TWI688658B - Non-oriented electrical steel sheet - Google Patents

Abstract

A non-oriented electrical steel sheet includes a silicon steel and an insulation coating. The silicon steel includes in the surface thereof an internal oxidation layer which includes a SiO ₂phase. An average thickness of the internal oxidation layer is 0.10 to 5.0 mm, and Vickers hardness of the internal oxidation layer is 1.15 to 1.5 times as compared with Vickers hardness of a thickness central area.

Description

Non-directional electromagnetic steel plate

The present invention mainly relates to a non-oriented electromagnetic steel sheet used as an iron core material for electrical equipment and having excellent fatigue strength and magnetic characteristics.

Background of the Invention In recent years, in the field of electrical equipment, especially in the fields of rotating machines, small and medium-sized transformers, electronic components, etc. that use non-oriented electromagnetic steel plates as iron core materials, global power saving, energy saving, and CO reduction Among the _second- class trends of global environmental protection, there is a growing demand for higher efficiency and miniaturization. Under the social environment as described above, it is a matter of course that the performance of non-oriented electromagnetic steel plates is also required to be improved.

Generally speaking, the motor system is composed of a stator (static stator) and a rotor (rotor). In recent years, as a driving motor for electric vehicles or hybrid vehicles, a built-in permanent magnet motor (hereinafter sometimes referred to as an "IPM motor") with a permanent magnet built into the rotor has been the mainstream. Efficiency, high output, high-speed rotation, and miniaturization are continuing to develop technology.

In order to improve the performance of the IPM motor, it is necessary to bring the stator and the permanent magnets inside the rotor closer. Therefore, the distance from the outer edge of the rotor core to the permanent magnets inside the rotor needs to be shortened. On the other hand, at the outer edge of the rotating rotor core, the centrifugal force caused by the permanent magnet is loaded during the rotation, and the load becomes larger under high-speed rotation. Therefore, the strength of the portion between the outer edge of the rotor core and the slot for permanent magnets (hereinafter sometimes referred to as "bridge portion"), especially the fatigue strength, becomes very important. Therefore, the following technology is disclosed, for example.

Patent Literature 1 discloses a technique that can increase the strength of the electromagnetic steel sheet used for the rotor core. As described above, the portion where the strength needs to be improved in the rotor core is the bridge portion, so Patent Document 2 discloses a technique that can be processed and hardened to strengthen only this portion. However, Patent Document 3 discloses a technique that uses a ring or the like to reinforce the rotor from the outside to improve the overall strength of the rotor core.

However, in the technique of Patent Document 1, since the strength of the electromagnetic steel sheet itself becomes high, there is a disadvantage that the punchability of the rotor core blank is reduced. Reduced punching performance will result in lower blank accuracy during punching, lower punching speed, or die wear during punching. In the technique of Patent Document 2, an additional step of strengthening only the bridge portion is required when manufacturing the rotor core, which leads to an increase in cost. In addition, in the technology of Patent Document 3, a ring or the like that can reinforce the rotor from the outside is required, which increases the cost.

Therefore, it is desired to develop a technique that can increase the strength of the target portion, especially the fatigue strength, without increasing the strength of the electromagnetic steel sheet itself and without adding new steps.

As described above, the bridge portion of the rotor core repeatedly loads the centrifugal force due to the rotation of the motor. Therefore, it is necessary to increase the fatigue strength of the bridge portion. As a representative technique for improving fatigue strength, there is a technique for hardening the surface of steel (plate).

As the surface hardening method, there are, for example, metamorphic strengthening of the steel itself as represented by quenching, precipitation strengthening of the second phase generated by nitriding or carburization, and work hardening by introducing strains such as bead strike, but no matter which Each one requires additional steps.

So far, regarding the non-oriented electrical steel sheet, there has not been established a technology that does not require additional steps and can balance fatigue strength and magnetic properties.

Prior technical literature Patent Literature Patent Document 1: Japanese Patent No. 5000136 Patent Document 2: Japanese Patent No. 4160469 Patent Document 3: Japanese Patent Application Publication No. 2013-115899 Patent Document 4: Japanese Patent No. 3307897 Patent Literature 5: Japanese Patent No. 4116748 Patent Document 6: Japanese Patent No. 4116749 Non-patent literature

Non-Patent Literature 1: Steel and Iron, Vol. 66 (1980), No. 7, p1000~p1009 Non-Patent Document 2: Materia Japan, Vol. 50 (2011), No. 3, p126~p128

Summary of the invention Problems to be solved by invention In view of the prior art, the object of the present invention is to provide a non-oriented electrical steel sheet without compromising additional steps in the conventional manufacturing method, taking into account both fatigue strength and magnetic properties. That is, an object of the present invention is to provide a non-oriented electrical steel sheet that is excellent in fatigue strength and magnetic properties and also excellent in cost.

Means to solve the problem In order to solve the above-mentioned problems, the present inventors deliberately studied the use of a manufacturing process of a non-oriented electrical steel sheet to form a surface hardened layer on a silicon steel sheet that is a base material steel sheet of the non-oriented electrical steel sheet. As a result, it was found that as long as the steel components and manufacturing conditions are properly combined, an internal oxide layer can be formed on the surface of the silicon steel plate, and the hardness of the internal oxide layer can be controlled to harden the surface, thereby improving fatigue strength.

In addition, as described in Patent Documents 4 to 6, if the thickness of the internal oxide layer becomes larger, the iron loss at high frequencies will be adversely affected in particular. Therefore, the present inventors have devoted themselves to the study of controlling the oxide in the internal oxide layer and the thickness of the internal oxide layer, and controlling the hardness of the internal oxide layer, so as to achieve both fatigue strength and magnetic properties.

As a result, it was found that for the steel plate after adjusting the steel composition, as long as the heat preservation treatment is performed during cooling after hot rolling and the heat preservation conditions are appropriately controlled, the average thickness of the oxide in the internal oxide layer and the internal oxide layer can be controlled. And can control the hardness of the internal oxide layer. That is, the following knowledge and insights were obtained: a non-oriented electrical steel sheet that combines fatigue strength and magnetic properties can be obtained without adding new steps.

The gist of the present invention is as follows.

