JPH0733550B2 - Stable manufacturing method of good primary insulating film of high magnetic flux density unidirectional electrical steel sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for stably obtaining a unidirectional electrical steel sheet having a high magnetic flux density and a good primary insulating film, which is used as an iron core of an electric device. .

(Prior Art) A grain-oriented electrical steel sheet is a so-called Goss orientation having a {110} plane on the steel sheet surface and a <001> axis in the rolling direction (indicated as {110} <001> orientation by Miller index). Is used for the iron core of transformers and rotating machines as a soft magnetic material.

The grain-oriented electrical steel sheet must have good magnetic properties and iron loss properties as magnetic properties. The quality of the magnetization characteristics is determined by the magnetic flux density induced in the iron core under a constant applied magnetic field. The iron core can be downsized by using the unidirectional electrical steel sheet having a high magnetic flux density. A grain-oriented electrical steel sheet with a high magnetic flux density can be obtained by highly aligning the crystal grain orientations with {110} <001>.

Iron loss is a power loss consumed as heat energy when a predetermined AC magnetic field is applied to the iron core. Therefore, a core loss having a low iron loss value has good iron loss characteristics. The magnetic flux density, the plate thickness, the amount of impurities in the steel, the specific resistance, the grain size, etc. affect the quality of the iron loss characteristics.

A unidirectional electrical steel sheet having a high magnetic flux density is desirable because it can downsize the iron core of electric equipment and also reduce the iron loss of the iron core. In particular, since the iron loss reducing effect when the magnetic domain refining treatment is applied to the grain-oriented electrical steel sheet is more remarkable as the magnetic flux density is higher, the magnetic flux density is also preferable from this point. Therefore, in this field, the development of a technique for manufacturing a grain-oriented electrical steel sheet having a magnetic flux density as high as possible at a low cost is an issue.

A unidirectional electrical steel sheet is a so-called secondary sheet that selectively grows primary recrystallized grains having a {110} <001> orientation by finishing and high-temperature annealing a steel sheet having a final thickness by hot rolling and cold rolling. Obtained by recrystallization. In order to cause secondary recrystallization, fine precipitates such as MnS, AlN, etc. are allowed to exist in the steel sheet before secondary recrystallization, and other than {110} <001> oriented grains during high temperature finishing annealing. It is necessary to suppress grain growth (inhibitor effect). Therefore,
The point of research and development in this technical field is to increase the abundance ratio of accurate {110} <001> oriented grains by using what kind of precipitates to cause secondary recrystallization.
How to achieve a proper precipitate distribution.

NFLittma as a typical manufacturing technology for grain-oriented electrical steel sheets
nn refers to Japanese Examined Patent Publication No. Sho 30-3651, JEMay and DT
urnbull is Trans.Met.Soc.AIME212 (1958) p.769 / 781 M
nS, Taguchi et al. in Japanese Patent Publication No. 33-4710 AlN and MnS, Konaka et al. in Japanese Patent Publication No. 51-13469 MnSe (S), Sb, HF
Fiedler in U.S. Pat. No. 3,905,843 discloses S, N, B combinations.

On the other hand, Kan et al. In Japanese Patent Publication No. 61-60869 and Komatsu et al. In Japanese Patent Publication No.
It has been disclosed that nS is a cause of linear secondary recrystallization failure in high-Si materials and thin materials, which adversely affects product characteristics. Disclosed is a manufacturing technique capable of obtaining a high magnetic flux density unidirectional electrical steel sheet only by heating.

Any product manufactured by these various methods
It has a primary insulating film composed mainly of forsterite formed by the reaction of SiO ₂ and MgO. As a method of forming a primary insulating film containing forsterite as a main component, a steel plate cold-rolled to a desired final plate thickness in a wet hydrogen atmosphere is used.
After decarburizing and annealing a steel sheet (strip) in the temperature range of 700 to 900 ° C to generate a subscale containing SiO ₂ on the surface of the steel sheet, apply an annealing separator containing MgO as a main component and wind it up. Strip coil, then finish high temperature annealing, Mg
The method of forming by reacting O and SiO ₂ is generally adopted.

