METHOD FOR MANUFACTURING GRAIN-ORIENTED ELECTRICAL STEEL SHEET, AND NITRIDING APPARATUS

Abstract

In a grain-oriented electrical steel sheet manufacturing process of processing a steel slab having a predetermined composition to a final sheet thickness and then performing primary recrystallization annealing and nitriding treatment, the nitriding treatment is performed in at least two stages of temperatures including high-temperature nitriding and low-temperature nitriding, and a residence time in the high-temperature nitriding is 3 seconds or more and 600 seconds or less. In this way, nitrogen is efficiently diffused into the steel of the steel sheet before secondary recrystallization to precipitate AlN. Such a method can manufacture a grain-oriented electrical steel sheet having excellent magnetic property.

Claims

1. A method for manufacturing a grain-oriented electrical steel sheet comprising: hot rolling a steel slab to obtain a hot rolled sheet, the steel slab having a chemical composition containing, in mass %: C: 0.10% or less; Si: 1.0% to 5.0%; Mn: 0.01% to 0.5%; one or two selected from S and Se: 0.002% to 0.040% in total; sol.Al: 0.01% to 0.08%; and N: 0.0010% to 0.020%, with a balance being Fe and incidental impurities; hot band annealing the hot rolled sheet if required; cold rolling the hot rolled sheet once or twice or more with intermediate annealing in between, to obtain a cold rolled sheet having a final sheet thickness; and performing primary recrystallization annealing and nitriding treatment on the cold rolled sheet, and then applying an annealing separator and performing secondary recrystallization annealing to obtain a grain-oriented electrical steel sheet, wherein the nitriding treatment is performed in at least two stages of temperatures including high-temperature nitriding and low-temperature nitriding that follows the high-temperature nitriding, and a residence time in the high-temperature nitriding is 3 seconds or more and 600 seconds or less.

2. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, one or more selected from: Ni: 0.005% to 1.50%; Sn: 0.01% to 0.50%; Sb: 0.005% to 0.50%; Cu: 0.01% to 0.50%; Cr: 0.01% to 1.50%; P: 0.0050% to 0.50%; Nb: 0.0005% to 0.0100%; Mo: 0.01% to 0.50%; Ti: 0.0005% to 0.0100%; B: 0.0001% to 0.0100%; and Bi: 0.0005% to 0.0100%.

3. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the high-temperature nitriding is performed at 850° C. or more, and the low-temperature nitriding is performed at less than 850° C.

4. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein in the primary recrystallization annealing, a heating rate between 500° C. and 700° C. is 50° C./s or more.

5. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 1, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

6. The nitriding apparatus according to claim 5, comprising a gas cooling zone between the high-temperature treatment portion and the low-temperature treatment portion.

7. The nitriding apparatus according to claim 5, serving to adjust a temperature of the high-temperature treatment portion to 850° C. or more and a temperature of the low-temperature treatment portion to less than 850° C.

8. The method for manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein the high-temperature nitriding is performed at 850° C. or more, and the low-temperature nitriding is performed at less than 850° C.

9. The method for manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein in the primary recrystallization annealing, a heating rate between 500° C. and 700° C. is 50° C./s or more.

10. The method for manufacturing a grain-oriented electrical steel sheet according to claim 3, wherein in the primary recrystallization annealing, a heating rate between 500° C. and 700° C. is 50° C./s or more.

11. The method for manufacturing a grain-oriented electrical steel sheet according to claim 8, wherein in the primary recrystallization annealing, a heating rate between 500° C. and 700° C. is 50° C./s or more.

12. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 2, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

13. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 3, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

14. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 4, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

15. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 8, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

16. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 9, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

17. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 10, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

18. A nitriding apparatus used in the method for manufacturing a grain-oriented electrical steel sheet according to claim 11, the nitriding apparatus comprising: a nitriding gas supply pipe for introducing gas including at least ammonia or nitrogen; and a nitriding treatment portion for successively performing high-temperature nitriding and low-temperature nitriding in nitriding treatment, wherein the nitriding treatment portion includes a high-temperature treatment portion for performing the high-temperature nitriding and a low-temperature treatment portion for performing the low-temperature nitriding, and the nitriding gas supply pipe to the high-temperature treatment portion includes a cooling device.

