GB2267715A

GB2267715A - Treating spray formed iron silicon alloys

Info

Publication number: GB2267715A
Application number: GB9211722A
Authority: GB
Inventors: Gordon Cunningham Eadie; Dennis Patrick Willia Campbell; Simon Andrew Hall
Original assignee: British Steel PLC
Current assignee: British Steel PLC
Priority date: 1992-06-03
Filing date: 1992-06-03
Publication date: 1993-12-15
Anticipated expiration: 2012-06-03
Also published as: WO1993024259A1; GB2267715B; GB9211722D0; AU4340193A

Abstract

A method of producing high silicon-iron alloys from a start material having a silicon content (with or without aluminium) of between 3.25 % and 6.5 % by weight comprises the steps of forming a melt of the start material, producing a product of the melted material by a spray forming technique, and heating the product to a temperature of the order of 700 DEG C to 900 DEG C in a moist hydrogen atmosphere to produce on the surface of the product a silicate layer.

Description

IMPROVEMENTS IN AND RELATING TO THE PRODUCTION OF HIGH SILICON-IRON ALLOYS This invention relates to the production of high silicon-iron alloys.

Magnetic cores of transformers, motors, generators and similar machines are usually fabricated from magnetically soft alloy strip such as 3% silicon-iron alloy and eddy currents circulate in the cores of these machines when alternating current flows in the windings.

In order to reduce such eddy currents which are wasteful of energy, the specific resistance of the strip alloy is raised when ever possible by increasing the alloying elements(s) such as silicon with or without aluminium. It is well known that using conventional processing routes, the tolerable combined level of such alloying elements(s) is about 3.25% by weight maximum otherwise the material is too brittle to process, cut up or bend due to excessively large grains formed during cooling for example.A further stratagem employed to limit eddy currents is to build up the machine core from thin sheets (lamination), both surfaces of which carry an electrical insulation medium which can be paper, paint, varnish, lacquer or even inorganic chemical compounds formed on the strip surfaces during processing and reinforced with further coatings such as cured on phosphoric acid employed to fill voids in the primary chemical compound coating, such voids being potential rust generating sites.

Our co-pending Patent Application 9116242.0 discloses a method by which silicon-iron alloy strip can be prepared with relatively very high levels of silicon with or without aluminium (3.25% -6.5% by weight) and because the strip has been prepared by sprayformin, the micro structure is such that brittleness is avoided and the material can be further processed if desired without breaking up.

During processing of conventional commercial strip having relatively low levels of alloying elements(s) (approximately 3.2% silicon or silicon with aluminium by weiaht) the material is subjected to a decarburising process involving treatment with moist hydrogen at temperatures of the order of 70Q-900 C in order to enhance the magnetic performance of the material by diffusing carbon out of the strip so that the residual level is not more than 0.005% by weight.

However it has been found that when the same decarburising treatment is applied to sprayformed strip where the total alloying element(s) level is of the order of 3.25% - 6.5% by weight, such decarburising treatment is inhibited and carbon is prevented from diffusing out of the strip by the formation of a silicate barrier layer on each surface. It has also been found that these silicate compounds form a sufficiently good electrical resistance layer to act as effective interlaminar resistance.

Preparation of spray formed electrotechnical steel with high levels of silicon with or without aluminium commences with already decarburised material such as finally processed conventional un-insulated strip scrap to which has been added clean, carbon free ferrosilicon with or without aluminium and the whole remelted under vacuum or other conditions where carbon cannot be picked up.

In accordance with this invention, after sprayforming, the material is subjected to a conventional decarburising treatment to encourage the formation of silicates at the surfaces. Subsequently these coatings are reinforced by passing the strip sequentially through a phosphoric acid bath and curing the insulation media by heating the strip to a temperature of about 7000 C in a non-oxidising atmosphere under tension to achieve thermal flattening as well as the benefits of stress relief annealing. To enhance the coating properties chromium and similar compounds can be added to the phosphoric acid bath before curing if required.

The material is produced having a high specific resistance due to relatively very high levels of silicon and/or aluminium good interlaminar electrical resistance due to the formation of silicate and similar compounds on the surfaces of strip prepared by sprayforming, and is capable of maintaining good magnetic properties due to the effective absence of carbon.

In the event that suitable scrap material is not available for remelting to form the starting composition of sprayformed strip, it is still possible to prepare material with all the essential properties as indicated by either using low carbon vacumm degassed strip and/or following the teachings of our United Kingdom Patents 2050421 and 2057500 whereby titanium or similar material is included in the composition so that carbides are formed within the grain structure of the ensuing material.

As is known, when carbon resides in the structure of the material in solid solution the magnetic and magnetic ageing performance is very poor whereas when the carbides are relatively large precipitated bodies the magnetic performance is barely affected if later high temperature treatment is applied. Titanium has been found to be a strong carbide former.

Accordingly, the present invention provides a method of producing high silicon an iron alloys from a start material having a silicon content (with or without aluminium) of between 3.25% and 6.5% by weight which comprises the steps of forming a melt of the start material, producing a product of the melted material by a spray forming technique, and heating the product to a temperature of the order of 7000C to 9000C in a moist hydrogen atmosphere to produce on the surface of the product a silicate layer.

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

Claims

1. A method of producing high silicon-iron alloys from a start material having a silicon content (with or without aluminium) of between 3.25% and 6.5 by weight which comprises the steps of forming a melt of the start material, producing a product of the melted material by a spray forming technique, and heating the product to a temperature of the order of 7000C to 9000C in a moist hydrogen atmosphere to produce on the surface of the product a silicate layer.

2. A high silicon-iron alloy produced from the method claimed in Claim 1.

3. A method of providing high silicon-iron alloys substantially as herein described.