GRAIN-ORIENTED ELECTRICAL STEEL SHEET, INTERMEDIATE STEEL SHEET FOR GRAIN-ORIENTED ELECTRICAL STEEL SHEET, AND METHODS FOR MANUFACTURING SAME

Abstract

This grain-oriented electrical steel sheet includes a base steel sheet, an intermediate layer that is formed on a surface of the base steel sheet and mainly contains silicon oxide, and an insulation coating that is formed on the surface of the intermediate layer. A number density of the oxide particles in a region from the surface of the base steel sheet to a depth of 10 μm toward an inside of the base steel sheet is 0.020 oxide particles/μm.sup.2 or less. In the grain-oriented electrical steel sheet, an area rate of an intermediate layer-remaining region n which the intermediate layer does not peel off but remains in a region in which the insulation coating peels off after a bend test performed using a mandrel according to JIS K 5600-5-1 (1999) is 20% or more.

Claims

1. A grain-oriented electrical steel sheet comprising: a base steel sheet; an intermediate layer that is formed on a surface of the base steel sheet and mainly contains silicon oxide; and an insulation coating that is formed on a surface of the intermediate layer, wherein a number density of oxide particles in a region from the surface of the base steel sheet to a depth of 10 μm toward an inside of the base steel sheet is 0.020 oxide particles/μm.sup.2 or less, and an area rate of an intermediate layer-remaining region in which the intermediate layer does not peel off but remains in a region in which the insulation coating peels off after a bend test performed using a mandrel according to JIS K 5600-5-1 (1999) is 20% or more.

2. A method for manufacturing a grain-oriented electrical steel sheet comprising: a hot rolling process of heating a slab at 1280° C. or lower and then performing hot rolling to manufacture a hot-rolled steel sheet; a hot-band annealing process of performing hot band annealing on the hot-rolled steel sheet to manufacture an annealed steel sheet; a cold rolling process of performing cold rolling on the annealed steel sheet to manufacture a cold-rolled steel sheet; a decarburization annealing process of performing decarburization annealing on the cold-rolled steel sheet to manufacture a base steel sheet; an annealing separator applying process of applying an annealing separator having a composition containing 50 mass % or more of alumina and, as a remainder, 0 to 50 mass % of magnesia to the base steel sheet; a final annealing process of performing final annealing on the base steel sheet after the annealing separator applying process; a cooling process of cooling the base steel sheet after the final annealing process in an atmosphere having an oxidation degree P.sub.H2O/P.sub.H2, which is a ratio of a water vapor partial pressure to a hydrogen partial pressure, within a temperature range of 1100° C. to 500° C. set to 0.30 to 100000; an intermediate layer forming process of performing, a heat treatment on the base steel sheet after the cooling process to form an intermediate layer mainly containing silicon oxide on a surface of the base steel sheet; and an insulation coating forming process of forming an insulation coating on a surface of the intermediate layer after the intermediate layer forming process.

3. An intermediate steel sheet for a grain-oriented electrical steel sheet comprising: a base steel sheet; and a film-shaped oxide formed on a surface of the base steel sheet, wherein the film-shaped oxide is present so as to cover the surface of the base steel sheet in a film shape, and a number density of the oxide particles in a region from the surface of the base steel sheet to a depth of 10 μm toward an inside of the base steel sheet is 0.020 oxide particles/μm.sup.2 or less.

4. A method for manufacturing an intermediate steel sheet for a grain-oriented electrical steel sheet comprising: a hot rolling process of heating a slab at 1280° C. or lower and then performing hot rolling to manufacture a hot-rolled steel sheet; a hot-band annealing process of performing hot band annealing on the hot-rolled steel sheet to manufacture an annealed steel sheet; a cold rolling process of performing cold rolling on the annealed steel sheet to manufacture a cold-rolled steel sheet; a decarburization annealing process of performing decarburization annealing on the cold-rolled steel sheet to manufacture a base steel sheet; an annealing separator applying process of applying an annealing separator having a composition containing 50 mass % or more of alumina and, as a remainder, 0 to 50 mass % of magnesia to the base steel sheet; a final annealing process of performing final annealing on the base steel sheet after the annealing separator applying process; and a cooling process of cooling the base steel sheet after the final annealing process in an atmosphere having an oxidation degree P.sub.H2O/P.sub.H2, which is a ratio of a water vapor partial pressure to a hydrogen partial pressure, within a temperature range of 1100° C. to 500° C. set to 0.30 to 100000.

