Method and device for pickling the edge portions of a sheet immersed in a reactive solution, in particular a hot rolled sheet

The device comprises an ultrasonic wave emitter (4) whose lower end portion (5) is immersed in the reactive solution (2) in which a sheet (3) to be pickled is immersed, and this end portion is provided with two emitting plates (6,7) respectively positioned on each side of an edge portion of the sheet (3) with their planar emitting surfaces parallel to the sheet and contained in two antinodal planes. Consequently, the two plates (6,7) vibrate in phase opposition and, if the vibratory amplitude is sufficiently large, these vibrations create a cavitation in the liquid in the space between the surfaces (3a,3b) of the edge portion and the plates (6,7) which considerably accelerates the rate of removal of scale from the edge portion.

The present invention relates to the pickling of the edge portions of a 
metal sheet immersed in a reactive solution, in particular the pickling of 
hot rolled steel sheets in metallurgical installations prior to their cold 
rolling. 
As is known, the strips of hot rolled steel are covered with a layer of 
oxides, namely scale, which is a hard, brittle and abrasive material which 
must be removed before cold rolling. Indeed, as it elongates less than the 
steel, the scale would become encrusted in the metal and would very 
substantially reduce its pressing capability. Being abrasive, it would 
rapidly deteriorate the surface quality of the rolls, the appearance of 
the sheets would be poor, non-bright, unsuitable for polishing and the 
various subsequent coatings. 
Scale, which is a mixture of three oxides FeO, Fe304 and Fe203, and also 
chromium oxides in the case of stainless steels, is conventionally pickled 
by immersion of the steel strips in tanks containing a reactive solution 
of hot acid, such as sulphuric acid, hydrochloric acid, etc. Consequently, 
a plurality of tanks are usually employed in order to attain sufficient 
levels of pickling, the product to be pickled remaining in these tanks for 
one to several minutes. 
For accelerating the pickling process, it has been proposed to generate in 
the reactive solution in which the product to be pickled is immersed, high 
frequency vibrations such as ultrasonic vibrations whose action is added 
to that of the bath to break down the layer of scale. There is then 
observed a large increase in the pickling rate, the best performances 
being obtained with very high vibratory powers injected in the solution 
and consequently capable of producing within the liquid acoustic 
cavitation phenomena. 
Now, the pickling of the edge portions of the product presents a particular 
problem, since the cooling of the edge portions is more rapid than that of 
the central zone. Consequently, the oxides formed on the edge portions are 
different from those of the central zone, which reduces the pickling rate 
on the edge portions relative to that of the centre of the product, at 
least in certain pickling lines. 
An object of the invention is therefore to improve the pickling rate on the 
edge portions of the product. 
In the method according to the invention, means are employed for emitting 
ultrasonic waves in the reactive solution in which the sheet to be pickled 
is immersed, these emitting means having an emitting surface. 
According to the invention, the ultrasonic waves are emitted in at least 
one plane parallel to the edge portion to be pickled and with sufficient 
power to create a cavitation in the volume of liquid between the edge 
portion and said plane. 
Preferably, the ultrasonic waves are emitted in two antinodal planes 
parallel to the edge portion and respectively located on each side of the 
latter. 
As an antinodal plane is a plane in which the vibration imparted to the 
liquid has its maximum amplitude, this means that the plane of one of the 
ultrasonic wave emitting surfaces must be located at a distance from the 
plane of the other emitting surface equal to 1/2 wavelength or a multiple 
of this value. It is found that, under these conditions, if this vibratory 
amplitude is sufficiently large, a cavitation is created in the liquid 
between the emitting plate and the surface of the edge portion. 
According to the invention, the device for carrying out this method 
comprises at least one plate element having a planar emission surface 
provided for being placed above an edge portion of the sheet and parallel 
to the latter. 
According to a preferred embodiment of the invention, the device comprises 
two emitting plates respectively placed on each side of an edge portion of 
the sheet with their emitting surfaces contained in two antinodal planes. 
As in the foregoing embodiment, if the amplitude of the vibrations, i.e. if 
the emission power of the pavilion elements is sufficient, and if the 
frequency range is suitably chosen, it is possible to create in the liquid 
a cavitation between the two plates which considerably accelerates the 
pickling of the edge portions. 
Further features and advantages of the invention will be apparent from the 
following description with reference to the accompanying drawings which 
illustrate an embodiment given as a non-limitative example.

There is shown in FIG. 1 and FIGS. 4A and 4B a tank 1 containing a reactive 
solution 2, such as hydrochloric acid, in which a travelling sheet 3 is 
immersed in the known manner for the purpose of removing its layer of 
scale. 
The device for pickling the surfaces 3a, 3b of an edge portion of the sheet 
3 comprises an ultrasonic wave emitter such as a piezoelectric emitter 4 
whose lower end, immersed in the solution 2, is a sonotrode 5 on which are 
fixed two emitting plates 6, 7 respectively on each side of the edge 
portion of the sheet 3 and parallel to the latter. The two plates 6, 7 are 
therefore immersed in the solution 2, the upper plate 6 being disposed 
above the surface 3a of the edge portion and the lower plate 7 below the 
lower surface 3b of the edge portion so that the latter is partly 
overlapped by the planar surfaces of the plates 6, 7. The distance d 
between the plates 6 and 7 is such that their emitting surfaces are 
located in two antinodal planes spaced apart from each other in the 
illustrated embodiment 1/2 wavelength d=.lambda./2 of the vibration V 
imparted to the solution 2. 
This distance d may also be a multiple of the 1/2 wavelength .lambda./2, 
i.e. d=n .lambda./2. 
Under these conditions, the two planar emitting surfaces of the plates 6, 7 
vibrate in phase opposition and, if the maximum amplitude A of the 
vibrations is large enough, a cavitation is obtained in the part of the 
liquid 2 between the two plates 6, 7 and facing the surfaces 3a, 3b of the 
edge portion. The pickling of the latter is in this way considerably 
accelerated. 
The sonotrode and the plates 6, 7 may be made for example from titanium so 
as to be little sensitive to corrosion by the solution 2. To obtain a 
cavitation field in the liquid 2 within the space between the plates 6, 7 
and the edge portion to be pickled, tests have shown that the ultrasonic 
waves emitted must have a frequency between about 10 and 50 kHz and that 
their emission power must be at least 400 Watts per square meter of the 
plate. The distance d may be for example 15 cm for a frequency of around 
20 kHz. 
In an advantageous embodiment, the plates 6, 7 are discs having a circular 
contour as shown in FIG. 4A. This contour has the advantage of 
homogenizing the reflections of the waves on the edge portions of the 
plate and therefore facilitates putting the disc constituting the plate 
into resonance, which in turn can produce cavitation in the liquid 2. 
In a modification, shown in FIG. 3, there may also be provided on the 
surface or surfaces 8 of the plates 6, 7 facing toward the product, extra 
thicknesses 9a, 9b, 9c, 9d, 9e which have a width d and are concentric 
with the circumference of the plate and are evenly spaced apart with a 
pitch equal to a 1/2 wavelength .lambda./2. This configuration 
substantially increases the vibratory efficiency. If these plates have a 
shape different from that of a circle or semi-circle, these extra 
thicknesses must be arranged to be parallel to the edge portions of the 
plates. 
The plates 6, 7 may optionally be constructed in the form of semi-circular 
discs, as shown in FIG. 4B. It will be understood that a pickling device 
such as that described hereinbefore is provided for each of the two edge 
portions of a sheet.