JP5423616B2 - Method for producing non-oriented electrical steel sheet with excellent magnetic properties and method for producing cast steel strip for producing non-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a non-oriented electrical steel sheet that is used as an iron core of electrical equipment and has high magnetic flux density and low iron loss.

In recent years, in electrical equipment in which non-oriented electrical steel sheets are used as iron cores, a trend toward higher efficiency has been rapidly progressing in the global environment conservation movement such as global CO ₂ reduction and Freon gas regulation.
In the Fourth Assessment Report (AR4) of the Intergovermental Panel on Climate Change (IPCC), a neutral position is that the cause of the rapid increase in CO ₂ and global warming since the Industrial Revolution is due to human activities. From the conclusion. As a result, reduction of CO ₂ emissions has become an urgent issue in order to suppress environmental destruction caused by global warming.

  For this reason, industrial motors, which have used low-grade non-oriented electrical steel sheets, are moving to legislation to IEC unified standards in Europe, the United States, etc. In the United States, IE (Industrial Engineering) 3 In Europe, IE2 was standardized in 2011, IE3 was standardized in 2015, and IE2 was also standardized in Japan.

Against this background, there is an increasing demand for non-oriented electrical steel sheets to improve their characteristics, that is, to achieve high magnetic flux density and low iron loss.
In order to improve the efficiency of equipment using non-oriented electrical steel sheets, it is important to improve the two magnetic properties of non-oriented electrical steel sheets. That is, the magnetic flux density is improved and the iron loss is reduced.

When the magnetic flux density is increased, the magnetizing force of the iron core during use is increased, and in motors, etc., the rotational torque is improved, and the same torque can be obtained with less current. The loss due to can be reduced.
By improving the rotational torque, the size of the motor required to obtain the same output can be reduced. As a result, when the rotating machine loaded on the moving body is downsized, the energy required by the moving body can be reduced. it can. This also leads to improved efficiency.
On the other hand, when the iron loss is reduced, Joule heat loss caused by eddy current flowing in the plate surface when the iron core is excited can be reduced.

In the prior art, in order to reduce iron loss, Si that increases electric low efficiency is added to iron to reduce eddy current and iron loss.
Also, in order to improve the texture of non-oriented electrical steel sheets and increase the magnetic flux density, the crystal structure before cold rolling is coarsened, the plate surface has a hard magnetization direction, and the magnetic flux density is reduced {111} In order to suppress the development of <112> texture, hot-rolled sheet annealing has been performed, or the temperature of the rear surface of finish hot-rolling has been increased to coarsen the crystal structure of the hot-rolled steel strip.

Inclusions that hinder grain growth during finish annealing include oxides such as silica and alumina, sulfides such as manganese sulfide, and nitrides such as aluminum nitride and titanium nitride.
Among these, with respect to oxides, it is possible to sufficiently add Al, which is a strong deoxidizing element, to take sufficient time to float the oxides, and to remove the oxides and make them harmless at the molten steel stage.
Regarding sulfides, there is a prior art in which desulfurization is performed by addition of Ca or REM described below.

For example, Patent Document 1 discloses a method of obtaining low iron loss by forming CaS and reducing sulfides by adjusting the steel composition to Ca and S having a suitable upper limit.
As described above, the sulfide is highly purified at the molten steel stage as described above, and fine and harmful TiN is added to the REM oxysulfide by adding REM as a desulfurization element as in Patent Document 2, Patent Document 3, and Patent Document 4. A method of fixing by composite precipitation is disclosed.
Regarding nitrides, Patent Documents 5 and 6 disclose a method of fixing N as coarse inclusions by adding B.

