Description

COATING SOLUTION FOR NON-ORIENTED ELECTRICAL STEEL SHEET, METHOD OF COATING NON-ORIENTED

ELECTRICAL STEEL SHEET USING THE SAME, AND COATING FILM OF NON-ORIENTED ELECTRICAL STEEL

SHEET Technical Field

[1] The present invention relates, in general, to a coating solution for a non-oriented electrical steel sheet, a method of coating a non-oriented electrical steel sheet using the same, and a coating film of the non-oriented electrical steel sheet, and, more particularly, to a coating solution for a non-oriented electrical steel sheet, which contains no chromium, to a method of coating a non-oriented electrical steel sheet using the same, and to a coating film of the non-oriented electrical steel sheet. Background Art

[2] Generally, a non-oriented electrical steel sheet, which is a rolled steel sheet having uniform magnetic properties in all directions thereon, is widely used for motors, iron cores of power generators, electromotors, and small transformers. In particular, a non- oriented electrical steel sheet tends to be high-grade in order to realize low iron loss (refrigerators or motors for plants) for electricity loss reduction, high magnetic flux density (motors for vacuum cleaners) for miniaturization and high efficiency, and ultra-thinness (OA instruments, motors for driving electrical vehicles) corresponding to an increase in frequency for high output.

[3] In such a high-grade non-oriented electrical steel sheet, a thick insulation coating film (thick film) is essential for high insulating properties in terms of efficiency of use of energy. For example, a non-oriented electrical steel sheet for use in medium and large electromotors, power generators and transformers requires an insulation coating film for imparting high insulating properties to minimize interlayer current loss in the case where a laminate formed from steel is used in a punched state. Such high insulating properties may be required even after heat treatment, such as stress-relief annealing (SRA).

[4] Because the high-grade non-oriented electrical steel sheet has high silicon content, the hardness of the substrate thereof is increased, undesirably causing problems related to poor processability by which a slitter and a press are subjected to high stress in the

course of slitting and punching. Accordingly, there is a need to form a thick coating film. The high-grade non-oriented electrical steel sheet is predicted to lead the future of electric/electronic industries.

[5] A solution for the formation of an insulation coating film for a non-oriented electrical steel sheet generally includes three types, that is, an inorganic coating solution, an organic coating solution, and an organic-inorganic composite coating solution, and the coating method thereof, including applying an inorganic coating solution and then applying an organic coating solution, is also under study.

[6] The inorganic coating solution is composed mainly of an inorganic material such as phosphate and so on, and enables the formation of a coating film having high heat resistance, weldability, and layering properties and is thus useful for EI cores. However, because the hardness of the insulation coating film is high, a mold is more quickly damaged upon punching compared to when using a coating film containing an organic material. Consequently, the inorganic coating solution is not desirable from the aspect of punching processability.

[7] The organic coating solution is composed mainly of an organic material and is very superior in terms of punchability. Even when the thickness of the film is increased, adherence is good, and thus the organic coating solution is chiefly used for large iron cores requiring good interlay er insulating properties. The weldability of the organic coating film is not good because resin-decomposing gas is generated upon welding.

[8] For this reason, in the interest of improving heat resistance and insulation properties, there has been developed an organic-inorganic composite coating solution using both an organic material and an inorganic material to make up for the poor punching processability of inorganic material such as phosphate or chromate. In the case of a coating film formed using such an insulation coating solution, heat resistance, which is characteristic of the inorganic material, and the lubrication effects of the organic material are realized at the same time, thus realizing a beautiful surface appearance.

[9] In addition, to improve the insulating properties of the non-oriented electrical steel sheet, the combination of various compositions is applied. The insulation coating film solution for a non-oriented electrical steel sheet, which has been produced by principal manufacturers to date, is mainly based on phosphate and chromate. The phosphate and chromate play a role in greatly improving heat resistance, insulating properties, and corrosion resistance of substrate metal.

[10] A method of forming an insulation coating film using an organic-inorganic coating agent is well-known and disclosed in Ktrean Patent Nos. 25106 and 31208 and US

Patent Nos. 4,316,751 and 4,4)8,936. Further, Japanese Examined Patent Publication No. Sho. 50-15013 discloses the formation of an insulation coating film using a treatment solution composed mainly of dchromate and an organic resin emulsion of vinyl acetate, a butadene-styrene copolymer, acrylic resin, etc. In the case where chromate is used, a hydrogen bond is formed between chromate and an Fe oxide layer of a base layer, thus attaining excellent coating film properties, including adherence and punchability, and furthermore, good coating film properties may be obtained even after SRA. However, because the composition of such a conventional coating solution essentially contains chromium oxide, adverse effects on the human body and environmental problems are caused. Attributable to the above problems, the use of heavy metals, including hexavalent chromium, is limited under the current strict control of environmental restrictions such as RoHS (restriction of the use of hazardous substances) between EU countries.

