Cold-Rolled Flat Steel Product and Method for the Production Thereof

Abstract

A cold-rolled flat steel product where Rm≧1400 MPa and A80≧5% and also a method for producing such product. The product includes, in addition to Fe and unavoidable impurities (in wt. %), 0.10-0.60% C, 0.4-2.5% Si, ≦3.0% Al, 0.4-3.0% Mn, ≦1.0% Ni, ≦2.0% Cu, ≦0.4% Mo, ≦% Cr, ≦1.5% Co, ≦0.2% Ti, ≦0.2% Nb, ≦0.5% V. The microstructure includes (in vol. %) ≧20% bainite, 10-35% residual austenite and martensite as the remainder. A slab, thin slab or a cast strip having said composition, is hot-rolled to form hot strip with a hot-rolling end temperature ≧830° C., coiled at a coiling temperature ≦560° C., cold-rolled at ≧30% reduction and heat-treated by firstly being heated to an annealing temperature ≧800° C., then being cooled at a cooling rate of ≧8° C./s to a holding temperature of 470° C. to greater than the martensite start temperature and then being held at the holding temperature until at least 20 vol. % bainite is present in the microstructure.

1 . A cold-rolled flat steel product, having a tensile strength Rm of at least 1400 MPa and an elongation A80 of at least 5% and comprising, in addition to iron and unavoidable impurities (in % by weight):
 C: 0.10-0.60%,   Si: 0.4-2.5%,   Al: up to 3.0%,   Mn: 0.4-3.0%,   Ni: up to 1.0%,   Cu: up to 2.0%,   Mo: up to 0.4%,   Cr: up to 2%,   Co: up to 1.5%,   Ti: up to 0.2%,   Nb: up to 0.2%, and   V: up to 0.5%,   wherein the microstructure of the flat steel product consists of bainite to an extent of at least 20% by volume, of residual austenite to an extent of 10-35% by volume and of martensite as the remainder.   
     
     
         2 . The flat steel product according to  claim 1 , wherein the C content thereof is at least 0.25% by weight. 
     
     
         3 . The flat steel product according to either of  claim 1 , wherein the C content thereof is at least 0.27% by weight. 
     
     
         4 . The flat steel product according to  claim 1 , wherein the Si content thereof is at least 1.0% by weight. 
     
     
         5 . The flat steel product according to  claim 1 , wherein the Al content thereof is at least 0.01% by weight. 
     
     
         6 . The flat steel product according to  claim 1 , wherein the Cu content thereof is at least 0.2% by weight. 
     
     
         7 . The flat steel product according to  claim 5 , wherein the Cu content thereof is at least 0.55% by weight. 
     
     
         8 . The flat steel product according to  claim 1 , wherein the Cr content thereof is at least 0.3% by weight. 
     
     
         9 . The flat steel product according to  claim 1 , wherein the Mn, Cr, Ni, Cu and C contents thereof satisfy the following condition:
   1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2, where   % Mn: respective Mn content in % by weight,   % Cr: respective Cr content in % by weight,   % Ni: respective Ni content in % by weight,   % Cu: respective Cu content in % by weight,   % C: respective C content in % by weight.   
     
     
         10 . The flat steel product according to  claim 1 , wherein the microstructure thereof comprises at least 50% by volume bainite. 
     
     
         11 . The flat steel product according to  claim 1 , wherein the microstructure thereof comprises 10-25% by volume residual austenite. 
     
     
         12 . A method for producing a flat steel product, said method comprising the following work steps:
 providing a preliminary product in the form of a slab, thin slab or a cast strip, which, in addition to iron and unavoidable impurities, comprises (in % by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%;   hot-rolling the preliminary product to form a hot strip in one or more rolling passes, wherein the hot strip obtained has a hot-rolling end temperature of at least 830° C. when it leaves the last rolling pass;   coiling the hot strip obtained at a coiling temperature which lies between the hot-rolling end temperature and 560° C.;   cold-rolling the hot strip to form a cold strip with a degree of cold-rolling of at least 30%;   heat-treating the cold strip obtained, wherein, during the course of the heat treatment, the cold strip   is heated to an annealing temperature amounting to at least 800° C.,   is cooled proceeding from the annealing temperature at a cooling rate amounting to at least 8° C./s to a holding temperature which lies in a holding temperature range having an upper limit of 470° C. and having a lower limit which is higher than the martensite start temperature (MS), from which martensite forms in the microstructure of the cold strip, and   is held at the holding temperature for a period of time which is sufficient to form at least 20% by volume bainite in the microstructure of the cold strip.   
     
     
         13 . The method according to  claim 12 , wherein the hot-rolling end temperature is 850-950° C. 
     
     
         14 . The method according to  claim 12 , wherein the holding temperature is 300-420° C. 
     
     
         15 . The method according to  claim 12 , wherein the cold strip is coated with a metallic protective layer after the heat treatment.