JP4389691B2 - Non-oriented electrical steel sheet for rotor and manufacturing method thereof - Google Patents

Description

  The present invention relates to a non-oriented electrical steel sheet used for a rotor of a high-efficiency motor such as a drive motor for an electric vehicle or a hybrid vehicle, a servo motor for a robot, a machine tool, and the like, and a method for manufacturing the same. In particular, the present invention relates to a non-oriented electrical steel sheet having excellent mechanical characteristics and magnetic characteristics suitable as a rotor of a permanent magnet embedded motor that rotates at high speed, and a method for manufacturing the same.

  Due to the recent increase in global environmental problems, energy conservation and environmental countermeasure technologies have been developed in many fields. The automobile field is no exception, and technologies for reducing exhaust gas and improving fuel efficiency are advancing rapidly. It is no exaggeration to say that electric vehicles and hybrid vehicles are the culmination of these technologies, and the performance of automobile drive motors (hereinafter also simply referred to as “drive motors”) greatly affects the performance of automobiles.

  Many drive motors use permanent magnets, and are composed of a stator (stator) portion provided with windings and a rotor (rotor) portion provided with permanent magnets. Recently, a shape in which a permanent magnet is embedded in a rotor (permanent magnet embedded motor; IPM motor) has become mainstream. Further, with the advancement of power electronics technology, the rotational speed can be arbitrarily controlled, and there is a tendency to increase the speed. Therefore, the rate at which the iron core material is excited in a high frequency range higher than the commercial frequency (50 to 60 Hz) is increased, and not only the magnetic characteristic at the commercial frequency but also the improvement of the magnetic characteristic at 400 Hz to several kHz is required. It has become like this. In addition, since the rotor is constantly subjected not only to centrifugal force during high-speed rotation but also to stress fluctuations associated with fluctuations in the rotational speed, the rotor core material is also required to have mechanical characteristics. In particular, since the IPM motor has a complicated rotor shape, the core material for the rotor needs to have mechanical characteristics sufficient to withstand centrifugal force and stress fluctuation in consideration of stress concentration. Also, in the field of servo motors for robots and machine tools, it is predicted that the rotation speed will increase in the same way as drive motors.

  Conventionally, the stator of the drive motor has been manufactured mainly by stacking non-oriented electrical steel sheets that have been stamped, but the rotor has also been manufactured by a lost wax casting method or a sintering method. This is because the stator requires excellent magnetic properties and the rotor requires robust mechanical properties. However, since the motor performance is greatly influenced by the air gap between the rotor and the stator, the above-described rotor has a problem in that the necessity for precision machining is required and the core manufacturing cost is significantly increased. From the viewpoint of cost reduction, it is only necessary to use a punched electrical steel sheet, but the current situation is that no non-oriented electrical steel sheet having both the magnetic and mechanical properties necessary for the rotor has been found. .

  As an electrical steel sheet having excellent mechanical properties, for example, in Patent Document 1, in addition to 3.5 to 7% Si, one or two of Ti, W, Mo, Mn, Ni, Co and Al are used. Steel sheets containing the above in a range not exceeding 20% have been proposed. In this method, solid solution strengthening is used as a steel strengthening mechanism. However, in the case of solid solution strengthening, the cold rolled base metal is also strengthened at the same time, so cold rolling is difficult, and in this method, a special process called warm rolling is indispensable. There is room for improvement such as improvement and yield improvement.

  Patent Document 2 discloses a steel sheet containing B and a large amount of Ni in addition to 2.0 to 3.5% Si and 0.1 to 6.0% Mn, and having a crystal grain size of 30 μm or less. Has been proposed. In this method, solid solution strengthening and strengthening by refinement of crystal grain size are used as the strengthening mechanism of steel. However, strengthening by grain refinement is relatively ineffective, so it is essential to contain a large amount of expensive Ni in addition to about 3.0% Si as shown in the example of Patent Document 2. However, the problem of frequent cracking during cold rolling and the problem of increased alloy costs remain.

  Furthermore, Patent Documents 3 and 4 propose steel sheets containing Nb, Zr, B, Ti, V, or the like in addition to 2.0 to 4.0% Si. In these methods, precipitation strengthening by precipitates of Nb, Zr, Ti or V is used in addition to solid solution strengthening by Si. However, since such strengthening by precipitates is relatively ineffective, it is necessary to make Si about 3.0% as shown in Examples of Patent Document 3 and Patent Document 4, and in particular, the method of Patent Document 3 Then, it is necessary to contain a large amount of expensive Ni. Therefore, the problem that cracks frequently occur during cold rolling and the problem of increased alloy costs remain.

