Magnetic defect detector for steel strip

Abstract: The invention provides a magnetic defect detector having a pair of rotatable cylinders (14a, 14b) made of nonmagnetic material which pinch a running steel strip (12), two pairs of bearings (22a, 22b, 23a,23b) being engaged with the end parts of the pair of cylinders, a pair of stationary shafts (21a, 21b) housed in the cylinders, the end parts of which are engaged with the bearings, a yoke (26) in one of the cylinders, the free ends of which are placed near to the pinched strip, a magnetizing coil (28) which surrounds the yoke, and a series of sensors (24) in the other of the cylinders, which are placed near to the pinched strip.

Description: The invention relates to the field of measuring technology. The invention relates to the field of detecting defects of steel strip by utilizing a magnetic defect detector. The apparatus of the invention is especially useful for inspection of a steel strip coated with tin and formed for the production of cans for foods by a deep drawing process. In the process of making steel strip, especially tin plate strip, it is required to detect defects on or in the strip. A recent technology for making cans for food is the process wherein the tin plate is deep drawn to form the part of the can corresponding to the body and the bottom end in one piece, in one step. This is a great advance in can making technology. However this technology puts a heavy burden on the quality of the tin plate used for can making. In deep drawing, the severe working stresses may cause breakage to the inside of, or on the outer surface of, the steel plate. Therefore minor defects which may be allowable in other can forming processes can be the ones for which detection is also required for tin plate to be utilized in a deep drawing process. The present improvement in measuring technology is also beneficial to the steel plate for general purpose. There are related arts in this field of technology wherein detection is performed of the defects of the steel strip such as inner defects or bubbles, or surface defects, when the steel plate is stationary or running. Japanese Utility Model laid open 107849/88 dicloses such an apparatus for the detection of the defects. Figures 3A and 3B are partially cutaway sectional views of a related art; Figure 3A is a side view and Figure 3B is a front view of the related art; and Figures 3A and 3B show an apparatus which continuously detects defects of a running steel strip. In these figures 1 denotes a hollow cylinder made of nonmagnetic material. The shaft 2 is placed at the center of the hollow cylinder 2. The shaft 2 is fixed to a stationary body, not shown. A pair of bearings 3a and 3b are attached between the shaft and the hollow cylinder at both ends of the hollow cylinder. Accordingly, the hollow cylinder can be freely rotated around the shaft. Inside of the hollow cylinder, the magnetizing yoke 4 is fixed to the shaft by the engagement of the supporting member 5, the free ends of the yoke being placed in the vicinity of the inner surface of the cylinder. The yoke is equipped with the magnetizing coil 6. Between both free ends of the yoke the magnetic detection elements 7 is placed and fixed to the shaft. A cable 8 which supplies the magnetizing electric current, and a cable 9 which feeds out the signal from the magnetic detection elements are led to the outside of the hollow cylinder via the shaft. Accordingly, the magnetizing yoke and the magnetic detection elements are fixed and the hollow cylinder rotates around the yoke and the element, with little clearance between the inner surface of the cylinder and the yoke or the elements. In this configuration of the cylinder, when the strip 10 running in the direction denoted as A, is pressed to the cylinder by a certain force, the cylinder rotates in the direction shown by the arrow. When electric current is supplied to the detection element 7, a closed magnetic circuit is formed between the yoke and the running steel strip 10. When a defect is in existence in or on the strip, the magnetic circuit is disturbed, and a magnetic flux leakage is generated which is detected by the magnetic detection elements as a defect signal. The detected defect signal corresponds to the magnitude of the defect at the inside or on the surface of the strip, and the the magnitude of the defect can be classified by the level of the signal. The level of the defect signal varies with the magnetic circuit condition which is determined by the magnetizer composed of the yoke and the magnetizing coil, and the strip, and with " lift off", or the distance between the magnetic detection element and the strip, denoted as " d" Figure 4 is a graph showing the relationship between "d " and the signal level of the defect signal. The abscissa denotes "d " and the ordinate denotes the signal level. As shown in Figure 4, the larger the distance between the magnetic detection elements