On the other hand, there is a CSP (Compact Strip Process) as a method of saving the equipment cost and reducing the process by producing a product close to the final product. Here, CSP is equivalent to a hot-rolled steel strip after steelmaking treatment of molten steel without going through the processes of steelmaking, continuous casting, rough rolling, and finish hot-rolling as in conventional hot-rolled steel strip manufacturing equipment. The equipment that directly obtains the cast steel strip is shown.
As a conventional technique of CSP, a twin roll method in which molten steel is poured between a pair of rolls and the molten steel is solidified from the gaps is put into practical use.
A cast steel strip made of CSP generally has a problem in cold rolling stability such that the crystal structure is likely to be coarsened, and stable plate-passability cannot be obtained during subsequent cold rolling.
For this reason, methods such as rolling down with a twin roll at the solidified contact portion of the twin roll have also been taken, but in the case of a highly pure non-oriented electrical steel sheet, crystal grain growth is fast and cold rolling stability is not always sufficient. There were issues that could not be improved.

JP 2001-271147 A JP 51-62115 A JP 56-102550 A Japanese Patent No. 3037878 Japanese Patent Publication No.58-1172 Japanese Patent Publication No.59-20731

With conventional technologies, when trying to manufacture non-oriented electrical steel sheets with high magnetic flux density and low iron loss, high-pure steel is produced at high cost by steelmaking, or hot-rolled sheet annealing is expensive for hot-rolled steel strips. It was necessary to give. On the other hand, when a thin slab is cast by CSP, the cost can be reduced and a crystal structure similar to that of hot-rolled sheet annealing can be obtained. Further, by appropriately controlling REM, O, and S, REM ₂ There is a merit that iron loss can be reduced by precipitating TiN on O ₂ S and preventing the formation of fine TiN to promote grain growth during finish annealing, but as described above, the cleanliness of steel Therefore, there is a problem in the stability of cold rolling such that the crystal structure of the high-temperature strip immediately after casting becomes coarse and the subsequent cold rolling does not provide a stable sheeting property.

As a means of controlling the crystal structure of the cast steel strip that grows until the crystal structure poses a problem in the stability of cold rolling due to an increase in the cleanliness of the steel, casting is performed by subjecting the hot strip immediately after casting to appropriate hot working It is intended to control the crystal structure of the steel strip.
The gist of the present invention is as follows.

(1) By mass% in steel,
0.3 ≦ Si ≦ 3.5,
0.15 ≦ Mn ≦ 1.5,
And the mass ppm in steel,
C ≦ 50,
S ≦ 40,
N ≦ 50,
Ti ≦ 50,
10 ≦ REM ≦ 500,
20 ≦ O ≦ 50
Containing the balance Fe and unavoidable impurities, and casting this by a twin roll casting method,
The strip immediately after casting is rolled at a rolling temperature of 900 ° C. or higher and 1100 ° C. or lower with a roll provided immediately below the twin rolls to give a reduction rate of 30% or more and 50% or less to obtain a cast steel strip,
A method for producing a non-oriented electrical steel sheet, wherein the cast steel strip is cold-rolled once and subjected to finish annealing.
(2) Furthermore, in steel,
0.3 ≦ Al ≦ 2.5
The method for producing a non-oriented electrical steel sheet according to (1) above, characterized by comprising:
(3) The method for producing a non-oriented electrical steel sheet according to (1) or (2), wherein a rolling rate of the cold rolling is 55% or more and 85% or less.
(4) The steel described in (1) or (2) above is melted and cast by a twin roll casting method, and the strip immediately after casting is rolled at a rolling temperature of 900 ° C. or higher and 1100 ° C. or lower at a twin roll. A method for producing a cast steel strip for producing a non-oriented electrical steel sheet, comprising rolling at a rolling reduction of 30% to 50% with a roll provided directly below.

  ADVANTAGE OF THE INVENTION According to this invention, there exists an effect which improves the subject of the cold rolling stability of the cast steel strip generate | occur | produced when casting the high purity steel by REM addition by the twin roll casting method.

It is a figure which shows the twin roll casting machine which attached DRM (Direct Rolling Mill).

  First, the components of steel used in the present invention will be described. In addition,% of element content and ppm mean mass% and mass ppm, respectively.

