Optimum Chemical Composition and Thermomechanical Processing Condition for Niobium-Boron Steel

Effects of chemical composition and variables in thermomechanical processing (TMP) with accelerated cooling were studied on the mechanical properties and microstructure of Nb-B steel. It was clarified that, in order to achieve a good strength and toughness balance by a combined addition of niobium and boron, it is necessary to reduce the carbon content, to dissolve niobium during slab-reheating and to roll with sufficient reduction in an unrecrystallization region of austenite. With these conditions, a fine-grained low-carbon bainitic structure is obtained. The changes in the mechanical properties and microstructure of Nb-B steel with composition and processing variables can be elucidated in terms of the distribution of boron and the amount of solute niobium of plates. The factors controlling strength and toughness of Nb-B steel were also discussed.