JP2007291519A - Electromagnetic bar steel and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic bar steel having sufficient magnetic properties and high yield strength as a rotor core material, and further having excellent machinability. <P>SOLUTION: The electromagnetic bar steel has a componential composition comprising, by mass, 0.04 to 0.12% C, ≤0.5% Si, 0.5 to 3.0% Mn, ≤0.1% Al, 0.03 to 0.35% Ti and 0.05 to 0.8% Mo, and the balance Fe with inevitable impurities, and has a structure of a ferrite single phase with the average crystal grain size of 30 to 80 μm; wherein the area ratio of the ferrite with a grain size of ≤10 μm is controlled to ≤20%, and also, fine precipitates with a grain size of <10 nm are dispersed into the ferrite. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electromagnetic steel bar having high yield strength, excellent magnetic properties, and excellent machinability, and a method for producing the same. The electromagnetic bar according to the present invention is also useful as a processing material used for cutting and forging.

As represented by main motors of electric vehicles and hybrid electric vehicles, in recent years, motors are strongly required to save energy and improve efficiency.
In order to save energy and increase efficiency, high frequency can be cited as one of the effective means, but as the frequency increases, the rotational speed of the motor also increases. When the number of rotations of the motor increases, the centrifugal force applied to the core constituting the rotor also increases, so that a high yield strength is required for the core material. That is, when the yield strength of the core material is insufficient, the core material undergoes plastic deformation due to centrifugal force, and the air gap between the rotor core and the stator core changes from the design value, and the motor performance deteriorates. Results in damage to the motor due to contact between the rotor and stator during rotation. For this reason, it is indispensable to increase the strength of the rotor core material in order to save energy and increase the efficiency of the motor by increasing the frequency.

  By the way, as a manufacturing method of a rotor core, until now, it was common to laminate electromagnetic steel sheets having a thickness of about 0.35 to 0.5 mm, but the electromagnetic steel sheets are punched one by one into a predetermined core shape. Since it takes a great deal of money to stack several hundred sheets, a motor that uses a magnetic bar steel that does not require lamination instead of a magnetic steel sheet has been put into practical use.

  However, the current electromagnetic bar steel is made of low carbon steel or silicon steel like the electromagnetic steel sheet, and the strength is not necessarily high because the main strengthening mechanism is the solid solution strengthening of ferrite. For example, even in the case of 3% Si steel, the yield strength is about 350 MPa, and although it is an example of a magnetic steel sheet, as disclosed in Patent Document 1, the yield strength is also intended for high strength. It is approximately 300 to 450 MPa, and sufficient yield strength is not obtained.

  Patent Document 2 discloses a hot-rolled steel sheet for a rotating core having both high magnetic flux density and high strength by dispersing and precipitating a carbide of less than 10 nm containing Ti, at least one of Mo and W in a ferrite structure. Is described.

In the hot-rolled steel sheet described in Patent Document 2, with respect to magnetic properties, the composition is defined, the structure is ferrite, fine precipitates are dispersed and precipitated in the ferrite, and the lengths of the long side and the short side of the carbide. However, no consideration is given to domain wall motion which is important in terms of magnetic characteristics, and it cannot be said that the magnetic characteristics are practically sufficient. In fact, in the example of Patent Document 2, the exciting current is extremely high as 30000 A / m, and only the value of the magnetic flux density B _{300 in} the vicinity of the saturation magnetic flux density determined only by the component (particularly Fe) is shown. However, for the nature of the motor or the like, as typified by the magnetic flux density B ₅₀ in the 5000A / m, it is important to the magnetic flux density at a lower magnetic field region, high B ₅₀ in the technique described in Patent Document 2 I can't get it.

Further, no consideration is given to the iron loss that governs the efficiency of the motor and the like, and since the iron loss value is high, it does not have practically sufficient magnetic properties. Furthermore, since no special consideration has been given to machinability among the properties required for steel bars, there remains a problem that the properties are not necessarily good in terms of tool life.
JP 2002-371340 A JP 2003-288509 A

  SUMMARY OF THE INVENTION An object of the present invention is to provide an electromagnetic steel bar having high yield strength and excellent machinability as well as sufficient magnetic properties as a rotor core material and a method for producing the same.

The inventors have found that when a fine precipitate having a particle size of less than 10 nm is dispersed and precipitated in a ferrite single-phase structure to enhance the precipitation of ferrite, the strength can be significantly increased. Conventionally, the precipitates were considered to be harmful to the magnetic properties, but it was newly found that when the precipitates are as fine as less than 10 nm, the magnetic properties are not adversely affected.
Furthermore, as a result of investigating the machinability of the above steel, in which ferrite was strengthened with very fine precipitates of less than 10 nm, the ferrite grain size was controlled within a predetermined range to achieve a uniform fine grain structure. It has also been found that an electromagnetic steel bar having both magnetic properties and machinability can be obtained.

The present invention has been made based on the above knowledge, and the gist of the present invention is as follows.
1. % By mass C: 0.04 to 0.12%,
Si: 0.5% or less,
Mn: 0.5-3.0%
Al: 0.1% or less,
Ti: 0.03-0.35% and
Mo: 0.05-0.8%
And the composition of the remaining Fe and inevitable impurities, the average crystal grain size is a ferrite single phase structure of 30μm to 80μm, the area ratio of ferrite with a particle size of 10μm or less is 20% or less An electromagnetic steel bar characterized by fine precipitates having a particle size of less than 10 nm dispersed in ferrite.

2. 2. The electromagnetic bar steel according to 1 above, wherein the component composition satisfies the following expression (1).
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (1)
However, the chemical component display indicates the content (% by mass) of the component.

3. 3. The electromagnetic steel bar according to 1 or 2 above, wherein the fine precipitate is a carbide of Ti and Mo.

4). The component composition is further mass%.
Nb: 0.08% or less,
The electromagnetic bar steel according to 1 above, comprising one or more of V: 0.15% or less and W: 1.5% or less.

5). The electromagnetic bar steel according to 4, wherein the component composition satisfies the following formula (2).
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)) ≦ 1.50… (2)
However, the chemical component display indicates the content (% by mass) of the component.

6). 6. The electromagnetic bar steel according to 4 or 5, wherein the fine precipitate is a carbide containing Ti and Mo and containing at least one of Nb, V and W.

7). The component composition further includes S: 0.01 to 0.1% by mass%, and
Pb: 0.2% or less,
Ca: 0.005% or less,
The electromagnetic bar steel according to any one of 1 to 6 above, containing one or more of Bi: 0.1% or less and B: 0.02% or less.

8). The electromagnetic bar steel according to any one of 1 to 7 above, wherein an area ratio of the ferrite having a particle size of 10 μm or less is 5% or less.

9. % By mass C: 0.04 to 0.12%,
Si: 0.5% or less,
Mn: 0.5-3.0%
Al: 0.1% or less,
Ti: 0.03-0.35% and
Mo: 0.05-0.8%
The steel material having the component composition of the remaining Fe and unavoidable impurities is heated to 1100 ° C. or higher, and then hot rolling is performed.The area reduction rate in the start pass is 25% or more, and the area reduction rate in the final pass is 15%. % To 35% and a finishing temperature of 880 ° C. or higher, and then cooling at a cooling rate of 1.0 ° C./s or lower.
Here, the area reduction rate (%) is
((Cross-sectional area before rolling in each pass− (cross-sectional area after rolling in each pass)) / (cross-sectional area before rolling in each pass) × 100
It can ask for.

10. 10. The electromagnetic bar steel according to 9, wherein the steel material satisfies the following expression (1).
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (1)
However, the chemical component display indicates the content (% by mass) of the component.

11. The steel material is further mass%.
Nb: 0.08% or less,
10. The electromagnetic bar steel as described in 9 above, comprising one or more of V: 0.15% or less and W: 1.5% or less.

12 The electromagnetic bar steel according to 11, wherein the steel material satisfies the following expression (2).
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50… (2)
However, a chemical component shows content (mass%) of the said component.

13. The steel material further includes S: 0.01 to 0.1% by mass%, and
Pb: 0.2% or less,
Ca: 0.005% or less,
The method for producing electromagnetic bar steel according to any one of 9 to 12 above, wherein one or more of Bi: 0.1% or less and B: 0.02% or less are contained.

14 Production of an electromagnetic steel bar characterized in that the hot rolling is annealed in the following temperature range T with respect to the steel bar produced by the method according to 9 to 13 with a reduction in area at the start pass of 30% or more. Method.
Record
When the Mn content is 1.7% or less: 600 ° C. ≦ T ≦ 800 ° C.
When Mn content exceeds 1.7%: 600 ℃ ≦ T ≦ 750 ℃

According to the present invention, an electromagnetic steel bar having sufficient magnetic properties and high yield strength is provided, so that the above-described problems can be avoided even if the rotational speed of the motor is increased. Therefore, the frequency in the motor can be further increased, and the energy saving and high efficiency of the motor can be realized. Therefore, the present invention is extremely useful industrially.
Furthermore, the machinability required for the steel bar can be given good machinability sufficient to realize the extension of the tool life.

