ArcelorMittal S.A.
ZINC-COATED STEEL SHEET WITH HIGH RESISTANCE SPOT WELDABILITY

Abstract:

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 in average not more than two Liquid Metal Embrittlement cracks per weld having a depth of 100 .mu.m or more, with steps of providing a cold-rolled steel sheet, heating cold-rolled steel sheet up to a temperature T1 between 550.degree. C. and Ac1+50.degree. C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, so that the iron is not oxidized, then adding in the furnace atmosphere, water steam or oxygen with an injection flow rate Q higher than (0.07%/h.times..alpha.), .alpha. being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T.gtoreq.T1, so to obtain an atmosphere (A2) with a dew point DP2 between -15.degree. C. and the temperature Te of the iron/iron oxide equilibrium dew point, then heating the sheet from temperature T.sub.1 up to a temperature T.sub.2 between 720.degree. C. and 1000.degree. C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen and more than 0.1% CO by volume, with an oxygen partial pressure higher than 10.sup.-21 atm., wherein the duration t.sub.D of heating of the sheet from temperature T.sub.1 up to the end of soaking at temperature T.sub.2 is between 100 and 500s., soaking the sheet at T.sub.2, then cooling the sheet at a rate between 10 and 400.degree. C./s, then coating the sheet with zinc or zinc-alloy coating.