US3562029A

US3562029A - Processing of fibrous magnesium silicate coated silicon steel

Info

Publication number: US3562029A
Application number: US722225A
Authority: US
Inventors: James J Goglio; Clarence L Miller Jr
Original assignee: Allegheny Ludlum Steel Corp
Current assignee: Allegheny Ludlum Corp; Pittsburgh National Bank
Priority date: 1968-04-18
Filing date: 1968-04-18
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09

Abstract

DESCRIBED HEREIN IS AN IMPROVEMENT IN THE PROCESSING OF SILICON STEEL WHERE THE STEEL IS COATED PRIOR TO HEAT TREATMENT WHICH INVOLVES THE APPLICATION OF A COATING OF FIBROUS MAGNESIUM SILICATE INSTEAD OF MAGNESIA WHICH HAS BEEN USED PREVIOUSLY AND WHICH RESULTED IN THE IN SITU FORMATION OF CRYSTALLINE MAGNESIUM SILICATE.

Description

United States Patent 3,562,029 PROCESSING OF FIBROUS MAGNESIUM SILICATE COATED SILICON STEEL James J. Goglio, Leechburg, and Clarence L. Miller, Jr., Pittsburgh, Pa., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed Apr. 18, 1968, Ser. No. 722,225

Int. Cl. C04b 35/58; H01f 1/18 US. Cl. 148-113 3 Claims ABSTRACT OF THE DISCLOSURE Described herein is an improvement in the processing of silicon steel where the steel is coated prior to heat treatment which involves the application of a coating of fibrous magnesium silicate instead of magnesia which has been used previously and which resulted in the in situ formation of crystalline magnesium silicate.

This invention relates to a method of treating silicon steel. More particularly, the invention concerns a method for minimizing or recluding the formation of an abrasive coating on the surface of said steel when exposed to elevated temperatures.

Silicon steel suitable for electrical applications is typically heat treated in coil form during the processing thereof to develop grain growth and magnetic properties. Prior to heat treating a coil of silicon steel, the surface is coated with magnesia to prevent welding and to allow proper magnetic properties to develop. During annealing the magnesia reacts with silica present on the surface of the steel to form a complex coating containing crystalline magnesium silicate. This coating, in the form normally occurrng, is moderately insulating, glassy in appearance, and typically bluish gray in coloring. It is also highly abrasive and can detrimentally affect the life of dies used in the fabrication of the silicon steel having such a coating on its surface. A common method of fabricating such steel is by stamping, and the abrasiveness of the glassy magnesium silicate coating causes excessive die wear in such an operation.

The present invention has an an object preventing the formation of the abrasive crystalline magnesium silicate coating on the surface of silicon steel during high temperature exposure as in heat treating. The formation of the abrasive coating is prevented by applying to the surface of the silicon steel, prior to exposure to elevated temperatures, a layer of fibrous magnesium silicate. This layer prevents the formation of crystalline magnesium silicate, and also prevents Welding of adjacent laps of the silicon steel coil together during annealing. The fibrous magnesium silicate does not interfere with the metallurgical treatment to develop the desired magnetic properties.

The effectiveness of the fibrous magnesium silicate in preventing or minimizing the formation of crystilline magnesium silicate can be determined by measuring the electrical resistance of the silicon steel surface after heat treatment. Since, as mentioned above, the crystalline magnesium silicate, which may form by reaction of the magnesia coating and silica present on the surface, is moderately insulating, the extent of its formation is reflected by measuring the surface electrical resistance. A useful test for this purpose is the so-called Franklin test of interlamination resistance which is described in ASTM Specification A344-60T (Method B). In this test, electrical conductivity of the surface is measured in amps, and the higher the amp value the greater the conductivity and consequently the lower the resistance caused by the presence of crystalline magnesium silicate on the surface. This test is used as an indication of the extent of the base reaction between silica and magnesia.

The following examples will illustrate the usefulness of fibrous magnesium silicate in reducing the formation of the crystalline magnesium silicate as measured by the aforementioned Franklin test of interlamination resistance.

A series of steel samples of uniform size and composition were provided with coatings of different concentration of fibrous magnesium silicate. The coated samples were then annealed at 2150 F. to simulate normal annealing of silicon steel for the purpose of developing grain growth. These samples were also tested for electrical properties to determine whether the fibrous magnesium silicate coating interfered with the development of the required properties. The results of the Franklin test on these samples are described in Table I.

As can be seen from the data in Table I, the resistance decreases markedly when sufiicient fibrous magnesium silicate is present. A convenient guide for indicating that the formation of crystalline magnesium silicate has not exceeded acceptable limits is a Franklin test value of greater than 0.9 amp. In accordance with the invention, sufficient magnesium silicate must be present in the coating applied to the silicon steel surface to provide a Franklin test result greater than 0.9 amp.

It has been mentioned above that the use of the fibrous magnesium silicate coating does not detrimentally affect the development of the required electrical properties. This is demonstrated by the data in Table II which reports the results of permeability and core loss tests on samples of the type described above containing fibrous magnesium silicate, and a control sample Without the fibrous silicate coating. The magnetic pro erties reported in Table II were made on Epstein packs of samples (3 cm. x 30.5 cm.).

1 Watts per pound. 2 B =gausses, KB =1,000 gausscs. 3 Oersteds.

The absence of crystalline magnesium silicate following heat treatment, i.e. annealing, is verified by visual observation as well as the Franklin test indication as discussed previously. Crystalline magnesium silicate which forms by reaction of magnesia with silica present on the surface of the silicon steel is typically bluish gray in color and glassy in appearance. The surface of the steel after treating with fibrous magnesium silicate and after high temperature exposure has a metallic appearance and remains clear and colorless.

Advantageously, the coating is applied substantially continuously on the metal surface and in coating weights of from about 0.01 to 0.04 oz./ft. Although any suitable method of application may be used, one presently preferred technique is to apply the coating by dipping and metering with approved rubber rolls.

'It is apparent from the above that various changes and modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims.

We claim:

1. In the processing of silicon steel, wherein said steel is exposed to elevated temperature prior to subsequent fabrication, the improvement comprising applying to a surface of said steel prior to exposure thereof to elevated temperature a coating cotaining fibrous magnesium silicate in an amount sufficient to produce, after exposure to said elevated temperature, Franklin test result of greater than about 0.9 amp and a metallic appearance on the surface.

2. An improvement according to claim 1 wherein said coating contains at least 50% by Weight fibrous magnesium silicate.

3. An improvement according to claim 1 wherein said fibrous magnesium silicate is applied at a coating weight of from about 0.01 to 0.04 oz./ft.

References Cited L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R.