EP0792949B1 - Process for pickling a steel workpiece, in particular stainless steel sheet strip - Google Patents

Abstract

In a steel pickling process, especially for stainless steel strip, employing an aqueous hydrochloric acid pickling solution, the Fe(III) ion concentration is maintained at 1-300 g/l, for constant pickling capacity of the solution of pH less than 1, by oxygenation (preferably aeration) to effect re-oxidation of the Fe(II) ions generated during pickling, the redox potential being maintained at a value of 0-800 (preferably 400-600) mV as measured between a Pt electrode and an Ag/AgCl reference electrode in the solution. Also claimed is a continuous rolled steel (especially stainless steel) strip production line employing the above process, in which the strip is subjected to mechanical treatment (e.g. stretch-levelling and/or peening), primary pickling, working (e.g. rolling), annealing, final pickling and finishing (e.g. skin pass rolling), at least one of the pickling operations being carried out by the above process.

Description

The invention relates to a method of pickling a steel part and in particular a strip of steel sheet stainless.

It is known from Japanese Patent No. JP S 56-171638 a process for descaling steel wire in which the wire is pickled in a hydrochloric acid bath for a time interval between 20 and 40 minutes, the bath having a hydrochloric acid concentration of between 10 and 20% by weight. To accelerate the pickling, the acid concentration of the bath is modified. However, it is taught that an excessively concentrated solution gives off smoke and obliges to oversize the size of the installations at the expense of cost. According to the technique proposed in this document, an amount of FeCl _{2 is added} at a concentration which can go up to saturation as well as an amount of FeCl ₃ so that the redox potentials of Fe ³⁺ / Fe ^{2 +} be measured during descaling. The potential is adjusted by adding FeCl ₃ via an injection nozzle. The pickling process consists of the HCl dissolution of the ferrous substrate, which gives off hydrogen.

It is also specified in this document that, generally, pickling baths based on hydrochloric acid generate corrosion of steel of the pitting type.
It is also known, e.g., JP H2-205692, a pickling bath for stainless steel wherein the hot-rolled or annealed are pickled in a sulfuric acid solution containing Fe ³⁺ and Fe ions ²⁺ , said solution containing at least 10 g / liter of iron, pickling being carried out by aerating the solution with blown air at a flow rate of at least 100 ml / min and per liter of solution.

In this type of bath, air blowing has the sole function of stirring the solution because it is well known that in a sulfuric medium the oxidation potential of iron is such that a simple stirring in air does can oxidize iron Fe ²⁺ to Fe ³⁺ .

It is also known from utility certificate No. FR 2551465, associated with patent EP N ° 0236 354, a process continuous pickling of a strip of stainless steel in which a compound pickling bath is used mainly hydrofluoric acid containing ions ferric, stripping of the steel strip being carried out keeping the ferric iron content at least 15 g / l by the oxidation of the bath by means of an air injection or the additional introduction of another oxidant while maintaining the REDOX potential of said bath in a range of values between 0 and 800 mV.

It is taught in these documents the oxidizing role of air which oxidizes Fe ²⁺ ions to Fe ³⁺ ions, the pickling reaction being carried out without release of hydrogen.

We also find in the field of pickling of stainless steels a substantial number of documents which describe pickling processes including baths consist of a mixture of two or three of acids mentioned above in order to avoid the conventional use of nitric acid which has the disadvantage of generating polluting derivatives of this NOx type acid.

• un traitement mécanique, par exemple de planage sous traction et/ou grenaillage,
• un décapage primaire,
• une opération de transformation comme par exemple de laminage,
• des opérations de recuit,
• un décapage final,
• une opération de finition comme par exemple un laminage à froid de type " skin pass".

In the context of the production of a strip of sheet steel rolled from steel and in particular stainless steel, on a line, continuously, the sheet is successively subjected to:

• mechanical treatment, for example leveling under tension and / or shot blasting,
• a primary pickling,
• a transformation operation such as for example rolling,
• annealing operations,
• a final stripping,
• a finishing operation such as for example cold rolling of the "skin pass" type.

A very significant reduction in the duration of the pickling treatment of steel is imposed to avoid an accumulation of tape between the different line devices. Such a line of development has the advantage of reducing intermediate handling and the stock.

There is the problem of pickling on this line and in particular a pickling in a short imposed time, to avoid, imperatively, an accumulation of the strip treated between the different devices in sequence on the tape production line.

