JPS6240703A - Manufacture of ultralow iron loss unidirectional silicon steel plate - Google Patents

Abstract

PURPOSE:To provide an ultralow iron loss by forming an extremely thin tension film of TiC, TiN or Ti(C, N) on the surface of a steel plate by carbonizing and/or nitriding after forming a Ti-surface layer on a mirror-finished surface. CONSTITUTION:After an oxide on the finish-annealed plate of a directional silicon steel plate is removed, the surface is mirror-polished in average roughness of center line of 0.4mum or less by chemically or electrolytically polishing. After a thin Ti layer of preferably approx. 0.1-2.0mum thick is coated, for example, by depositing on the surface of the mirror-finished surface, the layer is carbonized and/or nitrided to form an extremely thin tension film of TiC, TiN or Ti(C, N) on the surface of the steel plate. Thus, magnetic property and specially iron loss property is largely improved together with the improvement of a film bondability irrespective of the presence or absence of the application of high temperature heat treatment such as strain removing annealing at high temperature in the use as a transformer material of a winding core.

Description

[Detailed Description of the Invention] (Field of Industrial Application) In recent years, efforts have been made to improve the electrical and magnetic properties of unidirectional silicon steel sheets, and in particular to meet the extreme demands of reducing iron loss. The remarkable development efforts are gradually bearing fruit, but one serious drawback to their implementation is that after processing and assembly when using unidirectional silicon steel sheets,
It has been pointed out that when so-called strain-relief annealing is applied, it inevitably causes characteristic deterioration and is disadvantageous in that its usage is limited.

In this specification, the following is related to the results of research and development to open up a new method that can advantageously meet the above requirements, regardless of whether or not it undergoes a high-temperature thermal history such as strain relief annealing. state

As is well known, unidirectional silicon steel sheets are products in which secondary recrystallized grains are highly concentrated in the (110) (001), or Goss, orientation, and are mainly used in transformers and other equipment. The magnetic flux density (Bo.

It is required that the iron loss (represented by the W+tzso value) be high and the iron loss (represented by the W+tzso value) be low.

This unidirectional silicon steel sheet is manufactured through a wide variety of complicated processes, but numerous inventions and improvements have been made so far, and today the magnetic properties of a product with a thickness of 0.30 m have reached Bo.
o 1.90T or more, 1,7. . The magnetic properties of a product with a weight of less than 1.05 kg/kg or less and a plate thickness of 0.23 m are BIGl, 8
9T or more, 1. /,. Unidirectional silicon steel sheets with ultra-low core loss of 0.90 W/kg or less are being manufactured.

Particularly recently, there has been a marked increase in demand for power loss reduction features from an energy-saving perspective, and in Europe and the United States, when creating a transformer with low loss, the reduction in iron loss is converted into a monetary value and added to the transformer price.・The "evaluation" (iron loss evaluation) system is becoming widespread.

(Prior Art) Under these circumstances, recently, the surface of a unidirectional silicon steel sheet after final annealing is irradiated with a laser in a direction approximately perpendicular to the rolling direction to introduce local microstrain to subdivide the magnetic domains. It has been proposed to reduce the iron loss (see Japanese Patent Publication No. 57-2252, Japanese Patent Publication No. 57-53419, Japanese Patent Publication No. 58-26405, and Japanese Patent Publication No. 58-26406).

Although this magnetic domain refining technology is effective for transformer materials for laminated iron cores that are not subjected to strain relief annealing, it is difficult to introduce them by laser irradiation when applying strain relief annealing, mainly for wound core transformer materials. The local minute strain caused by the annealing process is released and the magnetic domain width becomes wider.
The disadvantage is that the laser irradiation effect is lost.

On the other hand, earlier in Japanese Patent Publication No. 52-24499, the surface of a unidirectional silicon steel sheet after finish annealing was mirror-finished, or the mirror-finished surface was coated with thin metal plating or an insulating coating was applied thereon. A method for manufacturing ultra-low core loss unidirectional silicon steel sheets has been proposed by coating and baking.

However, this method of improving iron loss through mirror finishing cannot be adopted from a process perspective because it does not make a sufficient contribution to reducing iron loss, although it increases the cost significantly, and especially after applying and baking the insulating film that is essential after mirror finishing. Due to problems with adhesion, it has not been adopted in current manufacturing processes. Japanese Patent Publication No. 56-4150 also proposes a method in which a steel plate surface is mirror-finished and then an oxide-based ceramic thin film is vapor-deposited. However, this method cannot be used in actual manufacturing processes because the steel sheet and the ceramic layer will separate when subjected to high-temperature annealing at 600° C. or higher.

