JP2008308727A - Non-oriented electromagnetic steel sheet for rotor and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-oriented electromagnetic steel sheet which has both excellent mechanical properties and magnetic properties that are required to a rotor of a rapidly rotating motor and contributes to reduce the weight of the rotor, and to provide a manufacturing method therefor. <P>SOLUTION: The non-oriented electromagnetic steel sheet for the rotor includes, by mass%, 0.06% or less of C, 3.5% or less of Si, 0.05 to 3.0% of Mn, more than 2.5 to 6.0% of Al, 0.30% or less of P, 0.04% or less of S, 0.02% or less of N, more than 0.02% Nb, further at least one element selected from the group consisting of Nb, Ti, Zr and V in such a range as to satisfy the expression of 0<Nb/93+Zr/91+Ti/48+V/51-(C/12+N/14)<5×10<SP>-3</SP>, and the balance Fe with impurities; wherein a recrystallized portion is controlled to less than 90% by an area ratio. <P>COPYRIGHT: (C)2009,JPO&INPIT

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 that has excellent mechanical characteristics and magnetic characteristics suitable for a rotor of a permanent magnet embedded motor that rotates at high speed, and that can contribute to weight reduction of the rotor, 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. In addition, all parts are being reduced in weight for the purpose of improving the fuel efficiency of automobiles, but reducing the weight of the drive motor is also an important issue. 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 sufficient to use punched electrical steel sheets, but non-oriented electrical steel sheets that have both the magnetic and mechanical properties necessary for rotors and that contribute to weight reduction have been found. There was no current situation.

  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 strengthening by such precipitates is relatively ineffective, it is necessary to contain about 3.0% of Si as shown in Examples of Patent Document 3 and Patent Document 4, In the method, 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 inventors have made various studies on the steel structure that should be possessed by non-oriented electrical steel sheets having both magnetic and mechanical properties suitable for rotors, and have focused on increasing the strength by dislocation strengthening, which has not been studied at all. did. The new knowledge that dislocations remaining in the recovery state have a relatively small effect on iron loss has been obtained, and this technology is completely opposite to the complete recrystallized ferrite structure, which is the technical recognition of conventional non-oriented electrical steel sheets. Based on the idea, the magnetic and mechanical characteristics required for the rotor can be obtained by making the steel sheet structure a recovered structure in which a large amount of dislocations remain (hereinafter referred to as “recovered structure”). I found. Furthermore, in order to obtain a recovery structure, it is important to contain Nb, Zr, Ti and V in a solid solution state, and it is necessary to set the contents of Nb, Zr, Ti and V within a predetermined range. I found out. Based on these findings, a non-oriented electrical steel sheet having excellent mechanical and magnetic properties necessary for a rotor of a motor that rotates at high speed and a manufacturing method thereof have been proposed (Japanese Patent Laid-Open No. 2006-9048, No. 2006-70296).

  Furthermore, the present inventors have studied in detail the conditions for obtaining a recovery tissue, and found that it is most effective to contain Nb especially among Nb, Zr, Ti and V. (Japanese Patent Laid-Open No. 2007-16278).

  However, these technologies lack the concept of contributing to weight reduction of parts, and improvement is also demanded from this point.

  The present invention has been made in view of the above circumstances, and has a non-oriented electrical steel sheet that has excellent mechanical characteristics and magnetic characteristics necessary as a rotor of a motor that rotates at high speed, and that contributes to weight reduction. The main purpose is to provide a manufacturing method.

  In order to contribute to the weight reduction of the rotor, it is necessary to reduce the weight of the non-oriented electrical steel sheet, which is an iron core material, and to reduce the specific gravity. The present inventors have found that it is extremely effective to contain Al positively in order to achieve reduction in specific gravity and securing desired magnetic properties and mechanical properties at the same time.

  However, in the non-oriented electrical steel sheet that contains Al positively, there is a concern that the surface properties are deteriorated more than before due to an increase in the recrystallization temperature accompanying the increase in alloying. In the non-oriented electrical steel sheet containing Nb, Zr, Ti and V, there is a further concern about deterioration of the surface properties.

  The inventors of the present invention have made extensive studies to improve surface properties and achieve magnetic properties, mechanical properties, and specific gravity reduction. As a result, for non-oriented electrical steel sheets containing Al and containing Nb, Zr, Ti and V, the surface properties are improved by optimizing the contents of Nb, Zr, Ti and V. It has been found that it can be stably improved, and it has been found that a non-oriented electrical steel sheet not only having good mechanical and magnetic properties but also having good surface properties and contributing to weight reduction of the rotor can be obtained. The present invention has been completed based on these new findings.

