Effect of Intercritical Annealing Time on Precipitations in a Hot Rolled Ultrafine Grained Metastable Automobile Steel

The size,morphology,distribution and components of precipitates in hot rolled ultrafine grained metastable automobile steel under hot rolling and intercritical annealing after different holding time were systematically investigated by using carbon extraction replica,transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDX).The results show that the precipitates are mainly composed of TiN,NbTi(CN) and NbC after hot rolling.The TiN precipitates during casting process,NbTi(CN) precipitates on the existing TiN particles and NbC precipitates in dislocations and sub-boundaries during hot rolling process.During intercritical annealing process,part of carbides dissolves into austenite,and remained precipitates merges with each other and coarsens according to typical Oswald ripening mechanism.In the course of initial annealing,some rectanular TiN precipitates from the matrix.With the increase of annealing time,the morphology of precipitates develop into circular and oval.After holding for 6hs,the precipitates are mainly composed of NbTi(CN) and NbC,the TiN is not found.