Method for producing a cold-rolled strip

Abstract

The invention relates to a method for producing a coated cold-rolled strip according to claim 1.

Description

[0001] Process for producing a cold-rolled strip

[0002] The invention relates to a method for producing a cold-rolled strip, in particular a coated one, comprising the steps of: - providing a hot-rolled strip; - electrolytically pickling the hot-rolled strip; - producing a cold-rolled strip from the hot-rolled strip; - electrolytically treating the cold-rolled strip; - optionally coating the cold-rolled strip.

[0003] The removal of surface defects, such as scale, during the production of hot strip is essential to provide a virtually defect-free surface on the hot strip for good (further) processability in the production sequence. A virtually defect-free surface can be advantageous because the amount of flaps, i.e. residues or elevations from hot rolling, and / or overrolls 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, particularly for the production of high- and ultra-high-strength steels, increased grain boundary oxidation can also occur in addition to flaps or overrolls 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 if the pickling time is too long.

[0004] WO 2021 / 105738 A1 discloses a method for electrolytic pickling of flat steel products. The key feature is that the pickling process uses an alternating current, which allows the scale on the surface of the flat steel product to be removed more quickly and with a shorter pickling time than with a direct current pickling process.

[0005] Furthermore, DE 10 2018 219 199 Al discloses a process for treating a surface of flat steel products in an acidic solution with an anodically polarized current density of at least 22 A / dm 2 .

[0006] Painted components formed from cold-rolled strip with a corrosion-protection coating, especially electrolytically coated cold-rolled strip, can exhibit undesirable surface defects, such as paint defects. A primary cause of paint defects is discontinuities on the cold-rolled strip surface. These can lead to the deposition of process gases such as hydrogen in the cold-rolled strip surface, thus causing unwanted outgassing during the painting process (e.c.p. process). This can lead to the formation of craters and other defects in a paint layer that has not yet fully solidified.

[0007] The object of the present invention is to provide a method for producing a coated cold-rolled strip with which the formation of surface defects during production can be substantially reduced or avoided.

[0008] This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

[0009] In order to achieve the object, the inventors have found that the formation of surface defects can be substantially reduced or avoided if the electrolytic treatment of the cold strip provides for at least cleaning of a surface of the cold strip with an aqueous solution of an inorganic acid, wherein the cleaning is carried out under the influence of current with an anodic current density between 1 and 120 A / dm 2 is carried out.

[0010] The cleaning of at least one, in particular both, surfaces during the treatment of the cold strip with an aqueous solution of an inorganic acid before coating under the influence of current with an anodic current density between 1 and 120 A / dm 2This has a positive impact on the desired result. By partially or completely reducing defects on the cold-rolled strip surface, paint defects and other defects in the paint layer can be reduced or even avoided on the subsequently painted component. To save energy, the anodic current density can be increased to a maximum of 100 A / dm 2 , preferably to a maximum of 90 A / dm 2 , preferably to a maximum of 85 A / dm 2 To promote the effect, the current density can be at least 5 A / dm 2 , preferably at least 15 A / dm 2 , preferably at least 25 A / dm 2 be.

[0011] Wetting the surface(s) of the cold-rolled strip with an acidic cleaner, i.e., an aqueous solution of an inorganic acid, is carried out using a process selected from the group consisting of spraying, atomizing, coating (coil coating process), and preferably dipping. Cleaning can be performed in one process step, two process steps, or more than two process steps.

[0012] The aqueous solution can be wetted at a temperature which, for example, corresponds at least to room temperature, approximately 20°C. In particular, the effectiveness and thus the cleaning effect can be improved by tempering the solution to at least 25°C, 30°C, preferably at least 35°C, 40°C, more preferably at least 45°C, 50°C and ensuring complete wetting of the cold strip surface. The temperature of the aqueous solution can be limited to a maximum temperature of 90°C. At a temperature above this, no positive effect can be discerned and this would only unnecessarily consume energy, which is required for heating the solution. Therefore, the temperature can be limited, in particular, to a maximum of 85°C, preferably a maximum of 80°C, more preferably a maximum of 75°C.

