JP6436316B2 - Method for producing grain-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a method for producing a grain-oriented electrical steel sheet, and particularly to a method for producing a grain-oriented electrical steel sheet having excellent iron loss characteristics.

  Electrical steel sheets are soft magnetic materials that are widely used as iron core materials for transformers and motors. Among them, grain oriented electrical steel sheets are highly integrated in the {110} <001> orientation, in which the crystal orientation is called the Goss orientation. Therefore, in order to show excellent magnetic properties, it is mainly used as a core material for large transformers. Therefore, the main development subject of the conventional grain-oriented electrical steel sheet is to reduce the loss caused when the steel sheet is excited, that is, the iron loss in order to reduce the no-load loss (energy loss) of the transformer.

  As a method for producing a grain-oriented electrical steel sheet, a method using a fine precipitate called an inhibitor such as AlN, MnS, MnSe, or CuS is generally used. In this method, the inhibitor is finely dispersed in the steel and the grain growth during finish annealing is suppressed, so that only the Goss orientation is preferentially recrystallized.

By the way, generally in the manufacturing method of a grain-oriented electrical steel sheet, the decarburization annealing which reduces C in a raw steel plate to the level which does not raise | generate a magnetic aging is performed. In this decarburization annealing, since the annealing atmosphere is oxidizing, an oxide layer mainly composed of oxides of Si and Fe is formed under the steel sheet surface (hereinafter, this oxide layer is referred to as “subscale”). Called). After applying an annealing separator mainly composed of MgO to the steel plate surface on which the subscale is formed, the subscale and MgO react with each other by subjecting the steel plate surface to forsterite (Mg ₂ SiO _{2). 4} ) A layer is formed. This forsterite layer plays an important role as an insulating coating when a product plate is used by being laminated, and to reduce iron loss by applying tensile tension to the surface of the steel plate.

  In addition, the subscale has a barrier action that suppresses nitrogen contained in the annealing atmosphere from entering the steel during the finish annealing. When AlN is used as the above-described inhibitor, if the above barrier action is weak, nitrogen easily penetrates into the steel and decomposition of AlN at high temperatures is suppressed, so that secondary recrystallization in the Goss orientation is appropriate. In addition, since the grains deviated from the Goss orientation are not regenerated at the proper temperature, and secondary recrystallization occurs, the magnetic properties of the product plate are deteriorated. Therefore, when AlN is used as an inhibitor, it is desirable to increase the barrier action against nitrogen intrusion by making the subscale dense.

As a method for controlling the form of the subscale, for example, in Patent Document 1, the thickness of Si is adjusted by setting P _H2O / P _H2 in the atmosphere of intermediate annealing to 0.4 to 2.0 (oxidizing). By suppressing the Si concentration gradient in the direction, the proportion of SiO _{2 in} the subscale formed by subsequent decarburization annealing is stably increased, so that a uniform and good forsterite film can be obtained. Is disclosed.

In Patent Document 2, the desiliconization layer on the surface of the steel plate before final cold rolling is set to 5 μm or more and less than 100 μm, and the maximum value of P _H2O / PH _{2 in} the atmosphere of decarburization annealing is set to 0.15 or more and less than 0.60. and then, by forming a dense subscale increase amount of oxidizing P _{H2O /} P _H2 atmosphere in the decarburization annealing as 0 to less than 0.15, local additional oxidation during annealing is suppressed It is disclosed that a uniform forsterite film can be obtained.

  However, the technology for controlling the subscale density by controlling the atmosphere of the intermediate annealing and the decarburization annealing is a technology mainly for the purpose of uniformly forming the forsterite film. It does not improve the magnetic properties by controlling the decomposition behavior of such inhibitors.

Further, in Patent Document 3, a silicon removal layer is formed on the surface of a steel plate before decarburization annealing, and the ratio of Si concentration to the Si concentration at the center of the plate thickness of this silicon removal layer is determined in the state of the final cold rolled sheet. In the region from the surface to the thickness direction of 1 μm, adjusting the region where the Si concentration ratio is 0.98 or less from the surface to the thickness direction of 5 μm, and the decarburization annealing, A technique is disclosed in which the P _H2O / P _H2 of the atmosphere in the soaking process is less than 0.70, and the P _H2O / P _H2 of the atmosphere in the heating process is set to a value lower than that in the soaking process. According to this technique, it is said that the grain-oriented electrical steel sheet having excellent coating properties and magnetic properties can be obtained by suppressing the oxidation / decomposition behavior of the inhibitor during finish annealing with H ₂ O contained in the annealing separator.

Japanese Patent Laid-Open No. 07-041861 JP-A-11-152517 Japanese Patent Laid-Open No. 10-060533

  However, the technique disclosed in Patent Document 3 is a technique for controlling the oxidation / decomposition behavior during finish annealing of an inhibitor when using MnS or MnSe in addition to AlN as an inhibitor. When used, it is not effective for the problem that nitrogen penetrates into steel during finish annealing and adversely affects the decomposition of AlN.

