HOT-ROLLED STEEL SHEET HAVING HIGH YIELD RATIO AND EXCELLENT LOW-TEMPERATURE IMPACT ENERGY ABSORPTION AND HAZ SOFTENING RESISTANCE AND METHOD FOR PRODUCING SAME.

摘要

Provided is a hot-rolled steel sheet having a high yield ratio and excellent low-temperature impact energy absorption and HAZ softening resistance wherein the maximum tensile strength is 600 MPa or higher, and also provided is method for producing the same. The steel is formed from, by mass %, C: 0.04 to 0.09%, Si: 0.4% or less, Mn: 1.2 to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02 to 0.09%, Ti: 0.02 to 0.07%, and N: 0.005% or less such that 2.0 Mn + 8 [%Ti] + 12 [%Nb] 2.6, with the remainder being Fe and inevitable impurities. The pearlite surface area percentage is 5% or less, and the total martensite and residual austenite surface area percentage is 0.5% or less, with the remainder being a ferrite and/or bainite metal structure. The average crystal grain size of the ferrite and bainite is 10 µm or smaller, the average grain size of phase-separated alloy carbonitride that contains Ti and Nb is 20 nm or less, and the yield ratio is 0.85 or greater.