[0013] The surface of the cold-rolled strip can be wetted with an aqueous solution of an inorganic acid for a time of 2 to 100 seconds. The surface can be wetted with an aqueous solution of an inorganic acid for a time of at least 5 seconds, in particular at least 7 seconds, preferably at least 8 seconds, 9 seconds, preferably at least 10 seconds, and a maximum of 90 seconds, in particular a maximum of 80 seconds, preferably a maximum of 70 seconds, 60 seconds, 50 seconds, and preferably a maximum of 45 seconds.

[0014] According to one embodiment, the aqueous solution can contain sulfuric acid at a concentration between 10 and 300 g / l, the remainder being water and unavoidable impurities. The concentration can be at least 15 g / l, preferably at least 20 g / l, and in particular at most 290 g / l, preferably at most 280 g / l, and most preferably at most 270 g / l.

[0015] According to one embodiment, the aqueous solution may additionally or alternatively contain nitric acid at a concentration between 10 and 300 g / l, the remainder being water and unavoidable impurities. The concentration may in particular be at least 15 g / l, preferably at least 20 g / l, and in particular a maximum of 290 g / l, preferably a maximum of 280 g / l, and more preferably a maximum of 270 g / l.

[0016] According to one embodiment, the aqueous solution may additionally or alternatively contain a phosphoric acid with a concentration between 10 and 300 g / l, the remainder being water and unavoidable impurities. The concentration may in particular be at least 15 g / l, preferably at least 20 g / l, and in particular a maximum of 290 g / l, preferably a maximum of 280 g / l, and more preferably a maximum of 270 g / l.

[0017] According to one embodiment, the aqueous solution may additionally or alternatively contain hydrochloric acid at a concentration between 10 and 300 g / l, the remainder being water and unavoidable impurities. The concentration may in particular be at least 15 g / l, preferably at least 20 g / l, and in particular a maximum of 290 g / l, preferably a maximum of 280 g / l, and more preferably a maximum of 270 g / l.

[0018] According to one embodiment, the treatment can additionally provide for at least one degreasing step in addition to the at least one cleaning step. In the at least one degreasing process step, which can also comprise multiple steps, the surface(s) of the cold-rolled strip can be wetted with an aqueous solution of alkali metal hydroxides for a time of 1 to 50 s and at a temperature of 20°C to 90°C. The alkaline degreasing serves to effectively remove the surface(s) of the cold-rolled strip that are contaminated and / or contaminated with emulsions, such as rolling oil and / or corrosion protection oil. This effectiveness can be positively influenced by increasing the temperature. The surface(s) are wetted with the aqueous solution of alkali metal hydroxides for a time of in particular at least 2 s, preferably at least 2.5 s, more preferably at least 3 s, and in particular a maximum of 45 s, preferably a maximum of 40 s, more preferably a maximum of 35 s.Wetting the surface(s) of the cold strip with an alkaline degreasing agent takes place at a temperature of 20 °C to 90 °C, in particular 30 °C to 85 °C, preferably 40 °C to 80 °C, more preferably 45 °C to 75 °C, particularly preferably 50 °C to 70 °C. Degreasing can also be carried out anodically and / or cathodically under the influence of current with a current density between 1 and 50 A / dm. 2 The current density can be at least 2 A / dm 2 , preferably at least 3 A / dm 2 and a maximum of 40 A / dm 2 , preferably to a maximum of 30 A / dm 2 , preferably to a maximum of 20 A / dm 2 Degreasing can preferably be carried out before cleaning.

[0019] Sodium hydroxide or potassium hydroxide are preferred alkali metal hydroxides. They are formed by the reaction of alkali metals or alkali metal oxides with water. Alkali metal hydroxides, either individually or in total, can have a concentration between 5 and 200 g / l, the remainder being water and unavoidable impurities. The concentration can be, in particular, at least 10 g / l, preferably at least 15 g / l, and especially not more than 150 g / l, preferably not more than 100 g / l, and more preferably not more than 50 g / l.