A large number of options in the pickling was considered taking into account the criterion the more important of the stripping which becomes the speed of stripping or the minimum period during which the strip should be in contact with the pickling solution.

The object of the invention is to propose a method of pickling which ensures, in a very short time, pickling a steel part and in particular a steel strip stainless, the pickling time being compatible with the durations imposed on a production line of the piece or strip of sheet metal.

The subject of the invention is a process for pickling a piece of stainless steel and in particular a strip of stainless steel sheet, consisting in applying an aqueous pickling solution consisting of hydrochloric acid and ferric and ferrous ions of pickling in solution having a hydrochloric acid concentration between 35 and 250 g / l and a total iron concentration between 30 and 250 g / l, at a temperature between 65 ° C and 85 ° C, and in order to preserve constant etching power of the aqueous hydrochloric acid solution having a pH of less than 1, to maintain the concentration of Fe ³⁺ ions at a value between 1 g / liter and 300 g / liter, by reoxidation using a oxygenation of Fe ²⁺ ions generated during pickling, the REDOX potential being maintained at a value between 400 and 600 mV potential measured between a platinum electrode and an Ag / AgCl reference electrode placed in the solution, te mps of application of the solution on the stainless steel part being less than 2 min.

• la réoxydation des ions Fe²⁺ par oxygénation est réalisée par aération de la solution décapante,
• l'aération de la solution décapante est réalisée par des moyens choisis parmi : pompage et refoulement de la solution à l'air libre, bullage, brassage, injection d'un gaz contenant l'élément oxygène, aspersion de la solution décapante dans une enceinte contenant de l'air,
• la réoxydation est complétée par addition de composés choisis parmi : les peroxydes et/ou les persels et de préférence le peroxyde d'hydrogène (H₂O₂) et/ou le permanganate de potassium (KMnO₄),
• on chauffe la pièce à traiter préalablement à l'application de la solution,
• on ajuste le potentiel REDOX entre 400 mV et 600 mV, par apport d'un composé et/ou d'un gaz oxydant.

The other characteristics of the invention are:

• the reoxidation of the Fe ²⁺ ions by oxygenation is carried out by aeration of the pickling solution,
• the stripping solution is aerated by means chosen from: pumping and delivery of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen element, spraying of the stripping solution into an enclosure containing air,
• reoxidation is completed by addition of compounds chosen from: peroxides and / or persalts and preferably hydrogen peroxide (H ₂ O ₂ ) and / or potassium permanganate (KMnO ₄ ),
• the part to be treated is heated prior to the application of the solution,
• the REDOX potential is adjusted between 400 mV and 600 mV, by adding a compound and / or an oxidizing gas.

The invention also relates to the use of the pickling process according to the invention in the field of accelerated stripping of a steel part and in particular of a stainless steel strip on an installation of on-line, continuous production of the steel strip as defined in claim 6.

• un traitement mécanique, par exemple de planage sous traction et/ou grenaillage,
• un décapage primaire,
• une opération de transformation comme par exemple de laminage,
• des opérations de recuit,
• un décapage final,
• une opération de finition comme par exemple un laminage à froid de type "skin pass", l'un au moins des décapages consistant à appliquer une solution aqueuse de décapage constituée d'acide chlorhydrique.et d'ions ferriques et ferreux de décapage en solution présentant une concentration en acide chlorhydrique comprise entre 35 et 250 g/l et une concentration de fer total comprise entre 30 et 250 g/l, à une température comprise entre 65°C et 85°c, et en vue de conserver un pouvoir décapant constant de la solution aqueuse d'acide chlorhydrique ayant un pH inférieur à 1, à maintenir la concentration en ions Fe³⁺, à une valeur comprise entre 1 g/litre et 300 g/litre, par réoxydation au moyen d'une oxygénation des ions Fe²⁺ générés lors du décapage, le potentiel REDOX étant maintenu à une valeur comprise entre 400 et 600 mV, potentiel mesuré entre une électrode de platine et une électrode de référence Ag/AgCl placées dans la solution le temps d'application de la solution sur la pièce en acier inoxydable étant inférieur à 2 mn.

