Changes of microstructure and mechanical properties of cold-rolled low carbon steel due to its surface treatment by Nd:glass pulsed laser

Abstract

Cold-rolled low-carbon rimming steel (i.e. not killed with Al or Si and not continuous but cast in a form) was surface-treated using a Nd:glass pulsed laser. The changes of the microstructure parameters were analysed by X-ray diffraction and optical microscopy. The modification of mechanical properties was evaluated by the changes of the residual macrostresses and Vickers microhardness values. It has been indicated that during the laser pulse the surface material of up to 80 μm thickness becomes melted and then resolidifies in a strongly non-equilibrium condition. This causes scattering of the crystallographic texture and introduction of significant tensile residual macrostresses in the steel surface mainly transverse to the rolling direction. At the same time, the residual macrostresses in the rolling direction of the investigated steel are not changed by the laser treatment and this fact is considered to be due to the anisotropy of the thermal conductivity in the both investigated directions. The performed laser treatment causes some surface oxidation and also an increase of the Vickers microhardness in the surface region of approximately 30%. Some particularities of the estimation of residual macrostresses in textured materials via the sin 2ψ X-ray diffraction method realised in grazing incidence beam geometry are discussed. It is shown that for cubic materials (and particularly for the investigated steel) with well pronounced crystallographic texture, it is advisable to apply the sin 2ψ method to more elastically isotropic crystallographic directions such as 〈100〉. It is also demonstrated that for the experiments performed in grazing incidence geometry the refraction correction is compulsory.