METHOD FOR PRODUCING A COATED FLAT STEEL PRODUCT AND COATED FLAT STEEL PRODUCT

The present invention relates to a method for producing a super-high-strength flat steel product provided with a metal coating and also to a coated flat steel product. The method comprises providing a hot-rolled flat steel product, which comprises a steel which consists of (in % by weight) 0.1-0.5% C, at least one element selected from the group consisting of Mn and Si, where the Mn content is 1.0-3.0% and the Si content is 0.7-2.5%, 0.05-1% Cr, up to 0.020% P, up to 0.005% S, up to 0.008% N, optionally one or more of the following elements 0.01-1.5% Al, 0.05-0.5% Mo, 0.0004-0.001% B and optionally in total 0.001-0.3% V, Ti and Nb, as the remainder iron and unavoidable impurities. The method also comprises pickling, cold rolling, heat-treating and hot-dip coating of the flat steel product with a zinc-based corrosion protection coating. The steel substrate has a microstructure which contains 5-20% by volume residual austenite, less than 5% by area bainite, less than 10% by area ferrite and at least 80% by area martensite, of which at least 75% by area is tempered martensite and less than 25% by area is untempered martensite. The coated flat steel product has in the boundary layer between the corrosion protection coating and the steel substrate a ratio of the sum of Si and Mn to Cr of at least 1.7 and at most 15. The ratio of the sum of Si+Mn to Cr is smaller in the boundary layer than in the base material.