The invention also relates to a continuous production line for a strip of rolled stainless steel sheet in which the following operations are implemented, the strip is successively subjected to:

• mechanical treatment, for example leveling under tension and / or shot blasting,
• a primary pickling,
• a transformation operation such as for example rolling,
• annealing operations,
• a final stripping,
• a finishing operation such as for example cold rolling of the "skin pass" type, at least one of the pickling operations consisting in applying an aqueous pickling solution consisting of hydrochloric acid. and of ferric and ferrous pickling ions in solution having a hydrochloric acid concentration between 35 and 250 g / l and a total iron concentration between 30 and 250 g / l, at a temperature between 65 ° C and 85 ° c, and in order to maintain a pickling power constant of the aqueous hydrochloric acid solution having a pH less than 1, to maintain the concentration of Fe ³⁺ ions, at a value between 1 g / liter and 300 g / liter, by reoxidation by means of oxygenation of the Fe ²⁺ ions generated during pickling, the REDOX potential being maintained at a value between 400 and 600 mV, potential measured between a platinum electrode and an Ag / AgCl reference electrode placed in the solution for the application time of the solution on the stainless steel part being less than 2 min.

The following description will make it clear the invention.

Among the various known pickling methods, pickling baths based on hydrochloric acid are considered to generate corrosion of the surface of pickled parts, this corrosion being of the type by sting.

Non-polluting pickling processes use, preferably, solutions comprising alone or in combination, in different concentrations, acids sulfuric and hydrofluoric.

Different pickling solutions have been tested in order to obtain a quick pickling especially on a strip of stainless steel, so that it can be usable, continuously, after pickling, in a production line installation of a sheet metal strip, the pickling installation being inserted in the line, between steel processing devices like for example a planer under tension and a rolling mill.

Preferably, the pickling tests were made with a single acid so as to avoid formation of insoluble complex compounds, the solution being intended to be recycled, that is to say reused pickling.

In a comparative test of pickling solutions containing sulfuric, hydrofluoric acid or hydrochloric acid it has been found that hydrochloric acid in concentrated solution had a surprising efficiency in the area of speed pickling.

• une solution décapante A aqueuse d'acide sulfurique de normalité 4 N, correspondant à une concentration de 196 g/litre d'acide et contenant 60 g/litre de fer total.
• une solution décapante B aqueuse d'acide fluorhydrique de normalité 4N, correspondant à une concentration de 80 g/litre d'acide et contenant 60 g/litre de fer total.
• une solution décapante C aqueuse d'acide chlorhydrique de normalité 4N correspondant à une concentration de 146 g/litre d'acide et contenant 60 g/litre de fer total.

It was compared:

• an aqueous stripping solution A of sulfuric acid of normality 4 N, corresponding to a concentration of 196 g / liter of acid and containing 60 g / liter of total iron.
• an aqueous pickling solution B of hydrofluoric acid of normality 4N, corresponding to a concentration of 80 g / liter of acid and containing 60 g / liter of total iron.
• an aqueous pickling solution C of hydrochloric acid of normality 4N corresponding to a concentration of 146 g / liter of acid and containing 60 g / liter of total iron.

These pickling solutions have been tested in pickling efficiency, taking into account their maximum efficiency by adjusting the REDOX potential corresponding to different acid concentrations used. The residence time is adapted to the composition steel and the type of oxide to be etched.

In the pickling test with the aqueous solution hydrochloric acid, the REDOX potential is fixed at 460 mV compared to an Ag / AgCl reference electrode, the potential being kept constant by air injection and additional supply of hydrogen peroxide.

Table 1 below presents the different pickling speeds measured in loss of material in grams per square meter and per second for different grades of steel: Steel Solution A Solution B Solution C The invention AISI 304 0.41; 0.37 0.42; 0.37 0.59 0.28; 0.27 0.31; 0.27 0.63-0.56 AISI 316L 0.31 0.40 0.53 AISI 430 0.46 - 0.46 0.92; 0.90 1.6 - 1.1 AISI 430 Ti 0.65 - 0.67 1.01; 0.99 1.5 - 1.8 AISI 409 0.55 - 0.58 0.93; 0.96 1.3 - 1.6

As a result, pickling in solution hydrochloric is remarkably effective in the pickling speed range. In addition, it is found that for the same pickling time the remains of oxides are significantly less after pickling in hydrochloric solution.

Furthermore, the surface of the strip stripped with a hydrochloric solution has a white appearance, without puncture, while the surface of the strip stripped with a aqueous sulfuric acid solution looks blackish.