(Problems to be Solved by the Invention) In order to more advantageously bring out the effect of improving iron loss aimed at by the above-mentioned mirror finish, the inventors have found that, especially from the viewpoint of the development of today's energy-saving materials, the above-mentioned disadvantages of increased costs should be avoided. In particular, we believe that it is important to overcome the problems of adhesion and durability of the insulating layer without deteriorating the characteristics during high-temperature treatment, and based on this basic understanding, we have developed A particularly advantageous ultra-low core loss has been achieved through drastic improvements in the steel sheet processing method after removing oxides, including the case where oxides on the surface of a silicon steel sheet are removed and then polished to a mirror-like state. It is an object of this invention to do so.

(Means for solving the problem) In order to achieve the above object, the inventors have made various efforts.
L f, knee 48, Kawa two, □ one. I, yo4□
□A 1 After removing the oxide on the surface, the surface is chemically polished or electrolytically polished to a center line average roughness of 0.4.
A mirror finish of μm J or less is to be applied, and then a mirror finish table is applied.

Preferably 0.1 to 2.0 μm on the surface, for example by vapor deposition.
After forming a thin Ti layer with a thickness of approximately m, carbonizing and/or nitriding the Ti surface layer to form an ultra-thin tensile coating of TiC, TiN or Ti(C,N) on the surface of the steel sheet. It has been found that (first invention) and furthermore, forming an insulating film mainly composed of phosphate and colloidal silica (second invention) is extremely effective in achieving the intended purpose.

Here, as a means for carbonizing and/or nitriding the Ti surface layer, C: 0.001 is added to the unidirectional silicon steel sheet after final annealing.
~0.010wtχ (shown in % below) and N:
Contains 0.0005 to 0.010 wto%,
After forming a surface layer of Ti on the mirror-finished surface of such a steel plate, annealing is performed in a non-oxidizing atmosphere to promote purification of C and N in the steel plate (third and fourth inventions); Similarly, after mirror polishing, a T4 surface layer is formed on the surface of a finish-annealed unidirectional silicon steel sheet containing a predetermined amount of C and N, and then a T4 surface layer is formed in a carbonizing gas and/or nitriding gas atmosphere. annealing in the atmosphere to promote purification of C and N in the fA plate as well as carburizing and/or nitriding from the atmosphere (fifth and sixth inventions); and further containing a predetermined amount of C and N. After mirror polishing, T1
After forming a surface layer of They also discovered that adding carburizing and/or nitriding effects (7th and 8th inventions) is particularly effective, leading to the completion of this invention.

The following explanation will be given in accordance with specific experiments that led to the success of each of the above inventions.

C: 0.042χ, Si: 3.36χ, Mn:
0.066X, Se: 0.021χ, Mo: 0.
025χ and 6sb: including 0.025χ and N
Various silicon steel slabs containing in the range of 0.001 to 0.015χ were hot rolled to a thickness of 2.4 dragons, and then homogenized annealed at 900°C for 3 minutes. Then, cold rolling was performed twice with intermediate annealing at 950° C. for 13 minutes to obtain a cold rolled sheet having a final thickness of 0.23 mm. Thereafter, after performing primary recrystallization annealing that also serves as decarburization in wet hydrogen at 820°C with the dew point varied in the range of 50 to 10°C, Ah03:60χ
, MgO: 25χ, Zr0z: 10χ and Ti0
z: After applying an annealing separator with a blending ratio of 5%,
Secondary recrystallization annealing was performed at 850°C for 50 hours, followed by purification annealing at 1200°C for 8 hours in saturated hydrogen.

After that, the oxides on the surface of each steel plate were removed by pickling, and then chemical polishing was performed to give a mirror finish with a center line average roughness of 0.01 μm, and then a vapor deposition device was used to deposit a 0.7 μm layer on the mirror finished surface. After depositing a thick Ti vapor layer,]
Annealing was performed in 1□ gas at 750°C for 3 hours.

Table 1 shows the results of an investigation into the relationship between the amount of C and N in the steel and the magnetic properties at the final annealing stage during the above manufacturing process, and the relationship between the amount of C and N and the magnetic properties of the final product.

Table 1 also shows that after annealing in H2 gas, NZ+1
(2 gas-, Nz+Ar, CH4”Hz, CH4+A
r and N2+C114+11□750 in gas atmosphere
The results of investigating the C and N contents and magnetic properties in the steel after annealing at ℃ for 5 hours are also listed.

