Method for the production and removal of a temporary protective layer for a cathodic coating

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/m.sup.2 to 0.8 g/m.sup.2 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.

Claims

1. A 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, comprising: subjecting a sheet steel. composed of a hardenable steel alloy to a preoxidization process; during the preoxidization process, a FeO layer with a thickness of 100 nm to 1,000 nm forms on the sheet steel; after the preoxidation process, carrying out a hot-dip coating process; during the hot-dip coating process, a zinc layer with a thickness of 5 to 20 μis deposited on the sheet steel; setting the hot-dip coating process and an aluminum content in a zinc bath so that during the hot-dip coating process, an aluminum content of 0.15 g/ m.sup.2 to0.8 g/m.sup.2 is produced in an inhibiting layer and the sheet steel or a component manufactured from the sheet steel is/are heated to a temperature above an austenitizing temperature and then cooled at a speed. that lies above a critical hardening speed, rn order to produce a hardening; adjusting the zinc bath for the hot-dip coating process to contain oxygen-affinity elements in a quantity of from 0.10 wt. % to 15 wt. % to form a thin skin composed of the oxide of the oxygen-affinity elements on the surface of a cathodic protective layer during the austenitizing and after hardening; and flaking off the oxide layer by blasting the sheet steel or the component with dry ice particles and without abrasive removal of the oxide layer.

2. The method as recited in claim 1, wherein the oxygen-affinity elements in the zinc bath comprise at least one of the group consisting of magnesium, silicon, titanium, calcium, aluminum, manganese, and boron.

3. The method as recited in claim 1, wherein at least one of the oxygen-affinity elements is aluminum and the aluminum forms a thin skin of aluminum oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a layer structure according to the invention, which responds well to being processed using a method according to the invention.

(2) FIG. 2 shows a comparative depiction of a surface that cannot be cleaned well.

(3) FIG. 3 shows a surface that can be cleaned well according to FIG. 1, in a scanning electron microscope image taken from above.

(4) FIG. 4 shows a surface according to FIG. 2 that cannot be cleaned well, in a scanning electron microscope image taken from above.

(5) FIG. 5 shows the surface of the sample according to FIG. 3 after the cleaning step according to the invention.

(6) FIG. 6 shows a surface according to FIG. 4 after a cleaning process is carried out.

(7) FIG. 7 schematically depicts the cleaning process according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The surface shown in FIG. 1, in which cracks and/or defects occur in the Al.sub.2O.sub.3 layer due to the heat treatment and hardening, is ideal for being cleaned with dry ice. The dry ice particles penetrate through the depicted cracks into the cavities beneath the Al.sub.2O.sub.3 layer and sublimate there as explained above. In this case, the dry ice cleaning is carried out in such a way that the dry ice particles do not attack the iron-zinc layer underlying the Al.sub.2O.sub.3 layer and also do not blast away the particles that adhere to the iron-zinc layer so firmly that they represent no problem for the paintability. As is evident in FIG. 1, the necessary requirements are met, namely that cavities must be present beneath the Al.sub.2O.sub.3 layer, the Al.sub.2O.sub.3 layer must have a certain thickness, and in addition, cracks must be present. Molten zinc can also vaporize through the cracks; it reacts with the oxygen in the air to form zinc oxide and recondenses on the protective Al.sub.2O.sub.3 layer. By contrast with this, in FIG. 2, not only does the iron-zinc layer undulate less, but also the Al.sub.2O.sub.3 layer has larger enclosed regions that extend beyond the cavities produced by the undulations in the iron-zinc layer. In addition, a correspondingly small amount of zinc oxide forms in the region of the cracks. Since parts of the cavities are covered by the Al.sub.2O.sub.3 layer, it is not possible to produce a blasting action through sublimation in the cavities.

(9) FIGS. 3 and 4 show electron microscope images, taken from above, of the states schematically depicted in FIGS. 1 and 2. Both cases involve a piece of sheet metal 1.0 mm thick, which has been annealed at 910° C. for 250 seconds in a radiation furnace and has then been hardened between cooled steel plates. FIG. 4 shows the surface after the hardening for the case of a thick inhibiting layer formation and/or an excessively thick zinc deposit. Since the Al.sub.2O.sub.3 layer in this case is comparatively thin, the electron beam is more easily able to penetrate it. The cavities situated beneath the Al.sub.2O.sub.3 layer are therefore visible as dark areas in the image since in these areas, fewer backscatter electrons from the Al.sub.2O.sub.3 layer contribute to the detector signal.

(10) If the aluminum oxide layer is thicker and has more cracks, then the scanning electron microscope image shows a continuous Al.sub.2O.sub.3 layer without dark patches. In the case shown in FIG. 3, the Al.sub.2O.sub.3 layer is approx. 150 nm to 200 nm thick. The state shown in FIG. 3 is the desired state, while the undesirable state shown in FIG. 4 corresponds to the conditions according to FIG. 2.

(11) FIG. 5 shows a surface according to FIG. 3, which has undergone the cleaning process according to the invention. The iron-zinc phases are clearly visible. An extensive Al.sub.2O.sub.3 and zinc oxide coverage is no longer visible. This surface produced according to the invention is very suitable for phosphating or some other form of aftertreatment and also demonstrates a very good paint adhesion.

