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

Abstract

A coated steel sheet includes 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. A structure of the steel sheet consists of, by volume fraction, 3% to 15% of residual austenite and 85% to 97% of martensite and bainite. The structure includes at least 65% of martensite and does not including ferrite. At least one face of the coated steel sheet includes a metallic coating. The steel 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%.

What is claimed is: 
     
         1 . A coated steel sheet comprising:
 a chemical composition of the steel including in weight %:
 13%≤C≤0.22%; 
 2%≤Si≤1.8%; 
 8%≤Mn≤2.2% 
 0.10%≤Mo≤0.20%; 
 Nb≤0.05%; 
 Al≤0.5%; and 
 Ti≤0.05%; 
 a remainder being Fe and unavoidable impurities; 
   a structure consisting of, by volume fraction, 3% to 15% of residual austenite and 85% to 97% of martensite and bainite, the structure including at least 65% of martensite, the structure not including ferrite;   at least one face of the coated steel sheet including a metallic coating; and   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%.   
     
     
         2 . The coated steel sheet according to  claim 1 , wherein the chemical composition of the steel satisfies at least one of the following conditions:
 16%≤C≤0.20%;   3%≤Si≤1.6%; and   1.9%≤Mn≤2.1%.   
     
     
         3 . The coated steel sheet according to  claim 1 , wherein the at least one face including a metallic coating is galvanized. 
     
     
         4 . The coated steel sheet according to  claim 1 , wherein the at least one face including a metallic coating is galvannealed. 
     
     
         5 . The coated steel sheet according to  claim 1 , wherein the chemical composition of the steel satisfies 0.02%≤Nb≤0.05%.