The component composition, microstructure and production conditions of the present invention are described in detail below. In addition, unless otherwise indicated, the "%" display regarding a component composition means "mass%".
[Ingredient composition]
C: 0.04-0.12%
If C is less than 0.04%, the amount of fine precipitates is insufficient and high yield strength cannot be obtained, so C needs to be 0.04% or more. On the other hand, if the content of C exceeds 0.12%, the precipitates become coarse and high yield strength cannot be obtained, so the upper limit of C needs to be 0.12%.

Si: 0.5% or less
Since Si decreases the cold workability, the addition amount should be 0.5% or less. More preferably, it is 0.15% or less.

Mn: 0.5-3.0%
In the present invention, the precipitation behavior of the precipitate is closely related to the progress of the transformation from austenite to ferrite (hereinafter referred to as ferrite transformation), and the transformation start temperature of the ferrite transformation that occurs during cooling after rolling and the precipitate When the difference from the precipitation start temperature is small and the ferrite transformation and precipitation compete with each other, the precipitate is finely dispersed and precipitated in the ferrite. That is, Mn contributes to competing ferrite transformation and precipitation by lowering the ferrite transformation temperature and reducing the difference between the transformation initiation temperature of the ferrite transformation and the precipitation initiation temperature of the precipitate. For that purpose, it is necessary to add 0.5% or more of Mn. On the other hand, when the amount of Mn exceeds 3.0%, a low-temperature transformation phase such as bainite is generated in addition to ferrite, and strengthening by fine precipitates is insufficient, and when a low-temperature transformation phase is generated, the magnetic flux density is also reduced. The upper limit of Mn is 3.0%. More preferably,

In addition, since especially high magnetic flux density _B50 is obtained when the amount of Mn is 1.7% or less, it is preferable to make it 1.7% or less in order to obtain high magnetic characteristics. More preferably, it is 0.6 to 1.65%. On the other hand, by adding Mn in an amount exceeding 1.7%, the effect of increasing the strength by solid solution strengthening of Mn becomes remarkable. Therefore, it is preferable to exceed 1.7% particularly when increasing the strength is desired. More preferably, it is 1.75 to 2.85%.

Al: 0.1% or more
Al may be added as a deoxidizing element. In this case, it is necessary to add at 0.01% or more. However, if it is added excessively, not only will the effect be saturated, but the amount of AlN that is a precipitate with N will increase, and AlN will not precipitate with a diameter of less than 10 nm, which will degrade the magnetic properties. . In order to avoid this, the amount of Al added is 0.1% or less. More preferably, it is 0.05% or less.

Ti: 0.03-0.35%
Ti precipitates finely including Ti-based carbides and Ti-Mo-based carbides to increase the hardness. In order to ensure a high yield strength, 0.03% or more is necessary. On the other hand, if added over 0.35%, the precipitates become coarse and the strength decreases, so Ti is made 0.03 to 0.35%. More preferably, it is 0.03 to 0.30%.

Mo: 0.05-0.8%
Mo is added to finely precipitate precipitates including Mo-based carbides and Ti-Mo-based carbides, and improve strength. Further, Mo has a slow diffusion rate, and when it precipitates together with Ti, it has an advantage that the growth rate of the precipitate is reduced and a fine precipitate is easily obtained. Here, in order to ensure high yield strength, it is necessary to add 0.05% or more of Mo. On the other hand, when adding over 0.8%, a low-temperature transformation phase such as bainite is generated in addition to ferrite. Precipitation strengthening due to fine precipitates is insufficient, resulting in a decrease in strength and a deterioration in magnetic properties. For this reason, Mo is made 0.05 to 0.8%. More preferably, it is 0.15-0.50%.

In the above component composition, particularly regarding the atomic ratio of the amounts of C, Ti, and Mo, satisfying the following formula (1) is advantageous for refinement of precipitates.
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (1)
This parameter affects the size of the precipitate, and when it is 0.50 or more and 1.50 or less, formation of fine precipitates having a particle size of less than 10 nm is facilitated, which is preferable.

  In addition, it was observed that fine Ti—Mo-based carbides have Ti / Mo in the carbide in an atomic ratio of Ti / Mo of 0.2 to 2.0, and finer carbides of 0.7 to 1.5.

Although the essential components have been described above, in the present invention, one or more of Nb, V and W can be added in order to further improve the strength and toughness.
Nb: 0.08% or less,
Nb contributes to the strength increase by forming fine precipitates together with Ti and Mo. Moreover, ductility and toughness are improved by adjusting the grain size of ferrite. In order to obtain these effects, 0.005% or more is preferably added. However, if the content exceeds 0.08%, the ferrite becomes finer and fine precipitates adversely affect the magnetic properties, so the addition amount should be 0.08% or less. More preferably, it is 0.04% or less.

V: 0.15% or less V also forms fine precipitates together with Ti and Mo and contributes to the increase in strength. Therefore, 0.005% or more is preferably added, but if the content exceeds 0.15%, the precipitates become coarse. Addition amount is 0.15% or less. More preferably, it is 0.10% or less.

W: 1.5% or less W also contributes to strength increase by forming fine precipitates with Ti and Mo, so preferably 0.01% or more is added, but if the content exceeds 1.5%, the precipitates become coarse. Addition amount is 1.5% or less. More preferably, it is 1.0% or less.

When these elements are added, the atomic ratio of these elements and the amounts of C, Ti, and Mo is defined as the following formula (2), which is advantageous for the refinement of precipitates.
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50… (2)
This parameter affects the size of the precipitate, and when it is set to 0.50 or more and 1.50 or less, it becomes easy to form a fine precipitate having a particle size of less than 10 nm.

  In addition, in the fine carbide | carbonized_material containing 1 type, or 2 or more types of Nb, V, and W, atomic ratio (Ti + Nb + V) / (Mo + W) of Ti, Mo, Nb, V, and W in carbide is 0.2- 2.0, and for finer carbides it was observed to be 0.7-1.5.

Further, in the present invention, in order to improve the machinability at the time of machining the part, S: 0.01 to 0.1%, Pb ≦ 0.2%, Ca ≦ 0.005%, Bi ≦ 0.1% and B ≦ 0.02% Seeds or two or more can be added.
Here, the reason why the S content is 0.01 to 0.1% is that if the S content is less than 0.01%, the machinability cannot be improved, and if it exceeds 0.1%, the ductility and toughness decrease. It is. S is less than 0.01% and is contained as an impurity. In the present invention, when the content is 0.1% or less, the strength and magnetic properties are not affected. Therefore, it can add actively and can be made into 0.01 to 0.1% of content.

Also, for Pb, Ca, Bi and B, when the addition amount exceeds the respective upper limit, ductility and toughness are lowered. Therefore, the addition amount is Pb ≦ 0.2%, Ca ≦ 0.005%, Bi ≦ 0.1%, B ≦ 0.02% is necessary.
In addition, for the purpose of improving ductility and toughness, one or more of Cr, Ni and Cu may be added in the range of Cr ≦ 0.5%, Ni ≦ 0.5% and Cu ≦ 0.5%.

The inevitable impurities P and N are elements that are not preferable for the magnetic properties, so it is desirable to reduce them as much as possible. Specifically, it is preferable to restrict P to 0.03% or less. N is preferably regulated to 0.01% or less, more preferably 0.005% or less.
In addition, the effect of this invention is not impaired by the presence or absence and content of these elements.

[Microstructure]
In the present invention, the microstructure is a ferrite single phase with an average crystal grain size of 30 μm or more and 80 μm or less, and the area ratio in the rolling direction cross section of ferrite grains with a grain size of 10 μm or less is 20% or less, and fine precipitation of less than 10 nm It is defined as the structure in which the material is dispersed and precipitated. Below, the reason for limitation of the organization will be described.
First, the ferrite single phase is used because the ferrite single phase is the most preferable structure for magnetic properties. The ferrite single phase structure in the present invention means that the area ratio of ferrite is 95% or more, preferably 98% or more in cross-sectional structure observation (observation with 200 times optical microscope structure).

  In the present invention, the area ratio of a ferrite single phase having an average crystal grain size of 30 μm or more and 80 μm or less and a fine ferrite having a grain size of 10 μm or less is 20% or less. In other words, the average grain size of ferrite is set to 30 μm or more, even if the precipitate is considered to be harmful to the magnetic properties, if the precipitate is as fine as 10 nm or less, the average grain size of ferrite This is because the adverse effect on the magnetic properties can be prevented by setting the diameter to 30 μm or more, making the crystal grain size uniform and not containing fine ferrite.

  From the viewpoint of such magnetic properties, the average grain size of ferrite needs to be 30 μm or more. However, if the grain size is excessively increased, the machinability during machining deteriorates. The upper limit is 80 μm. That is, it has been newly found that machinability in machining is improved by making the ferrite grain size uniform and appropriate.

