Method for producing a coated flat steel product and coated flat steel product

Abstract

The invention relates to a method for producing a very high-strength flat steel product provided with a metallic coating and to a coated flat steel product. The method comprises providing a hot-rolled flat steel product comprising steel (in weight %) consisting of 0.1-0.5% C, at least one element selected from the group consisting of Mn and Si, wherein the Mn content is 1.0- 3.0% with Si contents of 0.7‑2.5%, 0.05‑1% Cr, up to 0.020% P, up to 0.005% S, up to 0.008% N, and 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 V, Ti and Nb in a sum of 0.001‑0.3%, the balance being iron and unavoidable impurities. The method also includes pickling, cold rolling, heat treatment and hot-melt dip coating of the flat steel product with a zinc-based corrosion protection coating. The steel substrate has a microstructure comprising 5‑20% by volume retained austenite, less than 5% by area bainite, less than 10% by area ferrite, a minimum of 80% by area martensite, of which at least 75% by area Tempered martensite and less than 25% untempered martensite. The coated flat steel product has a ratio of the sum of Si and Mn to Cr of at least 1.7 and at most 15 in the boundary layer between the corrosion protective coating and the steel substrate. The ratio of the sum of Si+Mn to Cr in the boundary layer is smaller than that in the base material.

Description

technical field [0001] The invention relates to a method for producing a very high-strength metal-coated flat steel product and to a coated flat steel product. Background technique [0002] When flat steel products are mentioned here, steel strips, steel sheets or blanks produced therefrom, such as slabs, are understood. When metallic coatings are mentioned here, this is to be understood in particular as meaning metallic protective coatings and metallic corrosion protective coatings. [0003] Very high-strength steels are characterized by a high proportion of alloying elements that increase the strength of the material, such as silicon, manganese and chromium. For very high-strength steel applications, such as in air vehicle construction, surface finishing layers are often required to avoid corrosion of the material. The surface finishing layer can be applied, for example, electrolytically or by means of hot-melt dip coating, which is also referred to as hot-dip coating. ...

AI Technical Summary

Problems solved by technology

The disadvantage of these oxide layers consisting only of iron, manganese and silicon is that the adhesion of the metal coating can be deteriorated due to the lack of chromium and the oxide layer up to 5 μm thick

Furthermore, local formability degradation is expected between the soft, decarburized and well-formable ferrite layer and the harder and brittle oxide layer

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