DESCRIPTION
MANUFACTURING METHOD OF GRAIN-ORIENTED ELECTRICAL STEEL SHEET, TREATING METHOD OF STEEL STRIP, AND
NITRIDED STEEL STRIP
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, a treating method of a steel strip, and a nitrided steel strip.
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. Then, conventionally, a grain-oriented electrical steel sheet has been manufactured by producing fine precipitates by various methods. There is a method in which, for example, slab heating before hot rolling is performed at a high temperature of over 1350°C to completely solid-dissolve an inhibitor forming element, and at the time of hot rolling and subsequent hot-rolled sheet annealing, an inhibitor is precipitated. The method is sometimes called high-temperature slab heating. Further, there is a method in which slab heating is performed at a relatively low temperature, and from the start of decarburization annealing to the occurrence of secondary recrystallization in finish annealing, a nitriding treatment is performed to precipitate an inhibitor (Japanese Laid-open Patent Publication No. 62-40315, Japanese Laid-open Patent Publication No. 05-112827, Japanese Laid-open Patent Publication No. 02-294428, Japanese Laid-open Patent Publication No. 11-29824, and Japanese Laid-open Patent Publication No. 2001-152250). The method is sometimes called low-temperature slab heating. When these methods are compared, the low-temperature slab heating is superior in productivity, cost, a load on a facility, and so on.
[0004] However, in the case when the low-temperature slab heating is employed to manufacture a grain-oriented electrical steel sheet, there is sometimes a case that an edge crack, fracture, and so on are caused when a steel strip is coiled after a nitriding treatment. Thus, in the case when the low-temperature slab heating is employed, it is sometimes difficult to manufacture a grain-oriented electrical steel sheet with a high yield.
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0005] The present invention has an object to
provide a grain-oriented electrical steel sheet
capable of being manufactured with a high yield and a
manufacturing method thereof.
SOLUTION TO PROBLEM
[0006] 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; hot rolling the heated silicon steel material to obtain a steel strip; annealing the steel strip; cold rolling one time or more the steel strip after the annealing; decarburization annealing the steel strip after the cold rolling; performing a nitriding treatment in which an N content of the steel strip is increased after the decarburization
annealing is finished or in parallel with the decarburization annealing to obtain a nitrided steel strip in which primary recrystallization is caused; 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 performing the nitriding treatment comprises: performing annealing at a temperature of 700°C to 1000°C in gas containing ammonia while running the steel strip; and next performing cooling, and an average cooling rate in a temperature range of 700°C to 200°C in the cooling is set to 5°C/s or more.
[0007] 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 average cooling rate in the temperature range of 700°C to 200°C in the cooling is set to 10°C/s or more. [0008] 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, an average cooling rate in a temperature range of 700°C to 500°C in the cooling is set to 20°C/s or more. [0009] In a manufacturing method of a grain-oriented electrical steel sheet according to a fourth
aspect of the present invention, in the method according to the first or second aspect, the average cooling rate in the temperature range of 700°C to 500°C in the cooling is set to 30°C/s or more. [0010] A treating method of a steel strip according to a fifth aspect of the present invention includes: performing annealing at a temperature of 700°C to 1000°C in gas containing ammonia while running a steel strip for a grain-oriented electrical steel sheet; and next performing cooling, wherein an average cooling rate in a temperature range of 700°C to 200°C in the cooling is set to 5°C/s or more. [0011] In a treating method of a steel strip according to a sixth aspect of the present invention, in the method according to the first aspect, the average cooling rate in the temperature range of 700°C to 200°C in the cooling is set to 10°C/s or more. [0012] In a treating method of a steel strip according to a seventh aspect of the present invention, in the method according to the fifth or sixth aspect, an average cooling rate in a temperature range of 700°C to 500°C in the cooling is set to 20°C/s or more.
[0013] In a treating method of a steel strip according to an eighth aspect of the present invention, in the method according to the fifth or sixth aspect, the average cooling rate in the temperature range of 700°C to 500°C in the cooling is
set to 30°C/s or more.
[0014] A nitrided steel strip according to a ninth aspect of the present invention is characterized in that it has the grain boundary coverage by precipitates of 0.5% or less.
ADVANTAGEOUS EFFECTS OF INVENTION
[0015] According to the present invention, it is possible to manufacture the grain-oriented electrical steel sheet with a high yield by suppressing a fracture or the like of the nitrided steel strip at the time of winding.
