EP1297903B1 - Process for manufacturing painted automotive body parts from aluminium alloy - Google Patents

Abstract

Production of lacquered car chassis parts made from aluminum alloy comprises: cold working a hot-rolled strip; solution annealing the strip; cooling at specified rate; aging to produce the heat treatment state T4; cold rolling to a final thickness having a thickness reduction of 1.5-18%; applying a conversion layer to strip; applying one or more lacquer layers; and deforming to form chassis parts. <??>Production of lacquered car chassis parts made from an aluminum alloy containing alloying additions of 0.3-0.9 wt.% magnesium and 0.5-1.2 wt.% silicon comprises: cold working a hot-rolled strip; solution annealing the rolled strip for 10-100 seconds at 540-575 degrees C; cooling at a rate of 20-300 degrees C/second to 200 degrees C or less; aging to produce the heat treatment state T4; cold rolling to a final thickness having a thickness reduction of 1.5-18%; applying a conversion layer to the strip; applying one or more lacquer layers; and deforming to form the chassis parts. <??>Preferred Features: A surface texture is applied to the finished strip using a working roller roughened according electrical discharge texturing.

Description

The invention relates to a method for producing painted automobile body parts according to the preamble of claim 1.

In the production of painted automotive body parts made of aluminum alloys, it is generally known to bring body parts of sheet metal first into the final shape and then to finish the finished body part, i. to be painted only after the deformation of the sheet to the body part. In one or more steps then takes place a baking of the paint at temperatures that simultaneously lead to an increase in strength in the sheet.

From Murtha SJ et al., "Anisotropic Effects in the Forming of Aluminum Sheet," SAE Special Publications, XX, XX, 1995, pages 103-111, XP008005835, ISSN: 0099-5908 is a tape suitable for the manufacture of automotive body parts from alloy 6009 in state T4 with an EDT surface topography for improving the forming behavior.

The invention has for its object to provide a simplified method for producing painted automotive body parts of an aluminum alloy of the type mentioned, which is suitable for forming in the painted state.

For the inventive solution of the problem leads a method having the features of claim 1. The core of the invention lies in the combination of the heat treatment state T4 generated by the cold aging with the following, relatively weak cold working with a thickness decrease of max. 18%. This ensures that the softened as a result of solution annealing and therefore susceptible to surface damage of all kinds band to prevent such damage again has sufficient strength. At the same time the increase in strength is claimed in the claimed Kaltverfomungsbereich such that on the one hand still sufficient reserve for the deformation of the painted strip to the painted automotive body parts remains, on the other hand, the strength in the painted body parts is considerably higher than in the clean state T4.

The solution annealing of the rolled to the intermediate thickness strip is mandatory in a continuous band-pass annealing furnace.

The conversion layer here is the known pretreatment of aluminum surfaces such as e.g. Chromating or chromium-free process understood. With the conversion layer, the adhesion of the paint layers on the aluminum surface is significantly improved.

In certain cases, it may prove advantageous to strip the final thickness rolled strip before applying the conversion layer to a surface texture to provide. In the case of a thickness reduction caused by the texturing of the surface, if necessary by an embossing step, the reduction in thickness provided for cold rolling of the cold-laid strip to final thickness should be taken into account, ie the total reduction in thickness must not exceed the maximum value of 18%.

Application of a surface texture to the final thickness rolled strip is conveniently accomplished via a corresponding surface roughness pattern applied to the surface of work rolls which is transferred to the strip in a single rolling operation. As with any embossing operation, the surface topographies of the work roll and the tapes embossed therewith are complementary to one another, i. Tips or elevations on the roll surface become valleys or depressions in the strip surface and vice versa.

• Funkenerodierverfahren (EDT-Electrical Discharge Texturing). Beim EDT-Verfahren wird die Walze längs einer Reihe von Elektroden bei gleichzeitiger axialer Oszillation gedreht und dabei die Oberfläche auf eine vorgewählte Rauhtiefe texturiert. Durch die Entladung elektrischer Energie zwischen der Walzenoberfläche und den über eine dieelektrische Flüssigkeit getrennten Elektroden werden auf der Walzenoberfläche kleine Krater gebildet. Die Rauheit der Walze kann auf bekannte Weise durch eine Reihe von Maschinen-Parametern eingestellt werden. Das Rauheitsmuster ist eine statistische Verteilung von Spitzen und Tälern. Das EDT-Verfahren ermöglicht die Einstellung von Mittenrauhwerten R_a zwischen 0,5 und 2 µm. Mittenrauhwert und Spitzenzahl sind miteinander weitgehend verknüpft. Für das erfindungsgemässe Verfahren besonders geeignet ist eine Oberflächentextur mit niedriger Rauheit und hoher Spitzenzahl.
• Isomill-Verfahren. Beim Isomill-Verfahren sind in die Oberfläche der Arbeitswalzen in Umfangrichtung und quer dazu in Achsenrichtung oder in einem Winkel von je 45° zur Walzrichtung zur Erzeugung eines im wesentlichen quadratischen Rauheitsmusters Rillen eingraviert.
• Millfinish-Verfahren. Beim Millfinish-Verfahren sind in die Oberfläche der Arbeitswalzen in Umfangrichtung feine Rillen eingraviert. Bei Übertragung dieses Rauheitsmusters auf das Blech entsteht die Oberfläche "Millfinish".