The experiments conducted to determine the characteristics of the tissue are described in detail below.
According to the component composition range of the present invention, C: 0.072%, Si: 0.07%, Mn: 1.41%, Ti: 0.19%, Mo: 0.25%, P: 0.011%, S: 0.020%, Al: 0.039% and N: A steel containing 0.0028%, the remainder consisting of Fe and inevitable impurities is melted, heated to 1120 ° C, hot rolled to a 100mm diameter steel bar, and then the average cooling rate to 500 ° C is 0.18 ° C Cooled to room temperature at / s. At that time, in order to change the structure, the rolling pass schedule (temperature of each reduction pass and the area reduction ratio) in the hot rolling was variously changed.
The steel bar thus obtained was subjected to a structure observation and a tensile test value and a magnetic property were measured.

Here, with respect to the tensile test value, a test piece having a diameter of 6 mm and a length of 40 mm in the parallel part was taken from the position of 1/4 D of the steel bar (D: diameter of the steel bar 100 mm) in the longitudinal direction of the steel bar, and each measurement It was used for.
Regarding magnetic properties, a ring-shaped test piece having an inner diameter of 33 mm, an outer diameter of 45 mm, and a thickness of 5 mm was taken from the center of the obtained steel bar so that the ring plate surface was parallel to the steel bar cross section, and the primary winding 100 times and 100 secondary windings were performed, and the magnetic flux density B ₅₀ at a direct current excitation current of 5000 A / m and the iron loss W _10/50 when excited at a magnetic flux density of 1.0 T at an alternating current of 50 Hz were measured.

  Furthermore, as a structure observation, a structure observation test piece was collected from an arbitrary position of the steel bar, a total of 20 locations, and the structure was identified. For each test piece, select 100 grains each, and obtain the cross-sectional area of these by image processing. And the average crystal grain size of the whole steel bar was determined by calculating the average value of these. Furthermore, the size of the precipitate was evaluated by electron microscope observation described in detail later.

  As a result of the above structure observation, regardless of the rolling conditions, the structure was a ferrite single phase, but in order to change the crystal grain size, the pass schedule and annealing temperature in the hot rolling were changed variously, so that the individual ferrite grain size Changed from about 2 μm to about 100 μm. Moreover, regarding the size of the precipitate, it was as fine as about 5 nm regardless of the rolling conditions.

Moreover, when the tensile test values of these steel bars were measured, a yield strength as high as 500 MPa was obtained at the minimum, but the magnetic properties varied depending on the average crystal grain size of ferrite and the fraction of fine ferrite grains. It was. FIG. 1 shows the result of _dividing the values of magnetic flux density B ₅₀ and iron loss W _10/50 by the average grain size of ferrite and the area ratio of ferrite having a grain size of 10 μm or less. The area ratio of ferrite having a particle size of 10 μm or less is obtained by calculating the sum of the cross-sectional areas of ferrite grains having a particle size of 10 μm or less among the 2000 ferrites described above, and dividing this by the sum of the cross-sectional areas of 2000 ferrites. Was calculated.

  As can be seen from FIG. 1, when the average particle diameter of ferrite is 30 μm or more, the area ratio occupied by ferrite having a particle diameter of 10 μm or less is 25% or less, the magnetic flux density is 1.61 T or more, and the iron loss is 38 W / kg or less. Excellent magnetic properties can be obtained. However, when the ferrite area ratio with a grain size of 10 μm or less exceeds 25%, the magnetic flux density is lowered to 1.56 to 1.59 T, or the iron loss is increased to 42 to 44 W / kg, and at least one of the magnetic flux density and the iron loss is increased. Inferior. In addition, when the average particle diameter of ferrite is as fine as less than 30 μm, the magnetic flux density is as low as 1.56 to 1.59 T even if the ferrite area ratio with a particle diameter of 10 μm or less is 25% or less, and the iron loss is 42 to As high as 46W / kg, only low magnetic properties can be obtained.

  In this way, when the precipitate is as fine as 5 nm, if the average particle diameter of the ferrite is set to 30 μm or more and the fraction of fine ferrite having a particle diameter of 10 μm or less is reduced, the precipitate has been considered harmful to the magnetic properties. Can be prevented, and excellent magnetic properties can be obtained.

In order to confirm this point, a similar study was performed on steel with various changes in the component composition, precipitate diameter, and ferrite particle size. When the precipitate size was less than 10 nm, the average particle diameter of ferrite was It has been clarified that a high magnetic flux density B ₅₀ and a low iron loss W _10/50 can be obtained if the area ratio of ferrite having a grain size of 10 μm or less is 25% or less. Therefore, from the viewpoint of magnetic properties, it is necessary that the average particle diameter of ferrite is 30 μm or more and the area ratio of ferrite having a particle diameter of 10 μm or less is 25% or less.

  Here, when there are few fine ferrites with a grain size of 10 μm or less, the reason why the adverse effect of precipitates on magnetic properties is suppressed is not necessarily clear, but there is a relationship between the deterring force against domain wall motion and the crystal grain size. It is suggested.

  In general, it is preferable that the magnetic properties are such that the domain wall movement is easy, and the precipitates are considered to adversely affect the magnetic properties by preventing the domain wall migration. By the way, when the ferrite grain size increases, the size of the magnetic domain also increases, and the length of the domain wall that is the boundary of the magnetic domain also increases. Here, if the domain wall length is sufficiently long and the precipitates are sufficiently fine, the influence of the precipitates on the domain wall movement is a fact from the relative relationship between the domain wall movement deterring force due to the precipitates and the driving force of the domain wall movement. It is assumed that it will be negligible. For this reason, the larger the ferrite particle size, the more advantageous in terms of magnetic properties. However, considering the entire steel material, it is necessary to consider that the ferrite particle size is not necessarily the same, and the ferrite particle size varies to some extent.

If the ferrite grain size varies, the domain wall can move easily without being affected by precipitates in a large part of ferrite, but the domain wall movement is hindered by precipitates in a small part of ferrite. It will be rate-limiting.
Therefore, in addition to increasing the average size of the ferrite, it is important in terms of magnetic properties to reduce the proportion of fine ferrite that is the rate-determining domain wall motion. For this reason, if the proportion of fine ferrite with a particle size of 10 μm or less is reduced, the average particle size of ferrite to prevent the adverse effects of precipitates on the magnetic properties will be lower than when the proportion of fine ferrite is not specifically considered it is conceivable that.

  As described above, from the viewpoint of magnetic properties, the average grain size of ferrite needs to be 30 μm or more, but since the machinability deteriorates when the grain size increases excessively, the upper limit of the average grain size is 80 μm. To do. Hereinafter, this point will be described.

  The steel used in the experiment whose results are shown in FIG. 1 is heated to 1220 ° C., hot rolled to a 120 mm diameter steel bar, and then cooled to room temperature at an average cooling rate of up to 500 ° C .: 0.15 ° C./s. . In order to change the crystal grain size, the rolling pass schedule in the hot rolling was variously changed.

  As a result of observing the structure of the obtained steel bar, the steel bar structure was a ferrite single phase regardless of the rolling conditions, but various changes were made to the hot rolling pass schedule in order to change the crystal grain size. The ferrite grain size of the steel changed from about 4 μm to about 160 μm. As for the size of the precipitate, it was as fine as about 5 nm regardless of the rolling conditions. Moreover, the yield strength was a sufficiently high value of 500 MPa or more.

The machinability was investigated using these steel bars. For machinability, the tool life was defined as the time when the flank wear of the tool reached 0.2 mm after cutting the outer circumference of the bar steel using a carbide tool P10 at a cutting speed of 200 m / min and no lubrication.
FIG. 2 shows the relationship between the ferrite grain size and machinability. From the figure, when the average particle diameter of ferrite is 80 μm or less, the tool life is 15 min or more and a good value when the area ratio of ferrite having a particle diameter of 10 μm or less is 20% or less. However, when the ferrite area ratio with a grain size of 10 μm or less exceeds 20%, the tool life becomes less than 5 min. Furthermore, when the average grain size of ferrite is as large as more than 80 μm, a tool life of less than 5 minutes can be obtained even if the ferrite area ratio with a grain size of 10 μm or less is 20% or less.

  As described above, excellent machinability can be obtained when the average particle diameter of ferrite is 80 μm or less and there are few fine ferrites having a particle diameter of 10 μm or less. In order to confirm this point, the same study was conducted on steels with various chemical compositions, precipitate diameters, and ferrite particle sizes. When the precipitate size was less than 10 nm, the average particle size of ferrite was 80 μm. In addition to the following, it was confirmed that if the area ratio of ferrite having a particle size of 10 μm or less was 20% or less, the tool life was greatly extended as compared with those not satisfying these. Thus, from the viewpoint of machinability, the average grain size of ferrite is defined as 80 μm or less, and the area ratio of ferrite having a grain size of 10 μm or less is defined as 20% or less.

  Here, when the average grain size of ferrite exceeds 80 μm, the machinability deteriorates when the ferrite becomes excessively coarse, as the grain size increases, chips are formed in large units. This is probably because the surface is rough. Further, when the area ratio of ferrite having a particle size of 10 μm or less is increased, the influence of the hardness difference due to the difference in ferrite particle size becomes obvious, and it is assumed that this non-uniformity in hardness deteriorates the machinability.