19. The nitriding apparatus according to claim 6, serving to adjust a temperature of the high-temperature treatment portion to 850° C. or more and a temperature of the low-temperature treatment portion to less than 850° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In the accompanying drawings:

[0045] FIG. 1 is a diagram illustrating a suitable nitriding apparatus according to one of the disclosed embodiments; and

[0046] FIG. 2 (a) is a photograph of an SEM observation image of a section of a nitrided steel sheet formed under condition 3 in Examples, taken along the direction orthogonal to the rolling direction, and (b) and (c) are each a graph illustrating the result of analyzing texture in a designated part of the SEM observation image by energy-dispersive X-ray analysis (EDX).

DETAILED DESCRIPTION

[0047] Detailed description is given below.

[0048] The reasons for limiting the chemical composition of a steel slab are described first. In the following description, “%” denotes “mass %” unless otherwise noted.

[0049] C: 0.10% or less

[0050] C is an element useful in improving primary recrystallized texture. When the C content is more than 0.10%, however, the primary recrystallized texture degrades. The C content is therefore limited to 0.10% or less. The C content is desirably in the range of 0.01% to 0.08%, in terms of magnetic property. In the case where the required level of magnetic property is not so high, the C content may be 0.01% or less and 0.0005% or more in order to omit or simplify decarburization in primary recrystallization annealing.

[0051] Si: 1.0% to 5.0%

[0052] Si is an element useful in improving iron loss by increasing electrical resistance. When the Si content is more than 5.0%, however, cold rolling manufacturability decreases significantly. The Si content is therefore limited to 5.0% or less. Since Si is required to function as a nitride forming element, the Si content needs to be 1.0% or more. The Si content is desirably in the range of 1.5% to 4.5%, in terms of both iron loss property and cold rolling manufacturability.

[0053] Mn: 0.01% to 0.5%

[0054] Mn has an effect of improving hot workability during manufacture. When the Mn content is 0.01% or less, its effect is insufficient. When the Mn content is more than 0.5%, the primary recrystallized texture deteriorates and leads to lower magnetic property. The Mn content is therefore limited to 0.5% or less.

[0055] One or two selected from S and Se: 0.002% to 0.040% in total

[0056] S and Se are each a useful element that combines with Mn or Cu to form MnSe, MnS, Cu.sub.2−xSe, or Cu.sub.2−xS and thus exerts an inhibitor effect as a second dispersion phase in the steel. When the total content of S and Se is less than 0.002%, their effect is insufficient. When the total content of S and Se is more than 0.040%, not only dissolution during slab heating is incomplete, but also the product surface becomes defective. The total content of S and Se is therefore limited to the range of 0.002% to 0.040% whether they are added singly or in combination.

[0057] sol.Al: 0.01% to 0.08%

[0058] Al is a useful component that forms AlN in the steel and exerts an inhibitor effect as a second dispersion phase. When the Al content is less than 0.01%, a sufficient amount of precipitate cannot be ensured. When the Al content is more than 0.08%, AlN precipitates excessively after the steel sheet is nitrided. This makes the grain growth inhibiting capability too high, which hampers secondary recrystallization even when the steel sheet is annealed to high temperature.

[0059] N: 0.0010% to 0.020%

[0060] N is a component necessary to form AlN, as with Al. Nitrogen necessary as an inhibitor in secondary recrystallization can be supplied by nitriding in the subsequent step. When the N content is less than 0.0010%, however, crystal grain growth in the annealing step before the nitriding step is excessive, which may cause intergranular cracking in the cold rolling step or the like. When the N content is more than 0.020%, the steel sheet blisters or the like during slab heating. The N content is therefore limited to the range of 0.0010% to 0.020%.

[0061] In the case where AlN additionally formed as a result of the nitriding treatment is actively used as an inhibitor, it is preferable to control the sol.Al content to 0.01% or more and control the N content to less than 14/26.98 of sol.Al. This allows AlN to be newly precipitated by the nitriding.

