JP6801464B2 - Non-oriented electrical steel sheet - Google Patents

Description

The present invention relates to a method for manufacturing a high-strength non-oriented electrical steel sheet used as an iron core material for a drive motor such as an electric vehicle or a motor for various electric devices.

In recent years, the number of motors having a large capacity and rotating at high speed has been increasing for automobile applications and the like. The rotor material of the motor is required to have mechanical strength to withstand centrifugal force and stress fluctuations. In order to increase the mechanical strength of steel, methods such as fine grain strengthening and dislocation strengthening are used, but in general, these strengthening actions deteriorate the soft magnetic properties. The same material can be used for the rotor and stator as long as it can maintain excellent magnetic properties as well as mechanical strength. Patent Documents 1 to 8 and the like have proposed a method of finely precipitating metallic Cu after cold-rolled recrystallization for the purpose of achieving both low iron loss and high strength.

Japanese Unexamined Patent Publication No. 2004-084053 International Publication No. 2005/033349 Japanese Unexamined Patent Publication No. 2004-183066 International Publication No. 2004/050934 Japanese Unexamined Patent Publication No. 2008-22304 Japanese Unexamined Patent Publication No. 2010-24509 International Publication No. 2013/024899 International Publication No. 2013/146886

The technique of finely depositing Cu can obtain low iron loss and high mechanical strength, but there is a problem that the more Cu contained, the more easily the surface of the steel sheet is flawed during hot rolling. An object of the present invention is to provide a high-strength non-oriented electrical steel sheet in which Cu is finely deposited, and to suppress the occurrence of flaws and improve productivity in the manufacturing process.

The present inventors considered that it is effective to suppress the oxidation of steel during slab heating in order to suppress the defects of the hot-rolled plate caused by Cu, and investigated the relationship between various contained elements and oxidation. As a result, it was found that in the electromagnetic steel sheet containing Si, the content of Al leads to an increase in flaws, but the addition of Cr can suppress the flaws. Specifically, it is as follows.

(1) In mass%, C: 0.005% or less, Si: 1.0 to 4.0%, Mn: 0.05 to 1.5%, Al: 0.1 to 2.0%, Cu: It contains 0.5 to 2.5%, Cr: 0.1 to 4.0%, S: 0.004% or less, N: 0.004% or less, and the balance consists of Fe and impurities.
When the Al content is [Al] and the Cr content is [Cr], [Cr] -2 x [Al] ≥ 0.
It has a metal structure composed of ferrite grains that does not contain an unrecrystallized structure, and has a metal structure.
The average crystal grain size of the ferrite grains is 30 μm or more and 180 μm or less.
Metal Cu particles having a number density of 1.0 × 10 ^{4 to} 1.0 × 10 ⁷ / μm ³ are contained inside the ferrite particles, and the average particle size of the metal Cu particles inside the ferrite particles is 1. A non-oriented electrical steel sheet having a size of 0 nm or more and 10.0 nm or less.

According to the present invention, a high-strength non-oriented electrical steel sheet in which Cu is finely deposited can be produced with high productivity.

It is a figure which shows the temperature change of the oxidative increase in a typical alloy. It is a figure which shows the influence of Cr content on the temperature change of the oxidation increase. It is a figure which shows the influence of Cr addition on the oxidation increase at the time of heating at 1100 degreeC.

In order to prevent defects caused by Cu, it is important to suppress the oxidation of slabs during hot spreading heating. This is because when the steel containing Cu is oxidized, Fe, which is more base than Cu, is selectively oxidized, and Cu is concentrated in a metallic state at the interface between the scale and the base iron, which causes various defects. Is.

