Tensile properties of cold-rolled TWIP-cored three-layer steel sheets

Abstract

In this study, TWIP-cored three-layer steel sheets containing thin surface layers of low-carbon (LC) or interstitial-free (IF) steel were fabricated by solid-state hot-roll-bonding followed by cold-rolling. Interfaces were well bonded without any pores or voids, which indicated a successful fabrication of the TWIP-cored sheets. According to the tensile test results, both strength and elongation of TWIP-cored sheets increased proportionally with the volume fraction of TWIP-cored region, and were well matched with strength and elongation calculated by a rule of mixtures. However, yield point (YP) was observed in most of TWIP-cored sheets as well as mono-layer TWIP and LC sheets. After the sheets were quenched from 830 degrees C, the YP disappeared because the dislocation density was greatly increased by effects of thermal stresses formed during the rapid cooling. A bainitic microstructure, which was generally characterized by the continuous yielding, was formed along TWIP/LC or TWIP/IF interfaces, and also beneficially influenced to the elimination of YP. These TWIP-cored sheets covered a wide range of yield and tensile strength and ductility levels requiring in automotive steel sheets by controlling the volume fraction of TWIP-cored region, and thus present new applications to multi-functional automotive steel sheets.