UA128807C2 - Electro-assisted pickling of steel - Google Patents

Abstract

This invention relates to a method of etching a metal strip, which includes the following stages: - passing said metal strip through at least one etching bath, which has a temperature of 1 to 100 °C; - supply of alternating current with a current density of 1×102 to 1×105 A∙m-2 per unit surface of the indicated metal strip, which passes through at least one etching bath.

Description

This invention relates to a method that allows to increase the efficiency of the metal strip etching line. This is achieved by applying alternating current to a moving metal strip that moves through at least one pickling bath.

In the strip state of hot rolling, the metal surface is in contact with moist air at high temperatures, from 1200 "C (in the preheating furnace) to approximately 700 C (at the coil winding station). The specified conditions contribute to the formation of scale layer 1 on the metal strip 2, as shown in Fig. 1. When steel is smelted, the specified slag layer consists mainly of iron oxides, as illustrated in Fig. 2, while the steel sample is covered with 4 slag layer, which consists of ReO (wuestite) and HegOz (hematite).

The thickness of the scale can vary, as a rule, from 4 to 20 μm, depending on the conditions of the strip state of hot rolling, as shown in fig. 2 with the help of columns. After the hot rolling operation, the scale should be removed to obtain a metal surface suitable for further process steps such as cold rolling, annealing or hot dip coating. Usually, the indicated scale is broken up with the help of scale breakers on the input pickling line, and then removed in pickling tanks before cold rolling of metal strips and coating them.

As a rule, during the etching process in the etching line 5, the metal strip passes through several etching tanks (6, ba, 6B, bs), which contain an etching bath (7, 7a, 76, 7c), prepared at least from of a single acid or of a single etching salt, as shown in

Fig. 3. Successive pickling baths are not necessarily characterized by the same process parameters or the same composition. In addition, the nature and concentration of their pickling solution can change.

Patent 5 5472579 discloses an etching method in which a hot-rolled steel strip is continuously fed into at least one pickling tank and an electric current is passed through it.

However, when using the above methods and equipment, the etching time required to achieve a satisfactory surface quality is not optimal. Therefore, a more efficient method of digestion is necessary.

The purpose of the present invention is to develop a solution to overcome the above problems.

The specified goal is achieved by developing the method according to clause 1 of the claims. This method can also include any characteristics of paragraphs. 2-11 of the claims.

Other characteristics and advantages of the present invention will become apparent from the further detailed description of the invention.

To illustrate the invention, various implementations and tests of a non-limiting example will be described, specifically with reference to the following figures:

In Fig. 1 shows the presence of a scale layer on a steel sample.

Fig. 2 is an image of iron scale on a steel base.

In Fig. A version of etching line 5 is demonstrated.

In Fig. 4 shows an embodiment of the present invention, pickling tank 8.

In Fig. 5 shows another version of the etching line 5 using the method according to the invention.

In Fig. 6 shows the influence of the type of current on the etching time.

In Fig. 7 shows the influence of the bath temperature on the digestion time.

In Fig. 8 shows the influence of acid concentration in the bath on the digestion time.

In Fig. Figure 9 shows the effect of current density on etching time.

In Fig. 10-12 shows the influence of the current frequency on the etching time in different conditions.

In Fig. 13 and 14 demonstrate the effect of such a current characteristic as the ratio of the duration of the anodic and cathodic periods on the etching time.

As illustrated in Fig. 4, the invention relates to a method of etching a metal strip 9, which includes the following stages: - passing the indicated metal strip through at least one etching bath 7, which has a temperature from 1 to 100 "С, - supplying an alternating current, which is characterized by a current density from 1x102 to 105 Am? per unit surface of the specified metal strip, with respect to the specified metal strip passing through at least one specified etching bath, and characterized by an anodic and cathodic supply period to the metal strip passing through at least one specified bath for etching, which is characterized by the ratio of the duration of cathodic and anodic pulses from 0.1 to 5.0.

This method of etching is carried out mainly after the operation of hot rolling and even more preferably, after the operation of breaking the scale. The proposed method of etching is carried out preferably after a cold rolling operation and/or a coating operation, such as a hot dip coating process.

