DESCRIPTION
MANUFACTURING METHOD OF GRAIN-ORIENTED ELECTRICAL
STEEL SHEET
TECHNICAL FIELD
[0001] The present invention relates to a manufacturing method of a grain-oriented electrical steel sheet suitable for an iron core or the like of an electrical apparatus.
BACKGROUND ART
[0002] A grain-oriented electrical steel sheet is a soft magnetic material, and is used for an iron core or the like of an electrical apparatus such as a transformer. In the grain-oriented electrical steel sheet, Si of about 7 mass% or less is contained. Crystal grains of the grain-oriented electrical steel sheet are highly integrated in the {110}<001> orientation by Miller indices. The orientation of the crystal grains is controlled by utilizing a catastrophic grain growth phenomenon called secondary recrystallization.
[0003] For controlling the secondary recrystallization, it is important to adjust a structure (primary recrystallization grain structure) obtained by primary recrystallization before the secondary recrystallization and to adjust a fine
precipitate called an inhibitor or a grain boundary segregation element. The inhibitor has a function to preferentially grow the crystal grains in the {110}<001> orientation and suppress growth of the other crystal grains, in the primary recrystallization grain structure. [0004] Then, conventionally, there have been proposed techniques aimed at precipitating an inhibitor effectively (Japanese Laid-open Patent Publication No. 62-40315, Japanese Laid-open Patent Publication No. 02-294428, Japanese Laid-open Patent Publication No. 01-119621, Japanese Laid-open Patent Publication No. 02-200732, and Japanese Laid-open Patent Publication No. 02-305921).
[0005] Further, in the secondary recrystallization, a glass film having forsterite as its main component is formed. Good insulation performance, adhesiveness to a base iron, and so on are required for the glass film.
[0006] However, in the conventional techniques, the secondary recrystallization becomes non-uniform, thereby making it sometimes impossible to obtain a desired magnetic property and a desired glass film.
SUMMARY OF THE INVENTION TECHNICAL PROBLEM
[0007] The present invention has an object to provide a manufacturing method of a grain-oriented
Electrical steel sheet capable of stably
manufacturing a grain-oriented electrical steel sheet
having a good glass film and a high magnetic flux
density industrially.
SOLUTION TO PROBLEM
[0008] A manufacturing method of a grain-oriented electrical steel sheet according to a first aspect of the present invention includes: heating a silicon steel material at a temperature of 1280°C or less, the silicon steel material containing: Si: 0.8 mass% to 7 mass%; acid-soluble Al: 0.01 mass% to 0.065 mass%; N: 0.004 mass% to 0.012 mass%; Mn: 0.05 mass% to 1 mass%; and at least one selected from a group consisting of S and Se: 0.003 mass% to 0.015 mass% in total amount, a C content being 0.085 mass% or less, and a balance being composed of Fe and inevitable impurities; hot rolling the heated silicon steel material to obtain a hot-rolled steel strip; annealing the hot-rolled steel strip to obtain an annealed steel strip; cold rolling the annealed steel strip one time or more to obtain a cold-rolled steel strip; decarburization annealing the cold-rolled steel strip to obtain a decarburization-annealed steel strip in which primary recrystaliization is caused; performing a nitriding treatment in which an N content in the decarburization-annealed steel strip is increased to obtain a nitrided steel strip;
coating an annealing separating agent having MgO as its main component on the nitrided steel strip; and finish annealing the nitrided steel strip having the annealing separating agent coated thereon to cause secondary recrystallization. The decarburization annealing the cold-rolled steel strip includes: heating the cold-rolled steel strip in an atmosphere with an oxidation degree of 0.25 to 1.0 in a temperature range of 750°C to 800°C at an average heating rate of 2.5°C/s or more; next performing a first soaking treatment in an atmosphere with an oxidation degree of 0.25 to 1.0 at a temperature of 800°C to 900°C; and next performing a second soaking treatment in an atmosphere with an oxidation degree of 0.03 to 0.25 at a temperature of 800°C to 900°C. [0009] In a manufacturing method of a grain-oriented electrical steel sheet according to a second aspect of the present invention, in the method according to the first aspect, the silicon steel material further contains at least one selected from a group consisting of Cr: 0.3 mass% or less, Cu: 0.4 mass% or less, Ni: 1 mass% or less, P: 0.5 mass% or less, Mo: 0.1 mass% or less, Sn: 0.3 mass% or less, Sb: 0.3 mass% or less, B: 0.008 mass% or less, and Bi : 0.01 mass% or less.
[0010] In a manufacturing method of a grain-oriented electrical steel sheet according to a third aspect of the present invention, in the method
according to the first or second aspect, the finish annealing the nitrided steel strip includes: heating the nitrided steel strip in a mixed gas atmosphere of nitrogen and hydrogen; and next purifying a nitride in a hydrogen atmosphere, and in the heating the nitrided steel strip in the mixed gas atmosphere, in a temperature range of 800°C or less, a ratio of nitrogen in the mixed gas atmosphere is set to 25% to 75%, and an oxidation degree of the mixed gas atmosphere is set to 0.015 to 0.2.
ADVANTAGEOUS EFFECTS OF INVENTION
[0011] According to the present invention, it is possible to appropriately form the oxide layer in the decarburization annealing. Consequently, it becomes possible to stabilize the secondary recrystallization in the finish annealing to thereby obtain the good magnetic property, and further to form the good glass film. Further, these processes can be stably executed industrially.
BRIEF DESCRIPTION OF DRAWINGS [0012] Fig. 1 is a flowchart showing a manufacturing method of a grain-oriented electrical steel sheet;
Fig. 2 is a view showing a result of a first experiment (a relationship between an average heating
flate and a magnetic property obtained after finish annealing);
Fig. 3 is a view showing a result of the first experiment (a relationship between the average heating rate and a property of a glass film); and
Fig. 4 is a view showing infrared reflection spectra of decarburization-annealed steel strips.
