DESCRIPTION
TITLE OF INVENTION:DECARBURIZED NITRIDED STEEL SHEET
FOR GRAIN-ORIENTED ELECTRICAL STEEL SHEET AND
MANUFACTURING METHOD OF GRAIN-ORIENTED ELECTRICAL
STEEL SHEET
TECHNICAL FIELD
[0001] The present invention relates to a decarburized nitrided steel sheet for a grain-oriented electrical steel sheet and a manufacturing method of a grain-oriented electrical steel sheet for providing a grain-oriented electrical steel sheet suitable for an iron core material of a transformer or the like.
BACKGROUND ART
[0002] A grain-oriented electrical steel sheet is mainly used as an iron core material of a transformer or the like. Such a grain-oriented electrical steel sheet has been manufactured by a high-temperature slab heating method conventionally, but in recent years, there has been also conducted a low-temperature slab heating method in which a nitriding treatment is performed on a steel sheet before finish annealing to make fine dispersed precipitates such as AIN and (Al• Si)N that are inhibitors precipitate, as an industrial manufacturing process. The grain-oriented electrical steel sheets are used by being
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stacked, so that a good glass film is desirably formed.
[0003] In general, the glass film is formed in a manner that a steel sheet has decarburization annealing and nitridation annealing performed thereon, and then has an annealing separator applied on the surface, and MgO in the annealing separator reacts with an oxide layer on the steel sheet during finish annealing. Thus, the surface state of the steel sheet obtained by the nitridation annealing greatly affects the formation of the glass film. Thus, a technique of controlling atmospheres in the decarburization annealing and nitridation annealing has been disclosed in Patent Literatures 1 to 6, for example.
[0004] However, even such a conventional technique is not necessarily capable of forming a good glass film, which is insufficient.
CITATION LIST
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Laid-open
Patent Publication No. 01-119621
Patent Literature 2: Japanese Laid-open Patent
Publication No. 07-62440
Patent Literature 3: Japanese Laid-open Patent
Publication No. 04-183818
Patent Literature 4: Japanese Laid-open Patent Publication No. 04-329831
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Patent Literature 5: Japanese Laid-open Patent
Publication No. 2007-238984
Patent Literature 6: Japanese Laid-open Patent
Publication No. 11-29824
SUMMARY OF INVENTION TECHNICAL PROBLEM
[0006] In consideration of the previously described problem, the present invention has an object to provide a decarburized nitrided steel sheet for a grain-oriented electrical steel sheet and a manufacturing method of a grain-oriented electrical steel sheet that allow a good glass film to be formed.
SOLUTION TO PROBLEM
[0007] In order to solve the above-described problem, the present invention has the following characteristics.
(1) A decarburized nitrided steel sheet for a grain-oriented electrical steel sheet, in which when an area ratio of oxides occupying a steel sheet cross section perpendicular to a rolling direction at a position of a depth t from a surface of a steel sheet is set to b (t), an area ratio of Fe-based oxides is set to a (t), and a ratio of the Fe-based oxides to all the oxides at the position of the depth t is set to F (t) = a (t) /b (t) ,
F (t) has a maximum value at the depth t from the surface of the steel sheet of not less than 0.1 nm
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nor more than 1.2 jum, and an average value of F (t) at the depth t from the surface of the steel sheet falling within a range of 0.1 iJ.m. to 0.6 M m is not less than 0.05 nor more than 0.3.
(2) The decarburized nitrided steel sheet for the
grain-oriented electrical steel sheet according to
{1) , in which
F (t) has a maximum value at the depth t from the surface of the steel sheet of not less than 0.3 fx m nor more than 0.8 nm, and the average value of F (t) at the depth t from the surface of the steel sheet falling within the range of 0.1 /i m to 0.6 /x m is not less than 0.1 nor more than 0.25.
(3) A manufacturing method of a grain-oriented
electrical steel sheet includes: at 1280°C or lower,
heating a steel containing Si: 0.8 mass% to 7.0 mass%,
C: 0.085 mass% or less, Mn: 0.05 mass% to 1.00 mass%,
acid-soluble Al: 0.01 mass% to 0.065 mass%, N: 0.004
mass% to 0.012 mass%, at least one type selected from
a group consisting of S and Se: 0.003 mass% to 0.015 mass%, and a balance being composed of Fe and inevitable impurities and performing hot rolling to obtain a hot-rolled steel sheet;
performing annealing of the hot-rolled steel sheet to obtain an annealed rolled steel sheet;
performing cold rolling of the annealed steel sheet to obtain a cold-rolled steel sheet;
performing first decarburization annealing in which an oxide layer is formed on the cold-rolled
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steel sheet;
performing second decarburization annealing with a degree of oxidation smaller than that of the first decarburization annealing;
performing nitridation annealing on a steel sheet on which the second decarburization annealing has been performed to obtain a decarburized nitrided steel sheet; and
applying an annealing separator on a surface of the decarburized nitrided steel sheet and performing finish annealing of the decarburized nitrided steel sheet to form a glass film, in which
a degree of oxidation PH2O/PH2 in an atmosphere in the first decarburization annealing is set to be not less than 0.25 nor more than 1.0,
a degree of oxidation PH2O/PH2 in an atmosphere in the second decarburization annealing is set to be not less than 0.03 nor more than 0.25, and
a degree of oxidation PH2O/PH2 in an atmosphere in the nitridation annealing is set to be not less than 0.002 nor more than 0.023.
(4) The manufacturing method of the grain-oriented electrical steel sheet according to (3), in which the degree of oxidation PH2O/PH2 in the atmosphere in the second decarburization annealing is set to be not less than 0.11 nor more than 0.20.
ADVANTAGEOUS EFFECTS OF INVENTION [0008] According to the present invention, a good
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glass film can be formed in finish annealing.
