BRPI1101246A2 - grain oriented electric steel sheet, steel strip treatment method, and nitrided steel strip - Google Patents

Abstract

GRAIN-ORIENTED ELECTRIC STEEL SHEET, STEEL STRIP TREATMENT METHOD, AND NITRETE STEEL STRIP. In a nitriding treatment while performing a steel strip, annealing is performed at a temperature of 700 <198> C to 1000 <198> C in ammonia-containing gas, and then cooling is performed. An average cooling rate over a temperature range of 700 <198> C to 200 <198> C in cooling is set at 5 <198> C / s or more.

Description

Patent Descriptive Report for "GRAIN-ORIENTED ELECTRIC STEEL SHEET, STEEL STRIP TREATMENT METHOD, AND NITRETE STEEL STRIP". TECHNICAL FIELD

The present invention relates to a method of manufacturing

a grain-oriented electric steel sheet for an iron core or the like of an electrical appliance, a method of treating a steel strip, and a nitrided steel strip. BACKGROUND ART A grain oriented electric steel sheet is a material

soft magnetic, and is used for an iron core or similar of an electrical appliance such as a transformer. In grain-oriented electric steel sheet, Si of about 7% / mass or less is contained. The grain grains of the grain oriented electric steel sheet are highly integrated in orientation {100} <001> by the Miller indices. The orientation of crystal grains is controlled using a catastrophic grain growth phenomenon called recrystallization.

To control secondary recrystallization, it is important to adjust a structure (primary recrystallization grain structure) obtained by primary recrystallization prior to secondary recrystallization and to adjust a fine precipitate called an inhibitor of a grain boundary segregation element. Then, conventionally, a grain-oriented electric steel sheet is fabricated by producing fine precipitates by various methods. There is a method in which, for example, heating of the plate prior to hot rolling is carried out at a high temperature above 1350 ° C to completely solidify-dissolve an inhibitor forming element and at the time of hot rolling and subsequent annealing of the hot rolled sheet, an inhibitor is precipitated. The method is sometimes called high temperature plate heating. In addition, there is a method in which the plate is heated at a relatively low temperature, and from the beginning of decarburizing annealing to the occurrence of secondary recrystallization in the finishing annealing, a nitriding treatment is performed. to precipitate an inhibitor. (JP Patent Publication No. 62-40315, JP Patent Publication No. 05-112827, JP Patent Publication No. 02-294428, JP Patent Publication No. 11 29824, and JP Publication Publication No. 2001-152250). The method is sometimes called low temperature plate heating. When these methods are compared, low-temperature plate heating is superior in productivity, coating, a load in an installation, and so on. However, when the plate is heated

In order to fabricate a grain oriented electric sheet of steel, there is sometimes a case where an edge crack, fracture, and so on, are caused when a steel strip is coiled after a nitriding treatment. Thus, in the case when low temperature plate heating is employed, it is sometimes difficult to fabricate a high yield grain oriented electric steel sheet. SUMMARY OF THE INVENTION TECHNICAL PROBLEM

The present invention has an object of providing a grain oriented electric steel sheet capable of being manufactured with a high yield and a method of fabrication thereof. Solution to the problem

A method of fabricating a grain oriented electric steel sheet according to a first aspect of the present invention includes: heating a steel material to silicon; hot rolling the heated silicon steel material to obtain a steel strip; anneal the steel strip; cold rolling the steel strip once or more after annealing; anneal by decarburizing the steel strip after cold rolling; perform a nitriding treatment in which an N content of the steel strip is increased after the decarburising annealing is completed or in parallel with the decarburizing annealing to obtain a nitrided steel strip in which the primary recrystallization; coating the annealing release agent having MgO as its major component on the nitrided steel strip; and finishing annealing of the nitrided steel strip having the annealing separating agent coated thereon to cause secondary recrystallization, wherein said nitriding treatment embodiment comprises: annealing at a temperature of 700 ° C to 100 ° C. in ammonia-containing gas while running the steel strip; and then perform cooling, and an average cooling rate over a temperature range of 700 ° C to 200 ° C on cooling is set at 5 ° C / s or more.

In a method of manufacturing a grain-oriented electric 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 ° C on cooling is set at 10 ° C / s or higher.

In a method of manufacturing a grain-oriented electric steel sheet according to a third aspect of the present invention, in the method according to the first or second aspect, the average cooling rate over a temperature range of 700 ° C to 500 ° C on cooling is set at 20 ° C / s or higher.