  Patent Documents 5 and 6 propose steel sheets containing Ti, Nb and V, or P and Ni, respectively, after limiting Si and Al to 0.03 to 0.5%. In these methods, precipitation precipitation strengthening of carbide and solid solution strengthening of P are used rather than solid solution strengthening by Si. However, in these methods, there is a problem that a strength level necessary for a rotor of a drive motor, which will be described later, cannot be ensured, and as shown in Examples of Patent Documents 5 and 6, 2.0% There is a problem that the above Ni content is essential and the alloy cost is high.

  Further, Patent Document 7 proposes a non-oriented electrical steel sheet for embedded permanent magnet motors with Si: 1.6 to 2.8% and limited crystal grain size, internal oxide layer thickness, and yield point. Has been. However, at the yield point of the steel plate by this method, the strength is insufficient as a rotor of a drive motor that rotates at high speed.

  As non-oriented electrical steel sheets specified in JIS C 2552, so-called high-grade non-oriented electrical steel sheets (35A210, 35A230, etc.) have the highest alloy content and high strength, but the mechanical property level is high as described above. The strength is insufficient as a rotor of a drive motor that is below the tension electromagnetic steel plate and rotates at high speed.

JP 60-238421 A JP-A-1-162748 Japanese Patent Laid-Open No. 2-8346 JP-A-6-330255 JP 2001-234302 A JP 2002-146493 A JP 2001-172752 A

  As mentioned above, the solid solution strengthening and precipitation strengthening conventionally proposed as methods for increasing the strength of non-oriented electrical steel sheets also strengthens the base material of cold rolling, so cracks frequently occur during cold rolling. In the case of increasing the strength by refining crystal grains, the amount of strengthening is insufficient, so that it is not possible to realize a strength that can be practically used as a rotor. In addition, the present inventors have also examined transformation strengthening, but it has been found that the transformation structure such as martensite significantly increases iron loss in transformation strengthening, and realizes magnetic characteristics that can be practically used as a rotor application. I can't.

  The present invention has been made in view of the above problems, and provides a non-oriented electrical steel sheet having excellent mechanical characteristics and magnetic characteristics necessary as a rotor of a motor that rotates at high speed, and a method for manufacturing the same. The main purpose.

  The present inventors have made various studies on the steel structure that should be possessed by the non-oriented electrical steel sheet having both magnetic properties and mechanical properties suitable for rotors, and have achieved high strength by work hardening that has not been studied at all. Pay attention. The new knowledge that dislocations introduced during processing have a relatively small effect on iron loss has been obtained, and this technology is completely the opposite of the completely recrystallized ferrite structure that is the technical recognition of conventional non-oriented electrical steel sheets. Based on the idea, the magnetic structure and mechanical characteristics required for the rotor can be achieved by making the structure of the steel sheet a processed structure in which a large amount of dislocations remain and a recovered structure (hereinafter referred to as “recovered structure”). As a result, the present invention was completed.

  That is, the present invention is, in mass%, C: 0.04% or less, Si: 3.5% or less, Mn: 0.1% or more and 2.5% or less, Al: 2.5% or less, P: 0 .2% or less, S: 0.03% or less, N: 0.005% or less, the balance being substantially composed of Fe and impurities, and the area ratio of the recrystallized portion being less than 25% A non-oriented electrical steel sheet for a rotor is provided.

  In the present invention, the area ratio of the recrystallized portion is appropriately controlled, and the strength can be increased by making the steel structure in which many dislocations remain. Steel sheet. As a result, the magnetic characteristics and mechanical characteristics required for the rotor described above can also be satisfied.

Moreover, the non-oriented electrical steel sheet for rotor of the present invention contains at least one element selected from the group consisting of Nb, Ti, Zr and V, and has a steel composition satisfying the following formula (1). It is preferable.
Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14)> 0 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)

  The left side of the above formula (1) corresponds to the content of Nb, Zr, Ti, or V in a solid solution state where no precipitate such as carbide, nitride, or carbonitride is formed, and this value is If it is larger than zero, it means that Nb, Zr, Ti or V in a solid solution is contained. The larger the content of Nb, Zr, Ti or V in the solid solution state, the greater the effect of suppressing the disappearance and recrystallization of dislocations that proceed during soaking, which will be described later. Therefore, the above formula (1) is satisfied. It is preferable to do.

  The present invention also includes a hot rolling process in which hot rolling is performed on a steel ingot or steel slab having the above-described steel composition, and a hot rolled steel sheet obtained by the hot rolling process is subjected to one time or intermediate annealing. For a rotor characterized by not performing a soaking process for soaking a cold rolled steel sheet obtained by the above cold rolling process. A method for producing a non-oriented electrical steel sheet is provided.

