JPH0832929B2 - Method for producing unidirectional electrical steel sheet with excellent magnetic properties - Google Patents

Description

The present invention relates to a method for producing a grain-oriented electrical steel sheet used as an iron core of a transformer or the like.

[Prior Art] Unidirectional electrical steel sheets are mainly used as core materials for transformers and other electromagnetic equipment, and are required to have excellent magnetic characteristics such as excitation characteristics and iron loss characteristics. A magnetic flux density B ₈ at a magnetic field strength of 800 A / m is usually used as a numerical value indicating the excitation characteristic. Moreover, as a numerical value showing iron loss profitability, iron loss W17 / 50 per 1 kg when magnetized to 1.7 Tesler (T) at a frequency of 50 Hz is used. The magnetic flux density is the most dominant factor of the iron loss characteristics, and generally speaking, the higher the magnetic flux density, the better the iron loss characteristics. Generally, when the magnetic flux density is increased, the secondary recrystallized grains become large, which may result in poor iron loss characteristics. On the other hand, by controlling the magnetic domains, the iron loss characteristics can be improved regardless of the grain size of the secondary recrystallized grains.

This unidirectional electrical steel sheet undergoes secondary recrystallization in the final finish annealing process, resulting in {110} on the steel sheet surface and <001 in the rolling direction.
> Manufactured by developing a so-called Goth texture with axis. In order to obtain good magnetic properties, it is necessary to highly align <001>, which is the easy axis of magnetization, in the rolling direction. The directionality of the secondary recrystallized grains is significantly improved by the method of using MnS, AlN, etc. as inhibitors and performing the final strong reduction rolling, and accordingly the iron loss characteristics are also significantly improved.

[Problems to be Solved by the Invention]

By the way, in the production of the grain-oriented electrical steel sheet, various factors in each process influence the magnetic characteristics, and therefore extremely strict standards are set for the conditions in each process. Therefore, a great deal of labor is spent on process control. Nevertheless, it is not uncommon for secondary recrystallization defects, magnetic property defects, etc. of unknown cause to occur.

As a countermeasure against this, if the occurrence of secondary recrystallization defects and magnetic properties of the product can be predicted at an intermediate stage, problems due to manufacturing conditions, materials, surface properties, etc. can be solved, and secondary recrystallization is good and magnetic properties are improved. It is possible to manufacture under the conditions. However, so far, it has been difficult to predict the occurrence of secondary recrystallization failure and magnetic properties, despite various attempts.

Therefore, the present invention is based on a new finding that the crystal structure of the plate material before the final finish annealing after decarburization annealing has a large influence on the occurrence of secondary recrystallization defects and magnetic properties.
By specifying this crystal structure, it is an object to obtain a grain-oriented electrical steel sheet having excellent magnetic properties.

[Means for solving the problem]

The method for producing a unidirectional electrical steel sheet according to the present invention, in order to achieve the object, a slab for unidirectional electrical steel sheet, 1300 ° C
After heating to the following temperature, hot rolling is performed, and final cold rolling reduction of 80% or more is performed once or two or more times of cold rolling with intermediate annealing sandwiched, and further decarburization annealing is performed to determine the primary recrystallized grain size. The average diameter is 15 μm or more and 50 μm or less, the variation coefficient σ ^{* of the} diameter is 0.6 or less, and the inhibitor such as AlN or MnS is strengthened by nitrification or sulfurization before the secondary recrystallization of the final annealing after the completion of decarburization annealing. It is characterized by performing finish annealing.

Further, in addition to this feature, a final strong reduction rolling with a reduction rate of 80% or less can be performed to obtain a grain-oriented electrical steel sheet having more excellent magnetic properties.

[Action]

The unidirectional electrical steel sheet that is the subject of the present invention is a molten steel obtained by a conventional steelmaking method, is cast by a continuous casting method or an ingot making method, and is a slab by sandwiching the agglomeration step, if necessary. Successively hot rolled into a hot rolled sheet, annealed this hot rolled sheet as needed, and then cold rolled one time or two or more times with intermediate annealing sandwiched between them to obtain a final gauge cold rolled sheet,
This cold rolled sheet is decarburized and annealed.