  The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to appropriately control the form of the subscale after decarburization annealing, and to penetrate nitrogen into the steel during finish annealing. It is to propose a method of manufacturing a grain-oriented electrical steel sheet capable of stably obtaining excellent iron loss characteristics by suppressing the above.

  In order to solve the above-mentioned problems, the inventors have conducted intensive studies focusing on the form of a subscale effective for preventing nitrogen from entering into steel during finish annealing. As a result, in decarburization annealing, a dense subscale is formed below the steel sheet surface, nitrogen penetration into the steel sheet is suppressed, and good iron loss characteristics can be obtained. In order to form it, it has been found that it is effective to form a desiliconized layer on the steel sheet surface in an appropriate oxidizing atmosphere in the heating process of the intermediate annealing before the final cold rolling, and the present invention has been developed. It was.

That is, the present invention includes C: 0.002 to 0.10 mass%, Si: 2.5 to 6.0 mass%, Mn: 0.01 to 0.8 mass%, Al: 0.010 to 0.050 mass%, and N: 0.003 to 0.020 mass%, the remainder comprising Fe and inevitable impurities is hot-rolled, hot-rolled sheet annealed, cold-rolled twice with intermediate annealing in between, and decarburized In the method of manufacturing a grain-oriented electrical steel sheet comprising a series of steps of annealing and forming a sub-scale on the steel sheet surface, then applying an annealing separator mainly composed of MgO to the steel sheet surface, and finishing annealing, heating the intermediate annealing. The average heating rate between 700-900 ° C. in the process is 10 ° C./s or less, and the oxygen potential (P _H2O / P _H2 ) of the soaking process atmosphere is in the range of 0.20-0.80. Addition of charcoal annealing We propose a method for producing grain-oriented electrical steel sheets, characterized in that the oxygen potential (P _H2O / P _H2 ) in the thermal process is 0.20 or more.

In the method for producing the grain-oriented electrical steel sheet according to the present invention, the Fe emission intensity profile in the thickness direction obtained when glow discharge emission analysis is performed on the steel sheet surface after the decarburization annealing has a Fe-deficient region below the steel sheet surface. and a minimum value of Fe emission intensity of the Fe-deficient region _{I min,} the Fe emission intensity _{I max} of the bulk region, the _{I min} and between the _{I max,} Fe emission intensity _I min + 0.5 × (I _max -I _min) become until _t 50 the time from the start of analysis of (s), Fe emission intensity _I min + 0.95 × _(the I max _{-I min)} and the time from the start of analysis until _{t 95} When (s), t ₅₀ and t ₉₅ are
t ₅₀ / t ₉₅ ≦ 0.75
It is characterized by satisfying the relationship.

  Moreover, the manufacturing method of the said grain-oriented electrical steel sheet of this invention makes the average temperature increase rate between 500-700 degreeC in the heating process of the said decarburization annealing set to 80 degrees C / s or more.

  Moreover, in addition to the said component composition, the said slab used for the manufacturing method of the said grain-oriented electrical steel sheet of this invention is further among S: 0.002-0.03mass% and Se: 0.002-0.03mass% It contains 1 type or 2 types chosen from these.

  Moreover, in addition to the said component composition, the said slab used for the manufacturing method of the said grain-oriented electrical steel sheet of this invention is further Cr: 0.01-0.50mass%, Cu: 0.01-0.50mass%, P : 0.005-0.50 mass%, Ni: 0.01-1.50 mass%, Sb: 0.005-0.50 mass%, Sn: 0.005-0.50 mass%, Mo: 0.005-0 100% by mass, B: 0.0002 to 0.0025 mass%, Nb: 0.0010 to 0.0100 mass% and V: 0.001 to 0.01 mass%, or one or more selected from It is characterized by that.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide a directional electrical steel sheet with a low iron loss stably.

It is a figure which shows an example of the profile of Fe light emission intensity obtained by GDS analysis. Time ratio _(t 50 _/ t ₉₅₎ is a graph showing the effect on core loss property. Heating rate between 700-900 ° C. for intermediate annealing is a graph showing the effect on time ratio _(t 50 _/ t _{95). P} H2O _{/ P H2} atmosphere between 500-700 ° C. for decarburization annealing is a graph showing the effect on time ratio _(t 50 _/ t _95).

