HIGH-STRENGTH COLD ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME

摘要

A high-strength cold-rolled steel sheet having high chemical convertibility and a tensile strength of 590 MPa or more and a method for producing such a steel sheet are provided. The steel sheet contains, in terms of percent by mass, C: 0.05 to 0.3%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1%, N: 0.01% or less, and the balance being Fe and unavoidable impurities. The coverage ratio of reduced iron on a steel sheet surface is 40% or more. In order to produce such a steel sheet, an oxidation treatment is performed after cold rolling. In the oxidation treatment, first heating is conducted on a steel sheet in an atmosphere with an oxygen concentration of 1000 ppm or more until the steel sheet temperature reaches 630°C or higher and then second heating is conducted on the steel sheet in an atmosphere with an oxygen concentration of less than 1000 ppm until the steel sheet temperature reaches 700°C or higher. Subsequently, annealing is conducted in a furnace in a 1 to 10 vol% H 2 + balance N 2 gas atmosphere with a dew point of -25°C or less.