HIGH STRENGTH HOT-ROLLED STEEL SHEET EXCELLENT IN HOLE-EXPANDABILITY AND METHOD FOR MANUFACTURING THE SAME, AND HOLE-EXPANDABILITY EVALUATION METHOD

摘要

PROBLEM TO BE SOLVED: To provide a high strength hot-rolled steel sheet which has a tensile strength (TS) of 590 MPa or more and has high hole-expandability, and is excellent in press molding properties.SOLUTION: A high strength hot-rolled steel sheet excellent in hole-expandability contains, by mass%, C:0.02% or more and 0.30% or less, Si:2.0% or less, Mn:0.5% or more and 3.0% or less, Al:0.01% or more and 1.0% or less, Ti:0.01% or more and 0.20% or less, and the balance Fe with inevitable impurities, and has a composition structured of a double phase of ferrite and martensite. In the martensite, χ expressed by the following expression (1) in the martensite is smaller than 0.2. Expression (1):χ=(&Sgr;(b/V))/NIn the expression (1), bis the number of inlets of a through hole of a martensite i, i is an integer of 1 to N, Vis a volume [μm] of the martensite i, and Nis the total number of the martensite.SELECTED DRAWING: Figure 1