JP2000119824A - Grain oriented silicon steel sheet low in core loss - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively improve core loss in a steel sheet regardless of the steel sheet thickness and also without deteriorating the integration in the orientation of the secondarily recrystallized grains by providing the surface of a steel sheet contg. a specified amt. of Si with an oxide film consisting essentially of forsterite and limiting the contents of Al, B, Se and S on the whole. SOLUTION: The surface of a steel sheet using a stock of high purity contg. 1.0 to 8.0 wt.% Si is coated with a separation agent for annealing composed of MgO, and final finish annealing is executed. In this way, on the surface of the steel sheet, an oxide film consisting essentially of Mg2SiO4 (torsterite) is formed, and, moreover, secondarily recrystallized grains are formed to obtain a grain oriented silicon steel sheet. At this time, the contents of Al, B, Se and S on the whole of the steel sheet contg. the oxide film are respectively controlled to <=20 ppm. Moreover, as the stock for the steel sheet, a high purity material which does not use an inhibitor is preferably used, and, further, a phosphate tensile film contg. colloidal silica is preferably formed laminatingly onto the base film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grain-oriented electrical steel sheet which is suitable mainly for use as an iron core material of a power transformer.

[0002]

2. Description of the Related Art In the production of grain-oriented electrical steel sheets, it is common to use precipitates called inhibitors to generate secondary recrystallized grains having a Goss orientation ({110} <001>) during final finish annealing. Is used as a traditional method. For example, as typical techniques, Japanese Patent Publication No. 40-15644 discloses a method using AlN and MnS, and Japanese Patent Publication No. 51-13469 discloses a method using MnS and MnSe. It is practically used industrially. Apart from these, a technique for adding CuSe and BN is proposed in Japanese Patent Publication No. 58-42244, and a method using nitrides such as Ti, Zr, V is proposed in Japanese Patent Publication No. 46-40855. Many other techniques are known.

[0003] The method using these inhibitors is a useful method for stably developing secondary recrystallized grains. However, since it is necessary to finely disperse the precipitates, the slab heating temperature before hot rolling is required. Must be set to a high temperature of 1300 ° C. or higher. However, high-temperature heating of the slab not only increases the equipment cost, but also increases the amount of scale generated at the time of hot rolling, so that not only the yield is reduced but also the maintenance of the equipment becomes complicated, which causes many problems.

[0004] Another problem with the technique using inhibitors is that if these components remain after the final finish annealing, the magnetic properties are degraded. Therefore, in order to remove Al, B, Se, S, and the like, which are inhibitor components, from the steel, it is necessary to purify the steel for several hours in a hydrogen atmosphere at 1100 ° C. or higher following the completion of the secondary recrystallization. However, due to such high temperature purification annealing,
There is a problem that the mechanical strength of the steel sheet is reduced, the lower part of the coil buckles, and the yield of the product is significantly reduced. Therefore, a technique that does not use an inhibitor as described above is a means for solving such a problem.

A method for producing a grain-oriented electrical steel sheet without using an inhibitor is disclosed, for example, in JP-A-64-55339.
The techniques disclosed in Japanese Patent Application Laid-Open Nos. Hei 2-57635, Hei 7-76732 and Hei 7-197126 are known.
What is common to these techniques is that the {110} plane is preferentially grown using surface energy as a driving force. In order to effectively utilize the surface energy difference, it is inevitably required to reduce the plate thickness in order to increase the contribution of the surface.
In the technology disclosed in Japanese Patent Publication No. 339, the thickness is limited to 0.2 mm or less, and in the technology disclosed in Japanese Patent Application Laid-Open No. 2-57635, the thickness is limited to 0.15 mm or less. In this regard, the thickness disclosed in Japanese Patent Application Laid-Open No. 7-76732 is not particularly limited. However, according to the first embodiment, when the thickness is 0.30 mm, the magnetic flux density is B _8.
At 1.700T or less, the degree of azimuth integration is extremely poor. Further, in the examples, the plate thickness at which a good magnetic flux density is obtained is limited to 0.10 mm. Similarly, the thickness disclosed in Japanese Patent Application Laid-Open No. 7-197126 is not limited, but since this technology is a technology of performing tertiary cold rolling of 50 to 75%, the thickness is necessarily thin. In the embodiment, the thickness is 0.10 mm. Since the thickness of the grain-oriented electrical steel sheet currently used is almost 0.20 mm or more, it is difficult to obtain a normal product by the method using surface energy as described above.

