Grain-oriented electrical steel sheet

Abstract

Disclosed is a grain-oriented electrical steel sheet that has excellent high-frequency iron loss properties and blanking workability. The steel sheet includes: steel components including, by mass %, Si: 1.5-8.0%, Mn: 0.02-1.0%, and at least one selected from Sn: 0.010-0.400%, Sb: 0.010-0.400%, Mo: 0.010-0.200%, and P: 0.010-0.200%; and crystal grains including coarse secondary recrystallized grains having an average grain size of 5 mm or more and fine grains having a grain size of 0.1-2.0 mm, wherein at least some of the coarse secondary recrystallized grains penetrate the steel sheet in a thickness direction and are respectively exposed on front and back surfaces of the steel sheet such that projection planes of the exposed surfaces of these coarse secondary recrystallized grains on the front and back surfaces form an overlapping region, and the fine grains are present at a number density per unit area of 0.6-40 pieces/cm.sup.2.

Claims

1. A grain-oriented electrical steel sheet, comprising: a chemical composition containing, by mass %, Si: 1.5 to 8.0, Mn: 0.02 to 1.0, and at least one selected from the group consisting of Sn: 0.010 to 0.400, Sb: 0.010 to 0.400, Mo: 0.010 to 0.200, and P: 0.010 to 0.200, with the balance being Fe and inevitable impurities; and crystal grains including coarse secondary recrystallized grains having an average grain size of 5 mm or more and fine grains having a grain size of 0.1 mm to 2.0 mm, wherein at least some of the coarse secondary recrystallized grains penetrate the steel sheet in a thickness direction and are respectively exposed on front and back surfaces of the steel sheet such that projection planes of the exposed surfaces of these coarse secondary recrystallized grains on the front and back surfaces form an overlapping region by overlapping at least partially with each other, wherein an area ratio of an area of the overlapping region to an average area of the exposed surfaces is 80% or more, wherein the fine grains are present at a number density per unit area of 8.1 pieces/cm′ to 40 pieces/cm.sup.2, and wherein the grain-oriented electrical steel sheet has a forsterite film thereon.

2. The grain-oriented electrical steel sheet according to claim 1, wherein the fine grains have crystal orientations such that an average misorientation angle between the crystal orientations and Goss orientation is 15° or more.

3. The grain-oriented electrical steel sheet according to claim 2, wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cr: 0.01 to 0.50, Cu: 0.01 to 0.50, Ni: 0.01 to 0.50, Bi: 0.005 to 0.50, B: 2 ppm to 25 ppm, and Nb: 10 ppm to 100 ppm.

4. The grain-oriented electrical steel sheet according to claim 1, wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cr: 0.01 to 0.50, Cu: 0.01 to 0.50, Ni: 0.01 to 0.50, Bi: 0.005 to 0.50, B: 2 ppm to 25 ppm, and Nb: 10 ppm to 100 ppm.

5. The grain-oriented electrical steel sheet according to claim 1, wherein the fine grains are present at a number density per unit area of 10.5 pieces/cm.sup.2 to 40 pieces/cm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 is a schematic cross-sectional view for explaining crystal grains in a grain-oriented electrical steel sheet;

(3) FIG. 2 is a diagram illustrating the relation between the heating rate and the high frequency iron loss in secondary recrystallization annealing;

(4) FIG. 3 is a diagram illustrating the relation between the number of fine grains in a product sheet and the high-frequency iron loss; and

(5) FIG. 4 is a diagram illustrating the relation between the holding time and the blanking workability in the high temperature range in secondary recrystallization annealing.

DETAILED DESCRIPTION

(6) The present disclosure will be described in detail hereinafter. First, the reasons for limiting the chemical composition of the steel sheet to the aforementioned ranges according to the disclosure are explained.

(7) Si: 1.5 Mass % to 8.0 Mass %

(8) Si is an element necessary for increasing the specific resistance of the steel and reducing iron loss. However, a content below 1.5 mass % has no addition effect, while a content above 8.0 mass % deteriorates the processability of the steel, making rolling difficult. Therefore, the content is set in a range of 1.5 mass % to 8.0 mass %. The content is desirably in a range of 2.5 mass % to 4.5 mass %. Alternatively, the upper and lower limits may be placed independently on the content such that the lower limit is set at 2.99 mass % and the upper limit at 3.81 mass % independently from the lower limit.

