Method for manufacturing flat steel products from a steel forming a martensitic microstructure

Abstract

A method, which allows extremely high-tensile flat steel products to be manufactured with less effort includes a steel that forms a martensitic microstructure and contains (in wt. %) C: 0.15-0.19%, Mn: 0.80-1.20%, P: <0.030%, S: <0.004%, Si: 0.60-1.00%, Al: <0.05%, N: <0.0060%, Cr: 0.30-0.60%, Nb: 0.040-0.070%, remainder iron and unavoidable impurities, being cast into a cast strip having a thickness of 1-4 mm. The cast strip is hot-rolled in-line into a hot-rolled strip having a thickness of 0.5-3.2 mm in a continuous process at a final hot-rolling temperature ranging from 900 to 1050 ° C., the deformation degree being greater than 20%. The hot-rolled strip is coiled at a coiling temperature of at most 350° C., so as to obtain a hot-rolled strip, which has a minimum tensile strength R m of 1400 MPa at a minimum breaking elongation A 80 of 5%.

1 . Method for manufacturing flat steel products,
 wherein a steel that forms a martensitic microstructure with the following composition (in wt. %)   C: 0.15-0.19%   Mn: 0.80-1.20%   P: ≦0.030%   S: ≦0.004%   Si: 0.60-1.00%   Al: ≦0.05%   N: ≦0.0060%   Cr: 0.30-0.60%   Nb: 0.040-0.070%   
     remainder iron and unavoidable impurities is cast into a cast strip having a thickness of 1-4 mm,
 wherein the cast strip is hot-rolled in-line into a hot-rolled strip having a thickness ranging from 0.5 to 3.2 mm in a continuous process at a final hot-rolling temperature ranging from 900 to 1050° C., the deformation degree being greater than 20% and 
 wherein the hot-rolled strip is coiled at a coiling temperature of at most 350° C., 
 so as to obtain a hot-rolled strip, which has a minimum tensile strength R m  of 1400 MPa at a minimum breaking elongation A 80  of 5%. 
 
   
   
       2 . Method according to  claim 1 , wherein the width of the hot-rolled strip is greater than 1,200 mm. 
   
   
       3 . Method according to  claim 1 , wherein the thickness of the hot-rolled strip is 1.5 mm at most. 
   
   
       4 . Method according to  claim 1 , wherein the hot-rolled strip is cold-rolled into cold-rolled strip having a thickness of 0.5-1.4 mm. 
   
   
       5 . Method according to  claim 4 , wherein the cold-rolled strip is annealed at an annealing temperature of 750-850° C. 
   
   
       6 . Method according to  claim 4 , wherein a minimum tensile strength of the cold-rolled strip is 600 MPa. 
   
   
       7 . Method according to  claim 4 , wherein the cold-rolled strip has a minimum breaking elongation A 50  of 15%. 
   
   
       8 . Method according to  claim 1 , wherein the hot-rolled strip or cold rolled strip is provided with a metallic coating. 
   
   
       9 . Method according to  claim 8 , wherein the metallic coating is a zinc coating. 
   
   
       10 . Method according to  claim 4 , wherein the cold-rolled strip is provided with a metallic coating. 
   
   
       11 . Method according to  claim 10 , wherein the metallic coating is a zinc coating.