JP2779696B2 - Manufacturing method of grain-oriented high silicon steel sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a grain-oriented high silicon steel sheet used as a soft magnetic material for an iron core or the like of electric equipment, and more particularly to a conventional method for increasing the Si content. The present invention relates to a method for producing a soft magnetic material having unprecedented magnetic properties.

(Prior art) The grain orientation of grain-oriented silicon steel sheets is {110} <0 in Miller index.
01> Oriented silicon steel sheet with orientation or {100} <001>
This is a steel sheet composed of crystal grains strongly oriented in a certain crystal orientation, such as a bidirectional silicon steel sheet having an orientation. This steel sheet is required to have excellent excitation characteristics and iron loss characteristics as magnetic characteristics, and for that purpose, it is important to (1) add Si and (2) highly align crystal grains.

When the amount of Si is increased, the specific resistance increases and the iron loss characteristics are improved. In particular, it is widely known that when approximately 6.5% of Si is added, the magnetic permeability becomes extremely high and exhibits excellent magnetic properties.

However, increasing the amount of Si increases hardness and decreases elongation (TDYensen; Bozorth Ferromagnetism (1951)
P77), especially a steel sheet containing 4.8% or more of Si cannot be subjected to ordinary cold rolling. Various studies have been made on the rollability of high silicon steel sheets, and it is basically known that rolling can be performed by performing warm rolling (R.
C. Hall, JP-B-35-18709).

Further, integration of crystal grain orientation is achieved by utilizing a catastrophic grain growth phenomenon called secondary recrystallization.

In order to control the secondary recrystallization, it is essential to adjust the primary recrystallization structure before the secondary recrystallization and to adjust fine precipitates or elements of the grain boundary segregation type called inhibitors. The inhibitor has a function of suppressing the growth of general primary recrystallized grains in the primary recrystallized structure and selectively growing grains having a specific orientation.

Research on inhibitors has been extensively conducted on silicon steel sheets containing 3% of Si.

Representative examples of the precipitate-type inhibitors include MFLittmann (Japanese Patent Publication No. 30-3651) and JEM
ay, D.Turnbull uses MnS, Taguchi and Itakura (Japanese Patent Publication No. 40-15644) use AlN, and Imanaka and others (Japanese Patent Publication No. 51-13469) use MnS.
e is presented. On the other hand, as a grain boundary segregation type inhibitor, Saito (JIM 27 (1963) P186)
Although b, Nb, Ag, Te, Se, S, etc. are presented, they are only used industrially as an auxiliary of a precipitate type inhibitor.

The conditions necessary for these precipitates to function as inhibitors are not always clear, but Matsuoka (Iron and Steel 53 (1967) P1007) and Kuroki et al. (JIM 43 (1979) P175, 44 (1980) p. 419) can be summarized as follows.

(1) Before secondary recrystallization, a sufficient amount of fine precipitates to suppress the growth of general primary recrystallized grains must be present.

(2) The size of the precipitates is large to some extent and does not change very sharply during secondary recrystallization annealing.

The grain-oriented high-silicon steel sheet has excellent magnetic properties as described above, and although it is expected as a next-generation material, there is no clear disclosure about its manufacturing technology.

The reason is that in the high silicon steel sheet, the conventional inhibitor control technology of 3% Si-Fe cannot be applied as it is due to restrictions such as rollability.
For example, a grain boundary segregation type element remarkably induces cracking at the time of rolling, so that it is difficult to use it for a high silicon steel sheet.

The present inventors have found that, based on extensive research on inhibitor control for high silicon steel sheets, even in the case of precipitate type inhibitors, if formed before rolling, they are basically harmful to the rollability, and some of the present inventors have (Japanese Patent Publication No. 62-45285; unidirectional silicon steel sheet, Japanese Patent Publication No. 1-43818)
Patent Document: Bidirectional silicon steel sheet) It has been found that a manufacturing method in which an inhibitor is formed by nitriding after rolling is suitable.

What is important in this process is to uniformly precipitate and disperse the inhibitor in the steel sheet by nitriding. In particular, if there is non-uniform nitriding in the longitudinal direction and the width direction of the coil on an industrial scale, there arises a problem that the magnetic characteristics become non-uniform.

The rate-limiting step of nitriding is a reaction on the surface of the steel sheet, and in order to perform nitriding uniformly and stably, it is necessary to control an oxide layer formed on the surface during primary recrystallization annealing.

