Micromechanical Modelling of Damage Healing in Free Cutting Steel

Abstract

Continuous casting is used to solidify most of the steel produced in the world every year. The process reduces the number of required milling stages and results in qualitative semi-finished products such as billets, blooms and slabs which will later be rolled into more specific shapes. Extending the range of finished product sizes produced from a given concast bloom or billet section is often limited by the minimum area reduction required to ensure effective consolidation and final mechanical properties. Predicting effective consolidation or level of remnant porosity has always been an important issue for steel producers as it will affect the mechanical properties of final products (strength, ductility, etc.). It is known that partial or complete recovery of strength in such porous materials can be obtained by pore closure and diffusive healing processes at elevated temperature. Devising an appropriate healing process which does not cause discontinuity in the microstructure and mechanical properties at the healing sites and prevents distortion of the component during bonding requires an accurate choice of thermo-mechanical processing parameters. Although there has been considerable work on materials such as titanium alloys, aluminium alloys and copper, damage healing in free cutting steel has not received much attention. The main aim of this research is to develop a realistic damage healing computational approach that can predict damage healing or recovery during soaking under different compressive stress levels, and be used for hot rolling applications. This study investigates the void elimination process through two stages of void closure and healing. An Abaqus/UMAT subroutine has been developed for the analysis of the material porosity elimination process including two stages of void closure and healing. This study uses the Gurson-Tvergaard model under hydrostatic compression to predict the void closure. A novel approach has been developed in the present work to identify the Gurson-Tvergaard model parameters using a non-gradient based optimisation search method (Pattern Search Method). The healing process is modelled based on a combination of diffusion bonding, creep and plasticity following the Pilling model and can be adapted to any other healing/diffusion bonding model. The material model has been calibrated for free cutting steel and a stress state representative of the rolling process, and used to predict the closure and healing processes under rolling. The effect of parameters such as Roll Gap shape Factor (RGF), initial amount and distribution of void volume fraction on porosity elimination during rolling has also been investigated. An experimental technique has been developed to identify the conditions (temperature, pressure, time) required for void elimination in Free Cutting Steel (FCS). Different combinations of load and time were tested and optimum conditions have been obtained. Tensile tests on the bonded specimens have been carried out to measure the strength of the bonded region. The position of fracture on the specimen and also the cross section of the fracture surface have been inspected. The experimental results have been used to calibrate the developed void elimination model. Using the developed model, predictions of densification and healing can be made for optimisation of the rolling schedule.