Treatment solution for chromium-free tension coating, method for forming chromium-free tension coating, and grain oriented electrical steel sheet with chromium-free tension coating

Abstract

Provided is a treatment solution for chromium-free tension coating that can simultaneously achieve excellent moisture absorption resistance and a high iron loss reduction effect obtained by imparting sufficient tension, by using an inexpensive Ti source instead of expensive Ti chelate. The treatment solution for chromium-free tension coating contains: one or more of phosphates of Mg, Ca, Ba, Sr, Zn, Al, and Mn; colloidal silica in an amount of 50 parts by mass to 120 parts by mass per 100 parts by mass of the phosphate in terms of solid content of SiO.sub.2; Ti source in an amount of 30 parts by mass to 50 parts by mass per 100 parts by mass of the phosphate in terms of solid content of TiO.sub.2; and H.sub.3PO.sub.4, and the number of moles of metallic elements in the phosphate and of phosphorus in the treatment solution satisfy:
0.20([Mg]+[Ca]+[Ba]+[Sr]+[Zn]+[Mn]+1.5[Al])/[P]0.45(1).

Claims

1. A method of forming a chromium-free tension coating comprising: applying a treatment solution for chromium-free tension coating on a surface of a grain oriented electrical steel sheet subjected to final annealing; and performing baking treatment at a temperature of 800 C. or higher and 1000 C. or lower for 10 seconds to 300 seconds, wherein the treatment solution for chromium-free tension coating comprises: one or more of a Mg phosphate, Ca phosphate, Ba phosphate, Sr phosphate, Zn phosphate, Al phosphate, and Mn phosphate; colloidal silica in an amount of 50 parts by mass to 120 parts by mass per 100 parts by mass of the one or more phosphates in terms of solid content of SiO.sub.2; Ti source in an amount of 30 parts by mass to 50 parts by mass per 100 parts by mass of the one or more phosphates in terms of solid content of TiO.sub.2; and H.sub.3PO.sub.4, and the number of moles of metallic elements in the one or more phosphates and the number of moles of phosphorus in the treatment solution for chromium-free tension coating satisfy the relation of formula (1)
0.20([Mg]+[Ca]+[Ba]+[Sr]+[Zn]+[Mn]+1.5[Al])/[P]0.45(1) where each symbol of element shown in square brackets represents the number of moles of the element contained in the treatment solution for chromium-free tension coating.

2. The method of forming a chromium-free tension coating according to claim 1, wherein the Ti source contains TiO.sub.2 sol.

3. The method of forming a chromium-free tension coating according to claim 2, wherein the Ti source further contains titanium phosphate in a solid mass ratio of 0.1% to 50% with respect to TiO.sub.2 in the TiO.sub.2 sol.

Description

DETAILED DESCRIPTION

(1) Hereinbelow, reference will be made to the experimental results which served as the basis of the disclosure.

(2) First, samples were produced in the following way.

(3) Grain oriented electrical steel sheets subjected to final annealing with sheet thickness of 0.23 mm which were produced by a conventional method were sheared into a size of 300 mm100 mm to obtain sample pieces. The unreacted annealing separator remaining on the surfaces of the sample pieces were removed and then the sample pieces were subjected to stress relief annealing at 800 C. for 2 hours.
The sample pieces were then subjected to light pickling with 5% phosphoric acid, and then a treatment solution for tension coating was applied on the surfaces of the sample pieces. The treatment solution for tension coating was prepared by the following procedures. First, an aqueous solution of primary magnesium phosphate (Mg(H.sub.2PO.sub.4).sub.2), colloidal silica, and TiO.sub.2 sol were mixed to obtain a mixed solution. The mass ratios of each component in the mixed solution were set to be, in terms of solid content, primary magnesium phosphate: 30 g, colloidal silica: 20 g, and TiO.sub.2 sol: 12 g. Then, an aqueous solution of orthophosphoric acid (H.sub.3PO.sub.4) having a specific gravity of 1.69 with a concentration of 85% was added to the mixed solution in the amounts shown in Table 1 to obtain treatment solutions for tension coating. The ratios of the numbers of moles of Mg.sup.2+ to the numbers of moles of phosphorus (total number of moles of phosphorus derived from both phosphate and phosphoric acid) (P) in the obtained treatment solutions for tension coating i.e. Mg.sup.2+/P were set to be the values shown in Table 1.
The treatment solutions for tension coating were applied on the surfaces of the sample pieces so that the total coating amounts of both surfaces after drying were 10 g/m.sup.2. Then, the sample pieces were charged into the drying furnace and dried at 300 C. for 1 minute, and then subjected to heat treatment at 800 C. for 2 minutes in an atmosphere of N.sub.2: 100% for the purpose of both flattening annealing and baking for tension coating formation. Subsequently, the sample pieces were subjected to the second stress relief annealing at 800 C. for 2 hours.

