JP2003253336A - Process for manufacturing grain-oriented magnetic steel sheet having excellent surface quality and high magnetic flux density - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a process for manufacturing a grain-oriented magnetic steel sheet having an excellent surface quality and a high magnetic flux density. <P>SOLUTION: A Bi-containing material is introduced into molten steel which contains 0.01-0.15% C, 2.5-5.5% Si and 0.03-3.0% Mn and has a flow rate of ≥70 g/(scm<SP>2</SP>) to obtain a slab containing 0.002-0.08% Bi through continuous casting. The slab is subjected to rough rolling and subsequently to finish rolling by a tandem mill at 800-1,150°C wherein the cumulative rolling reduction in the first four stages comprising first to fourth stands is 70-95%. Next, the slab is successively subjected to cold rolling, primary recrystallization annealing and final finish rolling to obtain the grain-oriented magnetic steel sheet having an excellent surface quality and a high magnetic flux density. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a high magnetic flux density grain-oriented electrical steel sheet in which crystal grains are highly oriented in a (110) [001] orientation used for an iron core of a transformer or the like, and more particularly,
The present invention relates to a method for manufacturing a high magnetic flux density grain-oriented electrical steel sheet having excellent surface properties.

[0002]

2. Description of the Related Art In recent years, there has been an increasing social demand for high efficiency and low noise of transformers. For this reason, it is required to improve the magnetic flux density and reduce the iron loss of the grain-oriented electrical steel sheet, which is the core material. Has been. The magnetic flux density of grain-oriented electrical steel sheet is generally evaluated by B ₈ (the value of the magnetic flux density at a magnetic field strength of 800 A / m),
It is required to have a high value. In order to meet such demands, for example, as disclosed in Japanese Patent Publication No. 56-21331 and Japanese Patent Publication No. 62-50529, a technique of containing Bi in steel has been known for a long time, and recently, a technique disclosed in Japanese Patent Laid-Open No. As described in 8-269552, a grain-oriented electrical steel sheet having characteristics of values of 1.95 to 1.97T is obtained. Also, the iron loss is W
It was evaluated by _17/50 (loss per unit weight when magnetized up to 1.7 _{T at} 50 Hz), and in general, high magnetic flux density materials tended to have poor iron loss. 4-8812
By applying the magnetic domain subdivision technology described in Publication No. 1,
It has become possible to obtain a material with a low iron loss of 0.70 W / kg using a steel sheet with a thickness of 0.23 mm. As described above, grain-oriented electrical steel sheets containing Bi have been researched and developed for a long time, but Bi is difficult to dissolve in molten steel and has a specific gravity of about 10 and is larger than iron,
Since the boiling point is almost the same as the molten steel temperature, it is an element that is very difficult to add to molten steel.In addition, in the grain-oriented electrical steel sheet, the appropriate range of the inhibitor component is narrow, and it is more difficult to add Bi as the inhibitor component. Therefore, the grain-oriented electrical steel sheet containing Bi is not yet established as an industrial product.

To address such a problem, for example, Japanese Patent Laid-Open No. 2001-2001
-1116 discloses a method of adding Bi to molten steel by adding Bi to the molten steel flow moving from the ladle to the molten steel bath surface in the tundish. In addition, Japanese Patent Laid-Open No. 2001-150111 proposes a method of adding Bi to molten steel by adding Bi particles with a particle size of 0.5 to 5 mm to the molten steel flow that is moving from the ladle to the molten steel bath surface in the tundish. ing.

[0004]

[Problems to be Solved by the Invention] However, JP-A-2001-111
The technologies described in JP 6 and JP 2001-150111 A can certainly disperse Bi in steel with a high yield, but still have defects such as dents, scratches, and fine dents on the surface of the steel sheet. It often happens. When such steel sheets are laminated as an iron core of a transformer, such surface defects cause problems such as damage to the insulating properties of the steel sheets and a significant deterioration of the space factor. Further, the magnetic characteristics of the product also fluctuated greatly and were not stable.

The present invention provides a method for producing a grain-oriented electrical steel sheet having a high magnetic flux density, which is capable of suppressing the generation of surface defects in a grain-oriented electrical steel sheet containing Bi without impairing its excellent magnetic flux density. The purpose is to propose.

[0006]

Means for Solving the Problems The inventors of the present invention have made extensive studies on the reason why surface defects frequently occur in Bi-containing steel. As a result, the inventors of the present invention, as described in JP-A-7-150249, when Bi is present in a coarse state in the steel, bubbles or liquid during hot rolling. It has been found that when they are present in the surface layer of the steel sheet, the base metal portion of the skin is destroyed during rolling and appears as surface defects such as dents, scratches, and fine dents.

Further, the present inventors have investigated the cause by extending it to a general grain-oriented electrical steel sheet containing Bi. As a result, in order to eliminate the surface defects of grain-oriented electrical steel sheet containing Bi, firstly, the dispersion of Bi in the molten steel should be made extremely fine, or the dissolution in the steel should be promoted. Second, in the finish rolling of hot rolling, further Bi
It was newly discovered that it is effective to break down and finely and enhance the fine dispersion state. The finer dispersion of Bi in steel also enhances the inhibitory power of Bi as an inhibitor, and also brings about further improvement of magnetic properties.

First, the experimental results that form the basis of the present invention will be described. Molten steel containing 0.05 mass% C-0.02 mass% Mn-0.01 mass% O was melted by converter refining. Then, when the steel was tapped from the converter into the ladle, pure Bi grains having an average weight of 5.3 g / piece were continuously charged into the molten steel. At this time, the diameter of the molten steel outlet hole of the converter was changed to change the molten steel flow rate. As condition 1, steel is tapped out in 8 minutes (flow rate of molten steel = 320 g /
(S · cm ² ), and under the condition 2, steel was tapped (molten steel flow rate = 88.4 g / (s · cm ² )) in 18 minutes.

Next, these molten steels were subjected to RH degassing to kill the oxygen in the steel, and Si, C, Mn, Al, Sb, S.
Adjusting the content of e and N, 0.08% C-3.4% by mass%
3% Si-0.08% Mn-0.003% P-0.022% Al-0.030% S
b-0.008% N (condition 1) and 0.08% C-3.44% Si-
0.08% Mn-0.003% P-0.023% Al-0.029% Sb-0.00
After having a composition of 8% N (condition 2), it was poured into a tundish, and molten steel was poured into the mold from the tundish to form a slab by a continuous casting method. In addition, Bi in the slab
The respective contents were 0.025% (condition 1) and 0.023% (condition 2) in mass%.

