Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Ductility and Formability

A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the coated steel sheet having a yield strength YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%, comprising:

• providing a steel sheet having a chemical composition including in weight %:
• 0.15%≤C≤0.25%
• 1.2%≤Si≤1.8%
• 2%≤Mn≤2.4%
• 0.1%≤Cr≤0.25%
• Al≤0.5%
• a remainder being Fe and unavoidable impurities,
• annealing the sheet at an annealing temperature TA higher than Ac3 but less than 1000° C. for a time of more than 30 s,
• quenching the sheet by cooling it down to a quenching temperature QT between 250° C. and 350° C., at a cooling speed sufficient to obtain a final structure consisting of martensite, bainite and austenite just after quenching, and such that the final structure consists of between 3% to 15% of retained austenite and 85% to 97% of a sum of martensite and bainite, with a martensite content of at least 60%,
• heating the sheet up to a partitioning temperature PT between 430° C. and 480° C. and maintaining the sheet at this temperature for a partitioning time Pt between 10 s and 90 s,
• hot dip coating the sheet, and
• cooling the sheet down to the room temperature.