(1) The non-oriented electrical steel sheet according to one aspect of the present invention includes a silicon steel sheet and an insulating coating. The composition of the silicon steel sheet includes: in mass %, Si: more than 2.00% and 4.00% or less, and Al: 0.10% or more and 3.00% or less, Mn: 0.10% or more and 2.00% or less, C: 0.0030% or less, P: 0.050% or less, S: 0.005% or less, N: 0.005% or less, Sn: 0% or more and 0.40% or less, Cu: 0% or more and 1.00% or less, Sb: 0% or more and 0.40% or less, REM: 0% or more and 0.0400% or less, Ca: 0% or more and 0.0400% or less, Mg: 0% or more and 0.0400% or less, and remaining Partly composed of Fe and impurities; when viewed from a cutting plane parallel to the cutting direction and the thickness direction, the thickness range of the silicon steel plate is 5/8~3/8, that is, the Vickers hardness of the central part is 120Hv or more and 300Hv or less ; And when viewed from the above cut surface, the silicon steel plate has an internal oxide layer containing SiO ₂ phase on the surface, the average thickness of the internal oxide layer is 0.10 μm or more and 5.0 μm or less, and the Vickers hardness of the internal oxide layer relative to the center The Vickers hardness of the part is 1.15 times or more and 1.5 times or less. (2) In the non-oriented electrical steel sheet of (1) above, the silicon steel sheet may also contain at least one of the following elements as its component composition: in mass %, Sn: 0.02% or more and 0.40% or less, Cu: 0.10 % Or more and 1.00% or less, and Sb: 0.02% or more and 0.40% or less. (3) In the non-oriented electrical steel sheet of (1) or (2) above, the silicon steel sheet may also contain at least one of the following elements as its component composition: in mass %, REM: 0.0005% or more and 0.0400% or less , Ca: 0.0005% or more and 0.0400% or less, and Mg: 0.0005% or more and 0.0400% or less. (4) The non-oriented electrical steel sheet according to any one of (1) to (3) above, the internal oxide layer may have a Vickers hardness of 155 Hv or more. (5) The non-oriented electrical steel sheet according to any one of (1) to (4) above, the average thickness of the internal oxide layer may be 0.55 μm or more.

Invention effect According to the above aspect of the present invention, it is possible to provide a non-oriented electrical steel sheet that is excellent in fatigue strength and magnetic properties and also excellent in cost.

Embodiment of the invention The preferred embodiments of the present invention will be described in detail below. However, the present invention is not limited to the structure disclosed in this embodiment, and various changes can be made within the scope not departing from the gist of the present invention. In addition, the following numerical value limits the range, and the lower limit and the upper limit are included in the range. The value displayed as "greater than" or "less than" is not included in the value range. In addition, the symbol "%" related to the content of each element means "mass %".

First, regarding the non-oriented electrical steel sheet of the present embodiment (hereinafter sometimes referred to as "the electromagnetic steel sheet of the present invention"), the reason for limiting the composition of the silicon steel sheet that is the base material steel sheet will be described.

＜Composition of silicon steel sheet＞ In this embodiment, the composition of the silicon steel sheet contains basic elements, and optionally contains selective elements, and the remaining part is composed of Fe and impurities.

In this embodiment, among the composition of silicon steel sheets, Si, Al, and Mn are basic elements (mainly alloying elements).

Si: greater than 2.00% and less than 4.00% Si (silicon) is an element that can increase resistance and reduce eddy current loss, which helps reduce iron loss, and is a kind of element that can improve the yield ratio of the steel plate by solid solution strengthening, and Elements that help increase tensile strength and fatigue strength. Furthermore, as will be described later, Si is also an essential element for generating an SiO ₂ phase in the internal oxide layer to harden the surface of the steel sheet.

If Si is 2.00% or less, it is difficult to obtain the above effect, and it is difficult to increase the hardness of the internal oxide layer, so Si is set to be greater than 2.00%. It should be above 2.10%, more preferably above 2.30%, and more preferably above 2.60%. On the other hand, if Si exceeds 4.00%, the magnetic flux density will decrease, and the workability such as cold rolling will also decrease, resulting in an increase in manufacturing costs, so Si is made 4.00% or less. And it should be below 3.70%, more preferably below 3.40%.

Al: 0.10% or more and 3.00% or less Al (aluminum) is also an element that can increase resistance, reduce eddy current loss, and help reduce iron loss, just like Si. However, compared with Si, Al is also an element with a small increase in hardness. In addition, Al is an element that can increase the ratio of the magnetic flux density B ₅₀ to the saturated magnetic flux density Bs: B ₅₀ /Bs, and contribute to the improvement of the magnetic flux density.

If Al is less than 0.10%, the effect of addition cannot be sufficiently obtained, so Al is made 0.10% or more. It should be more than 0.30%, more than 0.50%, and more than 0.60%. On the other hand, if Al exceeds 3.00%, the saturation magnetic flux density decreases and the magnetic flux density decreases, and the yield ratio also decreases, thereby reducing the tensile strength and fatigue strength, so Al is set to 3.00% or less. And it should be below 2.70%, more preferably below 2.40%.

Mn: 0.10% or more and 2.00% or less Mn (manganese) is an element that can increase resistance, reduce eddy current loss, and can play a role in suppressing the formation of {111}<112> aggregate structure that is less ideal for magnetic properties.

If Mn is less than 0.10%, the effect of addition cannot be sufficiently obtained, so Mn is made 0.10% or more. It should be above 0.15%, more preferably above 0.20%, more preferably above 0.60%, and more preferably above 0.70%. On the other hand, if Mn exceeds 2.00%, the grain growth during annealing decreases and the iron loss increases, so Mn is set to 2.00% or less. And it should be below 1.70%, more preferably below 1.50%.

In this embodiment, the silicon steel sheet contains impurities as its component composition. In addition, the so-called "impurity" refers to materials mixed from ore and scrap as raw materials, or from the manufacturing environment, etc. when steel is manufactured industrially. And it refers to elements such as C, P, S and N. In order to fully exert the effects of the present embodiment, it is desirable to limit these impurities to the following. Moreover, it is ideal to have less content of impurities, so there is no need to limit the lower limit, and the lower limit of the impurities can also be 0%.