BRIEF DESCRIPTION OF DRAWINGS
[0009] [Fig. lA] Fig. lA is a schematic view illustrating a structure in the vicinity of a surface of a steel sheet obtained after first decarburization annealing (a heating and soaking step) is performed;
[Fig. IB] Fig. IB is a schematic view illustrating a structure in the vicinity of the surface of the steel sheet obtained after nitridation annealing;
[Fig. 2] Fig. 2 is a view illustrating a change in relationship between a depth from a surface of the steel sheet and a ratio of Fe-based oxides in second decarburization annealing (a reduction step of decarburization annealing);
[Fig. 3] Fig. 3 is a view illustrating a change in relationship between the depth from the surface of the steel sheet and the ratio of the Fe-based oxides by the nitridation annealing;
[Fig. 4] Fig. 4 is a photograph showing one example of a reflected electron image on a cross section of a decarburized nitrided steel sheet;
[Fig. 5] Fig. 5 is a view for explaining a method of calculating the ratio of the Fe-based oxides in the decarburized nitrided steel sheet; and
[Fig. 6] Fig. 6 is a view illustrating evaluation results of defects of the glass films in Example 1.
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DESCRIPTION OF EMBODIMENTS
[0010] A grain-oriented electrical steel sheet is obtained in a manner that casting of a silicon steel material composed of predetermined components is performed to manufacture a slab, and the slab is subjected to hot rolling, cold rolling,
decarburization annealing, nitridation annealing, and finish annealing. The magnetic property of a product changes according to a state of a glass film formed on a surface of the steel sheet by the finish annealing.
[0011] As a result of analysis of a mechanism that forms a good glass film, the present inventors found out that by controlling ratios of Fe-based oxides and non Fe-based oxides that are formed on a decarburized nitrided steel sheet obtained by the decarburization annealing and the nitridation annealing, occurrence of defects of the glass film formed in the finish annealing can be controlled. Then, they found out that in the case when an area ratio of oxides occupying a steel sheet cross section perpendicular to a rolling direction at a position of a depth t from a surface of the decarburized nitrided steel sheet is set to b (t), an area ratio of Fe-based oxides is set to a (t), and a ratio of the Fe-based oxides to all the oxides at the position of the depth t is set to F (t) = a (t)/b (t), and then the ratio F
(t) of the Fe-based oxides has a maximum value at the depth t from the surface of the steel sheet of not
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less than 0.1 fxm nor more than 1.2 /zm, and an average value of the ratio F (t) of the Fe-based oxides at the depth t from the surface of the steel sheet falling within a range of 0.1 i^ la to 0.6 At m satisfies a certain condition, the good glass film with reduced defects can be formed. [0012] Hereinafter, there will be explained the decarburized nitrided steel sheet being a semi¬manufactured product of the grain-oriented electrical steel sheet.
[0013] A decarburization annealing step is separated into, as the previous stage, first decarburization annealing in which C is eliminated from a cold-rolled steel sheet obtained by the cold rolling to form an oxide layer on the surface, (which will be called a heating and soaking step hereinafter), and, as the subsequent stage, second decarburization annealing in which a condition making a dew point smaller is made to reform the oxide layer, (which will be called a reduction step hereinafter) . In the heating and soaking step, as illustrated in Fig. lA, as an oxide layer 1, on a surface side, Fe-based oxides 2 such as 2FeO • Si02 and FeO are formed on a steel sheet 4, and on an inner side, non Fe-based oxides 3 such as Si02 are formed. On the other hand, in the reduction step, 2FeO • Si02 formed in the heating and soaking step is reformed into FeO • Si02, and further non Fe-based oxides such as Si02 are separated partly from FeO• Si02 and the ratio of the
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Fe-based oxides 2 on the surface side is suppressed. [0014] The Fe-based oxides such as 2FeO*Si02 and FeO that are formed in the heating and soaking step are low in sealing property, and thus when the finish annealing is performed without the reduction step being provided, in a temperature increasing process, additional oxidation ascribable to hydrated moisture of an annealing separator occurs to change the oxide layer 1. When the oxide layer 1 changes, absorption or discharge behavior of nitrogen through the oxide layer 1 becomes unstable to cause ununiformity in formation of inhibitors, resulting in that the formation of the glass film becomes unstable. Thus,
by providing the reduction step, 2FeO • Si02 and FeO that are low in sealing property are reformed into
FeO • Si02 high in sealing property and the additional oxidation in the finish annealing is suppressed. [0015] Next, in the nitridation annealing, the Fe-based oxides in the vicinity of the surface are further reduced, and on the decarburized nitrided steel sheet, as illustrated in Fig. IB, non Fe-based oxides 2' such as Si02 are newly formed in the vicinity of the surface. Then, under the non Fe-based oxides 2', the Fe-based oxides 2 based on FeO • Si02 high in sealing property are formed. [0016] Next, when an annealing separator having MgO as its main component is applied on the decarburized nitrided steel sheet, MgO, FeO'3102, and Si02 react by the finish annealing, and forsterite (Mg2Si04)
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being a main component of the glass film is formed. [0017] Fig. 2 is a view illustrating a change in relationship between the depth from the surface of the steel sheet and the ratio of the Fe-based oxides in the reduction step. Hereinafter, the ratio of the Fe-based oxides is defined as a ratio of volume of the Fe-based oxides to all the oxides.
In Fig. 2, the solid line indicates the ratio of the Fe-based oxides immediately after the heating and soaking step is performed, and the dotted line indicates the ratio of the Fe-based oxides immediately after the decarburization annealing is finished (immediately after the reduction step is performed). In the heating and soaking step, as it gets closer to the surface side, the ratio of the Fe-based oxides becomes higher, but when the reduction step starts, the oxide layer changes and the ratio of the Fe-based oxides on the uppermost surface side is suppressed. This is because the reform of the oxide layer is likely to advance in the vicinity of the surface and part of the oxide layer is reduced to Si02. [0018] Fig. 3 is a view illustrating a change in relationship between the depth from the surface of the steel sheet and the ratio of the Fe-based oxides by the nitridation annealing.