  In the present invention, since the soaking process is not performed, it is possible to obtain a processed structure without annihilating the dislocations introduced during processing to a predetermined thickness, thereby enabling high strength of the steel sheet. It is. In addition, since there is no need to increase the strength of the steel sheet used for cold rolling as in the conventional solid solution strengthening and precipitation strengthening, that is, the base material of cold rolling, it is possible to suppress breakage during cold rolling. . Furthermore, by using a steel ingot or steel slab having a predetermined steel composition, it is possible to produce a non-oriented electrical steel sheet for a rotor that has not only mechanical properties but also good magnetic properties.

  Furthermore, the present invention provides a hot rolling process in which hot rolling is performed on a steel ingot or steel slab having the steel composition described above, and a hot rolled steel sheet obtained by the hot rolling process is subjected to one time or intermediate annealing. A rotor for rolling, characterized by having a cold rolling process for performing cold rolling more than once, and a soaking process for soaking the cold-rolled steel sheet obtained by the cold rolling process at 780 ° C. or less. A method for producing a grain-oriented electrical steel sheet is provided.

  In the present invention, by setting the temperature in soaking to a predetermined range, recrystallization is suppressed, and the recovered structure that has remained without annihilating dislocations introduced during processing to a predetermined plate thickness Therefore, the strength of the steel sheet can be increased. Moreover, since it does not accompany the high intensity | strength of the steel plate which uses for cold rolling, ie, the base material of cold rolling, the fracture | rupture at the time of cold rolling can be suppressed. Furthermore, by using a steel ingot or steel slab having a predetermined steel composition, it is possible to produce a non-oriented electrical steel sheet for a rotor that has not only mechanical properties but also good magnetic properties. Furthermore, the flatness of the cold-rolled steel sheet can be improved by performing the soaking process, thereby improving the space factor when the rotor is configured and improving the motor efficiency.

  Moreover, the manufacturing method of the non-oriented electrical steel sheet for rotors of this invention may have the hot-rolled sheet annealing process which performs hot-rolled sheet annealing to the said hot-rolled steel sheet. This is because by performing hot-rolled sheet annealing, the ductility of the steel sheet is improved and breakage in the cold rolling process can be suppressed.

  According to the present invention, it is possible to stably manufacture a non-oriented electrical steel sheet having both excellent mechanical properties and magnetic properties necessary as a rotor of a motor that rotates at high speed without causing a significant increase in cost. is there. Therefore, it can sufficiently cope with the increase in the rotational speed in the field of drive motors of electric vehicles and hybrid vehicles, and its industrial value is extremely high.

  The characteristics necessary for the electrical steel sheet used for the rotor referred to in the present invention are mechanical characteristics, which are yield point and tensile strength. This is intended to suppress not only the deformation of the rotor during high-speed rotation but also the fatigue failure caused by stress fluctuations. In drive motors of recent electric vehicles and hybrid vehicles, the rotor receives a stress amplitude of about 150 MPa under an average stress of about 250 MPa. Therefore, from the viewpoint of suppressing deformation, the yield point must be 400 MPa or more, and considering the safety factor, it is necessary to satisfy 500 MPa or more. Preferably it is 550 MPa or more. Further, from the viewpoint of suppressing fatigue failure in the above-described stress state, the tensile strength is 550 MPa or more, and 600 MPa, preferably 700 MPa or more is necessary in consideration of the safety factor.

  The second characteristic necessary for the electromagnetic steel sheet used for the rotor is the magnetic flux density. In a motor that utilizes reluctance torque, such as an IPM motor, the magnetic flux density of the material used for the rotor also affects the torque. If the magnetic flux density is low, a desired torque cannot be obtained.

  Furthermore, the third characteristic necessary for the electromagnetic steel sheet used for the rotor is iron loss. Iron loss is composed of hysteresis loss due to irreversible domain wall motion and Joule heat (eddy current loss) due to eddy currents caused by magnetization changes. The iron loss of electrical steel sheets is the sum of these totals. It is evaluated by iron loss. Although the loss generated in the rotor does not dominate the motor efficiency itself, the permanent magnet is demagnetized due to the loss of the rotor, that is, heat generation, which indirectly deteriorates the motor performance. Accordingly, it is recalled that the upper limit of the iron loss value of the material used for the rotor is determined from the viewpoint of the heat resistance temperature of the permanent magnet, and is allowed even if the iron loss value is higher than the material used for the stator.

  Based on these ideas, the present inventors have made various studies on the steel structure that a non-oriented electrical steel sheet having both magnetic properties and mechanical properties suitable for a rotor should have. As a result, the strength of the cold-rolled base metal is strengthened by solid solution strengthening and precipitation strengthening, so it is inevitable to break during cold rolling. And it turned out that iron loss increases remarkably in transformation structures, such as martensite. Furthermore, as a result of examining work hardening as a strengthening mechanism, it was found that dislocations introduced during processing have a relatively small effect on iron loss. From these results, it is required for the rotor to have a processed structure and a recovered structure in which a large amount of dislocations remain, contrary to the complete recrystallized ferrite structure that is the technical recognition of conventional non-oriented electrical steel sheets. It was found that the magnetic properties and mechanical properties can be achieved.

