Reduction of Surface Crack by Modified Molten Metal Pouring Method on Al–Mg Alloy Strips Produced by Twin-Roll Casting

Abstract

In this study, cracks formed on the surface of Al-Mg alloy strips cast using a high-speed unequal-diameter twin-roll caster were observed. In addition, the effects of the roll load and the molten metal pouring method on the crack were investigated. Four types of Al-Mg alloys (AC7A, AC7B, Al-3%Mg, and Al-7%Mg) were cast into strips at roll speed of 30 m/min. The crack size decreased with decreasing roll load, and when the roll load was 4 N/mm, no cracks formed. However, the strips did not completely solidify under this low load, and conveyance was difficult due to the insufficient strength of as-cast strips. To reduce the load in the inner area along the width direction of the strips, the thickness of the solidification layer in this area was made shorter than that at the edges by modifying the molten metal pouring method. In the modified pouring method, the molten metal was poured onto the roll using a launder placed against the roll at a small angle. With this method, the edges completely solidified and had enough strength for conveyance. Moreover, cracking substantially improved, and when the distance between the edge of the molten metal pool and the pouring point was set, crack formation was almost completely eliminated. The band region at the center in the thickness direction of the strips was widened using the proposed pouring method. The tensile strength and limiting drawing ratio of strips cast using the proposed pouring method were both greater than those of the strips cast using the conventional pouring method.