The present invention relates to a method for producing a high strength coated steel sheet having improved strength, ductility and formability and to the sheets obtained with the method.

BACKGROUND

To manufacture various equipments such as parts of body structural members and body panels for automotive vehicles, it is usual to use galvanized or galvannealed sheets made of DP (dual phase) steels or TRIP (transformation induced plasticity) steels.

For example, such steels which include a martensitic structure and/or some retained austenite and which contain about 0.2% of C, about 2% of Mn, about 1.7% of Si have a yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of more than 8%. These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac3transformation point, down to an overaging temperature above Ms transformation point and maintaining the sheet at the temperature for a given time. Then the sheet is galvanized or galvannealed.

To reduce the weight of the automotive so as to improve their fuel efficiency in view of the global environmental conservation, it is desirable to have sheets having improved yield and tensile strength. But such sheets must also have a good ductility and a good formability and more specifically a good stretch flangeability.

In this respect, it is desirable to have sheets having a yield strength YS of at least 800 MPa, a tensile strength TS of about 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER according to ISO standard 16630:2009 of more than 25%. It must be emphasized that, due to differences in the methods of measure, the values of hole expansion ration HER according to the ISO standard are very different and not comparable to the values of the hole expansion ratio λ according to the JFS T 1001 (Japan Iron and Steel Federation standard).

SUMMARY

The present disclosure provides such sheet and a method to produce it.

A method is provided for producing a high strength coated steel sheet having an improved ductility and an improved formability, the 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%, by heat treating and coating a steel sheet wherein the chemical composition of the steel contains in weight %:0.15%≤C≤0.25%1.2%≤Si≤1.8%2%≤Mn≤2.4%0.1%≤Cr≤0.25%Al≤0.5%
the remainder being Fe and unavoidable impurities.

The heat treatment and the coating comprise the following steps: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 structure consisting of martensite and austenite just after quenching, the martensite content being at least 60% and the austenite content being such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite,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 coatings the sheet and,cooling the sheet down to the room temperature.

The chemical composition of the steel can, optionally, satisfy one or more of the following conditions: 0.17%≤C≤0.21%, 1.3%≤Si≤1.6% and 2.1%≤Mn≤2.3%.

In a particular embodiment, the hot dip coating step is a galvanizing step.

In another particular embodiment, the hot dip coating step is a galvannealing step with an alloying temperature TGA between 480° C. and 510° C.

Preferably, the cooling speed during the quenching is of at least 20° C./s, preferably at least 30° C./s.

Preferably, the method further comprises, after the sheet is quenched to the quenching temperature and before heating the sheet up to the partitioning temperature PT, a step of holding the sheet at the quenching temperature for a holding time comprised between 2 s and 8 s, preferably between 3 s and 7 s. In a particular embodiment, the at least one coated face is galvanized.

A coated steel sheet is also provided, the chemical composition of the steel containing in weight %:0.15%≤C≤0.25%1.2%≤Si≤1.8%2%≤Mn≤2.4%0.1≤Cr≤0.25%Al≤0.5%
the remainder being Fe and unavoidable impurities. The structure of the steel consists of 3% to 15% of residual austenite and 85% to 97% of martensite and bainite, without ferrite. A least one face of the sheet comprises a metallic coating. The sheet has a yield strength of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%.

The chemical composition of the steel can, optionally, satisfy one or more of the following conditions: 0.17%≤C≤0.21%, 1.3%≤Si≤1.6% and 2.1%≤Mn≤2.3%.

In another particular embodiment, the at least one coated face is galvannealed.

Preferably, the C content in the retained austenite is of at least 0.9%, still preferably of at least 1.0%, and up to 1.6%.

The average austenitic grain size, i.e. the average grain size of the retained austenite, is preferably of 5 μm or less.

The average size of the grains or blocks of martensite and bainite is preferably of 10 μm or less.

