Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
1999-09-20
2003-02-25
Black, Thomas (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S150000, C700S155000
Reexamination Certificate
active
06526328
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for rolling a metal product and applies more particularly to hot rolling of flat products such as slabs or bands originated from a shaping mill or from continuous casting.
2. Description of Related Art
Hot rolling usually takes place in successive rolling stages in a unit comprising one or several roll stands. Each roll stand can be used as a reversible mill performing a number of reducing passes, alternately in one direction and the other, until the desired thickness is achieved. But, a single rolling pass can be carried out in each stand. The unit operates then as a tandem mill, whereas the rolled product is taken simultaneously in all the stands and its thickness is reduced successively in each roll stand.
The invention applies especially to hot rolling of steels and their alloys, but can also be used, in certain conditions, for rolling non-ferrous metals such as aluminum and its alloys.
Generally, a mill comprises a rigid holding stand with two separate roll standards between which are provided at least two working rolls, superimposed, thus forming a gap enabling the product to be rolled to run through the said gap. In a conventional, so-called quarto arrangement, the working rolls rest each on a back-up roll of larger diameter. In a so-called sexto assembly, idling rolls are interposed between the working rolls and the back-up rolls.
At least the back-up rolls are fitted, at their ends, with journals rotating inside chocks that are mounted to slide into windows provided respectively on both standards of the stand, parallel to a clamping plane, generally vertical, passing more or less through the axes of the working rolls.
The mill is associated with means to control the running of the product between the rolls, at a certain forward speed. In the case of a reversible mill, which rolls alternately into two opposite directions, the forward control means consist, generally, of two roller tables, respectively, one roller table placed upstream of the stand, in the running direction, in order to control the engagement of the product and another roller table, placed downstream, in order to receive the product upon completion of the rolling operation.
In hot rolling, the product is heated, before rolling, up to a temperature of approx. 1200° C. in the case of steel, in order to facilitate deformation of the metal and its flow between the rolls. Generally, indeed, in a rolling process, the product exhibits at the inlet of the roll stand a thickness greater than the distance between the rolls and, when it contacts the said rolls, it is driven by a friction effect, then pinched between both rolls, whereas the metal continues flowing and being reduced in thickness, until a thickness more or less equal to the distance between the generatrices opposite both working rolls is achieved. Thus, a roll nip can be defined, delineated by the arcs of contact between each roll and the product.
Rolling, therefore, starts with a raw part such as a slab or a band of variable thickness, which may range between a few millimeters and several hundred millimeters and to each pass corresponds a reduction in thickness that may vary, for instance, from 50 mm to a few ten millimeters.
During rolling, the rolls tend to move away from one another and must therefore be held in place by an opposite roll load which, in a quarto mill, is applied to the chocks of the back-up rolls.
These clamping means are thus used, on the one hand, for prior adjustment of the distance between the rolls and, on the other hand, for maintaining the said distance during the roll pass. They generally consist of screws or hydraulic jacks mounted on the roll stand and resting respectively on both chocks of a back-up roll, whereas the other is blocked in its upward motion. However, other arrangements are possible. For instance, back-up rolls can be used, which comprise a shell mounted to rotate round a fixed shaft and resting on the said shaft via a series of jacks. These jacks then constitute clamping means exerting the rolling load, which is thus distributed over the whole length of the gap.
In all cases, under the effect of the rolling load, certain members of the roll stand will inevitably yield to some extent, thereby increasing slightly the distance between the rolls, which had been adjusted without any product, and therefore causes the crushing foreseen to be reduced. To realise accurately the requested reduction in thickness, the yielding value must be assessed, so that it can be compensated for as exactly as possible.
The rolling load to be applied for keeping a given distance between the rolls depends on how the product will be deformed in the nip between the rolls.
Conversely, the maximum reduction possible in thickness depends on the rolling load that can be applied, taking into account the capacities of the mill.
The reduction in thickness that can be achieved at each pass is therefore limited and this is why a raw product is rolled, normally, in several successive passes, each determining an elementary reduction in thickness, compatible with the capacity of the mill. The total reduction in thickness from a raw thickness e
o
down a final thickness e
n
can be achieved in n passes according to a progressive thickness reduction process, called a rolling scheme, which depends on the capacity of the mill and on the adjustment means available, on the mechanical and physical features of the roll stand and of the product, as well as the thickness and evenness tolerances to be adhered to.
According to the capacities of the unit available, a single rolling scheme can be defined in which, at each pass, the same average reduction in thickness is achieved. The number of passes to be carried depends then, simply, on the total reduction in thickness to be provided.
However, it may prove necessary to increase the number of passes since the selected average reduction in thickness must be determined so as to remain compatible, for all the passes, with the characteristics of the product and of the roll stand. Still, to improve the productivity, it obviously pays to reduce, as far as possible, the number of passes to be carried out.
But it has also appeared that the final quality of the product, and especially its evenness, depended on the conditions in which rolling is performed and that all the thickness reduction schemes are not equivalent when a product of a set quality should be achieved.
For example, even if a certain temperature of the product can be defined at the beginning of the rolling process, this temperature varies from one pass to the next. Indeed, the product cools down during the waiting time between two successive passes, but the deformation of metal causes, conversely, the product to be heated during the pass and it may prove necessary to cool the product down between two passes in order to prevent excessive cumulated heating.
Still, the deformation conditions of the product, which determine the rolling load to be exerted, depend obviously on the nature of the metal and its temperature.
It therefore seems judicious, in order to obtain a product with set qualities, to adhere to an optimum pattern that depends not only on the mechanical capacity of the installation, but also on the final quality requested for the product.
The last few years have witnessed attempts to automate the rolling process of a flat product enabling to achieve the thickness foreseen with good evenness and by using a minimum number of passes without overloading the roll stand(s).
In such a system, it is necessary to control, at each pass, the adjustment of the clamping means in order to apply between the working rolls, a rolling load enabling to realise the maximum reduction in thickness compatible with the capacity of the mill. This rolling load is assessed in relation to the different rolling parameters on which depend the running conditions of the metal in the chock, notably the reduction in thickness to be provided, the forward speed and the temperature of the p
Arbab Alireza
Biausser Hervé
Leclercq Yves
Maguin Nicolas
Arent Fox Kintner & Plotkin & Kahn, PLLC
Black Thomas
Hartman Jr. Ronald D
Vai Clecim
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