HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT DUCTILITY, AND METHOD FOR PRODUCING THE SAME

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet of which both the strength and ductility are improved, and, particularly, the ductility is excellent, and to provide a method for producing the same. SOLUTION: The high-strength steel sheet having excellent ductility has a metallic structure composed of ferrite as a soft phase, and one or more selected from pearlite, bainite, austenite and martensite as a hard phase(s). According to the structural observation of the sheet cross-section, a part or the whole of the respective soft phase crystal grains and hard phase crystal grains forms the group of the crystal grains with an orientation difference of ≤5 degrees, and, regarding both the crystal grain group of the soft phase and the crystal grain group of the hard phase(s), the aspect ratio of the groups of the crystal grains is ≥5.0. The method for producing the same is characterized in that the finish rolling of hot rolling is performed at a final draft of ≥10%, finishing temperature is controlled to ≤880°C, cooling is performed to the temperature region of 750 to 650°C at the average cooling rate of 1 to 100°C/s, successively, coiling is performed in the temperature range of 750 to 650°C, and the hot rolled steel sheet is pickled, cold-rolled, and then annealed. COPYRIGHT: (C)2008,JPO&INPIT