Three-Dimensional Distributions of Large-Sized Inclusions in the Surface Layer of IF Steel Slabs
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IF steels are commonly used in automobile panels and household appliances. Apart from the excellent formability and mechanical properties of this kind of steel, the characteristics of macro-inclusions should be strictly controlled to guarantee their surface quality.[1–3] Large-sized inclusions, such as bubbles plus alumina clusters or entrapped mold powder inclusions, especially distributed in the surface layer of the slabs, can be deformed, crushed and exposed to the surface during the cold rolling process. Therefore, the distribution of macro-inclusions on the continuously cast slabs is an important parameter that can directly guide the scarf operation before hot rolling.
XIAOXUAN DENG, CHENXI JI, HAIBO LI, XIAOJING SHAO, BAISONG LIU, and GUOSEN ZHU are with the Shougang Group Co., Ltd, Research Institute of Technology, No. 69, Yangzhuang Road, Shijingshan District, Beijing 100043, China and also with the Beijing Engineering Research Center of Energy Steel, No. 69, Yangzhuang Road, Shijingshan District, Beijing, 100043, China. Contact e-mails: [email protected], [email protected] QIANGQIANG WANG is with the College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China. Manuscript submitted September 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Macro-inclusions near the slab surface are mainly caused by the inclusion capture behavior in the mold, which is influenced by the so-called mold metallurgy.[4] Many factors affect inclusion entrapment in the mold, such as the interfacial tension gradient between the inclusion and solidification front, velocity of the advancing interface, inclusion size, morphology of the solidification front and electromagnetic force. Surfactants, such as Ti, S, B and O, affect the surface tension of liquid steel, and it was reported[5] that fine bubbles are more easily entrapped by the solidifying shell for Ti bearing ultra-low carbon (ULC) steel than low-carbon aluminum killed (LCAK) steel because of the effect of Ti on the surface tension. Furthermore, using confocal laser scanning microscopy, Shibata et al.[6] investigated the engulfment or pushing of inclusions by the advancing solid/liquid (S/L) interface and found that inclusions would be engulfed when the velocity of the advancing interface exceeded a critical value. At lower velocities or with smaller particles, the inclusions are pushed by the solidifying interface. In terms of inclusion size, Huang et al.[7] reported that there is a critical size for a given cooling rate of the steel. Large inclusions are more easily entrapped by the interface while smaller ones are pushed by the interface. The morphology of the solidification front depending on the undercooling of the steel is also a crucial parameter that affects the entrapment of
particles.[8] Juretzko et al.[9] reported four types of morphology pertaining to particle–interface interactions: planar, cellular, dendritic and equiaxed. In the actual continuous casting process, solidification morphology is commonly considered to be dendritic.[10] In
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