(12) FIG. 6 shows the surface according to FIG. 4, after the execution of the dry ice cleaning method. The darker areas show non-removed Al.sub.2O.sub.3 and a surface that only permits a low level of paintability.

(13) The method according to the invention is shown in FIG. 7; by means of a dry ice blasting gun, dry ice particles are shot at the Al.sub.2O.sub.3 layer, travel into the cavities, and sublimate therein. The enormous volume expansion that occurs upon sublimation detaches Al.sub.2O.sub.3 flakes along with zinc oxide residues adhering to them so that the iron-zinc layer, with its surface finish (see FIG. 5), remains behind.

(14) According to the invention, the pretreatment and hot-dip coating process are carried out so that during the preoxidation, a FeO layer of greater than 100 nm but less than 1,000 nm forms, and preferably an inhibiting layer forms, which has an aluminum content of 0.15 g/m.sup.2 to 0.4 g/m.sup.2. During the heating to a temperature above the austenitizing temperature in the radiation furnace, an intensified zinc-iron reaction occurs, which results in a breaking-up of the Al.sub.2O.sub.3 protective layer. Higher aluminum contents lead to a state of the type described in FIG. 4. Lower aluminum contents lead to an incomplete formation of the inhibiting layer and to a zinc-iron reaction that already takes place during the galvanizing process. This also results in the fact that the zinc can peel off during the cold forming.

(15) Preferably, the zinc layer deposit for carrying out the method according to the invention is between Z100 and Z200, i.e. between 7 μm and 14 μm per side. At higher deposits, the thorough reaction of the zinc-iron phases is delayed all the way to the surface as a result of which the Al.sub.2O.sub.3 layer is damaged only slightly and therefore remains thin. At lower deposits the cathodic corrosion protection can be insufficient.

(16) From a purely general standpoint, it can also be mentioned that through the proliferation of cracks and/or defects in the Al.sub.2O.sub.3 layer, this layer grows from underneath due to oxygen diffusion. Thicker Al.sub.2O.sub.3 layers, moreover, already tend to form cracks due to thermal stresses during the heating to a temperature above the austenitizing temperature. With a thinner Al.sub.2O.sub.3 layer, few cracks in the Al.sub.2O.sub.3 layer form during the heating to a temperature above the austenitizing temperature and the low level of oxygen diffusion results in only a thin Al.sub.2O.sub.3 skin over the zinc-iron mixed phases.

(17) The invention will be explained by means of examples.

EXAMPLE 1

(18) A sheet of 22MnB5 steel 1.0 mm thick is subjected to a preoxidation and a hot-dip coating with approx. 0.2 wt. % aluminum in a zinc bath. The preoxidation is carried out so that a FeO layer thickness of greater than 100 nm but less than 1,000 nm is produced. The galvanizing here is carried out so that a zinc deposit Z200, i.e. 14 μm per side, is achieved. The aluminum content of the inhibiting layer is set to 0.3 g/m.sup.2. The sheet is then placed for four minutes in a radiation furnace heated to 910° C., with a normal air atmosphere. As a result, a layer formation according to FIGS. 3 and 5 or according to FIG. 1 occurs. This layer responds favorably to cleaning with dry ice and yields the surface according to FIG. 5 and in subsequent trials, demonstrates the correspondingly favorable paint adhesion.

EXAMPLE 2

(19) A sheet of 22MnB5 steel 1.0 mm thick undergoes a preoxidation and a hot-dip coating process with approx. 0.2 wt. % aluminum in the zinc bath. The preoxidation of the blank sheet is carried out so that a FeO layer thickness of greater than 100 nm and less than 1,000 nm is produced. The galvanizing here is carried out so that a zinc deposit Z200, i.e. 14 μm per side, is achieved. The aluminum content of the inhibiting layer is set to 0.8 g/m.sup.2 and annealing conditions correspond to example 1. As a result, an aluminum oxide-rich surface with little zinc oxide is achieved, which only responds poorly to being cleaned with dry ice. As a result, the surface corresponds to FIG. 6 or before the cleaning, to FIG. 4, and in subsequent trials, demonstrates the poor paint adhesion due to extensive Al.sub.2O.sub.3 coverage.

EXAMPLE 3

(20) A steel sheet corresponding to examples 1 and 2 is embodied with a zinc deposit of Z300, i.e. 21 μm per side, instead of a zinc deposit of Z200. On the other hand, the preoxidation of the blank steel band is carried out so that a FeO layer thickness of greater than 100 nm and less than 1,000 nm is produced. The aluminum content of the inhibiting layer is set to 0.3 g/m.sup.2. The sheet is then placed for four minutes in a radiation furnace heated to 910° C., with a normal air atmosphere. Here, too, the Al.sub.2O.sub.3-rich surface not according to the invention forms with little zinc oxide; it responds poorly to being cleaned with dry ice and corresponds to the surface shown in FIG. 4. In subsequent paint trials, a poor paint adhesion is likewise achieved.

(21) The invention has the advantage that a method for producing and removing a temporary protective layer for a cathodic coating is created, which successfully creates a hardened steel component with a cathodic protection; the cathodic protective layer protects the steel—even during the heating—from oxidation and particularly from cinder formation and after a heat treatment and hardening of the steel component, a very highly paintable surface is produced with simple means.