JPH0756047B2 - Method for producing grain-oriented electrical steel sheet with excellent magnetic properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a grain-oriented grain-oriented electrical steel sheet having a {110} <001> orientation, or a grain having a {100} <001> orientation. The present invention relates to a method for producing a so-called grain-oriented electrical steel sheet that is strongly oriented in a certain crystal orientation, such as grain-oriented electrical steel sheet.

These steel sheets are mainly used as a soft magnetic material for iron cores of electric devices.

(Prior Art) As described above, a grain-oriented electrical steel sheet is a steel sheet composed of crystal grains having a certain orientation and usually containing 4.5% or less of Si and a thickness of 0.10 to 0.35 mm. .

This steel sheet is required to have excellent magnetic characteristics such as excitation characteristics and iron loss characteristics.

In order to obtain a grain-oriented electrical steel sheet having excellent magnetic properties, it is important to make the crystal grains highly oriented. This integration of crystal orientation is achieved by utilizing a catastrophic grain growth phenomenon called secondary recrystallization.

In order to control the secondary recrystallization, adjustment of the primary recrystallization structure before secondary recrystallization and adjustment of fine precipitates or grain boundary segregation type elements called inhibitors are essential. This inhibitor has the function of suppressing the growth of general primary recrystallized grains in the primary recrystallized structure and selectively growing certain oriented grains.

As a typical precipitate, MFLittmann (Japanese Patent Publication No. 30-3651)
Issue) and JEMay, D. Turnbull (Trans.Met.Soc.AI
ME 212 (1958) p.769 to 781) is MnS, Taguchi and Itakura (Japanese Patent Publication No. 40-15644) AlN, and Ima Junior and Senior College (Japanese Patent Publication 51-1346).
No. 9) presents MnSe, and Komatsu et al. Presents (Al, Si) N.

On the other hand, as a grain boundary segregation type element, Saito (Journal of Japan Institute of Metals)
27 (1963) p.186-195) presents Pb, Sb, Nb, Ag, Te, Se, S, etc., which are industrially used as auxiliary substances for precipitate-type inhibitors. It is only used as a thing.

The conditions required for the above-mentioned precipitate to function as an inhibitor are not necessarily clear, but Matsuoka ("Iron and Steel" 53 (1967) p.1007-1023), Kuroki et al. 43 (1979) p.419-424), the following can be considered.

(1) Prior to secondary recrystallization, there should be a sufficient amount of fine precipitates to suppress grain growth of primary recrystallized grains.

(2) The size of the precipitate is large to a certain extent, and the thermal change does not change very rapidly during the secondary recrystallization annealing.

(Problems to be Solved by the Invention) Currently, there are three types of manufacturing methods for typical industrially produced grain-oriented electrical steel sheets.

The first production method is proposed by TFLittmann in Japanese Patent Publication No. Sho 30-3651 and uses MnS as an inhibitor.
The second manufacturing method is Taguchi,
AlN presented in Japanese Patent Publication No. 40-15644 by Itakura
+ MnS is used as an inhibitor and the final cold rolling rate is 80
%, And the third production method is a double cold rolling step using MnS (or MnSe) + Sb as an inhibitor disclosed in Japanese Patent Publication No. 51-13469. It is a thing.

In all of these manufacturing technologies, in order to secure the amount of precipitates and meet the requirements for refinement, the slab is heated to a high temperature of around 1400 ° C before hot rolling to completely dissolve the inhibitor. The basic requirement is that

However, the high temperature slab heating has the following problems.

(1) A high temperature slab heating furnace dedicated to grain-oriented electrical steel is required.

(2) The energy intensity of the heating furnace is high.

(3) Oxidation of the slab surface progresses, and a molten material called slag is generated, which increases the frequency of heating furnace repairs, raises maintenance costs, and lowers equipment availability.

In order to solve such a problem, in order to realize the low temperature slab heating, a technique for producing an inhibitor that does not rely on the high temperature slab heating is required.

