DE102021200229A1 - Process for producing an electrolytically coated steel sheet - Google Patents

Abstract

The present invention relates to a method for producing an electrolytically coated steel sheet (1), the method comprising the following steps: providing a steel sheet; Electrolytic coating of the steel sheet (1). According to the invention, the coated steel sheet (1) undergoes a heat treatment (10) after the electrolytic coating, the coated steel sheet (1) being heated to a target temperature (Tz) of between 60° C. and 400° C. and for a holding time (tH) of between 0. 10 s and 300 s at target temperature (Tz).

Description

Technical Field

The invention relates to a method for producing an electrolytically coated steel sheet, the method comprising the following steps: - providing a steel sheet; - Electrolytic coating of the steel sheet.

Technical Background (Background Art)

The use of (metallically) coated sheet steel in vehicle construction is state of the art. Two coating systems, in particular zinc-based coatings, come into consideration here. Hot-dip coated and electrolytically coated steel sheets. Both coating variants have been established in vehicle construction for decades and have their advantages.

Coated steel sheets are formed into components in vehicle construction, which are usually fed to a painting process after shaping, often cathodic dip painting (KTL), which after dip painting involves heat treatment between 160 and 180°C for approx. 20 minutes to bake the paint is downstream. Painted components formed from electrolytically coated sheet steel have been found to have scratches on the paintwork, which are presumably caused by outgassing as a result of the temperature effect in the painting process and, as a result, the surface defects and the qualitatively unacceptable appearance mean that the affected components have to be scrapped or costly to rework would lead. There is therefore a need for optimization with regard to the electrolytically coated steel sheets.

Summary of Invention

The invention is therefore based on the object of providing a method for producing an electrolytically coated steel sheet, with which the aforementioned disadvantages can be essentially avoided or at least significantly reduced.

This problem is solved with the features of patent claim 1. Further advantageous configurations are listed in the dependent claims.

The method for producing an electrolytically coated steel sheet, in addition to the steps, comprises: providing a steel sheet; Electrolytic plating of the steel sheet, additionally the step that after the electrolytic plating, the plated steel sheet undergoes a heat treatment, wherein the plated steel sheet is heated to a target temperature between 60°C and 400°C and for a holding time between 0.10s and 300s target temperature is maintained.

According to the invention, it is provided that the electrolytically coated steel sheet is subjected to a heat treatment, so that the heat treatment, if present, recombines gaseous hydrogen in the steel sheet and can thus generate a high pressure, which allows the gas to pass through and thus escape through the closed electrolytically applied coating allows. As an alternative or in addition, other gaseous and/or liquid components that are present and embedded in the steel sheet can also escape or be expelled as a result of the heat treatment. These gaseous and/or liquid components were incorporated into the steel/base material or its near-surface structure during the preliminary stages of sheet steel production, e.g. during a pickling or annealing process. This effectively causes outgassing, which can no longer lead to paint irritation or surface defects in the painted state in the later painting process. The heat treatment according to the invention therefore does not correspond to the heat treatment in the painting process.

Here, electrolytic coating primarily means the deposition of a coating of zinc (ZE) or a zinc alloy, for example zinc-nickel, which can optionally be optimized with an organic top layer in the form of phosphating and/or chromating.

Steel sheet is to be understood as meaning a hot- or cold-rolled steel sheet which, in the planar undeformed state, can be provided in sheet form as a steel sheet blank or in strip form as a steel strip. A cold-rolled and, in particular, recrystallization-annealed steel strip is preferably provided as the steel sheet. The thickness of the steel sheet can be between 0.5 and 4 mm. The thickness is in particular at least 0.4 mm, preferably at least 0.5 mm and is in particular limited to a maximum of 3.5 mm, preferably to a maximum of 5 mm. The thickness of the electrolytic coating can be between 1.5 µm and 15 µm (per side).