[0062] While the essential components in the slab have been described above, the following elements may be contained as appropriate as components for improving the magnetic property industrially more stably. The balance in the steel slab is Fe and incidental impurities.

[0063] Regarding O as an incidental impurity, when the amount of O is 50 ppm or more, it causes an inclusion such as a coarse oxide, and hampers the rolling step. As a result, the primary recrystallized texture becomes non-uniform, or the formed inclusion itself degrades the magnetic property. Accordingly, the amount of O is desirably limited to less than 50 ppm.

[0064] Ni: 0.005% to 1.50%

[0065] Ni has a function of improving the magnetic property by enhancing the uniformity of the hot rolled sheet texture. To do so, the Ni content is preferably 0.005% or more. When the Ni content is more than 1.50%, secondary recrystallization is difficult, and the magnetic property degrades. Accordingly, the Ni content is desirably in the range of 0.005% to 1.50%.

[0066] Sn: 0.01% to 0.50%

[0067] Sn is a useful element that suppresses the nitriding or oxidation of the steel sheet during secondary recrystallization annealing and promotes the secondary recrystallization of crystal grains having favorable crystal orientation to improve the magnetic property. To do so, the Sn content is preferably 0.01% or more. When the Sn content is more than 0.50%, cold rolling manufacturability decreases. Accordingly, the Sn content is desirably in the range of 0.01% to 0.50%.

[0068] Sb: 0.005% to 0.50%

[0069] Sb is a useful element that suppresses the nitriding or oxidation of the steel sheet during secondary recrystallization annealing and promotes the secondary recrystallization of crystal grains having favorable crystal orientation to effectively improve the magnetic property. To do so, the Sb content is preferably 0.005% or more. When the Sb content is more than 0.50%, cold rolling manufacturability decreases. Accordingly, the Sb content is desirably in the range of 0.005% to 0.50%.

[0070] Cu: 0.01% to 0.50%

[0071] Cu has a function of suppressing the oxidation of the steel sheet during secondary recrystallization annealing and promoting the secondary recrystallization of crystal grains having favorable crystal orientation to effectively improve the magnetic property. To do so, the Cu content is preferably 0.01% or more. When the Cu content is more than 0.50%, hot rolling manufacturability decreases. Accordingly, the Cu content is desirably in the range of 0.01% to 0.50%.

[0072] Cr: 0.01% to 1.50%

[0073] Cr has a function of stabilizing the formation of a forsterite film. To do so, the Cr content is preferably 0.01% or more. When the Cr content is more than 1.50%, secondary recrystallization is difficult, and the magnetic property degrades. Accordingly, the Cr content is desirably in the range of 0.01% to 1.50%.

[0074] P: 0.0050% to 0.50%

[0075] P has a function of stabilizing the formation of a forsterite film. To do so, the P content is preferably 0.0050% or more. When the P content is more than 0.50%, cold rolling manufacturability decreases. Accordingly, the P content is desirably in the range of 0.0050% to 0.50%.

[0076] Nb: 0.0005% to 0.0100%, Mo: 0.01% to 0.50%

[0077] Nb and Mo each have an effect of suppressing a scab after hot rolling by, for example, suppressing cracking due to a temperature change during slab heating. When the Nb content and the Mo content are each less than the aforementioned lower limit, its scab suppression effect is low. When the Nb content and the Mo content are each more than the aforementioned upper limit, iron loss degradation results if Nb or Mo remains in the final product by forming, for example, a carbide or a nitride. Accordingly, the Nb content and the Mo content are each desirably in the aforementioned range.

[0078] Ti: 0.0005% to 0.0100%, B: 0.0001% to 0.0100%, Bi: 0.0005% to 0.0100%

[0079] These components may each have an effect of functioning as an auxiliary inhibitor and stabilizing secondary recrystallization, by forming a precipitate when nitrided, segregating, or the like. When the contents of these components are each less than the aforementioned lower limit, its effect as an auxiliary inhibitor is low. When the contents of these components are each more than the aforementioned upper limit, the formed precipitate may remain even after purification and cause magnetic property degradation, or embrittle grain boundaries and degrade bend property.

[0080] The following describes a manufacturing method according to one of the disclosed embodiments.