First, an experiment that triggered the development of the present invention will be described.
C: 0.06 mass%, Si: 3.0 mass%, Mn: 0.05 mass%, Al: 0.020 mass% and N: 0.006 mass%, with the balance being composed of Fe and inevitable impurities After reheating the steel slab having a temperature of 1400 ° C., it is hot-rolled to obtain a hot-rolled sheet having a thickness of 2.2 mm, subjected to hot-rolled sheet annealing of 1100 ° C. × 60 seconds, and then cold-rolled. The plate thickness was 1.5 mm, intermediate annealing was performed at 1100 ° C. for 80 seconds, and a cold rolled sheet having a final thickness of 0.23 mm was obtained by the second cold rolling.
In the intermediate annealing, the average temperature increase rate between 700 and 900 ° C. in the heating process is variously changed in the range of 1 to 50 ° C./s, and the oxygen potential P _H2O / P _{H2 of the} soaking process atmosphere. Was varied in the range of 0.001 to 3.0.
Next, the above-mentioned cold-rolled sheet is subjected to a primary recrystallization annealing in which the oxygen potential P _H2O / P _H2 = 0.40 (constant) is kept at a temperature of 850 ° C. for 120 seconds in a mixed wet atmosphere of H ₂ and N ₂ Decarburization annealing was also performed.
In the decarburization annealing, the average temperature increase rate between 500-700 ° C. in the heating process is 20 ° C./s (constant), and the oxygen potential P _H2O / P _H2 of the atmosphere in the temperature interval is 0.01 Various changes were made in the range of -1.0.

  A sample was taken from the center of the plate width of the steel plate after decarburization annealing thus obtained, and a profile of Fe emission intensity in the plate thickness direction was obtained by high-frequency glow discharge emission analysis GDS. The GDS analysis was performed using a Rigaku System 3860 under conditions of a discharge current of 20 mA and a purge gas flow rate of 200 ml / min.

  FIG. 1 shows an example of the measurement result of the Fe emission intensity profile. As can be seen from this figure, the Fe emission intensity is low on the outermost surface, but increases rapidly as the depth increases to show a maximum peak, then decreases to a minimum value, and then gradually increases. It shows the change to a bulk region where the intensity is constant. In the present invention, a region where the Fe emission intensity is low from the maximum peak of the Fe issue intensity to the bulk region is referred to as “Fe-deficient region”.

Here, in the Fe emission intensity profile, the minimum value of the Fe emission intensity in the Fe-deficient region is defined as I _min , and the convergence value of the Fe emission intensity in the bulk region is defined as I _max, and the above I _max is determined as follows.
In the Fe-deficient region, the time from the start of measurement at which Fe emission intensity becomes I _min is t _min , the Fe emission intensity at time t in the region satisfying time t> t _min is I (t), and time t to t + (1 / 4) When the standard deviation of the Fe emission intensity during t _min is σ (t), the Fe emission intensity at the minimum time t satisfying σ (t) / I (t) <0.01 is I _max . Define.
In addition, in a region satisfying time t> t _min , the time when the Fe emission intensity is I _min + 0.50 × (I _max −I _min ) is t ₅₀ and I _min + 0.95 × (I _max −I _min ). the becomes time defined as _{t 95,} and represent the time ratio of the both in the _(t 50 _/ t _95).

Next, after applying an annealing separator mainly composed of MgO to the surface of the steel sheet after the decarburization annealing and drying, a secondary recrystallization annealing and a finishing annealing consisting of a purification treatment held at 1150 ° C. for 6 hours are performed. did.
Test pieces were collected from the steel sheet after finish annealing thus obtained, and the iron loss W _17/50 at a magnetic flux density of 1.7 T and an excitation frequency of 50 Hz was measured in accordance with JIS C2550.
The result of the measurement is shown in FIG. From this figure, as the time ratio (t ₅₀ / t ₉₅ ) decreases, the iron loss W _17/50 tends to decrease, and a good iron loss of W _17/50 ≦ 0.82 W / kg can be obtained. Indicates that the time ratio (t ₅₀ / t ₉₅ ) is 0.75 or less.

The inventors consider the reason why a good iron loss is obtained by reducing the time ratio (t ₅₀ / t ₉₅ ) to 0.75 or less as follows.
The Fe-deficient region in the Fe emission intensity profile obtained from the GDS analysis of the decarburized annealing plate originated from the fact that Si in the steel formed a sub-scale mainly composed of SiO ₂ by internal oxidation during decarburization annealing. To do. Here, it can be considered that the time t ₉₅ corresponds to almost the entire thickness of the subscale, and t ₅₀ corresponds to the magnitude of Fe deficiency, that is, the thickness of the region mainly composed of SiO ₂ .

And, the fact that t ₅₀ is small with respect to the same t ₉₅ means that SiO ₂ is locally formed in a sub-scale having the same thickness, that is, a dense sub-scale is formed. On the other hand, a large t ₅₀ is considered to indicate that SiO ₂ is formed in a wide region, that is, a rough subscale is formed like dendritic SiO ₂ .
Here, in order to prevent nitrogen from entering the steel through the subscale during the finish annealing and to obtain good iron loss characteristics, a dense subscale is desirable. Therefore, it is considered that good iron loss was obtained under the condition of t ₅₀ / t ₉₅ ≦ 0.75.