DESCRIPTION OF EMBODIMENTS
[0013] The present inventors thought that in a manufacturing method of a grain-oriented electrical steel sheet in which slab heating before hot rolling is performed at a relatively low temperature, which is called low-temperature slab heating, conditions of decarburization annealing may affect stability of secondary recrystallization and formation of a glass film, and thus conducted various experiments. Here, an outline of the manufacturing method of the grain-oriented electrical steel sheet will be explained. Fig. 1 is a flowchart showing the manufacturing method of the grain-oriented electrical steel sheet. [0014] First, as shown in Fig. 1, in step SI, a silicon steel material (slab) having a predetermined composition is heated to a predetermined temperature, and in step S2, the heated silicon steel material is hot rolled. By the hot rolling, a hot-rolled steel strip is obtained. Thereafter, in step S3, the hot-rolled steel strip is annealed to normalize a
Structure in the hot-rolled steel strip and to adjust precipitation of an inhibitor. By the annealing, an annealed steel strip is obtained. Subsequently, in step S4, the annealed steel strip is cold rolled. The cold rolling may be performed only one time, or may also be performed a plurality of times with intermediate annealing performed therebetween. By the cold rolling, a cold-rolled steel strip is obtained. Incidentally, in the case of the intermediate annealing being performed, it is also possible to omit the annealing of the hot-rolled steel strip before the cold rolling to perform the annealing (step S3) in the intermediate annealing. That is, the annealing (step S3) may be performed on the hot-rolled steel strip, or may also be performed on a steel strip obtained after being cold rolled one time and before being cold rolled finally. [0015] After the cold rolling, in step S5, the cold-rolled steel strip is decarburization-annealed. In the decarburization annealing, primary recrystallization occurs. Further, by the decarburization annealing, a decarburization-annealed steel strip where an external oxide film having an Fe-based oxide as its main component and an internal oxide layer having SiO2 as its main component are formed on a surface thereof is obtained. Next, in step S6, a nitriding treatment in which an N content in the decarburization-annealed steel strip is
increased is performed to obtain a nitrided steel strip. Thereafter, in step S7, an annealing separating agent having MgO (magnesia) as its main component is coated on a surface of the nitrided steel strip, and finish annealing is performed. In the finish annealing, secondary recrystallization occurs, and a glass film having forsterite as its main component is formed on the surface of the steel strip, and the steel strip is purified. As a result of the secondary recrystallization, a secondary recrystallization grain structure arranged in the Goss orientation is obtained. By the finish annealing, a finish-annealed steel strip is obtained. [0016] In this manner, the grain-oriented electrical steel sheet can be obtained. [0017] Further, details will be described later, but as the silicon steel material, there is used one containing Si: 0.8 mass% to 7 mass%, acid-soluble Al: 0.01 mass% to 0.065 mass%, N: 0.004 mass% to 0.012 mass%, and Mn: 0.05 mass% to 1 mass%, and further at least one selected from a group consisting of S and Se: 0.003 mass% to 0.015 mass% in total amount, and a balance being composed of Fe and inevitable impurities.
[0018] Then, as a result of the various experiments, the present inventors found that it is important to adjust the conditions of the decarburization annealing (step S5) to then form, at
the time of decarburization annealing, the appropriate internal oxide layer to be used in forming the glass film. Concretely, the present inventors found that by adjusting a heating rate, an atmosphere, and so on, the composition of the oxide film to be formed at the time of decarburization annealing is adjusted, thereby enabling nitrogen to be uniformly introduced at the time of nitriding treatment (step S6) thereafter, and the secondary recrystallization occurs appropriately. Then, the present inventors obtained knowledge capable of stably manufacturing the grain-oriented electrical steel sheet having a good magnetic property, and completed the present invention.
[0019] Here, the experiments conducted by the present inventors will be explained. [0020] (First Experiment)
In the first experiment, first, various silicon steel slabs each containing Si: 3.3 mass%, C: 0.06 mass%, acid-soluble Al: 0.027 mass%, N: 0.007 mass%, Mn: 0.1 mass%, and S: 0.007 mass%, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at 1150°C and were hot rolled to obtain hot-rolled steel strips each having a thickness of 2.3 mm. Subsequently, the hot-rolled steel strips were annealed at 1120°C. Next, cold rolling was performed to obtain cold-rolled steel strips each having a
thickness of 0.22 mm. Thereafter, decarburization annealing was performed to obtain decarburization-annealed steel strips.
[0021] In the decarburization annealing, the temperature was increased up to 750°C from room temperature (0°C to 30°C) at a heating rate of 15°C/s, and next the temperature was increased up to 800°C from 750°C at a heating rate of l°C/s to 20°C/s. Incidentally, the range of room temperature is not limited to 0°C to 30°C. Temperatures of 50°C and less that do not practically affect recrystaliization behavior may also be said to be room temperature. That is also applied similarly in the following experiments and the like. Further, a soaking treatment at 830°C for 120 seconds was performed thereafter. Incidentally, while increasing the temperature from room temperature and for the period of the first 90 seconds out of 120 seconds in the soaking treatment, the atmosphere was set to a mixed gas atmosphere of nitrogen and hydrogen with an oxidation degree of 0.52. Further, for the period of the remaining 30 seconds out of 120 seconds in the soaking treatment, the atmosphere was set to a mixed gas atmosphere of nitrogen and hydrogen with an oxidation degree of 0.18. Incidentally, the oxidation degree of the atmosphere is expressed as "PH2O/PH2" when a partial pressure of H2O is
Represented as PH2O and a partial pressure of H2 is represented as PH2 •
[0022] After the decarburization annealing, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.022 mass%. Next, an annealing separating agent having MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200°C at a speed of 15°C/h to perform finish annealing and form a glass film thereon. In this manner, various samples were formed.