The present inventors focused their attention on the crystal structure after decarburization annealing,
The relationship between the crystal structure of the decarburized and annealed steel sheet (hereinafter referred to as the decarburized and annealed sheet) and the magnetic property (magnetic flux density) of the product was extensively studied from various viewpoints, and it was found that the relationship between the two was extremely close. I've found a relationship. The details will be described below based on the experimental results.

1 and 2 are the average diameter (corresponding to a circle) of primary recrystallized grains and the coefficient of variation of diameter obtained by image analysis of the crystal structure (total cross-section thickness) of the decarburized annealed plate observed with an optical microscope. FIG. 3 is a graph showing the effect of σ ^* on the magnetic flux density B _{8 of the} product. Further, FIG. 3 shows the crystal structure (total cross-section thickness) of decarburized annealed sheets having different average diameters and coefficient of variation σ ^{* of} diameters.
Is an example of.

Here, C0.020 ~ 0.090wt%, Si3.2 ~ 3.3wt%,
Slab containing acid-soluble Al 0.010-0.045% by weight, N0.0030-0.0100% by weight, S0.0030-0.0300% by weight, Mn0.070-0.500% by weight is heated to 1150-1400 ° C and has a thickness of 2.3 mm. Hot-rolled the hot-rolled sheet, annealing the hot-rolled sheet at a temperature of 900 ~ 1200 ℃,
Final cold reduction of about 88% was performed to make a cold-rolled sheet with a final thickness of 0.285 mm, decarburization was performed at a temperature of 830 to 1000 ° C, and subsequently an annealing separator containing MgO as the main component was applied. Final finish annealing was performed.

As is clear from FIGS. 1 and 2, the average diameter ≧
B ₈ ≧ 15 μm and diameter variation coefficient σ ^* ≦ 0.6
A high magnetic flux density of 1.88T is obtained. In addition, FIGS. 1 and 2 show that secondary recrystallization and magnetic properties can be improved by setting the average diameter of the decarburized annealed sheet and the coefficient of variation σ ^{* of the} diameter within an appropriate range. Is shown.

The reason why the relationship shown in FIGS. 1 and 2 is established between the average diameter of the decarburized annealed sheet and the coefficient of variation σ ^{* of the} diameter, the occurrence of secondary recrystallization failure of the product, and the magnetic flux density B ₈ is as follows. Although not always clear, the present inventors speculate as follows.

Factors that influence the secondary recrystallization phenomenon, including the orientation of secondary recrystallization, include the crystal structure (average diameter, particle size distribution) of primary recrystallization, texture, inhibitor strength, and the like. After the completion of primary recrystallization, the texture and grain size distribution change with grain growth, so the average diameter also indirectly represents the texture and grain size distribution. Further, the average diameter of the decarburized annealed plate itself is an amount inversely proportional to the total grain boundary area (per unit area), and these grain boundary energies serve as the driving force for grain growth of the secondary recrystallized grains. Therefore, the average diameter can be considered as a parameter that simultaneously describes the texture, the grain size distribution, and the sum total of the flow area, which are considered to influence the secondary recrystallization phenomenon.

By the way, the texture is oriented grains ({11
(0} <001> oriented grains, etc.), oriented grains in which secondary recrystallized grains are likely to grow ({111} <112> oriented grains, etc.) and other oriented grains. , Nucleation of secondary recrystallized grains,
It affects the non-uniformity of grain growth, and the total grain boundary area affects the nucleation of secondary recrystallized grains and the ease of grain growth. Therefore, it is estimated that the average diameter, which is a parameter that simultaneously describes the texture, the grain size distribution, and the total grain boundary area, has a strong correlation with the secondary recrystallization orientation.

On the other hand, the coefficient of variation sigma ^* The diameter represents the non-uniformity of particle size, variation coefficient sigma ^* increases when the secondary recrystallized grains of Kakuka diameter, the grain growth is difficult, the secondary recrystallization defect occurred It is presumed to do.

