Fire-resistant steel superior in weld joint reheat embrittlement resistance and toughness and method of production of same

A fire-resistant steel superior in weld joint reheat embrittlement resistance and toughness comprising a fire-resistant steel of a room temperature strength of the 400 to 600N/mm2 class having steel ingredients containing, 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%,
• having a balance of iron and unavoidable impurities, wherein
• among the elements forming said steel ingredients, the elements of Cr, Mo, Ni, Cu, and Mn satisfy a relationship expressed by the following formula (1):
• 4Cr[%]-5Mo[%]-10Ni[%]-2Cu[%]-Mn[%]>0 (1)
• {where, in formula (1), the units of the concentrations of the elements are mass %}