US2674251A - Apparatus for cleaning metal strip continuously - Google Patents

Description

April 6, 1954 R. DUNLEVY ET AL APPARATUS FOR CLEANING METAL STRIP CONTINUOUSLY Original Filed Sept. 5, 1950 INVENTOR HAROLD FR/CK JOHN H SHOEMAKER ATTORNEY Patented Apr. 6, 1954 APPARATUS FOR CLEANING METAL STRIP CONTINUOUSLY Ralph Dunlevy, Harold Frick, and John H. Shoemaker, Detroit, Mich, assignors to Kolene Corporation, Detroit, Mich.

Original application September 5, 1950, Serial No. 183,211. Divided and this application January 8, 1952, Serial No. 265,462

2 Claims. 1

Be it known that the undersigned have invented certain new and useful improvements in apparatus for cleaning metal strip continuously, of which the following is a specification.

This application is a divisional of our prior application, Serial No. 183,211, filed September 5, 1950, and discloses apparatus for continuously using a metal cleaning process of the molten alkali salt bath type, a preferred but not necessarily the only example of which is the process known commercially as the K1 process and described in Patent No. 2,458,661 of January 11, 1949, to Hugh G. Webster and Clarence J. Falter.

Generally speaking, the apparatus hereof employs molten alkali salts for cleaning metal strip continuously and includes means for preventing the directing and squeeze-out rollers employed in such apparatus from marring or scratching the cleaned surface or the surfaces being cleaned, and also means for preventing the deposit on such rolls of solid particles which would mar or scratch the steel strip.

It is understood that the term strip as used herein refers to a moving ribbon of any desired width, not only ribbon under 12" wide and known to the trade as strip, but also ribbon over 12" wide and. known to the trade as sheet, and also ribbon extremely narrow and known to the trade as wire.

It is also understood that while steel is the particular metal chosen for descriptive purposes herein, and is more commonly used for treatment by the process hereof, that copper may also be treated by the process hereof.

The process Patent No. 2,458,661 discloses a metal cleaning process of the molten alkali salt bath type wherein a first step is the immersion of the article being treated in a molten alkali salt bath at an operating temperature well above the melting point of the alkali salt, such immersion operating to transform impurities into easily removable oxides. The second step in the process of that patent is the immersion of the oxide coated article, coated with the oxide formed by the first step, into a bath which operates to remove the oxide coating. Such second bath may be itself an alkali metal salt bath or it may be a weak acid bath.

The time of immersion of the article being cleaned in either of these two baths is governed by operating conditions, which also govern the temperatures of the bath. The immersion in the first bath may be of any desired period, in some cases being well below one minute, and in other cases being well in excess of one minute. The immersion in the second bath is generally quite brief, only enough to remove the readily removed oxides formed by the first bath. The immersion in the second bath is a variable, depending on the metal being treated.

The temperature of the first bath is also as desired, ranging from somewhere above the melting point of the salt bath, to somewhere below its vaporization point, or its decomposition, point, whichever is lower. Where the bath is of the preferred formula, as set forth in Patent No. 2,458,661, the melting point is 550 F. approx, the decomposition point is 1100 F. approx, and the vaporization point is 2500 F. approx, and the bath temperature will be between 550 F. and 1100 F.

Between the first and second baths there is customarily employed a water rinse bath of any suitable type. This insures uniform oxide removal from the strip. It is well known that the presence of alkali (as from the first bath) on a metal surface when immersed in an acid (as in the second bath) tends to prevent oxide removal. The water rinse insures the absence of alkali on the strip from the first bath as the strip enters the second bath.

Inasmuch as the process of the foregoing named patent may readily be understood upon reference to that patent and need not be further described herein, reference to that process generally is here concluded with the observation that the preferred process hereof is identical with the process of that patent and with the understanding that the disclosure of that patent is incorporated into this application by reference, to avoid the necessity of incorporating that disclosure hereinto expressly.

While the process of said Patent 2,458,661 is the one preferred for use as the process hereof, other processes and variations of said process may also be used, to the extent disclosed herein.

For an understanding of the apparatus and process hereof, and on the assumption that the reader hereof will have become by this time familiar with the process of the aforesaid Patent 2,458,661, incorporated hereinto by reference, reference may now be had to the following specification and detailed description to be read in connection with the accompanying drawing.

