THERMOMECHANICAL PROCESSING OF DUAL PHASE STEEL AND ITS CORROSION BEHAVIOUR

Abstract

In present study an attempt has been made to introduce a potential processing route and most suitable processing conditions for the development of ultrafine grained dual phase steels by using thermomechanical processing. The thermomechanical processing was done employing a Gleeble 3800 thermomechanical simulator. To quantify the exact influence of thermomechanical processing on microstructural and electrochemical behaviour of dual phase steels, it was desired to develop DP steels by using two different paths i.e. via conventional thermal processing and via advanced thermomechanical processing. In conventional thermal processing three different kind of dual phase steels were developed by soaking at different temperatures in intercritical region followed by quenched in water. Dual phase steels by thermomechanical processing were developed via three different routes. In first two routes the soaking temperature were changed by keeping same deformation temperature while third route consisted of two stages of deformation. The comparisons between these two steels were made on microstructural and hardness basis. The microstructures were characterized by SEM and optical microscopy. It was found that after thermomechanical processing grain size and hardness of steels improves prominently in comparison to conventional thermal processed steels, while among thermomechanically processed steels the route having two stage of deformation revealed best property. Further to study the influence of various processing parameters, viz. strain, strain rate, deformation temperature and soaking time, a number of Dual Phase steels were developed by-varying the parameter which effect is to be quantified. It was found that an increase in strain increases grain refinement inside steel, higher strain rate leads to accelerate the nucleation of dynamic ferrite and an increase in deformation temperature and soaking time increases the amount of martensite inside dual phase steels. In the present study, a commonly used corrosive medium in the form of 3.5% NaCl solution was used to evaluate the performance of steels. The electrochemical behaviour of conventional thermal processed and thermomechanically processed dual phase steels were compared by initial ferrite- pearlite steel by combined study of tafel and linear polarization plots. It was concluded that all kind of dual phase steels shows better corrosion resistance than initial ferrite-pearlite steel.