Magnetic steel sheet with semi-organic insulation coating

This is a §371 of International Application No. PCT/JP2011/004123, with an international filing date of Jul. 21, 2011 (WO 2012/011286 A1, published Jan. 26, 2012), which is based on Japanese Patent Application No. 2010-165380, filed Jul. 22, 2010, the subject matter of which is incorporated herein by reference.

This disclosure relates to a magnetic steel sheet with a semi-organic insulation coating which has no deterioration of corrosion resistance and water resistance without containing a chromium compound and is excellent in powdering resistance, scratch resistance, sticking resistance, TIG weldability, punchability, and uniformity of coating appearance after annealing.

An insulation coating of a magnetic steel sheet used for motors, transformers and the like is required to have various properties such as not only interlaminar resistance, but also convenience in working, storage, stability in use and so on. Since the magnetic steel sheet is used in a variety of applications, it is attempted to develop various insulation coatings depending upon the application. When the magnetic steel sheet is subjected to punching, shearing, bending or the like, magnetic properties are deteriorated due to residual strain so that stress relief annealing at a temperature of about 700 to 800° C. is frequently conducted to solve this problem. In that case, therefore, the insulation coating must be resistant to the stress relief annealing.

The insulation coatings are roughly classified into three types:

(1) an inorganic coating placing great importance on weldability and heat resistance, and being resistant to stress relief annealing; (2) a resin-containing inorganic coating aiming to establish punchability and weldability, and being resistant to stress relief annealing (i.e., semi-organic coating); and (3) an organic coating that is unable to be subjected to stress relief annealing in a special application, but the coatings containing an inorganic component as shown in the above items (1) and (2) are resistant to the stress relief annealing as a general-purpose product, and they contain a chromium compound.

Particularly, chromate-based insulation coating of type (2) can dramatically improve the punchability in a 1-coat and 1-bake production as compared to the inorganic insulation coating and are widely used.

For example, in JP-B-S60-36476 is disclosed a magnetic steel sheet having an electrical insulation coating obtained by applying a treating solution formed by compounding a dichromate-based aqueous solution containing at least one divalent metal with 5-120 parts by weight as a solid content of a resin emulsion having a ratio of vinyl acetate/VeoVA as an organic resin of 90/10 to 40/60 and 10-60 parts by weight of an organic reducing agent based on 100 parts by weight of CrO 3 in the aqueous solution, onto a surface of a base iron sheet and then subjecting to baking according to the usual manner.

However, environmental awareness is rising in recent years. Hence, products having an insulation coating free of chromium compound are demanded by consumers even in the field of magnetic steel sheets.

There are developed magnetic steel sheets with an insulation coating containing no chromium compound. For example, as an insulation coating containing no chromium, but having a good punchability are described ones containing a resin and colloidal silica (alumina-containing silica) in JP-A-H10-130858. Also, an insulation coating comprising one or more of colloidal silica, alumina sol and zirconia sol and containing a water soluble or emulsion resin is described in JP-A-H10-46350, and an insulation coating composed mainly of a phosphate containing no chromium and containing a resin is described in Japanese Patent No. 2944849.

However, these magnetic steel sheets with the insulation coatings containing no chromium have problems that the bond between inorganic substances is relatively weak and the corrosion resistance is poor as compared to those containing a chromium compound. Also, when back tension is applied by rubbing the steel sheet surface with a felt in slit working (by using a tension pad), the occurrence of powdering comes into question. Furthermore, there is a problem that the coating becomes weaker after stress relief annealing and is easily scratched.

For instance, the above problems can not be solved by simply using one or more of colloidal silica, alumina sol and zirconia sol in the method of JP '350. Also, sufficient investigations are not made with respect to the case of mixing these components in combination and at a specific ratio. Furthermore, in the case of a phosphate coating with a composition containing no chromium as described in JP '849, sticking occurs and the water resistance tends to be deteriorated.