BRIEF DESCRIPTION OF DRAWINGS [0016] Fig. 1 is a flowchart showing a manufacturing method of a grain-oriented electrical steel sheet;
Fig. 2 is a view showing a relationship between an average cooling rate and brittleness of a nitrided steel strip; and
Fig. 3 is a view showing an optical micrograph of a sample with an average cooling rate of 0.1°C/s.
DESCRIPTION OF EMBODIMENTS
[0017] As a result of earnest studies on the cause of a crack or the like to be caused in coiling after a nitriding treatment, the present inventors thought that conditions of the nitriding treatment which is
necessary for low-temperature slab heating method may affect brittleness of a steel strip, and 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 based on low-temperature slab heating method.
[0018] First, as shown in Fig. 1, in step S1, 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. [0019] 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 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 steel strip obtained after the decarburization annealing is increased is performed to obtain a nitrided steel strip (steel sheet). Incidentally, it may also be designed that the nitriding treatment is not performed after the decarburization annealing but is performed in parallel with the decarburization annealing. That is, the nitriding treatment may also be performed on the cold-rolled steel strip. In the both cases, as a result of the decarburization annealing and the nitriding treatment, the nitrided steel strip in which the primary recrystallization is caused is obtained. 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 the nitrided steel strip having the annealing separating agent coated thereon is coiled. Subsequently, in step S8, finish annealing of the nitrided steel strip 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. For example, the decarburization annealing (step S5), the nitriding treatment (step S6), and the coating of the annealing separating agent (step S7) are continuously performed on the uncoiled and running steel strip, and the finish annealing (step S8) is performed on the coil-shaped steel strip.
[0020] In this manner, the grain-oriented electrical steel sheet can be obtained.
[0021] Then, as a result of the various experiments, the present inventors found that it is important to adjust a cooling condition in the nitriding treatment (step S6) to properly control the grain boundary coverage by precipitates in the nitrided steel strip. Concretely, the present inventors found that the adjustment of the cooling condition makes it possible to suppress
precipitations of (Si, Mn)N at the time of cooling to thereby suppress embrittlement of the steel strip, resulting that a crack, fracture, and so on caused at the time of coiling are suppressed. Then, the present inventors obtained the knowledge capable of manufacturing the grain-oriented electrical steel sheet with a high yield, and completed the present invention.
[0022] Here, the experiment conducted by the present inventors will be explained.
[0023] In the 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 hot rolled to obtain hot-rolled steel strips. 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 0.35 mm were. Thereafter, in mixed moist atmosphere gas of nitrogen and hydrogen, decarburization annealing was performed at a temperature of 830°C for 120 seconds. Subsequently, a nitriding treatment was performed on steel strips obtained after the decarburization annealing. [0024] In the nitriding treatment, atmosphere gas in which 5 vol% of ammonia was contained in the mixed
moist atmosphere gas of nitrogen and hydrogen was used. Further, at a temperature of 770°C, a soaking treatment for 30 seconds was performed, and thereafter a temperature range of 700°C to 200°C was cooled at various average cooling rates. Incidentally, the amount of N increased by the nitriding treatment in each of the steel strips was 500 ppm. In this manner, various samples were manufactured.
