JP2000038618A - Production of grain oriented silicon steel sheet good in magnetic property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the secondarily recrystallized grain size at a low cost and to improve the magnetic properties of a steel sheet by forming twin crystals on the surface layer of a silicon steel sheet before final cold rolling having a specified compsn. composed of C, Si, Mn, S or Se, Al, N, Sn, Cr and Fe and imparting high hardness thereto. SOLUTION: A silicon steel sheet slab composed of, by weight, 0.020 to 0.075% C, 2.5 to 4.5% Si, 0.05 to 0.45% Mn, <=0.15% S and/or Se, 0.020 to 0.035% acid soluble Al, 0.0035 to 0.012% N, 0.02 to 0.15% Sn, 0.003 to 0.20% Cr, and the balance Fe with inevitable impurities is heated at <=1,280 deg.C, is thereafter subjected to hot rolling and is, if required, subjected to hot rolled sheet annealing, cold rolling for one time or >= two times including annealing, decarburizing annealing, nitriding treatment and finish annealing to obtain a grain oriented silicon steel sheet. At this time, before the final cold rolling, the steel sheet is subjected to shot blasting or the like to form twin crystals on the surface layer, and the hardness of the surface layer is made higher than that of the center part by >=20 vickers hardness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet used for an iron core of electric equipment.
This makes it possible to produce a grain-oriented electrical steel sheet having good magnetic properties.

[0002]

2. Description of the Related Art A grain-oriented electrical steel sheet has a steel sheet surface of {110}.
So-called Gos whose rolling direction has the <100> axis
It is composed of crystal grains having s orientation (representing {110} <001> orientation by Miller index), and is used as a soft magnetic material in iron cores for transformers and generators. This steel sheet must have good magnetic properties such as magnetization properties and iron loss properties. The quality of the magnetization characteristics is determined by the level of the magnetic flux density induced in the core in the applied constant magnetic field, and a product with a high magnetic flux density can be downsized. The height of the magnetic flux density can be achieved by highly aligning the crystal grains of the steel sheet to {110} <001>.

[0003] Iron loss is a power loss consumed as heat energy when a predetermined alternating magnetic field is applied to an iron core. The quality of the iron loss depends on the magnetic flux density, plate thickness, coating tension, impurity amount, specific resistance, and crystal grain. Influences the size. Among them, high magnetic flux density and small crystal grain size are important in reducing iron loss, and the development of a method for manufacturing a product with as low an iron loss as possible at low cost is an issue. By the way, the following technology is known as a method of manufacturing a typical grain-oriented electrical steel sheet that is currently industrially produced.

The first technique is described in M. F. This is a two-time cold rolling process using MnS disclosed in Japanese Patent Publication No. 30-3651 by Littmann, and the obtained secondary recrystallized grains are small and stably developed, but a high magnetic flux density cannot be obtained. The second technique is to use a final cold rolling reduction of 8 using AlN + MnS disclosed in Japanese Patent Publication No. 40-15644 by Taguchi et al.
This is a process in which the rolling reduction is 0% or more. A high magnetic flux density can be obtained, but the secondary recrystallized grains are relatively large. In industrial production, strict control of production conditions is required. The third technology is now published by Tokuboku No. 51-1346
No. 9 MnS (and / or MnSe) + S
This is a process for producing silicon steel containing b twice by a cold rolling process, and a relatively high magnetic flux density and small secondary recrystallized grains are obtained.

[0005] The above three techniques have the following problems in common. In other words, each of the above techniques is a technique for controlling the precipitation finely and uniformly, by setting the slab heating temperature prior to hot rolling to a very high temperature of more than 1250 ° C., actually 1300 ° C. or more, so that the precipitation is coarse. This is a technique for forming an inhibitor by once dissolving a deposited precipitate in a solid solution and then depositing it during hot rolling or heat treatment.
Increasing the slab heating temperature increases the energy used during slab heating, increases the equipment damage rate, etc., and also causes linear secondary recrystallization failure due to the crystal structure of the slab. This problem becomes remarkable in high-Si materials.