Next, the inventors examined production conditions for forming the above-described dense subscale.
FIG. 3 shows the heating rate between 700 and 900 ° C. in the heating process of the intermediate annealing, which affects the time ratio (t ₅₀ / t ₉₅ ) of the decarburized annealing plate obtained in the above experiment, and the atmosphere of the soaking process. This shows the influence of the oxygen potential P _H2O / P _H2 . From this figure, the range of _P H2O _{/ P H2} atmosphere for intermediate annealing is 0.20 to 0.80, and the range rate of temperature increase below 10 ° C. / s between 700-900 ° C., the time ratio (t ₅₀ / t ₉₅ ) is 0.75 or less, which is the target of the present invention.

FIG. 4 shows the oxygen potential P _H2O / atmosphere between 500-700 ° C. in the heating process of decarburization annealing on the time ratio (t ₅₀ / t ₉₅ ) of the decarburized annealing plate obtained in the same experiment. This shows the effect of _PH2 . From this figure, by setting the oxygen potential P _H2O / P _H2 in the atmosphere between 500 and 700 ° C. in decarburization annealing to be 0.20 or more, the time ratio (t ₅₀ / t ₉₅ ) is the target 0 of the present invention. .75 or less.

The inventors consider the reason why a dense subscale can be formed by setting the intermediate annealing and decarburization annealing to the above-described conditions as follows.
In order to form a dense subscale after decarburization annealing, it is desirable to form a desiliconization layer having a reduced Si concentration on the steel sheet surface layer before decarburization annealing. The reason is that the desiliconized layer moderates the formation of subscales due to internal oxidation during the decarburization annealing and promotes the formation of lamella-like dense subscales. For this purpose, it is desirable to increase the oxygen potential P _H2O / P _H2 in the atmosphere during intermediate annealing to make it oxidizing, and selectively oxidize Si on the steel sheet surface layer to form a desiliconized layer. However, if the oxygen potential P _H2O / P _H2 in the atmosphere is increased too much, the oxidation of Fe is also promoted during the intermediate annealing and the external scale grows, which hinders the internal oxidation during the decarburization annealing, and the dense subscale It is thought that it becomes difficult to form.

  Further, the desiliconization reaction easily proceeds between 700-900 ° C. Therefore, in order to promote the formation of the desiliconization layer to form a dense subscale and to suppress the ingress of nitrogen into the steel during the finish annealing, a sufficient time for staying in the temperature section is ensured. It is considered preferable. Therefore, desiliconization can sufficiently proceed by setting the average temperature increase rate between 700-900 ° C. in the heating process of the intermediate annealing to 10 ° C./s or less.

The reason why the magnetic properties are improved by making the oxygen potential P _H2O / P _H2 of the atmosphere in the heating process of decarburization annealing more than 0.20 is related to the oxidation behavior during heating. I believe that.
That is, the formation of the subscale proceeds at a temperature of 500 ° C. or higher in the heating process of the decarburization annealing. At this time, if the oxygen potential P _H2O / P _{H2 of} the atmosphere is sufficiently high, a dense initial oxidation is performed on the steel sheet surface layer. Since a layer is formed and progress of internal oxidation at a soaking temperature of about 800 to 900 ° C. is suppressed, a dense subscale is formed after decarburization annealing. On the other hand, when the oxygen potential P _H2O / P _H2 of the atmosphere in the heating process is low, a dense initial oxide layer is not formed, and internal oxidation proceeds rapidly at a soaking temperature, so that a dendrite-like rough subscale is formed. Is done.
The present invention has been developed based on the above novel findings.

Next, the component composition of the steel material (slab) used for the material of the grain-oriented electrical steel sheet of the present invention will be described.
C: 0.002-0.10 mass%
When C is less than 0.002 mass%, the grain boundary strengthening effect due to C is lost, and slab cracking occurs, which causes problems in production. On the other hand, if it exceeds 0.10 mass%, it becomes difficult to reduce C to 0.005 mass% or less at which no magnetic aging occurs due to decarburization annealing. Therefore, C is in the range of 0.002 to 0.10 mass%. Preferably it is the range of 0.01-0.08 mass%.

Si: 2.5-6.0 mass%
Si is an element necessary for increasing the specific resistance of steel and reducing iron loss. If the effect is less than 2.5 mass%, it is not sufficient. On the other hand, if it exceeds 6.0 mass%, the workability deteriorates and it is difficult to roll and manufacture. Therefore, Si is set to a range of 2.5 to 6.0 mass%. Preferably, it is in the range of 2.9 to 5.0 mass%.