The electrolytic coating of steel sheets, in particular steel strips, is state of the art. The steel sheets can be electrolytically coated on one or both sides, depending on the requirements. Zinc-based coatings are preferably used, which ensure a certain cathodic protection of the steel sheet. Such coatings are also known in specialist circles under the designation "ZE" (zinc electrolytic) or "EG" (electrogalvanized) in English-speaking countries. The steel sheets coated with ZE are also called "steel sheet" in professional circles. This includes all conceivable compositions of carbon steels which can be coated electrolytically and are preferably used in vehicle construction. This can include soft steels for cold forming according to DIN EN 10152, e.g. B. DC01 to DC07; high and higher strength steels for cold forming according to DIN EN 10268, e.g. B. high-strength IF steels such as HC180Y and higher, e.g. B. isotropic steels such as HC220I and higher, e.g. B. Bake-hardening steels such as HC180B and higher, e.g. B. micro-alloyed steels such as HC260LA and higher; or multi-phase steels for cold forming according to DIN 10338, on the one hand cold-rolled, e.g. B. Dual phase steels such as HCT450X and higher, e.g. B. retained austenite steels such as HCT690T and higher, e.g. B. complex-phase steels such as HCT600C and higher, or on the other hot-rolled, z. B. Ferrite-bainite steels such as HDT450F and higher, e.g. B. Dual-phase steels such as HDT580X, e.g. B. Complex phase steels such as HDT760C and/or CP-W1000, e.g. B. Martensite phase steels such as HDT1180G1, see also https://ucpcdn.thyssenkrupp.com/legacy/UCPthyssenkruppBAMXProcessingEurope/assets.files/downloads/tkmpe electrolytically galvanized thin sheet de 20190807.pdf. This enumeration of the possible sheet steel grades and the associated composition of the carbon steels is not intended to be conclusive.

Since electrolytic coating has no significant thermal influence on the steel sheet compared to hot-dip coating, the selection of the target temperature and the selection of the holding time must be adapted to the composition (carbon steel) of the steel sheet in particular, so that aging processes in particular are essentially avoided during heat treatment . The target temperature/holding time to be set optimally for the steel sheet used and in particular taking into account the available system technology can be verified either via “trial and error” tests or alternatively or additionally can also be set using specialist knowledge.

The target temperature and holding time are to be selected according to the invention in such a way that outgassing/diffusion takes place effectively, but the mechanical properties of the steel/base material are not changed. Furthermore, the target temperature and the holding time are to be selected in particular in such a way that preferably no alloying of the coating with the steel/base material can take place.

Further advantageous configurations and developments emerge from the following description. One or more features from the claims, the description and the drawing can be combined with one or more other features from them to further refine the invention. One or more features from the independent claims can also be linked by one or more other features.

According to one embodiment of the method according to the invention, the target temperature is at least 80° C., in particular at least 100° C., preferably at least 130° C., preferably at least 150° C. and at most 350° C., in particular at most 300° C., preferably at most 280° C., preferably maximum 250 °C.

According to one embodiment of the method according to the invention, the holding time is at least 0.50 s, in particular at least 1.0 s, preferably at least 1.50 s, preferably at least 2.0 s and at most 200 s, in particular at most 100 s, preferably at most 50 s, preferably at most 30 s, more preferably at most 20 s.

According to one embodiment of the method according to the invention, the heating to the target temperature can be carried out inductively, in a continuous furnace or by means of radiation sources, depending on the existing system concept and/or installation space. The heating is preferably carried out inductively, for example by means of inductors, which can set a heat pulse in a targeted manner or can be controlled accordingly in order to heat the coated steel sheet relatively quickly. In the case of inductive heating, heating rates of at least 20 K/s, in particular at least 50 K/s, preferably at least 80 K/s, can be achieved, as a result of which the installation space can be selected to be relatively small, preferably when the heat treatment is considered inline in the process direction, is carried out after the electrolytic plating. From a process and cost point of view, the heat treatment is preferably carried out inline as a post-treatment of a continuous electrolytic coating of steel strips.

In particular, the target temperature can be maintained, for example, within an enclosure through which the heated, coated steel sheet is passed. Holding can thus preferably take place in a continuous furnace.

