EP2013372B1 - Method for the production and removal of a temporary protective layer for a cathodic coating - Google Patents

Abstract

The invention relates to a method for the production and removal of a temporary protective layer for a cathodic coating, particularly for the production of a hardened steel component with an easily paintable surface, wherein a steel sheet made of a hardenable steel alloy is subjected to a preoxidation, wherein said preoxidation forms a FeO layer with a thickness of 100 nm to 1,000 nm and subsequently a melt dip coating is conducted, wherein, during the melt dip coating, a zinc layer is applied having a thickness of 5 to 20 μm, preferably 7 to 14 μm, on each side, wherein the melt dip process and the aluminum content of the zinc bath is adjusted such that, during the melt dip coating, an aluminum content for the barrier layer results of 0.15 g/m2 to 0.8 g/m2 and the steel sheet or sheet components made therefrom is subsequently heated to a temperature above the austenitizing temperature and is then cooled at a speed greater than the critical hardening speed in order to cause hardening, wherein oxygen-affine elements are contained in the zinc bath for the melt dip coating in a concentration of 0.10 wt.-% to 15 wt.-% that, during the austenitizing on the surface of the cathodic protective layer, form a thin skin comprised of the oxide of the oxygen-affine elements and said oxide layer is blasted after hardening by irradiation of the sheet component with dry ice particles.

Description

The invention relates to a method for producing and removing a temporary protective layer for a cathodic coating on carrier metals.

From the EP 1 561 542 A1 For example, a method for removing a layer of a component is known. This is a layer of organic binder that is to be removed from a substrate without damaging the substrate. For this purpose, a jet of dry ice particles is guided over the surface, so that material is removed from the layer containing an organic binder by the action of the occurring dry ice particles. The removal of dry ice is intended to prevent contamination by foreign substances and to not impair the metallic basic body of the component.

From the EP 1 321 625 B1 For example, a method for removing a metal layer is known, wherein a layer system comprising the metal layer and a substrate coated by the metal layer and the removal process is a blasting process. In this case, the blasting process can be a sandblasting process, whereby the metal layer is strongly cooled in order to achieve low-temperature embrittlement of the layer with respect to the substrate. From the EP 1 034 890 A2 For example, a method and an apparatus for irradiating with different blasting agents are known. In this case, an abrasive blast treatment with blasting agents is to be shown, in which the abrasive effect of the blasting agent is between the blasting agents present in fluid form under normal conditions and the blasting agents present in solid state under normal conditions. Here, a mixture of a first blasting agent such as dry ice and a second abrasive blasting agent such as sand is used.

From the DE 199 46 975 C1 For example, an apparatus and method are known for removing a coating from a substrate which is said to be gentle on materials and suitable for removing both soft and hard coatings. Here is a cold treatment by irradiation with a refrigerant which leads to embrittlement of the coating and then an abrasive cleaning effect are performed with a machining tool, which can be performed by the cold treatment, the mechanical abrasive machining with tool parts lower hardness than in processing tools according to the prior art ,

From the DE 199 42 785 A1 a method for removing solid processing residues, surface coatings or oxide layers is known, only where a cleaning should take place where solid processing residues are. The cleaning can take place here with steam jets, dry ice blasting or cleaning with technically induced shock waves, so-called Lasercleanern. The CO ₂ purification can be done by known dry ice pellets.

From the DE 102 43 035 B4 For example, a method and apparatus for removing layers formed by heating and cooling on pieces of metal are known. Because when removing, for example, scale, oxydsilicate and slag layers on metal workpieces and especially metal workpieces with non-planar surfaces, such as axle and bodywork components for vehicles, the solid particles in abrasive pressurized gas jets are not intended to completely remove metal workpieces from the compressed gas flow with the aid of, for example, dry ice particles be applied to the metal workpiece to be cleaned, preheated and have a temperature which is greater than the temperature of the air surrounding the metal workpiece and / or as the surface temperature of the metal workpiece. This is intended to ensure that, on the one hand, the metal workpiece is not excessively overcooled and, on the other hand, that the compressed gas is at least substantially free of moisture and thus undesired formation of condensate is avoided. The layers to be removed from the surface of the piece of metal are removed by the mechanical action of the high velocity impacting dry ice particles and by the localized cooling of the surface and the layer due to the dry ice particles.