In Fig. 3, the solid line indicates the ratio of the Fe-based oxides immediately after the reduction step is performed, and the dotted line indicates the ratio of the Fe-based oxides immediately after the
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nitridation annealing is finished. By the nitridation annealing, many of the Fe-based oxides are reduced to Si02 in the vicinity of the surface. [0019] The decarburized nitrided steel sheet in this embodiment is designed such that when an area ratio of oxides occupying the steel sheet cross section perpendicular to the rolling direction at the position of the depth t from the surface of the decarburized nitrided steel sheet obtained as above is set to b (t), an area ratio of Fe-based oxides is set to a (t), and a ratio of the Fe-based oxides to all the oxides at the position of the depth t is set to F (t) = a (t)/b (t), the ratio F (t) of the Fe-based oxides has a maximum value at the depth t from the surface of the steel sheet of not less than 0.1 n m nor more than 1.2 Atm, and further an average value of the ratio F (t) of the Fe-based oxides at the depth t from the surface of the steel sheet falling
within a range of 0.1 iim to 0.6 iim is set to be not less than 0.05 nor more than 0.3, and preferably the ratio F (t) of the Fe-based oxides has a maximum value at the depth t from the surface of the steel
sheet of not less than 0.3 /x m nor more than 0.8 /zm, and further the average value of the ratio F (t) of the Fe-based oxides at the depth t from the surface of the steel sheet falling within the range of 0.1 ixxa. to 0.6 ju m is set to be not less than 0.1 nor more than 0.25. When the average value of the ratio F (t) of the Fe-based oxides is smaller than 0.05, FeO • Si02
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high in sealing property runs short, and thereby the reaction of MgO in the annealing separator and the oxide layer in the steel sheet does not go smoothly. As a result, the good glass film is not formed. On the other hand, the average value of the ratio F (t) of the Fe-based oxides exceeds 0.3, the oxide layer is not reformed sufficiently, and thus the Fe-based oxides such as 2FeO • Si02 and FeO that are low in sealing property remain. In the above case, as described previously, the oxide layer formed on the surface changes to make the formation of the glass film unstable.
[0020] Fig. 4 is a photograph showing one example of a reflected electron image on the cross section of the decarburized nitrided steel sheet according to this embodiment.
In Fig. 4, a spot 5 indicates Si02, which is observed in the blackest contrast in the reflected electron image. Further, a spot 6 indicates the Fe-based oxide, which is observed in the contrast lighter than that of Si02. When the reflected electron image is observed by a scanning electron microscope, in the decarburized nitrided steel sheet according to this embodiment, the ratio F (t) of the Fe-based oxides and the average value of the ratio F (t) satisfy the previously described conditions. [0021] The above ratio F (t) of the Fe-based oxides can be obtained by measurement and analysis processes that will be explained below. As a method of
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analyzing element concentrations, of the steel sheet, in a sheet thickness direction, for example. Auger electron spectroscopy in which a scanning electron microscope is used, electron probe for microanalysis, or the like is employed.
[0022] As the concrete measurement and analysis processes, first, the steel sheet obtained after the decarburization annealing and the nitridation annealing is cut out into an appropriate size, and a cross section of the steel sheet perpendicular to the rolling direction is mirror-polished and an analytical sample is made. Then, a figure photograph of an oxide layer of the made analytical sample is taken at, for example, a scanning electron microscope in an appropriate view. Next, distribution of oxides is confirmed in a reflected electron image or the like, and surface distributions of element concentrations of Fe, Si, and 0 are measured. Incidentally, as illustrated in Fig. 5, a measurement interval in a depth direction is made smaller than 0.1 /zm, and the surface distributions are measured with an interval of about 0.02 fj.m., for example. [0023] Next, a noise level of oxygen detected in a ground metal portion that is not oxidized is eliminated from values of the measured surface distributions of Fe, Si, and 0 with a certain threshold value, and the ground metal portion and the oxide portion are separated. Further, based on ratios of the value of Si and the value of 0 that are
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detected in the case of Si02, Si02 and the Fe-based oxides in the oxides are separated. After the separation, an area ratio of each of the oxides is calculated in a sheet thickness depth direction, and when the area ratio of the oxides at a sheet thickness t is set to b (t) and the area ratio of the Fe-based oxides is set to a (t) respectively, the ratio F (t) of the Fe-based oxides is defined by an expression below.
F (t) = a (t) /b (b) [0024] In this manner, the distribution of the ratio F (t) of the Fe-based oxides in the depth direction can be obtained. In this embodiment, it is designed such that the ratio F (t) of the Fe-based oxides measured by the previously described processes has a maximum value in an inner layer positioned at the depth t, of the steel sheet, in the sheet thickness direction of not less than 0.1 nm nor more than 1,2 p. va and further an average value of the ratio F (t) of the Fe-based oxides at the depth t from the surface of the steel sheet falling within a range of
0.1 Aim to 0.6 //m is set to be not less than 0.05 nor more than 0.3, and preferably the ratio F (t) of the Fe-based oxides has a maximum value in an inner layer positioned at the depth t, of the steel sheet, in the sheet thickness direction of not less than 0.3 //m nor more than 0.8 ^^ m, and further the average value of the ratio F (t) of the Fe-based oxides at the depth t from the surface of the steel sheet falling within
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the range of 0.1 iJ. m to 0.6 /z m is set to be not less than 0.1 nor more than 0.25, and thereby the good glass film with reduced defects can be formed. [0025] Next, the decarburized nitrided steel sheet explained above and a manufacturing method of a grain-oriented electrical steel sheet will be explained.
