Effect of rolling variables on precipitation strengthening in high-strength low-alloy steels containing vanadium and nitrogen

Abstract

An investigation has been made into the effects of thermo mechanical processing on precipitation strengthening in high-strength low-alloy steels containing vanadium with low and high nitrogen contents. It has been shown that to achieve maximum precipitation strengthening, strain-induced precipitation in the austenite should be prevented. Strain-induced precipitation of vanadium nitride occurs most rapidly at 900°–950°C, but vanadium carbide is strain induced at lower rolling temperatures of 830°–850°C. The effects of reheating temperature, rolling reduction, and holding temperature and time have also been examined. The intensity of precipitation strengthening has been shown to be a maximum at about the stoichiometric V/C ratio but, at this ratio also, strain-induced precipitation in the austenite is most pronounced. Precipitation of vanadium nitride in austenite detracts from precipitation strengthening in the ferrite, but if precipitation of vanadium nitride in austenite can be prevented, increased nitrogen contents in the steel can result in increased precipitation strengthening and a higher yield stress. Niobium appears to be most effective in a grain-refinement role, although under suitable conditions it can produce marked precipitation strengthening to an even greater extent than can vanadium, particularly at low equivalent concentrations. However, NbC is very readily strain induced to precipitate in the austenite, with a consequent loss of precipitation strengthening in ferrite. On the other hand, vanadium may be readily utilized to grain refine by vanadium nitride and to precipitation strengthen by vanadium carbide.