Detection device for detecting longitudinal cracks on slabs issu

Electricity: measuring and testing – Magnetic – Magnetic test structure elements

Patent

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Details

324225, 324238, 324242, 164451, G01N 2790, B22D 1112

Patent

active

054518725

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1Field of the Invention
The present invention relates to a device for detecting longitudinal cracks on slabs, in particular on steel slabs issuing from a continuous casting.
2. Discussion of the Background
It is known that continuous casting creates difficulties due to the appearance of surface defects or cracks on the faces of the slabs. These defects must be absolutely repaired before the slabs are drawn and rolled.
The solution consisting in systematically retreating all the slabs is too expensive and it is therefore necessary to detect the cracks on the slabs at the output end of the continuous casting so as to deviate to a subsequent retreatment solely the slabs which have cracks requiring said treatment.
These cracks extend longitudinally relative to the axis of the slab and may be located just as well on the lateral faces as on the main faces of the slab.
A device for detecting cracks on slabs by means of eddy current probes is known and comprises an induction coil fed with alternating voltage and means for detecting variations in said voltage indicating the presence of a crack.
This device comprises on each side of the passage of the slabs a vertical probe maintained in proximity to an edge, and a plurality of horizontal probes located at different levels and maintained in proximity to a lateral face of the slab.
The presence of a crack substantially modifies the impedance of the induction coil and the eddy currents and results in a variation in the amplitude of the voltage exciting the coil and in the phase shift between the voltage and the current exciting said coil.
The analysis of the excitation voltage of the coil by electronic circuits provides an electric signal which presents amplitude peaks in the presence of a crack.
But this device detects solely cracks located on the narrow faces of the slabs and does not permit detecting the presence of longitudinal cracks on the main faces of the slabs


SUMMARY OF THE INVENTION

An object of the invention is to provide a device for detecting longitudinal cracks on the main faces of a hot slab issuing from a continuous casting for the purpose of deviating the slabs toward a subsequent retreatment.
This is achieved in employing a detection device which comprises eddy current measuring probes and is so designed that, irrespective of the deformations of the slab and the roughness of its surface, the end of said probes are spaced away from the surface of the slab by a very small constant distance without this resulting in an excessive wear of these probes which must be capable of resisting wear and abrasion throughout the continuous casting procedure so that the wear of the probes does not require stoppage of the continuous casting.
The measuring probes must be held and guided at the short distance from the surface of the slabs without interposing between the end of the coil and said surface any metal part which would disturb the magnetic circuit.
The width of the air gap must remain constant whatever the shape and roughness of the surface to be checked, since very slight variations in the width of the air gap would result in variations in the coefficient of mutual induction which could result in variations in the electric signal having amplitudes of the same order as the variations due to the cracks, with the risk of deviating slabs exempt from defects by error.
The necessity to maintain an air gap of constant width between the measuring probe and the surface of the slab requires mounting the measuring probes on movable supports which must faithfully follow all the sinuosities of the surface and which temporarily move away from the slab in front of asperities having a steep leading edge.
The present invention permits attaining these objects and provides a device for detecting longitudinal cracks on slabs, in particular on steel slabs, issuing from a continuous casting, by means of measuring probes employing eddy currents fed with alternating voltage, and means for detecting variations in said voltage indicating the presence o

REFERENCES:
patent: 2537731 (1951-01-01), Angell
patent: 4534405 (1985-08-01), Hulek et al.
patent: 4855678 (1989-08-01), Kreiskorte

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