C: below 0.0030% C (carbon) is an impurity element, which will increase iron loss and also cause magnetic aging. If C is greater than 0.003%, iron loss will increase and significant magnetic aging will occur, so C is set to 0.0030% or less. And it should be below 0.0020%, more preferably below 0.0010%. Although the lower limit includes 0%, it is difficult to produce 0% in terms of production technology. In practice, 0.0001% is the substantial lower limit.

P: 0.050% or less Although P (phosphorus) sometimes helps to increase the tensile strength, it is also an impurity element that makes the steel plate brittle. If P is greater than 0.050%, the steel sheet containing Si of 2.00% or more becomes significantly weak, so P is set to 0.050% or less. And it should be below 0.030%, more preferably below 0.020%. Although the lower limit includes 0%, it is difficult to produce 0% in production technology. In practice, 0.002% is the substantial lower limit.

S: below 0.005% S (sulfur) is an impurity element that forms fine sulfides such as MnS, which hinders recrystallization and grain growth during finish annealing. If S is greater than 0.005%, it will significantly hinder recrystallization and grain growth during the finish annealing, so S is set to 0.005% or less. And it should be below 0.003%, more preferably below 0.002%. Although the lower limit includes 0%, it is difficult to make 0% in production technology. In practice, 0.0003% is the substantial lower limit.

N: below 0.005% N (nitrogen) impurity elements form fine nitrides such as AlN, which hinder recrystallization and grain growth during finish annealing. If N is greater than 0.005%, it will significantly hinder recrystallization and grain growth during the finish annealing, so N is set to 0.005% or less. And it should be below 0.003%, more preferably below 0.002%. Although the lower limit includes 0%, it is difficult to produce 0% in production technology. In practice, 0.0005% is the substantial lower limit.

In this embodiment, the silicon steel sheet may contain selective elements in addition to the basic elements and impurities described above. For example, as a selection element, Sn, Cu, Sb, REM, Ca, and Mg may be contained to replace a part of the remaining Fe. And as long as the selection elements are included depending on the purpose. Therefore, there is no need to limit the lower limit of these selected elements, and the lower limit can also be 0%. In addition, even if these selective elements are contained as impurities, the above effects will not be impaired.

Sn: 0% or more and 0.40% or less Cu: 0% or more and 1.00% or less Sb: 0% or more and 0.40% or less Sn (tin), Cu (copper), and Sb (antimony) can play the role of suppressing the formation of {111}<112> aggregate structure, which is not ideal for magnetic properties, and can control the oxidation of the surface of the steel plate, and can also An element that plays a role in crystal grain growth and granulation. In addition, Sn, Cu, and Sb are also elements that can appropriately control the thickness of the internal oxide layer in the hot-rolled steel sheet.

If Sn is greater than 0.40%, Cu is greater than 1.00%, and Sb is greater than 0.40%, the effect of addition will be saturated, and the grain growth during finish annealing will be suppressed, and the workability of the steel sheet will be reduced, resulting in delayed embrittlement during cold rolling, so Sn is 0.40% or less, Cu is 1.00% or less, and Sb is 0.40% or less. The more preferred Sn is 0.30% or less, Cu is 0.60% or less and Sb is 0.30% or less, and the more preferred Sn is 0.20% or less, Cu is 0.40% or less and Sb is 0.20% or less.

The lower limit of Sn, Cu, and Sb is not particularly limited, and may be 0%. In order to suitably obtain the above effect, it is sufficient to set Sn to 0.02% or more, Cu to 0.10% or more, and Sb to 0.02% or more. The preferred systems are Sn with 0.03% or more, Cu with 0.20% or more and Sb with 0.03% or more, and the more preferred systems are Sn with 0.05% or more, Cu with 0.30% and Sb with 0.05% or more.

In the present embodiment, the composition of the silicon steel sheet preferably contains at least one of the following elements: in mass %, Sn: 0.02% or more and 0.40% or less, Cu: 0.10% or more and 1.00% or less, and Sb: 0.02% or more And below 0.40%.

REM: 0% or more and 0.0400% or less Ca: 0% or more and 0.0400% or less Mg: 0% or more and 0.0400% or less REM (Rare Earth Metal), Ca (calcium), and Mg (magnesium) are used to fix S as sulfide or oxysulfide, suppress fine precipitation of MnS, etc., and promote recrystallization and grain growth during finish annealing Element of the role.

If REM, Ca and Mg are greater than 0.0400%, sulfide or oxysulfide will be excessively generated, which will hinder recrystallization and grain growth during the finish annealing, so REM, Ca and Mg are all set to 0.0400% or less. Preferably, all elements are below 0.0300%, more preferably below 0.0200%.

The lower limit of REM, Ca, and Mg is not particularly limited, and may be 0%. In order to suitably obtain the above-mentioned effects, all of REM, Ca, and Mg should be 0.0005% or more. Preferably, all elements are above 0.0010%, more preferably above 0.0050%.

In the present embodiment, the composition of the silicon steel sheet preferably contains at least one of the following elements: in mass %, REM: 0.0005% or more and 0.0400% or less, Ca: 0.0005% or more and 0.0400% or less, and Mg: 0.0005% or more And below 0.0400%.

Here, REM refers to a total of 17 elements of Sc, Y, and lanthanides, and at least one of them. The above REM content refers to the total content of at least one of these elements. If it is a lanthanide element, it is added industrially in the form of a rare earth metal alloy.

The above-mentioned steel composition can be measured by a general analysis method of steel. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) may be used to measure the steel composition. In addition, C and S may be measured by a combustion-infrared absorption method, N by an inert gas fusion-thermal conductivity method, and O by an inert gas fusion-non-dispersive infrared absorption method.

In addition, the above-mentioned component composition is a component composition of a silicon steel sheet, and if the silicon steel sheet as a measurement sample has an insulating film on the surface, it means the component composition measured after removing it.

As a method of removing the insulating coating of the non-oriented electromagnetic steel sheet, there are, for example, the following methods: immersing the non-directional electromagnetic steel sheet having the insulating coating and the like in a sodium hydroxide aqueous solution, a sulfuric acid aqueous solution, and a nitric acid aqueous solution in sequence, followed by washing, and Use warm air to dry. Through the above-mentioned series of treatments, the silicon steel sheet after the insulation coating is removed can be obtained.