From the WO 2005/021822 It is known to the Applicant, in order to protect a cathodic anti-corrosive layer, to add oxygen-affine elements into the metal forming the cathodic protective layer within certain limits in order to provide protection of the cathodic protective layer when curing a component made with the cathodically protected metal. For hardening such components, they must be heated above the austenitizing temperature of the base metal, in this case steel. Especially with highly hardenable steels this temperature is above 800 ° C. At such temperatures, most cathodic protective layers are destroyed by evaporation or oxidation, so that a component treated in this way would not have cathodic protection after curing. The addition of the oxygen-affine elements causes the oxygen-affinity elements diffuse from the composition of the cathodic protection layer to the surface and form there a very fine protective layer. This very fine protective layer may for example consist of magnesium oxide or aluminum oxide or mixtures thereof. From the WO 2005/021820 It is also known to apply such a method in roll profiling.

The object of the invention is to provide a method with which the paint adhesion can be improved on provided with a cathodic protective layer hardened steel components.

The object is achieved by a method having the features of claim 1.

Advantageous developments are characterized in the subclaims.

According to the invention, it has been recognized that, under certain conditions, the paint adhesion may not be optimal with cathodic anticorrosion coatings provided with a fine surface protection coating. On the other hand, there is no alternative to the formation of these thin layers, since otherwise only a post-galvanizing of these components could be carried out, which is very complex and expensive.

In addition, it has been found that under certain circumstances, such a protective layer for a cathodic protective layer Already a Phosphatierungsvorbehandlung for painting difficult.

According to the invention, therefore, the fine protective layer of one or more oxygen-affine elements is formed so that it can be removed again, ie only temporarily present, in order to protect the cathodic layer during heating above the austenitizing temperature, ie. H. of the glow, to ensure.

According to the invention, this thin protective layer is formed from at least one oxide of the oxygen-affine elements in such a way that cracks and / or defects form in this layer. These cracks allow to detach the scales of the oxide limited by the cracks and / or defects by means of dry ice irradiation.

Conventional sandblasting, however, is limited in use with the newest cathodic protective coatings having a protective layer of oxygen-affine element oxides because the conventional abrasive cleaning methods would eliminate most of the cathodic layer. In addition, the sandblasting also has a negative effect on the dimensional accuracy of the components and also requires a post-cleaning.

According to the invention, the radiation is carried out only with dry ice without additives, wherein the dry ice particles penetrate through the cracks and / or defects in the cavities under the protective layer and sublimate under up to 800-fold volume increase. As a result, the potentially loose or to be dissolved particles from the oxide of / the oxygen-affine elements / element along with any zinc oxide particles thereon are blasted off. The extra thermal shock from the cryogenic Dry ice particles lead to further thermal stresses in the oxide / oxygen-sensitive element / element layer and thus support the desired removal. However, an abrasive removal should and must be avoided, as this attacks the cathodic protective layer.

The desired and for the cathodic corrosion protection necessary zinc or zinc-iron layer is thereby not affected and not removed. With the method according to the invention thus a selective removal of poorly adhering oxides is possible. Good adhering oxides on the surface, on the other hand, remain on the surface and have no negative influence on the paintability.

According to the invention, it has been found that, for the formation of the cracks in the layer, it is necessary to carry out process steps which are to be carried out long before the formation of the cathodic layer on the component itself. While the voids under the fine protective layer are always formed due to the progressing iron-zinc reaction in the cathodic anti-corrosive layer upon annealing in the radiant oven, it has been found in the present invention that the thickness and the crackiness of the fine protective layer of the oxide of the / the oxygen affinity elements / elements arrive at the pretreatment of the bare steel strip and its influence on the interfacial kinetics or formation between zinc and steel substrate in the hot dip coating and on the zinc deposit.