As a fourth technique for such a high-temperature slab heating method, there is a low-temperature slab heating method disclosed in Japanese Patent Application Laid-Open No. Sho 62-40315 or Japanese Patent Application Laid-Open No. H5-112827. This is because the slab heating temperature is 1280 ° C or less, comparable to ordinary steel, by building the inhibitors required for secondary recrystallization from after the completion of decarburization annealing (primary recrystallization) to before the appearance of secondary recrystallization in finish annealing. Technology. In this technology, N acts as an inhibitor by infiltrating N into steel (Al,
A method of forming Si) N is used. N in steel
As a means for infiltrating N, the intrusion of N from the atmospheric gas during the finish annealing temperature raising process is used, or the strip after the decarburizing annealing or the strip after the decarburizing annealing is
This is performed using an atmosphere gas serving as a nitriding source such as H3. By such a method, an electromagnetic steel sheet having good magnetic properties (iron loss, magnetic flux density) has been obtained.

[0007]

In the above-mentioned manufacturing method using low-temperature slab heating, it is important to improve the magnetic flux density by strengthening the inhibitor by nitriding. However, the inhibitor due to nitriding has extremely high thermal stability, so that secondary recrystallized grains are likely to be large, and as a result, it is difficult to obtain good iron loss. Therefore, the present inventors
In the manufacturing process of the low-temperature slab heating-nitriding method, the process conditions for reducing crystal grains and improving iron loss were studied. The present invention provides a method for stably producing a grain-oriented electrical steel sheet having excellent magnetic properties by imparting strain and twins to the surface layer of a steel sheet before cold rolling.

[0008]

The gist of the present invention is as follows (1) to (3). (1) By weight ratio, C: 0.020 to 0.075%, S
i: 2.5 to 4.5%, Mn: 0.05 to 0.45%,
0.15% or less of S or Se alone or in combination, acid-soluble Al: 0.020-0.035%, N: 0.0035
0.012%, Sn: 0.02 to 0.15%, Cr:
An electromagnetic steel slab consisting of 0.003 to 0.20%, balance Fe and unavoidable impurities, is heated to a temperature of 1280 ° C. or lower and then hot-rolled, with or without hot-rolled sheet annealing,
In the production of unidirectional magnetic steel sheets that are subjected to a series of cold rolling, decarburizing annealing, nitriding, and finish annealing once or twice or more with annealing, twins are formed on the surface layer of the steel sheet before final cold rolling. A method for producing a grain-oriented electrical steel sheet having good magnetic properties, characterized by forming a surface layer hardness higher than that of a central part. (2) From the surface layer of the steel sheet before final cold rolling, the thickness ratio is 1
Average Vickers hardness of less than 0% is Hv (s), 10%
Hv is the average of Vickers hardness from
(1) The hardness of the surface layer is increased so that Hv (s) −Hv (c) ≧ 20 when (c), the unidirectional electrical steel sheet having good magnetic properties according to (1). Manufacturing method. (3) The hardness of the surface layer portion can be reduced by using any one of drive blast such as grid blast and sand blast, wet blast, wire brush polishing brush, and mechanical descaling of high pressure cleaning before final cold rolling. (1) The method for producing a grain-oriented electrical steel sheet having good magnetic properties according to (1) or (2), wherein the magnetic property is increased.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors made the secondary recrystallized grain size smaller by hardening only the surface layer portion of the steel sheet using means such as shot blasting between the hot-rolled sheet annealing and the cold rolling. And found that iron loss could be improved. Hereinafter, the present invention will be described in detail based on experiments. C: 0.055 by weight ratio
%, Si: 3.3%, Mn: 0.12%, S: 0.00
8%, Cr: 0.12%, acid-soluble Al: 0.027
%, N: 0.0077%, P: 0.030%, Sn:
An electromagnetic steel slab containing 0.05% was heated at 1150 ° C. and then hot rolled to produce a 2.3 mm hot rolled sheet. This one
After two-step annealing at a temperature of 120 ° C. + 900 ° C., it was rapidly cooled.

