GB1587147A

GB1587147A - Method of treating objects of silicon steel

Info

Publication number: GB1587147A
Application number: GB4494777A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1977-10-28
Filing date: 1977-10-28
Publication date: 1981-04-01

Description

(54) METHOD OF TREATING OBJECTS OF SILICON STEEL (71) We, ASEA AKTIEBOLAG, a Swedish Company of Västerås, Sweden, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: - In the manufacture of sheet material of silicon steel, so-called electrical sheet, with grain orientation, the sheet material is subjected after rolling and decarburisation to a heat treatment at about 850" to 13500C in order to achieve a grain growth of the crystals which is necessary for the sheet material to acquire the required magnetic properties.

Before the heat treatment the sheet material is coated with chemicals which are to form an electrically insulating protective coating on the sheet material during the heat treatment process. One such known protective coating may consist of a reaction product of silicon dioxide formed at the surface of the sheet material and an applied oxide or hydroxide of an alkaline earth metal, usually magnesium. The application of the protective coating on the surface of the sheet material is carried out by suspending the alkaline earth metal oxide or hydroxide in water and applying it into the sheet material in an even layer, after which the sheet is subjected to the previously mentioned heat treatment at a temperature of around 850" to 1350"C in a hydrogen atmosphere for several hours.However, in order to ensure that a welldeveloped, so-called "glass film" is formed on the sheet material, the temperature should amount to around 10000 to 13500C.

The hydroxide which is part of the suspension from the start, or which is formed from the oxide by reaction with water, liberates water while the sheet material is being heated. This liberated water, at temperatures below that mentioned above, is able to oxidise silicon in the steel to silicon dioxide without simultaneously oxidising the iron. The oxide which is formed from the hydroxide during the liberation of water, or which is possibly added from the start and has escaped hydration, reacts with the silicon dioxide during the heat treatment while forming the previously mentioned well-developed glass film on the surface of the sheet material. The glass film can also be obtained by using a carbonate of an alkaline earth metal. The carbon dioxide which the carbonate liberates during heating is able to oxidise silicon into silicon dioxide without the iron being oxidised.

When the silicon dioxide has been formed, the formation of glass proceeds in the manner described above. Any excess of the oxide which has not reacted during the glass formation acts as spacing material between adjacent layers of the sheet, whether these are present as turns in a roll or as laminae in a stack, and it prevents the layers from sticking or sintering together.

A protective coating of silicate as described above has an electrical insulating resistance which is insufficient for many purposes, and therefore the protective coating is often reinforced, earlier by treatment with phosphoric acid and metal phosphates, especially alkaline earth metal phosphates and aluminium phosphate, and latterly by treatment with such solutions which are provided with colloidal silica and chromic acid.

Incorporation of colloidal silica results in improve insulating resistance in the sheet, reduced dusting in the machining of the sheet and favourable magnetostriction.

The addition of chromic acid is made to neutralise excess phosphoric acids, which are present partly in original form in the solution and partly in such transformed state as is obtained during heat treatment after the phosphate has been applied. This function of the chromic acid is due to the fact that it is decomposed thermally at somewhat above 200 C while forming chrom- ium (III) ions which react to form chromium (III) phosphate. By nedtralising phosphoric acid in this way, flaking off of the silicate layer is prevented, which' is otherwise caused by phosphoric acid, during the heat treatment of the sheet in connection with the application of the phos phate. In addition, the retention of phosphoric acid on the sheet is prevented.This would otherwise occur to a significant extent because of the presence of the colloidal silica which hinders the evaporation of excess phosphoric acid. Such retained phosphoric acids will diffuse, after the finished sheet has been stored for some time, out to the surface of the phosphate layer where, after taking up water, it becomes strongly aggressive and destroys the insulation of the sheet.

The present invention is based on the observation that the chromium (III) phosphate mentioned in the preceding paragraph can be dissolved out from the phosphate layer if the sheet comes into contact with water.

There is a risk of such dissolution when the sheet is handled and used, among other things when used in the transformer where the oil is seldom entirely free from water.

Elimination of chromium (III) phosphate leads to breakthrough of the layer so that its insulating ability is destroyed.