  The processed structure and the recovered structure can be obtained by allowing dislocations introduced during processing to a predetermined plate thickness to remain without being eliminated during soaking or by suppressing the disappearance. Therefore, unlike the prior art mainly based on solid solution strengthening or precipitation strengthening, it is possible to increase the strength without increasing the strength of the cold-rolled base material, and to suppress breakage during cold rolling. In order to obtain such a processed structure and a recovered structure, the recrystallization and crystal grain growth are usually not subjected to a soaking treatment that is usually performed after cold rolling, or even if a soaking treatment is applied. It is necessary to carry out at a low temperature such that is insufficient. In order to suppress recrystallization during soaking, it is effective to contain appropriate amounts of Nb, Zr, Ti and V. Hereinafter, the knowledge that led to the completion of the present invention will be described.

  The main components are mass%, Si: 2.0%, Mn: 0.2%, Al: 0.3%, N: 0.002%, and the contents of C, S and Nb are respectively C: 0 0.001% to 0.04%, S: 0.0002% to 0.03%, Nb: 0.001% to 0.4%, hot rolled to 2.3mm, then 800 ° C Was subjected to hot rolling annealing for 10 hours, further cold-rolled to 0.35 mm, and subjected to soaking treatment under two conditions of holding at 700 ° C. for 20 seconds or holding at 750 ° C. for 20 seconds. The tensile strength of the steel sheet thus obtained was measured.

1 and 2, Nb ^* represented by the following formula (2) defined by the contents of Nb, C and N for each steel plate subjected to soaking at 700 ° C. or 750 ° C. for 20 seconds The relationship with the tensile strength of a steel plate is shown.
Nb ^* = Nb / 93-C / 12-N / 14 (2)
(Here, in the formula (2), Nb, C and N indicate the content (mass%) of each element.)

1 and 2, it was found that excellent mechanical properties can be obtained only when Nb ^* > 0. As a result of investigating the steel structure, recrystallization was suppressed only when Nb ^* > 0, and the steel structure was a processed structure and a recovered structure. Nb ^* corresponds to the solid solution Nb content, and it has been found that securing the solid solution Nb content is important for suppressing recrystallization. Furthermore, it has been found that when the soaking temperature in soaking is increased, the effect of suppressing recrystallization increases as Nb ^* increases.

Further, for Ti, Zr and V, the same examination as described above was performed, and the present invention was completed by combining these findings.
Hereinafter, the non-oriented electrical steel sheet for rotors of the present invention and the manufacturing method thereof will be described in detail.

A. Non-oriented electrical steel sheet for rotors The non-oriented electrical steel sheet for rotors of the present invention is mass%, C: 0.04% or less, Si: 3.5% or less, Mn: 0.1% or more. 5% or less, Al: 2.5% or less, P: 0.2% or less, S: 0.03% or less, N: 0.005% or less, the balance being substantially composed of Fe and impurities, The area ratio of the recrystallized portion is less than 25%.

“%” Indicating the content of each element means “mass%” unless otherwise specified .

In the present invention, the strength can be increased and the mechanical characteristics can be improved by appropriately controlling the area ratio of the recrystallized portion to obtain a processed structure and a recovered structure in which many dislocations remain. Moreover, it can be set as the non-oriented electrical steel sheet for rotors with favorable magnetic characteristics and mechanical characteristics by having a predetermined steel composition. Thereby, for example, the magnetic characteristics and mechanical characteristics required for the rotor of the drive motor can be satisfied.
Hereinafter, the steel composition and the area ratio of the recrystallized portion in the non-oriented electrical steel sheet for rotor of the present invention will be described.

1. Steel composition (1) C
Since C is combined with Nb, Zr, Ti or V to form a precipitate, it leads to a decrease in the content of solute Nb, Zr, Ti and V. Therefore, in order to secure the contents of solute Nb, Zr, Ti and V, it is preferable to reduce the C content. However, excessive reduction of the C content increases the steelmaking cost, and even if the C content is large, if the contents of Nb, Zr, Ti and V are increased accordingly, solid solution Nb, Zr, Ti In view of securing the V and V contents, the upper limit of the C content is set to 0.04%. Preferably it is 0.02% or less, More preferably, it is 0.01% or less.