Next, in mass% 0.05% C-0.02% Mn-0.01
After melting molten steel containing% O by converter refining,
Steel is put in a pan and the molten steel acid in the ladle is processed by RH degassing.
Killed element and content of Si, C, Mn, Al, Sb, Se, N
Was adjusted, and in% by mass, 0.08% C-3.45% Si-0.08%
Mn-0.003% P-0.023% Al-0.030% Sb-0.008% N
A molten steel containing the balance Fe and unavoidable impurities was prepared.
During the RH degassing refining process, the average weight was 5.3g / piece.
Pure Bi grains were continuously charged into the degassed reflux molten steel flow. this
At that time, the molten steel flow velocity of the reflux steel calculated from the diameter of the degassing immersion pipe
Is 817g / (s ・ cm ²) (Condition 3). Then this
Molten steel is poured into the tundish and then from the tundish
It was poured into a mold and made into a slab by a continuous casting method.
The Bi content in the slab is 0.023% (%
It was case 3).

Further, molten steel containing 0.05% C-0.02% Mn-0.01% O is smelted by converter refining, then tapped in a ladle and subjected to RH degassing to remove molten steel oxygen in the ladle. Killed and adjusted the content of Si, C, Mn, Al, Sb, Se, N, in mass% 0.08% C-3.44% Si-0.08% Mn-0.
003% P-0.024% Al-0.030% Sb-0.008% N and balance
A molten steel containing Fe and inevitable impurities was used. Then, this molten steel was further poured from a ladle into a tundish and from the tundish into a mold to form a slab by a continuous casting method. At this time, pure Bi particles having an average weight of 5.3 g / piece were continuously added to the molten steel flow at the lower nozzle of the ladle. By changing the diameter of the ladle lower nozzle, the flow rate of the molten steel injected is 654 g / (s · cm ² ) (condition 4), 543 g / (s · cm ² ) (condition 5), 433 g / (s · cm). ² ) (Condition 6). The Bi contents in the slabs were 0.024% (condition 4), 0.024% (condition 5), and 0.024% (condition 6) in mass%.

Further, in mass%, 0.05% C-0.02% Mn-
Molten steel containing 0.01% O is smelted by converter refining, then tapped in a ladle and subjected to RH degassing to kill oxygen in the steel and Si, C, Mn, Al, Sb, Se, N Content of 0.08% C-3.43% Si-0.08% Mn
Molten steel containing -0.003% P-0.023% Al-0.030% Sb-0.008% N, balance Fe and inevitable impurities. This molten steel was further poured into a tundish from a ladle and into a mold from the tundish, and made into a slab by a continuous casting method. At this time, pure Bi grains having an average weight of 5.3 g / piece were continuously charged into the tundish, but the width of the tundish was set to the normal size and a width narrower than that, and the molten steel flow rate in the tundish was changed to two conditions. . One is the molten steel flow rate of 18.3 g / (s ・ cm
² ) (Condition 7), and the other was a molten steel flow rate of 33.5 g / (s · cm ² ) (Condition 8). The Bi content in the slab is% by mass.
And 0.023% (condition 7) and 0.023% (condition 8).

Finally, in mass%, 0.05% C-0.02% Mn-
Molten steel containing 0.01% O is smelted by converter smelting, then tapped in a ladle, and RH degassing process kills oxygen in the steel and Si, C, Mn, Al, Sb, Se, The content of N was adjusted to be 0.08% C-3.43% Si-0.08% Mn in mass%.
Molten steel containing -0.003% P-0.023% Al-0.030% Sb-0.008% N and the balance Fe and unavoidable impurities was further injected by a continuous casting method by pouring molten steel into the tundish from the ladle and into the mold from the tundish. It was a slab. At this time, leave the minor axis of the mold at 200 mm and the major axis at 1000 mm.
mm and 520 mm, and increased the slab drawing speed. As a result, one of the molten steel flow velocity in the mold is
27.3g / (s ・ cm ² ) (Condition 9), the other is 57.3g / (s ・ cm ² )
(Condition 10). In this way, pure Bi grains having an average weight of 5.3 g / piece were continuously introduced into the mold in which the molten steel flow rate was changed. The Bi content in the slab is 0.025% by mass.
% (Condition 9) and 0.025% (condition 10).

The slabs manufactured under the above conditions 1 to 10 were subjected to a hot rolling experiment in a factory and a hot rolling experiment in a laboratory by cutting out a part of the slab. The hot rolling in the factory heats the slab at a temperature of 1420 ° C due to the solid solution of the inhibitor, followed by 40 ° C at 1350-1200 ° C.
Rough rolling was performed to a thickness of mm, and further, finish rolling was performed to a thickness of 2.4 mm in the temperature range of 1100 to 900 ° C and a coil winding temperature of 550 ° C. At this time, the reduction schedule for finish rolling was changed to two types.

That is, in the hot rolling experiment in the factory, the finish rolling pass schedule was set to 40 → 28 → 18 → 10 → 6 → 4.4 → 3.2 → 2.4 mm in the preceding stage strong rolling reduction condition (the preceding four rolling cumulative rolling reductions: 85%) 40 → 34 → 28 → 20 → 14 → 9.5 → 4.2 → 2.4 mm for the former stage light reduction condition (previous 4 stages cumulative reduction rate: 65%).

On the other hand, in the hot rolling experiment in the laboratory, the rolling pass schedule was 40 → 32 → 24 → 15 → 10 →
7->5.5->3.7->2.8-> 2.4mm 1st-stage strong reduction condition (previous 4-stage cumulative reduction rate: 75%) 40->36->30->26->22->18->12->9->6.5->4.6-> 3.
3 → 2.4mm, lighter pre-stage reduction conditions (previous 4 stages cumulative reduction rate:
45%). If the plate temperature is likely to drop from the target temperature in the middle of the rolling pass, the temperature was recovered by appropriately holding it in a heat retention furnace. After the rolling, it was placed in a furnace at 550 ° C and cooled in the furnace.