Description

EXAMPLES

[0245] Hereinafter, the present invention will be specifically described by proposing examples. Hereinafter, conditions in the examples are simply examples of conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to these examples of the conditions. The present invention is capable of adopting a variety of conditions within the scope of the gist of the present invention as long as the object of the present invention is achieved.

[0246] Slabs having a chemical com position containing Si: 3.30%, C: 0.050%, acid-soluble Al: 0.030%, N: 0.0080%, Mn: 0.10%, and S and Se: 0.005% in total with a remainder made up of Fe and an impurity were prepared.

[0247] The slabs were heated for soaking at 1150° C. for 60 minutes, and hot rolling was performed on the heated slabs, thereby manufacturing hot-rolled steel sheets having a sheet thickness of 2.6 mm. Hot band annealing was performed on the manufactured hot-rolled steel sheets, thereby manufacturing annealed steel sheets. As the conditions of the hot band annealing, the hot-rolled steel sheets were held at an annealing temperature of 900° C. for 120 seconds. Cold rolling was performed on the obtained annealed steel sheets, thereby manufacturing cold-rolled steel sheets having a final sheet thickness of 0.23 mm.

[0248] Decarburization annealing was performed on the obtained cold-rolled steel sheets. As the conditions of the decarburization annealing, the cold-rolled steel sheets were held in a wet atmosphere containing 75 vol % of hydrogen with a remainder made up of nitrogen and an impurity at 850° C. for 90 seconds.

[0249] An annealing separator containing MgO was applied to the surface of each of the obtained steel sheets at a proportion shown in Table 1. In the annealing separator, the remainder other than MgO was Al.sub.2O.sub.3.

[0250] Final annealing was performed on the steel sheets to which the annealing separator had been applied and dried, and cooling was performed, thereby obtaining base steel sheets. As the conditions of the final annealing, the steel sheets were heated up to 1200° C. at a temperature rise rate of 15 ° C./hour in a hydrogen-nitrogen mixed atmosphere and then held at 1200° C. for 20 hours in a hydrogen atmosphere. The heater in batch annealing was stopped, and the final-annealed base steel sheets were cooled as they were. The oxidation degrees (P.sub.H2O/P.sub.H2) represented by the ratio of the water vapor partial pressure to the hydrogen partial pressure within a temperature range in which the base steel sheets reached 1100° C. to 500° C. ere as shown in Table 1.

[0251] In addition, all of the chemical compositions of the final-annealed base steel sheets contained Si: 3.30%, C: 0.002% or less, acid-soluble Al: 0.0030% or less, N: 0.0020% or less, Mn: 0.10%, and S and Se: 0.0005% or less in total, and the remainder was made up of Fe and an impurity.