[0023] Then, a relationship between an average heating rate of 750°C to 800°C in the decarburization annealing and a magnetic property obtained after the finish annealing was examined. A result thereof is shown in Fig. 2. Further, a relationship between the average heating rate of 750°C to 800°C in the decarburization annealing and a property of the glass film was also examined. A result thereof is shown in Fig. 3. In the examination of the magnetic property, a magnetic flux density B8 was measured. The magnetic flux density B8 is a magnetic flux density generated in the grain-oriented electrical steel sheet when a magnetic field of 800 A/m is applied to the grain-oriented electrical steel sheet. Further, in the examination of the property of the glass film, an area ratio (%) of a defective area caused in the
glass film was measured, and in the case of the area ratio being less than 1%, the property was determined to be excellent (®), and in the case of the area ratio being 1% or more and less than 5%, the property was determined to be good (O) , and in the case of the area ratio being 5% or more, the property was determined to be poor (X) . Incidentally, the area of the glass film where a lack is caused, and the area where a color tone of the glass film is light, thereby allowing a base iron to be seen transparently, were determined to be defective areas. The reason why the color tone of the glass film is light, thereby allowing the base iron to be seen transparently is because forsterite crystal grains composing the glass film are formed coarsely. Each of the horizontal axes in Fig. 2 and Fig. 3 indicates the average heating rate of 750°C to 800°C in the decarburization annealing. The vertical axis in Fig. 2 indicates the magnetic flux density B8 obtained after the finish annealing, and the vertical axis in Fig. 3 indicates the property of the glass film. As shown in Fig. 2 and Fig. 3, in the case of the average heating rate being 2.5°C/s or more, the magnetic flux density B8 of 1.90 T or more was obtained, and the glass film having a good property was obtained. As a reason why such a result was obtained, facts as below can be considered.
[0024] When the steel strip is kept in the temperature range of 750°C to 800°C for a long time in the decarburization annealing, an external oxide film having FeSiO3 as its main component is formed on the surface of the steel strip non-uniformly. The external oxide film has a high atmosphere sealing property, so that an internal oxide layer having SiO2 as its main component to be formed in the soaking treatment thereafter becomes non-uniform. In a nitriding treatment thereafter, nitrogen permeates through the internal oxide layer to diffuse into the inside of the steel strip, but when the internal oxide layer is non-uniform, distribution of nitrogen introduced into the steel strip also becomes non¬uniform. Thus, distributions of (Al, Si)N functioning as inhibitors become non-uniform, and secondary recrystallization in the finish annealing becomes unstable. Accordingly, it is conceivable that in the case of the low average heating rate, the magnetic flux density B8 is reduced. Further, in the case of the low average heating rate, the internal oxide layer is likely to be formed non-uniformly in the soaking treatment, so that it is conceivable that in the finish annealing, the glass film formed by reaction of the internal oxide layer and MgO being the main component of the annealing separating agent becomes non-uniform and the property of the glass film deteriorates. Then, when the results shown in
fig. 2 and Fig. 3 are considered, it is important that the average heating rate is 2.5°C/s or more, and the average heating rate is preferably 5°C/s or more and is more preferably 7°C/s or more. [0025] (Second Experiment)
In the second experiment, first, cold-rolled steel strips were obtained similarly to the first experiment. Thereafter, decarburization annealing was performed to obtain decarburization-annealed steel strips. In the decarburization annealing, the temperature was increased up to 750°C from room, temperature at a heating rate of 15°C/s, and next the temperature was increased up to 800°C from 750°C at a speed of 10°C/s. Further, a soaking treatment at 830°C for 120 seconds was performed thereafter. Incidentally, while increasing the temperature from room temperature and for the period of the first 90 seconds out of 120 seconds in the soaking treatment (that will be sometimes called a previous stage period, hereinafter), the atmosphere was set to a mixed gas atmosphere of nitrogen and hydrogen with an oxidation degree of 0.15 to 1.2. Further, for the period of the remaining 30 seconds out of 120 seconds in the soaking treatment (that will be sometimes called a subsequent stage period, hereinafter), the atmosphere was set to a mixed gas atmosphere of nitrogen and hydrogen with an oxidation degree of 0.012 to 0.44. After the decarburization annealing,
processes similar to those of the first experiment were performed to form a glass film. In this manner, various samples were formed.
[0026] Then, a relationship between the atmosphere in the decarburization annealing and a magnetic property obtained after finish annealing was examined. A result thereof is shown in Table 1. Further, a relationship between the atmosphere in the decarburization annealing and a property of the glass film was also examined. A result thereof is shown in Table 2. In the examination of the magnetic property, the magnetic flux density B8 was measured, and in the case of the magnetic flux density being 1.92 T or more, the magnetic property was evaluated as ®, and in the case of the magnetic flux density being 1.90 T or more and less than 1.92 T, the magnetic property was evaluated as O, and in the case of the magnetic flux density being less than 1.90 T, the magnetic property was evaluated as X. In the examination of the property of the glass film, an area ratio (%) of a defective area caused in the glass film was measured, and similarly to the first experiment, in the case of the area ratio being less than 1%, the property was evaluated as excellent (O) , and in the case of the area ratio being 1% or more and less than 5%, the property was evaluated as good (O), and in the case of the area ratio being 5% or more, the property was evaluated as poor (X) .
[0027] [Table 1]
(Table Removed)
[0028] [Table 2]
(Table Removed)
[0029] As shown in Table 1 and Table 2, in the case of the oxidation degree of the atmosphere in the previous stage period being 0.25 to 1.0 and the oxidation degree of the atmosphere in the subsequent stage period being 0.03 to 0.25, the magnetic flux
density B8 of 1.90 T or more was obtained, and the glass film with a good property was obtained. As a reason why such a result is obtained, phenomena as below may be considered.