These problems are apt to be easily caused during baking at a relatively low temperature of not higher than 300° C., and the occurrence becomes particularly remarkable at a temperature of not higher than 200° C. Meanwhile, the baking temperature should be as low as possible from a viewpoint of the reduction of energy consumption and production cost, and so on.

In addition, when using the methods described in JP-A-2007-197820 and JP-A-2007-197824, i.e., when using a coating comprising a polysiloxane polymer obtained by copolymerizing polysiloxane with various organic resins and an inorganic compound such as silica, silicate or the like, there are problems that blowholes occur in the TIG welding, and that a spotted pattern occurs after annealing depending on the kind of the steel.

We discovered that, by compositely including a Zr compound and an Si compound containing a plate-like silica, or further a B compound as an inorganic component in the semi-organic coating, a number of the issues raised above can be addressed.

We thus provide:

1. A magnetic steel sheet provided on its surface with a semi-organic insulation coating comprising an inorganic component and an organic resin, characterized in that the inorganic component of the coating comprises a Zr compound and an Si compound containing a plate-like silica as a ratio in a dry coating of 20-70 mass % of Zr compound (converted to ZrO 2 ) and 10-50 mass % of Si compound containing a plate-like silica (converted to SiO 2 ), and the remainder of the coating is the organic resin. 2. The magnetic steel sheet with a semi-organic insulation coating according to the item 1, wherein the plate-like silica has an average particle size of 10 to 600 nm. 3. The magnetic steel sheet with a semi-organic insulation coating according to the item 1 or 2, wherein the plate-like silica has an aspect ratio (average length/average thickness ratio) of 2 to 400. 4. The magnetic steel sheet with a semi-organic insulation coating according to any one of the items 1 to 3, wherein the inorganic component further contains a B compound as a ratio in a dry coating of 0.1-5 mass % of B compound (converted at B 2 O 3 ). 5. The magnetic steel sheet with a semi-organic insulation coating according to any one of the items 1 to 4, wherein the coating further contains not more than 30 mass % in total of one or more selected from a nitric acid compound (converted to NO 3 ), a silane coupling agent (converted to a solid content) and a phosphorus compound (converted to P 2 O 5 ) as a ratio in a dry coating.

In other words, we provide a magnetic steel sheet having on its surface a semi-organic insulation coating comprising an inorganic component and an organic resin, wherein as the inorganic component is included

(1) a Zr compound and an Si compound containing a plate-like silica as a ratio in a dry coating of 20-70 mass % of Zr compound (converted to ZrO 2 ) and 10-50 mass % of Si compound containing plate-like silica (converted to SiO 2 ); (2) if necessary, a B compound as a ratio in a dry coating of 0.1-5 mass % of B compound (converted to B 2 O 3 ); and (3) if necessary, not more than 30 mass % in total of one or more selected from a nitric acid compound (converted to NO 3 ), a silane coupling agent (converted to a solid content) and a phosphorus compound (converted to P 2 O 5 ) as a ratio in a dry coating,

the remainder of the coating is substantially the organic resin (i.e., with an allowance of containing impurities and known additives), and moreover, preferably, the average particle size of the plate-like silica is 10 to 600 nm and/or the aspect ratio (average length/average thickness ratio) of the plate-like silica is 2 to 400.

Our magnetic steel sheets with a semi-organic insulation coating are excellent in various properties such as powdering resistance, scratch resistance, sticking resistance, TIG weldability and punchability, and has no deterioration of water resistance and corrosion resistance without containing a chromium compound, and also are excellent in the uniformity of coating appearance after annealing.

FIG. 1 is photographs showing coating appearance after stress relief annealing in comparison.

First, the reasons why the compounding ratio of the Zr compound and Si compound containing plate-like silica, and further B compound as the inorganic component of the semi-organic coating is limited to the above range will be described. Moreover, it should be noted that mass % of these components is the ratio in a dry coating.