Further, in order to utilize surface energy, high-temperature final finish annealing must be performed in a state where formation of surface oxides is suppressed. For example, JP-A-64-5553
The technique disclosed in Japanese Patent Application Publication No. 9 describes that a vacuum or an inert gas, or a mixed gas of a hydrogen gas and a nitrogen gas is used as an annealing atmosphere at a temperature of 1180 ° C. or more. In the technique disclosed in Japanese Patent Application Laid-Open No. 2-57635, it is recommended to reduce the pressure in an inert gas atmosphere or a mixed gas of hydrogen gas and inert gas at a temperature of 950 to 1100 ° C. Have been. Further, in the technique disclosed in Japanese Patent Application Laid-Open No. 7-197126, it is described that the final finish annealing is performed in a non-oxidizing atmosphere or a vacuum at a temperature of 1000 to 1300 ° C. and an oxygen partial pressure of 0.5 Pa or less. .

As described above, in order to obtain good magnetic properties by utilizing surface energy, the atmosphere of the final finish annealing requires an inert gas or hydrogen, and it is recommended that a vacuum be used as a recommended condition. Although required, compatibility between high temperature and vacuum is extremely difficult in terms of equipment, and costs are high.

Further, when the surface energy is used, only the {110} plane can be selected in principle, and the growth of goss grains having the <001> direction aligned with the rolling direction is selected. Not necessarily. Since the magnetic properties of a grain-oriented electrical steel sheet are improved only when the easy axis <001> is aligned in the rolling direction, good magnetic properties cannot be obtained in principle only by selecting the {110} plane. Therefore, rolling conditions and annealing conditions that can obtain good magnetic properties by a method utilizing surface energy are extremely limited, and as a result, the obtained magnetic properties must be unstable.

Further, in the method utilizing surface energy, the final finish annealing must be performed while suppressing the formation of a surface oxide layer. For example, since an annealing separator such as MgO cannot be applied and annealed, After finish annealing, an oxide film similar to that of a normal grain-oriented electrical steel sheet cannot be formed. For example, a forsterite film is a film formed when MgO is mainly used as an annealing separator, and this film not only gives tension to the steel sheet surface, but also is applied to the forsterite film by further coating and baking. It has the function of ensuring the adhesion of an insulating tension coating mainly composed of an acid salt. Therefore, when there is no forsterite film, the iron loss is significantly deteriorated.

[0010]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems. Even when an inhibitor is not used, the thickness of a steel sheet is not limited, and the secondary recrystallization orientation is not restricted. An object of the present invention is to propose a grain-oriented electrical steel sheet capable of effectively improving iron loss by positively forming a surface oxide film without deteriorating accumulation.

[0011]

The details of the invention will be described below. By the way, the present inventors have previously conducted intensive studies on the mechanism of secondary recrystallization of Goss-oriented grains.As a result, the grain boundary in which the misorientation angle in the primary recrystallized structure is 20 to 45 ° plays an important role. Acta M
material 45 (1997) p. 85 FIG. 1 shows that the misorientation angle in the primary recrystallized structure of grain-oriented electrical steel sheet is 20 to 45 °.
Is the existence frequency (%) of each grain boundary in each direction grain, and the Goss orientation has the highest frequency. According to the experimental data by CG Dunn et al. (AIME Transaction vol. 188 (1949) p. 368), the grain boundary having a misorientation angle of 20 to 45 ° is
High energy grain boundaries. This high energy grain boundary has a large free space in the grain boundary and has a random structure. Since the grain boundary diffusion is a process in which atoms move through the grain boundary, the grain boundary diffusion is faster in a high energy grain boundary having a large free space in the grain boundary. It is known that secondary recrystallization develops with the growth of a precipitate called an inhibitor by diffusion control. Precipitates on the high energy grain boundaries were preferentially coarsened during finish annealing, so they were preferentially unpinned, and started to move to the grain boundaries, indicating a mechanism by which goss grains grow.