[0030] When the annealing is performed in the atmosphere with an oxidation degree of 0.25 or more at 800°C to 900°C, an external oxide film having an Fe-based oxide (Fe2SiO4 and/or FeO) as its main component is formed on the surface of the steel strip, and further in the inside thereof, an internal oxide layer having SiO2 as its main component is developed. An atmosphere sealing property of the external oxide film having Fe2SiO4 and/or FeO as its main component is lower than that of the external oxide film having FeSiO3 as its main component. Thus, when the main component of the external oxide film is Fe2SiO4 and/or FeO, in the heating process of the finish annealing, additional oxidation ascribable to hydrated moisture of an annealing separating agent occurs and the internal oxide layer changes. Then, it is conceivable that the formation of the glass film becomes unstable due to the change in the internal oxide layer. Further, the change of the internal oxide layer affects absorption of nitrogen through the internal oxide layer and denitrification behavior to make distributions of (Al, Si)N functioning as inhibitors non-uniform and to make secondary recrystallization unstable. Due to such a
reason, it is conceivable that when the oxidation degree of the atmosphere in the annealing is always more than 0.25, the magnetic flux density B8 is reduced and the property of the glass film deteriorates. In contract to this, if the previous stage of annealing is performed in the atmosphere with an oxidation degree of 0.25 or more, and then the subsequent stage of annealing is performed in the atmosphere with an oxidation degree of 0.03 to 0.25, the main component of the external oxide film turns into FeSiO3, and thereby the atmosphere sealing property increases. Thus, it is conceivable that the above-described additional oxidation is suppressed to make the distributions of (Al, Si)N uniform and to make the secondary recrystallization stable, and further the glass film is also formed stably. Further, when the results shown in Table 1 and Table 2 are considered, it is important that the oxidation degree of the atmosphere in the previous stage period is 0.25 to 1.0, and it is preferable that the oxidation degree of the atmosphere in the previous stage period is 0.25 to 0.80, and it is important that the oxidation degree of the atmosphere in the subsequent stage period is 0.03 to 0.25, and it is preferable that the oxidation degree of the atmosphere in the subsequent stage period is 0.06 to 0.20 .
•0031] Further, the inventors of the present invention set the oxidation degree in the previous stage period to 0.44 and measured infrared reflection spectra of the decarburization-annealed steel strips by changing the oxidation degree in the subsequent stage period, and then obtained a result shown in Fig. 4. In Fig. 4, (a) shows the infrared reflection spectrum of the sample in which the oxidation degree in the subsequent stage period was set to 0.44, and (b) shows the infrared reflection spectrum of the sample in which the oxidation degree in the subsequent stage period was set to 0.20, and (c) shows the infrared reflection spectrum of the sample in which the oxidation degree in the subsequent stage period was set to 0.012. As shown in Fig. 4, as the oxidation degree in the subsequent stage period reduces, a main peak of the infrared ray reflection spectrum changes from Fe2SiO4 to FeSiO3, and changes from FeSiO3 to SiO2.
[0032] Next, an embodiment of the present invention made on the knowledge will be explained. [0033] First, limitation reasons of the components of the silicon steel material will be explained. [0034] The silicon steel material used in the embodiment contains Si: 0.8 mass% to 7 mass%, acid-soluble Al: 0.01 mass% to 0.065 mass%, N: 0.004 mass% to 0.012 mass%, Mn: 0.05 mass% to 1 mass%, and S and Se: 0.003 mass% to 0.015 mass% in total amount, a C
Content being 0.085 mass% or less, and a balance being composed of Fe and inevitable impurities. [0035] Si increases electrical resistance to reduce a core loss. However, when a Si content exceeds 7 mass%, the cold rolling becomes quite difficult to be performed, and a crack occurs easily at the time of cold rolling. Thus, the Si content is set to 7 mass% or less, and is preferably 4.5 mass% or less, and is more preferably 4 mass% or less. Further, when the Si content is less than 0.8 mass%, a y transformation is caused at the time of finish annealing to thereby make a crystal orientation of the grain-oriented electrical steel sheet deteriorate. Thus, the Si content is set to 0.8 mass% or more, and is preferably 2 mass% or more, and is more preferably 2.5 mass% or more.
[0036] C is an element effective for controlling a primary recrystallization grain structure, but adversely affects the magnetic property. Thus, in this embodiment, before the finish annealing (step S7), the decarburization annealing is performed (step S5). However, when the C content exceeds 0.085 mass%, a time taken for the decarburization annealing becomes long, and productivity in industrial production is impaired. Thus, the C content is set to 0.085 mass% or less, and is preferably 0.07 mass% or less.
[0037] Acid-soluble Al bonds to N to be precipitated as (Al, Si)N and functions as an inhibitor. In a case that a content of acid-soluble Al falls within a range of 0.01 mass% to 0.065 mass%, the secondary recrystallization is stabilized. Thus, the content of acid-soluble Al is set to not less than 0.01 mass% nor more than 0.065 mass%. Further, the content of acid-soluble Al is preferably 0.02 mass% or more, and is more preferably 0.025 mass% or more. Further, the content of acid-soluble Al is preferably 0.04 mass% or less, and is more preferably 0.03 mass% or less.
[0038] N bonds to Al to function as an inhibitor. When an N content is less than 0.004 mass%, a sufficient amount of the inhibitor cannot be obtained. Thus, the N content is set to 0.004 mass% or more, and is preferably 0.006 mass% or more, and is more preferably 0.007 mass% or more. On the other hand, when the N content exceeds 0.012 mass%, a hole called a blister occurs in the steel strip at the time of cold rolling. Thus, the N content is set to 0.012 mass% or less, and is preferably 0.010 mass% or less, and is more preferably 0.009 mass% or less. [0039] Mn, S and Se produce MnS and MnSe to be a nucleus for which A1N is preferentially precipitated. In a case that a Mn content falls within a range of 0.05 mass% to 1 mass%, the secondary recrystallization is stabilized. Thus, the Mn
content is set to not less than 0.05 mass% nor more than 1 mass%. Further, the Mn content is preferably 0.08 mass% or more, and is more preferably 0.09 mass% or more. Further, the Mn content is preferably 0.50 mass% or less, and is more preferably 0.2 mass% or less .
