Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Ductility and Formability

Abstract

A method for producing a high strength coated steel sheet having a yield stress YS>800 MPa, a tensile strength TS>1180 MPa, and improved formability and ductility. The steel contains: 15%≦C≦0.25%, 1.2%≦Si≦1.8%, 2%≦Mn≦2.4%, 0.1% 23 Cr≦0.25%, Al≦0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250° C. and 350° C., to obtain a structure consisting of at least 60% of martensite and a sufficient austenite content such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430° C. and 480° C. and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature.

1 - 16 . (canceled) 
     
     
         17 . A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the coated steel 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%, comprising the steps of:
 providing a steel sheet having a chemical composition including in weight %:
   0.15%≦C≦0.25%;
 
   1.2%≦Si≦1.8%;
 
   2%≦Mn≦2.4%;
 
   0.1%≦Cr≦0.25%; and
 
   Al≦0.5%;
 
 a remainder being Fe and unavoidable impurities; 
   annealing the steel sheet at an annealing temperature TA higher than Ac3 and less than 1000° C. for a time of more than 30 s;   quenching the steel sheet by cooling the sheet 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, with a martensite content of at least 60% and an austenite content such that the coated steel sheet has a final structure including between 3% and 15% of retained austenite and 85% to 97% of martensite and bainite, the final structure not including ferrite;   heating the steel sheet up to a partitioning temperature PT between 430° C. and 480° C. and maintaining the steel sheet at the partitioning temperature for a partitioning time Pt between 10 s and 90 s;   hot dip coating the steel sheet; and   cooling the steel sheet down to the room temperature.   
     
     
         18 . The method according to  claim 17 , wherein the chemical composition of the steel sheet includes 0.17%≦C≦0.21%. 
     
     
         19 . The method according to  claim 17 , wherein the chemical composition of the steel sheet includes 1.3%≦Si≦1.6%. 
     
     
         20 . The method according to  claim 17 , wherein the chemical composition of the steel sheet includes 2.1%≦Mn≦2.3%. 
     
     
         21 . The method according to  claim 17 , wherein the hot dip coating step is a galvanizing step. 
     
     
         22 . The method according to  claim 17 , wherein the hot dip coating step is a galvannealing step with an alloying temperature TGA between 480° C. and 510° C. 
     
     
         23 . The method according to  claim 17 , wherein the cooling speed during quenching is at least 20° C./s. 
     
     
         24 . The method according to  claim 23 , wherein the cooling speed during quenching is at least 30° C./s. 
     
     
         25 . The method according to  claim 17 , further comprising, after the step of quenching and before the step of heating, a step of holding the steel sheet at the quenching temperature QT for a holding time between 2 s and 8 s 
     
     
         26 . The method according to  claim 25 , wherein the holding time at the quenching temperature QT is between 3 s and 7 s. 
     
     
         27 . A coated steel sheet comprising:
 a chemical composition of the steel including in weight %:
   0.15%≦C≦0.25%;
 
   1.2%≦Si≦1.8%;
 
   2.1%≦Mn≦2.3%;
 
   0.10%≦Cr≦0.25%; and
 
   Al≦0.5%;
 
 a remainder being Fe and unavoidable impurities; 
   a structure consisting of 3% to 15% of retained austenite and 85% to 97% of martensite and bainite, 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%.   
     
     
         28 . The coated steel sheet according to  claim 27 , wherein the chemical composition includes 0.17%≦C≦0.21%. 
     
     
         29 . The coated steel sheet according to  claim 27 , wherein the chemical composition includes 1.3%≦Si≦1.6%. 
     
     
         30 . The coated steel sheet according to  claim 27 , wherein the at least one face including a metallic coating is galvanized. 
     
     
         31 . The coated steel sheet according to  claim 27 , wherein the at least one face including a metallic coating is galvannealed. 
     
     
         32 . The coated steel sheet according to  claim 27 , wherein the retained austenite has a C content of at least 0.9%. 
     
     
         33 . The coated steel sheet according to  claim 32 , wherein the retained austenite has a C content of at least 1.0%. 
     
     
         34 . The coated steel sheet according to  claim 27 , wherein the retained austenite has an average grain size of 5 μm or less.