According to one embodiment of the method according to the invention, the coated steel sheet is moved in the process direction during the heating, with the coated steel sheet being homogeneously heated transversely to the process direction. Homogeneous heating viewed transversely to the process direction means that over the (entire) width of the coated steel sheet, if possible no and only small temperature differences/differences are permitted during the heat treatment in order to ensure essentially "full-surface" outgassing over the entire surface of the coated steel sheet .

According to one embodiment of the method according to the invention, the coated steel sheet is actively cooled after the holding time. “Active” cooling means that the still warm coated steel sheet is cooled in a targeted manner using suitable means, for example known and common means that can bring about cooling, for example at a cooling rate of at least 5 K/s, in particular at least 10 K/s, preferably at least 20 K/s. For example, if the steel sheet is heat-treated inline after the electrolytic coating, it may be that the steel sheet should be cooled to a maximum temperature of 120 °C, in particular a maximum of 100 °C, in order to avoid other components in the process chain that are in contact with the still warm sheet steel, not to be thermally stressed.

Description of the preferred embodiments (Best Mode for Carrying out the Invention)

The only 1 shows a possible embodiment of the invention schematically. A steel sheet (1) has been provided in the form of a steel strip, which has been conventionally electrolytically coated or electrolytically galvanized in the opposite direction of the process, not shown, and in the process direction (P) a heat treatment (10th ) is supplied. Optionally, the electrolytic coated steel sheet (1) may be further phosphated and/or chromated before or after the heat treatment of the present invention. A phosphating and/or chromating to increase the paint adhesion and/or the corrosion resistance is not relevant for the implementation of the heat treatment according to the invention and is therefore not discussed further here. After the electrolytic coating, the coated steel sheet (1) undergoes a heat treatment (10), in this example inline in the process direction (P) after the electrolytic coating. The coated steel sheet (1) is heated to a target temperature (T _z ) of between 60° C. and 400° C. and held at the target temperature (T _z ) for a holding time (t _H ) of between 0.10 s and 300 s. The heating (11) to the target temperature (T _z ) is preferably carried out inductively, since the coated steel sheet (1) moved and coated in the process direction (P) is targeted and quick, in particular via an arrangement of an inductor (not shown) above and/or below the coated steel sheet (1 ) a homogeneous heating (11) transverse to the process direction (P) and thus over the (entire) width of the steel sheet (1) can also be ensured. Alternatively, a continuous furnace or a radiation source, not shown, can also be considered for the heating (11). After the coated steel strip (1), which has been heated to the target temperature (T _z ) and held at the target temperature (T _z ) for a holding time (t _H ), has passed through a holding zone (12) in the process direction (P), which is designed, for example, as a continuous furnace (12). active cooling (13) can take place, so that the still warm coated steel sheet (1) cannot have any negative thermal influences on the downstream components inline in the process chain. Active cooling (13) allows the temperature of the coated steel sheet (1) undergoing the heat treatment (10) to be reduced to a maximum of 120° C., for example. After the heat treatment (10) according to the invention, the coated steel sheet (1) can be subjected to further processing steps. Furthermore, the 1 a sketched temperature profile of an electrolytically coated steel sheet (1) heat-treated according to the invention in a time (t)-temperature (T) diagram. The temperature profile can be specified or defined individually and depending on the composition of the steel sheet as well as the existing system concept. The heat treatment can also take place offline.

Depending on the process conditions of the electrolytic coating, the deposited coating may have a more columnar or plate-like crystal structure. The method according to the invention is shown in tests to be independent of the morphology of the coating. Even in the case of a plate-shaped crystal structure with the associated lower porosity, which in particular does not favor outgassing, the task of the invention can still be achieved when the heat treatment is carried out according to the invention.

