11 th International Research/Expert Conference ”Trends in the Development of Machinery and Associated Technology” TMT 2007, Hammamet, Tunisia, 05-09 September, 2007. STUDY ON THE SURFACE PRESSURE IN THE SLAG SYSTEMS USED AT THE CONTINUOUS CASTING Hepuţ Teodor, Comşa Adriana, Osaci Mihaela, Petre Doina Faculty of Engineering Hunedoara Revolutiei, No.5, Hunedoara Romania ABSTRACT The intensity of the gas absorption processes (oxygen, nitrogen, hydrogen, sulphur, etc.) from the furnace environment, the assimilation of flux additions and of inclusions, the slaf foaming are greatly influenced by the surface pressure. The slag surface pressure is influenced by its components, an influence that is transmitted upon the slag-steel interface pressure too. The researches aimed to establish some correlation relationships between the surface pressure, temperature and chemical composition of the slag, analytically and graphically expressed, by processing the data in the MathCAD program. The best chemical compositions of the lubrication fluxes are established based on these correlations and by comparing them with those obtained for the viscosity variation. Keywords: surface pressure, steel, continuous casting, mould, slag. 1. INTRODUCTION The molten metallurgical slags being ionic melts have relatively high (300-600 mJ/m 2 ) surface pressure, but several times less than the metal one. It is considered that the surface pressure of the pure oxides in melted condition depends on the electrovalence link weight, being known that it varies from an oxide to another, function of the difference of electronegativity between the cation (basic ion) and anion (acid ion). Function of it the surface tension of the pure oxides varies too, decreasing from CaO to Al 2 O 3 and SiO 2 . This rule, though it is scientifically founded, is not verified integrally in practice, because of the miss of sure values of the surface pressure of the pure oxides. In the melts of two or several oxides the surface pressure is determined by the fact that the particles (ions) from the superficial layer are not subjected to some preponderant attractions of the phase where the slag is and that is why its value depends on the connection energy of the particles from the superficial layer with the neighbour particles from the same phase. The oxygen concentration in the superficial layer of the melted slags is approximately constant and the influence of replacing an oxide from the slag with another upon the surface pressure is explained by the appearance of some new cations - having another connection energy with the oxygen - in the superficial layer. The values of the connection energy in CaO, MgO, MnO and Al 2 O 3 are close and that is why their reciprocal replacement shall not lead to essential changes of the surface pressure, while introducing SiO 2 , B 2 O 3, P 2 O 5 , Na 2 O, having smaller connection energy values shall lead to the decrease of the slag surface pressure, fact that is confirmed into practice. The surface pressure shall provide an optimal meniscus shape. This is function of the chemical composition of the slag, of the steel being cast and of the temperature. 2. EXPERIMENTAL RESEARCHES In MathCAD it was obtaining dependences between superficial tension and chemicals composition for slags in CaO-SiO 2 -Al 2 O 3 system. Also is presented in fig.1-3 equations in analytical and graphical form. [1, 2] 331