JP6852965B2 - Electrical steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to an electromagnetic steel sheet having improved strength and a method for producing the same, and particularly to a high-strength non-oriented electrical steel sheet and a method for producing the same.

In recent years, drive motors used in hybrid electric vehicles (HEVs) and electric vehicles (EVs) have been remarkably rotated at high speeds, and non-oriented electrical steel sheets used for the iron cores of these drive motors have low iron loss. In addition to electromagnetic characteristics, there is an increasing demand for higher strength. Solid solution strengthening, precipitation strengthening, grain refinement strengthening, dislocation strengthening, transformation strengthening, etc. are applied to increase the strength of the steel sheet, but other than solid solution strengthening, the magnetic properties deteriorate, so it is preferable for electromagnetic steel sheets. Absent. In addition, solid solution strengthening has a great effect on increasing strength while avoiding deterioration of magnetic properties, but at the same time, there are problems of increasing rolling load and brittle fracture, and there is an upper limit to the amount of addition from the viewpoint of productivity.

Conventionally, regarding the increase in strength of such non-oriented electrical steel sheets, as shown in Patent Document 1, in mass%, C: 0.0400% or less, Si: 0.2 to 4.0%, Mn: 0.05 to 5.0%, P: It contains 0.30% or less, S: 0.020% or less, Al: 8.0% or less, N: 0.0400% or less, and consists of the balance Fe and unavoidable impurities. Disclosure of technology for improving the strength of electrical steel sheets mainly composed of ferrite phase within the range satisfying 50% or less by containing 20 intermetallic compounds with a diameter of 0.050 μm ^{or less at a density of 20 pieces / μm 3 or more inside the steel material.} ing.

Japanese Unexamined Patent Publication No. 2005-264315

However, there is a problem that the thermal conductivity of an electromagnetic steel sheet in which an intermetallic compound is precipitated inside a steel material to increase its strength tends to deteriorate. As described above, heat generation increases with the recent increase in high-speed rotation, so that the motor core is required to have excellent thermal conductivity. In order to rotate the drive motor at high speed, the thermal conductivity is not deteriorated. It is hoped that electromagnetic steel sheets with higher strength will appear.

An object of the present invention is to provide an electromagnetic steel sheet having improved strength without deteriorating electromagnetic characteristics and thermal conductivity, and a method for manufacturing the same.

The present inventors have conducted various experiments and studies in order to obtain an electromagnetic steel sheet having improved strength without deteriorating electromagnetic properties and thermal conductivity. In order to solve the above problems, the present inventors deposit fine intermetallic compounds mainly composed of Al and Ni in steel, and appropriately control the average value and standard deviation of the equivalent circle diameter and the precipitation density. As a result, it was found that the strength can be improved without deteriorating the electromagnetic characteristics and thermal conductivity. The main point of the technique of the present invention is to generate dislocations in a steel sheet after annealing, and to use the dislocations as precipitation sites to disperse and precipitate Al—Ni intermetallic compounds having a size as uniform as possible in the steel sheet. The purpose is to improve the strength of the electromagnetic steel sheet without deteriorating the electromagnetic characteristics and thermal conductivity. According to the present invention, the following electromagnetic steel sheets and a method for producing the same are provided.

(1)
By mass%, Si: 2-4%, Al: 1-3%, Ni : 1.5-4%, Cr: 0.01-4%, Cu: 0.01-4%, Sn: Containing 1 or 2 or more of 0.01 to 0.2%, consisting of the balance Fe and unavoidable impurities.
An electromagnetic steel plate in which ^{30,000 pieces / μm 3} or more of Al—Ni intermetallic compounds having an average value of the equivalent circle diameter determined on an area basis of 1 to 10 nm and a standard deviation of 1 or less are precipitated.
(2)
The method for producing an electromagnetic steel sheet according to (1), which contains Si: 2 to 4%, Al: 1 to 3%, Ni: 1.5 to 4% in mass%, and further comprises Cr: Hot-rolled steel slab containing 1 or 2 or more of 0.01 to 4%, Cu: 0.01 to 4%, Sn: 0.01 to 0.2%, and consisting of the balance Fe and unavoidable impurities. after cold rolling and annealing, plastic deformation to 1 × ¹⁰ 14 / ^{m 2} or more dislocation density was added, further performing aging treatment at 400 to 600 ° C., the manufacturing method of the electromagnetic steel sheets.

