DE1941469A1 - Method and device for descaling stainless steel - Google Patents

Description

Mitsubishi Jukogyo Kabushiki Kaisha- ti. in""""*

I 4th AUb. 1969

Tokyo, Japan

Method and device for descaling stainless steel

The invention relates to the descaling of steel, and more particularly relates to an improved method and method improved device for descaling stainless steel by annealing and pickling in a solution of an alkali metal compound, according to the German patent (patent application 19 21 305.9).

The inventive method is particularly suitable for continuous process in which steel in strip, strip or sheet form passes through a system in which it is is descaled.

In the process of manufacturing stainless steels If the process steps of rolling and wire drawing are usually followed by the AP operations (annealing and Pickling) designated process steps in which annealing and pickling are carried out in order to achieve the called scale, formed during annealing Remove oxide film. Pickling is usually done after an electrolytic process using a strong inorganic acid or a neutral salt or with a molten alkali system, in which the metal in a

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Molten bath is immersed in molten alkali, creating the scale without impairment-the steel body or the The base metal is removed and as a result a product with an excellent surface quality in a comparatively short time is delivered. The descaling is done by a Post-treatment completed, in which the steel body is briefly immersed in an acid bath, which is his Surface properties are further improved. The molten alkali treatment has therefore been used in the production of various Steel grades found widespread use, in particular because they accelerate the glow and Pickling operations permitted.

During the molten alkali treatment, the steel body from which the Zunda * is to be removed is immersed in a melt of mixed alkali metal hydroxides and / or salts, the main component of which is caustic soda. The melt is in the molten state at a temperature of about 400 - "held ■ 500 ⁰ C. It has been found, however ,.! that insufficient descaling can occur if the steel is simply immersed in the molten caustic soda bath. For this reason, it has proven to be required * to enrich the caustic soda bath with additives are · Two such decision 'zunderungsverfahren depending on the caustic compounds added as the "sodium-method" or the "Hooker traverse en ¹¹ indicates. When first mentioned method, the ^{steel body, referred to below as “workpiece 11} , which is to be descaled, is immersed for a certain period of time in a caustic soda melt which

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about 370 C and which is 1.5-2 wt .- # sodium hydride (NaH) are added as a reducing agent. With this one

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In the process, the scale component EeO-Cr ₂ O, which is characteristic of chromium steel, is converted into iron and chromium oxides that are easily soluble in acid, as indicated in equation (1) below, and is then washed with water or briefly in sulfuric acid, nitric acid, hydrochloric acid or dipped hydrofluoric acid to complete the descaling.

PeO-Cr ₂ O ₅ + 2HaOH "= Pe + 2 CrO + 2UaOH. (1)

In contrast, the workpiece during Hooker process is immersed in a Ätzalkallbad, maintained at a temperature of 400 - 500 ⁰ C is maintained and 5 - 40 wt .- ^ contains sodium nitrate as an oxidizing agent. The scale component is thereby, as indicated by equation (2) is converted into soluble salts. "The soluble salts are dissolved and the steel then subjected to a! Post-treatment consisting of a washing with an acid" may consist, zn to the Entzianderung

2 (PeO-Cr ₀ O ^.) + 14UaOH + 7UaUO ₃ - 2UTa-JPeO. * +

ά j j 5 5 ζ. -τ

-7UaUO ₂ . '(2)

Another modification of the "Sehmel Alkaline Treatment ^11" requires the descaling to be carried out while oxygen or air is blown through the melt of the alkali metal hydroxide, which is preferably caustic soda.

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Any process where caustic soda or another alkali metal hydroxide or salt is the primary descaling agent is applied, the alkali metal compound is always applied as a molten bath or melt and the steel workpiece is immersed in the melt. Such an approach is from the practical state - * - This is disadvantageous because there are difficulties in preparing the melt from the alkali metal compound, which require a considerable amount of equipment and facilities. Also, handling and Difficult to operate, and if special care is not taken, this mode of operation can become difficult for the operating personnel, absolutely dangerous. The main patent ".. * .." (Akt.-Z. P 19 21 305.9) describes a process for descaling of chromium-containing steel described and claimed, in which the surface of the steel to be descaled with a aqueous solution of an alkali metal compound is wetted, which is with the. Scale in the presence of oxygen under Formation of a layer soluble in acid or water Alkali metal compound can implement what the so coated steel is exposed to a temperature above the melting point of the alkali metal compound in the presence of an oxygen-containing atmosphere and the - The steel surface is then washed.

According to a preferred embodiment of this method the wetting is done while the steel is still is hot. It has now turned out that this Applying method specifically to the descaling of stainless steel that has been annealed. ,

In contrast, the invention creates a method for removing

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change of stainless steel, its peculiarity in it consists in first annealing the steel, then the surface of the steel from which the scale is to be removed should, with an aqueous solution of an alkali metal compound according to patent ....... (German patent application P 19 21 305.9) which is wetted with the tinder in the presence of oxygen to form a coating that is soluble in water or acid, the so coated Steel while it is still warm from the annealing treatment, is exposed to an oxygen-containing atmosphere and finally the steel surface is washed.

