Processing and Properties of Direct Reduced Iron Pellets Containing Material for Control of Steel Structure
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The thesis work was carried out at the Department of Materials Science and Engineering in Norwegian University of Science and Technology (NTNU) and in SINTEF. It is a part of a SINTEF /NTNU project called Dispersoids in Steel and Value-added Direct Reduced Iron (DISvaDRI), which has been financed by the Norwegian Research Council (NRC) since 2004. DISvaDRI aims to develop special addition alloys for steels that contain a fine dispersion of effective seed crystals, and promote grain refinement during steel solidification. Metallic materials with small grain sizes usually have improved mechanical properties, which depend upon the characteristics of the solidification microstructure. It has been well known that some non-metallic inclusions dispersing in steels can act as heterogeneous nucleation sites for various precipitates such as sulphides, nitrides and carbides. The multiphase particles are beneficial to catalyse formation of intragranular ferrite crystal during phase transformation which creates a finer grained microstructure, and consequently improve mechanical properties of steel. These beneficial inclusions are named dispersoids. Utilisation of dispersoids in steel metallurgy requires the controlled addition of the dispersoids to the steel, which can take place by employing an alloy containing a large number density of the desired dispersoids. The dispersoid alloy could be considered as a "smart master alloy", and directly added into the liquid steel. In this thesis work, both natural ilmenite ore and mixture of magnetite ore and cerium dioxide were used to produce smart master alloys. Several parameters were tested such as particle size and pellet size, reducing temperature and time, reducing agents, and gas composition and gas flow rate. In Chapter 3, the reduction of ilmenite pellets from MINPRO AB was investigated at 950°C and 1000°C under a CO-N2 gas mixture with 10 L/min gas flow rate. The reduction rate increases with temperature and the CO concentration in the gas mixture. In Chapter 4, the ilmenite powder obtained from TITANIA AS was jet milled down to average particle size of 14.61 μm, and then pelletized. The green ilmenite pellets were preoxidized at 800°C and 1200°C respectively. After that, the reduction of preoxidized ilmenite pellets was investigated respectively under both 70%CO-30%N2 gas mixture and H2 with 5 L/min gas flow rate. It was found out that the optimum preoxidation temperature was 800°C. In Chapter 5, the ilmenite powder was supplied by PI-KEM LTD. The reduction and nitridation of preoxidized ilmenite pellets were investigated under a H2-N2 gas mixture of 5 L/min with the prospect of creating grain refiners containing titanium oxides and titanium nitride dispersoids. The ilmenite pellets were reduced at 1100°C for 4 hours under 80%H2 - 20%N2 gas mixture, and then respectively nitrided at 700°C, 800°C, 900°C and 1000°C for 12 hours under 10%H2-90%N2 gas mixture. In Chapter 6, the raw materials of 20% CeO2 powder and 80% dried magnetite powder (weight percent) were mixed and pelletized. Green pellets were preoxidized in the chamber furnace under air at 1200°C for 2 hours. The reduction of preoxidized pellets was investigated at 700°C, 800°C, 900°C and 1100°C for 4 hours under a H2 gas of 5 L/min. Grain refining alloys (master alloys) containing cerium were produced by reducing preoxidized pellets, the purpose of this study is to reduce iron oxides into metallic iron and introduce cerium oxides as dispersoids into melt steel. All the reduction and nitridation experiments were conducted in a furnace with a thermogravimetric analysis system. The products were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM), and analyzed by Energy Dispersive Spectrometers (EDS) and Electron Probe Micro Analysis (EPMA), and a detailed understanding of the mechanism of reduction is obtained. In Chapter 7, thermodynamics and kinetics of the gaseous reduction of ilmenite were computed. The experimental results from Chapters 3-5 and simplified shrinking core model were used in the computation. These reduced pellets (master alloys) produced in the experiments will be selected and added into the melt steel in order to supply the nucleation sites and refine the grain of the steel during phase transformation in the future work.