That is, in the present invention, by mass%, C: 0.06% or less, Si: 3.5% or less, Mn: 0.05% or more and 3.0% or less, Al: more than 2.5% and 6.0% Hereinafter, P: 0.30% or less, S: 0.04% or less, N: 0.02% or less, Nb: more than 0.02%, and selected from the group consisting of Nb, Ti, Zr and V Non-directional for rotors, characterized in that at least one element is contained in a range satisfying the following formula (1), the balance is Fe and impurities, and the area ratio of the recrystallized portion is less than 90% An electrical steel sheet is provided.
0 <Nb / 93 + Zr / 91 + Ti / 48 + V / 51- (C / 12 + N / 14) <5 × 10 ^-3 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)

  In the present invention, the strength can be increased by positively containing Nb, appropriately controlling the area ratio of the recrystallized portion, and forming a steel structure with many dislocations remaining. It can be set as the non-oriented electrical steel sheet for rotors with a favorable characteristic. Furthermore, specific gravity can be reduced by positively containing Al. As a result, the magnetic properties and mechanical properties required for the rotor described above are also satisfied, and the rotor can be reduced in weight.

In addition, the non-oriented electrical steel sheet for rotors of the present invention may include at least one element selected from the group consisting of Cu, Ni, Cr, Mo, Co, and W instead of a part of the Fe described below. It is preferable to contain by mass%.
Cu: 0.01% to 8.0% Ni: 0.01% to 2.0% Cr: 0.01% to 15.0% Mo: 0.005% to 4.0% Co: 0.01% or more and 4.0% or less W: 0.01% or more and 4.0% or less The strength of the steel sheet can be further increased by the action of increasing the strength of the above elements.

Furthermore, the non-oriented electrical steel sheet for a rotor of the present invention contains at least one element selected from the group consisting of Sn, Sb, Se, Bi, Ge, Te and B instead of a part of the Fe. It is preferable to contain by the following mass%.
Sn: 0.5% or less Sb: 0.5% or less Se: 0.3% or less Bi: 0.2% or less Ge: 0.5% or less Te: 0.3% or less B: 0.01% or less This is because recrystallization can be effectively suppressed by grain boundary segregation of elements.

Furthermore, the non-oriented electrical steel sheet for rotors of the present invention contains at least one element selected from the group consisting of Ca, Mg and REM in the following mass%, instead of a part of the Fe. It is preferable.
Ca: 0.03% or less Mg: 0.02% or less REM: 0.1% or less Magnetic properties can be further improved by the sulfide form controlling action of the above elements.

  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 having a cold rolling step for performing cold rolling at least twice and a soaking step for soaking the cold rolled steel sheet obtained by the cold rolling step at 820 ° C. or less. A method for producing a non-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 and reduced specific gravity.

  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, a non-oriented electrical steel sheet that has both excellent mechanical and magnetic properties required as a rotor for a motor that rotates at high speed, and that can contribute to weight reduction, is stable without causing a significant increase in cost. Can be manufactured. Therefore, it can sufficiently cope with the high speed and light weight of the motor in the drive motor field 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 the yield point and the 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 stress state described above, the tensile strength is 550 MPa or more, and considering the safety factor, 600 MPa or more, preferably 700 MPa or more is required.

  Further, the second 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.

  The third characteristic required for the electrical 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 excessively low, a desired torque cannot be obtained. However, this is not the case when the output of the motor is secured not by torque but by rotation speed. Since recent drive motors and servo motors tend to rotate at high speed, it is recalled that the magnetic flux density of the material used for the rotor is acceptable even if the magnetic flux density is lower than that of the material used for the stator.

  The fourth characteristic necessary for the electrical steel sheet used for the rotor is specific gravity. Usually, the weight reduction of the motor is achieved by the miniaturization of the motor. For miniaturization of the motor, it is extremely effective to increase the magnetic flux density of the electromagnetic steel sheet used for the stator, and the influence of the magnetic flux density of the electromagnetic steel sheet used for the rotor on the miniaturization is small. Therefore, in order to reduce the weight of the motor as a rotor, there is no other way to reduce the weight of the rotor of the given shape, but it is necessary to reduce the weight of the electromagnetic steel sheet for the rotor, that is, to reduce the specific gravity of the electromagnetic steel sheet for the rotor Is done.