In this drawing:

Fig. 1 is a diagrammatic side view of a continuous strip treating apparatus, including a furnace, a molten salt bath, a water rinse bath, and an oxide removing bath.

Fig. 2 is a diagrammatic top plan view of the apparatus of Fig. 1.

Referring now to the drawing, it will be observed that the drawing shows at it a continuously moving strip being treated continuously by the process of the aforesaid patent.

The strip is first heated in a furnace 20 in order that its temperature be brought above the operating temperature of the molten salt bath, later to be described. Since the operating temperature of the molten salt bath may range anywhere from 500 F. up to approximately 900 F, depending upon operating conditions and the speed of operation, with the lower end of the range being determined by the operating temperature re quired for maintaining molten the salt bath, and with the maximum temperature being determined by the temperature above which the salt bath vaporizes or decomposes, whichever is lower, it is generally contemplated to heat the strip in the'furnace to a temperature around 1200 F. In some instances even higher temperatures, l900 F.-2200 F., are used. So heated the strip passes over a directing roller 2 i.

It is pointed out here that the aforesaid heating is to a temperature selected as proper for the particular alloy being treated and is not critical to the present invention.

In the event the steel strip leaving the furnace 20 is at too high a temperature, it may be cooled by the use of steam sprayed out of the steam pipes 22 just ahead of the roller 2|. It may be observed here that if the steel is too hot as it enters the molten salt bath, it will cause bubbling and splattering of the molten salt in the bath with undesirable effects. To prevent this result, the steel strip may be cooled by the steam from pipes 22. Air, water, or air-water-steam mixtures may also be used for cooling the strip in some cases.

In some instances the strip is cooler than the salt bath on entering it. For example, the strip might be entered at room temperature into the salt bath.

Next in line to the directing roller 2| is a molten salt bath which includes a'tub 32 containing the molten salt 33, the latter being of the molten alkali salt type. In the preferred process, it is generally of the formula of the aforesaid Patent 2,458,661, namely, 1 part by weight of alkali metal nitrate, 1.5-3.0 parts by weight of alkali metal hydroxide, and 0.1-0.5 part by weight of alkali metal chloride.

The salt 33 is maintained molten by suitable heating means. One suitable heating means are the burning gas'tubes 34 which are immersed in the molten salt.

The tubes 34 are merely cylindrical chambers immersed in the salt bath 33 which house elongated ducts or pipes 15. Pipes supply combustible gas which is burned in a series of small burner flames disposed along the length of each pipe 15 by forming a series of perforations 16 therein through which the combustible gas is emitted and burned. The combustion gas is supplied from any suitable source through a pipe 11 controlled in quantity by a valve 78, mounted thereon and it is mixed with air passed through a pipe 19 which joins pipe Tl both combustible gas and air mixing and passing into pipe 15, the air being in combustion supporting quantity as controlled by a valve 80 mounted on pipe 11. The waste gases after combustion within tubes 34 are led away through tubular ducts I0, each joining a tube 34 at right angles and thence into a manifold duct H common to all which leads to stack 65 of the vent for ultimate disposal of the waste gases.

Disposed on horizontal axes 35 above the normal level 36 of the molten salt 33 are directing rollers 3'|-38 which direct and move the strip ID from roller 2| down into the molten salt bath and then to a pair of rollers d0, later to be described, which in turn direct the steel strip out of the bath 33 and towards rollers 42, 43, and 44, 45, 46 which direct the strip under a water spray pipe 49 and through a water rinse bath 50 and an acid rinse bath 5|. The acid rinse is here disclosed as the preferred means for removing the oxides formed in the molten salt bath 33, and is of the character described in the aforesaid Patent 2,458,661, such acid rinse bath being a dilute acid. selected from the class consisting of hydrochloric and sulphuric acids, such that the bath will react chemically with the oxidized coating of the steel strip to remove such coating and leave the strip free of the original impurities and-of the oxidized coating as well. The immersion in the acid rinse bath is timed, however, to be such as not to expose at the surface other impurities not previously existent or not previously exposed and is sufficiently brief so as not to permit the acid to attack the metal of the steel strip, but merely to remove the oxide coating formed by the molten salt bath itself.

