*3.1. Increasing the Casting Speed in the Horizontal-Type TRC Process*

To maximize the cooling rate and raise the casting speed in the horizontal-type TRC process, Cu-Cr rolls were used and the castings were conducted in the absence of lubrication on the roll surface. The casting speed was increased to 5 m/min with a decreased RSF. For comparison, strips were also cast at 0.8 m/min using a conventional horizontal caster from a mass production line. Figure 1 shows the inverse pole figure (IPF) maps and pole figures (PFs) of the Al-5Mg strips fabricated at different casting speeds. In the case of the 0.8 m/min condition, the surface region exhibited a shear deformation texture (rot-Cube, {111}//ND), while a typical rolling texture (brass, S, Copper components) was dominant in the central region, thereby indicating that the strip was hot-rolled under high RSF conditions. In contrast, for the 5 m/min condition, the strip exhibited an equiaxed grain structure with an overall random texture. This implies that the rapid solidification structure instead of the hot-rolled grain structure can be obtained at an increased casting speed. It is therefore necessary to consider what role the RSF plays under high casting speeds during the TRC process.

**Figure 1.** Inverse pole figure (IPF) maps and pole figures (PFs) of twin-roll cast strips fabricated at casting speeds of (**a**) 0.8 m/min, and (**b**)5m/min.

## *3.2. Cooling Behavior of the Strip in HSTRC*

Using a vertical-type caster, HSTRC was carried out under various RSF conditions (i.e., 3, 20, and 60 kN) with a casting speed of 60 m/min. The temperature distribution from the center line of the melt pool to the strip center was then investigated using the direct temperature measurement technique. Figure 2 shows the change in temperature in the area of the roll nip. As indicated, the temperature began to decrease significantly at the kiss point where two solidifying shells encountered one another prior to the strip passing through the roll gap. At the roll nip, the cooling rate exhibited its maximum value, and the temperature of the strip center further decreased even after expulsion of the strip from the roll gap, due to the thermal gradient along the thickness direction. This was followed by the air cooling region where the strip temperature remained relatively constant. Using an RSF of 3 kN, the cooling rate at the roll nip was relatively low, resulting in the final strip temperature being equal to the eutectic temperature. As the RSF was increased to 20 kN, the cooling rate increased sharply, although no significant change in the cooling rate was observed upon increasing the RSF further to 60 kN. This result indicates that the contact condition between the strip and the roll surface was enhanced upon increasing the RSF, thereby resulting in the observed increased cooling rate. In the HSTRC process, the cooling behavior of the section from the kiss point to the position of the lowest strip temperature is important, as it determines the final strip temperature. If the cooling rate is decreased, i.e., the contact condition between the strip and the roll surface is poor due to solidification shrinkage, some liquid can remain at the mid-thickness region of the strip. This can cause severe internal cracking or tearing of the strip during the continuous casting process due to the low nature of the high temperature stiffness of aluminum alloys [4,5]. Moreover, if the final strip temperature is higher than or equal to the eutectic temperature, the solidification structure can become coarse under air cooling.

**Figure 2.** Changes in the (**a**) temperature and (**b**) cooling rate in the mid-thickness region of the strips under various roll separating forces.

Figure 3 shows the microstructures of the Al-7Si-3Mg alloy strips fabricated under RSF conditions of 3 and 20 kN. In the case of the 3 kN condition, relatively coarse plate-like eutectic Si structures were observed, implying that solidification of the strip proceeded under air cooling (i.e., under a low cooling rate) [6]. In contrast, fine dendrite and rod-like eutectic Si was observed in the strip fabricated at 20 kN, indicating that solidification took place during rapid cooling in the area of the roll nip. As such microstructural differences can greatly affect the mechanical properties of the strip [7], the application of an appropriate RSF is therefore critical for the HSTRC fabrication of sound aluminum strips with fine microstructures.

**Figure 3.** Microstructures of the Al-7Si-0.3Mg alloy strips fabricated using RSF (role separating force) values of (**a**) 3 kN and (**b**) 20 kN.
