Microstructure and strength of continuously cooled low alloy vanadium and niobium steels
The present study was concerned with the variation of strength with cooling rates in vanadium and niobium steels continuously cooled from austenite. The range of cooling rates varied from 0.05K. s-l to 500K-s⁻¹ approximately. It was found that a peak hardening was observed in the high vanadium steels (at = 2K.s⁻¹), whilst the low vanadium and niobium steels produced a less definite peak. An. attempt was also made to explain the variation of strength through an investigation into the strengthening mechanisms in steel. Finally, efforts were made to study changes in the microstructure in relation to cooling rates. To gain an understanding of the effects of vanadium and niobium additions to the strength and microstructure of low carbon steels, a study of the literature particularly relevant to the present study was carried out. For instance, the relationship between structure and mechanical properties was studied. Also, the mechanism of formation of microstructural components observed in the material was reviewed. Before a study of the findings was attempted, the instruments used and the experimental methods developed in the present study, were described. Strengths and weaknesses of these methods were pointed out. Following the observation of a peak hardening in the steels, interesting results were obtained on aspects which could explain the variation of strength with cooling rates in vanadium and niobium steels. For instance, two distinct groups of precipitates and a near constant of 10¹⁰ dislocations. cm/cm³ were found in vanadium and niobium steels. The study of the microstructure also revealed interesting features. In particular, it was observed that the peak hardening was obtained in a ferritic matrix. It was also shown that the solution treatment influences not only the precipitation hardening but also both the macrostructure and the microstructure of steels. (In this study, microstructure refers to features observed inside the grains, whereas macrostructure is generalized to include grain structure). The present study terminates with a discussion of the various strengthening mechanisms in the steels investigated. The strength of the materials was measured by means of hardness tests which were empirically related to the yield strength. The peak was thus ascribed to precipitation hardening. The largest increase in the height of the peak for a given amount of vanadium and niobium addition, corresponded to steels of a stoichiometric composition of (Nb, V) carbonitride. On the other hand, manganese was observed to reduce knd to spread the peak evenly. Twinning was found to be present in vanadium steels and was observed for the first time in niobium steels. The contribution to strength from microtwins was appraised, but they appeared to have negligible effect.