Each hot-rolled coil thus obtained was subjected to hot-rolled sheet annealing at 1000 ° C. for 45 seconds, pickled, and then 1.8 mm in a tandem rolling mill.
Cold rolled to thickness. Then, as an intermediate annealing, 1100 ℃
The temperature was raised at a heating rate of 15 ° C./s, then held for 30 seconds and sprayed with mist water to quench. At this time, the surface of the steel sheet was decarburized and annealed to reduce the C content by 0.020%. After that, pickling is carried out again, and a Zenzimer rolling mill is used for warm rolling where the maximum temperature of the stand exit side is 250 ° C and for 10 to 150 ° C to 230 ° C.
It was aged for 40 minutes between passes and rolled to a final thickness of 0.22 mm. Furthermore, the surface of these rolled plates has a depth of 25 μm and a width of 50 μm.
Grooves extending in the direction of 80 ° with respect to the rolling direction of μm were uniformly formed on the surface of the steel sheet by electrolytic polishing at a pitch of 3 mm in the rolling direction.

Furthermore, these coils are degreased and then
After decarburization annealing at 820 ° C. for 2 minutes, MgO containing 7% TiO ₂ and 2% strontium sulfate was applied to the coil surface as an annealing separator, and subjected to final finish annealing. The final finishing annealing conditions were a temperature rising rate of 35 ° C / h, an atmosphere of N ₂ alone until the temperature was raised to 900 ° C, and then 30% N ₂ and 70% until 1150 ° C.
A mixed atmosphere of H _2, and a condition for 5 hours after heating to 1180 ° C. in the subsequent H ₂ alone atmosphere. After the final finish annealing, the unreacted separating agent was removed.

These coils were further coated with an insulating coating containing aluminum phosphate containing 60% of colloidal silica as a main component and baked at 800 ° C.
(Factory test material) and laboratory test material. Epstein-sized test pieces were cut out from each product and laboratory test material along the rolling direction, subjected to strain relief annealing at 800 ° C for 3 hours, and then the iron loss value W _17/50 and magnetic flux at a magnetic flux density of _{1.7 T} were measured. The density B ₈ was measured. The results obtained are shown in Table 1. In addition, the occurrence rate of surface defects in product coils and laboratory test materials
Figure 1 shows the relationship with the flow rate of molten steel when Bi is added.

[0020]

[Table 1]

From Table 1, the molten steel flow having a flow velocity of 70 g / (s · cm ² ) or more, that is, the molten steel flow from the exit hole at the time of tapping (condition 1 and condition 2), the degassed reflux molten steel flow (condition 3) ), And the molten steel flow from the ladle lower nozzle (condition 4, condition 5 and condition 6) were charged with Bi, and the 4th-stage cumulative rolling reduction in the hot rolling finish rolling was performed under the condition of 75% or more. It can be seen that the magnetic properties of the steel sheet are excellent.

Further, from FIG. 1, under the condition that Bi is introduced into the molten steel flow having a flow rate of 70 g / (s · cm ² ) or more and the front four-stage cumulative reduction ratio in the finish rolling of hot rolling is 75% or more. It can be seen that in the steel sheet treated with, the surface defect occurrence rate of the final product is significantly reduced and the surface quality is improved. As described above, it is understood that the technique of regulating the molten steel flow velocity and the regulation of the 4th-stage cumulative reduction ratio before finish rolling simultaneously realize the improvement of the magnetic properties and the surface properties of the steel sheet.

The present invention has been completed by further studies based on the above findings. That is, the gist of the present invention is as follows. (1)% by mass, C: 0.01 to 0.15%, Si: 2.5 to 5.5
%, Mn: 0.03 to 3.0%, Bi: 0.002 to 0.08%,
A molten steel having a composition in which the balance consists of Fe and unavoidable impurities is formed into a slab by continuous casting, and the slab is hot-rolled to form a hot-rolled sheet, or the hot-rolled sheet is further annealed, or once or A high magnetic flux density direction in which cold rolling is performed two or more times with intermediate annealing between them to obtain a cold-rolled sheet having a final thickness, and then the cold-rolled sheet is subjected to primary recrystallization annealing and final finishing annealing to obtain a cold-rolled annealed sheet. In the method for producing a magnetic electrical steel sheet, Bi of the molten steel is contained by introducing a Bi-containing object into the molten steel having a flow rate of 70 g / (s · cm ² ) or more, and the hot rolling is performed.
Surface rolling characterized by finishing rolling with a tandem mill in a temperature range of 800 to 1150 ° C., and a cumulative rolling reduction of the preceding four stages up to the first to fourth stands in the finishing rolling being 70 to 95% A method for producing a high magnetic flux density grain-oriented electrical steel sheet having excellent properties. (2) In (1), in addition to the above composition, the molten steel further contains, by mass%, Al: 0.010 to 0.030% and B: 0.0010 to.
One of 0.0050% or the total of Al and B is 0.0010-0.
A method for producing a high magnetic flux density grain-oriented electrical steel sheet having excellent surface properties, which is a molten steel having a composition containing 030% and N: 0.003 to 0.012%. (3) In (1) or (2), in addition to the above composition, one or two kinds of S and Se in total of 0.010 to 0.030% by mass% and / or Sb: 0.005 to.
1.5%, Sn: 0.005-1.5%, Cr: 0.005-1.5%,
P: 0.005-1.5%, Ge: 0.005-1.5%, Ni: 0.005
To 1.5%, Cu: 0.005 to 1.5%, Mo: 0.005 to 1.5%, and a composition containing one or more selected from high flux density directionality with excellent surface properties. Manufacturing method of electrical steel sheet. (4) In any one of (1) to (3), the Bi
A method for producing a high magnetic flux density grain-oriented electrical steel sheet having excellent surface properties, characterized in that the contained object is a Bi-containing object having an average weight of 50 g / piece or less.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, C: 0.01% by mass.
~ 0.15%, Si: 2.5-5.5%, Mn: 0.03-3.0%, Bi:
Contains 0.002-0.08%, or Al: 0.010-
0.030%, B: One of 0.0010 to 0.0050% or Al
And B total 0.0010 to 0.030% and N: 0.003 to 0.01
A composition containing 2% and the balance being Fe and inevitable impurities, or in addition to the above composition, one or two kinds of S and Se in total of 0.010 to 0.030%, and / or
Or Sb: 0.005 to 1.5%, Sn: 0.005 to 1.5%, C
r: 0.005 to 1.5%, P: 0.005 to 1.5%, Ge: 0.005
~ 1.5%, Ni: 0.005-1.5%, Cu: 0.005-1.5%,
Mo: A molten steel having a composition containing one or more selected from 0.005 to 1.5% is made into a slab by continuous casting.

