EP4455374A1 - Method for electrolytic pickling of a hot-rolled strip - Google Patents

Abstract

Process for the electrolytic pickling of a hot strip, wherein the hot strip is first pickled in a first pickling bath containing an acidic medium with a cathodic polarization and then in a second pickling bath containing an acidic medium with an anodic polarization.

Description

The invention relates to a method for the electrolytic pickling of a hot strip, wherein the hot strip passes through at least two pickling baths.

The removal of surface defects, such as scale, during the production of hot strip is essential in order to be able to provide a virtually defect-free surface on a hot strip for good (further) processability in the production sequence and/or as a product. A virtually defect-free surface can be advantageous because the amount of flaps, i.e. residues or elevations from hot rolling and/or over-rolling from the hot strip stage, can be reduced and can subsequently lead to a minimization of surface defects in the as-rolled or annealed and optionally coated hot strip. Depending on the concept, in particular for the production of high-strength and ultra-high-strength steels, increased grain boundary oxidation can also occur in addition to flaps or over-rolling on the surface of the hot strip. The surface defects generated from the hot strip stage can be minimized by long pickling, but can lead to a different type of defect in the form of a fissured surface due to the pickling time being too long.

From the WO 2021/105738 A1 A process for electrolytic pickling of flat steel products is known. The key feature is that the pickling is carried out using an alternating current, which means that the scale on the surface of the flat steel product can be removed more quickly and with less pickling time than when pickling using direct current.

The object of the present invention is to provide a method for electrolytic pickling with which an essentially technically clean surface of the hot strip can be produced, in particular free of scale and free of damaged grain layers, such as grain boundary oxidation, discontinuities, carbon deposits, near-surface defects, etc., and/or embedded process gases, in particular diffusible hydrogen, can be reduced on the surface.

This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

According to the invention, the hot strip is first pickled in a first pickling bath containing an acidic medium with a cathodic polarization and then in a second pickling bath containing an acidic medium with an anodic polarization.

A very good result with the aim of an essentially technically clean surface is achieved by first electrolytic pickling in a first pickling bath containing an acidic medium. In this first pickling, hydrogen is specifically generated on the surface to be pickled by cathodic polarization (cathodic pickling), so that the hydrogen generation promotes an essentially "mechanical" flaking off of the scale. The large amount of hydrogen caused by the cathodic polarization can thus lead to an advantageous acceleration of the scale removal. With electroless pickling in a pickling bath containing an acid, however, the pickling process would not be completed for the same amount of time and thus the removal of the scale would be incomplete. This is followed by electrolytic pickling in a second pickling bath containing an acidic medium with anodic polarization (anodic pickling). This removes defects close to the surface, in particular damaged grain layers and imperfections on the surface. If "only" anodic pickling were carried out and no prior cathodic pickling were carried out, the scale layer would act as a temporary barrier layer and delay the anodic material removal.

For example, in order to bring about a targeted reduction of the diffusible hydrogen, cathodic pickling does not lead to a further introduction of hydrogen into the hot strip despite an increased hydrogen production near the surface or on the surface of the hot strip, but on the contrary causes a reduction of the stored hydrogen by escape during the "splintering".

The hot strip passes through at least two pickling baths, so that in the simplest embodiment the cathodic pickling bath is passed through first and then the anodic pickling bath in succession. In further embodiments the cathodic pickling bath (K) can also be carried out in two cathodic pickling baths (2K) or three cathodic pickling baths (3K) or more. In further embodiments the anodic pickling bath (A) can also be carried out in two anodic pickling baths (2A) or three anodic pickling baths (3A) or more. In further embodiments the hot strip can therefore be pickled in the following sequences: K + 2A; K + 3A; 2K + A; 3K + A; 2K + 2A; 3K + 2A and other analogous combinations. Other baths, in particular currentless or current-carrying baths, for example alkaline baths, can be passed through. The cathodic pickling bath and the anodic pickling bath can preferably be last or second to last if a rinsing step with optional drying step is provided for, in the sequence of a hot strip passing through several baths.

The surface properties achieved with the help of electrolytic pickling improve the forming properties, avoid flaps for the subsequent process sequence, such as cold rolling (cold strip), liquid metal embrittlement (LME), coatability/more uniform distribution and roughness/wsa value or others. In other words, the surface of the hot strip can be conditioned by the pickling according to the invention in such a way that process and product advantages result, in particular the fatigue strength or operational strength/service life can be improved/increased.

The hot strip consists of a steel material in a flat form as a hot-rolled flat steel product. The invention is particularly suitable for high-strength, higher-strength and ultra-high-strength steel materials which, among other things, have a high tendency towards LME and can thus be improved. A hot strip is preferably used for use as multi-phase steels which are intended for the production of components by means of cold forming, with a tensile strength of at least 800 MPa, in particular at least 900 MPa, preferably at least 950 MPa and more, for example up to 1800 MPa, in particular up to 1600 MPa, preferably up to 1400 MPa. The invention is preferably suitable for so-called and known DP, CP and Q&P steels with tensile strengths between 980 and 1400 MPa. In addition and alternatively, other grades can also be produced from hot-rolled strip using suitable chemistry, in particular with tensile strengths below 800 MPa, for example BHZ grades from cold-rolled strip.

