This invention relates to a method of and apparatus for effecting domain refinement of electrical steels and especially, but not exclusively, high-permeability grain- oriented electrical steels. In particular, but not exclusively, the invention concerns apparatus operable to produce within steel strip and sheet lines of local plastic deformation which refine the strip or sheet's domain structures and reduce power loss without causing damage to the insulation coating of the strip or sheet thereby removing the need for recoating after treatment.

It is known that the magnetic properties of grain- oriented electrical steels can be enhanced by processing in a known manner which effects preferential alignment of

grains within the steel.

One problem associated with known techniques for producing enhanced magnetic properties is that effecting optimum grain alignment tends simultaneously to produce larger than optimum grain sizes, this resulting in higher power losses than would have been the case with smaller grain sizes and domain wall spacings.

Known techniques for effecting domain refinement by creation of artificial grain boundaries include mechanical methods, lasers or high voltage discharge systems.

Hitherto, mechanical methods have not been found to be susceptible to commercial use with the result that relatively expensive laser systems are conventionally employed commercially. - A relatively inexpensive spark ablation method (high voltage discharge system) is being utilised but this system tends to be rather slow for successful commercial use.

Although domain re inement can be achieved by a method known as mechanical ball scratching in which very small balls of diameter of the order of 0.7mm are employed which are forced into contact with the surface of a sheet under treatment, the method is difficult to maintain and operate on a commercial basis. Also, scratching of the surface of the steel occurs, and recoating of the steel after treatment is necessary.

This invention sets out to provide a method and apparatus for refining the domain structure of high permeability grain-oriented electrical steels in which

high localised pressure is applied to the surface of the steel to produce lines of plastic deformation in the form of lines spaced approximately 5 mm apart in directions generally perpendicular to the rolling direction of the sheet or strip.

According to the present invention in one aspect there is provided a method of effecting domain refinement of a strip or sheet of electrical steel which comprises the steps of creating in the strip or sheet lines of local plastic deformation by moving a plurality of spaced balls of relatively large diameter into contact therewith.

In another aspect, the present invention provides apparatus for effecting domain refinement of electrical steels comprising an assembly of an. array of rotatable members mounted within a supporting structure and movable into direct contact with the surface of a strip or sheet of electrical steel to create lines of local plastic deformation which extend substantially across the width of the strip of sheet to effect domain refinement thereof, and means operable to impart relative linear movement between each member and the steel strip or sheet.

The rotatable member preferably comprises a ball of relatively large diameter in contact with a relatively large number of smaller balls, which is resiliently biased away from its supporting structure and into contact with the surface of the steel strip or ^" sheet. Each ball may be produced from any suitable hard wearing material, for example chrome steel or silicon nitride. It is preferable

that the electrical steel is supported on a relatively hard surface/substrate during the domain refinement process. The substrate may comprise or be coated with a resin bonded material or may or comprise a stainless steel plate. The substrate is preferably wider than the strip or sheet of electrical steel to be treated. In an alternative construction the substrate comprises a large diameter roller or a continuous belt.

The method and apparatus described in the preceding three paragraphs are readily amenable to commercial production and are comparatively cheap to operate. Additionally, the apparatus is relatively easy to maintain with no difficult adjustments to make during operation of the same. Further, recoating of the steel after treatment is not necessary and the process is amenable to continuous strip movement or strip indexing methods of production. Fast process speeds are also attainable.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings, in which:-

Figure 1 is a sectional view of apparatus in accordance with the invention;

Figures 2a, 2b, 2c, and 2d illustrate alternative embodiments of apparatus in accordance with the invention;

Figure 3 is a sectional view of a further embodiment of apparatus in accordance with the- invention.

Figures 4 and 5 are respectively plan and side views of alternative apparatus in accordance with the invention.

Figures 6 and 7 are respectively a plan view and a side view in section of further apparatus in accordance with the invention;

Figures 8 and 9 are respectively a plan view and a side view in section of still further apparatus in accordance with the invention; and

Figures 10A and 10B illustrate the surface of grain refined steel treated in accordance with the invention.

The apparatus illustrated in Figure 1 comprises an assembly 1 of a chrome-steel ball 2, typically of a diameter of between 12 and 32 mm, mounted for rotation within a steel support casing 3. Other diameter balls may be employed, these ranging from 10mm to 50mm in diameter. Also, the balls may be produced from other hard-wearing materials other than chrome-steel, eg silicon nitride. Bearings 4 are positioned between the opposed surfaces of the ball 2 and casing 3 to create a low friction assembly allowing the relatively large ball freely to rotate over the surface of a steel sheet under treatment. A threaded shaft 5 is upstanding from the casing for appropriate attachment to a carriage or the like.

