JPH041059B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a secondary recrystallization annealing method for producing grain-oriented silicon steel sheets with high magnetic flux density, particularly _B8 of 1.96T or higher. Unidirectional silicon steel sheets are used as core materials for transformers, and important features include high magnetic flux density and low iron loss at low excitations. The history of the development of unidirectional silicon steel sheets is a history of iron loss improvement, and iron loss improvement is mainly due to
This has been achieved by improving B ₈ (magnetic flux density at a magnetizing force of 800 AT/m). The origin of unidirectional silicon steel sheets lies in the NPGoss two-stage cold rolling process, and many improvements have been made to this manufacturing method to produce unidirectional silicon steel sheets with high _B8 and low iron loss. Special, special public service 1977
With the proposal of the method described in -15654 publication and Japanese Patent Publication No. 51-13469, a high _B8 unidirectional silicon steel sheet has now been developed, which has excellent unidirectionality with high _B8 and low core loss. Silicon steel sheets are currently being produced. On the other hand, in general, industrial products tend to have larger crystal grains as _B8 increases, and even if _B8 is improved beyond a certain level, the 180° magnetic domain width increases, resulting in increased eddy current loss and metallurgical problems. The current situation is that expectations for further reductions in iron loss are fading. In order to improve this difficulty, it is necessary to subdivide the 180° magnetic domain width of the unidirectional silicon steel plate with a high _B8 , and various attempts have been made in the past, and the effects of subdivision have been confirmed. The present inventors conducted research to manufacture a unidirectional silicon steel sheet with an extremely high _B8 , and
In Publication No. 2839, we proposed a method for performing secondary recrystallization under a temperature gradient. FIG. 1 shows the relationship between crystal grains and crystal orientation when a primary recrystallization plate containing AlN as a precipitated phase is subjected to secondary recrystallization under a temperature gradient. In other words, a in the figure shows the state of each crystal grain, and b shows the (100) state of each crystal grain.
It is a pole figure. From the figure, the one (crystal grain) that is closest to the ideal {110}[001] orientation (point X in figure b)
It can be seen that only 100% of the population grows preferentially. This is the basic phenomenon of temperature gradient secondary recrystallization, which is almost completely
Unidirectional silicon steel sheets with Goss orientation can be easily produced. In normal secondary recrystallization annealing, secondary nuclei first undergo nucleation and then grow into secondary recrystallized grains, but there is no clear distinction between nucleation and grain growth, and the final annealing process It is thought that they will be carried out at the same time. However, in temperature gradient annealing, sharp
Generates Goss nuclei and grows only them.
In other words, we believe that the feature is that nucleation and grain growth are performed under separate conditions. When a large temperature gradient can be obtained (for example, when temperature gradient annealing is performed on a veneer), sharp Goss grains can be grown without producing island grains at the grain growth front. This can be explained by the so-called thermal inhibition caused by the temperature gradient suppressing the growth of primary recrystallized grains and favoring the growth of Goss grains. However, industrially, final annealing is carried out using 5 to 20 ton coils, so it is quite difficult to secondary recrystallize the entire steel plate under a high temperature gradient. Therefore, from an industrial perspective, in addition to the thermal inhibition effect, it is assumed that the suppressing effect due to the precipitates is important. The present invention aims to provide a method for stably exhibiting the high _B8 effect of temperature gradient annealing, which was confirmed in the laboratory as described above, in industrial-scale coil annealing. That is, in the present invention, C: 0.010~0.10%, Si: 2.5~
A steel plate obtained by hot rolling, cold rolling, and decarburization annealing a silicon steel material containing 4.0%, acid-soluble Al: 0.010 to 0.065%, balance Fe and unavoidable impurities.
In a method for producing a grain-oriented silicon steel sheet, which comprises performing final annealing to develop and advance secondary recrystallization while applying a temperature gradient to a boundary region between a secondary recrystallization region and a secondary recrystallization region, the atmosphere in the final annealing step A method for producing a unidirectional silicon steel sheet with a high magnetic flux density, characterized by increasing the nitrogen content in the steel sheet so that the nitrogen content at the start of secondary recrystallization of the steel sheet is 130 to 200 ppm. It is. The present invention will be explained in detail below. First, the present inventors performed secondary recrystallization annealing 3 under a temperature gradient by heating 2 from the upper surface of the coil 1 as schematically shown in FIG. Figure 3 shows a cross-sectional view of the furnace and coil used. The furnace 7 has a ceiling heater 4 and a side heater 5 as heaters, and the input of these heaters can be controlled independently. The outer and inner circumferential parts of the coil 1 are insulated by a heat insulating material 6, and the coil is designed to receive heat only from the upper end surface as much as possible. As a result, a temperature gradient was created in the width direction of the coil while maintaining the upper end temperature of the coil higher than the lower end temperature. B ₈ of the unidirectional silicon steel sheet manufactured by the above method is always B ₈ , which is the aim of the inventors even though the heat treatment conditions are almost the same.
