Effect of Composition and Processing Variables on Thermomechanically Processed Niobium-Boron Steel

In the thermomechanical control process (TMCP) with accelerated cooling, the effect of composition and processing variables on the mechanical properties and microstructure of niobium-boron steel was studied. It was clarified that in order to achieve a good strength and toughness balance by the combined addition of niobium and boron, it is necessary to lower the carbon content, to dissolve niobium during slab-reheating and to controlled-roll with sufficient reduction in the non-recrystallization region γ. With these conditions, a fine-grained bainitic structure is obtained. The change of the mechanical properties and microstructure of niobium-boron steel with composition and processing variables can be explained in terms of the behavior of boron and the amount of niobium in solution. The strength and toughness controlling factors of niobium-boron steel were also discussed.