DE2041051A1 - Steel strips covered with an insulating layer of magnesium oxide and the means for their manufacture - Google Patents

Description

MORTON INTERNATIONAL, INCv 110 North Wacker Drive, Chicago, Illinois, USA.

Steel strips coated with an insulating layer of wagonslum oxide and the means for their production

The invention relates to magnesium oxide coatings on silicon; Mainly magnesium oxide and smaller proportions of boron compounds, sodium compounds and silicon compounds ₀

In transformers or similar electrical devices, the grains are generally constructed from soft magnetic steel sheets. These sheets are manufactured by Cold rolling of silicate-containing steels in sheet form, winding the steel sheets to cones or collars and tempering the steel

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ribbons under such conditions that grain orientation is achieved in view of the intended magnetic properties will.

Magnesium oxide is widely used as a refractory separation medium and protective coating for metal surfaces. In addition, it also serves as an electrically insulating coating on metals as a "pick-up" for the removal of contaminants and accompanying elements such as sulfur and carbon from the thin metal sheets and in particular as a protective coating on silicon steels. The electrically insulating coating is obtained by coating the steel sheet with magnesium oxide, which, when tempered, forms a film containing magnesium silicate MgpSiO ^, which is an effective electrical insulator is.

So far, on an industrial scale, silicon-containing steels have been cold-rolled into sheets, decarburized and then subsequently rolled up »The cold rolling builds up a potential in the steel, which when the steel is tempered later leads to a grain-oriented structure. In this context, a heat treatment of the steel to about 1200 C in essentially pure hydrogen or vacuum at a predetermined time and temperature. This starting leads to grain growth and a special grain orientation, which leads to the desired soft magnetic properties leads. Obviously, all excess carbon and sulfur from the steel is removed during tempering. This type of tempering is known as "box annealing".

In this way you get a long, thin, coiled one Steel belt, which can only be unwound and processed to the desired shape and size for end use got to. This includes, for example, stamping, embossing or bending necessary. These mechanical work processes

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there are tensions within the steel belt which have a negative effect on the magnetic properties. This tension must therefore be released again. This is generally achieved by tempering or annealing the steel blanks free of tension at a temperature of about 800 ^° C. in a reducing atmosphere for a sufficiently long time. '"' ^! · ·

During the glowing of the box, where the steel band is in large coils, the band windings can melt together in the absence of a suitable metal band, so that unwinding is no longer ensured is. In general, "this is prevented by applying a thin coating of magnesium oxide to the steel sheet before it is rolled up to form the coils. The magnesium oxide also has the effect of reducing the proportion of accompanying elements such as carbon and sulfur through chemical reaction. Finally, magnesium oxide leads." also to form an insulating layer of magnesium by reaction with the water contained in the steel Silcium _e where Eisensilieat is also formed in minor amounts. for some application areas it is also important to form an electrically insulating coating, for example, for transformer cores, which are composed of thin, magnetically soft steel sheets stacked on top of each other *

and each sheet is electrically isolated from the neighboring one have to be. In this way, the formation of eddy currents in the core can be largely reduced when this one is exposed to an alternating electric field. The middle density of soft iron in the core should be as high as possible. Consequently the insulation of the sheets should be as thin as possible in order to achieve maximum packing of steel sheets.

It is obvious that short circuits between the Sheet metal reduce the performance of the transformer and often lead to local overheating in the transformer core,.

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being able to lead. Soft magnetic sheet steel is often rated according to the number of short circuits per unit area, generally expressed as the electrical resistance of the insulating layer (dielectric strength). It is about a standardized test method according to ASTM, which is generally known as the "Franklin test".

Because the tension-free glow is sometimes in an atmosphere is carried out by not entirely pure hydrogen containing hydrocarbons, water and oxygen, so Carbides or oxide can form on the steel surface below the insulating magnesium oxide layer (at the grain boundaries) train leading to deterioration of the magnetic Properties of the steel sheet lead. This is a very serious problem for the industry, so that is quite a problem special insulation is required to maintain the magnetic properties.

