Grain-oriented electrical steel sheet and method for manufacturing same

Abstract

Provided is a grain-oriented electrical steel sheet having better transformer iron loss property than conventional grain-oriented electrical steel sheets. A grain-oriented electrical steel sheet comprises: a steel substrate; a forsterite film on a surface of the steel substrate; and a Cr-depleted layer at a boundary between the steel substrate and the forsterite film, the Cr-depleted layer having a Cr concentration that is 0.70 times to 0.90 times a Cr concentration of the steel substrate.

Claims

1. A grain-oriented electrical steel sheet comprising: a steel substrate; a forsterite film on a surface of the steel substrate; and a Cr-depleted layer at a boundary between the steel substrate and the forsterite film, the Cr-depleted layer having a Cr concentration that is 0.70 times to 0.90 times a Cr concentration of the steel substrate, wherein the steel substrate contains Cr: 0.01 mass % or more and 0.25 mass % or less.

2. The grain-oriented electrical steel sheet according to claim 1, wherein the steel substrate contains Cr: 0.02 mass % or more and 0.20 mass % or less.

3. The grain-oriented electrical steel sheet according to claim 1, wherein the steel substrate contains Cr: 0.15 mass % or more and 0.25 mass % or less.

4. The grain-oriented electrical steel sheet according to claim 1, wherein the steel substrate contains C: 0.08 mass % or less.

5. The grain-oriented electrical steel sheet according to claim 1, wherein the steel substrate contains Si: 2.0 mass % to 8.0 mass %.

6. The grain-oriented electrical steel sheet according to claim 1, wherein the steel substrate contains Mn: 0.005 mass % to 1.000 mass %.

7. A method for manufacturing the grain-oriented electrical steel sheet of claim 1, the method comprising: subjecting a grain-oriented electrical steel slab to hot rolling, to obtain a hot-rolled steel sheet; subjecting the hot-rolled steel sheet to cold rolling either once, or twice or more with intermediate annealing performed therebetween, to obtain a cold-rolled steel sheet having a final sheet thickness; subjecting the cold-rolled steel sheet to decarburization annealing to obtain a decarburization-annealed steel sheet; applying an annealing separator composed mainly of MgO to the decarburization-annealed steel sheet; thereafter subjecting the decarburization-annealed steel sheet in coil form to final annealing to obtain a final-annealed steel sheet comprising a steel substrate and a forsterite film on a surface of the steel substrate; and thereafter forming a tension coating on the final-annealed steel sheet, wherein oxidizability of atmosphere in at least a temperature range of 300° C. to 600° C. in a sheet passing process from after the final annealing to when baking the tension coating is controlled to form, at a boundary between the steel substrate and the forsterite film, a Cr-depleted layer having a Cr concentration that is 0.70 times to 0.90 times a Cr concentration of the steel substrate, wherein the grain-oriented electrical steel slab contains Cr: 0.01 mass % or more and 0.25 mass % or less.

8. The method for manufacturing a grain-oriented electrical steel sheet according to claim 7, wherein after the final annealing and before forming the Cr-depleted layer, the final-annealed steel sheet is passed through a pass line including at least one part that imparts bending in a direction opposite to coil set remaining in the final-annealed steel sheet, and the oxidizability of atmosphere when forming the Cr-depleted layer is controlled to an oxygen partial pressure P.sub.O2 of 0.01 atm to 0.25 atm.

9. The method for manufacturing a grain-oriented electrical steel sheet according to claim 7, wherein the grain-oriented electrical steel slab contains Cr: 0.02 mass % or more and 0.20 mass % or less.

10. The method for manufacturing a grain-oriented electrical steel sheet according to claim 7, wherein the grain-oriented electrical steel slab contains Cr: 0.15 mass % or more and 0.25 mass % or less.

11. The method for manufacturing a grain-oriented electrical steel sheet according to claim 8, wherein a curvature radius of the bending is 750 mm or less.

12. The method for manufacturing a grain-oriented electrical steel sheet according to claim 8, wherein the grain-oriented electrical steel slab contains Cr: 0.02 mass % or more and 0.20 mass % or less.

