The present invention relates to a method permitting to improve the effidency of a pickling line of a metallic strip. It is done by applying an alternating current to a metallic strip moved through at least a pickling bath.

A metallic surface is, in the hot strip mill, in contact with humid ait at high temperatures, from 1200°C (in the reheating furnace) to nearly 700°C (at the coiling station). These conditions favour the formation of a scale layer 1 on the metallic strip 2, as illustrated in Figure 1. In the case ofsteelmaking, said scale layer is mainly composed of iron oxides, as illustrated in Figure 2 wherein a steel piece 3 is covered by a scale layer 4 composed of FeO (Wustite), Fe ₃ O ₄ (Magnetite) and Fe ₂ O ₃ (Hematite). The scale thickness can vary, typically from 4 to 20 μm, depending on the hot strip mill conditions, as noted in Figure 2 by the bars. After the hot rolling operation, the scale should be removed to provide a metallic surface convenient for the following process steps such as cold rolling, annealing or hot-dip coating. Usually, this scale is broken by scale breakers at the pickling entry tine and then removed in pickling tanks prior to the metallic strip cold-rolling and/ or coating.

Generally, during a pickling process in a pickling line 5, the metallic strip passes through several pickling tanks (6, 6a, 6b, 6c) containing a pickling bath (7, 7a, 7b, 7c) made of at least one pickling acid or one pickling salt, as illustrated in Figure 3. The successive pickling baths do not have necessarily the same process parameters not the same composition. Moreover, their pickling liquor nature and concentration can vary.

Patent US 5472579 discloses a pickling method wherein a hot-rolled steel strip is continuously fed to at least a pickling tank and an electric current pass through said sted strip.

However, by using the above methods and equipment, the pickling time necessary to achieve a satisfying surface quality is not optimal. Consequently, a more efficient pickling method is necessary.

The purpose of this invention is to provide a solution solving the aforementioned problems.

This object is achieved by providing a method according to claim 1. The method can also comprise any characteristics of claims 2 to 11.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention. To illustrate the invention, various embodiment and trials of non-limiting example will be described, particularly with reference to the following figures:

Figure 1 illustrates the presence of a scale layer on a steel piece.

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

Figure 3 exhibits an embodiment of a pickling line 5.

Figure 4 exhibits an embodiment of the present invention, a pickling tank 8.

Figure 5 exhibits another embodiment of a pickling line 5 using the claimed process. Figure 6 exhibits the influence: of the current type on the pickling time.

Figure 7 exhibits the influence of the bath temperature on the pickling time.

Figure 8 exhibits the influence of the bath add concentration on the pickling time.

Figure 9 exhibits the influence of the current density on the pickling time.

Figures 10-12 exhibit the influence of the current frequency on the pickling time at different conditions.

Figures 13 and 14 exhibit the influence of the current ratio between the anodic and cathodic period on the pickling time.

As illustrated in Figure 4, the invention relates to a pickling process of a metallic strip 9 comprising the steps of:

- passing said metallic strip through at least a pickling bath 7 being at a temperature between 1 and 100ºC,

- applying an alternating current, having a current density of 1×10 ² to 1x10 ⁵ A.m ^-2 of unit surface of said metallic strip to said metallic strip passing through said at least one pickling bath and having an anodic period and a cathodic period, applied on said metallic strip passing through said at least one pickling bath has a cathodic/anodic pulse length ratio of 0.1 and 5,0.

This claimed pickling process is preferentially done downstream a hot rolling operation and even more preferentially downstream a scale breaking operation. The claimed pickling process is preferentially done upstream of a cold rolling operation and/or of a coating operation such as a hot-dip coating process. The pickling bath is contained in a pickling tank 6 as illustrated in Figure 4. Said pickling tank is preferably made of at least one of the followings: raw materials bricks, granite or ebonite bricks, polypropylene, high-density polyethylene (HOPE) and/or polypropylene homopolymer (PPHF) which permits to increase its lifetime in the pickling condition. The tank is preferably equipped with means capable of moving the strip through the bath, such as conveying tolls 10. As illustrated in Figure 4, the bath can be equipped with four conveying rolls 10, a pair on the entry side 11 and another pair on the exit side 12 of the tank. In each pair, one is completely immerged in said pickling bath and one is not immerged in said pickling bath. The pickling line is also preferably equipped with means to add and/or regenerate the pickling solution such as acid regeneration plant (ARP) which is not represented in Figure 3. Generally, the fresh/ regenerated pickling liquor is added into the last pickling tank, then it cascades from the last tank to the first tank, where the used pickling liquor is evacuated to the regeneration station (if it exists or to the storage tank). The acid flows are regulated by the pumps.

