Flat steel product and method for the production thereof

Abstract

A flat steel product may have a tensile strength R m ≥950 MPa, a yield point ≥800 MPa, and an elongation at break A 50 ≥8%. The flat steel product may comprise steel consisting of (in percent weight) 0.05%-0.20% C, 0.2%-1.5% Si, 0.01%-1.5% Al, 1.0%-3.0% Mn, ≤0.02% P, ≤0.005% S, ≤0.008% N, and in each case optionally 0.05%-1.0%, 0.05%-0.2% Mo, 0.005%-0.2% Ti, 0.001%-0.05% Nb, 0.0001%-0.005% B, the remainder being Fe and unavoidable impurities. A ratio ψ may conform to 1.5≤ψ≤3 with ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si), and % C, % Mn, % Cr, % Al, % Si being the respective C, Mn, Cr, Al, and Si contents of the steel. The flat steel product may have a microstructure consisting of (in percent area)≤5% bainite, ≤5% polygonal ferrite, ≥90% martensite, and ≤2% by volume of residual austenite, where at least half the martensite is annealed martensite.

1 .- 9 . (canceled) 
     
     
         10 . A flat steel product having a tensile strength R m  of at least 950 MPa, a yield point of at least 800 MPa, and an elongation at break A 50  of at least 8%, wherein the flat steel product comprises a steel consisting of iron, unavoidable impurities, and in percent by weight:
 0.05%-0.20% C;   0.2%-1.5% Si;   0.01%-1.5% Al;   1.0%-3.0% Mn;   up to 0.02% P;   up to 0.005% S; and   up to 0.008% N,   
       wherein a microstructure of the flat steel product consists of
 up to 5% by area bainite; 
 up to 5% by area polygonal ferrite; 
 up to 2% by volume residual austenite; and 
 at least 90% by area martensite, wherein at least half of the martensite is annealed martensite. 
 
     
     
         11 . The flat steel product of  claim 10  wherein the steel further includes in percent by weight one or more of
 0.05%-1.0% Cr; 
 0.05%-0.2% Mo; 
 0.005%-0.2% Ti; 
 0.001%-0.05% Nb; or 
 0.0001-0.005% B, 
 wherein a ratio ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si) conforms to 1.5≤ψ≤3, with % Mn being a respective Mn content of the steel, % Cr being a respective Cr content of the steel, % Al being a respective Al content of the steel, and % Si being a respective Si content of the steel. 
 
     
     
         12 . The flat steel product of  claim 11  wherein a sum total of Ti and Nb is at most 0.2% by weight. 
     
     
         13 . The flat steel product of  claim 11  comprising a carbon equivalent CE=% C+(% Si+% Mn)/5+(% Cr+% Mo)/6 that conforms to 0.254%≤CE≤1.1% by weight, with % C being a respective C content of the steel, % Si being a respective Si content of the steel, % Mn being a respective Mn content of the steel, % Cr being a respective Cr content of the steel, and % Mo being a respective Mo content of the steel. 
     
     
         14 . The flat steel product of  claim 13  wherein the carbon equivalent CE is at most 1.0% by weight. 
     
     
         15 . The flat steel product of  claim 10  wherein a sum total of Si and Al is at most 1.7% by weight. 
     
     
         16 . The flat steel product of  claim 10  further comprising a metallic protective coat applied by melt dip coating. 
     
     
         17 . A process for producing a flat steel product comprising:
 providing a flat steel product that is uncoated and comprises a steel consisting of iron, unavoidable impurities, and in percent by weight
 0.05%-0.20% C, 
 0.2%-1.5% Si, 
 0.01%-1.5% Al, 
 1.0%-3.0% Mn, 
 up to 0.02% P, 
 up to 0.005% S, and 
 up to 0.008% N; 
   heating the flat steel product to an austenitization temperature T HZ  that is above an A 3  temperature of the steel of the flat steel product and is not more than 950° C., wherein the heating is effected up to an inflection temperature T W  of 200-400° C. at a first heating rate θ H1  of 5-25 K/s and then up to the austenitization temperature T HZ  at a second heating rate θ H2  of at least 2-10° K/s;   holding the flat steel product at the austenitization temperature T HZ  over an austenitization period t HZ  of 5-15 seconds;   first cooling the flat steel product over a cooling period t k  of 50-300 seconds to an intermediate temperature T K  of not less than 680° C.;   quenching the flat steel product from the intermediate temperature T K  at a cooling rate θ Q  of more than 30 K/s to a cooling finish temperature T Q  where (T MS -175° C.)<T Q <T MS , wherein T MS  is a martensite start temperature of the steel;   maintaining the flat steel product at the cooling finish temperature T Q  for a holding period t Q  of 10-60 seconds;   treating the flat steel product that has been quenched to the cooling finish temperature T Q , wherein
 the flat steel product, over a total treatment period t B  of 10-1000 seconds, is kept at a treatment temperature T B  at least equal to the cooling finish temperature T Q  and not higher than 550° C., or 
 the flat steel product, proceeding from the cooling finish temperature T Q , is heated to a treatment temperature T B  of 450-500° C., wherein the flat steel product is kept under isothermal conditions at the treatment temperature T B  over a holding period t BI , wherein the heating to the treatment temperature T B  is effected at a heating rate θ B1  of less than 80 K/s and a total treatment period t BT , formed as a sum total of the heating period t BR  required for the heating and the holding time t BI , is 10-1000 seconds, wherein after the treatment the flat steel product is passed through a melt bath to overcoat the flat steel product with a metallic protective coating based on Zn; and 
   cooling, proceeding from the treatment temperature T B , at a cooling rate θ B2  of more than 5 K/s.   
     
     
         18 . The process of  claim 17  wherein the steel further includes in percent by weight one or more of
 0.05%-1.0% Cr; 
 0.05%-0.2% Mo; 
 0.005%-0.2% Ti; 
 0.001%-0.05% Nb; or 
 0.0001-0.005% B, 
 wherein a ratio ψ=(% C+% Mn/5+% Cr/6)/(% Al+% Si) conforms to 1.5≤ψ≤3, with % Mn being a respective Mn content of the steel, % Cr being a respective Cr content of the steel, % Al being a respective Al content of the steel, and % Si being a respective Si content of the steel. 
 
     
     
         19 . The process of  claim 18  further comprising coating the flat steel product with a Zn coating. 
     
     
         20 . The process of  claim 17  wherein the first heating rate θ H1  is the same as the second heating rate θ H2 . 
     
     
         21 . The process of  claim 17  wherein the treatment step of keeping the flat steel product, over the total treatment period t B  of 10-1000 seconds, at the treatment temperature T B  at least equal to the cooling finish temperature T Q  and not higher than 550° C. comprises heating the flat steel product from the cooling finish temperature T Q  at a heating rate θ B1  of less than 80 K/s to the treatment temperature T B .