HOT DUCTILITY OF HIGH-PURITY STEELS CONTAINING NIOBIUM

Ductility of high purity steels containing niobium was investigated at low strain rates at temperatures ranging from higher ferrite-pearlite region to lower austenite region. Ductility loss is observed at temperature where austenite phase is stable regardless of loading, and it increases with decreasing temperature. Ductility shows minimum at temperature where ferrite phase is induced to form by loading. Hot ductility reduces with increase of niobium content. At the temperature corresponding to minimum ductility, crack and thin film of ferrite are formed along grain boundary, but their location does not always coincide with each other. Grain boundary is easy to move at higher temperature with good ductility, whereas grain boundary keeps straight during deformation at temperature range with poor ductility. Very fine niobium carbide particles are homogeneously precipitated by deformation at austenite temperatures, and then no precipitate free zone can be found. Aluminum deepens and widens the ductility trough by coprecipitating with niobium during deformation, manganese shifts the ductility loss to lower temperature, and nitrogen slightly deteriorates hot ductility. The cause of the ductility loss is proposed on the basis of the role of vacancies. Both dynamic recovery and grain boundary migration are depressed by the fine particles, and some amount of dislocation pile up occurs at grain boundaries, to which vacancies diffuse to annihilate the dislocation, and the incompatibility of the strain. Microvoids are nucleated as far as the grain boundary is immobile due to locking by either segregation of impurities or fine precipitation, and microvoids coalesce to form crack along grain boundary. The ferrite formation relaxes the grain boundary incompatibility and recovers the ductility.