This requires the use of filtering processes and prevention of oxidation of liquid in the melting furnace and during liquid metal transfer to the headbox and to the inlet roll gap. Enhanced melt cleanliness can contribute significantly to reducing the likelihood of inclusions during TRC. The control of inclusions (typically oxide, fluoride and nitride) is very important to the production of high-quality commercial sheet. For example, there are two common types of internal strip defects: non-metallic inclusions and segregation defects. Various defects are possible in as- cast strips. Wells, in Advances in Wrought Magnesium Alloys, 2012 7.4.2 Strip defects Thus, twin roll casting is believed to be an excellent method for the generation of finegrained feedstock materials that can subsequently be warm rolled to thin sheets.Į. Although the first preparation of strip goes back to the early 1980s ( Brown, 2002), there are still issues to overcome that have to do with the homogeneity of the material. This type of material is very similar to a plate rolled from a slab and is therefore seen to have the potential to overcome economic limitations in the production of magnesium sheets. Micrographs of alloy AZ31 demonstrate this difference in Fig. 10.9. While in DC casting it is limited to 1–50 ☌/s, it reaches 500–1000 ☌/s in a twin roll caster, leading to greatly refined structures with small and finely distributed intermetallic phases, extended solid solutions and fine grain sizes. A key difference between twin roll casting and DC casting is the solidification rate. It is reported that twin roll cast strips are significantly more fine-grained compared to conventional castings as a result of the combined casting-rolling process and subsequent heat treatment of the material. The reduction in the number of processing steps to final gauge leads to shorter production times and a decrease in production costs. The overall process includes different heat and surface treatments, with some technical and economic advantages. In the next step the strip is rolled to final gauge. These strips can be rolled using conventional rolls in a continuous process. The metal solidifies upon contact with the cooled rolls and is rolled to a strip. The melt is then dragged into the roll gap of a pair of counter-rotating, internally cooled rolls.
In this process, liquid metal is pumped from a furnace or cast over a pipe into a tundish with a protective gas coverage. Fig. 10.8 shows the general principle of strip casting. Twin-roll casting is the only process that combines both solidification and rolling in a single step.
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