Fire-resistant steel material with excellent resistance to reheat embrittlement and with low-temperature toughness at welded heat-affected parts, and manufacturing method thereof

Abstract

Provided are a fire-resistant steel material to which is applied high heat input welding and that demonstrates excellent low-temperature toughness and resistance to reheat embrittlement when exposed to fire at welded heat-affected parts, and a method for manufacturing the same. The steel material contains C: 0.012-0.050%, Mn: 0.80-2.00%, Cr: 0.80-1.90%, and Nb: 0.01 to less than 0.05%; Cu: restricted to 0.10% or less; contains suitable amounts of Si, N, Ti, and Al; the content of Mo, B, P, S, and O are restricted; and the remainder comprises Fe and inevitable impurities. The content of C, Mn, Cr, Nb, and Cu satisfy -1200C - 20Mn + 30Cr -330Nb - 120Cu >= -80. The steel structure as observed using an optical microscope is 80% or more ferrite phase by surface area ratio, and the remainder of said steel material is made of a mixed structure comprising a bainite phase, a martensite phase, and a martensite-austenite phase.

Description

technical field [0001] The present invention relates to a fire-resistant steel material excellent in reheat embrittlement resistance and low-temperature toughness of a welded heat-affected zone and a manufacturing method thereof. Background technique [0002] Steel structures such as buildings are required to develop the necessary strength for a certain period of time in order to prevent collapse and allow occupants to escape when exposed to fire. However, in general, when a steel material is exposed to high temperature, its strength decreases. Therefore, conventionally, as a countermeasure against this, in order to suppress the temperature rise of the steel material during a fire, a method of covering the steel material with a refractory coating has been employed. [0003] On the other hand, in recent years, from the viewpoint of environmental issues, aesthetics, and the like, a technique of constituting a steel structure without using a refractory coating has been propose...

AI Technical Summary

Problems solved by technology

[0014] However, when left to cool after hot rolling, it is difficult to obtain a bainite structure, which is disadvantageous in obtaining high-temperature strength.

In addition, if the addition amount of alloying elements is increased in order to ensure high-temperature strength without accelerated cooling, there is a problem that reheat embrittlement of the HAZ portion becomes prominent due to grain boundary precipitation, etc.

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