First, the reasons for limiting the composition of molten steel will be described. In addition, mass% in a composition is only described as%. C: 0.01 to 0.15% If the C content is less than 0.01%, the crystal structure becomes coarse during slab heating prior to hot rolling, or the crystal structure becomes insufficiently fragmented due to transformation during hot rolling, resulting in secondary re-rolling. Causes crystal defects. On the other hand, if the content exceeds 0.15%, decarburization in the decarburization annealing process tends to be insufficient, resulting in deterioration of magnetic properties. Therefore, C is limited to the range of 0.01 to 0.15%.

Si: 2.5-5.5% If the Si content is less than 2.5%, the electric resistance will be insufficient and the iron loss will be deteriorated. On the other hand, if the content exceeds 5.5%, the frequency of plate breakage in the cold rolling process increases and the yield of the product decreases. Therefore, Si is limited to the range of 2.5 to 5.5%. Mn: 0.03 to 3.0% If the Mn content is less than 0.03%, the steel sheet is broken during hot rolling. On the other hand, if the content exceeds 3.0%, secondary recrystallization failure occurs during final finish annealing, deteriorating magnetic properties. Therefore, Mn is limited to the range of 0.03 to 3.0%.

Bi: 0.002 to 0.08% Bi is an element that has been known for a long time as an element having an action of suppressing the movement of crystal grain boundaries, and is essential in the present invention. If the Bi content is less than 0.002%, the above effect cannot be expected. On the other hand, if the content exceeds 0.08%, Bi cannot be finely dispersed even by the method of the present invention. Therefore, Bi is limited to the range of 0.002 to 0.08%. The action of Bi as an inhibitor is AlN, BN, Mn
When it coexists with an inhibitor such as S or MnSe, its action is strengthened, which is particularly preferable.

In addition to the above composition, it is possible to further contain an inhibitor component. As the inhibitor, AlN or BN is advantageous for achieving a high magnetic flux density, and for this reason, Al: 0.010 to 0.030%, B: 0.
One of 0010 to 0.0050%, or Al and B in total of 0.
It is preferable to contain 0010 to 0.030% and N: 0.003 to 0.012%.

In the case of Al alone, 0.010% or more of Al must be contained in order to act as the above-mentioned inhibitor. On the other hand, when Al is contained in excess of 0.030%, the secondary recrystallization temperature becomes too high, causing secondary recrystallization failure and deteriorating magnetic properties. Therefore, when Al is used alone, it is preferable that the Al content be 0.010 to 0.030%.

In the case of B alone, 0.0010% or more of B must be contained in order to act as the above-mentioned inhibitor. On the other hand, if the content exceeds 0.0050%, the BN is coarsened, and conversely, the suppressing force against the grain boundary movement is deteriorated, and the magnetic properties are deteriorated. Therefore, when B is used alone, B is preferably in the range of 0.0010 to 0.0050%. When both Al and B are contained, the total amount is 0.00
It is preferably 10 to 0.030%. In order to sufficiently maintain the restraint against the grain boundary migration, the total content of Al and B must be 0.0010% or more, but if it exceeds 0.030%, the secondary recrystallization temperature rises, This leads to poor magnetic properties.

N is one of the basic constituent elements of the inhibitor, but N can be contained in the steel sheet by nitriding during the manufacturing process, and 0.003% or more is sufficient at the molten steel stage. On the other hand, if the content exceeds 0.012%,
N ₂ will be present as bubbles in the steel, causing new surface defects. Therefore, N is 0.003 ~
It is preferable to limit it to the range of 0.012%.

If necessary, in addition to the above-mentioned compositions, as an MnS or MnSe inhibitor element,
One or two of S and Se in total of 0.010 to 0.030
% Can also be contained. By containing S and Se, more stable magnetic properties can be obtained. If the total of one or two of S and Se is less than 0.010%, the above effect cannot be expected. On the other hand, if the content exceeds 0.030%, S and Se remain in the steel even after purification by final finish annealing, resulting in deterioration of magnetic properties. Therefore, it is preferable that the total of one or two of S and Se is within the range of 0.010 to 0.030%.

If necessary, in addition to the above compositions, Sb: 0.005 to 1.5%, Sn: 0.005 to 1.5%,
Cr: 0.005-1.5%, P: 0.005-1.5%, Ge: 0.005
It is possible to contain one kind or two kinds or more selected from the range of up to 1.5%. Sb, Sn, P, Ge and Cr are elements that have an auxiliary function of strengthening the inhibitory force as an inhibitor, and are preferably contained in 0.005% or more each. On the other hand, if the content of each exceeds 1.5%, the rollability of the steel sheet is impaired and the magnetic properties are deteriorated.

If necessary, in addition to the above compositions, Ni: 0.005 to 1.5%, Cu: 0.005 to 1.5%,
Mo: One or more selected from 0.005 to 1.5% can be contained. Cu, Mo, and Ni are elements that have the effect of strengthening the inhibitor and improving the structure.
It is preferable to contain 0.005% or more. On the other hand, 1.5 each
If the content exceeds%, rollability of the steel sheet is impaired and magnetic properties are deteriorated.

The remaining molten steel other than the above-mentioned components is Fe and inevitable impurities. In the present invention, a Bi-containing body (Bi-containing body) is charged into molten steel having a flow rate of 70 g / (s · cm ² ) or more to obtain the molten steel having the above Bi content. The method of adding Bi to the molten steel is one of the major features of the present invention. By introducing the Bi-containing substance into the molten steel having a high flow rate of 70 g / (s · cm ² ) or more, the injected Bi-containing substance is dissolved and decomposed by the molten steel, and the droplets of Bi are finely dispersed in the molten steel. . As a result, Bi does not remain in the form of liquid or bubbles during hot rolling, and the occurrence of surface defects can be suppressed. The flow rate of molten steel is 70g / (s ・
If it is less than cm ² ), the above effect cannot be observed.