We have now found that it is possible to achieve the above mentioned neutralisation of phosphoric acids with iron and/or manganese compounds, the phosphoric acids then being transformed into phosphates which are insensitive or almost insensitive to water, and the insulation on the sheet then remaining intact when the sheet is handled and used. The use of these iron and manganese compounds has the great advantage that the use of chromium compounds, which are detrimental to the environment, is avoided.

According to the invention, a method of treating an object of silicon steel, for example a sheet or strip lamination for a motor, a generator or a transformer, provided with an electrically insulating protective coating of silicate, with a solution of phosphate-containing colloidal or suspended silica, is characterised in that the phosphatecontaining solution comprises an aqueous solution containing phosphate ions and iron and/or manganese ions and negative ions which convert to volatile products at a temperature below 400"C.

The grain size of the silica is preferably below 16 microns.

A particularly good water resistance is achieved in the insulated sheet when using solutions containing both iron and manganese ions with 0.75 to 1.25 moles of manganese ions per mole of iron ion.

Preferably, the solution also contains aluminium and/or magnesium ions because the presence of these ions makes the insulated sheet less sensitive to the conditions prevailing during the stress-relieving anneal to which the sheet is often subjected.

The solution, which is acid, preferably has a pH value of at the most 3.7 and of at least 0.8 and the phosphate ions preferably consist of monophosphate ions.

The negative ions which convert to volatile products at a temperature below 400"C may, among other things, consist of sulphate ions, acetate ions and nitrate ions, these three being preferred for price reasons. Other suitable negative ions are sulphite ions and ions of various organic acids, for example formic acid and propionic acid.

The application of the silicate coating on the object of silicon steel can take place in a conventional way, for example by applying on the object particles of an oxide, a hydroxide or a carbonate of an alkaline earth metal and heating the object with the coated particles to at least 8500 C, preferably from 1000" to 13500C in a vacuum, in nitrogen gas, in hydrogen gas or some other inert or reducing atmosphere. When using an oxide, a substance capable of oxidising silicon is also used simultaneously, usually water bound to the alkaline earth metal as hydroxide. As alkaline earth metal magnesium is particularly preferred, but also calcium, barium and strontium may be used.The thickness of the protective coating is from monomolecular thickness up to around 10 microns; for particularly favourable results 0.1 to 5 microns and preferably 0.1 to 1 micron. Before applying the phosphate the excess of alkaline earth oxide is brushed away from the object.

The application of the phosphate layer is made by applying the phosphate solution onto the object provided with the silicate coating, after which the object is heated to a temperature of at least 4000 C, suitably from 400" to 1 1000C and preferably from 700 to 850"C for at least t minute, preferably for a period of from t minute to 10 minutes. Longer periods are not harmful. The heating can be made in oxidising, reducing or inert atmosphere, i.e. the atmosphere is not critical. Air can be used with advantage.

To the phosphate solution there may be added insoluble fillers such as highly dispersed refractory boron-treated silica or mica powder, the grain size of which is normally below 10 microns. By adding fillers, the resistivity of the protective coating is increased.

The preferred contents of the components in the solution, per 100 parts by weight of silica (calculated as SiO2, without water) are: From 10 to 200, preferably from 50 to 150 parts by weight of phosphate ions (calculated as P043-), from 1 to 30, preferably from 2 to 20, parts by weight of iron or manganese ions or iron and manganese ions together, and from 0 to 25, preferably from 2 to 20 parts by weight of aluminium or magnesium ions, or aluminium and mag nesium ions together. The amount of nega tive ions which convert to volatile products at a temperature below 400"C is adjusted so that the solution contains a . sufficient amount of metal ions to react with the phosphate ions of the solution.Preferably there is used an amount of these negative ions which is equivalent to the amount of hydrogen ions in the solution, or which deviates therefrom by a maximum of 40, and preferably a maximum of 25, per cent.

If fillers are added to the phosphate solution, the content of fillers suitably amounts to from 5 to 50 parts by weight and preferably from 10 to 30 parts by weight per 100 parts by weight of silica (calculated as SiO?, without water).

The thickness of the layer of phosphate and silica may be from 0.1 to 20 microns; for especially favourable results from 0.5 to 5 microns and preferably from 1 to 3 microns.

