The ELO-SLAB induction heating machine for flat products features a modular architecture for economical adaptation to all types of rolling mills. Thanks to the targeted deployment of longitudinal and transverse flux heating with optimal frequency and power, specific temperature profiles over the strip width and thickness can be achieved.
The induction machine measures the infeed temperature of the material and equalises the surface and core temperature as necessary to ensure an optimal temperature distribution for the rolling process. This technique enables considerable cost and energy savings, regardless of whether gas or oil is used as the fuel, and reduces CO2emissions.
With longitudinal flux heating for slabs, the workpiece is fully surrounded by the coil or heated by an open inductor, depending on the process requirements. The resulting eddy currents flow within the current penetration depth parallel to the surface of the slab. For strips and slabs over a wide range of thicknesses, longitudinal flux heating is used both before and after the rolling furnace or after the pre-roll stand.
The longitudinal flux inductors are specifically designed to be very robust and suitable for integration into the rolling mill process. For easy maintenance, the inductors can also be individually withdrawn from the rollingline.
By contrast, transverse flux heating is used with thin stripsto obtain a specific temperature profile over the width of the material. This method with a special coil design avoids mutual cancellation of the induced currents on the upper and lower surfaces of the thin strip. Due to the operating principle, these inductors are designed as open, so they can also be withdrawn from the rolling line.
On modern rollinglines, intelligent process-optimised temperature elevation or temperature equalisation is needed at various positions. Induction heatingis used for this purpose, thereby creating added value for end users. Induction heatingcan be used after the rolling furnace to raise the slabtemperature in a controlled manner and flexibly regulate the input temperature in the rollingprocess at the optimal level, thereby enabling special processes such as thin-strip rolling.
Another advantage of induction heating after the rolling furnace is that it may be possible to lower the furnace temperature. Fossil fuel consumption can be sustainably reduced by using dry furnace rolls. In addition, reducing the furnace temperature prolongs the life of the refractory materials and the transport rolls. Scale formation is also reduced while the metallic yield is raised.
In Vario rolling mills, induction heating is used after the first roll stand. This is an innovative technology for production of micro-alloy steel grades in API quality or better.
Induction heating ensures a uniform temperature level between the first rolling mill stands, enabling greater relative thickness reduction. The high temperature level allows a correspondingly high degree of deformation to be achieved. Furthermore, the casting microstructure is fully eliminated by recrystallisation, resulting in a very uniform microstructure, even with slabs up to 20 mm thick.
The benefits of intermediate induction heating can also be seen in the rear area of compact hot rolling mills. In the endless rolling process for very thin strip grades or silicon steels, compact integrated induction units compensate for the temperature drop between finishing stands and maintain the right temperature level up to the end of the finish rolling mill.
For cold rolling of thin strips, the strip geometries and the material properties must be kept within close tolerances. The precise control capability of induction heating makes it particularly significant for thin strips. Along with full heating with boosters, partial heating is used in cold rolling mills.
To improve the deep-drawing properties of steel strip with full heating, boosters are used to warm the strip from room temperature to about 550 to 700°C. Partial heating is used for strip edge heating and other purposes.
Another application area is preheating of cut edges ahead of slitting or trimming shears. The heated cut edges have better surface properties and the service life of the knives is increased.