JPH0832928B2 - Method for producing unidirectional electrical steel sheet with excellent magnetic properties and glass film properties - Google Patents

Description

TECHNICAL FIELD The present invention relates to modification of an oxide film layer on a material surface obtained in a decarburizing annealing step of a production process of a grain-oriented electrical steel sheet, and particularly The present invention relates to a method for producing a grain-oriented electrical steel sheet that simultaneously satisfies the stabilization of secondary recrystallization of the material during final finish annealing and the improvement of the properties of the glass film.

(Prior Art) A grain-oriented electrical steel sheet is usually composed of crystal grains (goss-oriented grains) having a (110) [001] orientation in the rolling direction.
It is a steel plate containing 4.5% by weight or less of Si and having a plate thickness of 0.10 to 0.35 mm. The surface is usually covered with forsterite for the purpose of ensuring insulation.

That is, the grain-oriented electrical steel sheet has an extremely high degree of integration (11
0) A composite material composed of a Si-containing thin steel plate having a [001] texture (goss structure) and forsterite which is an oxide-based ceramic having a surface layer portion of 0.1 to several μm.

Achievement of such an extremely high degree of Goss organization,
In the current unidirectional electrical steel sheet manufacturing process, the two different processes of forming a thin forsterite insulating film on the surface layer of the material are performed almost at the same time during one box annealing process of final annealing. Be done. Of the former,
To improve the degree of Goss structure accumulation, catastrophic grain growth of rubber-oriented grains called secondary recrystallization (abnormal grain growth)
Is used industrially.

On the other hand, in the latter case, the formation of a thin forsterite insulating film on the surface layer of the material is caused by the solid-phase reaction between SiO ₂ in the oxide film previously formed on the surface of the material (steel plate) and the annealing separator applied on it. Done by

These two completely different phenomena of secondary recrystallization and generation of forsterite are the decarburization annealing conditions and annealing that determine the structure and thickness of the oxide film formed on the material surface during the decarburization annealing process. It is easily affected by the separating agent. In addition, it is considered that these two phenomena are actually the result of the progress of the reaction while interfering with each other inside the material (steel plate) and the surface layer interface.

From this point of view, much research has been conducted on decarburization annealing conditions and annealing separators.

On the other hand, in order to improve productivity, the unit weight of the strip coil handled in the final finish annealing step tends to increase, which causes the temperature or atmosphere composition (dew point, etc.) in the strip length direction and width direction. ), Especially when the temperature rises, is inevitable. In order to reduce the unevenness of the temperature and atmosphere conditions inside the material (strip coil) in the final finishing annealing process as much as possible, an oxide film layer formed on the material surface in the decarburizing annealing process or Annealing agents are important and have motivated research in this area.

The effect of the oxide film layer formed in the material direction in the decarburization annealing process is roughly divided into two.

One is the stable formation of a glass film (forsterite film) on the material surface, and the other is the stabilization of secondary recrystallization.

In the former case, the thickness of the oxide layer on the surface of the material, the constituents, etc. are important, and in the latter case, the mechanism by which secondary recrystallization is performed is important in terms of the material.
These are important criteria for selecting decarburization annealing conditions.

As is well known, in order to promote the secondary recrystallization of the material in the final finish annealing process, the presence of fine precipitates called inhibitors is essential. Usually, the secondary recrystallization is stabilized by strengthening and maintaining the dispersed phase of the fine precipitates in the high tone range of the final finishing annealing process.Therefore, when the inhibitor is mainly composed of nitride, nitrogen in the annealing atmosphere is used. If the main component of sulfide is (N ₂ ) partial pressure, maintain the sulfur partial pressure appropriately.

Nitrogen (N ₂ ) in the annealing atmosphere in the final finish annealing process
With respect to maintaining the partial pressure appropriately, for example, Japanese Patent Publication No. 46-937 discloses the usefulness of annealing an Al-containing silicon steel sheet in a nitrogen atmosphere. The process has been developed to the process disclosed in Japanese Patent Publication No. 46-40855, which proposes nitriding a silicon steel sheet containing Ti, Zr, V, etc. by various methods.

Further, JP-B-49-6455 has pointed out the usefulness of selectively nitriding the surface layer portion of an Al-containing silicon steel sheet, and JP-B-54-19850 discloses a suitable nitriding method. It has been proposed to set the dew point of the atmosphere of the final finish annealing in the range of -20 ° C to 30 ° C in order to cause absorption.

