JPH0941038A - Method for measuring size of primarily recrystallized grain of silicon steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the dispersion of core loss value and to improve measuring accuracy, at the time of measuring the size of primarily recrystallized grains based on core loss value measured by applying a magnetic field to a silicon steel sheet, by removing a nitrided layer on the surface of the steel sheet and measuring the core loss value. SOLUTION: Before the start of secondary recrystallization in final-finish annealing after hot rolling, as to the measurement of core loss value in a primarily recrystallized sheet subjected to nitriding treatment, the surface layer to 20μm from the surface of the steel sheet is removed by using a means such as grinding. This is because that the nitrogen content in the sheet width direction of the nitrided steel sheet varies to the average nitrogen content in the whole body, also, the sheet thickness distribution of the nitrogen content is not similar in the sheet width direction, the nitrogen content in the surface layer varies, and this difference causes the dispersion of the measured value of core loss, so the surface layer of the steel sheet in which nitriding variation is present is removed. Moreover, this measuring method is applicable to any parameter measurement related to secondary recrystallizing phenomenohs such as the average grain size, the ratio of coarsened grains and the distribution of the grain size.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring an average primary recrystallization grain size when manufacturing a grain-oriented electrical steel sheet used in industry.

[0002]

2. Description of the Related Art The unidirectional electrical steel sheets that have been put into practical use at present have a sharp texture formed by secondary recrystallization without exception. The secondary recrystallization texture is controlled to a crystal orientation that gives the best performance and efficiency when the steel sheet is used as electrical equipment.

Therefore, the higher the sharpness of the secondary recrystallization texture, the better the performance of the electric device. The sharpness of the secondary recrystallization texture generally strongly reflects on the magnetic flux density B ₈ (the strength of the magnetic flux density in a magnetic field of 800 AT / m). The magnetic flux density is
It changes extremely sensitively due to the secondary recrystallized texture. Therefore, proper secondary recrystallization growth is beneficial to manufacturers and users.

Secondary recrystallization is an abnormal grain growth phenomenon of primary recrystallized grains having extremely strong orientation selectivity. The main factors that control the magnetic flux density, which is an index of the degree of {110} <001> orientation integration of the secondary recrystallized grains, are the texture of the primary recrystallized structure, the crystal grain size, and the inhibitor strength (precipitates and grain boundaries). It is already well recognized that it is resistance to grain boundary migration due to segregation elements).

However, in the production of grain-oriented electrical steel sheets, there is a technique for measuring the above factors online to ensure proper primary recrystallization structure formation (Japanese Patent Publication No. 2-267).
No. 223), when a finer control is desired, satisfactory accuracy has not been achieved.

Reflecting such a situation, the industrial production of grain-oriented electrical steel sheet has the following problems.
Although extremely rare, a large amount of secondary recrystallization defects (the product is scrapped) may occur. The finish annealing of the grain-oriented electrical steel sheet is a batch annealing for a long time of 50 to 500 hours, and many similar coils are simultaneously annealed.

Therefore, when a secondary recrystallization defect is found after the finish annealing, a large amount of secondary recrystallization defect is already generated. Therefore, it takes a long time to investigate the cause of the secondary recrystallization defect and to establish a remedy for the defect, resulting in a large loss.

In addition to the above-mentioned secondary recrystallization failure, the magnetic flux density also varies in a normal material under normal manufacturing conditions. For example, although there is almost no difference in composition, there is some variation in the magnetic flux density between melted lots, between coils in the same melted lot, and depending on the part in the same coil, which stabilizes the magnetic flux density at a high level. Becomes an obstacle. This is because the secondary recrystallization behavior very sensitively reflects subtle variations in various process conditions.

If the cause of the secondary recrystallization defect or the variation of the magnetic flux density exists in the process before the finish annealing, the influence should be exerted on the magnetic flux density controlling factors such as the primary recrystallization average grain size. is there. Therefore, if these controlling factors can be accurately grasped by online measurement in the primary recrystallization annealing process, it is extremely useful in preventing a large amount of secondary recrystallization defects from occurring, and the adverse effects can be removed by changing the primary recrystallization annealing conditions. If possible, it is beneficial for high-order stabilization of the magnetic flux density.

The average particle size of primary recrystallization has an extremely large effect on the magnetic flux density and the occurrence of secondary recrystallization defects. Further, it is known that a magnetic flux density with high stability can be obtained by controlling the average particle diameter within the range of 18 to 23 μm. However, this 1
The optimum value of 8 to 23 μm is not common to the grain-oriented electrical steel sheet, but takes different values depending on the texture and the strength and type (heat resistance characteristics) of the inhibitor. Therefore, small fluctuations in the previous process greatly change the appropriate conditions in the subsequent process, and cause secondary recrystallization defects and variations in magnetic flux density.

As described above, if the average primary recrystallization grain size can be controlled within an appropriate range, secondary recrystallization defects and variations in magnetic flux density can be eliminated, and a unidirectional electrical steel sheet with excellent magnetic properties can be industrially produced. Can be manufactured stably.