The properties desired for the primary insulating film mainly composed of forsterite are that the tensile force applied to the steel sheet is large in order to improve the iron loss (decrease the iron loss value). In addition, IEEE Trans.Mag.Vol.MAG I
As shown in I, No.6, p.1655, the interface between the coating and the steel plate is smooth in order to smooth the movement of the magnetic domains without hindering the movement of the magnetic domain. Sometimes the adhesion is good so that the film does not peel off. In addition, it is required that there is no uneven scale-like film in appearance or a part of the film is missing in "marbling".

(Problems to be Solved by the Invention) The present invention is intended to provide a method for producing a high magnetic flux density grain-oriented electrical steel sheet which makes the appearance state of a primary insulating film containing forsterite as a main component extremely uniform and good. Is.

In particular, it is intended to provide an effective means for establishing a primary insulating film forming technique in a composition system of low S in steel that does not cause linear secondary recrystallization failure in high Si and thin materials. .

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

(1) By weight, Si: 0.8 to 4.8%, S ≤ 0.014%, acid-soluble Al: 0.008 to 0.048%, total N ≤ 0.0095%, starting from a silicon steel sheet consisting of the balance Fe and unavoidable impurities.
Annealing for a short time in the temperature range of 0 to 1120 ℃, cold rolling, decarburization annealing, applying an annealing separator containing MgO as a main component, and then finishing for the purpose of secondary recrystallization and purification of steel. In the method for producing a unidirectional electrical steel sheet, wherein the steel sheet is subjected to a nitriding treatment in any process from the decarburization annealing of the high temperature annealing process to the start of secondary recrystallization in the finishing high temperature annealing, the surface oxidation of the starting material is performed. After the film is removed, the material is annealed in an atmosphere in which the oxidation potential is controlled, and then the material is directly cold-rolled. Law.

(2) The annealing performed after removing the surface oxide film of the starting material is performed in an atmosphere having an oxidation potential such that the amount of surface C of the material after the annealing is not substantially reduced. High magnetic flux density Stable manufacturing method of good primary insulating film of grain-oriented electrical steel sheet.

(3) Starting material is, by weight, Si: 0.8-4.8%, S ≦ 0.014
%, Acid-soluble Al: 0.008 to 0.048%, total N ≦ 0.0095%,
A silicon steel slab consisting of the balance Fe and unavoidable impurities,
3. A method for stably producing a good primary insulating coating of a high magnetic flux density unidirectional electrical steel sheet according to the above 1 or 2, which is a silicon steel sheet obtained by heating to a temperature of 1280 ° C. or lower and hot rolling.

Hereinafter, the present invention will be described in detail.

The present invention is particularly effective in forming a good primary insulating film in the technology disclosed in Japanese Patent Publication No. 61-60896 and Japanese Patent Publication No. 62-45285, but it is the basis of the present invention. The basic phenomenon can be widely applied to the manufacturing process of the grain-oriented electrical steel sheet using a material having a small S content in steel.

The present inventors have disclosed in Japanese Examined Patent Publication No. 61-60896 and Japanese Examined Patent Publication No. 62-45285, which do not place importance on lowering the amount of S in steel and utilizing MnS as an inhibitor for the expression of secondary recrystallization. The state of formation of the forsterite-based primary insulating film in the existing technology is described in JP-B-30-3651 and JP-B-33.
No. 4710, the amount of S in steel is increased to increase the Mn
It was found that it is different from the state of formation of the primary insulating film in the manufacturing process in which S is utilized as an inhibitor for secondary recrystallization.

According to the research conducted by the present inventors, when cold-rolled sheets starting from steel sheets having different S contents in steel are annealed in a wet hydrogen atmosphere also for decarburization, the properties of the subscale on the surface of the steel sheet are It has been clarified that the formation reaction of the forsterite-based primary insulating film during finish high temperature annealing is greatly affected by the effect of the S content in the steel. Thus, the present inventors
In order to bring the properties of this sub-scale into a state suitable for forming a good primary insulating film, the scale is removed by the conventional, generally-annealed cold-rolling before cold rolling. It was later discovered that it is extremely effective to change to annealing that reduces changes in the surface composition of the steel sheet in an atmosphere in which the oxidation potential is controlled.