Thus, the variation coefficient σ ^{* of} diameter is closely related to the occurrence of defects in secondary recrystallization, and the average diameter of the decarburized annealed plate is closely related to the magnetic flux density when secondary recrystallization is good. .
Therefore, by controlling these parameters within a predetermined range, it becomes possible to manufacture a product having a high magnetic flux density B ₈ with high yield.

 Next, each requirement of the present invention will be described.

The sap component used in the present invention is not particularly limited, but in order to stabilize the magnetic properties, it is 0.025 to 0.
It preferably contains 100% by weight C and 2.5-4.5% by weight Si. Also, as an inhibitor constituent element,
If necessary, Al, N, Mn, S, Se, Sb, B, Cu, Bi, Nb, Cr, Sn, Ti and the like can be added.

The heating temperature of this slab is preferably 1300 ° C. or lower in terms of cost.

The heated slab is subsequently hot rolled into a hot rolled sheet. This hot rolled sheet is annealed if necessary. Then, one cold rolling or two or more cold rollings sandwiching intermediate annealing is performed to obtain a final gauge cold rolled sheet. In this case, reduction of the final cold rolling, be 80% or more, a magnetic flux density B ₈
It is necessary to increase By setting the rolling reduction within the above range, the sharpened {110} <001> oriented grains in the decarburized annealed sheet and the corresponding oriented grains ({111} <112) which are easily eroded by this
It is possible to obtain an appropriate amount of (> orientation grains, etc.).

The final cold-rolled cold-rolled sheet is decarburized and annealed, and then an annealing separation agent containing MgO as a main component is applied thereto, and finally final annealed. Before the final finish annealing, the average diameter of the primary recrystallized grains in the state of the decarburized annealed plate is usually 15 μm or more, and the variation coefficient σ ^{* of the} diameter is 0.6 or less.

The method for controlling the crystal structure in such a state is not particularly limited. For example, cold rolling reduction,
Method of adjusting the number of primary recrystallization nuclei by grain size before cold rolling, composition range of inhibitor constituent elements, slab heating temperature, hot rolling temperature, hot rolled sheet annealing temperature, etc. The method of controlling the grain growth at the time of decarburization annealing by adjusting the strength of No. 1 and the method of controlling the grain growth by adjusting the decarburizing annealing temperature can be collected. Alternatively, the crystal structure can be adjusted by additionally performing annealing between the decarburization annealing and the final finish annealing.

Regarding the annealing separator, final finish annealing, etc., the inhibitor strength during the final annealing is increased so that the proper crystal structure of the decarburized annealed sheet does not become an inappropriate crystal structure due to the grain growth during the temperature increase of the final annealing. It is necessary from the viewpoint of stable production to carry out treatments such as nitrification (formation of AlN), sulfurization (formation of MnS), etc. that raises the value. Also, relatively low temperature (~ 800
In order to obtain the desired crystal structure by decarburization annealing at (° C), the inhibitor strength during decarburization annealing must be reduced,
When the inhibitor strength is insufficient for stable secondary recrystallization, the inhibitor strengthening treatment in finish annealing is required. As an example of the inhibitor strengthening method, a method is known in which the nitrogen partial pressure of the finish annealing atmosphere gas is set to be high in the steel containing Al.

The average diameter ≧ 15 μm and the coefficient of variation of diameter σ ^* ≦ 0.6 are defined as 1.88 when the average diameter and the coefficient of variation of diameter σ ^* are in this range, as is clear from FIGS. 1 and 2.
This is because a product having a good magnetic flux density B ₈ of T or more can be obtained. Regarding the upper limit of the average diameter,
The upper limit of the average diameter obtained under the process conditions is 50 μm. When the average diameter is 50 μm or more, the cost is increased due to the high purity of the components and the increase of the annealing temperature, which is not preferable. On the other hand, the variation coefficient σ ^* of the diameter is allowed up to 0.