  The present inventors diligently studied non-oriented electrical steel sheets that satisfy these characteristics. First, based on the above idea, various studies were made on the steel structure that should be possessed by the non-oriented electrical steel sheet having both magnetic properties and mechanical properties suitable for the rotor. 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 dislocation strengthening as a strengthening mechanism, it was found that dislocations remaining in the recovery state have a relatively small effect on iron loss. From these results, the magnetic properties required for the rotor are achieved by using a recovery structure in which a large amount of dislocations remain, contrary to the completely recrystallized ferrite structure that is the technical recognition of conventional non-oriented electrical steel sheets. And the knowledge that the mechanical properties are achieved.

The recovery structure can be obtained by allowing dislocations introduced at the time of processing to a predetermined plate thickness to remain by suppressing annihilation by appropriate soaking. 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 recovery structure, it is necessary to suppress recrystallization in a soaking process usually performed after cold rolling. Moreover, in order to reduce specific gravity without deteriorating mechanical characteristics and magnetic characteristics, it is necessary to positively contain Al. In order to suppress recrystallization at the time of soaking in a steel sheet containing a large amount of Al, it is important to contain Nb, Zr, Ti and V in a solid solution state, and to contain Nb, Zr, Ti and V is required. In particular, since the contribution of Nb is large, it is necessary to contain an appropriate amount centering on Nb. However, if Nb, Zr, Ti and V are excessively contained in a steel sheet whose recrystallization temperature has increased due to the inclusion of a large amount of Al, the surface properties deteriorate. Therefore, in a steel sheet containing a large amount of Al, Nb, Zr , Ti and V content optimization is extremely important. Specifically, it is necessary to contain Nb, Zr, Ti and V in a solid solution state in an appropriate amount centering on solid solution Nb having a large recrystallization inhibiting effect, and it is necessary to satisfy the following (1). is there.
0 <Nb / 93 + Zr / 91 + Ti / 48 + V / 51- (C / 12 + N / 14) <5 × 10 ^-3 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)
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: 0.06% or less, Si: 3.5% or less, Mn: 0.05% or more. 0% or less, Al: more than 2.5%, 6.0% or less, P: 0.30% or less, S: 0.04% or less, N: 0.02% or less, Nb: more than 0.02% And containing at least one element selected from the group consisting of Nb, Ti, Zr and V in a range satisfying the above formula (1), the balance being Fe and impurities, and the area ratio of the recrystallized portion being It is characterized by being less than 90%.
“%” Indicating the content of each element means “mass%” unless otherwise specified.
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, it is preferable to reduce the C content in order to suppress the disappearance of dislocations and the progress of recrystallization that proceed in the soaking process after cold rolling by solute Nb, Zr, Ti, and V. 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 0.06%. Preferably it is 0.04% or less, More preferably, it is 0.02% or less. In particular, if the C content is 0.01% or less, Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14)> 0 necessary for satisfying the condition of Nb, Zr, Ti and V Since the content is small, it is desirable from the viewpoint of manufacturing cost.

(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. More preferably, it is less than 2.0%. When Si is used as a deoxidizing agent, it is necessary to contain 0.01% or more, but in the present invention, since Al is also actively contained as a deoxidizing agent, the lower limit value of the Si content Is not particularly limited. From the viewpoint of increasing the strength of the steel sheet by solid solution strengthening, it is desirably 0.2% or more, and more desirably 1.0% or more.

(3) Mn
Mn has the effect of increasing electrical resistance and reducing eddy current loss, similar to Si. However, if Mn is contained in a large amount, the alloy cost increases, so the upper limit of the Mn content is 3.0%. On the other hand, the lower limit of the Mn content is determined from the viewpoint of fixing S, and is 0.05%.

(4) Al
Al increases eddy current loss in the same manner as Si because it increases electric resistance. Furthermore, it has an effect of greatly reducing the specific gravity, and is an extremely important alloy element in the present invention aimed at contributing to the weight reduction of the rotor. However, when Al is contained in a large amount, the alloy cost increases, and cracks during cold rolling are induced as in the case of Si, and the manufacturing cost increases due to a decrease in the yield of the steel sheet. However, since the contribution to cold rollability deterioration is lower than that of Si, a large amount is allowed to be contained, and the Al content is more than 2.5% from the viewpoint of reducing specific gravity and eddy current loss. Preferably it is 3.0% or more. In order to obtain a desired electrical resistance and achieve a reduction in eddy current loss after securing a tensile strength of 800 MPa or more, 3.5% or more is preferable. From the viewpoint of suppressing the cost increase and the cold rolling deterioration, the upper limit value of the Al content is 6.0%.