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

By weight, C: 0.052%, Si: 3.27%, Mn: 0.14%, acid-soluble A
l: 0.027%, total N: 0.0078%, molten steel consisting of balance Fe and unavoidable impurities was dispensed into 6, and S contents were 0.0008%, 0.0060%, 0.0145%, 0.0223%, 0.0281%, 0.0
A slab whose ingredients were adjusted to 350% was created and each was divided into three parts. The slab divided into three is 1150 ℃, 1250 ℃,
After heating to 1370 ° C, it was hot-rolled into 18 kinds of hot-rolled sheets. These hot-rolled sheets were
After annealing at 1100 ° C. for 2 minutes in an atmosphere of% N ₂ + 10% H ₂ , the surface scale was removed by pickling, and then cold rolling was performed to obtain a 0.23 mm-thick cold-rolled sheet. The other one is 90% N ₂ + 10% after removing the surface scale by pickling.
It was annealed in a H ₂ atmosphere at 1100 ° C. for 2 minutes and then directly cold-rolled to obtain a cold-rolled sheet having a thickness of 0.23 mm. These cold-rolled sheets were annealed in a wet hydrogen atmosphere ((25% N ₂ + 75% H ₂ ), dew point: 59 ° C) at 830 ° C for 120 seconds and then 5%.
After applying an annealing separating agent containing MgO containing ferro-manganese as a main component, finishing high temperature annealing was performed at 1200 ° C for 20 hours. Table 1 shows the state of formation of the primary insulating film of the product thus obtained.

When the conventional hot-rolled sheet is annealed as it is with the scale and then the surface scale is removed by pickling, the better the primary insulating film is, the more S content in the steel is used as the starting material. On the other hand, when the amount of S in steel decreases, a part of the primary insulating film of the product is a defective film that is missing in the form of "marbling". In particular, the lower the slab heating temperature, the more the occurrence of this "marbling" defect. On the other hand, after the scale on the surface of the hot-rolled sheet was removed by pickling, the hot-rolled sheet was annealed and then cold-rolled as it was, regardless of the S content in the steel or the slab heating temperature. It turns out that a film is obtained.

As described above, when the S content in the steel is small, the cause of the primary insulating film defect is the property of the subscale containing SiO ₂ on the surface of the decarburized annealed plate although the S content in the steel is small. In Fig. 1, among the materials shown in Table 1, the slab heating temperature is 11
Total O (oxygen) corresponding to the amount of subscale on the surface of the decarburized and annealed sheet at 50 ° C and SiO ₂ contained in it
The relationship of the reflection intensity of Si by fluorescent X-rays corresponding to the amount of is shown.

In the case of a steel sheet that has been conventionally generally annealed by hot-rolled sheet with scale, then pickled and cold-rolled, the total O (oxygen) content increases sharply when the S content in the steel decreases. However, the amount of SiO ₂ does not change.

By the way, since the forsterite-based primary insulating film is formed by the reaction of SiO ₂ in the subscale and MgO in the annealing separator, it is necessary to obtain a uniform forsterite-based primary insulating film over the entire surface of the steel sheet. More than the required amount of SiO ₂ is required. On the other hand, the amount of total O (oxygen) indicates the amount of oxides of Fe, Mn and other components in the steel other than SiO _{2. If} this amount is above a certain level, it remains in the forsterite-based primary insulating film. To weaken the film strength, and further to form a composite oxide with forsterite, resulting in a low melting point and softening and chipping, resulting in a defective forsterite-based primary insulating film. Therefore, if the S content in steel decreases, Si
It is considered that the forsterite-based primary insulating film was defective because the amount of O ₂ remained almost unchanged and the amount of total O (oxygen) was too large.

Thus, the scale of the hot-rolled sheet was removed by pickling, then annealed, and the cold-rolled steel sheet was decarburized and annealed. The amount of O (oxygen) also corresponds to the amount of SiO _2.
There is almost no change in the fluorescent X-ray reflection intensity. Rather, SiO ₂
The fluorescence X-ray reflection intensity of Si corresponding to the amount of is high. This is the reason why a good forsterite-based primary insulating film was obtained regardless of the amount of S in steel when the process of annealing after removing the scale of the hot-rolled sheet by pickling was adopted. Conceivable. The present invention has been made based on the above new findings.

The molten steel used in the present invention may be produced by any means such as a converter or an electric furnace, but it is necessary that the content of the molten steel be within the following range.

