Obtenção de ferrita de grão utrafino em aços C - Mn e microligado com Nb-Ti através de ensaios de torção a quente

Abstract

In the last years, several studies concerning ultra refinement of ferrite grains have been conducted by different methods (ECAP, ARB, HPT) in order to provide an optmized relationship between mechanical properties and microstructure of steels. The ferrite strain induced dynamic transformation was studied in this work, through the application of plastic deformation by hot torsion experiments in two low carbon steels, a C-Mn and a Nb-Ti microalloyed respectively. The aim of this work is to obtain anultrafine grained ferrite microstructure, providing superior mechanical properties, if compared to traditional hot rolled industrial products, as demonstrated by the mechanical tests performed. After heating at a rate of 1ºC/s, austenitization at 900ºC and 1200ºC and 300s soaking, the samples were ice brine quenched. and then submitted to the torsion in temperatures of 700 and 740°C, involving 7 passes of deformation, up to a final true deformation of 1. The softening mechanisms were studied from the flow curves. Microstructure was analyzed by optical, scanning , transmission electron and scanning probe microscopy. The ferrite grain size evolution of the steels isothermally annealed at 800°C during six different times was verified. To evaluate the mechanical behavior of these steels tests of Vickers microhardness had been carried out, whose values at the end of the process reached approximately 200HV. The accumulation of deformation energy in the two-phase field provided favorable conditions for ferrite nucleation, leading to an ultrafine grained microstructure, in the order of 2m, without the necessity of implementing an extremely high accumulation of strain. The shape of stress-strain curves so obtained suggested that refinement of ferrite occurred by dynamic recrystallization during hot working stage. Thermal analysis was performed on cooling and heating with the aid of a thermocouple embebed in the samples, to evaluate the phase transformation temperatures. Empirical equations were developed in order to stablish a relationship between processing parameters, microstructures and mechanical properties. This processing route could be considered to be an industrial viable one for these steel grades, as long as there is a good control of these parameters.