In a method of manufacturing a grain-oriented electric 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 on cooling is set at 30 ° C / s or more.

A method of treating a steel strip in accordance with a fifth aspect of the present invention includes: annealing at a temperature of 700 ° C to 1000 ° C in ammonia-containing gas while performing a steel strip of a sheet of grain-oriented electric steel; and then perform cooling, where an average cooling rate over a temperature range of 700 ° C to 200 ° C on cooling is set at 5 ° C / s or more.

In a method of treating 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 cooling is set at 10 ° C / s or more.

In a method of treating a steel strip according to a seventh aspect of the present invention, in the method according to the fifth or sixth aspect, the average cooling rate over a temperature range of 700 ° C to 500 ° C in cooling is set at 20 ° C / s or higher.

In a method of treating 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 cooling is set at 30 ° C / s or more

A nitrided steel strip according to a ninth aspect of the present invention is characterized in that it has grain boundary coverage by precipitates of 0.5% or less. ADVANTABLE EFFECTS OF THE INVENTION According to the present invention, it is possible to manufacture the sheet

high performance grain-oriented electric steel material suppressing a fracture or the like of the nitrided steel strip at the time of winding. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a flow chart showing a method of fabricating a grain oriented electric steel sheet;

Figure 2 is a view showing a relationship between an average cooling rate and the brittleness of a nitrided steel strip; and

Figure 3 is a view showing an optical micrograph of a sample with an average cooling rate of 0.1 ° C / s. DESCRIPTION OF MODALITIES

As a result of serious studies on the cause of a crack or the like being caused in coiling after nitriding treatment, the present inventors believe that the nitriding treatment conditions that are required for the plate heating method in Low temperatures can affect the brittleness of a steel strip, and have conducted several experiments. At present, an outline of the grain-oriented sheet steelmaking method will be explained. Figure 1 is a flow chart showing the grain-oriented electric steel sheet fabrication method based on the low temperature plant heating method.

First, as shown in Figure 1, in step S1, a silicon steel material (plate) having a predetermined composition is heated to a predetermined temperature, and in step S2, the heated silicon steel material is Hot rolled. By hot rolling, a hot rolled steel strip is obtained. Therefore, in step S3, the hot-rolled steel strip is annealed to normalize a structure on the hot-rolled steel strip and adjust precipitation of an inhibitor. Upon annealing, an annealed steel strip is obtained. Subsequently, in step S4, the annealed steel strip is cold rolled. Cold rolling can be done only once, or it can be done a plurality of times with intermediate annealing between them. By cold rolling, a cold rolled steel strip is obtained. Incidentally, in the case of intermediate annealing that is performed, it is also possible to omit annealing of the hot rolled steel strip before cold rolling performs annealing (step S3) in the intermediate annealing. That is, annealing (step S3) may be performed on the hot-rolled steel strip, or it may also be performed on the steel strip obtained after being cold rolled once and before finally being cold rolled.

After cold rolling, in step S5, the cold rolled steel strip is annealed by decarburization. In decarburising annealing, primary recrystallization occurs. In addition, by decarbur annealing, a steel strip where an external oxide film having a Fe-based oxide as its main component and an internal oxide layer having SiO2 as its main component is formed on the surface of the surface. same is obtained. Then, in step S6, a nitriding treatment in which an N content of the steel strip obtained after decarburizing annealing is increased is performed to obtain a nitrided steel strip (steel sheet). Incidentally, it may be designated that nitriding treatment is not performed after decarburizing annealing, but is carried out in parallel with decarburizing annealing. That is, nitriding treatment can also be performed on the cold rolled steel strip. In both cases, as a result of decarburising annealing and nitriding treatment, the nitrided steel strip in which primary recrystallization is caused is obtained. Then, 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 on the surface. The same thing is foolish. Subsequently, in step S8, finishing annealing of the nitrided steel strip is performed. In the finishing 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 in the Goss orientation is obtained. By finishing annealing, a finished annealed steel strip is obtained. For example, decarburising annealing (step S5), nitriding treatment (step S6), and annealing separating agent coating (step S7) are continuously coated onto the uncoiled steel strip and in and the annealing finish (step 8) is performed on the coil-shaped steel strip.

In this way the grain oriented electric steel sheet can be

obtained.

