α+γ and γ Phases Annealing in Ultra Low-carbon Sheet Steels

• Published in: ISIJ International Vol. 34; no. 1; pp. 33 - 42
• Main Authors: Yoshinaga, Naoki, Ushioda, Kohsaku, Itami, Atsushi, Akisue, Osamu
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 01.01.1994
• Subjects: aging property; austenite phase annealing; bake-hardenability; cold rolled steel sheet; dual-phase annealing; manganese addition; transformation texture; ultra low-carbon steel
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.34.33

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.