(Problems to be Solved by the Invention) The present invention relates to a method for producing a grain-oriented high-silicon steel sheet, which is based on the control of an inhibitor by a nitriding treatment after a primary recrystallization annealing. An object of the present invention is to establish a method for forming a layer, and to uniformly and stably disperse an inhibitor in a steel sheet by a nitriding treatment after primary recrystallization annealing.

(Means for Solving the Problems) The gist of the present invention is as follows.

Si by weight: 4.8-7.1%, acid-soluble Al: 0.012-0.048%,
In a method for manufacturing a grain-oriented high silicon steel sheet which is to be a product by processing a high silicon steel sheet consisting of the balance Fe and inevitable impurities in each step of cold rolling, primary recrystallization annealing, application of an annealing separator, and finish annealing. The steel sheet after cold rolling is subjected to primary recrystallization annealing under the temperature specified by the following inequality: T (° C.) and the degree of oxidation of the atmosphere gas (P _H20 / P _H2 ): S, and then the primary recrystallization annealing is performed. A method for producing a grain-oriented high silicon steel sheet, wherein a nitriding treatment is performed during a period from before to before the start of secondary recrystallization in a finish annealing step.

700 ≦ T ≦ 850 0.2 ≦ S ≦ 0.9 S ≧ −0.008T + 6.19 S ≧ 0.0028T−2.05 Hereinafter, the present invention will be described in detail.

As described in Japanese Patent Application No. 1-94414, some of the present inventors have determined that the oxidation degree (P _H20 / P _H2 ) of the primary recrystallization annealing atmosphere in a steel sheet containing 0.8% to 4.8% of Si. It has been found that an oxide layer having an excellent nitriding ability can be formed by defining the content to be 0.15 to 0.80. Based on this knowledge, we conducted research on the oxide layer of high silicon steel sheets, and in high silicon steel sheets, it was not sufficient to specify only the degree of oxidation of the primary recrystallization annealing atmosphere. By doing so, it has been found that it is possible to form an oxide layer having excellent nitriding ability.

 This will be described in detail below.

The present inventors have found that Si: 6.6%, acid-soluble Al: 0.027%, N: 0.
A slab consisting of 007%, Mn: 0.16%, S: 0.007%, and substantially Fe is heated to 1200 ° C and hot-rolled to 2.3 mm
A thick hot rolled sheet was used. Next, after annealing at 1000 ° C. for 2 minutes, hot rolling was performed at a rolling temperature of 270 ° C. to a thickness of 0.30 mm. Considering the invention of Japanese Patent Application No. 1-94414, this steel sheet was
At 850 ° C. for 2 minutes in an atmosphere gas of, and then annealed at 750 ° C. for 5 minutes in an atmosphere gas containing ammonia gas to perform a nitriding treatment. After that, an annealing separator containing MgO as a main component was applied, followed by finish annealing for the purpose of secondary recrystallization and purification. When the steel sheet after annealing was examined, secondary recrystallization was not found. Further, when the amount of nitrogen increase after ammonia nitriding was examined, it was 5 ppm, and it was hardly nitrided. Although the primary recrystallization annealing was performed under the conditions of nitriding at 50 ppm or more in the steel sheet containing 3% of Si, the reason why the nitriding was not performed in the above experimental example is that the oxide layer depends on the amount of Si in the steel. It is considered that the material deteriorated and a difference occurred in the nitriding ability.

Therefore, the effect of the amount of Si in steel on the oxide layer was investigated. Si content 0.01%, 0.99%, 3.14%, 4.02%, 4.81
%, 6.56%, with the balance being substantially Fe and having a thickness of 0.28 mm by the same process as above, annealing at 850 ° C for 2 minutes in an atmosphere gas with an oxidation degree of 0.25, and then nitriding in an ammonia-containing gas Was done. FIG. 1 shows the oxygen content and the nitrogen increase content of these steel sheets.

According to FIG. 1, the oxygen amount increases as the Si amount increases up to the Si amount of 4.02%, but when the Si amount exceeds 4.81%, the oxygen amount decreases as the Si amount increases. The amount of nitridation is closely related to the oxidation behavior, and the amount of Si at which the amount of oxygen is maximized is 4.02
%, A stable nitriding occurs at a Si content of 4.02% or less, but hardly occurs at a Si content of 4.81% or more.
Regarding such oxidation and nitridation behavior, the present inventors have proposed that the outermost layer of the oxide layer, silica (SiO ₂ ) and firelite (Fe ₂ SiO
It is presumed to be due to the structure of ₄ ).

That, N _si, Si concentration in the steel sheet to N _o, the oxygen concentration immediately above steel sheet, D _si, D _o of Si in the steel, if the O diffusion rate, R.
Applying an oxidation theory such as A. Rapp (Corrosion 21 (1965) P382), the following is considered.

