High tensile strength cold-rolled steel sheet and method for production thereof

Abstract

The high tensile cold-rolled steel sheet consists essentially of 0.04 to 0.13% C, 0.3 to 1.2% Si, 1.0 to 3.5% Mn, 0.04% or less P, 0.01% or less S, 0.07% or less Al, by mass, and balance of Fe and inevitable impurities, has a microstructure containing 50% or larger area percentage of ferrite and 10% or larger area percentage of martensite, has 0.85 to 1.5 of ratio of intervals of the martensite in the rolling direction to those in the sheet thickness direction, and has 8 GPa or larger nano strength of the martensite. The high tensile cold-rolled steel sheet has a good strength-elongation balance, and shows excellent crashworthiness at about 10 s −1 of strain rate. Therefore, the high tensile cold-rolled steel sheet is suitable for reinforcing members for pillar and dashboard of automobile.

1 . A high tensile cold-rolled steel sheet: consisting essentially of 0.04 to 0.13% C, 0.3 to 1.2% Si, 1.0 to 3.5% Mn, 0.04% or less P, 0.01% or less S, 0.07% or less Al, by mass, and balance of Fe and inevitable impurities; having a microstructure containing 50% or larger area percentage of ferrite and 10% or larger area percentage of martensite, and having 0.85 to 1.5 of ratio of intervals of the martensite in the rolling direction to those in the sheet thickness direction; and having 8 GPa or larger nano strength of the martensite.  
   
   
       2 . The high tensile cold-rolled steel sheet as in  claim 1  further containing at least one element selected from the group consisting of 0.5% or less Cr, 0.3% or less Mo, 0.5% or less Ni, and 0.002% or less B, by mass.  
   
   
       3 . The high tensile cold-rolled steel sheet as in  claim 1  further containing at least one element selected from the group consisting of 0.05% or less Ti and 0.05% or less Nb, by mass.  
   
   
       4 . The high tensile cold-rolled steel sheet as in  claim 2  further containing at least one element selected from the group consisting of 0.05% or less Ti and 0.05% or less Nb, by mass.  
   
   
       5 . A method for manufacturing high tensile cold-rolled steel sheet, comprising the steps of: hot-rolling a steel slab consisting essentially of 0.04 to 0.13% C, 0.3 to 1.2% Si, 1.0 to 3.5% Mn, 0.04% or less P, 0.01% or less S, 0.07% or less Al, by mass, and balance of Fe and inevitable impurities, into a steel sheet, followed by coiling at coiling temperatures ranging from 450° C. to 650° C.; cold-rolling the coiled steel sheet at cold-rolling reductions ranging from 30 to 70%; annealing the cold-rolled steel sheet by heating to a temperature range of [the coiling temperature+the cold-rolling reduction percentage×4.5]−[the coiling temperature+the cold-rolling reduction percentage×5.5] (° C.); and cooling the annealed steel sheet to temperatures of 340° C. or below at average cooling rates of 10° C./s or higher.  
   
   
       6 . The method for manufacturing high tensile cold-rolled steel sheet as in  claim 5 , wherein the steel slab further contains at least one element selected from the group consisting of 0.5% or less Cr, 0.3% or less Mo, 0.5% or less Ni, and 0.002% or less B, by mass.  
   
   
       7 . The method for manufacturing high tensile cold-rolled steel sheet as in  claim 5 , wherein the steel slab further contains at least one element selected from the group consisting of 0.05% or less Ti and 0.05% or less Nb, by mass.  
   
   
       8 . The method for manufacturing high tensile cold-rolled steel sheet as in  claim 6  wherein the steel slab further contains at least one element selected from the group consisting of 0.05% or less Ti and 0.05% or less Nb, by mass.