Methods for Producing Coated High-Strength Steel Sheets with Improved Ductility and Formability and Obtained Coated Steel Sheets

摘要

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 of at least 12%, the product TSxTE of the tensile strength by the total elongation being at least 14200 MPa%, and a hole expansion ratio HER according to ISO standard 16630:2009 of at least 25%, comprising the following successive steps:- providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %:0.15% ≤ C ≤ 0.23%, 2.0 % ≤ Mn ≤ 2.7%, with C+Mn/10 ≥ 0.420%, 0 ≤ Cr ≤ 0.40%, with Mn + Cr ≥ 2.25%, 0.2% ≤ Si ≤ 1.6%, 0.02% ≤ Al ≤ 1.2%, with 1.0 % ≤ Si+Al ≤ 2.2%, 0 ≤ Nb ≤ 0.035 %, 0 ≤ Mo ≤ 0.1%, the remainder being Fe and unavoidable impurities,- annealing the steel sheet at an annealing temperature TAso as to obtain a structure comprising at least 65% of austenite and at most 35% of intercritical ferrite,- quenching the sheet from a temperature of at least 600°C at a cooling rate comprised between 20°C/s and 50°C/s down to a quenching temperature QT between 200°C and 270°C,- heating the sheet up to a partitioning temperature PT comprised between 400°C and 480°C and maintaining the sheet at this partitioning temperature PT for a partitioning time Pt comprised between 50s and 250s,- hot-dip coating the sheet at a temperature less than 515°C,- cooling the coated sheet down to the room temperature,the 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, the sum of the surface fractions of tempered martensite, fresh martensite and bainite being comprised between 55% and 92%, and between 5% and 35% of ferrite.