TABLE-US-00001 TABLE 1 MgO content Oxidation Number density Insulation in annealing degree in of internal Bending Evaluation coating Test separator cooling process oxide particles diameter area peeling area No. (mass %) (P.sub.H20/P.sub.H2) (oxide particles/μm.sup.2) (mm) (mm.sup.2) (mm.sup.2) 1 30 0.20 0.003 16 25.1 12.5 2 30 0.30 0.006 16 25.1 3.4 3 30 0.10 0.019 16 25.1 6.7 4 30 10000 0.010 16 25.1 2.9 5 40 0.0001 0.012 16 25.1 10.1 6 40 0.0001 0.012 10 15.7 15.0 7 40 50000 0.004 16 25.1 1.8 8 40 50000 0.004 10 15.7 7.1 9 40 200000 0.512 16 25.1 7.1 10 95 0.30 0.048 16 25.1 6.6 11 40 0.01 0.012 16 25.1 13.5 12 40 100 0.004 16 25.1 2.5 13 40 0.001 0.001 16 25.1 9.2 14 40 100 0.008 16 25.1 3.1 15 40 5000 0.006 10 15.7 4.1 16 40 150000 0.497 16 25.1 5.6 17 40 0.25 0.002 16 25.1 4.2 Intermediate layer Insulation coating Peeling residual rate of insulation peeling area after area due Iron Test coating-peeling portion water immersion to water loss No. (%) (mm.sup.2) (mm.sup.2) (W/kg) Note 1 10 21.5 9.0 0.82 Comparative Example 2 30 4.0 0.6 0.85 Invention Example 3 8 15.9 9.2 0.83 Comparative Example 4 54 3.6 0.7 0.87 Invention Example 5 12 18.1 8.0 0.81 Comparative Example 6 3 15.7 0.7 0.81 Comparative Example 7 66 1.9 0.1 0.81 Invention Example 8 38 8.3 1.2 0.81 Invention Example 9 15 19.3 12.2 1.02 Comparative Example 10 30 8.9 2.3 1.00 Comparative Example 11 3 25.1 11.6 0.81 Comparative Example 12 45 7.4 4.9 0.81 Invention Example 13 10 18.9 9.7 0.79 Comparative Example 14 46 4.2 1.1 0.80 Invention Example 15 61 5.5 1.4 0.83 Invention Example 16 17 16.1 10.5 1.12 Comparative Example 17 14 17.1 12.9 0.87 Comparative Example

[0252] In Test No. 1 to Test No. 12, a heat treatment for forming an intermediate layer and an insulation coating at the same time was performed on the final-annealed base steel sheet.

[0253] The conditions of an intermediate layer and insulation coating forming process were as described below.

[0254] A coating solution was applied to the surface of the steel sheet. The composition of the coating solution was, by mass %, a phosphate: 50%, colloidal silica: 45%, and chromic anhydride: 5% in Test No. 1 to Test No. 10. The composition of the coating solution in Test No. 11 and Test No. 12 was, by mass %, a phosphate: 55% and colloidal silica: 45%. The steel sheet to which the coating solution had been applied was heated up to 850° C. and held for 30 seconds in an atmosphere containing hydrogen, nitrogen, water vapor, and an impurity and having an oxidation degree (P.sub.H2O/P.sub.H2) of 0.1.

[0255] In Test No. 13 to Test No. 17, an intermediate layer forming process and an insulation coating forming process were separately performed. A heat treatment was performed on the final-annealed base steel sheet, thereby forming an intermediate layer. The conditions of the intermediate layer forming process were as described below. The final-annealed steel sheet was heated up to 850° C. and held for 30 seconds in an atmosphere having an oxidation degree (P.sub.H2O/P.sub.H2) of 0.01.

[0256] In addition, an insulation coating was formed on the base steel sheet on which the intermediate layer had been formed. In the insulation coating forming process, a coating solution was applied to the surface of the intermediate layer. The composition of the coating solution was a phosphate: 60% and colloidal silica: 40%. The steel sheet to which the coating solution had been applied was heated up to 850° C. and held for 30 seconds in an atmosphere containing 75 vol % of hydrogen with a remainder made up of nitrogen and an impurity to form an insulation coating and was cooled to room temperature.

Cross Section Observation

[0257] A test piece having a cross section perpendicular to a rolling direction was collected from the grain-oriented electrical steel sheet of each test number, and the cross section was observed with a scanning electron microscope (SEM). A region at a depth of 10 μm from the surface of the steel sheet was observed at a magnification of 10000 times in a range of 100 μm in a direction parallel to the surface of the steel sheet. In Test No. 1 to Test No. 8, Test No. 11 to Test No. 15, and Test No. 17, an internal oxide was rarely formed. That is, the number density of oxide particles having a circle-equivalent diameter of 0.1 μm or more in a region from the surface of the base steel sheet to a depth of 10 μm toward the inside of the base steel sheet was 0.020 oxide particles/μm.sup.2 or less.

