Influence of Ferroalloy Impurities on the Inclusion Characteristics in Liquid Steel (De invloed van onzuiverheden in de ferrolegeringen op de staalzuiverheid)
Influence of Ferroalloy Impurities on the Inclusion Characteristics in Liquid Steel
Pande, Manish Marotrao; S0206083
Ferroalloys are added to the liquid steel to impart special properties. The quality of ferroalloy can influence the steel quality as the ferroalloys are added during or towards the end of secondary refining. The steel industry has a strong evidence of nozzle clogging problem with the late addition of certain ferroalloy grades. The objectives of the present doctoral work were (i) to obtain the knowledge concerning the impurities of various ferroalloys added during the secondary metallurgy and (ii) to study the influence of ferroalloy impurities on inclusion characteristics in liquid steel on the industrial and laboratory scales. The research work was divided into three main phases.In the first phase, the investigation of eight different ferroalloy grades, viz., ferromolybdenum (FeMo), ferroniobium (FeNb), high carbon ferromanganese (HCFeMn), low carbon ferromanganese (LCFeMn), ferrotitanium70 (FeTi70), ferrotitanium35 (FeTi35), ferrosilicon75 (FeSi75) and ferrophosphorus (FeP) was carried out for their impurity assessments by using various characterization techniques. Among these ferroalloys, FeTi35 and FeP were found to be the most impure ferroalloys and studied further.In the second phase, ultra-low carbon (ULC) steel grades to which FeP and FeTi additions are made, were chosen for the statistical analysis of the industrial data. Pulse discrimination analysis by optical emission spectroscopy (PDA-OES) is an optimized technique at the industry to determine the online steel cleanliness in terms of the number of inclusions present in the steel sample. However, PDA-OES does not give any information about the inclusion size and its distribution. Therefore, two methodologies were developed to estimate the inclusion size on the basis of PDA-OES data and compared with the inclusion size measured by the extraction technique for some specific heats. It was found that the inclusion size difference was only marginal. The inclusion population data obtained by PDA-OES technique was used for the statistical analysis. In order to support the statistical analysis results, further evidence by means of industrial sample analysis and the thermodynamic calculations was provided. The main findings of this work were (i) FeTi70 was found to be the cleaner ferroalloy than FeTi35 while (ii) a new addition practice for FeP was proposed to maintain the balance between the steel cleanliness and the refining of FeP impurities without much elemental (phosphorus) losses.In the third, i.e. final phase, the laboratory study was carried out for three different Ti sources, viz., pure Ti, FeTi70 and FeTi35 to analyze the reaction zone formed on contacting the liquid iron. The experiments were carried out in a controlled atmosphere in a vertical tube furnace by using the novel liquid metal suction technique. The experimental procedure consisted of bringing the titanium sources in contact with the liquid iron for a predetermined time followed by quenching. The samples obtained in the above manner were further subjected to microstructural study. It was found that the impurities from FeTi70 contribute to the inclusion formation while FeTi35 directly introduces inclusions. The extent of inclusions introduction through FeTi35 seems far greater than the number of inclusions formed due to the impurities through the FeTi70 addition. This was in agreement with the observations made during the industrial study.