[0012]

DISCLOSURE OF THE INVENTION The present invention provides a method for accurately measuring the average primary recrystallization grain size in order to generate primary recrystallization having an appropriate grain size.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for eliminating and improving the above problems and for accurately measuring the average primary recrystallization grain size of a grain-oriented electrical steel sheet.
The place to be the gist of the present invention is to apply a magnetic field to an electromagnetic steel sheet, and at the time of measuring the primary recrystallized grain size by the measured iron loss value, the steel sheet surface is removed by polishing or the like, and the iron loss of the remaining portion is measured. The surface layer of 20 μm is removed from the plate thickness surface where many non-magnetic inclusions are present due to external gas, etc.
The present invention provides a method for measuring the primary recrystallized grain size of an electromagnetic steel sheet, which is used as a primary recrystallized sheet for a unidirectional electrical steel sheet.

The details will be described below based on the experimental results. As a result of various examinations on the method for measuring the average primary recrystallization grain size, the present inventors expect that there are many inclusions such as non-magnetic substances in the sheet thickness direction in comparison with the conventional means for measuring iron loss of a steel sheet. It has been found that a method of removing the surface layer that is formed and measuring the iron loss in the remaining portion to calculate the average primary recrystallization particle size is extremely effective in improving the particle size measurement accuracy. This measurement condition is not limited to online or offline.

In the measuring method of the present invention, general unidirectional electrical steel sheets are used. Among them, the primary recrystallized grain size calculated from iron loss causes a large error due to the gas touching the surface. Taking as an example a unidirectional electrical steel sheet that is subjected to nitriding treatment on the steel sheet before the start of secondary recrystallization of final finish annealing,
This will be described below.

The nitrogen amount in the plate width direction of the steel sheet nitrided with NH ₃ gas at the factory has a fluctuation of about 80 ppm with respect to the total average nitrogen amount (FIG. 1). Also, the distribution of nitrogen thickness is not the same in the width direction, and the nitriding amount of the surface layer changes,
This difference is considered to be an error factor that causes variations in iron loss measurement. Conventionally, it has been overlooked that the effect of nitrogen in the surface layer of about 20 μm from the surface is considered to have little effect on iron loss.

However, it was clarified in the present experiment that nitrogen having a non-uniform surface layer distribution locally acts as a non-magnetic material to cause variations in iron loss depending on the location of the steel sheet. In the past, iron loss was measured without considering this effect and the correspondence was made with the grain size, resulting in a large error.

Therefore, in the present invention, based on the results of various experiments, it was concluded as a solution to this that the surface nitriding layer which causes an error is removed. By measuring the iron loss using this method, the relationship between the iron loss and the grain size approaches a linear straight line, and the primary recrystallization average grain size can be measured more accurately.

Next, the reasons for limiting the constituents of the present invention will be described. In the measurement of the primary recrystallized grain size, the nitriding fluctuation of the steel sheet is expected to cause a local variation in the grain growth, and it is considered that it is greatly influenced by the place where the iron loss is measured. Therefore, the measurement is performed excluding the portion where the nitriding fluctuation occurs. The particle size can be measured by measuring any parameter related to the secondary recrystallization phenomenon, such as the average particle size, the ratio of coarse particles, and the particle size distribution.

Regarding the nitriding treatment, according to the present invention, the influence of nitrogen on the primary recrystallized grain size of the steel sheet subjected to the nitriding treatment is influenced by nitrogen between the end of hot rolling and the start of secondary recrystallization of final annealing. It is assumed that the measurement is made excluding.

By this nitriding treatment, a nitride is formed in the steel sheet, and the nitride is AlN when the temperature of the final annealing is increased.
In the state of (Al, Si) N, it functions as an inhibitor that emphasizes the grain boundary character dependence of grain boundary migration during secondary recrystallization. The nitriding method is not particularly limited,
Any method such as a method using NH ₃ gas, a method using plasma, a method of containing N ₂ in the annealing atmosphere, or the like may be used.

When the amount of nitridation in the steel sheet nitrided with NH ₃ gas at the factory deviates from its average value,
The average particle size estimation error becomes large. EP for the thickness distribution of nitrogen
When measured by MA, nitrogen is partially solidified and distributed in the plate thickness cross section, and particularly concentrated in the surface layer within about 20 μm from the surface (FIG. 2). It is expected that the average grain size estimation error will be reduced by removing the surface layer in which nitrogen is concentrated by chemical polishing.

For chemical polishing, there is a method of first immersing in a nitric acid aqueous solution diluted to about 5% for a predetermined time to dissolve the surface, then washing with water, replacing with hydrogen peroxide, and then drying with a dryer or the like. There is the best value for the depth of chemical polishing.When the depth is shallow, the influence of nitrogen remains and causes a measurement error.When the depth is deep, the influence of nitrogen can be removed, but the steel plate itself becomes thin,
It is considered that the unevenness of the surface largely controls the iron loss, and the variation (error) of the iron loss occurs due to the chemical polishing.

