In this paper, the development and production of nfiobium-titanium containing High Strength Low Alloy (HSLA) steels at Gallatin Steel Company (GSC) is described. The concept of High Temperature Conventional Controlled Rolling (HTCCR) was developed to hot roll 65 mm thin slabs at elevated temperatures. The HTCCR process was accomplished by using the optimal combination of niobium (Nb) and titanium (Ti) to raise the no recrystallization temperature (T(nr)), so that the critical deformation of austenite below the T(nr) temperature can be performed Lit a higher processing temperature, followed by an intense Accelerated Controlled Cooling (ACC) at cooling rates ranging from 20 degrees C/sec to 60 degrees C/sec. The use of HTCCR-ACC process has attained very fine ferrite-bainite microsturctures, con-sequently resulting, in yield strengths that have consistently achieved in excess of 550 MPa (80 KSI), while maintaining high toughness. The development of Gallatin Grade 100 (687.5 MPa minimum yield strength) will be discussed as well. The synergetic effect of titanium and niobium on mechanical properties will be discussed in terms of steel compositions and TMCP processing conditions in conjunction with their effects on forming as-cast equiaxed grain structures of thin slab, improving niobium solubility in austenite, controlling austenite grain coarsening during slab reheating, refining austenite grain during hot rolling, and enhancing post-deformation transformation of austenite-to-ferrite with optimal precipitation strengthening.