(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 0.30% or less. From the viewpoint of ensuring strength, the P content is 0.01% or more, preferably 0.02% or more.

(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 0.04%.

(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, in order to suppress recrystallization by the solid solution Nb, Zr, Ti and V, it is preferable to reduce the N content. However, in view of the fact that the contents of solute Nb, Zr, Ti and V can be secured if the contents of Nb, Zr, Ti and V are increased accordingly even if the N content is large, The upper limit is 0.02%. Preferably it is 0.01% or less. If the N content is 0.005% or less, the Nb, Zr, Ti and V contents necessary to satisfy the condition of Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14)> 0 This is desirable from the viewpoint of manufacturing cost.

(8) Nb, Zr, Ti and V
Steel sheets containing a large amount of Al exceeding 2.5% have an increased recrystallization temperature due to high alloying, and disclosed in JP-A-2006-9048, JP-A-2006-70296, and It is expected that the effect of suppressing recrystallization is higher than that of the steel sheet disclosed in JP 2007-16278. However, as a result of investigations by the present inventors, in order to obtain the mechanical and magnetic properties required for the rotor by suppressing the disappearance and recrystallization of dislocations during soaking and obtaining a recovery structure, precipitates are used. It has been found necessary to contain Nb, Zr, Ti or V in a solid solution state that is not formed. That is, it is necessary to contain at least one element selected from the group consisting of Nb, Zr, Ti, and V in a range that satisfies the following formula (2).
Nb / 93 + Zr / 91 + Ti / 48 + V / 51- (C / 12 + N / 14)> 0 (2)
(Here, in the formula (2), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)

  The left side of the above formula (2) represents the difference between the contents of Nb, Zr, Ti and V and the contents of C and N, and this value is positive when it is 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 described above, the solid solution Nb contributes particularly greatly among these elements, and the inclusion of the solid solution Nb in the center makes it easier to obtain a recovery structure even if the soaking temperature during soaking is high. In the invention, Nb is positively contained, and the Nb content exceeds 0.02%. Preferably it is 0.04% or more, more preferably more than 0.05%. The positive inclusion of Nb greatly contributes to productivity improvement as will be described later. On the other hand, the upper limit of the Nb content is set so as not to exceed the upper limit of the formula (1) described later.

When the soaking temperature at the time of soaking is relatively high, the greater the content of the solid solution Nb, Zr, Ti and V, the greater the effect of suppressing the disappearance and recrystallization of dislocations, thereby obtaining a recovery structure. Is effective.
However, when excessively containing solute Nb, Zr, Ti, and V, dislocation disappearance and recrystallization are suppressed during hot rolling and hot-rolled sheet annealing, so that the structure before cold rolling is not yet obtained. Recrystallized state. As a result, surface defects called ridging are generated, which is not preferable because the space factor when laminated on an iron core is lowered and the motor efficiency is lowered. In particular, in the present invention in which Al is contained in a large amount exceeding 2.5%, the recrystallization temperature is increased due to high alloying, and Japanese Patent Laid-Open Nos. 2006-9048 and 2006-70296 are disclosed. As compared with the steel sheet disclosed in Japanese Laid-Open Patent Publication No. 2007-16278, there is a greater concern about the deterioration of surface properties due to ridging. Moreover, a crack may arise at the time of cold rolling. Therefore, in the present invention in which Al is actively contained, the upper limit value of the content of solute Nb, Zr, Ti and V is determined from this viewpoint, and Nb, Zr, Ti and V are expressed by the following formula (1). It is necessary to contain in the range shown.
0 <Nb / 93 + Zr / 91 + Ti / 48 + V / 51- (C / 12 + N / 14) <5 × 10 ^-3 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.)

  In consideration of sulfide, the contents of Nb, Zr, Ti and V in the solid solution state are also affected by the S content. However, since the influence of S on the recrystallization suppressing effect was not recognized within the range of the S content of the present invention, the above formula (1) in which the term of S 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.

