coated steel sheet at a temperature between 600 to 1200° C. in an atmosphere comprising from 1 to 10% of H.sub.2 at a dew point between −60 and −30° C.; C) coating the steel sheet obtained in step B) with a second coating based on zinc; and D) subjecting the steel sheet obtained in step C) to an alloying heat treatment to form a galvannealed steel sheet.

2. The method according to claim 1, wherein in step A), the steel sheet further comprises 0.05<Al<1.0% and 0.75<(Si+Al)<3.0%.

3. The method according to claim 2, wherein in step A), the first coating has a thickness between 250 and 450 nm.

4. The method according to claim 1, wherein in step B), the annealing is a continuous annealing.

5. The method according to claim 1, wherein in step C), the second coating comprises above 50% of zinc.

6. The method according to claim 5, wherein in step C), the second coating comprises above 75% of zinc.

7. The method according to claim 6, wherein in step C), the second coating comprises above 90% of zinc.

8. The method according to claim 1, wherein the second coating does not comprise nickel.

9. The method according to claim 8, wherein in step C), the second coating consists of zinc.

10. The method according to claim 1, wherein in step D) the alloying treatment is performed by heating the coated steel sheet obtained in step C) at a temperature between 470 and 550° C.

11. The method according to claim 1, wherein the steel sheet obtained in step B) is cooled prior to step C).

12. The method according to claim 1, wherein the steel sheet obtained in step B) is cooled to quench temperature of 210° C.

13. The method according to claim 1, wherein the steel sheet obtained in step B) is cooled to room temperature.

Description

EXAMPLE

(1) For all samples, the steel sheets used have the following composition in weight percent: C=0.37%, Mn=1.95%, Si=1.95%, Cr=0.35% and Mo=0.12%.

(2) In Trial 1, steel was annealed in an atmosphere comprising 5% of H.sub.2 and 95% of N.sub.2 at a dew point of −45° C. The annealing was carried out at 900° C. for 132 seconds. After the annealing, the steel sheet was cooled to room temperature. On the annealed steel sheet Zinc coating was applied by electro-galvanizing method.

(3) In Trials 2 to 5, Ni was first deposited by an electro-plating method having a thickness of 150, 400, 650 and 900 nm respectively on full hard steel sheets before annealing. After that, the pre-coated steel sheets were annealed in an atmosphere comprising 5% of H.sub.2 and 95% of N.sub.2 at a dew point of −45° C. The annealing was carried out at 900° C. for 132 seconds. At the end of the annealing, the steel sheets were cooled to quench temperature of 210° C. and again heated at partitioning temperature of 410° C. Portioning was carried out for 88 s and then again heated up to galvanizing temperature of 460° C. and Zinc coating was applied by hot dip coating method using a liquid Zinc bath containing 0.12 wt. % Al maintained at 460° C. Just after the galvanizing, an alloying heat treatment was carried out at 520° C. for 20 seconds.

(4) The susceptibility of LME of above coated steel was evaluated by resistance spot welding method. To this end, for each Trial, two coated steel sheets were welded together by resistance spot welding. The type of the electrode was ISO Type B with a diameter of 16 mm; the force of the electrode was of 5 kN and the flow rate of water of was 1.5 g/min., the welding cycle was reported in Table 1:

(5) TABLE-US-00001 TABLE 1 Welding schedule Weld time Pulses Pulse (cy) Cool time (cy) Hold time (cy) Cycle 2 12 2 15

(6) The LME crack resistance behavior was also evaluated using a 3 layer stack-up condition. For each Trial, three coated steel sheets were welded together by resistance spot welding. The number of cracks 100 μm was then evaluated using an optical microscope as reported in Table 2.

(7) TABLE-US-00002 TABLE 2 LME crack details after spot welding (3 layer stack-up condition) Number Maximum of cracks per spot crack length Trials weld (>100 μm) (μm) Trial 1 6.8 850 Trial 2* 1.3 235 Trial 3* 2.2 215 Trial 4* 2.4 219.5 Trial 5 1 399.6 *according to the present invention.

(8) Trials 2, 3 and 4 according to the present invention show an excellent resistance to LME as compared to Trials 1 and 5. Indeed, the number of cracks above 100 μm is below 3 and the longest crack has a length below 300 μm. Moreover, Trials 2 to 4 having an optimum Ni coating thickness reduces the welding current. It results in a reduction of the amount of heat input during spot welding and thus causes a significant reduction of number of crack formations due to LME.