[0040] Further, in a case that a content of S and
Se falls within a range of 0.003 mass% to 0.015 mass%
in total amount, the secondary recrystallization is
stabilized. Thus, the content of S and Se is set to
not less than 0.003 mass% nor more than 0.015 mass%
in total amount. Further, in terms of preventing
occurrence of a crack in the hot rolling, an
inequation (1) below is preferably satisfied.
Incidentally, only either S or Se may be contained in
the silicon steel material, or both S and Se may also
be contained in the silicon steel material.
[Mn]/( [S] + [Se] ) ≥4 (1)
[0041] Ti forms coarse TiN to affect precipitation amount of (Al, Si)N functioning as an inhibitor. When a Ti content exceeds 0.004 mass%, the good magnetic property is not easily obtained. Thus, the Ti content is preferably 0.004 mass% or less. [0042] Further, one or more selected from a group consisting of Cr, Cu, Ni, P, Mo, Sn, Sb, B, and Bi may also be contained in the silicon steel material in ranges below.
[0043] Cr improves the oxide layer formed at the time of decarburization annealing, and is effective for forming the glass film when the oxide layer and MgO being the main component of the annealing separating agent react at the time of finish annealing. However, when a Cr content exceeds 0.3 mass%, decarburization is remarkably prevented. Thus, the Cr content is set to 0.3 mass% or less.
[0044] Cu increases specific resistance to reduce a core loss. However, when a Cu content exceeds 0.4 mass%, the effect is saturated. Further, a surface flaw called "copper scab" is sometimes caused at the time of hot rolling. Thus, the Cu content is set to 0.4 mass% or less.
[0045] Ni increases specific resistance to reduce a core loss. Further, Ni controls a metallic structure of the hot-rolled steel strip to improve the magnetic property. However, when a Ni content exceeds 1 mass%, the secondary recrystallization becomes unstable. Thus, the Ni content is set to 1 mass% or less .
[0046] P increases specific resistance to reduce a core loss. However, when a P content exceeds 0.5 mass%, a fracture occurs easily at the time of cold rolling due to embrittlement. Thus, the P content is set to 0.5 mass% or less.
[0047] Mo improves a surface property at the time of hot rolling. However, when a Mo content exceeds
0.1 mass%, the effect is saturated. Thus, the Mo content is set to 0.1 mass% or less. [0048] Sn and Sb are grain boundary segregation elements. The silicon steel material used in the embodiment contains Al, so that there is sometimes a case that Al is oxidized by moisture released from the annealing separating agent depending on the condition of the finish annealing. In this case, variations in inhibitor strength occur depending on positions in the grain-oriented electrical steel sheet, and the magnetic property also sometimes varies. However, in a case that the grain boundary segregation elements are contained, the oxidation of Al can be suppressed. That is, Sn and Sb suppress the oxidation of Al to suppress the variations in the magnetic property. On the other hand, when a content of Sn and Sb exceeds 0.3 mass%, the oxide layer is not easily formed at the time of decarburization annealing, and thereby the formation of the glass film when the oxide layer and MgO being the main component of the annealing separating agent react at the time of finish annealing becomes insufficient. Further, the decarburization is remarkably prevented. Thus, the content of each of Sn and Sb is set to 0.3 mass% or less, and the total content is preferably 0.3 mass% or less.
[0049] B bonds to N to be precipitated as BN and functions as an inhibitor. However, when a B content
exceeds 0.008 mass%, the cold rolling becomes difficult to be performed. Thus, the B content is set to 0.008 mass% or less.
[0050] Bi stabilizes a precipitate such as a sulfide to strengthen the function as an inhibitor. However, when a Bi content exceeds 0.01 mass%, the formation of the glass film is adversely affected. Thus, the Bi content is set to 0.01 mass% or less. [0051] Next, each process in the embodiment will be explained.
[0052] The silicon steel material (slab) having the above-described components may be formed in a manner that, for example, steel is melted in a converter, an electric furnace, or the like, and the molten steel is subjected to a vacuum degassing treatment according to need, and next is subjected to continuous casting. Further, the silicon steel material may also be formed in a manner that in place of the continuous casting, an ingot is made to then be bloomed. The thickness of the silicon steel slab is set to, for example, 150 mm to 350 mm, and is preferably set to 220 mm to 280 mm. Further, what is called a thin slab having a thickness of 30 mm to 70 mm may also be formed. In the case that the thin slab is formed, rough rolling performed when obtaining the hot-rolled steel strip may be omitted. [0053] After the silicon steel slab is formed, the slab heating is performed (step S1), and the hot
rolling (step S2) is performed to obtain the hot-rolled steel strip. Next, the hot-rolled steel strip is annealed (step S3). Thereafter, the cold rolling is performed (step S4). As described above, the cold rolling may be performed only one time, or the cold rolling may also be performed a plurality of times with the intermediate annealing performed therebetween. In the cold rolling, the final cold rolling rate is preferably set to 80% or more. This is to develop a good primary recrystallization texture. Subsequently, the decarburization annealing is performed (step S5).
[0054] In the decarburization annealing, in an atmosphere with an oxidation degree of 0.25 to 1.0, temperature is increased in the temperature range of 750°C to 800°C at an average heating rate of 2.5°C/s or more, and next in an atmosphere with an oxidation degree of 0.25 to 1.0, a first soaking treatment is performed at a temperature of 800°C to 900°C, and next in an atmosphere with an oxidation degree of 0.03 to 0.25, a second soaking treatment is performed at a temperature of 800°C to 900°C.