[0081] A steel slab adjusted to the aforementioned suitable chemical composition range is, after or without being reheated, hot rolled. In the case of reheating the slab, the reheating temperature is desirably about 1000° C. or more and 1300° C. or less. Since nitriding treatment is performed before secondary recrystallization annealing to reinforce the inhibitor in this embodiment, fine precipitate dispersion by complete dissolution in the hot rolling step is not necessarily required. Hence, ultrahigh-temperature slab heating exceeding 1300° C. is not suitable in this embodiment. It is, however, effective to increase the heating temperature to dissolve Al, N, Mn, S, and Se to some extent and disperse them during hot rolling so that the grain size will not be excessively coarsened in the annealing step before the nitriding. Besides, if the heating temperature is too low, the rolling temperature during hot rolling drops, which increases the rolling load and makes the rolling difficult. Accordingly, the reheating temperature is desirably 1000° C. or more.

[0082] Following this, the hot rolled sheet is hot band annealed if required, and then cold rolled once or twice or more with intermediate annealing in between, to obtain a final cold rolled sheet. The cold rolling may be performed at normal temperature. Alternatively, the cold rolling may be warm rolling with the steel sheet temperature being higher than normal temperature, e.g. about 250° C.

[0083] The final cold rolled sheet is further subjected to primary recrystallization annealing.

[0084] The aim of the primary recrystallization annealing is to cause the primary recrystallization of the cold rolled sheet having rolled microstructure to adjust it to an optimal primary recrystallized grain size for secondary recrystallization. For this aim, the annealing temperature in the primary recrystallization annealing is desirably about 800° C. or more and less than 950° C. The annealing atmosphere is preferably a wet hydrogen nitrogen atmosphere or a wet hydrogen argon atmosphere. Decarburization annealing may also be carried out by such an atmosphere.

[0085] In the primary recrystallization annealing, the heating rate between 500° C. and 700° C. is preferably 50° C./s or more in terms of improving the texture of the steel sheet. Annealing with such a heating rate enhances the amount of Goss orientation of the texture in the steel. As a result, the grain size after secondary recrystallization is reduced, with it being possible to improve the iron loss property of the steel sheet. The upper limit of the heating rate between 500° C. and 700° C. is not particularly limited, but is about 400° C./s in terms of apparatus.

[0086] In addition, the pertinent temperature range in the primary recrystallization annealing is the temperature range corresponding to the recovery of the texture, as the aim is to quickly heat the steel sheet in the temperature range corresponding to the recovery of the texture after the cold rolling and recrystallize the steel sheet microstructure.

[0087] The heating rate in this temperature range is preferably 50° C./s or more. When the heating rate is less than 50° C./s, the recovery of the texture in such temperature cannot be suppressed sufficiently.

[0088] These technical ideas are the same as those described in JP H7-62436 A and the like.

[0089] In this embodiment, nitriding treatment is performed during, following, or after the primary recrystallization annealing. Most importantly, nitriding treatment is performed at a temperature suitable for the precipitation of AlN, i.e. 850° C. or more, and then nitriding treatment is performed at a lower temperature suitable for the precipitation of Si.sub.3N.sub.4 or iron nitride, i.e. less than 850° C.

[0090] In the nitriding in this embodiment, high-temperature nitriding is performed first at the temperature suitable for the precipitation of AlN. In particular, by performing nitriding at 850° C. or more which is the temperature suitable for the precipitation of AlN, nitrogen supplied by the nitriding enters into the steel, and simultaneously precipitates as AlN. Here, since the precipitation of AlN occurs immediately after nitrogen enters into the steel, the precipitate forms only near the surface of the steel sheet. AlN is a thermodynamically stable nitride, so that the precipitation state is maintained even during the secondary recrystallization annealing and the grain growth near the surface is inhibited. After this, low-temperature nitriding is performed at the temperature suitable for the precipitation of Si.sub.3N.sub.4 or iron nitride. In particular, by performing nitriding at less than 850° C. which is the temperature suitable for the precipitation of Si.sub.3N.sub.4 or iron nitride, nitrogen supplied by the nitriding enters into the steel and simultaneously precipitates in the form of Si.sub.3N.sub.4 or the like. Such nitride is equally formed near the surface immediately after the nitriding, but is not as thermodynamically stable as AlN. Hence, the nitride is substituted by AlN during heating in the secondary recrystallization annealing. This results in such a state where AlN is dispersed through to the sheet thickness center.

