JP5085964B2 - Electromagnetic bar and its manufacturing method - Google Patents

Description

  The present invention relates to an electromagnetic steel bar having high strength and excellent magnetic properties and a method for producing the same.

  In recent years, as represented by main motors of electric vehicles and hybrid electric vehicles, motors are strongly required to save energy and increase efficiency.

  For example, in order to save energy and increase the efficiency of a motor, one of the effective means is to increase the frequency, but as the frequency increases, the rotational speed of the motor also increases, and the centrifugal force applied to the core constituting the rotor increases. Since the force also increases, the core material is required to have a high yield strength. That is, when the yield strength of the core material is insufficient, the core material causes plastic deformation due to centrifugal force, and the motor performance deteriorates because the air gap between the rotor core and the stator core changes from the design value, Furthermore, the rotor and stator come into contact during rotation, resulting in damage to the motor. Therefore, in order to save energy and increase the efficiency of the motor by increasing the frequency, it is essential to increase the strength of the rotor core material.

  By the way, the conventional manufacture of rotor cores was generally performed by laminating electromagnetic steel sheets with a thickness of 0.35 to 0.5 mm. However, hundreds of electromagnetic steel sheets were punched one by one into a predetermined core shape. Since a great deal of cost is required for lamination, motors for producing rotors using electromagnetic steel bars that do not require lamination instead of electromagnetic steel sheets have 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. Further, in the example of the electromagnetic steel sheet, as disclosed in Patent Document 1, even if the purpose is to increase the strength, the yield strength is approximately 300 to 450 MPa, and sufficient yield strength is not obtained.

  Further, Patent Document 2 discloses a heat for a rotating iron 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. It describes about a rolled steel sheet.

However, in Patent Document 2, after defining the components of steel, the steel structure is used as ferrite, fine precipitates are dispersed and precipitated in ferrite, and the ratio of the length of the long side to the short side of the carbide is specified. It is said that a hot-rolled steel sheet having both excellent workability and magnetic properties can be obtained, but no consideration is given to domain wall motion that is important in terms of magnetic properties, and it cannot be said that it has practically sufficient magnetic properties. . 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, although the magnetic flux density in a lower magnetic field region as represented by the magnetic flux density B ₅₀ at 5000 A / m is important for the properties of a motor or the like, the steel plate described in Patent Document 2 has a high B _50. Cannot be obtained. Further, no consideration was given to the iron loss that dominates the efficiency of the motor or the like, and the iron loss value was high, so that practically sufficient magnetic properties were not achieved.
JP 2002-371340 A JP 2003-268509 A

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

This invention is based on said knowledge, The summary structure 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%
Includes having a component composition of the balance Fe and unavoidable impurities, consists of the average crystal grain size 60μm or more ferrite area ratio is 95% or more of tissue, fine precipitates having a size of less than 10nm in the ferrite is dispersed An electromagnetic steel bar characterized by

2. 2. The electromagnetic bar steel according to 1, wherein the structure has an area ratio of ferrite having a particle size of 10 μm or less of 10% or less.

3. The electromagnetic bar steel according to 1 or 2, 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.

4). 4. The electromagnetic bar steel according to any one of 1 to 3 , wherein the fine precipitate is a carbide of Ti and Mo.

5. As the above component composition,
Nb: 0.08% or less,
The electromagnetic bar steel according to 1 or 2 above, comprising one or more of V: 0.15% or less and W: 1.5% or less.

6). The electromagnetic bar steel according to 5, 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.

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

8). As said component composition, it is further mass% S: 0.01-0.1%
And including
Pb: 0.2% or less,
Ca: 0.005% or less,
The electromagnetic bar steel according to any one of 1 to 7 above, which contains one or more of Bi: 0.1% or less and B: 0.02% 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%
Steel material with the remaining Fe and unavoidable impurity composition is heated to 1100 ° C or higher, then hot rolled under conditions of area reduction of 25% or less in the final pass and finishing temperature: 880 ° C or higher And then cooling at a cooling rate of 1.0 ° C./s or less.

Here, the area reduction rate in the final pass is as follows.
Area reduction rate (%) = (D−d) / D × 100
Where D: sectional area before rolling in the final pass
d: Cross-sectional area after rolling in the final pass

10. 10. The method for producing an electromagnetic steel bar according to 9, wherein the cooling is further performed in the following temperature range after the cooling.
Record
When Mn content (% by mass) is 0.5 to 1.7%: 600 ° C or higher and 800 ° C or lower
When Mn content (% by mass) is over 1.7% to 3.0%: 600 ° C or higher and 750 ° C or lower

11. The said component composition satisfy | fills following (1) Formula, The manufacturing method of the electromagnetic bar steel of said 9 or 10 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.