Mn: 0.01 to 0.8 mass%
Mn is an element necessary for improving the hot workability of steel. If the effect is less than 0.01 mass%, the effect is not sufficiently obtained. On the other hand, if it exceeds 0.8 mass%, the magnetic flux density of the product plate is lowered. Therefore, Mn is set to a range of 0.01 to 0.8 mass%. Preferably it is the range of 0.02-0.50 mass%.

Al: 0.010-0.050 mass%, N: 0.003-0.020 mass%
Both Al and N are elements necessary as inhibitor forming elements. If any element is less than the lower limit, the inhibitor effect cannot be sufficiently obtained.On the other hand, if the upper limit is exceeded, the solid solution temperature rises and remains undissolved during slab reheating. Inhibitor effect does not fully develop and magnetic properties deteriorate. Accordingly, Al is in the range of 0.010 to 0.050 mass%, and N is in the range of 0.003 to 0.020 mass%. Preferably, Al ranges from 0.015 to 0.035 mass%, and N ranges from 0.005 to 0.015 mass%.

  In addition to the above components, the steel material used in the present invention further contains any one or more of S: 0.002-0.03 mass% and Se: 0.002-0.03 mass% as inhibitor-forming elements. can do. If the respective contents are less than the lower limit value, the inhibitor effect cannot be sufficiently obtained.On the other hand, if the upper limit value is exceeded, the solid solution temperature becomes high, and remains undissolved during slab reheating. Degrading properties.

  In addition to the above components, the steel material used in the present invention further has Cr: 0.01 to 0.50 mass%, Cu: 0.01 to 0.50 mass%, and P: 0 in addition to the above components. One or two or more selected from 0.005 to 0.50 mass% can be contained. If the amount of each additive is less than the above lower limit value, the effect of reducing iron loss is small. On the other hand, if the amount exceeds the above upper limit value, the development of secondary recrystallized grains is inhibited, and on the contrary, the magnetic properties are reduced. In such a case, the above range is preferable.

  Moreover, in order to improve the magnetic flux density, the steel material used in the present invention is further provided with Ni: 0.01 to 1.50 mass%, Sb: 0.005 to 0.50 mass%, and Sn: 0 in addition to the above components. 0.005-0.50 mass%, Mo: 0.005-0.100 mass%, B: 0.0002-0.0025 mass%, Nb: 0.0010-0.0100 mass%, and V: 0.001-0.01 mass 1 type or 2 types or more chosen from% can be contained. When the content of each element is less than the above lower limit value, the effect of improving the magnetic flux density is small. On the other hand, when the above upper limit value is exceeded, the development of secondary recrystallized grains is hindered, and the magnetic properties are deteriorated. When added, the above range is preferable.

  In the steel material used in the present invention, the balance other than the above components is Fe and inevitable impurities. In addition, if it is in the range which does not impair the effect of this invention, inclusion of another component is not refused.

Next, the manufacturing method of the grain-oriented electrical steel sheet of this invention is demonstrated.
A steel material (slab) may be produced by a conventional ingot-bundling rolling method or continuous casting method after melting the steel having the above-described composition by a conventional refining process. Alternatively, a thin cast piece having a thickness of 100 mm or less may be manufactured by a direct casting method. The slab is subjected to hot rolling after reheating to a temperature of about 1400 ° C. according to a conventional method. In the case of a thin slab, the hot rolling may be omitted and the process may proceed as it is.

  Subsequently, the hot-rolled steel sheet (hot-rolled sheet) is subjected to hot-rolled sheet annealing as necessary. This hot-rolled sheet annealing preferably has a soaking temperature in the range of 800 to 1150 ° C. in order to obtain good magnetic properties. If it is less than 800 degreeC, the band structure | tissue formed by hot rolling will remain, it will become difficult to obtain the primary recrystallization structure | tissue of a sized particle, and the growth of a secondary recrystallized grain will be inhibited. On the other hand, when the temperature exceeds 1150 ° C., the grain size after the hot-rolled sheet annealing is excessively coarsened, so that it becomes difficult to obtain a primary recrystallized structure of sized particles.

  Thereafter, the steel sheet after hot rolling or after hot rolling sheet annealing is made into a cold rolled sheet having a final thickness by cold rolling twice with intermediate annealing. The annealing temperature of the intermediate annealing is preferably in the range of 900 to 1200 ° C. If the temperature is lower than 900 ° C., the recrystallized grains after the intermediate annealing become finer, and the Goss nuclei in the structure after the primary recrystallization are reduced to deteriorate the magnetic properties of the product plate. On the other hand, when it exceeds 1200 ° C., the crystal grains become too coarse as in the case of hot-rolled sheet annealing, and it becomes difficult to obtain a primary recrystallized structure of sized grains.