Further, Japanese Examined Patent Publication No. 54-22408 proposes to carry out the final finish annealing in a hydrogen / nitrogen mixed atmosphere containing 20% or less of hydrogen.

On the other hand, by adding a metal nitride to the annealing separator, there is a method of mitigating the influence of atmosphere variation in the length direction and width direction of the material (strip coil) in the final finish annealing process. It is disclosed in Japanese Patent No. 14568. Specifically, by adding chromium nitride, titanium nitride, and vanadium nitride to the annealing separator, the nitrogen partial pressure of the atmosphere in the strip width direction of the strip coil in the final finishing annealing process is made uniform, and the secondary recrystallization is performed. It is intended to realize the stability of.

On the other hand, there is a method proposed in Japanese Patent Laid-Open No. 53-50008 for the purpose of ensuring the partial pressure of sulfur in the atmosphere during the final finish annealing process. This is because in order to stabilize the secondary recrystallization of the silicon steel of the component system using Sb and the precipitated dispersed phase mainly composed of S and / or Se as an inhibitor,
In this method, a sulfur compound such as Fe ₂ S is added to the annealing separator, or final finishing annealing is performed in an atmosphere containing H ₂ S.

As seen in these prior arts, the secondary recrystallization of silicon steel tends to be stabilized by securing a nitrogen partial pressure and a sulfur partial pressure in the final finishing annealing process, and the additive in the annealing separator is also Often added for the purpose.

As described above, glass film formation, which is one of the functions of the oxide film formed on the material surface during decarburization annealing, occurs due to the reaction between MgO in the annealing separator and SiO ₂ in the oxide film. It is important that the constituent of the oxide film be rich in SiO ₂ and that the film thickness be above a certain level.

Regarding stabilization of secondary recrystallization in the final finish annealing process, which is the second function of the oxide film formed on the material surface in the decarburization annealing process, MnS, MnS as the conventional inhibitors in steel are used.
In the case of using e, AlN or the like, in order to prevent the change of the inhibitor due to the reaction with the atmospheric gas in the temperature rising process of the final annealing, an oxide film stable to the atmospheric gas, that is, a good sealing property ( It has been required to be an oxide film (which can function as a barrier).

For this purpose, for example, in Japanese Patent Publication No. 57-1575, the degree of oxidation (P _H20 / P _H2 ) of the atmosphere in the former region of the decarburization annealing process is set to 0.15 or more, and the degree of oxidation of the atmosphere in the succeeding latter region is It is disclosed that the value is 0.75 or less and smaller than that in the former region.

In recent years, attempts have been made to manufacture a grain-oriented electrical steel sheet in which the inhibitor is heated to a relatively low temperature without completely solutionizing the inhibitor during the slab heating process.

In such a process, it becomes necessary to precipitate and disperse the inhibitor in a process other than the hot-rolled sheet annealing process. As one of such inhibitor precipitation dispersion means, there is a means for nitriding precipitation of Al in steel, for example, at a stage before secondary recrystallization in final finish annealing.

When such means is adopted, the function of the oxide film formed on the material surface during the decarburization annealing process is completely changed. That is, the oxide film formed on the surface of the material during the decarburization annealing process should not be a stable oxide film against the atmospheric gas in the final finishing annealing as described above.

It is necessary to satisfy such conditions and to improve the characteristics of the glass film and stabilize the secondary recrystallization.

(Problems to be Solved by the Invention) The present invention stabilizes the nitriding reaction of the material in the final finishing annealing process when the unidirectional electrical steel sheet manufacturing process in which the slab heating temperature is set to a low level of less than 1280 ° C is adopted. The present invention was made for the purpose of providing a method for producing a grain-oriented electrical steel sheet capable of stabilizing secondary recrystallization and stably forming a glass film.