<Experiment 1>
The steel having the components shown in Table 1 was melted in vacuum, the resulting ingot was subjected to crude heat spreading, and a test piece having a size of 10 mm × 20 mm × 30 mm was cut out from the coarse bar and subjected to atmospheric annealing. The heating temperature was changed in the range of 1050 ° C to 1200 ° C, and the soaking time was 30 minutes. The weight of the sample before and after annealing was measured. The amount of increase is the amount of oxidation that accompanies oxidation. The relationship between the heating temperature and the increase in oxidation is shown in FIG. Oxidation at 1150 ° C. or lower is effectively suppressed in the alloy a2 in which Si is 3.2% with respect to the alloy a1 containing no Si and Al. In the alloy a3 containing 0.7% of Al, the amount of oxidation increase increases and the oxidation resistance deteriorates. Further, when the temperature of the alloys a2 and a3 exceeds 1160 ° C., the amount of oxidation increase sharply increases.

From the above results, Si has an action of preventing flaws caused by Cu, but when Al is compoundly contained, the flaw prevention action is lowered. Further, when the heating temperature of the slab becomes a high temperature of 1160 ° C. or higher, the precipitation of Cu becomes remarkable, which causes a defect.

<Experiment 2>
Since Al is an element effective for increasing the intrinsic resistance of the electrical steel sheet and reducing the iron loss, it is not preferable that the amount added is limited. An alloy in which Cr was added to steel containing Si and Al was investigated. The alloys having the composition shown in Table 2 were melted in vacuum and annealed in the air in the same manner as described above. FIG. 2 shows the change in the amount of oxidation increase when 0 to 3% of Cr is added to the alloy containing 3.2% of Si and 0.7% of Al. Oxidation is suppressed as the amount of Cr added increases. FIG. 3 shows an increase in oxidation in the case of heating at 1100 ° C. when Cr is added to an alloy containing 0.3% of Al. The higher the Al content, the higher the amount of Cr added to suppress the increase in oxidation. Table 2 also shows the amount of oxidation increase at 1100 ° C. for each component.

<Experiment 3>
Using the steel ingots having the composition shown in Table 2 as test materials, rough hot rolling at a heating temperature of 1100 ° C., finishing rolling at a heating temperature of 1150 ° C., a finishing temperature of 850 ° C. It was. Table 2 shows the presence or absence of scratches on the surface of the hot-rolled plate in each material. It was found that when the amount of Cr added increased and the amount of oxidation increased at 1100 ° C. was 0.05 mg / mm ² or less, no dents on the hot-rolled plate occurred. Looking at FIG. 3, the amount of Cr added required to reduce the oxidation increase at 1100 ° C. to 0.05 mg / mm ² or less is 0.5% when Al is 0.3% and 0.7% for Al. At the time of, it is 1.5%, which is about twice the amount of Al.

The constituent requirements of the present invention based on these findings are described below.
[Chemical composition and structure of steel]
In the following description, "%", which is a unit of the content of each element contained in steel, means "mass%" unless otherwise specified.

<C: 0.005% or less>
Since C deteriorates iron loss, the lower the C content, the better. Such a phenomenon is remarkable when the C content exceeds 0.005%. Therefore, the C content is 0.005% or less, preferably 0.003% or less, and more preferably 0.002% or less.

<Si: 1.0 to 4.0%>
As shown in the previous experiment, Si has an effect of suppressing the oxidation of steel during slab heating and suppressing the precipitation of Cu. Further, Si also has an action of increasing the natural resistance and reducing the iron loss. If the Si content is less than 1.0%, these effects cannot be sufficiently obtained. Therefore, the Si content is 1.0% or more, preferably 2.0% or more, and more preferably 2.5% or more.
On the other hand, when the Si content exceeds 4.0%, the steel becomes brittle and the rollability deteriorates. Therefore, the Si content is 4.0% or less, preferably 3.8% or less, and more preferably 3.5% or less.

<Mn: 0.05% to 1.5%>
Mn has an effect of increasing the intrinsic resistance of steel and an effect of preventing fine precipitation of sulfide during hot rolling by increasing the solution temperature of MnS. If the Mn content is less than 0.05%, these effects cannot be sufficiently obtained. Therefore, the Mn content is set to 0.05% or more, preferably 0.1% or more, and more preferably 0.2% or more. On the other hand, if the Mn content exceeds 1.5%, the steel may become embrittlement. Therefore, the Mn content is 1.5% or less, preferably 1.0% or less, and more preferably 0.5% or less.