As described above, the results of studies on the magnetic characteristics and machinability of the present invention are summarized as follows.
From the viewpoint of magnetic properties, it is necessary that the average particle size of ferrite is 30 μm or more and the area ratio of ferrite having a particle size of 10 μm or less is 25% or less. From the viewpoint of machinability, it is necessary that the average particle size of ferrite is 80 μm or less and the area ratio of ferrite having a particle size of 10 μm or less is 20% or less.

  The present invention aims to provide both excellent magnetic properties and machinability, and the average particle size of ferrite is 30 μm or more so that the requirements of ferrite particle size for magnetic properties and machinability are simultaneously satisfied. The area ratio of ferrite having a particle size of 80 μm or less and a particle size of 10 μm or less is defined as 20% or less.

Next, in the present invention, the particle size of the fine precipitate is less than 10 nm. When the particle size of the precipitate is 10 nm or more, the precipitation strengthening ability is insufficient.
The smaller the particle size of the fine precipitates, the more effective the strength increase. Desirably, the fine precipitates are 5 nm, more preferably 3 nm or less. As such fine precipitates, carbides containing a composite of Ti and Mo, and further Nb, V And a carbide containing one or more of W is preferred.

The number of fine precipitates is 1000 / μm ³ or more, more desirably 5000 / μm ³ or more, and it is preferable that high yield strength is easily obtained.

These fine precipitates are desirably dispersed and precipitated uniformly in the matrix phase. In the present invention, if the size of the precipitate satisfies 90% or more of the total precipitate, a high yield strength can be obtained.
However, since the precipitate having a size of 10 nm or more consumes the precipitate-forming elements and adversely affects the strength, the size is preferably suppressed to 50 nm or less.

  In addition to the precipitates described above, the effect of the present invention is not impaired even if a small amount of Fe carbide is contained. However, if a large amount of Fe carbide having an average particle size of 1 μm or more is contained, the magnetic properties are hindered. In this case, the upper limit of the size of the Fe carbide contained is preferably 1 μm, and the content is preferably 1% or less of the entire precipitate.

Here, the size of the precipitate and the ratio of the fine precipitate to the total precipitate are obtained by the following method.
An electron microscope sample is prepared by electropolishing using a twin jet method and observed at an acceleration voltage of 200 kV. At that time, the defocus method is used to control the crystal orientation of the parent phase so that the precipitate has a measurable contrast with respect to the parent phase, and to minimize the counting of the precipitates. Observe at. Moreover, the sample thickness of the area | region which measured the deposit particle | grains is evaluated by measuring an elastic scattering peak and an inelastic scattering peak intensity | strength using an electron energy loss spectroscopy.

By this method, the measurement of the particle diameter and the number of particles and the measurement of the sample thickness can be executed for the same region. The particle diameter and the number of particles are measured at four locations of a 0.5 μm × 0.5 μm region of the sample, and the precipitates distributed per 1 μm ² are calculated as the number for each particle diameter. Next, from this value and the thickness of the sample, the number of precipitates per particle diameter distributed per 1 μm ³ is calculated. Thereby, the size of the precipitate and the proportion of the precipitate having a particle size of less than 10 nm in the total precipitate are obtained.

[Production conditions]
Hereinafter, desirable manufacturing conditions will be described.
Heating temperature In the present invention, in order to precipitate precipitates finely during cooling after hot rolling, it is necessary to once dissolve the precipitates precipitated on the slab before hot rolling in a heating furnace. is there. At that time, if the heating temperature is less than 1100 ° C., Ti—Mo-based carbides and the like are not sufficiently dissolved, so the heating temperature is set to 1100 ° C. or higher.

In the present invention, in order to obtain excellent magnetic properties and machinability, it is necessary that the average grain size of ferrite is 30 μm or more and 80 μm or less and the area ratio of ferrite having a grain size of 10 μm or less is 20% or less. Among these, in order to reduce the area ratio of ferrite having a grain size of 10 μm or less to 20% or less, a strong reduction with a reduction in area of 25% or more was performed at the beginning of rolling, and austenite was introduced at the beginning of rolling by sufficiently introducing recrystallization nuclei. It is important to make the particle size uniform. By doing so, the ferrite structure finally obtained can be made uniform, and the production of fine ferrite can be suppressed to a certain amount or less. Here, in order to reduce the area ratio of ferrite having a grain size of 10 μm or less to 20% or less, a reduction in area of 25% or more is required.In the present invention, the reduction in area in the hot rolling start pass is 25 It is specified as% or more.

  In order to make the crystal grain size of ferrite 30 μm or more and 80 μm or less, it is effective to control the area reduction rate in the final pass of hot rolling. Specifically, if the area reduction rate is 15% or more and 35% or less, ferrite with a grain size of 30 μm or more and 80 μm or less is obtained. Therefore, the area reduction rate in the final pass of hot rolling is within the range of the area reduction rate. 15% to 35%.

Finishing temperature In the present invention, the precipitation behavior of the precipitate is closely related to the progress of the ferrite transformation, and the difference between the transformation start temperature of the ferrite transformation that occurs during cooling after rolling and the precipitation start temperature of the precipitate is small, When the ferrite transformation and precipitation compete, the precipitate is finely dispersed and precipitated in the ferrite. In order to compete with ferrite transformation and precipitation, it is necessary to lower the start temperature of ferrite transformation, but when the finishing temperature in hot rolling is low, the strain introduced by rolling increases the start temperature of ferrite transformation. Inhibits refinement of precipitates. In order to avoid this, the finishing temperature may be set to a high temperature at which the influence of distortion does not appear. From this point, the finishing temperature is set to 880 ° C. or higher.

Cooling rate In the present invention, fine precipitates are deposited during cooling after hot rolling. At that time, if the cooling rate after hot rolling exceeds 1.0 ° C./s, a low-temperature transformation phase is generated, precipitation does not proceed sufficiently, and high yield strength cannot be obtained. Therefore, the cooling rate after hot rolling needs to be 1.0 ° C./s or less. If the cooling rate is 1.0 ° C./s or less, the steel of the present invention has a low C, and thus a ferrite single phase structure can be obtained. In addition, since precipitation is substantially complete | finished by 500 degreeC, what is necessary is just to cool to 500 degreeC after a hot rolling with the cooling rate of 1.0 degrees C / s or less.

In addition, you may anneal further with respect to the steel bar of this invention obtained with the manufacturing method demonstrated above. By annealing after hot rolling, fine precipitates can be sufficiently precipitated, the structure can be made uniform, and high strength, magnetic properties and machinability can be combined at a higher level. In particular, the area ratio of fine ferrite grains can be further reduced by optimizing the annealing conditions, and the machinability can be further improved.
In the following, the results of investigations on the case of annealing will be described.

According to the component composition range of the present invention, C: 0.072%, Si: 0.07%, Mn: 1.41%, Ti: 0.19%, Mo: 0.38%, P: 0.011%, S: 0.020%, Al: 0.025% and N: A steel containing 0.0028%, the balance being Fe and inevitable impurities, is melted, heated to 1220 ° C, hot-rolled to a steel bar with a diameter of 100mm, and then the average cooling rate to 500 ° C is 0.18 ° C Cooled to room temperature at / s. At that time, in order to change the structure, the rolling pass schedule (temperature of each reduction pass and the area reduction rate) in hot rolling and the subsequent annealing temperature were changed variously.
The steel bar thus obtained was subjected to a structure observation and a tensile test value and a magnetic property were measured. The structure observation, tensile test, and magnetic property measurement method were the same as those described above.

  As a result of the structure observation, the structure was a ferrite single phase regardless of the rolling conditions, but in order to change the crystal grain size, the hot rolling pass schedule and the annealing temperature were variously changed. It changed from about 3 μm to about 110 μm. Moreover, regarding the size of the precipitate, it was as fine as about 5 nm regardless of the rolling conditions.

When the tensile test values of these steel bars were measured, a yield strength as high as 520 MPa was obtained at the minimum, but the magnetic properties depended on the average crystal grain size of ferrite and the fraction of fine ferrite grains. FIG. 3 shows the result of _dividing the values of magnetic flux density B ₅₀ and iron loss W _10/50 by the average grain size of ferrite and the area ratio of ferrite having a grain size of 10 μm or less. The area ratio of ferrite having a particle size of 10 μm or less is obtained by calculating the sum of the cross-sectional areas of ferrite grains having a particle size of 10 μm or less among the 2000 ferrites described above, and dividing this by the sum of the cross-sectional areas of 2000 ferrites. Was calculated.

  As can be seen from FIG. 3, when the average grain size of ferrite is 30 μm or more, the area ratio occupied by ferrite having a grain size of 10 μm or less is 10% or less, so that the magnetic flux density is 1.65 T or more and the iron loss is 36 W / Higher magnetic properties of kg or less can be obtained. Also in this case, when the average particle diameter of the ferrite is as fine as less than 30 μm, or when the area ratio of ferrite having a particle diameter of 10 μm or less exceeds 10%, the magnetic flux density is less than 1.65 T or the iron loss is 36 W. / kg was exceeded, and the magnetic flux density was low or the iron loss was high.

