The influence of hot forging conditions on the microstructure and mechanical properties of two microalloyed steels

With the aim of replacing quenched and tempered forging parts and eliminating by this way costly and time consuming operations; an industrial forging procedure was developed to evaluate the influence of thermomechanical processing parameters on the microstructure and mechanical properties of V and V–Ti microalloyed steels. After austenitization at 1200°C the microalloyed steel billets were forged in a 2500 t mechanical press. Forging was performed in three steps (roll forging, molding and finishing) after which the specimens were cooled according to various schedules in order to study the influence of cooling rate on the tensile, charpy V-notch, impact and hardness values. The variation of ferrite grain size, pearlite colony size, interlamellar spacing, ferrite/pearlite percentage and lath thickness as a function of forging conditions was followed by optical microscopy. The results indicated that by increasing the cooling rate both the strength and toughness increase. Under these conditions the microstructure is mainly composed of non-ferrite/pearlite transformation products. It was also found that under similar forging conditions the Ti-containing microalloy steel leads to a better strength/toughness combination than the V steel. The results are discussed in the framework of classical nucleation theory and the optimum forging conditions to reach the best strength/toughness ratio and microstructure for each steel were determined.