(1) Under the condition of D _si , N _si ≫D _o · N _o , the oxide layer is formed by outward diffusion of Si, and is formed in a film shape on the outermost surface of the steel sheet. The diffusion rate of oxygen in the oxide is very slow, and the oxidation is substantially stopped.

(2) Under the condition of D _si , N _si ≪D _o · N _o , oxidation is controlled by inward diffusion of oxygen into the inside of the steel sheet, and oxidation occurs continuously.

Considering the results in FIG. 1, the Si content is 4.
In the case of 02% or less (N _si small), it corresponds to the condition of (2) and Si
When the amount is 4.81% or more (N _si large), it corresponds to the condition of (1), and the nitridation behavior has a close relationship with the oxidation behavior in the form of a film under the condition of (1). This is considered to be because the diffusion rate of nitrogen in the formed oxide is low and the nitriding ability is substantially lost.

Therefore, in order to form an oxide layer with excellent nitriding ability on a high silicon steel sheet, it is necessary to suppress the formation of a film-like oxide by making the oxide layer an inward diffusion type of oxygen. For this purpose, in the above equation, N _o , that is, the degree of oxidation (P _H20 / P _H2 )
In addition, it is necessary to control D _si / D _o , that is, the diffusion rate of Si and O. Since the diffusion rate is greatly affected by the temperature, the influence of the temperature and oxidation degree of the primary recrystallization annealing on the nitriding of the above-mentioned high silicon steel sheet containing 6.56% Si was examined. FIG. 2 shows the results. Nitrogen increase 50pp
m or more, preferably secondary recrystallization at 100 ppm or more,
2, the degree of oxidation of the atmosphere gas in the present invention was limited.

The nitridation is suppressed at an oxidation degree of less than 0.2 because a uniform oxide layer of silica is formed in the outermost layer, and the nitridation is suppressed at an oxidation degree of more than 0.9 because the oxide layer is too thick. it is conceivable that.

It is considered that the lower limit of the temperature is determined by the diffusion rate of Si, and the upper limit is determined by the chemical reaction for forming silica (SiO ₂ ).

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

The molten steel used in the present invention is not limited to a smelting method such as a converter or an electric furnace, but the following content range is essential as a component.

Since the goal of the present invention is to establish a process for industrially producing a high silicon steel sheet containing approximately 6.5% Si having the maximum magnetic permeability, a range having a slight width centering on 6.5% It is good if there is. The lower limit of the amount of Si is set to 4.8% from the range which has not been commercially available, and a value as close to 6.5% as possible meets the purpose of the present invention. The upper limit of the amount of Si is set to 7.1% at which magnetic properties deteriorate.

Acid-soluble Al is an element necessary for forming an inhibitor essential for developing secondary recrystallization, and the limited range is 0.012 to 0.048% in which secondary recrystallization is likely to occur.

In the present invention, N, which is an element that combines with Al to form AlN, (Al, Si) N and functions as an inhibitor, is used in the present invention for any of the period from after the primary recrystallization annealing to the start of the secondary recrystallization in the finish annealing step. At the stage, it is supplied by nitriding the steel sheet. Therefore, it is not necessary to include N in the starting material. On the contrary, when N exceeding 0.01% is contained in the starting material, a defect called a blister is generated, and tends to be a starting point of a material crack at the time of rolling.

The starting material of the present invention is a hot-rolled sheet produced in a usual process from molten steel consisting of the above components and the balance substantially Fe,
Alternatively, the molten steel can be provided as a thin slab obtained by continuous casting.

This hot-rolled sheet or thin slab is subjected to warm rolling immediately or after a short annealing step.

The magnetic flux density of the product can be improved by performing the short-time annealing in a temperature range of 800 to 1100 ° C., but the production cost is increased.

Rolling should be performed at a final rolling reduction of 80% or more, as disclosed in Japanese Patent Publication No. 40-15644, basically for unidirectional electrical steel sheets to obtain a predetermined thickness and texture. For bi-directional electrical steel sheets, basically,
The cross rolling method disclosed in JP-A-657 or JP-B-38-8218 is applied.

120 to prevent cracking during rolling
It is necessary to roll at a temperature in the range of 380380 ° C.

The obtained steel sheet is subjected to primary recrystallization annealing in an atmosphere gas having a predetermined temperature and a predetermined oxidation degree in order to form an oxide film having excellent primary recrystallization and nitriding ability. At that time, when C is contained in the steel, the magnetic properties are deteriorated, so that the steel can also serve as decarburization.

