US2674250A - Apparatus for processing of steel strip continuously - Google Patents

Description

April 6, 1954 R. DUNLEVY ET AL 2,574,250

' APPARATUS FOR PROCESSING 0F STEEL STRIP CONTTNUOUSLY Filed Sept. 5, 195o I NVE NTOR @4L/DH DUNLE VY d a HAROLD FR/CK l-, t. TAM

Patented Apr. 6, 1954 APPARATUS FOR PROCESSING OF STEEL STRIP CONTINUOUSLY Ralph Dunlevy, Harold Frick, and John H. Shoemaker, Detroit, Mich., assignors to Kolene Corporation, Detroit, Mich.

Application September 5, 1950, Serial No. 183,211

7 Claims. 1

This application relates to apparatus for cleaning metal strip continuously and generally relates to apparatus for continuously using a metal cleaning process of molten alkali salt bath type, a preferred but not necessarily the only example of which is the process known commercially as the Kl process and described in Patent No. 2,458,661. of January 1l, 1949 to Hugh G. Webster and Clarence J. Falter.

Generally speaking, the apparatus under consideration employs molten alkali salts for cleaning metal strip continuously and includes the use of means for preventing 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 l2" wide and known to the trade as strip, but also ribbon over l2" 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, copper may also be treated l by the process hereof.

The process 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 fil.

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 inthe second bath is a variable, depending on the metal being treated.

The temperature of the rst 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. lil-here the bath is of the preferred formula, as set forth in Patent No. 2,458,561, 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. rihis insures uniform oxide removal from the strip. it is well known that the presence of alkali (as from the first bath) on a me tal 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 rst 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 lt is known to employ long troughs with directing rollers to treat strip continuously. Such apparatus is here used for continuously using the process above described on continuously moving strip. However, certain improvements have herein been disclosed and will now be described.

The improvements The improvements of this application relate to the treatment of continuously moving strip. Such improvements are aimed particularly towards the utilisation 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 removbaths, 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 lthe .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 diagrammaticside 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 l a continuously moving strip being treated continuously by the process of the aforesaid patent.

The strip is irst heated in a furnace 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 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, 1900n F.-2200 F., are used. So heated the strip passes over a directing roller 2|.

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 f 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 2l 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 is 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 15 supply combustible gas which is burned in a series of small .burner flames disposed along the length of each pipe l5 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 ll controlled in quantity by a valve 18, mounted thereon and it is mixed with air passed through a pipe 19 which joins pipe l1, 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 10, each joining a tube 34 at right angles, and thence into a manifold duct ll 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 31-38 which direct and move the strip l 0 from roller 2| 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, 4B which direct the strip under a Water spray pipe 49 and through a water rinse bath 50 and an acid rinse bath 5l. 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 coa-ting 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 suiciently brief so as not to permit the acid to attack the metal o1' 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 hydrouoric acids.

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.

l. First is the improvement in the nature of the rolls 3l, 38, and 40. These rolls 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 3l, 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 uid 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 ssured rolls functioning in a man- 5 ner similar 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 4i) for squeezing oir 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 l 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 2 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 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 tting insulating im cover 60 over the tub 32 the heat radiated from the surface of the molten salt 33, heated by the heating means 34 necessary to maintain such salt molten, operated to maintain the salt molten,

even at the rolls 40, and the molten salt on such 1F rolls acted as a lubricant to prevent scratching of the polished stainless steel strip by the rolls 40.

In addition, as long as the salt was maintained molten at the rolls 4c, the formation and dei* posit of alkali carbonates and other crystalline materials on the strip I0 at the rolls 40, and on the rolls 40 themselves, was inhibited and, thus, another cause for marring of the strip was eliminated. It was discovered that unless the salt a closely tting 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 5e, 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 5A 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 (is prevent spotting due to the condensed vapors but 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 discoloration 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 5l.

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 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 combustion of the burners 34 from coming in contact with the surface of the salt bath 33. The outlets of the burners 3e are connected through outlet pipes 'l0 and a manifold 1I to the Vent 64, so as to be exhausted without coming in contact with the molten salt bath 33.

It has been discovered that if the products of combustion in the burners 34 comev 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 'lil and 'll 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 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 snob 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. The apparatus hereof contains certain improvements listed as follows:

(1) The use of cast iron surfaces for the holddown, 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 iiuid 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.

