JP7601068B2 - Rotor core, its manufacturing method and motor - Google Patents

Description

The present invention relates to a rotor core used in high-speed motors such as drive motors for electric vehicles, a manufacturing method thereof, and a motor using the rotor core.

The drive motors used in electric vehicles are driven at high frequencies to achieve compact size and high efficiency. Many of these motors are internal permanent magnet motors (IPM motors), which have permanent magnets embedded inside the rotor core.

When the IPM motor rotates at high speed, a large centrifugal force acts on the magnets embedded in the rotor core, tending to jump out in the radial direction of the rotor. Furthermore, as the motor's rotation speed changes significantly with the acceleration and deceleration of the vehicle, the centrifugal force acting on the magnets also changes constantly. For this reason, in addition to having excellent magnetic properties, the rotor core is also required to have excellent strength properties such as tensile strength and fatigue strength.

In response to the above demands, efforts have been made to improve the strength characteristics mainly from the perspective of the material, such as by adding large amounts of elements such as Si and Al, or by controlling the crystal grain size and crystal orientation. However, improving the strength characteristics by adding such alloying elements makes the steel plate that constitutes the rotor core hard and brittle, which has a negative effect on the manufacturability of the steel plate itself. In addition, a method of improving the strength characteristics by reducing the crystal grain size can achieve a certain degree of effect by lowering the finish annealing temperature. However, strict control of the annealing conditions is required to stably obtain fine recrystallized grains of several tens of μm or less, and in some cases, the processed structure remains in the structure, which results in a problem that the magnetic properties and strength properties are not improved as much as expected. As mentioned above, there are limitations to improving the strength properties from the perspective of the material (steel plate) that constitutes the rotor core.

When manufacturing motor cores, punching is generally used to cut the core-shaped steel sheet from the base steel sheet. However, this punching process tends to result in large roughness on the cut surface. As a result, it is known that the fatigue properties of the material after punching are inferior to those of the original material.

To address this issue, for example, Patent Document 1 proposes a technique for improving the fatigue properties of a rotor core by chemically dissolving the end faces of holes formed by punching or the like. Patent Document 2 proposes a technique for obtaining a high-strength rotor by applying "laser peening," in which a laser is irradiated through a liquid onto the bridge side of the inner circumferential surface of the magnet insertion hole of the rotor core. Furthermore, Patent Document 3 proposes a method of performing a shaving process to shear and remove the plastic strain area generated by the punching process, and Patent Document 4 proposes a method of improving the fatigue strength of a rotor core by performing a coining process on the fracture surface of the punched end face to form a crushed surface.

JP 2009-219306 A JP 2005-124386 A International Publication No. WO 2011/129000 International Publication No. 2018/012599

However, the method disclosed in the above Patent Document 1 has problems in terms of productivity and cost, since holes are formed once by a method other than chemical dissolution, and then the end of the hole is treated by chemical dissolution. Furthermore, the dissolution rate of the chemical dissolution process differs depending on the orientation difference of the crystal grains at the end face. Therefore, a step (hereinafter, this step is also referred to as a "step at the crystal grain boundary") occurs at the end face at the position where the crystal grain boundary is located, and stress is concentrated at this part, so that the effect of improving fatigue characteristics cannot be sufficiently obtained. In addition, the method of performing laser peening disclosed in Patent Document 2 has a problem of poor productivity. In addition, when the electromagnetic steel sheet is melted across the laminations by laser irradiation, a short circuit occurs at the melted end face, increasing eddy current loss and reducing motor efficiency. In addition, the technology of arranging the cut surface of the steel sheet processed into the core shape by post-processing, disclosed in Patent Documents 3 and 4, can improve fatigue strength, but it is difficult to strictly control the dimensional accuracy of the core and to control the air gap, which is a design factor that greatly affects the characteristics of the motor. Therefore, there is a problem that the variation in motor characteristics becomes large, and it is not an optimal method for obtaining a rotor core with excellent fatigue strength. In addition, since the techniques disclosed in the above Patent Documents 3 and 4 manufacture rotor cores by mechanical processing of material steel plates, in the case of high-strength steel plates, there are problems such as severe wear of the mold and quality control.
As described above, even when improving the strength characteristics of the rotor core from the viewpoint of the motor manufacturing process, many problems remain in terms of motor characteristics, productivity, and cost.