According to one embodiment, the acidic medium in the first pickling bath and in the second pickling bath can comprise an aqueous solution of an organic or inorganic acid selected from the group containing or consisting of hydrochloric acid, phosphorous acid, phosphoric acid, perchloric acid, nitrous acid, nitric acid, hydrofluoric acid, sulphurous acid, sulphuric acid or a mixture of two or more of these acids. The inorganic acid can individually or in total have a concentration of between 50 and 600 g/l, the remainder being water and unavoidable impurities. The concentration can in particular be at least 80 g/l, preferably at least 100 g/l and in particular a maximum of 550 g/l, preferably a maximum of 500 g/l.

According to one embodiment, the cathodic and the anodic polarization can be carried out with a current density between 10 and 200 A/dm ² . Higher current densities offer the possibility of, among other things, accelerating the pickling process compared to classic electroless pickling. For example, the cathodic polarization can be carried out with a current density between 40 and 200 A/dm ² , in particular between 50 and 150 A/dm ² , preferably between 60 and 120 A/dm ² , preferably between 70 and 100 A/dm ² . For example, the anodic polarization can be carried out with a current density between 10 and 200 A/dm ² , in particular between 20 and 150 A/dm ² , preferably between 30 and 100 A/dm ² , preferably between 30 and 80 A/dm ² .

According to one embodiment, the pickling time, which corresponds to the residence/immersion time of the hot strip during pickling, can be between 1 and 100 s in the first pickling bath and in the second pickling bath. In particular, the pickling time can be at least 2 s, preferably at least 3 s, preferably at least 5 s and in particular a maximum of 80 s, preferably a maximum of 60 s, preferably a maximum of 50 s. The pickling baths each have a temperature between -5 and 105 °C, in particular between 10 and 90 °C, preferably a temperature between 25 and 75 °C, preferably a temperature between 40 and 60 °C.

According to one embodiment, the acidic medium in the first and also in the second pickling bath can contain sulphuric acid with a concentration between 100 and 300 g/l, the remainder being water and unavoidable impurities. The concentration can in particular be at least 130 g/l, preferably at least 150 g/l.

Through cathodic and anodic pickling, in particular with an acidic medium containing sulphuric acid, the surface of the hot strip can be influenced in such a way that surface-characteristic parameters, including the mean roughness R _a, can be reduced or adjusted and the waviness characteristic value Wsa, determined according to SEP 1941, can be lowered, preferably when the aforementioned conditions are met. The Wsa value can therefore be a maximum of 0.3 µm, in particular a maximum of 0.25 µm, preferably a maximum of 0.2 µm, preferably a maximum of 0.18 µm, particularly preferably a maximum of 0.15 µm. This value is > 0.01 µm.

Since the entire near-surface chemistry is influenced by electrolytic pickling, material is removed down to the base material, whereby the grain layers damaged by internal oxidation can be essentially completely removed.

Rinsing with water and/or an aqueous solution can be carried out between the individual pickling baths. For this purpose, rinsing with water and/or an alcohol, for example selected from the group containing or consisting of methanol, ethanol, propanol, isopropanol, ethanol, in particular isopropanol or an aqueous solution, can be interrupted. In an alternative, rinsing takes place in two steps, in a first step with water; in a second step with an alcohol or an aqueous solution of an alcohol as specified above. In another alternative, rinsing with water and an alcohol takes place in one step, preferably as a mixture of water with one of the alcohols specified above. Rinsing is preferably carried out continuously, in particular a method selected from the group consisting of spraying, spraying, application (coil coating) and preferably dipping can be used. Preferably, drying is carried out after rinsing, preferably the "rinsed" surface is dried by increasing the temperature (up to a maximum of 100 °C) or by a blower.

Alternatively, the "rinsed" surface can be air-dried, for example without any additional aids.

In the following, specific embodiments of the invention are explained in more detail:
Several samples were cut from a hot-rolled strip of grade Q&P, here specifically from a Q&P 1180, which were pickled differently.

A series of initial samples were electrolytically pickled in two process steps by successively immersing or flowing around an electrolyte, with both electrolytes containing sulfuric acid with a concentration of 250 g/l. Both electrolytes had a temperature of 55 °C and were operated under the influence of current. For the first electrolyte, the immersion time was 60 s with an anodic current density of 60 A/dm ² and for the second electrolyte, the immersion time was 30 s with an anodic current density of 40 A/dm ² .

A series of second samples were electrolytically pickled in three process steps by successive immersion or rinsing with an electrolyte, whereby all three electrolytes contained sulphuric acid with a concentration of 260 g/l. The electrolytes all had a temperature of 55 °C and were operated under the influence of current. For the first electrolyte, the immersion time was 30 s with a cathodic current density of 80 A/dm ² . For the second electrolyte, the immersion time was 60 s with an anodic current density of 60 A/dm ² and for the third electrolyte, the immersion time was 30 s with an anodic current density of 40 A/dm ² .