Here, what is important in the intermediate annealing is that the average heating rate between 700 and 900 ° C. in the heating process is 10 ° C./s or less, and the oxygen potential P _H2O / P _H2 of the atmosphere at the time of annealing is 0.00. That is, it is necessary to set the range of 20 to 0.80 (oxidizing atmosphere).
The reason for setting the average temperature rising rate between 700-900 ° C. to 10 ° C./s or less is that when the temperature rising rate is higher than 10 ° C./s, the residence time in the above temperature range where desiliconization is likely to proceed is shortened. This is because desiliconization becomes insufficient, and it becomes impossible to form a dense subscale during decarburization annealing, so that desiliconization proceeds sufficiently. A preferable temperature increase rate is 7 ° C./s or less.

In addition, the oxygen potential P _H2O / P _H2 of the atmosphere at the time of annealing is set to a range of 0.20 to 0.80 by forming a desiliconized layer on the surface of the steel sheet and gradually progressing internal oxidation at the time of decarburization annealing. This is because a lamellar dense subscale is formed. When P _H2O / _PH2 is less than 0.20, desiliconization does not proceed sufficiently, and a dense subscale cannot be formed during decarburization annealing. On the other hand, if P _H2O / P _H2 exceeds 0.80 and becomes too high, an external scale grows, making it difficult to remove the scale in the subsequent pickling process, or forming a dense subscale by decarburization annealing. It becomes difficult to let them. Preferably it is the range of 0.3-0.6.

Next, the cold-rolled sheet rolled to the final sheet thickness is subjected to decarburization annealing that also serves as primary recrystallization annealing in the range of a soaking temperature of 700 to 900 ° C. If the C content of the steel material is less than 0.005 mass%, decarburization is not necessary, but decarburization annealing is required to form a subscale.
The atmosphere in the heating process of the decarburization annealing needs to be oxidizable with an oxygen potential P _H2O / _PH2 of 0.20 or more. By making the atmosphere during heating oxidizable, an internal oxide layer is formed on the surface layer of the steel sheet, and this internal oxide layer rapidly advances the internal oxidation of the steel sheet in the subsequent soaking process. Is suppressed. If P _H2O / P _H2 is less than 0.20, the internal oxide layer is not sufficiently formed in the heating process, so that the internal oxidation proceeds rapidly in the soaking process, a rough subscale is formed, and good iron loss is achieved. Cannot be obtained. Preferably it is 0.3 or more. In addition, although the upper limit of P _H2O / P _H2 is not specified, it is preferably set to 1.2 or less in order to prevent scales such as FeO and Fe ₂ O _{3 from} being formed on the steel sheet surface due to external oxidation.

The oxygen potential in the soaking process atmosphere in the decarburization annealing is preferably in the range of 0.2 to 1.2 (oxidizing atmosphere) from the viewpoint of forming a dense subscale.
In addition, when the steel material used in the present invention contains C in an amount of 0.005 mass% or more, the decarburization annealing is performed to prevent the product plate from being magnetically aged and deteriorating the magnetic properties. However, it is necessary to reduce C to less than 0.005 mass%. This decarburization treatment is desirably performed at a temperature of 700 to 900 ° C. in a wet atmosphere containing hydrogen and nitrogen. From this viewpoint, the oxygen potential P _H2O / P _H2 is set to a range of 0.10 to 0.70. Is preferred.
The decarburization annealing may be performed in combination with the soaking process of the primary recrystallization annealing, or may be performed as a process independent of the primary recrystallization annealing.

  In the decarburization annealing, in order to obtain good magnetic properties, it is preferable to set the average temperature increase rate between 500-700 ° C. in the heating process to 80 ° C./s or more. By setting the average temperature increase rate between 500-700 ° C. to 80 ° C./s or more, the Goss orientation in the primary recrystallization texture increases, the crystal grains after secondary recrystallization become finer, and iron loss is improved. Is done. More preferably, it is 100 ° C./s or more.

  Increasing the rate of temperature increase in decarburization annealing is disadvantageous for the formation of the initial oxide layer because the residence time between 500-700 ° C. in the heating process is shortened. The above problem is solved by combining the formation and the oxidizing atmosphere of the heating process of decarburization annealing.

The subscale formed under the surface of the steel sheet decarburized and annealed as described above preferably has a time ratio (t ₅₀ / t ₉₅ ) in the Fe emission intensity profile of 0.75 or less when analyzed by GDS. As described above, the time ratio (t ₅₀ / t ₉₅ ) is an index representing the density of the subscale. When the time ratio is greater than 0.75, the subscale is a coarse subscale. This is because good iron loss cannot be obtained due to penetration. More preferably, it is 0.72 or less. The model of the GDS analyzer is not particularly limited.