Thus, as a result of various experiments, the present inventors have found it important to adjust a cooling condition in the nitriding treatment (step S6) to properly control the coverage of grain boundaries by precipitates in the nitrile strip. tada. Specifically, the present inventors have found that adjusting the cooling condition makes it possible to suppress precipitations of (si, Mn) N at the time of cooling to thereby suppress brittleness of the steel strip, resulting in a crack, fracture, and so on. onwards, caused at the time of cooling, are suppressed. Thus, the present inventors obtained the knowledge capable of manufacturing the high yield grain oriented electric steel sheet, and concluded the present invention.

At present, the experience conducted by the present inventors will be explained.

In the experiment, first, several silicon steel plates,

each containing Si: 3.3 wt%, C: 0.06 wt%, Acid soluble Al: 0.027 wt%, Mn: 0.1 wt% and S: 0.007 wt%, and a remainder being composed of Fe and unavoidable impurities, were obtained. The silicon steel plates were then heated to 1150 ° C and hot rolled to obtain hot rolled steel strips. Subsequently, the hot rolled steel strips were annealed at 1100 ° C. Thereafter, cold rolling was performed to obtain cold rolled steel strips each having a thickness of 0.35 mm. Then, in a humid atmosphere of mixed nitrogen and hydrogen gas, decarburising annealing was performed at a temperature of 830 ° C for 120 seconds. Subsequently, a nitriding treatment was performed on the steel strips obtained after decarburizing annealing.

In the nitriding treatment, a gas atmosphere was used in which 5% / v ammonia was contained in the wet atmosphere of mixed nitrogen and hydrogen gas. In addition, at a temperature of 770 ° C, a 30 second soaking treatment was carried out and thereafter a temperature range of 700 ° C to 200 ° C was cooled to various medium cooling rates. Incidentally, the amount of N increased by nitriding treatment in each of the steel strips was 500 ppm. Thus, several samples were fabricated.

Then, a relationship between the average cooling rate of 700 ° C to 200 ° C in nitriding treatment and the brittleness of a nitrided steel strip was examined. A result of the same is shown in Figure 2. On examination of frailty, a reverse bending test was conducted at a temperature of 0 ° C. That is, 90 degree reverse bending was performed to wind the nitrided steel strip around a round bar with a diameter of 20 mm, and the numbers of bends until the nitrided steel strips were fractured were measured. As shown in figure 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 bending number was 10 or more, and in the case of cooling average 10 ° C / s or more, the number of curves was 50 or more. As a reason why such a result was obtained, facts as above are conceivable.

When the present inventors observed an optimal micrograph at a depth of 30 μηι from a sample surface with an average cooling rate of 0.1 ° C / s, where the reverse bending numbers were quite small, a large Some of the coarse precipitates existed along the grain boundaries as shown in Figure 3. On the other hand, in samples with an average cooling rate of 5 ° C / s and more, such coarse precipitates hardly existed on the grain boundaries. It is therefore conceivable that coarse precipitates within the grain boundaries cause embrittlement. Also, as a result of the examination of coarse precipitates, it has become clear that they are (Si, Mn) N. It is conceivable that part of the nitrogen entering the steel strip in the nitriding treatment precipitates in the crystal grains or grain boundaries as (Al, Si) N, (Si, Mn) N, or the like during treatment. shrinkage, but a large amount of nitrogen exists as nitrogen in solid solution during soaking treatment and precipitates as nitrides during cooling. Also, in the low temperature range (700 ° C to 200 ° C or so) during cooling, a nitrogen diffusion rate is faster at the grain boundaries than at the crystal grain, so that nitrogen in solution solid preferably diffuses into the crystal grains. Moreover, in such a low temperature range, (Si, Mn) N are thermally stable between nitrides. Thus, it is conceivable that in the case of legendary average cooling rate, (Si, Mn) N preferably precipitates at the grain boundaries. So when the results shown in figure 2 and figure 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 is preferably 10 ° C / s or more. In addition, the frequency of coarse precipitate at the grain boundaries varies with position (depth) in one direction of leaf thickness, but it becomes clear that grain boundary coverage is high in one layer. depth of 20 μιη to 50 μηι, or so from the surface of the nitrided steel strip, and when a cover value of the grain boundary exceeds 0,5%, the number of reverse bends is markedly low, and when the value exceeds 1%, the numbers of reverse bends are lower.

Next, an embodiment of the present invention made upon knowledge will be explained.

Firstly, the reasons for limiting the components of the silicon steel material will be explained.