The flow velocity of the molten steel is measured directly, or when it is difficult to measure the molten steel directly, the weight of the molten steel per unit time of the molten steel flow flowing and passing at the place where Bi is introduced into the molten steel is defined as the location. It shall be calculated from the value divided by the minimum value of the cross-sectional area for the flow of the molten steel flow in. In addition, the weight of each Bi-containing object to be added to the molten steel is 50 g / on average.
It is preferable that the number is not more than the number. Dispersion in molten steel by setting the weight per Bi-containing object to 50 g or less
The droplet size of Bi becomes smaller and the amount of dissolution in molten steel becomes larger. As a result, Bi does not remain in the form of liquid or bubbles during hot rolling, and surface defects can be further reduced. On the other hand, when the weight of each Bi-containing object exceeds 50 g, the droplet size of Bi becomes large, and as a result, it tends to remain in the form of liquid or bubbles during hot rolling, causing surface defects. Increase the likelihood of. In the present invention, "Bi-containing object"
Includes granular or powdered Bi alloy such as pure Bi or Sb-Bi, and mixed grains of Bi and Sn. In the case of powder, it is desirable to perform various measures such as a gas blowing method so as to improve the fluidity of the powder at the time of charging.

The molten steel whose composition has been adjusted by the above-mentioned method is made into a slab by the continuous casting method, and then hot-rolled to obtain a hot-rolled sheet. During hot rolling, the slab is usually heated to a high temperature of 1350 ° C or higher due to the solid solution of the inhibitor in the steel. When the content of S or Se is less than 0.01%, it is possible to heat at a temperature of less than 1350 ° C.
In the hot rolling, known techniques such as thickness reduction treatment and width reduction treatment for homogenizing the structure before and after heating the slab can be added at any time. Further, when the slab is induction-heated, it is possible to raise the temperature to a high temperature of 1400 ° C. or higher in a very short time of 15 to 30 minutes.

The hot rolling is composed of rough rolling and finish rolling with a tandem mill having four or more stands. In the present invention, the temperature range for finish rolling is 800 to 11
It shall be in the range of 50 ℃. When finish rolling is performed at a temperature of less than 800 ° C, AIN and BN are coarsely precipitated during hot rolling, and recrystallization during rolling is delayed to deteriorate the steel structure and deteriorate magnetic properties. On the other hand, if the steel sheet is rolled at a temperature higher than 1150 ° C, the deformation resistance of the steel sheet is small,
Shear stress acting on the grains is weakened. Therefore, the fineness and dispersion of Bi during hot rolling become insufficient, and coarse air bubbles or agglomerates of liquid Bi are present in the surface layer of the steel sheet, increasing the surface defects of the product. Therefore, in the present invention, the temperature range of finish rolling in hot rolling is set to a temperature range of 800 to 1150 ° C.

Further, in the present invention, the rolling reduction at the time of finish rolling is 70-95% as the cumulative rolling reduction of the preceding four stages up to the first to fourth stands (hereinafter also referred to as the cumulative rolling reduction of the preceding four stages). To do. If the previous 4th-stage cumulative rolling reduction is less than 70%, Bi
Is not sufficiently miniaturized, and coarse air bubbles or agglomerates of liquid Bi are present in the surface layer of the steel sheet, and surface defects of the product increase. On the other hand, if the 4th-stage cumulative rolling reduction exceeds 95%,
Excessive stress is applied to the steel sheet, and surface defects such as cracks increase. Therefore, the reduction conditions in the finish rolling of the hot rolling should be the same as those before the first to fourth stands.
The cumulative rolling reduction of the step is 75 to 95%. In the latter half rolling after the fifth stand of finish rolling, the Bi refining ability is weak, and it is not necessary to control the reduction rate for the above purpose.

The hot-rolled sheet is preferably annealed and then cold-rolled once to obtain a cold-rolled sheet having a final thickness. Note that the hot-rolled sheet annealing may be omitted or the temperature may be lowered, and cold rolling may be performed twice or more with the intermediate annealing interposed therebetween. It is also possible to adopt a three-time cold rolling method in which cold rolling is performed three times to form a cold rolled sheet. As for the final reduction rate of cold rolling, as conventionally known, when Al or B is contained as an inhibitor component, the final reduction rate of final cold rolling is preferably in the range of 80 to 95%. When S or Se is contained as an inhibitor component, the final cold rolling reduction ratio (final reduction ratio) is 45 to 75%.
It is preferable to set it as the range.

In the present invention, cold rolling may be warm rolling as well known, and interpass aging may be performed. This further improves the magnetic characteristics. After the final cold rolling, it is possible to provide linear grooves on the surface of the steel sheet for subdividing the magnetic domains, as is known. The steel sheet formed into a cold rolled sheet by such a method is
After the subsequent recrystallization annealing, an annealing separator containing MgO as a component or an annealing separator for suppressing film formation is applied to the surface of the steel sheet and then subjected to final finish annealing. Further, it is also possible to perform a known nitriding treatment during the decarburization annealing or after the decarburization annealing.

After the final finish annealing, an insulating coating is applied and baked if necessary, and further flattening annealing is performed to obtain a product. In order to reduce iron loss, the flattened and annealed steel sheet should be subjected to linear plasma jet or laser irradiation as a well-known magnetic domain refining treatment, or to be provided with linear depressions by a projection roll. You can also

[0043]

EXAMPLES The present invention will be described in detail below based on examples. (Example 1) Molten steel having the composition shown in Table 2 (Steel No. A to N)
o.M) is poured from the ladle into the tundish, from the tundish into the mold, and cast by the continuous casting method.
It was a 50mm slab. At this time, half of the molten steel was continuously charged with pure Bi metal particles (average weight 15 g / piece) into the molten steel discharged from the ladle lower nozzle at a molten steel flow rate of 562 g / (s · cm ² ). Pure Bi metal particles (average weight: 15 g / piece) were continuously charged into a tundish molten steel with a molten steel flow rate of 30.5 g / (s · cm ² ).