The annealed hot rolled sheet was subjected to a shot blast treatment under various conditions. The conditions were untreated, and the shot projection amount was 400 to 1000 g / min. A sample was cut out from the treated steel sheet, the cross section was polished, and the hardness in the thickness direction was measured with a Macro Vickers hardness meter. The average hardness from the steel sheet surface to a depth of 10% and the average hardness of 10% or more were calculated from the cross-sectional hardness profile, and the difference was defined as the hardness difference between the surface layer and the center of the thickness. After the cross section was corroded with a nital etchant, the cross-sectional structure was observed.

Next, the steel sheet was pickled and cold rolled to 0.26 mm. This was decarburized and annealed. At this time, the grain size of the primary recrystallized grains was adjusted to 23 μm by changing the annealing temperature. A sample was cut out from the decarburized annealed plate, and the texture profile in the plate thickness direction was measured. Thereafter, nitriding annealing was performed on the steel sheet at 750 ° C. for 30 seconds in a mixed gas of hydrogen, nitrogen and ammonia to adjust the nitrogen amount of the steel sheet to approximately 200 ppm. Next, an annealing separator mainly composed of MgO and TiO ₂ was applied, heated to 1200 ° C. at 15 ° C./hr, and then subjected to finish annealing at 1200 ° C. for 20 hours.

After the finish annealed plate was subjected to SRA (strain relief annealing), the magnetic properties were measured by SST. In addition, the finish annealed plate was subjected to macro pickling, and the secondary recrystallized grain size was observed. FIG. 1 shows the relationship between the surface hardness and iron loss (W17 / 50) of the steel sheet before cold rolling, FIG. 2 shows the relationship between the secondary recrystallization average particle size, and FIG. 3 shows the relationship between the magnetic flux density (B8). From FIG. 1, it was found that the surface hardness of the steel sheet before cold rolling became harder and the iron loss was improved. At the same time, as shown in FIG.
It is considered that the iron loss was improved mainly because the secondary recrystallized grain size became smaller. At this time, as shown in FIG. 2, the magnetic flux density was slightly improved.

FIG. 4 shows the cross-sectional structure of the material with and without shot blasting and the hardness profile in the thickness direction. It was found that the material (A) that had been subjected to the shot blast had twins generated on the surface of the steel sheet, and was hardened corresponding to the twins. Also, as can be seen from FIG. 5, the hardness increased only in the surface layer portion, and the hardness at the center of the plate thickness was the same as that of the shot blast untreated material (B), and no change was observed.

It is presumed that the twinning or increase in hardness of the steel sheet surface has influenced the secondary recrystallization texture through the change of the primary recrystallization texture, and the primary recrystallization texture having undergone the processes (A) and (B) above. investigated. As a result, as shown in FIG.
The shot blasted material (A) had a deeper portion near the surface layer where the (110) plane strength was higher than the shot blasted material (B). On the other hand, as shown in FIG. 7, the difference between (A) and (B) in the (111) plane strength was small.

The mechanism by which twins or increased hardness formed on the surface of the steel sheet before cold rolling improves iron loss characteristics is not clear, but can be interpreted as follows from the above experimental results.
By hardening the steel sheet surface before cold rolling by twin formation, the metal flow on the steel sheet surface in cold rolling changes. That is, since the shear deformation near the surface of the steel sheet is restricted by twins, the shear deformation proceeds further inside, and the formation of the (110) plane orientation is brought deeper. It is believed that secondary recrystallization is formed by preferential growth of the GOSS nuclei orientation near the surface layer. Therefore, according to the present invention, GOSS
It is presumed that the number of secondary recrystallization nuclei increased due to the presence of the (110) plane, which is a nucleus, from the surface layer to a deeper depth. It is presumed that the secondary recrystallized grain size becomes smaller and the iron loss is improved as the number increases.

