ULTRASONIC DETECTORSThe invention relates to ultrasonic detectors, and in particular ultrasonic detectors for detecting impurities in liquid steel. It is well known that the quality of steel products depends in part on the purity of the liquid in the manufacturing process. The main sorts of impurities to be found are inclusions of a non-metallic nature, and bubbles of air and other gases within the liquid melt. Methods of monitoring the impurities in liquid steel are therefore very important, but, naturally, since the metal is at a high temperature the methods that can be used are somewhat limited.It has been proposed to use ultrasonic detectors but these have tended to be unsatisfactory in practice because they interfere with the free flow of steel partly because of the need to arrange for reflection of the ultrasonic signals from the transmitter to the receiver by way of a large reflecting target area. It is therefore an object of the present invention to provide an improved ultrasonic detector for detecting impurities (which comprise inclusion and bubbles) in liquid steel and other metals.
According to the invention there is provided an ultrasonic impurity detector for liquids including hot metals comprising a transmitting element and a receiving element arranged to be cooperatively partially lowered into a liquid and when so lowered to define between them a transmission path for ultrasonic signals said signals being used to detect the presence of impurities.
Said transmission path may either be for unreflected signals where the detection of impurities is by the attenuation of signals, or the transmission path may be by reflection of the signals from an impurity and the detection of the impurities is by the presence of a signal on the transmission path.
The transmitting and receiving elements may be rods, and may define a substantially horizontal transmitting path between them. Further, the ultrasonic signals may be reflected by reflecting faces at the bottom of said elements. The rods may further be made of silica or may be made of alumina mullite.
The invention will now be described with reference to the accompanying drawings of whichFigure 1 is an elevation of an ultrasonic detector loweredinto liquid steelFigure 2 is a plan view of the ultrasonic detector ofFigure 1,Figure 3 is a plan view of an ultrasonic detector used todetect by reflection.
Referring now to Figure 1 a melt of liquid steel 1 is to be tested for impurities, either inclusions of a non-metallic character or gaseous bubbles. Ultrasonics are used to do this and there are two type of ultrasonic detection that can be used. In the first case the direct ultrasonic pulses passing through the liquid steel would be attenuated by impurities, and the quantity of these can be deduced from the variations in the ultrasonic signal that has passed through these impurities. The advantage of the attenuation method is that it will detect clouds of small particles too small to be detected by reflection. The alternative method is to detect reflection of ultrasonic pulses from larger sized impurities over 200 micrometres in diameter. In these applications, a ultrasonic frequency of 10 MHzis required.The liquid steel inFigure 1 is shown covered by a layer of slag 2 as is usual in these applications partly in order to reduce heat loss from the liquid metal. Two rods 3 and 4 made of ultrasonic transmitting material are lowered together into the liquid steel so that they appear as shown in figure 1. Rod 3 is the transmitter and rod 4 is the receiver. A ultrasonic generator 10 drives an ultrasonic transducer 9 so as to launch an ultrasonic pulse shown diagramatically at 12 down the rod 3. At the lower end of the rod is provided a internally reflecting face 5. This face is designed to reflect the ultrasonic pulse at right angles into a substantially horizontal direction and launch it into the liquid steel through the transmitting window 7 in the rod.The pulse travels through the liquid steel as shown again diagramatically at 13 and enters the receiving rod 4 through a receiving window 8 in the receiving rod. The pulses are reflected again by a internally reflecting face 6, pass up the receiving rod as shown diagramatically at 14 are received by a receiving ultrasonic transducer at 15. The corresponding electrical signals are taken away and used in the processing circuitry shown diagramatically at 16. The operation of the detector will be clear to those skilled in the art, and so will only be described briefly. Various methods of operation are possible, one of which is to monitor the received signal over a period of time and record the fluctuations.The maximum received signal may be assumed to be that of the direct waves passing through the steel without meeting any impurities, and the fluctuations both in number and strength of the received signal is an indication of the quantity and size of impurities, including clouds of gaseous bubbles.
Turning now to figure 2, and using the same notation as in figure 1 where appropriate, there is shown a plan view of such a ultrasonic detector. The section is at the flat window faces 7 and 8. Indicated particularly is the straight line path of the ultrasonic signals 13 between the transmitter and receiver and it will be seen that this defines a volume shown in plan at 20 in which the presence of impurities will be detected and can be measured.
Turning now to Figure 3 the rods shown in Figure 2 have been rotated so that the faces 7 and 8 are no longer parallel.
Clearly waves transmitted from the rod 3 will not be received in the normal course by the receiver 4, and therefore the system will produce little information. However, should there be a large impurity over about 200 micrometres indiameter, for example an inclusion of non-metallic material as shown at 11, in the melt at the right position the ultrasonic pulses will be reflected according to the normal laws that apply to these situations, and there will be a reflected signal picked up by the receiver 4. The presence of such inclusions can there for be detected by using the appropriate frequency of 10 MHz or thereabouts, and using the detecting system in reflection mode.
It will be noticed that the transmitter and receiver are rotated about their vertical axes, and it is possible to scan the liquid metal melt over a limited range by appropriately rotating the two receiver and transmitter, so that the signals from the transmitter reflected by the impurity at the given point are received by the receiver. The rotation of the transmitter will of course be the main factor.