(9) Mn: 0.02 Mass % to 1.0 Mass %

(10) Mn is an element necessary for improving hot workability. However, a content below 0.02 mass % has no addition effect, while a content above 1.0 mass % decreases the magnetic flux density of the product sheet. Therefore, the content is set in a range of 0.02 mass % to 1.0 mass %. The content is desirably is in a range of 0.04 mass % to 0.20 mass %. Alternatively, the upper and lower limits may be placed independently on the content such that the lower limit is set at 0.06 mass % and the upper limit at 0.52 mass % independently from the lower limit.

(11) In addition, since C, Al, N, S, and Se may impair the magnetic properties, the contents of these elements are desirably lowered to an inevitable impurity level. For example, it is preferable that the contents of these elements are at a level of 50 mass ppm or less.

(12) To reduce the high-frequency iron loss, it is essential to contain at least one segregation element selected from the group consisting of Sn: 0.010 mass % to 0.400 mass %, Sb: 0.010 mass % to 0.400 mass %, Mo: 0.010 mass % to 0.200 mass %, and P: 0.010 mass % to 0.200 mass %. If the content of each added element is below the corresponding lower limit, there is no magnetic property improving effect, while if it exceeds the corresponding upper limit, the steel is embrittled, and the risk of occurrence of fracture or the like during manufacture increases. Desirable contents are Sn: 0.020 mass % to 0.100 mass %, Sb: 0.020 mass % to 0.100 mass %, Mo: 0.020 mass % to 0.070 mass %, and P: 0.012 mass % to 0.100 mass %.

(13) Further, for Sn, the upper and lower limits may be placed independently on the content such that the lower limit is set at 0.030 mass % and the upper limit at 0.250 mass % independently from the lower limit. For Sb, the upper and lower limits may be placed independently on the content such that the lower limit is set at 0.070 mass % and the upper limit at 0.360 mass % independently from the lower limit. For Mo, the upper and lower limits may be placed independently on the content such that the lower limit is set at 0.020 mass % and the upper limit at 0.440 mass % independently from the lower limit. For P, the upper and lower limits may be placed independently on the content such that the lower limit is set at 0.020 mass % and the upper limit at 0.160 mass % independently from the lower limit.

(14) Although the basic components of the present disclosure have been described, it is possible to further contain the following elements as appropriate in the present disclosure.

(15) At least one selected from the group consisting of Cr: 0.01 mass % to 0.50 mass %, Cu: 0.01 mass % to 0.50 mass %, Ni: 0.01 mass % to 0.50 mass %, Bi: 0.005 mass % to 0.50 mass %, B: 2 ppm to 25 ppm, and Nb: 10 ppm to 100 ppm

(16) Any of these elements may be added for the purpose of improving the magnetic properties. However, if the content of each added element is below the corresponding lower limit, there is no magnetic property improving effect, while it exceeds the corresponding upper limit, development of secondary recrystallized grains is suppressed, causing the magnetic properties to deteriorate.

(17) Referring back to FIG. 1 with respect to the crystal grains of the product sheet, it is essential for the above reasons that the crystal grain P includes coarse secondary recrystallized grains P1 having an average grain size of 5 mm or more and fine grains P2 having a grain size ranging from 0.1 mm to 2.0 mm. It is also essential for the above reasons that at least some of the coarse secondary recrystallized grains P1 penetrate a grain-oriented steel sheet 10 (hereinafter referred to as the steel sheet 10) in the direction of a thickness t and are exposed respectively on the front and back surfaces of the steel sheet 10, that projection planes of the exposed surfaces of these coarse secondary recrystallized grains P1 on the front and back surfaces of the steel sheet 10 form an overlapping region by overlapping at least partially with each other such that an area ratio of (S.sub.0/an average area) is 80% or more, where S.sub.0 denotes an area of the overlapping region, and the average area is calculated by averaging the areas of the exposed surfaces S.sub.1 and S.sub.2, and that fine grains P2 are present at a number density per unit area of 0.6 pieces/cm.sup.2 to 40 pieces/cm.sup.2. Note that the upper limit of the area ratio is theoretically 100%.

(18) Next, a method of manufacturing a steel sheet according to the present disclosure will be described.

(19) As the manufacturing method, a common method of manufacturing an electrical steel sheet can be used.

(20) That is, molten steel prepared to have the predetermined components may be made into a slab by typical ingot casting or continuous casting, or made into a thin slab or thinner cast steel with a thickness of 100 mm or less by direct casting.

(21) Since the above-mentioned components to be desirably added to steel are difficult to add during intermediate process steps, they are desirably added to the molten steel.

(22) Although a slab is hot rolled while being heated in a normal way to obtain a hot-rolled sheet, a chemical composition without inhibitors does not require high-temperature annealing for dissolving the inhibitors, and it is thus essential for cost-reduction purposes to perform hot rolling at temperatures as low as 1300° C. or lower, and desirably as low as 1250° C. or lower.