Thereafter, these slabs were held at a temperature of 180 ° C. for 3 hours and then subjected to width reduction for reducing the width of the slab by 40 mm. Next, the thickness was reduced to 230 mm, and the mixture was charged into an induction heating furnace, heated to 1410 ° C and soaked for 10 minutes. After soaking, hot rolling was started. Rough rolling of hot rolling is a sheet bar having a thickness of 35 mm in the temperature range of 1250 to 1200 ° C., and then the sheet bar is subjected to finish rolling in the temperature range of 1100 to 950 ° C. and 1.8 mm.
A thick hot rolled sheet was used. The pass schedule for finish rolling is
35 → 24 → 15 → 7.5 → 5.2 → 3.0 → 2.4 → 1.8mm (4th stage cumulative rolling reduction: 85%). The finish rolling finish temperature was 980 to 950 ° C. For hot rolled sheet after finishing rolling,
By jetting a jet water jet, it was rapidly cooled at a cooling rate of 55 to 65 ° C / s and wound into a coil at a temperature of 600 to 630 ° C.

Then, these hot-rolled sheets (coils) were annealed by hot-rolled sheet heating at a heating rate of 12 ° C./s up to 1100 ° C.
After holding at 1100 ° C. for 40 seconds, it was subjected to a treatment of rapidly cooling with mist water at a cooling rate of 35 ° C./s. The atmosphere for hot-rolled sheet annealing was 50% N ₂ and H ₂ gas with a dew point of 50 ° C., and 0.012 mass% C was decarburized from the surface layer of the steel sheet. After that, each hot-rolled sheet (coil) is pickled, and then warm-rolled with a Sendzimer rolling mill at a rolling pass outlet temperature of 170 to 250 ° C and a pass outlet temperature of 220 ° C or higher for at least two passes. The cold-rolled sheet had a final sheet thickness of 0.27 mm.

After degreasing these cold-rolled sheets (coils),
Decarburization annealing was performed at 2 ° C. for 2 minutes. Next, MgO containing 5% TiO ₂ was applied to the coil surface as an annealing separator, and N
Up to 850 ° C. in ₂ alone atmosphere and held for 15 hours at a temperature of heated then 850 ° C., then to 1150 ° C. and 25% N ₂ 75
% H ₂ mixed atmosphere, then H ₂ alone atmosphere was heated up to 1200 ° C., and then final finish annealing was carried out for 5 hours. After the final finish annealing, the unreacted separating agent was removed.

An insulating coating containing magnesium phosphate containing 50% of colloidal silica as a main component was applied to these cold-rolled sheets (coils) and baked at 800 ° C. to obtain products (oriented electrical steel sheets). Epstein test pieces with a size of 30 × 280 mm were cut out from each product along the rolling direction, subjected to strain relief annealing, and then the iron loss value W _17/50 and the magnetic flux density B ₈ at a magnetic flux density of _{1.7 T} were measured. It was measured.
Further, the appearance of each coil (product) was visually observed over the entire length to determine the occurrence rate of surface defects.

The results obtained are summarized in Table 3.

[0049]

[Table 2]

[0050]

[Table 3]

From Table 3, it can be seen that all of the examples of the present invention are grain-oriented electrical steel sheets excellent not only in magnetic properties but also in surface appearance. In comparative examples outside the scope of the present invention, surface defects frequently occur and surface appearance properties are deteriorated. (Example 2) Molten steel melted in a converter, tapped in a ladle, and subjected to RH degassing to have the molten steel (steel N
o.N to No. P) were poured from a ladle into a tundish and from the tundish into a mold, and cast by a continuous casting method to form a slab having a thickness of 250 mm. In addition, pure Bi particles (particle size 10 mm,
The average weight: 1.5 g / piece) was continuously charged to the steel output flow (molten steel flow rate: 270 g / (s · cm ² )) from the converter.

The obtained slab was heated to 1200 ° C. in a gas furnace, then charged into an induction heating furnace, heated to 1420 ° C. and soaked for 10 minutes. After soaking, hot rolling was started. Hot rolled 12
In the temperature range of 50 to 1200 ° C, rough rolling was performed to make a 45 mm thick sheet bar, and finish rolling was performed to roll the sheet bar into a hot rolled sheet having a thickness of 2.0 mm. The pass schedule for finish rolling is 45 → 32 →
20 → 14 → 9.0 → 5.5 → 3.2 → 2.0mm 1st stage deep reduction rolling (4th stage cumulative reduction: 80%). In finish rolling, the slab of each steel is divided into three, and 1/3 of the slab is rolled at a rolling start temperature of 1250 to 1200 ° C and a rolling finish temperature of 1120.
Rolling in the temperature range of ~ 1070 ℃ (high temperature rolling) 1
Rolling start temperature of finishing rolling of slab of / 3: 1140-9
Rolling is performed in a temperature range of 80 ° C and rolling end temperature: 960 to 840 ° C (medium temperature region rolling), and the remaining 1/3 of the slab is rolled starting temperature: 980 to 920 ° C, rolling ending temperature: 740 to 790 ℃
Rolling in the temperature range of (low temperature rolling). Each hot rolled sheet is
After hot finishing rolling was completed, it was rapidly cooled at a cooling rate of 55 to 65 ° C / s by applying a jet water jet, and wound into a coil at a temperature of 550 to 580 ° C.

Then, these hot-rolled sheets (coils) were annealed by hot-rolled sheet heating at a heating rate of 8 ° C./s up to 1150 ° C.
After holding at 1150 ° C. for 60 seconds, it was subjected to a treatment of rapidly cooling with mist water at a cooling rate of 30 ° C./s. The atmosphere for hot-rolled sheet annealing was 50% N ₂ and H ₂ gas with a dew point of 50 ° C., and 0.008 mass% of C was decarburized from the surface layer of the steel sheet. After that, each hot-rolled sheet (coil) is pickled, and then warm-rolled with a Sendzimer rolling mill at a rolling pass outlet temperature of 170 to 250 ° C and a pass outlet temperature of 220 ° C or higher for at least two passes. It was a cold rolled sheet (coil) with a final sheet thickness of 0.30 mm.

After degreasing these cold-rolled sheets (coils),
Decarburization annealing was performed at 2 ° C. for 2 minutes. Next, MgO containing 5% TiO ₂ was applied to the coil surface as an annealing separator, and N
₂ Raise the temperature to 850 ° C in a single atmosphere and then hold it at 850 ° C for 40 hours, then 25% N ₂ and 75% up to 1150 ° C.
% H ₂ mixed atmosphere, then H ₂ alone atmosphere was heated up to 1200 ° C., and then final finish annealing was carried out for 5 hours. After the final finish annealing, the unreacted separating agent was removed.