The alkali metal compound used is preferably a Alkali metal hydroxide, such as caustic soda or caustic ^ _Jtali. It It is also possible to use an aqueous solution of an alkali metal salt which, by hydrolysis, is caustic soda or caustic potash capable of forming, for example orthosilicate, nitrite, nitrate, chlorate or carbonates of sodium or potassium. Bas wetting the stainless steel with the alkali metal compound The solution containing it is used to form a thin layer of the solution on the steel, whereupon the water due to the high temperature of the steel with the formation of a thin alkali layer adhering to the metal surface evaporates. This layer is melted and, when it comes into contact with oxygen, causes descaling according to the following Equation (3):

2 (FeOOr ₂ O ₅ ) + HNaOH + 3JrO ₂ = 2Na ₃ FeO ₅ + 4Na ₂ CrO ₅ + 7H ₂ O.

When using an alkali metal salt such as sodium chlorate, instead of an alkali metal hydroxide, the salt is passed through

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Pyrolysis with release of oxygen and chlorine according to follow-up Equation (4) converted to caustic soda:

2NaGlO ₅ + H ₂ O = 2NaOH + 2 ^ O ₂ + Cl ₂ (4),

In this lall the Entzunderurigs reaction can because of that oxygen formed by the decomposition of the chlorate in an oxygen-free atmosphere or in a low one Oxygen concentration containing atmosphere take place.

Although when using sodium nitrate or sodium nitrite, as used in the Hooker method, a similar one Descaling reaction takes place, enables the joint Using sodium nitrate with an alkali metal hydroxide, such as caustic soda, provides a much faster process descaling than in the Hooker process, which indicates the combined effects of the internal oxidative effect the oxidizing agent and the effect of oxygen in the atmosphere surrounding the workpiece.

The descaled stainless steel surface can then with water or with a mild acid, for example an aqueous solution of nitric acid, sulfuric acid, hydrochloric acid, or hydrofluoric acid, to complete the descaling process complete.

In a further embodiment, the invention creates a device for surface treatment of stainless steel, which is characterized by an annealing furnace, a device for cooling the surface of the annealed steel, a device for wetting the cooled steel surface with an aqueous solution of an alkali metal compound, a facility for filling the

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Surface of the steel sprayed in this way, all in a row are arranged, wherein also a conveyor device for passing the steel through the device is provided"

The following is the invention in preferred embodiments explained in more detail with reference to drawings. It demonstrate:

Pig. 1 is a schematic representation of an annealing apparatus used in conventional methods and pickling of stainless steel and

Fig. 2 is a schematic representation of an apparatus performing as sur ¹ Treatment of stainless steel according to the inventive method.

According to Pig. 1, a strip 1 of stainless steel is fed in the indicated direction from numeral 1 towards numeral 12 through an annealing and pickling treatment device which is an ordinary annealing and pickling plant. The steel strip 1 first runs through an annealing furnace 2, in which it is heated to a temperature of about 1100 ° C., and then runs through a cooling device 3 t called a "spray quenching device" in which water is sprayed onto the steel surface to its temperature to about 200-300 ⁰ C under the temperature of a molten alkali

4 to lower that a mixed alkali salt with caustic soda and

5 - 40 wt .- # sodium nitrate and through a heater 5 to a temperature of 400 - 500 ⁰ C is maintained. The steel strip is drawn with the help of pulling rollers 6 and stripping rollers 7 «

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with the help of which excess salt adhering to the steel strip is removed, pulled through the molten salt 4. The rollers are generally made of iron because: they are immersed in the high temperature, molten salt or are in contact with it. The steel strip then runs through a washing device 8, in which water is sprayed onto the surface of the strip in order to remove any salt still adhering to the strip *. The steel strip then passes through a pickling tank 9 which contains an aqueous solution of 15% by weight nitric acid and 3% by weight hydrofluoric acid: - which is kept at a temperature of about 60 ^° C. by a heating device 10. All acid still adhering to the metal surface is removed in a washing device 11 by means of warm water. The steel strip is then dried and wound up at 12.

However, an annealing and pickling plant of this type has the following disadvantages:

1. Handling and operation of a high temperature are difficult Schmelzälkallbads \

2. The product can be crushed by the iron rollers and

3. The molten alkali bath must also be heated if the supply of the steel strip is interrupted because it otherwise solidifies at room temperature.

In the device according to the invention shown in FIG In turn, a steel strip 1 is passed through an annealing furnace 2

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passed, then cooled by a cooling device 3 ^{1 of} the forced "or air cooling type to about 600 - 900 ° O and then treated by an alkali spray device 1J, in which an aqueous solution of an alkali metal hydroxide or alkali metal salt, for example a 10 wt .- # ige solution of caustic soda located in a storage container 14 is sprayed onto the surface of the steel strip by means of a pump 15 via a number of nozzles provided in the spray device 13. The surface of the steel strip is evenly wetted in this way, while the excess aqueous alkali is returned to the storage container 14 via a liquid container 16. The aqueous alkali sprayed onto the steel strip in the spray device 13 is instantaneously concentrated because the water is evaporated by the heat remaining in the steel strip from the annealing treatment, see above that a PiIm melted alkali metal adhering to the metal surface allhydroxyds is formed. The steel strip 1 is then passed for about 10 seconds through an oxidation zone 17 in which the molten alkali metal hydroxide is brought into contact with air, so that a descaling reaction takes place which is assumed to be according to the above. Equation (3) runs. The steel strip is then washed in the washing device 8 with water. After washing in the washing device 8, chrome-type stainless steel such as SUS 24 is practically descaled. For the complete descaling of certain types of stainless steel of the nickel-chroa type, such as SUS 27 », a passage through the pickling tank 9 and through a further washing device 11 is required. The steel strip 1 descaled in this way is then wound up at 12.