Next, the reasons for limiting the components of the present invention will be described. When the content of C is less than 0.020%,
The secondary recrystallization becomes unstable, and the magnetic flux density of the product is as low as 1.80 T at B8 even when the secondary recrystallization is performed. On the other hand, when the content of C exceeds 0.075% and becomes too large, the decarburization annealing time becomes long, and the productivity is impaired. Preferably, 0.03 to 0.06% is good.

If the content of Si is less than 2.5%, it is difficult to obtain a product with a low iron loss, while if it exceeds 4.5%, there is a problem in the cold rolling property of the material. One of the features of the component system of the starting material of the present invention is that S or Se, alone or in combination, is 0.015% or less, preferably 0.007% or less.
The point is that it is 0% or less. As is well known, S and Se form MnS and MnSe, and act to suppress grain growth. In the present invention, the inhibitor required to develop secondary recrystallized grains is characterized by being built after decarburization annealing, so that fine precipitates are dispersed before cold rolling, and the primary recrystallized grains are dispersed. It is not preferable in the present invention to obtain a high magnetic flux density and a low iron loss by adjusting the diameter. Therefore, S or Se
Is set to 0.015% or less. Reducing S or Se is also highly effective in reducing edge cracks during hot rolling.

Al combines with N to form AlN,
In the present invention, it is essential to form (Al, Si) N by nitriding the steel after the post-process, ie, after the completion of the primary recrystallization, so that a certain amount of free Al is required. Therefore, as acid-soluble Al, 0.02
0-0.035% is added. N is 0.0035 to 0.0
Need to be 12%. If it exceeds 0.012%, blisters on the surface of the steel plate called blisters occur. Further, it becomes difficult to adjust the primary recrystallization structure. The lower limit is preferably 0.0035%. Below this value, it becomes difficult to develop secondary recrystallized grains.

If the content of Mn is too small, secondary recrystallization becomes unstable, while if it is too large, it becomes difficult to obtain a product having a high magnetic flux density. The proper content is 0.050
0.45%. Cr is an element that promotes oxidation during decarburization annealing, but the formation of a film after finish annealing is stabilized by the combined addition with Sn. Sn improves the texture after decarburizing annealing, and eventually improves the secondary recrystallized grains, and has a great effect on iron loss improvement in combination with stabilization of the coating.

An appropriate amount of Sn is 0.02 to 0.15%. If the amount is less than this, the effect is weak. On the other hand, if the amount is too large, nitriding becomes difficult and secondary recrystallized grains do not develop. Preferably 0.0
3-0.08% is good. The appropriate amount of Cr is 0.03 to 0.2
0% is good. If it is less than 0.03%, the above effects cannot be obtained. Addition of more than 0.20% is limited because the effect is not improved only by increasing the alloy cost. Preferably 0.
05-0.15% is good.

In addition, trace amounts of P, Cu, Sb, Ni, B
Including i or the like does not impair the gist of the present invention.
Next, the manufacturing process of the present invention will be described. Electromagnetic steel slabs are produced by melting steel in a melting furnace such as a converter or an electric furnace.
It is obtained by vacuum degassing if necessary, and then by continuous casting or by ingot-rolling. Thereafter, slab heating is performed prior to hot rolling. In the process of the present invention, the slab heating temperature is set to a low temperature of 1280 ° C. or less to reduce the consumption of heating energy and to make the AlN in the steel into an incomplete solid solution state without completely dissolving the AlN in the steel. Of course, MnS having a high solid solution temperature is also in an incomplete solid solution state. The slab is heated immediately after the rough hot rolling and the finish hot rolling by the usual method, with a sheet thickness of 2 to 3 m.
m.