The etching bath is placed in the etching tank 6, as shown in Fig. 4. The specified pickling tank is made, at least preferably, from one of the following materials: bricks from raw materials, granite or ebonite bricks, polypropylene, high-density polyethylene (HOPE) and/or polypropylene homopolymer (PPHR), which allows to increase its service life under digestion conditions. Preferably, the tank is equipped with devices capable of moving the strip through the bath, such as conveyor rolls 10. As illustrated in FIG. 4, the bath can be equipped with four conveyor rolls 10, one pair on the side of the entrance 11 and another pair on the side of the exit 12 from the tank. In each pair, one of the rolls is completely immersed in the indicated etching bath, and the other is not immersed in the indicated etching bath. Preferably, the etching line is also equipped with devices for adding and/or regenerating the etching solution, such as an acid regeneration unit (ACR), which is not shown in Fig. 3. As a rule, fresh/regenerated pickling solution is added to the last pickling tank, then it cascades from the last tank to the first tank, from where the spent pickling solution is pumped to the regeneration station (if it exists, or to a storage tank). The flow of acid flows is regulated with the help of pumps.

The pickling bath can be any pickling bath known to those skilled in the art. It is preferable that the pickling bath contains at least one acid and/or pickling salt in a concentration of 10 to 360 hl". More preferably, the pickling bath 7 contains at least one pickling acid or salt. Preferably, acid or pickling salts is one of the following substances: hydrochloric acid (HCI), sulfuric acid (He505), potassium chloride (KCI), sodium chloride (Mas), sodium sulfate (Mag504), potassium sulfate (K»2505) or nitric acid.As a result of etching and removing the substance in this way, the etching bath may also contain unwanted materials such as dissolved metals that appear as a result of the etching operation (iron ions, other typical alloying elements or impurities that are present in steels, for example, Mn, 5i, AI, St, Mi, Co, Ti, M, MB, Mo, Si, C, 5, P, B, M, ...), as well as solid particles of poorly soluble oxides that precipitate on the walls of tanks or in etching circuits, such as particles of silicon dioxide, aluminum oxide, mixed phases similar to fayalite (Beg25iO4), GeAi2Ox, Mp-containing spinels (MpgbiO»x, MpAgO4,...). In addition, depending on the operating conditions, the pickling bath may also contain an etching inhibitor that protects the steel surface by limiting the dissolution of the steel in the pickling bath.

The indicated alternating current is characterized by a current density of 102 to 105

Um? per unit surface of the specified metal strip. This means that a portion of the strip (and/or scale) will receive the alternating current described earlier as it passes through the etching bath 7, which helps remove the scale layer. For example, alternating current is supplied to the specified area, at least from c.

Alternating current is supplied in any possible way. Alternating current can have any waveform, such as square, triangle, sine, or complex. As illustrated in Fig. 4, it is preferable that the alternating current is supplied using a series of electrodes 13 with alternating anode 13a and cathode 1360 facing the metal strip. Alternation of the anode and cathode is carried out mainly by applying a positive or negative current to the specified electrodes. Preferably, both sides of the line face the electrodes. The electrodes are immersed in the etching bath and placed preferably at a distance of 1 to 30 cm from the metal moving strip. More preferably, the electrodes are placed at a distance of 1 to 10 cm from the moving metal strip.

For example, as illustrated in Fig. 5, on the etching line, which contains four etching tanks (6, ba, 60, bs), the metal strip can be subjected to a chemical etching process in the first three tanks and an etching process according to the invention in the fourth tank. Another possibility is to apply the process according to the invention in all baths of the pickling line. In addition, the number of etching tanks in line configurations can vary from 1 to b, and the etching process according to the invention can be carried out in at least one of them, that is, from one tank to all tanks.

The positive effect of alternating current on etching time, compared to direct current, can be seen in Fig. 6, where all experiments were carried out in a bath with a concentration of HCI acid of 100 g-l-1. The etching time is presented in the form of a dependence for two types of current (direct current or alternating current), which is applied to a steel sample that has a scale layer about 5 μm thick, at different values of the current density. In the case of alternating current (AC), the experiments were carried out using a current with a frequency of 50 Hz with the same duration of cathodic and anodic pulses, that is, a ratio of 1:1. All other parameters being equal, in the case of alternating current, it can be seen that the etch time is on average 33 95 less. Furthermore, the gain in etch time increases with increasing current density, and at 107 Am? AC gives a gain in etch time of about 4095 over DC. Therefore, the efficiency of the etching method according to the invention increases compared to electroetching using direct current.