13. The method for manufacturing a grain-oriented electrical steel sheet according to claim 11, wherein the grain-oriented electrical steel slab contains Cr: 0.02 mass % or more and 0.20 mass % or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 is a graph illustrating the relationship between the Cr concentration ratio of the Cr-depleted layer of the steel substrate surface layer to the steel substrate and the iron loss ratio;

(3) FIG. 2 is a graph illustrating the relationship between the Cr concentration ratio of the Cr-depleted layer of the steel substrate surface layer to the steel substrate and the nitriding quantity;

(4) FIG. 3 is a graph illustrating the relationship between the Cr concentration ratio of the Cr-depleted layer of the steel substrate surface layer to the steel substrate and the resistance to coating exfoliation;

(5) FIG. 4 is a graph illustrating an example of a Cr intensity profile; and

(6) FIG. 5 is a schematic diagram illustrating sheet passing patterns after final annealing.

DETAILED DESCRIPTION

(7) A method for manufacturing a grain-oriented electrical steel sheet will be described in detail below.

(8) [Chemical Composition]

(9) The chemical composition of a slab for a grain-oriented electrical steel sheet according to the present disclosure is a chemical composition capable of secondary recrystallization. In the case of using an inhibitor, for example, Al and N are added in appropriate amounts when using a AlN-based inhibitor, and Mn and Se and/or S are added in appropriate amounts when using a MnS/MnSe-based inhibitor. Both inhibitors may be used together. Preferable contents of Al, N, Mn, S, and Se in this case are Al: 0.010 mass % to 0.065 mass %, N: 0.0050 mass % to 0.0120 mass %, S: 0.005 mass % to 0.030 mass %, and Se: 0.005 mass % to 0.030 mass %.

(10) An inhibitorless grain-oriented electrical steel sheet in which the contents of Al, N, S, and Se are limited may be used in the present disclosure. In such a case, the contents of Al, N, S, and Se are preferably limited to Al: 100 mass ppm or less, N: 50 mass ppm or less, S: 50 mass ppm or less, and Se: 50 mass ppm or less.

(11) The basic components and optionally added components of a slab for a grain-oriented electrical steel sheet according to the present disclosure will be described in detail below.

(12) C: 0.08 Mass % or Less

(13) C is added to improve the hot-rolled sheet microstructure. If the C content is more than 0.08 mass %, it is difficult to reduce C to 50 mass ppm or less at which magnetic aging does not occur during the manufacturing process. The C content is therefore preferably 0.08 mass % or less. The lower limit is not particularly limited, as a material not containing C can still be secondary recrystallized. That is, the C content may be 0%.

(14) Si: 2.0 Mass % to 8.0 Mass %

(15) Si is an element effective in enhancing the electrical resistance of the steel and improving the iron loss. If the Si content is less than 2.0 mass %, the iron loss reduction effect is insufficient. If the Si content is more than 8.0 mass %, the workability decreases significantly, and the magnetic flux density decreases, too. The Si content is therefore preferably in a range of 2.0 mass % to 8.0 mass %.

(16) Mn: 0.005 Mass % to 1.000 Mass %

(17) Mn is an element necessary for achieving favorable hot workability. If the Mn content is less than 0.005 mass %, the addition of Mn is not effective. If the Mn content is more than 1.000 mass %, the magnetic flux density of the product sheet decreases. The Mn content is therefore preferably in a range of 0.005 mass % to 1.000 mass %.

(18) Cr: 0.02 Mass % to 0.20 Mass %

(19) Cr is an element that facilitates the formation of a dense oxide film at the interface between the forsterite film and the steel substrate. Although the oxide film formation is possible without Cr, the expansion of the suitable range and the like can be expected by adding Cr. If the Cr content is more than 0.20%, the oxide film is excessively thick, which decreases the resistance to coating exfoliation. The Cr content is therefore preferably in the foregoing range.

(20) In addition to the basic components described above, the following elements may be contained as appropriate.

(21) At least one selected from the group consisting of Ni: 0.03 mass % to 1.50 mass %, Sn: 0.010 mass % to 1.500 mass %, Sb: 0.005 mass % to 1.500 mass %, Cu: 0.02 mass % to 0.20 mass %, P: 0.03 mass % to 0.50 mass %, and Mo: 0.005 mass % to 0.100 mass %

(22) Ni is useful for improving the hot-rolled sheet microstructure and improving the magnetic property. If the Ni content is less than 0.03 mass %, the magnetic property improving effect is low. If the Ni content is more than 1.50 mass %, secondary recrystallization is unstable, and the magnetic property degrades. The Ni content is therefore preferably in a range of 0.03 mass % to 1.50 mass %.