[0055] With regard to increasing the temperature, the reason why the oxidation degree in the temperature range of 750°C to 800°C is set to 0.25 to 1.0, and the average heating rate is set to 2.5°C/s or more is because the first and second experiments were considered. That is, in the case that these
conditions are satisfied, it becomes possible to suppress the formation of the external oxide film having a high atmosphere sealing property and having FeSiO3 as its main component and to thereafter form the good internal oxide layer. That is, the average heating rate in the temperature range where the external oxide film having FeSiO3 as its main component is formed is increased, thereby reducing the time when the steel strip is kept in the temperature range to suppress the external oxide film having FeSiO3 as its main component. Incidentally, the reason why the lower limit of the temperature range where the oxidation degree and the average heating rate as above are controlled is set to 750°C is because the lower limit of a temperature range where oxidation of the steel strip practically occurs at a current heating rate is 750°C. Further, the reason why the upper limit is set to 800°C is because at a temperature of 800°C or more, FeSiO3, which is thermally metastable, is not produced but FeSiO4, which is thermally stable, is substantially to be produced. With regard to the first soaking treatment, the reason why the oxidation degree is set to 0.25 to 1.0 is because the second experiment was considered. That is, in the case that the condition is satisfied, it becomes possible to form the good internal oxide layer. That is, forming the external oxide film with a low atmosphere sealing property
takes it possible to uniformly form the good internal oxide layer. With regard to the second soaking treatment, the reason why the oxidation degree is set to 0.03 to 0.25 is because the second experiment was considered. That is, in the case that the condition is satisfied, it becomes possible to set the main component of the external oxide film to FeSiO3 to thereby secure a high atmosphere sealing property after forming the good internal oxide layer. Incidentally, the reason why the temperature of the first and second soaking treatments is set to 800°C to 900°C is because when the temperature is less than 800°C, the time required for the sufficient decarburization becomes too long and productivity reduces, and when the temperature exceeds 900°C, speeds of oxidation reaction and grain growth become too fast, thereby making it difficult to stably control grain structures and the formations of the internal oxide layer and the external oxide film. [0056] As a result of the decarburization annealing, as described above, the good external and internal oxide layer is formed and C contained in the steel strip is removed. Incidentally, the decarburization annealing is preferably performed at a time such that, for example, a crystal grain diameter obtained by the primary recrystallization becomes 15 urn or more. This is to obtain the good magnetic property. Further, the total time taken for
the first and second soaking treatments may be time suitable for removing C and controlling the crystal grain diameter and the internal oxide layer, and is set to 60 seconds to 180 seconds, for example. Particularly, the time taken for the second soaking treatment (time for the subsequent stage period) is preferably set for 5 seconds or more in terms of improving the external oxide layer. The upper limit of this time is not limited in particular, but if it becomes too long, it is not desirable in terms of productivity.
[0057] After the decarburization annealing (step S5), the nitriding treatment is performed (step S6). This is to form inhibitors of (Al, Si)N. In the nitriding treatment, annealing may be performed in an atmosphere containing gas having nitriding capability such as ammonia, for example. In order to perform the secondary recrystaliization more stably, it is desirable that the degree of nitriding in the nitriding treatment (step S6) is adjusted and the compositions of (Al, Si)N in the steel strip obtained after the nitriding treatment are adjusted. For example, according to the Al content, the B content, and a content of Ti existing inevitably, the degree of nitriding is preferably controlled so as to satisfy an inequation (2) below, and the degree of nitriding is more preferably controlled so as to satisfy an inequation (3) below.
[N] ≥14/27[Al]+14/11[B]+14/48 [Ti] (2)
[N] ≥2/3 [Al]+14/11[B]+14/48 [Ti] (3)
Here, [N] represents the N content (mass%) of the steel strip obtained after the nitriding treatment, [Al] represents the acid-soluble Al content (mass%) of the steel strip obtained after the nitriding treatment, [B] represents the B content (mass%) of the steel strip obtained after the nitriding treatment, and [Ti] represents the Ti content (mass%) of the steel strip obtained after the nitriding treatment.
[0058] After the nitriding treatment (step S6), coating of the annealing separating agent containing MgO and the finish annealing are performed (step S7). As a result, the crystal grains oriented in the {110}<001> orientation preferentially grow by the secondary recrystallization. Further, by the reaction of the internal oxide layer formed at the time of decarburization annealing and MgO in the annealing separating agent, the glass film having forsterite as its main component is formed. [0059] The method of the finish annealing (step S7) is also not limited in particular. However, in order to stabilize the secondary recrystallization and the formation of the glass film, it is preferable not to change the internal oxide layer formed in the decarburization annealing as much as possible while controlling the initial atmosphere gas of the finish
Annealing. From this point of view, it is preferable that at the time of heating in the finish annealing, in the temperature range of 800°C or less, the ratio of nitrogen (nitrogen fraction) in the atmosphere gas is set to 25% to 75% and the remaining in the atmosphere gas is set to hydrogen, and the oxidation degree is set to 0.015 to 0.2. With regard to the component of the atmosphere gas, an inert gas such as argon may also be used in place of the nitrogen gas, but the nitrogen gas is desirable in terms of cost. Further, the hydrogen gas is important to control the oxidation degree. Incidentally, when the nitrogen fraction is less than 25%, denitrification is likely to occur excessively, and when the nitrogen fraction exceeds 75%, the denitrification becomes difficult to be performed. Then, in the both cases, variations in behavior of the secondary recrystallization are likely to occur depending on the area of the steel strip wound in a coil shape. Thus, the nitrogen fraction is preferably 25% to 75%. Further, when the oxidation degree is less than 0.015, the external oxide film having FeSiO3 as its main component on the surface of the steel strip is likely to be reduced, and when the oxidation degree exceeds 0.2, the additional oxidation is likely to occur. Then, in the both cases, the secondary recrystallization and the formation of the glass film are sometimes adversely affected. Thus, the oxidation degree is
preferably 0.015 to 0.2. Further, in the temperature range of 800°C or more in the finish annealing, it is desirable that the oxidation degree is set to 0.015 or less in terms of prevention of the additional oxidation. In the temperature range of 800°C or more, the temperature is increased up to 1200°C in atmosphere gas containing, for example, hydrogen and nitrogen, and at 1200°C, the atmosphere gas is switched to hydrogen atmosphere gas, and the precipitates of (Al, Si)N and so on are purified. [0060] According to this embodiment as above, the grain-oriented electrical steel sheet excellent in the magnetic property can be manufactured stably.