According to the present invention, a non-oriented electrical steel sheet having excellent magnetic properties and thermal conductivity and improved strength, fatigue strength, and abrasion resistance can be obtained. As a result, a high-speed rotary motor or a motor in which a magnet is incorporated in a rotor that satisfies the characteristics required for a hybrid electric vehicle (HEV) or an electric vehicle (EV) in recent years can be obtained. Further, according to the present invention, for example, high efficiency, miniaturization, and long life of materials for electromagnetic switch can be achieved for other than motors.

It is a TEM observation photograph of an intragranular precipitate of an example.

Hereinafter, embodiments of the present invention will be described.

First, the component composition of the electromagnetic steel sheet of the present invention will be described. The electromagnetic steel sheet of the present invention basically contains Si: 2 to 4%, Al: 1 to 3% or less, Ni: 1.5 to 4% in mass%, and is composed of the balance Fe and unavoidable impurities. To do.

Si: 2-4%
Si increases the intrinsic resistance of steel, reduces eddy currents, reduces iron loss, and increases tensile strength, but the effect is small when the amount added is less than 2.0%. Further, since the work hardening ability is enhanced by the addition, there is also an effect of effectively increasing the dislocation density by the processing performed before the aging heat treatment. On the other hand, if Si exceeds 4%, the steel is embrittled and the magnetic flux density of the product is further reduced, so the content is set to 4% or less.

Al: 1-3%
In the present invention, Al is an important element positively added as a constituent element of an intermetallic compound, but if it exceeds 3%, embrittlement becomes a problem, so the upper limit is set to 3%. Al is usually added as a deoxidizing agent, but it is also possible to suppress the addition of Al and deoxidize with Si. On the other hand, in order to obtain the effect of strengthening the precipitation of the intermetallic compound, it is contained at least 1%. Further, the solid solution Al is known to have the effect of increasing the electric resistance and improving the iron loss, and for this purpose, it is effective to contain more Al than the Al—Ni precipitate is formed.

Ni: 1.5-4%
Conventionally, in general, Ni has been mainly used as a solid solution strengthening element or a precipitation strengthening element due to carbides, nitrides and the like. In the present invention, Ni is contained to form an intermetallic compound with Al and to develop precipitation strengthening by the intermetallic compound of Al—Ni. In order to develop precipitation strengthening due to the intermetallic compound of Al—Ni, 1.5% or more of Ni is required. On the other hand, excessive addition may deteriorate the ductility of the steel sheet and reduce the passability, reduce the magnetic flux density, and make it difficult to suppress the preferable formation of the intermetallic compound in the manufacturing process. Considering the addition cost, the upper limit is set to 4%.

Further, the electromagnetic steel sheet of the present invention is 1 or 1 of Cr: 0.01 to 4%, Cu: 0.01 to 4%, Sn: 0.01 to 0.2% in mass% as an optional content component. It may contain 2 or more. These elements are known as elements that form an intermetallic compound in the electromagnetic steel sheet targeted by the present invention, and can contain 1 or 2 or more as required. However, if it is excessively contained, the ductility of the steel sheet is deteriorated and the sheet-passability is lowered, and it may be difficult to suppress the preferable formation of the intermetallic compound in the manufacturing process. Further, considering the addition cost, Cr is 0.01 to 4%, Cu is 0.01 to 4%, and Sn is 0.01 to 0.2%.

The electromagnetic steel sheet of the present invention is based on the above component composition, and is composed of the balance Fe and unavoidable impurities.