The present inventors have further developed this research, and the essential factor of secondary recrystallization of Goss-oriented grains is the distribution of high energy grain boundaries in the primary recrystallized structure, and the role of the inhibitor is It has been found that there is a difference in the moving speed between the high energy grain boundaries and other grain boundaries. Therefore, according to this theory, without using an inhibitor,
If a difference in the moving speed of the grain boundary can be generated, secondary recrystallization can be performed.

[0013] Since impurity elements existing in steel are liable to segregate at grain boundaries, particularly at high energy grain boundaries, when a large amount of impurity elements is contained, there is a difference in the moving speed between the high energy grain boundaries and other grain boundaries. Is thought to be gone. In this regard, if the influence of the impurity element as described above can be eliminated by purifying the material, an inherent difference in the moving speed depending on the structure of the high-energy grain boundary becomes apparent, and the Goss-oriented grains are reduced. It is considered that the next recrystallization becomes possible.

The present inventors have conducted intensive studies based on the above-mentioned idea, and as a result, in a component system containing no inhibitor component, secondary recrystallization proceeds due to the high purity of the material and the action of trace nitrogen. The present inventors have newly found this fact, and have completed the present invention based on this finding. This technology is completely opposite to the conventional secondary recrystallization method in that it eliminates precipitates and impurities at the crystal grain boundaries, and is also different from the technology that uses surface energy. Even if present, secondary recrystallization can be performed well.

That is, according to the present invention, Si: 1.0 to 8.0 wt%
Containing forsterite (Mg ₂ S
Low iron loss characterized by having an oxide film mainly composed of iO ₄ ), and the content of Al, B, Se and S in the whole steel sheet including the oxide film is 20 ppm or less, respectively. It is a grain-oriented electrical steel sheet.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes experimental results that led to the success of the present invention. By the way, the inventors have studied the influence of trace components remaining on the steel sheet after the final finish annealing. As components, C is 0.07wt%, Si is 3.3wt%
%, Mn was fixed at 0.06 wt%, and a slab containing various inhibitor components was heated at 1400 ° C. for 30 minutes, and then hot-rolled into a 2.3 mm thick hot rolled sheet. Then 1100
After hot-rolled sheet annealing at 60 ° C for 60 seconds, cold rolling was performed to finish to a final sheet thickness of 0.35 mm. Then, hydrogen: 50%, nitrogen: 50%
After decarburizing annealing at 850 ° C. for 3 minutes in an atmosphere with a dew point of 60 ° C., MgO was applied as an annealing separator at a rate of 10 g / m ² , and then heated up to 1200 ° C. in a hydrogen atmosphere to 15 ° C.
A final finish annealing in which the temperature was increased at a rate of ° C./h was performed to produce a grain-oriented electrical steel sheet.

The relationship between the contents of Al, B, Se, and S and the magnetic properties in the entire magnetic steel sheet with a forsterite coating thus obtained was investigated. In addition, in the base steel from which the forsterite film was removed, each component was reduced to 5 ppm or less, but the analysis value of the entire steel plate with the forsterite film varies depending on the type and amount of the inhibitor component contained in the material. . Then, in order to investigate the influence of each element independently, each inhibitor was added alone. FIG. 2 shows the relationship between the analysis value of each component and the iron loss value for products having the same magnetic flux density. In the figure,
Except for the components shown, all of them are reduced to 5 ppm or less, and therefore, the effect of each component is shown alone in FIG.

As is clear from FIG. 2, when the content of each of Al, B, Se and S exceeds 20 ppm, the iron loss starts to deteriorate. In particular, when the content exceeds 50 ppm, the iron loss remarkably deteriorates. The results of this investigation clearly show that even if impurities are removed from steel, iron loss is significantly degraded when Al, B, Se, S, etc. remain in the oxide film. However, if a manufacturing method using no inhibitor component is used, Al, B, Se,
It has been newly found that it is possible to effectively reduce the S content, and particularly to obtain a good iron loss by reducing the content of each of these elements to 20 ppm or less.

