US3174491A - Molten salt spray apparatus for descaling stainless steel - Google Patents

Description

J- A. FALER March 23, 1965 MOLTEN SALT SPRAY APPARATUS FOR DESCALING STAINLESS STEEL Filed Oct. 23, 1963 R m 2 m G W T F E T L L "I. N m M m m M A A n m m E s E m 7 T s M 3 s 7 A 2 4 7 3 3 X I r a 9 6 2 L 4 Q a 3 Q w x14 \1 1/ JOHN A. FALER ATTORNEY United States Patent 3,174,491 MULTEN SALT SPRAY APPARATUS FOR DESALKNG STAKNLESS STEEL John A. Faler, Detroit, Mich assignor to Knlene Corporation, Detroit, Mich. Filed Oct. 23, 1963, Ser. No. 318,2% 2 Claims. (Ci. 134-1492) Stainless steel has been cleaned successfully in immersion baths. See Dunlevy UnitedStates Patent No. 2,635,- 062, of April 14, 1953, and Webster United States Patent No. 2,458,661, of January 11, 1949. The cleaning operation removes oxides and gives an excellent finish to the material, bright, high bufling quality, and high corrosion resistance.

This operation is more than a cleaning operation; it is a process which conditions the stainless steel strip by oxidizing scale, after which the thus oxidized scale may then be processed in accordance with the process of United States Patent No. 2,458,661 or any other process as, for example, through acid pickling, etc.

However, it was found that for various reasons, scratching and mar-ring of the strip occurred, explained because of the presence of insoluble particles on the strip and also because of the transverse movements of the strip on the metal'rolls. Rolls are needed to guide the strip through the bath, and metal has to be used for the rolls.

In addition to scratching and marring of the strip because of particles on the strip and transverse movements of the strip under metal guide rolls, or over metal guide rolls, another important fact to consider is that the more rolls used for starting strip under or over the metal guide rolls, the greater the amount of tension. This results in an actual elongation of strip under some conditions, or a certain amount of distortion, sometimes causing socalled cross breaks which run across the surface of the strip, or irregularities lengthwise, such as depressions or raised surfaces due to this tension. It is desirable to eliminate rolls that are used to guide strip and, theoretically, so the pass line could be straightwith no dipping into any tanks, Whether itbe salt, Water, acid or other solutions.

As a result, the immersion bath process is limited, by commercial factors, to intermediate grades of stainless steel. When it came to cleaning of very high quality stainless steel, the immersion bath process has not been widely adopted and instead, processors for these higher grades of stainless steel continue to resort to other'and older methods of cleaning, such as acid treatment.

The immersion process also was not widely adopted for very high quality stainless steel, nor extremely light gauges of stainless steel (such as below .010), particularly finished product where no additional cold rolling is used.

Hence, there has been developed and here disclosed a new process .for cleaning stainless steel, and this process is the subject matter of this application. In the process, the stainless steel instrip form comes out of an annealing oven and into a spray oven or muffle or box where it is cleaned or oxidized by molten salt spray and then emerges into the customary treatments such as water rinsing, quenching, dilute acid pickling, rinsing, and then rewinding. The novelty here is the spray oven and what happens in the spray oven where molten salt spray is thrown onto the strip as it leaves the annealing furnace.

The spray mufiie or oven under consideration comprises an enclosed spray zone which in practical structural form may be considered as a box into which the hotstrip enters at one end from the annealing furnace and from which the oxidized or cleaned strip emerges at the other end, thence passing into a rinsing zone which may be a rinsing bath or water spray.

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The oxidizing oven or spray box, as we may call it, receives the hot steel stripfrom the annealing furnacexand discharges it to the rinsing bath. In the oxidizing oven, there are no rolls to be engaged by the moving strip but the strip is moved freely under tension through the oven, entering a slot in one end and leaving through a slot in the other end and at no time in theoxidizing oven is'the strip in contact with any rolls or supports or guides.

The strip is not in contact with metal rolls as it passes through the spray box. Hence there is no electrolytic or no galvanic action, as would be encountered in an immersion bath containing salt and metal rolls over which a metal strip passes.