(23) Sn, Sb, Cu, P, and Mo are each an element useful for improving the magnetic property. If the content of each of these components is less than the corresponding lower limit, the magnetic property improving effect is low. If the content of each of these components is more than the corresponding upper limit, the development of secondary recrystallized grains is inhibited. The content of each of these components is therefore preferably in the foregoing range.

(24) The balance other than the components described above is Fe and inevitable impurities mixed in the manufacturing process.

(25) A method for manufacturing a grain-oriented electrical steel sheet according to the present disclosure will be described below.

(26) [Heating]

(27) A slab having the chemical composition described above is heated according to a conventional method. The heating temperature is preferably 1150° C. to 1450° C.

(28) [Hot Rolling]

(29) After the heating, the slab is hot rolled. Alternatively, the slab may be directly hot rolled without heating, after casting. In the case of a thin slab or thinner cast steel, it may or may not be hot rolled.

(30) In the case of hot rolling the slab, it is preferable to set the rolling temperature in the rough rolling final pass to 900° C. or more and the rolling temperature in the finish rolling final pass to 700° C. or more.

(31) [Hot Band Annealing]

(32) After this, the hot-rolled sheet is optionally hot band annealed. For high development of Goss texture in the product sheet, the hot band annealing temperature is preferably in a range of 800° C. to 1100° C. If the hot band annealing temperature is less than 800° C., band texture in the hot rolling remains, making it difficult to realize homogenized primary recrystallized microstructure and inhibiting the development of secondary recrystallized grains. If the hot band annealing temperature is more than 1100° C., the grain size after the hot band annealing is excessively coarse, making it difficult to realize homogenized primary recrystallized microstructure.

(33) [Cold Rolling]

(34) Following this, the hot-rolled sheet is cold rolled either once, or twice or more with intermediate annealing performed therebetween. The intermediate annealing temperature is preferably 800° C. or more and 1150° C. or less. The intermediate annealing time is preferably about 10 sec to 100 sec.

(35) [Decarburization Annealing]

(36) The cold-rolled sheet is then subjected to decarburization annealing to obtain a decarburization-annealed sheet. The decarburization annealing is preferably performed with an annealing temperature of 750° C. to 900° C., an oxidizing atmosphere PH.sub.2O/PH.sub.2 of 0.25 to 0.60, and an annealing time of about 50 sec to 300 sec.

(37) [Application of Annealing Separator]

(38) After this, an annealing separator is applied to the decarburization-annealed sheet. The annealing separator is preferably composed mainly of MgO, and applied in an amount of about 8 g/m.sup.2 to 15 g/m.sup.2.

(39) [Final Annealing]

(40) The decarburization-annealed sheet is then subjected to final annealing intended for secondary recrystallization and forsterite film formation. Preferably, the annealing temperature is 1100° C. or more, and the annealing time is 30 min or more. It is further preferable to, after the final annealing, pass the steel sheet through a pass line including at least one part that imparts bending in the direction opposite to coil set (residual curvature) remaining in the steel sheet.

(41) [Additional Oxidizing Treatment]

(42) Subsequently, continuous annealing for additional oxidizing treatment is performed after removing any unreacted separator and before applying an insulating coating. Alternatively, baking treatment also serving as additional oxidizing treatment is performed after applying an insulating coating. As a result of either one of these processes, an additional oxide film is formed at the interface between the forsterite film and the steel substrate.

(43) Specifically, in the additional oxidizing treatment, by controlling the oxidizability of atmosphere in at least one part of the process of performing the continuous annealing or the insulating coating baking treatment in a temperature range of 300° C. to 600° C., a Cr-depleted layer having a Cr concentration that is 0.70 times to 0.90 times the Cr concentration of the steel substrate is formed at the boundary between the steel substrate and the forsterite film. The oxidizability of atmosphere when forming the Cr-depleted layer is further preferably controlled to an oxygen partial pressure P.sub.O2 of 0.01 atm to 0.25 atm.

(44) [Flattening Treatment and Insulating Coating]

(45) In the foregoing insulating coating application and baking treatment, flattening treatment may be simultaneously performed for shape adjustment. The flattening annealing is preferably performed with an annealing temperature of 750° C. to 950° C. and an annealing time of about 10 sec to 200 sec.

(46) In the present disclosure, an insulating coating is formed on the steel sheet surface before or after the flattening annealing. This insulating coating is such a coating (tension coating) that imparts tension to the steel sheet for iron loss reduction. Examples of the tension coating include an inorganic coating containing silica and a ceramic coating by physical vapor deposition, chemical vapor deposition, or the like.