The invention will be described in greater detail by way of a number of Examples and with reference to the accompanying drawing, in which Figure 1 is a schematic sectional view of means for applying a protective coating of silicate onto a sheet, and Figure 2 is a schematic sectional view of means for applying a phosphate coating onto the sheet provided with the protective coating of silicate.

In Figure 1, the numeral 1 designates a sheet of silicon steel which has been pretreated to give grain orientation and which has been decarburised at a temperature of from 750" to 900"C, preferably at 820"C, in a wet hydrogen atmosphere. It has a thickness of 0.3 mm. The sheet is drawn from a coil on a reel 2 and passes under a roll 3 which rotates in a pan 4 containing a suspension 5 of the particulate material with which the sheet is to be coated. The suspension 5 can, for example, be manufactured by suspending 90 parts by weight of magnesium oxide in 1000 parts by weight of water, the magnesium oxide consisting of particles of which up to 95 per cent by weight have a grain size less than 5 microns and the rest a grain size less than 25 microns.After leaving the suspension 5 the sheet is passed between wiping rollers 6 and 7, which are suitably rubber-clad, and into a furnace 8 where it is dried at a temperature of about 100"C for about 30 seconds before it is coiled up on the reel 11 after having passed the transport rollers 9 and 10. Thereafter the sheet is annealed (at high temperature) in a batch annealing furnace at around 1000"C to 13500C in a hydrogen atmosphere for several hours after which a protective coating of silicate hav ing a thickness of 1 micron is present on the sheet.

When the sheet, which has been treated in the way described above with reference to Figure 1, has been liberated from excess coating by brushing, it is coated with phosphate in the means illustrated in Figure 2.

The sheet, which is there designated .21, is drawn from a reel 22 and passes under a roll 23 rotating in a pan 24 containing a solution 25 of phosphate in water, and possibly containing suspended filler. The sheet is then passed between wiping rollers 26 and 27, which are suitably rubber-clad and into a furnace 28, after which the sheet is cooled in a cooling device 29, before it is coiled up on the reel 30. The concentration of phosphate in the treatment liquid 25 is adjusted with regard to the profile of the rubber rollers 26 and 27 and to the roller pressure so that the desired thickness of the phosphate layer is obtained. For all the compositions of the solution 25 exemplified below, the furnace 28 has a temperature of 800"C and the time for the sheet to pass through the furnace is 2 minutes. The furnace atmosphere is air.The thickness of the phosphate layer in the exemplified cases is 2 microns. The following are Examples of suitable compositions of the solution 25, the parts being by weight: - Example 1 A solution is prepared from 20 parts of ferrous sulphate (FeSO4. 7H,O), 15 parts of phosphoric acid (d = 1.54), 100 parts of water and 180 parts of colloidal silica containing 300 grams of sio2 per litre and having a particle size of the silica of 100-200 Anstrom units and a specific surface of 250 mS per gram.

Example 2 A solution is prepared from 20 parts of manganese sulphate (MnSO4. H20), 15 parts of phosphoric acid (d=1.54), 100 parts of water and 180 parts of colloidal silica of the kind stated in Example 1.

Example 3 A solution is prepared from 6.7 parts of ferrous sulphate (FeSo4. 7H,O), 13.3 parts of manganese sulphate (MnSO4. H2O), 24 parts of phosphoric acid (d=1.54), 40 parts of aluminium phosphate solution (600 g A1PO,/1, pH 2), 25 parts of water and 180 parts of colloidal silica of the kind stated in Example 1.

Example 4 A solution is prepared from 70 parts of magnesium phosphate solution (400 g Mg(M.PO)2/1, pH 1.8), 67 parts of aluminium phosphate solution (600 g AlPO4/ 1, pH 2.0), 8 parts of manganese sulphate (MnSO4.H2O) and 180 parts of colloidal silica of the kind stated in Example 1.

Example 5 A solution is prepared from 42 parts of aluminium phosphate solution (600 g AlPO4i1, pH 2.0), 11 parts of ferrous sulphate (FeSO4. 7hay), 9 parts of manganese sulphate (MnSO4 .H3O), 24 parts of phosphoric acid (d=1.54), 180 parts of colloidal silica of the kind stated in Example 1, and also 70 parts of water.