  When the steel sheet subjected to decarburization annealing is important in terms of iron loss characteristics and transformer noise, an annealing separator mainly composed of MgO is applied to the surface of the steel sheet, dried, then subjected to finish annealing, and Goss. It is preferable to develop a secondary recrystallized structure highly accumulated in the orientation and to form a forsterite film. On the other hand, when emphasizing the punching processability and not forming the forsterite film, it is not necessary to apply an annealing separator or to perform a final annealing using an annealing separator mainly composed of silica, alumina or the like. preferable. In the case where the forsterite film is not formed, it is also effective to use an electrostatic coating that does not bring moisture into the annealing separator. Moreover, it may replace with application | coating of an annealing separator, and may wind the sheet | seat of a heat resistant inorganic material (a silica, an alumina, mica) between steel plates at the time of coil winding.

  Here, in the case where the forsterite film is not formed, the finish annealing can be completed only by holding for several hours or more in a temperature range of 850 to 950 ° C. where recrystallization occurs. On the other hand, when a forsterite film is formed or when iron loss characteristics are emphasized and a purification process is performed, after the secondary recrystallization is caused, the temperature is further increased to about 1200 ° C. preferable.

  After the finish annealing, the steel sheet is then washed with water, brushed, pickled, etc. to remove unreacted annealing separator adhering to the steel sheet surface, and then flattened to correct the shape. It is effective for reduction. This is because the finish annealing is normally performed in a coil state, so that deterioration of characteristics at the time of measuring iron loss due to coil curling is prevented.

  Further, when using steel sheets in a laminated manner as an application of the product plate, it is effective to deposit an insulating film on the surface of the steel plate in the above-described flattening annealing, or in its pre-process or post-process. In particular, in order to reduce iron loss, it is preferable to apply a tension-imparting coating that imparts tension to the steel sheet as the insulating coating. For the formation of the tension-imparting film, if a method of applying a tension film through a binder, or a method of depositing an inorganic substance on the surface of a steel sheet using a physical vapor deposition method, a chemical vapor deposition method, etc., excellent film adhesion is obtained. Insulating film having a large iron loss reduction effect can be formed.

C: 0.070 mass%, Si: 3.20 mass%, Mn: 0.09 mass%, Al: 0.025 mass% and N: 0.012 mass%, with the balance being composed of Fe and inevitable impurities A steel slab having a thickness of 2.4 mm was hot-rolled after being reheated to a temperature of 1420 ° C. by a continuous casting method, and subjected to hot-rolled sheet annealing at 1000 ° C. for 50 seconds. Thereafter, it was cold-rolled to an intermediate sheet thickness of 1.5 mm, subjected to intermediate annealing at 1100 ° C. × 20 seconds, and then cold-rolled for the second time to obtain a cold-rolled sheet having a final sheet thickness of 0.23 mm. At this time, in the intermediate annealing, the heating rate between 700 and 900 ° C. in the heating process and the oxygen potential P _H2O / P _H2 of the atmosphere in the soaking process were variously changed.
Next, the cold-rolled sheet was subjected to decarburization annealing that also served as primary recrystallization annealing, which was decarburized at a temperature of 840 ° C. for 100 seconds. At this time, the heating rate between 500-700 ° C. in the heating process was changed variously, and the oxygen potential P _H2O / P _H2 of the atmosphere in the heating process was changed variously. The oxygen potential P _H2O / P _H2 of the atmosphere during soaking was set to 0.35.

A sample is collected from the central part of the sheet width of the steel sheet after decarburization annealing obtained as described above, GDS analysis is performed, a profile of Fe emission intensity in the sheet thickness direction is collected, and the time ratio (t ₅₀ / _t95 ).

Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing and dried, and then a final annealing consisting of secondary recrystallization annealing and purification treatment at 1200 ° C. for 10 hours was performed. The atmosphere of the above-mentioned finish annealing was H ₂ when the purification treatment was held at 1200 ° C., and N ₂ when the temperature was raised (including secondary recrystallization annealing) and when the temperature was lowered.

Ten test pieces each having a width of 100 mm and a length of 280 mm in the width direction of the steel plate were collected from the steel plate after finish annealing obtained as described above under each condition, and the iron loss W _17/50 was measured by the method described in JIS _C2556. Was measured for each test piece, and the average value was determined.
The results of the above measurements are shown in Table 1 together with the measurement results of intermediate annealing conditions, decarburization annealing conditions, and time ratio (t ₅₀ / t ₉₅ ). From this table, it can be seen that the grain-oriented electrical steel sheet manufactured under conditions suitable for the present invention has excellent iron loss characteristics.