[0026] In this embodiment, first, casting of a molten steel for a grain-oriented electrical steel sheet with a predetermined composition is performed to manufacture a slab. A casting method is not limited in particular. The molten steel contains, for example. Si: 0.8 mass% to 7.0 mass%, C: 0.085 mass% or less, Mn: 0.05 mass% to 1.0 mass%, acid-soluble Al: 0.010 mass% to 0.065 mass%, N: 0.004 mass% to 0.012 mass%, and at least one type selected from a group consisting of S and Se: 0.003 mass% to 0.015 mass%. The balance of the molten steel is composed of Fe and inevitable impurities. Incidentally, in the inevitable impurities, there are also contained elements that are formed into inhibitors in processes of manufacturing the grain-oriented electrical steel sheet and remain in the grain-oriented electrical steel sheet after purification by high-temperature annealing. [0027] Here, limitation reasons of the numerical values of the composition of the above-described molten steel will be explained. [0028] Si is an element quite effective for
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increasing electrical resistance of the grain-oriented electrical steel sheet to thereby reduce an eddy current loss constituting part of core loss. When a Si content is less than 0.8 mass%, it is not possible to sufficiently suppress the eddy current loss. On the other hand, when the Si content exceeds 7.0 mass%, workability deteriorates. Thus, the Si content is set to 0.8 mass% to 7.0 mass%. [0029] Further, according to the Si content, a value of saturation magnetization Bs changes. The saturation magnetization Bs becomes smaller as the Si content is increased. Thus, a reference value of good magnetic flux density B8 also becomes smaller as the Si content is increased.
[0030] C is an element effective for controlling a structure obtained by primary recrystallization (primary recrystallization structure), so that C may also be contained. In the above case, a C content is set to 0.085 mass% or less. When the C content exceeds 0.085 mass%, a time required for the decarburization annealing becomes long and an emission amount of CO2 increases. Incidentally, unless the decarburization annealing is performed sufficiently, the grain-oriented electrical steel sheet having the good magnetic property is not easily obtained. Thus, the C content is set to 0.085 mass% or less. Further, in recent years, there is a request for reduction in emission amount of CO2, so that the time for the decarburization annealing is
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desirably shortened. From the above point, the C content is preferably set to 0.060 mass% or less. [0031] Mn increases specific resistance of the grain-oriented electrical steel sheet to reduce a core loss. Mn also exhibits a function of preventing occurrence of a crack during the hot rolling. When a Mn content is less than 0.05 mass%, these effects are not obtained sufficiently. On the other hand, when the Mn content exceeds 1.0 mass%, the magnetic flux density of the grain-oriented electrical steel sheet reduces. Thus, the Mn content is set to 0.05 mass% to 1.0 mass%.
[0032] Acid-soluble Al is an important element that forms AIN functioning as an inhibitor. When a content of acid-soluble Al is less than 0.010 mass%, it is not possible to form a sufficient amount of AIN and thus inhibitor strength becomes insufficient. On the other hand, when the content of acid-soluble Al exceeds 0.065 mass%, AIN coarsens to thereby reduce the inhibitor strength. Thus, the content of acid-soluble Al is set to 0.010 mass% to 0.065 mass%. [0033] N is an important element that reacts with acid-soluble Al to form AIN. As will be described later, a nitriding treatment is performed after the cold rolling, so that a large amount of N is not required to be contained in the steel for the grain-oriented electrical steel sheet, but when an N content is set to be less than 0.004 mass%, there is sometimes a case that a large load is required at the
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time of manufacturing the steel. On the other hand, when the N content exceeds 0.012 mass%, a porosity called a blister is caused in the steel sheet at the time of cold rolling. Thus, the N content is set to 0.004 mass% to 0.012 mass%. Further, the N content is preferably 0.010 mass% or less in order to further reduce blisters.
[0034] S is an important element that reacts with Mn to thereby form MnS precipitates, and Se is an important element that reacts with Mn to thereby form MnSe precipitates. The MnS and MnSe precipitates mainly affect the primary recrystallization to exhibit a function of suppressing locational change in grain growth of the primary recrystallization ascribable to the hot rolling. Thus, when a total amount of at least one type selected from a group consisting of S and Se is less than 0.003 mass%, the above effect is not sufficiently obtained. On the other hand, when the total amount exceeds 0.015 mass%, the magnetic property is likely to deteriorate. Thus, the total amount of at least one type selected from the group consisting of S and Se is set to 0.003 mass% to 0.015 mass%.
[0035] In this embodiment, the above elements are contained as the components of the molten steel, but about 0.01 mass% to 0.4 mass% of Sn, Sb, Or, Ni, P, B, Mo, and Cu may also be further contained. [0036] The slab is manufactured from the molten steel having such a composition, and then the slab is
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heated. Although in the process that the nitridation annealing is performed the precipitates are not required to be solid-dissolved when the slab is heated, the temperature of the above heating is set to 1280°C or lower in terms of the operation cost and so on,
[0037] Next, the hot rolling of the slab is performed, and thereby a hot-rolled steel sheet is obtained. The thickness of the hot-rolled steel sheet is not limited in particular, and is set to 1.5 mm to 3.5 mm, for example.
[0038] Thereafter, annealing of the hot-rolled steel sheet is performed, and thereby an annealed steel sheet is obtained. The condition of the annealing is not limited in particular, and the annealing is performed at a temperature of 750°C to 1200°C for 30 seconds to 10 minutes, for example. The magnetic property is improved by the above annealing. [0039] Subsequently, the cold rolling of the annealed steel sheet is performed, and thereby a cold-rolled steel sheet is obtained. The cold rolling may be performed only one time, or may also be performed a plurality of times while intermediate annealing being performed therebetween. The intermediate annealing is preferably performed at a
temperature of 750°C to 1200°C for 30 seconds to 10 minutes, for example.
