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Aspects of Production Hot Rolling of Nb Microalloyed High Al High Strength Steels

High Al low C – Mn steels are attracting growing interest because of their unique mechanical properties. The major efforts in studying these steels are focused on the effects of heat treatment routes on the final properties. Hot rolling, although being decisively important receive much less attention. The practical aspects of hot strip rolling of Nb microalloyed high Al steels are addressed in the paper. The data from production hot mill are analyzed in conjunction with the results of laboratory high temperature mechanical simulations. Mill data indicate that Al suppresses the effects of microalloying on softening of austenite. Al additions raise the austenite-to-ferrite transformation temperature under industrial mill conditions resulting in uncontrollable formation of ferrite in the surface layers during last hot rolling passes. This, in turn, leads to uncontrollable variation in tension, friction, rolling forces and mass flow. In general, high Al steels are much more sensitive to the variations in bar temperature than that of conventional HSLA steels, so that small variations in processing conditions can induce marked differences in the mill performance and the quality of the hot band. The severity of the related adverse effects also depends on hot mill configuration, capabilities and equipment.