The present invention relates to a plant for conductive electrical heating of elongated electrically conductive bodies, e.g. steel blanks for further treatment, e.g. in a roll mill.
The most important principle for heating blanks to be rolled is at present implemented in furnaces which are based on oil or gas. The blanks are fed through the furnace by the aid of moving beams or by pressure shift of the blank flow through the furnace.
The most important disadvantages of this kind of heating are:
Low efficiency, approximately 50%. PA1 Considerable loss of material by mill scale formation (approximately 2.5-4%). PA1 Long heating period in start-up of rolling. PA1 Considerable pollution of the environment, both air and water. PA1 Considerable maintenance costs. PA1 Low flexibility. PA1 Low efficiency, approximately 50%, and is complicated as well as needing bulky equipment. PA1 Due to heavy losses because of radiation at higher temperatures, and in order to reduce mill scale formation, and to increase the capacity of the plant, i.e. achievement of sufficient temperature in a short time, it was necessary to use a very high intensity of current (more than 100,000 amps). This causes great practical problems, e.g. problems and costs in connection with contact equipment, high magnetic fields, reactive voltage drop, etc. Grade problems may arise due to cracks forming in the blanks because of too rapid heating and, thus, a non-homogeneous temperature distribution in the blank. The plants are expensive, and show comparatively low efficiency. PA1 1. Reduced requirements of contact means, in practice very simple principles may be utilized (water cooled steel cups). PA1 2. A much more uniform distribution of temperature in the blank is achieved. PA1 3. Consequently cracking of the blank is avoided. PA1 4. The mill scale loss is reduced to a minimum due to the following:
Considerable effort was made to find improved methods of heating blanks for rolling, inter alia, by induction heating and resistance heating by current passage therethrough.
None of said concepts was much used due to considerable drawbacks in these cases as well. Induction heating shows:
Previously known concepts with direct current passage have the following disadvantages:
Advantages of such plants are: Short heating time, minimum mill scale formation, and the plant does not constitute a heavy load on the environment.
From U.S. Pat. No. 3,082,319 of Watson it is known to connect blanks serially in a circuit for heating to remove blanks for further treatment.
In order to permit removal of a blank without breaking the current passage of the circuit, two parallel blanks are provided between transmission contacts, said pairs of blanks being provided mutually in parallel and spaced from each other in a row, so that two and two blanks are serially connected with other correspondingsly connected blanks. One blank may, thus be removed from a pair of blanks without breaking the circuit, whereupon a new blank may be introduced. During such removal and insertion of blanks the resistance of the circuit will, obviously, increase during the time when a blank is not connected with the transmission contacts, and there will thus be less current in the circuit. The disadvantages of this known heating plant are that the blanks are not connected at their ends with transmission contacts, so that heating becomes uneven. Also, the passage of current in all pairs of blanks is equal, so that no individual adjustment of the heating of the blanks is possible. Furthermore, there will be high loss of heat at such a plant, since transmission contacts with blanks are not shielded against loss of heat.