Under pretreatment is a pre-oxidation of the bare steel strip to understand, as in the DE 100 59 566 B3 and described in EU Research Report No. 7210-PA / 118. This type of treatment is common to the property profile of To optimize steels with high strength. This improves the adhesion properties of the zinc coating in the hot-dip coating, in particular in steel strips with high alloy constituents.

As a result, the inhibitor layer formation can affect the thickness and the cracking of the fine protective layer. An inhibiting layer is a layer which occurs as a result of aluminum addition in the zinc bath between the steel substrate and the zinc layer during the continuous hot-dip coating and, if appropriate, subsequent heat treatment. The object of the inhibition layer in general is to brake an excessively strong alloy or reaction between iron and zinc.

If this inhibitor layer is formed too thick, the reaction of zinc with iron during heating over Austenitisierungstemperatur slows down and the overlying and further slightly increasing layer of the oxide of the oxygen-related element / element of the resulting iron-zinc phases only small or not damaged at all. Hereby, the thickness of the fine protective layer grows only slowly and there is also no severe cracking, as the now rather thin Al ₂ O ₃ layer lays like a thin skin over the iron-zinc phases. The same effect occurs when the zinc coating is too high.

Figur 1:

einen Schichtaufbau nach der Erfindung, der mit dem erfindungsgemäßen Verfahren gut bearbeitbar ist;

Figur 2:

eine vergleichende Darstellung einer nicht gut zu reinigenden Oberfläche;

Figur 3:

eine gut zu reinigende Oberfläche gemäß Figur 1 in einer rasterelektronenmikroskopischen Draufsicht;

Figur 4:

eine Draufsicht auf eine schlecht zu reinigende Fläche gemäß Figur 2 in einer rasterelektronenmikroskopischen Aufnahme;

Figur 5:

die Oberfläche der Probe gemäß Figur 3 nach dem erfindungsgemäßen Reinigungsschritt;

Figur 6:

eine Oberfläche nach Figur 4 nach der Durchführung eines Reinigungsverfahrens;

Figur 7:

schematisch der Reinigungsprozess nach der Erfindung.

The invention will be explained by way of example with reference to a drawing. It shows:

FIG. 1:

a layer structure according to the invention, which is easy to work with the inventive method;

FIG. 2:

a comparative representation of a surface that is not easy to clean;

FIG. 3:

a well-cleaned surface according to FIG. 1 in a scanning electron microscope top view;

FIG. 4:

a plan view of a poorly clean surface according to FIG. 2 in a scanning electron micrograph;

FIG. 5:

the surface of the sample according to FIG. 3 after the purification step according to the invention;

FIG. 6:

a surface after FIG. 4 after performing a cleaning process;

FIG. 7:

schematically the cleaning process according to the invention.

In the FIG. 1 The surface in which the Al ₂ O ₃ protective layer shows cracks and / or defects as a result of the heat treatment or hardening is ideally to be cleaned with dry ice. The dry ice particles penetrate through the cracks shown in the cavities under the Al ₂ O ₃ layer and sublimate there as already stated. Here, the dry ice cleaning is carried out such that the dry ice particles do not attack the underlying under the Al ₂ O ₃ layer iron-zinc layer and not even break off the particles that adhere so firmly to the iron-zinc layer that they are responsible for the Paintability does not pose a problem. As in FIG. 1 Obviously, the necessary requirements are met, according to which cavities must be present under the Al ₂ O ₃ layer, the Al ₂ O ₃ layer must have a certain thickness and cracks must also be present. The cracks also allow molten zinc to evaporate, reacting with the atmospheric oxygen to form zinc oxide and recondensing on the Al ₂ O ₃ protective layer. In contrast, one sees in FIG. 2 that both the ripple the iron-zinc layer is lower and the Al ₂ O ₃ layer has larger closed areas that go beyond the cavities caused by the waviness of the iron-zinc layer. Accordingly, little zinc oxide is formed in the region of the cracks. Since parts of the cavities are covered by the Al ₂ O ₃ layer, it is not possible to cause blistering by sublimation in the cavities.