[0040] Incidentally, when the cold rolling is performed without the intermediate annealing as
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described above being performed, there is sometimes a case that a uniform property is not easily obtained. Further, when the cold rolling is performed a plurality of times while the intermediate annealing being performed therebetween, a uniform property is easily obtained, but the magnetic flux density sometimes reduces. Thus, the number of times of the cold rolling and whether or not the intermediate annealing is performed are preferably determined according to the property and cost required for the grain-oriented electrical steel sheet to be obtained finally.
[0041] Further, even in any case, the draft ratio of the final cold rolling is preferably set to 80% to 95%.
[0042] Next, the decarburization annealing of the cold-rolled steel sheet is performed in order to eliminate C contained in the cold-rolled steel sheet to then cause the primary recrystallization. Further, in order to increase the N content in the steel sheet, the nitridation annealing is performed after the decarburization annealing, and thereby the decarburized nitrided steel sheet is obtained. [0043] As described previously, the decarburization annealing step is separated into the two stages, and is separated into the heating and soaking step as the previous stage and the reduction step as the subsequent stage. In the heating and soaking step, the decarburization annealing is performed, for
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example, at a temperature of 800°C to 900°C for 60 seconds to 180 seconds in a moist atmosphere containing hydrogen, nitrogen, and water vapor in order to obtain the targeted decarburized nitrided steel sheet. At this time, a degree of oxidation PH2O/PH2 in the atmosphere is set to be not less than 0.25 nor more than 1.0. When the degree of oxidation PH2O/PH2 is smaller than 0.25, the formation of the Fe-based oxides is not sufficiently performed and the formation of the glass film at the time of finish annealing results in a defect. On the other hand, when the degree of oxidation PH2O/PH2 exceeds 1.0, 2FeO • SiOa in the surface of the steel sheet is formed excessively. As a result, even though the reduction step is performed thereafter, the reform from 2FeO* Si02 to FeO• Si02 high in sealing property is not sufficiently made, and thus a lot of 2FeO• Si02 low in sealing property remains. As described previously, when a lot of 2FeO• Si02 low in sealing property remains, the oxide layer formed on the surface changes to thereby make the formation of the glass film unstable.
[0044] On the other hand, in the reduction step, the temperature is controlled to the same temperature as that achieved in the heating and soaking step, and the decarburization annealing is preferably performed for 5 seconds or more, for example. The upper limit of the annealing time is not limited in particular, but if the annealing time becomes very long, the long
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annealing time is not desirable in terms of productivity. At this time, in the atmosphere, the degree of oxidation PH2O/PH2 is made smaller than a value in the heating and soaking step, and the degree of oxidation PH2O/PH2 is adjusted so as to fall within a range of not less than 0.03 nor more than 0.25. Preferably, the degree of oxidation PH2O/PH2 is adjusted so as to fall within a range of not less than 0.11 nor more than 0.20. When the degree of oxidation PH2O/PH2 is smaller than 0.03, the reform of the oxide layer advances excessively, resulting in
that a lot of SiOa is separated from FeO• Si02 high in sealing property and the ratio of Si02 being the non Fe-based oxides is too increased. On the other hand, when the degree of oxidation PH2O/PH2 exceeds 0.25, the reform from 2FeO• Si02 formed by the annealing in the heating and soaking to FeO • Si02 high in sealing property is not sufficiently made, and thus a lot of 2FeO • Si02 low in sealing property remains. As described previously, when a lot of 2Fe0 • Si02 low in sealing property remains, the oxide layer formed on the surface changes to thereby make the formation of the glass film unstable.
[0045] The decarburization annealing is finished, and next the nitridation annealing is performed. In the case when the decarburization annealing and the nitridation annealing are performed continuously, the nitridation annealing is performed under an atmosphere further containing a gas having nitriding
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capability such as ammonia, and thereby the decarburized nitrided steel sheet is obtained. As for the nitridation annealing at this time, the annealing is performed at a temperature of 800°C or lower for 5 seconds or more, for example. The lower limit of the annealing temperature and the upper limit of the annealing time are not limited in particular, but when the annealing temperature is too low and the annealing time is too long, nitriding efficiency and productivity deteriorate. [0046] In the nitridation annealing, the degree of oxidation PH2O/PH2 is adjusted in the atmosphere so as to fall within a range of not less than 0.002 nor more than 0.023. When the degree of oxidation PH2O/PH2 is smaller than 0.002, a tight Si02 layer is made in the surface of the decarburized nitrided steel sheet to thereby prevent the reaction for forming the glass film. On the other hand, when the degree of oxidation PH2O/PH2 exceeds 0.023, decomposition of ammonia gas occurs to thereby deteriorate the nitriding efficiency and make it impossible to form the good glass film. When the decarburization annealing and the nitridation annealing are performed while adjusting the degree of oxidation PH2O/PH2 ^S described above, an oxygen amount of the decarburized nitrided steel sheet is preferably set to be not less than 1.0 g/m^ nor more than 3.0 g/m^, and is more preferably set to be not less than 1.7 g/m^ nor more than 2.1 g/m^ .
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[0047] Next, the annealing separator having MgO as its main component in a water slurry form is applied on the surface of the decarburized nitrided steel sheet, and the decarburized nitrided steel sheet is wound up in a coil shape. Then, in a temperature increasing process of the batch-type finish annealing, secondary recrystallization is caused in the coil-shaped decarburized nitrided steel sheet, and the glass film is formed on the surface of a finish-annealed steel sheet. In the temperature increasing
process, the temperature is increased up to 1200°C in an atmosphere gas containing hydrogen and nitrogen, for example.
[0048] The temperature is increased up to 1200°C, and then the atmosphere gas is switched to hydrogen and purification annealing for eliminating impurities is performed. A purification-annealed steel sheet obtained in this manner has, for example, a coating solution having phosphate and colloidal silica as its main component applied thereon and is baked, and thereby a product of the grain-oriented electrical steel sheet with an insulating coating film adhering thereto is obtained.