[0025] Then, a relationship between the average cooling rate from 700°C to 200°C in the nitriding treatment and brittleness of a nitrided steel strip was examined. A result thereof is shown in Fig. 2. In the examination of the brittleness, a reverse bending test was conducted under a temperature of 0°C. That is, 90-degree reverse bending was performed in a manner to wind the nitrided steel strip around a round bar with a diameter of 20 mm, and the bending numbers until the nitrided steel strips are fractured were measured. As shown in Fig. 2, in the case of the average cooling rate of 5°C/s or more in the temperature range of 700°C to 200°C, the reverse bending number was 10 or more, and in the case of the average cooling rate of 10°C/s or more, the reverse bending number was 50 or more. As one reason why such a result was obtained, facts as below are conceivable. [0026] When the present inventors observed an
optical micrograph at the depth of 30 urn from a surface of samples with the average cooling rate of 0.1°C/s, in which the reverse bending numbers were quite small, a lot of coarse precipitates existed along grain boundaries as shown in Fig. 3. On the other hand, in samples with the average cooling rate of 5°C/s and more, such coarse precipitates hardly-existed in grain boundaries. Consequently, it is conceivable that the coarse precipitates existing in the grain boundaries cause the embrittlement. Further, as a result of examining the coarse precipitates, it became clear that they are (Si, Mn)N. It is conceivable that part of nitrogen entering the steel strip in the nitriding treatment precipitates in crystal grains or grain boundaries as nitrides of (Al, Si)N, (Si, Mn)N, or the like during the soaking treatment, but a large amount of nitrogen exists as solid-solution nitrogen during the soaking treatment and precipitates as nitrides during the cooling. Further, in the low temperature range (700°C to 200°C or so) during the cooling, a diffusion rate of nitrogen is faster in the grain boundaries than in the crystal grain, so that the solid-solution nitrogen preferentially diffuses in the crystal grains. Further, in such a low temperature range, (Si, Mn)N are thermally stable among nitrides. Thus, it is conceivable that in the case of the slow average cooling rate, (Si, Mn)N preferentially
precipitate in the grain boundaries. Then, when the results shown in Fig. 2 and Fig. 3 are considered, it is important that the average cooling rate in the temperature range of 700°C to 200°C is 5°C/s or more, and the average cooling rate is preferably 10°C/s or more .
[0027] Further, the frequency of existence of the coarse precipitate in the grain boundaries varies according to the position (depth) in a sheet thickness direction, but it became clear that the grain boundary coverage is high in an inner layer at the depth of 20 µm to 50µm or so from the surface of the nitrided steel strip, and when a value of the grain boundary coverage exceeds 0.5%, the reverse bending numbers are noticeably low, and when the value exceeds 1%, the reverse bending numbers are lower.
[0028] Next, an embodiment of the present invention made on the knowledge will be explained. [0029] First, limitation reasons of the components of the silicon steel material will be explained. [0030] The content of a silicon steel material is not limited in particular. The silicon steel materials used for the conventional manufacturing methods for grain-oriented electrical steel sheet may be adopted. The silicon steel material contains, for example, 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. [0031] Si increases electrical resistance to reduce a core loss. However, when a Si content exceeds 7 mass%, the cold rolling becomes difficult to be performed, and a crack sometimes occurs at the time of cold rolling. Thus, the Si content is preferably 7 mass% or less, and is more preferably 4.5 mass% or less, and is still more preferably 4 mass% or less. Further, when the Si content is less than 0.8 mass%, there is sometimes a case that 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 preferably 0.8 mass% or more, and is more preferably 2 mass% or more, and is still more preferably 2.5 mass% or more.
[0032] 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 S8), 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 likely to be impaired. Thus, the C
content is preferably 0.085 mass% or less, and is more preferably 0.07 mass% or less. [0033] Acid-soluble Al bonds to N to be precipitated as (Al, Si)N and functions as inhibitors. 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 preferably not less than 0.01 mass% nor more than 0.065 mass%. Further, the content of acid-soluble Al is more preferably 0.02 mass% or more, and is still more preferably 0.025 mass% or more. Further, the content of acid-soluble Al is more preferably 0.04 mass% or less, and is still more preferably 0.03 mass% or less. [0034] N bonds to Al to function as an inhibitor. When an N content is less than 0.004 mass%, there is sometimes a case that a sufficient amount of the inhibitor cannot be obtained. Thus, the N content is preferably 0.004 mass% or more, and is more preferably 0.006 mass% or more, and is still more preferably 0.007 mass% or more. On the other hand, when the N content exceeds 0.012 mass%, a hole called a blister sometimes occurs in the steel strip at the time of cold rolling. Thus, the N content is preferably 0.012 mass% or less, and is more preferably 0.010 mass% or less, and is still more preferably 0.009 mass% or less. [0035] 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 preferably not less than 0.05 mass% nor
more than 1 mass%. Further, the Mn content is more
preferably 0.08 mass% or more, and is still more
preferably 0.09 mass% or more. Further, the Mn
content is more preferably 0.50 mass% or less, and is
still more preferably 0.2 mass% or less.
[0036] 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
preferably 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. In the
case that both S and Se are contained, it is possible
to promote precipitation of BN more stably and to
improve the magnetic property stably.
[Mn] /( [S] + [Se] ) ≥ 4 (1)
[0037] Ti forms coarse TiN to affect precipitation amounts of (Al, Si)N functioning as inhibitors. 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. [0038] 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.
