Patent ID: 7820099
Filing Date: 2010-10-26
Classification: C22C,C23C,C23G

Abstract:
1. Hot rolled steel sheet excellent in chemical convertibility and free from bald spots, characterized by the steel sheet containing, by mass %, C: 0.03 to 0.15%, Si: 1.3 to 3.0%, Mn: 0.5 to 3.0%, P: 0.07% or less, S: 0.01% or less, Al: 0.015 to 0.1%, N: 0.001 to 0.008%, the steel sheet optionally containing one or more of Ti: 0.02 to 0.3%, Nb: 0.01 to 0.5%, Ni: 0.1 to 2.0%, B: 0.0002 to 0.006%, and Ca: 0.0005 to 0.005%; and the steel sheet being free from containing Cu and Mo, and the balance of Fe and unavoidable impurities, the oxides on the steel sheet surface having, by mass %, a Si concentration of 3.5% or less and a Mn concentration of 3.5% or less, and an average roughness Ra of the steel sheet surface of 3.0 μm or less and an average number of pits of a diameter of 1 μm to 0.3 μm of 5 or less in squares of the steel sheet surface when dividing it into squares of 10 μm per side, the steel sheet obtained by pickling by dipping the steel sheet in an aqueous solution containing, by mass %, a HCl concentration of 7 to 15%, an Fe ion concentration of 4 to 12% and a balance of metal ions other than Fe and impurities, at a solution temperature of 80 to 98° C. for a dipping time such that the HCl concentration (mass %)×dipping time (sec) is 273 to 520.