EP0972854B1 - Process for pickling stainless steel - Google Patents

Description

The invention relates to a process for pickling stainless steel, wherein under stainless steel qualities AISI 200, 300 and 400 are understood by the US standards and the stainless steel in an aqueous solution of pure hydrochloric acid at temperatures of at least 40 ° C up to a maximum of the boiling point azeotropic hydrochloric acid is treated and an oxidizing agent is added to the aqueous hydrochloric acid and the oxidizing agent used is one or more of the following compounds chlorine, hydrogen peroxide, chlorine-oxygen compounds, other chlorine-containing acids such as perchloric acid and chromium oxygen compounds such as CrO ₃ , the addition being carried out such that at least one One-third, preferably half, of the metals dissolved in the pickling attack are in the form of Fe (III) chloride and the content of Fe (III) chloride in the pickling solution is determined by measuring the density, conductivity, redox potential or photometrically and a combination of these measuring methods based on the Measured value is set or regulated.

The pickling of mild steel with hydrochloric acid has been in recent years enforced worldwide and is used in virtually all countries and steel mills used. When pickling stainless steel, however, there has been the technical Prejudice that this is not because of the different scale training is possible. However, it has proved to be a surprising effect that very well with hydrochloric acid, a staining effect can be achieved. The necessary However, pickling times to achieve a scale-free surface are in the Ratio to standard steel extremely long. This is how a steel band becomes Quality St 12 pickled in a maximum of 30 sec. At 80 ° C, a comparable Tape of quality AISI 409 needed at 90 ° C for up to 10 minutes. These extremely long pickling times are unusable for practical operation, because the necessary pickling equipment would be too long.

It is known from EP-A-0 769 574, EP-A-0 792 949, DE 27 36 255 A, GB 444 865 A, EP-A-0 513 753, JP 58 199879 A, EP-A-0 814 180, EP-A-0 770 707 and CA Vol. 84/1976 page 223 those for pickling stainless steel hydrochloric acid at a temperature of above 40 ° C is used.

As a surprising effect has been shown that when adding an oxidizing agent to the hydrochloric acid used during pickling this pickling time can be reduced to 50% of the previous time. As the oxidizing agent while all conventional oxidizing agents such. As nitric acid, chlorine, hydrogen peroxide, chlorine - oxygen compounds, oxygen-containing gases, other chlorine-containing acids such as perchloric acid, chromium oxygen compounds such as CrO ₃ and ferric chloride are used. As a measure of the addition, the oxidation of the existing in the pickling acid and dissolved by the pickling iron to ferric chloride is used.

This concentration must according to the present invention at least a third, but preferably half of the one solved by the pickling attack Metals amount to a corresponding effect in terms of To achieve pickling time.

The invention is therefore characterized in that by means of brushes installed between the individual pickling vats, the surface of the Belt is mechanically cleaned, the brush preferably with in the bristles embedded abrasive particles may be provided. This allows a further reduction of the pickling time of 5 - 10% become. Brushes which have proved to be particularly advantageous are those which In their bristles abrasive particles such as silicon carbide or aluminum oxide had embedded.

Exemplary embodiments: example 1

1. Beizbottich 216 g Summe HCl, 85 g Eisen gelöst, davon 84 g als Fe ²⁺ und ca. 1 g Fe³⁺ , 9 g Cr³⁺ /I

2. Beizbottich 200 g Summe HCI, 60 g Eisen gelöst, davon 60 g als Fe ²⁺, 7 g Cr³⁺ /I

3. Beizbottich 195 g Summe HCI, 35 g Eisen gelöst, davon 33 g als Fe ²⁺, 2 g als Fe³⁺, 4 g Cr 3+ /I

Temperatur in allen Beizbottichen 80 °C ± 2 °C. Die Beizsäuren wurden dabei mit einer Menge von 150 m³/h aus den Beizbottichen zugeordneten Zirkulationstanks in den Beizbottich gepumpt und diese Menge lief über die Abläufe wieder in den Zirkulationstank zurück.At a pickling plant, consisting of 3 pickling tanks, each 20 m in length, pickling tests were carried out with a quality AlSl 409 stainless steel strip. The operating conditions for the plant were:

1. Beizbottich 216 g of total HCl, 85 g of iron dissolved, of which 84 g as Fe ²⁺ and about 1 g Fe ³⁺ , 9 g Cr ³⁺ / I.

2. Beizbottich 200 g of total HCl, 60 g of iron dissolved, of which 60 g as Fe ²⁺ , 7 g Cr ³⁺ / I.

3. pickling vat 195 g total HCI, 35 g iron dissolved, of which 33 g as Fe ²⁺ , 2 g as Fe ³⁺ , 4 g Cr 3+ / I

Temperature in all pickling tanks 80 ° C ± 2 ° C. The pickling acids were pumped at a rate of 150 m ³ / h from the Beizbottichen associated circulation tanks in the pickle and this amount ran back through the processes back into the circulation tank.