(9) Cu, Ni, Cr, Mo, Co and W
In the present invention, since the recrystallization itself is suppressed rather than the recrystallized grain size, the recrystallization itself is suppressed to achieve both magnetic properties and mechanical properties. Therefore, Cu, Ni, At least one element selected from the group consisting of Cr, Mo, Co and W can be contained. Since these elements have the effect of increasing the strength of the steel sheet, they are effective and preferable for further increasing the strength of the steel sheet.

Cu has the effect of increasing the specific resistance of the steel sheet and reducing iron loss. However, excessive inclusion of Cu leads to surface defects and cracking during cold rolling, so the Cu content is preferably 0.01% or more and 8.0% or less.
If Ni and Mo are excessively contained, cracks during cold rolling and an increase in cost are caused. Therefore, the Ni content is 0.01% or more and 2.0% or less, and the Mo content is 0.005% or more. It is preferable to make it 0% or less.
Cr has the effect of increasing the specific resistance of the steel sheet and reducing iron loss. It also has the effect of improving corrosion resistance. However, since the cost increases when Cr is excessively contained, the Cr content is preferably 0.01% or more and 15.0% or less.
When Co and W are excessively contained, the cost increases. Therefore, the Co content is preferably 0.01% or more and 4.0% or less, and the W content is preferably 0.01% or more and 4.0% or less. .

(10) Sn, Sb, Se, Bi, Ge, Te and B
Since the present invention attempts to achieve both magnetic properties and mechanical properties by suppressing recrystallization, it consists of Sn, Sb, Se, Bi, Ge, Te, and B, which have the effect of suppressing recrystallization by grain boundary segregation. It is preferable to contain at least one element selected from the group. When these elements are contained, the content of each element is Sn: 0.5% or less, Sb: 0.5% or less, from the viewpoint of suppressing the occurrence of cracks and cost increase in the hot rolling process. Preferably, Se is 0.3% or less, Bi is 0.2% or less, Ge is 0.5% or less, Te is 0.3% or less, and B is 0.01% or less. In order to reliably obtain the recrystallization suppressing effect by these elements, the content of each element is Sn: 0.001% or more, Sb: 0.0005% or more, Se: 0.0005% or more, Bi: 0.0005. % Or more, Ge: 0.001% or more, Te: 0.0005% or more, and B: 0.0002% or more are preferable.

(11) Ca, Mg and REM
Since the influence of S on the recrystallization suppression effect was not recognized within the range of the S content defined in the present invention, in the present invention, it is composed of Ca, Mg and REM for the purpose of improving magnetic properties by controlling the form of sulfide. At least one selected from the group can be contained.
Here, REM refers to a total of 17 elements of 15 elements having atomic numbers 57 to 71 and 2 elements of Sc and Y.

  When these elements are contained, the content of each element is preferably Ca: 0.03% or less, Mg: 0.02% or less, and REM: 0.1% or less. In order to reliably obtain the above effects, the content of each element is preferably set to Ca: 0.0001% or more, Mg: 0.0001% or more, and REM: 0.0001% or more.

(12) Others In the present invention, it is possible to contain elements other than the elements described above within a range not impairing the effects of the present invention. The present invention is different from the conventional technique based on the recrystallized structure, and is intended to increase the strength by using a recovery structure in which many dislocations remain. Therefore, the present invention is limited in the conventional technique based on the recrystallized structure. The inclusion of additional elements can be tolerated to a higher level. For example, Ta, Hf, As, Au, Be, Zn, Pb, Tc, Re, Ru, Os, Rh, Ir, Pd, Pt, Ag, Cd, Hg, and Po are contained in a total amount of 0.1% or less. Can do.

2. Next, the reason for limiting the area ratio of the recrystallized portion in the present invention will be described.
With the progress of recovery, which is the pre-recrystallization stage, the yield point and tensile strength decrease while the area ratio of the recrystallized portion remains zero. After the start of recrystallization, the yield point and the tensile strength further decrease with an increase in the area ratio of the recrystallized portion. Here, the area ratio of the recrystallized portion is determined from the viewpoint of securing the mechanical characteristics necessary for the rotor. Considering the safety factor, the area ratio of the recrystallized portion is less than 90% from the viewpoint of suppressing deformation during high-speed rotation. Preferably it is 70% or less. From the viewpoint of suppressing fatigue fracture, it is preferably 40% or less, more preferably less than 25%. From the viewpoint of mechanical properties, the area ratio of the recrystallized part is preferably as low as possible, and the area ratio of the recrystallized part is preferably set to zero so that it is completely in an unrecrystallized state (recovered structure).

