Method and apparatus for high-speed strip treatment

Abstract: A high-speed strip (1) pickling and other treatment such as rinsing and cleaning the acid (13), or other fluid treating medium, tends to be carried along with the strip (1), which reduces the efficiency of contact of the fluid with the strip (1). The invention addresses this problem by providing a segmented bath, each segment having a deflector (14) over the strip (1) to cause a flow reversal of the acid (13), which is then guided by an inclined separator panel (10) to the upstream end of the segment and deposited again on the strip (1). The exit of the segment also includes a constriction (15) to cause the fluid to accelerate, creating turbulence, and, on the exit side of the constriction (15), causing a decrease in pressure, whereby fluid is drawn down from above the separator panel (10), which in turn assists the circulation of fluid through the flow reversal.

Description: FIELD OF THE INVENTION This invention relates to treating steel, particularly to the pickling of steel, and more particularly to the control of the pickling bath used for pickling steel strip moving at high speeds. The present invention is not limited in its application to pickling baths, but may be used with other types of fluid treatment systems for high-speed strip, such as rinsing and cleaning. BACKGROUND OF THE INVENTION In the past ten to fifteen years there has been considerable evolution in the pickling of steel. The art has progressed from simple dip tanks to horizontal pickling tanks and on to the extremely advances turboflow systems, which led to the generation of more ecological and economical processes, all the while improving the quality of the treatment that the material had to undergo. Nevertheless, virtually all of the newer treatment facilities (primarily steel strip pickling plants) were designed for hot strips with thicknesses ranging from 1.8 to 6 millimeters. Average strip thickness throughout the world is about 3 millimeters. Today, thanks to the hot rolling technology combined with the thin slab casting technology, the hot strip sector produces thicknesses as low as 0.7 millimeters while retaining the ability to handle 6 millimeters in thickness. In order to produce substantially thinner hot strips in the plants, the strip must run at higher speeds during treatment. For example, with a production of 1.5 million tons per year and a 3 millimeters thick reference strip, strip speed is 250 metres per minute. With a 1.5 millimeters thick strip, processing speed at the same production capacity is 500 metres per minute; 400 metres per minute is to be expected for strip 2.4 millimeters thick at the same production rate. Pickling technologies currently available on the market are generally not compatible with such high speeds, since the facility is usually designed to propel the strip through a horizontal pickling plant under low tension. This presents guiding problems, and the added condition of high speed of the strip causes the strip to carry the pickling liquid on its surface. If the strip is propelled horizontally into the acid bath with considerable kinetics, the quantity of liquid carried away may be so great that adequate pickling and safe operation are difficult to guarantee. A boundary layer of acid tends to remain stationary with respect to the strip, resulting in poor contact efficiency. In the past, pickling tanks and their covers have been constructed to control acid vapors, as in US Patents 3.803.996 and 3.648.593 to Marshall and 4.592.784 to Ghizzi. Weirs have been used to create cascades of acid from one tank or zone to the next. See Hampel US patent 3.473.962 and Matiussi US Patent 5.179.967. Acid has been collected in separate vessels for recycling, as in Galloway US Patent 4.007.750 and Gravert et al US Patent 5.853.495; note also Zednicek et al US Patent 5.716.455, which discloses restrictions constructed to shear the acid on the strip, causing turbulence; the acid is recycled through drains. In pickling wire, Hone et al in US Patents 4.950.333 and 4.951.694 utilize the dynamics of the process by generating waves of acid, which are controlled by weirs. In particular, US Patent 5.853.495, which corresponds to EP-A-0795629, discloses a device for surface treatment of continuously moving strip, particularly for pickling rolled metal strip, and which includes a trough-shaped tank containing a treating liquid bath. The trough includes a panel mounted therein and submerged in the liquid, and aligned approximately parallel to the strip. The panel divides the treatment bath into a treatment area underneath the panel through which the strip being treated passes and a return flow sector for the liquid above the panel. The treating liquid is collected in the return flow sector at a discharge end of the trough and is directed along the return flow sector to a receiving end of the trough. Acid is recycled from a high end of an acid tank to a low end by gravity through a pipe, as configured by Lordo in US Patent 5.803.981. Kimura et al, in US Patent 5.116.447, recycles "wakes" of acid stripped by weir members shaped to direct the excess acid to the sides of the weirs for draining. As indicated above, the kinetics of the extremely high speed of the newer pickling lines results in rapid movement of the acid in the bath, particularly that above the strip, towards the downstream end of the process and apparatus. This causes increasing dephts of acid in the downstream ends of the pickling vessels, compounding the hazards for workers, and causing environmental problems due to the possibility of acid escaping from the apparatus, and economic loss from the underuse of the acid. SUMMARY OF THE INVENTION The present invention makes possible the efficient pickling of strip steel moving at high speeds while conserving acid and providing improved ecological and economic benefits. It is applicable not only for high-speed pickling, but also for other high-speed strip treatment, such as rinsing and cleaning. The present invention comprises a pickling apparatus for pickling steel strip travelling at high speed substantially horizontally, including a plurality of acid-containing sections in series for contacting said strip with acid, each