Method for Producing a Cold-Rolled Flat Steel Product Having a Bainitic Basic Structure and Cold-Rolled Flat Steel Product Having a Bainitic Basic Structure

Abstract

A process for producing a cold-rolled flat steel product having a bainitic base microstructure and a correspondingly cold-rolled flat steel product having a bainitic base microstructure.

1 . A process for producing a cold-rolled flat steel product having a bainitic base microstructure, comprising the steps of:
 a) casting a melt consisting of, aside from Fe and unavoidable impurities in % by weight:   C: 0.10 to 0.30%,   Si: 0.40 to 1.20%,   Mn: 1.00 to 2.00%,   Cr: 0.50 to 1.50%,   N: 0.0030 to 0.040%,   P: up to 0.10%,   S: up to 0.050%,   where P and S may count as impurities,   optionally, one or more alloy elements selected from the group consisting of Al, V, Ti, Nb, Ni, Mo, W, and Ca with   Al: up to 0.050%,   V: up to 0.20%,   Ti: up to 0.010%,   Nb: up to 0.10%,   Ni: up to 0.40%,   Cu: up to 0.80%,   Mo: up to 1.00%,   W up to 1.00%,   Ca: up to 0.0050%,   to give a precursor;   b) reheating the precursor to a temperature and/or keeping the precursor at a temperature between 1100 and 1350° C.;   c) optionally, hot-rolling the precursor in one or more roll stands at a final rolling temperature between 950 and 1250° C. to give a flat intermediate product;   d) hot-rolling the precursor or the optional flat intermediate product in one or more roll stands at a final rolling temperature between 800 and 1000° C. to give a hot-rolled flat steel product;   e) winding the hot-rolled flat steel product at a winding temperature between 400 and 650° C.;   f) optionally, annealing the hot-rolled flat steel product at an annealing temperature between 500 and 900° C.;   g) cold-rolling the hot-rolled and optionally annealed flat steel product in one or more roll stands with an overall degree of cold rolling of at least 30% to give a cold-rolled flat steel product;   h) heat-treating the cold-rolled flat steel product by austenitization at a temperature T_A between above 800 and 950° C., and quenching to a temperature T_B between 300 and 580° C., in such a way that a bainitic base microstructure is established in the cold-rolled flat steel product.   
     
     
         2 . The process as claimed in  claim 1 , wherein, in step h), the austenitization at T_A is conducted at a heating rate dTA of at least 1.0 K/s between 600 and 800° C. 
     
     
         3 . The process as claimed in  claim 1 , wherein, in step h), after attainment of temperature T_A, the cold-rolled flat steel product is kept at temperature T_A for a hold time t_A between 1 and 300 s. 
     
     
         4 . The process as claimed in  claim 1 , wherein, in step h), the quenching is effected in two stages, such that quenching is effected first to an intermediate temperature T_Z between 640 and 800° C. at a cooling rate dTZ of at least 0.50 K/s and then to temperature T_B between 300 and 580° C. at a cooling rate dTB of at least 10 K/s, where dTB is greater than dTZ. 
     
     
         5 . The process as claimed in  claim 1 , wherein, in step h), after attainment of temperature T_B, the cold-rolled flat steel product is kept at temperature T_B for a hold time t_B of at least 15 s. 
     
     
         6 . A cold-rolled flat steel product having a bainitic base microstructure produced according to  claim 1 , consisting of, aside from Fe and unavoidable impurities from the production, in % by weight:
 C: 0.10 to 0.30%,   Si: 0.40 to 1.20%,   Mn: 1.00 to 2.00%,   Cr: 0.50 to 1.50%,   N: 0.0030 to 0.040%,   P: up to 0.10%,   S: up to 0.050%,   where P and S may count as impurities,   optionally one or more alloy elements selected from the group consisting of Al, V, Ti, Nb, Ni, Mo, and Ca with   Al: up to 0.050%,   V: up to 0.20%,   Ti: up to 0.010%,   Nb: up to 0.10%,   Ni: up to 0.40%,   Cu: up to 0.80%,   Mo: up to 1.00%,   W: up to 1.00%,   Ca: up to 0.0050%,   where a microstructure of the cold-rolled flat steel product has the following proportions:   at least 0.5% martensite,   at least 5% residual austenite,   balance: bainite and unavoidable microstructure constituents.   
     
     
         7 . The cold-rolled flat steel product as claimed in  claim 6 , wherein bainite is also present in the form of lancet bainite, and the ratio of the proportions of lancet bainite to bainite is at least 60%. 
     
     
         8 . The cold-rolled flat steel product as claimed in  claim 6 , wherein the proportion of bainite is at least 60%. 
     
     
         9 . The cold-rolled flat steel product as claimed in  claim 6 , wherein the proportion of residual austenite is at least 8%. 
     
     
         10 . The cold-rolled flat steel product as claimed in  claim 6 , wherein the proportion of martensite is at least 10%. 
     
     
         11 . The cold-rolled flat steel product as claimed in  claim 6 , wherein the unavoidable microstructure constituents are up to a maximum of 10%. 
     
     
         12 . The cold-rolled flat steel product as claimed in  claim 6 , wherein the cold-rolled steel sheet has a tensile strength Rm of at least 1000 MPa. 
     
     
         13 . The cold-rolled flat steel product as claimed in  claim 10 , wherein the unavoidable microstructure constituents are fractions in the form of ferrite, perlite and/or cementite that are present apart from bainite and martensite.