Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-07-06
2001-10-30
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C034S062000, C266S113000
Reexamination Certificate
active
06309483
ABSTRACT:
The present invention relates to a method and to a device which are intended to eliminate vibration of strips moving continuously through zones of a heat treatment or coating line where gas is blown onto a continuously moving strip. The invention is most especially applicable, without however being restricted to this application, to cooling devices which work by blowing gas in jets with which the lines for the continuous heat treatment or coating of metal strip are equipped.
In order to permit a good understanding of the technical field to which the present invention applies, reference is first of all made to
FIG. 1
of the appended drawings which depicts, diagrammatically and in perspective, a rapid cooling zone
2
employing the blowing of gas in jets, according to the state of the prior art, through which a metal strip
1
passes, passing over rollers
3
and
12
. As it passes through the cooling zone
2
, the strip
1
is exposed to the jets of cooling gas blown out of a number of pairs of boxes such as
4
and
5
, each box being positioned one on each side of the strip
1
. The cooling boxes
4
and
5
are of limited length so as to allow one or a pair of stabilizing rollers such as
9
and
10
to be installed, these rollers being placed between two consecutive boxes as clearly shown in FIG.
1
and which are intended to guide and stabilize the strip and, in particular, to limit the extent to which the latter vibrates under the action of the cooling jets.
The blower boxes
4
and
5
may be split transversely into a number of boxes such as
6
,
7
and
8
equipped with independent gas supply means
13
,
14
and
15
and the supply flow rate and/or pressure characteristics of which can be regulated according to the level of cooling to be effected on the strip.
There are various embodiments of means that allow cooling gas to be blown onto strip. U.S. Pat. No. 3,068,586 describes a certain number of embodiments of these blowing means.
FIG. 2
of the appended drawings depicts, diagrammatically and in perspective, a cooling box
4
of known type, equipped with blow holes
16
, the diameter and pattern of which are tailored to the desired level of cooling.
FIG. 3
is a depiction similar to
FIG. 2
but in which the cooling box
4
of known type has cylindrical blower nozzles
17
arranged in a rectangular or diamond-shaped pattern over the entire surface of the box
4
. Finally,
FIG. 4
depicts, in a view similar to
FIGS. 2 and 3
, a known alternative form of blower box
4
which is equipped with blower nozzles in the form of slots
18
arranged across the entire width of the box.
The cooling gas blown onto the strip through the holes
16
, the nozzles
17
or the nozzles
18
is ducted transversely across the entire width of the box, between this box and the strip, so that it can be recycled through ducts located outside the cooling zone
2
. The means which fulfil these functions are well known to those skilled in the art and have not been depicted in FIG.
1
.
The increase in performance of lines for the continuous heat treatment of metal strip, and the search for increasing cooling gradients have entailed moving the holes
16
or the nozzles
17
or
18
closer to the strip
1
and the use of increasingly high cooling-gas blowing flow rates and/or pressure. This change has lead to the emergence of a new problem in this type of cooling zone, namely the vibration of the strip between the cooling boxes, this vibration phenomenon being limited or unknown in equipment produced according to the state of the prior art.
FIG. 5
of the appended drawings depicts sections on a horizontal plane through the blower boxes
4
and
5
. For a theoretical stable situation, the distance between the strip and the boxes
4
and
5
is equal to the distance denoted by the reference a, the blowing flow rates in the boxes
4
and
5
denoted by Vn and Vs are equal. Having been blown onto the strip, the gas is collected along Vn
1
and Vn
2
and also along Vs
1
and Vs
2
. This equilibrium is characterized by Vn
1
=Vn
2
and Vs
1
=Vs
2
.
FIG. 6
of the appended drawings depicts a strip which has a heterogeneous distribution of tension across its width, the tension being greater at the centre of the strip than at the edges, as a result, perhaps, of the rolling process, the profile of the rollers or heterogeneous heating or cooling, or as a result of some other phenomenon. In this configuration, the strip tension is concentrated in its central region, the longer edges of the strip being less taut. This difference in tension with “floppy” edges may give rise to a variation in the distance between the edges of the strip
1
and the boxes
4
and
5
, according to b and c, which leads to the variation in flow rates V′s
1
, V′s
2
, V′i
1
and V′i
2
. In this example, V′n
1
is smaller than V′n
2
and V′s
1
is greater than V′s
2
. Under this action, the strip shifts into a maximum position for which the pressure on side b increases and the pressure on side c decreases, and the opposite movement begins. This phenomenon causes torsional vibration of the strip, symmetrical or otherwise, which can be represented according to
FIG. 7
which illustrates this vibration between two consecutive rollers. This vibration in the zones where blowing is at a high flow rates may reach amplitude values such that they can cause contact between the holes
16
, the nozzles
17
or
18
and the strip, which of course causes surface defects to appear on the strip
1
, thus degrading the product obtained. Furthermore, strip vibration may be such that it leads to damage to the cooling boxes and to their blow holes or nozzles.
To solve this problem, attempts have been made, in the prior art, at limiting the vibration by reducing the length of the blower boxes so as to be able to bring the stabilizing rollers
9
and
10
(
FIG. 1
) closer together. However, this technique limits the useful blowing length and therefore the effectiveness of the cooling in the zone.
Another attempt at solving this problem has consisted in greatly increasing the strip tension, but this solution is possible only for greater strip thicknesses and cannot be used for high-temperature strip, thinner strip and great widths or on account of the mechanical strength properties of treated steels of which the strips are made.
The solution generally adopted in the state of the prior art for eliminating or at least reducing strip vibration consists in increasing the distance between the holes
16
or the nozzles
17
or
18
and the strip
1
or in limiting the blowing pressure in the boxes, these solutions leading to a limiting of the effectiveness of the cooling and resulting in a reduction in the productivity of the line in a proportion which may be as much as 40% of nominal productivity.
Furthermore, defects in the rolling of the strips that are to be treated, particularly the long edges, increase the risk that a strip exposed to a regime of recycled gases blown onto an unstable strip will begin to vibrate. Furthermore, the change in grades of steel currently treated is dictating increasingly steep cooling gradients cooling from increasingly high temperatures with low strip tensions, thus causing the appearance of strip torsional vibration to become more widespread.
The present invention has therefore set itself the objective of solving the above problem, that is to say of eliminating strip vibration in the cooling zones by improving the collection of the cooling gases between the strip and the blower box and by forcing the strip into a fixed position.
In consequence, the first object of the invention is a method for eliminating vibration of strip travelling continuously through zones of a heat treatment or coating line in which gas is blown onto a strip travelling continuously, particularly through devices which effect cooling by blowing gas in jets with which the lines for the continuous heat treatment or coating of metal strip are equipped, characterized in that this method consis
Mignard François
Wang Robert
Connolly Bove Lodge & Hutz
Stein Heurtey
Yee Deborah
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