MANUFACTURE OF HIGHHTOUGH AND HIGHHTENSILE TI STEEL EXCELLENT IN WELDABILITY

摘要

PURPOSE:To raise low-temperature toughness as well as keep excellent weldability at low carbon equivalent by improving the quality of steel by adding a specific amount of Ti to steel so as to cause a crystallization strengthening action to take place, combined to the effect of transformation zone-rolling. CONSTITUTION:A steel, containing 0.05 to 0.20% C, 0.70% Si, 0.5 to 2.0% Mn, 0.005 to 0.10% Al, and 0.005 to 0.10% Ti, is manufactured. The steel is excellent in weldability because of its low carbon equivalent but low in toughness. For this reason, the steel ingot is hot rolled in a gamma recrystallization zone at more than 1000 deg.C so that the gamma grain size becomes a gamma particle number of more than (-2 + 60Ti %). Afterwards, it is subject to a more than 10% drafting at Ar3 to 900 deg.C during air cooling and then to gammma-alpha transformation of a 5 to 60% transformation rate, followed by a transformation zone-rolling at a draft of 10 to 60%. The heat treatment after rolling is made by coiling it at a temperature below the Ar1 point or by cooling it with air. Thus, a hightensile Ti steel which is excellent in low-temperature toughness can be obtained in spite of low carbon equivalent.