Next, regarding the non-oriented electrical steel sheet of this embodiment, the internal oxide layer of the silicon steel sheet will be described.

FIG. 1 is a schematic cross-sectional view showing a non-oriented electrical steel sheet according to this embodiment. When viewed in a cutting plane parallel to the cutting direction and the plate thickness direction, the non-oriented electromagnetic steel sheet 1 of this embodiment includes a silicon steel sheet 11 and an insulating coating 15 disposed on the silicon steel sheet 11, and the silicon steel sheet has internal oxidation on the surface Layer 13. This internal oxide layer 13 contains SiO ₂ phase 131. In addition, the internal oxide layer refers to a region in which oxide phases such as Si are dispersed into particles or layers in the silicon steel sheet.

<SiO ₂ phase in the internal oxide layer> The internal oxide layer contains the SiO ₂ phase. In the present embodiment, the SiO ₂ phase is finely and finely deposited in the internal oxide layer to control the hardness of the internal oxide layer, and thereby the effect of improving fatigue strength can be obtained.

In order to finely and finely precipitate the SiO ₂ phase in the internal oxide layer, the steel sheet must contain Si greater than 2.00%. In addition, it is necessary to properly control the heat preservation treatment during the cooling after the hot rolling is delayed.

<Average thickness of internal oxide layer> The average thickness of the internal oxide layer: 0.10μm or more and 5.0μm or less If the average thickness of the internal oxide layer is less than 0.10 μm, the effect of improving fatigue strength cannot be obtained, so the average thickness of the internal oxide layer is set to 0.10 μm or more. It should be greater than 0.5 μm, more preferably 0.55 μm or more, more preferably 0.6 μm or more, more preferably 0.7 μm or more, and even more preferably 1.0 μm or more. On the other hand, if the average thickness of the internal oxide layer is greater than 5.0 μm, the magnetic properties will decrease, especially the iron loss will increase, so the average thickness of the internal oxide layer is set to 5.0 μm or less. It is preferably 4.0 μm or less, and more preferably 3.0 μm or less.

<Vickers hardness> In this embodiment, the Vickers hardness of the internal oxide layer is controlled to be higher than the Vickers hardness of the central portion of the steel plate. That is, in the present embodiment, the strength of the electromagnetic steel sheet itself is not increased, but only the strength of the target portion is increased to increase the fatigue strength.

<Vickers hardness at the center of the steel plate> Vickers hardness of the central part of the steel plate: 120Hv or more and 300Hv or less When looking at the cutting plane parallel to the cutting direction and the thickness direction, the thickness range of 5/8~3/8 of the silicon steel plate is the central part. If the Vickers hardness of the central portion is less than 120Hv, sufficient fatigue strength cannot be obtained, so the Vickers hardness of the central portion is set to be 120Hv or more. And it should be above 150Hv, more preferably above 170Hv.

On the other hand, if the Vickers hardness of the central portion is greater than 300 Hv, the entire steel sheet will be too hard, resulting in a reduction in punching workability, so the Vickers hardness of the central portion is set to 300 Hv or less. And it should be below 270Hv, more preferably below 250Hv.

In addition, the Vickers hardness of the central portion can be controlled by the solid solution strengthening of Fe by Si, Al, and Mn, or by the crystal grain size after completion of annealing. As long as the required magnetic properties, cold-rolling delay processability and manufacturing cost are weighed to determine the contents of Si, Al and Mn, and determine the crystal grain size after the finish annealing. In addition, the crystal grain size will affect the magnetic properties, especially the iron loss.

<Vickers hardness of the internal oxide layer> Vickers hardness of the internal oxide layer: 1.15 times or more of the Vickers hardness of the central portion The SiO ₂ phase is finely and finely deposited in the internal oxide layer to control the hardness of the internal oxide layer , You can further increase the fatigue strength. That is, in this embodiment, the Vickers hardness of the internal oxide layer becomes larger than the Vickers hardness of the central portion of the steel plate.

If the Vickers hardness of the internal oxide layer is less than 1.15 times the Vickers hardness of the central portion, the effect of sufficiently improving the fatigue strength cannot be obtained, so the Vickers hardness of the internal oxide layer is set to 1.15 times the Vickers hardness of the central portion the above. And it should be more than 1.20 times, more preferably more than 1.25 times.

From the viewpoint of improving fatigue strength, the upper limit of the Vickers hardness of the internal oxide layer is not specifically defined. However, the Vickers hardness of the inner oxide layer actually obtained is about 1.5 times the Vickers hardness of the central portion.

Since the Vickers hardness of the internal oxide layer only needs to be 1.15 times or more of the Vickers hardness of the central portion, it may be 138 Hv or more. However, the Vickers hardness of the internal oxide layer should be above 155Hv, more preferably above 180Hv, and more preferably above 200Hv. In addition, the Vickers hardness of the internal oxide layer may be 400Hv or less, and preferably 300Hv or less.

The observation of the internal oxide layer and the structure of the central portion of the silicon steel sheet or the hardness measurement can be carried out using general observation and measurement methods. For example, it may be performed by the following method.

The test piece is cut out from the non-oriented electromagnetic steel sheet in such a way that the cutting direction is parallel to the plate thickness direction (specifically, the cutting surface is parallel to the plate thickness direction and perpendicular to the rolling direction) Test piece), and using SEM (Scanning Electron Microscope; Scanning Electron Microscope), the cross-sectional structure of the cut surface was observed at a magnification that each layer could enter the observation field. As long as it is observed using, for example, a reflected electron composition image (COMPO image), the structural phase of the cross-sectional structure can be inferred. For example, in the COMPO image, the silicon steel plate can be identified as a light color, the SiO ₂ phase in the internal oxide layer can be identified as a dark color, and the insulating coating can be identified as a middle color. And if necessary, use SEM-EDX (Energy Dispersive X-ray Spectroscopy; Energy Dispersive X-ray Spectroscopy) to carry out quantitative analysis of the composition of the composition, in order to clarify the constituent phases in detail.