[0020] The wetting of the surface(s) of the cold rolled strip with an alkaline degreaser can be carried out by a method selected from the group consisting of spraying, spraying, application and preferably dipping.

[0021] In addition to at least one cleaning step, the treatment may also include at least one pickling step. In the at least one pickling process step, which may also comprise multiple steps, the surface(s) of the cold-rolled strip can be wetted with an aqueous solution of an inorganic acid for a time of 1 to 50 s and at a temperature of 10°C to 70°C. Acidic pickling removes parts of the surface(s) of the cold-rolled strip, with the aim of avoiding excessive material loss by reducing the temperature, so that the wetting of the surface(s) of the cold-rolled strip with acidic pickling takes place at a temperature of 10°C to 65°C, in particular 10°C to 60°C, preferably 10°C to 55°C, more preferably 15°C to 50°C.The surface(s) are wetted with the aqueous solution of an inorganic acid for a time of, in particular, at least 2 s, preferably at least 5 s, more preferably at least 10 s, and in particular, a maximum of 45 s, preferably a maximum of 40 s, more preferably a maximum of 35 s. Pickling can preferably be carried out before cleaning. Furthermore, it is possible to perform electrolytic cleaning, which is carried out under the influence of current with an anodic current density between 1 and 120 A / dm. 2 is carried out in at least one step during the pickling process.

[0022] An inorganic acid for pickling is selected from the group containing or consisting of: hydrochloric acid, phosphorous acid, phosphoric acid, nitrous acid, nitric acid, hydrofluoric acid, sulfurous acid, sulfuric acid or a mixture of two or more of these acids used as an aqueous solution. The inorganic acid can individually or in total have a concentration between 20 and 200 g / l, the remainder being water and unavoidable impurities. The concentration can in particular be at least 30 g / l, preferably at least 40 g / l and in particular a maximum of 150 g / l, preferably a maximum of 120 g / l, more preferably a maximum of 100 g / l. For example, the treatment can each comprise at least one process step of alkaline degreasing, acidic pickling, and acidic cleaning, preferably in the sequence of degreasing, pickling, and cleaning.

[0023] The wetting during optional degreasing, optional pickling, and cleaning can be terminated by rinsing with water and / or an aqueous solution. For this purpose, the wetting is interrupted by 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. In one alternative, the rinsing takes place in two sub-steps: in a first sub-step with water, and in a second sub-step with an alcohol or an aqueous solution of an alcohol as specified above. In another alternative, the rinsing takes place with water and an alcohol in one step, preferably as a mixture of water with one of the alcohols specified above. Rinsing is preferably carried out continuously, and in particular a method selected from the group consisting of spraying, atomizing, dipping, and application can be used.Preferably, after wetting by rinsing, drying is carried out, whereby the “rinsed” surface is preferably dried by increasing the temperature (up to a maximum of 100 °C) or by a fan.

[0024] Alternatively, the rinsed surface can be air-dried, for example without any additional aids.

[0025] In order to provide a surface of the hot strip which is substantially free of defects or reduced, according to one embodiment of the invention the hot strip is electrolytically pickled and passes through at least two pickling baths, 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.

[0026] A very good result with the goal of a substantially technically clean surface can be 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 through cathodic polarization (cathodic pickling), so that the hydrogen generation promotes an essentially "mechanical" spalling of the scale. The large amount of hydrogen caused by the cathodic polarization can thus advantageously accelerate scale removal. With electroless pickling in a pickling bath containing an acid, however, the pickling process would not be completed for the same duration, and thus the scale removal would be incomplete. This is followed by electrolytic pickling in a second pickling bath containing an acidic medium with anodic polarization (anodic pickling).In this process, near-surface defects, especially damaged grain layers and discontinuities, are removed. If only anodic pickling were performed and no prior cathodic pickling were performed, the scale layer would likely temporarily act as a barrier layer and delay the anodic material removal.

[0027] For example, in order to achieve a targeted reduction of diffusible hydrogen, cathodic pickling does not lead to a further introduction of hydrogen into the hot strip despite increased hydrogen production near or on the surface of the hot strip, but on the contrary causes a reduction of the stored hydrogen by escape during the "scaling off".

