Effect of cooling rate on microstructure and mechanical properties of a low-carbon low-alloy steel

The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.