ELECTRON BACKSCATTER DIFFRACCION APPLICATION ON STEEL CONTINUOUS CASTING NOZZLES TO INCREASE THE KNOWLEDGE OF WEAR MECHANISMS

E. BRANDALEZE; M. AVALOS

Viena

Conferencia; Unified International Technical Conference 2015; 2015

UNITERC

Therequirements for submerged nozzles are particularly severe because they need tohave good resistance to thermal shock and chemical attack during the nozzleimmersion into molten steel and mould flux at 1530ºC. The study of the corrosionand aggregates processes developed under those conditions are mandatory to preventthe expensive production losses due to break down during the service. One of thenozzle refractory materials with optimum performance is the alumina-graphite(Al2O3-C) bonded with phenolic resin. The mould flux -nozzle contact during continuous casting process causes localized corrosion atthe flux line and determines the life in service of the submerged nozzle. Newmould fluxes are formulated in order to replace the traditional Ca2Fto prevent gas emissions and environmental problems. In that sense, theevaluation of the corrosion mechanism and the impact of the new fluxes on thenozzle corrosion mechanism is very important to improve the life of the nozzlesin service. Inparticular, in this paper the influence of a new flux on the nozzle structuralintegrity is evaluated considering chemical and structural aspects. In this newflux, a combination of Li2O and B2O3 is usedas a Ca2F substitution compounds The characterization of the nozzle degradationincludes chemical composition determination by different techniques correlatedwith X ray diffraction results, structural study of the material by differentmicroscopy techniques and the evaluation of changes in microestructure inrelation with temperature was carried out by DTA TG, changes, were studied. In this work two mould fluxes wereconsidered such as corrosive agents, a commercial one with Ca2Fidentified as PC with 10.4% of CaF2 content and other formulated inthe laboratory that contains Li2O-B2O3 as Ca2Freplacement. The viscosity behaviour of both fluxes at process conditions wassimulated by Fact Sage software and the melting behaviour was determined by hotstage microscopy (HSM).Two different situations wereanalized: a post mortem nozzle corroded in service in contact with with the PCmould flux and a cup testing  nozzlerefractory material in contact with Li2O-B2O3flux at 1450°C. The last one is so useful to analize corrosion mechanism sincein the post mortem nozzle the dinamic of the process releases and drag  the damage material removing the evidence ofthe intermediate stages of corrosion. The cristallographic and compositional aspects of the cup testingcorroided nozzle were analized by Electron Backscatter Diffraction technique (EBSD)combined with Energy Dispersive Spectroscopy (EDS) technique in order to obtaininformation of the corrosion mechanisms associated with the crystalline natureof the aggregates present in the nozzle. The nature of the chemical reactionsbetween flux and nozzle were stablished using scanning electron microscopy SEMwith EDS analysis and corroborated by thermochemical simulation, applying theFact Sage software. In addition, the thermochemicalsimulations provide the possibility to corroborate the chemical reactions inthe system between nozzle and mould flux.