The content of a silicon steel material is not limited in particular. The silicon steel materials used for conventional grain oriented electric sheet steelmaking methods can be adopted. The silicon steel material contains, for example, Si: 0.8% / mass 7% / mass, acid-soluble Al: 0.01% / mass 0.065% / mass, N: 0.004% / mass 0.012 % / mass, Mn: 0,05% / mass at 1% / mass, and S and Se: 0,003% / mass at 0,015% / mass in the total quantity, a C content being 0,085% / mass or less, and a remainder being composed of Fe and unavoidable impurities.

Si increases electrical resistance to reduce a loss of core. However, when a Si content exceeds 7% by mass, cold rolling becomes difficult to perform, and cracking sometimes occurs at the time of cold rolling. Thus, the Si content is preferably 7 wt% or less, and is more preferably 4.5 wt% or less, and is even more preferably 4 wt% or less. In addition, when the Si content is less than 0.8% / mass, it is sometimes the case that a γ transformation is caused at the time of finishing annealing to thereby deteriorate the orientation of the sheet crystals. grain-oriented electric steel. Thus, the Si content is preferably 0.8 wt.% Or more, and is more preferably 2 wt.% Or more, and is even more preferably 2.5 wt.% Or more.

C is an effective element for controlling a primary recrystallization grain structure, but adversely affects magnetic property. Thus, in this embodiment, prior to finishing annealing (step S8), decarburising annealing is performed (step S5). However, when the C content exceeds 0.085% / mass, a time taken for decarburizing recoil becomes long, and productivity in industrial production is likely to be impaired. Thus, the C content is preferably 0.085 wt% or less, and is more preferably 0.07 wt% or less. Acid soluble Al binds to N to be precipitated as (Al,

Sn) N e acts as inhibitors. In the case where the acid soluble Al content falls within a range of 0.01% / mass to 0.065% / mass, the secondary recrystallization is stabilized. Thus, the acid soluble Al content is preferably not less than 0.01% / mass and no more than 0.065% / mass. In addition, the acid soluble Al content is more preferably 0.02 wt.% Or more, and even more preferably 0.025 wt.% Or more. In addition, the acid soluble Al content is more preferably 0.05 wt% or less, and is even more preferably 0.03 wt%.

N binds to Al to function as an inhibitor. When an N content is less than 0.004% / mass, it is sometimes the case when a sufficient amount of 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 even more preferably 0.007% / mass or more. On the other hand, when the N content exceeds 0.012% / mass, a hole called a hole sometimes occurs in the steel strip at the time of cold rolling. Thus, the N content is preferably 0.012 wt% or less, and more preferably 0.010 wt% or less, and is even more preferably 0.009 wt% or less.

Mn, S and Se produce MnS and MnSe to be a nucleus for which AIN is preferably precipitated. In the case where an Mn content is 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 and not more than 1% / mass. Furthermore, the Mn content is more preferably 0.08% / mass or more, and even more preferably 0.09% / mass or more. In addition, the Mn content is more preferably 0.50 wt% or less, and even more preferably 0.2 wt% or less.

Also, in the case where an S and Se content is in the range of 0.003% / mass to 0.015% / mass in the 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 the total amount. In addition, in terms of preventing the occurrence of a crack in hot rolling, an inequality (1) below is preferably satisfied. Incidentally, only either S or Se may be contained in the silicon steel material, or both S or Se may also be contained in the silicon steel material. Where S and Se are contained, it is possible to promote BN precipitation more stably and to improve the magnetic property stably.

[Mn] / ([S] + [If]) £ 4 (1)

Ti forms coarse TiN to affect the precipitation amounts of (Al, Si) N acting 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.

In addition, 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 the ranges below. Cr improves the oxide layer formed at the time of annealing.

decarburization, and is effective in forming the vitreous film when the oxide and MgO layer being the major component of the annealing separator react at the time of the annealing finish. However, when a Cr content exceeds 0.3% / mass, decarburization is markedly prevented. Thus, the Cr content is preferably 0.3 wt.% Or less.

Cu increases specific strength to reduce a core loss. However, when a Cu content exceeds 0.4% by mass, the effect is saturated. In addition, a surface defect called "copper crust" is sometimes caused at hot rolling. Thus, the Cu content is preferably 0.4% by weight or less.