EXAMPLE
[0061]
In Example 1, the effect of the conditions of the decarburization annealing was confirmed in a component base containing Mn and Se.
[0062] First, silicon steel slabs containing, in mass%, Si: 3.2%, C: 0.05%, acid-soluble Al : 0.025%, N: 0.007%, Mn: 0.14%, and Se : 0.01%, and a balance being composed of Fe and inevitable impurities were formed. Next, the silicon steel slabs were heated at a temperature of 1200°C and were hot rolled to obtain hot-rolled steel strips each having a thickness of 2.3 mm. Subsequently, the hot-rolled steel strips were annealed at 1120°C. Next, cold rolling was
Performed to obtain cold-rolled steel strips each having a thickness of 0.22 mm. Thereafter, decarburization annealing was performed in a mixed gas atmosphere of N2 gas: 15% to 90% and H2 gas: 85% to 10% to obtain decarburization-annealed steel strips. In the decarburization annealing, the temperature was increased in a temperature range of room temperature to 750°C at a heating rate of 15°C/s, and thereafter the temperature was increased in a temperature range of not less than 750°C nor more than 800°C at a heating rate shown in Table 3, and thereafter a soaking treatment for 120 seconds was performed at a temperature of 850°C. Further, an oxidation degree of an atmosphere while increasing the temperature from room temperature (oxidation degree in a heating period), an oxidation degree of an atmosphere for the period of the first 90 seconds out of 120 seconds in the soaking treatment (oxidation degree at the previous stage), and an oxidation degree of an atmosphere for the period of the remaining 30 seconds out of 120 seconds in the soaking treatment (oxidation degree at the subsequent stage) were changed as shown in Table 3. [0063] After the decarburization annealing, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.022 mass%. Next, an annealing separating agent having MgO as its main
component was coated on the steel strips, and then finish annealing was performed. In the finish annealing, the steel strips were heated up to 1200°C at a speed of 15°C/h.
[0064] Then, a relationship between the oxidation degree of the atmosphere in the decarburization annealing, a magnetic property obtained after the finish annealing, and a property of a glass film was examined. A result of the examination is shown in Table 3. In the examination of the magnetic property, the magnetic flux density B8 was measured. In the examination of the property of the glass film, evaluation similar to that of the first experiment was performed.
[0065]
[Table 3]
(Table Removed)
[0066]
In Example 2, the effect of the conditions of the decarburization annealing was confirmed in a component base containing Mn, S and Se. [0067] First, silicon steel slabs containing, in mass%, Si: 3.3%, C: 0.06%, acid-soluble Al: 0.028%, N: 0.007%, Mn: 0.12%, S: 0.006%, and Se: 0.007%, and a balance being composed of Fe and inevitable impurities were formed. Next, the silicon steel slabs were heated at a temperature of 1150°C and were hot rolled to obtain hot-rolled steel strips each having a thickness of 2.3 mm. Subsequently, the hot-rolled steel strips were annealed at 1100°C. Next, cold rolling was performed to obtain cold-rolled steel strips each having a thickness of 0.22 mm. Thereafter, decarburization annealing was performed in a mixed gas atmosphere of N2 gas: 15% to 90% and H2 gas: 85% to 10% to obtain decarburization-annealed steel strips. In the decarburization annealing, the temperature was increased in a temperature range of room temperature to 750°C at a heating rate of 15°C/s, and thereafter the temperature was increased in a temperature range of not less than 750°C nor more than 800°C at a heating rate shown in Table 4, and thereafter a soaking treatment for 120 seconds was performed at a temperature of 850°C. Further, an oxidation degree of an atmosphere while increasing the temperature from room temperature (oxidation
degree in a heating period), an oxidation degree of an atmosphere for the period of the first 90 seconds out of 120 seconds in the soaking treatment (oxidation degree at the previous stage), and an oxidation degree of an atmosphere for the period of the remaining 30 seconds out of 120 seconds in the soaking treatment (oxidation degree at the subsequent stage) were changed as shown in Table 4. [0068] After the decarburization annealing, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.021 mass%. Next, an annealing separating agent having MgO as its main component was coated on the steel strips, and then finish annealing was performed. In the finish annealing, the steel strips were heated up to 1200°C at a speed of 15°C/h.
[0069] Then, a relationship between the oxidation degree of the atmosphere in the decarburization annealing, a magnetic property obtained after the finish annealing, and a property of a glass film was examined. A result of the examination is shown in Table 4. In the examination of the magnetic property, the magnetic flux density B8 was measured. In the examination of the property of the glass film, evaluation similar to that of the first experiment was performed. [0070]
[Table 4]
(Table Removed)
[0071]
In Example 3, the effect of added components was confirmed.
[0072] First, silicon steel slabs containing components shown in Table 5 and a balance being composed of Fe and inevitable impurities were formed. Next, the silicon steel slabs were heated at a temperature of 1100°C and were hot rolled to obtain hot-rolled steel strips each having a thickness of 2.3 mm. Subsequently, the hot-rolled steel strips were annealed at 1120°C. Next, cold rolling was performed to obtain cold-rolled steel strips each having a thickness of 0.22 mm. Thereafter, decarburization annealing was performed in a mixed gas atmosphere of N2 gas: 15% to 90% and H2 gas: 85% to 10% to obtain decarburization-annealed steel strips. In the decarburization annealing, the temperature was increased in a temperature range of room temperature to 750°C at a heating rate of 20°C/s, and thereafter the temperature was increased in a temperature range of not less than 750°C nor more than 800°C at a heating rate of 15°C/s, and thereafter a soaking treatment for 120 seconds was performed at a temperature of 830°C. Further, an oxidation degree of an atmosphere while increasing the temperature from room temperature (oxidation degree in a heating period), and an oxidation degree of an atmosphere for
the period of the first 90 seconds out of 120 seconds in the soaking treatment (oxidation degree at the previous stage) were set to 0.44, and an oxidation degree of an atmosphere for the period of the remaining 30 seconds out of 120 seconds in the soaking treatment (oxidation degree at the subsequent stage) was set to 0.15.