In addition, SEM and SEM-EDX can also be used to clarify whether there is an internal oxide layer on the surface area of the silicon steel plate. Specifically, it is confirmed from the interface between the silicon steel sheet and the upper layer toward the depth direction of the silicon steel sheet that there is a region where the SiO ₂ phase can be observed. The SiO ₂ phase only needs to specify precipitates with an atomic ratio of Si to O of about 1:2 in the observation field by EDX. For example, in the observation field, along a straight line in the thickness direction is set as a reference line, to confirm whether the region of the SiO ₂ phase can be observed in the presence of the reference line, if there exists in the silicon steel sheet was observed with SiO ₂ The area is determined as the internal oxide layer. In addition, the line segment (length) on the reference line of this region may be used as the thickness of the internal oxide layer.

The average thickness of the internal oxide layer can be determined by the following method. In the SEM image, an area of approximately 100 μm or more was observed in the plane direction of the steel sheet. Next, 10 or more of the above reference lines are set at equal intervals, and the thickness of the internal oxide layer on each reference line is calculated. In addition, the average value of the thickness of the internal oxide layer obtained is taken as the average thickness of the internal oxide layer.

In addition, when recognizing the SiO ₂ phase or determining the average thickness of the internal oxide layer, if you need to observe a region that is more microscopic than the resolution of the SEM, you can also use a transmission electron microscope (TEM: Transmission Electron Microscope) .

The Vickers hardness can be measured by the method described in JIS Z 2244:2009. Regarding the Vickers hardness of the internal oxide layer, the Vickers hardness indentation must stay in the internal oxide layer, and the measured load at this time should be between 9.8×10 ^-5 and 9.8×10 ^-2 N.

The Vickers hardness of the internal oxide layer can be measured in a form corresponding to the thickness of the internal oxide layer. Within the thickness of the internal oxide layer, as long as the load of the indentation with the largest size can be set appropriately, it can be performed more accurately Determination. In order to accurately measure the Vickers hardness of the internal oxide layer, the load may also be a load greater than the above load range.

In the measurement of the Vickers hardness, the indentation diameter is usually measured with an optical microscope, but for high-precision measurement, the indentation diameter can also be measured with an electron microscope such as SEM at a magnification of 1000 times or more.

On the other hand, the Vickers hardness of the central portion of the steel plate should be measured with the same load as the Vickers hardness of the internal oxide layer. In this case, the indentation diameter will be smaller than the crystal grain size of the steel plate. Therefore, it is more desirable to set the indentation away from the crystal grain boundary and measure the indentation diameter.

In the Vickers hardness test stipulated by JIS, although the measured load is set from 1gf (9.8×10 ^-2 N), it is better to precisely control the load, reduce the load, and set the load so that the indentation stays inside the internal oxide layer, to Determine the Vickers hardness. In addition, when measuring Vickers hardness, if it is necessary to observe a region that is more microscopic than the resolution of an optical microscope or SEM, the nanoindentation method can also be used to convert the measured value to Vickers hardness.

Next, a method of manufacturing the non-oriented electrical steel sheet of this embodiment will be described.

FIG. 2 is a flowchart showing the method of manufacturing the non-oriented electrical steel sheet according to this embodiment. In this embodiment, the molten steel with the adjusted composition is cast, hot rolled, and then subjected to heat treatment, pickling, and cold rolling during cooling after hot rolling, followed by finish annealing to produce silicon Steel plate. In addition, an insulating coating is provided on the silicon steel sheet to produce a non-oriented electromagnetic steel sheet.

Here, the formation of the internal oxide layer will be described. FIG. 3 is a schematic cross-sectional view showing a case where an internal oxide layer is formed on a base material steel plate. FIG. 3(A) shows the state after hot rolling, FIG. 3(B) shows the state after heat preservation treatment, FIG. 3(C) shows the state after pickling, and FIG. 3(D) shows the cold The state after rolling.

As shown in FIG. 3(A), through hot rolling, an external oxide layer 17 is formed on the surface of the base material steel plate 11. Next, as shown in FIG. 3(B), by the heat preservation treatment after cooling after hot rolling, oxygen can be diffused from the outer oxide layer 17 into the base material steel plate 11 to form the inner oxide layer 13. At this time, by controlling the conditions of the heat preservation treatment, the SiO ₂ phase 131 is finely and finely precipitated in the internal oxide layer 13.

Next, as shown in FIG. 3(C), the external oxide layer 17 on the surface of the base material steel plate 11 is removed by pickling. At this time, for the purpose of improving the magnetic characteristics, a part of the internal oxide layer 13 may also be removed by pickling to control the thickness of the internal oxide layer 13. Then, as shown in FIG. 3(D), the internal oxide layer 13 on the surface of the base material steel plate 11 is extended in the rolling direction L by cold rolling. After cold rolling, the internal oxide layer 13 may be left in its original state. If the thickness of the internal oxide layer 13 is too thick, a part of the internal oxide layer 13 may also be removed by pickling or the like to control the internal oxide layer 13 thickness.

After that, finish annealing is performed in a gas environment containing nitrogen and hydrogen, for example, to recrystallize the base material steel plate and grow grains, and a silicon steel plate having an internal oxide layer containing SiO ₂ phase on the surface can be produced .

An insulating film can also be applied on the surface of the silicon steel plate. The insulating coating is generally referred to as a semi-organic coating. Generally, for example, it is a film composed of chromic acid and an organic resin disclosed in Non-Patent Document 1, or a film composed of phosphate and an organic resin disclosed in Non-Patent Document 2. The adhesion amount of the insulating coating should be 0.1~5gm ^-2 per side.

The non-oriented electrical steel sheet of this embodiment is characterized in that the silicon steel sheet has an internal oxide layer containing an SiO ₂ phase, the average thickness of the internal oxide layer is 0.10 μm or more and 5.0 μm or less, and the dimension of the central portion of the steel sheet When the Vickers hardness is 120 Hv or more and 300 Hv or less, the Vickers hardness of the internal oxide layer is 1.15 times or more and 1.5 times or less the Vickers hardness of the central portion.

The silicon steel sheet having the above-mentioned characteristics may be manufactured by, for example, the following method.

<Hot rolling> The cast piece with adjusted composition is heated and hot rolled. At this time, in order not to deteriorate the iron loss due to the solid solution and precipitation of sulfides and the like in the steel, the heating temperature is set to 1200° C. or lower. In order to ensure a completion temperature of 900°C or higher, the heating temperature is set to 1080°C or higher.

