**1. Introduction**

Near net shape casting (NNSC) processes can be regarded as an ideal method for metal sheet production. Apart from their low energy requirements, lower capital and operating costs, and smaller plant footprints than those associated with slab casters (Fe), and DC casters (Al), they also have promising metallurgical characteristics associated with much higher cooling rates possible [1–5]. However, they can also have drawbacks, regarding surface quality, which conventional plants can overcome, by scarfing and multi-pass rolling, to the final quality strip [6]. An advantage of NNSC processes is that a homogenous microstructure with almost zero macrosegregation and fine grain sized products can be attained, provided cooling rate conditions are met, as the cast product's dimensions approach the desired sheet specifications [4].

Therefore, fewer hot/cold reduction passes are required as opposed to conventional methods i.e., slab, or thin slab, casting (TSC) of steels, or direct chill (DC) casting for aluminum alloys. This undoubtedly brings down the overall cost of the operation. On the other hand, the high surface area-to-cast thickness is much higher (~100×), making surface oxidation more problematic, as well as surface quality and surface dimensions [3]. The three most commonly used NNSC processes, commercialized to date, are the twin roll casting (TRC), the earlier twin belt caster, and, most recently, the single belt caster [6]. Historically, the commercialization of the Hazelett twin belt caster in the 1930s by Clarence Hazelett, was an early breakthrough for NNSC. It has since been used around the world, ever since, for casting non-ferrous alloys, mainly aluminum, copper, and zinc, down to cast thicknesses of 13–21 mm [7]. All attempts with steel have failed.

As the name suggests, TRC utilizes two contra-rotating water-cooled rolls, onto which the molten metal is fed and cast as a thin strip. Invented by Henry Bessemer back in 1857 [8], and despite extensive research and development activities that followed, the world's first strip caster for aluminum was only commercialized in 1954, approximately 100 years after Bessemer's initial idea. Currently, FATA Hunter and Novelis PAE (previously SCAL-Pechiney) are leading companies manufacturing TRC equipment for aluminum strip production [9]. The cast strip thickness is almost 6 mm. The first TRC strip caster commercialized for steel was by Nippon Steel and Mitsubishi Heavy Industries Corporation in 2000. The caster could produce stainless steel strips, 2–5 mm thick, 0.76–1.5 m wide, in NSC's Hikari Works in Southern Japan, but was not a commercial success and was abandoned. Later, in 2002, NUCOR started the production of a low carbon steel strip, 1.7–1.9 mm thick via a TRC process, CASTRIP, and this continues to operate, commercially [6]. However, TRC, though a proven aluminum/steel strip manufacturing process, suffers from low annual production of steel vs. that of a conventional slab casting process, i.e., 400,000 tpy and 2,000,000 tpy, respectively [10].
