.ALPHA.+.GAMMA. and .GAMMA. Phases Annealing in Ultra Low-carbon Sheet Steels.

Abstract

α+γ and γ phases annealing in Ti and Nb-added ultra low-carbon sheet steels with and without Mn, P, and/or Cr has been investigated. In high-strength steels with Mn, P and/or Cr, BH increases as the annealing temperature increases, whereas YP-EI after aging at 100°C for 1 hr clearly decreases. It is speculated that the coexistence of BH and nonaging properties stems from the high dislocation density introduced by γ→α transformation. In contrast, YP-EI in mild steel without Mn, P, and/or Cr increases as BH increases because dislocation density is not high enough to provide non-aging property. In mild steel, r-value distinctly decreases and texture is randomized by γ phase annealing, while r-value increases and {111} component develops by intercritical annealing. On the contrary, in high-strength steel, recrystallization texture in α is assumed to be inherited even after α→γ→α transformation takes place. Apparently, variant is distinctly selected when at least γ transforms into α in high-strength sheet steels. The variant selection in γ→α transformation is speculated to be caused by residual stress introduced by α→γ transformation.