US2674551A - Method of processing of steel strip continuously - Google Patents

Description

April 6, 1954 R. DuNLEvY ET AL METHOD OF PROCESSING OF' STEEL STRIP CONTINUOUSLY Filed Sept. 5, 1950 IN VE NT( )R RALPH DUN/ E V Y HAROLD FR/CK JOHN H SHOEMA/(ER ATTORNEY Patented Apr. 6, 1954 METHOD OF PROCESSING OF STEEL STRIP CONTINUOUSLY Ralph Dunlevy, Harold Frick, and John H. Shoemaker, Detroit, Mich., poration, Detroit, Mich assignors to Kolene Cor- Application September 5, 1950, Serial No. 183,215

l Claim.

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

This application relates to a method of cleaning metal strip continuously and generally relates to 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 Kl process and described in Patent No. 2,458,661 of January 11, 1949, to Hugh G. Webster and Clarence J. Falter.

Generally speaking, the method under `consideration employs molten alkali salts for cleaning metal strip continuously and includes the use of means for preventing directing and squeezeout 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 12l 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 rst step,

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 rst 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 bat-h temperature will be between 500 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 rst 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 d the strip from the nrst bath as the strip enters the second bath.

Inasmuch as the patent process of the foregoing named may readily be understood upon reference 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 The improvements The improvements of this application 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 specication and detailed description to be read in connection with the accompanying drawing.

In this drawing:

i Fig. 1 is a diagrammatic side View of a continuous strip treating apparatus, including a fur` nace, 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 i8 a continuously moving strip being treated continuously by the process of the aforesaid patent.

The strip is first heated in a furnace 2i! in order that its temperature 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, 1900c 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 I. 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.

The molten 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 be brought above the 4 molten alkali salt type. In the preferred proc'- ess, 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 311i are merely cylindrical chambers immersed in the salt bath 33 which house elongated ducts or pipes l5. Pipes l5 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 I6 therein through which the combustible gas is emitted and burned. The combustion gas is supplied froin any suitable source through a pipe 'l1 controlled in quantity by a valve 18, mounted thereon and it is mixed with air passed through a pipe l5 which joins pipe 'H both combustible gas and air mixing and passing into pipe '15, the air being in combustion supporting quantity as controlled by a valve S0 mounted on pipe T1. The waste gases after combustion within tubes 34 are led away through tubular ducts 10, each joining a tube 35i at right angles, and thence into a manifold duct 'H common to all which leads to stack 55 of the vent for ultimate disposal of the waste gases.

Disposed on horizontal axes 35 above the normal level 30 of the molten salt 33 are directing rollers Sil-38 which direct and move the strip i0 from roller El 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 Liri, 5, 46 which direct the strip under a water spray pipe 43 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 yPatent 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. rlhe 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.

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 31, 38, and 48. 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,

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 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 4l; for squeezing off excess molten salt from the strip Il) 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.

2. However, it was found necessary to provide means for heating the rolls 4l! 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 insulating 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 4l?, and the molten salt on such rolls acted as a lubricant to prevent scratching of the polished stainless steel strip by the rolls 4l).

In addition, as long as the salt was maintained molten at the rolls 4l), the formation and deposit of alkali carbonates and other crystalline materials on the strip l at the rolls 4l), and on the rolls All themselves, was inhibited and, thus, another cause for marring of the strip was eliminated. It was discovered that unless the salt was maintained molten at the rolls 4Q, then solid carbonates and other crystalline materials were deposited on such rolls and these caused the scratching of the strip. The provision of the insulating hood 50 operated in the manner above described to prevent the formation of these solid deposits on the rolls 4eand, 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 50 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 5l), 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 54 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 S4, 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 pos- 'sible 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 d2, which is rubber covered. This is accomplished by passing the strip through the bath 5E 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 3,3. The outlets of the burners 34 are connected through outlet pipes in and a manifold 'H 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 3l and 33 and cause marring of the surface of the strip. The provision of the outlets 'In and 1| 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 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.

M "Summary We have here disclosed apparatus for produc-,- ing cleaned andscratch-free surfaces on continuously moving strip, such as polished stainlessy steel strip. The process herein disclosed is the K1 process described in the aforesaid vPatent 2,458,661. The apparatus hereof containscertain improvements listed as follows:

(l) The use of cast iron surfaces for the holddown, directing, and squeeze-out rollers in the molten salt bath. v

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

V(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 combustioniof `the burners Vused 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 bat Now having described the invention of this 1 application and the construction shown in the 0 appended drawing, reference should now be had tothe claim which follows.

" We claim: n

"In the cleaning of metal strips by running continuous lengths thereof through a hot molten alkali metal salt in'an'atmospherically open bath,

saidV stripsbeing guided through said b'ath by at least one guide roller in contact with said molten salt and said metal strip, said molten alkali metal salt being reactive with carbon dioxide of the atmosphere above the surface of said bath to form insoluble 'abrasively solid carbonates therein tending to coat said guide roller in a manner to scratch and mar the clean metal strip in contact therewith, said bath being maintained in a molten state by combustion of gas in chambers' sub'- merged beneath the surface of said'molte'n salt bath, said gas comprising a carbonaceous gas forming substantial quantities of carbon dioxide in the gas resulting from the combustion thereof, the improvement comprising Venting all of the waste carbon dioxide containing combustion gases from said submerged combustion chambers to withdraw all of said 'gases away from the area of the molten salt bathv thereby to prevent contamination of said molten salt on said roller with insoluble abrasive carbonate.

y References cited in the me of this patent UNITED sTATEs PATENTS Number Name Date 131,681 Hill Sept. 24, 1872 279,580 Midgley June 19, 1883 505,132 Sague Sept. 19, 1893 42,092,139 Ramer Sept, 7, 1937 .2,254,864 Welstead Sept. 2, 1941 ,2,311,099 Tainton Feb. 16, 1943 .2,311,139 Tainton Feb. 16, 1943 Webster Jan. 11, 1949

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