Patent ID: 8470102

Claim:
A process for manufacturing a hot-dip galvanized or galvannealed steel sheet having a TRIP microstructure comprising ferrite, residual austenite and optionally martensite, bainite or a mixture thereof, wherein a composition of the steel sheet comprises Fe and: by weight 0.01≦C≦0.22% 0.50≦Mn≦2.0% 0.2≦Si≦2.0% 0.005≦Al≦2.0% Mo<1.0% Cr≦1.0% P<0.02% Ti≦0.20% V≦0.40% Ni≦1.0% Nb≦0.20% and unavoidable impurities resulting from smelting, wherein the process comprises: oxidizing said steel sheet in a direct flame furnace where an atmosphere comprises air and fuel with an air-to-fuel ratio between 0.80 and 0.95; forming a layer of iron oxide having a thickness from 0.05 to 0.2 μm on the surface of the steel sheet; forming at least one internal oxide of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide of Si and Al, complex oxide comprising Mn and Al, and complex oxide comprising Si, Mn and Al; reducing said layer of iron oxide, at a reduction speed from 0.001 to 0.01 μm/s; growing the internal oxide in depth of steel sheet; completing the reduction of the layer of iron oxide; hot-dip galvanizing said reduced steel sheet to form a zinc-coated steel sheet; and optionally, subjecting said hot-dip coated steel sheet to an alloying treatment to form a galvannealed steel sheet.