EP0972854A2 - Process for pickling stainless steel - Google Patents

Abstract

A special steel pickling process, employing a hot aqueous hydrochloric acid solution, is new. A special steel pickling process uses an aqueous hydrochloric acid solution at ≥ 40 degrees C. Preferred Features: The solution contains one or more oxidizing agents selected from nitric acid, chlorine, hydrogen peroxide, chlorine-oxygen compounds, other chlorine-containing acids (e.g. perchloric acid), chromium-oxygen compounds (e.g. CrO3) and ferric chloride, in amount such that at least half of the dissolved metal is in the form of ferric chloride.

Description

The invention relates to a method for pickling stainless steel, wherein under Stainless steel qualities AlSl 200, 300 and 400 according to the US standards be understood.

The pickling of normal steel with hydrochloric acid has changed in recent years enforced worldwide and is practically in all countries and steelworks used. Until now, this was the case with pickling stainless steel technical prejudice that this is because of the different scale formation not possible. However, it has a surprising effect shown that a pickling effect can very well be achieved with hydrochloric acid. The are necessary pickling times to achieve a scale-free surface however, extremely long compared to normal steel. This is how a steel belt is made quality St 12 stained at 80 ° C in a maximum of 30 seconds, a comparable one AlSl 409 quality tape takes up to 10 minutes at 90 ° C. This extremely long pickling times are unusable for practical operation, because the necessary pickling lines would be too long.

It has been shown as a surprising effect that when an oxidizing agent is added to the hydrochloric acid used during pickling, this pickling time can be shortened to 50% of the previous time. All the usual oxidizing agents such as, for. B. 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 ferrichloride can be used. The oxidation of the iron present in the pickling acid and dissolved by the pickling attack to ferrichloride is used as a measure of the addition.

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

The invention is therefore characterized in that the stainless steel is treated in an aqueous solution of hydrochloric acid at temperatures of at least 40 ° C.
A favorable embodiment of the invention is characterized in that the stainless steel is treated in an aqueous solution of hydrochloric acid at temperatures up to a maximum of the boiling point of an azeotropic hydrochloric acid.
An advantageous development of the invention is characterized in that an oxidizing agent is added to the aqueous hydrochloric acid.
A favorable further development of the invention is characterized in that one or more substances from the following compounds nitric acid, chlorine, hydrogen peroxide, chlorine-oxygen compounds, other chlorine-containing acids such as perchloric acid, chromium oxygen compounds such as CrO ₃ and ferric chloride are used as the oxidizing agent.
An advantageous embodiment of the invention is characterized in that the addition takes place in such a way that at least one third, preferably half, of the metals dissolved in the pickling attack is in the form of ferric chloride.
An advantageous development of the invention is characterized in that the content of ferrichloride in the pickling solution is determined by measuring the density, conductivity, redox potential or photometrically and a combination of these measurement methods, the content of ferrichloride being able to be set or regulated on the basis of the measured value .

Examples: example 1

1. Beizbottich 216 g Summe HCl, 85 g Eisen gelöst, davon 84 g als Fe ²⁺ und ca. 1 g Fe ³⁺ , 9 g Cr ³⁺ /l

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

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

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.On a pickling line consisting of 3 pickling tanks, each 20 m long, a stainless steel band of quality AlSl 409 pickling tests were carried out. The operating conditions for the plant were:

1. Pickling tub 216 g total HCl, 85 g iron dissolved, of which 84 g as Fe ²⁺ and approx. 1 g Fe ³⁺ , 9 g Cr ³⁺ / l

2. Pickling tub 200 g total HCl, 60 g iron dissolved, of which 60 g as Fe ²⁺ , 7 g Cr ³⁺ / l

3rd pickling tank 195 g total HCl, 35 g iron dissolved, of which 33 g as Fe ²⁺ , 2 g as Fe ³⁺ , 4 g Cr 3+ / l

Temperature in all pickling tanks 80 ° C ± 2 ° C. The pickling acids were pumped with a quantity of 150 m ³ / h from the circulation tanks assigned to the pickling tanks into the pickling tank and this amount ran back into the circulation tank via the processes.

The first belt was moved through the pickling tanks at 30 m / min (pickling time 120 sec.) And was almost unpickled after pickling. The same band was pulled through the pickling tanks again and was after only about 50% descaled. Only after another transport through the pickling tub, this time at a speed of 15 m / min the tape is scale-free. The total pickling time was therefore 480 sec.