[0258] On the other hand, in Test No. 9 and Test No. 16, a large amount of an internal oxide mainly containing silicon oxide that was to form unevenness on the surface of the base steel sheet was formed. That is, the number density of oxide particles having a circle-equivalent diameter of 0.1 μm or more in region from the surface of the base steel sheet to a depth of 10 μm toward the inside of the base steel sheet was more than 0.020 oxide particles/μm.sup.2. In Test No. 10, a large amount of an internal oxide mainly containing forsterite that was to form unevenness on the surface of the base steel sheet and having a circle-equivalent diameter of 0.1 μm or more was formed.

[0259] Regarding the grain-oriented electrical steel sheet of each test number, it was also confirmed from an electron beam diffraction pattern and an energy dispersive X-ray analysis (EDX) in the cross section observation with a transmission electron microscope (TEM) that the composition of the intermediate layer was a Fe content of less than 30 atom %, a P content of less than 5 atom %, a Si content of less than 50 atom % and 20 atom % or more, an O content of less than 80 atom % and 50 atom % or more, and a Mg content of 10 atom % or less.

Adhesion Test

[0260] An adhesion test was performed according to the bend resistance test of JIS K 5600-5-1 (1999). A test piece that was 80 mm long in the rolling direction and 40 mm long in a direction perpendicular to the rolling direction was collected from the grain-oriented electrical steel sheet of each of Test No. 1 to Test No. 17. The collected test piece was coiled around a mandrel having a diameter of 10 mm or 16 mm. In the adhesion test, the test piece was bent 180° using a type 1 testing device described in the bend resistance test of JIS K 5600-5-1 (1999). The total area of portions in which the insulation coating peeled off on the inner side surface of the bent test piece (insulation coating-peeling area) was measured.

[0261] After that, the area rate of the intermediate layer-remaining region was obtained by the above-described method. The results are shown in Table 1. The bending diameter in Table 1 indicates the diameter of the mandrel.

[0262] In a case where the diameter of the coiled mandrel was 10 mm, the adhesion was determined as excellent when the insulation coating-peeling area was 7.5 mm.sup.2 or less. In addition, in a case where the diameter of the coiled mandrel was 16 mm, the adhesion was determined as excellent when the insulation coating-peeling area was 5.0 mm.sup.2 or less.

[0263] The evaluation area of insulation coating peeling, in the bend test was defined by the following expression. In a case where the insulation coating peeling area was less than 5% of the evaluation area, the insulation coating peeling area was re-evaluated by decreasing the bending diameter (the diameter of the mandrel). As a result of the re-evaluation, in a case where the insulation coating peeling area was 5% or more of the evaluation area, the area rate of the intermediate layer-remaining region was obtained.

(Evaluation area)=(bending diameter)×(ratio of circumference to diameter)÷2

[0264] Regarding the area rate of the intermediate layer-remaining region, a specified insulation coating-peeling region was mapped using an energy dispersive X-ray spectroscope (SEM-EDS), a Si concentration distribution was obtained, in the obtained Si concentration distribution, the maximum value of the Si concentration and the minimum value of the Si concentration were specified, and a region that satisfied the following expression was defined as the intermediate layer-remaining region.

(Si concentration of region)>{(maximum value of Si concentration)+(minimum value of Si concentration)}/2

[0265] In addition, the proportion of the total area of the defined intermediate layer-remaining region in the EDS mapping total area of the coating peeling portion was defined as the area rate (%) of the intermediate layer-remaining region. In a case where the area rate of the intermediate layer-remaining region was 20% or more, the adhesion was regarded as satisfying the requirement regulated by the present invention and determined as pass. On the other hand, in a case where the area rate of the intermediate layer-remaining region was less than 20%, the adhesion as regarded as not satisfying the requirement regulated by the present invention and determined as fail.

[0266] In a case here the maximum value of the Si concentration and the minimum value of the Si concentration satisfied the following expression, the area rate of the intermediate layer-remaining region was regarded as 0%.

