US3522108A - Method of forming electric insulating films on al - containing silicon steel sheet and surface-coated al-containing silicon steel sheet - Google Patents

Description

July 28,' 1970 T K YAMAMQTO ET AL 3,522,108

METHOD OF FORMING ELECTRIC INSULATING FILMS ON A] -GONTAINING SILICON STEEL SHEET AND SURFACE-COATED I Al-CONTAINING SILICON STEEL SHEET Filed March 17, 1967 Al- CONTAINING SILICON STEEL SHEET GLASSY FILM HAVING HIGH ELECTRIC INSULATING PROPERTY INVENTORS TAKAAKI YAMAMOTO KANEO AKANUMA BY WWWZ, fldgm ATTORNEYS United States Patent Office 3,522,108 METHOD OF FORMING ELECTRIC INSULATING FILMS ON Al-CONTAINING SILICON STEEL SHEET AND SURFACE-COATED Al-CONTAIN- ING SILICON STEEL SHEET Takaaki Yamamoto and Kaneo Akanuma, Kitakyushu, Japan, assignors to Nippon Steel Corporation, Tokyo,

Japan Filed Mar. 17, 1967, Ser. No. 624,068 Claims priority, application Japan, Mar. 18, 1966, 41/ 16,902 Int. Cl. C23f 7/04 US. Cl. 148-6 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of forming a glassy film having a high electric insulation adhesion, space factor and heat resistance on the surface of a steel sheet in a process for producing an oriented silicon steel sheet containing Al as a constituent.

An Al-containing oriented steel sheet (the term oriented shall included single oriented and double oriented hereinafter) is generally obtained by subjecting a hot-rolled plate or strip to a step including one or more cold-rollings and, if required, one or more annealings to obtain a steel sheet of final gauge, then subjecting the so-obtained sheet to a continuous short time annealing (simultaneously decarburizing when so required) and subsequently to a final-annealing (box annealing) at a high temperature of about 1200 C.

The annealing separator, that is, a film forming agent to be used in the box annealing is preferably a material which can immediately act as a separator during the final annealing and combines with the surface oxide layer to form a highly adhesive glassy film high in electric insulation, value, space factor and heat resistance, because the surface film for a material to be used for laminated sheets or wound cores for use in electric devices is required to have such high adhesion that the film will not peel off when subjected to a processing, high space factor and such high heat resistance that the characteristics of the film will not be deteriorated when subjected to high temperature stress-relieving annealing. Such material which can be used as an annealing separator and a film forming agent are known, for example MgO+SiO (U.S.P 2,354,123), Mg(OH) +Ca(OH) (U.S.P. 2,492,682) and oxides of alkaline earth metals (U.S.P. 2,533,351).

According to known electric insulating film forming methods, when Si is selectively oxidized during the decarburizing annealing in a wet reducing atmosphere so that SiO will be formed on the surface of a steel sheet and then the steel sheet is coated on the surface with a suspension consisting of MgO(Mg(OI-I) )H O, is dried and is then final annealed at a high temperature in a reducing atmosphere, MgO or the like will prevent the sticking between the steel sheets and will act as an annealing separator and at the same time a part of it will react with the above mentioned SiO to form a glassy electric insulating film.

3,522,108 Patented July 28, 1970 The present inventors have found that a highly adhesive glassy film high in the electric insulation value, space factor and heat resistance can be obtained even with a known annealing separator, but have also found that with a known annealing separator, no highly adhesive film high in electric insulation value can be obtained on some electric steel sheets. That is to say, it has been found that, when more than 0.01% of sol. Al is contained in a hot-rolled silicon steel sheet, even if the steel sheet is annealed in a decarburizing atmosphere for a short time, then is coated with the above mentioned conventional annealing separator in a decarburizing atmosphere, is dried and is final annealed by using dry H or a mixture gas of H and N as in the conventional practice, no highly adhesive glassy film high in electric insulation value will be obtained. This is understood to be because, if A1 is contained in a silicon steel sheet, the Al will be preferentially oxidized to produce A1 0 on the surface of the steel sheet, while it is being annealed, consequently the reaction which occurs in a conventional silicon steel sheet containing no Al in which the SiO will react with MgO to produce a favorable glassy film (for example, in U.S.P. 2,533,331) is interrupted. Further an attempt was made to add SiO into MgO in an annealing separator, but no highly adhesive glassy film was obtained thereby.