The hot steel strip enters at a temperature of about 1000 F. and emerges at about the same temperature, there being no substantial drop in temperature of the strip until it enters the water rinse and quench. It is important to maintain the strip at substantially the same temperature all the way through the oxidizing oven. As long as the strip is at high temperature, in the presence of a moiten salt spray, the oxidizing action will take place without the formation of any solid particles that might otherwise abrade or scratch the strip. The strip enters hot and is maintained at substantially the same temperature all the way through the oxidizing oven.

The oxidizing action is obtained by subjecting the strip and the atmosphere within the oven to a molten salt spray, such as Kolene No. 1, see United States Patent No; 2,458,- 661, for example, and the spray is formed byatornizing the molten salt, using large quantities of an inert gas such as superheated steam at about 1000 R, which passes into nozzles, into which is fed in very tiny quantities and in small increments the molten salts which, at that point, is at a temperature of1000 F. The exact temperature is not critical, except that it exceed the melting point of the salt spray, about 550 F., a substantial range being permitt'ed.

The-interior of the oven is also maintained charge'diwith an atmosphere of superheated steam coming from different outlets from the steam super-heater. In other words, some of the steam goes out through steam nozzles into the atmosphere for heating the atmosphere and filling it full of steam, and another part of the steam goes through the atomizing nozzles.

The steam maintains a hot steam atmosphere in the oven so that in the event of a possibility of some of the salt coming through in unatomized condition from the nozzles, then this salt will be carried around by the steam and be maintained molten by the steam so that at no point can any solidified salt particle engage the strip as it goes through the oven.

Another important reason for loading the oven atmosphere with steam is to prevent the possibility of any gas, such as air, entering the oven. Steam is inert tothe salt. Other atmospheres, such as air, would react with the salt and form solid particles which would impair the operation. By using steam, or similar inert gas, we insure against anything else coming into the oven, and-the steam coming in under superheated conditions seals the slots through which-the strip is passing and prevents air from entering the oven at such slots. Otherwise, the oven is maintainted sealed whenever possible, and even where there are breaks in the seal, the fact that "the oven is loaded with steam seals these breaks against the entrance of air into the oven from outside.

The pressure of 1000 F. steam is greaterthan thepressure of the atmosphere outside of the oven. It seems that when atmospheric air comes in contact witha heated steel surface, certain higher forms of oxides are promoted. Since it is a desire and intention, by the use'of'the molten salt spray process, to form no oxide or a form of oxide more easily soluble, it would appear that, since the pressure of steam does not allow atmosphere to enter the oven, while the strip is hot, the lower forms of oxide formed in the presence of steam with the exclusion of air is more desirable, and this is another attribute of the molten salt spray process.

In addition, to insure the temperature within the oven remaining at the desired 1000 F., and in order to provide a safety factor in the event the steam does not function, additional heating means may be employed to maintain the atmosphere at the desired temperature. Such additional or auxiliary heating means may or may not be used in the actual operation from time to time, but is initially provided as equipment so that it is available when needed. This additional means could comprise combustion burners for discharging large quantities of hot combustion vapors into a jacket around the oven, and such jacket is sealed from the oven so that no combustion vapors can enter the oven, and the oven is maintained free of any atmosphere except inert gas, such as steam.

The auxiliary heating means, of course, could be electric heaters or any other type of heater, just so long as it heats from the outside and does not discharge any harmful vapors into the oven. If air or combustion vapors were to enter the oven, harm might result. The air or vapors contain CO and because CO by reaction with the salt may form carbonates (CO and because these carbonates are solidified, there might result a plugging of the nozzles through which the molten salt is atomized and, also a marring of the strip by the solid particles.

The superheated steam moves the atomized salt spray with very high velocity onto the strip and by so doing, concentrates the scouring and chemical action of the molten salt onto the strip and, thus, reduces possible wastage of molten salt. It is estimated that at least 90% of the molten salt is actually used in the reaction that takes place of the salt on the scale and converts it into oxides, and no more than is unused in the process.

The salt spray also has a scale loosening, scouring, or scrubbing action. Impinging on the strip, it facilitates oxidizing of the scale.

At the discharge end, and preferably at the lowest point of the discharge end, there is a condenser through which is exhausted steam and excess salt vapors. The condenser removes and exhausts the oven, which is constantly receiving a supply of fresh salt and fresh steam, and at least 90% of the salt enters into the chemical change.