[0049] As described above, the value of the degree of oxidation PH2O/PH2 in the atmosphere is adjusted during the decarburization annealing in the heating and soaking step and the reduction step and during the nitridation annealing, and thereby when the area ratio of the oxides occupying the steel sheet cross
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section perpendicular to the rolling direction at the position of the depth t from the surface of the steel sheet is set to b (t), the area ratio of the Fe-based oxides is set to a (t), and the ratio of the Fe-based oxides to all the oxides at the position of the depth t is set to F (t) = a (t)/b (t), it becomes possible to make the ratio F (t) of the Fe-based oxides have a maximum value at the depth t from the surface of the steel sheet of 0.1 /U m or more, and to further set an average value of the ratio F (t) of the Fe-based oxides at the depth t from the surface of the steel
sheet falling within the range of 0.1 iJ. m to 0.6 iJ. m to be not less than 0.05 nor more than 0.3. As a
result, the good glass film can be formed by FeO • Si02 high in sealing property. Further, the distribution of inhibitors formed by the nitridation annealing becomes uniform relatively, so that the grain-oriented electrical steel sheet provided with the good magnetic property can be manufactured stably. [Example 1]
[0050] Hereinafter, experiments based on the knowledge as above will be explained. Conditions employed in examples are condition examples employed for confirming the applicability and effects of the present invention. Thus, the present invention is not limited to these examples, and can employ various conditions as long as the object of the present invention is achieved without departing from the spirit of the present invention.
- 25 -

[0051] Steel ingots containing Si: 3.3 mass%, C: 0.06 mass%, acid-soluble Al: 0.027 mass%, N: 0.08 mass%, Mn: 0.1 mass%, S: 0.07 mass%, and a balance being composed of Fe and inevitable impurities were manufactured in a vacuum melting furnace. Then,
annealing of the steel ingots was performed at 1150°C, and thereafter hot rolling was performed, and thereby hot-rolled steel sheets each having a thickness of 2.3 mm were obtained.
[0052] Subsequently, annealing of the hot-rolled steel sheets was performed at 1100°C, and thereby annealed steel sheets were obtained. Next, pickling of the annealed steel sheets was performed, and thereafter cold rolling was performed, and thereby cold-rolled steel sheets each having a thickness of 0.2 3 mm were obtained.
[0053] Next, in a gas atmosphere containing water vapor, hydrogen, and nitrogen, in first decarburization annealing being the heating and soaking step, the degree of oxidation PH2O/PH2 was changed from 0.2 to 1.2 according to samples, and the first decarburization annealing of the cold-rolled steel sheets was performed at 850°C for 90 seconds to 180 seconds. Thereafter, in second decarburization annealing being the reduction step, while maintaining the temperature at the same temperature, the degree of oxidation PH2O/PH2 was changed from 0.02 to 0.3 according to the samples, and the second decarburization annealing was performed for 5 seconds
- 26 -

to 60 seconds. Further, in a gas atmosphere containing ammonia, the degree of oxidation PH2O/PH2 was set to 0.01 and nitridation annealing was performed at 750°C for 30 seconds, and thereby decarburized nitrided steel sheets were obtained.
[0054] Some of the decarburized nitrided steel sheets obtained in this manner were collected as analytical samples of decarburized oxide layers, and by the previously described method, the ratio F (t) of Fe-based oxides to all the oxides at the position of the depth t from the surface of each of the decarburized nitrided steel sheets was calculated. Then, a depth at which the ratio F (t) of the Fe-based oxides in each of the decarburized nitrided steel sheets becomes a maximum value, (which will be denoted as tpmax hereinafter), and an average value of the ratio F (t) of the Fe-based oxides at the depth t = 0.1 iJ. m to 0.6 Aim, (which will be denoted as Fave. hereinafter), were obtained.
[0055] Thereafter, an annealing separator having MgO as its main component in a water slurry form was applied on each of the surfaces of the decarburized nitrided steel sheets corresponding to the above-described analytical samples. Then, finish annealing
was performed at 1200°C for 20 hours, and thereby finish-annealed steel sheets each having a glass film formed thereon were obtained.
[0056] In Fig. 6, results of grades of the glass films corresponding to the depths tFmax and the average
- 27 -

values Fave. of the obtained decarburized nitrided steel sheets are illustrated. Incidentally, the device used for calculating the depths tpmax of the maximum values and the average values Fave. is an Auger electron spectroscopy device with a scanning electron microscope attached thereto ( (manufactured by ULVAC-PHI, Incorporated), device name: PHI700). Further, as an evaluation method of defects in the glass film of the finish-annealed steel sheet, an area ratio (%) of defect points caused in each of the glass films was measured, and in the case of the area ratio being less than 1%, the evaluation was determined to be
excellent (@), and in the case of the area ratio being 1% or more and less than 5%, the evaluation was
determined to be good (O), and in the case of the area ratio being 5% or more, the evaluation was
determined to be poor (X) .
[0057] As illustrated in Fig. 6, in the decarburized nitrided steel sheets in which the depth tpmax is not less than 0.1 //m nor more than 1.2 //m and the average value Fave. is 0.05 to 0.3, the ratio of defects was less than 5%, so that the good glass film was obtained. Particularly, in the decarburized nitrided steel sheets in which the depth trmax is not less than 0.3 /z m nor more than 0.8 ii m and the average value Fave. is 0.1 to 0.25, the ratio of defects was less than 1%, so that the quite good glass film was obtained. [Example 2]
- 28 -

[0058] Steel ingots containing Si: 3.2 mass%, C: 0.05 mass%, acid-soluble Al: 0.028 itiass%, N: 0.08 mass%, Mn: 0.1 mass%, S: 0.06 mass%, and a balance being composed of Fe and inevitable impurities were manufactured in a vacuum melting furnace. Then, annealing of the steel ingots was performed at 1150°C, and thereafter hot rolling was performed, and thereby hot-rolled steel sheets each having a thickness of 2.3 mm were obtained.