The present invention was made in consideration of the above problems with the conventional technology, and its purpose is to provide a rotor core with excellent strength characteristics, particularly fatigue characteristics, to propose a manufacturing method thereof, and to provide a motor that uses the rotor core to facilitate air gap control.

In order to solve the above problem, the inventors noticed that in a steel plate processed into a rotor shape (hereinafter also referred to as "rotor core material"), the part where stress concentration due to centrifugal force is greatest is the bridge part of the rotor core (the part sandwiched between the holes for embedding magnets). They then conducted extensive research into ways to improve the fatigue properties of the bridge part. As a result, it was found that it is effective not only to smooth the cut surface of the steel plate at the bridge part, i.e., the cut surface of the hole for embedding the magnet, but also to reduce the step that exists on the cut surface and is located at the position of the grain boundary to 0.5 μm or less. In addition, they discovered that in order to achieve this, it is necessary to form the holes by thermal cutting and to optimize the heat beam irradiation conditions for the thermal cutting, which led to the development of the present invention.

The present invention, based on the above findings, is a rotor core constructed by laminating steel plates that are taken from base steel plates and processed into the outer peripheral shape of the rotor core, and the rotor core is characterized in that the steel plates have a plurality of holes formed by thermal cutting, and the maximum step value at the grain boundaries present on the cut surfaces of the holes is 0.5 μm or less.

The material steel sheet in the rotor core of the present invention is characterized in that it is a non-oriented electrical steel sheet containing C: 0.0005-0.01 mass%, Si: 7 mass% or less, Mn: 0.05-3 mass%, and Al: 3 mass% or less, and optionally containing at least one selected from P: 0.001-0.1 mass%, S: 0.01 mass% or less, Sn: 0.001-0.1 mass%, Sb: 0.001-0.1 mass%, Ca: 0.0002-0.005 mass%, and Mg: 0.0002-0.005%, with the balance being Fe and unavoidable impurities.

The material steel plate in the rotor core of the present invention is characterized in that, when the surface layer is from the front and back surfaces of the steel plate to 1/3 of the plate thickness, and the center is from the surface of the steel plate to inside 1/3 of the plate thickness, the average Si content of the front and back surfaces of the surface layer is 1 mass% or more higher than that of the center.

The rotor core of the present invention is also characterized in that the thermal cutting process is a laser cutting process.

The rotor core of the present invention is also characterized in that the outer periphery of a steel plate processed into the shape of the rotor core is thermally cut.

The rotor core of the present invention is also characterized in that the thermal cutting process of the outer periphery is a laser cutting process.

The rotor core of the present invention is also characterized in that permanent magnets are inserted into some or all of the holes.

The present invention also proposes a method for manufacturing a rotor core, which involves forming multiple holes in a rotor core-shaped steel plate taken from a base steel plate having the above-mentioned composition by thermal cutting, and then stacking the steel plates to assemble the rotor core. In the thermal cutting process, the heat beam is irradiated at an angle of 1 to 5 degrees toward the hole side to be cut off with respect to the perpendicular line to the steel plate surface, thereby making the maximum step value of the grain boundary present on the cut surface 0.5 μm or less.

The steel sheet material used in the rotor core manufacturing method of the present invention is characterized in that, when the surface layer is from the front and back surfaces of the steel sheet to 1/3 of the sheet thickness, and the center is from the surface of the steel sheet to inside 1/3 of the sheet thickness, the average Si content of the front and back surfaces of the surface layer is 1 mass% or more higher than that of the center.

The rotor core manufacturing method of the present invention is also characterized in that the thermal cutting process is a laser cutting process.

The manufacturing method of the rotor core of the present invention is characterized by thermally cutting the outer periphery of a steel plate processed into the shape of the rotor core.

