Hot-rolled steel sheet having high yield ratio and excellent low-temperature impact energy absorption and HAZ softening resistance and method for producing same

Abstract

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 [mu]m 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.

Description

technical field [0001] The present invention relates to a high-yield-ratio hot-rolled steel sheet having a maximum tensile strength (tensile strength) of 600 MPa or more, excellent in impact energy absorption characteristics at low temperatures and HAZ (Heat-Affected Zone: heat-affected zone) softening characteristics, and its Manufacturing method. This steel plate is suitable as blanks (raw materials) for booms and frames of construction machinery, and as blanks for frames and members of trucks and automobiles that are mainly formed by bending. It is also suitable as a blank for line pipe. Background technique [0002] Frames of construction machines and trucks are assembled by forming hot-rolled steel sheets mainly by bending, and arc welding the formed parts. Therefore, for blanks used for these parts, excellent bending workability and arc weldability are required. Furthermore, since construction machines and trucks are sometimes used in low-temperature environments, e...

AI Technical Summary

Problems solved by technology

However, these steel sheets utilizing precipitation strengthening have the following problems: the softening of the heat-affected zone of arc welding may be large, and the joint strength may be lowered, and brittle fracture may occur at low temperatures, or the impact energy absorption may be reduced.

However, these methods have the following problems: brittle fracture may occur due to the billet at low temperature, or the impact energy absorption amount may become small

However, this technique is to disperse fine precipitates in the HAZ region of the steel sheet during arc welding, but it does not optimize the size of the precipitated particles in the steel, so as a result, the impact energy absorption characteristics of the steel sheet are not good. The problem

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