Zr Compound: 20 to 70 Mass % as Converted to ZrO 2

The Zr compound has a strong bonding force to oxygen and can be strongly bonded to an oxide, a hydroxide and the like on the Fe surface. Also, since the Zr compound has three or more chemical bonds, a strong coating can be formed by forming a network between mutual Zrs or between Zr and other inorganic compound without using chromium. However, when the ratio of the Zr compound in the dry coating is less than 20 mass % as converted to ZrO 2 , not only the adhesion property, corrosion resistance and powdering resistance are deteriorated, but also the deterioration of appearance after annealing due to an Si compound occurs. On the other hand, when it exceeds 70 mass %, the corrosion resistance and powdering resistance are deteriorated, and also the scratch resistance on the surface of the sheet after the stress relief annealing is deteriorated. Therefore, the amount of the Zr compound is 20 to 70 mass % as converted to ZrO 2 .

As such, a Zr compound are mentioned, for example, zirconium acetate, zirconium propionate, zirconium oxychloride, zirconium nitrate, zirconium ammonium carbonate, zirconium potassium carbonate, zirconium hydroxychloride, zirconium sulfate, zirconium phosphate, zirconium sodium phosphate, zirconium potassium hexafluoride, tetra-n-propoxy-zirconium, tetra-n-butoxy-zirconium, zirconium tetra-acetylacetonate, zirconium tributoxy acetylacetonate, zirconium tributoxy stearate, and so on. They may be used alone or in a combination of two or more.

Si Compound Containing Plate-Like Silica: 10 to 50 Mass % as Converted to SiO 2

The plate-like silica is also called “leaf-like” silica or “scale-like” silica, has a layered silicate structure formed by laminating many SiO 2 thin layers, and is mostly non-crystalline or microcrystalline. Such a plate-like silica is excellent in inhibitive performance of permeating corrosive substance due to the layered form, excellent in adhesion property due to the presence of many hydroxyl groups, and excellent in slidability due to flexibility as compared to a commonly-used silica particle, for example, colloidal silica or the like. Therefore, it is more effective in the improvement of corrosion resistance and punchability.

The plate-like silica can be obtained by preparing agglomerated particles of laminated primary particles of the thin layers, and pulverizing these agglomerated particles.

The average particle size of the plate-like silica is preferably about 10 to 600 nm. More preferably, it is 100 to 450 nm. In addition, the aspect ratio (length/thickness ratio) of the plate-like silica is preferably about 2 to 400. More preferably, it is 10 to 100.

Moreover, since the plate-like silica tends to be laminated by orienting parallel to the surface of the steel sheet, the length (length in the longitudinal direction) of the plate-like silica can be determined by observing the surface of the coating with SEM (Scanning Electron Microscope). Also, the thickness of the plate-like silica can be determined by observing the cross section of frozen fracture, FIB (focused ion beam) or the like with SEM. Therefore, the length and the thickness of the plate-like silica are measured at several places by SEM observation, and the aspect ratio (average length/average thickness ratio) is determined from an average of these measured values.

Moreover, when silica is oval or polygonal, the average particle size in the sheet plane and the average thickness in the cross section are similarly measured at several places by SEM observation, and the aspect ratio is determined by (average particle size)/(average thickness).

As the Si compounds other than the plate-like silica may be mentioned colloidal silica, fumed silica, alkoxysilane, siloxane and the like. They contribute to the improvement of corrosion resistance, and adhesion property after stress relief annealing. Generally, colloidal silica and fumed silica are spherical or an ellipsoidal body with an aspect ratio of less than 2, while alkoxysilane and siloxane are amorphous.

When the compounding ratio of the plate-like silica in the whole of the Si compound is less than 50 mass %, good corrosion resistance and punchability cannot be obtained so that the compounding ratio of the plate-like silica is preferably not less than 50 mass %. Of course, all of the Si compounds may be plate-like silica. A particularly preferable range is 50 to 100 mass %.

Moreover, the compounding ratio of the plate-like silica can be determined, for example, by measuring SiO 2 amount (B) in the plate-like silica region and SiO 2 amount (A) in the other regions at the FIB cross section and calculating B/(A+B)×100. Furthermore, the SiO 2 amount in each region can be determined from the average Si concentration and the area of the region measured by EDS (Energy Dispersion Spectroscopy) and the like.

