On the Fine Structures and the Strength of Continuously Cooled Vanadium Steels

Abstract

A study by the transmission electron microscope was carried out to clarify the strengthening mechanism due to the addition of a small amount of vanadium to 0.2% carbon steels which were subjected to continuous cooling from 1200°C at various rates. The results are summarized as follows: The strength of the proeutectoid ferrite of the steels containing 0.2∼0.5% vanadium and 0.2% carbon does not monotonously decrease with the decreasing cooling rates, but shows a minimum at about 600°C/min and then increases with decreasing cooling rates, reaching a maximum at about 80°C/min. Various substructures are found in the proeutectoid ferrite of the vanadium steels cooled at rates ranging 5∼200°C/min. These substructures can be classified into the following three categories; (i) dispersed dislocation loops and elongated dislocation dipoles, (ii) dispersed and lined-up vanadium carbide particles, and (iii) microtwins. The mechanism of the formation of these substructures was discussed. It was concluded that the strengthening of proeutectoid ferrite was caused by the peculiar substructures with fine vanabium carbide particles of 50 Å or less which were precipitated on dislocations.