Table 1 also shows the results of examining the adhesion of each product board.

As is clear from the results shown in Table 1, samples with C and Nfi of 100 ppm or less each at the time of final annealing are mirror-polished, deposited with Ti, and then annealed in H2 gas or further carbonized. When annealing is performed in a nitriding and/or nitriding gas atmosphere, the magnetic flux density B,
. is 1, 90T or more, and iron loss, /,. is 0.8
Excellent characteristics of 5 W/kg or less were obtained.

It is noteworthy that the amounts of C and N in the final product are both significantly reduced compared to after final annealing.

All products with good magnetic properties also had excellent adhesion.

Next, a steel plate coated with a Ti evaporated layer in the same manner as in the manufacturing process described above was coated with N! +H2, Nz+Ar, CHa
+H,, Ctl, +Ar and Nz + CHa +
Table 2 shows the results of investigating the C and N contents in the steel and the magnetic properties of the products obtained by annealing at 800° C. for 5 hours in a Hz gas atmosphere, together with the results of the adhesion investigation.

As is clear from Table 2, the samples with C3Nil of 100 ppm or less after final annealing were evaluated for carbonization and/or
Or, when annealing is performed in a nitriding gas atmosphere, the magnetic flux density B1° is 1.90T or more, and the iron loss WI? Excellent characteristics with /s11 of 0.83 W/kg or less were obtained.

Furthermore, all of the above-mentioned C and N content materials also had excellent adhesion.

(Function) The above-mentioned improvement in magnetic properties can be achieved by applying carbonization and/or nitriding treatment after forming the Ti surface layer to form an ultra-thin layer made of TiC, TiN or Ti(C,N) on the surface of the m-plate. A coating is formed and the coating effectively applies tension to the steel plate.

In particular, by utilizing the diffusion of C and N in the steel sheet during the carbonization and/or nitriding treatment of the Ti surface film, the degree of bonding between the steel sheet and the film is increased, further improving the adhesion of the film. It is possible.

Next, the manufacturing process of the unidirectional silicon steel sheet will be described in more detail, including a general explanation.

First, the starting material is a conventionally known unidirectional silicon steel material, such as ■c: o, oa to 0.050χ, St: 2.
50~4.5χ, Mn: 0.01~0.2χ,
Mo: 0.003~0.1χ, Sb? 0.00
5~0.2χ, N: 0.0005~0.01χ,
The total of one or two types of S and Se is 0.005~
Composition containing 0.05%, ■C: 0.03~0.08χ, Si: 2.0~
4.0χ, S: 0.005-0.05χ, N:
0.001~0.01χ, Al: 0.01~0.0
6χ, Sn: 0.01~0.5χ, Cu: 0
．． 01~0.3χ, Mn: 0.01~0.2χ
Composition containing, ■C70,03~0.06χ, St: 2.0~4
．． 0χ, s: o, oos~0.05χ, B: 0
．． 0QO3~0.0040χ, N: 0.001~0
．． 01χ, Mn: Composition containing 0.01-0.2K, ■C: 0.03-0.05X, Si: 2.0-
4.0! .

sb: o, oos~0.2χ, N: 0.00
Composition containing 0.05 to 0.01χ, any one or two of S and Se: 0.005 to 0.05χ, ■
C: 0.03~0.05χ, Si: 2.0~4
．． 0χ, N: 0.0005-0.01χ, any one or two of S and Ss 70.005-0.
It is applicable in compositions containing 05χ.

Next, the hot-rolled sheet undergoes uniform annealing at 800 to 1100°C.
One-time cold rolling method, in which the final plate thickness is obtained by two cold rolling steps, or two-step cold rolling method, in which intermediate annealing is usually performed at 850°C to 1050°C, and then further cold rolling is performed.In the latter case, the first rolling rate is 50
% to about 80%, the final rolling reduction is about 50% to 85%, and the final cold rolled plate thickness is from 0.151 m to 0.35+n thickness.

After finishing the final cold rolling, the steel plate finished to the product thickness is surface degreased and then decarburized and primary recrystallization annealed in wet hydrogen at 750°C to 850°C.