(2) Si
Si is an element that has the effect of increasing electrical resistance and reducing eddy current loss. However, when a large amount of Si is contained, cracks during cold rolling are induced, and the manufacturing cost increases due to a decrease in the yield of the steel sheet. Therefore, the Si content is 3.5% or less. Moreover, 3.0% or less is preferable from a viewpoint of crack suppression. Furthermore, when using Si as a deoxidizing agent, it is necessary to contain 0.01% or more, but since Al may be used as a deoxidizing agent, the lower limit of the Si content is not particularly limited.

(3) Mn
Mn, like Si, has the effect of increasing electrical resistance and reducing eddy current loss. However, since an alloy cost increases when Mn is contained in a large amount, the upper limit of the Mn content is set to 2.5%. On the other hand, the lower limit of the Mn content is determined from the viewpoint of fixing S, and is set to 0.1%.

(4) Al
Al increases eddy current loss in the same manner as Si because it increases electric resistance. However, when Al is contained in a large amount, the alloy cost increases and the leakage of magnetic flux occurs due to the decrease of the saturation magnetic flux density, so that the motor efficiency decreases. From these viewpoints, the upper limit of the Al content is 2.5%. Moreover, when using Al as a deoxidizer, it is necessary to contain 0.01% or more, but since Si may be used as a deoxidizer, the lower limit of the Al content is not particularly limited.

(5) P
P has the effect of increasing the strength of the steel sheet by solid solution strengthening, but when it contains a large amount of P, it induces cracks during cold rolling. Therefore, the P content is 0.2% or less.

(6) S
S is an impurity inevitably mixed in the steel. However, since the cost increases to reduce it in the steelmaking stage, the upper limit of the S content is 0.03%.

(7) N
Since N is combined with Nb, Zr, Ti, or V to form a precipitate, the content of solute Nb, Zr, Ti, and V is reduced. Therefore, it is preferable to reduce the N content from the viewpoint of securing the contents of the solute Nb, Zr, Ti and V. Therefore, the N content is 0.005% or less.

(8) Nb, Zr, Ti and V
In order to obtain a processed structure and a recovered structure after soaking, it is effective to contain Nb, Zr, Ti, or V in a solid solution state in which no precipitate is formed. Therefore, Nb, Zr It is preferable to contain at least one element selected from the group consisting of Ti and V in a range satisfying the following formula (1).

Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14)> 0 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)

  The left side of the above formula (1) represents the difference between the contents of Nb, Zr, Ti and V and the contents of C and N, and this value is positive that carbide, nitride or carbonitride It corresponds to containing Nb, Zr, Ti or V in a solid solution state in which no precipitate is formed.

  As shown in FIGS. 1 and 2, when the soaking temperature during soaking is high, the more the content of solid solution Nb, Zr, Ti and V is, the more effective the effect of suppressing dislocation disappearance and recrystallization. It is effective for obtaining a processed structure or a recovered structure. Therefore, the upper limit of the value represented by the above formula (1) is not particularly limited. In order to improve the flatness of the steel sheet, it is preferable to perform soaking, and in order to suppress the disappearance and recrystallization of dislocations that proceed, Nb, Zr, Ti and V are within the range satisfying the above formula (1). It is preferable to make it contain.

  Further, when considering sulfide, the contents of Nb, Zr, Ti and V in a solid solution state are also affected by the S content. However, since the influence of S was not recognized within the range of the S content described above, the above formula (1) in which the S term was omitted was adopted in the present invention. The reason why the influence of S was not recognized is not clear, but it is considered that S was fixed by Mn by, for example, crystallization as MnS from a region where S at the end of solidification was concentrated.

2. Next, the area ratio of the recrystallized portion in the present invention will be described.
In the present invention, even if the steel composition is adjusted to be in the above-described range, if the area ratio of the recrystallized portion is 25% or more, the strength suddenly decreases, so that desired mechanical properties may not be obtained. is there. Therefore, the area ratio of the recrystallized portion is set to less than 25%. From the viewpoint of mechanical properties, the area ratio of the recrystallized portion is preferably as low as possible, and is preferably 20% or less. In addition, it is preferable that the area ratio of the recrystallized portion is zero and it is completely non-recrystallized state (worked structure and recovery structure). For this purpose, soaking is not performed or if soaking is performed, soaking is performed. It is important to adjust the heat temperature and the like.

  Here, the area ratio of the recrystallized portion indicates the ratio of the recrystallized grains in the visual field in the longitudinal sectional structure photograph of the non-oriented electrical steel sheet for rotors of the present invention. It can be measured originally. As the longitudinal cross-sectional structure photograph, an optical microscope photograph can be used. For example, a photograph taken at a magnification of 100 times may be used.