There has been already disclosed a method of producing an oriented silicon steel sheet from a rolled silicon steel sheet containing, for example 0.01 to 0.09 wt. percent sol. Al. As a result of research on the annealing separator (film forming agent) most adapted to obtain a favorable glassy film in the final annealing in a process for producing an oriented electric steel sheet from a silicon containing Al, the present inventors have discovered that a mixture of MgO and MnO is excellent as an annealing separator.

The present invention is characterized by applying an inorganic oxide film forming agent consisting of a solution containing an Mg compound and an Mn compound mixed so that the weight ratio of MgO to MnO will be 98:2 to 20:80 in the coating slurry or in the film after being dried to an annealing separator in the final annealing in a process for producing oriented silicon steel sheet from a hot-rolled silicon steel plate or steel strip containing 0.01 to 0.09 wt. percent sol. Al so that a surface layer containing A1 0 produced during the short time annealing or the final annealing and said film forming agent will react with each other to form a glassy electric insulating film.

The Mg compound or Mn compound so called here designates a compound which will become MgO or MnO when heated, such as its oxide, peroxide, hydroxide or carbonate.

The fact that the A1 0 which is to react with the inorganic oxide film forming agent containing the Mg compound and Mn compound, is obtained specifically by the oxidization of Al contained in the silicon steel instead of being in the form of an added mixing agent is a feature of the present invention. When a silicon steel sheet is merely coated on the surface a mixture of MgO, MnO and A1 0 and is annealed at a high temperature, no uniform smooth electric insulating film will be obtained.

The formation of a surface layer containing A1 0 on the suiface of an oriented silicon steel sheet containing Al as one of constituents and the reaction of this surface layer with an inorganic oxide film forming agent containing an Mg compound and an Mn compound are carried out industrially in such an annealing step as short time annealing or box-annealing used in a process for produc ing silcon steel sheets.

That is to say, when a silicon steel plate containing, for example, 0.010 to 0.090 wt. percent Al and 2.0 to 4.0 wt. percent Si is annealed for a short time after being cold-rolled so that a surface layer containing A1 will be produced on the surface, is coated on the surface with a suspension in which are mixed, for example, MaO, MnO and H 0 so that the coating of the annealing is present after the drying in an amount of from 2 to 10 g./m. or preferably 4 to 8 g./m. is dried and is then box annealed for to 30 hours in a reducing atmosphere, for example, at 1000 to 1200 C., a part of the MgO and MnO will react with the surface of the steel sheet to produce a glassy film. Needless to say, it is not always necessary to produce a necessary and sufficient amount of A1 0 during the annealing for a short time. Even when it is produced by properly selecting the atmosphere of the final annealing or is produced in a required amount in both the short time annealing and final annealing and is then made to react with MgO-MnO the object of the present invention will be able to be attained. Such a glassy film is highly adhesive and has a high electric insu lation value and space factor and its chemical composition is of an Al O -SiO -FeO-MgO-MnO series when a Mn compound is to be used. Further, when the annealing separator remaining after the final annealing is analyzed, it can be seen that the content of the Mn compound has decreased and that of Mn in the steel has somewhat increased. It is thus presumed that the reaction therein has contributed to the formation of a highly adhesive glassy film and other precedingly enumerated properties. When the Mg compound and Mn compound are, however, mixed so that the weight ratio of MgO:MnO in the separator after the drying is more than 98 of MgO and less than 2 of MnO, a highly adhesive and electric insulating film is not obtained. Further, even if the Mg compound and Mn compound are so mixed that the weight ratio of MgOzMnO in the separator after the drying is less than 20 of MgO and more than 80 of MnO, the obtained film after the annealing becomes so rough that the space factor is reduced and the adhesion is impaired. The mixing weight ratio of MgO to MnO effective for the production of a glassy film is in the range of 98:2 to 20:80. Specifically, in the range of 70:30 to 90:10, a favorable result can be obtained. For the Mg compound there can be used MgO and for the Mn compound manganese oxides and manganese hydroxides in such various forms as Mg(OH) Mn O MnO-OH and Mn(OH) can be used; also compounds which will become MgO and MnO when heated can be employed. It is considered that oxygen liberated from Mn0 by the thermodecomposition during the annealing or water produced by the reaction of this liberated 0 with the reducing atmosphere will be effective to form a glassy film. Thus, among the Mn compounds, MnO is a compound specifically desirable in practice. Further it is desirable that the Mn compound have a fine granularity such as more than 100 meshes per inch. Such a Mn compound has an act to refine compounds contained in the silicon steel such as S and A1 during the box-annealing and also has an advantage of not deteriorating the magnetic property of the silicon steel sheet. Even in case the Mg compound and Mn compound are not mixed together but the steel sheet is coated first with the Mn compound as an under layer and then with the Mg compound thereon so as to form double layers, a favorable glassy film will be obtained. In such case, the ratio of the coated amounts of the under layer and the upper layer of the coating is so made that the ratio of MgOzMnO in the separator after drying will be in the above mentioned range the same as in the case of mixing them.