The particle size estimation accuracy is determined by the variation in the nitriding amount and the variation in the processing conditions for obtaining the same nitriding amount.
This is because nitrogen affects the iron loss of the steel sheet. Iron loss increases when non-magnetic nitride is present in the steel sheet. Further, the iron loss also changes depending on the distribution of the nitride in the plate thickness direction.

This fluctuation of nitrogen in the plate thickness direction is particularly remarkable in the case of the steel sheet subjected to the nitriding treatment which is the target material of the present invention, and should be particularly noted. As a result, variations in iron loss cause variations, which affects the estimation accuracy.
In this method, by polishing the surface, the particle size can be accurately measured at any place of the steel sheet.

[0026]

According to the present invention, it is possible to improve the accuracy of primary recrystallization average particle size measurement by iron loss measurement, which could not be accurately measured in the past. This is because the nitrided portion of the steel sheet, which was the cause of the variation in iron loss, was removed. For the above reason, by measuring the primary recrystallization average grain size and controlling it appropriately, it has become possible to stably produce the grain-oriented electrical steel sheet having excellent magnetic properties.

[0027]

【Example】

(Example 1) The iron loss and primary recrystallization average grain size of a steel sheet nitrided with NH ₃ after decarburization annealing in a factory have a certain width and are represented by a linear function as shown in FIG. Magnetic flux density 1.0
In the measurement of iron loss (W10 / 50) at T and an excitation frequency of 50 Hz, the estimation error of the average primary recrystallization grain size was about 6.7 μm. Next, the surface layer was chemically polished to a depth of 20 μm and the iron loss was measured. As a result, this estimation error was reduced to about 3.3 μm, and the accuracy was improved.

(Embodiment 2) Frequency 2 for the same steel sheet
When the iron loss was measured at 00 Hz, the iron loss and the average primary recrystallization grain size have the relationship shown in FIG. Magnetic flux density 1.0
In the measurement of iron loss (W10 / 200) at T, the estimation error of the average primary recrystallization grain size was about 5.8 μm. Next, as a result of removing the surface layer by chemical polishing to 20 μm and measuring the iron loss, this error was reduced to about 3.3 μm and the accuracy was improved.

(Example 3) C: 0.052% by weight,
Si: 3.30%, Mn: 0.14%, S: 0.005
%, Acid-soluble Al: 0.018%, N: 0.0074%
After heating the slab containing slab to a temperature of 1150%, it is hot-rolled, cold-rolled at a reduction rate of about 85% to the final plate thickness, decarburized and annealed, and a primary recrystallized plate for unidirectional electrical steel sheet is prepared. did.
Then, after nitriding, the surface of the steel sheet was polished to examine the relationship between iron loss and grain size (FIG. 5).

The polishing of 10 μm does not eliminate the influence of nitrogen, and the estimation error is not much different from that before polishing. The polishing error of 20 μm reduced the measurement error. Further, when the surface was polished to a depth of 50 μm, the measurement error increased. It is expected that the iron loss is greatly affected by the surface condition when the polishing depth is 50 μm.

[0031]

According to the present invention, the primary recrystallized grain size of the grain-oriented electrical steel sheet can be accurately measured.
Since the primary recrystallized grain size has a great influence on the magnetic flux density through the secondary recrystallization behavior, by maintaining it in an appropriate range using the present invention, it is possible to prevent the occurrence of secondary recrystallization defects and increase the magnetic flux density. The stabilization has a very remarkable effect, and the industrial benefit is extremely large.

[Brief description of drawings]

FIG. 1 is a chart showing a nitrogen amount of a steel plate in a plate width direction.

FIG. 2 is a schematic diagram showing the amount of nitrogen in the steel plate in the plate thickness direction.

FIG. 3 is a chart showing the relationship between iron loss at 50 Hz and average particle size.

FIG. 4 is a table showing the relationship between iron loss at 200 Hz and average particle diameter.

FIG. 5 is a table showing the relationship between the depth of chemical polishing and the average particle size estimation error.

Claims (3)

[Claims] 1. When measuring the primary recrystallized grain size based on the iron loss value measured by applying a magnetic field to an electromagnetic steel sheet, a predetermined thickness of the steel sheet surface is removed by polishing or the like, and the iron loss in the remaining portion is measured. A method for measuring the primary recrystallized grain size of an electrical steel sheet, which is characterized in that 2. The method for measuring the primary recrystallized grain size of an electromagnetic steel sheet according to claim 1, which is used for a primary recrystallized sheet obtained by nitriding a steel sheet after hot rolling and before the start of secondary recrystallization in final annealing. . 3. The primary recrystallized grain size measuring method according to claim 1, wherein the surface layer having a thickness of 20 μm is removed from the surface of the plate thickness.