The system operates in such a way that any scale or sludge formed is removed in the form of vapors suspended in the steam and exhausted through the condenser and the exhaust system.

The dragout losses are extremely small here because instead of having a molten salt film of excess amounts on the strip, as is conventional with bath type processes, such excess molten salt film being needed to lubricate the strip as it passes over the rolls, here we use only as much salt as is absolutely necessary to create the desired reactions and, hence, the dragout loss is reduced to an absolute minimum.

The fact that the salt approaches the strip in atomized condition, and under high pressure, creates an almost instantaneous or flash-type conversion of the oxide which means that the reaction takes place much quicker and is believed to result in a greatly improved chemical reaction. It is well known that a fast reaction in small increments is often highly superior in its chemical action to a slow, prolonged reaction.

Not only is there a saving in salt because of the reduced wastage due to dragout losses, but also a considerably greater amount of the salt enters into the reaction and gives a considerably greater degree of cleaning action, or oxidizing action, than might otherwise be obtained where a strip passes through a bath. Here, because of the 4 atomized condition of the salt impinging upon the strip under high pressure, there is a highly improved cleaning action as contrasted with the action that takes place in immersion baths.

The small amount of salt that is melted for use in the molten salt spray process hereof is always free of buildups of complex metallic impurities and metallic salts such as chrome oxides, iron oxides, nickel oxides, manganese oxides, titanium oxides and others, as might be found in immersion baths. While, to the best of our knowledge, no complete analysis has been made as to how the buildup of these metallic salts affects descaling in salt baths, we are reasonably sure they do have a deleterious effect and that they impede the reaction.

The salt may be anhydrous. It might also contain some water; or it can be a concentrated solution which is heated, pressurized and superheated before it is sprayed onto the strip. In such case, flashing off the water allows the salt to impinge as usual on the metal in anhydrous form.

Now having described the process hereof, reference is had to the appended drawings showing a typical apparatus.

In these drawings:

FIG. 1 shows diagrammatically one form of such apparatus in side view, with parts cut away for purposes of clarity.

FIG. 2 is a section on line 22 of FIG. 1.

The drawing shows an insulated steam box, mutfie, or oven 10, supported on legs 11 and having entrance and exit slots 12-14. The metal strip 20 being cleaned enters slot 12 after leaving an uncoiled means not shown and a pre-spray heating or annealing means 16, and leaves slot 14 on its way to rinsing means, acid treatment means, heating means, recoiling means, etc., also not shown. In the straight through pass in chamber or housing 10 from slot 12 to slot 14 the strip 20 is processed by the process of this application.

The strip is supported outside the box 10 by rollers 21, but is supported inside the box, not by contact with rollers therein, but by its being under tension between rollers 21.

The box It is inclined as shown so that products formed or deposited in the box collect at a drain or discharge point 22 from where they are removed by a suitable removal means, such as a condenser not shown, including an exhausting fan.

Solid salt, in the instance here shown is fed into hoppers 24 and passes into melting chambers 26 where it is melted by steam admitted from outside the box 10 through a steam line 27, a steam manifold 28, and pipe terminals 30. In the steam lines connecting manifold 28 to terminals 30 are siphon spray nozzles 32 which siphon the molten salt from chambers 26 and spray it onto strip 20 immediately after the strip leaves annealing means 16, as soon thereafter as possible. The timing is such that when the strip reaches the salt spray nozzles, the strip is at a maximum annealed temperature; but by the time it leaves exit slot 14 and passes out of the steam atmosphere and into the room air atmosphere, it has cooled down to a point where it is not seriously affected by room air.

Additional steam nozzles or lines 34 admit steam into the box to spray live steam into the box to form a hot steam atmosphere therein at above atmospheric pressure.

Inside box 10 is a steam chest 36 supplied by lines 37 which maintains a live steam heated wall inside the box.

