High strength hot rolled steel sheet and method for manufacturing the same

Abstract

The present invention relates to a high strength hot rolled steel sheet containing 0.15% or less C, 0.02 to 0.35% Ti, and 0.05 to 0.7% Mo by weight percentage and consisting essentially of a matrix of ferrite structure single phase and fine precipitates with a grain size of smaller than 10 nm dispersed in the matrix, for example, a high strength hot rolled steel sheet which consists essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo by weight percentage, and the balance being Fe, wherein fine precipitates with a grain size of smaller than 10 nm are dispersed in a matrix of ferrite structure single phase at a number per unit volume of 5×10 4 /μm 3 or higher. This steel sheet, which has tensile strength of not lower than 550 MPa, high elongation and excellent stretch flangeability, is suitable for intricately shaped automotive chassis parts such as a suspension arm.

What is claimed is:  
     
         1 . A method for manufacturing a high strength hot rolled steel sheet, comprising the steps of: 
 producing a steel slab which consists essentially of 0.06% or less C, 0.5.% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, 0.05 to 0.6% Mo, 0.02 to 0.10% Ti by weight percentage, and the balance being Fe, and satisfies the equation of 0.8≦(C/12)/[(Ti/48)+(Mo/96)]≦1.3;    producing a hot rolled steel sheet by hot rolling said steel slab at a temperature of Ar3 transformation point or higher; and    coiling said hot rolled steel sheet at a temperature of 550 to 700° C.    
     
     
         2 . A method for manufacturing a high strength hot rolled steel sheet, comprising the steps of: 
 producing a steel slab which consists essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, 0.05 to 0.6% Mo, 0.02 to 0.10% Ti, at least one element selected from 0.08% or less Nb and 0.15% or less V by weight percentage, and the balance being Fe;    producing a hot rolled steel sheet by hot rolling said steel slab at a temperature of Ar3 transformation point or higher; and    coiling said hot rolled steel sheet at a temperature of 550 to 700° C.    
     
     
         3 . A method for manufacturing a high strength hot rolled steel sheet, comprising the steps of: 
 producing a steel slab which consists essentially of more than 0.06% and 0.15% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, more than 0.10% and 0.35-% or less Ti, 0.3 to 0.7% Mo by weight percentage, and the balance being Fe, and satisfies the equation of 0.8≦(C/12)/[(Ti/48)+(Mo/96)] s 1.3;    producing a hot rolled steel sheet by hot rolling said steel slab at a temperature of Ar3 transformation point or higher; and    coiling said hot rolled steel sheet at a temperature of 550 to 700° C.