**1. Introduction**

In aluminum sheet production, the twin-roll casting (TRC) process has attracted increasing attention due to its economic advantages when compared with conventional direct-chill slab casting and hot rolling processes. Due to the rapid cooling rate of >100 K/s that is employed in the TRC process, the production of 3–10 mm thick strips can be achieved directly from the molten metal. The traditional TRC process was designed to combine the metal casting and hot rolling processes into a single operation, and in general mass production, a horizontal-type (or slightly tilted) caster is typically employed, where the casting speed is optimized to give a range of <2 m/min. However, high-speed twin-roll casting (HSTRC) has recently been developed using a vertical-type caster [1], with the primary aim being to improve the casting speed by rapid cooling rather than through applying the hot rolling effect. Therefore, copper rolls with relatively higher thermal conductivities compared to their steel roll counterparts were employed for this purpose, and the roll separating force (RSF) was reduced to one tenth of the level of the conventional horizontal-type TRC. These conditions allowed the casting speed to be increased to 180 m/min [2].

In the solidification stage of TRC, solid shells begin to grow on the roll surfaces after the molten metal is supplied between two rotating rolls. At a certain point, typically referred to as the "kiss point," two solidifying shells encounter one another, and joint shells are introduced to the roll gap to produce a single strip. In this process, the RSF is applied to the hot strip when the solidifying shells widen the roll gap. Since the strip is subjected to external stress during solidification, the RSF plays an important role in determining the cast strip microstructure [3].

Thus, the influence of the RSF on the cooling behavior and on the solidification structure of such strips is investigated in this study, and in particular in the high casting speed range. Through microstructural and textural observations, the role of the RSF is clarified, and the mechanism of strip formation is discussed.
