Dual-phase steel, flat product made of a dual-phase steel of this type and processes for the production of a flat product

Abstract

A dual-phase steel, a flat product produced therefrom and a process for the production thereof. The dual-phase steel has, in addition to a strength of at least 950 MPa and good deformability, a surface finish which, when a simple production process is used, makes it possible for the flat product produced from this steel to be formed into a complexly formed component, such as a part of a car bodywork, in an uncoated state or in a state provided with an anti-corrosion coating. The steel according to the invention comprises 20-70% martensite, up to 8% retained austenite and the remainder ferrite and/or bainite and comprises (in % by weight): C: 0.10-0.20%, Si: 0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20-0.80%, Ti: 0.02-0.08%, B: <0.0020%, Mo: <0.25%, Al: <0.10%, P: ≦0.2%, S: ≦0.01%, N: ≦0.012%, the remainder iron and unavoidable impurities.

1 - 30 . (canceled) 
     
     
         31 . A dual-phase steel, having a structure comprising 20-70% martensite, up to 8% retained austenite and the remainder of ferrite and/or bainite and a tensile strength of at least 950 MPa, comprising (in % by weight): 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C: 
                   0.10-0.20%, 
                 
                     
                   Si: 
                   0.10-0.60%, 
                 
                     
                   Mn: 
                   1.50-2.50%, 
                 
                     
                   Cr: 
                   0.20-0.80%, 
                 
                     
                   Ti: 
                   0.02-0.08%, 
                 
                     
                   B: 
                   <0.0020%,  
                 
                     
                   Mo: 
                   <0.25%, 
                 
                     
                   Al: 
                   <0.10%, 
                 
                     
                   P: 
                    ≦0.2%, 
                 
                     
                   S: 
                   ≦0.01%, 
                 
                     
                   N: 
                   ≦0.012%  
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
         the remainder being iron and unavoidable impurities. 
       
     
     
         32 . The dual-phase steel according to  claim 31 , wherein the yield strength thereof is at least 580 MPa. 
     
     
         33 . The dual-phase steel according to  claim 31 , wherein the elongation A 80  thereof is at least 10%. 
     
     
         34 . The dual-phase steel according to  claim 31 , wherein the P content thereof is <0.1% by weight. 
     
     
         35 . The dual-phase steel according to  claim 31 , wherein the C content thereof is from 0.12 to 0.18% by weight. 
     
     
         36 . The dual-phase steel according to  claim 31 , wherein the Si content thereof is from 0.20 to 0.40% by weight. 
     
     
         37 . The dual-phase steel according to  claim 31 , wherein the Mn content thereof is from 1.50 to 2.35% by weight. 
     
     
         38 . The dual-phase steel according to  claim 31 , wherein the Cr content thereof is from 0.30 to 0.70% by weight. 
     
     
         39 . The dual-phase steel according to  claim 31 , wherein the Ti content thereof is from 0.030 to 0.055% by weight. 
     
     
         40 . The dual-phase steel according to  claim 31 , wherein in the presence of N, the Ti content of said dual-phase steel is more than 5.1 times greater than the respective N content. 
     
     
         41 . The dual-phase steel according to  claim 31 , wherein the B content thereof is from 0.0005 to 0.0020% by weight. 
     
     
         42 . The dual-phase steel according to  claim 41 , wherein the B content thereof is from 0.0007 to 0.0016% by weight. 
     
     
         43 . The dual-phase steel according to  claim 31 , wherein the Mo content thereof is from 0.05 to 0.22% by weight. 
     
     
         44 . The dual-phase steel according to  claim 43 , wherein the Mn content thereof is 1.5 to 1.7% by weight. 
     
     
         45 . The dual-phase steel according to  claim 43 , wherein the Cr content thereof is 0.20 to 0.4% by weight. 
     
     
         46 . The dual-phase steel according to  claim 31 , wherein the Mo content thereof is from 0.065 to 0.150% by weight. 
     
     
         47 . The dual-phase steel according to  claim 31 , wherein the Al content thereof is from 0.01 to 0.06% by weight. 
     
     
         48 . The dual-phase steel according to  claim 31 , wherein the S content thereof is <0.003% by weight. 
     
     
         49 . The dual-phase steel according to  claim 31 , wherein the N content thereof is <0.007% by weight. 
     
     
         50 . The dual-phase steel according to  claim 31 , wherein the retained austenite content thereof is less than 7%. 
     
     
         51 . A flat product comprising a dual-phase steel according to  claim 31 . 
     
     
         52 . The flat product according to  claim 51 , wherein it is a hot strip which has only been hot-rolled. 
     
     
         53 . The flat product according to  claim 51 , wherein it is a cold strip obtained by cold rolling. 
     
     
         54 . A flat product according to  claim 51 , further comprising a protective metallic coating. 
     
     
         55 . The flat product according to  claim 54 , wherein the protective metallic coating is produced by hot-dip galvanisation. 
     
     
         56 . The flat product according to  claim 54 , wherein the protective metallic coating is produced by galvannealing. 
     
     
         57 . A process for the production of a hot strip having a tensile strength of at least 950 MPa and a dual-phase structure comprising 20-70% martensite, up to 8% retained austenite and the remainder ferrite and/or bainite, comprising the following steps:
 melting a dual-phase steel obtained according to  claim 31 ,   casting the melt into a pre-product, such as slab or thin slab,   reheating to or keeping the pre-product at a starting hot rolling temperature of 1100-1300° C.,   hot rolling the pre-product at a final hot rolling temperature of 800-950° C. into a hot strip, and   reeling the hot strip at a reeling temperature of up to 570° C.   
     
     
         58 . A process for the production of a cold strip having a tensile strength of at least 950 MPa and a dual-phase structure comprising 20-70% martensite, up to 8% retained austenite and the remainder ferrite and/or bainite, comprising the following steps:
 melting a dual-phase steel composed according to  claim 31 ,   casting the melt into a pre-product, such as slab or thin slab,   reheating to or keeping the pre-product at a starting hot rolling temperature of 1100-1300° C.,   hot rolling the pre-product at a final hot rolling temperature of 800-950° C. into a hot strip,   reeling the hot strip at a reeling temperature of 500-650° C.,   cold-rolling the hot strip after reeling,   annealing the cold strip at an annealing temperature of 700-900° C., and   cooling the annealed cold strip in a controlled manner.   
     
     
         59 . A process according to  claim 58 , wherein the hot strip is cold-rolled into a cold strip with a degree of cold-rolling of from 40 to 70%. 
     
     
         60 . A process according to  claim 58 , wherein the controlled cooling is carried out within a temperature range of from 550 to 650° C. at a cooling rate of at least 10 K/s.