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

The formation of the micropores in mould flux is further investigated employing solid-state 1 H 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 H 2 O during the crystallization of mould flux in the flux channel. According to the proposed mechanism, mould flux with a lower activity of SiO 2 has been developed for Si-K steel, and it successfully prevents the sticker breakouts.

Effect of Water Vapor on Evaporation and Melt Crystallization of Mold Fluxes

“…Afterward excessive hydroxyl will cause enhancement of fluoride evaporation as explained in Eq. [6], hence acceleration of crystallization of Ca 2 SiO 4 . The larger number density of Ca 2 SiO 4 nuclei will bring excessive decrease of horizontal mold heat transfer rate and deterioration of mold lubrication at elevated temperature, which finally induces sticking breakout of the steel shell near the meniscus.…”

“…As mentioned earlier, most previous studies [3][4][5][6][7][8] suggested that the enhancement of heterogeneous nucleation of crystalline mold flux on evolved gas species would be the main reason for the hydrogen-induced breakout during continuous casting under humid atmosphere. However, above results and discussions in this study showed that accelerated evaporation of fluorine due to hydroxyl in molten mold flux should be considered as a potential mechanism for the hydrogen-induced breakout.…”

“…[1,2] Under humid atmosphere, increase of the occurrence of so-called hydrogen-induced sticking-type breakout of steel shell has been frequently observed due to the accelerated crystallization of mold slag film, which brings the reduction of mold heat transfer rate and the deterioration of lubrication. As the sticking breakout is one of the most serious troubles in a continuous casting process, many previous studies [3][4][5][6][7][8] had been carried out to clarify the main reason for the acceleration of crystallization.…”

Ações De Controle Para Evitar O Rompimento De Pele Nas Máquinas De Lingotamento Contínuo Da Aciaria 2 Da Usiminas, Usina De Ipatinga

“…O que agravou-se com o passar dos anos. Esse vapor desprendido, ao encontrar a superfície fria do molde, condensava-se e umidificava o pó fluxante a ponto de molhar e criar incrustações na face do molde e/ou pelotas, condição essa que pode alterar o desempenho térmico da escória do fluxante [2] e potencializar a formação de trincas longitudianais em placa. Pontualmente eram observados picos de umidade nas pelotas de até 24%, teor esse indevido para um processo estável e seguro de lingotamento.…”

Mechanism and Control of Hydrogen Induced Abnormal Sticky Behavior in Slab Casting Mould

“…[11][12][13] Kajitani et al 14) established a mechanism of formation of micro pores in slag film due to increase in hydrogen content in molten steel during the casting of Si killed steels.…”