(23) Then, the hot-rolled sheet is optionally subjected to hot-rolled sheet annealing. The temperature for hot-rolled sheet annealing is preferably about 950° C. to 1150° C. If the temperature is lower than this range, non-recrystallized portions remain, whereas if the temperature is higher than this range, crystal grains excessively coarsen after the annealing, making the subsequently-obtained primary recrystallized texture inappropriate. The temperature is preferably 1000° C. or higher and 1100° C. or lower.

(24) The steel sheet after subjection to the hot rolling or hot-rolled sheet annealing is subjected to cold rolling once, or twice or more with intermediate annealing performed therebetween, to obtain a cold-rolled sheet having a final sheet thickness. The annealing temperature for intermediate annealing is preferably in a range of 900° C. to 1200° C. At temperatures below 900° C., finer recrystallized grains will be obtained after the intermediate annealing and there will be less nuclei with Goss orientation in the primary recrystallized texture, resulting in deterioration of the magnetic properties of the product sheet. On the other hand, at temperatures above 1200° C., as in the hot-rolled sheet annealing, crystal grains excessively coarsen, making it difficult to obtain a primary recrystallized texture with uniformly-sized grains.

(25) In addition, in the cold rolling (final cold rolling) to obtain a final sheet thickness, it is effective for improving the primary recrystallized texture and the magnetic properties to perform warm rolling while raising the steel sheet temperature during the cold rolling to 100° C. to 300° C., or to perform aging treatment once or multiple times at a temperature of 100° C. to 300° C. partway through the cold rolling.

(26) The cold-rolled sheet having a final sheet thickness is then subjected to primary recrystallization annealing that also serves as decarburization annealing. The annealing temperature for this primary recrystallization annealing is, if accompanied by decarburization annealing, preferably in a range of 800° C. to 900° C. from the viewpoint of allowing the decarburization reaction to proceed rapidly, and the atmosphere is preferably a wet atmosphere. However, this does not apply if a steel material having a C content of 0.005 mass % or less and without requiring decarburization is used. Alternatively, primary recrystallization annealing and decarburization annealing may be performed separately.

(27) In the heating process during the primary recrystallization annealing, it is desirable to perform rapid heating at 50° C./s or higher within a temperature range of 400° C. to 700° C. because the magnetic properties are improved.

(28) Then, the steel sheet is subjected to secondary recrystallization annealing where it is applied with an annealing separator mainly composed of MgO to develop a secondary recrystallization texture and to form a forsterite film. The temperature for the secondary recrystallization annealing is desirably 800° C. or higher for ensuring secondary recrystallization. Further, for the reasons described above, the heating rate within a temperature range of the room temperature to 1000° C. is desirably set in a range of 15° C./h to 100° C./h, and the holding temperature in a higher temperature range is desirably set to 1150° C. or higher. Further, at the time of holding in a high temperature range, a desirable holding time is 8 hours or more.

(29) After the secondary recrystallization annealing, performing water washing, brushing, or pickling is effective for removing the adhered annealing separator. Then, flattening annealing is performed to correct the shape, which is useful for iron loss reduction.

(30) In the case of using the steel sheet in a stacked state, in order to improve iron loss properties, it is effective to apply an insulation coating to the steel sheet surface before or after the flattening annealing. For iron loss reduction, it is desirable to apply a coating capable of imparting tension to the steel sheet. It is desirable to apply a tension coating applying method with a binder, or a method that allows inorganic materials to be deposited as a coating on the surface layer of the steel sheet through physical vapor deposition or chemical vapor deposition, because it may provide excellent coating adhesion and significant iron loss reduction effects.

EXAMPLES

Example 1

(31) Steel slabs containing, by mass %, C: 0.051%, Si: 3.45%, Mn: 0.16%, Al: 22 ppm, N: 13 ppm, S: 16 ppm, Se: 20 ppm and P: 0.09%, with the balance being Fe and inevitable impurities, were prepared by continuous casting. Then, each steel slab was subjected to slab heating where it was subjected to 80 minutes of soaking at 1200° C., and then hot rolled to obtain a hot-rolled sheet having a thickness of 2.2 mm. Then, each hot-rolled sheet was subjected to hot-rolled sheet annealing for 20 seconds at 1000° C. in a dry nitrogen atmosphere. Then, after being cold rolled to obtain a cold-rolled sheet having a thickness of 0.23 mm, the cold-rolled sheet was subjected to primary recrystallization annealing that also served as decarburization for 70 seconds at 840° C. in a 52% H.sub.2-48% N.sub.2 wet atmosphere with a dew point of 60° C. Further, each cold-rolled sheet was subjected to secondary recrystallization annealing where it was applied with an annealing separator mainly composed of MgO and then held at 1225° C. in a hydrogen atmosphere. At this time, the heating rate for the secondary recrystallization annealing and the holding time at 1225° C. were varied in the ranges presented in Table 1.

(32) For each product sheet thus obtained, the high-frequency iron loss W.sub.10/200 (i.e., the iron loss when excited to 1.0 T at 200 Hz) was measured by the method prescribed in JIS C 2550. In addition, in order to evaluate the blanking workability, a continuous punching test was conducted using a steel die with a die diameter of 15 mmϕ, and the number of punching times until the burr height of the punched samples reached 50 μm was counted. In addition, each product sheet was subjected to pickling and macroetching to expose secondary recrystallized grains. Then, the average grain size, the area of grain boundaries of individual crystal grains on the front and back surfaces, and the area of grain boundaries of individual crystal grains overlapping on the front and back surfaces were determined, and the area ratio was obtained for the overlapping area. In addition, the number of fine grains having a grain size ranging from 0.1 mm to 2.0 mm was counted to determine the number density per unit area. Samples were judged as satisfactory when the high-frequency iron loss W.sub.10/200 was 4.50 W/kg or less and the number of punching times was 6.0×10.sup.3 or more. The evaluation criteria also apply to Example 2. The obtained results are listed in Table 1.

(33) TABLE-US-00001 TABLE 1 Area ratio for Holding time in Average grain size overlapping area Heating high temperature Number of of secondary on front and back Iron loss Number of rate range fine grains recrystallized grains surfaces W.sub.10/200 punching times No. (° C./h) (h) (pcs/cm.sup.2) (mm) (%) (W/kg) (×10.sup.3) Remarks 1 7 5  0.2 5.6 71.2 4.98 3.5 Comparative Example 2 7 10  0.2 6.3 90.4 4.96 6.7 Comparative Example 3 15 5  1.4 7.2 72.4 4.45 3.4 Comparative Example 4 15 10  1.4 7.1 89.5 4.41 6.5 Example 5 15 15  1.5 7.5 91.2 4.43 7.1 Example 6 50 5 17.5 11.4  72.5 4.27 3.7 Comparative Example 7 50 10 14.8 12   89.4 4.29 6.6 Example 8 100 5 33.3 9.8 74.8 4.16 3.5 Comparative Example 9 100 10 36.7 9.6 84.6 4.09 7.1 Example 10 125 5 98.5 3.7 70.8 6.55 2.9 Comparative Example 11 125 10 102.3  3.2 80.3 6.70 4.8 Comparative Example

(34) As can be seen from the table, good high-frequency iron loss properties and blanking workability can be obtained when the secondary recrystallized grains are within the scope of the present disclosure in terms of the average grain size, the number of fine grains, and the area ratio for the area of individual crystal grains overlapping on the front and back surfaces.

Example 2

(35) Steel slabs containing the components listed in Table 2, with the balance being Fe and inevitable impurities, were prepared by continuous casting. Then, each steel slab was subjected to slab heating to 35 minutes of soaking at 1150° C., and hot rolled to obtain a hot-rolled sheet having a thickness of 1.8 mm. Then, each hot-rolled sheet was subjected to hot-rolled sheet annealing for 20 seconds at 1100° C. in a dry nitrogen atmosphere. Then, after being cold rolled to obtain a cold-rolled sheet having a thickness of 0.23 mm, the cold-rolled sheet was subjected to primary recrystallization annealing that also served as decarburization for 170 seconds at 825° C. in a 38% H.sub.2-62% N.sub.2 wet atmosphere with a dew point of 48° C. Further, each cold-rolled sheet was subjected to secondary recrystallization annealing where it was applied with an annealing separator mainly composed of MgO and then held at 1200° C. for 10 hours in a hydrogen atmosphere. The heating rate for the secondary recrystallization annealing was 20° C./h.

(36) For each product sheet thus obtained, the high-frequency iron loss W.sub.10/200 (i.e., the iron loss when excited to 1.0 T at 200 Hz) was measured by the method prescribed in JIS C 2550. In addition, in order to evaluate the blanking workability, a continuous punching test was conducted using a steel die with a die diameter of 15 mmϕ, and the number of punching times until the burr height of the punched samples reached 50 μm was counted. Further, the results of identifying the steel substrate components of each product sheet are listed in Table 3 together with the iron loss and the number of punching times. Table 3 also lists the results of, after subjecting each product sheet to pickling and macroetching to expose secondary recrystallized grains, determining the average grain size and the area ratio for the area of individual crystal grains overlapping on the front and back surfaces, and counting the number of fine grains with a grain size ranging from 0.1 mm to 2.0 mm.