If the finishing temperature of hot rolling is low, the hot workability will be reduced and the thickness accuracy of the steel sheet in the width direction will be reduced. Therefore, the lower limit of the finishing temperature is set to 900°C. On the other hand, if the completion temperature is higher than 1000°C, the {100} aggregate structure beneficial to magnetism will be reduced, so the upper limit of the completion temperature is set to 1000°C.

In addition, in order to properly form an internal oxide layer during the heat treatment after hot rolling, it is preferable to form an external oxide layer with a thickness of 1 μm or more on the surface of the hot rolled steel sheet during hot rolling. The formation of the external oxide layer can be controlled by the temperature or the holding time of the hot rolling delay.

<Insulation treatment> During the cooling after the hot rolling, the hot rolled steel plate is kept warm. In the heat preservation treatment, the crystal grain size is coarsened to 20 μm or more, and the oxygen contained in the outer oxide layer generated on the surface of the hot-rolled steel sheet is diffused into the hot-rolled steel sheet to form an inner oxide layer.

The internal oxide layer is an external oxide layer formed by hot rolling delay. Specifically, it uses an external oxide layer mainly containing magnetite and containing wurtzite or hematite as an oxygen source to diffuse oxygen to Formed inside the steel plate.

In the middle of cooling after hot rolling, in a gas environment with an oxygen partial pressure of 10 ^-15 Pa or more, within a temperature range of 850°C or less and 700°C or more, and for a time of 10 minutes or more and 3 hours or less The thermally insulated hot-rolled steel sheet can form an internal oxide layer in which SiO ₂ phase is finely and finely precipitated, and the hardness of the internal oxide layer can be appropriately controlled.

If the holding temperature is higher than 850℃, the average thickness of the internal oxide layer will become thicker. Therefore, even after cold rolling, the average thickness of the internal oxide layer is still greater than 5.0 μm, so it may sometimes burden pickling to reduce the thickness of the internal oxide layer. In addition, if the holding temperature is higher than 850°C, the SiO ₂ phase will not precipitate finely and finely. Therefore, the holding temperature should be below 850℃. On the other hand, although the holding temperature also depends on the Si concentration in the steel, in order to precipitate the SiO ₂ phase finely and finely, it is preferably 700°C or higher, and more preferably 750°C or higher. Above 800℃.

In order to allow the grains of the hot-rolled steel sheet to grow to more than 20 μm, the holding time should preferably be more than 10 minutes. In addition, in order to precipitate the SiO ₂ phase finely and finely, the holding time is preferably 10 minutes or more, 20 minutes or more, and 30 minutes or more. On the other hand, although the upper limit of the holding time is not particularly limited, if the holding time is too long, the crystal grain boundaries near the surface of the steel plate will become brittle, and subsequent pickling and cold rolling will easily cause cracking or It is broken, so the holding time should be less than 3 hours.

The gas environment for heat preservation is preferably based on oxygen partial pressure above ^10-15 Pa. The gas environment is preferably a mixed gas environment of nitrogen and other inert gases.

In addition, it is advisable to form an external oxide layer of 1 μm or more during the hot rolling delay, and adjust it to keep the steel plate surface in contact with the gas environment during heat preservation during the heat preservation treatment before heat preservation. For example, if the hot-rolled steel sheet is coiled and then heat-preserved, in addition to the outermost surface of the coil, the plate surfaces of the steel plate are in contact with each other. Therefore, the contact between the surface of the steel plate and the gas environment during heat preservation can be suitably blocked.

If the steel sheet contains Sn, Cu, and Sb, these elements will inhibit the formation and growth of the internal oxide layer, so the heat retention temperature can be increased within the above range. At this time, excessive growth of the internal oxide layer can be suppressed, and the crystal grain size can be appropriately coarsened. In addition, when the steel sheet contains Sn, Cu, and Sb, if the holding temperature is above 800°C, an internal oxide layer with an appropriate thickness can be formed, and the magnetic flux density can be suitably improved.

However, even if the steel plate contains Sn, Cu and Sb, if the holding temperature is too high, although the magnetic properties will be improved, the internal oxide layer sometimes becomes too thick. In this case, the amount of pickling can be controlled during the pickling process to adjust the internal oxide layer to an appropriate thickness.

In addition, when the steel sheet contains Sn, Cu, and Sb, the formation and growth of the internal oxide layer will be inhibited. It is presumed that these elements will segregate between the external oxide layer and the steel, thereby hindering the inclusion of the external oxide layer. The reason why the oxygen diffuses into the steel plate.

In the conventional technique, the hot rolled steel sheet is cooled to near room temperature after hot rolling, and then heated again to anneal the hot rolled sheet that is maintained in the temperature range of 800 to 1000°C for about 1 minute. However, in this embodiment, in order to appropriately control the internal oxide layer, the hot-rolled steel sheet is kept under the above conditions during the cooling process after the hot rolling. And after cooling the steel plate after heat preservation to near room temperature, it is supplied to pickling and cold rolling without performing hot-rolled sheet annealing.

<Pickling> The base material steel plate after the heat treatment is pickled. The amount of pickling (weight loss after pickling) will vary depending on the state of the outer and inner oxide layers on the surface of the steel plate, and the acid species, concentration, or temperature used for pickling. In pickling, it is only necessary to dissolve the outer oxide layer and reduce the thickness of the inner oxide layer to the target thickness.

As an adjustment method for reducing the amount of pickling, an effective method is, for example, a method of shortening the pickling time, lowering the temperature of the pickling solution, or adding a commercially available pickling inhibitor (polyamine, etc.). The pickling inhibitor contains, for example, polyamine as the main component, and the polymer has the property of non-shared electron pairs that are easily adsorbed to iron atoms. By attaching the polymer to the surface of the steel plate, the area contacting the acid is reduced, and the pickling speed can be suppressed. As additives for enhancing this effect, for example, formic acid and the like are known.

On the other hand, as an adjustment method for increasing the pickling amount, for example, a method of extending the pickling time, raising the temperature of the pickling solution, or adding a commercially available pickling accelerator (sodium thiosulfate, etc.). The pickling accelerator is a chelating agent for iron atoms, that is, it has the property of easily forming coordination bonds of iron ions. If the pickling accelerator is added, the iron dissolved in the pickling solution will be chelated. Therefore, the concentration of iron ions dissolved in the pickling solution is difficult to increase, so pickling can be done when the dissolution rate of iron does not become low Proceed.

