elongation and hole expandability and high-strength hot-rolled steel sheet and a method of manufacturing the same excellent in fatigue characteristics

摘要

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which is used for car parts being press-worked, and has a strength of ≥590 N/mm² class, and excellent elongation, hole expansibility and fatigue characteristics, and to provide a method for producing the same.;SOLUTION: A high strength hot rolled steel sheet having excellent elongation, hole expansibility and fatigue characteristics has a component composition including, by mass, 0.01-0.35% of C, ≤2.0% of Si, 0.1-4.0% of Mn, 0.001-2.0% of Al, ≤0.2% of P, 0.0005-0.02% of S, ≤0.02% of N, and 0.0003-0.01% of O, and is characterized by having a steel structure that, in phase fraction, the tempered martensite fraction is ≥5%, the retained austenite fraction is 2%, the martensite fraction is 1%, the pearlite fraction is 5%, and the remainder is composed of ferrite and bainite, wherein the average grain diameter of the tempered martensite phase is within the range of 0.5 to 5 μm.;COPYRIGHT: (C)2013,JPO&INPIT