Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
2002-09-23
2004-02-03
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S225000, C324S227000
Reexamination Certificate
active
06686735
ABSTRACT:
The invention relates to a method and measuring device for the in situ detection of the degree of transformation of a nonmagnetic phase into a ferromagnetic phase of a metallic workpiece after a thermomechanical treatment. More specifically, the invention relates to the detection of the degree of &ggr;-&agr; phase transformation of a steel workpiece in strip form using a measuring device which is arranged in close proximity to the workpiece. The detection takes place on the basis of the eddy currents of the nonmagnetic phase, which are caused when an electromagnetic excitation field is applied to the workpiece, and on the basis of response fields which are generated by the magnetic phase which may be present. At least one electromagnetic excitation field which acts on the workpiece is generated by means of at least one excitation coil, and a response field which results from the application of the excitation field is measured by means of at least one detector measurement coil. The resulting measurement signal, which is dependent on the distance between the measuring device and the workpiece, is used to detect the degree of transformation.
BACKGROUND OF THE INVENTION
The mechanical properties of high-quality steel sheet which is produced during a hot-rolling process is dependent not only on the composition of the steel, but also on the way in which the hot steel sheet is cooled when it leaves the rolling train. Cooling initiates a recrystallization process which is dependent on the prevailing temperature and the holding time, with the nucleus density of a phase which forms being particularly time-dependent. When the steel sheet is being cooled, in addition to a nonmagnetic phase, known as the &ggr;-phase, a ferromagnetic phase, known as the &agr;-phase, is also formed. Depending on the cooling rate during cooling, the phases are frozen without the phases being in an equilibrium state, i.e. the proportions of phases can be varied as a function of the cooling rate, i.e. the cooling ramp which is passed through. In this context the steel composition also plays a role, since the temperatures at which a new phase is formed are dependent on the steel compositions. The phase transitions which result for a defined steel composition are to be found in the iron-carbon phase diagram.
The possibility of varying the phase proportions by means of a defined cooling process, however, presupposes that the temperature of the metal can be determined as accurately as possible, so that it can be detected when the formation of a certain phase commences. Although it is possible to determine the temperature, for example on the basis of measurement of the heat radiated by the metal, or by means of infrared scanning, the measured values which these measurement processes supply are highly inaccurate, since the oxide layer which forms on the metal strip surface distorts the measurement results. Accordingly, it would be desirable to measure the temperature of the metal below the oxide layer, but this is not possible with the above mentioned processes.
The phase transition from the nonmagnetic &ggr;-phase to the ferromagnetic &agr;-phase, which takes place as a function of the specific steel composition at a precisely determined temperature (which can be worked out on the basis of the iron-carbon phase diagram,) can be effected by means of an eddy-current measuring method. As part of this method, an electromagnetic excitation field is generated and acts on the metallic workpiece. The nonmagnetic phase and the ferromagnetic phase react differently to this field. In the nonmagnetic phase, the electrons of the metallic workpiece move as a function of the electromagnetic field which is present and screen the field. The movement of electrons leads to eddy currents, which in turn generate an opposite field which attenuates the outer excitation field which is applied. The response of the nonmagnetic phase therefore leads to attenuation of the electromagnetic field which is present.
The magnetic phase, by contrast, reacts in the opposite way. The coupling of the magnetic moments means that they are oriented in the direction of the excitation or primary field which is present. The magnetic field resulting from the orientation amplifies the excitation field. Overall, therefore, two opposite processes occur when the nonmagnetic and ferromagnetic phases are present.
If a suitable detector coil which is arranged close to the metallic workpiece is now used to measure the response of the metallic workpiece, at a workpiece temperature which lies above the temperature of phase transition to the &agr;-phase, a low resulting overall field is measured, which results from the attenuation of the primary field by the eddy current response. As soon as the nucleation of the &agr;-phase commences, the resulting field which can be measured by means of the detector coil changes on account of the field fraction of the magnetic phase, which boosts the primary field. This jump can be measured very accurately, and consequently a precise temperature can be determined therefrom. On account of the increasing proportion of magnetic phase during the cooling operation, the resulting field also changes to an increasing extent, so that the temperature-dependent degree of transformation can be determined on the basis of the measured value obtained.
In practice, however, it is not possible to determine the degree of transformation by means of the eddy-current measuring method described above. This results from the movement of the steel sheet during the measurement. The hot steel sheet which comes out of the rolling mill is moved continuously past the stationary measuring device toward a cooling section. However, the movement is not uniform since the steel sheet wobbles and clatters over the conveying rollers. This non-uniform movement results in a number of problems. The strength of the magnetic field which is generated by the eddy currents is dependent on the distance from the workpiece surface to the detector coil, and decreases very considerably as the distance increases. On account of the wobbling of the sheet with respect to the stationary detector coils, the recorded measurement signal changes as a function of the movement of the steel sheet, since the eddy-current field changes continuously as the strip vibrates. Furthermore, the electromagnetic excitation field not only penetrates into the workpiece but also is reflected thereby. The wobbling alone results in an interfering signal which affects the measurement signal.
A further problem is the high metal temperature itself. The temperature of the relevant phase transition is approximately 900° C. The high temperature leads to a certain temperature-dependent change in the shape of the measurement configurations and the detector coils. If a plurality of coils are distributed over the length of the cooling section, it is scarcely possible to obtain results which are suitable for comparison, since the change in shape results in a change in the distance between the individual distributed coils and the metal strip. Changes in shape may even cause difficulties in relation to the individual coil, since the temperature of the metal sheet in the region of the detector coils may change as a function of the cooling operation which is being carried out at that particular time.
U.S. Pat. No. 4,686,471 discloses a measuring system for recording the degree of transformation, which operates in accordance with the method described in the introduction, with one excitation coil and two detector coils being provided in this system. The excitation coil and the detector coils all lie in one plane, and the detector coils are at different distances from the excitation coil.
SUMMARY OF THE INVENTION
The present invention therefore provides a method which enables the in situ detection of the degree of transformation which is extremely precise, and which comprises the steps of measuring a response field, which results from the application of the excitation field, by means of two distance-measuring coils, which are at
Daalmans Gabriel
Doell Ruediger
Weinzierl Klaus
Baker & Botts LLP
Patidar Jay
Siemens Aktiengesellschaft
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