It was not always possible to obtain a value of 1.96T or higher, and depending on the annealing conditions, there were cases where the value was 1.96T or lower. FIG. 4 shows a portion of the grain structure and the corresponding B ₈ value when a 5-ton coil is annealed under the temperature gradient under the heat treatment conditions shown in FIG. 3. In the case of Figure a, the crystal grains grow almost linearly from the top of the coil, and the _B8 value (with a surface coating) is very high and almost the same throughout the sample. On the other hand, in case b of the same figure, unlike in case a, a single grain does not cover the entire sample, and different grains occur here and there, and the presence of island grains left behind among large grains is characteristic. be. _B8 in the region where single grains grow linearly is extremely high;
It was found that the area _B8 where other grains were mixed was slightly inferior. Figure 5a is an enlarged view of Figure 4b, showing large grains (A, B,
C, G, F, etc.), grains that appeared before the growth front of these grains (D, E, etc.), and grains left behind in the grains (,, etc.) are observed. These small grains have a much worse orientation than the larger grains 3 and 9, as shown in the pole figure and the first orientation table shown in _FIG . That is, obtaining the highest B ₈ by temperature gradient annealing is achieved by not allowing grains with other orientations to form before the growth front of grains with good orientation that begin to grow first.

[Table] The present inventors further conducted research and considered technical means for stabilizing the magnetic flux density ( _B8 value) of products obtained through industrial production in the form of strip coils. As a result, it was found that by increasing the amount of N contained in the steel at the start of secondary recrystallization in the final annealing process, it was possible to stably obtain a product with a high magnetic flux density ( _B8 value). That is, for example, by reducing the B content in the annealing separator mainly composed of MgO applied to the strip after the primary recrystallization annealing that also serves as decarburization annealing, Increase the partial pressure of _N2 in the atmosphere (increase the content)
In particular, it was concluded that a large amount of N was absorbed into the steel at the start of secondary recrystallization, making it possible to stably obtain a product with a high magnetic flux density (B ₈ value). Next, the process that led to the above conclusion will be explained. First, the present inventors investigated the influence of the atmosphere and annealing separator on B ₈ during two-hour recrystallization annealing under a temperature gradient. Sample size is 21cm (RD) x 84
cm, and about 20 such steel plates were stacked and charged into a temperature gradient furnace with 6 heating zones, and about 5 cm
Grain growth was performed in a direction perpendicular to the rolling direction under a temperature gradient of °C/cm. Figure 6 shows the relationship between annealing atmosphere and _B8 , which shows that _B8 strongly depends on the annealing atmosphere.
It can be seen that the larger the _N2 %, the higher the _B8 . FIG. 7 shows the relationship between the boron (B) content in the annealing separator MgO and _B8 . This shows that B ₈ is higher when MgO with a lower B content is used. Next, the present inventors subjected about 5 tons of coils to temperature gradient annealing in the equipment shown in FIG. 4 while changing the atmosphere and the type of MgO. The results are shown in FIG. The figure shows the average value of B ₈ in a region where the temperature gradient is about 5° C./cm or more at a plate temperature of about 950° C. Low B content in MgO and 100% _N2
B ₈ becomes higher when , and N ₂ 25% +
This shows that even in a 75% H ₂ atmosphere, B ₈ becomes high when B is 100 PPM. Figure 9 shows the amount of N in the steel sheet at the start of secondary recrystallization.
This shows the relationship of _B8 . The higher the amount of N
_B8 tends to be high. The amount of N required for temperature gradient secondary recrystallization is as follows: _N2 % of the working atmosphere, B of MgO
The amount of N required to fully exhibit the _B8 improvement effect of temperature gradient annealing is 200 PPM or more in the steel sheet during secondary recrystallization annealing. It is difficult to absorb stably, and at 180 to 200 PPM.
