High Silicon Bearing Dual Phase Steels With Improved Ductility and Method

Abstract

A dual phase steel sheet is provided. The steel sheet has a microstructure containing ferrite and tempered martensite, a tensile strength of at least 980 MPa, a total elongation of at least 15%, and a hole expansion ratio of at least 15%. The dual phase steel sheet has a composition including 0.2 to 0.3 wt. % C, 1.6 to 2.5 wt. % Si and 1.75 to 2.5 wt. % Mn. A method for producing the dual phase steel sheet is also provided.

What is claimed is: 
     
         1 . A dual phase steel sheet comprising:
 a microstructure including ferrite and tempered martensite;   a tensile strength of at least 980 MPa, a total elongation of at least 15% and a hole expansion ratio of at least 15%; and   a composition including:
 0.1 to 0.3 wt. % C; 
 1.5 to 2.5 wt. % Si; and 
 1.75 to 2.5 wt. % Mn. 
   
     
     
         2 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has at total elongation of at least 18%. 
     
     
         3 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has a tensile strength of at least 1180 MPa. 
     
     
         4 . The dual phase steel sheet of  claim 1 , wherein the composition includes from 0.14 to 0.21 wt. % C. 
     
     
         5 . The dual phase steel sheet of  claim 4 , wherein the composition includes less than 0.19 wt. % C. 
     
     
         6 . The dual phase steel sheet of  claim 4 , wherein the composition includes about 0.15 wt. % C. 
     
     
         7 . The dual phase steel sheet of  claim 1 , wherein the composition includes from 1.8 to 2.2 wt. % Mn. 
     
     
         8 . The dual phase steel sheet of  claim 1 , further comprising:
 from 0.05 to 1 wt. % Al.   
     
     
         9 . The dual phase steel sheet of  claim 8 , further comprising:
 from 0.005 to 0.1 wt. % total of one or more elements selected from the group consisting of Nb, Ti, and V.   
     
     
         10 . The dual phase steel sheet of  claim 1 , further comprising:
 from 0 to 0.3 wt. % Mo.   
     
     
         11 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has a hole expansion ratio of at least 20%. 
     
     
         12 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has a hole expansion ratio of at least 25%. 
     
     
         13 . A method for producing a dual phase steel sheet comprising the steps of:
 providing a dual phase hot rolled steel sheet having a microstructure including ferrite and martensite having a composition including:
 0.1 to 0.3 wt. % C; 
 1.5 to 2.5 wt. % Si; and 
 1.75 to 2.5 wt. % Mn; 
   annealing the hot rolled steel sheet at a temperature from 750 to 875° C.;   water quenching the hot rolled steel sheet to a temperature from 400 to 420° C.; and   overaging the steel sheet at the temperature from 400 to 420° C. to convert the martensite in the hot rolled steel sheet to tempered martensite;   the overaging sufficient to provide the hot rolled steel sheet with a hole expansion ratio of at least 15%.   
     
     
         14 . The method for producing a dual phase steel sheet as recited in  claim 13 , further comprising the step of:
 grinding the hot rolled steel sheet to remove decarburized layers.   
     
     
         15 . The method for producing a dual phase steel sheet as recited in  claim 13 , further comprising the step of:
 cold rolling the hot rolled steel sheet.   
     
     
         16 . The method for producing a dual phase steel sheet as recited in  claim 13 , wherein said dual phase steel sheet has a hole expansion ratio of at least 20%.