Rapid annealing of an ultralow carbon steel: in-depth exploration of the microstructure-mechanical properties evolution

Abstract

The elaborate comparisons of microstructure, mechanical properties and texture evolution between rapid annealing and conventional industrial annealing in an ultralow carbon plate cold commercial steel were performed. And in-depth exploration of the microstructure-mechanical properties evolution by different rapid annealing parameters was investigated. Results manifested that optimal balance between tensile strength (494.07 MPa) and elongation (24.7%) accompanied with a product of strength and elongation of 12.20 GPa·% was achieved in steel rapid annealed at 650 °C for 15 s. The increased yield strength (171.03 MPa) was attributed to the refined ferrite grains (7.96 μm) providing the better grain refinement effect by the rapid annealing. While the decreased ductility (24.70%) in rapid annealed steel was ascribed to the deteriorated work hardening and {111} texture. The deteriorated work hardening was synergistically ascribed to the refined ferrite grains accompanied by suppressed cementite particles, and the stored geometry necessary dislocations. Furthermore, the intensity of {111} texture increased with the annealing time accompanied by the decreased α texture, thus generating a weak γ texture by rapid annealing although α and γ texture both formed. In addition, the ferrite grains were refined and uniformized by rapid annealing, manifesting the strong grain refinement effect by the recrystallization nucleation density and nucleation driving force provided by rapid annealing although with a high annealing temperature. More importantly, the greatly increased tensile strength and enough elongation by rapid annealing process highlighted one novel method to further improve the strength in ultra-low carbon steel with reasonable elongation. What's more, the whole annealing process was radically shortened from tens of hours (16–17 h) into seconds (2–15 s) by rapid annealing.