[0073] After the decarburization annealing, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.020 mass% to 0.024 mass%. Next, an annealing separating agent having MgO as its main component was coated on the steel strips, and then finish annealing was performed. In the finish annealing, the steel strips were heated up to 1200°C at a speed of 15°C/h.
[0074] Then, the magnetic flux density B8 was measured as a magnetic property obtained after the finish annealing. Further, by evaluation similar to that of the first experiment, a property of a glass film was examined. A result of the measurement and examination is shown in Table 5. [0075] [Table 5]
(Table Removed)
[0076]
In Example 4, the effect of the atmosphere gas in the finish annealing was confirmed.
[0077] First, silicon steel slabs containing, in mass%, Si: 3.3%, C: 0.06%, acid-soluble Al : 0.027%, N: 0.007%, Mn: 0.1%, and S: 0.007%, and a balance being composed of Fe and inevitable impurities were formed. Next, the silicon steel slabs were heated at a temperature of 1170°C and were hot rolled to obtain hot-rolled steel strips each having a thickness of 2.3 mm. Subsequently, the hot-rolled steel strips were annealed at 1100°C. Next, cold rolling was performed to obtain cold-rolled steel strips each having a thickness of 0.22 mm. Thereafter, decarburization annealing was performed in a mixed gas atmosphere of N2 gas: 15% to 90% and H2 gas: 85% to 10% to obtain decarburization-annealed steel strips. In the decarburization annealing, the temperature was increased in a temperature range of room temperature to 750°C at a heating rate of 20°C/s, and thereafter the temperature was increased in a temperature range of not less than 750°C nor more than 800°C at a heating rate of 15°C/s, and thereafter a soaking treatment for 120 seconds was performed at a temperature of 840°C. Further, an oxidation degree of an atmosphere while increasing the temperature from room temperature (oxidation degree in a heating period), and an oxidation degree of an atmosphere for
the period of the first 90 seconds out of 120 seconds in the soaking treatment (oxidation degree at the previous stage) were set to 0.59, and an oxidation degree of an atmosphere for the period of the remaining 30 seconds out of 120 seconds in the soaking treatment (oxidation degree at the subsequent stage) was set to 0.15.
[0078] After the decarburization annealing, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.022 mass%. Next, an annealing separating agent having MgO as its main component was coated on the steel strips, and then finish annealing was performed. In the finish annealing, the steel strips were heated up to 1200°C at a speed of 15°C/h. In the finish annealing, at the time of heating, the composition and oxidation degree of atmosphere gas in the temperature of 800°C or less were changed as shown in Table 6.
[0079] Then, the magnetic flux density B8 was measured as a magnetic property obtained after the finish annealing. Further, by evaluation similar to that of the first experiment, a property of a glass film was examined. A result of the measurement and examination is shown in Table 6.
[0080]
[Table 6]
(Table Removed)
It should be noted that the above embodiments merely illustrate concrete examples of implementing the present invention, and the technical scope of the present invention is not to be construed in a restrictive manner by these embodiments. That is, the present invention may be implemented in various forms without departing from the technical principles or main features thereof.
INDUSTRIAL APPLICABILITY
[0081] The present invention can be utilized in, for example, an industry of manufacturing electrical steel sheets and an industry in which electrical steel sheets are used.

CLAIMS
1. A manufacturing method of a grain-oriented electrical steel sheet, comprising:
heating a silicon steel material at a temperature of 1280°C or less, the silicon steel material containing:
Si: 0.8 mass% to 7 mass%;
acid-soluble Al: 0.01 mass% to 0.065 mass%; N: 0.004 mass% to 0.012 mass%; Mn: 0.05 mass% to 1 mass%; and at least one selected from a group consisting of S and Se: 0.003 mass% to 0.015 mass% in total amount,
a C content being 0.085 mass% or less, and a balance being composed of Fe and inevitable impurities;
hot rolling the heated silicon steel material to obtain a hot-rolled steel strip;
annealing the hot-rolled steel strip to obtain an annealed steel strip;
cold rolling the annealed steel strip one time or more to obtain a cold-rolled steel strip;
decarburization annealing the cold-rolled steel strip to obtain a decarburization-annealed steel strip in which primary recrystallization is caused;
performing a nitriding treatment in which an N content in the decarburization-annealed steel strip is increased to obtain a nitrided steel strip;
coating an annealing separating agent having MgO as its main component on the nitrided steel strip; and
finish annealing the nitrided steel strip having the annealing separating agent coated thereon to cause secondary recrystallization,
wherein
said decarburization annealing the cold-rolled steel strip comprises:
heating the cold-rolled steel strip in an atmosphere with an oxidation degree of 0.25 to 1.0 in a temperature range of 750°C to 800°C at an average heating rate of 2.5°C/s or more;
next performing a first soaking treatment in an atmosphere with an oxidation degree of 0.25 to 1.0 at a temperature of 800°C to 900°C; and
next performing a second soaking treatment in an atmosphere with an oxidation degree of 0.03 to 0.25 at a temperature of 800°C to 900°C.
2. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein the silicon steel material further contains at least one selected from a group consisting of Cr: 0.3 mass% or less, Cu: 0.4 mass% or less, Ni: 1 mass% or less, P: 0.5 mass% or less, Mo: 0.1 mass% or less,
Sn: 0.3 mass% or less, Sb: 0.3 mass% or less, B: 0.008 mass% or less, and Bi: 0.01 mass% or less.
3. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1 or 2, wherein said finish annealing the nitrided steel strip comprises:
heating the nitrided steel strip in a mixed gas atmosphere of nitrogen and hydrogen; and
next purifying a nitride in a hydrogen atmosphere, and
in said heating the nitrided steel strip in the mixed gas atmosphere, in a temperature range of 800°C or less, a ratio of nitrogen in the mixed gas atmosphere is set to 25% to 75%, and an oxidation degree of the mixed gas atmosphere is set to 0.015 to 0.2.