(47) The resultant steel sheet may be irradiated with a laser, plasma, an electron beam, or the like to undergo magnetic domain refining, for further iron loss reduction. Moreover, an etching resist may be attached to the steel sheet after the final cold rolling by printing or the like, and then the region without the etching resist attached thereto may be subjected to treatment such as electrolytic etching to form linear grooves.

(48) The other manufacturing conditions may comply with typical grain-oriented electrical steel sheet manufacturing methods.

EXAMPLES

Example 1

(49) Steel slabs having a composition containing the components shown in Table 7 with the balance being substantially Fe were each produced by continuous casting, heated to 1420° C., and then hot rolled to obtain a hot-rolled sheet with a sheet thickness of 1.8 mm. The hot-rolled sheet was then subjected to hot band annealing at 1000° C. for 100 sec. Following this, the hot-rolled sheet was cold rolled to an intermediate sheet thickness of 0.45 mm, and subjected to intermediate annealing under the conditions of oxidizability: PH.sub.2O/PH.sub.2=0.40, temperature: 1000° C., and time: 70 sec. Subsequently, after removing subscale from the surface by pickling with hydrochloric acid, the steel sheet was cold rolled again to obtain a cold-rolled sheet with a sheet thickness of 0.23 mm.

(50) The cold-rolled sheet was then subjected to decarburization annealing in which the cold-rolled sheet was held at a soaking temperature of 830° C. for 300 sec. After this, an annealing separator composed mainly of MgO was applied, and final annealing intended for secondary recrystallization, forsterite film formation, and purification was performed at 1200° C. for 30 hr. After removing any unreacted separator, the cold-rolled sheet was subjected to continuous annealing for forming a dense oxide film at the interface between the forsterite film and the steel substrate. The end-point temperature, the atmosphere, and the line tension in the continuous annealing are shown in Table 8. Lastly, an insulation coating containing 60% of colloidal silica and aluminum phosphate was applied, and baked at 800° C. This coating application process also serves as flattening annealing. The resultant product was then used to produce a wound core, and the wound core was subjected to stress relief annealing in a N.sub.2 atmosphere at 860° C. for 10 hr.

(51) TABLE-US-00007 TABLE 7 Steel Chemical composition (mass %) No. C Si Mn Ni Cr P Mo Sb Sn Al N Se S O A 0.08 3.4 0.030 0.01 0.08 0.03 0.030 0.030 0.001 0.030 0.0100 0.001 0.002 0.0015

(52) The results of performing the same measurements as in the foregoing Experiment 1 are shown in Table 8. As can be seen from Nos. 1 to 12 in Table 8, even in the case where the products were made using the same product sheets and the same manufacturing conditions, if the treatment conditions of the dense oxide film formation at the interface between the forsterite film and the steel substrate changed, the Cr concentration ratio of the Cr-depleted layer to the steel substrate (oxide film formation state) changed. In the case where the oxide film formation amount was excessively low (the Cr concentration ratio of the Cr-depleted layer to the steel substrate was excessively high), nitriding in the stress relief annealing was not suppressed. In the case where the oxide film formation amount was excessively high (the Cr concentration ratio of the Cr-depleted layer to the steel substrate was excessively low), an increase in the film thickness of the oxide film led to lower adhesion of the steel substrate, thus causing a decrease in the resistance to coating exfoliation. These results demonstrate that it is important to control the two parameters, i.e. the oxide film formation temperature and the treatment atmosphere (oxygen partial pressure), in combination.

(53) Nos. 13 to 24 correspond to results when passing the steel sheet through a pass line (pattern I in FIG. 5) including at least one part that imparts, by a roller of Φ1000 mm, bending in the direction opposite to coil set (residual curvature) which occurs in the steel sheet when annealed in coil form. In Nos. 1 to 12, the two parameters, i.e. the oxide film formation temperature and the treatment oxygen partial pressure, need to be controlled in combination in the range according to the present disclosure. In Nos. 13 to 24, even when the end-point temperature was different, the appropriate oxygen partial pressure was the same (comparison of Nos. 16, 17, 18, 19, 20, and 21), and favorable results were obtained by controlling only the oxygen partial pressure.