The hot rolled sheet is annealed in a usual manner. Annealing may be performed after preliminary cold rolling to adjust the rolling reduction. Annealing may be performed by a known method, but is usually performed at a temperature of 900 to 1170 ° C. and then rapidly cooled. Even if the annealing of the hot-rolled sheet is omitted, a product with sufficient commercial value can be manufactured. The gist of the present invention is to perform surface treatment immediately before the final cold rolling step to form twins only on the surface and to harden the surface layer. As such a surface hardening method, a drive blast (shot blast, grid blast, sand blast), a wet blast, a wire brush, a polishing brush, a mechanical descaling device of high water pressure cleaning is effective and effective as a pretreatment for pickling. is there. In addition, press, thermal spraying, skin pass rolling, and a leveler surface effect method can also be applied. Of these methods, shot blasting, which is usually used as descaling, is the most suitable method at a low cost.

The twin formation can be determined by observing the cross-sectional structure as shown in FIG. Also, quantitatively, FIG.
As shown in FIG. 3, it is more preferable that the difference in hardness between the surface layer and the center of the plate thickness be 20 or more in order to stably secure good magnetic properties. Although it is preferable to measure the cross section as shown in FIG. 5, the hardness may be simply measured from the surface of the steel sheet. The surface hardness can be controlled by adjusting the shot blast projection amount as shown in FIG. Although this projection amount can be achieved by ordinary shot blasting equipment, it is not unique due to the composition and material strength of the steel sheet, so it is necessary to adjust it during production. Care must be taken because if the hardness is too high, it may break during cold rolling. After the shot blasting, pickling is performed to such an extent that the twin forming portion is not removed.

The final cold rolling is performed by a usual method. High B
In order to obtain eight values, it is preferable to increase the rolling reduction or to perform aging treatment between passes. The steel sheet rolled to the final thickness is subjected to decarburization annealing. The decarburization annealing has a role of adjusting the primary recrystallization structure and generating an oxide layer necessary for forming a film, in addition to performing decarburization. This is usually performed in a temperature range of 800 to 900 ° C. in wet hydrogen or nitrogen gas.

Next, a nitriding treatment is performed following the decarburizing annealing. The conditions of the nitriding treatment are known conditions, and the annealing temperature is 65
Good nitriding in a temperature range of 0 to 850 ° C. In order to stably develop good secondary recrystallized grains, the amount of nitrogen is 120 pp
m, preferably 150 ppm or more. Thereafter, a slurry mainly containing MgO and TiO ₂ is applied by a known method, and finish annealing is performed at a temperature of 1100 ° C. or more.

[0026]

EXAMPLE C: 0.056%, Si: 3.5 by weight
%, Mn: 0.10%, S: 0.007%, Cr: 0.
12%, acid-soluble Al: 0.030%, N: 0.007
An electromagnetic steel slab having a composition of 5%, P: 0.025%, and Sn: 0.05% was hot-rolled at 1150 ° C. to produce a 2.3-mm hot-rolled sheet. This was subjected to two-step annealing at a temperature of 1120 ° C. + 900 ° C. and then rapidly cooled.

Thereafter, surface treatment was performed at two levels: (A) without shot blast and (B) with shot blast.
The surface layer hardness is Vickers hardness (A) 300, (B) 25
0 and the center of the plate thickness was 250 for both (A) and (B). Therefore, the difference in hardness between the surface layer and the center of the plate thickness is (A) 5
0, (B) 0. Then, it was pickled and cold rolled to 0.23 mm. This was subjected to decarburization annealing at an annealing temperature of 820 ° C. in a wet hydrogen and nitrogen atmosphere. The particle size of the primary recrystallized grains was about 23 μm.