[0028] The hot strip passes through at least two pickling baths, so that in the simplest embodiment, the cathodic pickling bath is passed through first, followed by the anodic pickling bath. 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. Thus, in further embodiments, the hot strip can 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 electroless or electrostatic baths, for example alkaline baths, can be passed through.The cathodic pickling bath and the anodic pickling bath may preferably be last or second to last if a rinsing step with optional drying step is provided, in the sequence of a hot strip passing through several baths.

[0029] According to one embodiment, the acidic medium in the first pickling bath and 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, sulfurous acid, sulfuric acid, or a mixture of two or more of these acids used as an aqueous solution. The inorganic acid can individually or in total have a concentration 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 at most 550 g / l, preferably at most 500 g / l.

[0030] According to one embodiment, the cathodic as well as the anodic polarization can be carried out with a current density between 10 and 200 A / dm 2Higher current densities offer the possibility of accelerating the pickling process compared to electroless conventional pickling. For example, cathodic polarization can be carried out with a current density between 40 and 200 A / dm 2 , especially 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, anodic polarization can be carried out with a current density between 10 and 200 A / dm 2 , especially between 20 and 150 A / dm 2 , preferably between 30 and 100 A / dm 2 , preferably between 30 and 80 A / dm 2 be performed.

[0031] According to one embodiment, the pickling time, which corresponds to the residence time / immersion time of the hot strip during pickling, can be between 1 and 100 s in the first pickling bath and the second pickling bath, respectively. 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.

[0032] According to one embodiment, the acidic medium in the first and second pickling baths can each contain sulfuric 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.

[0033] Since the entire near-surface chemistry is influenced by electrolytic pickling, material removal can occur down to the base material of the hot strip, whereby the grain layers damaged by internal oxidation can be essentially completely removed.

[0034] Rinsing with water and / or an aqueous solution can be carried out between the individual pickling baths. For this purpose, the rinsing can be interrupted 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. In one alternative, the rinsing takes place in two sub-steps: in a first sub-step with water; in a second sub-step with an alcohol or an aqueous solution of an alcohol as specified above. In another alternative, the 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, and in particular a method selected from the group consisting of spraying, atomizing, application (coil coating) and preferably dipping can be used.Preferably, drying is carried out after rinsing, whereby the “rinsed” surface is preferably dried by increasing the temperature (up to a maximum of 100 °C) or by a fan.

[0035] Alternatively, the rinsed surface can be air-dried, for example without any additional aids.

[0036] The electrolytic cleaning of the cold strip surface as well as the electrolytic pickling of the hot strip surface can be carried out either with a DC current for the duration of the treatment or with an AC current, as disclosed, for example, in WO 2021 / 105738 A1.

[0037] The cold-rolled strip can be further processed uncoated.

[0038] The cold-rolled strip can also be further processed in the so-called “as-rolled” state, either coated or uncoated.

[0039] For example, after electrolytic treatment, the cold-rolled strip can be subjected to heat treatment to achieve the desired material properties, which depend particularly on the material composition. Known annealing equipment is used for heat treatment, such as continuous annealing or batch annealing. Other annealing equipment, which operates inductively and / or conductively, preferably in conjunction with rapid cooling, can also be used.

[0040] According to a preferred embodiment, the cold-rolled strip can be electrolytically coated, in particular with a zinc-based coating, also known in specialist circles as "ZE" or "EG." Methods and devices for electrolytic coating are state of the art.

