Global systems provider for the metallurgical industry SMS Group has been contracted to supply a 20-roll cold-rolling mill and a bright annealing line to US-based stainless steel company North American Stainless (NAS), a subsidiary of Spain-based stainless steel manu- facturer Acerinox Group.

NAS is erecting a modern, high- performance production facility for bright-annealed stainless steel strips in Ghent, in the US state of Kentucky. The installed plants will be characterised by high efficiency, flexibility and product quality. The output capacity of the plant will amount to 95 000 t/y and commissioning will take place in early 2017.

With this investment, NAS is entering the market for bright- annealed stainless steel, as there is a market of stable growth for this material, which is partly imported, in the US.

The cold-rolled stainless steel strip of surface-quality bright- annealed steel is characterised by a shiny, reflective appearance and, while the typical areas of application are household appliances and kitchen utensils, bright-annealed stainless steel is also used in many other areas where, in addition to the material properties of stainless steel, high-grade, reflective surfaces are required, such as in the medical engineering and transportation industries.

NAS’s bright-annealed stainless steel plants will manufacture both austenitic and ferritic steel grades (the AISI 200, 300 and 400 series). The end material will be between 600 mm and 1 350 mm wide, and between 0.15 mm and 1.5 mm thick.

In addition to the supply of the mechanical, process, furnace, electrical and automation equipment, the scope of SMS Group’s contract includes supervising the installation, commissioning support and training the operating personnel. However, the complete strip steering systems will be supplied by automation technology provider EMG Automation.

The 20-roll cold rolling mill, a Monoblock mill of type MB 22B-54", will process up to 6.2-mm-thick hot strip. The compact and robust plant is characterised by high millstand stiffness. With a maximum rolling speed of 800 m/min and a maximum rolling force of 8 000 kN, thickness reductions of up to 90% can be achieved. The millstand will be equipped with hydraulic gap control and roll-crown adjustment systems for the B/C and F/G axes and with inner intermediate roll shifting.

Additionally, the auxiliary and service facilities are part of the supply scope. For example, the 100% regenerative Supafine filter system will ensure environment-friendly purification and cooling of the rolling oil. The fume exhaust system will also contribute to an environment-friendly plant operation, as it will meet the most stringent environmental requirements imposed by the local authorities in Kentucky. To reduce nonproductive times, an uncoiling group with a straightener and crop shear will be integrated into the system, in addition to the two reversing coilers.

SMS Siemag notes that, at the bright-annealing line plant, the cold-rolled strip will first be cleaned of rolling oil and other contaminants in a two-stage cleaning section. This will be followed by recrystallisation annealing in an oxygen-free, inert atmosphere with a high hydrogen content, creating a high-grade, reflective strip surface.

A four-high skin pass mill will be integrated into the line for post-treatment, improving not only the surface and flatness of the strips but, as a result of low-strain hardening, the metallurgical characteristics as well. The downstream tension leveller will perfect the strip flatness and, in addition to tension levelling, in the case of thin-gauge strips, this plant can also operate as a pure stretcher.

Subsequently, the strips can be trimmed on both sides. In the entry and exit sections, the strips will achieve a maximum speed of 75 m/min, while the maximum speed in the process section will be 50 m/min.

SMS Group notes that the technological highlight of the bright- annealing line is the entirely electrically powered vertical furnace from Drever, an SMS Group company. Owing to the furnace’s high operating efficiency, the energy consumption, at about 220 kWh/t, is 60% lower than in a conventional design with a muffle furnace. Further, the shorter heating section enables the bay height to be reduced by 50%.

The high efficiency of the system is achieved by means of direct heating through electrical heating elements and dynamic annealing, which reaches temperatures of up to 1 250 °C. Further, the annealing process takes place in a pressurised inert atmosphere (up to 90% hydrogen), which ensures a low dew point and produces an immacu- lately reflective surface. A special strip stabilisation system allows for contactless positioning of the strip, preventing surface damage.

SMS Group notes that the total furnace construction is made gas-tight and special sealing elements at the entry and exit reduce losses. Further, to keep the hydrogen requirement low, a recycling unit will be integrated to condition the used hydrogen and return it to the process. For strip cooling, the furnace will be fitted with two inert gas recirculation units. To enhance production and reduce resource consumption, all process parameters will be adjusted automatically, based on a mathematical model.