Ni increases specific resistance to reduce loss of

core. In addition, Ni controls a metal structure of the hot rolled steel strip to improve magnetic property. However, when a Ni content exceeds 1% / mass, secondary recrystallization becomes unstable. Thus, the Ni content is preferably 1 wt% or less. P increases specific resistance to reduce a loss of

core. However, when a P content exceeds 0.5% / mass, a fracture occurs easily at the time of cold rolling due to brittleness. Thus, the P content is preferably 0.5% / mass or less.

Mo improves a surface property at the time of hot rolling. However, when a Mo content exceeds 0.1% by mass, the effect is saturated. Thus, the Mo content is preferably 0.1 wt.% Or less.

Sn and Sb are elements of segregation of the grain boundaries. The silicon steel material used in the embodiment contains Al, so there is sometimes a case where Al is oxidized by the moisture released from the annealing separating agent depending on the finishing annealing condition. In this case, variations in inhibitor resistance occur depending on the grain-oriented sheet steel positions, and the magnetic property also varies sometimes. However, where grain boundary segregation elements are contained, the oxidation of Al may be suppressed. That is, Sn and Sb suppress the oxidation of Al to suppress variations in magnetic property. On the other hand, when a Sn and Sb content exceeds 0.30% / mass, the oxide layer is not easily formed at decarburizing annealing, and thus the formation of the vitreous film when the oxide layer and MgO being the major component of the annealing separating agent reacting at the time of finishing annealing becomes insufficient. In addition, decarburization is markedly 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.

B binds to N to be precipitated as BN and functions as an inhibitor. However, when a B content exceeds 0.008% / mass, hot lamination becomes difficult to perform. Thus, the B content is preferably 0.008% / mass or less.

Bi stabilizes a precipitate such as a sulfide to enhance function as an inhibitor. However, when a Bi content exceeds 0.01% / mass, vitreous film formation is adversely affected. Thus, the Bi content is preferably 0.01 wt% or less.

Then each process in the mode will be explained.

The silicon acid material (plate) having the above components may be formed such that, for example, the steel is cast in a converter, an electric furnace, or the like, and the cast steel is subjected to a vacuum degassing treatment as required and then subjected to continuous casting. In addition, the silicon steel material can also be formed so that instead of continuous casting, an ingot is made to be then roughed. The thickness of the silicon steel plate is fixed, for example, at 150 mm to 350 mm, and is preferably set at 220 mm to 280 mm. In addition, what is called a thin plate having a thickness of 30 mm to 70 mm can also be formed. In the case when the thin plate is formed, the raw rolling when obtaining the hot rolled steel strip may be omitted. After the silicon steel plate is formed, the heating of the

plate is made (step S1), and hot rolling (step S2) is performed to obtain the hot rolled steel strip. Then the hot rolled steel strip is annealed (step S3). Then hot rolling is performed (step S4). As described above, cold rolling can be performed only once, or cold rolling can also be performed a number of times with the intermediate annealing between them. In cold rolling, the cold rolling rate is preferably set at 80% or more. This is to develop a good texture of primary recrystallization.

Subsequently, decarburising annealing is performed (step S5). As a result, C contained in the steel strip is removed. Decarburising annealing is performed in a humid atmosphere containing hydrogen and nitrogen, for example. In addition, decarburising annealing is preferably performed at a time so that, for example, a crystal grain diameter obtained by primary recrystallization becomes 15 μηι or more over a temperature range of 750 ° C to 900 ° C. This is to get the good magnetic property. In addition, nitriding treatment is performed (step S6)

after the decarburising annealing is over, or in parallel with the decarburizing annealing. In nitriding treatment, annealing (soaking treatment) is performed on the unembedded steel strip in an atmosphere of hydrogen gas, mixed nitrogen, and ammonia while running the steel strip, thereby making nitrogen. not enter the steel strip. For example, an annealing temperature range is set at 700 ° C to 100 ° C, and a time taken for annealing is set at 5 seconds to 180 seconds.