(4) The iron loss reduction effect obtained by imparting tension and moisture absorption resistance of the samples thus obtained were examined. The iron loss reduction effect was evaluated based on magnetic properties measured using an SST (Single Sheet Test) tester (single sheet magnetism tester). Measurement of magnetic properties was performed for each sample right before applying the treatment solution for tension coating, after baking for tension coating formation, and right after subjecting the samples to the second stress relief annealing.

(5) Moisture absorption resistance was evaluated by performing an elution test of phosphorus. Three sample pieces for using in the elution test were prepared by cutting steel sheets right after baking for tension coating formation into a size of 50 mm50 mm. These sample pieces for the elution test were boiled in distilled water at 100 C. for 5 minutes, and the amounts of phosphorus eluted during the process were measured. Based on the amount of eluted phosphorus, the solubility of tension coating to water can be determined.

(6) Table 1 shows the measurement results of magnetic properties and elution amounts of phosphorus.

(7) The criteria in the table are as follows.

(8) B.sub.8 (R) before application: magnetic flux density right before application of treatment solution for tension coating

(9) B after application=B.sub.8 (C)B.sub.8 (R) where B.sub.8 (C): magnetic flux density right after baking for tension coating formation

(10) B after stress relief annealing=B.sub.8 (A)B.sub.8 (R) where B.sub.8 (A): magnetic flux density right after second stress relief annealing

(11) W.sub.17/50 (R) before application: iron loss right before application of treatment solution for tension coating

(12) W after application=W.sub.17/50 (C)W.sub.17/50 (R) where W.sub.17/50 (C): iron loss right after baking for tension coating formation

(13) W after stress relief annealing=W.sub.17/50 (A)W.sub.17/50 (R) where W.sub.17/50 (A): iron loss right after second stress relief annealing

(14) Elution amount of phosphorus: amount measured right after baking for tension coating formation

(15) Coating appearance: degree of transparency of coating after stress relief annealing determined by visual observation

(16) TABLE-US-00001 TABLE 1 B after additive amount B.sub.8 (R) stress W.sub.17/50 (R) of 85% before B after relief before W after W after stress elution orthophosphoric application application annealing application application relief annealing amount of P coating No. acid (ml) Mg.sup.2+/P (T) (T) (T) (W/kg) (W/kg) (W/kg) (g/150 cm.sup.2) appearence 1 0 0.50 1.910 0.010 0.009 0.832 0.032 0.035 80 clouded 2 1 0.45 0.010 0.009 0.030 0.035 80 transparent 3 5 0.33 0.010 0.009 0.031 0.032 80 transparent

(17) From the experimental results presented in Table 1, it can be seen that by adding phosphoric acid and reducing Mg.sup.2+/P, it is possible to suppress crystallization when adding a large amount of Ti, and both iron loss and moisture absorption resistance can be improved.

(18) Reasons for limitations on the features of the disclosure will be explained below.

(19) The steel types of the steel sheets contemplated herein are not particularly limited as long as they are grain oriented electrical steel sheets. Generally, such grain oriented electrical steel sheets are produced by subjecting silicon-containing steel slabs to hot rolling with a known method to obtain hot rolled steel sheets, subjecting the hot rolled steel sheets to cold rolling once or multiple times with intermediate annealing performed therebetween to obtain cold rolled steel sheets with final sheet thickness, subjecting the cold rolled steel sheets to primary recrystallization annealing, applying an annealing separator thereon, and then subjecting the cold rolled steel sheets to final annealing.

(20) Regarding the insulating coating treatment liquid components, one or more of a Mg phosphate, Ca phosphate, Ba phosphate, Sr phosphate, Zn phosphate, Al phosphate, and Mn phosphate are used as the phosphate.

(21) While it is normal to use one of the above phosphates, two or more of them may be mixed and used to precisely control the property values of the insulating coating. As the phosphate, primary phosphate (biphosphate) is easily available and is therefore preferable. Since phosphates of alkali metal (Li, Na or the like) significantly deteriorate the moisture absorption resistance of the coating, they are unsuitable.