(Means for Solving Problems) The gist of the present invention is that, in% by weight, C: 0.0010 to 0.10.
%, Si: 2.5 to 4.0%, acid-soluble Al: 0.010 to 0.06%, S ≦
0.014%, balance: Fe and unavoidable impurities slabs for unidirectional electrical steel sheets are heated to a temperature not exceeding 1280 ° C, then hot-rolled and hot-rolled sheet annealed, then once or annealed. In the method for producing a unidirectional electrical steel sheet, after performing cold rolling twice or more to obtain a final plate thickness, decarburizing annealing in a wet hydrogen atmosphere, and further applying an annealing separating agent and finally finishing annealing, When the material staying time in the soaking zone in the decarburization annealing process is a and the material staying time in the latter stage area is b, b ≦ a / 3 and steam for the hydrogen partial pressure of the atmosphere in the latter stage area _Partial pressure ratio P _H20
/ P _H2 is 0.02 or less, decarburization annealing is performed, then an annealing separator is applied, and then the temperature rising process up to 1000 ℃ contains N ₂ P _H20 / P
This is a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and glass film properties, which is characterized by performing final finish annealing in an atmosphere of _H2 ≤ 0.02.

 Hereinafter, the present invention will be described in detail.

The present invention, as described above, by combining the control conditions of the atmosphere in the decarburization annealing and the final finishing annealing,
The purpose is to stabilize the secondary recrystallization and the stable formation of the glass film by stabilizing the nitriding reaction of the material in the final finishing annealing process.

First, the material (strip) after decarburization annealing according to the present invention
As a result of the intervening surface oxide layer, the point that the secondary maximum crystal is stable will be described.

The inventors of the present invention, as a starting material, in% by weight, C:
0.055%, Si: 3.30%, Mn: 0.15%, Oxide soluble Al: 0.030
%, N: 0.0070%, balance: a slab consisting essentially of Fe,
This material was processed in the process of hot rolling-hot rolled sheet annealing-pickling-cold rolling to a final sheet thickness of 0.30 mm, and this material was changed by changing P _H20 / P _H2 in the atmosphere in the latter region of the decarburization annealing process. Decarburized and annealed.

The surface oxide layer of the material thus obtained was analyzed by measuring the potential difference curve using GDS analysis and the pseudo electrode potential method. As a result, by annealing a specific region in the subsequent stage within the soaking time of the decarburization annealing process in an extremely dry atmosphere, a strong concentrated layer of Si and Mn is formed on the outermost surface of the oxide layer formed on the material surface. It was found that the outermost layer had almost no oxide layer corresponding to the M _x O _y layer. This pattern is shown in FIG.

In addition, even in the result of the potential difference curve, when P _H20 / P _H2 ≦ 0.02 in the latter region during decarburization annealing, the solubility of the oxide layer was very fast, and there was no formation of a tight M _x O _y layer. . This pattern is shown in FIG.

The annealing separator was applied to this material, at a heating rate of 20 ° C. / hr, using a N ₂ + H ₂ mixed dry atmosphere N ₂ mainly annealed,
The state of nitriding on the way, the magnetic properties of the product after the final finish annealing and the state of secondary recrystallization were investigated.

As a result, the amount of N in the steel at the temperature rising process of 900 ° C has a strong correlation with the decarburization annealing condition, and as shown in Table 1, P _H20 / P _{H2 in} the latter part of the soaking region is low. In this case, it was found that the N content in the steel increased steadily. This pattern is shown in FIG. 3 and Table 4.

Further, the magnetic properties of the product after the final finish annealing are better as the P _H20 / P _H2 in the post-heating soaking region in the decarburizing annealing process is lower, and the secondary recrystallization in the final finishing annealing is stable, The coating was found to be extremely uniform and excellent.

As described above, it was confirmed that by controlling the decarburization annealing conditions, a remarkable improvement effect can be obtained in both the magnetic properties of the product and the glass film. As for the improvement effect on both sides, it was observed that nitriding of the material in the final finish annealing process was essential in addition to the control of the decarburization annealing conditions.

 Next, the reasons for limiting the requirements of the present invention will be described.

First, the atmospheric conditions in the soaking region in decarburization annealing will be described.

The ratio of the material staying time a in the first stage of the soaking amount region to the material staying time b in the latter stage is set to b ≦ a / 3, because the material staying time in the latter stage region is longer than a / 3, industrial production In consideration of the above, in addition to inhibiting decarburization and oxide film formation, it is difficult to obtain products with excellent magnetic properties because the absorption amount of nitrogen in the material during the final finish annealing process increases too much, and the magnetic properties and film are rather This is because it tends to deteriorate.

