The present invention relates to a mechanical descaling device for continuously descaling hot rolled sheet metal before it is subjected to the cold rolling.
In the cold rolled sheet metal production lines, a coil of hot rolled sheet metal is first uncoiled and scales on the surfaces of sheet metal must be completely removed before the sheet metal is subjected to the cold rolling which further reduces the sheet metal to a desired gage.
In general, in the conventional cold rolled sheet metal production lines, the uncoiled hot-rolled sheet metal is made to pass through a body of an acid such as hydrochloric or sulfuric acid to chemically remove the scales on the surfaces of sheet metal. Such chemical descaling process takes a long time and is inefficient. Furthermore, it requires additional installations for recovering and neutralizing the used acid so that the overall length of the chemical descaling line (to be referred to as "pickling line" in this specification) becomes very long. Moreover, the pollution problems due to the use of strong acids must be taken into consideration. Therefore because of high capital cost and running cost, the production cost of cold rolled sheet metal becomes very expensive.
There exists a further disadvantage of the pickling line. That is, the pickling line is in general not continuous with the cold rolling line. As a result, after passing through the pickling line, the sheet metal must be recoiled and transported to a storage area, from where the coils of sheet metal must be transported again by cranes or trucks to the cold rolling line. Therefore, there must be provided a large space between the pickling and cold rolling lines in order to store the coils of sheet metal temporarily. Furthermore, during the storage of coils, rust may be formed on the sheet metal due to the attack by the pickling acid remaining on them. To overcome this problem, a device is disposed downstream of the pickling line so as to apply an anti-rusting oil to the sheet metal as it emerges from the pickling line.
Various methods have been proposed in order to directly connect the pickling line with the cold rolling line, thereby attaining a high productivity. All of the methods however have encounted the following problems.
(1) The rate with which the pickling acid dissolves the scales on the sheet metal is generally constant so that the sheet metal must continuously be moved at a constant velocity through a huge pickling vessel which is considerably long in length. On the other hand, the velocity of the sheet metal entering the first cold rolling stand in the cold rolling line must be varied depending upon various cold rolling conditions and requirements such as a reduction ratio. Thus, it is next to impossible to continuously connect the pickling line and the cold rolling line through which the sheet metal must be moved at different velocities.
(2) When the sheet metal passes through the pickling line at a velocity lower than a predetermined velocity, over-descaling occurs and the poor yields result. Furthermore, the degradation in quality and variations in properties of the sheet metal result because of the strong attack by the pickling acid. The overconsumption of the pickling acid also results in the resultant increase in pickling cost.
Since the cold rolling line must be shut down periodically to replace the rolls according to a pass schedule, a considerably hugh accumulator for temporarily accumulating a suitable length sheet metal must be installed both upstream and downstream of the pickling line.
(3) The pickling acid remaining on the sheet metal may attack the parts of the devices following the pickling line. Fumes of pickling acid evolving from the pickling line may attack the equipment and devices, especially the rolling motors, around the pickling line.
As described above, the realization of a continuous cold rolled sheet metal production line with a pickling line is very difficult. Therefore, many efforts have been made to provide mechanical descaling devices combined with a cold rolling line for the purpose of continuous production of cold rolled sheet metal from a coil of hot-rolled sheet metal. In one of the mechanical descaling devices so far proposed, a descaling slurry made of abrasive particles is blasted together with water under high pressure against the surfaces of sheet metal through nozzles. However, these nozzles are in general fixed so that not only the angle between the axis of the nozzle and the centerline of the sheet metal being descaled cannot be varied, but also the angle between the nozzle axis and the plane of the sheet metal cannot be varied at all. Moreover, the distance between the nozzle tip and the surface of the sheet metal cannot be varied. As a result, the conditions of descaling operations cannot be varied depending upon the dimensions, properties and desired surface conditions of sheet metal, thus resulting in waste in energy and undesired surface quality. Furthermore, with the fixed nozzles, the inspection and maintenance become difficult. When it is desired to change the angles of the nozzles and the distance between the nozzle tip and the sheet metal, the production line must be completely shut down, thus resulting in the decrease in productivity.