No abstract

Steels containing large carbon concentrations are used particularly when a high hardness is required, for example, in the manufacture of components such as bearings. This, however, makes it difficult to shape or machine the alloys during the process of component manufacture unless they are first heat-treated into a softened condition. One method of achieving this economically is to generate a microstructure known as divorced pearlite, in which ferrite and cementite grow from the austenite in a non-cooperative manner, leading to a final microstructure that consists of coarse, spherical particles of cementite dispersed in a matrix of ferrite. This is in contrast to the harder lamellar pearlite which normally develops when high-carbon steels are cooled. The theoretical framework governing the transition from the divorced to the lamellar form is developed and validated experimentally.

A theory for the diffusion-controlled growth of pearlite in steels containing manganese is presented and assessed in the light of experimental data. Given the overwhelmingly rapid diffusivity that a substitutional solute has within the transformation front, the growth rate is found to be dominated by diffusion parallel to the interface with austenite as the rate controlling step. The relevant interfacial diffusion parameters have been derived by fitting experimental data to kinetic theory. All reported measurements of pearlite growth where the full set of necessary parameters have been listed, are shown to be inconsistent with mechanisms that do not involve the partitioning of substitutional solutes. The method adopted here, which is based on the local equilibrium at the transformation interface with the long-range partitioning of substitutional solutes by migration within the interface, has been shown to reasonably explain experimental data over a range of temperatures and chemical compositions.

A kinetic theory for the diffusion-controlled growth of pearlite is presented, which accounts simultaneously for diffusion through the austenite and via the transformation front. The simplified method abandons the need for mechanical equilibrium at the phase junctions and yet is able to explain experimental data on the growth rate of pearlite. Furthermore, unlike previous analyses, the deduced value for the activation energy for the interfacial diffusion of carbon is found to be realistic when compared with corresponding data for volume diffusion.

Roll wear in a hot strip mill (HSM) is a detrimental process, which progressively worsens the rolls. This necessitates replacement of the rolls after rolling a number of strips. Roll wear adversely affects the strip shape and the performance of a mill. It is important, therefore, to quantify roll wear during the rolling process. The present paper proposes a wear model for prediction of the wear profile of the rolls at the finishing stands of a hot strip mill. Initially, a roll force model was developed to calculate the roll force at each stand by considering various factors such as the strip thickness and width, mean flow stress (MFS), and roll diameter for each pass. The roll force calculated by the model was subsequently integrated with the roll wear model for calculation of the wear along the roll barrel. The effects of the continuous variable crown (CVC) profile of the roll and roll shift were also included in the wear calculation. The wear profile predicted by this model was validated experimentally in the plant. Predicted values match closely with those obtained from experiment. The model has the potential to be used as an efficient online method to predict the wear profile within a reasonable computational time.