While in the preferred embodiment the acid rinse bath is a dilute acid of the group comprising hydrochloric and sulphuric acids, it has been found, in some cases, practical to use other acids, such as nitric or nitric hydrofluoric acids.

The rolls 3'1, 38 and 40 are not steel rolls, as are customarily contemplated in baths used for treating continuously moving steel strip, but rather are of cast iron. It was discovered that when steel rolls were utilized, as the rolls 3'5, 38 and 40, the steel rolls scratched the surface of the steel strip I0, particularly objectionable if that steel strip was polished stainless steel. It was further discovered that when cast iron was used for the surface of such rolls, as by using a cast iron shell on a steel roll, there was formed in the surfaces of the cast iron rolls, minute fissures produced by the removal of the graphitic carbon from the cast iron rolls due to the action of the molten salt on the cast iron rolls. These minute fissures provided excellent reservoirs or pockets for the fluid or molten salt on the rolls and the molten salt was observed to function as a lubricant on the ferritic surface of the cast iron rolls, the fissured rolls functioning in a manner similer to a porous metal roll. The molten salt clinging to the surfaces of these rolls acts as an excellent lubricant and prevents the rolls from scratching the polished stainless steel strip passing by and engaging them.

Similarly, the rolls 40 for squeezing off excess molten salt from the strip l0 leaving the salt bath were also made of cast iron.

While the use of cast iron for the rollers is of special value because of the effect on such rollers of the molten salt bath of Patent 2,458,661, it may here be pointed out that the same beneficial results may be obtained with other salt baths, provided they be of a class or nature as to remove graphitic carbon from cast iron. The latter is the essential or determining factor in the cooperative relationship of the bath and the rollers, cooperating to the end that the rollers will not scratch the strip.

At is shown a means for heating the squeeze-off rolls 40 so as to maintain the salt at such rolls in a molten and fiuid condition. Such means is a rather close fitting insulating cover 60 over the tub 32 whereby the heat radiated from the surface of the molten salt 33, heated by the heating means 34 necessary to maintain such salt molten, operates to maintain the salt molten, even at the rolls 40, and the molten salt on such rolls act as a lubricant to prevent scratching of the polished stainless steel strip by the rolls 43.

In addition, as long as the salt is maintained molten at the rolls 40, the formation and deposit of alkali carbonates and other crystalline materials on the strip 13 at the rolls ii), and on the rolls in themselves, is inhibited and, thus, another cause for marring of the strip is eliminated. It was discovered that unless the salt is maintained molten at the rolls 40, then solid carbonates and other crystalline materials are deposited on such rolls and these cause the scratching of the strip. The provision of the insulating hood 60 operates in the manner above described to prevent the formation of these solid deposits on the rolls 40 and, thus, prevents marring of the strip due to the presence of such solids.

The water rinse bath 56 is vented as by the provision of a closely fitting venting hood 64 connected to an exhaust fan or the like to outlet at 35. It was observed that in the absence of any vent, such as the vent 64, alkali vapors from the surface of the molten salt, particularly in the water rinse bath 5t, condense, with the condensate settling on the surface of the finished strip passing through such rinse bath in or near such rinse bath and causing surface imperfections. The provision of the vent 3t prevents such alkali vapors from condensing and coming into contact with the strip and settling on the strip and prevents the marring that would otherwise be the case. Not only does the vent 64 prevent spotting due to the condensed vapors but also vent 64 accelerates removal of the vapors themselves. These vapors, coming into contact with the strip, may cause discoloration. Their rapid removal, by vent 64, inhibits such action.

The specific improvement of this application is in the location of the water rinse tank. This tank is located as close to the molten salt bath as possible. It has been discovered that improved results, with respect to the preventing of imperfections and discolorations on the stainless steel strip, may be obtained by decreasing the distance between the point where the strip leaves the molten salt bath and the point where it enters the Water rinse bath.

By providing the water rinse bath as close to the molten salt bath as possible, it becomes possible to rinse the strip at the highest possible temperature, and this insures superior rinsing with less facilities. The quench action of the rinse is more rapid than otherwise, and this aids in loosening the oxide at the surface and facilitates the removal of the oxide in the weak acid bath 5!.

By locating the rinse bath 50 very close to the squeeze-off rollers 40 at the output of the molten salt bath 33, there is no time for the molten salt on the strip at rollers do to solidify before the strip reaches rinse bath 50 and roller 42 therein. If the salt were solidified in this interval, the strip would be scratched. Remaining molten, the salt lubricates rather than scratches the strip.