The manufacturing method of the rotor core of the present invention is also characterized in that the thermal cutting process of the outer periphery is a laser cutting process.

The present invention also provides a motor having a rotor core and a stator core, the rotor core being made of any of the cores described above, and characterized in that the shortest distance between the outer diameter of the rotor core and the inner diameter of the stator core is 0.30 mm or less.

The present invention makes it possible to provide a rotor core with excellent fatigue properties without excessively restricting the freedom of design of the air gap, which not only increases the efficiency of the motor but also greatly improves its reliability.

FIG. 2 is a diagram showing an example of a shape of a steel plate constituting a rotor core. FIG. 2 is a diagram illustrating an inclination angle θ in laser cutting processing. FIG. 2 is a diagram illustrating a fatigue test piece. FIG. 1 is a diagram illustrating a method for observing a cut surface of a fatigue test piece.

First, the rotor core for high-speed motors that is the subject of this invention is a rotor core to which centrifugal force is repeatedly applied. This rotor core is usually made by laminating steel plates (rotor core material) that have been processed into the outer periphery shape of the rotor core by punching or thermal cutting using a heat beam such as a laser beam from a raw steel plate, and assembling them by fixing them with rivets or the like. From the viewpoint of high efficiency, multiple permanent magnets are embedded in the rotor core of an IPM motor.

In the case of a rotor core having a shape as shown in Figure 1, it is known that the centrifugal force caused by high-speed rotation is concentrated in the bridge portion (part A or part B in Figure 1) sandwiched between the holes into which the magnets are inserted (magnet insertion holes). Therefore, such parts are prone to fatigue failure due to the centrifugal force that is repeatedly applied.

However, because the shape of the rotor core is determined from the standpoint of efficiency, it is difficult to change it. Therefore, as proposed in the above-mentioned Patent Documents 1 to 4, efforts have been made to improve the cut surfaces of the magnet insertion holes to prevent stress concentration in the bridge section. However, the above-mentioned conventional technology leaves much room for improvement, not only in terms of the effect of improving fatigue properties, but also in terms of manufacturability.

The inventors therefore conducted extensive research to solve the above problems. As a result, as described below, they discovered that in order to improve the fatigue properties of the rotor core with good productivity, it is extremely effective to form the magnet insertion holes by thermal cutting and to control the step at the grain boundaries present on the cut surface by the thermal cutting to 0.5 μm or less. They also discovered that in order to achieve the step of 0.5 μm or less, when forming the magnet insertion holes by thermal cutting, it is important to perform the cutting process by irradiating the heat beam used for cutting at an angle of 1 to 5 degrees toward the hole side to be cut off with respect to the perpendicular line of the steel plate surface, as shown in Figure 2.

The thermal beam used in the thermal cutting process for cutting steel plates in this invention can be a laser beam, an electron beam, a plasma beam, or gas cutting, but hereafter, the present invention will be explained using an example in which laser cutting is used as the thermal cutting process.

The experiments that led to the development of the present invention will now be described.
Using an electromagnetic steel sheet having a thickness of 0.20 mm and containing 0.0015 mass% C, 3.7 mass% Si, 0.5 mass% Mn, 1.0 mass% Al, 0.002 mass% S, with the balance being Fe and unavoidable impurities, rotor core materials having the shape shown in FIG. 1 were produced by the following five methods.
Method 1: Punching using a die (clearance: 5% of plate thickness)
Method 2: The cut surface of the rotor core material obtained by Method 1 is smoothed by mechanical polishing. Method 3: The cut surface of the rotor core material obtained by Method 1 is treated with a ferric chloride aqueous solution (45 Baume, liquid temperature 45°C) for 10 seconds. Method 4: Laser cutting (single mode fiber laser, output: 300 W, scanning speed: 10 m/min, laser beam inclination angle θ = 0°).
Method 5: Laser cutting (single mode fiber laser, output: 300 W, scanning speed: 10 m/min, inclination angle θ of laser beam = 2°)
(The inclination angle θ of the laser beam in the above methods 4 and 5 refers to the inclination angle of the laser beam from the perpendicular line of the steel plate surface toward the hole side to be cut off, as shown in FIG. 2.)

