GB2268272A

GB2268272A - Detecting regions of fine grain in grain oriented silicon steel

Info

Publication number: GB2268272A
Application number: GB9213777A
Authority: GB
Inventors: Robert John Taylor
Original assignee: ORB Electrical Steels Ltd
Current assignee: ORB Electrical Steels Ltd
Priority date: 1992-06-29
Filing date: 1992-06-29
Publication date: 1994-01-05
Anticipated expiration: 2012-06-29
Also published as: GB2268272B; GB9213777D0

Abstract

Regions of fine-grain are detected by applying an alternating magnetic field by means of a magnetic yoke 10 to a surface of the steel substrate 1, placing a magnetic sensor head 14 having a pole gap in close proximity to the surface and aligned so that the pole gap is aligned with the direction of relative movement between the steel signals the presence of fine grain within the steel substrate. Preferably the alternating magnetic field is applied along the length of the substrate and the sensor head 14 scans transversely to the substrate which also moves linearly. The sensor head may be positioned within the span of the yoke 10 and on the same side of the substrate. <IMAGE>

Description

MONITORING APPARATUS AND METHOD This invention relates to apparatus for and a method of detecting and monitoring fine grain regions in grain oriented silicon steel and, especially, rolled strips and sheet of such steel.

As is generally known, during secondary recrystallisation in the production of grain oriented silicon steel grains which are favourably aligned with the direction of rolling grow at the expense of other grains. This recrystallisation process is normally completed when all the grains in the strip or sheet material have their most magnetically permeable crystallographic directions closely aligned with the rolling direction. For reasons as yet unclear, this process does not always proceed to completion, with the result that bands of fine, relatively non-orientated material, are sometimes found to remain. These bands are generally found to lie parallel to the direction of rolling. and material containing the bands has markedly inferior magnetic properties compared with properly recrystallised silicon steel.At this stage in the production process, the material has usually acquired an opaque coating which makes visual detection impossible. A conventional power loss measuring apparatus is normally used to test the material as it is produced. Such a method gives only an average figure for the whole width of the strip or sheet; the bands of fine grain are thus not detected.

The strip or sheet material produced is rarely used in its full width form but is usually cut to a width (by slitting in the direction of rolling) suitable for use in the construction of, for example. electrical transformers.

It is thus possible for a strip of silicon steel to be cut which contains a high percentage of fine grained material.

The magnetic properties of this portion will be markedly inferior to the average for the whole sheet width (as determined by the conventional loss tester method).

Currently, material is tested for the presence of fine grain bands by the simple expedient of cutting samples and removing the surface coating with a sulphuric and hydrofluoric acid mixture. A visual inspection can then be made. However, this process is hazardous, owing to the highly corrosive nature of the acids employed, and is also very time consuming. Furthermore, it is a destructive method which tests only a tiny area of the total strip or sheet produced. The need for a nondestructive method for detecting the presence of fine grained material "on line" is therefore clearly apparent.

It is an object of the present invention to provide such a non-destructive "on-line" method of detecting fine grained material.

According to the present invention in one aspect, there is provided a method of detecting regions of fine grain in a grain oriented silicon steel substrate, which method comprises applying to a surface of the steel substrate an alternating magnetic field; placing a magnetic sensor head in close proximity to the said surface of the steel substrate; imparting relative movement between the steel substrate and the sensor head; aligning the sensor head such that a pole gap thereof is aligned with the direction of relative movement between the steel substrate and the sensor head: monitoring signals produced by the magnetic sensor head; and detecting from said signals the presence of fine grain within the steel substrate.

Preferably, the applied alternating magnetic field is imposed along the length of steel substrate, that is in the direction of relative movement between the substrate and the sensor head. The applied alternating magnetic field may, however, be imposed across the width of the steel substrate, that is in a direction normal to the direction of relative movement.

In another aspect, there is provided apparatus for detecting in a grain oriented silicon steel strip regions of relatively fine grain, the apparatus comprising a magnetic yoke operable to generate in a steel strip an alternating magnetic field. and a magnetic sensor having a pole gap which is so positioned that in use of the apparatus it can readily be aligned with the direction of travel of strip in which regions of relatively fine grain are to be detected.

In a further aspect, the invention provides apparatus for detecting regions of fine grain in a moving grain oriented silicon steel substrate, which apparatus comprises means for applying an alternating magnetic field to a surface of the steel substrate; means for scanning across the width of the substrate in a direction generally perpendicular to the direction of travel of the steel substrate, the scanning means comprising a magnetic sensor head arranged to detect variations in the magnetic fields generated in the steel by a magnetic yoke at locations across the width of the substrate; means for amplifying the signals produced by the magnetic sensor head; and means for converting the signals into a detectable form.

