Tensile and impact behaviour of a microalloyed medium carbon steel: effect of the cooling condition and corresponding microstructure

The effect of cooling rate after hot rolling on the final microstructure and mechanical properties of a microalloyed medium C steel was investigated. The microstructure was characterized by optical microscopy; the mechanical behavior was studied by hardness, tensile and instrumented Charpy V-notch impact tests carried out at room and sub-zero temperatures. The results of microstructural analysis indicate that a low cooling rate of 0.7 C/s led to a mixed microstructure consisting of perlite, pro-eutectoid ferrite and bainite, while an increase of the cooling rate to 7.5 C/s favored the formation of martensite and acicular ferrite. This latter microstructure, in turn, induced an increase in the tensile strength of the steel, with a reduction of its elongation to failure, and superior impact toughness. Analyses of the fracture surfaces with scanning electron microscopy confirmed the influence of the two microstructures on the failure mechanisms of the steel.