Next, a micro fatigue test piece having the shape shown in FIG. 3 (width of parallel part: 1.0 mm) was cut out from the bridge part (part A in FIG. 1) of the rotor core material and subjected to a fatigue test. At this time, the arithmetic mean roughness Ra of the cut surface (part C) of the parallel part, which is the laser cut surface, was measured in accordance with JIS B 0601-2001. The fatigue test was performed under tension-tension (pulsating) conditions of stress ratio: 0.1 and frequency: 20 Hz, and the maximum stress at which no breakage occurred even after ¹⁰⁷ repetitions was taken as the fatigue strength. The results are shown in Table 1.

From Table 1, the following can be seen:
A comparison of Method 1 and Method 2 shows that fatigue strength is improved by mechanical polishing, and a comparison of Method 2 and Method 3 shows that fatigue strength is further improved by chemical polishing. This is thought to be because mechanical polishing and chemical polishing reduce the surface roughness Ra of the punched cut surface, mitigating stress concentration.
On the other hand, when laser cutting is performed using method 4 (tilt angle θ: 0°), the surface roughness Ra is at the same level as method 2, but the fatigue strength does not reach that of method 2. However, method 5, in which the tilt angle θ is set to 2°, shows the highest fatigue strength among methods 1 to 5, even though the surface roughness Ra is at the same level as methods 2 and 4.

To investigate the cause of this, a more detailed investigation was carried out on the cut surface of the test piece. Specifically, the cut surface of the parallel part of the fatigue test piece (part C in Figure 3) was embedded in a resin mold, and the cut surface was observed using an optical microscope. Specifically, as shown in Figure 4, the test piece was embedded in a resin mold, and a cross section perpendicular to the cut surface was observed. In the observation surface of the embedded sample, the end corresponding to the cut surface was observed in the plate thickness direction, and the steps occurring at the grain boundary positions were measured at more than 30 points. As a result, the maximum step of the test piece of method 4 was more than 0.5 μm, while the maximum step of the test piece of method 5 was 0.5 μm or less. From this result, it became clear that simply reducing the surface roughness of the cut surface is insufficient to improve fatigue strength, and that it is only by further reducing the steps at the grain boundaries present on the cut surface that a rotor core with unprecedented high fatigue strength can be obtained.

The mechanism by which the steps at the grain boundaries can be reduced by performing laser cutting with a laser beam tilt angle θ in the range of 1 to 5° is not fully understood at present, but the inventors believe it to be as follows: Multiple physical phenomena, namely melting of the steel plate by laser irradiation, removal of the molten part by assist gas, and solidification of the molten part, affect each other in a complex way. Therefore, by setting the angle at which the laser is irradiated to the steel plate at a specified angle rather than perpendicular, conditions are met that do not create steps at discontinuous boundaries such as grain boundaries, improving the quality of the cut surface.
The present invention was developed based on the above novel findings.

Here, the type of laser used to form the magnet insertion hole is not particularly limited, and may be a commonly used _CO2 laser, fiber laser, etc. However, in order to control the step at the grain boundary present on the cut surface to 0.5 μm or less, a continuous laser is preferable to a pulsed laser because it can perform continuous and uniform processing along the processing surface.

The laser output and scanning speed are not particularly limited as long as they are conditions that can suppress the generation of dross and excessive sputtering. In the laser cutting process of the present invention, it is preferable to blow an assist gas in order to blow away the molten metal and prevent dross adhesion. There is no limit to the type of gas to be blown, and compressed air, _N2 , Ar gas, etc. can be used.

Next, the steel sheet used in the rotor core of the present invention is preferably a non-oriented electrical steel sheet having the following chemical composition.
C: 0.0005~0.01mass%
Since C is an element that causes magnetic aging and deteriorates the magnetic properties of the product sheet, it is desirable to reduce it as much as possible, and in the present invention, it is preferable to limit it to 0.01 mass% or less. However, since an extreme reduction in C leads to an increase in refining costs, the lower limit is set to 0.0005 mass%, and more preferably, it is in the range of 0.0010 to 0.0040 mass%.