  To control the area ratio of the recrystallized portion, it is important to adjust the soaking temperature and soaking time during soaking. Since the present invention is characterized by positively containing Nb having a large effect of suppressing recrystallization, such area ratio control of the recrystallized portion is easier than when Nb is not positively contained. It also leads to productivity improvement.

  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 this invention is demonstrated.
The method for producing a non-oriented electrical steel sheet for a rotor of the present invention 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 process obtained by the hot rolling process. A cold rolling process in which cold rolling is performed twice or more by sandwiching the rolled steel sheet once or by intermediate annealing, and a soaking process in which the cold rolled steel sheet obtained by the cold rolling process is soaked at 820 ° C. or less. It is characterized by having.

According to the present invention, by setting the temperature in the soaking process to a predetermined range, recrystallization is suppressed, and disappearance of dislocations introduced during processing to a predetermined plate thickness is suppressed, and a large amount of The recovery structure in which dislocations are left can be the main component, 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 the present invention, since a steel ingot or steel slab having a predetermined steel composition is used, and the temperature in the soaking process is set within a predetermined range as described above, the strength is increased. Non-rotatability for rotors that does not use expensive steel components and does not go through special processes, for example, satisfies the magnetic and mechanical properties required for rotors of drive motors and contributes to weight reduction An electromagnetic steel sheet can be manufactured stably.
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 invention is a process of hot-rolling 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 piece, since it is the same as that of what was described in the term of the "A. non-oriented electrical steel sheet for rotors" mentioned above, 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.
The slab heating temperature is not particularly limited, but is preferably 1000 ° C. to 1300 ° C. from the viewpoint of cost and hot rolling properties. More preferably, it is 1050 degreeC-1250 degreeC.
Moreover, various conditions of hot rolling are not specifically limited, For example, what is necessary is just to carry out according to general conditions, such as finishing temperature of 700 to 950 degreeC and coiling temperature of 750 degrees C or less.

  A hot-rolled steel sheet is usually subjected to cold rolling after removing scales generated on the surface of the steel sheet during hot rolling by pickling. When hot-rolled sheet annealing described later is applied to the hot-rolled steel sheet, it may be pickled either before hot-rolled sheet annealing or after hot-rolled sheet annealing.

(2) Cold rolling process The cold rolling process in the present invention is a 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. 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.

  In the present invention, the strength is increased by suppressing the disappearance of the dislocations introduced by cold rolling, so that sufficient strength cannot be ensured when the amount of dislocations introduced is small. Since the amount of dislocation introduced can be determined by the strength of the steel sheet, in the present invention, the tensile strength in the previous stage to be subjected to the soaking process is used as an index, and the measured value with the rolling direction as the longitudinal direction is 850 MPa or more. It is preferable to do.

  Since the effect of the present invention can be obtained if dislocations are sufficiently introduced, various conditions for cold rolling, such as the steel sheet temperature during cold rolling, the rolling reduction, and the rolling roll diameter, are not particularly limited, The material is appropriately selected depending on the steel composition of the material to be rolled, the thickness of the target steel sheet, and the like. Even when light processing is performed for the purpose of correcting the flatness of the steel sheet before being subjected to the soaking process, the effects of the present invention can be obtained as long as the above-described tensile strength is satisfied after the light processing.

(3) Soaking step The soaking step in the present invention is a step of soaking the cold-rolled steel sheet obtained by the cold rolling step described above at 820 ° C or lower.

  The gist of the present invention is to suppress the recrystallization that proceeds in the soaking process and leave the dislocations. Therefore, when the recrystallization suppressing effect is small, it is necessary to make the soaking temperature significantly lower than the soaking temperature of a normal non-oriented electrical steel sheet. Assuming soaking in a continuous annealing line of a normal non-oriented electrical steel sheet, it cannot be subjected to soaking until the furnace temperature is lowered and stabilized. Furthermore, once the furnace temperature is lowered, the normal non-oriented electrical steel sheet cannot be subjected to soaking treatment until the furnace temperature rises to the soaking temperature of the normal non-oriented electrical steel sheet and stabilizes. . From these facts, it can be easily imagined that the productivity is remarkably lowered when the recrystallization suppressing effect is small.