[0041] According to an alternative preferred embodiment, the cold-rolled strip can be hot-dip coated, in particular with a zinc-based coating. In addition to zinc and unavoidable impurities, the coating can contain additional elements such as aluminum with a content of up to 8 wt.%, in particular up to 5 wt.% and / or magnesium with a content of up to 8 wt.%, in particular up to 5 wt.%. Cold-rolled strip with a zinc-based coating has very good cathodic corrosion protection and has been used in automotive construction for years. If improved corrosion protection is required, the coating additionally has magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt. Aluminum can be used alternatively or in addition to magnesium with a content of at least 0.1 wt.%, in particular at least 0.3 wt.-%, for example, to improve the bonding of the coating to the cold-rolled strip. The coating thickness per side can be between 1.5 and 30 pm, in particular between 2 and 20 pm, preferably between 3 and 15 pm. Below the minimum limit, adequate cathodic corrosion protection cannot be guaranteed, and above the maximum limit, joining problems can arise when connecting a component made from it to another component. In particular, if the coating thickness exceeds the specified maximum limit, a stable process cannot be ensured during thermal joining or welding.

[0042] If the coating contains magnesium and aluminum in addition to zinc and unavoidable impurities, the coating or coating is known in the technical world as zinc-magnesium, ZM or Zn-Al-Mg.

[0043] The aluminum content can be between 1.1 and 8 wt.%, in particular 1.2 to 5 wt.%.

[0044] The magnesium content can be between 1.1 and 8 wt%, in particular 1.2 to 5 wt%.

[0045] The coating may also contain only zinc with small amounts of aluminum (> 0 up to 0.5 wt.%) in addition to unavoidable impurities, also known in technical circles as "Z" or "Gl." See, for example, DIN EN ISO 10346:2009-07, Chapter 3, Section 3.1. If an additional heat treatment is carried out afterwards, also known in technical circles as "galvannealing," this is referred to as "ZF" or "GA." See, for example, DIN EN ISO 10346:2009-07, Chapter 3, Section 3.2.

[0046] The coating can also be a “ZA” or “AZ” or “AS” known in specialist circles, see for example DIN EN ISO 10346:2009-07, Chapter 3, Sections 3.2 to 3.5.

[0047] As unavoidable impurities, for example, elements from the group silicon, antimony, lead, titanium, calcium, manganese, tin, lanthanum, cerium and chromium may be present individually or in combination with a total of up to 0.5 wt.%, in particular up to 0.3 wt.% in the coating.

[0048] The hot-rolled strip or cold-rolled strip is a carbon steel.

[0049] The hot-rolled strip or cold-rolled strip consists of a steel material, comprising a steel containing, in addition to Fe and unavoidable impurities (all values ​​in wt.%):

[0050] 0.0010 to 1.00, preferably 0.0100 to 0.500, particularly preferably 0.110 to 0.200, C, 0.0010 to 5.00, preferably 0.0100 to 0.750, preferably 0.120 to 0.500, Si, 0.500 to 10.00, preferably, 0.500 to 4.00, preferably 1.00 to 3.00, Mn, up to 0.1500, preferably 0.0002 to 0.100, particularly preferably 0.0002 to 0.0500, P, up to 0.100, preferably 0.0005 to 0.0200, particularly preferably 0.0005 to 0.0100, S, up to 10.00, preferably 0.0010 to 2.00, particularly preferably 0.0150 to 1.00, Al, up to 5.00, preferably 0.0001 to 2.00, particularly preferably 0.0010 to 1.00, Cr, up to 2.00, preferably 0.0001 to 0.500, particularly preferably 0.0001 to 0.250, Cu, up to 2.00, preferably 0.0001 to 0.500, particularly preferably 0.0001 to 0.250, Mo, up to 0.0100, preferably 0.0002 to 0.0100, particularly preferably 0.0002 to 0.0090, N, up to 3.00, preferably 0.0001 to 0.400, particularly preferably 0.0001 to 0.200, Ni, up to 1.00, preferably 0.0001 to 0.200, particularly preferably 0.0001 to 0.100, Nb, up to 1.500,preferably 0.0001 to 0.200, particularly preferably 0.0025 to 0.100, Ti, up to 1.00, preferably 0.0001 to 0.0800, particularly preferably 0.0001 to 0.0400, V, up to 0.0050, preferably 0.0005 to 0.0050, particularly preferably 0.0005 to 0.0040, B, up to 3.00, preferably 0.0001 to 0.500, particularly preferably 0.0001 to 0.200, W, up to 0.0080, preferably 0.0001 to 0.0080, particularly preferably 0.0001 to 0.0060, Ca, up to 0.500 rare earth metals (SEM).,