[11] Accordingly, thorough research into chromium- free coating agents for electrical steel sheets is being conducted these days. The preparation method of such a coating agent is largely classified into, in order to improve on the low corrosion resistance and adherence resulting from the absence of chromate, a method of introducing phosphate, and a method of inducing barrier effects through the addition of colloidal silica. The former method utilizes metal phosphate, in which aluminum phosphate (A1(H ₂ PO ₄ ) ₃ ), calcium phosphate (Ca(H ₂ PO ₄ ) ₂ ), and zinc phosphate (Zn(H ₂ PO ₄ ) ₂ ) are mixed at an appropriate ratio, thus increasing adherence and corrosion resistance, as disclosed in Japanese Unexamined Patent Publication No. 2004-322079. However, in the case where metal phosphate is utilized, free phosphoric acid present in the phosphate may cause a coating film to be sticky.

[12] On the other hand, as typical examples in which barrier effects are increased with the addition of colloidal silica, Korean Unexamined Patent Publication No. 1999-026911 and Japanese Patent No. 3370235 disclose a technique for ensuring corrosion resistance, adherence and smoothness after SRA, using one or more inorganic materials selected from among colloidal silica, alumina sol, and zirconium oxide, and for improving adherence or solvent resistance with the addition of a silane coupling agent. Further, the improvement of adherence and corrosion resistance thanks to the probability of formation of a thin dispersion coating film under conditions in which a surface area ratio of resin and silica is appropriate is disclosed in Japanese Patent No. 3320983. However, the chromium-free coating solution based on phosphate or colloidal silica, as noted above, suffers because the use of phosphate causes stickiness

and colloidal silica has limited ability to improve corrosion resistance, and thus it is still difficult to commercialize techniques for completely replacing chromium oxide using such a coating solution.

[13] Other than the use of the coating agent mentioned above, non-oriented electrical steel sheet products having high functionality have been manufactured using a coating solution for a thick film comprising a functional resin and an inorganic filler for surface functionality to impart adherence between a first coating layer and a second coating layer after secondary coating for blocking most interlayer current flow on the surface of the non-oriented electrical steel sheet at temperatures of 15O ⁰ C or higher or for realizing complete interlayer insulation of the non-oriented electrical steel sheet, and such products are available from European steelmakers (Cogent, TKS, etc.).

[14] These products are mainly utilized for manufacturing motors for large power generators (water power, fire power, wind power), which are highly valuable and environmentally friendly, and for high-speed trains, and also require corrosion resistance of the surface of a non-oriented electrical steel sheet, adherence to a substrate, and MAG (metal argon gas) weldability, as well as the aforementioned high insulating properties, heat resistance, and secondary coating properties.

[15] In order to minimize current loss (eddy current loss) in proportion to the square of the thickness of a substrate, a thick insulation coating film is formed on both surfaces of the non-oriented electrical steel sheet so that high surface resistivity is realized and the surface is simultaneously imparted with high functionality, which is representatively disclosed in Korean Patent Application Nos. 1998-0056329 and 1998-1193287 submitted by Armco. The insulation coating solution disclosed in these patents is composed of aluminum phosphate, inorganic silicate particles, and acrylic resin containing a water-soluble organic solvent. The size of silicate particles is 0.3-60 μm, and the particle size of acrylic resin in an emulsion type is 1 μm or less. However, these patents are also disadvantageous because metal phosphate is used, and thus, the free phosphoric acid present in phosphate may cause problems related to the stickiness of a coating film and the deposition of free phosphate. Disclosure of Invention Technical Problem

[16] Accordingly, the present invention provides a coating solution for a non-oriented electrical steel sheet, having superior insulating properties, heat resistance, corrosion resistance, adherence, and secondary coating properties, a method of coating a non-

oriented electrical steel sheet using the same, and a coating film of the non-oriented electrical steel sheet. Technical Solution

[17] According to the present invention, a coating solution for a non-oriented electrical steel sheet may comprise 20-40 wt% of barium sulfate (Ba ₂ SO ₄ ), 10-20 wt% of titanium oxide (TiO ₂ ), 15-30 wt% of ionized water, and 30-50 wt% of melamine- based resin containing ethylene glycol and glycerin as high boiling point solvents.

