钒：改造未来

Importance of nitrogen for precipitation phenomena in V-microalloyed steels

Abstract:

The importance of the interaction of nitrogen with vanadium in HSLA steels is discussed in the present paper which analyses quantitatively the theoretical basis for achieving maximum grain refinement and precipitation strengthening with a minimum of V addition. The effect of nitrogen, in particular, has been studied with regard to precipitate evolution and the precipitation kinetics of V(C,N) in association with the isothermal and continuous cooling γ-α transformation. Temperature ranges within which interphase precipitation and random precipitation occur were established for isothermal transformation. The influence of various compositional (V, N and C) and process parameters (cooling rate, transformation temperature) on the characteristics of the V(C,N) dispersion, the particle size distributions, distance between particles and their chemical composition have been determined. The precipitation strengthening contribution to the yield strength from V(C,N) has been evaluated and compared with Orowan theory. It was shown that the principal factors influencing the size distribution of V(C,N) precipitates are nitrogen content and the ratio of V:N. It was demonstrated that the effect of enhanced nitrogen is manifested in smaller particle size of precipitates, smaller interparticle distances and greater resistance to particle coarsening. At lower nitrogen content precipitates coarsen readily and become less effective with respect to their precipitation strengthening. The large strengthening effect of nitrogen in vanadium steels was found to be due to greater number of refined V(C,N) precipitates which is partly a result of lower solubility of VN and partly of faster γ-α transformation and shorter time available for the growth of precipitate particles in high N steels. It is concluded that the interaction of vanadium with nitrogen is of key importance in the grain refinement and precipitation strengthening of HSLA steels