<Al: 0.1 to 2.0%>
Al has the function of increasing the intrinsic resistance of steel, fixing nitrogen as AlN, and reducing iron loss. If the Al content is less than 0.2%, these effects cannot be sufficiently obtained. Therefore, the Al content is 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more. On the other hand, when the Al content exceeds 2.0%, the steel becomes brittle and the rollability deteriorates. Therefore, the Al content is 2.0% or less, preferably 1.5% or less, more preferably 1.2% or less, and further preferably 1.0 or less.

<Cu: 0.5-2.5%>
Cu is finely precipitated in the grains after cold-rolled recrystallization, so that the mechanical strength is increased without deterioration of iron loss. If the Cu content is less than 0.5%, this effect cannot be sufficiently obtained. Therefore, the Cu content is 0.5% or more, preferably 0.7% or more, and more preferably 1.0% or more. On the other hand, when the Cu content exceeds 2.5%, flaws are likely to occur during hot rolling and embrittlement is likely to occur. Therefore, the Cu content is 2.5% or less, preferably 2.0% or less, and more preferably 1.5% or less.

<Cr: 0.1-4.0%>
As shown in the previous experiment, Cr suppresses oxidation and suppresses the occurrence of flaws in the hot-rolled plate in steel containing Cu. If Cr is less than 0.1%, this effect cannot be sufficiently obtained. Therefore, the Cr content is set to 0.1% or more. It is preferably 0.2% or more, and more preferably 0.5% or more. On the other hand, when Cr exceeds 4.0%, the hysteresis loss deteriorates. Therefore, the Cr content is set to 4.0% or less. It is preferably 3.5% or less, more preferably 3.0% or less, still more preferably 2.5%, and even more preferably 2.0%.

<S: 0.004% or less>
Since S produces fine sulfides and deteriorates the grain growth property, the lower the S content, the better. Such a phenomenon is remarkable when the S content exceeds 0.004%. Therefore, the S content is 0.004% or less, preferably 0.003% or less, and more preferably 0.002% or less.

<N: 0.004% or less>
Since N produces fine nitrides and deteriorates grain growth, the lower the N content, the better. Such a phenomenon is remarkable when the N content exceeds 0.004%. Therefore, the N content is 0.004% or less, preferably 0.003% or less, and more preferably 0.002% or less.

<[Cr] -2 x [Al] ≧ 0>
As seen in FIG. 3, the amount of Cr added to suppress oxidation differs depending on the Al content. In this figure, the amount of Cr added to reduce the oxidation increase at 1100 ° C to 0.05 mg / mm ² or less is 0.5% when Al is 0.3% and 0.7% when Al is 0.7%. Was 1.5%. Therefore, in the present invention, when the contents of [Al] and Cr are [Cr], [Cr] -2 x [Al] ≥ 0.

<Other elements>
In order to fix S by forming coarse sulfate or sulfide and suppress the formation of fine sulfide, REM may be added in the range of 0.03% or less. REM is a general term for a total of 17 elements, which is obtained by adding Sc with an atomic number of 21 and Y with an atomic number of 39 to 15 elements from La with an atomic number of 57 to Lu with an atomic number of 71. Since Ca also has the same effect, it may be contained in the range of 0.005% or less.

For the purpose of improving the magnetic properties, Sn and Sb can be added in the range of 0.05% or less, respectively.
Further, P can be added in the range of 0.1% or less in order to increase the mechanical strength and improve the texture.

Other harmful impurity elements are preferably reduced as much as possible, and Ti, Nb, V, and 0 are particularly preferably 0.005% or less.
The rest are unavoidable impurities and Fe.

