Method for producing high strength, high ductility steel strip

Abstract

A method is provided for the manufacture of steel steel strip having high strength and high ductility. The method includes hot rolling steel consisting essentially of in weight percent C 0.05/0.20, Mn 3.0/8.0, Si less than 0.5, Al less than 0.1, the balance Fe and inevitable impurities at a minimum finishing temperature of Ar1+50 C, cooling the hot rolled steel strip at a rate sufficient to ensure that the microstructure of the strip consists of greater than 50 percent by volume of martensite, and then annealing the steel strip at a temperature within the range of Ac1 to Ac1+50° C. for a minimum time at temperature of one hour. The steel has a microstructure after annealing that consists essentially of ferrite and retained austenite.

1 . A method for producing a high strength, high ductility steel strip, said method comprising: 
 (a) providing a steel slab consisting essentially of in weight percent carbon 0.05/0.20, manganese 3.0/8.0, silicon less than 0.5, aluminum less than 0.1, the balance iron and incidental impurities;    (b) hot rolling the steel slab to strip with a finish rolling temperature above Ar1+50° C.;    (c) cooling the hot rolled steel strip at a rate sufficient to form a microstructure that consists of greater than 50 percent by volume of martensite;    (d) then box annealing the steel strip in coil form for a minimum of one hour at a temperature within a range of Ac1 to Ac1+50° C.; and    (e) cooling the steel to room temperature.    
   
   
       2 . The method of  claim 1 , wherein in the cooling step, the hot rolled steel strip is cooled at a rate sufficient to form a microstructure that consists of greater than 90 percent by volume of martensite;  
   
   
       3 . The method of  claim 1 , wherein the box annealing step is carried out at a temperature within the range of 593° C. to 654° C.  
   
   
       4 . The method of  claim 1 , wherein the annealing cycle time is controlled so that the maximum difference in temperature between an outer lap of the coil and an inner lap of the coil is less than 28° C. during the anneal cycle.  
   
   
       5 . The method of  claim 1 , wherein the hot rolled strip is coiled at a temperature within the range of 566° C. to 621° C.  
   
   
       6 . The method of  claim 1 , wherein the temperature in the box annealing step is within the range of 635° C. to 663° C.  
   
   
       7 . The method of  claim 1 , wherein the hot roll finishing temperature is within the range of 871° C. to 982° C., the hot rolled strip is coiled at a temperature within the range of 566° C. to 621° C., and the temperature in the box annealing step is within the range of 635° C. to 663° C.  
   
   
       8 . The method of  claim 7 , wherein the steel slab consists essentially of C 0.09/0.11, Mn 5.7/6.0, Si less than 0.5, and Al less than 0.1, the balance iron and incidental impurities.  
   
   
       9 . The method of  claim 7 , wherein the steel slab consists essentially of C 0.09/0.11, Mn 7.0/7.2, Si less than 0.5, Al less than 0.1, the balance iron and incidental impurities.  
   
   
       10 . A hot rolled and box annealed steel strip produced according to the method of  claim 1 , said steel strip having an ultimate tensile strength of at least 800 MPa and total elongation of at least 25%.  
   
   
       11 . A hot rolled and box annealed steel strip produced according to the method of  claim 8 , said steel strip has an ultimate tensile strength of at least 800 MPa and a total elongation of at least 35%.  
   
   
       12 . A hot rolled and box annealed steel strip produced according to the method of  claim 9 , said steel strip has an ultimate tensil strength of at least 900 MPa and a total elongation of at least 30%.  
   
   
       13 . A method for producing a high strength, high ductility steel strip, said method comprising: 
 (a) providing a steel slab consisting essentially of in weight percent carbon 0.05/0.20, manganese 2.0/8.0, silicon less than 0.5, and aluminum less than 2.0, the balance iron and incidental impurities;    (b) hot rolling the steel slab to strip with a finish rolling temperature above Ar1+50° C.;    (c) cooling the hot rolled steel strip at a rate sufficient to form a microstructure that consists of greater than 50 percent by volume of martensite;    (d) then box annealing the steel strip in coil form for a minimum of one hour at a temperature within a range of Ac1 to Ac1+50° C.; and    (e) cooling the steel to room temperature.