[0039] 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 noticeably prevented. Thus, the Cr content is preferably 0.3 mass% or less. [0040] 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 preferably 0.4 mass% or less.
[0041] 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 recrystaliization becomes unstable. Thus, the Ni content is preferably 1 mass% or less . [0042] 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 preferably 0.5 mass% or less.
[0043] 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 preferably 0.1 mass% or less. [0044] 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 0.30 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 noticeably prevented. Thus, the content of Sn and Sb is preferably 0.3 mass% or less, and the total content is more preferably 0.3 mass% or less.
[0045] 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 preferably 0.008 mass% or less.
[0046] 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 preferably 0.01 mass% or less .
[0047] Next, each process in the embodiment will be explained.
[0048] 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 when the thin slab is formed, rough rolling performed when obtaining the hot-rolled steel strip may be omitted. [0049] 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.
[0050] Subsequently, the decarburization annealing is performed (step S5). As a result, C contained in the steel strip is removed. The decarburization annealing is performed in a moist atmosphere containing hydrogen and nitrogen, for example. Further, 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 in a temperature range of 750°C to 900°C. This is to
obtain the good magnetic property. [0051] Further, the nitriding treatment is performed (step S6) after the decarburization annealing is finished, or in parallel with the decarburization annealing. In the nitriding treatment, annealing (the soaking treatment) is performed on the uncoiled steel strip in a mixed gas atmosphere of hydrogen, nitrogen, and ammonia while running the steel strip, thereby making nitrogen enter the steel strip. For example, a temperature range of the annealing is set to 700°C to 1000°C, and a time taken for the annealing is set to 5 seconds to 180 seconds.
[0052] If the temperature of the annealing is 500°C or more, it is possible to uniformly diffuse nitrogen into the steel strip to obtain the good secondary recrystallization, but if it is less than 700°C, there is sometimes a case that a crack, fracture, and so on are caused at the time of subsequent coiling. Further, the decarburization annealing is performed at 750°C to 900°C or so as described above, and thereby in a case that the nitriding treatment is performed after the decarburization annealing is finished and the temperature of the nitriding treatment exceeds 1000°C, a grain structure obtained by the decarburization annealing sometimes changes greatly. Further, in a case that the nitriding treatment is performed in parallel with the
decarburization annealing and the temperature of the nitriding treatment exceeds 1000°C, a grain structure obtained after the decarburization annealing and the nitriding treatment sometimes does not become proper. [0053] The time taken for the annealing in the nitriding treatment is not limited in particular, and may be the degree to which a sufficient amount of nitrogen is allowed to enter the steel strip so as to perform the secondary recrystallization stably. A nitridation rate may be adjusted according to an amount of ammonia in the atmosphere gas and the annealing temperature in the nitriding treatment. However, when the amount of ammonia is high to reduce the time taken for the annealing in the nitriding treatment, variations in the degree of nitridation are likely to be increased between the positions and between the front and rear surfaces of the steel strip. Then, if the time taken for the annealing is less than 5 seconds, the variations are likely to be noticeable. On the other hand, as the time taken for the annealing becomes longer, the variations in the degree of nitridation are reduced, but productivity is likely to reduce. Then, if the time taken for the annealing exceeds 180 seconds, the reduction in the productivity is likely to be noticeable. [0054] As described above, in the nitriding treatment, the cooling condition after the soaking treatment is important. This is to suppress a crack,
fracture, and so on at the time of coiling. Then, in the embodiment, the average cooling rate in the temperature range of 700°C to 200°C is set to 5°C/s or more, and is preferably set to 10°C/s or more. When the cooling is performed at such an average rate, as shown in Fig. 2, the reverse bending number is high, namely a reverse bending property is improved. Further, the grain boundary coverage by the precipitates in the nitrided steel strip obtained by the nitriding treatment as above becomes 0.5% or less .
[0055] In order to further suppress the production of coarse inclusions in the grain boundaries, the average cooling rate in the temperature range of 700°C to 500°C is more preferably set to 20°C/s or more, and is still more preferably set to 30°C/s or more. When the cooling is performed at such an average rate, a mechanical property is improved. Incidentally, the upper limit of the cooling rate is not limited in particular in terms of the mechanical property, but if the cooling rate is accelerated excessively, there is a possibility that thermal strain is caused in a coil width direction (sheet width direction) to affect the secondary recrystallization.