Generally, in the prior art using an inhibitor such as AlN, MnSe, MnS, Al, B, Se, S remains in the forsterite film, but Se,
For the purpose of removing S and the like, for example,
JP-A-58-144429 discloses a technique of performing pickling after final finish annealing. However, pickling not only roughens the surface of the steel sheet but also inevitably erodes the forsterite film, so that the adhesion is reduced and the iron loss is significantly deteriorated accordingly. In addition, in pickling, since erosion occurs from the surface, it is substantially impossible to remove impurities existing near the interface of the coating film of the base iron.

In fact, according to the technique disclosed in Japanese Patent Application Laid-Open No. 58-144429, even when strong pickling is performed, the amounts of S and Se are not reduced to 20 ppm or less. Therefore, in order to reduce the content of Al, B, Se, and S in the forsterite coating to 20 ppm or less, a manufacturing method in which secondary recrystallized grains are developed using a high-purity material that does not use an inhibitor is much more excellent. I have.

The above experimental results suggest that a better iron loss can be obtained by the secondary recrystallization method using a material without using an inhibitor according to the present invention than the conventional technique using an inhibitor. .

When MgO is applied as an annealing separating agent, an oxide base coat mainly composed of forsterite is formed on the surface of the steel sheet after the final finish annealing. However, it is generally more advantageous to form a phosphate-based tension coating containing colloidal silica on such an undercoating in order to reduce iron loss.

[0023]

The reason why low iron loss can be obtained by the present invention has not always been clearly elucidated, but the inventors consider as follows. When an inhibitor component is used as a material, AlN, BN, MnSe, and MnS are used in steel to develop secondary recrystallized grains during final annealing.
It is necessary to suppress the grain boundary movement of the primary recrystallized grains by such precipitates. After the completion of the secondary recrystallization, such precipitates become unnecessary. On the other hand, when the precipitates remain, iron loss deteriorates to suppress the movement of the domain wall. For this reason, a technique of removing inhibitor components such as Al, B, Se, S, etc. from steel by hydrogen annealing in a high temperature region of 1100 ° C. or higher, which is called purification annealing, is generally implemented.

By such high-temperature purification annealing, it is possible to reduce the contents of Al, B, Se, S and the like in the steel to 20 ppm or less, respectively. However, components such as Al, B, Se, and S that are removed from the steel are rather concentrated in the forsterite film. For this reason, simple substances or compounds such as Al, B, Se, and S inevitably remain at the interface between the coating and the base iron. The presence of such a substance hinders the movement of the domain wall and causes an increase in iron loss. Therefore, even in the oxide film, the inhibitor components Al, B,
It is considered that the iron loss of the electromagnetic steel sheet is improved by reducing Se, S, and the like. In order to reduce Al, B, Se, and S in the forsterite film, a manufacturing method that uses secondary materials that do not use an inhibitor to develop secondary recrystallized grains is excellent.

Next, the reasons for limiting the configuration requirements of the present invention will be described. It is necessary to contain Si as a component of the magnetic steel sheet of the present invention to increase electric resistance and reduce iron loss. For this purpose, at least 1.0 wt% must be added, but if it exceeds 8.0 wt%, not only does the magnetic flux density decrease, but also the secondary workability of the product deteriorates significantly. %. A particularly preferable range of the amount of Si is 2.0 to 4.5 wt%.

Further, in the present invention, in order to effectively reduce iron loss, it is necessary to reduce trace components in the steel sheet as much as possible. That is, as shown in FIG. 2 above, it is important to suppress the respective contents of Al, B, Se and S to 20 ppm or less in the entire steel sheet including the oxide film.
If the content is more than this, the iron loss deteriorates.

