Zinc-coated steel sheet with high resistance spot weldability

1. A method for producing a zinc or zinc-alloy coated steel sheet with a tensile strength higher than 900 MPa, for the fabrication of resistance spot welds containing on average not more than two Liquid Metal Embrittlement cracks per weld having a depth of 100 μm or more, comprising the successive steps of:

providing a cold-rolled steel sheet, a nominal composition of the steel sheet containing, in weight percent:

0.07%≤C≤0.5%

0.3%≤Mn≤5%

0.010%≤Al≤1%

0.010%≤Si≤2.45%

with 0.35%≤(Si+Al)≤2.5%,

0.001%≤Cr≤1.0%

0.001%≤Mo≤0.5%

and optionally

0.005%≤Nb≤0.1%

0.005%≤V≤0.2%

0.005%≤Ti≤0.1%

0.0001%≤B≤0.004%

0.001%≤Cu≤0.5%

0.001%≤Ni≤1.0%,

a remainder being iron and unavoidable impurities from smelting;

heating said cold-rolled steel sheet up to a temperature T1 between 550° C. and Ac1+50° C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, the balance being nitrogen and unavoidable impurities, so that the iron is not oxidized, then

adding to the atmosphere (A1) at an injection location in a furnace, at least one element selected from the group consisting of water steam and oxygen with an injection flow rate Q higher than (0.07%/h×α), α being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T≥T1, so to obtain an atmosphere (A2) with a dew point DP2 between −15° C. and the temperature Te of the iron/iron oxide equilibrium dew point, wherein the injection flow rate Q is the injected volume of water steam or oxygen per hour divided by the volume of the furnace between the injection location of water steam or oxygen and an end of a section of the furnace heated at a soaking temperature T2;

heating the sheet from said temperature T1 up to the soaking temperature T2 between 720° C. and 1000° C. in a furnace zone under the atmosphere (A2), wherein the atmosphere (A2) contains nitrogen, from 2 to 15% hydrogen, and more than 0.1% CO by volume, with an oxygen partial pressure higher than 10 −21 atm., wherein the duration to of said heating of the sheet from temperature T1 up to the end of soaking at temperature T2 is between 100 and 500 s;

soaking the sheet at T2; then

cooling the sheet at a rate between 10 and 400° C./s; and then

coating the sheet with zinc or zinc-alloy coating.

2. The method according to claim 1 , wherein the dew point DP2 is between −10 and +20° C.

3. The method according to claim 1 , wherein the said atmosphere (A2) contains more than 0.2% CO by volume.

4. The method according to claim 1 , wherein said temperature T2 is between 750 and 900° C., and wherein the atmosphere (A2) contains between 3 to 5% in volume of H2.

5. The method according to claim 1 , wherein after said soaking, the steel sheet is cooled down to a temperature T3 between Ms and Ms+150° C., and maintained at T3 for at least 40 s, so to obtain a Carbide Free Bainite steel sheet.

6. The method according to claim 5 , wherein said temperature T3 is between Ms+10° C. and Ms+150° C.

7. The method according to claim 1 , wherein after said cooling, the steel sheet is cooled down to a temperature QT between Ms−5° C. and Ms−170° C., optionally held at QT for a duration comprised between 2 and 8 s, then reheated up to a temperature T4 between 350 and 550° C. so to obtained partitioned martensite.

8. The method according to claim 7 , wherein the steel sheet is reheated up to the temperature T4 between 350 and 490° C.

9. The method according to claim 1 , wherein the steel microstructure contains retained austenite in quantity not more than 20%.

10. The method according to claim 1 , wherein said coating is performed by hot-dipping.

11. The method according to claim 1 , wherein said coating is performed by electroplating.

12. The method according to claim 1 wherein the impurities include S<0.003%, P<0.02% and N<0.008%.

13. A method for fabrication of structural parts of automotive vehicles comprising performing the method according to claim 1 .