<Structure consisting of ferrite grains that do not contain an unrecrystallized structure>
If the unrecrystallized structure remains in the steel sheet, the iron loss of the steel sheet increases remarkably. Therefore, in the present invention, the structure is composed of ferrite grains that do not contain an unrecrystallized structure.

<Average crystal grain size of ferrite grains: 30 to 180 μm>
The average crystal grain size of the ferrite grains needs to be 30 μm or more in order to reduce the hysteresis loss of the steel sheet. However, if the average crystal grain size of the ferrite grains is too large, the iron loss may deteriorate due to the increase in the eddy current loss. Therefore, the average crystal grain size of the ferrite grains is 180 μm or less. The lower limit of the average crystal grain size of the ferrite grains is preferably 30 μm, more preferably 50 μm, and even more preferably 70 μm. The upper limit of the average crystal grain size of the ferrite grains is preferably 170 μm, more preferably 160 μm, and even more preferably 150 μm. The average crystal grain size of the ferrite grains can be determined according to JIS G 0551 “Steel-Crystal Grain Size Microscopic Test Method”.

<Average particle size of metal Cu particles: 1.0 nm or more and 10.0 nm or less>
Cu particles precipitated in the recrystallized particles hinder the movement of dislocations. Metal Cu particles having too small a particle size have a small resistance to dislocation movement. On the other hand, metal Cu particles having a large particle size have a large resistance to the movement of dislocations, but the number density of the metal Cu particles decreases, so that the distance between the particles becomes large and the movement of dislocations becomes easy. Further, the metal Cu particles having a particle diameter of about 100 nm or more having a domain wall thickness hinder the movement of the domain wall and increase the hysteresis loss. Therefore, the average particle size of the metal Cu precipitated particles is 1.0 nm or more and 10.0 nm or less. It is preferably 2.0 nm or more and 5.0 nm or less, more preferably 2.0 nm or more and 4.0 nm or less, and further preferably 2.0 nm or more and 3.0 nm or less.

The average particle size of the metal Cu particles is determined using a bright field image of a transmission electron microscope (TEM). The area of each Cu particle in the image is obtained, and the diameter of the circle having that area (circle equivalent diameter) is regarded as the diameter of each particle.

<Number density of metal Cu particles: 1.0 × 10 ^{4 to} 1.0 × 10 ⁷ / μm ³ >
In the present invention, the number of metal Cu particles per 1 μm ³ volume in the ferrite particles is 1.0 × 10 ⁴ / μm ³ or more. It is preferably 1.0 × 10 ⁵ / μm ³ or more, and more preferably 5.0 × 10 ⁵ / μm ³ or more. On the other hand, if the number density of the metal Cu particles is too large, the magnetic properties of the steel sheet may be deteriorated. Therefore, the upper limit of the number density of metal Cu particles in the ferrite grains and 1.0 × 10 ⁷ / μm ^3.

The number density of metal Cu particles is the number density of metal Cu particles having a particle size of 1.0 nm or more in all ferrite particles. Metal Cu particles having a particle size of less than 1.0 nm are not included in the measurement because they are difficult to detect and are considered to have almost no effect on the characteristics of the steel sheet according to the present embodiment. The number density N of metal Cu particles in the ferrite particles of the steel plate according to the present embodiment is such that the area of the electron microscope observation image is A, the number of Cu particles observed therein is n, and the average particle size (circle equivalent diameter) thereof. When d is the arithmetic mean), it is calculated based on the following formula.
N = n / (A × d)

[Production method]
The non-oriented electrical steel sheet of the present invention can be manufactured by, for example, the following method. That is, after the steel having the component composition is melted, it is made into a slab by continuous casting or the like, and the slab is heated to 1180 ° C. or lower and hot-rolled to obtain a hot-rolled steel sheet, and if necessary, hot-rolled sheet is annealed. The hot-rolled steel sheet or the hot-rolled fired steel sheet was cold-rolled to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet was recrystallized and annealed, and then Cu was precipitated. This is a method for manufacturing a non-directional electromagnetic steel sheet as a non-directional electromagnetic steel sheet product.

