Effect of Water Vapor on Evaporation and Melt Crystallization of Mold Fluxes
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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[3–8] had been carried out to clarify the main reason for the acceleration of crystallization. Firstly, it is suggested that heterogeneous nucleation of crystalline mold flux could be accelerated on the hydrogen gas bubbles evolved from solidification front[3] or at the mold side surface of steel shell.[4,5] Hydrogen gas can be evolved in a casting mold from the following sequences. Entrapment of water into mold slag: H2 OðgasÞ þ ðOÞ2 ¼ 2ðOHÞ ½1 Hydrogen pickup into steel: 2ðOH) ¼ ðOÞ2 þ 2½H þ ½O Evolution of hydrogen gas during cooling: 1 ½H] = H2 ðgasÞ 2
½2
½3
Secondly, water gas bubble generated by reduction of hydroxyl will also induce heterogeneous nucleation of mold fluxes during their melt crystallization.[6] Reduction of hydroxyl by aluminum in steel: 6ðOH) þ 2½Al ¼ ðAl2 O3 Þ þ 3H2 O(gas)
During continuous casting of steels, the mold flux infiltrates into the gap between copper mold and solidifying steel shell to form a slag film. This mold slag film plays the decisive role in controlling both the horizontal heat transfer and lubrication in the casting mold wall, which are key functions to determine the quality and productivity of continuous casting process. It should be emphasized that these functions are strongly dependent on the degree of crystallinity.[1,2] Under humid atmosphere, increase of the occurrence of
JI-YEON BAEK, Ph.D. Candidate, and SEON-HYO KIM, Professor, are with Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. JUNG-WOOK CHO, Research Associate Professor, is with Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH). Contact e-mail: [email protected] Manuscript submitted February 28, 2015. Article published online December 7, 2015. 32—VOLUME 47B, FEBRUARY 2016
½4 Lastly, hydroxyl itself can accelerate crystallization of cuspidine by participating in crystallization reaction.[7–9] Formation of hydroxyl containing cuspidine ðCa4 Si2 O7 F2 Þ: haiF þ h2 aiðOHÞ þ 4Ca2þ þ 3Si4þ þ 7O2 ¼ Ca4 Si2 O7 ðF þ OHÞ2
½5
The above mechanisms suggested are largely based on the reduction of effective activation energy for nucleation of primary crystalline phase, cuspidine. However, there has been no systematic investigation to confirm the effect of evolved gas species on the crystallization behavior. Also, there may be another reason for the acceleration of crystallization under humid atmosphere, such as, for example, composition change due to water METALLURGICAL AND MATERIALS TRANSACTIONS B
vapor. Therefore, in this study, in situ observations on melt crystallization have been carried out for commercial mold fluxes using a single hot
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