Computational modeling of phase transformations and mechanical properties during the cooling of hot rolled rod

Recently, great emphasis has been placed on controlled the cooling of hot rolled steel rods to achieve the desired microstructure and final mechanical properties. The approach is to develop the desired microstructure and temperature in the hot rolled rod and then transform it from the austenite phase to different volume fractions of martensite, bainite, pearlite and ferrite phases. This paper presents a computational approach to grain size evolution and finishing temperature predictions during the rolling process using Sellar’s evolution equations. This is followed by digitization of the continuous cooling curves to determine the volume fraction of various phases as the hot rolled rod transforms during the post rolling cooling. The mechanical properties are evaluated using the volume fractions of transformed phases using standard relationships for steels. These evolution—transformation—property relations are implemented in a 3D finite element program that uses a rigid–viscoplastic materials model and has a microstructure-based flow stress module. The efficacy of this approach is demonstrated by applying it to an eight pass square-to-round bar rolling sequence. Such an approach has been shown to be a powerful analysis tool that can be used as an inexpensive alternative to the traditional trial-and-error approach in roll pass design.