Next, a range of components suitable for producing the magnetic steel sheet of the present invention will be described. C is a useful element that improves magnetic properties by improving the structure, but has a content of 0.12 wt.
%, It becomes difficult to remove by decarburization annealing.
The upper limit is about 0.12 wt%. The lower limit is not particularly set, since secondary recrystallization is possible even with a material containing no C. In particular, if C is reduced to 30 ppm or less from the material stage, decarburization annealing can be omitted, which is advantageous in terms of production cost. Therefore, when manufacturing low-grade products, use a material with reduced C. Is advantageous.

It is necessary to reduce other impurity elements as much as possible. However, not only harmful to the generation of secondary recrystallized grains but also Al, which remains in the base iron and deteriorates iron loss,
Since it may be difficult to remove B, S, Se, O, etc. in the subsequent steps, B, S, Se, O,
It is preferable to reduce the content to 30 ppm or less and Al to 70 ppm or less.

In the present invention, in order to improve magnetic characteristics,
Ni can be contained. That is, Ni is a useful element that improves the structure and improves the magnetic properties, and can be added as necessary. However, the content
If it is less than 0.005 wt%, the amount of improvement in magnetic properties is small, while if it exceeds 1.50 wt%, secondary recrystallization becomes unstable and magnetic properties deteriorate, so Ni should be contained in the range of 0.005 to 1.50 wt%. Is preferred.

Next, a preferred production process of the steel sheet of the present invention will be described. First, a slab is manufactured from molten steel adjusted to the above preferable component composition. Such a slab may be manufactured by a usual ingot-bulking method, a continuous casting method, or 100 mm.
A thin slab having the following thickness may be manufactured by a direct casting method. The slab is usually hot-rolled by heating, but may be hot-rolled immediately after casting without heating. In the case of a thin cast slab, the hot rolling may be omitted and supplied directly to the subsequent steps. As for the slab heating temperature, a minimum of about 1100 ° C., at which hot rolling is possible, is sufficient because no inhibitor component is contained in the material.

Then, after hot-rolled sheet annealing is performed as necessary, cold rolling is performed once or twice or more with intermediate annealing, followed by decarburizing annealing as necessary, and then MgO
Is applied, and then a final finish annealing is performed. Here, the magnetic properties can be improved by performing hot-rolled sheet annealing. Further, sandwiching the intermediate annealing between the cold rolling is also useful for stabilizing the magnetic properties. However, since all of them increase production costs, it is determined from the economical point of view whether hot rolled sheet annealing or intermediate annealing is to be selected. The preferred temperature range for hot-rolled sheet annealing and intermediate annealing is 700 ° C or more and 1200 ° C or less. The reason is that if the annealing temperature is lower than 700 ° C, recrystallization during annealing does not proceed, so the effect is thin, while if it exceeds 1200 ° C, the mechanical strength of the steel sheet decreases and it becomes difficult to pass through the line .

[0032] Decarburization annealing is not particularly necessary when a material containing no C is used. In addition, since oxidation of the steel sheet surface is performed by oxides and hydroxides in the annealing separator at the time of final finish annealing, it is not always necessary to oxidize before the final finish annealing. Further, prior to the final finish annealing, a technique of increasing the amount of Si after cold rolling by a siliconizing method may be used in combination.

In the present invention, in order to obtain good iron loss characteristics,
And forsterite (Mg _TwoSiO_Four)
Steel sheet with an oxide film formed
The content of Al, B, Se, S in the whole is 20ppm or less each
Needs to be reduced. Here, steel containing oxide film
The content of Al, B, Se, S in the whole plate
In order to reduce it to pm or less,
It is important to reduce elements sufficiently at the material stage
However, such elements may not be contained in the annealing separator.
is important.

In order to further improve iron loss, it is effective to form a tension coating on the surface of the steel sheet. For this purpose, a multilayer structure composed of two or more kinds of films may be used. Further, depending on the application, a coating in which a resin or the like is mixed may be applied. Furthermore, a magnetic domain refinement technique can be used to obtain good iron loss. Here, as a method for refining magnetic domains, a method of irradiating a product plate with a pulse laser described in JP-B-57-2252,
The method of irradiating a product plate with a plasma flame described in Japanese Patent Application No. 9-96617 and the method of providing grooves by etching before decarburizing annealing disclosed in Japanese Patent Publication No. 3-69968 are effective.