Actual dimensions, in inches, in one instance are here given, it being observed that the optimum results are obtained by as little time interval as possible between rollers 40 and 42:

Distance between rollers 40 and the surface of the rinse bath36-40"; speed of strip-l0-35 min.

It is noted that the strip is cooled before it reaches roller 42, which is rubber covered. This is accomplished by passing the strip through the bath 5!) before it reaches roller 42, or by providing water spray means ahead of roller 42.

For preventing the products of combustion of the burners 34 from coming in contact with the surface of the salt bath 33, the outlets of the burners 34 are connected through outlet pipes 10 and a manifold ill to the vent 64, so as to be exhausted without coming in contact with the molten salt bath 36.

It has been discovered that if the products of combustion in the burners 34 come in contact with the surface of the salt bath 33, there are formed carbonates which deposit on the rolls 31 and 38 and cause marring of the surface of the strip. The provision of the outlets l0 and II for these products of combustion eliminates such carbonates and eliminates this cause for marring the strip.

The salt bath is as close to the furnace as possible so as to reduce the heat loss in the strip as it passes from the furnace to the salt bath, and thus reduces the cost of fuel for maintaining both the strip and the salt bath at the operating temperature for the bath, whatever that temperature is.

It is also noted that the salt bath, generally below 900 F. in temperature, functions as a quench for the higher temperature steel strip and is located close enough to the exit end of furnace 20 as to receive the strip within two minutes from the time the strip leaves such furnace. Thus it functions to prevent carbide precipitation, when the strip is of stainless steel of the nickel chrome type. Such precipitation, an undesirable phenomenon, occurs when strip of stainless steel of the nickel chrome type is left at 900 F. or above for more than two minutes. The quench action of the salt bath occurs within two minutes because the bath is located so close to the furnace and prevents such carbide precipitation.

Summary We have here disclosed apparatus for producing cleaned and scratch-free surfaces on continuously moving strip, such as polished stainless steel strip. The process herein disclosed is the K1 process described in the aforesaid Patent 2,458,661.

This application is directed to one certain improvement, namely, locating the rinse bath as close as possible to the salt bath.

Now having described the invention of this application and the construction shown in the appended drawing, reference should now be had to the claims which follow.

We claim:

1. In apparatus for continuously cleaning metal strip, the combination of an atmospherically open tank containing molten alkali metal salt for cleaning metal strip continuously passed therethrough, an atmospherically open tank containing relatively cold water comprising a bath for quenching and washing off the molten alkali metal salt coated on the metal strip as it emerges from said molten salt bath, said tanks being disposed so closely contiguous in a series respectively containing said molten salt and quenching water-that saidstrip emerging hot from said molten salt bath with a-coating otmolten .salt thereon has inadequate time to cool-and carries itssalt coatingin its moltencondition. directly into the aqueous quench bath, a pair of tangential rollers mounted near the after end of said molten salt tank above the contiguous ends and liquid levels of both baths to squeeze off excess molten-salt carriedby said metalstr-ip passing through the nip of said rollers to dripthe-salt back-into said molten salt cleaning tank a, and simultaneously to guide said strip and thin molten salt film thereon directly into the quench water tank, and means to heat the rollers, metal strip and salt coating on both to maintain the salt coating molten as the strip passes from the molten salt bath, as it passes through the rollers and as it moves between the two baths.

2. The apparatus as defined in claim 1 wherein the means for heating the tangential rollers is a heat reflecting metallic vshield mounted above said rollers adapted to absorb heat radiated from the open molten salt bath and reflect the same upon the rollers.

defer-enemas Cited in the fileof this patent UNITED STATES PATENTS Number Name Date 783,569 Edlich Feb. 28, 1905 1,544,506 Tytus June 30, 1925 1,765,498 Naugle June 24, 1930 2,135,713 Higgins Nov. 8, 1938 2,166,583 Critten July 18, 1939 2,216,544 True Oct. 1, 1940 2,234,593 Fern Mar. 11, 1941 2,311,099 Tainton Feb. 16, 1943 2,311,139 Tainton Feb. 16,1943 2,458,661 Webster Jan. 111, 1949

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