Cold rolled and annealed steel sheet, method of production thereof and use of such steel to produce vehicle parts

Abstract

A cold rolled and annealed steel sheet includes by weight: 0.6≤C≤1.3%, 15.0≤Mn≤35%, 5≤Al≤15%, Si≤2.40%, S≤0.03%, P≤0.1%, N≤0.1%, possibly one or more optional elements chosen among Ni, Cr and Cu in an respective amount of up to 4.0%, up to 3.0% and up to 3.0% and possibly one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, the microstructure of said sheet comprising optionally up to 3% of kappa carbides, optionally up to 10.0% of granular ferrite, the remainder being made of austenite, the average grain size and average aspect ratio of the austenite being respectively below 6 μm and comprised between 1.5 and 6 and the average grain size and average aspect ratio of the ferrite, when present, being respectively below 5 μm and below 3.0.

1 - 14 . (canceled) 
     
     
         15 . A cold rolled and annealed steel sheet comprising by weight:
   0.6≤C≤1.3%,
     15.0≤Mn≤35%,
     5≤Al≤15%,
     Si≤2.40%
     S≤0.03% a,
     P≤0.1% a,
     N≤0.1%,
   the remainder of the composition making up of iron and inevitable impurities resulting from elaboration, the microstructure of said sheet comprising optionally up to 3% of kappa carbides, optionally up to 10% of granular ferrite, the remainder being made of austenite, an average grain size of the austenite being below 6 μm, an average aspect ratio of the austenite being between 1.5 and 6, an average grain size of the ferrite, when present, being below 5 μm, and an average aspect ratio of the ferrite, when present, being below 3.0.   
     
     
         16 . A steel sheet according to  claim 15 , wherein the steel sheet further comprises one or more elements chosen among Ni, Cr and Cu in an respective amount of up to 4.0%, up to 3.0% and up to 3.0%. 
     
     
         17 . A steel sheet according to  claim 15 , wherein the steel sheet further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%. 
     
     
         18 . A steel sheet according to  claim 16 , wherein the steel sheet further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%. 
     
     
         19 . A steel sheet according to  claim 15 , wherein the carbon content is between 0.8 and 1.0%. 
     
     
         20 . A steel sheet according to  claim 15 , wherein the manganese content is between 20 and 30%. 
     
     
         21 . A steel sheet according to  claim 15 , wherein the aluminum content is between 8.5 and 10%. 
     
     
         22 . A steel sheet according to  claim 15 , wherein the steel sheet has a ultimate tensile strength of at least 900 MPa, a yield strength of at least 700 MPa and a uniform elongation of at least 28%. 
     
     
         23 . A steel sheet according to  claim 15 , wherein the steel sheet is covered by a metallic coating. 
     
     
         24 . A steel sheet according to  claim 15 , wherein the steel sheet is covered by an aluminum-based coating or a zinc-based coating. 
     
     
         25 . A method for producing a steel sheet comprising:
 feeding a slab having a composition including, by weight: 0.6≤C≤1.3%, 15.0≤Mn≤35%, 5≤Al≤15%, Si≤2.40%, S≤0.03%, P≤0.1%, N≤0.1%, the remainder of the composition making up of iron and inevitable impurities resulting from elaboration;   reheating the slab at a temperature above 1000° C. and hot rolling the slab with a final rolling temperature of at least 800° C. to obtain a hot rolled steel sheet;   coiling the hot rolled steel sheet at a temperature below 600° C.;   cold-rolling such hot rolled steel sheet at a reduction comprised between 30 and 80%;   annealing such cold rolled sheet by heating it up to an annealing temperature between 700 and 1000° C., holding it at such temperature during less than 5 minutes and cooling it at a rate of at least 30° C./s.   
     
     
         26 . The method according to  claim 25 , wherein the composition further comprises one or more elements chosen among Ni, Cr and Cu in an respective amount of up to 4.0%, up to 3.0% and up to 3.0%. 
     
     
         27 . The method according to  claim 25 , wherein the composition further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%. 
     
     
         28 . The method according to  claim 27 , wherein the steel sheet further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%. 
     
     
         29 . The method according to  claim 25 , wherein the annealing temperature is between 800 and 950° C. 
     
     
         30 . The method according to  claim 25 , wherein the coiling temperature is between 350 and 500° C. 
     
     
         31 . The method according to  claim 25 , comprising further a final coating step. 
     
     
         32 . A structural or safety part of a vehicle comprising the steel sheet of  claim 15 . 
     
     
         33 . The part according to  claim 32 , obtained by flexible rolling of said steel sheet. 
     
     
         34 . A vehicle comprising a part according to  claim 32 .