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

A method for producing a high strength coated steel sheet having an improved strength 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 the steps of:

• providing a steel sheet having a chemical composition including in weight %:
• 0.13%≤C≤0.22%;
• 1.2%≤Si≤1.8%;
• 1.8%≤Mn≤2.2%;
• 0.10%≤Mo≤0.20%;
• Nb≤0.05%;
• Al≤0.5%;
• Ti≤0.05%;
• and
• 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 the sheet down to a quenching temperature QT between 325° C. and 375° C. at a cooling speed higher than 30° C./s in order to obtain a structure consisting of austenite and at least 60% of martensite, with an austenite content such that the coated steel sheet has a final structure after coating and cooling to room temperature, including between, by volume fraction, 3% and 15% of residual austenite and between 85% and 97% of a sum of martensite and bainite, the final structure including at least 65% of martensite, the final structure not including ferrite;
• holding the sheet at the quenching temperature QT for a holding time between 2 s and 8 s;
• heating the sheet up to a partitioning temperature PT between 430° C. and 480° C. and maintaining the sheet at the partitioning temperature PT for a partitioning time Pt between 10 s and 90 s;
• hot dip coating the sheet;
• and
• cooling the sheet down to room temperature.