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:

l. In a metal strip cleaning apparatus for cleaning the surfaces of metal strip by passing the same through a liquid bath comp-rising a mol'- ten alkali metal salt, the combination of an atmospherically open tank containing abath of said molten alkali metal salt, a pair of tangential rollers mounted above the surface of said bath in a manner to support metal strip passing through said molten salt bath and squeeze eX- cess molten salt from the surfaces thereof, a metal shield overlying the entire length of said tank and said tangential rollers whereby to trap hot gases between said shield and the surface of said bath and reflect suflicient heat upon said rollers to maintain the salt on the surfaces thereof in its molten state while exposing the salt thereon to said hot gases, burners mounted submerged beneath the surface of the bath for heating the bath to maintain the same in a highly fluid state whereby the gases trapped beneath said shield above said bath would normally tend to be highly contaminated with carbon dioxide from the waste combustion gases reactive to form solid abrasive` carbonates in the salt film about said rollers, and vent pipes surrounding said burners mounted to receive and vent all of the waste combustion gases away from said cleaning apparatus.

2. In a metal strip cleaning apparatus for cleaning the surfaces of metal strip by passing the same through a liquid bath comprising a molten alkali metal salt, the combination of an atmospherically open tank containing a bath of said molten alkali metal salt, several guide rollers mounted in the upper portion of said tank, at least two of said guide rollers being partially submerged in the molten salt of said bath, each disposed near the inlet and outlet ends thereof respectively to guide the strip through the bath, and a pair of tangential rollers mounted above the surface of said bath in the outlet end of said tank in a manner to guide said strip and squeeze excess molten salt from the surfaces of said metal strip as it passes through the nip of said tangential rollers, whereby all of the rollers are exposed to the atmosphere above the bath and become coated with salt, a metal shield overlying the entire length of said tank and said rollers whereby to trap hot gases beneath said shield and the surface of said bath reflecting sufficient heat radiated from the bath upon said rollers to maintain the salt on the surface thereof in its molten state, burners mounted submerged beneath the surface of said bath for heating the bath to maintain the same in a highly iiuid state whereby the gases trapped beneath said shield above said bath would normally tend to be highly contaminated with carbon dioxide from waste combustion gases reactive to form solid abrasive carbonates in the salt nlm about said rollers, and vent pipes surrounding said burners mounted to receive and vent all of the waste combustion gases away from said cleaning apparatus.

3. In a metal strip cleaning apparatus for cleaning the surfaces of the metal strip by passing the same through a liquid bath comprising a molten alkali metal salt, the combination of an atmospherically open tank containing said molten alkali metal salt as a bath, several guide rollers mounted in the upper portion of said tank, at least two of said guide rollers being partially submerged in the molten salt of said bath each disposed near the inlet and outlet ends thereof respectively, and a pair of tangential rollers mounted above the surface of said bath in the outlet end of said tank in a manner to guide said strip and squeeze excess molten salt from the surfaces of said metal strip as it passes through the nip of said tangential rollers, whereby all of said rollers become exposed to the atmosphere above said tank and become coated with salt, a second tank mounted closely contiguous in a series with said molten salt tank whereby metal strip passed therethrough passes from the first tank directly into said second tank in a series, said second tank containing rinse and wash water for quenching and washing oii molten alkali metal salt coated on the metal strip as it emerges from said molten salt bath, said pair of tangential rollers being mounted high enough above the liquid levels of both baths to act as a single metal strip support and guide means for both tanks, adapted to squeeze excess salt off of said strip passing through said tangential rollers said salt being allowed to drip back into said molten salt tank and said metal strip with a thin even molten salt film thereon being guided directly into said aqueous quench tank, a metal shield overlying the entire length of said molten salt tank and said rollers, a vent entirely overlying said aqueous quench bath in contact with said metal shield, whereby to trap hot gases above both baths beneath said shield and vent, said shield portion being adapted to absorb heat radiated from the molten salt bath and reflect the same upon said rollers and metal strip coated with the molten salt to maintain the salt on the surfaces thereof in its molten state while exposing the salt thereon to said hot gases, burners mounted submerged beneath the surface of said bath for heating the bath to maintain the same in a highly fluid state whereby the gases trapped beneath said shield would normally tend to be contaminated with carbon dioxide from the waste combustion gases reactive to form solid abrasive carbonates in the salt film about said rollers, and vent pipes surrounding said burners and mounted to receive and pass all of the waste combustion gases to the outlet of the vent above said quench water tank and thence away from said cleaning apparatus.

4. An apparatus as defined in claim 3 wherein the molten salt bath contains an oxidizing agent and the rollers are all formed of cast iron containing graphitic carbon reactable with the oxidizing agent of the bath to remove graphitic carbon from the surfaces of said rollers and form fissures therein in which a molten salt film is entrained about the surface of each roller to act as a lubricating film in non-abrasive guiding and supporting contact with said metal strip.