(54) Nos. 25 to 30 correspond to evaluation results of products with different manufacturing conditions. Even in the case where the oxide film formation conditions were the same, if other manufacturing conditions were different, the Cr depletion proportion varied. This indicates the need to control a combination of a plurality of parameters, i.e. normal conditions such as the oxidizing atmosphere in the decarburization annealing and the amount of MgO applied and the oxygen partial pressure in the oxide film formation. Nos. 31 to 36 correspond to results when passing the steel sheet through a pass line including at least one part that imparts, by a roller of Φ500 mm, bending in the direction opposite to coil set (residual curvature) which occurs in the steel sheet when annealed in coil form. No dependence on the other manufacturing conditions was recognized here, and favorable properties were obtained if the oxide film formation conditions satisfied the conditions according to the present disclosure.

(55) TABLE-US-00008 TABLE 8 Number of parts that impart bending in Oxygen direction End-point partial Cr Iron loss Oxide opposite to temperature pressure in concentration ratio ratio Oxidizability Amount film coil set in oxide film of (wound core Resistance in decarburization of MgO formation treatment which occurs continuous formation Cr-depleted layer to iron loss/product to coating annealing applied line tension when annealed annealing treatment steel Nitriding quantity sheet exfoliation Sample No. Steel No. PH.sub.2O/PH.sub.2 (g/m.sup.2) (kgf/mm.sup.2) in coil form (° C.) (atm) substrate (ppm) iron loss) (mmϕ) Remarks 1 A 0.45 12.0 1.1 0 200 0.05 1.00 70  1.18 20 Comp. Ex. 2 A 0.45 12.0 1.1 0 0.1 1.00 70  1.18 20 Comp. Ex. 3 A 0.45 12.0 1.1 0 0.3 1.00 70  1.18 20 Comp. Ex. 4 A 0.45 12.0 1.1 0 420 0.05 0.92 9 1.04 20 Comp. Ex. 5 A 0.45 12.0 1.1 0 0.1 0.85 4 1.04 20 Ex. 6 A 0.45 12.0 1.1 0 0.3 0.78 4 1.04 20 Ex. 7 A 0.45 12.0 1.1 0 520 0.05 0.88 2 1.01 20 Ex. 8 A 0.45 12.0 1.1 0 0.1 0.78 4 1.03 20 Ex. 9 A 0.45 12.0 1.1 0 0.3 0.65 2 1.01 65 Comp. Ex. 10 A 0.45 12.0 1.1 0 650 0.05 1.00 67  1.16 20 Comp. Ex. 11 A 0.45 12.0 1.1 0 0.1 1.00 66  1.17 20 Comp. Ex. 12 A 0.45 12.0 1.1 0 0.3 1.00 68  1.16 20 Comp. Ex. 13 A 0.45 12.0 1.1 1 200 0.05 1.00 70  1.18 20 Comp. Ex. 14 A 0.45 12.0 1.1 1 0.1 1.00 70  1.18 20 Comp. Ex. 15 A 0.45 12.0 1.1 1 0.3 1.00 70  1.18 20 Comp. Ex. 16 A 0.45 12.0 1.1 1 420 0.05 0.85 9 1.04 20 Ex. 17 A 0.45 12.0 1.1 1 0.1 0.81 4 1.04 20 Ex. 18 A 0.45 12.0 1.1 1 0.3 0.68 4 1.04 60 Comp. Ex. 19 A 0.45 12.0 1.1 2 520 0.05 0.88 2 1.01 20 Ex. 20 A 0.45 12.0 1.1 2 0.1 0.78 4 1.03 20 Ex. 21 A 0.45 12.0 1.1 2 0.3 0.65 2 1.01 65 Comp. Ex. 22 A 0.45 12.0 1.1 1 650 0.05 1.00 67  1.16 20 Comp. Ex. 23 A 0.45 12.0 1.1 1 0.1 1.00 66  1.17 20 Comp. Ex. 24 A 0.45 12.0 1.1 1 0.3 1.00 68  1.16 65 Comp. Ex. 25 A 0.4 5.5 1.5 0 480 0.1 0.83 2 1.03 20 Ex. 26 A 0.4 8.5 1.5 0.91 45  1.14 20 Comp. Ex. 27 A 0.4 13 1.5 0.95 50  1.15 20 Comp. Ex. 28 A 0.2 8.5 1.5 0 480 0.1 0.78 2 1.01 20 Ex. 29 A 0.3 8.5 1.5 0.85 4 1.03 20 Ex. 30 A 0.5 8.5 1.5 0.93 45  1.14 20 Comp. Ex. 31 A 0.4 5.5 1.5 1 480 0.1 0.81 2 1.03 20 Ex. 32 A 0.4 8.5 1.5 0.81 5 1.04 20 Ex. 33 A 0.4 13 1.5 0.81 4 1.04 20 Ex. 34 A 0.2 8.5 1.5 1 480 0.1 0.78 2 1.01 20 Ex. 35 A 0.3 8.5 1.5 0.78 4 1.03 20 Ex. 36 A 0.5 8.5 1.5 0.78 8 1.05 20 Ex.

Example 2

(56) Steel slabs having a composition containing the components shown in Table 9 with the balance being substantially Fe were each produced by continuous casting, heated to 1400° C., and then hot rolled to obtain a hot-rolled sheet with a sheet thickness of 2.6 mm. The hot-rolled sheet was then subjected to hot band annealing at 950° C. for 10 sec. Following this, the hot-rolled sheet was cold rolled to an intermediate sheet thickness of 0.80 mm, and subjected to intermediate annealing under the conditions of oxidizability: PH.sub.2O/PH.sub.2=0.35, temperature: 1070° C., and time: 200 sec. Subsequently, after removing subscale from the surface by pickling with hydrochloric acid, the steel sheet was cold rolled again to obtain a cold-rolled sheet with a sheet thickness of 0.20 mm.

(57) The cold-rolled sheet was then subjected to decarburization annealing in which the cold-rolled sheet was held at a soaking temperature of 860° C. for 30 sec. After this, an annealing separator composed mainly of MgO was applied, and final annealing intended for secondary recrystallization, forsterite film formation, and purification was performed at 1150° C. for 10 hr. After removing any unreacted separator, a coating liquid containing 50% of colloidal silica and aluminum phosphate was applied, and tension coating baking treatment (baking temperature: 850° C.) also serving as flattening annealing was performed. In a temperature range in the heating process in this tension coating baking treatment, a DX gas atmosphere (CO.sub.2: 15%, CO: 3%, H.sub.2: 0.5%, and the balance being N.sub.2, dew point: 30° C.) was used, thus performing oxide film formation treatment. The oxide film formation treatment conditions and the other manufacturing conditions are shown in Table 9. Lastly, a coating liquid containing 50% of colloidal silica and aluminum phosphate was applied, and baked at 800° C. This coating application process also serves as flattening annealing. The resultant product sheet was then used to produce a wound core, and the wound core was subjected to stress relief annealing in a DX gas atmosphere (CO.sub.2: 15%, CO: 3%, H.sub.2: 0.5%, and the balance being N.sub.2, dew point: 30° C.) at 860° C. for 10 hr.

(58) TABLE-US-00009 TABLE 9 Sample Chemical composition (mass %) No. C Si Mn Ni Cr P Mo Sb Sn Al N Se S O A 0.08 3.4 0.030 0.01 0.01 0.03 0.030 0.030 0.001 0.030 0.0100 0.001 0.002 0.0015 B 0.05 3.4 0.030 0.07 0.08 0.05 0.001 0.020 0.001 0.025 0.0080 0.013 0.004 0.0010 C 0.07 3 0.050 0.01 0.08 0.01 0.001 0.001 0.030 0.007 0.0040 0.001 0.001 0.0010 D 0.02 2.5 0.010 0.05 0.15 0.02 0.010 0.001 0.014 0.008 0.0038 0.008 0.001 0.0013

(59) The results of performing the same measurements as in the foregoing Experiment 1 are shown in Table 10. As can be seen from Nos. 1 to 16 in Table 10, even in the case where the manufacturing conditions were the same, the proportion of the Cr-depleted layer varied if the steel composition was different. Thus, there is no specific suitable range in controlling the proportion of the Cr-depleted layer to be in the range according to the present disclosure, but the proportion needs to be adjusted according to the manufacturing conditions (the combination of influential factors) each time. Even with different conditions, favorable product property was achieved if the proportion of the Cr-depleted layer was limited to the range according to the present disclosure.

(60) Nos. 16 to 32 correspond to results when passing the steel sheet through a pass line including at least one part that imparts bending in the direction opposite to coil set (residual curvature) which occurs in the steel sheet when annealed in coil form. In the case where the oxygen partial pressure was in a range of 0.01 atm to 0.25 atm, the proportion of the Cr-depleted layer was suitable regardless of the steel composition (Nos. 21 to 28), and favorable product property was achieved.

