Strip Processing LinesBig River Steel
The universal annealing and hot-dip galvanizing line is designed to process two different products: annealed and hotdip galvanized cold strip. Thanks to a Drever furnace with ultra- fast cooling system generating high cooling rates, the line can produce high-strength dual-phase grades up to DP 800.
The new line permits Big River Steel to produce steel grades such as CQ, DQ, DDQ and IF and also high-strength steels like HSLA, DP and CP. The total capacity of galvanized and annealed material is about 476,000 tons. The strips processed have thicknesses from 0.36 to 3.0 millimeters whereas their width range is between 900 and 1,850 millimeters. Process speed may attain 150 meters per minute.
The strips are pre-cleaned in a horizontal cleaning section. The annealing process in the Drever furnace marks the start of the actual strip treatment. The furnace facility comprises a vertical pre-heating section and a vertical, directly fired furnace. Thereafter, the strip runs through a horizontal radiant tube furnace with electrically heated holding area. Cooling is accomplished in a vertical section with slow cooling and ultra-fast cooling systems.
After the annealing process, there are two ways of strip processing. In the galvanizing mode, the strip travels through a furnace snout into the hot zinc bath. A FOEN® air knife system precisely sets the required thickness of the zinc layer. In the annealing mode, however, this galvanizing process is bypassed. To this end, the zinc pot is lowered with the aid of a guide roll arranged above the zinc pot. A skin-pass stand, a tension leveler, a horizontal passivation system and an oiling machine are installed for the purpose of post-treatment. The Drever furnace complies with strict environmental standards and comprises equipment to recover energy. In the pre-heating zone, the hot exhaust gases from the burners are used for strip heating. The energy of burner and radiant tube exhaust air serves to heat the combustion air. A special system for Selective Catalytic Reduction (SCR) is provided to reduce the nitrogen oxides present in the exhaust gasses. This measure helps achieve NOx emission values of less than 60 mg/Nm³.
The annealing process in the continuous furnace is controlled by a mathematical model from Drever International. For nondestructive online strength measurement the line is equipped with an IMPOC measuring unit from EMG Automation GmbH. In addition, SMS group has developed an annealing model to optimize, using numerical methods, individual process parameters on the basis of line data, product data and information on preceding processes.
Linking these three elements to an I-Furnace (Intelligent Furnace) offers the opportunity of maximizing throughput and increases the yield of strip with a consistently high quality level. Thanks to an improved process control, the concept offers the potential to enhance energy efficiency or throughput once again by adapting nominal temperatures within the process windows.
Equipped with hydraulic adjustment, work roll bending system and variable work roll diameters, the facility is able to cope with all skin-passing tasks. This applies also and specifically to highstrength grades.
Integrated in the exit section of the strip processing line is an Inline Skin-Pass Mill (ISPM) which comes in four-high design and operates in the wet mode. Furthermore, the equipment includes a high-pressure roll cleaning system.
The ISPM has a maximum annual capacity of 484,000 tons. It processes strips that are up to 1,850 millimeters wide and have thicknesses ranging from 0.28 to 1.4 millimeters. Elongation is maximum 2.5 percent.
Apart from the continuous annealing and hot-dip galvanizing line, SMS group also installed batch-annealing furnaces for cold strip processing at Big River Steel. A total of 413,000 tons of steel per year can be heat treated in the 24 annealing bases installed. Being a new SMS group development, the batch-annealing furnaces at Big River Steel are the first of their kind in practical operation.
OPTIMIZED GAS FLOWS
Cold strip coils inside the hoods are annealed or cooled in what is called a batch-type process. Each base can accommodate several coils at a time. The stacked coils are separated by convector plates which provide better heating and cooling results and protect the coil edges. The gas circulation within the protective covers was optimized in Computational Fluid Dynamics(CFD) simulations.
The treatment of carbon steel grades (CQ, DQ, EDDQ, HSLA) and of non-grain-oriented silicon steels (NGO-semi finished and motor laminated) takes place in a pure hydrogen atmosphere at temperatures of up to 750 degrees centigrade. To conform to the strict U.S. environmental standards, the gas-fired heating hoods use special low-NOx technology. The batches are cooled with air and water.
The automation system is based on WINCC process control and MOBAF process intelligence systems. The process intelligence system comprises the level 3 link, the mathematical annealing model and functionalities like material tracking, throughput optimization by taking into account the availability of the plant units and optimized batch planning. Plant utilization and annealing times are optimized with a view to mix coil grades in production.
With a maximum rolling force of 18 meganewtons, Big River Steel’s new skin-pass mill is a real power pack permitting very high elongations of up to 8.5 percent to be attained.
The four-high skin-pass mill is designed for an annual capacity of 400,000 tons and rerolls strips that are between 914 and 1,880 millimeters wide and have thicknesses ranging from 0.28 up to 1.4 millimeters.
As it is part of the discontinuous process route, the skinpass mill is arranged downstream the batch annealing furnaces. Its main function is to provide the annealed carbon strips and part of the non-grain-oriented silicon strips with the material properties required for subsequent processing. These properties include yield point harmonizing and setting of strip roughness and flatness.
When it comes to meeting different material requirements, the skin-pass mill is very flexible as it can be operated at several work roll diameters. The high rolling force of 18 meganewtons is needed, in particular, to skin-pass silicon steel grades.