Dual phase hot rolled steel sheet having excellent formability and stretch flangeability

Abstract

The present invention provides a process of producing a family of hot rolled dual phase steel sheets having excellent formability and stretch flangeability, with yield strengths of from about 500 MPa to about 900 MPa from a single steel chemistry consisting of, by weight percent, 0.02-0.15% of C, 0.3-2.5% of Mn, 0.1-2.0% of Cr, 0.01-0.2% Al, 0.001-0.01% Ca, not more than 0.1% P, not more than 0.03% S, not more than 0.2% Ti, not more than 0.2% V, not more than 0.2% Nb, not more than 0.5% Mo, not more than 0.5% Cu, not more than 0.5% Ni, the balance being Fe and unavoidable impurities. A slab or ingot of this composition is reheated to a temperature of between 1050° C. and 1350° C. and held at this temperature for at least 10 minutes, then hot rolled, completing the hot rolling at between 800° C. and 1000° C. The sheet is cooled, immediately after completion of hot rolling, at a rate of not lower than 10 C/sec., without requiring specific cooling patterns, and coiled at a temperature of not less than 450 C. The cooling temperature is controlled to produce the desired yield strength within the range of from about 500 MPa to about 900 MPa.

We claim:  
     
         1 . A method of producing hot rolled dual phase steel sheet having excellent formability and stretch flangeability comprising the steps of 
 (a) providing as a starting material a steel slab or ingot containing, by weight percent, 0.02-0.15% of C, 0.3-2.5% of Mn, 0.1-2.0% of Cr, 0.01-0.2% Al, 0.001-0.01% Ca, not more than 0.1% P, not more than 0.03% S, not more than 0.2% Ti, not more than 0.2% V, not more than 0.2% Nb, not more than 0.5% Mo, not more than 0.5% Cu, not more than 0.5% Ni, the balance being Fe and unavoidable impurities;    (b) reheating the steel slab or ingot to a temperature in the range between 1050° C. and 1350° C. and holding at this temperature for a time of not less than 10 minutes;    (c) hot rolling the reheated steel slab into a steel sheet, completing the hot rolling process at a temperature in the range between 800° C. and 1000° C.;    (d) cooling the hot rolled steel sheet, immediately after completion of hot rolling, at a mean rate not lower than 10° C./sec. without requiring specific cooling patterns, and    (e) coiling the hot rolled steel sheet at a temperature not lower than 450° C. to produce a dual phase steel sheet having a yield strength of between about 500 MPa and about 900 MPa, depending on the coiling temperature.    
     
     
         2 . The method as defined in  claim 1 , wherein the coiling temperature is between 450° C. and 500° C. to produce a steel having a yield strength of at least about 800 MPa.  
     
     
         3 . The method as defined in  claim 1 , wherein the coiling temperature is between 500° C. and 550° C. to produce a steel having a yield strength of at least about 600 MPa.  
     
     
         4 . The method of  claim 1 , wherein the coiling temperature is within the range of about 550° C. to about 650° C. to produce a steel sheet having a yield strength of at least about 550 MPa.  
     
     
         5 . The method of  claim 1 , wherein the steel sheet contain at least about 0.03% C.  
     
     
         6 . The method of  claim 1 , wherein the steel sheet contain at least about 0.5% Mn.  
     
     
         7 . The method of  claim 1 , wherein the steel sheet contain at least about 0.3% Si.  
     
     
         8 . The method of  claim 1 , wherein the steel sheet contain at least about 0.3% Cr.  
     
     
         9 . The method of  claim 1 , wherein the dual phase hot rolled steel sheet has a micro-structure consisting of about 3-30%, by volume, martensite islands as a hard secondary phase embedded in a fine-grained ferrite matrix.  
     
     
         10 . The method of  claim 1 , wherein step (d) comprises cooling the steel sheet at a rate of at least about 30° C./sec.  
     
     
         11 . A hot rolled dual phase steel sheet produced by the method of  claim 1.