In this way, the state of the primary recrystallized grains before final finish annealing is defined by the fact that even if the crystal structure of the decarburized annealed plate is inappropriate, after the decarburization annealing, before the final finish annealing, After heat treatment, change the state of primary recrystallized grains to an average diameter ≧ 15μ
This is because good magnetic characteristics can be obtained by adjusting the variation coefficient σ ^* of the m and diameter to be in the range of ≦ 0.6.

〔Example〕

 Examples will be described below.

-Example 1-A slab containing C0.054 wt%, Si3.25 wt%, Mn0.15 wt%, S0.005 wt%, acid-soluble Al0.027 wt%, N0.0078 wt%, 1150 ° C 2.3m after hot rolling after heating to the temperature of
It was a hot rolled sheet of m. This hot-rolled sheet is annealed at temperatures of 1150 ° C and 950 ° C, then cold-rolled to a final thickness of 88% with a reduction rate of 88% to obtain a cold-rolled sheet of 0.285 mm, and held at 810 ° C for 150 seconds. Then, decarburization annealing was performed at 830 ° C, 890 ° C and 950 ° C for 20 seconds each. The obtained decarburized annealed plate is coated with an annealing separating agent containing MgO as a main component, and it is applied in an atmosphere gas of N225% and H275% to 1
A final finish annealing was carried out by raising the temperature to 1200 ° C. at a rate of 0 ° C./hour and then maintaining the temperature at 1200 ° C. for 20 hours in an atmosphere gas of H 2 100%.

After decarburization annealing, the average diameter of the decarburized annealed plate (total cross-section thickness) and the variation coefficient σ ^{* of the} diameter were measured using an image analyzer. Table 1 shows the image analysis results and the magnetic properties of the products at this time.

Example 2 A slab containing C0.058% by weight, Si3.28% by weight, Mn0.14% by weight, S0.007% by weight, acid-soluble Al0.025% by weight, N0.0075% by weight at 1150 ° C. 2.3m after hot rolling after heating to the temperature of
It was a hot rolled sheet of m. The hot rolled sheet was annealed at 1150 ° C. for 30 seconds and then at 900 ° C. for 30 seconds. Next, cold rolling to a final thickness of 0.28
A 5 mm cold-rolled sheet was decarburized and annealed at 850 ° C for 150 seconds.

The obtained decarburized annealed plate is coated with an annealing separator containing MgO as a main component, and the temperature is kept at 10 ° C / 25 ° C in an atmosphere gas of N ₂ 25% and H ₂ 75%.
The final finishing annealing was carried out by raising the temperature to 1200 ° C at an hourly rate and then maintaining the temperature at 1200 ° C for 20 hours in an atmosphere gas containing 100% H2.

After decarburization annealing, the average diameter of the decarburized annealed plate (total cross-section thickness) and the variation coefficient σ ^{* of the} diameter were measured using an image analyzer.
Table 2 shows the processing conditions, the image analysis results, and the magnetic properties of the product at this time.

-Example 3-The decarburized annealed plate obtained under the conditions of the slab heating temperature of 1250 ° C of Example 2 is heat-treated at 950 ° C for 30 seconds, and then an annealing separator containing MgO as a main component is applied. Then, final finish annealing was performed under the conditions of Example 2.

Table 3 shows the average diameter and the variation coefficient σ ^{* of the} diameter of the steel sheet (total cross-section thickness) after the additional heat treatment, together with the magnetic flux density B _{8 of the} product.

-Example 4-C: 0.056 wt%, Si: 3.27 wt%, Mn: 0.14 wt%, S: 0.00
A slab containing 6% by weight, acid-soluble Al: 0.027% by weight, N: 0.0078% by weight was heated to a temperature of 1150 ° C and then hot rolled.
A 2.0 mm hot rolled sheet was used. This hot rolled sheet is held at 1120 ° C for 30 seconds, and subsequently hot rolled sheet is annealed at 900 ° C for 30 seconds, and then cold rolled to a final sheet thickness of 0.220 mm at a rolling reduction of 89%, Decarburization annealing was carried out by holding at 830 ° C for 90 seconds and then at 890 ° C and 920 ° C for 20 seconds respectively. The decarburized and annealed sheet thus obtained was coated with an annealing separator containing MgO as a main component, and N ₂ 2
Raise the temperature to 880 ° C in the atmosphere gas of 5%, H ₂₂ 75%, and 880 ° C
To 1200 ° C in the atmosphere gas of N ₂ 75% and H ₂ 25%, and then the final finish annealing was carried out in the atmosphere gas of H ₂ 100% at 1200 ° C for 20 hours. At this time, the rate of temperature increase up to 1200 ° C was set to 10 ° C / hour and 25 ° C / hour.

