*3.3. Microstructure of the HSTRC Strip*

In the HSTRC process, the final solidification region of the strips (generally the mid-thickness region) is greatly affected by the RSF. Especially in the case of aluminum alloys exhibiting a wide range of freezing temperatures, solidification proceeds with the formation of a mushy layer at the growth front of the solidifying shell on the roll surface. Consequently, when two shells encounter one another at the kiss point, the microstructure of the semi-solid mid-thickness region is influenced by the external stress. Thus, Figure 4 shows the macro- and microstructures of the Al-2Si strip fabricated at a casting speed of 60 m/min and an RSF of 20 kN. As shown, an equiaxed grain macrostructure was apparent at the outer shell region, while a band of fine globular grains formed in the mid-thickness region. The formation of this globular grain band is likely related to the compression mode under the different RSF conditions. It is noted that Suery and Flemings conducted a simple compression test on semi-solid dendritic alloys and demonstrated the fragmentation of dendrite arms under a high compression rate [8]. In the HSTRC process, the high casting speed and the RSF conditions caused the active fragmentation of dendrite arms in the semi-solid mushy layer, resulting in the formation of a band structure in the mid-thickness region. This microstructural feature is significant in relation to center segregation control.

**Figure 4.** (**a**) Anodized grain structure, and normal etched microstructures of (**b**) the outer shell, and (**c**) the central band region.