12 As the above component composition,
Nb: 0.08% or less,
11. The method for producing electromagnetic bar steel according to 9 or 10 above, comprising one or more of V: 0.15% or less and W: 1.5% or less.

13. 13. The method for producing an electromagnetic steel bar according to 12, 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.

14 As said component composition, in mass% S: 0.01-0.1%
And including
Pb: 0.2% or less,
Ca: 0.005% or less,
14. The method for producing an electromagnetic steel bar according to any one of 9 to 13 above, comprising one or more of Bi: 0.1% or less and B: 0.02% or less.

  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.

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 a 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 amount of Si is 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 and precipitate of the ferrite transformation generated during cooling after rolling. 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. Here, 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, but the effect is obtained. It is necessary to add 0.5% or more of Mn. On the other hand, if the amount of Mn exceeds 3.0%, a low-temperature transformation phase such as bainite is generated in addition to ferrite, resulting in insufficient precipitation strengthening due to fine precipitates, resulting in a decrease in strength. Furthermore, when the low temperature transformation phase is generated, the magnetic properties are deteriorated. For this reason, the upper limit of Mn is set to 3.0%.
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 2.80%.

Al: 0.1% or less
Al may be added as a deoxidizing agent, but if it is added excessively, not only the effect is saturated, but also the amount of AlN that is a precipitate with N increases, and this AlN precipitates with a diameter of less than 10 nm. Therefore, the magnetic characteristics are deteriorated. In order to avoid this, the amount of Al added is 0.1% or less. More preferably, it is 0.05% or less. When used as a deoxidizer, 0.01% or more is preferable.

Ti: 0.03-0.35%
Ti is added to finely precipitate precipitates including Ti-based carbides and Ti-Mo-based carbides, and improve strength. In other words, 0.03% or more is necessary to ensure a high yield strength. On the other hand, if it exceeds 0.35%, precipitates become coarse and the strength decreases. Therefore, Ti is added in the range of 0.03 to 0.35%. And 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, and strength is lowered and magnetic properties are deteriorated. For this reason, the addition of Mo is set to 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 in making the precipitate finer.
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, in the fine Ti-Mo type carbide, Ti and Mo in the carbide were observed to have an atomic ratio Ti / Mo of 0.2 to 2.0, and in the finer carbide, 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 forms a fine precipitate together with Ti and Mo and contributes to an increase in strength. Moreover, ductility and toughness are improved by adjusting the grain size of ferrite. In order to obtain these effects, 0.005% or more is preferable. However, if the content exceeds 0.08%, the ferrite becomes finer and fine precipitates adversely affect the magnetic properties. Therefore, 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 increasing the strength. Therefore, V is preferably added at 0.005% or more, but if it exceeds 0.15%, the precipitates become coarse. Therefore, the addition amount is 0.15% or less. More preferably, it is 0.10% or less.

W: 1.5% or less W also forms fine precipitates together with Ti and Mo and contributes to increasing the strength. Therefore, W is preferably added at 0.01% or more, but if it exceeds 1.5%, the precipitates become coarse. Therefore, the addition amount is 1.5% or less. More preferably, it is 1.0% or less.

When these elements are added, if the atomic ratio of these elements and the amounts of C, Ti, and Mo is defined as shown in the following formula (2), it is advantageous for 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, formation of fine precipitates having a particle size of less than 10 nm is facilitated, which is preferable.

  In addition, in the fine carbide containing one or more of Nb, V, and W, the atomic ratio (Ti + Nb + V) / (Mo + W) of Ti, Mo, Nb, V, W in the carbide is 0.2 to 2.0, and is finer It was observed to be 0.7 to 1.5 for the fine carbides.

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% or Two or more kinds 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 deteriorate. 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 it can be set as 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 reduced. Therefore, the addition amount is Pb ≦ 0.2%, Ca ≦ 0.005%, Bi ≦ 0.1% and It is necessary to set B ≦ 0.02%.

  In addition, for the purpose of improving strength, 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%.

P and N, which are inevitable impurities, are elements that are not preferable for the magnetic properties, so it is desirable to reduce P and N. 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, it is important to define the microstructure as a structure in which the area ratio of ferrite having an average crystal grain size of 60 μm or more is 95% or more and fine precipitates having a grain size of less than 10 nm are dispersed and precipitated. This is due to the following reason.

