JP2007162096A - Non-oriented electrical steel sheet for rotor, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-oriented electrical steel sheet which combines excellent mechanical properties and magnetic properties both required of a rotor of a rapidly rotating motor while suppressing the increase of alloy cost and also to provide its manufacturing method. <P>SOLUTION: The non-oriented electrical steel sheet for a rotor has a composition containing, by mass, ≤0.06% C, ≤3.5% Si, 0.05 to 3.0% Mn, ≤2.5% Al, ≤0.30% P, ≤0.04% S, ≤0.02% N and >0.02% Nb, and comprising Nb, Ti, Zr, V and Al in the ranges satisfying Nb/93+Zr/91+Ti/48+V/51-(C/12+N/14)≤0 and Zr/91+Ti/48+Al/27-N/14>0, and the balance substantially Fe with impurities. Further, the area ratio of a recrystallized portion is <90%. <P>COPYRIGHT: (C)2007,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 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 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.

  Patent Document 8 proposes a high-strength electrical steel sheet having excellent magnetic properties. However, since it is based on making the content of Ti and Nb into an inevitable impurity level or reducing it, high intensity | strength cannot be obtained stably.

  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 sheet 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 JP-A-2005-113185

  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 suppresses an increase in alloy cost, and has a non-oriented electrical steel sheet having excellent mechanical characteristics and magnetic characteristics required as a rotor for a motor that rotates at high speed. And a method for manufacturing the same.

  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 the rotor. Pay attention. And we obtained new knowledge that dislocations remaining in the recovery state have a relatively small effect on iron loss, which is completely opposite to the complete recrystallized ferrite structure, which is the technical recognition of conventional non-oriented electrical steel sheets. Based on this technical concept, the magnetic properties and machine required for the rotor are obtained by making the steel sheet structure a processed structure in which a large amount of dislocations remain and a recovered structure (hereinafter referred to as “recovered structure”). It has been found that characteristics can be obtained. Furthermore, in order to obtain a recovery structure, it has been found that the contents of Nb, Zr, Ti and V need to be within a predetermined range, and based on these findings, Japanese Patent Application Nos. 2004-183554 and 2004 are disclosed. -252395 proposes 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 method for manufacturing the same. However, in the preferred embodiment described in Japanese Patent Application No. 2004-183554 and the invention described in Japanese Patent Application No. 2004-252395, Nb, Zr, Ti and V in an amount exceeding the content of C and N are contained. There is room for improvement from the viewpoint of alloy costs.

  Therefore, the present inventors examined a method for stably obtaining a recovery structure in consideration of cost reduction, and positively containing Nb having a large recrystallization suppressing effect, and then using Zr, Ti, and Al. By fixing N as precipitates, even if the contents of Nb, Zr, Ti and V are equal to or less than the contents of C and N, the disappearance and recrystallization of dislocations during the soaking treatment performed after the final cold rolling It has been found that the progression of is suppressed. The present invention has been completed based on these new findings.

That is, the present invention is, in mass%, C: 0.06% or less, Si: 3.5% or less, Mn: 0.05% or more and 3.0% or less, Al: 2.5% or less, P: 0 30% or less, S: 0.04% or less, N: 0.02% or less, Nb: more than 0.02%, and at least one selected from the group consisting of Nb, Ti, Zr and V The element is contained within a range satisfying the following formula (1), and at least one element selected from the group consisting of Zr, Ti and Al is contained within a range satisfying the following formula (2), and the balance is There is provided a non-oriented electrical steel sheet for a rotor which is substantially composed of Fe and impurities and has an area ratio of a recrystallized portion of less than 90%.
Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14) ≦ 0 (1)
Zr / 91 + Ti / 48 + Al / 27-N / 14> 0 (2)
(Here, in the formulas (1) and (2), Nb, Zr, Ti, V, Al, C and N indicate the content (mass%) of each element.)

  According to the present invention, since the content of Zr, Ti and Al is optimized after positively containing Nb having a large recrystallization suppressing effect, N is fixed as a precipitate by Zr, Ti and Al. Even if the contents of Nb, Zr, Ti, and V are less than or equal to the contents of C and N, the dissolution of dislocation and the progress of recrystallization during soaking can be suppressed by the solid solution Nb. In the present invention, since the steel composition is optimized and the area ratio of the recrystallized portion is appropriately controlled to obtain a steel structure in which many dislocations remain, the strength can be increased. And it can be set as the non-oriented electrical steel sheet for rotors with a favorable magnetic characteristic. 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 rotors according to the present invention contains 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.

Moreover, it is preferable that the non-oriented electrical steel sheet for rotors of this invention satisfies the following formula (3) in steel composition.
Nb / 93−C / 12> 0 (3)
(Here, in the formula (3), Nb and C indicate the content (% by mass) of each element.)
This is because solid solution Nb can suppress the disappearance of dislocations and the progress of recrystallization during soaking, and can effectively increase the strength of the steel sheet.

  The present invention also includes a hot rolling process in which a steel ingot or steel slab having the above steel composition is hot-rolled, 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.

  According to the present invention, by using a steel ingot or steel slab having a steel composition satisfying Nb / 93-C / 12> 0, the recrystallization is suppressed by setting the temperature in soaking to a predetermined range. The strength of the steel sheet can be increased because the recovered structure that remains without annihilating the dislocations introduced during the processing to a predetermined plate thickness can be mainly used. 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 the above 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.

  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. Moreover, it is because the effect of high intensity | strength may arise by optimization of hot-rolled sheet annealing conditions.

