Patent ID: 8715432
Filing Date: 2014-05-06
Classification: C21D,C22C

Abstract:
1. A fire-resistant steel having weld joint reheat embrittlement resistance and toughness and a room temperature strength of 400 to 600N/mm 2 class, consisting of, by mass %, C: 0.010% to less than 0.05%, Si: 0.01 to 0.50%, Mn: 0.80 to 2.00%, Cr: 0.50% to less than 2.00%, V: 0.03 to 0.30%, Nb: 0.01 to 0.10%, N: 0.001 to 0.010%, and Al: 0.005 to 0.10%, limiting contents of Ni, Cu, Mo, and B to Ni: less than 0.10%, Cu: less than 0.10%, Mo: 0.10% or less, and B: less than 0.0003%, further limiting contents of impurity ingredients of P, S, and O to P: less than 0.020%, S: less than 0.0050%, and O: less than 0.010%, and a balance of iron and unavoidable impurities, wherein Cr, Mo, Ni, Cu, and Mn satisfy the following formula (1): where, in formula (1), the units of the concentrations of the elements are mass %, wherein a dislocation density in a ferrite phase of said steel material is 10 10 /m 2 or more, said steel material structure has an occupancy of bainite or martensite in a structure viewed under an optical microscope of 20% or more and is comprised of a quenched structure, and in said steel material, carbides or nitrides comprising one or more of Nb, V, and Cr are precipitated by a 2/μm 2 or higher density.