Global view for grain refinement in ultra-low-C IF steel during high-strain deformation at various temperatures and strain rates

Abstract

Single-phase ferrite of an interstitial free (IF) steel was continuously deformed by torsion to high strains (>5.5) at various temperatures and strain rates; i.e., under various Zener–Hollomon parameter (Z) conditions (Z = 1030 s−1–1015 s−1). Accordingly, different microstructures composed of ultrafine, fine, and coarse grains (0.6 µm–46 µm) were obtained under the conditions where steady-states were achieved at high strains. The microstructures examined by Electron Backscattering Diffraction (EBSD) were categorized into three types: (I) ultrafine lamellae (with thickness <1 µm) formed under high-Z conditions, (II) transition-type fine microstructures formed under intermediate-Z conditions, and (III) relatively coarse and equiaxed microstructures (with grain sizes >10 µm) under low-Z conditions. The type III microstructures formed in the present bcc-ferrite were heterogeneous and different from typical DRX microstructures frequently reported in fcc-metals. Such variations in the microstructures (types I, II, and III) were also confirmed by the Derby-type plot between the torsion flow stress (σ) and the grain size (D): σ∝D−P. That is, three different p (power-law exponent) values were determined for the conditions under which three different microstructures formed. It was concluded that grain subdivision was the dominant mechanism in the formation of the ultrafine lamellae (type I) under high-Z conditions, whereas migration of high angle boundaries (grain growth) was the dominant mechanism for the formation of the equiaxed microstructures (types II and III) under intermediate-Z and low-Z conditions. Global view for understanding the formation of ultrafine and fine microstructures during high-strain deformation at various temperatures and strain rates were acquired in the present study.