Furthermore, the present invention includes a hot rolling process in which a steel ingot or a steel piece having the steel composition described above and the steel composition satisfies the following formula (4) is hot-rolled, and the hot rolling process. The obtained hot-rolled steel sheet is subjected to cold rolling twice or more with one or intermediate annealing, and the steel sheet before the final cold-rolling is performed at a temperature of 850 ° C. to 1200 ° C. for 10 seconds to 5 minutes. Non-directionality for a rotor, comprising: a cold rolling process for performing the following continuous annealing; and a soaking process for soaking the cold rolled steel sheet obtained by the cold rolling process at 820 ° C. or less. A method for manufacturing an electrical steel sheet is provided.
Nb / 93−C / 12 ≦ 0 (4)
(Here, in the formula (4), Nb and C indicate the content (mass%) of each element.)

  According to the present invention, even when Nb / 93−C / 12 ≦ 0, Nb is re-dissolved by optimizing the annealing conditions for the steel sheet before the final cold rolling, and proceeds during soaking. It is possible to suppress the disappearance of dislocations and the progress of recrystallization, so that the strength of the steel sheet can be increased.

  In the present invention, it is possible to stably manufacture a non-oriented electrical steel sheet having both excellent mechanical characteristics and magnetic characteristics required as a rotor of a motor that rotates at high speed without causing a significant increase in cost. . 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 in the rotor referred to in the present invention are mechanical characteristics, and 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 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.

  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 consists 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 present inventors diligently studied non-oriented electrical steel sheets that satisfy these characteristics. First, based on the above-mentioned idea, various studies were made on the steel structure that the non-oriented electrical steel sheet having both magnetic characteristics and mechanical characteristics suitable for the 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. Further, as a result of studying work hardening as a strengthening mechanism, it was found that dislocations remaining in a recovered state 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 are obtained by allowing dislocations introduced during processing to a predetermined plate thickness to remain without disappearing during 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 processed structure and a recovered structure, it is necessary to suppress recrystallization in a soaking process that is usually performed after cold rolling. In order to suppress recrystallization by soaking, as described in Japanese Patent Application No. 2004-183554 and Japanese Patent Application No. 2004-252395, Nb, Zr, Ti in an amount exceeding the contents of C and N, respectively. Although it is necessary to contain V and V, the alloy cost increases. For the purpose of reducing the alloy cost, the present inventors proceeded with the premise that the contents of Nb, Zr, Ti, and V are not more than the contents of C and N. As a result, if Nb having the greatest recrystallization inhibiting effect is positively contained among Nb, Zr, Ti, and V and N is fixed by Zr, Ti, and Al, even if the above preconditions are satisfied. It was found that molten Nb can suppress the disappearance of dislocations and the progress of recrystallization during soaking performed after the final cold rolling.
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: 2.5% or less, P: 0.30% or less, S: 0.04% or less, N: 0.02% or less, Nb: more than 0.02%, Nb, Ti And at least one element selected from the group consisting of Zr and V within a range satisfying the following formula (1), and at least one element selected from the group consisting of Zr, Ti and Al It contains in the range which satisfies following formula (2), the remainder consists essentially of Fe and an impurity, and the area ratio of a recrystallized part is less than 90%, It is characterized by the above-mentioned.
Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14) ≦ 0 (1)
Zr / 91 + Ti / 48 + Al / 27-N / 14> 0 (2)
(Here, in the formulas (1) and (2), Nb, Zr, Ti, V, Al, C and N indicate the content (mass%) of each element.)

“%” Indicating the content of each element means “mass%” unless otherwise specified. Further, in the present invention, “the balance is substantially composed of Fe and impurities” means that it includes a case where other elements are contained within a range that does not impair the effects of the present invention.
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 precipitates, solid solution Nb, Zr, Ti having an effect of suppressing the disappearance of dislocations and the progress of recrystallization that proceed in the soaking process after cold rolling. And lead to a decrease in the V content. Therefore, 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 high, if the precipitate is re-dissolved by annealing under predetermined conditions before the final cold rolling. In view of obtaining the effect of suppressing the disappearance and recrystallization of dislocations during soaking after cold rolling, 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, the Nb content necessary to satisfy the condition of> 0 for Nb / 93-C / 12 is reduced, which 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. 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. From the viewpoint of increasing the strength of the steel sheet by solid solution strengthening, the desirable lower limit is 1.0%.

(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. Al also has an effect of fixing N. 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. From the viewpoint of increasing the strength of the steel sheet by solid solution strengthening, the desirable lower limit is 0.2%.

(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.

(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 precipitates, solid solution Nb, Zr, Ti having an effect of suppressing the disappearance of dislocations and the progress of recrystallization that proceed in the soaking process after cold rolling. And lead to a decrease in the V content. However, even if the N content is large, if the Zr, Ti, and Al contents are increased accordingly, N can be fixed as precipitates, and the effect of suppressing the disappearance of dislocations and the progress of recrystallization is extremely high. In view of securing a large content of solid solution Nb, the upper limit of the N content is 0.02%. Preferably it is 0.01% or less. On the other hand, if the N content is too low, the Nb content necessary for satisfying the relationship of the above formula (1) is drastically reduced, so that the effect of suppressing the disappearance and recrystallization of dislocations during soaking is not obtained. There is a fear. Therefore, the N content is preferably more than 0.0015%. More preferably, it is over 0.0026%.