Si: 7 mass% or less Si is an element that increases the resistivity of steel and reduces iron loss, so it is an effective element for reducing motor loss. It is also an effective element for increasing the strength of steel. In order to obtain the above effect, it is preferable that it is contained in an amount of 0.5 mass% or more. However, the addition of more than 7 mass% not only reduces the magnetic flux density, but also hardens the steel, making it difficult to reduce the properties of the cut surface, especially the steps at the grain boundaries present on the cut surface, when manufacturing a core material by machining such as punching. In addition, it is not preferable because it also increases the cost of raw materials. Therefore, it is preferable to limit Si to 7 mass% or less. More preferably, it is in the range of 2.5 to 6.5 mass%, and even more preferably, it is in the range of 3.5 to 6.5 mass%.

Mn: 0.05~3mass%
Mn, like Si, is an element that is effective in increasing the resistivity of steel to reduce iron loss and increase the strength of steel. It is also an element that improves hot workability. In order to obtain the desired effect, the content of Cr is preferably 0.05 mass% or more. However, if the content exceeds 3 mass%, the magnetic flux density decreases, the steel becomes hard, the workability decreases, and the raw material cost increases. Therefore, the Mn content is preferably in the range of 0.05 to 3 mass%, and more preferably in the range of 0.06 to 1.5 mass%.

Al: 3 mass% or less Like Si, Al is an element that is effective in increasing the resistivity of steel to reduce iron loss and increase the strength of steel. In order to obtain the above effect, it is desirable to contain 0.0010 mass% or more. However, if it exceeds 3 mass%, it will cause a decrease in magnetic flux density, as well as harden the steel, resulting in a decrease in workability and an increase in raw material costs, so it is preferable to limit it to 3 mass% or less. More preferably, it is in the range of 0.1 to 1.5 mass%.

The steel sheet used in the present invention contains the balance of Fe and unavoidable impurities other than the above-mentioned components, but may contain the following components as necessary.
P:0.001~0.1mass%
P has the effect of increasing the strength of steel and improving punching workability. In addition, P segregates at grain boundaries and has the effect of lowering the melting point of the grain boundaries and the steel in their vicinity during laser cutting, and is effective in reducing the step of the grain boundaries, so it is preferable to add 0.001 mass% or more. However, if it exceeds 0.1 mass%, the steel becomes too hard, making it difficult to roll, and in addition, it leads to an increase in raw material costs. Therefore, when P is added, it is preferable to set the upper limit at 0.1 mass%. More preferably, it is in the range of 0.02 to 0.08 mass%.

S: 0.01 mass% or less S is an element contained as an inevitable impurity, but it segregates at the grain boundary and has the effect of lowering the melting point of the grain boundary and the steel in its vicinity during laser cutting, and is effective in reducing the step of the grain boundary. However, if it exceeds 0.01 mass%, it forms precipitates with Mn etc. due to heating accompanying the laser cutting process, which leads to deterioration of magnetic properties, so the upper limit is preferably 0.01 mass%. More preferably, it is 0.0020 mass% or less.

Sn: 0.001 to 0.1 mass%, Sb: 0.001 to 0.1 mass%
Sn and Sb have the effect of lowering the melting point of the grain boundary and the steel in its vicinity during laser cutting by segregating at the grain boundary, and are effective in reducing the step of the grain boundary. Therefore, the content of each is 0.001 mass% or more. However, if the content exceeds 0.1 mass%, the rolling property deteriorates and the raw material cost increases, so the upper limit is preferably set to 0.1 mass%. More preferably, each of them is 0.02 to 1.0 mass%. The range is 0.08 mass%.

Ca: 0.0002 to 0.005 mass%, Mg: 0.0002 to 0.005 mass%
Ca and Mg form stable sulfides and have the effect of improving the grain growth of the base steel sheet, so it is preferable to include each of them in an amount of 0.0002 mass% or more. However, if the content exceeds 0.005 mass%, Since the above effects are saturated, the upper limit is preferably set to 0.005 mass%, and more preferably, each of them is in the range of 0.001 to 0.004 mass%.