<Cold rolling extension> The base material steel plate after pickling is cold rolled. From the viewpoint of improving the magnetic flux density, the cold rolling reduction ratio is preferably 50 to 90%. In addition, the cold-rolling shrinkage ratio is the cumulative cold-rolling shrinkage ratio, which can be calculated from the following: (plate thickness before cold-rolling-plate thickness after cold-rolling)÷plate thickness before cold-rolling×100. The more ideal system is to calculate the thickness of the final product and calculate the cold rolling shrinkage and cold rolling ductility.

<Complete Annealing> The base steel sheet after cold rolling is finished and annealed. Finishing annealing is a step to recrystallize the cold-rolled steel sheet and adjust the crystal grain size to obtain magnetic properties, and is particularly a step of good magnetic flux density and iron loss characteristics. During finishing annealing, the gas environment is very important. If the steel plate is oxidized, the magnetic properties will be reduced, so it is better to make the oxygen concentration of the gas environment in the finish annealing below tens of ppm.

The gas system of the gas environment is preferably a nitrogen environment or an argon environment, and hydrogen can also be added as necessary to prevent oxidation of the steel plate. In addition, if the hydrogen concentration is excessively increased, the internal oxide layer will be reduced, and the fine SiO ₂ phase that contributes to the improvement of fatigue strength will be reduced.

The finish annealing temperature is preferably 700°C or more, which can produce recrystallization of the steel plate. If the finish annealing temperature is too low, recrystallization will be insufficient. On the other hand, if the finish annealing temperature is too high, the fine SiO ₂ phase contained in the internal oxide layer will grow, and the effect of improving fatigue strength cannot be obtained. Therefore, the annealing temperature should be below 1150℃.

After finishing annealing, the silicon steel sheet forms an insulating coating. The insulating film may be, for example, a film composed of chromic acid and organic resin, or a film composed of phosphate and organic resin. The adhesion amount of the insulating coating should be 0.1~5gm ^-2 per side.

Examples Next, the effects of one aspect of the present invention will be described in more detail through examples, but the conditions in the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this One condition example. As long as the purpose of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

<Example 1> After casting molten steel with adjusted composition, the production conditions in each step are controlled to produce silicon steel sheets. The chemical composition is shown in Table 1 and Table 2, and the manufacturing conditions are shown in Table 3 and Table 4. In addition, in the above-mentioned manufacturing, heating was performed at a heating temperature of 1180°C, and hot rolling was performed under the condition that the output side temperature of the finishing rolling was 970°C, and a hot-rolled steel sheet having a thickness of 2.0 mm was produced. At this time, a layer mainly composed of Fe ₃ O _{4 of} about 10 μm was formed on the surface as an external oxide layer.

For the produced hot-rolled steel sheet, in the midway of cooling after hot-rolling, in a gas atmosphere with an oxygen partial pressure of 10 ^-15 Pa or more, heat preservation treatment is performed at the temperatures and times described in Table 3 and Table 4, The crystal grains grow to 20 μm or more, and an internal oxide layer is formed. In addition, for the sample described as "Hot rolled sheet annealing" in the "Heat preservation" column in Table 4, it was cooled to room temperature without heat preservation in the middle of cooling after hot rolling, and thereafter, 100% nitrogen gas The hot-rolled sheet was annealed at 800°C for 60 seconds in the environment.

The steel sheet subjected to heat preservation or hot-rolled sheet annealing after hot rolling is immersed in hydrochloric acid (10% by mass) at 85° C. with the additives (0.05% by mass) described in Tables 3 and 4 for 30 seconds. Pickling. And the steel plate after pickling was cold-rolled by a reduction ratio of 75% to produce a cold-rolled steel plate with a thickness of 0.5 mm. In a furnace with a gas atmosphere of 10% hydrogen + 90% nitrogen, the cold-rolled steel sheet was finish annealed at 1000°C for 30 seconds. At this time, the dew point of the gas environment in the furnace is -30℃. In addition, a phosphoric acid-based insulating film having an average thickness of 1 μm is formed on the silicon steel sheet after the finish annealing.

Thereafter, the magnetic properties (B ₅₀ and W _15/50 ) and fatigue strength were measured, and the Vickers hardness of the internal oxide layer and the central portion of the steel plate was measured. The results are shown in Table 5 and Table 6 together.

Magnetic properties (B ₅₀ and W _15/50 ) _Cut from the prepared non-directional electromagnetic steel plate and take a sample of 55 mm square, and use the Single Sheet Tester (single-sheet tester; SST) to measure B ₅₀ (at 5000A/m The magnetizing force of the steel plate is the magnetic flux density of the steel plate when the steel plate is magnetized, unit: T (Tesla) and W _15/50 (iron loss when the steel plate is magnetized at 50 Hz to a magnetic flux density of 1.5 T). Evaluation criteria for B ₅₀ : Qualified above 1.65T: Failed: less than 1.65T W _{15/50 Qualified} for assessment: Less than 3.0W/kg: Failed: Greater than 3.0W/kg

Fatigue strength Using electrical discharge machining from the rolling direction of the steel plate to take a sample from the produced non-oriented electromagnetic steel plate, this sample is equivalent to the test piece No. 5 specified in JIS Z 2241: 2011 Appendix B, and under the following conditions stress test. Fix the stress ratio, and change the corresponding minimum stress and maximum stress to carry out the experiment, find out the stress condition that does not rupture more than 2 of the number of samples, and the number of repeats is 2 million times. The average stress ((minimum stress + maximum stress) ÷ 2) is taken as the fatigue strength.

The fatigue test was carried out under the condition of the average stress ±10 MPa, and the stress conditions that did not break more than 2 of the number of samples with 2 million times of repeated times were obtained, and the average strength at this time was used as fatigue strength. Test conditions Test method Partial pulsation test Stress ratio 　　0.05 Frequency 　　20Hz Repeat times 　 2 million times Number of samples: 3 per stress level Evaluation criteria for fatigue strength Pass: The average stress is more than 200MPa 　Fail: The average stress is less than 200MPa

Analysis of the average thickness of the internal oxide layer and the internal oxidation chromatography The cross section of the non-oriented electromagnetic steel plate produced by grinding was used to capture the SEM image at a magnification of 1000 times using a reflection electron image. The surface of the steel plate and the back surface were observed in an area of about 100 μm or more in the plane direction of the steel plate. If necessary, the cross section of the non-oriented electrical steel sheet produced is observed by TEM.