[0051] Depending on the composition and heat treatment, cold-rolled strip made of a steel material can be produced with a tensile strength Rm of at least 120 MPa up to 1800 MPa, in particular up to 1600 MPa, preferably up to 1500 MPa. The tensile strength can be determined according to DIN EN ISO 6892-1:2020-06. The produced cold-rolled strip is intended for the manufacture of components, in particular automotive components, by cold forming. Depending on the composition of the steel material and a desired property such as tensile strength, yield strength, elongation at break, hole expansion ratio, etc., the expert knows the annealing conditions after cold-rolled strip production in order to specifically adjust these properties depending on the composition (chemistry) by heat treatment before the preferred coating. These range from soft steels (deep-drawing steels) to high-strength, higher-strength, and ultra-high-strength steels, see, for example, material data sheet VDA 239-100, May 2016 edition.The latter may, among other things, have a high tendency to LME and may preferably be improved by applying the invention.

[0052] The cold-rolled strip may be a multi-phase steel with a tensile strength Rm of at least 400 MPa, in particular at least 500 MPa, preferably at least 600 MPa and more, for example up to 1500 MPa, in particular up to 1400 MPa, preferably up to 1300 MPa, see for example also DIN EN ISO 10346:2009-07, Table 4 in conjunction with Table 9.

[0053] The invention is preferably suitable for so-called and known DP, CP and Q&P steels with a tensile strength Rm between 780 and 1500 MPa.

[0054] Q&P steels are third-generation "advanced high strength steels," or AHSS for short (see also http: / / ahssinsights.org / ?s=QD+steel). The production of this type is well known in the industry.

[0055] In the following, specific embodiments of the invention are explained in more detail.

[0056] Several samples were cut from a hot-rolled strip with a special composition for Q&P, in this case specifically for producing a cold-rolled strip made of Q&P 1180, and then pickled differently. A series of initial samples were electrolytically pickled in two process steps, each involving successive immersion or circulation with an electrolyte, with both electrolytes containing sulfuric acid at a concentration of 250 g / l. Both electrolytes had a temperature of 55 °C and were operated under current. For the first electrolyte, the immersion time was 60 s with an anodic current density of 60 A / dm³. 2 and for the second electrolyte, the immersion time was 30 s with an anodic current density of 40 A / dm 2 .

[0057] A series of second samples were electrolytically pickled in three process steps, each consisting of successive immersion or flushing with an electrolyte. All three electrolytes contained sulfuric acid at a concentration of 260 g / l. The electrolytes all had a temperature of 55 °C and were operated under current. For the first electrolyte, the immersion time was 30 s with a cathodic current density of 80 A / dm³. 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 .

[0058] After pickling, the samples were immersed in water at approximately 20 °C (room temperature) for 2 to 15 seconds. The rinsed samples were then dried using a hot air blower for 30 seconds.

[0059] The pickling results of the differently pickled samples are shown in Figure 1, with a first sample on the left and a second sample pickled according to the invention on the right. It is clearly visible that the samples pickled according to the invention produce a better pickling result.

[0060] GDOES was also used to determine element enrichments of Al, Si, Cr, and Cu, which shift due to the pickling according to the invention, partly increasing and partly decreasing the alloying elements Al, Si, Cr, and Cu. Initial cathodic polarization is therefore necessary to obtain an acceptable scale removal pattern. Furthermore, a microscopic surface examination showed that the invention results in a flat and more uniform (less roughness) hot strip surface and no pickled grain boundaries. This is shown by cross-sections and SEM top 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 understood using light spectra.Grain boundary oxidation can be completely removed by the inventive combination of cathodic and anodic pickling. Furthermore, complete scale removal is achieved, see left illustration in Figure 3.

[0061] The hot-rolled strip samples were cold-rolled on a laboratory scale and then subjected to a known annealing treatment to produce a Q&P 1180 under laboratory conditions. Several samples were cut from the cold-rolled and heat-treated cold-rolled strip samples.