[0059] Subsequently, annealing of the hot-rolled steel sheets was performed at 1120°C, and thereby annealed steel sheets were obtained. Next, pickling of the annealed steel sheets was performed, and thereafter cold rolling was performed, and thereby cold-rolled steel sheets each having a thickness of 0.23 mm were obtained.
[0060] Next, in order to adjust the depth tFmax and the average value Fave. that are described previously, in a gas atmosphere containing water vapor, hydrogen, and nitrogen, in first decarburization annealing, the degree of oxidation PH2O/PH2 was changed from 0.2 to 1.2 according to samples, and the first decarburization annealing of the cold-rolled steel
sheets was performed at 830°C to 860°C for 90 seconds to 180 seconds. Thereafter, in second decarburization annealing, while maintaining the temperature at the same temperature, the degree of oxidation PH2O/PH2 was changed from 0.02 to 0.3 according to the samples, and the second
- 29 -

decarburization annealing was performed for 5 seconds to 60 seconds. Further, in a gas atmosphere containing ammonia, the degree of oxidation PH2O/PH2 was set to 0.01 and nitridation annealing was
performed at 780°C for 20 seconds, and thereby decarburized nitrided steel sheets were obtained. [0061] Some of the decarburized nitrided steel sheets obtained in this manner were collected as analytical samples of decarburized oxide layers, and by a method similar to that of Example 1, the ratio F (t) of Fe-based oxides was calculated, and the depth tpmax and the average value Fave. in each of the decarburized nitrided steel sheets were obtained. [0062] Thereafter, an annealing separator having MgO as its main component in a water slurry form was applied on each of the surfaces of the decarburized nitrided steel sheets corresponding to the above-described analytical samples. Then, finish annealing was performed at 1200°C for 20 hours, and thereby finish-annealed steel sheets each having a glass film formed thereon were obtained.
[0063] Then, by an evaluation method similar to that of Example 1, defects of the glass films of the finish-annealed steel sheets were evaluated. A result thereof is shown in Table 1.
- 30 -

[0064]
[Table 1]
F (t) VALUE
MAXIMUM AVERAGE GLASS FILM
No. VALUE VALUE IN DEFECTIVE
POSITION RANGE OF EVALUATION
0.1 TO 0.6
liiri
]im
Al O^J 0. 15 o
A2 1^2 0. 05 o
A3 0_J[ 0 . 05 o
INVENTION _^i Oj^J 2JJ °
EXAMPLE A5 0.5 0.1 (Q)
_A6 0^^8 0.25 @
A7 0^_3 0^^2 @
A8 0. 65 0. 15 @
al 0. 05 0_^2 X
COMPARATI a2 1.3 0^_08 x
EXA^M^PLE -^ ^JLI °-^^ ^
I a4 I 0^^_5 I 0_;_4 I ><
[0065] As shown in Table 1, in the case of the decarburized nitrided steel sheets in which the depth tpmax is not less than 0.1 u m. nor more than 1.2 Aim and the value of the average value Fave. is 0.05 to 0.3, the ratio of defects was less than 5%, so that the good glass film was obtained. Particularly, in the case of the decarburized nitrided steel sheets in which the depth tpmax is not less than 0.3 fx xu. nor more than 0.8 /x m and the average value Fave. is 0.1 to 0.25, the ratio of defects was less than 1%, so that the quite good glass film was obtained. [Example 3]
[0066] Steel ingots containing Si: 3.4 mass%, C: 0.06 mass%, acid-soluble Al: 0.028 mass%, N: 0.07 mass%, Mn: 0.1 mass%, S: 0.07 mass%, and a balance
- 31 -

being composed of Fe and inevitable impurities were manufactured in a vacuum melting furnace. Then,
annealing of the steel ingots was performed at 1150°C, and thereafter hot rolling was performed, and thereby hot-rolled steel sheets each having a thickness of 2.3 mm were obtained. [0067] Subsequently, annealing of the hot-rolled
steel sheets was performed at 1120°C, and thereby annealed steel sheets were obtained. Next, pickling of the annealed steel sheets was performed, and thereafter cold rolling was performed, and thereby cold-rolled steel sheets each having a thickness of 0.2 3 mm were obtained.
[0068] Next, in order to adjust the depth trmax and the average value Fave. that are described previously, in a gas atmosphere containing water vapor, hydrogen, and nitrogen, in first decarburization annealing, the degree of oxidation PH2O/PH2 was changed from 0.2 to 1.1 according to samples, and the first decarburization annealing of the cold-rolled steel sheets was performed at 850°C for 110 seconds. Thereafter, in second decarburization annealing, while maintaining the temperature at the same temperature, the degree of oxidation PH2O/PH2 was changed from 0.02 to 0.3 according to the samples, and the second decarburization annealing was performed for 15 seconds. Further, in a gas atmosphere containing ammonia, the degree of oxidation PH2O/PH2 was changed from 0.001 to 0.03
- 32 -

according to the samples, and nitridation annealing
was performed at 770°C for 20 seconds, and thereby decarburized nitrided steel sheets were obtained. [0069] Some of the decarburized nitrided steel sheets obtained in this manner were collected as analytical samples of decarburized oxide layers, and by a method similar to that of Example 1, the ratio F (t) of Fe-based oxides was calculated, and the depth trmax and the average value Fave. in each of the decarburized nitrided steel sheets were obtained. [0070] Thereafter, an annealing separator having MgO as its main component in a water slurry form was applied on each of the surfaces of the decarburized nitrided steel sheets corresponding to the above-described analytical samples. Then, finish annealing
was performed at 1200°C for 20 hours, and thereby finish-annealed steel sheets each having a glass film formed thereon were obtained.