If the annealing temperature is 500 ° C or higher, it is possible to uniformly diffuse nitrogen into the steel strip to obtain good secondary recrystallization, but if it is less than 700 ° C, it is sometimes the case that a crack, fracture, and so on, are caused at the subsequent coiling. In addition, decarburising annealing is performed at 900 ° C or so as described above, and thus in the case where nitriding treatment is performed after decarburizing annealing is finished and the treatment temperature With nitriding exceeding 1000 ° C, a grain structure obtained by decarburizing annealing sometimes changes greatly. In addition, where nitriding treatment is carried out in parallel with decarburizing annealing and the temperature of nitriding treatment exceeds 1000 ° C, a grain structure contained after decarboning annealing and nitriding treatment at sometimes do not become appropriate. The time taken for annealing in 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 to effect secondary recrystallization stably. A nitriding rate can be adjusted according to the amount of ammonia in the gas atmosphere and the annealing temperature in the nitriding treatment. However, when the amount of ammonia is high to reduce the time taken for annealing in nitriding treatment, variations in the degree of nitration are likely to be increased between positions and between the front and rear surfaces of the strip. of steel. So if the time taken for baking is less than 5 seconds, the variations are likely to be marked. On the other hand, as the time taken for annealing becomes longer, variations in the degree of nitriding are reduced, but productivity is likely to decrease. So if the time taken for annealing exceeds 180 seconds, the reduction in productivity is likely to be marked.

As described above, in nitriding treatment, the cooling condition after soaking treatment is important. This is suppressing a crack, fracture, and so on, at the moment of concealment. Thus, in the embodiment, the average cooling rate in the temperature range of 700 ° C to 200 ° C is set at 5 ° C / s or higher, and is preferably set at 10 ° C / s or higher. When cooling at this average rate, as shown in figure 2, the number of reverse bends is high, namely a reverse bending property is improved. In addition, the coverage of the grain boundaries by the precipitates in the nitrided steel strip obtained by nitriding treatment as above becomes 0.5% or less.

In order to further suppress the production of coarse grain boundary inclusions, the average cooling rate in the temperature range of 700 ° C to 500 ° C is more preferably set at 20 ° C / s or more, and is even more preferably. set at 30 ° C / s or more. When cooling is performed at this average rate, a mechanical property is improved. Incidentally, the upper limit of the cooling rate is not particularly limited in terms of mechanical property, but if the cooling rate is excessively accelerated, there is a possibility that the thermal voltage will be caused in one direction of the cooling. coil width (sheet width direction) to affect secondary recrystallization. Particularly, in the case of the average cooling rate being above 100 ° C / s, such an effect is of concern.

The cooling method is not limited in particular. For example, cooling can be done using nitrogen gas or hydrogen gas, or cooling can also be done using mixed nitrogen gas and hydrogen gas. It should be noted that cooling is preferably performed in a non-oxidizing atmosphere to prevent oxidation of the steel strip.

After nitriding treatment (step S6), the MgO-containing annealing separating agent is coated on the surface of the nitrided steel strip, and the nitrided steel strip having the annealing separating agent coated on it. (step S7). Subsequently, the annealing finish is performed for the nitrided steel strip (step S8). As a result, the {110}? Orientation oriented crystal grains preferably grew by secondary recrystallization. In addition, by reacting the internal oxide layer formed at the time of decarburizing annealing and MgO in the annealing separation agent, the vitreous film having forsterite as its main component is formed.

The finishing annealing method (step S8) also

It is not limited in particular. For example, the temperature may be increased to 1200 ° C in the atmosphere containing hydrogen gas and nitrogen, and the gas atmosphere may be switched to hydrogen gas atmosphere at 1200 ° C, and precipitates may be purified. According to this embodiment above, it is possible to manufacture the

High performance grain-oriented electric steel wire suppressing a fracture or the like of the nitrided steel strip at the time of embedding. EXAMPLE EXAMPLE 1

Firstly, silicon steel plates containing by weight% Si: 3.1%, C: 0.05%, Acid-soluble Al: 0.025%, N: 0.007%, Mn: 0.14%, and S : 0.007%, and a remainder consisting of Fe and unavoidable impurities, were heated to 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. Then cold rolling was performed to obtain cold rolled steel strips having a thickness of 0.22 mm. Then decarbur annealing was performed in a humid atmosphere of nitrogen and hydrogen gas mixed at 850 ° C for 120 seconds to obtain decarbonised annealed steel strips. After decarburizing annealing as a treatment

With nitriding, the decarburized 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 were then cooled over a temperature range of 700 ° C. ° C to 200 ° C under various conditions shown in table 1. Then, a retaining agent having MgO as its main component was coated onto the steel strips, and the steel strips were heated to 1200 ° C. ° C at a rate of 15 ° C / h in order to perform the annealing finish. Then, after finishing firing, an examination of the reverse bending property (brittleness) was conducted as an examination of the mechanical property. In table 1, the conditions of nitriding treatment and mechanical property are shown. TABLE 1

