Cleavage initiation in Ti–V–N and V–N microalloyed ferritic–pearlitic forging steels

The effects of silicon (0.53 and 1.05 wt.%) and titanium (<0.002 and 0.022 wt.%) on microstructure and mechanical properties of vanadium microalloyed medium carbon steels heat treated after rolling to simulate the thermal cycle of hot forging have been determined using room temperature tensile tests, impact tests, optical microscopy and scanning electron microscopy (SEM). Silicon was found to increase strength values whilst titanium had a strong refining action on prior austenite grain size. Room temperature Charpy ‘U’ notch impact energies were all on the lower shelf; ductile–brittle transition temperatures, determined from fracture appearance in Hounsfield impact tests, ranged from 100 to 145°C, scaling with material strength. Initiation in the Charpy tests was by microcracking of coarse (Ti,V)(C,N)-containing single or multi-phase inclusions except in the low strength, titanium-free case when the absence of a completely continuous grain boundary ferrite layer allowed matrix microstructure initiation by interfacing pearlite colonies to occur.