[0062] The samples were treated using different process steps and then coated conventionally with a zinc-based coating in a coating system.

[0063] Degreasing was performed in two process steps by successively immersing the parts in an electrolyte. Both electrolytes contained sodium hydroxide at a concentration of 50 g / l each, with the remainder being water and unavoidable impurities. Both electrolytes had a temperature of 68 °C and were operated under current. For the first electrolyte, the immersion time (wetting) was 30 s with an anodic current density of 4 A / dm. 2 and for the second electrolyte, the immersion time (wetting) was 30 s with a cathodic current density of 4 A / dm 2 . -> Treatment A

[0064] Alternatively, only electrolyte could be used, whereby a polarity reversal can take place after a defined immersion time, so that degreasing can also be carried out in one process step and thus in one tank.

[0065] After treatment A, immersion rinsing was carried out for 2 to 4 s in water with a temperature of approximately 20 °C (room temperature).

[0066] Pickling was performed in a single process step by immersion in an electrolyte containing hydrochloric acid at a concentration of 80 g / l, the remainder being water and unavoidable impurities. The electrolyte temperature was 20 °C (room temperature), and the immersion time (wetting) was 21 s. -> Treatment B: After treatment B, the sample was immersed in water at a temperature of approximately 20 °C (room temperature) for 2 to 4 s.

[0067] Cleaning was performed in a single process step by immersion in an electrolyte containing sulfuric acid at a concentration of 25 g / l, the remainder being water and unavoidable impurities. The electrolyte temperature was 55 °C and the immersion time (wetting) was 15 s with an anodic current density of 80 A / dm³. 2 . -> Treatment C

[0068] Cleaning was performed in a single process step by immersion in an electrolyte containing sulfuric acid at a concentration of 25 g / l, the remainder being water and unavoidable impurities. The electrolyte temperature was 55 °C and the immersion time (wetting) was 30 s with an anodic current density of 80 A / dm³. 2 . -> Treatment D

[0069] Cleaning was carried out in two process steps by successive immersion in an electrolyte, each containing sulfuric acid at a concentration of 25 g / l, the remainder water, and unavoidable impurities. Both electrolytes had a temperature of 55 °C and were operated under current. For the first electrolyte, the immersion time (wetting) was 15 s with an anodic current density of 80 A / dm³. 2 and for the second electrolyte, the immersion time (wetting) was 15 s with an anodic current density of 20 A / dm 2. -> Treatment E

[0070] Cleaning was performed in a single process step by immersion in an electrolyte containing sulfuric acid at a concentration of 250 g / l, the remainder being water and unavoidable impurities. The electrolyte temperature was 55 °C and the immersion time (wetting) was 15 s with an anodic current density of 80 A / dm³. 2 . -> Treatment F

[0071] Cleaning was performed in a single process step by immersion in an electrolyte containing sulfuric acid at a concentration of 250 g / l, the remainder being water and unavoidable impurities. The electrolyte temperature was 55 °C and the immersion time (wetting) was 30 s with an anodic current density of 80 A / dm³. 2 . -> Treatment G

[0072] Cleaning was carried out in two process steps by successive immersion in an electrolyte, each containing sulfuric acid at a concentration of 250 g / l, the remainder water, and unavoidable impurities. Both electrolytes had a temperature of 55 °C and were operated under current. For the first electrolyte, the immersion time (wetting) was 15 s with an anodic current density of 80 A / dm³. 2 and for the second electrolyte, the immersion time (wetting) was 15 s with an anodic current density of 20 A / dm 2 . -> Treatment H

[0073] After treatments C to H, the strip was immersed in water at approximately 20 °C (room temperature) for 2 to 4 seconds. The rinsed cold strip was then dried for 30 seconds using a hot air blower.

[0074] The differently treated cold strip samples were then uniformly electrolytically galvanized by immersion in two consecutive electrolytes for 32 s each, each with a 60 °C warm electrolyte with zinc as the anode and a current density of 50 A / dm 2 . In the process, a zinc layer approximately 7 pm thick was deposited in the first electrolyte on the first surface and in the second electrolyte on the second surface of the cold strip.