The specified metal strip is made mainly of steel.

The specified alternating current has a frequency preferably from 0.5 to 100 Hz.

Preferably, said metal strip is passed through the bath at a speed of 10 m"min" to 450 m-min'.

Preferably, the specified alternating current is applied to the specified metal strip, which passes through at least one specified pickling bath, for at least 5 seconds, which allows increasing the efficiency of scale dissolution. Preferably, said alternating current is applied to said metal strip which passes through at least one etch bath for a maximum of 600 seconds. More preferably, the specified alternating current is applied to the specified metal strip, which passes through at least one specified pickling bath, for a maximum of 300 s, which allows to reduce electricity consumption while simultaneously achieving a satisfactory rate of scale dissolution.

Preferably, the pickling bath 7 contains only one acid or only one pickling salt. This allows to suppress the interaction between pickling acids and pickling salts and, thus, to have a more stable pickling bath.

Preferably, said pickling bath contains hydrochloric acid with a concentration of 10 to 360 hl". Preferably, said pickling bath contains sulfuric acid with a concentration of 10 to 360 hl.

Several experiments were conducted to assess the impact of selected process parameters on digestion efficiency. The tests were performed on steel samples with the same surface condition: steel covered with a layer of iron oxides (scale) 5 μm thick. The etching time was recorded as a function of the selected process parameters. Then the brightness of the surface was evaluated using a spectrophotometer SM-26004 of Ye Kopis-MipoKa. The etching time corresponds to the time required to achieve a brightness of 60 to 75, which, without regard to any theory, indicates that all (or almost all) of the oxide layer has been removed.

The shorter the digestion time, the better the digestion efficiency. It should be noted that the brightness of the surface covered with scale before etching is about 30 units, and the brightness of metallic steel without scale is usually in the range of 60 to 75 units, depending on the chemical properties of the product and the morphology of the surface (roughness). Therefore, the increase in brightness during etching is associated with the removal of scale.

Preferably, the etching bath has a temperature of at least 40 "C. This increases the etching efficiency compared to the corresponding indicator of the etching bath at temperatures below 39.5 "C. In Fig. 7, the etching time is presented in the form of a graph of the dependence on the temperature of the bath for experiments performed in a bath with a concentration of NOSI of 100 hl", at different temperatures and current densities. The frequency of oscillations of alternating current (AC) is 50 Hz, and the ratio of the duration of cathodic and anodic pulses is equal to 1:1.

All other parameters being equal, it can be seen that the higher the temperature of the etching bath, the shorter the etching time.

Preferably, the said pickling bath is characterized by a concentration of acid or pickling salt of at least 30 hl", more preferably at least 20 hl". This increase in the lower limit improves digestion efficiency. In Fig. 8, the etching time is presented in the form of a graph of the dependence on the acid concentration in the etching bath at 40 "С for the experimental conditions implemented when using alternating current (AC) with a density of 0.5x107 A:m2 and an oscillation frequency of 50 Hz with the ratio of the duration of the cathodic and anodic pulses equal to 1:1.Since all other parameters are the same, it can be seen that the higher the acid concentration for etching, the shorter the etching time.

The specified current density is preferably at least 103 Abm? per unit surface of the specified metal strip with respect to the specified metal strip that passes through at least one specified etching bath, and more preferably, at least 1x101 AUm--. This makes it possible to increase the etching efficiency compared to the corresponding indicator at a lower current density. In Fig. 9, the etching time is presented in the form of a graph of the dependence on the current density with a frequency of 50 Hz, which is applied to the metal strip with a ratio of the duration of the cathode and anode pulses equal to 1:1, for a bath with a concentration

NSI 100 goals. All other parameters being equal, it can be seen that the higher the current density, the shorter the etching time.