In the present invention, Si is added in the range of 0.3% to 3.5% in order to ensure electrical resistivity. If it is less than 0.3%, the effect of reducing iron loss is insufficient, and if it exceeds 3.5%, cold rolling becomes difficult.
Al is not necessarily added in the present invention, but when added, Al is added in the range of 0.3% to 2.5% in order to ensure electrical resistivity. If it is less than 0.3%, the effect of reducing iron loss is insufficient, and if it exceeds 2.5%, cold rolling becomes difficult.

  In the present invention, Mn is added in the range of 0.15% to 1.5% in order to improve electrical resistivity and hot brittleness during casting. If it is less than 0.15%, the iron loss reduction effect is insufficient, and if it exceeds 1.5%, the effect is saturated.

  C, S, N, and Ti, which are inevitable impurities in the steel, promote reduction of iron loss by reducing them to the following ranges.

If the C content increases, magnetic aging occurs during use as an iron core and iron loss increases, so C is set to 50 ppm or less.
If the S content is increased, the B-based inclusions of sulfides are increased to significantly hinder crystal grain growth and the iron loss is deteriorated, so S is set to 40 ppm or less.
If the N content increases, nitrides increase, which prevents grain growth and deteriorates iron loss. Therefore, N is set to 50 ppm or less.
When Ti content increases, compounds such as TiN increase and iron loss deteriorates, so Ti is set to 50 ppm or less.

In the present invention, fine TiN is complex-deposited on REM ₂ O ₂ S to improve the cleanliness of the steel. Therefore, REM is an important element. REM combines with S and O to form coarse oxysulfide inclusions. After REM becomes an oxysulfide, it becomes the core of precipitates such as TiN and scavenging the steel to reduce fine precipitates in the steel, remarkably promote grain growth immediately after casting and during final annealing, One or two or more of them are added in an amount of 10 ppm or more and 500 ppm or less because they are effective in improving iron loss.
If it is less than 10 ppm, the effect is not sufficient, and if it exceeds 500 ppm, the effect is saturated.
Here, REM is a generic name for a total of 17 elements including 15 elements of lanthanum having an atomic number of 57 to 15 elements of lutesium having an atomic number of 57 plus yttrium having an atomic number of 21 and yttrium having an atomic number of 39.

  O is combined with REM and S to form coarse oxysulfide inclusions, and becomes a nucleus of precipitates such as TiN, scavenging, and reduces fine precipitates harmful to magnetism in steel. . If the addition amount is less than 20 ppm, the effect is insufficient, and if it is more than 50 ppm, an excessive oxide-based precipitate is deposited, which is harmful to the magnetism. Therefore, the range of O is set to 20 ppm or more and 50 ppm or less.

Next, process conditions for manufacturing the non-oriented electrical steel sheet will be described.
The feature of the present invention is to improve the cold rolling property of the cast steel strip by the twin roll casting method which has a coarse structure and has many problems in the stability of cold rolling. For this reason, the main focus of the technology is to obtain a cast steel strip with improved cold rolling stability by performing controlled light reduction immediately after casting by twin rolls, improving the structure of the strip.

First, steel having the above composition is cast into a strip by a twin roll casting method. It is an apparatus feature of the present invention that there is a roll directly under the twin rolls that can immediately roll down the cast strip. Hereinafter, this reduction device is referred to as DRM (Direct Rolling Mill).
Further, the steel strip in the section from the twin-roll caster to this DRM is called a strip, and the steel strip after receiving the reduction by DRM is called a cast steel strip.
FIG. 1 shows a schematic diagram of a twin roll casting machine used in the present invention.

In DRM, a cast strip is rolled at a temperature of 900 ° C. or higher and 1100 ° C. or lower. If this temperature is less than 900 ° C., the crystal grain growth of the strip after rolling is insufficient, and the magnetic properties of the non-oriented electrical steel sheet after cold rolling and annealing are not good, so the temperature is set to 900 ° C. or higher.
Further, if the temperature exceeds 1100 ° C., there remains a problem in the stability of cold rolling of a cast steel strip obtained by recrystallization after rolling by DRM, and then the grain growth remarkably proceeds. For this reason, since the effect of rolling by DRM is lost, it is set to 1100 ° C. or lower.