[18] In the present invention, barium sulfate may have a particle size of 1-3 μm and an or- thorhombic shape, and may exhibit dspersibility with the melamine-based resin and solution stability, and titanium oxide may have a spherical shape and a particle size of 50-200 nm.

[19] In addition, a method of coating the non-oriented electrical steel sheet may comprise applying the above coating solution on the steel sheet and then heat treating it at 300~600°C for 10-30 sec.

Advantageous Effects

[20] According to the present invention, a coating solution for a non-oriented electrical steel sheet contains no chromium, and can also exhibit superior solution stability, insulating properties, heat resistance, corrosion resistance, adherence, and secondary coating properties. In particular, in order to ensure high insulating properties and heat resistance of the non-oriented electrical steel sheet, two types of inorganic fillers having different shapes and sizes with high heat resistance, that is, barium sulfate and titanium dioxide, are used. Further, to realize working stability, melamine-based resin, containing ethylene glycol and glycerin as high boiling point solvents and exhibiting superior solution stability, heat resistance, and force of adhesion between a substrate for an electrical steel sheet and a filler, is used. Brief Description of the Drawings

[21] FIGS. 1 to 6 illustrate the superior properties of the coating film of the non-oriented electrical steel sheet according to the present invention. Mode for the Invention

[22] Hereinafter, preferred embodiments of the present invention are described with reference to the appended drawings so that the technical scope of the present invention can be easily understood by one ordinarily skilled in the art. The present invention is not limited to these embodiments, but may be embodied in another form. The embodiments of the present invention are merely set forth so that the disclosure thereof is

thorough and complete and the technical scope of the present invention is sufficiently communicated to one skilled in the art.

[23] The present invention is characterized in that an optimal coating solution for a non- oriented electrical steel sheet comprises 20-40 wt% of barium sulfate (Ba ₂ SO ₄ ) having a particle size of 1-3 μm and an orthorhombic shape, 10-20 wt% of TiO ₂ having a particle size of 50-200 nm and a spherical shape, 15-30 wt% of ionized water, and 30-50 wt% of melamine-based resin containing small amounts of ethylene glycol and glycerin as high boiling point solvents and having high force of adhesion.

[24] The solution composition having the above weight proportions is very stable even without gelation for a long period of time at room temperature, and is applied in the thickness range of 4-8 μm per surface using a coater and is then subjected to heat treatment at 300~600°C for 10-30 sec, thus forming an insulation coating film. Thereby, a coating composition having excellent insulating properties, heat resistance, corrosion resistance, adherence, and secondary coating properties is provided on the surface of the non-oriented electrical steel sheet.

[25] The present inventors have conducted research into the development of a solution able to form an insulation coating film having high functionality in order to improve the surface quality of a high-grade non-oriented electrical steel sheet having high functionality. Based on such research results, the present invention has been devised.

[26] The present invention is described in detail with reference to the following examples.

[27] (Example)

[28] As mentioned above, the basic concept of the present invention is that the coating solution for a non-oriented electrical steel sheet is capable of imparting high functionality to the surface of the electrical steel sheet while eliminating the use of Cr, which is environmentally hazardous, and metal phosphate, which entails problems pertaining to free phosphate deposition. The coating solution of the present invention was prepared through two steps.

[29] In a first step, to evaluate compatibility between an inorganic filler and an emulsion resin, the type of filler and the emulsion resin are determined. The compatibility between the inorganic filler and the emulsion resin was evaluated in a manner such that 50 wt% of an emulsion resin was blended with 50 wt% of an inorganic filler using a high-speed agitator (1000-3000 rpm) for 1 hour, the solution thus obtained was maintained at room temperature for 24 hours, and then the compatibility of the solution was measured depending on the degree of gelation and phase separation of the solution.

[30] The inorganic fillers used in the present invention were barium sulfate (Ba ₂ SO ₄ ), titanium doxide (TiO ₂ ), calcium carbonate (CaCO ₃ ), silicon dioxide (SiO ₂ ), and talc (3MgO.4SiO ₂ .H ₂ O). The shape and basic properties of each inorganic filler are summarized in Table 1 below.

[31] Table 1 [Table 1] [Table ]

[32] As the emulsion resin, ester-based resin, melamine-based resin, epoxy -based resin, and acryl-based resin were used. The basic properties of each resin are shown in Table 2 below.

