Hot Rolled Precipitation Strengthened and Grain Refined High Strength Dual Phase Steel Sheet Possessing 600 MPa Minimum Tensile Strength and a Process Thereof

Abstract

A process for producing dual phase steel sheet including steps of making a liquid steel having a chemical composition in wt % of C: 0.03-0.12. Mn: 0.8-1.5. Si: <0.1, Cr: 0.3-0.7, S: 0.008 maximum, P: 0.025 maximum, Al: 0.01 to 0.1, N: 0.007 maximum. Nb: 0.005-0.035. and V: 0.06 maximum, remainder Fe; continuous casting the liquid steel into a slab; hot rolling the slab into a hot rolled sheet at finish rolling temperature (FRT) 840±30 ° C.; cooling the hot rolled sheet on the run out table at a cooling rate 40 −70° C./s to an intermediate temperature (Tint) of 720° C.≤Tint≤650° C.; natural cooling the hot rolled sheet for a duration of 5-7 seconds and rapidly cooling the hot rolled sheet to transform remaining carbon enriched austenite to martensite, at cooling rate of 40-70 ° C./s to a coiling temperature below 400° C.

1 . A process for producing dual phase steel sheet, comprising steps of:
 making a liquid steel having chemical composition in wt % of   C: 0.03-0.12, Mn: 0.8-1.5, Si:<0.1, Cr: 0.3-0.7, S: 0,008 maximum, P: 0.025 maximum, Al: 0.01 to 0.1, N: 0.007 maximum, Nb: 0.005-0.035, and V: 0.06 maximum remainder Fe and inevitable impurities;   continuous casting the liquid steel into a slab;   hot rolling the slab into a hot rolled sheet at finish rolling temperature (FRT) of 810-870° C.   cooling the hot rolled sheet on the run out table at a cooling rate 40-70° C./s to an intermediate temperature (TINT) of 720° C.≤TINT≤650° C.;   natural cooling the hot rolled sheet for a duration of 5-7 seconds; and   rapidly cooling the hot rolled sheet to transform remaining carbon enriched austenite to martensite, at cooling rate of 40-70° C./s to a coiling temperature below 400° C. to produce a dual phase steel sheet.   
     
     
         2 . The process as claimed in  claim 1 , wherein the slab is re-heated to a temperature of 1100-1200° C. to dissolve precipitates before hot rolling. 
     
     
         3 . The process as claimed in  claim 1 , wherein a yield stress of the dual phase steel sheet is 350-500 MPa. 
     
     
         4 . The process as claimed in  claim 1 , wherein the dual phase steel sheet has a 600 MPa minimum tensile strength. 
     
     
         5 . The process as claimed in  claim 1 , wherein the dual phase steel sheet has 16% minimum uniform elongation. 
     
     
         6 . The process as claimed in  claim 1 , wherein the dual phase steel sheet has 22% minimum total elongation. 
     
     
         7 . The process as claimed in  claim 1 , wherein a strain hardening exponent (n) of the dual phase steel sheet is 0.15-0.16 
     
     
         8 . The process as claimed in  claim 1 , wherein a yield strength to tensile strength ratio of the dual phase steel is 0.6-0.8. 
     
     
         9 . The process as claimed in  claim 1 , wherein the dual phase steel sheet has a hole expansion ratio in a punched condition is of about 40%. 
     
     
         10 . The process as claimed in  claim 1 , wherein the dual phase steel sheet 75-90% ferrite, 10-25% martensite and <5% bainite by volume. 
     
     
         11 . The process as claimed in  claim 1 , wherein a grain size of the dual phase steel sheet is 2-5 μm. 
     
     
         12 . A dual phase steel sheet, comprising:
 a chemical composition in wt % C: 0.03-0.12, Mn: 0.8-1.5, Si:<0.1, Cr: 0.3-0.7, S: 0,008 maximum, P: 0.025 maximum, Al: 0.01 to 0.1, N: 0.007 maximum, Nb: 0.005-0.035, and V: 0.06 maximum remainder Fe and inevitable impurities;   
     
     
         13 . The dual phase steel sheet as claimed in  claim 12 , wherein a yield stress of the dual phase steel sheet is 350-500 MPa. 
     
     
         14 . The dual phase steel sheet as claimed in  claim 12 , wherein the dual phase steel sheet has a 600 MPa minimum tensile strength. 
     
     
         15 . The dual phase steel sheet as claimed in  claim 12 , wherein the dual phase steel sheet has 16% minimum uniform elongation. 
     
     
         16 . The dual phase steel sheet as claimed in  claim 12 , wherein the dual phase steel sheet has 22% minimum total elongation. 
     
     
         17 . The dual phase steel sheet as claimed in  claim 12 , wherein a strain hardening exponent (n) of the dual phase steel sheet is 0.15-0.16. 
     
     
         18 . The dual phase steel sheet as claimed in  claim 12 , wherein a yield strength to tensile strength ratio of the dual phase steel sheet 0.6-0.8. 
     
     
         19 . The dual phase steel sheet as claimed in  claim 12 , wherein a hole expansion ratio in a punched condition is about 40%. 
     
     
         20 . The dual phase steel sheet as claimed in  claim 12 , wherein the dual phase steel sheet has 75-90% ferrite, 10-25% martensite and <5% bainite by volume. 
     
     
         21 . The dual phase steel sheet as claimed in  claim 12 , wherein a grain size of the dual phase steel sheet is 2-5 μm.