Strip reduction by DRM is performed within a range of a reduction rate of 30% to 50%.
If the reduction ratio is less than 30%, recrystallization does not occur after the strip is reduced with DRM. Therefore, the effect of the present invention is not expected. In addition, when the rolling temperature is low, strain-induced grain growth occurs and huge crystals are formed, and the rolling stability is lowered. For the above reasons, the strip reduction by DRM is determined to be 30% or more.
On the other hand, if the rolling reduction ratio of the strip by DRM exceeds 50%, the recrystallized structure after rolling becomes too fine, and the grain growth occurs until the strip is cooled to the final cast steel strip after receiving the rolling by DRM. Becomes insufficient, and the magnetic properties after cold rolling and annealing deteriorate. For the above reasons, the strip reduction by DRM is determined to be 50% or less.

The roll material in DRM, the drive mechanism of the roll, etc. may be a mechanism having a known rolling function.
The cast steel strip that has undergone rolling by DRM is cooled by a known method, or is wound without being cooled. The winding method may be a known method.

  In the present invention, a cast steel strip is manufactured under the above conditions. Thereby, as shown in the below-mentioned Example, the result of the repeated bending test of a cast steel strip improves. As a result, the handleability of the steel strip is improved, the plate-passability in cold rolling is stabilized, and the problems with the conventional CSP are improved.

In the present invention, in particular, when high-purity steel to which REM is added is subjected to 30-50% reduction by DRM to the strip immediately after casting, the reason why the results of the repeated bending test of the cast steel strip are improved, It is inferred as follows.
That is, the twin roll cast slab made of high purity steel by adding REM has a remarkable crystal grain growth after casting, and in some cases, has a coarse crystal structure such that one grain penetrates the plate thickness. When this is subjected to a repeated bending test, twins are likely to be generated particularly in a high Si material, which is likely to be a starting point of a crack. The same applies to cold rolling, and cracks are likely to occur because there is little freedom of dislocation slipping during processing. Five or more independent crystal systems are required for free deformation, and coarse crystals generate a transition zone to ensure the degree of freedom of deformation.
By appropriately adjusting the rolling by DRM, it is presumed that the crystal grain size of the cast steel strip of high-purity steel is appropriately adjusted, the handling properties thereafter are improved, and the results of the repeated bending test are improved.

The cast steel strip after being reduced by DRM is subsequently cold-rolled and finish-annealed. The conditions at that time may be those employed in the production of ordinary non-oriented electrical steel sheets, but from the viewpoint of magnetic properties, the cold rolling rate should be 55% or more and 85% or less.
When the cold rolling rate is less than 55%, the development of the GOSS orientation that improves the magnetic flux density is suppressed and the magnetic flux density is lowered. On the other hand, if the cold rolling rate exceeds 85%, a gamma fiber texture unfavorable for magnetic properties develops and the magnetic flux density decreases. From the viewpoint of developing the GOSS orientation, the cold rolling rate is more preferably 60% or more and 80 or less.

  Hereinafter, the present invention will be further described by examples.

Steels having the components shown in Table 1 were melted, and strips having a predetermined thickness were cast by a twin roll casting method.
The strip is subjected to reduction at a predetermined reduction ratio (DRM%) by a roll for direct hot rolling (DRM; Direct Rolling Mill) provided directly below the twin roll drum, and a cast steel strip having a predetermined thickness (HGmm). Finished.
Next, the obtained strip was immersed in oil at a predetermined temperature, and a bending test was repeatedly performed by a method defined in JISC2550. If the repeated bending test exceeds 5 times, it becomes an index for obtaining stable plate-passability in cold rolling. The results are shown in Table 2.
From Table 2, when the rolling reduction of DRM is 30% or more, the number of repeated bending tests at 25 ° C. exceeds 5 times, and the stability of the plate in cold rolling is excellent. There was no problem with rolling.
As described above, the casting immediately after casting is rolled at a rolling temperature of 900 ° C. or higher and 1100 ° C. or lower, and rolled at a rolling reduction of 30% or more and 50% or less by a roll provided with a rolling ratio of 30% to 50%. It can be seen that a steel strip is obtained.

