HIGH-STRENGTH HOT-ROLLED STEEL PLATE HAVING EXCELLENT STRETCH PROPERTIES, STRETCH FLANGING PROPERTIES AND TENSION FATIGUE PROPERTIES, AND METHOD FOR PRODUCTION THEREOF

摘要

Disclosed is a high-strength hot-rolled steel plate which is produced by effectively utilizing Ti, which is an inexpensive element and has a large amount of precipitation strengthening, instead of using an expensive element Mo, which shows improved stretch properties and stretch flanging properties at TS780 Mpa or greater, and which is excellent in tension fatigue properties. Also disclosed is a method for production of the steel plate. The steel plate has the following chemical composition by mass%: C: 0.06 to 0.15% inclusive; Si: 1.2% or less; Mn: 0.5 to 1.6% inclusive; P: 0.04% or less; S: 0.005% or less; Al: 0.05% or less; Ti: 0.03 to 0.20% inclusive; with the remainder being Fe and unavoidable impurities. With respect to the volumetric occupation rate, the steel plate comprises 50 to 90% inclusive of ferrite, with the remainder being substantially bentonite, wherein the total volumetric occupation rate of ferrite and bentonite is 95% or more. In the ferrite, a precipitate containing Ti appears. The steel plate has a texture composed of the precipitates having an average diameter of 20 nm or less. In the steel plate, 80% or more of the Ti contained in the steel is precipitated.