of said acid-containing sections comprising an upstream end and a downstream end, comprising (a) means for maintaining a desired level of strip therein while permitting acid to flow on the bottom as well as the top thereof, (b) en exit for the strip in the downstream and including an arcuate deflector for acid flowing with the strip and on top thereof, the arcuate deflector being shaped so as to scoop the flowing acid and reverse its direction of flow at a level higher than that of the acid on the strip, and (c) an inclined separator panel for receiving acid from the deflector in the reverse direction of flow and guiding it downwardly to the upstream end of the section. Below the deflector, it is placed a constriction on the flow of acid on top of the strip, followed by a divergent zone. The constriction, combined with the divergent zone, accelerates the flow of the acid remaining on top of the strip so that it is no longer merely carried by the strip. The constriction also aids the function of the deflector by urging the higher levels of acid upwardly towards the deflector so they can be recirculated. The present invention optionally includes the use of a special cover for the pickling tanks and other optional features and variations, as will be seen below. BRIEF DESCRIPTION OF THE DRAWINGS The attached figures are given as a non-restrictive example and show a preferred embodiment of the invention as follows: Fig. 1 is a side sectional view of a portion of a high-speed strip pickling line including the present invention. No acid is shown in this depiction. Fig. 2 is an enlarged view of just one section of the pickling line of figure 1, permitting a better understanding of its effect. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to Figure 1, steel strip 1 moves through a pickling facility 2 at a high speed - that is, at least 150 metres per minute, and possibly 400 or 500 metres per minute - brought about by any conventional means, not shown. The steel strip 1 is initially held at the desired level by wringer rolls 3 in advancing chamber 4 and then is held at a desired level by slabs 5 interspersed in the facility 2. Slabs 5 may be made of granite, polypropylene, or other material more or less impervious to the acid 13, as is known in the art. Slabs 5 divide the acid bath into bath sections 8. Tank cover 12 covers three consecutive bath sections 8. Wringer rolls, not shown, similar to wringer rolls 3, may be located at the downstream end of facility 2. Pickling acid 13 or any other fluid may be introduced through sprays 6 directly onto the upper side of moving strip 1 and preferably may substantially fill the bath section 8, thoroughly immersing strip 1. The pickling acid 13 forms a bath 7 which contacts the under side of the strip 1. In figure 2, the upstream end of bath section 8 is defined by lower slab 5A and upper slab 5B, and the downstream limits are defined by lower slab 5C and upper slab 5D. The acid 13 is swept along with the moving strip within each bath section 8 so that it flows towards the downstream end of bath section 8. The upper levels of the acid 13 in the downstream end are caught by deflector 14, which advantageously has an arcuate configuration similar to a snowplow, tending to reverse the direction of flow of the acid 13, as shown by arrow 9. Acid 13 emerging from the top edge of deflector 14 is carried by its kinetic energy in the direction of the upstream end of bath section 8, and flows onto the surface of separator panel 10. Separator panel 10 has a slight incline downwards towards the upstream end of bath section 8; the acid 13 thus flows back to the upstream end of bath section 8 and flows, drains, or is drawn downwardly through openings 11 onto the surface of strip 1. The separator panel 10 may be built into each bath section 8 or may be built into the tank cover 12, which may also include upper slabs 5B and 5D. The separator panel 10 may contain channels or corrugations to ensure that the acid 13 does not tend to flow to one side or the other of the separator panel 10. Opening 11 may be built into the separator panel 10 or be continuous across the width of bath section 8. It should be noted that rounded slabs 5B and 5D form a constriction 15 on the flow of the portion of acid 13 not subject to flow reversal as shown by arrow 9. The constriction 15 tends to accelerate the flow on the top of and underneath the strip 1 and is followed downstream by a diverging zone 16, which accelerates and adds turbulence to the flow of acid 13 on top of and underneath strip 1 while recirculated acid 13 is added to it through openings 11. The acceleration of the acid 13 in diverging zone 16 tends to create a negative pressure with respect to the acid 13 above openings 11, helping to draw the acid down and through openings 11, which in turn assists the overall circulation pattern above the strip 1 illustrated by arrows 9 and 18. The constriction 15, together with diverging,zone 16, contributes to the efficiency of the treatment step by enhancing the contact of the acid 13 with the strip 1 both by impressing contact of the acid 13 on the strip and by causing turbulence within it. By reversing the flow of a significant portion of the acid 13 in each segment (tank) and recirculating it within the segment, the present invention helps to reduce cross contamination from tank (segment) to tank (segment) wherein it is desired to maintain different acid concentrations in the segment. It will be seen that the invention includes a method of recycling strip treatment fluid in a high-speed strip treatment bath, wherein the treatment fluid is carried at high speed along with and on top of a strip, comprising (a) guiding the treatment fluid carried along with and on top of the strip upwardly and in a flow reversal path designed to employ the momentum of the treatment fluid to reverse the direction of flow of the treatment fluid, (b) guiding the treatment fluid, preferably using its kinetic energy, onto a receiving surface above the treatment fluid being carried with the strip, and (c) guiding the treatment fluid flowing in a reversed direction to a point upstream of the flow reversal path and recycling the treatment fluid by depositing the treatment fluid on the strip.

