The first roll stand of the rolling mill of any sheet, in which the fed original aluminum ingot, It called roughing stand. This usually freestanding four-high reversing stand hot rolling.
Rough rolling mill
Its main function - "roll out" Bar, reduce its thickness to thick sheet dimensions - Plate.
Sometimes roughing stand itself may produce manufactured articles to a predetermined final thickness. More often - the production of the roughing stand goes as an initial billet to subsequent rolling stands (hot or cold rolling) to further reduce the thickness to an aluminum strip or even aluminum foil (less 0,15 mm).
Mill rolls - workers and support
In the roughing stand working rolls are driven, and back-up rolls, who support them, rotated by contact with the work rolls. Typical roughing stand may have a capacity of about 3750 kilowatts on each work roll, and all - around 7500 kilowatt. This allows the push bar through the nip, wherein the contact occurs along a line compressive force of about 3700 tonnes.
Rolling stand generally designated by barrel length – working length – the work roll. This - the maximum width, they can be rolled. In the US, this length is measured in inches, and we have - in millimeters. for instance, "120-inch rolling mill" or "mill 3000".
The design of the rolling stand
Operator roughing stand, which is located in the premises of the stand at the controls, raises the upper rollers of their last setting and sets the gap for the first pass - somewhere on 50-70 mm less than the thickness of the ingot.
For, to compress the thick original ingot thickness, for example, 760 mm, roughing stand must have a large and strong structure. Her lower rolls - work and support - are in a fixed position. Upper rollers - the working and reference - can be raised or lowered on screw or hydraulic actuators, which are mounted on side columns stand. Therefore, the operator can gradually change the gap between the work rolls.
The gap between the work rolls is only one of many rolling parameters, which was necessary to obtain a product with desired properties. The rollers should have an appropriate diameter and surface. Acceleration of the roll and the speed must be calculated on the aluminum alloy properties, as well as the specified compression. In modern rolling mills control computers involved.
Lubrication and cooling of the work rolls
Initial preform - bar - is included in the roughing stand under strictly predetermined temperature. The rolling process - successive reduction in the thickness of the ingot - itself generates additional heat: from friction between the ingot and the work rolls, as well as the severe deformation of the metal - substantially, internal friction.
This extra heat must be removed, to maintain the optimum rolling temperature. This is necessary not only to, to prevent excessive thermal distortion of the workers mill rolls, but also to, to monitor the state of aluminum alloy - Structure and Mechanical Properties - during rolling.
Moreover, work rolls should be lubricated, to prevent sticking of hot aluminum to the rolls and damage the leaf surface. The amount and method of applying lubricant strictly control. A typical lubricant circuit roughing rolls of the rolling stand shown in Figure.
Grease typically turned off when a backlash problem ingot rolling stand, to provide a good friction "biting" ingot work rolls, and when rolling begins, her again include. Lubricants should be enough, to prevent sticking, but not too much. And too little, and too much grease adversely affects the quality of the surface of the sheet.
Both of these functions - heat dissipation and lubrication - are carried out on the rough rolling stand one supply liquid, typically aqueous emulsion-five percent oil. This liquid is continuously filtered and recirculated.
The process of the rough rolling sheets
The ingot is applied to the work rolls through the supply roller conveyor through the conveyor rolls, which are divided into right and left. Each of them has an independent drive and control. By controlling the speed of the right or left roll, the operator can align the ingot or even expand it.
The rollers feeding roller table lead bullion into contact in the working rolls, and the work rolls themselves "snack" ingot due to friction and drive off the ingot through a gap therebetween.
For a few passes through the cage bar is reduced in thickness to several centimeters. for instance, thick ingot 380 mm may be crimped in a single pass to 330 mm. Then the gap between the work rolls and the ingot is reset again forced through a compression cage, for example, to 280 mm. The gap between the work rolls tapers after each passage, until a thickness of the rough rolling sheet.
sheet thickness – a sheet length
When an ingot is rolled in the usual longitudinal direction, the reduction of its thickness is almost completely compensated by the increase of its length. Friction along the contact line between the work rolls and the ingot is usually sufficient, to prevent substantial expansion of the ingot.
To prevent the broadening of the ingot, and cracking its edges, often used rolling ingot edges rolls with a vertical axis of rotation.