Here, the decarburization treatment usually develops secondary recrystallized grains that are strongly accumulated in the Goss orientation in the subsequent secondary recrystallization annealing, and also reduces the amount of C in the steel in order to further reduce C in the steel in the purification annealing. This is done in order to reduce the iron loss as much as possible, and as already shown in this invention, the purification of C along with N is promoted in the annealing after depositing Ti. In this decarburization annealing step, it is not necessary to carry out severe decarburization as in the past, and about 0.01% or less (preferably 0.001% or more) is sufficient.

After that, an annealing separator mainly composed of 90 is applied to the surface of the steel sheet, but in this invention, it is better not to form forsterite, which is generally required to be formed after finish annealing, so that the subsequent steel sheet AIZO31Z is used as an annealing separator because it is effective in simplifying the mirror finishing of
It is preferable to use 50% or more of r0z+ TiOz or the like mixed with MgO.

After that, secondary recrystallization annealing is performed, but this step is (110)
This is done to sufficiently develop secondary recrystallized grains with <001> orientation, and is usually box annealed to immediately
This is done by raising the temperature above ℃ and maintaining it at that temperature.

In this case, in order to develop a highly uniform secondary recrystallized grain structure in the (110) <001> orientation, the
It is more advantageous to carry out retention annealing at a low temperature of 0.000C, and in addition, heat removal annealing at a temperature increase rate of 0.5 to 15C/h may also be used.

Purification annealing is then performed in a saturated hydrogen atmosphere, but in order to further improve the film adhesion of the product sheet, c:o is added to the steel sheet.
, ooi~0.01% and N: 0.0005~0
．． It is important that 0.1% remain. For this purpose, appropriate annealing conditions may be selected from among the conditions of 1100° C. or higher and 1 to 20 hours in purification annealing.

Thereafter, the oxide film on the surface of the steel plate is removed by known chemical removal methods such as pickling, mechanical removal methods such as cutting and grinding, or a combination thereof.

After this oxide removal treatment, conventional methods such as chemical polishing such as chemical polishing and electrolytic polishing, mechanical polishing such as puff polishing, or a combination thereof are used to polish the steel plate surface to a mirror-like state, that is, to a center line average roughness of 0.4 μm. Finish as below.

Thereafter, a Ti surface layer is formed on the mirror-finished surface, and the thickness of the Ti surface layer at this time is preferably about 0.1 to 2.0 μm.

Further, the method for forming the Ti surface layer may be, in addition to the vapor deposition described above, a method such as a CVD method, an ion blasting method, or an ion implantation method.

The grain-oriented silicon steel sheet with a Ti surface layer is then annealed in a non-oxidizing atmosphere, a carbonizing and/or nitriding atmosphere, and further annealing in a non-oxidizing atmosphere and then a carbonizing and/or nitriding atmosphere. However, these annealing treatments are performed in the following manner.

i) Annealing in a non-oxidizing atmosphere As the atmospheric gas, ■2 gas or Ar gas is particularly advantageously suitable, and annealing is carried out in such an atmosphere at a temperature of 500°C or higher, and the surface of C and H in the steel sheet is This promotes the spread of the virus. At this time, if the amount of C in the steel is greater than the amount of N, an extremely thin film consisting mainly of TiC will be formed on the surface of the steel sheet, and in the case of one-way feeding, an extremely thin coating mainly consisting of TiN will be formed. .

ii) Annealing in a carbonizing and/or nitriding gas atmosphere Carbonizing gases include hydrocarbon gases such as CH and C2H4, CO gas, and mixed gases of these gases and N2 and Ar gases. , and as a nitriding gas, N
2 gas and Nlh gas as well as these gases and N2 and Ar
By performing annealing in such an atmosphere at a temperature of 500°C or higher, the purification of C and N in the steel and carburizing and/or nitriding from the atmospheric gas are advantageously suited. , a mixed thin film consisting of carbide and/or nitride is formed on the surface of the steel plate.

Furthermore, it is natural to apply and bake an insulating film containing phosphate and colloidal silica as main components on the ultra-thin tension film formed in this way when using a large capacity transformer of up to 1 million KV^. This is necessary, and conventionally known methods may be used as they are for forming the insulating coated and baked layer.

(Example) (A) C: 0.041χ, St: 3.48χ9
, Mn: 0.062χ, Mo: 0.025X,
Se: 0.022χ, Sb: 0.025χ and N: 0.0038χ (B) C: 0.053X, St: 3.32χ,
Mn: 0.072χ, S: 0.018χ, Al:
0.025χ and N: 0.0066χ(C)
C: 0.039χ, Si: 3.31χ, Mn
: 0.059χ, S: 0.030χ, B: 0.
0019χ, N: 0.0068χ and Cu: 0.
15χ (D) C: 0.046χ, Si: 3.09χ
, Mn: 0.063χ, Se: 0.019χ and Sb: 0.025χ(E)C: 0.038
χ, St: 3.08χ, Mn: 0.071χ
, S: 0.019%.

