THE OPTIMISATION OF TITANIUM AND NITROGEN CONCENTRATION IN MICRO-ALLOYED STEEL

The paper deals with the control of the concentrations of titanium and nitrogen content in micro-alloyed steel with the aim of preserving sufficient amounts of these components for precipitation of relevant amount of TiN inclusions in austenite. Fine TiN inclusions inhibit the growth of austenite grains during soaking prior to hot rolling. The analysis of three stages of secondary metallurgy treatment: deoxidization with aluminum, titanium addition and cooling down to liquidus temperature was based on the parameters of thermodynamic equilibrium between metallic phase and non-metallic phases present in the system. The equilibrium compositions of phases were calculated by means of the FactSage software. The low alloyed carbon steel containing 0.02% C, 1.5% Mn and 0.2% Si was the subject of calculations. It was determined that deoxidization by means of aluminum reduces the oxygen concentration below 20 ppm. The TiN addition causes the formation of solid TiN and the modification of composition of liquid oxide precipitates with dissolved Ti(2)O(3). The composition of obtained phases as well as the titanium distribution were determined for various initial nitrogen contents, steel deoxidization levels and titanium additions. It was stated that the most effective use of titanium from the view point of TiN formation in austenite appears at relatively low nitrogen concentration in steel, approximately 25 ppm.