Exact setting of the properties of high-strength und ultra-high-strength steel strip
Kobelco Angang Auto Steel, China, has started up its new continuous annealing line with the successful processing of the first coil. The final strip properties were achieved by treating the strip in the cleaning section, the furnace and the subsequent skin-pass mill. As systems provider, SMS group (www.sms-group.com) supplied the complete plant installed in Anshan in the Province of Liaoning on a single-source basis. Besides the design of the mechanical, process and thermal equipment and the manufacture of key components, SMS group was also responsible for the complete electrical and automation systems.
After completion of the commissioning phase, the plant will produce around 600,000 tons of steel strips, mostly for the automotive industry. Strain hardening resulting from rolling is removed from the material by annealing, rapid cooling and subsequent skin-pass rolling, while setting the superior mechanical properties exactly as required. The new line will produce, among others, ultra-high-strength grades for light-weight, high-safety automotive structural components.
The heart of the processing line is the high-performance, eco-friendly Drever radiant-tube annealing furnace which comes with two alternative cooling systems. The ultra-fast cooling system based on gas cooling is used for grades such as high-strength TRIP, dual-phase, complex-phase and other high-strength steels as well as drawing and deep-drawing grades. The water-spray cooling system, which achieves even higher cooling rates, can be used to also produce ultra-high-strength martensitic grades and multi-phase steels. Both systems provide very uniform cooling guaranteeing that the strip geometry and surface quality satisfy most exacting requirements.
With a rolling force of up to 18 MN, the six-high inline skin-pass mill is designed to skin-pass strips with high tensile strengths of up to 1,600 MPa. Such strength values require skin-passing under very high strip tension. Therefore the inline skin-pass mill is able to apply strip tensions of up to 350 kN. Work rolls of two different diameter ranges have been provided in order to optimally cover all material strengths and skin-passing rates for the entire product spectrum. Technological features such as work roll and intermediate roll bending, strip-edge-oriented roll shifting and a strip-blowing device arranged in the exit section of the stand assure high quality of the strips.
The line has been designed for a strip thickness range between 0.5 and 2.5 millimeters and widths from 750 to 1,650 millimeters. The maximum process speed is 180 meters per minute, while at the entry and exit, speeds of up to 280 meters per minute may be achieved.