Particularly, in the case of the average cooling rate being over 100°C/s, such an effect is concerned. [0056] The cooling method is not limited in particular. For example, the cooling may be
performed by using nitrogen gas or hydrogen gas, or the cooling may also be performed by using mixed gas of nitrogen gas and hydrogen gas. It should be noted that the cooling is preferably performed in a nonoxidation atmosphere in order to prevent oxidation of the steel strip.
[0057] After the nitriding treatment (step S6), the annealing separating agent containing MgO is coated on the surface of the nitrided steel strip, and the nitrided steel strip having the annealing separating agent coated thereon is coiled (step S7). Subsequently, finish annealing is performed for the nitrided steel strip (step S8). 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.
[0058] The method of the finish annealing (step S8) is also not limited in particular. For example, the temperature may be increased up to 1200°C in atmosphere gas containing hydrogen and nitrogen, and the atmosphere gas may be switched to hydrogen atmosphere gas at 1200°C, and the precipitates may be purified. [0059] According to this embodiment as above, it is
possible to manufacture the grain-oriented electrical
steel sheet with a high yield by suppressing a
fracture or the like of the nitrided steel strip at
the time of coiling.
[Example]
[0060]
First, silicon steel slabs containing, in mass%, Si: 3.1%, C: 0.05%, acid-soluble Al: 0.025%, N: 0.007%, Mn: 0.14%, and S: 0.007%, and a balance being composed of Fe and inevitable impurities were heated at 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 moist mixed gas atmosphere of nitrogen and hydrogen at 850°C for 120 seconds to obtain decarburization-annealed steel strips.
[0061] After the decarburization annealing, as a nitriding treatment, the decarburization-annealed steel strips were annealed at 750°C for 30 seconds in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.025 mass%, and then were cooled in a temperature range of 700°C to 200°C under various conditions shown in Table 1. 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 rate of 15°C/h so as to perform finish annealing. Then, after the finish annealing, an examination of the reverse bending property (brittleness) was conducted as an examination of the mechanical property. In Table 1, the conditions of the nitriding treatment and the mechanical property are shown. [0062] [Table 1]
(Table Removed)
[0063] In the examination of the reverse bending property (brittleness) , under a temperature of 0°C, 90-degree reverse bending was performed in a manner to wind a nitrided steel strip around a round bar with a diameter of 20 mm and the bending number until the nitrided steel strip is fractured was measured. Evaluation by the reverse bending was set that the reverse bending number until the fracture is less than 30, which is I, and the reverse bending number until the fracture is 30 to 39, which is II, and the reverse bending number until the fracture is 40 to 50, which is III, and the reverse bending number until the fracture is 50 or more, which is IV.
[0064]
In Example 2, first, cold-rolled steel strips were obtained similarly to Example 1. Next, in a moist mixed gas atmosphere of ammonia, nitrogen, and hydrogen, the cold-rolled steel strips were decarburization annealed at 850°C for 120 seconds, and a nitriding treatment was performed in parallel with the decarburization annealing to increase nitrogen in the steel strips to 0.023 mass%, and thereafter the steel strips were cooled in a temperature range of 700°C to 200°C under various conditions shown in Table 2. Subsequently, 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 rate of 15°C/h so as to perform finish annealing. Then, similarly to Example 1, after the finish annealing, an examination of the reverse bending property (brittleness) was conducted as an examination of the mechanical property. In Table 2, the conditions of the nitriding treatment and the mechanical property are shown. [0065] [Table 2]
(Table Removed)
[0066]
First, silicon steel slabs containing, in mass%, Si: 3.2%, C: 0.05%, acid-soluble Al: 0.024%, N: 0.005%, Mn: 0.04%, and S: 0.01%, and a balance being composed of Fe and inevitable impurities were heated at 1320°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 0.26 mm. Thereafter, decarburization annealing was performed in a moist mixed gas atmosphere of nitrogen and hydrogen at 850°C for 150 seconds to obtain decarburization-annealed steel strips.