B. Next, the manufacturing method of the non-oriented electrical steel sheet for rotors of the present invention will be described. The method for producing a non-oriented electrical steel sheet for a rotor of the present invention can be divided into two modes depending on the presence or absence of a soaking process. Hereinafter, each aspect will be described.

1. 1st aspect The 1st aspect of the manufacturing method of the non-oriented electrical steel sheet for rotors of this invention is a steel ingot provided with the steel composition described in the term of the "A. non-oriented electrical steel sheet for rotors" mentioned above. Or a hot rolling process in which hot rolling is performed on a steel slab, and a cold rolling process in which the hot rolled steel sheet obtained by the hot rolling process is subjected to cold rolling twice or more with one or intermediate annealing. And the soaking process in which soaking process is performed on the cold-rolled steel sheet obtained by the cold rolling process is not performed.

In conventional non-oriented electrical steel sheet manufacturing methods, a soaking process is usually performed after cold rolling for the purpose of recrystallization and grain growth, whereas in this embodiment, the soaking process is performed. Therefore, the dislocations introduced at the time of processing to a predetermined plate thickness are allowed to remain without annihilation, and the steel structure is made as it is during cold rolling. Strengthening is possible. In addition, the strengthening of steel sheets by such a processed structure is accompanied by the strengthening of steel sheets used for cold rolling, that is, the base material of cold rolling, unlike conventional strengthening mechanisms such as solid solution strengthening and precipitation strengthening. Therefore, breakage during cold rolling can be suppressed. Further, in this embodiment, steel ingots or steel slabs having a predetermined steel composition are used, and as described above, high strength is achieved by not performing the soaking process, so that expensive steel components are conventionally used. It is possible to stably produce a non-oriented electrical steel sheet for a rotor that satisfies the magnetic characteristics and mechanical characteristics required for a rotor of a drive motor, for example, without using a special process.
Hereinafter, each process in the manufacturing method of such a non-oriented electrical steel sheet for rotors is demonstrated.

(1) Hot rolling process The hot rolling process in this aspect is a process which hot-rolls the steel ingot or steel slab (henceforth "slab") provided with the steel composition mentioned above.

  In addition, about the steel composition of a steel ingot or a steel slab, since it is the same as that of what was described in the above-mentioned "A. Non-oriented electrical steel sheet for rotors", description here is abbreviate | omitted.

  In this step, the steel having the above-described composition is made into a slab by a general method such as a continuous casting method or a method of rolling a steel ingot, and is charged in a heating furnace and subjected to hot rolling. At this time, when the slab temperature is high, hot rolling may be performed without charging the heating furnace.

  Moreover, although the slab heating temperature is not specifically limited, it is preferable to set it as 1000-1300 degreeC from a viewpoint of cost and hot rolling property. More preferably, it is 1050-1250 degreeC.

  Hot rolling is not particularly limited, and may be performed according to general conditions such as a finishing temperature of 700 to 950 ° C. and a winding temperature of 750 ° C. or less.

(2) Cold rolling step The cold rolling step in this embodiment is a step of subjecting the hot rolled steel sheet obtained by the hot rolling step to cold rolling twice or more with one or intermediate annealing. By performing such a cold rolling process, the steel sheet is finished to a predetermined thickness.

  In this step, the sheet thickness may be finished by one cold rolling or may be finished by two or more cold rollings including intermediate annealing.

  Various conditions for cold rolling are not particularly limited, and are appropriately selected depending on the steel composition of the material to be rolled, the thickness of the target steel sheet, and the like.

(3) Hot-rolled sheet annealing process In this aspect, you may perform the hot-rolled sheet annealing process which performs hot-rolled sheet annealing to the hot-rolled steel plate obtained by the said hot-rolling process. This is because hot-rolled sheet annealing is not necessarily an essential process, but by performing the hot-rolled sheet annealing process, the ductility of the steel sheet is improved and breakage in the cold rolling process can be suppressed.

(4) Others In this embodiment, after the cold rolling step, a coating step of applying an insulating film made of only an organic component, only an inorganic component, or an organic-inorganic composite to a steel sheet surface according to a general method may be performed. preferable. Further, the coating process may be a process of applying an insulating coating that exhibits adhesive ability by heating and pressurizing. As a coating material exhibiting adhesive ability, an acrylic resin, a phenol resin, an epoxy resin, a melamine resin, or the like can be used.

  The non-oriented electrical steel sheet for rotors manufactured according to the present invention is the same as that described in the section “A. Non-oriented electrical steel sheet for rotors” described above. Omitted.

2. 2nd aspect The 2nd aspect of the manufacturing method of the non-oriented electrical steel sheet for rotors of this invention is a steel ingot provided with the steel composition described in the term of the "A. non-oriented electrical steel sheet for rotors" mentioned above. Or a hot rolling process in which hot rolling is performed on a steel slab, and a cold rolling process in which the hot rolled steel sheet obtained by the hot rolling process is subjected to cold rolling twice or more with one or intermediate annealing. And a soaking treatment step of soaking the cold-rolled steel sheet obtained by the cold rolling step at 780 ° C. or lower.

