Dispersion strengthening in vanadium microalloyed steels processed by simulated thin slab casting and direct charging: Part I - Processing parameters, mechanical properties and microstructure

Abstract

A study simulating thin slab continuous casting followed by direct charging into an equalisation furnace has been undertaken based on six low carbon (0.06wt-%) vanadium microalloyed steels. Mechanical and impact test data showed properties were similar or better than those obtained from similar microalloyed conventional thick cast as rolled slabs. The dispersion plus dislocation strengthening was estimated to be in the range 80-250MPa.A detailed TEM/EELS analysis of the dispersion sized sub-15nm particles showed that in all the steels, they were essentially nitrides with little crystalline carbon detected. In the Steels V-Nb, V-Ti and V-Nb-Ti, mixed transition metal nitrides were present. Modelling of equilibrium precipitates in these steels, based on a modified version of ChemSage, predicted that only vanadium rich nitrides would precipitate in austenite but that the C/N ratio would increase through the two phase field and in ferrite. The experimental analytical data clearly points to the thin slab direct charging process, which has substantially higher cooling rates than conventional casting, nucleating non-equilibrium particles in ferrite which are close to stoichiometric nitrides. These did not coarsen during the final stages of processing, but retained their highly stable average size of ~7nm resulting in substantial dispersion strengthening. The results are considered in conjunction with pertinent published literature