Steel having various composition described in Table 2 was melted and made into a steel slab by a continuous casting method, then reheated to a temperature of 1400 ° C., and then hot-rolled to a hot-rolled sheet having a thickness of 2.2 mm Then, after hot-rolled sheet annealing at 1050 ° C. for 30 seconds, it was cold-rolled to an intermediate sheet thickness of 1.5 mm and subjected to intermediate annealing at 1100 ° C. for 20 seconds. At this time, in the heating process of the intermediate annealing, the temperature increase rate between 700-900 ° C. was 3 ° C./s, and the oxygen potential P _H2O / P _H2 of the soaking process atmosphere was 0.40. Then, the cold rolling of the 2nd time was carried out, and it was set as the cold rolled sheet of final board thickness 0.23mm.
Next, decarburization annealing was performed, which also performed primary recrystallization annealing, which was decarburized at a temperature of 850 ° C. for 150 seconds. At this time, the heating rate between 500 and 700 ° C. in the heating process was changed to three levels of 20 ° C./s, 100 ° C./s and 200 ° C./s. Further, the oxygen potential P _H2O / P _H2 of the atmosphere in the heating process was set to 0.40 oxidizing property, and the oxygen potential P _H2O / P _H2 of the atmosphere in the subsequent soaking process was set to 0.40.

A sample is collected from the central part of the sheet width of the steel sheet after decarburization annealing obtained as described above, GDS analysis is performed to collect a profile of Fe emission intensity, and the time ratio (t ₅₀ / t _{95 is determined} by the method described above. ) Was found to be in the range of 0.60 to 0.70.

Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing and dried, and then a final annealing consisting of secondary recrystallization annealing and purification treatment at 1200 ° C. for 10 hours was performed. The atmosphere in the finish annealing was H _{2 at} the time of maintaining at 1200 ° C. for purification, and N ₂ at the time of temperature rise (including secondary recrystallization annealing) and at the time of temperature fall.

Ten pieces of test pieces each having a width of 100 mm and a length of 280 mm in the width direction of the steel plate were collected from each of the steel plates after the finish annealing obtained as described above in each condition, and each test piece was subjected to the method described in JIS C2556. The iron loss W _17/50 was measured and the average value was determined. The results are also shown in Table 2. From the table, the grain-oriented electrical steel sheet having a component composition suitable for the present invention is excellent in iron loss characteristics, in particular, the grain-oriented electrical steel sheet subjected to rapid heating at 80 ° C./s or more by decarburization annealing, It can be seen that the iron loss characteristics are more excellent.

Claims (4)

C: 0.002-0.10 mass%, Si: 2.5-6.0 mass%, Mn: 0.01-0.8 mass%, Al: 0.010-0.050 mass%, and N: 0.003- A slab containing 0.020 mass%, the balance being Fe and inevitable impurities is hot-rolled, hot-rolled sheet annealed, cold-rolled twice with intermediate annealing, decarburized and annealed to the steel sheet surface After forming the subscale, applying an annealing separator mainly composed of MgO to the steel sheet surface, in the method for producing a grain-oriented electrical steel sheet comprising a series of steps of finish annealing,
The average heating rate between 700 and 900 ° C. in the heating process of the intermediate annealing is 10 ° C./s or less, and the oxygen potential (P _H2O / P _H2 ) of the atmosphere in the soaking process is 0.20 to 0.80. Range and
By setting the oxygen potential (P _H2O / P _H2 ) in the heating process of the decarburization annealing to 0.20 or more ,
The Fe emission intensity profile in the plate thickness direction obtained when glow discharge emission analysis of the steel sheet surface after decarburization annealing has a Fe deficient region below the steel sheet surface, and the minimum of the Fe emission intensity of the Fe deficient region The value is I _min , the Fe emission intensity in the bulk region is I _max , and from the start of analysis until the Fe emission intensity becomes I _min + 0.5 × (I _max −I _min ) between the above I _min and I _max . When the time from the start of analysis until the time t ₅₀ (s) and the Fe emission intensity become I _min + 0.95 × (I _max −I _min ) is t ₉₅ (s), the above t ₅₀ and t ₉₅ But,
t ₅₀ / t ₉₅ ≦ 0.75
A method for producing a grain-oriented electrical steel sheet characterized by satisfying the above relationship . 2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein an average temperature increase rate between 500 and 700 ° C. in the heating process of the decarburization annealing is set to 80 ° C./s or more. In addition to the above component composition, the slab further contains one or two selected from S: 0.002-0.03 mass% and Se: 0.002-0.03 mass%. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1 or 2 . In addition to the above component composition, the slab further comprises Cr: 0.01 to 0.50 mass%, Cu: 0.01 to 0.50 mass%, P: 0.005 to 0.50 mass%, Ni: 0.01 -1.50 mass%, Sb: 0.005-0.50 mass%, Sn: 0.005-0.50 mass%, Mo: 0.005-0.100 mass%, B: 0.0002-0.0025 mass%, One or more types chosen from Nb: 0.0010-0.0100mass% and V: 0.001-0.01mass% are contained , The any one of Claims 1-3 characterized by the above-mentioned. The manufacturing method of the grain-oriented electrical steel sheet described in 1.

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