The pickling bath can be any pickling bath known by the skilled in the art. Preferably, the pickling bath comprises at least a pickling add and/ or a pickling salt at a concentration between 10 and 360 g-L ^-1 . Even more preferably, the pickling bath 7 comprises at least a pickling add or a pickling salt. The pickling adds or salts are preferentially one of the following: hydrochloric add (HQ), sulfuric add (H ₂ SO ₄ ), potassium chloride (KCl ), sodium chloride (NaQ), sodium sulfate (Na ₃ SO ₄ ), potassium sulfate (K ₃ SO ₄ ) or nitric add. Due to the pickling and thus matter removal, the pickling bath can also comprise undesired materials such as dissolved metals resulting from the pickling operation (iron ions, other typical alloying elements or impurities in steel as Mn, Si, Al, Cr, Ni, Co, Ti, V, Nb, Mo, Cu, C, S, P, B, N,....) as well as solid particles of low dissolving oxides depositing in the tanks walls or in the circuits as silica, alumina, mixed phases like fayalite (Fe ₄ SiO ₄ ), FeAl ₂ O ₄ , Mn containing spinels (Mn ₂ SiO ₄ , MnAl ₂ O ₄ ,...)· Moreover, due to the working condition, the pickling bath can also comprise an over-pickling inhibitor which protects the sted surface by limiting the sted dissolution in the pickling bath.

Said alternating current has a current density of 10 ³ to 10 ⁵ Am ^-2 of unit surface of said metallic strip. It means that a spot of the strip (and/or of the scale) will receive an alternating current, as previously defined, when it passes through the pickling bath 7 which helps the removal of the scale layer. For example, an alternating current is applied on said spot during at least 3 seconds.

The alternating current is applied by any possible means. The alternating current can be of any waveform such as square, triangular, sine or complex. Preferably, as illustrated in Figure 4, the alternating current is applied using a series of electrodes 13, forming an alternation of anode 13 a and cathode 13 b, facing the metallic strip. The alternation of anode and cathode is preferably made by applying cither a positive Or negative current to said electrodes. Preferably, both strip faces are faced with electrodes. The electrodes are immerged in the pickling bath and preferably positioned at a distance between 1 and 30 cm from the moving metallic strip. Even more pteferahly, the electrodes are positioned at a distance between 1 and 10 cm from the moving metallic strip.

For example, as illustrated in Figure 5, in a pickling line comprising four pickling tanks (6 6a, 6b, 6c), the metallic strip can undergo a chemical pickling process in the three first tanks and the claimed pickling process in the fourth tank. Another possibility is to apply the claimed process in all the baths of a pickling line. Furthermore, the number of pickling tanks in the line configurations can vary from 1 to 6 and the claimed pickling process can be canied in at least one of them to all of them.

The positive influence, on the pickling time, of an alternating current compared to a direct current can be observed in Figure 6 wherein all experiments arc carried out in a bath having a HCl acid at the concentration of 100 g.L ^-1 . The pickling time is plotted in function of the current type (direct or alternating) applied to a steel sample having a scale layer of around 5 μm, for various current density: The AC experiments are carried out at 50Hz oscillating current with equal cathodic/anodic pulse length, i.e. ratio 1:1). It can be observed that all other parameters being equal, for an alternating current, the pickling time is smaller of 33% in average. Moreover, the gain in pickling time increases with the increase of the current density and at 10 ⁴ A.m ^-2 , the alternating current gives a gain in pickling time of about 40% relatively to the direct current. Consequently, the efficiency of the claimed pickling process is improved compared to an electro-assisted pickling using a direct current

Preferably, said metallic strip is made of steel.

Pteferahly, said alternating current has a frequency between 0.5 and 100 Hz.

Preferably, said metallic strip is passing through the bath at a speed comprised between 10 m.min ^-1 and 450 m-rnin ^-1 . Preferably, said alternating current is applied during at least 5 seconds to said metallic strip passing through said at least one pickling bath which permits to increase the scale dissolution efficiency. Preferably, said alternating current is applied during maximum 600 seconds to said metallic strip passing through said at least one pickling bath. Even more preferably, said alternating current is applied during maximum 300 seconds to said metallic strip passing through said at least one pickling bath which permits to lower the electrical consumption while achieving satisfying scale dissolution rate.

Preferably, the pickling bath 7 comprises only one pickling add or only one pickling salt It permits to suppress the interaction between the pickling adds and pickling salts and thus have a more stable pickling bath.