Another problem now is that magnesium oxide is applied to the steel strip in the form of an aqueous slurry. At the beginning of box annealing escaping water vapor from the coating and can under normal conditions with the superficial iron of the steel sheet react unterlßildung of conductive iron oxides _β again adversely affect the electrical insulation areas with such harmful iron oxides on the sheet surface is referred to as Glühmuster. Finally, it was found that magnesium oxide does not form a tightly bonded adhesive layer which does not easily lift off to the touch. "

It has now been found that magnesium oxide as such is not able to meet the high requirements To meet insulation and maintenance of the magnetic properties, i.e. not to eliminate the above disadvantages able. The magnesium oxide coatings on the steel sheets or

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j-bands must therefore be modified in such a way that they adhesive and tight are the insulating properties after box annealing and stress relieving and recrystallizing and, moreover, allow the desired magnetic properties of the steel to be maintained after the stress relief annealing. ■ "■ '■ · ■

The magnesium oxide layers according to the invention on silicon-containing steel sheets or strips are thus modified in such a way that they produce a satisfactory electrical insulation, not to the undesirable cause | patterns as they can occur due to the formation of iron oxides during the annealing of the box. The layers according to the invention are compared with not quite pure protective gas in the form of hydrogen during the stress relief anneal, much less sensitive than thieves knew "According to the invention one may thus silicon-containing steel sheets or strips provided with Magnesiumoxidüberzügen _f · with excellent insulating effect with respect to the separating effect in wound steel strips to satisfy.

The invention relates thus optionally annealed steel strips with an insulating magnesium oxide layer exceeds the like subjected to which small amounts of a boron compound, namely boric acid, boron oxide or magnesium borate, a sodium compound, sodium hydroxide, sodium oxide, sodium peroxide or. Sodium borate and a silicon compound, namely silicon dioxide or alkaline earth silicate. The specific surface of the magnesium oxide should be more than about 20m ² / g and the iodine number should be higher than about 2k (2 * 1 mg iodine per g dry magnesium oxide). The iodine number corresponds approximately to 1 to 1.25 times the specific surface area (this is determined using the standard Brunauer-Emmet-Teller method). '■' \

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The iodine number is determined as follows.

1 ») A 2 g sample of magnesium oxide is weighed to the nearest mg.

2.) The sample is placed in a dry 200 cm sealable Glass flask transferred

3.) There are 100+ 0.2 mg of a 0.1 η iodine solution in carbon tetrachloride (free of traces of sulfur or carbon disulfide) added.

4.) The flask is closed and at room temperature for 30 min shaken (since this test is relatively insensitive to temperature, no temperature control is required) ο

5 ») The flask is left to stand for at least 5 roin and then a

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20 cm aliquot taken from the clear solution

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and placed in a 250 cnr Erlenmeyer basket containing 50 cm of a 0.03 η potassium iodide solution in 75 f ° ethanol.

3 6.) The aliquot part of the clear solution of 20 cm is with a standard sodium thiosulphate solution 0.05 η titrated. The sodium thiosulfate solution should be used at least once all 2 weeks against a potassium iodate solution. A sharp end point is obtained without the need a strength indicator.

7.) The iodine number is calculated as mgj / g according to the following equation

318 (V ₂ -V ₁ ) N ₁ = MgI ₂ / g = iodine number,

where V _{2 is} the volume of thiosulfate solution required for

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20 cnr iodine solution - before adsorption of the iodine on the oxide, V ₁ is the thiosulphate volume for the 20 cm sample after Adsorptin, N ₁ is the concentration of the thiosulphate solution.

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all known ones fall under the designation "boric acids" Boric acids, such as metha-, ortho- or pyroboric acid.

The term magnesium borate includes the entire magnesium borate, like Magnesiumort ho-, -metha- or -pyroborate.

The term sodium borate includes the '.-Known sodium borates., such as sodium tetra-, -metha- or -or thobo rat. Alsr silica one can use all Eoimen, if they have a grain size below about 7 ^ / um, z, B. like them as a filter aid or the like ("Cabosil") are on the market. Particularly Alkaline earth silicates such as calcium or magnesium silicates are suitable

In the layers according to the invention, the components are present in the following molar ratios »

MgO γ SiO ₂ about 280 ι 1 to 2900: 1, preferably * about 1000 si to 2000 s 1 (this corresponds to ^ about 0.03 ^ to 0.357 MoI ^ SiO ₂ based on MgO _1, preferably 0.05 to 0.1

The molar ratio MgO to Na should be between about 2000 and 1 and 4-000: 1, preferably around 2800: 1 (dies . corresponds to about 0.025 to 0.05 mol- ^ Na based on MgO, preferably around 0.035

The molar ratio B ^ O ^. SiO ₀ should be between about 3 and 5 1, preferably 3: 1 to ht 1.