  In the present invention, among Nb, Zr, Ti and V, Nb is particularly positively contained, so that the effect of suppressing recrystallization is great. Therefore, even if the soaking temperature in the soaking process is high, a recovery structure can be obtained, and it is not necessary to provide a special soaking temperature opportunity, so that productivity can be improved. Specifically, if the soaking temperature in the soaking process is 820 ° C. or less, desired mechanical properties can be obtained. From the viewpoint of mechanical properties, it is preferably 780 ° C. or lower, more preferably 750 ° C. or lower. This soaking temperature is within the range to be implemented with a normal non-oriented electrical steel sheet, and does not hinder productivity.

  On the other hand, the lower the soaking temperature, the lower the progress of recrystallization. However, if the soaking temperature is too low, the flatness of the steel sheet is not corrected and the space factor when laminated on the rotor may decrease. . Moreover, since the soaking heat treatment has an effect of improving the iron loss as compared with the cold rolling state, if the soaking temperature is too low, the iron loss is increased. Furthermore, if the soaking temperature is too low, the productivity is significantly reduced as described above. Therefore, from the viewpoint of flatness correction and iron loss improvement, a preferable lower limit value of the soaking temperature is set to 500 ° C. More preferably, it is 600 degreeC or more.

The soaking process may be carried out by box annealing, but the flatness of the steel sheet decreases or the shape deteriorates due to winding of the coil (also called coil set) due to the annealing in the coil state. In some cases, a straightening process for correcting the flatness and shape of the steel sheet after the soaking process may be necessary. Therefore, it is desirable to carry out in a continuous annealing line from the viewpoint of productivity.
If recrystallization proceeds by soaking at a high temperature and the mechanical properties are lowered due to this, the number of steps is unavoidable, but processing may be performed after the soaking step to ensure strength.

(4) Hot-rolled sheet annealing process In this invention, 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 hot-rolled sheet annealing process is a process performed between a hot rolling process and a cold rolling process.
The hot-rolled sheet annealing process 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.
Hot-rolled sheet annealing may be performed by any method of box annealing and continuous annealing. Moreover, the various conditions of hot-rolled sheet annealing are not specifically limited, It shall select suitably by the steel composition etc. of a hot-rolled steel plate.

(5) Others In the present invention, after the soaking step, according to a general method, 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 may be performed. preferable. From the viewpoint of reducing environmental burden, an insulating film not containing chromium may be applied. 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.

  Note that the non-oriented electrical steel sheet for rotors manufactured according to the present invention is the same as that described in the above-mentioned section “A. Non-oriented electrical steel sheet for rotors”, so the description here is as follows. 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 26]
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. For some hot-rolled steel sheets, after the above-described hot-rolled sheet annealing, after cold rolling to an intermediate sheet thickness, box annealing that is held at 750 ° C. or 800 ° C. for 10 hours in a hydrogen atmosphere, or 1000 Intermediate annealing was performed by continuous annealing held at 60 ° C. for 60 seconds, 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, in Examples 1 to 9 and 11 to 26, soaking was performed by continuous annealing that was held at various temperatures for 30 seconds. In Example 10, soaking was performed by box annealing held at 500 ° C. for 10 hours. In this way, a steel plate was produced.

[Comparative Examples 1 to 9]
A steel plate was produced in the same manner as in Examples 1 to 26, using steel having the steel composition shown in Table 1 above.

[Evaluation]
About the steel plates of Examples 1 to 26 and Comparative Examples 1 to 9, the mechanical properties of the steel plates in the stage before soaking, and the area ratio, mechanical properties, magnetic properties and specific gravity of the recrystallized portion after soaking 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.
The mechanical properties were evaluated by performing a tensile test using a JIS No. 5 test piece. The steel plate before the soaking was evaluated by tensile strength: TS, and the steel plate after the soaking was evaluated by yield point: YP and tensile strength: TS.
Regarding the magnetic characteristics, the iron loss W _10/400 at a maximum magnetic flux density of 1.0 T and an excitation frequency of 400 Hz was measured with a 55 mm square single plate test piece. The measurement was carried out in the rolling direction and the direction perpendicular to the rolling, and the average value thereof was adopted.
About specific gravity, the weight in water and the weight in air were measured, and it evaluated by the Archimedes method which considered the density of the water in measurement temperature.