The film forming and annealing separator may be applied by such general means as a roller or spray. However, specifically, in the case of coating the steel sheet with the Mn compound as an under layer, the steel sheet should be dipped in an aqueous solution such as a nitrate or 4 sulfate of Mn for electrodeposition, then the oxide or hydroxide of Mn can be securely and uniformly deposited on the surface of the steel and therefore a better glassy film can be obtained.

The above explained glassy film may be well utilized, as it is, for laminated core materials. However, the electric insulation and anticorrosion properties can be improved if the surface is further coated, for example, with an inorganic film made by applying, drying or baking a solution prepared by adding at least one part of Water glass, magnesium oxide, calcium oxide, zinc oxide and silicic anhydride to an aqueous solution containing 5 to 40 wt. percent phosphoric acid, 1 to 10 wt. percent of a compound containing hexavalent chromium as chromic anhydride or dichromic acid and less than 8 wt. percent of boric acid or borate, or with an inorganic film made by applying, drying or baking an aqueous solution prepared by adding 1 to 4 wt. percent glycerine and 2 to 5 wt. percent boric acid to an aqueous solution of 10 to 60 wt. percent zinc dichromate. Therefore, after a glassy insulaing film is formed by the present invention, it may be further coated with a film of one or more of such inorganic compounds as P, Cr and Si. Further, in case the steel sheet is to be used without any stress relieving annealing, the heat resistance of the film will not be required and therefore the glassy film can be coated thereon with an organic film or a mixed organic and inorganic film.

As above mentioned, for a film forming agent and annealing separator, a mixture of a Mg compound and a Mn compound has been proved to be excellent. However, even when using any one or more of Zn, Cu and Cr compounds instead of the Mn compound, a glassy film can also be obtained.

The mixing ratio of such Zn, Cu or Cr compound to the Mg compound is, however, in the same range as that of MgOzMnO.

EXAMPLE 1 In the production of a silicon steel sheet (with the range of about 2.0 to 4.0 wt. percent Si and about 0.010 to 0.040 wt. percent Al and for whose particulars U.S.P. 3,159,511 should be referred to) containing 0.017 wt. percent of total Al and 3 wt. percent Si, and which was subjected to a cold-rolling with a reduction rate of 65 to after being hot-rolled, the steel sheet was in order to be decarburized at 800 C. (by using a range of about 750 to 950 C.) for 3 minutes in an atmosphere of a decomposed ammonia gas having a dew point of 65 C., was thinly coated on the surface with a suspension in which the mixing ratio of an annealing separator was g. of MgO, 10 g. of MnO and 1 liter of H 0 (MgOzMnO: 91.7 :8.3) so that the coated amount of the annealing separator after the drying was 8 g./m. was dried and was then final annealed at 1200 C. for 10 hours in a reducing atmosphere (for which was used a range of 1000 to 1200 C.) by introducing a dry mixture gas of H and N (of a dew point having about 30 C.) for the final annealing atmosphere to produce an electric insulating film by the reaction of the surface layer containing A1 0 produced during the annealing with the annealing separator.

The results as compared with those of the case of applying only MgO were as follows:

Interlayer resistance (ASTM Process No, 2) in 9 cm]! sheet (35 kg./cm.

Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

During the process of producing a silicon steel sheet (with the range of 0.025 to 0.085 wt. percent C, 2.5 to 4.0

wt. percent Si, 0.010 to 0.065 wt. percent of acid-soluble Al and 0.005 to 0.050 wt. percent SiO and for whose particulars U.S.P. 3,287,183 should be referred to) contain ing 0.028 percent sol. Al and 3% Si, the steel sheet was annealed for a short time of .5 minutes at 850 C. in an atmosphere of a decomposed ammonia gas having a dew point of 60 C. so as to make a surface layer containing A1 was thinly uniformly coated on the surface with a suspension consisting of 70 g. of MgO, 30 g. of MnO and 1 liter of H 0 (MgO:MnO:74.l:25.9) so that the coated amount of the annealing separator after the drying was 6 g./m. was dried and was then final annealed at 1200 C. for 20 hours in a reducing atmosphere by introducing a dry H gas (of dew point of about 30 C.) so that an electric insulating film was produced.