The sum of these additives, namely SiO ₂ , boron compound and sodium compound based on MgO and additives, makes up to about 2.5, in particular 2. Mol- $ out of ₀

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An aqueous slurry of magnesium oxide and additives with the desired rheological properties is used to coat the steel sheets or strips. The solids content is generally about 5 to 20 percent by weight, based on water. Such a slurry has adequate viscosity and fluidity for the job.

A frit can be made by fusing the magnesium oxide with the boron, sodium and silicon compounds. The frit is crushed to a grain size below about 7 ^ / um and then stirred up in water, during manufacture the order in which the components are added is immaterial to the slurry.

The slurry is then applied to any one of them Way applied to the magnetic tape or sheet metal! such as dipping, brushing or spraying »The dipping process is generally like that. carried out that the steel sheet is pulled through a container containing the sludge. Afterward is heated to remove the water and the layer is dried »Now the sheet or tape can be rolled up and used in the box annealing furnace. During the glowing process, an adhesive, electrically insulating layer forms Corrosion-resistant coating that also acts as a separating medium between the windings of the federal government.

The enclosed flow chart shows a production process for the coated steel strips according to the invention. Magnesium oxide mass is fed to the mixer via the funnel 10 and water via the funnel 12 via the metering lines 13 »13a. The slurry goes via line 18 with valve 20 into trough 22. The hot sheet metal coming from the rolling mill runs in the direction indicated by arrows over roller Zk and under roller 26 within a quenching bath 28

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then over the roller 25 into the trough 22, where the aqueous Slurry is applied. The layer thickness can be adjusted with the aid of the adjustable roller pair 30 the coated sheet is then dried 32, runs around the heating roller 33 and becomes bundles at 3 ^ wrapped. The sheet metal coils 23a are annealed in the furnace 36 ' and then put into stock. '- ..'. ,.

In the preferred embodiment, the coating on the steel sheet consists of magnesium oxide, boric acid »■ sodium hydroxide and silicon dioxide, the molar ratio λ MgO t SiO ₂ about 1500 * 1, MgO: Ma about 2800: 1 and ΒΛ j SiO ₂ about 3» 5 «is 1. Approximately 10 to 12% by weight of this mass are mixed with water during the production of the coated sheets. It was found that the bundles coated and annealed as described above could be easily unrolled. The steel surface showed relatively low electrical conductivity and was resistant to corrosive gas and smoke.

Another preferred embodiment consists in a coating containing magnesium oxide, boron oxide, sodium tetraborate and silicon dioxide, the molar ratio of MgO s SiO _{2 being} about 1500 1 I _5, the molar ratio of MgO to Na about 2800 si ™ and of B 0o s SiO ₂ about-3.5 1 1 is «

The invention is further explained in the following examples. Example 1

A means for coating the steel sheets was prepared by adding 18 g ( _{0.5 mol) of MgO 1 to} 150 cm.sup.2 of an aqueous boric acid solution which contained 1.05 mmol of boron, expressed as BgOo. 16 mEJol sodium hydroxide and O ₁ 3 rnlol SiO ₉ with a ^ _n

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Crystallite size of about 70 α added. In this way, an aqueous slurry with a solids content of about 10 ^a / o was obtained, the molar ratio of the components being «

MgO: SiO ₂ molar ratio = 1500

B ₂ O ₃ : SiO ₂ = 3.5 11
MgO ϊ Na = 2812 t 1

This mass was applied, with constant stirring, to 26 silicon steel sheets or plates 3 x 15/3 cm in a layer thickness of 0.3 to 0.355 mm (12 to 1.5 mils). This wet application was smoothed and dried in the air at a temperature of about 500 ° C. The dry coated sheets were stacked and asserstoffatmosphäre at 1177 ⁰ C 4.3 h annealed in V /, then cooled and excess magnesium oxide or loose from each sheet by brushing in flowing water ₀

In a modification of this method, a compound was applied in which the molar ratio of MgO 1 SiO 2 was increased to 2960 t 1. With the same ratio of B ₉ Oo «SiO ₉ , the molar ratio of MgO to Na was 2800 11.