Example 2

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

2. Beizbottich 200 g Summe HCl, 60 g Eisen gelöst, davon 38 g als Fe ²⁺, 22 g Fe ³⁺, 7 g Cr ³⁺ /l

3. Beizbottich 195 g Summe HCl, 35 g Eisen gelöst, davon 22 g als Fe ²⁺, 13 g als Fe ³⁺, 4 g Cr ³⁺ /l

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, gaseous chlorine was introduced into the circulation tanks or pickling tanks to adjust the concentration of ferrichloride in the pickling tanks to the following values:

1. Pickling tub 216 g total HCl, 85 g iron dissolved, 54 g as Fe ²⁺ and approx. 31 g Fe ³⁺ , 9 g Cr ³⁺ / l

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

3rd pickling tank 195 g total HCl, 35 g iron dissolved, of which 22 g as Fe ²⁺ , 13 g as Fe ³⁺ , 4 g Cr ³⁺ / l

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

Example 3

1. Beizbottich 216 g Summe HCl, 85 g Eisen gelöst, davon 38 g als Fe ²⁺ und ca. 47 g Fe ³⁺, 9 g Cr ³⁺ /l

2. Beizbottich 200 g Summe HCl, 60 g Eisen gelöst, davon 26 g als Fe ²⁺, 34 g Fe ³⁺, 7 g Cr³⁺ /l

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

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 ferrichloride was therefore further increased before the next bunch by adding hydrogen peroxide to the suction lines of the circulation pumps and adjusted to the following values:

1. Pickling tub 216 g total HCl, 85 g iron dissolved, of which 38 g as Fe ²⁺ and approx. 47 g Fe ³⁺ , 9 g Cr ³⁺ / l

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

3rd pickling tank 195 g total HCl, 35 g iron dissolved, 15 g as Fe ²⁺ , 20 g as Fe ³⁺ , 4 g Cr ³⁺ / l

Temperature in all pickling tanks 80 ° C ± 2 ° C
After the first pass through the pickling tubs, the belt was at a speed of 15 m / min. scale-free. The total pickling time in this experiment was only 240 seconds. The addition of oxidizing agents so that all of the iron was in the form of ferric chloride could not significantly shorten the pickling time. (Example 4) The diagram in FIG. 1 shows the results of these tests with the corresponding reduction in pickling time according to Examples 1, 2, 3 and 4.
By increasing the pickling temperature to 90 ° C, the speed could be increased to 18 m / min. at the concentrations given above, so that the total pickling time in Example 3 was only 200 seconds. The experiments were carried out with the pickling acids as indicated in Examples 1, 2, 3 and 4, the temperature being raised to 90 ° C. (Fig. 2)
Further tests with the addition of other oxidizing agents showed that at least 50% of the dissolved metals must be present as ferrichloride in order to achieve a corresponding reduction in the pickling time. For example, the addition of perchloric acid to the pickling tanks in each case 10% by volume of the amount present resulted in a reduction in 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.

Tests with other stainless steel strips of qualities AlSl 304, 316 and
AlSl 430 showed similar reductions in pickling times under the conditions listed above. The results are summarized in the diagrams in FIGS. 4 and 5.

In a further series of experiments between the individual Pickling tanks brushes installed that loosen the surface of the belt free adhering tinder. This meant that in all experiments a further reduction in the pickling time of 5 - 10% can be achieved. Brushes that have proven to be particularly advantageous in their Bristle abrasive particles such as silicon carbide or aluminum oxide are embedded had.

Claims (12)

Process for pickling stainless steel, characterized in that the stainless steel in an aqueous solution of hydrochloric acid Temperatures of at least 40 ° C is treated. A method according to claim 1, characterized in that the stainless steel in an aqueous solution of hydrochloric acid at temperatures up to treated to a maximum of the boiling point of an azeotropic hydrochloric acid becomes. A method according to claim 1 or 2, characterized in that an oxidizing agent is added to the aqueous hydrochloric acid. A method according to claim 3, characterized in that one or more substances from the following compounds nitric acid, chlorine, hydrogen peroxide, chlorine-oxygen compounds, other chlorine-containing acids such as perchloric acid, chromium oxygen compounds such as CrO ₃ and ferric chloride are used as the oxidizing agent. A method according to claim 1 to 3, characterized in that the The addition is such that at least one third, preferably the Half of the metals dissolved in the pickling attack in the form of ferrichloride is present. Method according to one of the above claims, characterized characterized in that the content of ferrichloride in the pickling solution by measuring the density, conductivity, redox potential or photometrically and a combination of these measurement methods is determined. A method according to claim 6, characterized in that the Ferrichloride content adjusted based on the measured value or is regulated. Method according to one of claims 3 to 7, characterized characterized in that the addition of the oxidizing agent is proportional the enforced belt surface takes place. Method according to one of claims 3 to 7, characterized characterized in that the addition of the oxidizing agent is proportional the tons passed through the pickling plant. Method according to one of claims 3 to 9, characterized in that that the addition of the oxidant to the suction line the circulation pump of each individual pickling tub. Method according to one of claims 3 to 9, characterized in that that the addition of the oxidizing agent in each pickling tub assigned circulation tank. Method according to one of claims 1 to 11, characterized in that that brushes between the individual pickling tanks installed that mechanically clean the surface of the belt.

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