On these cold rolled sheets (coils), an insulating coating containing magnesium phosphate containing 50% of colloidal silica as a main component was applied and baked at 800 ° C.
Further, a plasma jet was linearly irradiated on the surface of the steel sheet at intervals of 5 mm in the rolling direction to obtain a product (oriented electrical steel sheet). From each product, SST with a size of 100 × 400 mm along the rolling direction
(Single-plate magnetic test) A test piece was cut out, and an iron loss value W _17/50 and a magnetic flux density B ₈ at a magnetic flux density of _{1.7 T} were measured. In addition, the appearance of each coil (product) was visually observed over the entire length to determine the occurrence rate of surface defects. The results obtained are summarized in Table 4.

[0056]

[Table 4]

From Table 4, it can be seen that all of the examples of the present invention are grain-oriented electrical steel sheets excellent not only in magnetic properties but also in surface appearance. In comparative examples outside the scope of the present invention, surface defects frequently occur and surface appearance properties are deteriorated, or magnetic properties are deteriorated. (Example 3) Molten steel melted in a converter, tapped in a ladle, and adjusted to the composition shown in Table 2 by RH degassing (steel N
o. Q to No. S) were poured from a ladle into a tundish and from the tundish into a mold, and cast by a continuous casting method to form a slab having a thickness of 250 mm. In addition, Sb-Bi alloy particles (60ma
molten steel flow rate during RH degassing refining of ss% Bi-40 mass% Sb 7
It was continuously charged into molten steel of 10 g / (s · cm ² ). In addition, steel No. Q
In No. R and No. R, average weight: 3.4 g / piece of Sb-Bi alloy particles was used, and in Steel No. S, average weight: 68 g / piece of Sb-Bi alloy particles.

The obtained slab was heated to 1250 ° C. in a gas furnace, and then hot rolling was started. Hot rolling is 1240 ~ 1180
In the temperature range of ℃, a sheet bar with a thickness of 45 mm was rough-rolled and a sheet bar was rolled into a hot-rolled sheet with a thickness of 2.3 mm. The pass schedule for finish rolling is 45 → 34 → 23 → 17
→ 10 → 6.8 → 3.6 → 2.3mm (4th stage cumulative rolling reduction: 78%)
And The starting temperature for finish rolling is 1100 to 900 ° C.
And the finish rolling finish temperature was 920 to 900 ° C. By jetting a jet water jet to the hot-rolled sheet after finishing rolling, it is rapidly cooled at a cooling rate of 55-65 ° C / s to 550-
It was wound into a coil at a temperature of 580 ° C.

Then, these hot rolled sheets (coils) were heated to 1150 ° C. at a heating rate of 8 ° C./s as a hot rolled sheet annealing.
After holding at 1150 ° C. for 50 seconds, a treatment of rapidly cooling with mist water at a cooling rate of 30 ° C./s was performed. The atmosphere of hot-rolled sheet annealing was 50% N ₂ and H ₂ gas with a dew point of 10 ° C., and 0.002 mass% of C was decarburized from the surface of the steel sheet. After that, each hot-rolled sheet (coil) is pickled, and then warm-rolled with a Sendzimer rolling mill at a rolling pass outlet temperature of 170 to 250 ° C and a pass outlet temperature of 220 ° C or higher for at least two passes. It was a cold-rolled sheet (coil) with a final sheet thickness of 0.35 mm. After degreasing, these cold-rolled sheets (coils) were subjected to decarburization annealing at 850 ° C. for 2 minutes, and subjected to nitriding treatment to make steel contain N of 120 to 150 ppm.

Next, 10% TiO 2₂Annealing of MgO containing
Apply as a release agent to the coil surface, N ₂In a single atmosphere
 25% N up to 850 ℃, then 1150 ℃₂And 75%
H₂Mixed atmosphere, then H₂Raises to 1200 ℃ in a single atmosphere
A final finish anneal was performed, warming and then holding for 5 hours.
After the final finish annealing, the unreacted separating agent was removed. These cold
The rolled plate (coil) further contains 50% colloidal silica.
Insulating coating containing magnesium phosphate as a main component
Coating, bake at 800 ° C,
Zumajet is irradiated linearly at intervals of 5 mm in the rolling direction.
(Oriented electrical steel sheet).

From each of the obtained products, 1 along the rolling direction
An SST test piece having a size of 00 × 400 mm was cut out, and an iron loss value W _17/50 and a magnetic flux density B ₈ at a magnetic flux density of _{1.7 T} were measured. In addition, the appearance of each coil (product) was visually observed over the entire length to determine the occurrence rate of surface defects. The results obtained are summarized in Table 5.

[0062]

[Table 5]

It can be seen from Table 5 that all of the examples of the present invention are grain-oriented electrical steel sheets which are excellent not only in magnetic characteristics but also in surface appearance. In particular, it can be seen that the smaller the average weight of the Bi-containing substance (grains) added to the molten steel flow, the better the magnetic properties and surface appearance. In Comparative Examples outside the scope of the present invention, surface defects frequently occur and the surface appearance deteriorates. (Example 4) Molten steel having the composition shown in Table 2 (Steel No. A and Steel No. T to No. V) was cast by a continuous casting method to a thickness of 250 m.
It was a slab of m. When pouring molten steel into the tundish from the ladle, the molten steel flow at the ladle lower nozzle (
Pure Bi metal particles (average weight 25.3 g) were continuously charged into 10 g / (s · cm ² ).

Among the obtained slabs, the slab of steel No. A was heated to 1350 ° C. in a gas furnace, and then hot rolling was started. The hot rolling was rough rolling to make a 40 mm thick sheet bar in the temperature range of 1260 to 1220 ° C and finish rolling to make a 2.2 mm thick hot rolled sheet bar. The pass schedule for finish rolling is 40 → 32 → 22 → 16 → 10 → 6.5 → 3.5 → 2.2mm
(4th stage cumulative rolling reduction: 75%). The starting temperature of finish rolling was set to a temperature range of 1180 to 1000 ° C, and the finishing temperature of finish rolling was set to 900 to 840 ° C. By applying a jet water jet to the hot-rolled sheet after finishing rolling, 45-55 ℃ / s
It was rapidly cooled at a cooling rate of, and coiled at a temperature of 580 to 650 ° C.