[0035]

EXAMPLES Example 1 After steel slabs having the composition shown in Table 1 were produced by continuous casting, each slab was heated at 1250 ° C. for 20 minutes and then hot-rolled into a hot-rolled sheet having a thickness of 2.8 mm. . Then, after hot-rolled sheet annealing at 1000 ℃, 60 seconds, 0.29mm by cold rolling
Finished to the final thickness. Then, hydrogen: 75%, nitrogen: 25
After decarburizing annealing at 850 ° C for 120 seconds in an atmosphere with a dew point of 40 ° C for 120 seconds to reduce C in steel to 0.0020wt%, an annealing separator mainly containing the components shown in Table 2 was added. After application, a final finish annealing was performed. The final finish annealing was performed at 20 ° C./50% in a mixed atmosphere of nitrogen: 50% and hydrogen: 50%.
The temperature was raised to 1180 ° C. at a speed of h, and the temperature was kept at this temperature in a hydrogen atmosphere for 5 hours. The magnetic flux density B ₈ and the iron loss W _17/50 were measured for each of the product sheets thus obtained. In addition, the steel sheet after the final finish annealing was subjected to the component analysis with the coating, and the amounts of Al, B, Se and S were investigated. The results obtained are also shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

As is clear from the table, according to the present invention, Al,
When the amounts of B, Se, and S were reduced to 20 ppm or less, a product having good iron loss was obtained.

Example 2 After steel slabs having the composition shown in Table 3 were produced by continuous casting, each slab was heated at 1100 ° C. for 20 minutes, and then hot-rolled into a hot-rolled sheet having a thickness of 2.4 mm. Next, after an intermediate thickness of 1.8 mm was obtained by cold rolling, intermediate annealing was performed at 1100 ° C. for 30 seconds, and a final thickness of 0.22 mm was obtained by warm rolling at 200 ° C. Then, hydrogen: 75%, nitrogen: 25%,
Dew point: After decarburizing annealing at 880 ° C for 100 seconds in an atmosphere of 60 ° C to reduce C in the steel to 0.0020wt%, apply an annealing separator mainly composed of MgO, and then finish Annealed. Here, the final finish annealing is as follows: nitrogen: 50%, hydrogen:
In a mixed atmosphere of 50%, the temperature was raised to 1100 ° C. at a rate of 20 ° C./h.

After the above-mentioned final finish annealing, a magnesium phosphate containing 50% of colloidal silica was applied and baked at 800 ° C. for 2 minutes for flattening annealing. Then, after baking, a magnetic domain refining treatment was performed by irradiating a pulse laser at intervals of 15 mm in a direction perpendicular to the rolling direction. Thus obtained was measured magnetic flux density B ₈ and iron loss W _17/50 of the product sheet. In addition, the steel sheet after the final finish annealing was subjected to the component analysis with the coating, and the amounts of Al, B, Se and S were investigated. Table 3 shows the obtained results.

[0041]

[Table 3]

As shown in the table, after the final finish annealing,
When the content of Al, B, Se, and S in the coated magnetic steel sheet was reduced to 20 ppm or less, a product with good iron loss was obtained.

[0043]

As described above, according to the present invention, the content of Al, B, Se, and S in the entire steel sheet including the oxide film on the surface is suppressed to 20 ppm or less, respectively. An electromagnetic steel sheet can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the frequency (%) of grain boundaries having a misorientation angle of 20 to 45 ° before finish annealing with respect to each orientation grain.

FIG. 2 is a graph showing the effect of trace components on iron loss in a coated electromagnetic steel sheet.

Claims (1)

[Claims] 1. A composition containing Si: 1.0 to 8.0 wt%,
The steel sheet surface has an oxide coating mainly composed of forsterite (Mg ₂ SiO ₄ ), and the total content of Al, B, Se and S in the entire steel sheet including the oxide coating is 20 ppm or less. A grain-oriented electrical steel sheet with low iron loss.