**2. Materials and Methods**

Al-2Si, Al-7Si-0.3Mg, and Al-5Mg (wt%) alloys were melted in an electric furnace, and degassing was conducted using Ar gas for 10 min prior to casting. Lab-scale horizontal- and vertical-type twin-roll casters were used for strip fabrication. The horizontal-type caster consists of two Cu-Cr rolls, which are fixed firmly. Using this caster with minimized lubrication on the roll surface, the casting speed could be increased to 5 m/min. To compare the strip microstructure and texture, the steel roll caster in the mass-production line was also used to fabricate 1250 mm wide strips, which exhibited the typical microstructure of traditional TRC strips. For the vertical-type caster, pure Cu rolls with no lubrication on the roll surface were used to maximize the cooling rate, and one of the rolls was loosely supported by a series of springs. This condition makes it possible to maintain a low RSF at a high-speed casting range. The roll size of both lab-scale casters was 300 mm in diameter, and the rolls were internally cooled by running water during the strip casting process. The strip width fabricated by both casters was 100 mm. A direct temperature measurement technique [4] was adopted to investigate the cooling behavior of the strip during vertical-type HSTRC. A K-type thermocouple (ANBE SMT Co, Yokohama, Japan) was flown directly into the molten pool to measure the temperature from the center line of the upper side of the melt pool to the strip center. The casting speed was 60 m/min, and the RSF was varied from 3 to 60 kN. The temperature change was recorded every 0.5 ms using a data logger (NR600, KEYENCE, Osaka, Japan). For the cast strip, longitudinal cross-sectioned samples were mounted in epoxy resin prior to mechanical polishing. The polished samples were then etched using a 2% solution of Hydrogen fluoride (HF) in distilled water for microstructural observations using an optical microscope. The samples were also anodized at 40 V in a 3.3% solution of HBF4 in distilled water to reveal their grain structures. The strip textures were observed by electron backscattering diffraction (EBSD) using a TESCAN MIRAII FE-SEM (Brno, Czech Republic) equipped with a Hikari EBSD detector at an accelerating voltage of 20 kV. The EBSD results were analyzed using OIM analysis 7 software provided by TSL, Co., Ltd. An automatic serial sectioning machine (UES Inc., Robo-Met. 3D, Dayton, OH, USA) and 3-D analysis software (FEI, Avizo Fire 7, Hillsboro, OR, USA) were used for three-dimensional characterization of the center segregation [3]. For the as-cast Al-5Mg strips, tensile tests along both the casting direction (CD) and the transverse direction (TD) were conducted using flat tensile specimens with a 25 mm gauge length and 6 mm gauge width. The fracture surfaces were observed using scanning electron microscopy (SEM; JSM-6610LV, JEOL, Tokyo, Japan).