[0091] By performing the nitriding treatment with heating pattern of two stages or more including high-temperature nitriding and low-temperature nitriding in this way, a state in which the amount of AlN precipitate is intentionally increased near the surface of the steel sheet is created to suppress secondary recrystallization from the texture near the surface. The magnetic property can be improved stably in this way. The upper limit of the temperature of high-temperature nitriding is not particularly limited, but is about 1050° C. in terms of technology. The lower limit of the temperature of low-temperature nitriding is not particularly limited, but is about 450° C. in terms of productivity.

[0092] The nitriding treatments at the respective temperatures may be performed in two or more separate steps to achieve the same advantageous effects. Performing soaking in each temperature range eases the control of the precipitation state. However, even when soaking (a state without any temperature change) is not performed, the advantageous effects can be achieved as long as the residence time in the corresponding temperature range is ensured.

[0093] It is essential to ensure a residence time of 3 seconds or more in the temperature range of 850° C. or more. In the temperature range of 850° C. or more, AlN, while precipitating, simultaneously undergoes Ostwald ripening and increases in precipitates size, and so the residence time is limited to 600 seconds or less. Meanwhile, nitriding in the temperature range of less than 850° C. is intended to obtain the grain growth inhibiting capability throughout the sheet thickness, and a residence time until the required nitriding quantity is obtained is necessary.

[0094] The nitriding quantity in the nitriding treatment ((the amount of nitrogen after nitriding)−(the amount of nitrogen contained in the slab)) is preferably in the range of 100 mass ppm to 500 mass ppm which is a typical range in nitriding technology for grain-oriented electrical steel sheets. When the nitriding quantity is 100 mass ppm or less, nitriding is insufficient for the precipitation of AlN. When the nitriding quantity is more than 500 mass ppm, the supply of nitrogen is excessive and a secondary recrystallization failure may occur.

[0095] In the nitriding treatment, reaction efficiency decreases with a decrease in temperature, so that the required residence time varies widely depending on the temperature. For example, when the treatment is performed at about 750° C. at which Si.sub.3N.sub.4 precipitates, the required nitriding quantity can be obtained in a residence time of 1 minutes or less. When the treatment is performed at a low temperature such as 450° C. at which iron nitride precipitates, on the other hand, the reaction rate is very low, and so at least several hours may be necessary to obtain the required nitriding quantity.

[0096] Applying the nitriding treatment following the primary recrystallization annealing is efficient because energy necessary to heat the steel sheet can be saved. While the same advantageous effects can be achieved even when the treatment is performed by a plurality of annealing operations from the high temperature side, performing the treatment by one operation further enhances energy efficiency.

[0097] The following describes a suitable nitriding apparatus in this embodiment.

[0098] FIG. 1 illustrates a suitable nitriding apparatus. In FIG. 1, reference sign 1 is a nitriding apparatus, 2 is a steel strip, 3 is a nitriding gas supply pipe including a cooling device, 4 is a cooling device, 5 is a cooling gas supply pipe, 6 is a nitriding gas supply pipe, 7 is a high-temperature nitriding treatment portion, 8 is a gas cooling zone, 9 is a low-temperature nitriding treatment portion, and 10 is an exhaust port.

[0099] The nitriding apparatus 1 does not require any complex structure, and only needs to have the apparatus length corresponding to the sheet passing rate of the steep strip 2, and to be a heat treatment apparatus including front and rear heaters capable of separate temperature controls and the predetermined exhaust port 10. The nitriding apparatus 1 includes a gas introduction portion with a nitriding gas supply pipe (3 and 6) for introducing gas including at least ammonia or nitrogen with which a nitriding atmosphere can be maintained, and a nitriding treatment portion (7 and 9) capable of high-temperature nitriding and low-temperature nitriding in the nitriding treatment.