The results as compared with those of the case of MgO only were as follows:

Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

Substantially no film. The film did not peel off.

MgO only MgO+MnO2 EXAMPLE 3 During the process (the same as in Example 2) of producing a silicon steel sheet containing 0.028 percent sol. Al and 3% Si, the steel sheet was annealed for a short time of minutes at 900 C. in an atmosphere of a decomposed ammonia gas of a dew point of 60 C. so as to make a surface layer containing A1 0 was thinly uniformly coated on the surface with a suspension consisting of 85 g. of MgO, g. of MnO and 1 liter of H 0 (MgO:MnO:87.4:12.6) mixing ratio so that the coated amount of the annealing separator after the drying was 6.5 g./m. was dried, was then final annealed at 1200 C. for hours in a dry mixture gas of H and N (of a dew point of about C.) so that a glassy film was produced. After the residual annealing separator was removed, the steel sheet was again coated on the coated film with an equeous solution consisting of 20 g. of H PO 3.5 g. of MgO, 4.5 g. of CrO 2.5 g. of H BO and 100 cc. of H 0 in the mixing ratio so that the coated amount of the film after the baking was 3 g./cm. and was baked so that an electric insulating film was formed. The results as compared with those of the case of MgO only were as follows:

15. l The film peeled off greatly. 100 The film did not peel off.

MgO only MgO-i-MnOz EXAMPLE 4 During the process of producing a silicon steel sheet (with the range of 2.0 to 4.0 wt. percent Si and 0.010 to 0.090 wt. percent of acid-soluble Al) containing 0.045% of sol. Al and 29% Si, and subjecting it to cold-rolling with a reduction rate of 60 to 90% in thickness after hotrolling it, the sheet was annealed to be decarburized at 800 C. (for which could be used a range of 700 to 900 C.) for 3 minutes in an atmosphere of a decomposed ammonia gas of a dew point of 70 C. so as to make a surface layer containing Al O was thinly uniformly coated on the surface with a suspension consisting of 30 g. of MgO, 50 g. of MnO and 1 liter of H 0 (MgOzMnO: 42.4:57.6) mixing ratio so that the coated amount of the separator after the drying was 7 g./m. was then dried and then final annealed at 1200 C. (for which a temperature above 900 C. could be used) for 15 hours in a dry H gas atmosphere (of a dew point of 30 C.) so that a glassy film was formed. Needless to say, A1 0 produced not only in the short time annealing but also during the final annealing reacted. Thereafter, the residual annealing separator was removed. The steel sheet was then coated with an aqueous solution consisting of 1.5 g. of ZnCr O 1.5 g. of H BO 1.5 g. of glycerine and cc. of H 0 in the recited mixing ratio so that the coated amount of the film after the baking was 2.5 g./m. and was baked so that an electric insulating film was produced. The results as compared with those of the case in which MgO only was used were as follows:

Adhesion (when bent by degrees with a diameter of 20 mm.)

MgO only MgO +MnO z EXAMPLE 5 During the process of producing a silicon steel sheet containing 0.025 percent sol. Al and 3% Si according to U.S.P. 3,287,183, the steel sheet was annealed for a short time of 4 minutes at 850 C. in an atmosphere of a dew point of 65 C., was thinly uniformly coated on the surface with a suspension in which the annealing separator consisted of 80 g. of MgO, 20 g. of ZnO and 1 liter of H 0 in the recited mixing ratio so that the coated amount of separator after the drying was 6 g./m. was dried and was then final annealed at 1200 C. for 200 hours by introducing a dry H gas (of dew point of about 30 C.) so that a glassy electric insulating film was produced. The results as compared with the case of MgO only were as follows:

Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

Substantially no film. Th1; film did not peel Interlayer resistance (ASTM Process No. 2) in 90111. sheet (35 kgJcmJ) MgO only MgO+ZnO EXAMPLE '6 The steel sheet in Example 5 was treated under the same conditions as in Example 5 by using a suspension consisting of 80 g. of MgO, 20 g. of CuO and 1 liter of H 0 instead of the MgO-ZnO series suspension.

EXAMPLE 7 The steel sheet in Example 5 was treated under the same conditions as in Example 5 by using a suspension consisting of 80 g. of MgO, 20 g. of G0,, and 1 liter of H 0 instead of the MgO-ZnO series suspension.