(8) Nb, Zr, Ti and V
In order to suppress dislocation annihilation and recrystallization during soaking, and to obtain a processed structure and a recovered structure, it is necessary to contain Nb, Zr, Ti, or V in a solid solution state in which precipitates are not formed. However, if these elements are contained in a large amount, the alloy cost increases. In order to reduce the cost, it is necessary to contain at least one element selected from the group consisting of Nb, Zr, 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. The negative value means that Nb, Zr in a solid solution state , Ti or V is considered not to be contained. However, even if the value on the left side of the above formula (1) is negative, if the contents of Zr, Ti, and Al that are likely to bond with N are appropriate amounts as will be described later, N is a value of these elements. The content of solid solution Nb, which is fixed as nitride or the like and has a large effect of suppressing the disappearance of dislocations and the progress of recrystallization, is ensured. Therefore, in the present invention, Nb is positively contained, and the Nb content exceeds 0.02%. Preferably it is 0.04% or more. The upper limit of the Nb content is determined from the viewpoint of cost, and is the range represented by the above formula (1).

  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 on the recrystallization suppressing effect 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.

For the purpose of reducing the cost, when Nb, Zr, Ti, and V are contained within a range that satisfies the above formula (1), Zr, It is necessary to fix N as a precipitate by Ti and Al. For that purpose, it is necessary to contain at least one element selected from the group consisting of Zr, Ti and Al in a range satisfying the following formula (2).
Zr / 91 + Ti / 48 + Al / 27-N / 14> 0 (2)
(Here, in the formula (2), Zr, Ti, Al and N indicate the content (mass%) of each element.)

  The left side of the above formula (2) shows the difference between the contents of Zr, Ti and Al and the N content, and this value being positive corresponds to fixing N as a precipitate. . Thereby, the content of solid solution Nb having a large effect of suppressing the disappearance of dislocations and the progress of recrystallization is ensured. By controlling the content of Zr, Ti and Al in the range of the above formula (2), the effect of suppressing the disappearance and recrystallization of dislocations during soaking even if the value of the left side of the above formula (1) is negative Is presumably because Zr, Ti, and Al form precipitates with N more easily than Nb.

  For Zr and Ti, the upper limit values of the Zr content and the Ti content are determined by the above formula (1), and since the upper limit value of the Al content is determined for Al as described above, the above formula ( In 2), there is no particular upper limit.

Furthermore, in the present invention, the steel composition preferably satisfies the following formula (3).
Nb / 93−C / 12> 0 (3)
(Here, in the formula (3), Nb and C indicate the content (% by mass) of each element.)

  The left side of the above formula (3) is the difference between the Nb content and the C content, and this value is positive when N is fixed by optimizing the contents of Zr, Ti and Al described above. This corresponds to the solid solution Nb content. By setting the Nb content in the range of the above formula (3), it is possible to effectively suppress the disappearance of dislocations and the progress of recrystallization during the soaking process after cold rolling with the solid solution Nb.

  Even in the case of Nb / 93−C / 12 ≦ 0, in the present invention, N is fixed by optimizing the contents of Zr, Ti and Al, so that “B. As described in the second aspect of “Method for producing non-oriented electrical steel sheet”, if the annealing conditions before the final cold rolling are optimized, the dislocation disappearance and the progress of recrystallization during soaking are suppressed. Can do.

(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) Other components In this invention, it is possible to contain elements other than the element mentioned above in the range which does not impair the effect of this invention. Unlike the prior art based on the premise of a recrystallized structure, the present invention increases strength by using a processed structure and a recovered structure in which many dislocations remain. Inclusion of limited elements can be tolerated to higher levels. 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 together with the experimental results.
In mass%, C: 0.002%, Si: 2.8%, Mn: 0.2%, Al: 1.2%, S: 0.006%, N: 0.009%, P: 0.00. 01%, Nb: 0.07% steel was hot-rolled to 2.3 mm, hot-rolled sheet annealed at 800 ° C. for 10 hours, further cold-rolled to 0.35 mm, 680-800 A soaking treatment was performed by holding at various temperatures of 1050 ° C. for 10 seconds. The tensile strength of the steel sheet thus obtained was measured.
FIG. 1 shows the relationship between the area ratio of the recrystallized portion, the yield point, and the tensile strength. 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 portion is preferably as low as possible, and it is preferable that the area ratio of the recrystallized portion is zero and the state is completely unrecrystallized (worked structure and recovery 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 disclosed in Japanese Patent Application Nos. 2004-183554 and 2004-252395. It is easier than the developed technology and leads to improved productivity.

  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 manufacturing method of the non-oriented electrical steel sheet for rotors of this invention can be divided into two aspects with steel composition. Hereinafter, each aspect will be described.

1. 1st aspect The manufacturing method of the non-oriented electrical steel sheet for rotors of this aspect is hot-rolled to the steel ingot or steel piece which is provided with the steel composition mentioned above, and the said steel composition satisfies following formula (3). A hot rolling process for applying a cold rolling process, a cold rolling process for subjecting the hot rolled steel sheet obtained by the hot rolling process to one or more cold rollings sandwiching intermediate annealing, and the cold rolling process. And a soaking process for soaking the obtained cold-rolled steel sheet at 820 ° C. or less.
Nb / 93−C / 12> 0 (3)
(Here, in the formula (3), Nb and C indicate the content (% by mass) of each element.)

According to this aspect, by using a steel ingot or steel slab having a steel composition that satisfies Nb / 93-C / 12> 0, the recrystallization is suppressed by setting the temperature in the soaking process to a predetermined range. In addition, the recovery structure in which a large amount of dislocations remain by suppressing the disappearance of dislocations introduced during the processing to a predetermined plate thickness can be mainly used, and thereby the strength of the steel plate can be increased. . 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, 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. For example, a non-oriented electrical steel sheet for a rotor that satisfies the magnetic and mechanical characteristics required for a rotor of a drive motor can be stably manufactured without using expensive steel components or special processes. be able to.
Hereinafter, each process in the manufacturing method of the non-oriented electrical steel sheet for rotors of this aspect is demonstrated.

(1) Hot rolling process The hot rolling process in this aspect is also referred to as a steel ingot or steel slab (hereinafter referred to as “slab”) having the above-described steel composition and the steel composition satisfying the above formula (3). ) Is hot-rolled.
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 to 1300 ° C. from the viewpoint of cost and hot rolling properties. More preferably, it is 1050-1250 degreeC.
Moreover, various conditions of hot rolling are not specifically limited, For example, what is necessary is just to perform according to general conditions, such as finishing temperature 700-950 degreeC and coiling temperature 750 degrees 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.

  In the present invention, the dislocations introduced before the soaking process are suppressed and disappeared in the soaking process so as to achieve high strength, so when the amount of introduced dislocations is small. Cannot secure sufficient strength. 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. That is, it is preferable that the cold-rolled steel sheet obtained by this process has a strength of 850 MPa or more.

The reason for this will be described based on experimental results.
In mass%, C: 0.002%, Si: 2.9%, Mn: 0.2%, Al: 1.1%, S: 0.001%, N: 0.01%, P: 0.00. Steel having a composition of 01%, Nb: 0.001% (0.001% Nb steel), C: 0.002%, Si: 2.8%, Mn: 0.2%, Al: 1.2 %, S: 0.006%, N: 0.01%, P: 0.01%, and Nb: 0.08% steel (0.08% Nb steel) with hot rolling. After 2.0 mm, hot rolled sheet annealing is performed at 750 ° C. for 10 hours, finished to various sheet thicknesses of 0.35 to 1.2 mm by one cold rolling, and held at 700 ° C. for 20 seconds. Soaking was performed. For some hot-rolled plates, after hot-rolled sheet annealing, the intermediate sheet thickness is 0.4 to 1.8 mm, and the intermediate annealing conditions are maintained at 750 ° C. for 10 hours until cold rolling is performed to 0.35 mm Finishing was similarly performed soaking at 700 ° C. for 20 seconds. These steel sheets were subjected to a tensile test with the rolling direction as the longitudinal direction before and after soaking.

  FIG. 2 shows the tensile strength before and after the soaking process, and FIG. 3 shows the relationship between the tensile strength before the soaking process and the yield point after the soaking process. From FIG. 2, only 0.08% Nb steel, regardless of the number of cold rolling, the tensile strength before the soaking process, that is, the tensile strength as it is in the cold rolling process is increased, and the tensile after the soaking process is increased. It can be seen that the strength also increases. From FIG. 3, as in the above case, only 0.08% Nb steel has the tensile strength before the soaking step, that is, the tensile strength as it is in cold rolling, regardless of the number of cold rollings. It can be seen that the yield point after the soaking process increases as the value increases.

  Here, the cold-rolled tensile strength is an index of the total amount of dislocations introduced before cold rolling and dislocations introduced by cold rolling, that is, introduced before the soaking process. It is an indicator of the amount of dislocations that have been made. The present invention is intended to increase the strength of a steel sheet by suppressing the dislocations introduced before the soaking process from disappearing during the soaking process. Therefore, in order to leave dislocations sufficiently after soaking, it is necessary to introduce a large amount of dislocations before the soaking step, and it is important to introduce a large amount of dislocations during cold rolling.

  Since 0.001% Nb steel cannot suppress the disappearance of dislocations during soaking, a large amount of dislocations are introduced before the soaking process, that is, the tensile strength before soaking process is increased. However, the amount of dislocations remaining after soaking is reduced, and sufficient strength cannot be ensured after soaking. On the other hand, in 0.08% Nb steel, the disappearance of dislocations during soaking is suppressed, so if there is a large amount of dislocations introduced before the soaking process, that is, the tension before the soaking process. If the strength is large, the amount of dislocations remaining after the soaking is increased, and the strength can be stably secured after the soaking.

The 0.08% Nb steel satisfies the following formulas (2) and (3), and is a steel containing solute Nb.
Zr / 91 + Ti / 48 + Al / 27-N / 14> 0 (2)
Nb / 93−C / 12> 0 (3)
(Here, in the formulas (2) and (3), Nb, Zr, Ti, Al, C and N indicate the content (mass%) of each element.)
Therefore, in steel containing solute Nb, the tensile strength before soaking is used as a measure of the amount of dislocations to be introduced in order to ensure the strength such as tensile strength and yield point of the steel plate after soaking. It turns out that it can be adopted. Based on these findings, steel sheets that have the effect of suppressing the disappearance and recrystallization of dislocations should adopt the tensile strength before the soaking process as an index of the tensile strength and yield point after the soaking process. Turned out to be possible. And, it became clear that it was important to secure a tensile strength of 850 MPa or more in the previous stage of the soaking process as a necessary condition for sufficiently securing the strength such as tensile strength and yield point after soaking process. is there.
Here, the tensile strength in the previous stage of the soaking process can be measured with a tensile specimen taken with the rolling direction as the longitudinal direction.

  Since the effects of the present invention can be obtained if sufficient dislocations are introduced, various conditions for cold rolling, such as the temperature of the steel sheet during rolling, the rolling reduction, and the diameter of the rolling roll, are not particularly limited. The material is appropriately selected depending on the steel composition of the material, the thickness of the target steel plate, 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 if the above-described tensile strength is satisfied after the light processing.

In this aspect, the steel sheet before the final cold rolling may be subjected to continuous annealing at a temperature of 850 ° C. to 1200 ° C. for 10 seconds to 5 minutes. Thereby, the strength of the steel sheet can be further improved.
In addition, since it describes in the below-mentioned 2nd aspect about the detail with respect to the steel plate before the last cold rolling, description here is abbreviate | omitted.

(3) 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 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 ordinary non-oriented electrical steel sheets, 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 a soaking process 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 positively contained. Therefore, the effect of suppressing recrystallization is great. Therefore, even if the soaking temperature in the soaking process is high, a processed structure and a recovered structure can be obtained, and productivity can be improved because there is no need to provide a special soaking temperature opportunity. 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. The lower the soaking temperature is, the more the recrystallization progress is suppressed. However, when the soaking temperature is low, the flatness of the steel sheet is not corrected, and the space factor when laminated on the rotor may decrease. Moreover, since it has the effect of improving the iron loss as compared with the state of cold rolling by soaking, it leads to an increase in iron loss when the soaking temperature is low. Furthermore, when the soaking temperature is 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 any method of box annealing and continuous annealing, but it is desirable to carry out in a continuous annealing line from the viewpoint of productivity. In the box annealing, the flatness of the steel sheet may be lowered or the shape may be deteriorated due to coil winding (also referred to as a coil set) due to being subjected to annealing in a coil state. There is a case where a straightening process for correcting the flatness and shape of the steel sheet is necessary after the process.
In addition, when recrystallization progresses by the soaking process at high temperature and the mechanical characteristics are lowered due to the progress, the increase in the process is unavoidable, but it may be processed after the soaking process to ensure the strength.

(4) 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 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. Moreover, the effect of high strength may arise by the optimization of hot-rolled sheet annealing conditions.
In this aspect, the 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) Other steps In this embodiment, after the soaking step, according to a general method, a coating step is performed in which an insulating film composed of only an organic component, only an inorganic component, or an organic-inorganic composite is applied to the steel sheet surface. It is 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.

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

2. Second Aspect Next, a second aspect of the method for producing a non-oriented electrical steel sheet for rotors of the present invention will be described.
The manufacturing method of the non-oriented electrical steel sheet for rotors of this aspect is hot which comprises the steel composition mentioned above, and hot-rolls the steel ingot or steel slab in which the said steel composition satisfies following formula (4). The hot-rolled steel sheet obtained by the rolling process and the hot-rolling process is subjected to cold rolling at least once with one or two intermediate sandwiches, and the steel sheet before the final cold rolling is 850 ° C. or higher and 1200 ° C. A cold rolling process in which continuous annealing is performed at a temperature of ℃ or less for 10 seconds or more and 5 minutes or less, and a soaking process of soaking the cold-rolled steel sheet obtained by the cold rolling process at 820 ° C or less. The manufacturing method of the non-oriented electrical steel sheet for rotors characterized by these is provided.
Nb / 93−C / 12 ≦ 0 (4)
(Here, in the formula (4), Nb and C indicate the content (mass%) of each element.)

Hereinafter, the knowledge that led to the present invention will be described.
The main components in mass% are C: 0.002%, Si: 2.0%, Mn: 0.2%, Al: 0.3%, N: 0.01%, P: 0.01%, S: It is 0.002%, Nb content is changed to 0.001 to 0.15%, and the main component is mass% C: 0.02%, Si: 2.0%, Mn: 0.00. Steel with 2%, Al: 0.3%, N: 0.01%, P: 0.01%, S: 0.002%, and Nb content varied from 0.001 to 0.15% Were subjected to hot rolling to have a sheet thickness of 2.3 mm and subjected to hot-rolled sheet annealing of box annealing held at 800 ° C. for 10 hours or continuous annealing held at 1050 ° C. for 3 minutes. Then, it cold-rolled to plate | board thickness 0.35mm, and performed the soaking | uniform-heating treatment which hold | maintains at 700 degreeC for 0 to 160 second for various time. And the tensile strength of the steel plate after soaking was measured. All the steels satisfy the following formula (2).
Zr / 91 + Ti / 48 + Al / 27-N / 14> 0 (2)
(Here, in the formula (2), Zr, Ti, Al, and N indicate the content (mass%) of each element.)

  FIG. 4 shows a steel sheet having a C content of 0.002%, and a box annealing in which hot-rolled sheet annealing is held at 800 ° C. for 10 hours. FIG. 5 shows a steel sheet having a C content of 0.002%. In the case where the plate annealing is a continuous annealing that is held at 1050 ° C. for 3 minutes, in FIG. FIG. 7 shows a steel sheet having a C content of 0.02%, and a continuous annealing in which hot-rolled sheet annealing is held at 1050 ° C. for 3 minutes, and the soaking time and the tensile strength of the steel sheet after soaking. The relationship with each is shown.

4 and 5, it was found that a steel sheet having a C content of 0.002% can obtain a sufficient tensile strength after soaking by positively containing Nb. For steel sheets with a relatively high Nb content, hot-rolled sheet annealing is performed by box annealing that is held at 800 ° C. for 10 hours when hot-rolled sheet annealing is performed at 1050 ° C. for 3 minutes. It was found that the tensile strength was increased as compared with the case of the above. As a result of investigating the steel structure, the steel sheet positively containing Nb was not recrystallized even if held for 160 seconds during soaking regardless of the hot-rolled sheet annealing method.
Moreover, from FIG. 6, when hot-rolled sheet annealing by box annealing which hold | maintains for 10 hours at 800 degreeC is implemented to the steel plate whose C content is 0.02%, even if it contains Nb actively, soaking | uniform-heating treatment is carried out. It was found that the tensile strength suddenly decreased as the holding time increased. As a result of investigating the steel structure, all the steel plates were recrystallized all over when the soaking time was 40 seconds.
Furthermore, from FIG. 7, when hot-rolled sheet annealing by continuous annealing which hold | maintains at 1050 degreeC for 3 minutes to the steel plate whose C content is 0.02% was implemented, by making Nb contain actively, soaking | uniform-heating treatment is carried out. It was found that sufficient tensile strength can be obtained when the holding time is short. As a result of investigating the steel structure, the steel sheet positively containing Nb was not recrystallized until the holding time of soaking was 40 seconds.

Next, the relationship between the following formula (5) defined by the contents of Nb and C and the tensile strength of the steel sheet was examined.
x = Nb / 93−C / 12 (5)
(Here, in the formula (5), Nb and C indicate the content (mass%) of each element.)

For steel sheets subjected to hot-rolled sheet annealing by box annealing held at 800 ° C. for 10 hours, sufficient tensile strength can be obtained only from Nb / 93-C / 12> 0 from FIG. 4 and FIG. I understood. As a result of investigating the steel structure, recrystallization was suppressed only when Nb / 93-C / 12> 0, and the steel structure was a processed structure and a recovered structure. That is, when hot-rolled sheet annealing by box annealing held at 800 ° C. for 10 hours is performed, dislocation disappearance and recrystallization are suppressed only when Nb / 93−C / 12> 0, and sufficient strength is obtained. Can be secured.
On the other hand, when hot-rolled sheet annealing is performed by continuous annealing held at 1050 ° C. for 3 minutes, a certain amount of Nb is obtained even if Nb / 93−C / 12 <0 from FIGS. It has been found that sufficient strength can be secured if it is contained and the soaking time is relatively short. Further, it was found that when Nb / 93-C / 12> 0, it is possible to further increase the strength.

Although it is not clear about these reasons, the present inventors presume as follows.
That is, even if Nb / 93-C / 12 <0, sufficient strength can be secured because the Nb-based precipitates are re-dissolved substantially by performing hot-rolled sheet annealing at a relatively high temperature. It is presumed that this is because dislocation disappearance and recrystallization suppression effect due to solute Nb was obtained. In addition, when Nb / 93-C / 12> 0, the strength is further improved, as described above, by performing hot-rolled sheet annealing at a relatively high temperature, the Nb-based precipitates are re-dissolved. This is presumably because the amount of solute Nb increased. Further, since solid solution C and solid solution Nb generated by re-dissolution of precipitates have an interaction, dislocation and solid solution C, and dislocation and solid solution Nb also have an interaction. The effect of suppressing the disappearance of dislocations due to the optimization of the annealing conditions and the Nb content is higher than the effect of suppressing the disappearance of dislocations due to the optimization of only the Nb content, and this is presumed to have contributed to the further increase in strength. Is done.

  Hereinafter, the hot rolling process and the cold rolling process in the manufacturing method of the non-oriented electrical steel sheet for rotors of this aspect will be described. Note that the soaking process and other processes are the same as in the first embodiment, and a description thereof is omitted here.

(1) Hot rolling process The hot rolling process in this aspect is a process which hot-rolls to the steel ingot or steel piece which is provided with the steel composition mentioned above and whose steel composition satisfies the said Formula (4). .
In addition, about the steel composition of a steel ingot or a steel slab, 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, About hot rolling conditions, it is the said 1st aspect. The description here is omitted.

(2) Cold rolling step The cold rolling step in the present invention is performed by subjecting the hot rolled steel sheet obtained by the hot rolling step to cold rolling twice or more with one or intermediate annealing, and the final step. In this step, the steel sheet before cold rolling is subjected to continuous annealing at a temperature of 850 ° C. to 1200 ° C. for 10 seconds to 5 minutes. 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.

Here, “the steel plate before the final cold rolling” means that the hot rolled steel plate is used when the product is subjected to cold rolling only once without intermediate annealing to finish the product sheet thickness. This means that when the hot-rolled steel sheet is subjected to two or more cold rolling sandwiching intermediate annealing to finish the product sheet thickness, it means the cold-rolled steel sheet before the final cold rolling.
In addition, there are cases where the mechanical properties and thickness of the product are adjusted by performing cold working after the above-mentioned soaking process, but the cold working is not cold rolling but the number of cold rolling described above. Does not count.

  In addition, “the steel sheet before the final cold rolling is subjected to continuous annealing at a temperature of 850 ° C. or more and 1200 ° C. or less for 10 seconds or more and 5 minutes or less” is performed only once without performing intermediate annealing. In this case, it means that hot-rolled sheet steel is subjected to hot-rolled sheet annealing under predetermined conditions, and when performing cold rolling more than once with intermediate annealing, before the final cold-rolling. Means that the cold-rolled steel sheet is subjected to intermediate annealing under predetermined conditions. That is, in this embodiment, when cold rolling is performed only once without performing intermediate annealing, the hot-rolled sheet annealing conditions are optimized, and cold rolling is performed twice or more with intermediate annealing. In such a case, dislocation disappearance and recrystallization during soaking can be effectively suppressed by optimizing the conditions of the intermediate annealing before the final cold rolling.

  In this embodiment, it is important to subject the steel sheet before the final cold rolling to continuous annealing at a temperature of 850 ° C. to 1200 ° C. for 10 seconds to 5 minutes. That is, when cold rolling is performed only once without intermediate annealing, it is performed by hot-rolled sheet annealing, or when cold rolling is performed twice or more with intermediate annealing, the final cold In the intermediate annealing before rolling, continuous annealing is performed at a temperature of 850 ° C. or higher and 1200 ° C. or lower for 10 seconds to 5 minutes. The precipitate is re-dissolved by this annealing, and dislocation disappearance and recrystallization during soaking are suppressed by the solid solution Nb. If the annealing temperature is less than the above range, the time required to dissolve the precipitates is prolonged, resulting in poor productivity, and if the annealing temperature exceeds the above range, the equipment is overloaded and the steel plate is overloaded. A scale is formed on the surface layer, and the surface properties of the product deteriorate due to poor pickling. Further, if the holding time is less than the above range, the solid solution of the precipitate becomes insufficient, so the effect of the present invention cannot be obtained, and if the holding time exceeds the above range, a scale is formed on the surface of the steel sheet, resulting in poor pickling. As a result, the surface properties of the product deteriorate. In order to improve the effect of suppressing the disappearance of dislocations and recrystallization, it is preferable that the annealing temperature is 900 ° C. or higher and the holding time is 30 seconds or longer. A more preferable holding time is 60 seconds or more.

  The reason why the annealing performed before the final cold rolling is continuous annealing is that the cooling rate after annealing is larger in the continuous annealing than in the case of box annealing. This is because it can be suppressed. The effect of this invention can be acquired if the cooling rate in the continuous annealing line used for the hot-rolled sheet annealing of a non-oriented electrical steel sheet is generally employ | adopted. A preferable lower limit of the cooling rate is 10 ° C./s from the viewpoint of suppressing reprecipitation during cooling and from the viewpoint of suppressing a long cooling time. From the viewpoint of suppressing reprecipitation, the larger the cooling rate, the better. Therefore, the upper limit of the cooling rate is not particularly limited, but 100 ° C./s or less is preferable from the viewpoint of suppressing an excessive load on the equipment. You may use the equipment which has a cooling rate larger than the above-mentioned value, such as a water cooling equipment and a roll cooling equipment.

  Moreover, when performing cold rolling only once without performing intermediate annealing, it is necessary to subject hot-rolled steel sheet to hot-rolled sheet annealing under predetermined conditions, but hot-rolled sheet annealing is an essential process, When performing cold rolling twice or more with intermediate annealing, hot-rolled sheet annealing may or may not be performed before cold rolling. When hot-rolled sheet annealing is performed, there is an advantage that the ductility of the steel sheet is improved and breakage in cold rolling can be suppressed.

  When performing cold rolling at least twice with intermediate annealing and performing hot-rolled sheet annealing before cold rolling, the conditions for intermediate annealing before final cold rolling may be optimized. Therefore, various conditions of hot-rolled sheet annealing such as annealing temperature, holding time, method (continuous annealing / box annealing) at the time of hot-rolled sheet annealing are not particularly limited, depending on the steel composition of the hot-rolled steel sheet, etc. Shall be selected. In addition, when performing intermediate annealing twice or more, it is only necessary to optimize the conditions of intermediate annealing before the final cold rolling as described above, so intermediate annealing other than the intermediate annealing before the final cold rolling is performed. As for, various conditions of intermediate annealing such as annealing temperature, holding time, method (continuous annealing / box annealing) during intermediate annealing are not particularly limited, and are appropriately selected depending on the steel composition of the material to be rolled, etc. To do.

In this aspect, similarly to the first aspect, the cold-rolled steel sheet obtained by this step preferably has a strength of 850 MPa or more.
The reason for this will be described. In the experiment described in the first aspect, the same examination was performed by changing the C content and the like. As a result, even when Nb / 93-C / 12 ≦ 0 steel is also actively contained, by appropriately optimizing the annealing conditions before the final cold rolling, substantially solid solution Nb is obtained. As an indication of the amount of dislocations that should be introduced to ensure the strength such as tensile strength and yield point of the steel sheet after soaking, It has been found that tensile strength can be employed. Based on these findings, steel sheets that have the effect of suppressing the disappearance and recrystallization of dislocations should adopt the tensile strength before the soaking process as an index of the tensile strength and yield point after the soaking process. Turned out to be possible. And it became clear that it was important to ensure the tensile strength of 850 MPa or more in the previous stage of the soaking process as a necessary condition for sufficiently securing the strength such as the tensile strength and the yield point after the soaking process.

  Since the effects of the present invention can be obtained if sufficient dislocations are introduced, various conditions for cold rolling, such as the temperature of the steel sheet during rolling, the rolling reduction, and the diameter of the rolling roll, are not particularly limited. The material is appropriately selected depending on the steel composition of the material, the thickness of the target steel plate, 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 if the above-described tensile strength is satisfied after the light processing.

  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 for various times at various temperatures, The plate thickness was finished to 0.35 mm by rolling. For some hot-rolled steel sheets, after the above-mentioned 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 various Intermediate annealing was performed by continuous annealing that was held at various temperatures for various times, 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, Examples 1 to 9 and 11 to 26 were subjected to soaking treatment by continuous annealing held at various temperatures for 30 seconds. Example 10 was subjected to soaking treatment by box annealing held at 500 ° C. for 10 hours. In this way, a steel plate was produced.

[Comparative Examples 1-8]
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 8, the mechanical properties of the steel plates in the stage before soaking, the area ratio of recrystallized portions after soaking, mechanical properties, and magnetic 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 conducting a tensile test using a JIS No. 5 test piece with the rolling direction as the longitudinal direction. 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 magnetic characteristics, a 55 mm square single plate test piece has a maximum magnetic flux density of 1.0 T, an excitation frequency of 400 Hz, an iron loss W _10/400, and a magnetic flux density B ₅₀ of 5000 A / m. It was measured. The measurement was carried out in the rolling direction and the direction perpendicular to the rolling, and the average value thereof was adopted.