Some of the inventors of the present invention have disclosed a method for producing a grain-oriented electrical steel sheet in which an inhibitor is formed by nitriding a steel sheet after the material has a final thickness, and for a unidirectional electrical steel sheet, Japanese Patent Publication No. 62-45285. In addition, regarding the grain-oriented electrical steel sheet, it is proposed in Japanese Patent Application No. 62-297825.

In particular, when the nitriding treatment of the steel sheet is performed after the primary recrystallization annealing, not only the problems caused by high temperature slab heating can be solved, but the primary recrystallization is released from the restriction of the inhibitor, and the grains can be freely grained. It becomes possible to control the texture and texture, stabilize the secondary recrystallization, and manufacture a product having a high magnetic flux density.

However, according to the above-mentioned technique, when production is performed on an industrial scale, if there is uneven nitriding in the length direction and width direction of the strip, there arises a problem that the magnetic characteristics become non-uniform. Therefore, it is necessary to uniformly and stably nitride the steel sheet. The rate-determining step when nitriding a steel sheet (strip) is a reaction on the surface of the steel sheet, and it is important to control the oxide layer formed on the surface during primary recrystallization annealing. Further, this oxide layer chemically reacts with MgO applied to the surface of the steel sheet as an annealing separator to form a forsterite film during finish annealing. This forsterite film has a function of securing insulation between plates and applying a tension to the steel plate to improve iron loss characteristics when the product is laminated and used as a transformer, for example.

Therefore, the oxide layer formed on the surface of the steel sheet in the primary recrystallization annealing must function to stabilize the nitriding and the formation of the forsterite coating.

The present invention has been made for the purpose of providing a method for forming such an oxide layer on the surface of a steel sheet.

(Means for Solving the Problem) In the present invention, when the steel sheet (strip) is subjected to primary recrystallization annealing, the degree of oxidation (PH ₂ O / PH ₂ ) of the atmosphere in the temperature rising process
And a method of forming a surface oxide layer that forms a forsterite film with excellent nitriding ability and iron loss characteristics by defining the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere during the soaking process It is a thing.

That is, by weight, Si: 0.8-6.8%, acid-soluble Al: 0.008-0.0
A slab consisting of 48%, balance Fe and unavoidable impurities,
Hot rolling, after annealing if necessary, cold rolling to the final plate thickness, then primary recrystallization annealing is performed, then an annealing separator is applied, and finish annealing is applied. In the method for producing a grain-oriented electrical steel sheet, after which a nitriding treatment of the steel sheet is performed before the secondary recrystallization starts in the finish annealing, and an inhibitor necessary for the secondary recrystallization is formed.
Divided primary recrystallization oxidation of the annealing atmosphere (PH ₂ O / PH ₂₎ to the heated stage and soaking stage, the degree of oxidation of atmosphere in the soaking step (PH ₂ O
/ PH ₂ ): x in a temperature range of 650 to 800 ° C in the temperature rising stage for at least 5 seconds in an atmosphere having an oxidation degree (PH ₂ O / PH ₂ ): y within the region defined by the following inequality. It is a method of manufacturing a grain-oriented electrical steel sheet having excellent magnetic properties, which is characterized by annealing.

0.15 ≦ x ≦ 0.80 0.15 ≦ y ≦ 0.80 0.16x + 0.11 ≦ y ≦ −0.41x + 0.78 Hereinafter, the present invention will be described in detail.

As a result of extensive research on the influence of primary recrystallization annealing conditions on the nitriding of steel sheets, the present inventors have defined the nitriding ability by defining the degree of oxidation (PH ₂ O / PH ₂ ) of the atmosphere during primary recrystallization annealing. It has been found that an excellent surface oxide layer can be formed.

Such knowledge is based on the following experiment.