Condition Cooling Rate- Cooling Rate- Property Ownership Average Mean Bending- Cooling (0CIs) (0CIs) Re- (700 ° ca 200 ° C) (700 ° ca 500 ° C) verse 1A 1 4 I 1B 6 13 11 1C 6 20 Ill 1D 6 33 IV 1E 13 17 IV

In the examination of the reverse bending property (brittleness) under an atmosphere of 0 ° C, 90 degree reverse bending was performed in such a way as to wrap a strip of nitrided steel around a bar.

round with a diameter of 20 mm and the number of bends until the nitrided steel strip was fractured was measured. The assessment by reverse bending was defined as the number of reverse bending until fracture is less than 30, which is I, and the number of reverse bending until fracture is 30 to 39, which is II, and the number of reverse bending 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. EXAMPLE 2

In example 2, the cold rolled steel strips were obtained similar to example 1. Then, in a humid atmosphere of mixed ammonia, nitrogen and hydrogen gas, the cold rolled steel strips were annealed by decarburizing at 850 ° C. ° C for 120 seconds, and nitriding treatment was performed in parallel with decarburizing annealing to increase nitrogen in the 0.023% / mass steel strips, and then the steel strips were cooled to a temperature range of 700 ° C. at 200 ° C under various conditions shown in table 2. Subsequently, an annealing separating agent having MgO as its main component was coated onto the steel strips, and the steel strips were heated to 1200 ° C at a rate of 15 ° C. ° C / h to finish annealing. Then, similarly to Example 1, after finishing annealing, an examination of the reverse bending property (brittleness) was conducted as an examination of the mechanical property. In table 2, nitriding treatment conditions and mechanical property are shown.

TABLE 2

Condition Cooling Rate- Cooling Rate- Property Ownership Average Mean Bending- Cooling (° C / s) (0CIs) Re- (700 ° C to 200 ° C) (700 ° C to 500 ° C) back 2A 1 4 I 2B 6 14 11 2C 6 22 Ill 2D 6 33 IV 2E 13 18 IV

EXAMPLE 3

Firstly, silicon steel plates containing by weight% Si: 3.2%, C: 0.05%, Acid-soluble Al: 0.024%, N: 0.005%, Mn: 0.04% and S : 0.01%, and a remainder being composed of Fe, and unavoidable impurities were heated to 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. Then cold rolling was performed to obtain cold rolled steel strips each having a thickness of 0.26 mm. Afterwards, decarburising was performed in a humid atmosphere of nitrogen and hydrogen gas mixed at 850 ° C for 150 seconds to obtain decarbonised annealed steel strips.

After decarburising annealing as a nitriding treatment, the decarbonised annealed steel strips were annealed at 750 ° C for 30 seconds at a temperature containing ammonia to increase nitrogen in the steel strips to 0.024% / mass, and then cooled to a temperature range of 700 ° C to 200 ° C under various conditions shown in Table 3. Then an annealing separating agent having MgO as its main component was coated onto the steel strips, and the strips The steel plates were heated to 1200 ° C at a rate of 15 ° C / h in order to perform the annealing finish. Then, similarly to example 1, after finishing annealing, an examination of the reverse bending property (brittleness) was conducted as an examination of the mechanical property. In table 3, nitriding treatment conditions and mechanical property are shown.

TABLE 3

Condition Cooling Rate- Cooling Rate- Building Property Average Average Curve-Cooling (0CIs) (0CIs) Rever- (700 ° C to 200 ° C) (700 ° C to 500 ° C) so 3A 1 4 I 3B 6 13 11 3C 6 20 Ill 3D 6 33 IV 3E 13 17 IV

EXAMPLE 4

Firstly, silicon steel plates containing by weight% Si: 3.3%, C: 0.06%, Acid-soluble Al: 0.027%, N: 0.008%, Mn: 0.11% and S: 0.06%, and a remainder being composed of Fe, and unavoidable impurities were heated to 1150 ° C and hot-rolled to obtain hot-rolled steel strips each 2.0 mm thick. Subsequently, the hot rolled steel strips were annealed at 1100 ° C. The cold rolling was then performed to obtain cold rolled steel strips each having a thickness of 0.20 mm. Afterwards, decarburising annealing was carried out in a humid atmosphere of nitrogen and hydrogen gas mixed at 850 ° C for 100 seconds to obtain decarbonised annealed steel strips.

