Abstract
Effects of hot rolling conditions on the mechanical properties of extra low carbon cold rolled steel sheets containing niobium have been investigated. The mechanical properties of this steel depend on the hot working conditions and heat history after hot rolling, and the former effect is stronger than the latter. Hot rolling with high reductions, high speeds or low finishing temperatures (≥Ar3) improves the Lankford-value and the ductility of cold rolled and continuously annealed steel sheets. The reason can be explained as follows. When carbo-nitride precipitation is accelerated through the hot rolling with the above conditions, the grain size of the hot rolled sheets becomes amaller and the density of fine carbo-nitrides precipitating after hot rolling decreases extremely. The smaller grain size of hot rolled sheets contributes to the improvement of Lankford-value. The low density of fine carbo-nitrides in hot rolled sheets increases the ductility of cold rolled and rapidly heated steel sheets.
