0.16%炭素鋼の熱間加工組織に対するひずみとひずみ速度と温度の影響

抄録

The high temperature tensile deformation of a 0.16% carbon steel was studied over a wide range of strain rate from 18 sec^-1 to 2.6×10^-4 sec^-1 in the austenite range. From the metallographic investigation of the specimens quenched by hydrogen gas instantaneously after deformation, it was observed that the structure rapidly deformed (ε=18 sec^-1), which was very unstable at deformation temperature and underwent static recrystallization in less than 0.2 seconds, was retained to room temperature by the instantaneous quenching.
The relation between the strain at which the structure changes distinctly (ε_c), and the strain at the firstmaximum flow stress (ε_M) is expressed by the following equation; ε_c= (0.72±0.06) ·ε_M. The average grain size varied with the increasing strain in the range over ε_c and approached to a value determined by the strain rate (and the temperature) in high strain region. The final grain size (D) is dependent solely on the first maximum flow stress (σ_M) regardless of the deformation conditions (i. e. initial structure, temperature and strain rate), and expressed by σ_M=σ₀+K·D^-N, where σ₀, K and N are constants, and 17σ0 and N are equal to zero and 0.70, respectively. The deformed structure in high strain region showed a mixture of uniformly distributed isolated fine grains and comparatively coarse grains with serrated boundaries whose period was about the smaller grain size, and this state of mixture was almost the same regardless of the deformation conditions.
The apparent activation energy for high temperature deformation obtained from the temperature and the strain rate dependence of σ_M was approximately equal to that for the self-diffusion of Fe atom in the austenite range of 0.16% C-Fe. It is concluded, therefore, that the deformation of 0.16% C-Fe in the austenite range is controlled by the dynamic recrystallization process in terms of the self-diffusion of the Fe atom. This fact suggests that whether the observed stress-strain curve shows a peak stress type or a stress oscillation type depends on the difference between an initial grain size (D₀) and a dynamically recrystallized grain size (D), and the peak stress type curve is observed when a grain size decreases with deformation (D<D₀), and the stress oscillation type curve appears when a grain size increases with deformation (D>D_O).