[33] Table 2 [Table 2] [Table ]

[34] The evaluation results of compatibility of solutions prepared by 20 combinations consisting of the inorganic fillers and the emulsion resins of Tables 1 and 2 are shown

in Table 3 below. The compatibility of the inorganic filler and the emulsion resin is indicated by θ to signify excellent, O good, δ poor, and x very poor.

[35] Table 3 [Table 3] [Table ]

[36] As is apparent from the results of evaluation shown in Table 3, the ester-based resin, the melamine-based resin, and the acryl-based resin exhibited good compatibility with barium sulfate (Ba ₂ SO ₄ ), titanium dioxide (TiO ₂ ), and calcium carbonate (CaCO ₃ ). In particular, the melamine-based resin showed excellent compatibility with barium sulfate (Ba ₂ SO ₄ ), titanium dioxide (TiO ₂ ), and calcium carbonate (CaCO ₃ ).

[37] Based on the results of Table 3, four combinations of inorganic fillers and emulsion resins having excellent solution compatibility were adapted, and solutions were prepared. Table 4 below shows the basic properties of a solution, including ds- persibility and workability, and the basic properties of the surface of the non-oriented electrical steel sheet on which the solution was applied and then dried. Specifically, the solution was prepared using each of four combinations of inorganic fillers and emulsion resins, including barium sulfate and melamine-based resin, titanium dioxide and melamine-based resin, calcium carbonate and melamine-based resin, and titanium dioxide and acryl-based resin, was applied to a thickness of 4-6 μm on a non-oriented electrical steel sheet sample using various bar coaters, and was then dried in an oven at 300~500°C for 20-30 sec, after which the surface state, adherence, and corrosion resistance were evaluated.

[38] Table 4 [Table 4] [Table ]

[39] As is apparent from the results of evaluation of Table 4, the workability and ds- persibility of the solution were excellent when using the combination of barium sulfate and melamine-based resin and the combination of calcium carbonate and melamine- based resin. This is considered to be because the size of the filler, such as barium sulfate or calcium carbonate, is 1-3 μm, which is somewhat larger than 50-100 nm of titanium dioxide, thus facilitating dispersion, and also in addition, aggregation or clogging of the bar coater in the coating process does not occur, resulting in good workability. After the drying process, the surface state, adherence, and corrosion resistance were superior when using the combination of titanium dioxide and melamine-based resin and the combination of titanium dioxide and acryl-based resin compared to when using the combination of barium sulfate and melamine-based resin and calcium carbonate and melamine-based resin. The reason is that the size of the filler is somewhat smaller, and thus the coating film becomes compact, resulting in superior surface properties. From these results, it was confirmed that dispersibility and stability, in terms of the coating solution, and adherence and high insulating properties, in terms of the surface properties, were both realized at the same time when using the solution composed of barium sulfate and melamine-based resin or titanium dioxide and melamine-based resin.

[40] In a second step, in order to ensure excellent surface properties, the optimal composition ratio of the inorganic filler and the resin is set on the basis of the above-

determined inorganic filler and resin. Further, in the present invention, to improve compactability in the coating film, two types of inorganic fillers having different sizes are used. Table 5 below shows the properties of blended solutions using combinations of two types of inorganic fillers having different sizes with the melamine -based resin having high dspersibility and stability, as seen in the results of Table 4. As such, because barium sulfate and calcium carbonate have a similar particle size, the combination of barium sulfate and calcium carbonate is inferior from the point of view of compactability of the coating film. Hence, in the present invention, the combination of barium sulfate and titanium dioxide and the combination of calcium carbonate and titanium dioxide were used. The composition ratio of the melamine-based resin to the total inorganic filler was set at 4:6, and the workability, dspersibility, and surface properties of the electrical steel sheet were measured.

[41] Table 5 [Table 5] [Table ]

[42] Based on the composition shown in Table 5, two types of inorganic fillers having dfferent sizes were dspersed in the melamine-based resin to thus prepare the coating solution, after which the coating solution was applied to a thickness of 4-6 μm on a non-oriented electrical steel sheet sample using a bar coater and then dried in an oven at 300~500°C for 20-30 seς followed by measuring the surface properties thereof. The

results are shown in Table 6 below. As such, the coating thickness was measured using a thickness gauge (delta scope), and corrosion resistance was determined in a manner such that a salt spray test (35 ⁰ C, 5% NaCl, 95% humidity, 8 hr) was conducted and then an iron rust state was observed. Further, the insulating properties of the coating film were measured using a Franklin insulation tester, and adherence was measured by observing whether the internal coating film peeled off when bending the coating sample to 180° at 10 mφ using a bending tester. The secondary coating properties were measured by applying a secondary coating solution on the first coating layer, drying it, and measuring the surface state and adherence of the first coating layer and the second coating layer. The heat resistance was determined by measuring the adherence of the surface and checking whether the surface peeled off after aging (thermal level H 18O ⁰ C, 24 hr).