Next, an object to which the laser cutting process of the present invention is applied will be described.
The non-oriented electrical steel sheet used in the present invention preferably has a Si concentration gradient of 1 mass% or more in the sheet thickness direction. Since the eddy current loss of the steel sheet is concentrated in the surface layer of the sheet thickness due to the skin effect, increasing the Si concentration of the sheet thickness surface layer to increase the resistivity is effective in reducing the eddy current loss. In order to fully obtain this effect, it is preferable that the Si concentration difference between the surface layer part and the center part of the steel sheet is 1 mass% or more (the surface layer part is 1 mass% or more higher than the center part). Here, the above-mentioned Si concentration difference refers to the difference between the average value of the Si concentration of the surface layer parts of the front and back and the Si concentration of the center part when the steel sheet is divided into three parts in the sheet thickness direction, the layers from the surface of the front and back steel sheets to 1/3 of the sheet thickness are the surface layer parts, and the layer sandwiched between them at 1/3 of the sheet thickness in the center of the sheet thickness is the center part. The Si concentration difference may be measured by measuring the Si concentration distribution in the cross section of the steel sheet by EPMA, or by wet analyzing the Si concentration of the steel sheet across its entire thickness and of the remaining ⅓ of the sheet thickness after ⅓ of the sheet thickness is dissolved and removed from both sides of the steel sheet by chemical polishing or the like, and determining the difference therebetween.

While electrical steel sheets with a high-Si surface layer have excellent magnetic properties such as iron loss in the high-frequency range, they are difficult to control the properties of the cut surface by machining such as punching. For this reason, it is extremely effective to apply the laser cutting process of the present invention to electrical steel sheets with a high-Si surface layer.

From the viewpoint of improving motor efficiency, it is effective to reduce the gap between the outer diameter of the rotor core and the inner diameter of the stator core when the motor is assembled. However, in conventional punching, some additional processing is required on the outer surface of the rotor core to smooth the cut surface, making it difficult to reduce the gap. On the other hand, when the laser cutting process of the present invention is applied, the outer surface of the rotor core can be made extremely smooth with little unevenness. As a result, when the laser cutting process of the present invention is applied, it is possible to make the shortest distance between the outer diameter of the rotor core and the inner diameter of the stator core 0.30 mm or less, which can greatly contribute to improving motor efficiency. In principle, it is possible to manufacture a motor with an unprecedented narrow air gap as long as it is equal to or greater than the kerf width during laser cutting.

In addition, when manufacturing a rotor core, it is most effective to apply the laser cutting process of the present invention to the areas where stress concentration due to centrifugal force occurs most when the rotor core rotates. However, by also applying it to the outer periphery of the rotor core, the fatigue strength of the rotor core can be increased more reliably.

Furthermore, the laser cutting process of the present invention can be applied not only to the formation of magnet insertion holes, but also to the formation of cooling holes through which coolant flows to cool the rotor core, which is subject to stress concentration due to centrifugal force, just like the magnet insertion holes. Therefore, in the present invention, it is not necessary for permanent magnets to be inserted into all of the holes created by the laser cutting process. Furthermore, it is sufficient for the permanent magnets to be inserted into a portion of the opening area of the magnet insertion hole, and they do not need to occupy the entire area.

As mentioned above, the above explanation has been given with an example of cutting using a laser beam as a thermal beam (laser cutting), but thermal cutting using an electron beam, plasma beam, gas cutting, or the like other than a laser beam may also be used. However, from the standpoint of reducing manufacturing costs, improving productivity, and reducing capital investment, laser cutting is the most preferable.

Steel having the composition shown in Table 2, with the balance being composed of Fe and unavoidable impurities, was melted by a commonly known refining process, and continuously cast to obtain a steel material (slab), which was then hot-rolled to obtain a hot-rolled sheet having a thickness of 2.3 mm. Next, this hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C x 10s, pickled, and cold-rolled to obtain a cold-rolled sheet having the final thickness shown in Table 2. Next, the cold-rolled sheet was subjected to finish annealing in an atmosphere of 20 vol% H ₂ -80 vol% N ₂ at various temperatures shown in Table 2 for 10s. Thereafter, an insulating coating of an organic/inorganic mixture was formed on the surface of the steel sheet to obtain a non-oriented electrical steel sheet (product sheet).