Tests were carried out on electrolytically galvanized steel sheets (1). In order to be able to carry out a comparison, two steel strips were provided as steel sheet (1), each an identical cold-rolled bake-hardening steel with a thickness of 0.8 mm, each electrolytically coated on both sides with the same parameters with a thickness of 5 μm per side have been galvanized. The first steel sheet was further processed conventionally as a reference. The second steel sheet (1) became inline, analogous to the sketched 1 , fed after the electrolytic coating of a heat treatment (10). The heating (11) took place on both sides via an inductor arranged across the width above and below the steel sheet (1), which in each case applied the second steel sheet (1) at a heating rate of 200 K/s and to a target temperature (T _z ) of heated to 220 °C. The holding time (t _H ) was 4.50 s at 220 °C and the holding was done in a continuous furnace (12) to maintain the target temperature (T _Z ) of 220 °C. Downstream of the holding zone (12) in the process direction (P), the hot coated steel sheet (1) was actively cooled using a water spray (13) and the hot coated steel sheet (1) was cooled to 70 °C at a cooling rate of 120 K/ s cooled down so that it could be further processed without thermally negatively influencing the other components in the process chain.

Several samples were taken from the two processed steel sheets (1), which were supplied undeformed to a standard phosphating and a standard KTL process on a laboratory scale with the same parameters, with a subsequent baking treatment of 20 minutes at 170 °C after the immersion bath. On the surfaces of the samples, flaws were digitally qualified and quantified on a test area of 600 cm ² so that all samples that were taken from the second steel sheet (1) and had undergone the heat treatment (10) according to the invention were surprisingly less qualitative and quantitative surface defects than the samples taken from the reference.

The number of faults caused by outgassing in the KTL process in the reference samples based on 600 cm ² was determined as follows for all samples: over 5000 holes and over 100 pustules. The defect pattern in the samples from the second steel sheet (1) based on 600 cm ² was determined as follows for all samples: under 50 holes and under 15 pustules.

The result was thus clear that the inventive heat treatment (10) of electrolytically coated steel sheet (1) leads to fewer surface defects during the painting process, so that the quality requirements of electrolytically coated and painted steel sheets (1) can be met and, in particular, improved .

The features described can all be combined with one another as far as technically possible. The invention does not have to be carried out inline on a steel strip, rather it can also be carried out on sheet-like steel sheet blanks before these are further processed. It has also been shown within the scope of the invention that the heat treatment does not necessarily have to include outgassing/effusion of the entire steel/base material. On the other hand, it is sufficient to expel the gaseous and/or liquid components in a near-surface area, for example up to 20 μm, of the steel/base material, which typically also contains the inclusions from the preliminary stages of sheet steel production, in order to meet the requirements.

Claims (10)

Method for producing an electrolytically coated steel sheet (1), the method comprising the following steps: - providing a steel sheet; - Electrolytic coating of the steel sheet (1); characterized in that after the electrolytic coating, the coated steel sheet (1) undergoes a heat treatment (10), the coated steel sheet (1) being heated to a target temperature (T _z ) between 60 °C and 400 °C and for a holding time (t _H ) is kept at the target temperature (T _z ) between 0.10 s and 300 s. procedure after claim 1 , where the target temperature (T _z ) is between 80 °C and 350 °C procedure after claim 2 , where the target temperature (T _z ) is between 100 °C and 300 °C. procedure after claim 3 , where the target temperature (T _z ) is between 130 °C and 280 °C. Method according to one of the preceding claims, in which the holding time (t _H ) is between 0.50 s and 200 s. procedure after claim 5 , where the holding time (t _H ) is between 1.0 s and 100 s. procedure after claim 6 , where the holding time (t _H ) is between 1.50 s and 50 s. Method according to one of the preceding claims, in which the heating (11) to the target temperature (T _z ) is carried out inductively, in a continuous furnace or by means of radiation sources. Method according to one of the preceding claims, wherein the coated steel sheet (1) is moved in a process direction (P) during the heating (11), the coated steel sheet (1) being heated homogeneously transversely to the process direction (P). Method according to one of the preceding claims, in which the coated steel sheet (1) is actively cooled after the holding time (t _H ).

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