High strength steel sheet and method for manufacturing the same

Abstract

The invention provides a high strength steel sheet which exhibits excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content, and a method for manufacturing such steel sheets. The method includes continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at ≦0.01%, the balance being represented by Fe and inevitable impurities, while controlling the dew-point temperature of the atmosphere to become not less than −10° C. when the heating furnace inside temperature is in the range of not less than A° C. and not more than B° C. during the course of heating (A: 600≦A≦780, B: 800≦B≦900).

1 . A method for manufacturing high strength steel sheets, comprising continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at ≦0.01%, the balance being represented by Fe and inevitable impurities, while controlling the dew-point temperature of the atmosphere to become not less than −10° C. when the heating furnace inside temperature is in the range of not less than A° C. and not more than B° C. during the course of heating wherein A is 600≦A≦780 and B is 800≦B≦900. 
     
     
         2 . The method for manufacturing high strength steel sheets according to  claim 1 , wherein the chemical composition of the steel sheet further includes one or more elements selected from B at 0.001 to 0.005%, Nb at 0.005 to 0.05%, Ti at 0.005 to 0.05%, Cr at 0.001 to 1.0%, Mo at 0.05 to 1.0%, Cu at 0.05 to 1.0% and Ni at 0.05 to 1.0% in terms of mass %. 
     
     
         3 . The method for manufacturing high strength steel sheets according to  claim 1 , further comprising, after the continuous annealing, electrolytically pickling the steel sheet in an aqueous solution containing sulfuric acid. 
     
     
         4 . A high strength steel sheet manufactured by the method described in  claim 1  in which a surface portion of the steel sheet extending from the steel sheet surface within a depth of 100 μm contains an oxide of at least one or more selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu and Ni at 0.010 to 0.50 g/m 2  per single side surface, and in which with respect to a region extending from the steel sheet surface within a depth of 10 μm, a crystalline Si/Mn oxide is present in grains that are within 1 μm from crystal grain boundaries of the steel sheet.