First, the area ratio of ferrite of 95% or more, to preferably 98% or more, since ferrite phase is the most preferred tissues for magnetic properties. In addition, the area ratio of the ferrite in this invention is calculated | required by cross-sectional structure | tissue observation (200 times optical microscope structure | tissue observation) . Hereinafter , a structure having a ferrite area ratio of 95% or more is referred to as a ferrite single-phase structure .

  Furthermore, in the present invention, the average crystal grain size of ferrite is set to 60 μm or more. This is because even if the precipitate has been considered to be harmful to the magnetic properties in the past, if it is fine as less than 10 nm, the adverse effect can be eliminated by increasing the average crystal grain size of ferrite. . This point will be described in detail below.

  Component composition is within the scope of the present invention, C: 0.068%, Si: 0.11%, Mn: 1.48%, Ti: 0.24%, Mo: 0.28%, P: 0.018%, S: 0.023%, Al: 0.029% and N : Steel containing 0.0041% and the balance being iron and inevitable impurities (steel 1), and the composition was outside the scope of the present invention, C: 0.072%, Si: 0.08%, Mn: 1.38%, Ti: 0.41%, Mo: 0.16%, P: 0.015%, S: 0.017%, Al: 0.031% and N: 0.0030%, the balance being iron and inevitable impurities (steel 2) Steel was melted. These were heated to 1180 ° C., hot-rolled into a steel bar having a diameter of 100 mm, and air-cooled to room temperature after rolling (the average cooling rate up to 500 ° C. was 0.18 ° C./s). At that time, in order to change the crystal grain size, the rolling pass schedule (temperature of each reduction pass and the area reduction rate) in 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, the tensile test value was measured by taking a specimen having a parallel part diameter of 6 mm and a parallel part length of 40 mm 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. It was used for.

Regarding the 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. In addition, 100 times of secondary windings were applied, 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 structure observation, specimens for structure observation were collected from 20 positions at any position on the steel bar, and the structure was identified, and the average cross-sectional area of the crystal grains was determined for each specimen by the cutting method of JIS G 0552. From this, the crystal grain size of each test piece was calculated as the diameter of the equivalent circle, and the average value of a total of 20 locations was determined to determine the average crystal grain size of the entire steel bar. Furthermore, the magnitude | size of the deposit was evaluated by the electron microscope observation explained in full detail behind.

  As a result of the structure observation, both steels 1 and 2 had a ferrite single phase structure regardless of the rolling conditions. However, since the pass schedule in hot rolling was changed to change the crystal grain size, Changed from about 30 μm to about 100 μm. Further, regarding the size of the precipitate, the steel 1 having the component composition within the range of the present invention had a fineness of about 5 nm, but the steel 2 having the component composition outside the range of the present invention had a coarseness of 20 to 30 nm. It was.

Next, FIG. 1 shows the relationship between the ferrite grain size, the yield strength YS, the magnetic flux density B ₅₀ and the iron loss W _10/50 .
As can be seen from the figure, both the steel 1 whose component composition falls within the scope of the present invention and the steel 2 whose component composition falls outside the scope of the present invention both show that the yield strength YS tends to decrease as the ferrite grain size increases. Steel 1 with a fineness of 5 nm gives a sufficiently high yield strength of 500 MPa or more, whereas steel 2 with a coarse precipitate of 20 to 30 nm gives a yield strength of less than 500 MPa.

The magnetic properties also vary depending on the ferrite grain size, but the grain size dependence differs greatly between Steel 1 and Steel 2. In Steel 1, which has a component composition within the range of the present invention and a precipitate diameter of 5 nm, when the ferrite grain size is small, the magnetic flux density is as low as around 1.59 T and the iron loss is as high as about 44 W / kg. When the particle size is 60 μm or more, both the magnetic flux density and the iron loss are remarkably improved. The magnetic flux density is 1.63 T or more and the iron loss is 40 W / kg or less, and excellent magnetic properties are exhibited.
On the other hand, in Steel 2, which has a component composition that is out of the scope of the present invention and the precipitate is 20 to 30 nm, the magnetic flux density increases and the iron loss decreases as the ferrite grain size increases. The magnetic flux density is only about 1.59 T, and the iron loss is 40 W / kg or more, showing only low magnetic properties.

