US2800420A - Method of cleaning metal strip continuously - Google Patents

Description

y 1957 R. DUNLEVY ETAL METHOD OF CLEANING METAL STRIP CONTINUOUSLY Original Filed Sept. 5, 1950 INVENTOR ATTORNEY Uited Sta es Patent '1 2,300,420 M H D F LEANl M TA S R CONTINUOUSLY R lph D e y, Ha a r k an 101 .H- Shoemaker,

Detroit, Mich., assignors to Kolene Corporation, DetroitMich'. H

Qrigin ai application September 5, 1950, Serial No.

' 183,211, nowlatent No. 2,674,250, dated April 6, 1954. Divided and this application March 1, 1954, Serial No. 413,393

2 Claims. (Cl. 13415) This application relates tomethod for cleaning metal strip'continuously and generally relates'to a method for continuously using a metal cleaning process of the molten alkali metal 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 N 2,458,661 of January 11,1949, Hii nowebster and Clarence I. Falter. A i ii Generally speaking, the apparatus and method under consideration of which only the method is herein claimed employs molten alkali salts for cleaning metal strip continuously and includes the use of means for preventing directing and squeeze-out rollers employed'in suchapparatus from marring or scratching thecleaned 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. i

It is understood that the term fstrip 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 nanbwa aifibwn to the trade as wire. I i I I It is also understood that while steel is the particular metal chosen for descriptive purposes herein, and is r nore 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 metal salt bath type wherein a first step is the immersion of the article being treated in a molten alkali metal salt bath at an operating temperature well above the melting point of thealk'ali 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'rnetal salt bath or it'may be a weak acid bath. w

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 m y 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 500 F. and 1100 F.

2,800,420 Patented July 23, 1957 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.

The apparatus scribedon continuously moving strip. However, certain improvements have herein been disclosed and will now be as ri s The improvements The improvements of this and our parent application having elaims directedto apparatus relate to the treatment of'continuously moving strip. Such improvements are aimed particularly towards the utilization of rollers for directing and moving strip continuously through the baths and to the use of means for preventing the deposit on such rollers of solid particles which would mar the finish of the strip as it passes by and engages such rollers.

It can readily be understood to those skilled in the art that the use of rollers for directing and moving strip continuously through molten salt baths, water rinse baths, and oxide removing baths, is a prerequisite. This application specifically relates to the apparatus and the use of such apparatus for preventing deposits on such rollers from marring the surface of the strip engaging and passing by such rollers.

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 speciiication 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 10 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 deter mined by the operating temperature required 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, 1900 F.2200 F.,

are used. So heated the strip passes over a directing roller 21.

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 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 21. 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 airwater-steam mixtures may also be used for cooling the Y 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.

The molten salt bath Next in line to the directing roller 21 is a molten salt 1 r 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;

i a series of small burner flames disposed along the length of each pipe 75 by forming a series of perforations 76; therein through which the combustible gas is emitted and burned. The combustion gas is supplied from any suitable source through a pipe 77 controlled in quantity by a valve 78, mounted thereon and it is mixed with air passed through a pipe 79 which joins pipe 77 both combustible gas and air mixing and passing into pipe 75, the air being in combustion supporting quantity as controlled by a valve 80 mounted on pipe 77. The waste gases after combustion within tubes 34 are led away through tubular ducts 70, each joining a tube 34 at right angles, and thence into a manifold duct 71 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 377-38 which direct and move the strip 10 from roller 21 down into the molten salt bath and then to a pair of rollers 40, 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 51. 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, 1 however, to be such as not to expose at thesurface other impurities not previously existent or not previously exposed and is sufliciently 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 (111se other acids, such as nitric or nitric hydrofluoric aci s.

Thus far we have described nothing more than a continuous strip treating process for carrying out the process of the aforesaid patent, such as might be developed by anyone skilled in the art. Now we turn to the improvements of this application.

1. First is the improvement in the nature of the rolls 37, 38, and 40. These rolls are not steel rolls, as are custornarily 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 37, 38, and 40, the steel rolls scratched the surface of the steel strip 10, 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 fromthe 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 7 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 similar to a porous metal roll. The molten salt clinging to the surfaces ofthese rolls acts as an excellent lubricant and prevents thei'r-olls from scratching the polished stainless steel strip passing by and engaging them.

Similarly, the rolls 40 for squeezing oif excess molten salt from the strip 10 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 sa-lt baths, provided they may 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.

2. However, it was found necessary to provide means for heating the rolls 40 so as to maintain the salt at such rolls in a molten and fluid condition. 'Obviously, any suitable heating means could be employed. However, it was observed that by providing a rather close fitting insu-lating cover 60 over the tub 32, the heat radiated from .the surface of the molten salt 33, heated by the heating at the rolls 40, the formation and deposit of alkali car- "bonates and other crystalline materials on the strip 10 at the rolls 40, and on the rolls 40 themselves, was inhibited and, thus, another cause for marring of the strip was elimi- -nated. It was discovered that unless the salt was main- I tained molten at the rolls 40, then solid carbonates and other crystalline materials were deposited on such rolls 7 and these caused the scratching of the strip. The provision of the insulating hood 60 operated in the manner above described to prevent the formation-of these solid deposits on the rolls 40 and, thus, prevented marring of the strip due to the presence of such solids.