and the strip, the smaller the signal level. To reduce this problem, "d " is maintained constant by using a hollow cylinder having a thickness of "t " as shown in Figures 3A and 3B. The hollow cylinder is made of a nonmagnetic material since otherwise the formation of the magnetic circuit is not possible. The magnetic circuit is most effective when the magnetic gap between the free ends of the yoke and the strip is small or, if "t " is small. When "t " is reduced, then "d " is reduced and the signal level becomes larger, and signal noise level ratio also becomes larger which leads to the enhancement of the accuracy of the detection. Accordingly it is desirable to make the thickness of the hollow cylinder as thin as possible. To carry out the detection when the strip is running, the hollow cylinder and the strip should be constantly contacted. As the result of the application of the tension to the strip to make contact with the cylinder, the downward force due to the tension and the weight of the strip is applied on the hollow cylinder which may deform the cylinder or may break it. When the cylinder is deformed, the distance between the magnetic detection element and the strip "d " can not be maintained constant. Therefore the thickness of the hollow cylinder "t " can not be made thinner than a predetermined restriction to maintain the circular shape of the hollow cylinder as long as possible. Accordingly the apparatus of the related art shown in Figures 3A and 3B is incapable of performing the detection of minor defects, of the strip running in the production line or in the processing line because the thickness of the cylinder can not be as thin as is necessary to ensure that the quality requirements set for the strip can be successfully monitored. It is an object of the present invention to provide an apparatus which detects the defects of the running steel strip. It is an object of this invention to provide a magnetic defect detector for steel strip. The present invention provides a magnetic defect detector comprising: a pair of rotatable cylinders made of nonmagnetic material which pinch a running steel strip; two pairs of bearings being engaged with the end parts of said pair of cylinders; a pair of stationary shafts housed in the pair of cylinders, the end parts of said pair of stationary shafts being engaged with said two pairs of bearings; a yoke housed in one of the pair of cylinders, the free ends of said yoke being placed in the vicinity of the inner surface of a part of the cylinder which contacts with the strip; a magnetizing coil surrounding a part of the yoke; the yoke and the strip together forming a magnetic circuit; a supporting element which supports the yoke and fixed to the one of said pair of shafts; a series of sensors for detecting signals of defects of the strip caused by a magnetic leakge flux due to said defects, housed in the other one of said pair of cylinders and arranged parallel to the axis of the cylinder in the vicinity of the inner surface of a part of the cylinder which contacts with the strip; a second supporting element which supports said series of sensors and fixed to the other one of said pair of shafts; and means for determining, from said signals, defects on or in said strip. The thickness of the cylinder which houses the magnetic defect detector can be decreased by making the thickness of the cylinder under the magnetic detecting elements thinner than at the parts of the cylinder between the magnetic detecting elements without affecting the structural strength of the cylinder. This invention can be utilized with a surface defect detector which detects a surface defect of the strip. When the surface defect detector detects a surface defect at a certain spot of the strip and the magnetic defect detector of this invention does the detection, the defect is defined as the surface defect of the strip. When the surface defect detector does not detect a surface defect at a certain spot of the strip and the magnetic defect detector of this invention generates a defect signal, the defect is defined as the inner defect of the strip. This invention can be utilized with a weld detector which detects the welded part of the strip. When the weld detector detects the welded part of the strip and the magnetic defect detector of this invention detects the welded part, the weld is not classified as the defect of the strip. The material of the cylinder should be nonmagnetic, but it should not be one which can generate an eddy current in the rotating cylinder which may disturb the accuracy of the detection of the magnetic detecting elements. Therefore the material should have high electric resistance. In this embodiment, an austenitic steel is utilized. However titanium, synthetic resin, fiber reinforced plastic, and ceramics are a good substitute. Figure 1A is a partially cutaway side view of an embodiment of the invention. Figure 1B is a partially cutaway front