The Effect of Ti and Ti-Nb Additions on .ALPHA. Formation and Restoration during Intercritical Rolling and Holding of C-Mn Structural Steels.

Abstract

Single-pass rolling in the (γ+α) two-phase region has been carried out to investigate the effect of Ti and Ti-Nb additions on the γ→α transformation and the restoration characteristics of deformed α in a 0.13%C-1.45%Mn base steel. The microalloyed steels consisted of 0.016%Ti, and 0.019%Ti plus 0.024%Nb.Experimental results showed that rolling in the (γ+α) two-phase region accelerated the γ→α transformation, and the presence of Ti or Ti plus Nb enhanced this accelerative effect. In undeformed and lightly deformed samples, α grains nucleated mainly at γ grain boundaries during and/or after rolling. In samples rolled with reductions higher than a critical value, α grains nucleated at γ grain boundaries as well as interiors, mainly at the boundaries of deformation bands and deformed annealing twins. Depending on the rolling reductions, holding time and alloy composition, deformed α grains developed into cell and/or subgrains, or recrystallization occurred resulting in equiaxed grains.Recovery and recrystallization of deformed α proceeded rapidly in the C-Mn steel, but was sluggish in the Ti and Ti-Nb steels. The incubation time for recrystallization of deformed α was retarded by 1-2 orders of magnitude in the Ti and Ti-Nb steels compared with the base C-Mn steel, because of the presence of alloy carbide/nitride precipitate particles.Ti and Ti-Nb additions also produced finer initial γ grains after reheating before rolling, and stabilized the dislocation substructures during and after rolling. The substructures resulted in increased nucleation sites for the γ→α transformation. These effects led to finer α grains and a higher hardness (and strength) in the microalloyed steels than in the C-Mn steel after the same rolling and holding treatments.