3. Still another improvement is the venting of the water rinse bath 5 0 as by the provision of a closely fitting venting hood 64 connected to an exhaust fan or the like to outlet at 65. 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 50, condensed, 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 64 prevented such alkali vapors from condensing and coming into contact with the strip and settling on the strip and preventing 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.

4. Still another improvement 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 51.

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 50 before it reaches roller 42, or by providing Water spray means ahead of roller 42.

5. Still another improvement is in the provision of means for preventing the products of combusion 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 70 and a manifold 71 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 37 and 38 and cause marring of the surface of the strip. The provision of the outlets 7t) and 71 for these products of combustion eliminates such carbonates and eliminates this cause for marring the strip.

6. It is noted that 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 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 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 described is the K1 process described in the aforesaid 6 Patent 2,458,661. The apparatus hereof contains certain improvements listed as follows:

(1) The use of cast iron surfaces for the hold-down, directing, and squeeze-out rollers in the molten salt bath.

(2) The use of an insulated cover or hood for the salt bath at the squeeze-out rollers to maintain the molten salt fluid at such rollers.

(3) The use of a shield and ventilator for the water rinse immediately following the molten salt bath.

(4) The outletting of the products of combustion of the burners used for heating the molten salt bath away from the salt bath so as to prevent these products from coming into contact with the salt bath and forming carbonates and the like to deposit on the rollers and cause scratching of the strip.

(5) The locating of the salt bath as close as possible on the one hand to the strip heating furnace, and on the other hand to the water rinse following the salt bath.

This is a divisional application of our copending application Ser. No. 183,211 filed September 5, 1950, now Patent 2,674,250, issued April 6, 1954.

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

We claim:

1. The method of continuously cleaning metal strip comprising continuously passing said strip through a molten bath of an alkali metal inorganic compound reactive with the surface of the metal and impurities therein; whereby the cleaned strip emerges from said bath with a molten film coating of said alkali metal compound, squeezing the excess of said molten compound from said strip as it emerges from said fused compound bath While maintaining the strip hot by reflecting thereon heat radiated from said bath to prevent cooling of said strip and simultaneous congealing of said molten compound film thereon and guiding said strip into an aqueous quench bath to quickly cool the hot strip and dissolve said compound film, said aqueous quench bath being mounted closely contiguous to said molten compound bath whereby said strip heated as it passes between the two baths enters said quench bath at a rate timed to maintain its molten compound film in a liquid state.

2. The method of cleaning metal strip comprising passing the same through an atmospherically open hot molten bath comprising an alkali metal hydroxide by guiding the same through said bath over the surfaces of rollers mounted to be at least partially exposed to the hot gases above said bath, whereby the strip emerging from said bath tends to cool and freeze the molten caustic film thereon in contact with said guide rollers, heating said guide rollers and strip as it leaves the bath by heat radiated from said bath and reflected upon said rollers and strip to maintain the molten caustic film thereon molten, and immediately washing and quenching the hot molten caustic coated strip in a bath of water before the strip and its molten caustic film thereon has frozen, said molten bath being heated by combustion of gases in chambers beneath the surface thereof to produce a substantial content of carbon dioxide in the waste gases tending to react with the atmospherically exposed molten alkali metal hydroxide coated upon the hot metal strip and rollers to form abrasive carbonates thereon, and venting all of said waste combustion gases away from the system to prevent contamination of the atmosphere above the tank with carbon dioxide.

References Cited in the file of this patent UNITED STATES PATENTS 1,392,781 Marsh Oct. 4, 1921 2,234,593 Ferm Mar. 11, 1941 2,259,277 Theiss Oct. 14, 1941 2,334,177 Dishauzi Nov. 16, 1943 2,717,845 Carter Sept. 13, 1955

Claims (1)

1. THE METHOD OF CONTINUOUSLY CLEANING METAL STRIP COMPRISING CONTINUOUSLY PASSING SAID STRIP TROUGH A MOLTEN BATH OF AN ALKALI METAL INORGANIC COMPOUND REACTIVE WITH THE SURFACE OF THE METAL AND IMPURITIES THEREIN; WHEREBY THE CLEANED STRIP EMERGES FROM SAID BATH WITH A MOLTEN FILM COATING OF SAID ALKALI METAL COMPOUND, SQUEEZING THE EXCESS OF SAID MOLTEN COMPOUND FROM SAID STRIP AS IT EMERGES FROM SAID FUSEDCOMPOUND BATH WHILE MAINTAINING THE STRIP HOT BY REFLECTING THEREON HEAT XXDIATED FROM SAID BATH TO PREVENT COOLING OF SAID STRIP AND SIMULTANEOUS CONGEALING OF SAID MOLTEN COMPOUND FLIM THEREON AND GUIDING SAID STRIP INTO AN AQUEOUS QUENCH BATH TO QUICKLY COOL THE HOT STRIP AND DISSOLVE SAID COMPOUND FILM, SAID AQUEOUS QUENCH BATH BEING MOUNTED CLOSELY CONTIGOUS TO SAID MOLTEN COMPOUND BATH WHEREBY SAID STRIP HEATED AS IT IS PASSES BETWEEN THE TWO BATHS ENTERS SAID QUENCH BATH AT A RATE TIMED TO MAINTAIN ITS MOLTEN COMPOUND FILM IN A LIQUID STATE.

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