  In this way, when the precipitate is as fine as 5 nm, it is harmful to the magnetic properties if the average particle diameter of ferrite is set to 30 μm or more and the fraction of fine ferrite having a particle diameter of 10 μm or less is further reduced to 10% or less. It is possible to prevent the influence of precipitates that have been considered to be excellent, and to obtain excellent magnetic properties.

In order to confirm this point, a similar study was performed on steel with various changes in the component composition, precipitate diameter, and ferrite particle size. When the precipitate size was less than 10 nm, the average particle diameter of ferrite was It is clear that a high magnetic flux density B ₅₀ and a low iron loss W _10/50 can be obtained by _setting the area ratio of ferrite having a grain size of 10 μm or less to 10% or less. Therefore, from the viewpoint of magnetic characteristics, it is particularly preferable that the average particle size of ferrite is 30 μm or more and the area ratio of ferrite having a particle size of 10 μm or less is 10% or less.
It has also been found that further reducing the area ratio of ferrite having a grain size of 10 μm or less by annealing is effective in improving machinability. Hereinafter, this point will be described.

  The steel used in the experiment whose results are shown in FIG. 3 is heated to 1230 ° C., hot rolled to a 120 mm diameter steel bar, and then cooled to room temperature at an average cooling rate of up to 500 ° C .: 0.15 ° C./s. . In order to change the crystal grain size, the rolling pass schedule in the hot rolling and the subsequent annealing temperature were variously changed.

  As a result of observing the structure of the obtained steel bar, the steel bar structure was a ferrite single phase regardless of the rolling conditions, but in order to change the crystal grain size, the hot rolling pass schedule and the annealing temperature were variously changed. Therefore, the particle diameter of each ferrite changed from about 4 μm to about 140 μm. As for the size of the precipitate, it was as fine as about 5 nm regardless of the rolling conditions. Moreover, the yield strength was a sufficiently high value of 520 MPa or more.

The machinability was investigated using these steel bars. For machinability, the tool life was defined as the time when the flank wear of the tool reached 0.2 mm after cutting the outer circumference of the steel bar using a carbide tool P10 under cutting conditions of 200 m / min and no lubrication.
FIG. 4 shows the relationship between the ferrite grain size and machinability. From the figure, when the average particle diameter of ferrite is 80 μm or less, the tool life is 30 minutes or more and particularly good when the area ratio of ferrite having a particle diameter of 10 μm or less is 5% or less. Here, too, when the average grain size of ferrite is as large as more than 80 μm, the tool life of 30 min or more is not improved even if the ferrite area ratio having a grain size of 10 μm or less is 5% or less.

  As described above, when the average particle diameter of the ferrite is 80 μm or less, the machinability can be further improved by further reducing the fine ferrite having a particle diameter of 10 μm or less. In order to confirm this point, the same study was conducted on steels with various chemical compositions, precipitate diameters, and ferrite particle sizes. When the precipitate size was less than 10 nm, the average particle size of ferrite was 80 μm. In addition to the following, it was confirmed that if the area ratio of ferrite having a particle size of 10 μm or less is 5% or less, the tool life is greatly extended as compared with those not satisfying these. Thus, from the viewpoint of machinability, it is particularly preferable to reduce the area ratio of ferrite having a particle size of 10 μm or less to 5% or less.

FIG. 5 and FIG. 6 are examples in which similar investigations were performed for other component compositions. That is, C: 0.075%, Si: 0.09%, Mn: 2.15%, Ti: 0.24%, Mo: 0.48%, P: 0.018%, S: 0.023%, Al: 0.029% and N: 0.0041%, the balance FIG. 5 shows the results of investigating the relationship between the average grain size of ferrite and the area ratio of ferrite having a grain size of 10 μm or less, the magnetic flux density B ₅₀ , and the iron loss W _{10/50 for steels} that _contain Fe and inevitable impurities. Shown in

  Here, the structure was a ferrite single phase regardless of the rolling conditions, but in order to change the crystal grain size, the pass schedule and the annealing temperature in the hot rolling were changed variously, so the individual ferrite grain size was about 2 μm. It changed to about 100μm. Moreover, regarding the size of the precipitate, it was as fine as about 5 nm regardless of the rolling conditions. Further, when the tensile test values of these steel bars were measured, a yield strength as high as 550 MPa was obtained at a minimum, but the magnetic properties depended on the average crystal grain size of ferrite and the fraction of fine ferrite grains.

  As is apparent from FIG. 5, in this steel, the magnetic flux density is 1.63 T by setting the area ratio of ferrite having an average grain size of 30 μm or more and 10 μm or less to 10% or less. As described above, the iron loss is 38 W / kg or less. However, when the average grain size of ferrite is as small as less than 30 μm, or when the area ratio of ferrite with a grain size of 10 μm or less exceeds 10%, the magnetic flux density is less than 1.65 T or the iron loss is 38 W / kg. The magnetic flux density was low or the iron loss was high.

  Further, FIG. 6 shows the results of investigating the relationship between the average grain size of ferrite and the area ratio of ferrite having a grain size of 10 μm or less and machinability. From the figure, when the average grain size of ferrite is 80 μm or less, if the area ratio of ferrite having a grain size of 10 μm or less is 5% or less, the tool life is as good as 30 min or more. However, when the ferrite area ratio with a grain size of 10 μm or less exceeds 5%, the tool life becomes less than 30 min. Furthermore, it can be seen that when the average grain size of ferrite is as large as more than 80 μm, only a tool life of less than 30 min can be obtained even if the ferrite area ratio with a grain size of 10 μm or less is 5% or less.

As described above, a particularly preferable range can be summarized from the examination results on the magnetic characteristics and machinability of the present invention as follows.
From the viewpoint of magnetic properties, it is necessary that the average particle size of ferrite is 30 μm or more and the area ratio of ferrite having a particle size of 10 μm or less is 10% or less. From the viewpoint of machinability, it is necessary that the average particle size of ferrite is 80 μm or less and the area ratio of ferrite having a particle size of 10 μm or less is 5% or less.

  Therefore, it is particularly preferable that the ferrite average particle size is 30 μm or more and 80 μm or less, and the area ratio of ferrite having a particle size of 10 μm or less is 5% or less so that the requirements of the ferrite particle size for magnetic properties and machinability are simultaneously satisfied. preferable.

In order to make the area ratio of ferrite having a particle diameter of 10 μm or less 5% or less, it is possible to apply the annealing conditions shown below in addition to the above-described manufacturing method.
Annealing temperature In the present invention, high strength, magnetic properties and machinability can be combined at a higher level by sufficiently depositing fine precipitates by annealing after hot rolling and further homogenizing the structure. . At this time, if the annealing temperature is less than 600 ° C., fine precipitates cannot be precipitated, so that the strength cannot be sufficiently increased. In addition, the area ratio of ferrite having a particle size of 10 μm or less cannot be reduced to 5% or less, and the magnetic properties and machinability cannot be improved. Accordingly, the annealing temperature is preferably 600 ° C. or higher. On the other hand, if the annealing temperature is too high, fine precipitates are coarsened, and the second phase is precipitated during cooling after annealing, thereby degrading the magnetic properties. If the Mn content in the steel is 1.7% or less, good magnetic properties can be secured by setting the annealing temperature to 800 ° C or less, and if the Mn content in the steel exceeds 1.7%, annealing is performed. It was found that good magnetic properties can be secured if the temperature is 750 ° C. or lower.
As a result of the above examination, the annealing temperature when annealing is set to the following temperature range T.
Record
When the Mn content is 1.7% or less: 600 ° C. ≦ T ≦ 800 ° C.
When Mn content exceeds 1.7%: 600 ℃ ≦ T ≦ 750 ℃

[Example 1]
Steels having the compositions shown in Table 1 were melted, and these were hot-rolled into steel bars having predetermined dimensions in accordance with the conditions described in Tables 2 and 3. In hot rolling, the heating temperature, pass schedule, finishing temperature, and cooling rate to 500 ° C. after rolling were changed. Here, the cooling finish was changed, and the cooling rate was changed by air cooling after the rolling.

The steel bar thus obtained was subjected to a structure observation and a tensile test, and magnetic characteristics and tool life were measured.
In the structure observation, specimens for tissue observation were collected from 20 arbitrary position gauges of the steel bar to identify the structure, and 100 grains for each specimen were arbitrarily selected, and their cross-sectional areas were imaged. The total grain size of 2000 crystal grains was calculated individually as the diameter of an equivalent circle having a cross-sectional area equivalent to this, and the average value of these was obtained to determine the average grain size of the entire steel bar. Also, the sum of the cross-sectional areas of ferrite with a grain size of 10 μm or less out of a total of 2000 crystal grains, and dividing this by the sum of the cross-sectional areas of 2000 crystal grains, the area ratio of ferrite with a grain size of 10 μm or less Was calculated.

  Furthermore, a thin film sample was prepared by electrolytic polishing, and the particle size of the precipitate was measured by observation with a transmission electron microscope (TEM) according to the method described above, and an energy dispersive X-ray spectrometer (EDX) was used in combination. Thus, the precipitate was identified.