The lower limit of Si is that the α → γ transformation does not substantially occur in order to prevent the crystal orientation from being destroyed in the finishing high temperature annealing step.
8% On the other hand, when the Si content exceeds 4.8%, the material is significantly cracked during cold rolling.

C may be in the range of 0.01 to 0.06%, but is not particularly limited in the present invention.

In the present invention, (Si, Al) N formed by nitriding the steel sheet is mainly used as the precipitation-dispersed phase necessary for secondary recrystallization, and AlN is used as the auxiliary precipitation-dispersed phase as necessary. Therefore, in order to secure the required precipitation dispersed phase, 0.008% or more of acid-soluble Al is contained in the steel. However, when the content of acid-soluble Al exceeds 0.048%, the manifestation of secondary recrystallization becomes unstable.

Regarding the total N, when it exceeds 0.0095%, swelling defects called plisters occur on the surface of the steel sheet, so it is 0.
0095% or less. The lower limit of the total N amount can be ensured by nitriding the steel sheet during the manufacturing process, and is not particularly limited. However, unless special treatment is performed by a usual melting method, the content of the impurities is about 0.0025%.

The upper limit of the S content is 0.014 as the upper limit that does not cause linear secondary recrystallization failure because the basic technology of the present invention aims to realize a high Si and thin material having the best iron loss characteristics. % Specified. Thus, the smaller the S content is, the more the characteristics of the present invention will be exhibited. However, if the de-S treatment is performed at the time of melting, the cost is usually increased. Therefore, the lower limit is about 0.0009%. It is advisable to select the S content that matches the purpose.

Other than the above-mentioned limited components, the balance is Fe and unavoidable impurities, but addition of elements for other purposes is acceptable as long as it does not change the gist of the present invention. The molten steel having the components in the above range is cast into a slab, hot rolled into a hot rolled sheet, or cast directly into a hot gauge ribbon.

The present invention is different from the conventional mainstream technology, which is disclosed in Japanese Patent Publication No. Sho 30-3651, Japanese Patent Publication No. 33-4710, and Japanese Patent Publication No. 51-13469, in which a steel sheet is nitrided in a post-process. Since the process of forming the precipitation-dispersed phase by treatment is adopted, it is not necessary to adjust the precipitation-dispersed phase at the production stage of the steel sheet (hot gauge strip obtained by hot rolling or casting) as a raw material. However, the improvement effect of the forsterite-based primary insulating film formation, which is the aim of the present invention, is greater as the slab heating temperature during hot rolling is lower, so that the temperature range where "slag" (slag) does not occur during slab heating is Combined, the desired slab heating temperature range is 1280 ° C or lower.

The thin steel sheet that is the starting material needs to be annealed in a short time continuous type in order to obtain a product having a high magnetic flux density. The temperature is preferably in the range of 800 to 1120 ° C. In this temperature range, the higher the temperature, the higher the magnetic flux density of the product.

A feature of the present invention is that during this short-time continuous annealing, after removing the scale on the surface of the steel sheet, the steel sheet (strip) is annealed in an atmosphere in which the oxidation potential is adjusted.

When the conventional process of annealing with a scale on the surface is adopted, C, Si, Mn, and S, which are oxidizable elements in the base iron of the steel sheet, are oxidized by the surface scale, and the surface of the base steel is oxidized. Decreases in the vicinity. In particular, C diffuses quickly, so that C is greatly reduced.

When such a steel sheet is cold-rolled and then a surface subscale is formed in an annealing step which also serves as decarburization, it has the properties shown by the solid line in FIG. Moreover, the degree of reduction of the steel sheet surface component varies depending on the nature of the subscale, and due to this, the forsterite-based primary insulating coating cannot be stably formed.

On the other hand, after removing the surface scale of the starting material (hot gauge ribbon obtained by hot rolling or casting),
If the steel sheet is annealed in an atmosphere in which the oxidation potential is reduced to the extent that all components on the surface of the steel sheet are not reduced by oxidation, decarburization annealing is performed after the final thickness is reached, and the surface has the properties shown by the broken line in Fig. 1. A subscale is formed, and a good forsterite-based primary insulating film is obtained. The behavior of C, which is most sensitive to the degree of oxidation, is taken as an index showing the reduction amount of the steel sheet surface component. If it does not decrease much, there is no problematic decrease with other components. However, it is not possible to analyze the C content only on the surface of the steel sheet. Therefore,
When the C of the total thickness is analyzed for the 2 mm thickness (actually, the C of the outermost surface is the smallest, and it increases in the direction of the plate thickness direction, there is a gradient), C: within about 0.003% If so, the condition of the present invention is satisfied.

