Flow behaviour of a medium carbon microalloyed steel during a hot working process

Over the past few decades, medium carbon microalloyed steels have aroused considerable interest in physicists and metallurgical engineers because of the fact that these materials find wide application in car components industry These kinds of steels have a high strength to weight ratio, superior toughness and weldability. These beneficial properties have been achieved by careful control of composition and by adopting suitably controlled thermomechanical processes. The design of deformation processing of materials in car components industry has assumed considerable importance since a knowledge of the interaction between process parameters, microstructure, and the products property will help in achieving products giving longer service with reliability. From this point of view, an appropriate understanding of the mechanical and microstructural behaviour of the steel under consideration is required. As a first step towards characterizing the mechanical behaviour, constitutive relations describing material flow must be determined. These can then be used to carry out computer simulations before conducting any industrial tests. The high temperature flow behaviour of a medium carbon microalloyed steel was characterized. For this purpose, uniaxial compression tests were carried out on samples with diverse initial microstructural states, in the strain rate range 10(-4)-3 s(-1) and at different temperatures. After each experiment, the true stress - true strain curve was obtained. The general features of the experimental flow curves under hot working conditions were typical of dynamic recovery processes at relatively low temperatures and/or high strain rates, i.e. the rate of work hardening decreases with increasing strain until it drops to zero and a constant value of flow stress is attained. The true stress - true strain curves observed at higher temperatures or lower strain rates exhibits additional flow softening once a peak stress is attained; a lower steady state stress is then achieved at strains well in excess of the peak strain. This type of flow behaviour is associated with the occurrence of dynamic recrystallization. The high temperature flow behaviour of the studied steel can be accurately described by the classical Garofalo-Sellars-Tegart hyperbolic sine relation between stress and strain rate, provided that the stresses are normalized by Young's modulus and the strain rates by the selfdiffusion coefficient. In recent publications, the present authors proposed that in order to maintain the general validity of this relation, an internal stress should be included, especially when the initial grain size is fine. The determined internal stresses are strain rate dependent, and also depend on the initial grain size according to a Hall-Fetch type of relation. The internal stress was not sensitive to chemical composition. The reason for the observed strain rate dependence of the internal stresses may be attributable to the operating deformation mechanism.