[0100] In this embodiment, high-temperature nitriding is performed first. Here, gas such as ammonia which is typically known as gas having nitriding ability is susceptible to high-temperature decomposition. If decomposed, the gas such as ammonia loses nitriding ability. In other words, if the gas changes in property in the gas supply pipe to the nitriding furnace, the nitriding efficiency of the gas decreases significantly. Accordingly, it is important to provide the nitriding gas supply pipe 3 including the cooling device 4 having cooling function in the high-temperature treatment portion 7 for high-temperature nitriding (the front half of the nitriding apparatus), in order to prevent the property change of the gas. The cooling device may be a cooling device typically used for gas cooling, such as a cooling device with a nozzle for blowing nitriding gas or inert gas of 400° C. or less onto the steel sheet.

[0101] Regarding the other parts, the following structures can be used to realize more effective nitriding treatment.

[0102] For example, the low-temperature treatment portion 9 for low-temperature nitriding (the rear half of the apparatus) may utilize natural cooling as long as heat insulation is sufficient. In the case where the uniformity of temperature cannot be maintained isothermally, however, the nitriding control level drops significantly. In such a case, it is preferable to use a heater capable of soaking the steel sheet at a slightly lower temperature or suppressing a decrease in temperature of the steel sheet. Moreover, the nitriding apparatus 1 desirably has a function of adjusting the temperature of the high-temperature treatment portion to 850° C. or more and adjusting the temperature of the low-temperature treatment portion to less than 850° C.

[0103] In the case of a single apparatus, the cooling zone 8 for cooling the steel strip 2 by the introduction of cooling gas from the cooling gas supply pipe 5 is preferably provided between the high-temperature treatment portion and the low-temperature treatment portion, to shorten the apparatus length. Such an apparatus can cool the steel strip 2 to an appropriate temperature in a short time while performing separate temperature adjustments in the front and rear of the furnace.

[0104] The gas introduced from the gas introduction portion is not limited as long as it is a gas typically used for nitriding such as NH.sub.3 in electrical steel sheet manufacture. An oxynitriding atmosphere in which O.sub.2 is slightly added to NH.sub.3, a softnitriding atmosphere in which a slight amount of C is contained, or the like is also applicable. The gas used in the cooling zone is, for example, inert gas such as N.sub.2 or Ar or the aforementioned nitriding gas.

[0105] FIG. 2 illustrates a SEM image obtained by SEM observation on a section of a nitrided steel sheet formed under condition 3 in the below-mentioned Examples, taken along the direction orthogonal to the rolling direction. As is clear from FIG. 2, MN and Si.sub.3N.sub.4 have precipitated in grain boundaries or in grains near the surface after nitriding treatment. In the case of condition 12 in which nitriding treatment is performed at a lower temperature, on the other hand, not Si.sub.3N.sub.4 but iron nitride has formed near the surface.

[0106] Thus, when high-temperature nitriding and then low-temperature nitriding are performed in the nitriding atmosphere of the nitriding treatment, a non-uniform precipitation state can be intentionally formed in the sheet thickness direction, with it being possible to enhance the grain growth inhibiting capability near the surface of the steel sheet.

[0107] An annealing separator is applied to the surface of the steel sheet after the aforementioned primary recrystallization annealing and nitriding treatment. To form a forsterite film on the surface of the steel sheet after the secondary recrystallization annealing, the main agent of the annealing separator needs to be magnesia (MgO). In the case where the formation of a forsterite film is unnecessary, on the other hand, the main agent of the annealing separator may be an appropriate oxide whose melting point is higher than the secondary recrystallization annealing temperature, such as alumina (Al.sub.2O.sub.3) or calcia (CaO).

[0108] One or more selected from sulfates and sulfides of Ag, Al, Ba, Ca, Co, Cr, Cu, Fe, In, K, Li, Mg, Mn, Na, Ni, Sn, Sb, Sr, Zn, and Zr may be added to the annealing separator as sulfate and/or sulfide. The content of the sulfate and/or sulfide in the annealing separator is preferably about 0.2% or more and 15% or less. When the sulfate and/or sulfide content is in this range, sulfur enters into the steel by the separator during secondary recrystallization, thus reinforcing the grain growth inhibition especially near the surface of the steel sheet. When the sulfate and/or sulfide content is less than 0.2%, the sulfur increase amount in the steel matrix is small. When the sulfate and/or sulfide content is more than 15%, the sulfur increase amount in the steel matrix is excessive. In either case, the magnetic property improving effect is low.

