Dynamic softening during hot deformation of an extra low carbon steel (St.15)

Abstract

The softening behavior of an extra low carbon steel (St . 15) during deformations in austenitic, ferritic and dual-phase ranges was investigated using hot compression tests. Two different austenitizing conditions were chosen, once at 1000 ℃ and the other at 1250 ℃ in order to investigate the effect of initial grain size. The deformation temperatures were between 700 ℃ and 1250 ℃. The strain rates were 0.01, 0.1, 1 and 10/s. The critical flow stress is defined as the stress equal to the stress at 0.8 ɛ[subscript p], where ɛ[subscript p] is the strain at maximum or peak stress during deformation. The critical flow stress in the ferritic range increases more rapidly with decreasing deformation temperatures than that in the austenitic range. In the case of deformation in the austenitic range, a large initial grain size affects the dynamic recrystallizAtion by shifting the strain at the peak stress to a higher value. Decreasing deformation temperatures or increasing strain rates delays the onset of dynamic recrystallization by shifting the peak stress and also the steady state stress to higher values. In the case of ferritic deformation, the strain at the onset of steady state seems to be independent of deformation temperatures and strain rates.