After pickling, the samples were immersed in water at a temperature of approximately 20 °C (room temperature) for 2 to 4 seconds. The rinsed samples were then dried using a warm air blower for 30 seconds.

A pickling result of the differently pickled samples is shown in Figure 1 shown, on the left using the example of a first sample, on the right using the example of a second sample pickled according to the invention. It can be clearly seen that the samples pickled according to the invention produce a better pickling result.

Using GDOES, element enrichments of Al, Si, Cr and Cu were also determined, which shift due to the pickling according to the invention, partly increasing and partly decreasing the alloying elements Al, Si, Cr and Cu. An initial cathodic polarization is therefore necessary in order to obtain an acceptable scale removal image. Furthermore, a microscopic surface examination found that according to the invention a flat and more uniform (less roughness) hot strip surface and no pickled grain boundaries are present, shown by cross sections and SEM views in Figure 2 , on the left using the example of a second sample pickled according to the invention and on the right using the example of a first sample.

Furthermore, residual scale was detected in the first samples using GDOES, in particular grain boundary oxidation, which can be traced using light spectra. The grain boundary oxidation can be completely removed by the inventive combination of cathodic pickling with anodic pickling. In addition, complete scale removal is achieved, see left illustration in Figure 3 .

Some of the aforementioned hot strip samples were cold rolled and several samples were separated from the cold rolled samples (cold strip) and pickled (again). All cold strip samples were subjected to electrolytic pickling by immersion or rinsing with an electrolyte containing sulfuric acid with a concentration of 250 g/l. The temperature of the electrolyte was 20 °C (room temperature) and the immersion time was 60s. Pickling was carried out without electricity. After pickling, immersion rinsing was carried out for 2 to 4s in water with a temperature of approx. 20 °C (room temperature). The rinsed samples were then dried using a hot air blower for 30s.

Figure 4 shows the surfaces of the cold strip samples, using the example of a first sample on the left and the example of a second sample pickled according to the invention on the right. It can be clearly seen that the pickling of the hot strip according to the invention makes it possible to maintain an almost defect-free surface, even on the cold strip.

The cold-rolled strip samples were each coated with a zinc layer approximately 7 µm thick in a hot-dip coating process.

Subsequently, identical samples were MIG soldered in order to be able to carry out a joining characterization. The MIG soldering process, as a one-sided joining process, shows a tendency towards liquid metal induced cracking (LME). The tendency to cracking is characterized, for example, by soldering the lap joint with subsequent X-ray radiographic testing. The number of cracks can be reduced by pickling according to the invention, which showed an average of around 13 cracks in the solder joints compared to the first samples with an average of around 28 cracks. The invention is therefore suitable for reducing or even preventing LME.

Furthermore, hydrogen uptake could be reduced by up to 60% by the pickling process according to the invention. This was demonstrated by thermal desorption mass spectrometric analysis (TD-MS) as follows:
The first samples had a diffusible hydrogen content of mean/standard deviation approximately 0.033 ppm/0.001 ppm and the second samples had a diffusible hydrogen content of mean/standard deviation approximately 0.010 ppm/0.005 ppm.

Claims (9)

Method for the electrolytic pickling of a hot strip, wherein the hot strip passes through at least two pickling baths, characterized in that the hot strip is first pickled in a first pickling bath containing an acidic medium with a cathodic polarization and then in a second pickling bath containing an acidic medium with an anodic polarization. A method according to claim 1, wherein the acidic medium in the first pickling bath and in the second pickling bath comprises an aqueous solution of an inorganic acid selected from the group containing or consisting of hydrochloric acid, phosphorous acid, phosphoric acid, perchloric acid, nitrous acid, nitric acid, hydrofluoric acid, sulfurous acid, sulfuric acid or a mixture of two or more of these acids. Method according to one of the preceding claims, wherein the cathodic as well as the anodic polarization is carried out with a current density between 10 and 200 A/dm ² . Method according to one of the preceding claims, wherein the pickling times in the first pickling bath and in the second pickling bath are each between 1 and 100 s, wherein the pickling baths each have a temperature between -5 and 150 °C. Process according to one of the preceding claims, wherein the acidic medium in the first and also in the second pickling bath contains a sulphuric acid with a concentration between 100 and 300 g/l, the remainder being water and unavoidable impurities. Method according to one of the preceding claims, wherein the surface of the hot strip has a waviness characteristic value Wsa, determined according to SEP 1941, of maximum 0.3 µm. Method according to one of the preceding claims, wherein the hot strip consists of a steel material with a tensile strength of at least 800 MPa up to 1800 MPa. Method according to one of the preceding claims, wherein the hot strip consists of a DP or CP steel with a tensile strength between 980 MPa and 1400 MPa. A method according to any one of claims 1 to 7, wherein the hot strip consists of a Q&P steel having a tensile strength between 980 MPa and 1400 MPa.

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