  In this embodiment, by setting the temperature in the soaking process to a predetermined range, recrystallization is suppressed, and the disappearance of dislocations introduced during processing to a predetermined plate thickness is suppressed, so that a large amount of dislocations The main body is a recovery structure in which the steel remains, which makes it possible to increase the strength of the steel sheet. In addition, since there is no need to increase the strength of the steel sheet used for cold rolling as in the conventional solid solution strengthening and precipitation strengthening, that is, the base material of cold rolling, it is possible to suppress breakage during cold rolling. . Furthermore, in this embodiment, since the strength is increased by using a steel ingot or steel slab having a predetermined steel composition and setting the temperature in the soaking process to a predetermined range as described above, the magnetic properties and A non-oriented electrical steel sheet for rotors having good mechanical properties can be stably produced. Furthermore, the flatness of the cold-rolled steel sheet can be improved by performing the soaking process, thereby improving the space factor when the rotor is configured and improving the motor efficiency.

  In addition, about a hot rolling process, a cold rolling process, and a hot-rolled sheet annealing process, since it is the same as that of what was described in the 1st aspect mentioned above, description here is abbreviate | omitted. Hereinafter, the soaking process in the manufacturing method of the non-oriented electrical steel sheet for rotors of this embodiment will be described.

(1) Soaking process The soaking process in this aspect is a process of soaking the cold-rolled steel sheet obtained by the cold rolling process described above at 780 ° C. or lower.

  The soaking process can be performed by a continuous annealing line. At this time, if the soaking temperature is high, recrystallization proceeds and sufficient mechanical properties may not be obtained. Therefore, the upper limit of the soaking temperature is 780 ° C. Preferably it is 750 degrees C or less, More preferably, it is 700 degrees C or less. In addition, the lower the soaking temperature, the more the progress of recrystallization is suppressed, so the lower limit is not particularly limited. When the flatness of the cold-rolled steel sheet is poor, the space factor when the flatness is not corrected and the lamination is performed on the rotor may be lowered if the soaking temperature is low. Therefore, the lower limit of the soaking temperature is preferably 500 ° C. from the viewpoint of flattening.

  Such a soaking process can correct the flatness of the cold-rolled steel sheet as described above, and is therefore effective when the flatness of the cold-rolled steel sheet is poor.

(2) Others In this embodiment, after the soaking step, it is preferable to perform a coating step in which an insulating coating composed of only an organic component, only an inorganic component, or an organic-inorganic composite is applied to the steel sheet surface according to a general method. . Further, the coating process may be a process of applying an insulating coating that exhibits adhesive ability by heating and pressurizing. As a coating material exhibiting adhesive ability, an acrylic resin, a phenol resin, an epoxy resin, a melamine resin, or the like can be used.

  The non-oriented electrical steel sheet for rotors manufactured according to the present invention is the same as that described in the section “A. Non-oriented electrical steel sheet for rotors” described above. Omitted.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits the same effects. Are included in the technical scope.

  Hereinafter, the present invention will be described specifically by way of examples and comparative examples.

[Examples 1 to 18]
Steels having the steel compositions shown in Table 1 below are vacuum-melted, these steels are heated to 1150 ° C., hot-rolled at a finishing temperature of 820 ° C., wound up at 580 ° C., and a thickness of 2.0 mm. A hot rolled steel sheet was obtained. Except for some of these hot-rolled steel sheets, box annealing that is held for 10 hours in a hydrogen atmosphere or hot-rolled sheet annealing by continuous annealing that is held at 1000 ° C. for 60 seconds is performed for one cold rolling. And finished to a plate thickness of 0.35 mm. In addition, some hot-rolled steel sheets are subjected to box annealing that is held in a hydrogen atmosphere for 10 hours after the above hot-rolled sheet annealing and then cold-rolled to an intermediate thickness, or held at 1000 ° C. for 60 seconds. Intermediate annealing by continuous annealing was performed and finished to 0.35 mm by the second cold rolling. Furthermore, some hot-rolled steel sheets were finished to 0.35 mm by performing cold rolling once or twice including intermediate annealing without performing hot-rolled sheet annealing. Thereafter, a soaking treatment was performed for 30 seconds at various temperatures. In this way, a steel plate was produced.

[Comparative Examples 1 to 7]
Steel plates were produced in the same manner as in Examples 1-18.

[Evaluation]
About the steel plates of Examples 1-18 and Comparative Examples 1-7, the area ratio of the recrystallized portion, mechanical properties, magnetic properties, and fatigue properties were evaluated.