[43] Table 6 [Table 6] [Table ]

[44] As is apparent from the results of Table 6, the surface properties resulting from the combination of barium sulfate and titanium dioxide exhibited corrosion resistance, adherence, insulating properties, and secondary coating properties superior to those

resulting from the combination of calcium carbonate and titanium doxide. This is considered to be because barium sulfate inorganic filler particles are smaller than calcium carbonate inorganic filler particles, and thus the compactability with titanium doxide is high. Therefore, the two types of inorganic fillers most useful in the present invention are determined to be barium sulfate and titanium doxide. As shown in Table 1, barium sulfate has a particle size of 1-3 μm and an orthorhombic shape, and titanium doxide has a particle size of 50-100 nm with a spherical shape. Referring to the process of dspersing barium sulfate and titanium doxide as two types of inorganic fillers, titanium doxide and barium sulfate were sequentially added to the melamine resin and then stirred using a high-speed agitator at 2000 rpm or more for 1 hour or longer to thus be dspersed.

[45] Using two types of inorganic fillers selected from the above experimental results and melamine-based resin, experiments were conducted to determine the optimal composition of resin and inorganic filler for satisfying both coating workability and surface properties. Table 7 below shows the experimental results of workability and surface properties dependng on the composition of the resin and the filler. In particular, in Table 7, in order to ensure workability suitable for a roll coater, water was added in a predetermined amount to the solution so that the viscosity of the solution was 45-50 cP, after which the resultant solution was applied on the non-oriented electrical steel sheet.

[46] Table 7

[Table 7] [Table ]

[47] As is apparent from the results of table 7, when the mass ratio of the melamine-based resin and the total inorganic filler was in the range of 3:7-5:5, the surface properties were excellent. In the case where the ratio of the resin and the total inorganic filler was less than 3:7, due to an insufficient amount of melamine resin, adherence was poor. Conversely, in the case where the ratio was greater than 5:5, the compactability of the fillers in the coating film was low, undesirably resulting in low corrosion resistance.

[4S] Further, in the case where the mass ratio of barium sulfate and titanium dioxide was greater than 3: 1, the compactability of the coating film composed of two types of inorganic fillers was low, leading to low corrosion resistance. Conversely, at mass ratios less than 1: 1, the aggregation of titanium dioxide powder having a nanoscale particle size occurred, resulting in poor workability. Therefore, workability was excellent when the mass ratio of barium sulfate and titanium dioxide was in the range of 3:1-1:1.

[φ] From the above results, in the present invention, the inorganic fillers (Ba ₂ SO ₄ , TiO ₂ ) were used to ensure heat resistance, and also, two types of inorganic fillers having difference shapes and sizes were used to simultaneously satisfy high insulating properties, adherence, and corrosion resistance. The solution was prepared by uniformly blending the resin and the filler using a high-speed agitator without the use of a dspersant.

[50] FIGS. 1 to 6 are photographs showing the surface, cross-section and surface properties of the non-oriented electrical steel sheet on which the solution prepared at the composition ratio of Example 6 was applied and then dried. FIG. 1 is a scanning electron micrograph showing the surface of the coating film, in which the surface roughness Ra is very low, to the level of about 0.25 μm, and pencil hardness is very high, to the level of 9H. FIG. 2 shows surface adherence. There was no peeling-off after a cross cut test and a tape peel test, and adherence was superior, to the level of 5B. FIG. 3 shows the results of a salt spray test (5% NaOH, 35 ⁰ C, 8 hr), in which rust occurred on part of the surface but corrosion resistance was evaluated to be very good. FIG. 4 shows the second coating layer (varnish) applied on the first coating layer, in which the second coating layer appeared to be superior even at a coating thickness of 11 μm or more. FIG. 5 shows the results of a cross cut test and a tape peel test after secondary coating, in which adherence was very good, to the level of 5B.

[51] FIG. 6 is a photograph showing the coating film 20 applied on the substrate 10. From this, inorganic fillers having different shapes and sizes could be seen to be very compactly distributed in the coating film, and the coating film was 5-6 μm thick.