[0109] Following this, secondary recrystallization annealing is performed. In the heating process of the secondary recrystallization annealing, iron nitride decomposes and N diffuses in the steel. As the annealing atmosphere, N.sub.2, Ar, H.sub.2, or any mixture thereof is applicable.

[0110] The grain-oriented electrical steel sheet manufactured by the aforementioned steps from the grain-oriented electrical steel sheet slab has the following features. In the heating process of the secondary recrystallization annealing before the start of secondary recrystallization, the amount of nitride present near the surface of the steel sheet is increased, and also nitride is precipitated through to the sheet thickness center. As a result, favorable magnetic property can be obtained by effectively suppressing secondary recrystallization from the surface that tends to have inferior texture.

[0111] After the secondary recrystallization annealing, an insulating coating may be applied to the surface of the steel sheet and baked. The type of the insulating coating is not particularly limited, and may be any conventionally well-known insulating coating. For example, a method of applying an application liquid containing phosphate-chromate-colloidal silica described in JP S50-79442 A and JP S48-39338 A to the steel sheet and baking it at about 800° C. is suitable.

[0112] Moreover, flattening annealing may be performed to arrange the shape of the steel sheet. This flattening annealing may also serve as the insulating coating baking treatment.

EXAMPLES

[0113] Each type of grain-oriented electrical steel sheet slab shown in Table 1 was heated at 1230° C., hot rolled into a hot rolled sheet of 2.5 mm in sheet thickness, and then hot band annealed at 1050° C. for 1 minute. After this, the sheet was cold rolled to a final sheet thickness of 0.27 mm. A sample of 100 mm×400 mm in size was collected from the center portion of the obtained cold rolled coil, and subjected to annealing serving as both primary recrystallization and decarburization in a laboratory.

[0114] Following this, nitriding treatment was performed under the nitriding condition shown in Table 1, in a mixed atmosphere of ammonia, hydrogen, and nitrogen. In the primary recrystallization annealing, the heating rate between 500° C. and 700° C. was any of two levels of 20° C./s and 150° C./s.

[0115] Moreover, 21 or 20 steel sheets of the same condition were produced per condition. In each condition for which 21 steel sheets were produced, one of the steel sheets was used for the analysis of the nitrided sample. For the remaining 20 steel sheets, an annealing separator mainly containing MgO, to which the annealing separation additive shown in Table 1 was added in an aqueous slurry state, was applied and dried, and baked on the steel sheet. Subsequently, final annealing with a maximum temperature of 1200° C. was performed to cause secondary recrystallization. Following this, a phosphate-based insulating tension coating was applied and baked, and the magnetic flux density (B.sub.8, T) with a magnetizing force of 800 A/m and the iron loss (W.sub.17/50, W/kg) with 50 Hz and an excitation magnetic flux density of 1.7 T were evaluated. As the magnetic property, the magnetic flux density was evaluated based on the average value and minimum value of 20 steel sheets in each condition, and the iron loss was evaluated based on the average value of 20 steel sheets in each condition.

[0116] The evaluation results are shown in Table 1.