Then, these hot rolled plates (coils) were
After hot-rolled sheet annealing that holds at 50 ℃ for 30 seconds, pickled,
The coil was split in two. One of the divided coils was cold-rolled to an intermediate thickness of 65 mm, the other coil was an intermediate thickness of 75 mm, and then the second cold-rolling was performed to cold-rolled steel sheet with a final thickness of 0.34 mm. And On the other hand, the steel No. T to No. V slabs were heated to 1200 ° C. in a gas furnace, and then hot rolling was started for each two slabs.

Hot rolling is 45 mm in the temperature range of 1180 to 1120 ° C.
Rough rolling was used as a thick sheet bar and finish rolling was performed by rolling the sheet bar into hot rolled sheets of 2.2 mm thickness and 1.7 mm thickness, respectively. The pass schedule for finish rolling is 45 → 34 → 23 → 17
→ 10 → 6.6 → 3.5 → 2.2mm (4th stage cumulative rolling reduction 78%),
And 45 → 33 → 20 → 15 → 9 → 6.5 → 3.2 → 1.7mm (front 4
Staged rolling reduction of 80%). The starting temperature of finish rolling was set to a temperature range of 1050 to 900 ° C, and the finishing temperature of finish rolling was set to 860 to 820 ° C. The hot-rolled sheet after finishing rolling was rapidly cooled at a cooling rate of 55 to 65 ° C / s by applying a jet water jet, and wound into a coil at a temperature of 550 to 580 ° C.

Then, these hot rolled plates (coils) were
After hot-rolled sheet annealing was carried out at 50 ° C for 50 seconds, it was rapidly cooled with mist water at a cooling rate of 30 ° C / s, and then pickled. After pickling, each hot-rolled sheet (coil) was subjected to warm rolling in a tandem rolling mill at a rolling pass outlet temperature of 80 to 200 ° C, and at least 2 passes at a pass outlet temperature of 180 ° C or higher.
It was a cold rolled plate (coil) with a final plate thickness of 34 mm.

Each of the obtained cold-rolled sheets (coils) was degreased, decarburized and annealed at 850 ° C. for 2 minutes, and then 5%.
Of MgO containing TiO ₂ as an annealing separator is applied to the coil surface, and the temperature is raised to 850 ° C. in an atmosphere of N ₂ alone, and then to 1150 ° C., a mixed atmosphere of 25% N ₂ and 75% H ₂ , After that, final finish annealing was carried out in which the atmosphere was H ₂ alone and the temperature was maintained at 1150 ° C. for 10 hours. After the final finish annealing, it was cooled to remove the unreacted separating agent.

Furthermore, an insulating coating containing magnesium phosphate containing 50% of colloidal silica as a main component was applied to each of these cold-rolled sheets (coils), and the temperature was set to 800 ° C.
It was baked into a product (oriented electrical steel sheet). Epstein test pieces of size 30 × 280 mm were cut out from each of the obtained products along the rolling direction and subjected to strain relief annealing, and then 1.
The value W _17/50 and the magnetic flux density B ₈ of iron loss in the magnetic flux density of 7T was measured. In addition, the appearance of each coil (product) was visually observed over the entire length to determine the occurrence rate of surface defects.
The obtained results are summarized in Table 6.

[0070]

[Table 6]

From Table 6, it can be seen that all of the examples of the present invention are grain-oriented electrical steel sheets which are excellent not only in magnetic properties but also in surface appearance.

[0072]

As described in detail above, according to the present invention,
It is possible to obtain a grain-oriented electrical steel sheet having very few surface defects and extremely excellent magnetic properties, which is very industrially effective.

[Brief description of drawings]

[Figure 1] Bi on molten steel, which affects the incidence of surface defects in products
It is a graph which shows the influence of the molten steel flow velocity at the time of addition, and the preceding 4th stage rolling reduction at the time of hot finish rolling.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroshi Yamaguchi             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works F-term (reference) 4K033 AA02 BA01 BA02 CA00 CA01                       CA02 CA03 CA04 CA06 CA07                       CA09 EA02 FA03 FA05 FA08                       HA01 HA03 RA04                 5E041 AA02 CA02 NN01 NN18

Claims (4)

[Claims] 1. By mass%, C: 0.01 to 0.15%, Si: 2.5 to 5.5%, Mn: 0.03 to 3.0%, Bi: 0.002 to 0.08%, and the balance being Fe and inevitable impurities. A molten steel having the composition In a method for producing a high magnetic flux density grain-oriented electrical steel sheet, which is cold-rolled to obtain a cold-rolled sheet having a final thickness, and then subjected to primary recrystallization annealing and final finish annealing to the cold-rolled sheet to obtain a cold-rolled annealed sheet, Bi of the molten steel, is contained by placing the Bi content object to a molten steel having a 70g / (s · cm ²⁾ or more flow rates, the hot rolling, the temperature range of the finish rolling by the tandem mill 800-1150 C, and the first to the first in the finish rolling
A method for producing a high magnetic flux density grain-oriented electrical steel sheet having excellent surface properties, which comprises rolling in which the cumulative rolling reduction of the preceding four stages up to the fourth stand is 70 to 95%. 2. The molten steel, in addition to the above composition, further comprises, by mass%, Al: 0.010 to 0.030% and B: 0.0010 to 0.0050%.
One of the above or Al and B in total of 0.0010 to 0.030%,
And N: 0.003 to 0.012%, which is a molten steel having a composition. A method for producing a high magnetic flux density grain-oriented electrical steel sheet having excellent surface properties according to claim 1, wherein the molten steel is a molten steel. 3. In addition to the composition, further in mass%,
One or two of S and Se in total of 0.010 to 0.030
% And / or Sb: 0.005 to 1.5%, Sn: 0.005
~ 1.5%, Cr: 0.005-1.5%, P: 0.005-1.5%,
Ge: 0.005-1.5%, Ni: 0.005-1.5%, Cu: 0.005
To 1.5%, Mo: 0.005 to 1.5%, and a composition containing one or more selected from the group consisting of Mo and 0.005 to 1.5%. 3. High magnetic flux density direction excellent in surface properties according to claim 1 or 2, characterized in that For manufacturing high-performance electrical steel sheet. 4. The Bi-containing body has an average weight of 50.
The method for producing a high magnetic flux density grain-oriented electrical steel sheet having excellent surface properties according to any one of claims 1 to 3, which is a Bi-containing object of g / piece or less.