(22) Colloidal silica is contained in the treatment solution in the amount of 50 parts by mass to 120 parts by mass per 100 parts by mass of the above phosphate in terms of solid content of SiO.sub.2. Colloidal silica has an effect of reducing the thermal expansion coefficient of the coating. However, if the content of colloidal silica is less than 50 parts by mass, the effect of reducing the thermal expansion coefficient is limited, and sufficient tension cannot be imparted to the steel sheet. As a result, a sufficient iron loss reduction effect cannot be obtained by forming a tension coating. By contrast, if the content exceeds 120 parts by mass, not only will the coating easily crystallize during baking, but the moisture absorption resistance of the coating will decrease as well.

(23) Further, the treatment solution described herein contains a Ti source in an amount of 30 parts by mass to 50 parts by mass to 100 parts by mass of the above phosphate in terms of TiO.sub.2. If the content of the Ti source is less than 30 parts by mass, the moisture absorption resistance of the coating deteriorates. By contrast, if the content exceeds 50 parts by mass, it becomes difficult to prevent crystallization even if phosphoric acid is added to control M/P.

(24) Further, the treatment solution described herein contains phosphoric acid (H.sub.3PO.sub.4). In the disclosure, it is important that the number of moles of metallic elements in the phosphate and the number of moles of phosphorus contained in the treatment solution satisfy the relation of formula (1).
0.20([Mg]+[Ca]+[Ba]+[Sr]+[Zn]+[Mn]+1.5[Al])/[P]0.45 (1)
Here, each symbol of element shown in square brackets in formula (1) represents the number of moles of the element contained in the treatment solution for chromium-free tension coating. The number of moles of metallic elements which are not added to the treatment solution as phosphate is regarded as zero. The coefficient for [Al] is 1.5 due to the fact that, while metallic elements other than Al are bivalent, Al is trivalent. Hereinafter, the middle part of the above formula i.e. ([Mg]+[Ca]+[Ba]+[Sr]+[Zn]+[Mn]+1.5 [Al])/[P] will be referred to as M/P.
When M/P is less than 0.20, the P in the coating is excessive and therefore the elution amount of phosphorus from the coating increases, and the moisture absorption resistance decreases. On the other hand, if M/P is over 0.45, it is not possible to contain Ti of an amount required to obtain a sufficient moisture absorption resistance without causing crystallization in the coating.

(25) As the Ti source to be contained in the treatment solution for the chromium-free tension coating described herein, TiO.sub.2 sol is preferable in terms of availability, costs and the like. Although the TiO.sub.2 sol may be acidic, neutral or alkaline, pH is preferably 5.5 to 12.5.

(26) Further, it is preferable for the TiO.sub.2 sol to contain titanium phosphate in a solid mass ratio of 0.1% to 50% with respect to TiO.sub.2. By adding titanium phosphate, the dispersibility of TiO.sub.2 particles can be enhanced. Further, titanium phosphate has the effect of enhancing the compatibility between TiO.sub.2 and phosphate and enhancing the stability of the coating liquid. With a titanium phosphate content of less than 0.1%, the effect of enhancing compatibility is poor. On the other hand, titanium phosphate content exceeding 50% leads to an increase in costs. The amount of phosphoric acid in the treatment solution in formula (1) is the total amount of phosphoric acid in the treatment solution and this includes the amount of phosphoric acid added as titanium phosphate.

(27) Further, fine powdery inorganic mineral particles such as silica and alumina can be added to the treatment solution described herein. These inorganic mineral particles are effective for improving sticking resistance of the coating. The content of the inorganic mineral particles is preferably 1 part by mass with respect to 20 parts by mass of colloidal silica at most in order to prevent a decrease in the stacking factor.

(28) The above treatment solution is applied to the surface of the electrical steel sheet and then baked to form tension coating. The total coating amount of both sides of the steel sheet after drying the coating is preferably 4 g/m.sup.2 to 15 g/m.sup.2. This is because if the coating amount is less than 4 g/m.sup.2, the interlaminar resistance decreases, whereas if it is more than 15 g/m.sup.2, the stacking factor decreases. In the examples described herein, coating is formed so that the coating amount is substantially the same on both sides. However, when laminating steel sheets to form an iron core, such steel sheets are normally laminated in a manner that the front side and the back side are in contact with each other. Therefore, it is not necessary for the coating amounts of the front and back sides to be equal and there may be a difference between the coating amounts of the front and back sides.