After decarburizing annealing, as a nitriding treatment, decarburized annealed steel strips were annealed at 750 ° C for 20 seconds in an atmosphere containing ammonia to increase nitrogen in the steel strips to 0.03% / mass, and then cooled to a temperature range of 700 ° C to 200 ° C under various conditions shown in table 4. Then, a retaining agent having MgO as its main component was coated onto the strips. steel, and the steel strips were heated to 1200 ° C at a rate of 15 ° C / s in order to perform the annealing finish. Then, similarly to example 1, after the finished annealing, an examination of the reverse bending property (frailty) was conducted as an examination of mechanical property. In addition, the coverage of grain boundaries by precipitation in an inner layer at the depth of 20 μιτι from the surface of each of the nitrided steel strips was measured. In table 4, the nitriding treatment conditions, the coverage of the grain boundaries by the precipitates and the mechanical property are shown. TABLE 4

Condition- Res- Rate of Coverage Ownership Cooling Cooling of Limiting Resil Limits (0CIs) Average (0CIs) of Curved Grains (700 ° C to (700 ° C a per Precipitation Re - at 200 ° C) 500 ° C) data (%) versa 4A 0,1 1 1,1 I 4B 6 17 0,44 Il 4C 13 40 0,18 IR

In measuring the grain boundary coverage by the precipitates

Each of the nitrided steel strips was polished at the position of the inner layer at a depth of 20 μιη to 50 μιη from the front surface, and then subjected to a caustic treatment with a caustic reagent such as a nital solution and a metal structure. (grain boundaries and precipitates) were observed. Then, by image processing, the extent of grain boundaries where precipitates exist and the total extent of grain boundaries were measured, and the ratio of the extent of grain boundaries where precipitates exist to the full extent of precipitates. Grain boundaries were calculated as the coverage of grain boundaries by the precipitates.

It should be noted that the above embodiments merely illustrate concrete examples of implementing the present invention, and the scope of the technique of the present invention should not be construed narrowly 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

The present invention may be used, for example, in a

electric steel sheet industry and an industry in which electric steel strip is used.

Claims (9)

A method of manufacturing a grain oriented electric steel sheet, comprising: heating a steel material to silicon; hot rolling the heated silicon steel material to obtain a steel strip; anneal the steel strip; cold rolling the steel strip once or more after annealing; decarburize the steel strip after cold rolling; perform a nitriding treatment in which an N content of the steel strip is increased after the decarburising annealing is over or in parallel with the decarburising annealing to obtain a nitrided steel strip in which primary recrystallization is caused; coating an annealing separating agent having MgO as its major component on the nitrided steel strip; and finally annealing the nitrided steel strip having the annealing separating agent thereon to cause secondary recrystallization, wherein said execution of the nitriding treatment comprises: annealing at a temperature of 700 ° C to 100 ° C in gas containing ammonia while running the steel strip; and then perform cooling, and an average cooling rate over a temperature range of 700 ° C to 200 ° C on cooling is set to 5 ° C / s or more. A method of fabricating a grain oriented electric steel sheet according to claim 1, wherein the average cooling rate in the temperature range of 700 ° C to 200 ° C in cooling is set at 10 ° C / I'm more. A method of fabricating a grain-oriented electric steel sheet according to claim 1 or 2, wherein the average cooling rate over a temperature range of 700 ° C to 500 ° C in cooling is set at 20 ° C / s or more. A method of fabricating a grain oriented electric steel sheet according to claim 1 or 2, wherein the average cooling rate in the temperature range of 700 ° C to 500 ° C on cooling is set at 30 ° C / s or more. A method of treating a steel strip, comprising: annealing at a temperature of 700 ° C to 100 ° C in ammonia-containing gas while performing a steel strip for a grain oriented electric steel sheet; and then perform cooling, where an average cooling rate over a temperature range of 700 ° C to 200 ° C on cooling is set at 5 ° C / s or more. A steel strip treatment method according to claim 5, wherein the average cooling rate in the temperature range of 700 ° C to 200 ° C in the cooling is set at 10 ° C / s or more. A method of treating a steel strip according to claim 5 or 6, wherein an average cooling rate in the temperature range of 700 ° C to 500 ° C in cooling is set at 20 ° C / s or more. A steel strip treatment method according to claim 5 or 6, wherein the average cooling rate in the temperature range of 700 ° C to 500 ° C in cooling is set at 30 ° C / I'm more. 9. Nitrided steel strip with coverage of grain boundaries by precipitates 0.5% or less.