Metal treatment – Process of modifying or maintaining internal physical... – With measuring – testing – or sensing
Reexamination Certificate
1999-08-19
2001-03-27
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With measuring, testing, or sensing
C148S533000, C266S100000
Reexamination Certificate
active
06206986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for monitoring and controlling the quality of the galvannealed coating of steel strip immediately following the galvannealing step, wherein the hot galvanized coating of the steel strip produced by hot dipping is converted into FeZn phases by diffusion reactions through a thermal treatment in an annealing furnace at temperatures above the melting point of zinc.
2. Description of the Related Art
In order to improve the corrosion protection, the protection against wear, the surface hardness or the sliding behavior of metals, but also for reasons of a better appearance, it is known in the art to provide the metals with a coating with another metal, for example, to provide iron with a coating of zinc or chromium.
Various known methods have been found for applying the coating. They are:
Galvanic coatings, wherein the metal coating is applied in suitable baths containing acids or aqueous solutions by electrolysis of the metal immersed in the baths. When this method is used, coating thicknesses of up to about 10 &mgr;m are achieved;
Hot dip coatings, wherein the coatings are produced by immersing the metal to be coated in baths of liquid molten metal. In this method, a thin alloy layer is formed as a result of reactions between the metal atoms of the liquid coating metal and the atoms of the base metal, wherein a layer of pure coating metal is then placed on the alloy layer. The coating layer thicknesses achieved by this method are significantly higher than those achieved by galvanizing and are, for example, in the case of galvanizing iron (hot galvanizing) 4-50 &mgr;m; an improvement of the hot dip coatings can be achieved by
Galvannealing, wherein the term galvannealing is coined by combining the words galvanizing and annealing.
In the case of galvannealing, the steel strip which emerges from the molten metal bath and is now hot galvanized is thermally aftertreated by annealing at temperatures above the melting point of zinc. During this annealing step, diffusion processes take place which also penetrate through the upper layer composed of pure coating metal, i.e., zinc, so that different FeZn phases are formed.
When the galvannealing process is carried out in an optimum manner, the coating contains about 10-12% Fe. Higher Fe contents of greater than 12% Fe lead to an unfavorable deformation behavior of the steel strip, wherein increased powdering occurs during a later deformation.
In the case of lower Fe contents of less than 9% Fe, the coating is not yet completely alloyed through and residual zinc remains at the surface of the coating. This leads to an undesirable non-uniform appearance with inhomogeneous product properties.
In order to obtain a galvannealed coating which is reacted through in an optimum manner, the thermal treatment must be controlled with respect to the duration of the treatment and with respect to the temperature level to values which are adjusted to the layer thickness. Possible control variables for the duration of the treatment are the length of the annealing zone of the annealing furnace and the speed of the steel strip, and a possible control variable for the temperature is the temperature of the annealing furnace.
At the present time, monitoring and controlling the quality of the galvannealed coating is carried out on a large technical scale in such a way that the temperature of the steel strip above the galvannealing furnace is used as the parameter for the quality of the coating. However, this type of measurement, which is carried out without contact by means of pyrometers, has the problem that a change of the emission behavior of the strip surface at the moment of complete alloying of the coating and also with continuing formation of the alloy in the galvannealed state takes place. There is also the danger that an indicated temperature difference is not the result of an actual difference, but is caused by a changed emission. Consequently, a properly functioning control based on this measured temperature cannot be carried out satisfactorily.
Another disadvantage of the temperature measurement is the fact that the ideal temperature for reaching the desired galvannealed state depends on the reaction behavior of the steel, on the bearing weight, on the aluminum content in the zinc bath and on the temperatures during galvanizing. The galvannealing temperature mentioned above constitutes only an indirect measurement and must be considered in relation to the material used and the galvanizing conditions. The statement of an optimum galvannealed temperature can only always refer to a specific material and a specific combination of the galvanizing parameters which have an influence on the galvanizing process.
Another possibility for determining the quality of the galvannealed coating by measurement technology in order to carry out a control is the combination of temperature measurement and the Fe measurement at the cold measuring location. A disadvantage is the position of the measuring location which may be up to 100 m behind the galvannealing furnace and another disadvantage is the unsatisfactory accuracy of the iron measuring device. Also, it is not possible to recognize differences in the alloying condition over the strip width.
SUMMARY OF THE INVENTION
Therefore, it is the primary object of the present invention to provide a method and an apparatus for carrying out the method which make it possible to measure the quality of the galvannealed coating by realistic measurement technology as closely as possible to the annealing furnace, so that it is possible to monitor and safely control the quality of the coating.
In accordance with the present invention, the measurement value used for the quality of the galvannealed coating of the steel strip is the visual appearance of the galvannealed coating recorded by means of a visual recording device in the form of measurement signals as the quality measurement value, wherein the measurement signals are processed by measurement technology and the resulting measurement values are used after conversion into control variables for the direct control of the annealing furnace.
Residual zinc is clearly visible to the naked eye because of its metal shine. The galvannealed coating has a light grey dull or mat appearance. With an increasing degree of alloying toward higher Fe contents, a change in color toward dark grey takes place. With the appropriate experience and from an appropriate location, the condition of the galvannealed coating can be predetermined and evaluated purely visually.
In accordance with the present invention, this visual appearance is measured as a quality measurement value and is processed by measurement technology in such a way that a direct control of the annealing furnace is possible. For this purpose, the visual appearance of the galvannealed coating is recorded by means of a suitable visual recording device and is compared to a predetermined standard. Using suitable software, the occurring differences are converted into measurement signals which can then be used for the direct continuous control of the annealing furnace.
The visual recording devices used may be, for example, light-optical devices, such as a video camera. However, other conventional recording devices can be used with the appropriate software and a suitable standard if they are capable of recording with appropriate sensitivity the special surface changes (metal shine of the residual zinc or shades of grey) which indicate the quality of the galvannealed coating.
In accordance with a feature of the present invention, recording of the visual appearance by means of a recording device can be carried out immediately following the thermal treatment, i.e., immediately behind the annealing furnace. This provides the advantage that the control of the annealing furnace can be carried out more quickly. However, it is possible to carry out the recording only after cooling which takes place after the thermal treatment because the visual
Coy Nicole
Jenkins Daniel J.
Kueffner Friedrich
SMS Schloemann--Siemag Aktiengesellschaft
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