High-strength steel sheet and method of manufacturing the same

Abstract

A high-strength steel sheet has excellent workability, excellent phosphatability, and excellent corrosion resistance after electrodeposition coating has been performed, even when the contents of Si and Mn are high. Condition 1 through Condition 3 below are used when continuous annealing is performed on a steel sheet. Condition 1: In a heating process of continuous annealing, a steel sheet is heated at a heating rate of 7° C./s or more in a temperature range in an annealing furnace of 450° C. or higher and A° C. or lower (A: 500≦A). Condition 2: The maximum end-point temperature of a steel sheet is 600° C. or higher and 750° C. or lower in continuous annealing. Condition 3: In a steel sheet temperature range of 600° C. or higher and 750° C. or lower in continuous annealing, the traveling time of the steel sheet through the temperature range is 30 seconds or more and 10 minutes or less, and the dew point of the atmosphere is −10° C. or higher in the temperature range.

1 .- 4 . (canceled) 
     
     
         5 . A method of manufacturing a high-strength steel sheet, comprising:
 when a steel sheet having a chemical composition containing, by mass %, C: 0.03% or more and 0.35% or less, Si: 0.01% or more and 0.50% or less, Mn: 3.6% or more and 8.0% or less, Al: 0.01% or more and 1.0% or less, P: 0.10% or less, S: 0.010% or less, and the balance being Fe and inevitable impurities is annealed in a continuous annealing process,   heating the steel sheet in a heating process of the continuous annealing process at a heating rate of 7° C./sec. or more in a temperature range in an annealing furnace of 450° C. or higher and A° C. or lower (A: 500≦A),   controlling a maximum end-point temperature of a steel sheet in the continuous annealing process to 600° C. or higher and 750° C. or lower, and   controlling, in a steel sheet temperature range of 600° C. or higher and 750° C. or lower in the continuous annealing process, a traveling time of the steel sheet through the temperature range to 30 seconds or more and 10 minutes or less and a dew point of an atmosphere to −10° C. or higher in the temperature range.   
     
     
         6 . The method according to  claim 5 , the steel sheet having the chemical composition further containing, by mass %, one or more chemical elements selected from among B: 0.001% or more and 0.005% or less, Nb: 0.005% or more and 0.05% or less, Ti: 0.005% or more and 0.05% or less, Cr: 0.001% or more and 1.0% or less, Mo: 0.05% or more and 1.0% or less, Cu: 0.05% or more and 1.0% or less, Ni: 0.05% or more and 1.0% or less, Sn: 0.001% or more and 0.20% or less, Sb: 0.001% or more and 0.20% or less, Ta: 0.001% or more and 0.10% or less, W: 0.001% or more and 0.10% or less, and V: 0.001% or more and 0.10% or less. 
     
     
         7 . The method according to  claim 5 , further comprising performing electrolytic pickling in an aqueous solution containing sulfuric acid after the continuous annealing process has been performed. 
     
     
         8 . A high-strength steel sheet having the chemical composition according to  claim 5 , the oxides of at least one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V in an amount of 0.010 g/m 2  or more and 0.050 g/m 2  or less per side in total in a region within 100 μm from a surface of the steel sheet, and oxides containing Mn in an intra-grain region within 1 μm from a grain boundary of a steel sheet crystal grain existing in a region within 10 μm from the surface of the steel sheet. 
     
     
         9 . The method according to  claim 6 , further comprising performing electrolytic pickling in an aqueous solution containing sulfuric acid after the continuous annealing process has been performed. 
     
     
         10 . A high-strength steel sheet having the chemical composition according to  claim 6 , the oxides of at least one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V in an amount of 0.010 g/m 2  or more and 0.050 g/m 2  or less per side in total in a region within 100 μm from a surface of the steel sheet, and oxides containing Mn in an intra-grain region within 1 μm from a grain boundary of a steel sheet crystal grain existing in a region within 10 μm from the surface of the steel sheet.