5. In a metal strip cleaning apparatus for cleaning the surfaces of metal strip by passing the same through a liquid bath comprising molten alkali metal salts containing an oxidizing agent reactable with graphitic carbon, the combination of an atmospherically open tank containing a bath of said alkali metal salts, several guide rollers mounted in the upper portion of said tank, at least two of said rollers being partially submerged in said molten salt bath each disposed in forward and after ends thereof respectively to guide metal strip through the bath, and a pair of tangential rollers mounted above the after yend of the molten salt level of said bath in a manner to squeeze excess molten salt from the surfaces of the metal strip passing through the nip thereof, whereby all of the rollers are exposed to the atmosphere above the bath and become coated with salt, said rollers being formed of cast iron having a normal content of graphitic carbon which has been ssured by reaction with the oxidizing component of said bath and removal of carbon from said fissures and entraining in said fissures the molten salt of said bath as a film about each roller in guiding and lubricating contact with said metal strip while exposing said molten salt film to the atmosphere above said bath, a metal shield overlying the length of said tank and said rollers whereby to trap hot gases between said shield and the surface of said bath, absorb heat radiated from the bath and reflect sufficient heat upon said rollers to maintain salt on the surfaces of said bath for heating the bath in its molten state exposed to said hot gases, burners mounted submerged beneath the surface thereof to maintain the same in a highly fluid state whereby the gas trapped beneath said shield above said bath would normally tend to be contaminated With carbon dioxide from the waste combustion gases reactive to form solid abrasive carbonates in the salt film about said rollers, and vent pipes surrounding said burners mounted to receive and vent all of the waste combustion gases away from said cleaning apparatus.

6. In apparatus for producing cleaned surfaces on metal strip of the type comprising a molten alkali salt bath, a water rinse bath, and an oxide removing bath, arranged in succession and through which metal strip passes continuously in the order named, with said salt bath being of a nature that operates to transform impurities on the surface of the metal strip into easily removable oxides, said bath also being of a nature that removes graphitic carbon from cast iron, and with said oxide removing bath containing means for chemically removing the oxides formed in the salt bath, that improvement which comprises rolls in the salt bath against and below which the metal strip passes for moving the strip through such bath, these rolls being of cast iron and being at least partly immersed in the molten salt, roller means at the exit end of the bath for removing excess molten salt from the strip comprising cast iron roller means Well above the molten salt against which passes the strip, and means for maintaining the salt on the strip molten until at least after the strip passes such roller means and thus inhibit the formation of solid salts on the strip and l0 on the roller means which would scratch the strip leaving the bath, comprising means for heating the salt in the bath to maintain it molten and an insulating hood above the bath and hooding the roller means, whereby heat radiated from the surface of the salt bath will heat the roller means sufficiently to maintain molten the salt on the strip at such roller means, means for venting the water rinse bath to inhibit alkaline vapors therein from condensing and spotting the strip in or near the water rinse bath.

7. In apparatus for producing cleaned surfaces on metal strip of the type comprising a molten alkali salt bath, a water rinse bath, and an oxide removing bath, arranged in succession and through which metal strip passes continuously in the order named, with said salt bath being of a nature that operates to transform impurities on the surface of the metal strip into easily removable oxides, said bath also being of a nature that removes graphitic carbon from cast iron, and with said oxide removing bath containing means for chemically removing the oxides formed in the salt bath, that improvement which comprises rolls in the salt bath against and below which the metal strip passes for moving the strip through such bath, these rolls being of cast iron and being at least partly immersed in the molten salt, roller means at the exit end of the bath for removing excess molten salt from the strip comprising cast iron roller means Well above the molten salt against which passes the strip, and means for maintaining the salt on the strip molten until at least after the strip passes such roller means and thus inhibit the formation of solid salts on the strip and on the roller means which would scratch the strip leaving the bath, comprising means for heating the salt in the bath to maintain it molten and an insulating hood above the bath and houding the roller means, whereby heat radiated from the surface of the salt bath will heat the roller means sufficiently to maintain molten the salt on the strip at such roller means, means for venting the water rinse bath to inhibit alkaline vapors therein from condensing and spotting the strip in or near the water rinse bath, combustible fuel heating means for the salt bath, and means for Venting the outlet of such heating means to prevent the products of combustion from coming into contact with the salt surface and forming carbonates which would deposit on strip directing rollers in the bath and cause scratching of the strip passing through the bath.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 131,681 Hill Sept. 24, 1872 279,589 Midgley June 19, 1883 505,132 Sague Sept. 19, 1893 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 2,212,588 Csanyi Aug. 27, 1940 2,216,544 True Oct. l, 1940 2,234,593 Ferm Mar. 11, 1941 2,311,099 Tainton Feb. 16, 1943 2,311,139 Tainton Feb. '16, 1943 2,458,661 Webster Jan. 11, 1949

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