First, hot rolled sheets (A), (C), (D), (E
), uniform annealing was performed at 900°C. On the other hand, the hot rolled sheet (B) was homogenized at 1050°C for 3 minutes and then heated to 90°C.
It was rapidly cooled from 0°C.

After that, (A), (D), ([Regarding 9
Cold rolling was performed twice with intermediate annealing at 50°C to reduce the temperature to 0.
The final plate thickness was 23 m, and (B) and (C) were finished in a final cold-rolled plate with a thickness of 0.23 mm by one round of strong cold rolling, but warm rolling at 300°C was interposed during cold rolling. .

Next, after degreasing the surface of these cold-rolled plates, the dew point was reduced to 25℃.
After decarburization annealing at 830°C in wet hydrogen, A1zO3
Near 0! , MgO:25χ, zrOt=5%.

Thereafter, (8) and (D) were subjected to secondary recrystallization annealing at 850° C. for 50 hours, and then subjected to purification annealing at 1200° C. for 6 hours in saturated hydrogen. On the other hand, (B), (C), (
E) was heated from 850°C to 1050°C at a rate of 5°C/h for secondary recrystallization, and then subjected to purification annealing at 1200°C for 8 hours in saturated hydrogen.

Thereafter, each steel plate obtained was pickled to remove the oxide film on the surface, and then chemically polished to a center line average roughness of 0.
Finished with a mirror surface of 0.03 μm or less.

Then, a 0.7 μm thick Ti vapor deposited layer was formed on the mirror-finished surface.

After that, for (A), (B), and (C), N
Annealing was performed at 800° C. for 5 hours in a two-gas atmosphere, and some samples were further annealed at 700° C. for 3 hours in an atmosphere containing NZ and/or CH gas.

For (D) and (E), immediately conduct the test at 800°C for 50 minutes in an atmosphere containing N2 and/or CH gas.
Subjected to time annealing.

TiC, TiN or Ti(C,N) thus obtained
C of grain-oriented silicon steel plate with an ultra-thin coating consisting of
, the results of investigating the amount of N, magnetic properties and adhesion.
Table 3 shows a comparison of the C and N contents and magnetic properties in the steel after final annealing.

Table 3 also includes the results of a study on the magnetic properties of a product in which the surface of the grain-oriented silicon steel sheet with the ultra-thin coating described above is further coated with a coating film containing phosphate and colloidal silica as the main components. show.

1 ": ), 5 As is clear from the results shown in Table 3, according to the present invention, the unidirectional silicon steel sheet after finish annealing was polished to a mirror surface after the surface oxide was removed, and then Ti was applied. After forming the thin film, annealing is performed in a non-oxidizing atmosphere, or in a carbonizing and/or nitriding gas atmosphere, or further in a non-oxidizing atmosphere, and then in a carbonizing and/or nitriding gas atmosphere to improve the surface of the steel sheet. TiC, TiN or Ti(C
, N), a remarkable improvement in magnetic properties, particularly iron loss properties, was achieved while maintaining good film adhesion.

(Effects of the Invention) Thus, according to the present invention, the winding core direction can significantly improve magnetic properties, especially iron loss properties, regardless of whether high-temperature treatment such as high-temperature strain relief annealing is applied when used as a transformer material. It is possible to achieve significant improvements in combination with improved film adhesion.

Claims (1)