The P _H20 / P _H2 in the latter stage region was set to 0.02 or less, because if it exceeds this, the intercalation reaction of the outermost surface phase of the material in decarburization annealing becomes weaker, and the nitriding reaction in the final finish annealing process is suppressed M _x
Residual amounts of O _y phase is to increase. Especially, _PH20 / _PH2 = 0.15
When the value is up to 0.35, the densification of the oxide film on the surface of the material becomes remarkable with the steel composition in the present invention, and the secondary absorption is caused by the inferior proper N absorption of the material in the final finishing annealing temperature rising process. Recrystallization becomes unstable. Further, the concentration phenomenon of Mn and Si to the surface layer of the material is weakened, and an appropriate nitriding promotion effect of the material in the final finish annealing temperature rising process cannot be obtained.

According to the research results of the inventors, N of the oxide film on the material surface is
Considering the absorbability, b: a / 4 to a / 10 and P _H20 / P _H2 ≦ 0.01 are the most desirable conditions.

The atmospheric conditions in the final finish annealing process are particularly important under the component conditions in the present invention.
It is important to have an annealing atmosphere containing N ₂ except when using a nitriding agent as in the technique disclosed in Japanese Patent Laid-Open No. 8 (1999).

The atmosphere gas in the final finish annealing process is N ₂
It may be alone, N ₂ + H ₂ , N ₂ + Ar, N ₂ + H ₂ + Ar, or the like.

In the present invention, another important requirement is P _H20 / P _H2 in the atmosphere during the temperature rising process of final finish annealing, and this value needs to be 0.02 or less. P _H20 / P _H2 is 0.02
If it exceeds, the surface oxide film having no M _x O _y layer other than SiO ₂ , which is the outermost surface layer of the material modified by the atmospheric conditions in decarburization annealing, is deteriorated by additional oxidation in the temperature rising process. .

In the component system in the present invention, M _x O _y layer such as Fe _x O _y Mn _x O _y which occurs when the PH _H20 / P _H2 of the atmosphere in the final annealing is high is P _H20 / P during decarburization annealing. _As with the high _H2 phenomenon, it has adverse effects.

When a gas that does not contain H ₂ , such as N ₂ and N ₂ + Ar, is used in the temperature rising process of final finish annealing, P _H20 / P _H2 ≤
It is arguable that it needs to have a low dew point equivalent to 0.02.

I will explain the reasons for limiting the ingredients. Since (Si, Al) N is used as the precipitate required for the 2 o'clock recrystallization in the present invention, the inclusion of Si and Al is essential. If Si is less than 2.5%, the specific resistance of the material is too low and the low iron loss required for the transformer core material cannot be obtained. Further, if it exceeds 4.0%, cracking during cold rolling becomes significant, so the content was made 4.0% or less. AlN is the AlN or (S
In order to secure i, Al) N, 0.010% or more is required as acid-soluble Al. However, if it exceeds 0.06%, the AlN of the hot-rolled sheet becomes unsuitable and the secondary recrystallization becomes unstable, so the content was made 0.06% or less.

S is 0.014% or less because if S is high, secondary recrystallization defects called linear fine grains tend to occur. If nitriding treatment is sufficient to prevent this, 0.014% or less Is desirable. If S exceeds this amount, no matter how nitriding treatment is used to form (Si, Al) N in steel to strengthen the inhibitor, the probability of secondary recrystallization failure is increased, which is not preferable.

When C is less than 0.0010%, the secondary recrystallization becomes unstable, and even when the secondary recrystallization is performed, only a low magnetic flux density can be obtained, so the content was made 0.0010% or more. On the other hand, if the amount of C is too much, the decarburization annealing time becomes long and it is not economical, so 0.10%
Below.

 Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example) Example 1 C: 0.048%, Si: 3.30%, S: 0.008%, N: 0.0% by weight
Molten steel consisting of 075%, Mn: 0.015%, balance: Fe and unavoidable impurities, continuous casting-hot rolling-hot rolled sheet annealing-pickling-
Cold rolled to a final thickness of 0.29 mm.

Then, decarburization annealing was performed under the condition that the atmosphere gas of N ₂ 25% + H ₂ 75% was used and the P _H20 / P _H2 in the latter region was changed, and then MgO: 100 parts by weight, TiO ₂ : 3 After applying the burn-up separator consisting of parts by weight, the final finish annealing is performed in the atmosphere N ₂ during the heating process.
25% + H ₂ 75%, the atmosphere H ₂ 10 hot holding time of the 1200 ° C. × 20 hr
It was performed under the condition of 0% dry. At this time, the same sample was pulled out at a temperature rising process of 900 ° C. in the final annealing to measure the N absorption amount from the atmosphere to the material.

