Microstructure and mechanical property improvement of X70 in asymmetrical thermomechanical rolling

Abstract

Asymmetrical thermomechanical rolling (ATMR) process of API X70 microalloy steel was investigated to evaluate of rolling force and distribution of material property in the roll gap. Applying a user-defined VUMAT subroutine, the equations of material flow and microstructure kinetics, as material genome package, were implemented into finite element (FE) solver of ABAQUS for simulation of X70 multi-pass rolling. A rough rolling schedule was carried out to verify the accuracy of the material genome by evaluation of the computed rolling force of FE model and experimental results. Considering the interaction of the asymmetrical parameters such as rolls speed ratio (VA), pass height (PH) and rolls diameter ratio, it was found that coinciding of the high level PH with high range of rolls speed ratio and diameter ratio lead to a rise of the rolling force. Strain distribution of the roll gap indicated that an increase of the rolls speed ratio can improve the dislocation density and the strength of rolled material. The study of strain distribution contour in the length of slab thickness revealed that the homogeneity of deformation can be enhanced in the mechanism of asymmetrical rolling of microalloy steel. Experimental test reveals that in asymmetry condition in TMR process of X70, increase of the speed ratio (1.02–1.05) in roughing passes leads to a considerable grain refinement and growth of yield strength and ultimate tensile strength of 3.12 and 2.05% of final plate structure, respectively.