TABLE-US-00001 TABLE 1 Heating rate in primary recrystallization Nitriding treatment condition Slab component (%) between 500° C. High-temperature Low-temperature Condition Si C Mn S Se sol. Al N Others and 700° C. nitriding nitriding 1 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s N/A N/A 2 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s N/A 750° C. × 30 sec 3 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s 900° C. × 2 sec  750° C. × 30 sec 4 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s 900° C. × 10 sec 750° C. × 30 sec 5 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s 900° C. × 60 sec 750° C. × 30 sec 6 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s 860° C. × 90 sec 750° C. × 30 sec 7 3.40 0.06 0.02 0.001 0.010 0.020 0.004 N/A 20° C./s  860° C. × 720 sec 750° C. × 30 sec 8 3.15 0.04 0.04 0.010 Tr. 0.015 0.007 N/A 20° C./s N/A N/A 9 3.15 0.04 0.04 0.010 Tr. 0.015 0.007 N/A 20° C./s N/A 750° C. × 30 sec 10 3.15 0.04 0.04 0.010 Tr. 0.015 0.007 N/A 150° C./s  950° C. × 5 sec  750° C. × 30 sec 11 3.15 0.04 0.04 0.010 Tr. 0.015 0.007 N/A 20° C./s 950° C. × 5 sec  750° C. × 30 sec 12 3.15 0.04 0.04 0.010 Tr. 0.015 0.007 N/A 20° C./s 950° C. × 5 sec   480° C. × 1200 sec 13 3.20 0.04 0.05 0.004 0.005 0.023 0.006 Ni: 0.03, Sn: 0.02 20° C./s 900° C. × 10 sec 750° C. × 30 sec 14 3.20 0.04 0.05 0.004 0.006 0.022 0.005 Sb: 0.03, Mo: 0.03 20° C./s 900° C. × 10 sec 750° C. × 30 sec 15 3.15 0.04 0.05 0.003 0.006 0.024 0.005 P: 0.02, B: 0.0005 20° C./s 900° C. × 10 sec 750° C. × 30 sec 16 3.10 0.04 0.05 0.004 0.004 0.022 0.006 Nb: 0.001, P: 0.01 150° C./s  900° C. × 10 sec 750° C. × 30 sec 17 3.15 0.04 0.05 0.003 0.004 0.023 0.005 Bi: 0.001 150° C./s  900° C. × 10 sec 750° C. × 30 sec 18 3.15 0.04 0.05 0.005 0.005 0.025 0.005 Cu: 0.03 150° C./s  900° C. × 10 sec 750° C. × 30 sec 19 3.10 0.04 0.05 0.004 0.006 0.024 0.006 Cr: 0.02, Ti: 0.002 150° C./s  900° C. × 10 sec 750° C. × 30 sec Magnetic property Annealing Magnetic property W.sub.17/50 separator B.sub.8 (T) average Condition additive average minimum (W/kg) Remarks 1 TiO.sub.2 1.89 1.87 1.03 Comparative Example 2 TiO.sub.2 1.92 1.90 0.96 Comparative Example 3 TiO.sub.2 1.92 1.90 0.96 Comparative Example 4 TiO.sub.2 1.93 1.92 0.96 Example 5 TiO.sub.2 1.92 1.91 0.96 Example 6 TiO.sub.2 1.92 1.92 0.96 Example 7 TiO.sub.2 1.91 1.90 1.00 Comparative Example 8 TiO.sub.2 1.89 1.86 1.04 Comparative Example 9 TiO.sub.2 1.92 1.90 0.98 Comparative Example 10 TiO.sub.2 1.92 1.91 0.93 Example 11 MgSO.sub.4 1.93 1.92 0.97 Example 12 TiO.sub.2 1.93 1.91 0.96 Example 13 TiO.sub.2 1.93 1.92 0.96 Example 14 MgSO.sub.4 1.93 1.92 0.96 Example 15 MgSO.sub.4 1.93 1.92 0.94 Example 16 MgS 1.93 1.92 0.90 Example 17 MgS 1.94 1.92 0.90 Example 18 TiO.sub.2 1.93 1.92 0.91 Example 19 TiO.sub.2 1.93 1.92 0.91 Example

[0117] As shown in Table 1, in Examples, the minimum value of B.sub.8 improved as compared with Comparative Examples. The average value of B.sub.8 also improved to some extent. In the case where S was contained in the annealing separator, the magnetic flux density was a little higher. Moreover, each raw material with a higher heating rate in primary recrystallization had excellent iron loss property.

REFERENCE SIGNS LIST

[0118] 1 nitriding apparatus

[0119] 2 steel strip

[0120] 3 nitriding gas supply pipe including cooling device

[0121] 4 cooling device

[0122] 5 cooling gas supply pipe

[0123] 6 nitriding gas supply pipe

[0124] 7 high-temperature nitriding treatment portion

[0125] 8 gas cooling zone

[0126] 9 low-temperature nitriding treatment portion

[0127] 10 exhaust port