In the tensile test, a specimen having a parallel part diameter of 6 mm and a parallel part length of 40 mm was sampled in the longitudinal direction of the steel bar from any 1 / 4D position of the steel bar, and the yield stress YS was measured.
Regarding magnetic properties, a ring-shaped test piece having an inner diameter of 33 mm, an outer diameter of 45 mm, and a thickness of 5 mm was taken from the center of the obtained steel bar so that the ring plate surface was parallel to the cross section of the steel bar, and the primary winding was 100 times. The secondary winding was applied 100 times, and the magnetic flux density B ₅₀ at a direct current excitation current of 5000 A / m and the iron loss W _10/50 when excited to a magnetic flux density of 1.0 T at an alternating current of 50 Hz were measured.
For machinability, the tool life was defined as the time when the flank wear of the tool reached 0.2 mm after cutting the outer circumference of the bar steel using a carbide tool P10 at a cutting speed of 200 m / min and no lubrication.

Tables 2 and 3 show the results of the above-described structure observation, tensile test, magnetic measurement, and tool life measurement.
The numbers in the table are for classifying the individual results, and the steel numbers and hot rolling conditions were combined to indicate the combination of the test steel and hot rolling conditions. For example, when steel No. 1 was hot-rolled under condition A, it started as 1-A.
Regarding the structure, ferrite is abbreviated as T when a low temperature transformation phase such as F, bainite or martensite is generated and its volume fraction exceeds 5%. The average particle size is described for the precipitate. The variation in particle diameter was ± 1 nm at the maximum for precipitates of less than 10 nm, and ± 3 nm to ± 5 nm for precipitates larger than that. In the case where a low temperature transformation phase was generated in the structure, the measurement of the crystal grain size and the particle size of the precipitate was omitted.

Table 2 shows the hot rolling conditions within the scope of the present invention and shows the influence of the steel composition. As is apparent from the table, in the invention examples in which both the steel composition and the hot rolling conditions satisfy the scope of the present invention. Yield strength (yield stress) of 500 MPa or more has been obtained, and the magnetic properties are also high, with a magnetic flux density B ₅₀ of 1.60 T or more and an iron loss W _{10/50 of 38} W / kg or less at an excitation current of 5000 A / m. Show the characteristics. Moreover, the tool life at the time of cutting also shows a value of 15 min or more.

On the other hand, in the comparative example in which the steel composition is out of the range of the present invention, at least one of the yield strength and the magnetic property shows a low value.
No. 17-A has low C, the amount of fine precipitates is insufficient, and the yield strength is low.
No. 18-A has high C, coarse precipitates, and low yield strength. When the precipitate is coarse, as described above, the precipitate adversely affects the magnetic properties, so the magnetic flux density B ₅₀ is 1.57 T and the iron loss W _10/50 is about ₄₃ W / kg. Inferior in characteristics.

In No. 19-A, since the Mn was low, the ferrite transformation and precipitation did not sufficiently compete with each other, and as a result of coarse precipitation, the yield strength was low. Further, due to the coarsening of the precipitates, the magnetic flux density B ₅₀ is as low as 1.59 T, and the iron loss W _10/50 is as high as 46 W / kg.
No. 20-A having a high Mn has a low yield strength because a low-temperature transformation phase is generated and precipitation strengthening due to fine precipitates is insufficient. In addition, the magnetic flux density B ₅₀ is as low as 1.55 T, which is probably due to the generation of the low temperature transformation phase.
No. 21-A has a low yield strength due to a low amount of fine precipitates because Ti is low. On the other hand, No. 22-A with high Ti has coarse precipitates, low yield strength, and is inferior in _both magnetic flux density B _{50 and} iron loss W _10/50 .
No. 23-A has a low yield strength because Mo is low, resulting in an insufficient amount of fine precipitates. On the other hand, No. 24-A, which has a high Mo, has a low-temperature transformation phase and a low yield strength due to insufficient precipitation strengthening by fine precipitates. In addition, the magnetic flux density B ₅₀ is as low as 1.56 T, as in No. 20-A, which has a high Mn and also produced a low-temperature transformation phase.

Table 3 shows the results of hot pressing steel No. 2 which is the steel of the present invention under various conditions. As is apparent from the table, Steel No. 2 which is the steel of the present invention is within the scope of the present invention. In the invention example that was hot-rolled under the above conditions, a yield strength as high as 500 MPa or higher was obtained, and the magnetic properties were also such that the magnetic flux density B ₅₀ was 1.60 T or more and the iron loss W _10/50 was approximately ₃₈ W / kg or less. Excellent value. Moreover, the tool life at the time of cutting also shows a value of 15 min or more.

On the other hand, in the comparative example in which the hot rolling conditions are out of the scope of the present invention, at least one of yield strength, magnetic characteristics, and tool life is low.
That is, No. 2-G and No. 2-M have a low area reduction rate of the rolling start pass in hot rolling, and the area ratio of ferrite having a grain size of 10 μm or less exceeds the upper limit of 20% of the present invention. As a result, the precipitate adversely affects the magnetic properties, and as a result, either the magnetic flux density B ₅₀ or the iron loss W _10/50 is inferior.
No.2-H and No.2-N have a low area reduction rate in the final rolling pass in hot rolling, the average grain size of ferrite exceeds the upper limit of 80 μm of the present invention, and the tool life is short.
In No.2-I and No.2-O, the area reduction rate of the final rolling pass in hot rolling is high, the average grain size of ferrite is below the lower limit of 30 μm of the present invention, and the precipitates have magnetic properties. As a result of adverse effects, the magnetic flux density B ₅₀ and the iron loss W _10/50 are inferior.

In addition to the area reduction ratio in the hot rolling, it is necessary to optimize the heating temperature, the finishing temperature, and the cooling rate, and in these comparative examples that are outside the scope of the present invention, the yield strength is low and is 500 MPa or less.
In other words, No.2-P has a low heating temperature, and the precipitate deposited on the slab before hot rolling does not sufficiently dissolve in the heating furnace, so that the fine precipitation of the precipitate is hindered. The yield strength is low, and the magnetic properties are also low. As for the precipitate, there are a mixture of what appears to be finely precipitated during cooling after rolling and what is considered to be the undissolved residue of the precipitate deposited on the slab, and the average particle size of the precipitate is 100 nm or more. It was.
In No.2-T, the finishing temperature is low, and the strain introduced by rolling increases the starting temperature of the ferrite transformation, preventing the competition between the ferrite transformation and precipitation, resulting in coarse precipitates and reduced yield strength. At the same time, the magnetic properties have deteriorated.
No. 2-U is an example in which the cooling rate after hot rolling is excessive, but in order to increase the cooling rate only for this, mist cooling after rolling was performed. It can be seen that when the cooling rate is high, a low-temperature transformation phase is generated and the precipitation of fine precipitates is prevented, so that the yield strength decreases.

[Example 2]
Steels having the compositions shown in Table 4 were melted, and these were hot-rolled into steel bars having predetermined dimensions in accordance with the conditions described in Tables 5 and 6.
In hot rolling, the heating temperature, pass schedule, finishing temperature, and cooling rate to 500 ° C. after rolling were changed. Here, the cooling finish was changed, and the cooling rate was changed by air cooling after the rolling. Further, after rolling, annealing was performed at a temperature of 0 to 850 ° C.

The steel bar thus obtained was subjected to a structure observation and a tensile test, and magnetic characteristics and tool life were measured.
Tissue observation was carried out by collecting 20 tissue observation specimens from arbitrary positions of the steel bar and identifying the structure. For each test piece, obtain the average cross-sectional area of the crystal grains by the cutting method of JIS G 0552, calculate the crystal grain size of each test piece as the diameter of the equivalent circle, and then calculate the average value of 20 pieces each. Thus, the average crystal grain size D at each position and the area ratio in the cross section in the rolling direction with a grain size of 10 μm or less were obtained.

  Furthermore, by preparing a thin film sample by electropolishing and observing it with a transmission electron microscope (TEM) according to the method described above, the particle size of the precipitate is measured and an energy dispersive X-ray spectrometer (EDX) is used in combination. Then, the precipitate was identified.

In the tensile test, a specimen having a parallel part diameter of 6 mm and a parallel part length of 40 mm was sampled in the longitudinal direction of the steel bar from any 1 / 4D position of the steel bar, and the yield stress YS was measured.
Regarding magnetic properties, a ring-shaped test piece having an inner diameter of 33 mm, an outer diameter of 45 mm, and a thickness of 5 mm was taken from the center of the obtained steel bar so that the ring plate surface was parallel to the cross section of the steel bar, and the primary winding was 100 times. Measured the iron loss W _10/50 when the secondary winding was applied 100 times and the magnetic flux density B ₁₀ , B _{50 at} DC excitation current 1000 and 5000 A / m and the magnetic flux density 1.0 T at 50 Hz AC were excited. .
The tool life was defined as the tool life when the peripheral surface of the steel bar was cut with a carbide tool P10 at a cutting speed of 200 m / min and no lubrication, and the flank wear of the tool reached 0.2 mm.