In the FIGS. 3 and 4 are the in FIGS. 1 and 2 schematized states shown in a plan view by electron microscope. In both cases it is a sheet of 1.0 mm thickness, which was annealed at 910 ° C for 250 sec. In a radiant oven and then cured between cooled steel plates. FIG. 4 shows the surface after curing in the case of a thick inhibitor layer formation and / or too high a zinc deposit. Since the Al ₂ O ₃ protective layer is comparatively thin in this case, the electron beam can penetrate it more easily. The cavities located under the Al ₂ O ₃ protective layer are therefore recognizable in the recording as dark areas, since less backscattered electrons from the Al ₂ O ₃ protective layer contribute to the detector signal.

If the aluminum oxide layer is thicker and has more cracks, a continuous Al ₂ O ₃ layer without dark spots can be recognized in the scanning electron microscope. Im in FIG. 3 In the case shown, the thickness of the Al ₂ O ₃ layer is approximately 150 nm to 200 nm FIG. 3 shown state is the desired state while in FIG. 4 shown unwanted condition according to the conditions FIG. 2 equivalent.

In FIG. 5 is a surface according to FIG. 3 shown, which has been subjected to the cleaning process according to the invention. The iron-zinc phases are very evident. A large area Al ₂ O ₃ - and zinc oxide occupancy is no longer recognizable. This surface produced according to the invention can be phosphated very well or aftertreated in another way and shows a very good paint adhesion.

In FIG. 6 is the surface after FIG. 4 after performing the dry ice cleaning process. The darker areas show uncoated Al ₂ O ₃ and a surface that only allows poor paintability.

The inventive method is in FIG. 7 in which dry ice particles are brought onto the Al ₂ O ₃ layer with a dry ice blasting gun, enter the cavities and sublimate there. As a result of the enormous volume expansion during sublimation, Al ₂ O ₃ flakes are removed together with zinc oxide constituents adhering to them, so that the roughness of the iron-zinc layer (see US Pat FIG. 5 ) stays behind.

According to the invention, the pretreatment and hot-dip coating is performed so that in the pre-oxidation a FeO layer is greater than 100 nm but less than 1000 nm and preferably forms an inhibiting layer having an aluminum content of 0.15 g / m ² to 0.4 g / m ² has. When heated above the austenitizing temperature in the radiation furnace, there is an increased zinc-iron reaction, which leads to the break-up of the Al ₂ O ₃ protective layer. Higher aluminum contents lead to a condition as in FIG. 4 described. Lower aluminum contents lead to incomplete formation of the inhibiting layer and to a zinc-iron reaction already during the galvanizing process. This in turn means that the zinc can chip off during cold forming.

In addition, the zinc layer support for carrying out the method according to the invention preferably lies between Z100 and Z200, which means between 7 μm and 14 μm per side. For longer runs, the reaction of the zinc-iron phases can be delayed to the surface, whereby the Al ₂ O ₃ layer is only slightly damaged and thus remains thin. For lower runs, cathodic protection may be too low.

In general, it can also be stated that increased cracks and / or defects in the Al ₂ O ₃ protective layer increase them by oxygen diffusion from below. Thicker Al ₂ O ₃ protective layers are more prone to cracking due to thermal stress during heating above austenitizing temperature. With a thinner Al ₂ O ₃ protective layer, few cracks form in the Al ₂ O ₃ protective layer during the heating above the austenitizing temperature and the low oxygen diffusion leads only to a thin Al ₂ O ₃ skin over the zinc-iron mixed phases.

The invention will be explained by way of examples.