B ₈ almost reaches saturation, even 200PPM
Even if it could contain more than
And so. Moreover, B ₈ ≧ which is the object of the present invention
Approximately 130 PPM is required to stably achieve 1.96T, and this was set as the lower limit. The reason why the B ₈ improvement effect by temperature gradient secondary recrystallization annealing becomes more pronounced with an increase in the amount of N in the steel sheet is not necessarily clear at present, but the following can be inferred from the experimental examples described above. In other words, since N absorption during secondary recrystallization annealing becomes easier, many
AlN is formed, the inhibitor effect becomes stronger, and the temperature at which secondary recrystallization starts increases. This means that the generation of new secondary recrystallized grains at the secondary recrystallized growth front is suppressed. In this way, the secondary recrystallized grains with the sharp Goss orientation that have already been generated grow towards the low temperature region and become extremely high.
This results in crystal grains with _B8 . (Example) Example 1 By weight, C: 0.058%, Si: 2.95%, Mn: 0.083
%, S: 0.025%, Al: 0.025%, N: 0.008%, balance Fe and unavoidable impurities.The slab obtained by continuous casting was subjected to hot rolling, annealing, cold rolling, and decarburization annealing. After performing primary recrystallization annealing, which also serves as a primary recrystallization annealing, a 5-ton strip coil was made by applying an annealing separator mainly composed of MgO containing about 100 ppm of B (boron). Ten samples with a width of 21 cm (RD (rolling direction)) x 80 cm were cut from this strip coil. Five of them were processed by scraping off the annealing separator on the surface, washing with water,
After drying, MgO with B content of about 1300ppm
An annealing separator mainly composed of was applied and dried. These 10 samples with different B contents in the annealing separator were bundled, a patch plate was applied, and 6 thermocouples were pasted at equal intervals along the length of the samples, each with a furnace length of 1 m. H ₂ : 75vol in a temperature gradient annealing furnace with six temperature-controlled heating zones.
Finish annealing (secondary recrystallization) was performed in an atmosphere of %+N ₂ :25 vol % under a temperature gradient of about 5° C./cm. When the secondary recrystallization of about 20 cm of the tip of the sample was completed, annealing was stopped and the sample was pulled out of the furnace. Chemical analysis samples were cut out from six locations in the longitudinal direction of the sample, and the total N content was analyzed. The boundary area between primary recrystallization and secondary recrystallization of the sample coated with an annealing separator containing 100 ppm B was approximately 1050°C, and the total N content on the primary recrystallization side of this boundary area was approximately 190 ppm. .
On the other hand, the boundary area between primary recrystallization and secondary recrystallization of the sample coated with an annealing separator containing 1300 ppm of B is approximately
980℃, and the temperature on the primary recrystallization side of this boundary area is
The total N amount was about 110 ppm. The relationship between the amount of B in the annealing separator and the amount of N in the steel before secondary recrystallization is considered as follows. In other words, the greater the amount of B in the annealing separator, the easier it is to form a tight oxide film on the surface of the steel plate (strip) at low temperatures (experimental fact), and therefore the easier it is to prevent N from diffusing into the steel. It is estimated that The above strip coil, H ₂ : 75% + N ₂ : 25
% atmosphere, temperature gradient annealing (finish annealing) was performed. The heating rate is 50℃/hr from room temperature to 650℃,
The rate was 20°C/hr between 650 and 1200°C. To create a temperature gradient, a heat insulating material was wrapped around the outer circumferential surface of the strip coil, and the strip coil was heated from the top. The average value of the magnetic flux density ( _B8 value) of the products in the region where the temperature gradient was 5° C./cm or more was 1.975 Tesla. On the other hand, a strip coil with the same composition was coated with an annealing separator containing 1300 ppm of B, and the temperature gradient (finish annealing) obtained was 5°C/cm under the same conditions as above. The average value of magnetic flux density ( _B8 value) for products in the above range is
It was 1.945 Tesla. Example 2 By weight, C: 0.062%, Si: 2.98%, Mn: 0.075
%, S: 0.028%, Al: 0.027%, N: 0.0082%,
One of the two 5-ton strip coils obtained by applying the same treatment as in Example 1 to a slab obtained by continuous casting consisting of the balance Fe and unavoidable impurities had a B content of about An annealing separator with a B content of about 1300 ppm was applied to the other 5 ton strip coil. A sample for analysis was cut from each strip coil in the same manner as in Example 1, and N ₂ :
Secondary recrystallization annealing was performed in a 100% atmosphere, and the relationship between strip temperature and total N content was investigated.