Table 1 shows the conditions for final finish annealing.
Table 2 shows the N absorption measurement results of the material at 0 ° C. and the magnetic properties and coating properties of the product.

As is clear from Table 2, the magnetic properties of Comparative Example 1 were relatively good, but the film properties were non-uniform and many pinhole-like defects were observed.

On the other hand, in the case where P _H20 / P _H2 in the region after the decarburization annealing process was lowered, the film was uniform and good. In addition, the magnetic properties showed a significant improvement effect at P _H20 / P _H2 ≤0.008.

Further, the N content in the steel during the temperature rising process of the final finish annealing is 210 when P _H20 / P _H2 ≦ 0.008 in the latter region of the decarburizing annealing process.
~250ppm many, the least when the P _H20 / P _H2 = 0.19,
It was 170 ppm when P _H20 / P _H2 : 0.44 was uniform over the entire area.

As described above, it was revealed that there is a strong correlation between the magnetic properties of the product and the N content in the steel during the temperature rising process of final finish annealing.

Example 2 After starting materials having the same components as in Example 1 were treated in the same process to give a final plate thickness of 0.22 mm, decarburization annealing was carried out under the conditions shown in Table 3 and the time distribution of the latter stage was changed. I went. After applying the annealing separator to the material thus obtained,
The same final finishing annealing as in Example 1 was performed. Table 3 shows the magnetic properties and coating properties of the product after the completion of the final finish annealing.

In Comparative Example 1, the glass film was non-uniform, a large number of pinhole-like defects were generated, and the adhesion was also poor.

On the other hand, when the P _H20 / P _H2 in the post-decarburization annealing region was set as low as 0.012 and the treatment time was set to 15 to 50 seconds, the magnetic properties were remarkably improved as well as the uniformity of the glass coating and the improvement of adhesion. There was a tendency to change.

(Effect of the invention) Since the present invention is configured and operated as described above, the slab heating temperature is set to a low level of less than 1280 ° C, and secondary recrystallization in the final finish annealing process is stabilized. The final product can be made excellent in both magnetic properties and coating properties, so the energy for heating the slab can be greatly reduced, resulting in a great effect in terms of equipment maintenance, equipment availability, and workability. Play.

[Brief description of drawings]

Fig. 1 shows the formation of a strong Si and Mn concentrated layer at the outermost surface of the oxide layer of the material by setting the extremely dry atmosphere in the specific region in the latter part of the soaking time during the decarburization annealing process. In the figure showing the pattern to be formed, (a) shows a comparative example and (b) shows an example of the present invention. Fig. 2 shows the post-stage region within the soaking time during the decarburization annealing process.
When P _H20 / P _H2 ≦ 0.02, the potential difference curve showing that the material surface layer has good solubility and there is no formation of a tight M _x O _y layer. Figure 3 shows soaking during the decarburization annealing process. In the latter part of the time
It is a figure which shows the relationship between P _H20 / P _H2 and the amount of N absorption in the final finishing annealing temperature rising process 900 ° C.

Claims (1)

[Claims] 1. By weight%, C: 0.0010 to 0.10%, Si: 2.5 to 4.
0%, acid-soluble Al: 0.010 to 0.06%, S ≦ 0.014%, balance: F
After heating the slab for unidirectional electrical steel sheet consisting of e and unavoidable impurities to a temperature not exceeding 1280 ° C, hot rolling and hot rolled sheet annealing are performed, and then once or twice or more by annealing. After performing cold rolling to the final plate thickness, decarburizing and annealing in a wet hydrogen atmosphere, and further applying an annealing separating agent and finally finishing annealing, in the method for producing a unidirectional electrical steel sheet, in the decarburizing and annealing process. When the material staying time in the former stage region of the soaking zone is a and the material staying time in the latter stage region is b, b ≦ a / 3 and the ratio P of the steam partial pressure to the hydrogen partial pressure of the atmosphere in the latter stage region is set. After decarburization annealing with _H20 / P _H2 of 0.02 or less, an annealing separator is applied, and then a final temperature annealing process up to 1000 ° C. is finally finished in an atmosphere of N ₂ containing P _H20 / P _H2 ≦ 0.02. It has excellent magnetic properties and glass film properties. Method for manufacturing grain-oriented electrical steel sheet.