(61) TABLE-US-00010 TABLE 10 Number of parts that Oxide impart film bending in formation Oxide direction treatment Oxidizability film opposite to Oxide film atmosphere in Amount formation coil set formation oxygen decarburization of MgO treatment which occurs treatment partial Sample annealing applied line tension when annealed temperature pressure No. Steel No. PH.sub.2O/PH.sub.2 (g/m.sup.2) (kgf/mm.sup.2) in coil form range (atm) 1 A 0.4 10.0 1.5 0 100 to 0.005 2 B 0.4 10.0 1.5 0 420° C. 3 C 0.4 10.0 1.5 0 4 D 0.4 10.0 1.5 0 5 A 0.4 10.0 1.5 0 100 to 0.05 6 B 0.4 10.0 1.5 0 420° C. 7 C 0.4 10.0 1.5 0 8 D 0.4 10.0 1.5 0 9 A 0.4 10.0 1.5 0 100 to 0.1 10 B 0.4 10.0 1.5 0 420° C. 11 C 0.4 10.0 1.5 0 12 D 0.4 10.0 1.5 0 13 A 0.4 10.0 1.5 0 100 to 0.3 14 B 0.4 10.0 1.5 0 420° C. 15 C 0.4 10.0 1.5 0 16 D 0.4 10.0 1.5 0 17 A 0.4 10.0 1.5 1 100 to 0.005 18 B 0.4 10.0 1.5 1 420° C. 19 C 0.4 10.0 1.5 1 20 D 0.4 10.0 1.5 1 21 A 0.4 10.0 1.5 3 100 to 0.05 22 B 0.4 10.0 1.5 3 420° C. 23 C 0.4 10.0 1.5 3 24 D 0.4 10.0 1.5 3 25 A 0.4 10.0 1.5 2 100 to 0.1 26 B 0.4 10.0 1.5 2 420° C. 27 C 0.4 10.0 1.5 2 28 D 0.4 10.0 1.5 2 29 A 0.4 10.0 1.5 4 100 to 0.3 30 B 0.4 10.0 1.5 4 420° C. 31 C 0.4 10.0 1.5 4 32 D 0.4 10.0 1.5 4 Cr concentration Iron loss ratio of ratio Cr-depleted (wound core Resistance layer to Nitriding Carburizing iron loss/product to coating Sample steel quantity quantity sheet exfoliation No. substrate (ppm) (ppm) iron loss) (mmφ) Remarks  1 1.00 35  70  1.28 20 Comp. Ex.  2 0.95 25  45  1.24 20 Comp. Ex.  3 0.92 25  45  1.24 20 Comp. Ex.  4 0.88 2 10  1.02 20 Ex.  5 0.95 30  50  1.25 20 Comp. Ex.  6 0.92 15  35  1.19 20 Comp. Ex.  7 0.88 0 5 1.02 20 Ex.  8 0.85 0 5 1.02 20 Ex.  9 0.92 20  45  1.24 20 Ex. 10 0.88 3 9 1.05 20 Ex. 11 0.85 0 3 1.02 20 Ex. 12 0.83 0 3 1.02 20 Ex. 13 0.8  0 8 1.03 20 Ex. 14 0.72 0 2 1.02 20 Ex. 15 0.65 0 2 1.02 50 Comp. Ex. 16 0.55 0 2 1.02 60 Comp. Ex. 17 1.00 35  70  1.28 20 Comp. Ex. 18 0.95 25  45  1.24 20 Comp. Ex. 19 0.92 25  45  1.24 20 Comp. Ex. 20 0.88 2 10  1.02 20 Ex. 21 0.82 6 15  1.04 20 Ex. 22 0.82 6 11  1.04 20 Ex. 23 0.82 2 5 1.02 20 Ex. 24 0.82 2 5 1.02 20 Ex. 25 0.8  3 11  1.04 20 Ex. 26 0.8  0 5 10.2 20 Ex. 27 0.8  0 2 1.02 20 Ex. 28 0.8  0 2 1.02 20 Ex. 29 0.55 2 8 1.03 60 Comp. Ex. 30 0.55 0 2 1.02 60 Comp. Ex. 31 0.55 0 2 1.02 60 Comp. Ex. 32 0.55 0 2 1.02 60 Comp. Ex.