Said alternating current preferably has a frequency of at least 15 Hz. It is obvious that such a lower limit makes it possible to increase the etching efficiency compared to the corresponding indicator at a lower frequency. Said alternating current preferably has a maximum frequency of 50

Hz. It is obvious that such an upper limit allows to increase the etching efficiency compared to the corresponding indicator at a higher frequency. In Fig. 10-12, the etching time is presented in the form of a graph of the dependence of time on the frequency of the current supplied to the metal strip (under the specified conditions, the ratio of the duration of the cathodic and anodic pulses is 1:1) in an etching bath with an acid concentration of 100 gOl" at 40 "C in in the case of different acids for etching and current density.

Without being bound by any theory, it can be seen that it is preferable to increase the lower frequency limit to 15 Hz and lower the upper frequency limit to 50 Hz.

The specified alternating current, characterized by a certain anodic and cathodic supply period to the specified metal strip, which passes through at least one specified etching bath, is preferably characterized by a ratio of the duration of the cathodic and anodic pulses from 0.3 to 4.0. More preferably, the specified alternating current, characterized by a certain anodic and cathodic supply period to the specified metal strip, which passes through at least one specified etching bath, is characterized by a ratio of the duration of cathodic and anodic pulses from 1.1 to 2 Optimally specified alternating current, characterized by a certain anodic and cathodic supply period to the specified metal strip, which passes through at least one specified etching bath, characterized by a ratio of the duration of the cathodic and anodic pulses from 1.5 to 2.4. In Fig. 13 and 14, the etching time is presented in the form of a graph of the dependence of time on the ratio of periods of alternating current, which is supplied to the metal strip, at 40 "С in the case of an etching bath with an acid concentration of 100 gL" and a current density of 0.5 A:"cm7?. All other parameters being equal, it can be clearly observed that when the AC period ratio is in the specified range, the etching efficiency is higher.

This invention is set forth above in the form of an embodiment which is believed to be in practice and which is presently preferred. However, it should be understood that this invention is not limited to the embodiment disclosed in the description, and may be suitably modified within a range that does not go beyond the basic meaning or essence of the invention, which can be discovered based on the appended claims and the entire description.

Claims (10)

FORMULA OF THE INVENTION 1. The method of etching a metal strip (9), which includes the stages in which: - the specified metal strip is passed through at least one etching bath (7), which has a temperature from 1 to 100 "C, - an alternating current is supplied to the specified metal strip , which passes through said at least one etching bath, while said alternating current has a current density of 1x102 to 1x105 Am2 per unit surface area of said metal strip and is characterized by the presence of an anodic period and a cathodic period, and said alternating current applied to said metal strip , which passes through the indicated at least one etching bath, has a ratio of the duration of the cathodic and anodic pulses from 1.1 to 2.7. 2. The etching method according to claim 1, in which said metal strip is made of steel. 3. The etching method according to claim 1 or 2, in which the specified metal strip is passed through the bath at a speed of 10 to 450 m'min". 4. The etching method according to any one of claims 1-3, in which said etching bath has a temperature of at least 40 "С. 5. The pickling method according to any one of claims 1-4, in which said pickling bath has a concentration of acid or pickling salt that is at least 30 hl". b. The etching method according to any one of claims 1-4, in which said etching bath has a concentration of acid or etching salt that is at least 60 hl". 7. The method of etching according to any of claims 1-6, in which the specified current density is at least 1x103 D.m? per unit surface of said metal strip that passes through said at least one etching bath. 8. The etching method according to any one of claims 1-7, in which said alternating current has a frequency of 0.5 to 100 Hz. 9. The etching method according to any one of claims 1-8, in which said alternating current has a frequency of at least 15 Hz. 10. The etching method according to any of claims 1-9, in which said alternating current has a frequency that does not exceed 50 Hz. min Fig. 1 "a zhi b, mk 5 MEM B ih she ro; s 0. 1-3 х хх ОК х Z СХ MKK 5 MZHK o o. i" p. o what o vyh what VAR YAH - "Ou MeV m DY HNA OO Z o o nshnNnnnychnny PE what POT PE ENN y POCKET EE mkm x Fig. 2 5 Same poof pesos today. today it is not like Zhe and NN Ye EK mk KY viy shi yaz). 76 7b and Ta ba 7 in 8 Fig. WITH