Tables 5 and 6 show the results of the above-described structure observation, tensile test, magnetic measurement, and tool life measurement.
The numbers in the table are for classifying the individual results, and the steel numbers and hot rolling conditions were combined to indicate the combination of the test steel and hot rolling conditions. For example, when steel No. 1 was hot-rolled under condition A, it started as 1-A.
Regarding the structure, ferrite is abbreviated as T when a low temperature transformation phase such as F, bainite or martensite is generated and its volume fraction exceeds 5%. The average particle size is described for the precipitate. The variation in particle diameter was ± 1 nm at the maximum for precipitates of less than 10 nm, and ± 3 nm to ± 5 nm for precipitates larger than that. In the case where a low temperature transformation phase was generated in the structure, the measurement of the crystal grain size and the particle size of the precipitate was omitted.

Table 5 shows hot rolling conditions within the scope of the present invention and shows the influence of the steel composition. As is apparent from the table, in the invention examples in which both the steel composition and hot rolling conditions satisfy the scope of the present invention. Yield strength (yield stress) of 500 MPa or more has been obtained, and the magnetic properties of magnetic flux density B _{10 at} an excitation current of 1000 A / m are 1.0 T or more, magnetic flux density B _{50 at} 5000 A / m is 1.65 T or more, and frequency. The iron loss W _10/50 when excited to a magnetic flux density of 1.0T at 50Hz is 36W / kg or less, showing excellent magnetic properties. Moreover, the tool life at the time of cutting also shows a value of 30 min or more.

On the other hand, in the comparative example in which the steel composition is out of the range of the present invention, at least one of the yield strength and the magnetic property shows a low value.
No. 13-A has a low C and a small amount of fine precipitates, resulting in a low yield strength.
No. 14-A is high in C, the precipitates are coarsened, and the yield strength is low. If precipitate coarse, since an adverse effect on precipitate magnetic properties as described above, the magnetic flux density B ₁₀ is 0.87T, B ₅₀ is as low as 1.57T, iron loss W _10/50 is 40W / kg Thus, the magnetic properties are inferior.
In No. 15-A, since Mn was low, ferrite transformation and precipitation did not compete sufficiently, and as a result of precipitation of coarse precipitates, yield strength was low and magnetic properties were inferior. On the other hand, in No. 16-A having a high Mn, a low-temperature transformation phase is generated and the yield strength is low because precipitation strengthening due to fine precipitates is insufficient. Although seems to be due to formation of the low-temperature transformation phase, the magnetic flux density B ₁₀ is 0.91T and low.

No. 17-A has a low yield strength due to a low amount of fine precipitates because Ti is low. On the other hand, in No. 18-A where Ti is high, precipitates are coarsened, yield strength is low, magnetic flux density B ₅₀ is low, and iron loss W _10/50 is also inferior.
No. 19-A has a low Mo, so the amount of fine precipitates is insufficient and the yield strength is low. On the other hand, in No. 20-A with high Mo, a low-temperature transformation phase is generated, and precipitation strengthening due to fine precipitates is insufficient, resulting in low yield strength and poor magnetic properties.

Table 6 shows the result of annealing after hot rolling steel No. 4 which is the steel of the present invention under various conditions. As is clear from the table, in the inventive example, a high yield strength of 500 MPa or more was obtained. As for the magnetic characteristics, the magnetic flux density B ₁₀ is 0.93 or more, B ₅₀ is 1.61 T or more, and the iron loss W _10/50 is 40 W / kg or less. The tool life is also excellent machinability of 25 min or more. Among the inventive examples, No. 4-H steel and No. 4-N steel are not annealed, so the area ratio of ferrite having a particle size of 10 μm or less is high, and the magnetic flux density B ₁₀ is higher than that of the annealed steel. Low level.
On the other hand, in a comparative example in which the hot rolling condition or the annealing condition is out of the range of the present invention, at least one of yield strength, magnetic characteristics, and machinability shows a low value.

Since No. 4-G steel has a high annealing temperature, precipitates are dissolved, and the second phase is precipitated during cooling. As a result, the strength is low and the magnetic properties are inferior.
No. 4-I steel and No. 4-O steel have a low area reduction ratio of fine ferrite with a grain size of 10 μm or less and a low magnetic flux density B ₁₀ because the reduction in area of the rolling start path in hot rolling is low. It is.
No. 4-J steel has a low reduction in area in the final rolling pass in hot rolling, and the average crystal grain size of ferrite exceeds the upper limit of 80 μm of the present invention, so that the machinability is inferior.
No. 4-P steel has a high area reduction rate in the final rolling pass in hot rolling, the average crystal grain size of ferrite is below the lower limit of 30 μm of the present invention, and the area ratio of ferrite of 10 μm or less is also low. Since it is high, the magnetic properties are low.

In addition to the area reduction ratio in hot rolling, it is necessary to optimize heating temperature, finishing temperature, and cooling rate. In the comparative example in which these manufacturing conditions are outside the scope of the present invention, the yield strength is low and the magnetic properties are low.
Since No. 4-Q steel has a low heating temperature, the slab precipitate before hot rolling does not sufficiently dissolve in the heating furnace, and the precipitate becomes coarse. As a result, in addition to the low yield strength, the magnetic properties are inferior.
In No. 4-U steel, the finishing temperature is low, and the strain introduced by rolling increases the starting temperature of the ferrite transformation and inhibits the competition between the ferrite transformation and precipitation. As a result, the precipitates become coarse and the yield strength decreases, and the magnetic properties deteriorate.
No. 4-V steel is an example in which the cooling rate after hot rolling is excessive, and mist cooling was performed after rolling in order to increase the cooling rate. When this cooling rate is fast, a low-temperature transformation phase is generated, and fine precipitates are not sufficiently precipitated even after annealing, so that the yield strength is lowered. Also, the magnetic properties were low.

[Example 3]
Steels having the compositions shown in Table 7 were melted, and these were hot-rolled into steel bars having predetermined dimensions in accordance with the conditions described in Tables 8 and 9.
In hot rolling, the heating temperature, pass schedule, finishing temperature, and cooling rate to 500 ° C. after rolling were changed. Here, the cooling finish was changed, and the cooling rate was changed by air cooling after the rolling. Further, after rolling, annealing was performed at a temperature of 0 to 850 ° C.

The steel bar thus obtained was subjected to a structure observation and a tensile test, and magnetic characteristics and tool life were measured.
Tissue observation was carried out by collecting 20 tissue observation specimens from arbitrary positions of the steel bar and identifying the structure. For each test piece, obtain the average cross-sectional area of the crystal grains by the cutting method of JIS G 0552, calculate the crystal grain size of each test piece as the diameter of the equivalent circle, and then calculate the average value of 20 pieces each. Thus, the average crystal grain size D at each position and the area ratio in the cross section in the rolling direction with a grain size of 10 μm or less were obtained.

  Furthermore, by preparing a thin film sample by electropolishing and observing it with a transmission electron microscope (TEM) according to the method described above, the particle size of the precipitate is measured and an energy dispersive X-ray spectrometer (EDX) is used in combination. Then, the precipitate was identified.

In the tensile test, a specimen having a parallel part diameter of 6 mm and a parallel part length of 40 mm was sampled in the longitudinal direction of the steel bar from any 1 / 4D position of the steel bar, and the yield stress YS was measured.
Regarding magnetic properties, a ring-shaped test piece having an inner diameter of 33 mm, an outer diameter of 45 mm, and a thickness of 5 mm was taken from the center of the obtained steel bar so that the ring plate surface was parallel to the cross section of the steel bar, and the primary winding was 100 times. Measured the iron loss W _10/50 when the secondary winding was applied 100 times and the magnetic flux density B ₁₀ , B _{50 at} DC excitation current 1000 and 5000 A / m and the magnetic flux density 1.0 T at 50 Hz AC were excited. .
For machinability, the tool life was defined as the time when the flank wear of the tool reached 0.2 mm after cutting the outer circumference of the bar steel using a carbide tool P10 at a cutting speed of 200 m / min and no lubrication.

Tables 8 and 9 show the results of the above-described structure observation, tensile test, magnetic measurement, and tool life measurement.
The numbers in the table are for classifying the individual results, and the steel numbers and hot rolling conditions were combined to indicate the combination of the test steel and hot rolling conditions. For example, when steel No. 1 was hot-rolled under condition A, it started as 1-A.
Regarding the structure, ferrite is abbreviated as T when a low temperature transformation phase such as F, bainite or martensite is generated and its volume fraction exceeds 5%. The average particle size is described for the precipitate. The variation in particle diameter was ± 1 nm at the maximum for precipitates of less than 10 nm, and ± 3 nm to ± 5 nm for precipitates larger than that. In the case where a low temperature transformation phase was generated in the structure, the measurement of the crystal grain size and the particle size of the precipitate was omitted.

Table 8 shows the influence of the steel composition on the hot rolling conditions within the scope of the present invention, and as is clear from the table, in the invention examples in which both the steel composition and the hot rolling conditions satisfy the scope of the present invention. Yield strength (yield stress) of 550 MPa or more has been obtained, and the magnetic properties of magnetic flux density B _{10 at} an excitation current of 1000 A / m are 1.0 T or more, magnetic flux density B _{50 at} 5000 A / m is 1.65 T or more, and frequency. The iron loss W _10/50 when excited to a magnetic flux density of 1.0T at 50Hz is 36W / kg or less, showing excellent magnetic properties. Moreover, the tool life at the time of cutting also shows a value of 30 min or more.