When heating the slab in the hot rolling step, it is preferable to control the temperature of the surface of the slab so that it does not exceed 1180 ° C. As shown in FIG. 2, for any steel component, when a certain temperature is exceeded, the steel tends to be rapidly oxidized. The temperature was 1180 ° C. or higher within the component range of the present invention. Therefore, it is preferable to control the surface temperature so that it does not exceed 1180 ° C. during heating.

Other manufacturing conditions are not particularly limited, but it can be manufactured under the following conditions.
The slab heating temperature during hot spreading is preferably 1000 ° C. or higher. If the slab heating temperature is less than 1000 ° C., hot rolling becomes difficult. As mentioned above, the surface temperature of the slab is controlled so as not to exceed 1180 ° C. The hot-rolled finishing temperature FT is preferably 900 ° C. or lower. If the winding temperature CT of the hot-rolled steel sheet is high, Cu is deposited in the coil after winding and the toughness of the hot-rolled steel sheet is lowered, so that the temperature is preferably 500 ° C. or lower. The thickness of the hot-rolled finished plate is preferably 2.7 mm or less in order to prevent the texture from deteriorating due to the high reduction rate during cold rolling. However, if it is too thin, hot spreading becomes difficult and productivity decreases. Therefore, the finished hot-rolled plate thickness is preferably 1.6 mm or more.

In order to improve the texture of the final product and obtain a high magnetic flux density, the hot-rolled steel sheet may be annealed. The preferred soaking temperature is 750 to 1100 ° C., and the soaking time is 10 seconds to 5 minutes. When the soaking temperature is less than 750 ° C. or the soaking time is less than 10 seconds, the effect of improving the texture is small. If the soaking temperature exceeds 1100 ° C., or if the soaking time exceeds 5 minutes, the manufacturing cost will increase due to an increase in energy consumption and deterioration of ancillary equipment. After cold spreading, the Cu in the steel sheet before recrystallization is made fine, and Cu is resolidified during recrystallization annealing after cold spreading. Therefore, in the cooling section of 800 to 400 ° C, the average cooling rate is 10 ° C / sec or more. Cooling. The average cooling rate is preferably 20 ° C./or or higher, more preferably 40 ° C./sec or higher. The high average cooling rate also helps to ensure the toughness of the hot-spread annealed plate.

Further, in the manufacturing method of the present invention, a hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Cold rolling may be performed once, or may be performed twice or more including intermediate annealing. The final reduction rate is preferably in the range of 60 to 90%. As a result, high magnetic flux density and low iron loss can be obtained. The intermediate annealing temperature is preferably 900 to 1100 ° C. In this case as well, it is desirable to cool the cooling section of 800 to 400 ° C. at an average cooling rate of 10 ° C./sec or more.

In the recrystallization step, the metal structure of the steel sheet is recrystallized and Cu is dissolved. The soaking temperature is preferably 850 ° C. or higher in order to make the average crystal grain size of the ferrite grains, which is one of the above-mentioned requirements, 30 μm or more, and to dissolve Cu in a solid solution. On the other hand, if the soaking temperature is too high, the average crystal grain size of the ferrite grains tends to exceed the specified 180 μm, energy consumption increases, and ancillary equipment such as hearth rolls are easily damaged. The temperature is preferably 1100 ° C. or lower. The soaking time is preferably 10 seconds or more and 2 minutes or less. The average cooling rate from 800 ° C. to 400 ° C. in the cooling process is preferably 10 ° C./sec or more so that the Cu once solid-solved does not precipitate in the cooling process.

Next, the recrystallized steel sheet obtained in the recrystallization step is annealed to precipitate Cu in the crystal grains. The number density of Cu particles precipitated in the ferrite particles is 1.0 × 10 ^{4 to} 1.0 × 10 ⁷ / μm ³ , and the average size is 1.0 nm or more and 10.0 nm or less. Is preferably 450 ° C. or higher and 650 ° C. or lower.