Since C may deteriorate the magnetic characteristics, it is preferably 0.0400% or less. On the other hand, it also has the effect of increasing the work hardening ability and effectively increasing the dislocation density due to the processing performed before the aging heat treatment. From the viewpoint of manufacturing cost, it is advantageous to reduce the amount of C by degassing equipment at the molten steel stage. If it is 0.0030% or less, the effect of suppressing magnetic aging is remarkable, and carbides, etc. are the main means of increasing the strength. In the steel of the present invention which does not use the non-metal precipitate of the above, it is more preferably 0.0020% or less, further preferably 0.0015% or less. It may be 0%.

Mn is also effective as an element for increasing strength by solid solution, increasing electrical resistance, and improving iron loss, and the steel of the present invention can also be used according to a known technique. It also has the effect of increasing the work hardening ability and effectively increasing the dislocation density due to the processing performed before the aging heat treatment. From the viewpoint of increasing the strength, it is not particularly required in the present invention utilizing the fine intermetallic compound. 0% may be used, but in an industrial manufacturing method using iron ore as a raw material, about 0.01% is inevitably contained.

Since N deteriorates the magnetic characteristics like C, it is preferably 0.0400% or less. The content also has the effect of increasing the work hardening ability and effectively increasing the dislocation density due to the processing performed before the aging heat treatment. In particular, in the present invention, it is preferable that N is low in order to avoid the formation of a strong nitride with Al, and if it is 0.0027% or less, the effect of suppressing the deterioration of characteristics due to magnetic aging and the formation of fine nitrides is remarkable, which is more preferable. May be 0.0022%, more preferably 0.0015% or less, or 0%.

Cu significantly lowers the saturation magnetic flux density Bs of iron, and also significantly lowers B50 (magnetic flux density [T] when the magnetization force is 5000 [A / m]). Since a decrease in Bs and B50 leads to a decrease in motor torque, in the present invention, the inclusion of Cu is not essential, and the non-oriented electrical steel sheet having high strength and low iron loss is not accompanied by a decrease in Bs and B50. The manufacturing method can be realized. On the other hand, it is also known that the strength is increased by Cu precipitation, and the steel of the present invention can be used according to the known technique.

Nb is not necessary to be added for this purpose because precipitates such as NbC are effective for increasing the strength, but these precipitates hinder the movement of the domain wall and significantly deteriorate the iron loss. On the other hand, the solid solution Nb is effective not only for solid solution strengthening but also for increasing the strength and improving the high frequency characteristics by refining the crystal grains, and the steel of the present invention can also be used according to the known technique.

P is an element having a remarkable effect of increasing the tensile strength by strengthening the solid solution, but it is not necessary to add it for this purpose. It may be 0%. On the other hand, the addition also has the effect of increasing the work hardening ability and effectively increasing the dislocation density due to the processing performed before the aging heat treatment. If it exceeds 0.3%, embrittlement becomes severe and processing such as hot spreading and cold spreading on an industrial scale becomes difficult. Therefore, the upper limit is preferably 0.30%, more preferably 0.10%. It is as follows.

Since S may form sulfides and deteriorate magnetic properties, particularly iron loss, the content of S is preferably as low as possible and may be 0%. In the present invention, it is preferably 0.020% or less, more preferably 0.0040% or less, still more preferably 0.0020% or less, still more preferably 0.0010% or less.

Next, the method for manufacturing the electromagnetic steel sheet of the present invention will be described. First, the steel containing the above components is melted in a converter in the same manner as an ordinary electromagnetic steel plate, continuously cast into steel slabs, and then hot-rolled, cold-rolled and annealed, and then the dislocation density is 1 × 10. ^{Plastic deformation to 14} / m ² or more is added, and further aging treatment is performed at 400 to 600 ° C. In addition to these steps, the process of forming an insulating film, decarburizing, etc. does not impair the effect of the present invention.