[Claims] 1. After removing surface oxides from a finish-annealed unidirectional silicon steel plate, polishing it to a mirror finish with a center line average roughness of 0.4 μm or less; Next, after forming a Ti surface layer on the mirror-finished surface, the Ti surface layer is carbonized and/or nitrided to form an ultra-thin tensile coating of TiC, TiN or Ti(C,N) on the surface of the steel sheet. A method for producing an ultra-low iron loss unidirectional silicon steel sheet, characterized by forming a unidirectional silicon steel sheet. 2. After removing surface oxides from the finish-annealed unidirectional silicon steel sheet, it is polished to a mirror-like state with a center line average roughness of 0.4 μm or less, and then the mirror-finished surface is After forming a Ti surface layer thereon, the Ti surface layer is carbonized and/or nitrided to form an ultra-thin tensile coating of TiC, TiN or Ti (C, N) on the surface of the steel sheet, and then A method for producing an ultra-low core loss unidirectional silicon steel sheet, which is characterized by forming an insulating film containing phosphoric acid salt and colloidal silica as main components. 3, C: 0.001 to 0.010 wt% and N: 0.
For finish annealed unidirectional silicon steel sheets with a composition containing 0.0005 to 0.0100 wt%, surface oxides are removed and then polished to a mirror surface with a center line average roughness of 0.4 μm or less. After forming a Ti surface layer on the mirror-finished surface, annealing is performed in a non-oxidizing atmosphere to promote the purification of C and N in the steel sheet.
A method for producing an ultra-low core loss unidirectional silicon steel sheet, which comprises forming an ultra-thin tensile coating of C, TiN or Ti (C, N). 4, C: 0.001 to 0.010 wt% and N: 0.
For finish annealed unidirectional silicon steel sheets with a composition containing 0.0005 to 0.0100 wt%, surface oxides are removed and then polished to a mirror surface with a center line average roughness of 0.4 μm or less. TiC on the surface of the steel plate is polished to a fine state, and then the surface layer of Ti is removed on the mirror-finished surface, and then annealed in a non-oxidizing atmosphere to promote the purification of C and N in the steel plate.
, an ultra-low core loss unidirectional silicon that forms an ultra-thin tensile film of TiN or Ti(C,N), and forms an insulating film whose main components are phosphate and colloidal silica. Method of manufacturing steel plates. 5, C: 0.001-0.010 wt% and N: 0.
For finish annealed unidirectional silicon steel sheets with a composition containing 0.0005 to 0.0100 wt%, surface oxides are removed and then polished to a mirror surface with a center line average roughness of 0.4 μm or less. After forming a Ti surface layer on the mirror-finished surface, annealing is performed in a carbonizing and/or nitriding gas atmosphere to promote the purification of C and N in the steel sheet and remove it from the atmosphere. A method for producing an ultra-low iron loss unidirectional silicon steel sheet, which is characterized by forming an ultra-thin tensile film of TiC, TiN or Ti(C,N) on the surface of the steel sheet by carburizing and/or nitriding. . 6, C: 0.001-0.010 wt% and N: 0.
For finish annealed unidirectional silicon steel sheets with a composition containing 0.0005 to 0.0100 wt%, surface oxides are removed and then polished to a mirror surface with a center line average roughness of 0.4 μm or less. After forming a Ti surface layer on the mirror-finished surface, annealing is performed in a carbonizing and/or nitriding gas atmosphere to promote the purification of C and N in the steel sheet and remove it from the atmosphere. By carburizing and/or nitriding, an ultra-thin tensile film of TiC, TiN or Ti(C,N) is formed on the surface of the steel sheet, and an insulating film mainly composed of tiphosphate and colloidal silica is formed. A method for producing an ultra-low core loss unidirectional silicon steel sheet. 7, C: 0.001-0.010 wt% and N: 0.
For finish annealed unidirectional silicon steel sheets with a composition containing 0.0005 to 0.0100 wt%, surface oxides are removed and then polished to a mirror surface with a center line average roughness of 0.4 μm or less. After forming a surface layer of Ti on the mirror-finished surface, annealing is performed in a non-oxidizing atmosphere to promote purification of C and N in the steel sheet, and further carbonization and/or nitridation is performed. It is possible to form an ultra-thin tensile coating of TiC, TiN or Ti(C,N) on the surface of a steel sheet by annealing in a hostile gas atmosphere and adding carburizing and/or nitriding effects from the atmosphere. A method for manufacturing unidirectional silicon steel sheets with a feature of ultra-low iron loss. 8, C: 0.001-0.010 wt% and N: 0.
For finish annealed unidirectional silicon steel sheets with a composition containing 0.0005 to 0.0100 wt%, surface oxides are removed and then polished to a mirror surface with a center line average roughness of 0.4 μm or less. After forming a surface layer of Ti on the mirror-finished surface, annealing is performed in a non-oxidizing atmosphere to promote purification of C and N in the steel sheet, and further carbonization and/or nitridation is performed. By carrying out annealing in a hostile gas atmosphere and adding carburizing and/or nitriding effects from the atmosphere, an ultra-thin tensile coating of TiC, TiN or Ti (C, N) is formed on the surface of the steel sheet, and then A method for producing an ultra-low core loss unidirectional silicon steel sheet, which is characterized by forming an insulating film containing phosphoric acid salt and colloidal silica as main components.