The following example illustrates the practice of this invention:

Example 1 A salt composition comprising 2 parts by weight of caustic soda, 1 part by weight sodium nitrate and 0.3 part by weight of sodium chloride are heated with steam at 1000 F. to fusion, picked up as an atomized liquid, and sprayed in a high pressure blast upon both sides of a stainless 8-18 chromium nickel steel strip as it leaves an annealing oven maintained at 1500 F. the metal strip moving at a rate of about 1 ft. per minute and slowly cooling from the annealing temperature to the steam cleaning temperature. The strip is then sprayed with steam alone to clean off any adhering molten salts. The strip, as it leaves the steam cleaning chamber box 10, as described above, is finally quenched with a spray of steam at a temperature of 250 F. At no point in the continuous passage from the annealing oven through the salt spray and quench is the steel strip contacted with rolls. It is finally dipped in a dilute 5% hydrochloric pickling bath as a bright dip to remove the readily soluble lower oxide scale formed thereon in the salt spray oven, and is finally dried in air as clean, bright stainless-steel metal.

Now having described the embodiment here shown, reference should be had to the claims which follow.

I claim: 1. In the continuous spray descaling of metal strip With molten salt;

a descaling device comprising a housing enclosing a chamber; inlet and outlet slots in opposite ends of said housing through Which hot metal strip to be descaled may enter and leave While passing through said chamber Without marring contact With the walls; means outside said chamber for supporting and passing said metal strip entering and leaving said chamber through said inlet and outlet slots;

the length of the chamber being such, in relation to the Weight and stifiness of the metal strip and its speed of passage through the chamber and the location of the strip supporting means, that the strip is out of contact with the surfaces of the slot;

the chamber being free of members which might engage or make contact with such strip in its normal passage through the chamber;

a supply of steam under pressure;

a supply of molten salt;

and nozzles in said chamber connected to both supplies, steam and molten salt, for atomizing and spraying molten salt on to the strip passing therethrough and for filling said chamber with steam under pressure to seal the inlet and outlet slots against entrance of outside air into said chamber.

2. Apparatus according to claim 1 including steam spray nozzles Within the chamber near the outlet slot, connected to the steam supply, for spraying salt-free steam on the strip as it approaches the outlet slot.

References Cited by the Examiner UNITED STATES PATENTS 1,818,041 8/31 Chapin 68-54 1,907,429 5/ 33 Masland 685 .4 X 2,372,599 3/45 Nachtman.

CHARLES A. WILLMUTH, Primary Examiner. GEORGE J. NORTH, Examiner.

Claims (1)

1. IN THE CONTINUOUS SPRAY DESCALING OF METAL STRIP WITH MOLTEN SALT; A DESCALING DEVICE COMPRISING A HOUSING ENCLOSING A CHAMBER; INLET AND OUTLET SLOTS IN OPPOSITE ENDS OF SAID HOUSING THROUGH WHICH HOT METAL STRIP TO BE DESCALED MAY ENTER AND LEAVE WHILE PASSING THROUGH SAID CHAMBER WITHOUT MARRING CONTACT WITH THE WALLS; MEANS OUTSIDE SAID CHAMBER FOR SUPPORTING AND PASSING SAID METAL STRIP ENTERING AND LEAVING SAID CHAMBER THROUGH SAID INLET AND OUTLET SLOTS; THE LENGTH OF THE CHAMBER BEING SUCH, IN RELATION TO THE WEIGHT AND STIFFNESS OF THE METAL STRIP AND ITS SPEED OF PASSAGE THROUGH THE CHAMBER AND THE LOCATION OF THE STRIP SUPPORTING MEANS, THAT THE STRIP IS OUT OF CONTACT WITH THE SURFACES OF THE SLOT; THE CHAMBER BEING FREE OF MEMBERS WHICH MIGHT ENGAGE OR MAKE CONTACT WITH SUCH STRIP IN ITS NORMAL PASSAGE THROUGH THE CHAMBER; A SUPPLY OF STEAM UNDER PRESSURE; A SUPPLY OF MOLTEN SALT; AND NOZZLES IN SAID CHAMBER CONNECTED TO BOTH SUPPLIES, STEAM AND MOLTEN SALT, FOR ATOMIZING AND SPRAYING MOLTEN SALT ON TO THE STRIP PASSING THERETHROUGH AND FOR FILLING SAID CHAMBER WITH STEAM UNDER PRESSURE TO SEAL THE INLET AND OUTLET SLOTS AGAINST ENTRANCE OF OUTSIDE AIR INTO SAID CHAMBER.

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