Effect of Heterogeneous Surface Structure on Mechanical Properties of Interstitial-Free Steel Subjected to Laser Surface Treatment

To investigate the effect of heterogeneous gradient surface structure on mechanical properties of metals, a Ti-added interstitial-free steel with coarse-grained matrix and ultrafine-grained matrix were applied and were treated by laser surface treatment using liner-scanning. The distances between the scanning lines were modified to alter the space between the hard-treated zone and soft matrix. The laser-treated zone contains a melting zone, comprised of fine grains, and a heat-affected zone featured by coarse columnar grains grown radically from the laser beam center. For coarse-grained matrix sample, after proper laser surface treatment significant improvement (about 100%) in yield strength and moderate improvement (22.8%) in ultimate tensile strength were achieved with little loss in elongation. For ultrafine-grained matrix sample, gradual decrease in strength together with recovering of elongation was realized. Electron channeling contrast imaging was applied to investigate the structure of the sample which was then compared with the untreated one to reveal the origin of the mechanical behavior of the laser surface-treated steel.