[0067] After the decarburization annealing, as a nitriding treatment, the decarburization-annealed steel strips were annealed at 750°C for 30 seconds in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.024 mass%, and then were cooled in a temperature range of 700°C to 200°C under various conditions shown in Table 3. 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 rate of 15°C/h so as to perform finish annealing. Then, similarly to Example 1, after the finish annealing, an examination of the reverse bending property
(brittleness) was conducted as an examination of the mechanical property. In Table 3, the conditions of the nitriding treatment and the mechanical property are shown. [0068] [Table 3]
(Table Removed)
[0069]
First, silicon steel slabs containing, in mass%, Si: 3.3%, C: 0.06%, acid-soluble Al: 0.027%, N: 0.008%, Mn: 0.11%, and S: 0.06%, and a balance being composed of Fe and inevitable impurities were heated at 1150°C and were hot rolled to obtain hot-rolled steel strips each having a thickness of 2.0 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 0.20 mm. Thereafter, decarburization annealing was performed in a moist mixed gas atmosphere of nitrogen and hydrogen at 850°C for 100 seconds to obtain decarburization-annealed steel strips.
[0070] After the decarburization annealing, as a nitriding treatment, the decarburization-annealed
steel strips were annealed at 750°C for 20 seconds in an ammonia containing atmosphere to increase nitrogen in the steel strips to 0.03 mass%, and then were cooled in a temperature range of 700°C to 200°C under various conditions shown in Table 4. 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 rate of 15°C/h so as to perform finish annealing. Then, similarly to Example 1, after the finish annealing, an examination of the reverse bending property
(brittleness) was conducted as an examination of the mechanical property. Further, the grain boundary coverage by precipitates in an inner layer at the depth of 20 µm from the surface of each of nitrided steel strips was measured. In Table 4, the conditions of the nitriding treatment, the grain boundary coverage by the precipitates, and the mechanical property are shown.
[0071]
[Table 4]
(Table Removed)
[0072] In the measurement of the grain boundary coverage by the precipitates, each of the nitrided steel strips was polished to the position of the
inner layer at the depth of 20 µm to 50 µm from the front surface, and then was subjected to an etching treatment with an etchant such as a nital solution, and a metallic structure (grain boundaries and precipitates) were observed. Then, by image processing, the length of the grain boundaries in which the precipitates exist and the total length of the grain boundaries were measured, and the ratio of the length of the grain boundaries in which the precipitates exist to the total length of the grain boundaries was calculated as the grain boundary coverage by the precipitates. [0073] 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 spirit or main features thereof.
INDUSTRIAL APPLICABILITY
[0074] 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 What is claimed is:
1. A manufacturing method of a grain-oriented electrical steel sheet, comprising:
heating a silicon steel material;
hot rolling the heated silicon steel material to obtain a steel strip;
annealing the steel strip;
cold rolling one time or more the steel strip after the annealing;
decarburization annealing the steel strip after the cold rolling;
performing a nitriding treatment in which an N content of the steel strip is increased after the decarburization annealing is finished or in parallel with the decarburization annealing to obtain a nitrided steel strip in which primary recrystallization is caused;
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 performing the nitriding treatment comprises:
performing annealing at a temperature of 700°C to
1000°C in gas containing ammonia while running the steel strip; and
next performing cooling, and
an average cooling rate in a temperature range of 700°C to 200°C in the cooling is set to 5°C/s or more.
2. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1, wherein the average cooling rate in the temperature range of 700°C to 200°C in the cooling is set to 10°C/s or more .
3. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1 or 2, wherein an average cooling rate in a temperature range of 700°C to 500°C in the cooling is set to 20°C/s or more.
4. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the average cooling rate in the temperature range of 700°C to 500°C in the cooling is set to 30°C/s or more.
5. A treating method of a steel strip, compris ing:
performing annealing at a temperature of 700°C to 1000°C in gas containing ammonia while running a steel strip for a grain-oriented electrical steel sheet; and
next performing cooling, wherein
an average cooling rate in a temperature range of
700°C to 200°C in the cooling is set to 5°C/s or more.
6. The treating method of a steel strip
according to claim 5, wherein the average cooling
rate in the temperature range of 700°C to 200°C in the
cooling is set to 10°C/s or more.
7. The treating method of a steel strip
according to claim 5 or 6, wherein an average cooling
rate in a temperature range of 700°C to 500°C in the
cooling is set to 20°C/s or more.
8. The treating method of a steel strip
according to claim 5 or 6, wherein the average
cooling rate in the temperature range of 700°C to
500°C in the cooling is set to 30°C/s or more.
9. A nitrided steel strip whose grain boundary
coverage by precipitates is 0.5% or less.