The aforementioned Si compound containing the plate-like silica is useful in solving the problems caused when the Zr compound is added alone. More specifically, although the use of the Zr compound alone tends to deteriorate the corrosion resistance and powdering resistance and considerably deteriorate the scratch resistance on a stress relief annealed steel sheet, powdering resistance and scratch resistance can be largely improved by compounding a proper amount of the Si compound.

When the amount of the Si compound included in the dry coating is less than 10 mass % as converted to SiO 2 , sufficient corrosion resistance cannot be obtained, while when it exceeds 50 mass %, the powdering resistance is deteriorated, and also the scratch resistance on the stress relief annealed steel sheet is deteriorated, so that the amount of the Si compound is 10 to 50 mass %.

A B compound may be further included as the inorganic component in addition to the aforementioned Zr compound and the Si compound containing the plate-like silica.

B Compound: 0.1 to 5 Mass % as Converted to B 2 O 3

The B compound advantageously contributes to solve the problems caused when the Zr compound is added alone. That is, when the Zr compound is added alone, a tendency of deteriorating the corrosion resistance and powdering resistance and considerably deteriorating the scratch resistance on the stress relief annealed steel sheet is seen. The reason is assumed due to the fact that when the Zr compound is used alone, the volume constriction is large in the baking and hence coating cracking easily occurs, and a place of exposing the substrate is partially caused.

In contrast, when a proper amount of the B compound is compounded to the Zr compound, the coating cracking caused in the case of using the Zr compound alone can be effectively mitigated to considerably improve the powdering resistance.

When the ratio of the B compound in the dry coating is not less than 0.1 mass % as converted to B 2 O 3 , the addition effect is exerted, while when it is not more than 5 mass %, unreacted substances do not remain in the coating, and adhesive between the coatings (i.e., a defect of fusing the coatings to each other) after stress relief annealing (sticking) does not occur, so that the amount of the B compound is preferably 0.1 to 5 mass % as converted to B 2 O 3 .

As the B compounds are mentioned boric acid, orthoboric acid, metaboric acid, tetraboric acid, sodium metaborate, sodium tetraborate and the like. They may be used alone or in a combination of two or more. However, it is not limited to them, and may include, for example, a compound dissolving in water to generate a boric ion, and also the boric ion may be polymerized in a linear or a cyclic state.

In addition to the aforementioned inorganic components, one or more selected from a nitric acid compound, a silane coupling agent and a phosphorous compound described below can be further included in the dry coating at a ratio of not more than 30 mass % in total. Moreover, the ratios of the nitric acid compound, silane coupling agent and phosphorous compound in the dry coating are represented by a value converted to NO 3 (nitric acid compound), a value converted to a solid content (silane coupling agent) and a value converted to P 2 O 5 (phosphorous compound), respectively.

The nitric acid compound, silane coupling agent and phosphorous compound effectively contribute to the improvement of corrosion resistance. When the ratio in the dry coating is not more than 30 mass % in total, unreacted substances do not remain in the coating and the water resistance is not deteriorated, so that the content is preferable to be not more than 30 mass % in total. To sufficiently develop the effect of these components, it is preferable to include these components at a ratio of not less than 1 mass % in the dry coating.

Nitric acid-based or nitrous acid-based compounds and further hydrates thereof as shown below are advantageously suitable as the nitric acid compound.

Nitric Acid-Based:

nitric acid (HNO 3 ), potassium nitrate (KNO 3 ), sodium nitrate (NaNO 3 ), ammonium nitrate (NH 4 NO 3 ), calcium nitrate (Ca(NO 3 ) 2 ), silver nitrate (AgNO 3 ), iron (II) nitrate (Fe(NO 3 ) 2 ), iron (III) nitrate (Fe(NO 3 ) 3 ), copper (II) nitrate (Cu(NO 3 ) 2 ), barium nitrate (Ba(NO 3 ) 2 ), aluminum nitrate (Al(NO 3 ) 3 ), magnesium nitrate (Mg(NO 3 ) 2 ), zinc nitrate (Zn(NO 3 ) 2 ), nickel (II) nitrate (Ni(NO 3 ) 2 ), zirconium nitrate (ZrO(NO 3 ) 2 ).