(29) The baking treatment for tension coating formation may be performed for the purpose of flattening annealing. The baking treatment is performed in a temperature range of 800 C. to 1000 C. for a soaking time of 10 seconds to 300 seconds. If the temperature is too low or the soaking time is too short, the flattening will be insufficient. As a result, shape failure is caused and leads to a decrease in yield. On the other hand, if the temperature is too high, the effect of flattening annealing becomes excessive and therefore causes creep deformation of the steel sheet to deteriorate magnetic properties.

EXAMPLES

Example 1

(30) Grain oriented electrical steel sheets subjected to final annealing with sheet thickness of 0.23 mm were prepared. The magnetic flux density B.sub.8 of the grain oriented electrical steel sheets at this time was 1.912 T. The grain oriented electrical steel sheets were subjected to pickling in phosphate acid and then chromium-free tension coating was formed on the surfaces thereof. For the formation of the tension coating, treatment solutions for chromium-free tension coating of various compositions shown in Table 2 were used. The treatment solutions were applied on both sides of the grain oriented electrical steel sheets so that the total coating amounts of both sides after drying at 300 C. for 1 minute were 10 g/m.sup.2. Then, in an atmosphere of N.sub.2: 100%, baking treatment was performed at 850 C. for 30 seconds. Then, the steel sheets were subjected to stress relief annealing in an atmosphere of N.sub.2: 100% at 800 C. for 2 hours.
As phosphate, primary phosphate solutions were used for each sample. The amounts of the phosphate in terms of solid content are shown in Table 2. As Ti source, TiO.sub.2 sol TKS-203 manufactured by Tayca Corporation was used. As phosphoric acid, an 85% phosphoric acid solution was used.
The results of examining the characteristics of the grain oriented electrical steel sheets thus obtained are shown in Table 3.

(31) The evaluation of each characteristic was performed in the following way.

(32) W.sub.17/50 (R) before application: iron loss right before application of treatment solution for tension coating

(33) W after application=W.sub.17/50 (C)W.sub.17/50 (R) where W.sub.17/50 (C): iron loss right after baking for tension coating formation

(34) W after stress relief annealing=W.sub.17/50 (A)W.sub.17/50 (R) where W.sub.17/50 (A): iron loss right after stress relief annealing

(35) Elution amount of phosphorus: three sample pieces with a size of 50 mm50 mm and a coating surface area of 150 cm.sup.2 were boiled in distilled water at 100 C. for 5 minutes and then examined

(36) Coating appearance: degree of transparency of coating after stress relief annealing determined by visual observation

(37) TABLE-US-00002 TABLE 2 phosphate in terms of solid content (g) magnesium calsium barium strontium zinc aluminum manganese No. phosphate phosphate phosphate phosphate phosphate phosphate phosphate 1 100 2 100 3 70 30 4 80 20 5 100 6 100 7 100 8 100 9 50 50 10 50 50 11 100 12 100 13 100 14 100 15 60 40 16 100 17 100 18 30 70 19 50 50 20 100 21 100 22 100 colloidal silica TiO.sub.2 sol 85% in terms of solid in terms of solid orthophosphoric content of SiO.sub.2 content of TiO.sub.2 acid No. (g) (g) (ml) M/P remarks 1 60 40 0 0.50 comparative example 2 60 40 4 0.44 example 3 60 40 10 0.38 example 4 60 40 20 0.31 example 5 60 40 40 0.22 example 6 60 40 60 0.17 comparative example 7 50 25 10 0.38 comparative example 8 50 30 10 0.38 example 9 50 35 10 0.38 example 10 50 40 10 0.37 example 11 50 50 10 0.38 example 12 50 60 10 0.38 comparative example 13 50 60 40 0.22 comparative example 14 40 40 20 0.31 comparative example 15 100 40 20 0.31 example 16 120 40 20 0.31 example 17 140 40 20 0.31 comparative example 18 50 35 10 0.37 example 19 50 35 10 0.38 example 20 50 35 10 0.34 example 21 50 35 10 0.36 example 22 50 35 10 0.37 example

(38) TABLE-US-00003 TABLE 3 W.sub.17/50 (R) before W after W after stress elution amount of application application relief annealing phosphorous No. (W/kg) (W/kg) (W/kg) (g/150 cm.sup.2) coating appearance remarks 1 0.840 0.029 0.001 80 clouded (crystalized) comparative example 2 0.031 0.029 82 transparent example 3 0.032 0.030 85 transparent example 4 0.029 0.026 85 transparent example 5 0.033 0.031 87 transparent example 6 0.031 0.031 500 transparent comparative example 7 0.034 0.033 350 transparent comparative example 8 0.028 0.028 68 transparent example 9 0.028 0.027 75 transparent example 10 0.035 0.033 58 transparent example 11 0.012 0.010 63 transparent example 12 0.035 0.002 60 clouded (crystalized) comparative example 13 0.038 0.002 52 clouded (crystalized) comparative example 14 0.001 0.000 56 transparent comparative example 15 0.035 0.035 60 transparent example 16 0.018 0.032 70 transparent example 17 0.005 0.000 80 clouded (crystalized) comparative example 18 0.033 0.029 70 transparent example 19 0.033 0.030 65 transparent example 20 0.028 0.030 75 transparent example 21 0.028 0.032 73 transparent example 22 0.032 0.029 76 transparent example