In addition, the observation of the internal oxide layer and the structure of the central portion of the silicon steel plate and the hardness measurement are carried out according to the above method. The average thickness of the internal oxide layer is calculated from a total of 20 locations. In addition, the Vickers hardness is measured with a load of 0.03gf (2.94×10 ^-3 N), and a total of 10 indentations are formed in the internal oxide layer and the central part, and the diagonal length of each indentation (diamond) is measured by SEM , The average value is calculated from a total of 10 locations. And if necessary, convert the value measured by the nanoindentation method to Vickers hardness.

Table 1 and Table 2 show the chemical composition of the manufactured silicon steel sheet, and Table 3 to Table 6 show the manufacturing conditions and evaluation results. Moreover, the chemical composition of molten steel is essentially the same as that of silicon steel sheet. In the table, the value with the bottom line indicates that it is outside the scope of the present invention. In addition, in the table, regarding the composition of the silicon steel sheet, the symbol "-" indicates that no alloying elements were intentionally added.

As shown in Tables 1 to 6, the invention examples of Test Nos. B1 to B26 are suitable for controlling the composition of silicon steel sheet, the internal oxide layer and the central portion of the steel sheet, so as a non-directional electromagnetic steel sheet, the magnetic properties and fatigue Excellent strength. That is, in these test Nos. B1 to B26, without adding a new step for hardening the surface, a non-oriented electrical steel sheet excellent in fatigue strength and magnetic properties can be produced.

On the other hand, as shown in Table 2, Table 4 and Table 6, in the comparative examples of Test Nos. b1 to b14, since any of the composition of the silicon steel sheet, the internal oxide layer or the central portion of the steel sheet is not properly controlled, Therefore, as a non-oriented electrical steel sheet, it is impossible to satisfy any of the magnetic properties or fatigue strength.

[Table 1]

[Table 2]

[table 3]

[Table 4]

[table 5]

[Table 6]

Industrial availability According to the above aspect of the present invention, it is possible to provide a non-oriented electrical steel sheet that is excellent in fatigue strength and magnetic properties and also excellent in cost. Therefore, a non-oriented electromagnetic steel sheet can be provided, which is suitable as a core material for electrical equipment, particularly suitable as a core material for rotating machines, small and medium-sized transformers, or electronic components, and is particularly suitable as a rotor core for IPM motors. In addition, a non-oriented electromagnetic steel sheet can be provided, which can fully respond to the demands for higher efficiency, higher speed, and miniaturization of rotating machines in the field of electrical equipment. Therefore, the industrial availability is high.

1: Non-directional electromagnetic steel sheet 11: Silicon steel sheet (base material steel sheet) 13: Internal oxide layer 15: Insulation coating (tension coating) 17: External oxide layer 131: SiO ₂ phase L: Rolling direction

1 is a schematic cross-sectional view showing a non-oriented electrical steel sheet according to an embodiment of the present invention. FIG. 2 is a flowchart showing the method of manufacturing the non-oriented electrical steel sheet according to this embodiment. FIG. 3 is a schematic cross-sectional view of the non-oriented electrical steel sheet of the present embodiment in which an internal oxide layer is formed on the base material steel sheet.

11: Silicon steel plate (base material steel plate)

13: Internal oxide layer

17: External oxide layer

131: SiO ₂ phase

L: rolling direction

Claims (9)

A non-oriented electrical steel sheet, comprising a silicon steel sheet and an insulating coating, the non-oriented electrical steel sheet is characterized in that the composition of the aforementioned silicon steel sheet includes: in mass %, Si: more than 2.00% and 4.00% or less, Al: 0.10 % Or more and 3.00% or less, Mn: 0.10% or more and 2.00% or less, C: 0.0030% or less, P: 0.050% or less, S: 0.005% or less, N: 0.005% or less, Sn: 0% or more and 0.40% or less , Cu: 0% or more and 1.00% or less, Sb: 0% or more and 0.40% or less, REM: 0% or more and 0.0400% or less, Ca: 0% or more and 0.0400% or less, and Mg: 0% or more and 0.0400% or less , And the remaining part is composed of Fe and impurities; when viewed in a cutting plane parallel to the cutting direction and the thickness direction, the thickness range of the 5/8~3/8 of the silicon steel plate, that is, the Vickers hardness of the central part is 120Hv Above and below 300Hv; and when viewed from the cut surface, the silicon steel sheet has an internal oxide layer containing SiO ₂ phase on the surface, the average thickness of the internal oxide layer is 0.10 μm or more and 5.0 μm or less, and the internal oxide layer The Vickers hardness is 1.15 times or more and 1.5 times or less with respect to the Vickers hardness of the central portion. The non-oriented electrical steel sheet according to claim 1, wherein the foregoing composition of the silicon steel sheet includes at least one of the following elements; in mass %, Sn: 0.02% or more and 0.40% or less, Cu: 0.10% or more and 1.00% The following and Sb: 0.02% or more and 0.40% or less. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the foregoing composition of the silicon steel sheet includes at least one of the following elements: in mass %, REM: 0.0005% or more and 0.0400% or less, Ca: 0.0005% or more and 0.0400% or less and Mg: 0.0005% or more and 0.0400% or less. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the Vickers hardness of the internal oxide layer is above 155Hv. The non-oriented electrical steel sheet according to claim 3, wherein the Vickers hardness of the internal oxide layer is above 155Hv. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the average thickness of the aforementioned internal oxide layer is 0.55 μm or more. The non-oriented electrical steel sheet according to claim 3, wherein the average thickness of the aforementioned internal oxide layer is 0.55 μm or more. For the non-oriented electromagnetic steel sheet of claim 4, where the front The average thickness of the internal oxide layer is above 0.55 μm. The non-oriented electrical steel sheet according to claim 5, wherein the average thickness of the aforementioned internal oxide layer is 0.55 μm or more.

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