Finite element investigation of instability in hot bar rolling process

Abstract

In steel company, reduction of surface cracks is of importance in multi-pass bar rolling process. The surface crack or instability which is frequently encountered in bar rolling of steel A can easily develop into fatal cracks during the cold heading process of steel A, and it is therefore necessary to minimize inherent cracks on the surface of hot rolled bars. Many studies have been carried out to reduce these surface cracks. Many researchers focused on experimental findings to improve the process mechanism for reducing cracks. Some used FE analyses to suggest better processing conditions in the process for reducing cracks or instability. The processing maps were also used to reduce instability but the developed processing maps in the literature were not consistent as compared to the production data in terms of instability of the surface. In this approach, an integrated technique was developed to study the effect of processing parameters on instability of steel A during bar rolling process, in which processing map (to judge instability) was merged into FE analysis. The processing map was formulated based on the plastic work approach available in the literature. The effects of roll pass geometry and temperature on rolled bars were examined with the help of a parametric study by employing integrated technique. Based on these FE results from integrated technique, the improved roll pass design was determined and the initial processing temperature in the hot bar rolling process recommended for reducing flow instability of the process. The laboratory rolling mill was used to validate the effect of processing conditions suggested from FE results on flow instability. The effect of initial temperature on surface cracks or instability was observed and it was concluded that with the increase in initial temperature in the process the flow instability reduced. Since at initial temperature of $943^\circ C$ the surface cracks did not appear but cracks appeared when the...