| 摘要 |
PURPOSE:To improve impact toughness and strength at low temperature by adequately controlling rolling conditions and the heat treatement after this with respect to the steel of the specific composition. CONSTITUTION:The steel containing C<0.2%, Si; 0.03-0.8, Mn; 0.5-4.5%, Ni; 0.5-6.0%, Al-sol<0.07%, Nb; 0.005-0.2% as basic components is added, if necessary, with one or two or more kinds of Cr<1%, Mo<0.8%, Cu<0.5%, V< 0.2%, Ti<0.1%, La<0.03%, Ce<0.03%. This steel ingot is hot rolled at a draft of 60% or more in the temperature region between 1100 deg.C to Ar3 transformation point. Next, it is held for 30sec-60min in the temperature region beteen Ar3-Ar1 transormation point, after which it is hardened and further it is tempered at the temperature lower than the Ac1 transformation point. The structure assumes the form wherein the tempered martensite is encircled by fine ferrite grains and this results in improved impact toughness and strength at low temperature. |