Thereafter, nitriding annealing was performed at 750 ° C. for 30 seconds in a mixed gas of hydrogen, nitrogen and ammonia to adjust the amount of nitrogen in the steel sheet to approximately 210 ppm. Next, MgO, Ti
Apply an annealing separator mainly composed of O2,
Finish annealing was performed for 20 hours. After the strain-annealed finish annealed plate, the magnetic properties were measured. As a result, (A) [Comparative Example] magnetic flux density (B8): 1.92 T, iron loss (W
17/50): 0.87 w / kg (B) [Invention] Magnetic flux density (B8): 1.92 T, iron loss (W
17/50): 0.82 w / kg Under the condition (B) satisfying the conditions of the present invention, good iron loss was obtained.

[0029]

According to the present invention, it is possible to reduce the secondary recrystallized grain size without imposing a cost in a process on the premise of low-temperature slab heating-nitriding, which has a high cost merit, and has good magnetic characteristics. We can manufacture unidirectional electrical steel sheets.

[Brief description of the drawings]

Fig. 1 Surface hardness and iron loss of steel sheet before final cold rolling (W17 / 50)
FIG.

FIG. 2 Surface hardness and magnetic flux density (B8) of steel sheet before final cold rolling
FIG.

FIG. 3 is a graph showing the relationship between the surface hardness of the steel sheet before final cold rolling and the average secondary recrystallized grain size.

FIG. 4 is a view showing a sectional structure of a steel sheet before final cold rolling.

FIG. 5 is a view showing a profile in a thickness direction of a sectional hardness of a steel sheet before final cold rolling.

FIG. 6 is a view showing a profile in a sheet thickness direction of a texture of a decarburized annealed sheet.

FIG. 7 is a view showing a profile in a thickness direction of a texture of a decarburized annealed sheet.

FIG. 8 is a diagram showing a relationship between a shot blast projection amount and a surface hardness.

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[Procedure amendment]

[Submission date] September 9, 1998 (1998.9.9)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 4

[Correction method] Change

[Correction contents]

[Fig. 4]

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) // C21D 7/06 H01F 1/16 B (72) Inventor Tomoji Kumano Tobata-ku, Tobata-ku, Kitakyushu-shi, Fukuoka 1-1 Nippon Steel Corporation Yawata Works (72) Inventor Katsuro Kuroki F-term 4K033 AA02 BA02 FA01 FA12 at 59 Nippon Steel Plant Design Co., Ltd., 46-46 Nakahara, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture HA03 JA04 5E041 AA02 AA11 AA19 CA02 HB05 HB07 HB11 NN01 NN17

Claims (3)

[Claims] 1. C: 0.020 to 0.07 by weight ratio
5%, Si: 2.5-4.5%, Mn: 0.05-0.
45%, S or Se alone or in combination with 0.15% or less, acid-soluble Al: 0.020 to 0.035%, N:
0.0035 to 0.012%, Sn: 0.02 to 0.1
5%, Cr: 0.003 to 0.20%, electromagnetic steel slab consisting of balance Fe and unavoidable impurities is heated to a temperature of 1280 ° C. or less, then hot-rolled, and hot rolled sheet annealing is performed or not. In the production of unidirectional electrical steel sheets subjected to a series of cold rolling, decarburizing annealing, nitriding treatment, and finish annealing one or more times including annealing, twinning is applied to the surface layer of the steel sheet before final cold rolling. And a method of manufacturing a grain-oriented electrical steel sheet having good magnetic properties, wherein the surface layer hardness is higher than that of the central part. 2. The average of Vickers hardness of less than 10% in the sheet thickness ratio from the sheet thickness of the steel sheet before final cold rolling is defined as Hv.
(S) Hv (s) −Hv (c) where Hv (c) is the average of Vickers hardness from 10% to the center of the plate thickness.
The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the hardness of the surface layer portion is increased so as to satisfy ≥20. 3. As a method for increasing the hardness of the surface layer, shot blast, grid blast,
3. The one-way magnetic medium according to claim 1, wherein any one of drive blast such as sand blast, wet blast, wire brush, polishing brush, and mechanical descaling of high-pressure cleaning is used. Manufacturing method of electrical steel sheet.