By weight, Si: 3.3%, acid-soluble Al: 0.027%, N: 0.008%, M
Steel hot-rolled steel sheet containing n: 0.14% and the balance Fe and unavoidable impurities was annealed at 1100 ° C for 2 minutes and then cold-rolled.
The final plate thickness was 0.20 mm. The degree of oxidation (PH ₂ O / P
Primary recrystallization annealing was performed in an atmosphere in which H ₂ ) was changed in the range of 0.02 to 1.0. Then, an annealing separator containing MgO as a main component was applied and finish annealing was performed. Finish annealing is N ₂ : 25% + H
_2: After raising the temperature in 75% atmosphere until 120 ° C., H _2: switching to 100% of the atmosphere, for 20 hours purification annealing, nitriding behavior and product steel sheet in the temperature raising process (strip coil) properties I checked.

Figure 2 shows the amount of nitrogen increase at 850 ° C, which is the maximum nitrogen content in the steel sheet, and the degree of oxidation (PH ₂ O / P) in the atmosphere during primary recrystallization annealing.
H ₂ ).

As is clear from Fig. 2, the degree of oxidation of the atmosphere (PH ₂ O / P
H ₂₎ is from 0.15 to 0.80, preferably has nitride stable steel in the range of 0.25 to 0.70, often increasing the nitrogen content and correspondingly also the magnetic flux density of the product ₍₈ value B) shown in FIG. 3 If it is higher. However, if the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere is increased, dot-like defects may occur in the forsterite film,
Residual oxides in the steel directly under the forsterite film, that is,
Al ₂ O ₃ remains, causing problems such as deterioration of iron loss characteristics,
It has been found that it is extremely difficult to achieve both nitriding and forsterite film formation.

As a result of investigating the problem related to the formation of the forsterite film, it was found that there is a close relationship with the oxygen content of the steel sheet after the primary recrystallization annealing, and the above problem occurs when the oxygen content increases.

The reason for this is that excess oxygen in excess of the amount necessary to react with MgO to form a forsterite film is gasified from defects in the steel during finish annealing, or Al in the steel is oxidized.
It is believed that this is because it reacts with Al ₂ O ₃ to form Al ₂ O ₃ .

Therefore, it is necessary to form an oxide layer having excellent nitriding ability while controlling the oxygen content of the steel sheet after the primary recrystallization annealing to a certain level or less. As a result of various studies, the inventors of the present invention have found that the oxidation behavior of the steel sheet during the temperature rise process of primary recrystallization annealing plays an important role, and the temperature rise cycle during temperature rise and the degree of oxidation of the atmosphere (PH ₂ O
/ PH ₂ ) and the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere during soaking are controlled separately to find that a surface oxide layer that can achieve both nitriding and forsterite film formation can be created. It was

Such knowledge is based on the following experiment.

First, in order to know the important temperature range in the temperature rising process of primary recrystallization annealing, the above-mentioned cold-rolled sheet was used to measure the degree of oxidation (PH
₂ O / PH ₂ ): A steel plate is rapidly heated in an atmosphere of 0.25 to a predetermined temperature in the temperature range of 500 to 850 ° C. at a temperature rising rate of 100 ° C./s, held at that temperature for 5 seconds, and then 100 ° C. It was rapidly heated again at a heating rate of / s and annealed at 850 ° C.

After that, an annealing separator was applied to the steel sheet and finish annealing was performed.

FIG. 4 is a diagram showing the relationship between the holding temperature of the steel sheet, the amount of oxygen after the primary recrystallization annealing, and the amount of nitrogen increase at the 850 ° C. point during the finish annealing process. From this figure, by holding the steel plate in the temperature range of 650 to 800 ° C for at least 5 seconds at the time of temperature rise to form the primary oxide layer, the oxidation in the subsequent soaking process is suppressed, resulting in the primary recrystallization annealing. Although the amount of oxygen afterward decreases, on the other hand, it can be seen that the amount of nitriding is almost constant and does not deteriorate.

Therefore, next, the heating rate is set to 25 so that the above heating conditions are satisfied.
The effect of the degree of oxidation (PH ₂ O / PH ₂ ) in each atmosphere during temperature soaking was investigated in a temperature-time cycle of heating and annealing the steel sheet to 850 ° C at ℃ / s.

Figure 1 shows the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere during temperature rise: y, the degree of oxidation in the atmosphere during soaking (PH ₂ O / PH ₂ ): x, and the properties of the product forsterite film. Shows the relationship. From FIG. 1, it can be seen that nitriding and forsterite film formation are compatible within the range of the following inequality.