The conductivity of these two coatings A and B was then determined using the "Franklin test" (see ASTM method A 33 ^ 52, test method for electrical and mechanical Properties of magnetic materials). In a nutshell The test method consists in supplying electrical current through brass contacts, with the contacts approximately Covering 0.65 cm (0.1 sq.in.). From one electrode the current flows through the coating and then the steel sheet, which connected directly to the other electrode. The current strength flowing through is a measure of the insulating effect

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of the coating. Some 100 contacts are required for an evaluation. A complete short circuit is shown by measured values in the order of magnitude of 1.55 »A / dm (| .00 mA / 0, 1 sq _o in). The coatings according to the invention show satisfactory test values of about 0.7 mA / dm (45 mA / 0.1 sq.in) or below, namely the coating A has a value of 0, Jj-O and the coating B of 0.8l mA / dm ² (26 or “52 mA / 0.1 sq.in) ₀

F or game .1 2 . "

The measures of Example 1 were repeated using a compound in which the molar ratio MgO _{1 "-SiO 2.} - | 1480 s 1, MgO ι Na 2830 ι 1 and BgOo ι SiO ₂ 4.6 s 1 The coating C obtained in this way and the coating D _s which has been produced with a compound which, in a modification of the above composition, has a ratio of B ₉ Oo s SiO "of 8.7 ι 1," were tested in the Franklin test " see that the coating

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C had a value of 0 _f 40 and the coating D of 0.9 mA / dm (26 or 58 mA / 0.1 sq.in) »

The procedures of Example 1 were repeated _r but in this case a mole ratio of MgO for coating E s g siop ~ 1480: 1, MgO: Na = 2800 rl'und B _o 0 _Q: SiO ₀ ^ 4 «6 1 1 complied with. When viewed with the naked eye, the coatings showed a uniform area _o-

In a modification of Example 1, a compound was used in which the ratio of B ₂ O _{3 to} SiO ₂ was 2.3 si, where in the coating F the molar ratio of MgO s SiO was 1480 j 1 and MgO 1 ha as above Was 2800 sl. It was found that the smoothness of layer E was excellent, but layer F had bare spots.

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Example k

The measures of Example 1 were repeated and a _{molar ratio of MgO: SiO 2} = 1Λ80: 1, MgO: Na = ίΛΐ5 i 1 and B ₂ O ₃ = 3.5 J 1 was maintained in the coating G. This mass was modified for coating H in such a way that the molar ratio MgO: Na was increased to 56βΟ t 1. The test panels with the coatings A, G and H were examined with regard to the uniformity of the layer and it was found that the coating A showed good uniformity, whereas the coatings G and H showed only a very moderate uniformity.

example 5

In a modification of Example 1, a coating was prepared I, wherein the ratio Mo MgO SiO t 2830:, j MgO Na 2800 I 1 and B ₂ O ₃ was 1 j SiO ₃ 3.5. "1 Together with these samples, a comparative product was investigated which had a layer of only magnesium oxide without additives, and the rate of corrosion was determined using a chlorine test method.

When testing the corrosion resistance to chlorine, a test strip of the steel sheet 30 cm ^{2 was held} in contact with a thermocouple in the middle of the hot zone of an automatically controlled tube furnace. Helium flowed through the tube at a flow rate of 65 cm / min. The temperature was raised to 816 ⁰ C in order to imitate conditions in stress relief annealing and the system for 10 minutes left to disposition of equilibrium after reaching this temperature. The furnace atmosphere was then switched very quickly to a humidified chlorine flow with a flow rate of 15 cm / min and switched back to helium after 60 seconds, the helium flow now being used

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had a speed of 1 cm / min. Under this Atmosphere was cooled to 315 ° Gι the test platelets taken out of the oven, quenched under running water, brushed off, dried and balanced. The weight loss as a result of the volatilization of iron chloride and silicon chloride is a measure of the corrosion resistance,

In these tests, it was shown that the weight loss of the coating was 1-0.18 g and that of the comparative coating was 0.32 g. In other words, the weight loss of the comparative product was OyI4 'g higher, so that the invention Corresponding coating showed an improvement in corrosion resistance of 43.8 <fo. .

By i s pie I 6

In a modification of Example 1, a coating J was produced in which the molar ratio of MgO s SiO ₂ 943 si, MgO «Na 2800 i 1 and B ₂ O ₃ » SiO _{2 was} 3.5 ι 1. The test plaques were treated in the same way as in Example I _1, but here a stress-free annealing for 8.6 h followed. The corrosion resistance to chlorine was then determined in accordance with Example 5. It was found that the coating J ä resulted in a weight loss of 0.21 and the comparative product of 032 g _/ m; in other words, the weight loss of the coating _{according to the invention was O 1} H g less than that of the comparative material, corresponding to an improvement in corrosion resistance, by 34.4 %.

