TOPICAL COLLECTION: ADVANCES IN MATERIALS MANUFACTURING AND PROCESSING In Situ Observation of Calcium Aluminate Inclusions Dissolution into Steelmaking Slag KEYAN MIAO, ALYSSA HAAS, MUKESH SHARMA, WANGZHONG MU, and NESLIHAN DOGAN The dissolution rate of calcium aluminate inclusions in CaO-SiO 2 -Al 2 O 3 slags has been studied using confocal scanning laser microscopy (CSLM) at elevated temperatures: 1773 K, 1823 K, and 1873 K (1500 °C, 1550 °C, and 1600 °C). The inclusion particles used in this experimental work were produced in our laboratory and their production technique is explained in detail. Even though the particles had irregular shapes, there was no rotation observed. Further, the total dissolution time decreased with increasing temperature and decreasing SiO 2 content in the slag. The rate limiting steps are discussed in terms of shrinking core models and diffusion into a stagnant fluid model. It is shown that the rate limiting step for dissolution is mass transfer in the slag at 1823 K and 1873 K (1550 °C and 1600 °C). Further investigations are required to determine the dissolution mechanism at 1773 K (1500 °C). The calculated diffusion coefficients were inversely proportional to the slag viscosity and the obtained values for the systems studied ranged between 5.64 9 10 12 and 5.8 9 10 10 m 2 /s. https://doi.org/10.1007/s11663-018-1303-y Ó The Minerals, Metals & Materials Society and ASM International 2018 I. INTRODUCTION INCLUSION formation during liquid steel refining is an unavoidable consequence of current steel making processes. The nature and quantity of the inclusions formed in steel is critical, as it affects both productivity and in-service properties of steel. These inclusions can be controlled by two approaches; the modification of their composition and morphology, or the removal of inclusions to the waste slag phase. Oxide and sulfide inclusions can be modified with the addition of calcium using powder injection or wire feeding. A well-known example is the modification of solid alumina inclusions into liquid, or partially liquid, calcium aluminate inclusions. There has been a debate about the correct reaction between dissolved calcium [Ca] and Al 2 O 3 inclusions. Traditionally, the reactions of Al 2 O 3 inclusions in calcium treatment were proposed as Eqs. [1] and [2] to form various types of calcium aluminate inclusions [1–3] x Ca ½ þ 1  2 3 x   Al 2 O 3 ¼ xCaO ð1  xÞAl 2 O 3 þ 2 3 x Al ½ ; ½1 xCaO þ yAl 2 O 3 ¼ xCaO  yAl 2 O 3 : ½2 The melting point of calcium aluminates increases with increasing alumina concentration, to form stable solid inclusions at steelmaking temperatures. Insufficient or superfluous addition of calcium leads to incomplete or excessive modification of alumina inclu- sions and the formation of unwanted inclusions such as CaOÆ2Al 2 O 3 and CaOÆ6Al 2 O 3 that can cause clogging of submerged entry nozzles at continuous casting. The resulting inclusions would ideally be removed from the liquid steel to slag in the ladle, tundish, or caster mold prior to solidification. Therefore, it is important to understand the inclusion–slag interaction towards improving the removal of inclusions from liquid steel. During ladle refining, inclusions are transported to the steel–slag interface where they are separated from steel and dissolved in the slag. Fast dissolution kinetics helps to prevent the re-entrainment of inclusions into liquid steel. Inclusions are similarly removed by tundish/mold flux during casting, where rapid dissolu- tion also prevents early crystallization within the slag that may lead to sticker breakouts. [4,5] Research on inclusion removal in steel refining falls into three main categories: (i) flotation of inclusions to the steel/slag KEYAN MIAO, ALYSSA HAAS, MUKESH SHARMA, and NESLIHAN DOGAN are with the Steel Research Centre, Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada. Contact e-mail: miaok@mcmaster.ca WANGZHONG MU is with the Department of Materials Science and Engineering, Royal Institute of Technology in Stockholm, Brinellva¨gen 8, Stockholm, 100 44, Sweden. Manuscript submitted August 10, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B