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

A method for producing a coated steel sheet having a tensile strength TS of at least 1100 MPa, a total elongation TE according to ISO standard 6892-1:2009 of at least 12%, a product TS×TE of the tensile strength by the total elongation of at least 14200 MPa %, and a hole expansion ratio HER according to ISO standard 16630:2009 of at least 25%, the method comprising the following successive steps:

• providing a cold-rolled steel sheet made of a steel having a chemical composition containing by weight:
• 0.15
• %
• ≤
• C
• ≤
• 0.23
• %
• ,
• 2.
• %
• ≤
• Mn
• ≤
• 2.7
• %
• ,
• with
• ⁢
• C
• +
• Mn
• /
• 10
• ≥
• 0.42
• %
• ,
• 0
• ≤
• Cr
• ≤
• 0.4
• %
• ,
• with
• ⁢
• Mn
• +
• Cr
• ≥
• 2.25
• %
• ,
• 0.2
• %
• ≤
• Si
• ≤
• 1.6
• %
• ,
• 0.02
• %
• ≤
• Al
• ≤
• 1.2
• %
• ,
• with
• 1.
• %
• ≤
• Si
• +
• Al
• ≤
• 2.2
• %
• ,
• 0
• ≤
• Nb
• ≤
• 0.035
• %
• ,
• 0
• ≤
• Mo
• ≤
• 0.1
• %
• ,
• and
• a remainder, the remainder being Fe and unavoidable impurities;
• annealing the steel sheet at an annealing temperature TA lower than Ac3 so as to obtain a structure comprising at least 65% of austenite and between 5% and 35% of intercritical ferrite;
• quenching the steel sheet from the annealing temperature down to a quenching temperature QT between 200° C. and 270° C., with a cooling rate, from the annealing temperature down to the quenching temperature, of more than 20° C./s and up to 50° C./s;
• heating the steel sheet up to a partitioning temperature PT comprised between 400° C. and 480° C. and maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt comprised between 50 s and 250 s;
• hot-dip coating the steel sheet at a temperature less than 515° C.;
• and
• cooling the coated steel sheet down to room temperature;
• the coated steel sheet having a microstructure consisting of, in surface fraction:
• between 3% and 15% of retained austenite,
• at least 30% of tempered martensite,
• at most 5% of fresh martensite,
• at most 35% of bainite,
• a sum of the surface fractions of tempered martensite, fresh martensite and bainite being comprised between 55% and 92%, and
• between 5% and 35% of intercritical ferrite.