The steel sheet thus annealed is cold-rolled,
Annealing that also serves as decarburization is performed in a wet hydrogen atmosphere to form a surface subscale. Next, after applying an annealing separating agent containing MgO as a main component, finishing high temperature annealing is performed. In the finishing high-temperature annealing process, secondary recrystallization, forsterite-based primary insulating film formation, and steel purification are performed to obtain a product.

The precipitate formation method which is the basis of the present invention, by nitriding the steel sheet at any stage after the completion of primary recrystallization to the start of secondary recrystallization, it may contain some Mn (Si, A
l) A method of forming a multi-component precipitate containing N as a main component. As a method of nitriding the steel sheet, a method of heat-treating the steel sheet after decarburization annealing in an atmosphere having a nitriding ability, for example, an atmosphere containing NH ₃ for a short time or before starting secondary recrystallization in a finishing high temperature annealing step There is a method of nitriding the steel sheet in the temperature rising process of 1. Especially, in the latter case, since annealing is performed in the form of a strip coil, if the method of nitriding the steel sheet is performed by adjusting the nitrogen partial pressure of the atmosphere, nitriding is difficult to be uniform, so that the annealing separator has a nitriding ability. By adding a compound, a method of forming a multi-component precipitate mainly composed of (Si, Al) N by nitriding the steel sheet in the temperature rising process before the start of secondary recrystallization in the finishing high temperature annealing step is adopted. Is appropriate.

(Example 1) C: 0.056%, Si: 3.32%, Mn: 0.16%, acid-soluble Al: 0.027
%, TN: 0.0027%, molten steel consisting of balance Fe and unavoidable impurities was poured for 3 minutes, and the S was 0.005% and 0.013, respectively.
%, 0.026% adjusted slab was made, heated to 1150 ° C., and made into a 2.0 mm thick hot rolled sheet. One of these hot-rolled sheets was annealed at 1100 ° C for 60 seconds with the scale still attached, the scale was removed by pickling, and then cold-rolled to a thickness of 0.23 mm. In the other one, after removing the scale by pickling, it was annealed at 1100 ° C for 60 seconds and cold-rolled to a thickness of 0.23 mm. The annealing atmosphere at this time was performed in two types of (A) 90% N ₂ + 10% H ₂ , dew point -20 ° C or lower (B) 90% N ₂ + 10% H ₂ , dew point + 20 ° C. These 12 types of cold rolled sheets (75%
H ₂ + 25% N ₂ ), decarburization annealing at 840 ° C for 120 seconds in a dew point + 60 ° C atmosphere, and Mg containing 5% ferromanganese nitride
After applying O, finish high temperature annealing was performed at 1200 ° C for 20 hours. Table 2 shows the state of formation and magnetic properties of the forsterite-based primary insulating film of this product. In the examples of the present invention, both the primary insulating film, B ₈ and W _17/50 are good. In contrast,
The magnetism is poor when the amount of S in the steel outside the present invention is large. Further, even when annealing with scale or annealing after removing scale is carried out, the oxidizing potential of the atmosphere is large, and when C is removed, a part of the primary insulating film is missing in a "marbling" shape.

(Example 2) C: 0.056%, Si: 3.32%, Mn: 0.16%, S: 0.005%, acid-soluble Al: 0.027%, TN: 0.0072%, and a slab consisting of the balance Fe and inevitable impurities at 1220 ° C. After heating, make a 2.0 mm thick hot-rolled sheet and remove the scale by pickling, (90% N ₂ + 10%
1100 ° C. × 60 seconds in an atmosphere of a dew point of -5 ° C. with H ₂₎ and 950 ° C.
Two types of annealing were performed for 60 seconds. After that, cold rolling to a thickness of 0.23 mm (75% H ₂ + 25% N ₂ ) and decarburization annealing at 840 ° C for 120 seconds in an atmosphere with a dew point of + 60 ° C and a 0.010% increase in TN in an atmosphere containing ammonia After treatment, MgO was applied and finish high temperature annealing was performed at 1200 ° C for 20 hours. Table 3 shows the formation status and magnetic properties of the forsterite-based primary insulating film of this product.