view of an embodiment of the invention. Figure 2 is an explanatory view showing the flow system of an embodiment of the invention. Figures 3A and 3B are patially cutaway sectional views of a related art. Figure 4 is a graph showing the relationship between the distance from the free ends of the yoke to the strip and the signal level of the defect signal. Figures 5A, 5B, 5C, and 5D are schematic illustrations showing the arrangement of the hollow cylinders and the pinch rolls. Figure 6 is a partially cutaway view of an embodiment of the invention. The problem encountered in the related art is that the distance between the sensor and the strip cannot be minimized sufficienty to assure the desired accuracy of the sensor due to the fact that the thickness of the hollow cylinder cannot be made as thin as required. The problem lies in the structure of the sensing system. The present invention solves the problem by adopting a two cylinder system; one encloses the magnetizing unit and the other encloses the sensor. The two cylinders pinch the strip between them and the applied tension on the strip and the weight of the strip act on the cylinder which encloses the magnetizer. The design is such that no significant force is applied on the cylinder which encloses the sensor unit. The thickness of the cylinder which encloses the magnetizer unit may be increased if the magnetizing power is increased. The thickness of the cylinder which encloses the sensor unit can be as thin as possible so that the desired accuracy can be attained by minimizing the distance between the sensor and the strip. It is required for the hollow cylinder which houses the magnetizing unit to have an angle of contact for the strip in applying the tension on the hollow cylinder. Moreover this angle of contact is favorable for the magnetizer to magnetize the strip enough for the detection of the defect of the strip. The arrangement of the two hollow cylinders in the strip production lines or in the strip processing lines wherein high line tension is applied on the strip, is quite simple since high tension is applied to the strip and a sufficient angle of contact can be achieved by a slight deviation in the line of the contact off the otherwise straight line of the strip. However when the line tension is low, two pinch rolls are required at the front and back sides of the hollow cylinders. Figures 5A, 5B, 5C, and 5D are schematic illustrations showing arrangements of the hollow cylinders and the pinch rolls. Figure 5A shows the most standard type of arrangement. As shown in Figure 5A, the hollow cylinder 50 which houses the sensor is placed on top of the hollow cylinder 52 which houses the magnetizer. The strip 58 is bent by the front pinch roll 54, hollow cylinder 52, and the back pinch roll 56. The plane which passes both center lines of the hollow cylinders is vertical. In this case the burden of the tension of the strip 58 and the weight thereof is supported by the hollow cylinder 50, front pinch roll 54, and back pinch roll 56. In Figure 5B, the hollow cylinder 52 is placed on top of the hollow cylinder 50. The components under load are the same with in the case of Figure 5A. In Figures 5C and 5D, the plane which passes both center lines of the hollow cylinders is slanted with respect to a vertical plane. In these cases an angle of contact is procured also for the hollow cylinder 50 which ensures the contact thereof by a small contact force compared with the cases in Figures 5A and 5B in order to rotate the cylinder by the running of the strip. In case of Figure 5C, the hollow cylinder 50 is placed on top of the hollow cylinder 52. In case of Figure 5D, the hollow cylinder 52 is placed on top of the hollow cylinder 50. Examples Figure 2 is an explanatory view showing the flow system of an embodiment of the invention. The strip 12 running out of the pay off reel 11 is introduced between a pair of the cylinders 14a and 14b via the forward pinch rolls 13a and 13b, travels through the rearward pinch rolls 15a and 15b, and is coiled by the tension reel 16. The power supply 17 for magnetization is connected to the inside of the cylinder 14b by the cable 17a. The control circuit 18 is connected to the inside of the cylinder 14a by the cable 18a. The control circuit is connected to the defect counter 20 and the alarm circuit 19. Figure 1A is a partially cutaway side view of an embodiment of the invention. The section of the embodiment is cut by a plane parallel to the travelling direction of the strip. Figure 1B is a partially cutaway front view of the embodiment of the invention. The section of the embodiment of the invention is cut by a plane perpendicular to the travelling direction of the strip. The hollow cylinders 14a and 14b protect the instrumentations from directly contacting the strip. Said cylinders are made of nonmagnetic materials to enable the magnetic flux to penetrate from inside of the cylinders to the strip. The hollow