After the decarburization annealing, the average diameter of the decarburized annealed plate (total cross-section thickness) and the variation coefficient σ ^{* of the} diameter were measured using an image analyzer.
Table 4 shows the processing conditions, image analysis results, and magnetic characteristics at this time.

- the decarburization annealed sheet obtained under the conditions of Example 5 Example 4, MgO and coated with an annealing separator mainly comprised of, N ₂ 25%, H ₂ in 75% of the atmosphere gas, and N ₂ A final finish annealing was carried out by raising the temperature to 1200 ° C. at a rate of 15 ° C./hour in an atmosphere gas of 50% and H ₂ 50%, and then maintaining the temperature at 1200 ° C. for 20 hours in an atmosphere gas of H ₂ 100%.

After decarburization annealing, the average diameter of the decarburized annealed plate (total cross-section thickness) and the variation coefficient σ ^{* of the} diameter were measured using an image analyzer.
Table 5 shows the processing conditions, the image analysis results, and the magnetic characteristics of the product at this time.

[Effects of the Invention] As described above, in the present invention, a steel piece for a grain-oriented electrical steel sheet is heated to a temperature of 1300 ° C or lower and then hot rolled to obtain a final cold rolling reduction of 1% or more. In the method for producing a unidirectional electrical steel sheet, which is subjected to two or more cold rollings sandwiching intermediate or intermediate annealing, and further subjected to decarburizing annealing, strengthening the inhibitor of AlN or MnS by nitriding or sulfurization, and finishing annealing. By controlling the average diameter of primary recrystallized grains before charcoal annealing and before final finishing annealing and the variation coefficient σ ^{* of the} diameter, it is possible to stably manufacture unidirectional electrical steel sheets with excellent magnetic properties. Becomes Further, the average diameter and the coefficient of variation σ ^{* of the} diameter can also be used as a parameter for predicting the magnetic flux density of the product, and the magnetic flux density of the product can be determined by adjusting the conditions of the subsequent steps such as final finishing annealing. It is possible to set it within the target value.

[Brief description of drawings]

Fig. 1 is a graph showing the effect of the average diameter of the decarburized annealed plate on the magnetic flux density B _{8 of the} product, and Fig. 2 is the graph showing the effect of the variation coefficient σ ^{* of the} diameter on the magnetic flux density B _{8 of the} product. FIG. 3 is a graph showing the average diameter and the variation coefficient σ ^{* of the} diameter ^.
3 is a micrograph showing an example of a crystal structure of a decarburized annealed plate having different values.

Front page continued (72) Inventor Yoshiyuki Ushigami 1-1-1, Emitsu, Hachimanto-ku, Kitakyushu, Fukuoka Prefecture, Nippon Steel & Co., Ltd. Third Technical Research Institute (56) Reference Japanese Patent Laid-Open No. 62-290824 (JP) , A) JP-A-52-6329 (JP, A)

Claims (1)

[Claims] 1. A slab for unidirectional electrical steel sheet is heated to a temperature of 1300 ° C. or lower and then hot-rolled, and cold rolling is performed once at a final cold rolling reduction of 80% or more or at least twice between intermediate annealings. Cold rolling,
Further, decarburization annealing was performed to reduce the average diameter of the primary recrystallized grains to 15
μm or more and 50 μm or less, diameter variation coefficient σ ^* is 0.6 or less, and after completion of decarburization annealing and before the secondary recrystallization of finish annealing, strengthening the inhibitor such as AlN or MnS by nitriding or sulfurization, and finish annealing. A method for producing a grain-oriented electrical steel sheet having excellent magnetic properties, which comprises: