The effect of warm rolling on the structure and properties of a low-carbon steel

Abstract

In order to increase the strength of hot-rolled low-carbon steel products there have been changes in hot-rolling practice which have led to situations where either partially of fully ferritic, rather than fully austenitic, structures are being deformed in the last passes of rolling schedules. The effects that this may have on the structure and properties of the product are not fully understood. The work described in this paper was therefore initiated to obtain information relevant to this situation. The results show that, by rolling a 0.017% plain-carbon steel to natural strains of up to 1.6 at 650°C, product strenght increases of up to 60% can be achieved, but only to the detriment of ductility. However, the transition temperature, although initially increasing with rolling strain, then improves so that at the highest rolling strain it is better than that of the as-hot-rolled material. The increase in strength and loss of ductility is associated with the formation of a recovered substructure, and the improvement in transition temperature with the formation of a {100} texture. These property changes are similar in both longitudinal and transverse testing directions. By lowering the deformation temperature to 500°C, higher strengths can be achieved but greater losses in toughness and ductility occur. This appears to be the results of a higher dislocation density. The significance of these initial results to industrial hot-rolling practice, is to indicate that if finishing temperatures are such that the microstructure of the steel is mainly ferrite in the finishing passes, then large increases in strength can be obtained. These strength increases are, however, at the expense of ductility and toughness, as the resultant structure is recovered, rather than recrystallised.