HIGH-STRENGTH STEEL PLATE WITH EXCELLENT FORMABILITY, WARM WORKING METHOD, AND WARM-WORKED AUTOMOTIVE PART

摘要

A high-strength steel sheet has a chemical composition including 0.05% to 0.3% of C, 1% to 3% of Si, 0.5% to 3% of Mn, 0% to 0.1% ofP, 0.001% to 0.1% of Al, and 0.002% to 0.03% ofN, in mass percent; further includes iron and impurities; and has a structure including 50% to 90% of bainitic ferrite, 5% to 20% of retained austenite (³ R ), a total of 10% to 50% of martensite and the retained austenite, and 0% to 40% of polygonal ferrite, in area percent based on the entire structure. The retained austenite has a carbon content (C³ R ) of 0.5% to 1.2% by mass, an average equivalent circle diameter of 0.2 to 2 µm, and an average aspect ratio (maximum diameter/minimum diameter) of less than 3.0. The high-strength steel sheet excels both in elongation and deep drawability while having a strength of 980 MPa or more.