In making this invention, it was recognised that fine grained regions and larger oriented grain regions of a steel substrate exhibit different magnetic behaviour. For purposes of explanation, a given grain can be envisaged as containing magnetic domains which can be regarded as equivalent to tiny bar magnets, each domain being magnetised in the opposite sense to its adjacent neighbours. The region separating adjacent domains is known as the "domain wall"; this is a transition region where the direction of the magnetic field changes through 1800 giving rise to a "leakage" of magnetic flux. In unmagnetised material, an exact balance exists between domains pointing in the two magnetisable directions.An applied external magnetic field causes the sympathetically aligned domains to grow in concert, expanding their width by domain wall movement and consuming the oppositely oriented domains. If the field direction is reversed, however, the domains which previously grew will shrink, and those which shrank will grow. Under the application of an alternating field, domains swell and shrink repeatedly in a cyclical manner.

The larger grains present in silicon steel are generally oriented such that their magnetic domains are aligned in a direction parallel to the rolling direction of the steel. However, this is not the case with the magnetic domains within the fine grain regions of the steel, which tend to be somewhat randomly oriented. In the present invention, an alternating field is applied to a length of steel by a magnetic yoke and the steel is then scanned across its width by a magnetic head having a pole gap. In the oriented large grain regions of the steel, the aligned magnetic domains swell and shrink cyclically under the influence of the alternating magnetic field, and are therefore in constant motion along a line of movement which is normal to the rolling direction of the steel.The magnetic head is placed close to the sheet with its pole gap aligned with the rolling direction and is sensitive to the moving domains and produces electrical signals of the same frequency as that of the magnetised field, However, when the head is positioned over a region of fine grain, the signal is greatly reduced because: (i) the magnetic domains are smaller; (ii) the grains are generally not aligned with the rolling direction with the result that the domain walls do not sweep past the head gap in the most sensitive direction; and (iii) the domain walls do not move so far or so fast as in properly aligned material because the applied field (imposed through continuity of contact with the surrounding material) is rarely aligned with the direction of easy magnetisation within the grain.

It is this difference in signal levels which allows the fine grain to be detected through the coating applied to the steel, and whilst the material is in motion. A single magnetic head or an array of such heads can be used to scan back and forth across the width of the steel substrate or a larger fixed array of magnetic heads can be set up to monitor constantly the whole of the width of the steel substrate.

The invention will now be described by way of reference to the accompanying diagrammatic drawings, in which:- Figure 1 is a schematic view of the crystal structure of a length of steel strip; Figure 2 is an enlarged schematic view of the crystal structure of a portion of the steel strip; Figures 3a to 3c are schematic views illustrating the orientation of magnetic domains within the steel strip in the absence and presence of a magnetising field Figure 4 is a schematic view illustrating detection apparatus in accordance with one embodiment of the invention; Figure 5a is a front elevation of the magnetic head shown in Figure 4; Figure 5b is an isometric view of the magnetic head illustrated in Figure 5a; Figure 5c is a view from beneath of the magnetic head shown in Figures 5a and 5b;; Figure 6 is a schematic view illustrating the alignment of the magnetic head of Figures 5a to 5c with the steel strip; Figure 7 is a perspective view of alternative apparatus in accordance with the invention, and Figures 8 and 9 schematically show the apparatus of Figure 7 in use and a plan view from below of this apparatus.

Turning now to the drawings, it can be seen that Figure 1 illustrates a length of rolled silicon steel strip 1 in which relatively large grains 2 are aligned with the direction D of rolling. Along the centre region of the strip (although it need not be along the centre), and generally parallel to the direction D of rolling, is a region of fine grain 3. In the larger grains 2, the magnetic domains which are defined by domain walls, are aligned relatively closely with the rolling direction D.

By comparison, the magnetic domains defined by domain walls within the fine grain regions 3 are randomly oriented, as is shown with greater clarity in Figure 2.

As illustrated in Figure 3a, in the absence of a magnetic field,. adjacent domains within a given grain are aligned in opposite directions, and are generally balanced so as to neutralise one another. However, in the presence of an applied magnetic field, the magnetic domains aligned with the applied field grow at the expense of the magnetic domains opposed to the field. This phenomenon is illustrated schematically in Figures 3b and 3c, the field directions being indicated by arrows 4b and 4c respectively.

In the method of the present invention, an alternating magnetic field is applied to the steel strip and the resulting movements of the magnetic domain walls are monitored.