Preferably, said pickling bath comprises hydrochloric add at a concentration between 10 and 360 g.L ^-1 . Preferably, said pickling bath comprises sulfuric add at a concentration between 10 and 360 g.L ^-1 .

Several experiments have been conducted to assess the influence of the selected process parameters on the pickling efficiency. The tests have been conducted on steel samples having the same surface state: a steel covered by a 5 μm thick layer of iron oxides (scale). Their pickling time, in function of the selected process parameters, were recorded. Then their brightness has been assessed by a spectrophotometer, the CM-2600d from ©Konica-Minolta. The pickling time corresponds to the time necessary to reach a brightness comprised between 60 and 75 which, without to be bound by any theory, indicates that the all (or almost all) the oxide layer has been removed. Smaller is the pickling time, better is the pickling efficiency. It should be noted that the brightness of the surface covered by the scale, prior to its pickling, is of about 30 units and the brightness of a metallic steel without scale is typically in a range between 60 and 75 units depending on the product chemistry and surface morphology (roughness). Thus, the brightness increase during pickling is linked to the removal of the scale.

Preferably, said pickling bath has a temperature of at least 40°C. It improves the efficiency of the pickling compared to pickling bath temperatures lower than 39.5°C, In Figure 7, the pickling time is plotted in function of the bath temperature for experiments carried out in bath having a HQ concentration of 100 g.L ^-1 at different temperatures and current densities. The oscillation frequency of AC current is of 50Hz and cathodic/anodic pulse length ratio of 1:1). It can be observed that all other parameters being equal, higher is the pickling bath temperature, smaller is the pickling time.

Preferably, said pickling bath has a pickling acid or pickling salt concentration of at least 30 g.L ^-1 , even more preferentially of at least 60 g.L ^-1 . Such an increased lower limit improves the pickling efficiency. In figure 8, the pickling time is plotted in function of the pickling bath add concentration at 40°C for an experimental condition carried out with AC current at a current density of 0.5x10 ⁴ A.m ^-2 and 50Hz: oscillation frequency with cathodic/ anodic pulse length ratio of 1:1), It can be observed that all other parameters being equal, higher is the pickling add concentration, smaller is the pickling time.

Preferably, said current density is of at least of 1 ×10 ³ A.m ^-2 of unit surface of said metallic strip to said metallic strip passing through said at least one pickling bath and even more preferably of at least 1x10 ⁴ A.m ^-2 . It permits to increase the pickling efficiency compared to lower current density. In Figure 9, the pickling time is plotted in function of the current density applied to the metallic strip at a 50 Hz frequency and a 1:1 cathodic/anodic pulse length ratio for a bath having a HQ concentration of 100 g.L ^-1 . It can be observed that all other parameters being equal, higher is the current density, smaller is the pickling time.

Preferably, said alternating current has a frequency of at least 15 Hz. Apparently, such a lower limit permits to increase the pickling efficency compared to lower frequency. Preferably, said alternating current has a frequency of maximum 50 Hz. Apparently, such an upper limit permits to increase the pickling efficiency compared to higher frequency. In Figures 10 to 12, the pickling time is plotted in function of the current frequency applied to the metallic strip (cathodic/ anodic pulse length ratio is 1:1 in these conditions) in pickling bath at 40°C having an add concentration of 100 g.L ^-1 for various pickling add and current density.

It can he observed that, without to be bound by any theory, it is preferable to increase the lower frequency limit to 15 Hz and to lower the upper frequency limit to 50 Hz.

Preferably, said alternating current, having an anodic period and a cathodic period, applied on said metallic strip passing through said at least one pickling bath has a cathodic/ anodic pulse length ratio of 0.3 and 4.0. Even more preferably, said alternating current, having an anodic period and a cathodic period, applied on said metallic strip passing through said at lest one pickling bath has a cathodic/anodic pulse length ratio of 1.1 and 2.7. Optimally, said alternating current, having an anodic period and a cathodic period, applied on said metallic strip passing through said at least one pickling bath has a cathodic/anodic pulse length ratio of 1.5 and 2.4. In Figures 13 and 14, the pickling time is plotted in function of the alternating current period ratio applied to the metallic strip for pickling bath at 40°C having an acid concentration of100 g.L ^-1 and a current density of 0.5 A.cm ^-2 . Apparently, it can be observed that all other parameters being equal, when the alternating current period ratio is in the claimed range, the pickling efficiency is improved.

The invention has been described above as to the embodiment which is supposed to be practical as well as preferable at present However, it should be understood that the invention is not limited to the embodiment disclosed in the specification and can be appropriately modified within the range that does not depart from the gist or spirit of the invention, which can be read from the appended claims and the overall specification.