As the area ratio of the recrystallized portion, the ratio of the recrystallized grains in the visual field was calculated using an optical micrograph of the longitudinal section of the steel sheet taken at a magnification of 100 times.
Mechanical properties were evaluated by a yield test: YP and a tensile strength: TS in a tensile test using a JIS No. 5 test piece.
Magnetic properties were measured in the Epstein test specified in JIS C 2550 by measuring iron loss W _10/400 at a maximum magnetic flux density of 1.0 T and excitation frequency of 400 Hz and a magnetic flux density B ₅₀ at a magnetizing force of 5000 A / m. did.
In the fatigue test, a test piece was collected by punching, and subjected to a single-sided electromagnetic resonance test at a frequency of 60 Hz while being punched without grinding the end face. In this fatigue test, the safety factor was taken into consideration with respect to the stress state of the drive motor, and those that did not undergo fatigue failure under the conditions of average stress: 300 MPa and stress amplitude: 180 MPa were judged to be good. The number of repetitions was up to 10 ⁷ , and the determination was made based on the presence or absence of destruction at this number of repetitions. In Table 2, those with no fatigue failure are indicated with “◯”, and those with fatigue failure are indicated with “x”.

  Table 2 shows hot rolled sheet annealing conditions, cold rolling conditions, soaking conditions, and evaluation results for the steel sheets of Examples 1 to 18 and Comparative Examples 1 to 7, respectively.

  The steel plate of Comparative Example 1 broke during cold rolling because of the high Si content. Moreover, since the steel plate of Comparative Example 2 had a high Al content, the magnetic flux density was low. Since the steel plate of Comparative Example 3 had a high P content, it broke during cold rolling. Further, the steel sheet of Comparative Example 4 had a high C and Mn content, and the steel structure was a martensite structure, so that the iron loss was remarkably increased and the magnetic flux density was low. Moreover, since the steel sheets of Comparative Examples 5 to 7 had a high area ratio of the recrystallized portion, both the yield point and the tensile strength were inferior. On the other hand, in the steel plates of Examples 1 to 18 that satisfy the requirements defined in the present invention, both the yield point and the tensile strength were excellent, and fatigue failure did not occur under the stress conditions described above.

  Further, by comparing Examples 14 to 17 having the same soaking conditions, it is a measure of the content of solute Nb, Zr, Ti and V (*; Nb / 93 + Zr / 91 + Ti / 48 + V / 51− ( C / 12 + N / 14)) It was found that the higher the value of the steel, the lower the recrystallization and the better the mechanical properties. Furthermore, by comparing Examples 14 and 15, it was found that the mechanical properties did not change even when the S content increased.

It is a figure which shows the relationship between Nb ^* (= Nb / 93-C / 12-N / 14) and tensile strength about the steel plate which performed the soaking | uniform-heating treatment hold | maintained at 700 degreeC for 20 second. It is a figure which shows the relationship between Nb ^* (= Nb / 93-C / 12-N / 14) and tensile strength about the steel plate which performed the soaking | uniform-heating treatment hold | maintained at 750 degreeC for 20 second.

Claims (4)

In mass%, C: 0.04% or less, Si: 3.5% or less, Mn: 0.1% or more and 2.5% or less, Al: 2.5% or less, P: 0.2% or less, S : 0.03% or less, N: 0.005% or less, and further containing at least one element selected from the group consisting of Nb, Ti, Zr and V, the balance being Fe and inevitable impurities A non-oriented electrical steel sheet for a rotor having a steel composition satisfying the following formula (1) and having an area ratio of a recrystallized portion of less than 25%.
0.0064 ≧ Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14)> 0 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)   The hot rolling process which hot-rolls the steel ingot or steel slab provided with the steel composition of Claim 1, and the hot rolling steel plate obtained by the said hot rolling process once or twice which inserts intermediate annealing A non-direction for a rotor having a cold rolling process for performing the above cold rolling and not performing a soaking process for soaking the cold rolled steel sheet obtained by the cold rolling process. Method for producing an electrical steel sheet.   The hot rolling process which hot-rolls the steel ingot or steel slab provided with the steel composition of Claim 1, and the hot rolling steel plate obtained by the said hot rolling process once or twice which inserts intermediate annealing A non-direction for a rotor, comprising: a cold rolling process for performing the above cold rolling; and a soaking process for soaking the cold-rolled steel sheet obtained by the cold rolling process at 780 ° C. or less. Method for producing an electrical steel sheet.   The method for producing a non-oriented electrical steel sheet for a rotor according to claim 2 or 3, further comprising a hot-rolled sheet annealing step for subjecting the hot-rolled steel sheet to hot-rolled sheet annealing.

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