As will be seen from Figure 2, the assembly when used in an indexing mode, may include a plurality of chrome- steel ball units 2, the relative positions of these on the respective support carriage being a matter of choice depending upon the length of the sheet or strip to be treated and its manner of movement relative to the

assembly. In the embodiment illustrated in Figure 2a, the positions of several chrome-steel ball units 2 are shown, the balls of each row being offset with respect to its neighbours such that the spacing between lines created at right angles to the rolling direction of the strip on traverse are of the order of 5mm. In this embodiment, in use the assembly of the balls and casing are caused to traverse the full width of the strip which is stationary, the latter then being indexed laterally in the direction indicated by arrow 'A' by a distance equivalent to the length taken up by the ball units employed. Conventional linear motion units may be employed as the traverse drive mechanism.

In the embodiment illustrated in Figure 2b, an assembly of several side-by-side chrome-steel ball units is employed, the ball units being caused to traverse to a limited extent in a reciprocating manner at an appropriate speed across the width of a continuously or periodically moving strip of high-permeability electrical steel to create lines of local plastic deformation spaced typically 5mm apart. The direction of strip movement is indicated again by arrow 'A' .

In the arrangement illustrated in Figure 2c, the assembly comprises an arrangement of chrome steel balls 6 which are supported so as to traverse continuously across the width of the upper or lower surface of the strip whereby each ball unit creates lines of stress spaced approximately 5mm apart normal to the rolling

direction of the strip. A lower or upper return path is provided in which the ball units do not come into contact with the strip surface. A typical production arrangement in accordance with this particular embodiment is illustrated in Figure 6.

In the embodiment of Figure 2d, chrome-steel ball units are again employed, these being made to traverse continuously the strip in a circular-like manner whereby slightly curved lines of stress are created across the strip width creating the domain refinement on traversing the strip in one direction and missing the strip on the return path. Several circular motion arrangements may be employed instead of one in order to reduce the size of the assembly. A typical production arrangement in accordance with this particular embodiment is illustrated in Figures 4, 5 and 7.

As will be seen from Figure 3, each assembly 1 is supported within a carriage 8 secured by a bolt 9 to a drive member 10. A spring 11 is located about a non- threaded attachment shaft 12 to urge the ball unit 2 into contact with the strip or sheet under treatment. Alternatively, a pneumatic method of urging the ball unit 2 into contact with the strip or sheet may be employed.

The apparatus illustrated in Figures 4 and 5 comprises a circular array of chrome-steel balls 12 supported within a rotable carriage 14 including an upstanding shaft 15 rotatable by an electric motor 16. The strip is indicated by reference numeral 17 and its

intended direction of movement is indicated by arrow 'A' . A flat-surfaced steel substrate 18 is positioned below the strip 17. As will be seen from Figure 4, the strip 17 is masked from the strip surface during one of its paths across the strip width.

In operation, steel strip from a coil 19 moves continuously over the substrate 18 and into contact with the chrome-steel balls 12 to create the required lines of stress. The speeds of movement of the strip 17 and the carriage 14 are selected to ensure that the lines of stress created lie generally transverse of the strip. The apparatus illustrated in Figures 6 and 7 is similar to that illustrated in Figures 4 and 5 and like integers have been given the same reference numerals. In this embodiment, however, the chrome-steel balls 12 are replaced by chrom-steel rings 21 mounted on shafts 22. In other respects, the apparatus illustrated in Figures 6 and 7 is the same as that illustrated in Figures 4 and 5.

The apparatus illustrated in Figures 8 and 9 comprises a pair of rotatable wheels 22 about which tracks an array of articulated carriages 23 each supporting a chrome-steel ball 24. The balls 24 are moved transversely across the width of the steel strip 25 moving continuously in the direction of arrow 'A' .

The refinement produced by the steel balls or rings of the present invention can be seen from Figures 10A and 10B. Clear areas of domain refinement are indicated by reference numbers 25.

Examples of initial trials conducted using apparatus in accordance with the invention will now be described by way of example only.

Numerous samples of phosphate coated, finally processed high permeability grain-oriented electrical steel sheets, 610mm x 305mm in size, were obtained and power loss (B = 1.7T, 50 H _z ) and permeability 031kA/m) values were determined.