Thereafter, an annealing separator is applied, and finish annealing is performed for the purpose of secondary recrystallization and purification.

At this time, nitridation is performed after the primary recrystallization annealing until the onset of the secondary recrystallization in the finish annealing to form an inhibitor essential for the secondary recrystallization.

The nitriding method is not particularly limited. For example,
A method of increasing the partial pressure of nitrogen in the atmosphere during finish annealing, a method of performing a nitriding treatment with a gas having a nitriding ability such as ammonia gas, and adding a nitriding metal nitride such as manganese nitride and chromium nitride to the annealing separator. Can be used.

(Example) Example 1 Si: 6.55%, N: 0.0065%, Mn: 0.16%, S: 0.005% by weight%
The molten steel containing and substantially the remainder Fe was dispensed, and the acid-soluble Al was adjusted to 0.005%, 0.026%, and 0.059%, respectively.
Individual slabs.

These slabs were heated to 1230 ° C. and then hot-rolled to a thickness of 2.0 mm. Thereafter, annealing was performed at 1000 ° C. for 2 minutes, followed by warm rolling at 220 ° C. to obtain a 0.20 mm thick plate.

This is placed in an atmosphere gas with an oxidation degree of 0.11, 0.33, 0.59, 1.01 for 8
Annealed at 00 ° C for 2 minutes. Thereafter, nitriding treatment is performed in an ammonia atmosphere, and MgO is applied as an annealing separator,
For 10 hours. Table 1 shows the results.

Example 2 The same hot-rolled sheet as in Example 1 containing 0.026% of acid-soluble Al was used in an atmosphere gas having an oxidation degree of 0.59 at 650 ° C, 700 ° C, and 75 ° C.
Annealing was performed at 0 ° C, 800 ° C, 850 ° C, and 900 ° C for 2 minutes.
Thereafter, annealing was performed at 800 ° C. for 5 minutes in an ammonia atmosphere to perform a nitriding treatment. Thereafter, MgO was applied as an annealing separator, and subjected to finish annealing at 1200 ° C. for 10 hours. The result is
It is shown in the table.

Example 3 After heating a slab containing Si: 6.4%, acid-soluble Al: 0.032%, and N: 0.008% by weight, and the balance substantially consisting of Fe to 1150 ° C
It was hot rolled to a thickness of 1.6 mm. Then, after annealing at 1000 ° C. for 2 minutes, a 0.23 mm thick plate was formed by warm rolling. This steel sheet was annealed at 750 ° C. for 1 minute in an atmosphere gas having an oxidation degree of 0.59. Thereafter, an annealing separator was applied and finish annealing was performed.
At that time, a sample in which 5% ferromanganese nitride was added to the annealing separator and a sample in which 5% ferromanganese nitride was not added were compared. Table 3 shows the results.

(Effects of the Invention) According to the present invention, as described above, by defining the atmosphere gas and the temperature at the time of the primary recrystallization annealing, the formation of the inhibitor by the nitridation performed in the subsequent step can be performed stably. It is possible to industrially stably produce a grain-oriented high silicon steel sheet having excellent magnetic properties.

[Brief description of the drawings]

Fig. 1 shows the effect of the amount of Si in the steel on the oxidation and nitriding behavior. Fig. 2 shows the temperature during primary recrystallization annealing: T (° C) and the degree of oxidation (P _H20 / P _H2 ): S. It is a figure which shows the relationship of nitriding ability.

Continuation of the front page (72) Inventor Yozo Suga 1-1-1 Edamitsu, Yawatahigashi-ku, Kitakyushu-city, Fukuoka Prefecture Inside the Nippon Steel Corporation 3rd Technical Research Institute (58) Field surveyed (Int. Cl. ⁶ , DB name ) C21D 8/12

Claims (1)

(57) [Claims] (1) Si: 4.8-7.1% by weight, acid-soluble Al: 0.012
Directional high-silicon steel sheet, which is processed by cold rolling, primary recrystallization annealing, application of an annealing separating agent, and finish annealing, to a high silicon steel sheet consisting of ~ 0.048%, balance Fe and unavoidable impurities The steel sheet after cold rolling is subjected to primary recrystallization annealing under the temperature: T (° C.) and the degree of oxidation of the atmosphere gas (P _H20 / P _H2 ): S, which are defined by the following inequalities. A method for producing a grain-oriented high silicon steel sheet, comprising performing a nitriding treatment after primary recrystallization annealing and before starting secondary recrystallization in a finish annealing step. 700 ≦ T ≦ 850 0.2 ≦ S ≦ 0.9 S ≧ −0.008T + 6.19 S ≧ 0.0028T−2.05