The product plate thus obtained was used as a material to process a steel plate (rotor core material) having the shape shown in FIG. 1 by various methods shown in Table 2. Then, fatigue test pieces having the shape shown in FIG. 3 were cut out from part A (see FIG. 1) of the rotor core material and subjected to a fatigue test. At this time, the steps at the grain boundaries present on the laser cut surface (part C, see FIG. 3) of the parallel part of the test piece were measured in the same manner as in the above-mentioned experiment. The fatigue test was performed under the conditions of tension-tension (pulsating), stress ratio: 0.1, and frequency: 20 Hz, and the maximum stress at which no fracture occurred even after ¹⁰⁷ repetitions was taken as the fatigue strength.

The results of the above measurements are shown in Table 2 together with the steel components and manufacturing conditions. From these results, it can be seen that high fatigue strength is obtained when the maximum difference in grain boundary height on the laser cut surface is 0.5 μm or less, and this difference in height is obtained when the inclination angle θ is set in the range of 1 to 5° in the laser cutting process. On the other hand, it can be seen that in laser cutting processes and punching processes that do not meet the above conditions, the difference in grain boundary height cannot be made 0.5 μm or less, and high fatigue strength is not obtained. Among them, in the invention examples in which the inclination angle θ is set in the range of 3 to 5°, the difference in grain boundary height on the laser cut surface is 0.33 μm or less.

A steel containing 0.01 mass% C, 2 mass% Si, 0.03 mass% Mn, 0.005 mass% Al, and the balance being Fe and unavoidable impurities, was melted by a commonly known refining process and continuously cast into a steel material (slab). Next, the slab was hot rolled to obtain a hot rolled sheet having a thickness of 2.0 mm. Next, the hot rolled sheet was subjected to hot rolled sheet annealing at 1000 ° C. × 10 s, pickled, and cold rolled to obtain a cold rolled sheet having a final thickness of 0.1 mm. Next, the cold rolled sheet was subjected to a siliconization treatment to increase the Si concentration of the steel sheet surface layer by heat treatment at 1200 ° C. in a mixed gas of silicon tetrachloride and N _2. At this time, various non-oriented electrical steel sheets (product sheets) with different Si concentration distributions were manufactured by adjusting the concentration of silicon tetrachloride and the holding time.

The Si concentration distribution and C content of the product plate thus obtained were measured. As in Example 1, a rotor core material having the shape of FIG. 1 was taken from the product plate under laser cutting processing conditions of output: 200 W, scanning speed: 30 m/min, and inclination angle θ: 2°, and a fatigue test piece was cut out from the bridge part (part A) and subjected to a fatigue test. At this time, the grain boundary step present on the laser cut surface (part C, see FIG. 3) of the parallel part of the test piece was measured. The fatigue test was performed under the conditions of tension-tension (pulsating), stress ratio: 0.1, and frequency: 20 Hz, and the maximum stress at which no breakage occurred even after 10 ⁷ repetitions was taken as the fatigue strength.

The above measurement results are shown in Table 3. These results show that the fatigue strength of steel plates that have been siliconized is greatly improved. In particular, steel plates with a difference in Si concentration of 1 mass% or more between the surface layer and the center of the plate thickness have a higher fatigue strength than steel plates with a uniform Si concentration in the plate thickness direction, even if the average Si content is the same.

The technology of the present invention is not limited to rotor cores of IPM motors in which centrifugal force is applied to magnets, but can also be applied to rotor cores of synchronous reluctance motors and the like that do not have magnet insertion holes, and to fields in which magnetic circuits are constructed to improve motor characteristics.