stationary shafts 21a and 21b are placed at the center of the hollow cylinders 14a and 14b. A pair of bearings 22a and 22b are inserted between the end parts of the shaft 21a and the hollow cylinder 14a. A pair of bearings 23a and 23b are inserted between the end parts of the shaft 21b and the hollow cylinder 14b. The shaft 21b is fixed to a stationary body, not shown. The shaft 21a is engaged with the shaft 21b by a weak spring, not shown. Accordingly the hollow cylinders 14a and 14b can freely be rotated around the shafts 21a and 21b. Inside of the hollow cylinder 14a, the magnetic detection elements 24 are arrayed parallel to the axial direction of the hollow cylinder 14a and the array is placed in the vicinity of the part of the hollow cylinder which contacts to the strip 12. The magnetic detection elements are fixed to the shafts 21a by the engagement of the supporting element 25. The clearance between the tip of the magnetic detection elements 24 and the inner surface of the hollow cylinder 14a is very small. The defect signal generated by the magnetic detection elements is sent to the control circuit 18 via the cable 18a. Inside of the hollow cylinder 14b, the magnetic yoke 26 is fixed to the shaft 21b by the engagement of the supporting element 27. The free ends 26a of the magnetic yoke 26 is placed in the vicinity of the part of the hollow cylinder 14b which contacts to the strip. The magnetic yoke is magnetized by the magnetizing coil 28 attached to the yoke. The electric current for the magnetic coil is supplied by the power supply 17 by the cable 17a led into the hollow shaft 21b. The magnetic detection elements 24 are placed at the opposite side of the free ends of the yoke 26a with respect to the contact point of the strip 12 and the hollow cylinders 14a and 14b. The distances between the magnetic detection elements and the strip 12 , denoted as "d " , and between the free ends of the yoke 26a, are fixed. When the strip runs in the direction denoted in Figure 2A, the hollow cylinder 14b is rotated in the direction denoted in Figure 2A driven by the frictional force generated through the contact force which is created by the tension of the strip and the weight thereof. The hollow cylinder 14a is also rotated since the cylinder is pressed to the strip by a spring mechanism, not shown. When the power supply 17 supplies electric current to the magnetic coil, a closed magnetic circuit is formed between the magnetic yoke 26 and the strip 12 since the hollow cylinders are made of nonmagnetic material. When a defect inside of or on the surface of the strip passes through the magnetic circuit, the magnetic circuit is disturbed and a magnetic flux leakage is generated which is detected by the magnetic detection elements 24 as a defect signal. The defect signal outputted by the magnetic detection elements is inputted to the control circuit 18. The control circuit specifies the level of the signal and classifies as the defect when the level is higher than a predetermined value. The specified defect signal is sent to the alarm circuit 19 which generates an alarm to operators. The specified defect signal is also sent to the defect counter 20 which renews the counted number of defects. The thickness of the hollow cylinders which house the instrumentation is determined by the deformation of the cylinder and the capabilities of the instrumentation. As for the thickness of the hollow cylinder 14b denoted as t₂ in Figure 1A, consideration should be given to the. deformation of the cylinder 14b due to the tension of the strip acting on it and the weight of the strip which is also applied on the hollow cylinder 14b. There should be enough thickness to avoid deformation of said cylinder so that the free ends of the yoke 26a does not touch the inner surface of the hollow cylinder. To generate enough magnetic flux to detect a defect of the strip, the free ends of the yoke should be as close to the strip as possible. However this problem can be solved by providing a strong magnetizing capacity. As for the consideration on the thickness of the hollow cylinder 14a denoted as t₁ in Figure 1A, no significant external force is applied on the hollow cylinder 14a. However some external force should be applied on the hollow cylinder 14a and the hollow cylinder should be pressed to the strip in order that the hollow cylinder may be driven by the strip. In spite of the above-mentioned facts the thickness of the hollow cylinder can be decreased to the extent wherein the magnetic detecting elements are enabled to detect a defect of the strip since no significant external forces are applied on the hollow cylinder. The smaller the value t₁ , the more improved the S/N ratio of the defect signal, which enhances the accuracy of the detection. In this embodiment is desirable for t₁ to be in the range of at least 1mm to provide the structural strength of the cylinder, and at most 5 mm to provide the sufficient