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 AlMg and AlSi 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 Al4.8%Mg and AlSi 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 AlMg 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 Al4.8%Mg alloy than in the AlSi alloys. The degree of cracking could not be reduced by cold-rolling. In the AlSi alloys, the ripple marks were affected by the Si content, degree of superheating of the molten metal and the roll load. In the Al3%Si alloy, ripple marks were produced, and worsened as the roll load and the superheating of the molten metal were 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 AlSi consisted of globular crystals, and that in AlMg consisted of equiaxed dendrites and Mg-rich areas.