A pilot line utilising a single ball unit, of diameter 12.5mm, was used to treat these samples, the force applied by the spring arrangement being of the order of between 2 and 6kgf (20-60N). Typical applied force values may be of the order of 4 1/2 and 5 l/2kgf.

Application of this force created within the steel lines of stress enabling domain refinement to be clearly detectable on both sides of the sheets. The domain refinement achieved, observed using a magnetic domain viewer, can be clearly seen from Figure 9. The insulation coatings of the samples were essentially undamaged during the trials with the result that the applied pressure lines were frequently difficult to observe, the effect however being clearly visible when using a domain viewer. It should be noted that effects of refinement achieved with spark ablation or lasers are not always clearly detectable on both sides of a treated sheet, whilst with the method described above refinement is almost always clearly detectable on both sides of the sheet.

Power loss and permeability values were then

remeasured as was the insulation resistance by means of the dual electrode method of BS 6404, Part 2, appendix D. Typical values of loss reduction achieved and the effect of the treatment on permeability values are given in Table 1 below.

TABLE 1

Initial Results of Large Ball Unit Domain Refinement

Sample Initial Domain % Initial Domain Loss Refined Loss " W/kg Loss W/kg Reduction

1 1.006 0.941 6.5 2 1.015 0.929 8.5 3 0.981 0.889 9.4 4 0.964 0.919 4.7 5 0.951 0.907 4.6 6 1.076 1.014 5.8 7 1.005 0.937 6.8 8 0.975 0.925 5.1 9 1.059 0.996 5.9 10 1.046 0.987 5.6

It can be seen from the tabulated results that for appropriate samples excellent values of loss reduction and final loss are achieved eg 9.4% loss reduction, final loss 0.889W/kg for 9.27mm material.

The results show a small reduction in permeability value but this is insignificant. The range of results are typical of those achieved with the spark ablation system, results being dependent on, for example, the starting material grain size, orientation, and coating characteristics.

Insulation data given in Table 2 below indicates that use of the method of this invention does not degrade the insulation resistance significantly, making recoating unnecessary.

TABLE 2

Insulation Resistance of Large Ball Unit Domain Refined Samples (Non-Recoated)

Sample % Number of Readings

>10 ohms >25 ohms

4

5

6

11

9

14

15

13

16

The following examples are provided further to explain and exemplify features of the present invention.

EXAMPLE 1

Numerous samples of high permeability grain oriented electrical steel, 0.27mm x 610mm x 220mm, were obtained and were domain refined using a range of applied forces to a ball unit 19.1mm in diameter, the stress line spacing being 10mm.

The results given in Table 3 clearly show the effect

of increasing applied force on reduction of power loss eg. an applied force of 3.39kg results in only 0.51% loss reduction whereas increasing the applied force to 5.8kg results in a loss reduction value of 6.88%.

A similar result can be seen in Table 3 when using a ball unit of diameter 31.8mm in which an applied force of 4.94kg results in 2.42% loss reduction whilst an applied force of 5.87kg results in a loss reduction value of 5.24%.

EXAMPLE 2

Numerous samples of high permeability grain oriented electrical steel were obtained as in Example 1 and were domain refined using a range of applied force values, for a range of stress line spacings and for ball unit assemblies covering a range of diameters.

The results are given in Table 4 where it can be clearly seen that line spacings of < 5mm are undesirable.

TABLE 4

10 7. 6

12.7 5 4.55 3. 75 3

10 7.5 6

19.1 5 4.68

3.75 3

10 7.5

25.4 6 5.86 5 3 75

10 7.5

31.8 6 6.14 5 3 75

10

39.7 7.5 6.48 6.0

EXAMPLE 3

Further examples of loss reduction achieved for samples domain refined employing various ball unit assemblies and applied force values with the stress line spacing being 10mm, are given in Table 5 where it can be seen that loss reduction values of up to 9.65% are achieved.

TABLE 5

19.1 10 5.21

31.8 10 5.55

4. 61 5.21

19.1 10 5.21 5.21

4.56 4.56

12.7 10 4. 86

4.86

19.1 10 5.21 0.994 0.927 6.74

EXAMPLE 4

Insulation measurements were carried out on numerous samples domain refined using ball units of various diameters, various line spacings and applied force values higher than those which would normally be employed.

Samples were treated using tufnol and stainless steel backing plates.

The results of insulation measurements are given in Table 6 where it can be seen that in all cases excellent insulation resistance was maintained after treatment.

TABLE 6

Comments

Tufnol

Backing

Plate

Stainless Steel Backing Plate

It will be appreciated that the foregoing is merely exemplary of methods and apparatus in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the inventio .