The resistivity of the protective coating may be increased by adding an insoluble filler to the phosphate solutions prepared according to any of Examples 1 to 5. As examples of insoluble fillers may be mentioned highly dispersed refractory borontreated silica powder or mica powder, the grain size of which is normally below 10 microns.

Electrical sheet insulated with the solutions prepared according to any of Examples I to 5 is insensitive to water. The insulating layer has an excellent adhesion to the sheet and the magnetostrictive properties are very good.

WHAT WE CLAIM IS: - 1. A method of treating an object of silicon steel provided with an electrically insulating protective coating of silicate, with a solution of phosphate-containing colloidal or suspended silica, characterised in that the phosphate-containing solution comprises an aqueous solution containing phosphate ions and iron and/or manganese ions and negative ions which convert to volatile products at a temperature below 400"C.

2. A method according to claim 1, in which the aqueous solution also contains aluminium and/or magnesium ions.

3. A method according to claim 1 or 2, in which the phosphate solution has a pH value of 3.7 at the most.

4. A method according to any of claims 1 to 3, in which the phosphate ions consist of monophosphate ions.

5. A method according to any of claims 1 to 4, in which the negative ions which convert to volatile products at a temperature below 400"C consist of sulphate ions, acetate ions and/or nitrate ions.

6. A method according to any of claims 1 to 5. in which the object with the applied phosphate solution is heated to a temperature of at least 400"C to secure a layer of phosphate and silica to the protective coating of silicate.

7. A method according to claim 6, in which the object with the applied phosphate solution is heated to a temperature of from 700" to 850"C.

8. A method according to any of claims 1 to 6, in which an insoluble filler is added to the phosphate.

9. A method according to claim 8, in which the insoluble filler is boron-treated silica or mica powder.

10. A method of treating an object of silicon steel substantially as hereinbefore described with reference to the accompanying drawings and any of the foregoing Examples 1 to 5.

11. An object of silicon steel when treated by the method claimed in any of the preceding claims.

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Claims

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aluminium phosphate solution (600 g AlPO4i1, pH 2.0), 11 parts of ferrous sulphate (FeSO4. 7hay), 9 parts of manganese sulphate (MnSO4 .H3O), 24 parts of phosphoric acid (d=1.54), 180 parts of colloidal silica of the kind stated in Example 1, and also 70 parts of water.

The resistivity of the protective coating may be increased by adding an insoluble filler to the phosphate solutions prepared according to any of Examples 1 to 5. As examples of insoluble fillers may be mentioned highly dispersed refractory borontreated silica powder or mica powder, the grain size of which is normally below 10 microns.

Electrical sheet insulated with the solutions prepared according to any of Examples I to 5 is insensitive to water. The insulating layer has an excellent adhesion to the sheet and the magnetostrictive properties are very good.

WHAT WE CLAIM IS: - 1. A method of treating an object of silicon steel provided with an electrically insulating protective coating of silicate, with a solution of phosphate-containing colloidal or suspended silica, characterised in that the phosphate-containing solution comprises an aqueous solution containing phosphate ions and iron and/or manganese ions and negative ions which convert to volatile products at a temperature below 400"C.
2. A method according to claim 1, in which the aqueous solution also contains aluminium and/or magnesium ions.
3. A method according to claim 1 or 2, in which the phosphate solution has a pH value of 3.7 at the most.
4. A method according to any of claims 1 to 3, in which the phosphate ions consist of monophosphate ions.
5. A method according to any of claims 1 to 4, in which the negative ions which convert to volatile products at a temperature below 400"C consist of sulphate ions, acetate ions and/or nitrate ions.
6. A method according to any of claims 1 to 5. in which the object with the applied phosphate solution is heated to a temperature of at least 400"C to secure a layer of phosphate and silica to the protective coating of silicate.
7. A method according to claim 6, in which the object with the applied phosphate solution is heated to a temperature of from 700" to 850"C.
8. A method according to any of claims 1 to 6, in which an insoluble filler is added to the phosphate.
9. A method according to claim 8, in which the insoluble filler is boron-treated silica or mica powder.
10. A method of treating an object of silicon steel substantially as hereinbefore described with reference to the accompanying drawings and any of the foregoing Examples 1 to 5.
11. An object of silicon steel when treated by the method claimed in any of the preceding claims.