AbstractAt high temperatures, materials display rate dependent plasticity, which in global terms is controlled by the glide and climb of dislocations. Classical descriptions of the flow behaviour under hot working conditions generally take no account of the internal microstructure, leading to apparent rather than actual values in the constants considered. An expression which takes into account the initial microstructure is proposed in the present work. In order to study the flow behaviour of a commercial medium carbon steel containing vanadium, titanium, and aluminium, uniaxial hot compression tests were carried out. True strain rates ranged from 10−4to 10 s−1and testing temperatures varied from 1150 to 850°C. Various austenitising conditions were employed in order to produce different initial microstructures. The results obtained were then correlated with the initial austenite grain size.MST/3265