The beneficial roles of nitrogen in non-quenched and tempered steel were described.Among Nb,V,Ti,the solubility of vanadium is high in medium carbon steel.Therefore,vanadium is the most effective and common use strengthening element for non-quenched and tempered steel.Vanadium added into the steel plays two essential roles-grain refinement and precipitation strengthening by the fine carbo-nitrides precipitates. Compared with carbon,the affinity of vanadium to nitrogen is higher and the nitrides are more stable than carbides.Hence,nitrogen play an important role in controlling precipitation of vanadium carbonitrides in microalloyed steel.The experimental results show the increase in the nitrogen content in non-quenched and tempered steel changes vanadium interphase distribution,promotes precipitation of V(C,N),decreases the particle size of V(C,N) precipitates,hence,enhances precipitation strengthening effect of vanadium in the steel and improves strength of the steel by a big margin.Because nitrogen promotes precipitation of V(C,N),pins austenite-ferrite boundary effectively and refines the ferrite grain.The fine V(C,N) precipitates promote intragranular ferrite nucleation,furthe,refine ferrite.In non-quenched and tempered steel treated by microalloying element Ti,the increase in nitrogen content makes TiN particles more stable,retards the growth of austenite grain effectively.The full use of cheap and plentiful nitrogen can save expensive vanadium alloy,further decrease the production cost of non-quenched and tempered steel,provided a given strength level of the steel is ensured.Therefore,the nitrogen is a very cost-effective microalloying element in the steel.
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