Effect of Cooling Rate on Microstructure and Mechanical Properties of a Microlaminated Low-Density Steel

In this study, the effect of the cooling rate on the microstructure and mechanical properties of Fe-4Al-2.5Mn-2Ni-0.18C (wt.%) low-density steel was investigated through scanning electron microscopy, electron backscatter diffraction, dilatometer and Thermo-Calc software. It was established that δ -ferrite always exists after solidification, and a laminated microstructure was formed during hot working. During the cooling for the cooling rate in the range of 0.5-50 °C/s, the experimental steel underwent α -ferrite and martensite transformation. The α -ferrite transformation primarily occurred above 670 °C. As the cooling rate decreased, the ferrite fraction increased, and the starting temperature of martensitic transformation decreased. The α -ferrite transformation was accompanied by the diffusion of alloying elements, which affected the subsequent martensitic transformation. C, Mn, and Ni diffused from ferrite to austenite at the interface, whereas Al did the opposite. The C content of austenite increased with decreasing cooling rate, whereas the content of other alloys remained almost unchanged. The hardness of bulk ferrite is significantly lower than that of martensite. The cooling rate had little effect on the microhardness of both the bulk ferrite and martensite. However, the strength and elongation decreased with decreasing cooling rate.