Method for producing a high strength coated steel sheet having improved strength, formability and obtained sheet

Procede de production d'une tole d'acier revetue a haute resistance ayant une resistance et une aptitude au formage ameliorees et feuille obtenue

Abstract

A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the chemical composition of the steel containing: 0.13% = C = 0.22%, 1.9% = Si = 2.3%, 2.4% = Mn = 3%, Al =0.5%, Ti < 0.05 %, Nb < 0.05 %, the remainder being Fe and unavoidable impurities. The sheet is annealed at temperature TA higher than Ac3 but less than 1000°C for a time of more than 30 s, quenched by cooling to a quenching temperature QT between 200°C and 280°C in order to obtain a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite, without ferrite, heated up to a partitioning temperature PT between 430°C and 490°C and maintained at this temperature for a time Pt between 10 s and 100 s, hot dip coated and cooled to the room temperature.

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