Steels having the components shown in Table 3 were melted and finished into strips having a predetermined thickness by a twin roll casting method. Next, a direct hot rolling (DRM) roll provided directly below the twin roll drum is used to directly reduce the 1030 ° C. strip immediately after casting at a reduction rate (DRM%) of 35%, and a temperature range of 930 ° C. to 980 ° C. And finished to a cast steel strip having a predetermined thickness (HG mm).
Further, as a comparative example, a 200 mm thick continuous cast slab manufactured from steel 45 to steel 52 by a known method is heated at 1150 ° C. to finish a 40 mm rough bar, and has a predetermined thickness (HGmm) at a hot rolling end temperature of 880 ° C. Finished in hot rolled sheet.
These hot-rolled sheets were subjected to repeated bending tests. As a result, the number of bendings was 5 or more at any temperature, and there was no problem even in actual cold rolling.

As described above, the casting immediately after casting is rolled at a rolling temperature of 900 ° C. or higher and 1100 ° C. or lower, and rolled at a rolling reduction of 30% or more and 50% or less by a roll provided with a rolling ratio of 30% to 50%. It can be seen that a steel strip is obtained.
The cast steel strip or hot-rolled sheet obtained by these processes was finished to a sheet thickness of 0.35 mm by cold rolling, subjected to finish annealing at 950 ° C. for 30 seconds, and magnetic measurements were performed.
The process conditions and magnetic measurement results are shown together in Table 3. From Table 3, it can be seen that excellent magnetic properties can be obtained at the cold rolling rate specified in the present invention by the process specified in the present invention.

Steels having the components shown in Table 4 were melted and finished into strips having a predetermined thickness by a twin roll casting method. Next, a direct hot rolling (DRM) roll provided immediately below the twin-roll drum was used to directly reduce the strip at 1020 ° C. immediately after casting at a reduction rate of 35%, and at 950 ° C., thickness (HG) 1. Finished into a 4 mm cast steel strip.
The cast steel strip obtained by this process was finished by cold rolling to a cold-rolled sheet thickness of 0.35 mm, subjected to finish annealing at 950 ° C. for 30 seconds, and magnetic measurements were performed.
The process conditions and magnetic measurement results are shown together in Table 4.
From Table 4, it can be seen that a low excellent iron loss value can be obtained by appropriately controlling the REM addition amount and the combined addition amount of Ti and O.

Claims (4)

% By weight in steel
0.3 ≦ Si ≦ 3.5,
0.15 ≦ Mn ≦ 1.5,
And the mass ppm in steel,
C ≦ 50,
S ≦ 40,
N ≦ 50,
Ti ≦ 50,
10 ≦ REM ≦ 500,
20 ≦ O ≦ 50
Containing the balance Fe and unavoidable impurities, and casting this by a twin roll casting method,
The strip immediately after casting is rolled at a rolling temperature of 900 ° C. or higher and 1100 ° C. or lower with a roll provided immediately below the twin rolls to give a reduction rate of 30% or more and 50% or less to obtain a cast steel strip,
A method for producing a non-oriented electrical steel sheet, wherein the cast steel strip is cold-rolled once and subjected to finish annealing. Furthermore, in steel,
0.3 ≦ Al ≦ 2.5
The method for producing a non-oriented electrical steel sheet according to claim 1, comprising:   The method for producing a non-oriented electrical steel sheet according to claim 1 or 2, wherein a rolling rate of the cold rolling is 55% or more and 85% or less. Smelted steel according to claim 1 or 2, which was cast by twin-roll casting method, immediately after the cast strip, at a rolling temperature of 900 ° C. or higher 1100 ° C. temperature below a roll provided directly under the twin roll A method for producing a cast steel strip for producing a non-oriented electrical steel sheet, comprising rolling at a rolling reduction of 30% to 50%.

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