Nozzle Clogging during Continuous Casting of Ti-Bearing Al-killed Ultra Low Carbon Steel and Its Countermeasure

Abstract

It is well known that Ti causes serious nozzle clogging problem during continuous casting of Ultra Low C (ULC) steel, even severe than continuous casting of Ti-free ULC steel. However, nozzle clogging mechanism of Ti-ULC steel is still uncertain compared with that of Ti-free grades. Nozzle clogging of the Ti-free steel casting is less serious, and most of the clogging material is composed of alumina inclusions. On the other hand, a clog material are composed of mainly frozen “Fe” containing inclusions in case of Ti-ULC steel casting. In order to figure out why nozzle clogging issues become severe in the presence of Ti, cross-section of actual worked nozzles in POSCO Gwangyang works were analyzed by SEM-EDS. From observations of the nozzle, complex oxide mainly composed of calcium aluminate with limited amount of TiOx was identified in the vicinity of inner wall contacting with molten steel after Ti-ULC steel casting. It should be noted that the complex oxide was adhered with frozen “Fe” on the periphery of nozzle outports, but inclusions in frozen “Fe” consisted of mainly alumina. The presence of TiOx containing phase only on the inner wall of the nozzle indicates that there is a source of Ti oxidation by refractory. A series of experiment using finger rotating method by stirring a refractory sample into liquid steel also represented similar tendency to actual process. The present author established a kind of hypothesis that CO gas generated by a carbothermic reaction of nozzle constituents (SiO2 and graphite) is detrimental to the nozzle clogging. The CO gas oxidizes the Ti-ULC steel at the interface between the steel and the nozzle, and forms FetO-Al2O3-TiOx liquid oxide temporarily. The fact was proven by both thermodynamic calculation and a series of experiments for the interfacial reactions between CO gas and steel was conducted employing semi-levitation methods. The FetO-Al2O3-TiOx liquid oxide acts as a binder between frozen “Fe” and refractory due to its wettable characteristics both steel and refractory. The FetO-Al2O3-TiOx is initial form of CaO-Al2O3-TiOx phase in the field operation, and Ti oxidation of Ti-ULC on the nozzle can be a trigger to initiate nozzle clogging involving frozen “Fe” adhesion. Also, dissolution behavior of CO gas into pure molten steel was investigated prior to understand how fast the liquid oxide is formed. However, FetO-Al2O3-TiOx was not found after both actual process and lab scale test. Instead, CaO-Al2O3-TiOx phase with frozen “Fe” was found that on the refractory side since FeO in the liquid oxide was reduced quickly by Al and Ti. When a refractory coated by FetO-Al2O3-TiOx was dipped and stirred during few seconds, entire FetO-Al2O3-TiOx was changed into CaO-Al2O3-TiOx phase. For this reason, the final form of accretion on the nozzle would be, frozen “Fe” in contact with CaO-Al2O3-TiOx phase. In summary, nozzle clogging mechanism is composed of 1) CO gas formation from carbothermic reaction of nozzle constituents, 2) FetO-Al2O3-TiOx formation by interfacial reaction between the CO gas and Ti-bearing steels at the interface, 3) the FetO-Al2O3-TiOx highly wettable characteristics with molten steel is reduced by alloying elements (Al and Ti), and 4) is converted into the CaO-Al2O3-TiOx phase with “Fe” deposit.On the basis of revealed clogging mechanism, countermeasures to cope with nozzle clogging could be established and were tried in various type of laboratory scale tests. All of the countermeasures were focused on decreasing amount of FetO-Al2O3-TiOx which causes frozen “Fe” adhesion by optimizing composition of both refractory and steels. From application of these countermeasures for lab scale tests, they are effective to lessen amount of frozen “Fe” attached on the refractory sample by suppressing interfacial reaction between the nozzle material and the liquid steel.