Se i gle 7

In a modification of Example 1, a coating K was produced in which sodium tetraborate was used instead of sodium hydroxide and the molar ratio of MgO j SiO ₂ was 2830. Here, too, the stress-free annealing was carried out for 8.6 hours and the "

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Corrosion resistance determined in accordance with Example 5. The coating K according to the invention suffered a weight loss of 0.17 g compared to 0.32 g of the comparative product, i.e. 0.15 g less, corresponding to an improvement in $ 46.9 corrosion resistance

Example 8

The coating L was produced as a modification of example 1 by _{melting magnesium metkaborate Mg (BOp) 2} and silicon dioxide together and producing a frit from the melt. Die'pulveriform frit was slurried in an aqueous solution containing sodium hydroxide and boric acid, the MoI ratio of the components corresponded to coatings I and K. The coated sheets obtained in this way were annealed for 8.6 h stress-free and examined for corrosion resistance Coating L suffered a weight loss of 0.22 g and was therefore 0.1 g better than the comparative product with 0.32 g, corresponding to an improvement in corrosion resistance of 31.2 ° />,

Example 9

In a modification of Example 1, a coating M was produced in which the molar ratio MgO ι Si0 "283 j I, MgO ι Na 2800: 1 and B ,, 0 ~ »SiO 3i5» 1 was. HfLe loading

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laminated sheets were annealed for 8.6 h stress-free and then the corrosion resistance was determined in accordance with Example 5. The coating M resulted in a weight loss of 0.19 g and is therefore 0.13 g better than the comparison product with 0.32 g, corresponding to an improvement in corrosion resistance of 40.9 ##

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A coating N was produced by mixing magnesium oxide, boron oxide, sodium tetraborate and silicon dioxide at a _{molar ratio of MgO t SiO 2} .1480 s 1, MgO Y Na 2800: 1 and BA: SiO ₉ 3t 5 '· 1. The tempered test plates were again examined for corrosion resistance. It was found that the coating-N showed a weight gradient of 0.09 g compared to 0 _f 32 g of the comparative product. This is a difference of 0.23 g, corresponding to a 72 .Ji improvement in corrosion resistance.

Claims

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Claims (1)

Patent claims 1. Covered with an insulating layer of magnesium oxide if necessary, tempered steel belts ο der -machine, characterized that the layer of magnesium oxide with a specific surface area of at least 20 in / g and small amounts of a boron compound such as boric acid, boron oxide or magnesium borate, a sodium compound such as sodium hydroxide, sodium oxide, sodium peroxide or sodium borate, and a silicon compound in the form of silicon dioxide or alkaline earth metal silicate, the molar ratio being MgO to SiO ₂ about 280 5 1 to 29ΟΟ s 1 and from MgO to Na about 2000 μl to ^ 000: 1 and the molar ratio B "0" to SiO _{9 is} up to about 3: 1 to 5 * 1. 2. Steel strips according to claim 1, characterized in that the magnesium oxide layer is boric acid, sodium hydroxide or contains sodium tetraborate and silicon dioxide. 3 · Steel strips according to claim 1 or 2, characterized in that the molar ratio of MgO to SiO _{2 is} approximately 1000 1 to 2000: 1. ** - · Steel strips according to claims 1 to 3 »characterized in that the molar ratio B _? 0 ^ to SiO _? approx. 3 ι to 4 ι 1 is. 5o steel strips according to claims 1 to k, characterized in that the molar ratio of MgO to Na is about 2800 1 " 109817/119 Ö 6. Steel strips according to claim 1 Ms 5 »thereby geke η η ζ e ι ch net _{that the} jjol-Verhältriis MgO to SiO ₂ about 1500» 1, MgO to Na about 2800 ι 1 and BA to SiO ₂ about 3, 5 ι is 1. 7. Steel strips according to claim 1 to 6 _r characterized ge k'.e η η that the sum of the compounds of boron, silicon and sodium make up a total of about 2.5 mol-fo of the coating, preferably up to 2 8. Means for producing the steel strips according to claim 1 to 7, g e k e η η ze i without t through a slurry of the ingredients with a solids content of 5 to 20 9f means according to claim 8, characterized in that the silicon dioxide has a grain size <7 ^ / um possesses. 109βΐ7 / 11δΰ it ι Blank page