The Effect of Restoration Process on the Mechanical Behavior of Ultra-Fine Grain Size Nb-Ti Steel Processed by Warm Rolling and Sub and Intercritical Annealing

Abstract

The ferrite grain size refining is the unique mechanism for increasing both mechanical strength and toughness of steels. Steel with an ultra-fine ferrite grained structure must show a good relation between mechanical strength, ductility and toughness, while the low carbon content enhances good welding characteristics. The objective of this work is to investigate the influence of restoration process on the mechanical behavior of a microalloyed low carbon-manganese (0.11%C, 1.41%Mn, 0.028%Nb and 0.012%Ti) steel with ultra-fine grains produced through out quenching, warm rolling, followed by sub and intercritical annealing. The evolution of restoration process -recovery and recrystallization - was followed by optical and scanning microscopy. The x-rays diffraction was employed to evaluate the degree of restoration after subcritical annealing. The mechanical behavior of the steel was estimated using microhardness tests. After subcritical annealing, the microstructure was formed by spheroidal iron carbides and a ferritic recovered matrix. Otherwise, after intercritical annealing, the microstructure was composed mainly by ultra-fine grain polygonal ferrite, MA constituent and carbides. The hardness after processing has shown a 20% increase compared with the steel under hot rolling condition.