0.15 ≤ x ≤ 0.80 0.15 ≤ y ≤ 0.80 0.16x + 0.11 ≤ y ≤ -0.41x + 0.78 Steel plate heating rate and degree of oxidation of the atmosphere during heating (PH ₂ O / PH ₂ )
Results of investigation of the case heating rate is high, it is necessary to increase the degree of oxidation of atmosphere _{(PH 2 O / PH 2)} , when the heating rate is low, the degree of oxidation of atmosphere (PH ₂ O I found that / PH ₂ ) can be low. That is, the oxidation amount of the steel sheet increases as the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere increases, so that an oxidation layer having a certain thickness or more is formed in the temperature range of 650 to 800 ° C during the temperature rising process. It is thought that it should be made.

Although the theoretical basis for these is not always clear, the present inventors have found that the outermost layer of the steel sheet surface oxide layer, which is silica (SiO ₂ ) and firelite (Fe ₂ S).
It is presumed that this is due to the structure of iO ₄ ).

That is, FIG. 5 shows an equilibrium diagram of oxides, and the limited range of the present invention substantially corresponds to the formation region of firelite. However, as a result of actually investigating by infrared spectroscopy, CDS, and other analysis methods, it was found that silica and firelite coexist, and that the structure is non-uniform, which does not actually reach an equilibrium state. It was.

Oxidation degree of atmosphere (PH ₂ O / PH ₂ ) for nitriding behavior of steel sheet
It is considered that the reason why the nitriding is suppressed when is less than 0.15 is that the outermost layer is made of uniform silica as can be inferred from the equilibrium diagram of FIG. Further, the oxidation degree of the atmosphere (PH ₂ O / PH ₂₎ is deteriorated nitriding ability of the steel sheet at 0.80 greater, when the degree of oxidation of atmosphere (PH ₂ O / PH ₂₎ is high, the outermost layer of fayalite The higher the ratio, the faster the oxidation,
It is considered that this is because the oxide layer becomes too thick. Therefore, the upper limit of the atmospheric oxidation degree (PH ₂ O / PH ₂ ) is considered to vary depending on the time of the primary recrystallization annealing, but considering the time sufficient to complete the primary recrystallization, the upper limit is 0.80. did.

The outermost layer is formed when the temperature of the primary recrystallization annealing is increased. The diffusion rate of Fe, Si, O, etc. forming the oxide layer largely changes depending on the temperature, and the structure of the oxide layer is greatly affected by the movement of these elements. Therefore, it is considered that the oxidation behavior of the steel sheet during the temperature rising process in the primary recrystallization annealing has a great influence on the structure formation of the outermost oxide layer and controls the oxidation behavior during the subsequent soaking process. To be

As described above, in the primary recrystallization annealing, the temperature rising process and the soaking process are separately controlled, that is, in the temperature rising process, the residence time and the atmosphere of the temperature rising range of 650 to 800 ° C. The degree of oxidation (PH ₂ O / PH ₂ ) is specified to control the primary oxide layer,
In the soaking process, the growth of the oxide layer is controlled by defining the degree of oxidation (PH ₂ O / PH ₂ ) of the atmosphere with respect to the primary oxide layer created at the time of temperature rise, thus stabilizing nitriding overall. The main purpose of the present invention is to form a surface oxide layer that allows the forsterite film formation to proceed well and to progress.

Next, an embodiment of the present invention will be described.

In the present invention, the component composition of the slab as the starting material is, by weight, Si: 0.8 to 6.8%, acid-soluble Al: 0.008 to 0.048%, the balance Fe and inevitable impurities, and these as essential components There is no particular limitation.

Si is effective for improving the properties by increasing the electrical resistance of the product and reducing the iron loss, but if the content exceeds 4.8%, cold rolling becomes impossible, and if it exceeds 6.8%, the warm rolling is impossible. Even rolling causes the material to crack easily, making rolling impossible.