The results of the above examples as compared with those of the case of MgO only were as follows:

Interlayer resistance (ASTM Process No. 2) in item?! sheet (35 kgJemJ) Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

MgO only 0. 49 Substantially no film. Example 6 4.12 Th1? film did not peel o Example 7 7. 27 Th5?f film did not peel EXAMPLE 8 H and was dried. The coated amount of the Mg compound after the drying was 6.5 g./m. (MgO:MnO=about 83:17). The steel sheet was then final annealed at 1200 C. for hours by introducing a dry H gas (of a dew point of about C.) so that an electric insulating film was produced. This film was a uniform glassy film having a mirror-like luster and was higher in the luster, uniformity, adhesion and electric insulation than in the case where MgO and MnO were used as mixed. The results as compared with the case of MgO only were as follows:

Intcrlayer resistance Adhesion (when (ASTM Process bent by 180 N0. 2) in ncmfi/ degrees with a sheet kg./cm. diameter of 20 mm.)

0. 49 Substantially no film.

Mn compound (in the under layer) plus MgO (in the upper layer).

EXAMPLE 9 When the steel sheet Was painted with 1.5 g./m. of MnO by spraying instead of the electrodeposition of the Mn compound in Example 8 and was then treated the same as in Example 8, the same results as in Example 8 were obtained.

Having thus described the invention, what is claimed is:

1. A method of producing an electric insulating film on the surface of an Al-containing silicon steel sheet con taining soluble Al in an amount of 0.01 to 0.09% by weight, comprising the steps of applying to the surface of the silicon steel sheet a compound selected from the group consisting of oxides, peroxides, hydroxides and carbonates and which produces MnO upon heating to form an under coating layer of said compound, applying an upper layer of MgO to said under coating layer in an amount so that the ratio of MgO to MnO is from 98:2 to 20:80, and annealing the thus coated steel sheet at a temperature of above 1000 C.

2. A method of keeping Al-containing silicon steel sheets containing soluble Al in an amount of from 0.01 to 0.09% by weight'separate during final annealing while simultaneously producing an electric insulating film on the surface of the sheets, comprising the steps of applying to the surface of the silicon steel sheets which have a surface layer of A1 0 thereon at least partly formed during a prior annealing a mixture of at least one Mg compound selected from the group consisting of oxides, peroxides, hydroxides and carbonates thereof and which produces MgO upon heating, and at least one further compound taken from the group consisting of Mn, Zn, Cu and Cr compounds which produce oxides of said further compounds when heated, the mixing ratio of the MgO to the oxide of the further compound being from a ratio of 98:2 to a ratio of 20:80, stacking the sheets, and further annealing the stacked sheets in a reducing atmosphere at a temperature of more than 1000 C. for causing the thus obtained mixture to react with the surface layer of the steel sheet so as to form a glassy electric insulating film.

3. A method of producing an electric insulating film on the surface of an Al-containing silicon steel sheet containing soluble Al in an amount of from 0.01 to 0.09% by weight, comprising the steps of applying to the surface of the silicon steel sheet which has a surface layer of A1 0 thereon at least partly formed during a prior annealing a mixture of at least one Mg compound selected from the group consisting of oxides, peroxides, hydroxides and carbonates thereof and which produces MgO' upon heating, and at least one further compound taken from the group consisting of Mn, Zn, Cu and Cr compounds which produce oxides of said further compounds when heated, the mixing ratio of the MgO to the oxide of the further compound being from a ratio of 98:2 to a ratio of 20:80, and causing the thus obtained mixture to react with the surface layer of the steel sheet in a reducing atmosphere of more than 1000 C. so as to form a glassy electric insulating film.

4. The method according to claim 1 wherein the step of applying the mixture comprises applying the Mn compound as an under layer on the surface of the Al-containing silicon steel sheet and then applying the Mg compound as an upper layer over the Mn compound.

5. A method as claimed in claim 1 in which said further compound is a Mn compound.

6. The method as claimed in claim 5 in which said ratio of MgO to M110 is in the range of :30 to :10.

7. The method as claimed in claim 5, wherein the mixture further includes at least one compound from the group consisting of a Zn compound, a Cu compound and a Cr compound, which are in the form of a compound taken from the group consisting of oxides, peroxides, hydroxides and carbonates of the respective elements and produce oxides of Zn, Cu and Cr upon heating.

References Cited UNITED STATES PATENTS 3,051,589 8/1962 Sanford et a1 106-48 X 3,105,781 10/1963 Walter 148l13 X 3,132,056 5/1964 McQuade 148113 X 3,151,000 9/1964 Schmidt et al 148-113 X 3,207,636 9/1965 Wada et al 10648 X 3,259,526 7/1966 Walker et a1. l486.35 3,368,712 2/1968 Sanford et al 10648 X ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R.

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