[0075] Alternatively, a zinc-based hot-dip coating could also be used.

[0076] The electrolytically coated cold-rolled strip samples (samples VR to VF) were subjected to automotive-standard phosphating and cathodic dip coating.

[0077] The painted samples were tested. The data were recorded on a measuring area of ​​100 x 300 mm 2 determined and set to 600 cm 2The extrapolated area of ​​600 cm 2 Up to 10 paint defects are permissible; between > 10 and 25 paint defects are marginal and conditionally acceptable; and > 25 paint defects are unacceptable. The paint defects essentially corresponded to the sum of three parallel samples in terms of pustules and holes.

[0078] Table 1 shows the results of the evaluation of the individual samples with the corresponding pretreatment.

[0079] Table 1

[0080] It is clearly evident that treatment with acidic cleaning under the influence of current can reduce paint defects on painted components. In particular, the combination of the various treatments with acidic cleaning under the influence of current results in synergistic effects that lead to significantly better results than the theoretical sum of the individual treatments. This effect can be further enhanced if the hot-rolled strip is previously electrolytically pickled and passes through at least two pickling baths, with the hot-rolled strip first being pickled in a first pickling bath containing an acidic medium with cathodic polarization and then in a second pickling bath containing an acidic medium with anodic polarization.

Claims

Patent claims 1. A method for producing a cold-rolled strip, in particular a coated one, comprising the steps: - Providing a hot-rolled strip; - Electrolytic pickling of the hot strip; - Producing a cold strip from the hot strip; - Electrolytic treatment of the cold strip; - Optional coating of the cold strip; characterized in that the treatment provides for at least one cleaning of a surface of the cold strip with an aqueous solution of an inorganic acid, wherein the cleaning is carried out under the influence of current with an anodic current density between 1 and 120 A / dm 2 is carried out.

2. The process according to claim 1, wherein the acidic aqueous solution contains sulfuric acid, nitric acid, phosphoric acid and / or hydrochloric acid at a concentration between 10 and 300 g / l.

3. Method according to one of the preceding claims, the treatment in addition to the at least one cleaning additionally provides for at least one degreasing.

4. A method according to claim 3, wherein the degreasing is carried out under the influence of current anodically and / or cathodically with a current density between 1 and 50 A / dm 2 is carried out.

5. A method according to any one of the preceding claims, wherein the hot strip is electrolytically pickled and passes through at least two pickling baths, 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.

6. The method according to claim 3, 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, sulphurous Acid, sulfuric acid or a mixture of two or more of these acids as an aqueous solution.

7. A method according to claim 3 or 4, wherein the cathodic and anodic polarization are carried out with a current density between 10 and 200 A / dm 2 is carried out.

8. The method according to any one of claims 3 to 5, wherein the pickling time in the first pickling bath and in the second pickling bath is each between 1 and 100 s, wherein the pickling baths each have a temperature between -5 and 150 °C.

9. A process according to any one of claims 3 to 6, wherein the acidic medium in the first and also in the second pickling bath contains sulfuric acid with a concentration between 100 and 300 g / l, the remainder being water and unavoidable impurities.

10. Method according to one of the preceding claims, wherein the pretreatment, in addition to the at least one cleaning, additionally provides for at least one pickling.

11. Method according to one of the preceding claims, wherein the cold strip is electrolytically coated.

12. The method according to any one of claims 1 to 10, wherein the cold strip is hot-dip coated.

13. A method according to any one of the preceding claims, wherein a cold-rolled strip is produced from a steel material having a tensile strength Rm of at least 120 MPa up to 1800 MPa.

14. A method according to any one of the preceding claims, wherein a cold strip is produced from a DP or CP steel with a tensile strength Rm between 780 MPa and 1500 MPa.

15. A method according to any one of claims 1 to 13, wherein a cold-rolled strip is produced from a Q&P steel having a tensile strength Rm between 700 MPa and 1500 MPa.

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