Moreover, the soaking time needs to be 10 seconds or more. It is preferably 30 seconds or longer, more preferably 40 seconds or longer. Within the above temperature range, it is possible to perform annealing in batch annealing with a soaking time of several hours. The optimum conditions for the soaking temperature and the soaking time vary slightly depending on the composition of the steel sheet, particularly the amount of Cu, but are generally included in the above range.

When recrystallization annealing and Cu precipitation annealing are performed simultaneously on one continuous annealing line, the soaking temperature is 850 ° C or higher and 1050 ° C or lower, the soaking time is 10 seconds or longer, and the temperature of 600 ° C to 450 ° C during the cooling process. The time for the steel plate to stay in the region is 10 seconds or more.

If necessary, the steel sheet obtained by the production method of the present invention is coated with an insulating film to obtain a non-oriented electrical steel sheet having high strength and low iron loss.

<Example 1>
The steel having the composition shown in Table 3 was melted in vacuum, and the obtained ingot was hot-rolled at a heating temperature of 1150 ° C., a finishing temperature of 850 ° C., a winding temperature of 400 ° C., and a finishing thickness of 2.3 mm. The presence or absence of dents on the hot-rolled plate was visually confirmed, and the results are shown in Table 3. The obtained hot-rolled plate was annealed with a hot-rolled plate having a soaking temperature of 1000 ° C. and a holding time of 30 seconds, and then subjected to cold rolling to obtain a cold-rolled plate having a plate thickness of 0.35 mm. The cold-rolled plate was subjected to recrystallization annealing at a soaking temperature of 1000 ° C., a holding time of 30 seconds, and an average cooling rate of 20 ° C./sec from 800 ° C. to 400 ° C., followed by a soaking temperature of 550 ° C. and a holding time of 60 seconds. Cu precipitation annealing was performed to obtain a product plate of a non-directional electromagnetic steel plate. The average ferrite crystal grain size, the number density and average particle size of precipitated Cu, the mechanical properties and the magnetic properties of the obtained product board were investigated, and each is shown in Table 3. Good product characteristics were W10 / 400 of 20 W / kg or less, B50 of 1.60 T or more, and YP and TS of 400 MPa or more and 500 MPa or more, respectively. According to the present invention, both good mechanical properties and good iron loss can be achieved without flaws in the hot-rolled plate.

<Example 2>
Using the cold-rolled plate of alloy c15 in Table 3 as a test material, finish annealing was performed at a soaking temperature of 950 to 1050 ° C. and a soaking time of 30 to 90 seconds, and further, a soaking temperature of 550 to 650 ° C. and a soaking time. Cu precipitation annealing was performed for 30 seconds to obtain a non-directional electromagnetic steel plate. In the same manner as in Example 1, the average ferrite crystal particle size, the number density and average particle size of precipitated Cu, and the mechanical and magnetic characteristics were investigated. Each is shown in Table 4. According to the present invention, both good mechanical properties and good iron loss can be achieved at the same time.

Claims (1)

By mass%
C: 0.005% or less Si: 1.0 to 4.0%,
Mn: 0.05-1.5%,
Al: 0.1 to 2.0%,
Cu: 0.5-2.5%,
Cr: 0.1-4.0%,
S: 0.004% or less,
N: 0.004% or less,
Containing, the balance consists of Fe and impurities,
When the Al content is [Al] and the Cr content is [Cr], [Cr] -2 x [Al] ≥ 0.
It has a metal structure composed of ferrite grains that does not contain an unrecrystallized structure, and has a metal structure.
The average crystal grain size of the ferrite grains is 30 μm or more and 180 μm or less.
Metal Cu particles having a number density of 1.0 × 10 ^{4 to} 1.0 × 10 ⁷ / μm ³ are contained inside the ferrite particles, and the average particle size of the metal Cu particles inside the ferrite particles is 1. A non-oriented electrical steel sheet having a size of 0 nm or more and 10.0 nm or less.

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