The plastic deformation of the annealed steel sheet so that the dislocation density is 1 × 10 ¹⁴ / m ² or more can be performed by, for example, skin pass rolling or leveler straightening. Further, the dislocation density can be set to ^{1 × 10 14} / m ² or more by processing when punching into the shape of the motor core. Then, the steel sheet is subjected to a dislocation density of 1 × 10 ¹⁴ / m ² or more by skin pass rolling, leveler straightening, punching, etc., and then further subjected to aging treatment at 400 to 600 ° C. By performing such plastic deformation and aging treatment, the average value of the equivalent circle diameter determined on an area basis is 1 to 10 nm, and the standard deviation is 1 or less. The number of Al—Ni intermetallic compounds is 30,000 / μm. ^{It is possible to obtain a non-oriented electrical steel sheet having three} or more precipitates, excellent electromagnetic characteristics such as low iron loss and high magnetic flux density, and high strength, and the thermal conductivity is not deteriorated.

Most of the elements conventionally used for high strength in electrical steel sheets not only have a problem of addition cost, but also have a considerable adverse effect on magnetic characteristics, but the high strength effect is small and cost performance is a problem. was there. The present invention also contains Si, Al, and Ni for the purpose of increasing the strength, but its technical effect and technical purpose are completely different from the conventional ones. That is, while the conventional additive element is mainly used as a solid solution strengthening element or a precipitation strengthening element due to carbides, nitrides, etc., in the present invention, an intermetallic compound of Al—Ni is formed, and the precipitation strengthening effect due to the formation is formed. It is contained for expression.

It should be noted that the iron loss is not a little deteriorated by the solid solution strengthening by Ni and the miniaturization of the crystal grains. Further, in order to secure the strength satisfying the customer only by solid solution strengthening without making the crystal grains finer, the contents of Si, Al and Ni must be increased, and the alloy cost increases. Further, there are problems such as an increase in rolling load and a decrease in productivity due to embrittlement.

The reason why the dislocation density of the steel plate after plastic working is set to 1 × 10 ¹⁴ / m ² or more is that the dislocations are sufficiently generated in the steel plate by the plastic working, and the dislocations are used as precipitation sites to make the size as uniform as possible in the steel plate. This is because the intermetallic compound of Al—Ni is dispersed and precipitated. If the dislocation density is ^{less than 1 × 10 14} / m ² , the precipitation sites of the intermetallic compounds are insufficient, and after aging, an intermetallic compound of Al—Ni having a ^{number density of 30,000 / μm 3 or more cannot be obtained.} It ends up. Further, when the number density of the intermetallic compounds of Al-Ni is small, the average value of the equivalent circle diameters of the intermetallic compounds becomes larger than 10 nm, and the size variation of each intermetallic compound becomes large, and the standard deviation becomes large. Exceeds 1. If the dislocation density of the steel sheet after annealing ^{exceeds 3 × 10 16} / m ² , the electromagnetic characteristics will deteriorate, and after aging, the size, standard deviation, and number density of the Al—Ni intermetallic compound will be reduced. It may be out of the scope of the invention. Therefore, it is desirable that the dislocation density of the steel sheet after annealing is 3 × 10 ¹⁶ / m ^{2 or less.}

The aging treatment is performed at 400 to 600 ° C. If the temperature is lower than 400 ° C, a sufficient intermetallic compound cannot be obtained, while if the temperature exceeds 600 ° C, the formed intermetallic compound becomes coarse. The holding time at this time is preferably 1 to 120 minutes. If it is too short, a sufficient intermetallic compound cannot be obtained, while if it is too long, the formed intermetallic compound becomes coarse. In addition, the heating rate, cooling rate, and the like may also affect the state of the precipitate, which is a feature of the present invention. These heat treatment conditions are determined in consideration of components, productivity, etc. according to the desired characteristics. It is easy for a person skilled in the art to determine appropriate conditions by several trials according to the technical idea of the present invention.