Example 1:

A sheet of 22MnB5 steel 1.0mm thick is subjected to pre-oxidation and hot dip coating of about 0.2% by weight aluminum in the zinc bath. The pre-oxidation is carried out so that a FeO layer thickness of greater than 100 nm but less than 1,000 nm is set. The galvanizing is carried out in such a way that a zinc coating Z200, that means 14 μm per side is achieved. The aluminum content of the inhibiting layer is adjusted to 0.3 g / m ² . The sheet is then placed for four minutes in a 910 ° C hot air oven with normal air atmosphere. As a result, layer formation is according to Figures 3 and 5 or according to FIG. 1 to recognize. This layer is easy to clean with dry ice and it results in the surface according to FIG. 5 and in subsequent experiments, the correspondingly good paint adhesion.

Example 2:

A sheet of 22MnB5 steel of 1.0 mm thickness is subjected to preoxidation and hot dip coating of about 0.2 wt% aluminum in the zinc bath. The pre-oxidation of the bare steel sheet is carried out so that a FeO layer thickness of greater than 100 nm and less than 1,000 nm is set. The zinc coating is carried out in such a way that a zinc coating of Z200, ie 14 μm per side, is achieved. The aluminum content of the inhibitor layer is set to 0.8 g / m ² and the annealing conditions are as in Example 1. As a result, an aluminum oxide-rich surface is achieved with little zinc oxide, which is difficult to clean with dry ice. As a result, the surface corresponds FIG. 6 or before cleaning FIG. 4 and in subsequent Lackierversuchen results in the poor paint adhesion due to large-area Al ₂ O ₃ occupancy.

Example 3:

A steel sheet according to Examples 1 and 2 is formed instead of a zinc coating Z200 with a zinc coating Z300, ie 21 microns per side. Again, the pre-oxidation of the bare steel strip is carried out so that a FeO layer thickness of greater than 100 nm and less than 1,000 nm is set. The aluminum content of the inhibiting layer is adjusted to 0.3 g / m ² . The sheet is then placed for four minutes in a 910 ° C hot air oven with normal air atmosphere. Again, the non-inventive Al ₂ O ₃ -rich surface is formed again with little zinc oxide, which is poorly cleansed with dry ice and the illustrated Surface in FIG. 4 equivalent. In subsequent Lackierversuchen also a poor paint adhesion is achieved.

In the invention, it is advantageous to provide a method for producing and removing a temporary protective layer for a cathodic coating, with which it is possible to provide a hardened steel component with a cathodic protection, wherein the cathodic protective layer protects the steel against oxidation during the heating process and in particular scale formation protects and wherein after a heat treatment and hardening of the steel component with simple means a very good paintable surface is created.

Claims (3)

1. Method for producing and removing a temporary protective layer for a cathodic coating, particularly for manufacturing a hardened steel component with a highly paintable surface, wherein a steel sheet consisting of a hardenable steel alloy is subjected to a preoxidization process, wherein, during the preoxidization process, an FeO layer with a thickness of 100 nm to 1,000 nm is formed and a hot-dip coating process is then carried out, wherein, during the hot-dip coating process, a zinc layer with a thickness of 5 to 20 µm, preferably 7 to 14 µm per side is deposited, wherein the hot-dip coating process and the aluminium content in the zinc bath is set so that during the hot-dip coating process, an aluminium content of 0.15 g/m² to 0.8 g/m², preferably 0.2 g/m² to 0.5 g/m² is realized in the inhibiting layer and the steel sheet or the components manufactured therefrom is/are then heated to a temperature above the austenitizing temperature and then cooled at a rate that lies above the critical hardening rate, in order to effect a hardening, wherein elements with an oxygen affinity are contained in the zinc bath for the hot-dip coating process in a quantity of from 0.10 wt. % to 15 wt. %, which form a thin skin consisting of the oxide of the elements with an oxygen affinity on the surface of a cathodic protective layer during the austenitization and this oxide layer is blown off after hardening by blasting the sheet metal component with dry ice particles.
2. Method according to claim 1, characterized in that magnesium and/or silicon and/or titanium and/or calcium and/or aluminium and/or manganese and/or boron are used as elements with an oxygen affinity in the zinc bath.
3. Method according to any one of the preceding claims, characterized in that the element with an oxygen affinity is aluminium and the aluminium forms a thin aluminium-oxide skin.

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