FIRE-RESISTANT STEEL MATERIAL WITH EXCELLENT RESISTANCE TO REHEAT EMBRITTLEMENT AND WITH LOW-TEMPERATURE TOUGHNESS AT WELDED HEAT-AFFECTED PARTS, AND MANUFACTURING METHOD THEREOF

摘要

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.