Through the above manufacturing process, the number of Al—Ni intermetallic compounds having an average circle-equivalent diameter of 1 to 10 nm and a standard deviation of 1 or less, which is obtained on an area basis, is 30,000 / μm ³ or more. High-strength electrical steel sheets can be obtained by precipitating and hardly impairing magnetic properties and thermal conductivity. The magnetic steel sheet of the present invention has a tensile strength increased by 100 MPa or more or a hardness increased by 1.1 times or more by aging heat treatment for hardening. Further, the object of the present invention is one having a final strength of 600 MPa or more after the aging treatment.

If a large amount of coarse compounds having an average value of the equivalent circle diameter of the intermetallic compound exceeding 10 nm are produced, the efficiency of increasing the strength may decrease and the magnetic characteristics may also deteriorate. In the present invention, by producing a fine-sized intermetallic compound at a high density, the strength of the electrical steel sheet is improved while maintaining excellent magnetic properties and thermal conductivity. On the other hand, if the average value of the equivalent circle diameters of the intermetallic compounds is less than 1 nm, the reinforcing ability becomes small. In order to further increase the strength reliably, it is necessary that the sizes of the individual intermetallic compounds are uniform so that the standard deviation of the equivalent circle diameter is 1 or less. If this standard deviation exceeds 1, the size of the intermetallic compound becomes non-uniform, and it becomes difficult to improve the strength of the electrical steel sheet while maintaining the magnetic properties and thermal conductivity. As the intermetallic compounds of Al-Ni to form in the steel material, NiAl, such as _Ni 3 Al is commonly known. Further, it is known that the elemental ratios of these compounds fluctuate considerably, and those containing some impurity elements also correspond to the present invention.

From the viewpoint of increasing the strength, the number density of the intermetallic compound needs to be precipitated at ^{30,000 / μm 3 or more.} Controlling the intermetallic compound size and number density is extremely important from the viewpoint of achieving both excellent strength and magnetic properties and preventing deterioration of thermal conductivity. The reason is that not only do they affect the strength, magnetic properties and thermal conductivity, respectively, but the behavior of changing the strength, magnetic properties and thermal conductivity when they are changed is different. That is, it is necessary to control the region where the strength increasing effect is high and the deterioration of magnetic characteristics and the deterioration of thermal conductivity are small. For this purpose, it is effective to appropriately control the components and heat treatment conditions, as well as the dislocation density before the aging treatment, and to set the size and number density of the intermetallic compound within a desired range as described above.

Since the present invention does not utilize the miniaturization of the crystal structure as the main means for increasing the strength, the crystal grain size can be adjusted to the optimum range from the viewpoint of magnetic properties. Since the size and density of the intermetallic compound that contributes to high strength can be controlled not only by the components but also by the final heat treatment, the crystal grain size is the maximum reached temperature of recrystallization annealing and its temperature before this heat treatment, for example. It is possible to control the intermetallic compound independently of the control of the intermetallic compound depending on the retention time in the region and the like. The crystal grain size is usually 300 μm or less, and is preferably controlled to 30 to 250 μm. More preferably, it is 60 to 200 μm. Generally, when the frequency of the exciting current when using the steel sheet is high, it is preferable to keep the crystal grains fine. Further, even if the grain size is coarsened to several cm by using secondary recrystallization or the like as in the grain-oriented electrical steel sheet, the effect of the present invention is not impaired at all.

It should be noted that the effect of the present invention can be applied to non-oriented or directional electromagnetic steel sheets because it does not depend on the presence and type of the surface film normally formed on the surface of the electrical steel sheet and further does not depend on the manufacturing process. The application is not particularly limited, and is applied to all applications requiring strength and magnetic characteristics, in addition to motor rotor applications used in home appliances or automobiles.

The vacuum-melted hot-rolled steel sheet shown in Table 1 was pickled, cold-rolled to a thickness of 0.20 mm, and annealed. Then, plastic deformation with the dislocation density as the condition shown in Table 1 was added, and aging treatment was performed at 350 to 750 ° C. for 2 hours to evaluate the magnetic properties and mechanical properties. The results are shown in Table 1. In Table 1, the numerical values outside the scope of the present invention are underlined.