Roughness patterns suitable for carrying out the method according to the invention are obtained by embossing a surface texture with work rolls whose surface has been roughened by the following methods:

• Spark EDM (EDT Electrical Discharge Texturing). In the EDT process, the roll is rotated along a series of electrodes with simultaneous axial oscillation, thereby texturing the surface to a preselected roughness. By discharging electrical energy between the roll surface and the electrodes separated by a dielectric liquid, small craters are formed on the roll surface. The roughness of the roll can be adjusted in a known manner by a number of machine parameters. The roughness pattern is a statistical distribution of peaks and valleys. The EDT method allows the adjustment of average roughness values R _a between 0.5 and 2 μm. Mean roughness and number of peaks are largely linked. Particularly suitable for the process according to the invention is a surface texture with low roughness and a high number of tips.
• Isomill method. In the Isomill process grooves are engraved in the surface of the work rolls circumferentially and transversely thereto in the axial direction or at an angle of 45 ° to the rolling direction to produce a substantially square roughness pattern.
• Millfinish method. In the Millfinish process, fine grooves are circumferentially engraved in the surface of the work rolls. When this roughness pattern is transferred to the sheet, the surface "Millfinish" is created.

The aforementioned painted automotive body parts are for example modules, such as roof modules for sunroofs or complete roofs or other components that are brought from the painted strip by cold forming in the desired shape.

Claims (8)

1. Process for the production of lacquered car body parts from an aluminium alloy containing 0.3 to 0.9 wt.% of magnesium and 0.5 to 1.2 wt.% of silicon, wherein

   - a hot-rolled strip is cold-rolled in one or more reduction passes,

   - the cold-rolled strip is subjected to a solution heat treatment,

   - the strip subjected to the solution heat treatment is cooled, and

   - the cooled strip is naturally aged to attain heat-treated condition T4,

   wherein

   - the hot-rolled strip is cold-rolled to an intermediate thickness,

   - the strip cold-rolled to the intermediate thickness is subjected to a solution heat treatment in a continuous strip annealing furnace for 10 to 100 sec at a temperature of 540 to 575°C, and

   - the strip subjected to the solution heat treatment is cooled to 200°C or below at a rate of 20 to 300°C/sec,

   characterized in that

   - the strip naturally aged to attain heat-treated condition T4 is cold-rolled to the final thickness with a thickness reduction of 1.5 to 18%,

   - a conversion layer is applied to the strip cold-rolled to the final thickness,

   - one or more coats of lacquer are applied to the strip provided with the conversion layer, and

   - the lacquered strip is shaped into the lacquered car body parts.
2. Process according to Claim 1, characterized in that, as the last step, a surface texture is applied to the strip cold-rolled to the final thickness.
3. Process according to Claim 2, characterized in that the surface texture is transferred to the strip in a single rolling operation by means of a roughness pattern applied to the surface of working rolls.
4. Process according to Claim 3, characterized in that the surface of the working rolls is roughened by the EDT (Electrical Discharge Texturing) process.
5. Process according to Claim 3, characterized in that grooves are engraved in the surface of the working rolls in the peripheral direction and transversely thereto in the axial direction, or at an angle of 45° to the rolling direction in each case, in order to produce a substantially square roughness pattern (isomill process).
6. Process according to Claim 3, characterized in that grooves are engraved in the surface of the working rolls in the peripheral direction in order to produce a millfinish pattern.
7. Process according to one of Claims 1 to 6, characterized in that sheets cut out of the lacquered strip are shaped into the lacquered car body parts by cold forming.
8. Process according to one of Claims 1 to 7 wherein the lacquered car body parts are used as modules such as roof modules for sun roofs or complete roofs.