Claims (20)

A rotor core formed by laminating steel plates obtained by cutting a material steel plate and processing the steel plate into an outer peripheral shape of the rotor core,
The steel plate has a plurality of holes formed therein by thermal cutting, and
A rotor core characterized in that the maximum difference in level at the grain boundary present on the cut surface of the hole is 0.33 μm or less. The rotor core according to claim 1, characterized in that the material steel sheet is a non-oriented electrical steel sheet containing C: 0.0005-0.01 mass%, Si: 7 mass% or less, Mn: 0.05-3 mass%, and Al: 3 mass% or less, and optionally containing at least one selected from P: 0.001-0.1 mass%, S: 0.01 mass% or less, Sn: 0.001-0.1 mass%, Sb: 0.001-0.1 mass%, Ca: 0.0002-0.005 mass%, and Mg: 0.0002-0.005%, with the balance being Fe and unavoidable impurities. The rotor core described in claim 1, characterized in that the surface layer portion of the material steel sheet is from the front and back surfaces of the steel sheet to 1/3 of the sheet thickness, and the center portion is from the steel sheet surface to inside 1/3 of the sheet thickness, and the average Si content of the front and back surfaces of the surface layer portion is 1 mass% or more higher than that of the center portion. The rotor core described in claim 2, characterized in that the surface layer portion of the material steel sheet is from the front and back surfaces of the steel sheet to 1/3 of the sheet thickness, and the center portion is from the steel sheet surface to inside 1/3 of the sheet thickness. A rotor core as described in any one of claims 1 to 4, characterized in that the thermal cutting process is a laser cutting process. A rotor core as described in any one of claims 1 to 4, characterized in that the outer periphery of the steel plate processed into the shape of the rotor core is thermally cut. The rotor core described in claim 5, characterized in that the outer periphery of the steel plate processed into the shape of the rotor core is thermally cut. A rotor core as described in claim 6, characterized in that the thermal cutting process of the outer periphery is a laser cutting process. The rotor core according to claim 7, characterized in that the thermal cutting process of the outer periphery is a laser cutting process. A rotor core according to any one of claims 1 to 4, characterized in that permanent magnets are inserted into some or all of the holes. A rotor core as described in claim 5, characterized in that permanent magnets are inserted into some or all of the holes. A manufacturing method of a rotor core, comprising the steps of forming a plurality of holes by thermal cutting in a rotor core-shaped steel plate obtained from a base steel plate having the composition according to claim 2, and then laminating the steel plates to assemble the rotor core,
In the above-mentioned thermal cutting process, the heat beam is irradiated at an angle of 3 to 5 degrees toward the hole to be cut off with respect to the perpendicular line to the steel plate surface, thereby making the maximum step value of the grain boundary on the cut surface 0.5 μm or less. The manufacturing method of the rotor core described in claim 12, characterized in that the surface layer portion of the material steel sheet is from the front and back surfaces of the steel sheet to 1/3 of the sheet thickness, and the center portion is from the steel sheet surface to inside 1/3 of the sheet thickness, and the average Si content of the front and back of the surface layer portion is 1 mass% or more higher than that of the center portion. The method for manufacturing a rotor core according to claim 12 or 13, characterized in that the thermal cutting process is a laser cutting process. The manufacturing method for a rotor core according to claim 12 or 13, characterized in that the outer periphery of the steel plate processed into the shape of the rotor core is thermally cut. The manufacturing method for a rotor core according to claim 14, characterized in that the outer periphery of the steel plate processed into the shape of the rotor core is thermally cut. The manufacturing method for a rotor core according to claim 15, characterized in that the thermal cutting process of the outer periphery is a laser cutting process. The manufacturing method for a rotor core according to claim 16, characterized in that the thermal cutting process of the outer periphery is a laser cutting process. A motor having a rotor core and a stator core, the rotor core being formed using the core according to any one of claims 1 to 4,
A motor characterized in that the shortest distance between the outer diameter of the rotor core and the inner diameter of the stator core is 0.30 mm or less. A motor having a rotor core and a stator core, the rotor core being formed using the core according to claim 5,
A motor characterized in that the shortest distance between the outer diameter of the rotor core and the inner diameter of the stator core is 0.30 mm or less.

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