Carbide characterization in a Nb-microalloyed advanced ultrahigh strength steel after quenching–partitioning–tempering process

Based on the observations of scanning electron microscopy and transmission electron microscopy, four kinds of carbides were identified in a Nb-microalloyed steel after quenching–partitioning–tempering treatment. In addition to transitional epsilon carbide that usually forms in silicon-free carbon steel, other three types of niobium carbides (NbC) formed at various treatment stages respectively. They are incoherent NbC inclusion that nucleated at solidification mainly, fine NbC that nucleated in lath martensite at tempering stage and regular polygonal NbC that nucleated in austenite before quenching. Their formation mechanisms on steel were discussed briefly based on thermodynamics.