Effect of casting conditions on surface defect and segregation of strips cast by a high-speed twin-roll caster

Abstract

In this study, the effects of the roll load and superheating on the appearance of surface defects in Al-Mg and Al-Si alloy strips cast using a high-speed twin-roll caster were investigated. The band zone that existed in the center of the thickness direction was investigated. The alloy compositions were Al-4.8%Mg and Al-Si with Si contents of 1, 2, 3 and 11%. The diameter and width of the copper roll in the twin-roll caster were 300 and 50 mm, respectively. The superheating temperature of the molten metal was 5 and 50°C, and the roll load was varied from 2 to 88 N/mm. A roll speed of 30 m/min was utilized. The existence of surface cracks was investigated by penetrant inspection and bending test. The amount of surface cracking in the Al-Mg alloy was affected by the roll load, and the number of surface cracks decreased with decreasing roll load. Surface cracks occurred more easily in the Al-4.8%Mg alloy than in the Al-Si alloys. The degree of cracking could not be reduced by cold-rolling. In the Al-Si alloys, the ripple marks were affected by the Si content and the roll load. In the Al-3%Si alloy, ripple marks were produced, and worsened as the roll load increased. A specimen for observation of the microstructure near the roll bite was obtained by performing a roll-stop during casting, and observed using Weck’ s reagent. A band zone was found to exist. The band zone in Al-Si consisted of globular crystals, and that in Al-Mg consisted of equiaxed dendrites and Mg-richareas.