On the other hand, if the Si content is low, the α → γ transformation occurs during finish annealing, and the crystal orientation is destroyed. Therefore, 0.8% or more, which does not cause α → γ transformation at 950 ° C., which is considered to be the lower limit of the secondary recrystallization temperature, is set as the limited range of Si content.

Acid-soluble Al combines with N to become AlN or (Al, Si) N, and functions as an inhibitor. In particular, for inhibitor formation by nitriding the steel sheet after primary recrystallization annealing,
It is necessary to exist as free Al. The content range of the acid-soluble Al is set to 0.008 to 0.04% at which the magnetic flux density of the product is increased by the process of the present invention.

In addition, Mn, S, Se, B, Bi, Nb,
Sn, Ti, etc. can also be added.

As described in Japanese Patent Publication No. 62-45285, the heating temperature of the slab is preferably a temperature range in which Al and N are not completely dissolved from the viewpoint of inhibitor formation by the nitriding treatment of the steel sheet. However, if the temperature is lower than 1000 ° C, it becomes difficult to secure the shape (flatness) of the steel sheet (strip) in the hot rolling process. On the other hand, when the temperature exceeds 1270 ° C, the above-mentioned problems such as slag formation occur. Therefore, the range of 1000 to 1270 ° C is preferable.

The heated slab is subsequently hot rolled. The obtained hot-rolled sheet may be heated in the temperature range of 750 to 1200 ° C for 30 seconds to 3
Anneal for 0 minutes.

Then, in order to obtain a predetermined final thickness and texture, cold rolling is performed once or twice or more with intermediate annealing.

For unidirectional electrical steel, basically
As disclosed in the 5644 publication, the final cold rolling rate
80% or more, and for bi-directional electrical steel sheets, as disclosed in JP-B-35-2657 or JP-B-38-8218, a reduction rate of 40-80%. Cross cold rolling is applied.

After that, if carbon is contained in the steel, double as decarburization,
Perform primary recrystallization annealing.

Here, by defining the thermal cycle at the time of temperature rise in primary recrystallization annealing, the degree of oxidation of the atmosphere (PH ₂ O / PH ₂ ) and the degree of oxidation of the soaking atmosphere (PH ₂ O / PH ₂ ), A surface oxide layer is formed to control the amount of oxygen in a steel sheet after recrystallization annealing and to stably and reliably perform the nitriding treatment of the steel sheet performed after the primary recrystallization annealing and before the start of secondary recrystallization in finish annealing. The material thus obtained is coated with an annealing separator containing MgO as a main component, and then subjected to secondary recrystallization and finish annealing for the purpose of blunting.

Here, for the nitriding treatment of the steel sheet performed after the primary recrystallization annealing and before the start of the secondary recrystallization in the finish annealing,
It is not limited in any way. For example, a method of increasing the nitrogen partial pressure in the atmosphere during finish annealing, a method of adding a gas having a nitriding ability such as ammonia gas to the atmosphere, and a metal nitride having a nitriding ability such as manganese nitride and chromium nitride as an annealing separator. A method of adding or the like can be used.

(Example) Example 1 By weight, Si: 3.2%, acid-soluble Al: 0.027%, N: 0.007%, M
A slab consisting of n: 0.13%, S: 0.007%, C: 0.05%, the balance Fe and unavoidable impurities was heated to 1150 ° C. and then hot rolled to obtain a hot rolled sheet having a thickness of 1.8 mm.

The hot-rolled sheet was annealed at 1120 ° C. for 2 minutes and then at 900 ° C. for 2 minutes (two-step annealing), and then cold-rolled to a final sheet thickness of 0.20 mm. This cold-rolled sheet was heated to 830 at a heating rate of 10 ℃ / s, 20 ℃ / s, 30 ℃ / s, 40 ℃ / s in an atmosphere with an oxidation degree (PH ₂ O / PH ₂ ) of 0.35.
Primary recrystallization annealing was performed by heating to 830 ° C and holding at 830 ° C for 90 seconds.

Then, after applying an annealing separator having MgO as a main component to which 5% ferromanganese nitride was added for the purpose of nitriding, finish annealing was performed.

