Effect of the chemical composition on the peak and steady stresses of plain carbon and microalloyed steels deformed under hot working conditions

Two different behaviours are classically observed during the high temperature deformation of metals: i) power law creep and ii) exponential law creep. The first is observed at relatively low stresses and is considered as a deformation process controlled by diffusion. At higher stresses the above behaviour is converted into an exponential one, i.e. the power law breaks down. Both phenomena can be described by a single expression of the form: (epsilon) over dot = A(sinh alpha sigma)(n).exp(- Q/RT) Here the parameters A, n, alpha and Q depend on the material being considered, and are usually referred to as apparent values because no account is generally taken of the internal microstructural state. In the particular case of microalloyed steels, a broad range of values have been reported in the literature for the latter constants, and clear trends have not always been evident. In recent work, it has been shown that the high temperature behaviour of medium carbon microalloyed steels can be accurately described by the classical hyperbolic sine relation provided the stresses are normalised by Young's modulus E(T) and the strain rates by the self-diffusion coefficient D(T). According to this formulation, only two parameters need to be determined to characterise the hot flow behaviour: A and alpha (n can be set equal to 5 for carbon steels). In the present work, the latter expression is extended to plain carbon and low carbon microalloyed steels, and applied to the peak and steady stresses of the flow curve. To attain this goal, experimental results corresponding to several different steels reported by many authors are employed. The effect of chemical composition on the above constants is derived statistically.