(39) As shown in Tables 2 and 3, by using the treatment solutions satisfying the conditions of the disclosure, chromium-free tension insulating coating, a small elution amount of phosphorus and excellent moisture absorption resistance and good appearance could be obtained.

Example 2

(40) Grain oriented electrical steel sheets subjected to final annealing with sheet thickness of 0.23 mm were prepared. The magnetic flux density B.sub.8 of the grain oriented electrical steel sheets at this time was 1.912 T. The grain oriented electrical steel sheets were subjected to pickling in phosphate acid and then chromium-free tension coating was formed on the surfaces thereof. For the formation of the tension coating, treatment solutions containing 100 g of primary magnesium phosphate in terms of solid content as phosphate with the other components being various compositions shown in Table 4 were used. The treatment solutions were applied on the surfaces of the grain oriented steel sheets so that the total coating amount of both sides after drying at 300 C. for 1 minute were 15 g/m.sup.2. Then, in an atmosphere of N.sub.2: 100%, baking treatment was performed at 950 C. for 10 seconds. Then, the steel sheets were subjected to stress relief annealing in an atmosphere of N.sub.2: 100% at 800 C. for 2 hours.
The results of examining the characteristics of the grain oriented electrical steel sheets thus obtained are shown in Table 5.
The evaluation of each characteristic was conducted with the same method as example 1.

(41) TABLE-US-00004 TABLE 4 colloidal silica in terms of 85% solid orthophosphoric Ti source and additive amount thereof in terms of TiO.sub.2 (g) content of acid No. Ti(OH).sub.4 TiOCl.sub.2 Ti.sub.2(SO.sub.4).sub.3 TiSO.sub.4 [(OH).sub.2Ti(C.sub.3H.sub.5O.sub.3)].sup.2(NH.sub.4.sup.+).sub.2 TiPO.sub.4 SiO.sub.2 (g) (ml) M/P remarks 1 20 80 4 0.44 comparative example 2 40 80 10 0.38 example 3 50 80 10 0.38 example 4 60 80 10 0.38 comparative example 5 30 60 10 0.38 example 6 30 60 10 0.38 example 7 10 50 10 0.38 comparative example 8 30 50 10 0.38 example 9 5 50 10 0.38 comparative example 10 30 50 10 0.38 example 11 30 30 50 10 0.34 example

(42) TABLE-US-00005 TABLE 5 W.sub.17/50 (R) before W after W after stress application application relief annealing elution amount of P No. (W/kg) (W/kg) (W/kg) (g/150 cm.sup.2) coating appearance remarks 1 0.840 0.024 0.025 250 transparent comparative example 2 0.031 0.029 82 transparent example 3 0.028 0.029 85 transparent example 4 0.002 0.000 78 clouded (crystalized) comparative example 5 0.024 0.031 87 transparent example 6 0.031 0.031 83 transparent example 7 0.031 0.030 520 transparent comparative example 8 0.026 0.028 68 transparent example 9 0.028 0.028 690 transparent comparative example 10 0.029 0.028 58 transparent example 11 0.030 0.030 61 transparent example

(43) As shown in Tables 4 and 5, by using the treatment solutions satisfying the conditions of the disclosure, chromium-free tension insulating coating with a small elution amount of phosphorus, excellent moisture absorption resistance and good appearance could be obtained.

INDUSTRIAL APPLICABILITY

(44) According to the disclosure, it is possible to prevent crystallization of the coating which occurs when adding Ti for the purposes of improving moisture absorption resistance of the chromium-free tension coating. As a result, it is possible to avoid the adverse effect of the reduction in the tension imparted to the steel sheet and add a sufficient amount of Ti. Therefore, by using the treatment solution described herein, chromium-free tension coating with excellent moisture absorption resistance and iron loss improving effect can be obtained.

(45) Further, by coating the above chromium-free tension coating, grain oriented electrical steel sheets with excellent moisture absorption resistance and low iron loss can be obtained.