Temperature rising rate in primary recrystallization annealing, oxygen content of steel sheet after primary recrystallization annealing, and magnetic properties of products (magnetic flux density and iron loss value after magnetic domain refinement treatment by irradiating product with laser at 5 mm intervals) Is shown in Table 1.

Example 2 Using the same cold-rolled sheet as in Example 1, the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere during primary recrystallization annealing was 0.15 to 0.8.
Under various conditions in the range, the temperature is raised to 830 ° C at a heating rate of 20 ° C / s, and the oxidization degree (PH ₂ O / PH ₂ ) of the atmosphere is changed to 0.35 at 830 ° C and held for 90 seconds. Was applied.

Next, after performing a nitriding treatment in the atmosphere gas containing ammonia such that the amount of nitrogen increase of the steel sheet was 0.012%, an annealing separator having MgO as a main component was applied and finish annealing was performed.

Oxidation degree of atmosphere (PH ₂ O
/ PH ₂ ), oxygen content of the steel sheet after primary recrystallization annealing and magnetic properties of the product are shown in Table 2 (iron loss after laser irradiation).

(Effects of the Invention) As described above, the present invention provides a method for producing a grain-oriented electrical steel sheet in which an inhibitor required for secondary recrystallization is formed by nitriding the steel sheet after primary recrystallization. By controlling the temperature oxidation process of the atmosphere and controlling the surface oxidation layer of the steel sheet by separating the temperature increasing process and the soaking process in (1), it is possible to stably manufacture the grain-oriented electrical steel sheet having excellent magnetic properties.

[Brief description of drawings]

Figure 1 is a degree of oxidation of atmosphere in the Atsushi Nobori process of the primary recrystallization annealing (PH ₂ O / PH ₂₎ y, the degree of oxidation of atmosphere in the soaking process (PH ₂ O / PH ₂₎ of the x and forsterite coating FIG. 2 is a diagram showing the relationship of properties, and FIG. 2 is a diagram showing the relationship between the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere during primary recrystallization annealing and the amount of nitrogen increase in the steel sheet at 850 ° C. during finish annealing, Figure 3 shows the relationship between the degree of oxidation (PH ₂ O / PH ₂ ) in the atmosphere during primary recrystallization annealing and the magnetic properties of the product. Figure 4 shows the holding temperature during the temperature rise process of primary recrystallization annealing. And Fig. 5 is a diagram showing the relationship between the oxygen content of the steel sheet after primary recrystallization annealing and the nitrogen content of the steel sheet during finish annealing. Fig. 5 is an equilibrium diagram of oxides (FeO, Fe ₂ SiO ₄ , SiO ₂ ). is there.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Toshio Nishiyama Inventor Toshio Nishiyama 1 Fuji-machi, Hirohata-ku, Himeji-shi, Hyogo Nippon Steel Stock Company, Hirohata Works (56) Reference JP-A-2-274813 (JP, A) Japanese Patent Publication Sho 57-1575 (JP, B2)

Claims (1)

[Claims] 1. Si: 0.8-6.8% by weight, acid-soluble Al: 0.00
A slab consisting of 8 to 0.048% and the balance Fe and unavoidable impurities is treated in each step of hot rolling, cold rolling, primary recrystallization annealing, annealing separator application, and finish annealing. In the method for producing a grain-oriented electrical steel sheet in which the steel sheet is nitrided between the time of crystal annealing and the start of secondary recrystallization in finish annealing, the degree of oxidation (PH ₂ O / PH) in the atmosphere during the soaking process of the primary recrystallization annealing step is used. ₂ ): For x, 650 to 800 ℃ in the temperature rising process for at least 5 seconds, in an atmosphere having an oxidation degree (PH ₂ O / PH ₂ ): y within the region defined by the following inequality: A method for producing a grain-oriented electrical steel sheet having excellent magnetic properties, the method comprising: 0.15 ≤ x ≤ 0.80 0.15 ≤ y ≤ 0.80 0.16x + 0.11 ≤ y ≤ -0.41x + 0.78