(37) TABLE-US-00002 TABLE 2 Steel Chemical composition (mass % or mass ppm) sample C Si Mn N Sol. Al S Se Sn Sb Mo P Others ID (%) (%) (%) (ppm) (ppm) (ppm) (ppm) (%) (%) (%) (%) (% or ppm) A 0.012 3.29 0.18 22 32 11 20 0.03 0.36 0.13 0.08 — B 0.055 3.42 0.06 23 55 12 — 0.25 — — — — C 0.005 3.19 0.19 21 89 18 20 — 0.07 — — — D 0.013 2.75 0.22 19 18 9 — — — 0.03 — — E 0.070 3.33 0.15 21 24 39 — — — — 0.16 — F 0.046 3.81 0.52 18 28 6 30 — 0.13 — 0.03 — G 0.011 1.38 0.17 36 19 9 — 0.23 — — — — H 0.018 8.55 0.19 12 29 12 30 0.14 — — — — I 0.022 3.50 0.01 21 28 14 — 0.18 — — — — J 0.017 3.26 1.24 15 30 15 — 0.08 — — — — K 0.018 3.27 0.20 18 22 14 — — 0.03 0.44 0.04 Cr: 0.05%, Cu: 0.12% L 0.023 3.48 0.19 20 20 42 40 — 0.11 0.02 0.02 Ni: 0.08%, Nb: 38 ppm M 0.018 2.99 0.22 19 17 14 20 — 0.07 — Bi: 0.014%, B: 15 ppm

(38) TABLE-US-00003 TABLE 3 Steel Chemical composition (mass % or mass ppm) Iron loss sample Si Mn Sn Sb Mo P Others W.sub.10/200 ID (%) (%) (%) (%) (%) (%) (% or ppm) (W/kg) A 3.29 0.18 0.03 0.36 0.13 0.08 — 4.35 B 3.42 0.06 0.25 — — — — 4.33 C 3.19 0.19 — 0.07 — — — 4.28 D 2.75 0.22 — — 0.03 — — 4.39 E 3.33 0.15 — — — 0.16 — 4.20 F 3.81 0.52 — 0.13 — 0.03 — 4.28 G 1.38 0.17 0.23 — — — — 9.11 H 8.55 0.19 0.14 — — — — 12.33 I 3.50 0.01 0.18 — — — — 12.12 J 3.26 1.24 0.08 — — — — 11.97 K 3.27 0.20 — 0.03 0.44 0.04 Cr: 0.05%, 4.09 Cu: 0.12% L 3.48 0.19 — 0.11 0.02 0.02 Ni: 0.08%, 4.11 Nb: 38 ppm M 2.99 0.22 — 0.07 — Bi: 0.014%, 4.16 B: 15 ppm Average grain Area ratio for Number of size of secondary overlapping Steel punching Number of recrystallized area on front sample times fine grains grains and back surfaces ID (×10.sup.3) (pcs/cm.sup.2) (mm) (%) Remarks A 7.2 3.6 11.6 88.8 Example B 6.8 4.8 13.8 90.4 Example C 7.1 15.6 13.9 95.8 Example D 8.8 30.4 11.5 90.2 Example E 7.3 18.2 10.6 94.6 Example F 6.2 10.5 8.4 84.1 Example G 8.5 without secondary recrystallization Comparative Example H 4.2 without secondary recrystallization Comparative Example I 6.7 without secondary recrystallization Comparative Example J 6.5 without secondary recrystallization Comparative Example K 7.2 20.9 15.5 98.7 Example L 6.6 17.7 14.8 95.4 Example M 8.0 8.1 15.9 92.1 Example

(39) As can be seen from Table 3, good high-frequency iron loss properties and blanking workability can be obtained when the chemical compositions are within the scope of the present disclosure, and the secondary recrystallized grains satisfy the appropriate ranges specified in the present disclosure in terms of the average grain size, the number of fine grains, and the area ratio for the area of individual crystal grains overlapping on the front and back surfaces.

REFERENCE SIGNS LIST

(40) 10 grain-oriented electrical steel sheet

(41) 20 forsterite film

(42) P crystal grain

(43) P1 coarse secondary recrystallized grain

(44) P2 fine grain

(45) S.sub.0 area of overlapping region

(46) S.sub.1 area of exposed surface on front surface

(47) S.sub.2 area of exposed surface on back surface

(48) t sheet thickness