• Variation de la concentration en acide chlorhydrique: Des essais de décapage ont été effectués avec des solutions décapantes C aqueuses d'acide chlorhydrique de normalité comprise entre 3N et 5N correspondant à une concentration comprise entre 108 g/litre et 180 g/litre d'acide et contenant 60 g/litre de fer total.Dans ces essais de décapage, le potentiel REDOX est fixé à 460 mV par rapport à une électrode de référence Ag/AgCl, le potentiel étant maintenu constant par injection d'air et apport complémentaire de peroxyde d'hydrogène, la température de la solution étant maintenue à 80°C.Dans ces conditions, la vitesse de décapage augmente avec la concentration en acide et peut atteindre des valeurs jusqu'à deux à trois fois supérieures à ce qu'elles sont avec les bains de référence A ou B.Pour parvenir à une telle efficacité de décapage, on contrôle le potentiel d'oxydo réduction du bain selon l'invention, contrôle effectué par addition notamment de peroxyde d'hydrogène en complément d'injection d'air, ce qui permet de garder un haut niveau d'agressivité et de traiter en continu, notamment sur une ligne de laminage, les bandes de tôle sur lesquelles est appliquée la solution.
• Variation de la concentration en fer total : Des essais de décapage ont été effectués avec des solutions décapantes C aqueuses d'acide chlorhydrique de normalité 4N correspondant à une concentration de 146 g/litre d'acide et contenant de 30 g/litre à 250 g/litre de fer total.Avec une concentration en fer d'environ 250 g/litre, le fer est en limite de solubilité dans la solution d'acide.Dans ces essais de décapage avec la solution chlorhydrique, le potentiel REDOX est fixé à 460 mV par rapport à une électrode de référence Ag/AgCl, le potentiel étant maintenu constant par injection d'air et apport complémentaire de peroxyde d'hydrogène, la température de la solution étant maintenue à 80°C.Dans ces conditions, la vitesse de décapage augmente avec la concentration en fer et atteint des valeurs jusqu'à trois fois supérieures à ce qu'elles sont avec les bains classiques A ou B.
• Variation de la température: Les températures de la solution de décapage peuvent varier de 10°C à 95°C et varient de préférence entre 65 °C et 85 °C. La vitesse de décapage croit avec la température. Au-dessous de 70°C, la vitesse de décapage croit modérément avec la température, lorsque la température atteint de 70°C à 85°C, la vitesse de décapage augmente de près de 20%. Bien qu'au-dessus d'une certaine température, il peut se produire une évaporation non négligeable, on peut envisager dans une installation, une enceinte fermée, qui permet l'utilisation de la solution à une température supérieure à 85°C, associée à un dispositif de condensation de la solution pour un retour en utilisation.Pour mettre à profit l'effet de la température, il est possible de chauffer la pièce ou la bande de tôle pour compenser les effets d'inertie thermique.
• Variation du potentiel REDOX:

After this remarkable comparison, various tests were carried out to delimit the different parameters for optimizing the pickling solution:

• Variation of the hydrochloric acid concentration: Pickling tests were carried out with aqueous pickling solutions C of hydrochloric acid of normality between 3N and 5N corresponding to a concentration between 108 g / liter and 180 g / liter of acid and containing 60 g / liter of total iron. In these pickling tests, the REDOX potential is fixed at 460 mV relative to an Ag / AgCl reference electrode, the potential being kept constant by injection of air and additional supply of peroxide. of hydrogen, the temperature of the solution being maintained at 80 ° C. Under these conditions, the etching rate increases with the acid concentration and can reach values up to two to three times higher than they are with the reference baths A or B. To achieve such pickling efficiency, the oxidation-reduction potential of the bath according to the invention is controlled, control carried out by addition in particular of per hydrogen oxide in addition to air injection, which allows to keep a high level of aggressiveness and to process continuously, in particular on a rolling line, the sheet metal strips on which the solution is applied.
• Variation in total iron concentration: Pickling tests were carried out with aqueous pickling solutions C of 4N hydrochloric acid corresponding to a concentration of 146 g / liter of acid and containing from 30 g / liter to 250 g / liter of total iron. With an iron concentration of around 250 g / liter, the iron is at the limit of solubility in the acid solution. In these pickling tests with hydrochloric solution, the REDOX potential is fixed at 460 mV relative to an Ag / AgCl reference electrode, the potential being kept constant by injection of air and additional supply of hydrogen peroxide, the temperature of the solution being maintained at 80 ° C. Under these conditions, the speed of pickling increases with iron concentration and reaches values up to three times higher than with conventional baths A or B.
• Temperature variation: The temperatures of the pickling solution can vary from 10 ° C to 95 ° C and preferably vary between 65 ° C and 85 ° C. The pickling speed increases with temperature. Below 70 ° C, the pickling speed increases moderately with temperature, when the temperature reaches from 70 ° C to 85 ° C, the pickling speed increases by almost 20%. Although above a certain temperature, significant evaporation may occur, we can consider in an installation, a closed enclosure, which allows the use of the solution at a temperature above 85 ° C, associated to a device for condensing the solution for return to use.To take advantage of the effect of temperature, it is possible to heat the part or strip of sheet metal to compensate for the effects of thermal inertia.
• Variation of REDOX potential:

Measuring REDOX potential is one way to quality control of pickling solution in an industrial installation, for optimization of pickling speed and quality maintenance constant at constant pickling efficiency.

Pickling speed can increase from 30% to 50% when the REDOX potential is increased from 400 mV to 600 mV.

The pickling speed can be adjusted by adding in the hydrochloric acid bath according to the invention a amount of another acid such as acid hydrofluoric in a proportion less than 40 g / liter.

Claims (7)

1. Process for pickling a stainless steel workpiece and particularly a stainless steel sheet strip, which comprises applying an aqueous pickling solution made of hydrochloric acid and ferric and ferrous ions for pickling in solution, having a concentration of hydrochloric acid between 35 and 250 g/l and a total concentration of iron between 30 and 250 g/l, at a temperature between 65°C and 85°C and aiming to preserve a constant pickling power in the aqueous solution of hydrochloric acid having a pH less than 1, to maintain the concentration of Fe³⁺ ions at a value between 1g/l and 300 g/l, by reoxidation by means of an oxygenation of the of Fe²⁺ ions which are generated during the pickling, the REDOX potential being maintained at a value between 400 and 600 mV, the potential being measured between a platinum electrode and an Ag/AgCl reference electrode, which electrodes are placed in the solution, the duration of application of the solution on the stainless steel workpiece being less than 2 mn.
2. Process for pickling according to claim 1, characterised in that the reoxidation of the Fe²⁺ ions by oxygenation is achieved by aeration of the pickling solution.
3. Process according to any of the claims 1 and 2, characterised in that the aeration of the pickling solution is achieved by means chosen from: pumping and banking up of the solution in an open atmosphere, bubbling through, agitation, injection of a gas containing the element oxygen, spraying of the pickling solution in an enclosed space containing air.
4. Process according to any of the claims 1 to 3, characterised in that the reoxidation is completed by addition of compounds chosen from: peroxides, persalts, and preferably hydrogen peroxide H₂O₂, potassium permanganate KMnO₄.
5. Process according to any of the claims 1 to 4, characterised in that the workpiece to be treated is heated prior to application of the solution.
6. Use of the pickling process according to any of the claims 1 to 5, in the sphere of accelerated pickling of a workpiece and in particular of a stainless steel strip in a continuous production line plant for the strip.
7. Continuous production line for a rolled stainless steel sheet strip using the pickling process according to any of the claims 1 to 6, in which the following operations take place, the strip is successively subjected to:

   a mechanical treatment, for example rolling under tension and/or shot blasting,

   a primary pickling,

   a manufacturing operation such as for example rolling,

   annealing operations,

   a final pickling

   a finishing operation such as for example cold rolling of the "skin pass" type,

   at least one of the pickling operations comprising applying an aqueous pickling solution made of hydrochloric acid and ferric and ferrous ions for pickling in solution, having a concentration of hydrochloric acid between 35 and 250 g/l and a total concentration of iron between 30 and 250 g/l, at a temperature between 65°C and 85°C and, aiming to preserve a constant pickling power in the aqueous solution of hydrochloric acid having a pH less than 1, to maintain the concentration of Fe³⁺ ions at a value between 1g/l and 300 g/l, by reoxidation by means of an oxygenation of the Fe²⁺ ions which are generated during the pickling, the REDOX potential being maintained at a value between 400 and 600 mV, the potential being measured between a platinum electrode and an Ag/AgCl reference electrode, which electrodes are placed in the solution, the duration of application of the solution on the strip being less than 2 mn.