The invention refers to a method for treating a metal element for an automobile.
A metal element can be a component of the automobile body or automobile frame made out of a metal sheet as well as a massive metal element of the automobile, such as a bolt, a spring or another massive component not made out of a metal sheet. The metal element has to have a high strength and good anti-corrosive qualities.
For increasing the metal element strength, the metal element is heated in an oven to a temperature of 550°C to 1200°C, and is subsequently quickly cooled down to a temperature below 220°C in an oil bath or by cooled air. In a so called press-hardening process, the metal element is heated to a temperature of 800°C to 1000°C and is formed into its end shape by pressing and cooled in a press subsequent to the heating. The hardened metal element has a very high strength and allows to produce relatively light metal elements for the automobile. Subsequent to the hardening, the hardened high strength metal element is made anti-corrosive by means of a surface treatment or a surface coating step.
On the way from the hardening oven to the oil bath or to the press, the heated metal element is, because the high temperature difference with the environment, subject to scaling so that the scale has to be removed from the metal element before the metal element is passed to the surface treatment or surface coating step. A classic method for scale removing is pickling in a cleaning liquid with a strong acid, for example with sulphuric acid. With this scale removing method, even the smallest structures, such as slits, hollow spaces, undercuts etc. are removed from scale. But using a strong acid has the disadvantage that the treated high strength metal element becomes brittle. An alternative method for scale removing is shot blasting which does not effect the inner structure of the metal element but is not suitable to remove scale from small structures, slits, undercuts etc. In addition, the shot blasting stream can mechanically deform a sheet-made metal element.
It is an object of the invention to improve the scale removing from a automobile metal element between the hardening step and the subsequent coating step.
This object is, according to the invention, solved with the features of claim 1.
According to claim 1, the metal element is first hardened. The hardening step can be accomplished by heating the metal element in an oven to a temperature of about 550°C to 1200°C. After heating, the metal element is quickly cooled down to a temperature below 200°C in an oil bath or by cooled air. On the way from the heating oven to the oil bath or to a press for providing a press-hardening step, the metal element is subject to scaling.
Subsequently to the hardening step, the scale removing step is provided by an ultrasonic process, whereby the ultrasonic liquid is a water solution with an organic carboxylic with the concentration of 0,1 to 10 vol.%.
As experiments have shown, such a water solution does not significantly affect the metal elements surface, and in particular does not make the metal element brittle. On the other hand, the ultrasonic cleaning method guarantees that the scale Is completely removed even from small structures, slits, hollow spaces, undercuts etc.
Subsequent to the scale removing step, the metal element is coated in a coating process with an anticorrosion coating. This coating process can be realized, for example, by a so-called thermo diffusion process with zinc powder.
According to the preferred embodiment of the invention, the organic carboxylic acid concentration is between 1,0 and 6,0 vol.%, and preferably between 2,0 and 6,0 vol.%. According to a preferred embodiment, the organic carboxylic acid is a citric acid.
Preferably, the ultrasonic liquid has a pH value between 3,0 and 5,0.
According to a preferred embodiment of the invention, is the ultrasonic frequency during the ultrasonic cleaning sequence is between 18 kHz and 60 kHz, whereby the ultrasonic frequency can be a varied during the complete ultrasonic scale removing sequence. Preferably, the ultrasonic activity is provided in intervals with numerous interruptions of less than 30% during the ultrasonic process step. The scale removing sequence can take between one and eight minutes.
Preferably, the temperature of the ultrasonic liquid is between 30 and 70°C. In this temperature range, the ultrasonic liquid is very effective.
According to a preferred embodiment of the invention, the metal element is moved with respect to the ultrasonic liquid during the ultrasonic cleaning sequence. Preferably, the metal element is rotated in the ultrasonic liquid during the ultrasonic treatment. Preferably, the metal element is rotated by a rotor in the ultrasonic liquid during the ultrasonic treatment. The rotor holds the metal element in a rotor cradle or with rotor fingers attached to the metal element.
Preferably, the metal element is a complex structural automobile element.
The following is a description of one embodiment of the method according to the invention.
A metal element to be treated can be any metal automobile component which has to have high strength and good anticorrosive qualities. Especially automobile components made out of thin metal sheets but also massive parts as springs and bolts are predestinated to be treated accordingly. The following example refers to a sheet-like metal element which is formed to a component of the automobile body or automobile frame.
In a hardening step, the metal sheet element is transferred to a hardening oven wherein the metal sheet element is heated to a temperature of 700°C to 1000°C in an oven, and subsequently is formed into its end shape by pressing under high pressure in a press-hardening press. As soon as the heated metal sheet element is removed from the oven, the metal sheet element is subject to scaling on the way to the press so that the metal sheet element is covered with a layer of scale after leaving the press-hardening oven. After the press-hardening step in the press, the press is opened, and the metal sheet element is removed from the press.
Before the metal sheet element is transferred to a coating step for coating the high strength metal sheet element with an anticorrosion coating, the scale layer has to be removed from the metal sheet element.
The scale removing step is carried out in a scale removing device, wherein the high strength metal sheet element is fixed to a rotor which rotates the metal sheet element in an ultrasonic liquid. The ultrasonic liquid is a water solution with an organic carboxylic acid with a concentration of about 4,0 vol%. Preferably, the acid is a citric acid. The ultrasonic liquid has a temperature of about 50°C and a pH value of 4.
As soon as the metal sheet element is fixed to the rotor and immersed into the ultrasonic liquid, the scale removing process starts: an ultrasonic element induces ultrasonic waves into the ultrasonic liquid. The frequency of the ultrasonic waves varies between 20 kHz and 40 kHz, and is provided in intervals of for example 50 seconds separated by interruptions of 10 seconds. During the complete ultrasonic treatment step, the metal sheet element is rotated by the rotor in the ultrasonic liquid. The ultrasonic treatment of one metal sheet element takes about 5 minutes in total.
After the scale removing step is finished, the metal sheet element is removed from the ultrasonic scale removing device, and is transferred to a coating process step whereby the metal sheet element is provided with an anticorrosion treatment by a so-called thermo diffusion process with zinc powder.
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