CPC C23C 2/06 (2013.01) [B23K 11/11 (2013.01); B23K 11/166 (2013.01); C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0273 (2013.01); C21D 9/46 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/12 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] | 31 Claims |
1. A method for producing a coated steel sheet having a yield strength of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation according to ISO standard 6892-1 of at least 13% and a hole expansion ratio HER according to ISO standard 16630:2009 of at least 30%, the method comprising the following successive steps:
providing a cold-rolled steel sheet made of a steel having a chemical composition containing by weight:
15% ≤C≤0.23%
1.4% ≤Mn≤2.6%,
0.6% ≤Si≤1.3%
with C+Si/10≤0.30%,
0.4% ≤Al≤1.0%,
with Al≥6(C+Mn/10)−2.5%,
0.010% ≤Nb≤0.035%,
0.1% ≤Mo≤0.5%,
a remainder being Fe and unavoidable impurities,
annealing the cold-rolled steel sheet at an annealing temperature TA between 860° C. and lower than Ac3, so as to obtain an annealed steel sheet having a structure consisting of at least 90% of austenite and at least 2% of intercritical ferrite;
quenching the annealed steel sheet from the annealing temperature TA down to a quenching temperature QT between Ms-10° C. and Ms-60° C. at an average cooling rate Vc higher than 30° C./s, to obtain a quenched steel sheet;
heating the quenched steel sheet from the quenching temperature QT to a partitioning temperature PT between 410° C. and 470° C., and maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt between 60 s and 130 s,
hot-dip coating the steel sheet in a bath; and
cooling the steel sheet down to room temperature, to obtain a coated steel sheet having a microstructure comprising, in area fraction:
between 45% and 68% of martensite, the martensite comprising partitioned martensite and fresh martensite, between 85% and 95% of the martensite being partitioned martensite, the partitioned martensite having a C content of at most 0.45%,
between 10% and 15% of retained austenite,
between 2% and 10% of intercritical ferrite, and
between 20% and 30% of lower bainite.
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