Claims: An apparatus for steel strip (1) treatment bath wherein said strip (1) travels substantially horizontally at high speed, comprising a plurality of fluid-containing sections (8) in series for contacting said strip (1) with a treatment fluid, each of said fluid-containing sections (8) comprising an upstream end and a downstream end, characterized in that said fluid-containing section (8) comprises means (5) for maintaining a desired level of strip (1) therein while permitting fluid to contact both the top and bottom of said strip (1), said means (5) comprising rounded slabs (5B, 5D) able to define a constriction (15) on the flow of said treatment fluid, an exit for said strip in said downstream end including deflector means (14) for fluid flowing with said strip (1) and on top thereof, said deflector means (14) being shaped so as to scoop at least a portion of said flowing fluid and reverse its direction of flow at a level higher than that of said fluid on said strip (1), and an inclined separator panel (10) for receiving fluid from said deflector means (14) in said reverse direction of flow and guiding said fluid to said upstream end of said section (8). An apparatus according to claim 1, wherein said fluid is an acid (13) for pickling said steel strip (1). An apparatus according to claim 1, wherein said deflector means comprise an arcuate deflector (14). An apparatus according to claim 1, further including a cover (12) and wherein said inclined separator panel (10) is built into said cover (12). An apparatus according to claim 1, wherein said separator panel (10) is corrugated. An apparatus according to claim 1, wherein said exit for said strip (1) includes a constriction (15) and a divergent zone (16) for enhancing turbulence of said fluid on said strip (1). An apparatus according to claim 2, wherein said means for maintaining a desired level of strip (1) comprise acid-resistant blocks on which said strip (1) passes. An apparatus according to claim 3, wherein said arcuate deflector (14) is also shaped to define said restriction above said strip (1). An apparatus according to claim 7, wherein one of said acid-resistant blocks is oriented beneath said restriction. An apparatus according to claim 9, further including means for placing acid (13) on top of said strip (1). An apparatus according to claim 10, wherein said acid-resistant block beneath said restriction also creates a restriction. A method of recycling strip treatment fluid in a high-speed strip (1) treatment bath using an apparatus according to claim 1, wherein said strip treatment fluid is carried at high speed along with a strip (1), characterized by the following steps: (a) guiding said treatment fluid carried along with and on top of said strip in a flow reversal path designed to employ the momentum of said treatment fluid to reverse the direction of flow of said treatment fluid, (b) receiving said treatment fluid on a separator panel (10) between said treatment fluid being carried with said strip (1) and said treatment fluid flowing in a reversed direction, and (c) guiding said treatment fluid flowing in a reversed direction on said separator panel (10) to a point upstream of said flow reversal path and recycling said treatment fluid by depositing said treatment fluid from said separator panel (10) onto said strip (1). A method according to claim 12, wherein, prior to step (b), said fluid flows in a reverse direction. A method according to claim 12, wherein said separator panel (10) is substantially planar. A method according to claim 12, wherein said receiving surface is corrugated. A method according to claim 12, further including, at a point below said flow reversal path, guiding a portion of said fluid on top of said strip (1) into a constriction (15) and then into a divergent zone (16), whereby the flow of said portion of said fluid accelerates and becomes turbulent in said divergent zone (16).

Inventors: LORDO RICHARD KENNETH, PERSSON THOMAS SVEN, LORDO, RICHARD KENNETH, PERSSON, THOMAS SVEN
Applicants: DANIELI TECHNOLOGY INC, DANIELI TECHNOLOGY, INC.
Application year: 1999
Main classification: C23G3
IPC codes: C23G3, B08B3