The total N amount on the primary recrystallization side in the boundary area between primary recrystallization and secondary recrystallization is approximately 170 ppm in the case of a strip coil coated with an annealing separator with a B content of approximately 390 ppm. For strip coils coated with annealing separator, which was about 1300 ppm, it was about 120 ppm. These strip coils were temperature gradient annealed in the same manner as in Example 1 in a 100% N ₂ atmosphere. The average value of magnetic flux density ( _B8 value) for products in areas where the temperature gradient is 5℃/cm or more is
The latter was 1.983 Tesla, and the latter was 1.967 Tesla. Example 3 By weight, C: 0.062%, Si: 2.90%, Mn: 0.075
%, S: 0.026%, Al: 0.026%, N: 0.008%, balance Fe and unavoidable impurities. After performing the same treatment as in Example 1, a slab containing B Approximately 390 ppm of annealing separator was applied and wound into a 5 ton strip coil. This strip coil was subjected to finish annealing (secondary recrystallization) under a temperature gradient in an atmosphere of 100% N ₂ . The temperature increase rate is 50℃/from room temperature to 650℃.
hr, was 15°C/hr between 650 and 1200°C. In order to anneal the entire strip coil under a predetermined temperature gradient, the strip coil was raised during final annealing. That is, until the upper end of the strip coil reaches the secondary recrystallization temperature, the upper end of the strip coil is submerged to the same height as the hearth, and when the upper end of the strip coil begins to undergo secondary recrystallization, the The strip coil was raised at a rate corresponding to the growth rate of the recrystallized grains. During this time, in order to create a temperature gradient, cooling gas was flowed into the base plate of the final annealing furnace to remove heat from the lower end surface of the strip coil. The plate temperature (strip temperature) in this final annealing is
The average value of the magnetic flux density ( _B8 value) of products in the area where the temperature gradient is approximately 3.5℃/cm or more at 1050℃ is
It was 1.985 Tesla. Figures 10a, b, and c show macrostructures of secondary recrystallized grains according to the above embodiment. a is the outer periphery of the coil winding thickness, b is the same center, and c is the macro structure of the inner periphery. Regardless of whether you look at the outer periphery, center, or inner periphery of the coil winding thickness, the grains generated at the upper end of the coil (upper in the figure) grow linearly in the direction perpendicular to the rolling direction, and there are almost no island grains in between. With such grains, a Goss orientation close to the ideal one can be obtained, except for slight orientation fluctuations (the inclination of the [001] axis with respect to the rolled surface) caused by coil set and flattening annealing during secondary recrystallization annealing. . (Effects of the Invention) According to the present invention, when producing grain-oriented electrical steel sheets on an industrial scale using the temperature gradient annealing method, it is possible to stably obtain products with extremely high magnetic flux density ( _B8 value). , which has great industrial effects.

[Brief explanation of drawings]

Figure 1 shows the relationship between crystal grains and crystal orientation when a primary recrystallization plate containing AlN as a precipitated phase is subjected to secondary recrystallization under a temperature gradient, and a indicates the state of each crystal grain. Schematic diagram shown, b is (100) of each grain
Pole figure, Figure 2 is a diagram showing the mode of heating from the top surface of the coil when performing secondary recrystallization annealing under a temperature gradient, and Figure 3 is an example of an apparatus for performing secondary recrystallization annealing under a temperature gradient. Figures 4a and 4b, which show the cross section of the coil, show part of the macrostructure of the crystal grains of the coil obtained by secondary recrystallization annealing under a temperature gradient using the heating device shown in Figure 3, and the corresponding B Figure _5a is an enlarged view of the macrostructure of Figure 4b, Figure 5b is its pole figure, and Figure 6 is the relationship between the annealing atmosphere and _B8 . Figure 7 shows the relationship between the boron content in the annealing separator MgO and _B8 , and Figure 8 shows the annealing atmosphere and annealing separator.
A diagram showing the relationship between boron content in MgO and _B8 ,
FIG. 9 is a diagram showing the relationship between the N content and B ₈ during secondary recrystallization, and FIGS. 10 a, b, and c are diagrams showing the macrostructure of secondary recrystallized grains of the coil obtained in Example 3. It is.

Claims (1)

[Claims] 1 A silicon steel material containing C: 0.010 to 0.10%, Si: 2.5 to 4.0%, acid-soluble Al: 0.010 to 0.065%, the balance Fe and unavoidable impurities is hot rolled, cold rolled, and decarburized annealed. The primary recrystallization area and the secondary recrystallization area are
In a method for producing a grain-oriented silicon steel sheet, which comprises performing final annealing to develop and advance secondary recrystallization while applying a temperature gradient to the boundary region of the secondary recrystallization region, the nitrogen content in the atmosphere in the final annealing step is A method for producing a unidirectional silicon steel sheet with a high magnetic flux density, characterized in that the nitrogen content at the start of secondary recrystallization of the steel sheet is 130 to 200 ppm.