Deformation Behaviours of a 0.16% Carbon Steel in the Austenite Range

Abstract

High temperature tensile deformation of a 0.16% carbon steel was studied over a wide range of strain rates from 1.43sec^-1 to 2.73×10^-5sec^-1 in the austenite (γ) range. The shape of the true stress-true strain (σ-ε) curve showed a sharp maximum or regular oscillation in the low strain region and then reached to a steady state deformation in the higher strain region. With a given initial grain size, D₀, the σ-ε curve is expressed solely in terms of the first maximum flow stress, σ_M, or Z in the following equation. σ_M which is independent of D₀ can be correlated with temperatuer, T, and strain rate, ε, approximately by the following deformation equation;
in which n (=5.4) and Q (=68.5kcal/mol) are nearly equal to those obtained in creep experiments. The energy value is almost the same as the activation energy for self-diffusion in the γ phase of the 0.16% carbon steel.
The austenite grain size (D) in the deformed structure is unchanged in the high strain region. D is a function of σ_M or Z in the above equation and is independent of the actual D₀ and T or ε. From these results and the metallographic observations reported previously, it is considered that the high temperature deformation of the 0.16% carbon steel in the γ range is controlled by the dynamic recrystallization process assisted by the diffusion of atoms.
The mono-peak stress type curve is observed when the grain size decreases with deoformation (D₀>D), and the oscillated stress type curve appears when the grain size increases with deformation (D₀<D). It is concluded, therefore, that whether the observed σ-ε curve shows a peak stress type or an oscillated stress type depends on the relative difference between the initial grain size, D₀, and the dynamically recrystallized grain size, D. This suggests that the shape of σ-ε curves is dependent on not only Z but also D₀ and the change in the shape with Z and D₀ is explained qualitatively in terms of the nucleation and growth model applied to the dynamic recrystallization.