magnetizing capability. The thickness of the cylinder t₁ can be further decreased when the thickness of the cylinder under the magnetic detecting elements is made thinner than the part of the cylinder between the magnetic detecting elements; in other word a stepped inner surface of the hollow cylinder is possible as shown in Figure 6 that is a partially cutaway view of an embodiment of the invention. In Figure 6 the thickness of the hollow cylinder 14a under the magnetic detecting elements 24, denoted as t₃, is thinner than the thickness of the cylinder 14a betweem the magnetic detecting elements 24, denoted as t₄. In this embodiment, an austenitic steel is utilized. However titanium, synthetic resin, fiber reinforced plastic, and ceramics are the good substitute. In this embodiment, the material of the magnetizing coil is a pure iron which has high performance in magnetizing of the yoke. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A magnetic defect detector comprising: a pair of rotatable cylinders (14a, 14b) made of nonmagnetic material which pinch a running steel strip (12); two pairs of bearings (22a, 22b, 23a, 23b) being engaged with the end parts of said pair of cylinders; a pair of stationary shafts (21a, 21b) housed in the pair of cylinders, the end parts of said pair of stationary shafts being engaged with said two pairs of bearings; a yoke (26) housed in one of the pair of cylinders, the free ends (26a) of said yoke being placed in the vicinity of the inner surface of a part of the cylinder which contacts with the strip; a magnetising coil (28) surrounding a part of the yoke; the yoke and the strip together forming a magnetic circuit; a supporting element (27) which supports the yoke and fixed to one of the pair of shafts; a series of sensors (24), for detecting signals of defects of the strip caused by a magnetic leakage flux due to said defects, housed in the other one of said pair of cylinders and arranged parallel to the axis of the cylinder in the vicinity of the inner surface of a part of the cylinder which contacts with the strip; a second supporting element (25) which supports said series of sensors and fixed to the other one of the pair of shafts; and means (18, 19, 20) for determining, from said signals, defects on or in said strip. The magnetic defect detector of Claim 1, characterized in that the thickness of parts of the cylinder under the sensor is smaller than the thickness of parts of the cylinder laterally situated between the sensors. The magnetic defect detector of Claim 2, characterized in that the thickness of parts of the cylinder under the sensor is not less than 1mm and not more than 5 mm. The magnetic defect detector of Claim 1, characterized in that said means for determining defects from said signals comprises; a signal treating circuit wherein a signal from the magnetic defect detector is compared with a second signal from a surface defect detector; and a signal specifying circuit which specifies the signal from the magnetic defect detector as the surface defect of the strip when the both signals are generated at the same spot of the strip and as the inner defect of the strip when the signal from the surface defect detector is not generated at the spot and the signal from the magnetic defect detector is generated. The magnetic defect detector of Claim 1, characterized in that said means for determining defects from said signals comprises; a signal treating circuit wherein a signal from the magnetic defect detector is compared with a second signal from a weld detector; and a signal specifying circuit which specifies that the signal from the magnetic defect detector is a welded part of the strip when the signal from weld detector is generated. The magnetic defect detector of Claim 1, characterized in that the material of the cylinders is an austenitic stainless steel. The magnetic defect detector of Claim 1, characterized in that the material of the cylinders is a titanium alloy. The magnetic defect detector of Claim 1, characterized in that the material of the cylinders is a synthetic resin. The magnetic defect detector of Claim 1, characterized in that the material of the cylinders is a fiber reinforced plastic. The magnetic defect detector of Claim 1, characterized in that the material of the cylinders is a ceramics. The magnetic defect detector of Claim 2, characterized in that the material of the cylinders is an austenitic stainless steel.

Inventors: FURUKAWA TAKATO C O PATENT LIC, IWANAGA KENICHI C O PATENT LIC, TAKEGOSHI ATSUHISA C O PATENT, FURUKAWA, TAKATO, C/O PATENT & LICENSE DEPT., IWANAGA, KENICHI, C/O PATENT & LICENSE DEPT., TAKEGOSHI, ATSUHISA, C/O PATENT & LICENSE DEPT., Furukawa, Takato, c/o Patent &amp; License Dept., Iwanaga, Kenichi, c/o Patent &amp; License Dept., Takegoshi, Atsuhisa, c/o Patent &amp; License Dept.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1990
Main classification: G01N27
IPC codes: G01N27, G01V3