Mechanism of a hydrogen-induced sticker breakout in continuous casting of steel: Influence of hydroxyl ions in mould flux on heat transfer and lubrication in the continuous casting mould

Abstract

Mechanism of a hydrogen-induced sticker breakout is studied paying attention to the observation that the frequency of the breakout is higher during continuous casting of Si-K steel than Al-K steel. During the casting of Si-K steel, the micropores, which are present in mould flux film, reduces heat transfer at the mould meniscus. In such a case, mould flux consumption is decreased, because the oscillation marks that provide mould flux into the flux channel become less pronounced, and it results in a sticker breakout. The formation of the micropores in mould flux is further investigated employing solid-state 1H NMR with CRAMPS technique that has successfully enabled the quantitative analysis of hydroxyl ions in mould flux. The spectra have revealed that hydroxyl ions exist in mould flux and they increase in molten flux pool especially during the casting of Si-K steel. The result provides the new mechanism for the formation of the micropores: in a continuous casting mould, water vapour in the atmospheric air is absorbed into molten flux pool as hydroxyl ions, and they are usually reduced by soluble Al in molten steel pool. However, during the casting of Si-K steel where Si is a weaker deoxidizing element, the hydroxyl ions are not easily reduced, and they form the micropores of the gas including H2O during the crystallization of mould flux in the flux channel. According to the proposed mechanism, mould flux with a lower activity of SiO2 has been developed for Si-K steel, and it successfully prevents the sticker breakouts. © 2008 ISIJ.