[0071] Then, by an evaluation method similar to that of Example 1, defects of the glass films of the finish-annealed steel sheets were evaluated. A result thereof is shown in Table 2.
- 33 -

[0072] [Table 2]
DEGREE OF OXIDATION PH20/PH2 F (t) VALUE
VERAGE
MAXIMUM
FIRST SECOND VALUE IN GLASS FILM
No. NITRIDATION VALUE
DECARBURIZATION DECARBURIZATION RANGE OF DEFECTIVE
ANNEALING POSITION
ANNEALING ANNEALING 0.1 TO 0.6 EVALUATION
(urn)
yim
Bl 0.25 0.15 0.012 0.35 0.06 o
B2 1_;_0 0.08 0.005 0.73 0.27 o
B3 0_;_6 0.03 0.016 1.04 0.14 o
B4 0_;5 0.25 0.008 0.16 0_;_2 o
B5 O^J 0.09 0.002 0.88 0.06 o
INVENTION
B6 0.4 0.21 0.023 0.18 0.12 o
EXAMPLE
B7 O^J 0.11 0.01 0.53 0.24 @
B8 0_;_4 0_;_2 0.006 0.32 0.14 @
B9 0_;_5 0.11 0.002 0.79 0.11 @
BIO 0_;_6 0.11 0.008 0.65 0.16 @
BU 0^ 0.14 0.023 0.31 0.25 ©
bl 0_;_2 0.14 0.016 0.52 0.02 «
b2 1^ 0.06 0.008 0.79 0.32 x
COMPARATIVE ^3 0_;_6 0.02 0.008 1.22 0.13 x
EXAMPLE ^4 O^J 0_J 0.012 0.06 0.13 x
_b5 0^_5 0.03 0.001 1.25 0.05 x
b6 0^^8 0_^ 0.03 0^ 0_;_4 x
[0073] As shown in Table 2, in the case when the conditions in which the degree of oxidation PH2O/PH2 of the first decarburization annealing is 0.25 to 1.0, the degree of oxidation PH2O/PH2 of the second decarburization annealing is 0.03 to 0.25, and the degree of oxidation PH2O/PH2 of the nitridation annealing is 0.002 to 0.023 were satisfied, the ratio of defects was less than 5% and thus the good glass film was obtained. Particularly, in the case of the degree of oxidation PH2O/PH2 of the second decarburization annealing being 0.11 to 0.2, the ratio of defects was less than 1% and thus the quite good glass film was obtained.
- 34 -

INDUSTRIAL APPLICABILITY
[0074] According to the present invention, it is possible to provide a grain-oriented electrical steel sheet having a good glass film, and the grain-oriented electrical steel sheet can be utilized as an iron core material of a transformer or the like.
- 35 -

CLAIMS
[Claim 1] A decarburized nitrided steel sheet for a grain-oriented electrical steel sheet, wherein
when an area ratio of oxides occupying a steel sheet cross section perpendicular to a rolling direction at a position of a depth t from a surface of a steel sheet is set to b (t), an area ratio of Fe-based oxides is set to a (t), and a ratio of the Fe-based oxides to all the oxides at the position of the depth t is set to F (t) = a (t)/b (t),
F (t) has a maximum value at the depth t from the surface of the steel sheet of not less than 0.1 ^u m nor more than 1.2 Atm, and an average value of F (t) at the depth t from the surface of the steel sheet falling within a range of 0.1 /i m to 0.6 /x m is not less than 0.05 nor more than 0.3.
[Claim 2] The decarburized nitrided steel sheet for the grain-oriented electrical steel sheet according to claim 1, wherein
F (t) has a maximum value at the depth t from the surface of the steel sheet of not less than 0.3 /z m nor more than 0.8 Aim, and the average value of F (t) at the depth t from the surface of the steel sheet falling within the range of 0.1 /z m to 0.6 yU m is not less than 0.1 nor more than 0.25.
[Claim 3] A manufacturing method of a grain-oriented electrical steel sheet comprising:
at 1280°C or lower, heating a steel containing
- 36 -

Si: 0.8 inass% to 7.0 mass%, C: 0.085 mass% or less, Mn: 0.05 mass% to 1.00 mass%, acid-soluble Al: 0.01 mass% to 0.065 mass%, N: 0.004 mass% to 0.012 mass%, at least one type selected from a group consisting of S and Se: 0.003 mass% to 0.015 mass%, and a balance being composed of Fe and inevitable impurities and performing hot rolling to obtain a hot-rolled steel sheet;
performing annealing of the hot-rolled steel sheet to obtain an annealed rolled steel sheet;
performing cold rolling of the annealed steel sheet to obtain a cold-rolled steel sheet;
performing first decarburization annealing in which an oxide layer is formed on the cold-rolled steel sheet;
performing second decarburization annealing with a degree of oxidation smaller than that of the first decarburization annealing;
performing nitridation annealing on a steel sheet on which the second decarburization annealing has been performed to obtain a decarburized nitrided steel sheet; and
applying an annealing separator on a surface of the decarburized nitrided steel sheet and performing finish annealing of the decarburized nitrided steel sheet to form a glass film, wherein
a degree of oxidation PH2O/PH2 in an atmosphere in the first decarburization annealing is set to be not less than 0.25 nor more than 1.0,
- 37 -

a degree of oxidation PH2O/PH2 in an atmosphere in the second decarburization annealing is set to be not less than 0.03 nor more than 0.25, and
a degree of oxidation PH2O/PH2 in an atmosphere in the nitridation annealing is set to be not less than 0.002 nor more than 0.023.
[Claim 4] The manufacturing method of the grain-oriented electrical steel sheet -according to claim 3, wherein
the degree of oxidation PH2O/PH2 in the atmosphere in the second decarburization annealing is set to be not less than 0.11 nor more than 0.20.