The first belt was moved at 30 m / min through the pickling vats (pickling time 120 sec.) And was almost unpicked after pickling. The same band was once again pulled through the Beizbottiche and was afterwards only about 50% descaled. Only after another transport through the Beizbottiche, this time with a speed of 15 m / min was the band without scale. The total pickling time was 480 seconds.

Example 2

1. Beizbottich 216 g Summe HCI, 85 g Eisen gelöst, davon 54 g als Fe ²⁺ und ca. 31 g Fe ³⁺ ,9 g Cr³⁺ /I

2. Beizbottich 200 g Summe HCI, 60 g Eisen gelöst, davon 38 g als Fe ²⁺, 22 g Fe ³⁺, 7 g Cr³⁺ /I

3. Beizbottich 195 g Summe HGI, 35 g Eisen gelöst, davon 22 g als Fe²⁺, 13 g als Fe³⁺, 4 g Cr³⁺/I

Temperatur in allen Beizbottichen 80 °C ± 2 °C
Auch bei der reduzierten Geschwindigkeit von 15 m/min mußte das Band zwei Mal durch die Beizbottiche transportiert werden, um eine vollständig zunderfreie Oberfläche zu erzielen. Die gesamte Beizzeit betrug daher unverändert 480 sec.Before the next bundle, the concentration of ferric chloride in the pickling tubs was adjusted to the following values by introducing gaseous chlorine into the circulation tanks or into the pickling tubs:

1. Beizbottich 216 g total HCI, 85 g iron dissolved, of which 54 g as Fe ²⁺ and about 31 g Fe ³⁺ , 9 g Cr ³⁺ / I.

2. Beizbottich 200 g of total HCl, 60 g of iron dissolved, of which 38 g as Fe ²⁺ , 22 g Fe ³⁺ , 7 g Cr ³⁺ / I.

3. pickling vat 195 g total HGI, 35 g iron dissolved, of which 22 g as Fe ²⁺ , 13 g as Fe ³⁺ , 4 g Cr ³⁺ / I

Temperature in all pickling tanks 80 ° C ± 2 ° C
Even at the reduced speed of 15 m / min, the tape had to be transported twice through the pickling tubs to achieve a completely scale-free surface. The total pickling time was therefore unchanged at 480 seconds.

Example 3

1. Beizbottich 216 g Summe HCI, 85 g Eisen gelöst, davon 38 g als Fe ²⁺ und ca. 47 g Fe³⁺, 9 g Cr³⁺ /I

2. Beizbottich 200 g Summe HCI, 60 g Eisen gelöst, davon 26 g als Fe ²⁺, 34 g Fe³⁺, 7 g Cr³⁺ /I

3. Beizbottich 195 g Summe HCI, 35 g Eisen gelöst, davon 15 g als Fe ²⁺, 20 g als Fe ³⁺, 4 g Cr³⁺ /I

Temperatur in allen Beizbottichen 80 °C ± 2 °C
Bereits nach dem ersten Durchlauf durch die Beizbottiche war das Band bei einer Geschwindigkeit von 15 m/min. zunderfrei. Die gesamte Beizzeit betrug bei diesem Versuch nur 240 sec. Durch weitere Zugabe von Oxidationsmitteln, so daß alles Eisen in Form von Ferrichlorid vorlag, konnte die Beizzeit nicht mehr wesentlich verkürzen. ( Beispiel 4 ) Das Diagramm in Fig. 1 zeigt die Ergebnisse dieser Versuche mit der entsprechenden Beizzeitverkürzung gemäß den Beispielen 1, 2, 3 und 4.
Durch eine Erhöhung der Beiztemperatur auf 90 °C konnte die Geschwindigkeit auf 18 m/min. bei den oben angegebenen Konzentrationen gesteigert werden, so daß die gesamte Beizzeit im Beispiel 3 nur mehr 200 sec. betrug. Die Versuche wurden mit den Beizsäuren wie in den Beispielen 1, 2, 3 und 4 angegeben gemacht, wobei die Temperatur auf 90 °C erhöht wurde. (Fig. 2)
Weitere Versuche mit Zugabe von anderen Oxidationsmitteln zeigten, daß mindestens 50 % der gelösten Metalle als Ferrichlorid vorliegen müssen, um eine entsprechende Reduktion der Beizzeit zu erzielen. So ergab zum Beispiel der Zusatz von Perchlorsäure zu den Beizbottichen in jeweils 10 Vol. % der vorhandenen Menge eine Reduktion der Beizzeiten gemäß Fig. 3.
Als besonders vorteilhaft hat es sich auch bei den Versuchen herausgestellt, die Konzentrationen laufend zu überwachen, um sicherzustellen, daß stets ausreichend Oxidationsmittel dem Beizbad zugegeben wird.The concentration of ferric chloride was therefore further increased before the next collar by adding hydrogen peroxide in the suction lines of the circulation pump and set to the following values:

1. Beizbottich 216 g total HCI, 85 g iron dissolved, of which 38 g as Fe ²⁺ and about 47 g Fe ³⁺ , 9 g Cr ³⁺ / I.

2. Beizbottich 200 g of total HCl, 60 g of iron dissolved, of which 26 g as Fe ²⁺ , 34 g Fe ³⁺ , 7 g Cr ³⁺ / I.

3. pickling vat 195 g total HCI, 35 g iron dissolved, of which 15 g as Fe ²⁺ , 20 g as Fe ³⁺ , 4 g Cr ³⁺ / I

Temperature in all pickling tanks 80 ° C ± 2 ° C
Already after the first pass through the Beizbottiche the tape was at a speed of 15 m / min. scale-free. The entire pickling time in this experiment was only 240 sec. By further addition of oxidizing agents, so that all iron in the form of ferric chloride was present, the pickling time could not be significantly shortened. (Example 4) The diagram in FIG. 1 shows the results of these tests with the corresponding pickling time reduction according to Examples 1, 2, 3 and 4.
By increasing the pickling temperature to 90 ° C, the speed was 18 m / min. be increased at the above concentrations, so that the total pickling time in Example 3 was only 200 sec. The experiments were carried out with the pickling acids as in Examples 1, 2, 3 and 4, the temperature being raised to 90 ° C. (Fig. 2)
Further experiments with the addition of other oxidizing agents showed that at least 50% of the dissolved metals must be present as ferric chloride in order to achieve a corresponding reduction in pickling time. For example, the addition of perchloric acid to the pickling tubs in each case in 10% by volume of the amount present resulted in a reduction of the pickling times according to FIG. 3.
It has also proven to be particularly advantageous in the experiments to continuously monitor the concentrations in order to ensure that sufficient oxidizing agent is always added to the pickling bath.

Try other stainless steel bands of the qualities AISI 304, 316 as well
AISI 430 gave similar reductions in pickling times under the conditions listed above. The results are summarized in the diagrams in FIGS. 4 and 5.

In another series of experiments were between the individual Beizbottichen brushes installed, which loose the surface of the tape liberated tinder. This could generally in all experiments a further reduction of the pickling time of 5 - 10% can be achieved. In this case, brushes have proven to be particularly advantageous in their Bristles embedded in abrasive particles such as silicon carbide or aluminum oxide had.

Claims (5)

1. Process for pickling stainless steel strip, where the stainless steel is treated in pickling tanks in an aqueous solution of hydrochloric acid at temperatures of at least 40°C, but not exceeding the boiling point of an azeotropic hydrochloric acid, and where an oxidant is added to the aqueous hydrochloric acid, the oxidant being one or more substances from the following compounds - chlorine, hydrogen peroxide, chlorine oxygen compounds, other acids containing chlorine, such as perchloric acid, or chromium oxygen compounds like CrO₃ - with the oxidant being added in such a way that at least one third, but preferably half of the metals dissolved during pickling is present in the form of ferric chloride and the ferric chloride content in the pickling solution is determined by measuring the density, conductivity, oxidation reduction potential, or photometrically, or using a combination of these measuring methods, and is set or controlled on the basis of the value measured, characterised by brushes which are mounted between the individual pickling tanks cleaning the strip surface mechanically, with the brushes being preferably provided with abrasive particles imbedded in the bristles.
2. Process according to Claim 1, characterised by the oxidant being added in proportion to the strip surface being treated.
3. Process according to Claim 1, characterised by the oxidant being added in proportion to the tonnage being treated in the pickling plant.
4. Process according to one of Claims 1 to 3, characterised by the oxidant being added into the suction pipe leading to the circulating pump for each individual pickling tank.
5. Process according to one of Claims 1 to 3, characterised by the oxidant being added into the circulation tank assigned to each of the pickling tanks.

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