| 摘要 |
<p>A high tensile and fire-resistant steel which has a chemical composition, in mass %, that C: 0.04 to 0.14 %, Si: 0.50 % or less, Mn: 0. 50 to 2.00 %, P: 0.020 % or less, S: 0.010 % or less, Nb: 0.01 to 0.05 %, Mo: not less than 0.30 % and less than 0.70 %, Al: 0.060 % or less, N: 0. 0010 to 0.0060 %, with the proviso that a weld cracking sensitive composition: PCM = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B takes a value of 0.25 % or less, and the balance: iron and inevitable impurities, and has a structure wherein a finally rolled steel plate has an area percentage of polygonal ferrite or pseudo polygonal ferrite at the 1/4 position in the plate thickness direction thereof of 10 % or less.</p> |