(Maximum value of Si concentration)−(minimum value of Si concentration)<5 atom %

Water Resistance Test

[0267] A bend test was performed on the test pieces of Test No. 1 to Test No. 17 under the same conditions as in the adhesion test. While the bent portions (bending regions) of the test piece remained bent, the entire bending regions were immersed in pure water for one minute. After one minute elapsed, the test pieces were lifted. After that, the test pieces were dried. The test pieces were bent back, and the insulation coating peeling areas after the water immersion were calculated by image analysis. The peeling areas due to water were calculated by the following expression. The results are shown in Table 1.

(Peeling area due to water)=(insulation coating peeling area after water immersion)−(insulation coating peeling area)

[0268] When the peeling area, due to water was 5.0 mm.sup.2 or less, the water resistance was determined as excellent. On the other hand, when the peeling area due to water was more than 5.0 mm.sup.2, the water resistance was determined as poor.

Measurement of Iron Losses

[0269] Regarding iron losses, the iron loss W17/50 (W/kg) at an excited magnetic flux density of 1.7 T and a frequency of 50 Hz was measured by the Epstein test based on JIS C 2550-1. In a case where the iron loss W17/50 was less than 1.00, the iron loss was determined as favorable. On the other hand, in a case where the iron loss W17/50 was 1.00 or more, the iron loss was determined as poor.

[0270] With reference to Table 1, in Test No. 2, Test No. 4, Test No. 7, Test No. 8, Test No. 12, Test No. 14, and Test No. 15, the insulation coating peeling areas were small, in addition, the area rates of the intermediate layer-remaining regions reached 20% or more, and the adhesion and the water resistance were excellent. Particularly, in Test No. 2, Test No. 4, Test No. 7, Test No. 8, Test No. 14, and Test No. 15, the peeling areas due to water became smaller than the insulation coating peeling areas, and the water resistance was superior. Furthermore, in these invention examples, the number densities of the internal oxide particles having a circle-equivalent diameter of 0.1 μm or more were 0.020 oxide particles/μm.sup.2 or less, and the iron losses were favorable. In addition, in these invention examples, the arithmetic average roughness Ra of the surfaces of the base steel sheets was 1.0 μm or less, and the thicknesses of the intermediate layers were 2 to 400 nm. The arithmetic average roughness Ra and the thicknesses of the intermediate layers were measured by the above-described method.

[0271] On the other hand, in Test No. 1, Test No. 3, Test No. 5, Test No. 6, Test No 11, and Test Nos. 13 and 17, the oxidation degrees in the cooling process were less than 0.30. Therefore, the area rates of the intermediate layer-remaining regions were less than 20%, and the adhesion was poor. In Test No. 1, Test No. 3, Test No. 5, Test No. 11, Test No. 13, and Test No. 17, the peeling areas due to water were more than 5.0 mm.sup.2, and the water resistance was poor.

[0272] In Test No. 9 and Test No. 16, the oxidation degrees in the cooling process after the final annealing exceeded 100000. Therefore, an internal oxide mainly containing silicon oxide was formed, and the number densities of the internal oxide particles exceeded 0.020 oxide particles/μm.sup.2. Therefore, it was not possible to obtain a low iron loss, which is necessary for grain-oriented electrical steel sheets.

[0273] In Test No. 10, the MgO content in the annealing separator was high. Therefore, an internal oxide mainly containing forsterite was formed, and the number densities of the internal oxide particles having a circle-equivalent diameter of 0.1 μm or more exceeded 0.020 oxide particles/μm.sup.2. Therefore, it was not possible to obtain a low iron loss, which is necessary for grain-oriented electrical steel sheets.

[0274] In Test No. 1, Test No. 3, Test No. 5, Test No. 6, Test No. 9, Test No. 10, Test No. 11, Test No. 13, and Test No. 16, the peeling areas of the insulation coatings were also large.

[0275] Hitherto, the embodiment of the present invention has been described. However, the above-described embodiment is simply an exemplary example for performing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and performed within the scope of the gist of the present invention.

INDUSTRIAL APPLICABILITY

[0276] According to the present invention, it is possible to provide a grain-oriented electrical steel sheet having an intermediate layer mainly containing silicon oxide in which the adhesion of an insulation coating and the water resistance are favorable. In addition, it is possible to provide an intermediate steel sheet for a grain-oriented electrical steel sheet and a method for manufacturing the same.