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Although in the embodiment of the device according to FIG. 2 described above, the annealed steel strip is cooled by a cooling device 3 'of the air cooling type, this can also be done by a water cooling device. The task of the cooling device 3 ¹ is to set the temperature of the steel strip 1 near the inlet to the spray device 13 to about 600 ^° C., ie to a temperature at which an aqueous solution of an alkali metal hydroxide is converted into a molten hydroxide film.

Although wetting the stainless steel workpiece and its contact with an oxygen-containing atmosphere described above as separate process steps are, these operations can also be carried out simultaneously, for example by removing the workpiece sprayed in the presence of air or oxygen.

The method according to the invention has the following advantages over the known methods for carrying out the annealing and pickling treatment of stainless steel:

1. Since no molten salt bath or molten salt is required whose handling is difficult and complicated, the plant costs are reduced}

2. The cost of the treatment itself is also reduced, since the descaling can be carried out using the heat remaining in the stainless steel from the annealing |

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3. The addition of an oxidizing agent can result utilization of the effect of atmospheric oxidation of the molten film of the alkali metal hydroxide or salt can be dispensed with, whereby the speed of descaling is increased}

4. Due to the lack of iron rollers, there is no scratching Steel product more

5. The handling of the aqueous solutions is special simply because these substances do not have the undesirable property the alkali melts have themselves at ambient temperature to solidify}

6. the waste of alkali is minimized, because the amount of solution required for wetting is easily achieved by changing its concentration can be regulated depending on the steel in question and its amount of scale j

7. The alkali metal hydroxide or salt is in the form of a aqueous solution of this substance applied, the standing in contact with these substances, respectively applied nozzles and devices without further ado can be selected, which leads to cost reductions for the plant, and

8. since the alkali metal hydroxide or salt is in the form of a aqueous solution is used, the descaling and the "spray quenching" by the zurückge-. holding heat of the stainless steel can be carried out at the same time, with the addition of the rapid Evaporation of the water from the scale surface

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precalled mechanical shock the chipping of the Can favor scale, so that a higher descaling speed is guaranteed than in the pail of a molten bath of alkali metal hydroxide.

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Claims (11)

19AU69 Pat e η t a η s ρ r ü c h- e 1. Process for descaling stainless steel according to Patent (German patent application P 19 21 305.9), characterized ,, that the steel is first annealed, then the surface of the steel from which the scale is to be removed * with an aqueous Solution of an alkali metal compound according to patent ..., (German patent application P 19 21 305.9) is wetted, which with the scale in the presence of oxygen to form a soluble in water or acid Able to convert the coating, the coated steel while it is still warm from the annealing treatment, is exposed to an oxygen-containing atmosphere and finally the steel surface is washed. 2. The method according to claim 1, characterized in that that caustic soda and / or caustic potash or an alkali metal salt is used as the alkali metal compound, the capable of forming caustic soda or caustic potash under the process conditions. ', 3 »Method according to claim 2, characterized in that that the alkali metal salt is the silicate, carbonate, nitrate, nitrite or chlorate of sodium or potassium is used. 4. The method according to any one of claims 1 to 3, characterized in that that the wetting is done by spraying the solution onto the steel surface. -H-009815/1273 5. The method according to any one of the preceding claims, characterized in that the wetting by immersion of the steel in the solution happens. 6ο Method according to one of the preceding claims, characterized characterized in that the descaled steel is then washed with an inorganic acid. 7. The method according to claim 6, characterized in that the inorganic acid nitric acid, sulfuric acid, Hydrochloric acid or hydrofluoric acid is used » 8. The method according to ein.der preceding claims, characterized in that the descaled steel is finally is washed with water. 9. Device for the surface treatment of stainless steel, characterized by an annealing furnace (2), a device (3 ¹ ) for cooling the surface of the annealed steel, a device (13) for wetting the cooled steel surface with an aqueous solution of an alkali metal compound, means (11) for washing the surface of the steel thus sprayed, all of which are arranged one behind the other, wherein a conveyor device is also provided for conveying the steel through the device. 10. Apparatus according to claim 9 »characterized» that a spray device (13) for wetting the cooled Steel surface is provided. 0 0 9815/1273 . . . ORJGJNAL INSPECTED 11. The device according to claim 9 or 10, characterized in, that zv / ischen the device (13) for wetting the cooled steel surface and the device (11) for washing the steel surface an acid bath (9) for pickling the descaled Steel surface is provided. 009815/127 Blank page