AbstractThe effects of precipitates formed by the addition of vanadium, nitrogen, and aluminium on the strength and toughness in 0·15%C steels were studied by cooling suitable steels at varying rates from temperatures giving a constant austenite grain size. Yield strength increments resulting from precipitation were derived from the experimental results. Similar increments were produced by vanadium carbide (VC) and vanadium carbonitride (V(C,N)) and in each case a reduction in cooling rate resulted in reduced strengthening because of increased particle coarsening. In the presence of aluminium rather less strengthening was obtained from V(C,N) precipitates. No strengthening resulted from the presence of aluminium nitride (AIN) particles. When the carbon content of the steel is greatly in excess of the stoichiometric quantity, the carbonitride formed is relatively low in nitrogen and the yield strength increments caused by low nitrogen V(C,N) and VC are similar. Impact transition temperature increments resulting from precipitation were also derived from the experimental data. These showed that V(C,N) precipitates are less detrimental to transition temperatures than are VC precipitates giving comparable yield strength increments. Shelf energies were also reduced to a greater extent by VC than by V(C,N). The presence of AIN particles had little effect on shelf energy, but had a detrimental effect on transition temperature. The high nitrogen V(C,N) precipitates are less detrimental to both impact transition temperatures and upper shelf energies of the steels than are low nitrogen V(C,N) or VC precipitates. Removal of soluble nitrogen in the form of nitrides has been confirmed as a means of improving the toughness of steels. Aluminium has been found to be more effective than vanadium in the removal of soluble nitrogen.MST/935