Process for manufacturing high strength steel

Abstract

A method of making high strength steel sheet with a tensile strength of 800 to 1000 MPa and a hole expansion ratio of at least 50%, comprising the steps of reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3; hot rolling the slab to final desired thickness; cooling the steel sheet at a rate of 50° C. per second to a temperature less than 400° C.; and winding the steel sheet into a coil.

What is claimed is: 
     
         1 . A method of making high strength steel sheet with a tensile strength of 800 to 1100 MPa and a hole expansion ratio of at least 50%, comprising the steps of:
 reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3;   hot rolling the slab to final desired thickness; cooling the steel sheet at a rate of 50° C. per second to a temperature less than 400° C.; and   winding the steel sheet into a coil.   
     
     
         2 . The method of making high strength steel according to  claim 1 , wherein the temperature Ar3 is a temperature greater than the austenite-to-ferrite transformation temperature. 
     
     
         3 . The method of making high strength steel according to  claim 1 , further comprising cooling the steel sheet to an acicular ferrite structure. 
     
     
         4 . The method of making high strength steel according to  claim 1 , further comprising cooling the steel sheet to a bainitic structure. 
     
     
         5 . The method of making high strength steel according to  claim 1 , further comprising the application of a secondary treatment to the steel sheet to promote precipitation reactions for strength preservation or an increase in strength. 
     
     
         6 . The method of making high strength steel according to  claim 5 , further comprising reheating the steel coil to a temperature below Ac1. 
     
     
         7 . The method of making high strength steel according to  claim 6 , wherein the temperature Ac1 is a temperature above 500° C. and below the ferrite-to-austenite phase transformation temperature. 
     
     
         8 . The method of making high strength steel according to  claim 6 , wherein the temperature depends on a time duration anticipated for a process employed. 
     
     
         9 . The method of making high strength steel according to  claim 6 , further comprising continuous annealing of the steel sheet to achieve reduced heating times. 
     
     
         10 . The method of making high strength steel according to  claim 9 , wherein the reduced duration of the heating time during the continuous annealing allows for the steel sheet to approach a temperature Ac1 temperature while achieving the desired properties. 
     
     
         11 . A method of making high strength steel sheet having a tensile strength of approximately 800 MPa and a composition of 0.06 weight percent of Carbon, 1.0 weight percent of Mn and 0.1 weight percent of Si, 0.03 weight percent of Ti and 0.0020 weight percent of boron, comprising the steps of:
 reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3;   than 400° C.; and   hot rolling the slab to final desired thickness;   cooling the steel sheet at a rate of 50° C. per second to a temperature less winding the steel sheet into a coil.   
     
     
         12 . The method of making high strength steel according to  claim 11 , further comprising cooling the steel sheet to an acicular ferrite. 
     
     
         13 . The method of making high strength steel according to  claim 11 , further comprising a applying a secondary treatment to the steel sheet to promote precipitation reactions for strength preservation or an increase in strength. 
     
     
         14 . The method of making high strength steel according to  claim 13 , further comprising reheating the steel coil to a temperature below Act. 
     
     
         15 . The method of making high strength steel according to  claim 14 , wherein the temperature Ac1 is a temperature above 500° C. and below the ferrite-to-austenite phase transformation temperature. 
     
     
         16 . The method of making high strength steel according to  claim 11 , wherein the temperature depends on a time duration anticipated for a process employed. 
     
     
         17 . The method of making high strength steel according to  claim 11 , further comprising continuous annealing of the steel sheet to achieve reduced heating times. 
     
     
         18 . The method of making high strength steel according to  claim 17 , wherein the reduced duration of the heating time during the continuous annealing allows for the steel sheet to approach a temperature Ac1 temperature while achieving the desired properties. 
     
     
         19 . A method of making high strength steel sheet a tensile strength of approximately 1000 MPa and a composition 0.06 weight percent of C, 1.0 weight percent of Mn, 0.1 weight percent of Si, 0.03 weight percent of Ti and 0.0020 weight percent of B, comprising the steps of:
 slab, above Ar3;   than 400° C.; and   reheating a previously cast slab, or retaining the heat from a directly cast hot rolling the slab to final desired thickness;   cooling the steel sheet at a rate of 50° C. per second to a temperature less winding the steel sheet into a coil.