Nitrous Acid-Based:

nitrous acid (HNO 2 ), potassium nitrite, calcium nitrite, silver nitrite, sodium nitrite, barium nitrite, ethyl nitrite, isoamyl nitrite, isobutyl nitrite, isopropyl nitrite, t-butyl nitrite, n-butyl nitrite, n-propyl nitrite.

As the silane coupling agent are advantageously suitable those shown below.

Vinyl-Based:

vinyl trichlorosilane, vinyl trimethoxysilane, vinyl triethoxysilane.

Epoxy-Based:

2-(3,4-epoxycyclohexyl)ethyl trimethoxysilane, 3-glycidoxypropyl trimethoxysilane, 3-glycidoxypropylmethyl diethoxysilane, 3-glycidoxypropyl triethoxysilane.

Styryl-Based:

p-styryl trimethoxysilane.

Methacryloxy-Based:

3-methacryloxy propylmethyl dimethoxysilane, 3-methacryloxy propyl trimethoxysilane, 3-methacryloxy propylmethyl diethoxysilane, 3-methacryloxy propyl triethoxysilane.

Acryloxy-Based:

3-acryloxy propyl trimethoxysilane.

Amino-Based:

N-2-(aminoethyl)-3-aminopropylmethyl dimethoxysilane, N-2-(aminoethyl)-3-aminopropyl trimethoxysilane, N-2-(aminoethyl)-3-aminopropyl triethoxysilane, 3-aminopropyl trimethoxysilane, 3-aminopropyl triethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine and a partial hydrolyzate thereof, N-phenyl-3-aminopropyl trimethoxysilane, hydrochloride of N-(vinylbenzyl)-2-aminoethyl-3-aminopropyl trimethoxysilane, special aminosilanes.

Ureido-Based:

3-ureidopropyl triethoxysilane.

Chloropropyl-Based:

3-chloropropyl trimethoxysilane.

Mercapto-Based:

3-mercaptopropylmethyl dimethoxysilane, 3-mercaptopropyl trimethoxysilane.

Polysulfide-Based:

bis(triethoxysilylpropyl)tetrasulfide.

Isocyanate-Based:

3-isocyanatepropyl triethoxysilane.

Furthermore, phosphoric acid and phosphates as shown below are advantageously suitable as the phosphorous compound.

Phosphoric Acid:

orthophosphoric acid, phosphoric acid anhydride, linear polyphosphoric acid, cyclic metaphosphoric acid.

Phosphate:

Magnesium phosphate, aluminum phosphate, calcium phosphate, zinc phosphate.

Hf, HfO 2 , TiO 2 , Fe 2 O 3 and so on may be incorporated into the inorganic component as an impurity. However, a problem does not particularly occurs when the total amount of these impurities is not more than 1 mass % in the dry coating.

It is preferable that the organic resin is compounded at a ratio of 5 to 40 mass % to render the content of the inorganic component as mentioned above in the dry coating into 60 to 95 mass %.

The organic resin is not particularly limited and any of known ones conventionally used are advantageously suitable. The organic resin includes, for example, aqueous resins (emulsion, dispersion, water soluble) of acryl resin, alkyd resin, polyolefin resin, styrene resin, vinyl acetate resin, epoxy resin, phenol resin, polyester resin, urethane resin, melamine resin and so on. Emulsions of acryl resin and ethylene acrylate resin are particularly preferable.

The organic resins effectively contribute to the improvement of the corrosion resistance, scratch resistance and punchability. When the compounding ratio in the dry coating is not less than 5 mass %, the addition effect is large, while when it is not more than 40 mass %, the scratch resistance after stress relief annealing and TIG weldability are not deteriorated, so that the compounding ratio of the organic resin is preferably about 5 to 40 mass % as converted to a solid content.