C after annealing the hot-rolled sheet is almost the same as that of the raw material, and both of the primary insulating films B ₈ and W _17/50 are good. It can be seen that a material having a high hot-rolled sheet annealing temperature has a high B ₈ .

(Example 3) C: 0.044%, Si: 3.27%, Mn: 0.14%, S: 0.007%, acid-soluble Al: 0.025%, TN: 0.0068%, balance: Fe and molten steel consisting of inevitable impurities are twin drums. It was cast into a thin plate with a thickness of 2.0 mm by a conventional casting machine and cooled in the atmosphere. One of the thin cast plates was annealed at 1070 ° C for 90 seconds with the scale still attached, the scale was removed by pickling, and then cold-rolled to a thickness of 0.23 mm. The other one is 1070 ℃ × 60 after removing the scale by pickling.
Second annealing was performed and cold rolling was performed to a thickness of 0.23 mm. The annealing atmosphere at this time was (A) 90% N ₂ + 10% H ₂ , dew point −5 ° C. (B) 90% N ₂ + 10% H ₂ , and dew point + 20 ° C.

Decarburization annealing of these 4 types of cold-rolled sheets was performed at (75% H ₂ + 25% N ₂ ) dew point + 60 ° C for 840 ° C x 120 seconds.
After applying MgO containing% ferromanganese nitride, 1200 ℃ × 20
A high temperature annealing was performed for a finishing time. Table 4 shows the formation and magnetic properties of the forsterite-based primary insulating film of this product.

In the example of the present invention, C after the thin cast piece annealing is almost the same as the material C, and the primary insulating film, B ₈ and W _{17/50 are} all good. On the other hand, even in the case of annealing with scales outside the present invention, or even when annealing is performed after scale removal, if the oxidizing potential of the atmosphere is large and there is decarbonization, a part of the primary insulating film is missing in a "marbling" state, and Is also inferior.

(Effect of the Invention) According to the present invention, a steel plate containing a small amount of S in steel is used as a starting plate,
Manufacture unidirectional electrical steel sheet with high magnetic flux density and good forsterite-based primary insulating coating by nitriding in any process from decarburization annealing to before secondary recrystallization of finishing high temperature annealing You can

[Brief description of drawings] Fig. 1 shows the amount of surface subscale of decarburized annealed sheet.
FIG. 3 is a diagram showing the effect of S in steel on the Si reflection intensity by fluorescent X-rays showing the amount of SiO ₂ therein.

Claims (3)

[Claims] 1. By weight, Si: 0.8-4.8%, S ≦ 0.014%, acid-soluble Al: 0.008-0.048%, total N ≦ 0.0095%, balance Fe
And a silicon steel sheet consisting of inevitable impurities as a starting material, annealed in a temperature range of 800 to 1120 ° C for a short time, cold rolled, decarburized and annealed, and an annealing separator having MgO as a main component is applied, and then One-way property of nitriding the steel sheet in any process from the decarburization annealing in the process of performing secondary high temperature annealing for the purpose of secondary recrystallization and steel purification to the start of secondary recrystallization in high temperature finishing annealing In the method for producing an electromagnetic steel sheet, after removing the surface oxide film of the starting material, the material is annealed in an atmosphere in which the oxidation potential is controlled, and then the material is directly cold-rolled. A stable method for producing a good primary insulating coating of grain-oriented electrical steel. 2. The annealing performed after removing the surface oxide film of the starting material is performed in an atmosphere having an oxidation potential such that the amount of surface C of the material after annealing is not substantially reduced. 1. A stable method for producing a good primary insulating coating of a high magnetic flux density unidirectional electrical steel sheet according to 1. 3. The starting material is, by weight, Si: 0.8 to 4.8%, S ≦
0.014%, acid-soluble Al: 0.008-0.048%, total N ≦ 0.009
The high magnetic flux density unidirectional electromagnetic field according to claim 1 or 2, which is a silicon steel sheet obtained by heating a silicon steel slab consisting of 5%, balance Fe and unavoidable impurities to a temperature of 1280 ° C or lower and hot rolling. A stable method for producing a good primary insulating film for steel sheets.