  Table 2 shows hot rolled sheet annealing conditions, cold rolling conditions, soaking conditions, and evaluation results for the steel sheets of Examples 1 to 26 and Comparative Examples 1 to 9, respectively.

  Since the steel plate of Comparative Example 1 has a high Si content, the steel plate of Comparative Example 2 has a high Al content, and the steel plate of Comparative Example 3 has a high P content. Furthermore, 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. In the steel sheet of Comparative Example 5, since the contents of Nb, Zr, Ti and V are outside the scope of the present invention, recrystallization is not suppressed, the area ratio of the recrystallized portion is increased, and the yield point and tensile strength are inferior. It was. The steel sheet of Comparative Example 6 was inferior in yield point and tensile strength because the amount of dislocations introduced by cold rolling was not sufficient. The steel plate of Comparative Example 7 was inferior in yield point and tensile strength because the area ratio of the recrystallized portion was high. The steel plate of Comparative Example 8 broke during cold rolling because the Nb, Zr, Ti and V contents exceeded the upper limit of the range of the present invention. The steel plate of Comparative Example 9 had a high specific gravity because of the low Al content.

  On the other hand, in the steel plates of Examples 1 to 26 that satisfy the requirements specified in the present invention, regardless of the hot-rolled sheet annealing method and the number of cold rolling, the magnetic properties and mechanical properties showed excellent values. The specific gravity was as low as 7.31 to 7.35. These specific gravity levels can reduce the weight of the rotor having the same shape by 3 to 4% as compared with the case where the steel plate of Comparative Example 9 is used.

In addition, it was found that even under conditions where the soaking temperature is relatively high, the recrystallization suppression effect is large, and thus the magnetic properties and mechanical properties are excellent.
Furthermore, by comparing Examples 13 and 14, it was found that the mechanical properties did not change even when the S content changed.

Claims (6)

In mass%, C: 0.06% or less, Si: 3.5% or less, Mn: 0.05% or more and 3.0% or less, Al: more than 2.5% and 6.0% or less, P: 0.0. 30% or less, S: 0.04% or less, N: 0.02% or less, Nb: more than 0.02%, and at least one element selected from the group consisting of Nb, Ti, Zr and V In a range that satisfies the following formula (1), the balance is Fe and impurities, and the area ratio of the recrystallized portion is less than 90%.
0 <Nb / 93 + Zr / 91 + Ti / 48 + V / 51- (C / 12 + N / 14) <5 × 10 ^-3 (1)
(Here, in the formula (1), Nb, Zr, Ti, V, C and N indicate the content (mass%) of each element.) The at least one element selected from the group consisting of Cu, Ni, Cr, Mo, Co, and W is contained in the following mass%, instead of a part of the Fe. Non-oriented electrical steel sheet for rotors.
Cu: 0.01% to 8.0% Ni: 0.01% to 2.0% Cr: 0.01% to 15.0% Mo: 0.005% to 4.0% Co: 0.01% or more and 4.0% or less W: 0.01% or more and 4.0% or less 2. Instead of a part of the Fe, at least one element selected from the group consisting of Sn, Sb, Se, Bi, Ge, Te and B is contained in the following mass%. Or the non-oriented electrical steel sheet for rotors of Claim 2.
Sn: 0.5% or less Sb: 0.5% or less Se: 0.3% or less Bi: 0.2% or less Ge: 0.5% or less Te: 0.3% or less B: 0.01% or less 4. Instead of a part of Fe, at least one element selected from the group consisting of Ca, Mg, and REM is contained in the following mass%, and any one of claims 1 to 3 The non-oriented electrical steel sheet for rotors according to claim 1.
Ca: 0.03% or less Mg: 0.02% or less REM: 0.1% or less   A hot rolling process in which hot rolling is performed on a steel ingot or steel slab having the steel composition according to any one of claims 1 to 4, and hot rolling obtained by the hot rolling process A cold rolling process in which the steel sheet is subjected to cold rolling at least once with intermediate or intermediate annealing, and a soaking process in which the cold rolled steel sheet obtained by the cold rolling process is soaked at 820 ° C. or less. A method for producing a non-oriented electrical steel sheet for a rotor, comprising:   The method for producing a non-oriented electrical steel sheet for a rotor according to claim 5, further comprising a hot-rolled sheet annealing step for subjecting the hot-rolled steel sheet to hot-rolled sheet annealing.