Apparatus in accordance with the invention for magnetising the steel strip and monitoring the movement of the domain walls is illustrated in Figures 4 and 5. This apparatus comprises an electromagnetic yoke 10 comprising a core 11 and a coil 12 linked to an oscillator and amplifier 13. The electromagnetic yoke 10 magnetises the steel strip 1 by subjecting it to an applied alternating field typically in the frequency ranae of 0.5 to 10 KHz (preferably 5 KHz). The alternating field is preferably applied in the lenghwise direction of the strip. It may, however, be applied in a direction across the strip width.

As will be seen from Figures 5a to Sc a magnetic head 14 is provided which comprises a C-shaped core 15 and a coil 16. The gap 17 in the core of the magnetic head 14 is aligned with the direction of travel D of the steel strip 1. The magnetic head 14 is connected to a pre-amplifier 18 which in turn is connected via a filter 19 to amplifier 20. Signals received and amplified by amplifier 20 are then directed to loudspeaker 21 and /or meter 22. The signals can be further processed by a computer which may be capable of constructing a "map of fine grain sites for each length of strip processed. The steel strip 1 is transported by rollers 23 and is supported both before and after the detection zone by smaller diameter rollers 24 or guides.

In this embodiment, the magnetic head 14 is fitted with a sliding trammel or carriage assembly (not shown) which enables the head 14 to scan back and forth along a line perpendicular to the direction of travel D. Typical 'scanning frequencies of the order of 0.1 to 1 cycle per second.

Turning now to the apparatus illustrated in Figures 7 to 9 of the drawings, it will be seen that the sensing head 14 is positioned within the span of the yoke 10 and on the same side of the strip 1 as the yoke 10. This single sided apparatus is likely to lead to reduced installation problems. The sensing head 14 is not swamped by the alternating magnetic field because its sensitive direction is perpendicular to the field, In alternative embodiments to that illustrated, an array of scanning heads 14 or an array of fixed magnetic heads extending across the entire width of the strip may be used.

In use, the silicon steel strip 1 is moved along in the direction D by means of he rollers 23, and a magnetic field is applied to the strip 1 by the electro-magnetic yoke 10. Typically, the speed of the strip is of the order of between 65 and 90 metres per minute. Application of the alternating magnetic field causes cyclical growth and shrinkage of the magnetic domains in the strip, and this is detected by the magnetic head 14 which produces a signal which is amplified and made audible and/or visible by means of the speaker 21-and/or the meter 22. As the magnetic head passes over a region of fine grain material, the intensity of the signal generated by the magnetic head is reduced considerably and this shows up on the meter and/or loudspeaker.

It will be readily apparent that the foregoing is simply exemplary of the apparatus and method of this invention and that various modifications can readily be made thereto without departing from the features of the invention.

Claims

1. A method of detecting regions of fine grain in a grain oriented silicon steel substrate, which method comprises applying to a surface of the steel substrate an alternating magnetic field; placing a magnetic sensor head in close proximity to the said surface of the steel substrate; imparting relative movement between the steel substrate and the sensor head; aligning the sensor head such that a pole gap thereof is aligned with the direction of relative movement between the steel substrate and the sensor head; monitoring signals produced by the magnetic sensor head; and detecting from said signals the presence of fine grain within the steel substrate.

2. A method as claimed in Claim 1 wherein the applied alternating magnetic field is imposed along the length of steel substrate in the direction of relative movement between the substrate and the sensor head.

3. Apparatus for detecting in a grain oriented silicon steel strip regions of relatively fine grain, the apparatus comprising a magnetic yoke operable to generate in a steel strip an alternating magnetic field, and a magnetic sensor having a pole gap which is so positioned that in use of the apparatus it can readily be aligned with the direction of travel of strip in which regions of relatively fine grain are to be detected.

4. Apparatus for detecting regions of fine grain in a moving grain oriented silicon steel substrate, which apparatus comprises means for applying an alternating magnetic field to a surface of the steel substrate; means for scanning across the width of the substrate in a direction generally perpendicular to the direction of travel of the steel substrate, the scanning means comprising a magnetic sensor head arranged to detect variations in the magnetic fields generated in the steel by a magnetic yoke at locations across the width of the substrate; means for amplifying the signals produced by the magnetic sensor head; and means for converting the signals into a detectable form.

5. Apparatus as claimed in Claim 4 wherein the

head comprises a core aligned with the direction of travel of the substrate.

6. Apparatus as claimed in Claim 5 wherein the sensor head is fitted with a carriage assembly movable back and forth across the width of the substrate.

7. Apparatus as claimed in any one of Claims 3 to 6 wherein the sensor head is positioned within the span of the yoke and on the same side of the substrate as the yoke.

8. Apparatus for detecting regions of fine grain in a grain oriented steel substrate substantially as herein described with reference to Figures 4 to 6 or Figures 7 to 9 of the accompanying drawings.