On the other hand, in the comparative example in which the steel composition is out of the range of the present invention, at least one of the yield strength and the magnetic property shows a low value.
No. 13-A has a low C and a small amount of fine precipitates, resulting in a low yield strength.
No. 14-A is high in C, the precipitates are coarsened, and the yield strength is low. When the precipitate is coarse, the precipitate adversely affects the magnetic properties as described above. Therefore, the magnetic flux density B ₁₀ is as low as 0.86 T, B ₅₀ is as low as 1.56 T, and the iron loss W _10/50 is 40 W / kg. Thus, the magnetic properties are inferior.
In No. 15-A, since Mn was low, ferrite transformation and precipitation did not compete sufficiently, and as a result of precipitation of coarse precipitates, yield strength was low and magnetic properties were inferior. On the other hand, in No. 16-A having a high Mn, a low-temperature transformation phase is generated and the yield strength is low because precipitation strengthening due to fine precipitates is insufficient. Although seems to be due to formation of the low-temperature transformation phase, the magnetic flux density B ₁₀ is 0.94T, B ₅₀ is 1.55T and low.

No. 17-A has a low yield strength due to a low amount of fine precipitates because Ti is low. On the other hand, in No. 18-A where Ti is high, precipitates are coarsened, yield strength is low, magnetic flux density B ₅₀ is low, and iron loss W _10/50 is also inferior.
No. 19-A has a low Mo, so the amount of fine precipitates is insufficient and the yield strength is low. On the other hand, in No. 20-A with high Mo, a low-temperature transformation phase is generated, and precipitation strengthening due to fine precipitates is insufficient, resulting in low yield strength and poor magnetic properties.

Table 9 shows the results of annealing after hot rolling steel No. 2 which is the steel of the present invention under various conditions. As is clear from the table, in the inventive examples, a high yield strength of 600 MPa or more was obtained. As for magnetic characteristics, the magnetic flux density B ₁₀ is 0.90 T or more, B ₅₀ is 1.60 T or more, and the iron loss W _10/50 is 40 W / kg or less. The tool life is also excellent machinability of 25 min or more. Among the inventive examples, No. 2-H steel and No. 2-N steel are not annealed, so the area ratio of ferrite with a grain size of 10 μm or less is high, and B ₁₀ is lower than annealed steel. is there.
On the other hand, in the comparative example where the hot rolling conditions or annealing conditions are out of the scope of the present invention, the yield strength,
At least one of magnetic properties and machinability exhibits a low value.

Since No. 2-G steel has a high annealing temperature, precipitates are dissolved, and the second phase is precipitated during cooling. As a result, the strength is low and the magnetic properties are inferior.
No. 2-I steel and No. 2-O steel have a low area reduction ratio of the rolling start path in hot rolling, so the area ratio of fine ferrite with a grain size of 10 μm or less is large, and magnetic density B ₁₀ is high. Low level.
No. 2-J steel has a low reduction in area in the final rolling pass in hot rolling, and the average crystal grain size of ferrite exceeds the upper limit of 80 μm of the present invention, so that the machinability is inferior.
No. 2-P steel has a high area reduction ratio in the final rolling pass in hot rolling, the average grain size of ferrite is below the lower limit of 30 μm of the present invention, and the area ratio of ferrite of 10 μm or less is also low. Since it is high, the magnetic properties are low.

In addition to the area reduction ratio in hot rolling, it is necessary to optimize heating temperature, finishing temperature, and cooling rate. In the comparative example in which these manufacturing conditions are outside the scope of the present invention, the yield strength is low and the magnetic properties are low.
Since No. 2-Q steel has a low heating temperature, the slab precipitate before hot rolling does not sufficiently dissolve in the heating furnace, and the precipitate becomes coarse. As a result, in addition to the low yield strength, the magnetic properties are inferior.
In No. 2-U steel, the finishing temperature is low, and the strain introduced by rolling increases the starting temperature of the ferrite transformation and inhibits the competition between ferrite transformation and precipitation. As a result, the precipitates become coarse and the yield strength decreases, and the magnetic properties deteriorate.
No. 2-V steel is an example in which the cooling rate after hot rolling is excessive, and mist cooling was performed after rolling in order to increase the cooling rate. When this cooling rate is fast, a low-temperature transformation phase is generated, and fine precipitates are not sufficiently precipitated even after annealing, so that the yield strength is lowered. Also, the magnetic properties were low.

It is a figure which shows the relationship between the average particle diameter of a ferrite, the area ratio of a ferrite with a particle size of 10 micrometers or less, magnetic flux density _B50 , and iron loss _{W10 / 50} . FIG. 3 is a diagram showing the relationship between the average particle diameter of ferrite and the area ratio of ferrite having a particle diameter of 10 μm or less and the cutting tool life. It is a figure which shows the relationship between the average particle diameter of a ferrite, the area ratio of a ferrite with a particle size of 10 micrometers or less, magnetic flux density _B50 , and iron loss _{W10 / 50} . FIG. 3 is a diagram showing the relationship between the average particle diameter of ferrite and the area ratio of ferrite having a particle diameter of 10 μm or less and the cutting tool life. It is a figure which shows the relationship between the average particle diameter of a ferrite, the area ratio of a ferrite with a particle size of 10 micrometers or less, magnetic flux density _B50 , and iron loss _{W10 / 50} . FIG. 3 is a diagram showing the relationship between the average particle diameter of ferrite and the area ratio of ferrite having a particle diameter of 10 μm or less and the cutting tool life.

Claims (14)

% By mass C: 0.04 to 0.12%,
Si: 0.5% or less,
Mn: 0.5-3.0%
Al: 0.1% or less,
Ti: 0.03-0.35% and
Mo: 0.05-0.8%
And the composition of the remaining Fe and inevitable impurities, the average crystal grain size is a ferrite single phase structure of 30μm to 80μm, the area ratio of ferrite with a particle size of 10μm or less is 20% or less An electromagnetic steel bar characterized by fine precipitates having a particle size of less than 10 nm dispersed in ferrite. The electromagnetic bar steel according to claim 1, wherein the component composition satisfies the following formula (1).
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (1)
However, the chemical component display indicates the content (% by mass) of the component.   The electromagnetic bar steel according to claim 1 or 2, wherein the fine precipitate is a carbide of Ti and Mo. The component composition is further mass%.
Nb: 0.08% or less,
The electromagnetic bar steel according to claim 1, comprising one or more of V: 0.15% or less and W: 1.5% or less. The electromagnetic bar steel according to claim 4, wherein the component composition satisfies the following formula (2).
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50… (2)
However, the chemical component display indicates the content (% by mass) of the component.   The electromagnetic bar steel according to claim 4 or 5, wherein the fine precipitate is a carbide containing Ti and Mo and containing at least one of Nb, V and W. The component composition further includes S: 0.01 to 0.1% by mass%, and
Pb: 0.2% or less,
Ca: 0.005% or less,
The electromagnetic bar steel according to any one of claims 1 to 6, comprising one or more of Bi: 0.1% or less and B: 0.02% or less.   The electromagnetic bar steel according to claim 1, wherein an area ratio of the ferrite having a particle size of 10 μm or less is 5% or less. % By mass C: 0.04 to 0.12%,
Si: 0.5% or less,
Mn: 0.5-3.0%
Al: 0.1% or less,
Ti: 0.03-0.35% and
Mo: 0.05-0.8%
The steel material having the component composition of the remaining Fe and unavoidable impurities is heated to 1100 ° C. or higher, and then hot rolling is performed.The area reduction rate in the start pass is 25% or more, and the area reduction rate in the final pass is 15%. % To 35% and a finishing temperature of 880 ° C. or higher, and then cooling at a cooling rate of 1.0 ° C./s or lower. The said steel raw material satisfy | fills following (1) Formula, The manufacturing method of the electromagnetic bar steel of Claim 9 characterized by the above-mentioned.
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (1)
However, the chemical component display indicates the content (% by mass) of the component. The steel material is further mass%.
Nb: 0.08% or less,
The method for producing an electromagnetic steel bar according to claim 9, comprising one or more of V: 0.15% or less and W: 1.5% or less. The said steel raw material satisfy | fills following (2) Formula, The manufacturing method of the electromagnetic bar steel of Claim 11 characterized by the above-mentioned.
Record
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50… (2)
However, a chemical component shows content (mass%) of the said component. The steel material further includes S: 0.01 to 0.1% by mass%, and
Pb: 0.2% or less,
Ca: 0.005% or less,
The method for producing an electromagnetic steel bar according to any one of claims 9 to 12, comprising one or more of Bi: 0.1% or less and B: 0.02% or less. The hot rolling is performed in the following temperature range T with respect to the steel bar manufactured by the method according to any one of claims 9 to 13, with a reduction in area in the start pass being 30% or more. A method for producing electromagnetic steel bars.
Record
When the Mn content is 1.7% or less: 600 ° C. ≦ T ≦ 800 ° C.
When Mn content exceeds 1.7%: 600 ℃ ≦ T ≦ 750 ℃

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