  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 8, 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. 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. In the steel plate of Comparative Example 5, since the Nb content was outside the range of the present invention, recrystallization was not suppressed, the area ratio of the recrystallized portion was high, and both the yield point and tensile strength were inferior. The steel sheet of Comparative Example 6 was inferior in yield point and tensile strength because N content could not be fixed because the contents of Zr, Ti and Al were outside the range of the present invention, and recrystallization was not suppressed. The steel plate of Comparative Example 7 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 8 was inferior in yield point and tensile strength because of the high area ratio of the recrystallized portion.
On the other hand, in the steel plates of Examples 1 to 26 that satisfy the requirements defined in the present invention, even if the contents of Nb, Zr, Ti and V are less than the contents of C and N, the method of hot-rolled sheet annealing Regardless of the number of cold rolling operations, both magnetic properties and mechanical properties were excellent. Further, it was found that even under conditions where the soaking temperature is relatively high, the recrystallization suppressing effect is large, and thus the magnetic properties and mechanical properties are excellent.

By comparing Examples 2 and 3, the effect of suppressing recrystallization by increasing the annealing (here, hot-rolled sheet annealing) on the steel sheet before the final cold rolling is increased, and the same soaking temperature. However, it was found that the yield point and tensile strength were improved.
Further, by comparing Examples 4, 5, 6 and 7, the effect of suppressing recrystallization by increasing the annealing (here intermediate annealing) to the steel sheet before the final cold rolling is increased, and the leveling is uniform. It was found that the same yield point and tensile strength can be obtained even when the heat temperature is increased.
Further, according to Examples 12 and 26, even when the steel is Nb / 93-C / 12 <0, annealing to the steel sheet before the final cold rolling (in Example 12, hot rolled sheet annealing, in Example 26) It was found that excellent magnetic properties and mechanical properties can be obtained by setting the intermediate annealing) to continuous annealing.

By comparing Examples 13 and 14, it was found that the mechanical properties did not change even when the S content changed.
Further, as shown in Examples 17 to 25, when Cu, Ni, Cr, Mo, Co, W, Sn, Sb, Se, Bi, Ge, Te, B, Ca, Mg, and REM are contained in appropriate amounts It was also found that the effects of the present invention can be obtained. Furthermore, the content of Ta, Hf, As, Au, Be, Zn, Pb, Tc, Re, Ru, Os, Rh, Ir, Pd, Pt, Ag, Cd, Hg, and Po is appropriate from Example 26. In some cases, it has been found that the effects of the present invention can be obtained.

It is a figure which shows the relationship between the area ratio of a recrystallized part, a yield point, and tensile strength. It is a figure which shows the relationship of the tensile strength before and behind a soaking process. It is a figure which shows the relationship between the tensile strength before a soaking process, and the yield point after a soaking process. The relationship between the holding time of soaking and the tensile strength of the steel plate after soaking is shown in the case of a steel plate having a C content of 0.002% and box annealing in which hot rolling annealing is held at 800 ° C. for 10 hours. FIG. The relationship between the holding time of soaking and the tensile strength of the steel plate after soaking is shown in the case of a steel plate having a C content of 0.002% and continuous annealing in which hot rolling annealing is held at 1050 ° C. for 3 minutes. FIG. The relationship between the holding time of soaking and the tensile strength of the steel plate after soaking is shown in the case of a steel plate having a C content of 0.02% and box annealing in which hot rolling annealing is held at 800 ° C. for 10 hours. FIG. The relationship between the holding time of soaking and the tensile strength of the steel plate after soaking is shown in the case of a steel sheet with a C content of 0.02% and hot-rolled sheet annealing that is kept at 1050 ° C. for 3 minutes. FIG.

Claims (8)

In mass%, C: 0.06% or less, Si: 3.5% or less, Mn: 0.05% or more and 3.0% or less, Al: 2.5% or less, P: 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 is represented by the following formula (1 ) And at least one element selected from the group consisting of Zr, Ti and Al in a range satisfying the following formula (2), the balance being substantially Fe and impurities A non-oriented electrical steel sheet for a rotor, wherein the area ratio of the recrystallized portion is less than 90%.
Nb / 93 + Zr / 91 + Ti / 48 + V / 51− (C / 12 + N / 14) ≦ 0 (1)
Zr / 91 + Ti / 48 + Al / 27-N / 14> 0 (2)
(Here, in the formulas (1) and (2), Nb, Zr, Ti, V, Al, 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 The non-oriented electrical steel sheet for rotors according to any one of claims 1 to 4, wherein the steel composition satisfies the following formula (3).
Nb / 93−C / 12> 0 (3)
(Here, in the formula (3), Nb and C indicate the content (% by mass) of each element.)   The hot rolling process which hot-rolls the steel ingot or steel slab provided with the steel composition of Claim 5, 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 820 ° 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 6, further comprising a hot-rolled sheet annealing step for subjecting the hot-rolled steel sheet to hot-rolled sheet annealing. A hot rolling comprising the steel composition according to any one of claims 1 to 4 and hot rolling the steel ingot or the steel slab, wherein the steel composition satisfies the following formula (4): The hot-rolled steel sheet obtained by the process and the hot-rolling process is subjected to cold rolling twice or more sandwiching one or intermediate annealing, and the steel sheet before the final cold rolling is 850 ° C. or higher and 1200 ° C. A cold rolling step of performing continuous annealing for 10 seconds to 5 minutes at the following temperature, and a soaking step of 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 for rotors, which is characterized.
Nb / 93−C / 12 ≦ 0 (4)
(Here, in the formula (4), Nb and C indicate the content (mass%) of each element.)

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