It should be noted that the ratio in the dry coating refers to a ratio of each component in a coating formed on a surface of a steel sheet by applying a treating solution containing the aforementioned components onto the steel sheet, and drying by baking. It can also be determined from residual components (solid content) after drying the treating solution at 180° C. for 30 minutes.

Furthermore, it is possible to include additives usually used and other inorganic or organic compounds in addition to the aforementioned components.

The additive is added to further enhance performance and uniformity of the insulation coating, and includes a surfactant, an antirust agent, a lubricant, an antioxidant and the like. Moreover, the amount of the additive compounded in the dry coating is preferably not more than about 10 mass % from a viewpoint of the maintaining of sufficient coating properties.

The magnetic steel sheet as a starting material is not particularly limited, and any known ones are suitable.

That is, any of so-called soft (magnetic) iron sheets (electric iron sheets) having a high magnetic flux density, common cold-rolled steel sheets such as SPPC and the like, non-oriented magnetic steel sheets containing Si and/or Al for increasing specific electrical resistance and so on are advantageously suitable for reducing iron loss. Particularly, it is preferable to use a steel sheet containing about 0.1 to 10 mass % of Si or (Si+Al) and/or a steel sheet having an iron loss W 15/50 of not more than about 7 W/kg.

Next, the method for forming the insulation coating will be described.

A pretreatment of a magnetic steel sheet as a starting material is not particularly defined. More specifically, the steel sheet may be untreated, but it is advantageous to be subjected to a degreasing treatment with alkali or the like and a pickling treatment with hydrochloric acid, sulfuric acid, phosphoric acid or the like.

Then, to the surface of this magnetic steel sheet is applied a treating solution formed by compounding a Zr compound and an Si compound containing a plate-like silica, and optionally a B compound and optionally one or more selected from a nitric acid compound, a silane coupling agent and a phosphorous compound, and, if necessary, an additive and the like together with an organic resin at given ratios and then baked to form an insulation coating. As a method for coating the treating solution for insulation coatings are applicable various methods usually and industrially used such as roll coater, flow coater, spray, knife coater and so on. Also, as the baking method are possible a hot air type, an infrared type, an induction heating type and the like as generally conducted. The baking temperature may be a normal level, and may be about 150 to 350° C. as a maximum reaching temperature on the steel sheet.

The magnetic steel sheet with an insulation coating can remove strain due to punching, for example, by subjecting to stress relief annealing. As a preferable stress relief annealing atmosphere is applied an atmosphere hardly oxidizing iron such as N 2 atmosphere, DX gas atmosphere or the like. The corrosion resistance can be further improved by setting a dew point high, for example, Dp: about 5 to 60° C. to slightly oxidize the surface and cut end surface. Also, the stress relief annealing temperature is preferably 700 to 900° C., more preferably 700 to 800° C. The holding time of the stress relief annealing temperature is preferably longer, and more preferably not shorter than 2 hours.

The amount of the insulation coating adhered is not particularly limited, but is preferably about 0.05 to 5 g/m 2 per side. The coating amount or the total solid content mass of the insulation coating can be measured from weight reduction after the removal of the coating through dissolving with alkali. If the coating amount is small, it can be determined by using a standard sample having a known coating amount and measuring a relationship between a detection intensity of Zr or Si by a fluorescent X-ray analysis and the coating amount per coating composition and converting the fluorescent X-ray analyzed intensity of Zr or Si into a coating amount depending upon the coating composition based on this calibration curve. When the coating amount is not less than 0.05 g/m 2 , the insulation property can be satisfied with the corrosion resistance, while when it is not more than 5 g/m 2 , not only the adhesion property is improved, but also blistering is not caused in the coat baking, and the deterioration of coatability is not incurred. More preferably, it is 0.1 to 3.0 g/m 2 . Although it is preferable to form insulation coatings on the both sides of the steel sheet, the coating may be formed on only one side depending on the purpose. In addition, the coating is formed on only one side and another insulation coating may be formed on the other side depending on the purpose.

Although the effect of our steel sheets will be concretely described based on examples below, this disclosure is not limited to these examples.