Materials whose components are in the range of the present invention were subjected to aging treatment at 550 ° C. for 2 hours after normal finish annealing to obtain an increase in YP: 140 to 160 MPa and TS: 160 to 180 MPa without iron loss deterioration ( Reach YP: 570 to 910 MPa, TS: 710 to 1130 MPa). As shown in FIG. 1, by performing dark-field TEM observation considering the difference in crystal structure between α-Fe and β'-NiAl using a high-resolution TEM, fine precipitates of about 5 nm are formed in the crystal grains. It was observed in large quantities at a density of 3,000 grains / μm ² , and it was found that this was the cause of the increase in strength. In FIG. 1, the white-looking precipitate was an Al—Ni intermetallic compound having a diameter of about 5 nm, and the number density was about 3,000 / μm ² . This is the mechanism of strength increase that was first discovered by using this observation technique. The thickness of the observation site for dark-field TEM observation is about 0.1 μm, and 3,000 / μm ² corresponds to 30,000 / μm ^3.

When the dislocation density after plastic deformation was about 5 × 10 ¹³ / m ² , the average circle-equivalent diameter of the precipitate was as small as about 3 nm, but the standard deviation was larger than 1, and the number density was about 5000 / μm. ^It became very small with 3. In this case, no increase in strength was obtained after the aging treatment, and YP = 480 MPa. Even when the dislocation density after plastic deformation is about 1 × 10 ¹⁴ / m ² , when the aging treatment temperature is 350 ° C., almost no Al—Ni precipitates are observed, and the strength after aging treatment is increased. There wasn't. On the other hand, when the dislocation density after plastic deformation is about 1 × 10 ¹⁴ / m ² and the aging treatment temperature is 750 ° C., precipitates with an average circle diameter of 10 nm or more increase, and the number density is about 20000 pieces / μm. ^It was as small as 3. In this case, not only was the strength not increased after the aging treatment, but the iron loss W10 / 800 was also significantly deteriorated to 32.0 W / kg.

When 10 disc samples having a diameter of 10 mm were cut out from each material and the thermal conductivity was measured by the laser flash method, the variation in the measured thermal conductivity was larger in the comparative example than in the material of the present invention. .. It is presumed that this is because the size distribution of the precipitates is large, and especially when the standard deviation exceeds 1, the variation of the measured thermal conductivity values becomes large as an average value of about ± 10%.

Further, YP was improved by adding 0.01% or more of Cu in mass%, and in the example of the present invention, YP of 830 MPa or more was obtained by adding 1% or 2%. The addition of Cr was effective in reducing the iron loss W10 / 800, and the addition of Sn improved the magnetic flux density B50. The effect was obtained by adding 0.01% or more in mass% of each.

Claims (2)

By mass%, Si: 2-4%, Al: 1-3%, Ni : 1.5-4%, Cr: 0.01-4%, Cu: 0.01-4%, Sn: Containing 1 or 2 or more of 0.01 to 0.2%, consisting of the balance Fe and unavoidable impurities.
An electromagnetic steel plate in which ^{30,000 pieces / μm 3} or more of Al—Ni intermetallic compounds having an average value of the equivalent circle diameter determined on an area basis of 1 to 10 nm and a standard deviation of 1 or less are precipitated. The method for manufacturing an electromagnetic steel sheet according to claim 1.
By mass%, Si: 2-4%, Al: 1-3%, Ni: 1.5-4%, Cr: 0.01-4%, Cu: 0.01-4%, A steel slab containing 1 or 2 or more of Sn: 0.01 to 0.2% and composed of the balance Fe and unavoidable impurities is hot-rolled, cold-rolled and annealed, and then the dislocation density is 1 × 10. ¹⁴ / M ² A method for manufacturing an electromagnetic steel sheet, which is subjected to the above plastic deformation and further subjected to an aging treatment at 400 to 600 ° C.

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