Impact of the heating rate on the annealing behavior and resulting mechanical properties of UFG HSLA steel

The formability of materials with ultrafine grained (UFG) microstructures produced by severe plastic deformation processes is often limited due to low strain hardening capabilities and strain localizations. Most commonly, heat treatments are used to regain the formability. However, conventional furnace heat treatments can lead to grain growth and significant losses in strength. One approach to address this issue is to increase the heating and cooling rates of the heat treatments. The present work focuses on the heating rate dependencies of the microstructure evolution and resulting mechanical properties of a UFG high strength low alloy (HSLA) steel. Electron backscatter diffraction (EBSD) measurements are used to characterize the microstructure and texture. Bending tests and uniaxial tensile tests are conducted to provide insight into the mechanical properties that are related to the obtained microstructures, with an emphasis on the tendency for strain localizations. The investigations reveal a distinct effect of the heating rate on the annealing behavior, i.e., continuous vs. discontinuous growth. Thus, resulting microstructures and mechanical properties are not only a function of time and temperature but also depend on the heating rate. In this context, the application of laser annealing is shown to be a suitable approach to impede strain localizations in the form of shear bands without sacrificing much of the strength of the UFG material obtained by severe plastic deformation.