Thus, when the precipitate is made as fine as 5 nm, high yield strength is obtained, and when the average crystal grain size is increased to 60 μm or more, the influence of the precipitate, which has been considered harmful to magnetic properties, is suppressed. Excellent magnetic properties can be obtained.
The above examination was performed on steels with various changes in the size of the precipitates. When the size of the precipitates was less than 10 nm, the ferrite particle size was set to 60 μm or more, so that high magnetic flux density B ₅₀ and iron Since the loss W _10/50 was obtained, the ferrite grain size is defined as 60 μm or more in the present invention.

  Although it is not always clear why the grain size of the ferrite is refined and the ferrite grain size is increased, the adverse effect of the precipitate is suppressed and excellent magnetic properties are obtained. The relationship is suggested.

That is, in general, it is preferable that the domain wall movement is easy in terms of magnetic characteristics, and precipitates are considered to adversely affect the magnetic characteristics by preventing the domain wall movement. 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, when the domain wall length is sufficiently long and the precipitates are sufficiently fine, the influence of the precipitates can be ignored from the relative relationship between the domain wall movement deterring force due to the precipitates and the driving force itself of the domain wall movement. It is presumed that For this reason, it is considered that if the ferrite grain size is increased after the precipitates are refined, excellent magnetic properties are provided.

Furthermore, in the present invention, the particle size of the fine precipitate is less than 10 nm. When the grain size of the precipitate is 10 nm or more, the precipitation strengthening ability is insufficient and a high yield strength cannot be obtained.
That is, the smaller the particle size of the fine precipitates, the more effective the strength increase, and it is less than 10 nm, preferably 5 nm, and more preferably 3 nm or less. As such fine precipitates, carbides containing a composite of Ti and Mo, and carbides further containing one or more of Nb, V and W are preferable. The fine precipitate is precipitated during cooling after hot rolling.

The number of fine precipitates is preferably 1000 / μm ³ or more, and more preferably 5000 / μm ³ or more from the viewpoint of securing strength.

  These fine precipitates are desirably dispersed and precipitated uniformly in the matrix phase. In the present invention, if the size of the precipitates satisfies 90% or more of the total precipitates, the high yield strength can be obtained. However, since the precipitate having a size of 10 nm or more consumes the precipitate-forming element 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 Fe carbide contained is preferably 1 μm, and the content is preferably 1% or less of the entire precipitate.

The size of the precipitate and the ratio of the fine precipitate to the total precipitate are obtained by the following method.
As an electron microscope sample, a sample prepared by an electropolishing method using a twin jet method is used and observed at an acceleration voltage of 200 kV. At that time, the focus is shifted from the normal focus in order 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 number of precipitates. Observe by method. 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 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 measurement of the particle diameter and the number of particles is performed on four places 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, the number per particle diameter distributed per 1 μm ^{3 of the} precipitate is calculated from this value and the thickness of the sample. 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.
Further, the area ratio of ferrite having a particle size of 10 μm or less is preferably 10% or less in order to obtain a higher magnetic flux density. This point will be described in detail below.

  Component composition is within the range of the present invention, C: 0.095%, Si: 0.15%, Mn: 1.50%, Ti: 0.22%, Mo: 0.44%, P: 0.022%, S: 0.018%, Al: 0.015% and N : Steel containing 0.0053%, the balance being iron and inevitable impurities (steel 3), and the composition was outside the scope of the present invention, C: 0.060%, Si: 0.05%, Mn: 1.25%, Ti: 0.65%, Mo: 0.12%, P: 0.02%, S: 0.008%, Al: 0.020% and N: 0.0045%, the balance with steel (steel 4) having a component composition of iron and inevitable impurities Two types of steel were melted. These were heated to 1200 ° C. and hot-rolled into a steel bar having a diameter of 100 mm, and after rolling, the steel was cooled to room temperature at an average cooling rate of up to 500 ° C. at 0.18 ° C./s. At that time, in order to change the crystal grain size, various rolling schedules (temperature of rolling pass and area reduction ratio) and subsequent annealing temperatures in hot rolling were changed. The obtained steel bars were observed for the structure, and the tensile test values and magnetic properties were measured.

  Here, the tensile test value was measured by taking a specimen having a parallel part diameter of 6 mm and a parallel part length of 40 mm 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. It was used for.

Regarding the 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. In addition, the magnetic flux density B ₅₀ at a DC excitation current of 5000 A / m and the iron loss W _10/50 when excitation was performed up to a magnetic flux density of 1.0 T at an AC 50 Hz were measured.

  In addition, for structure observation, specimens for tissue observation were collected from arbitrary positions 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 area ratio of ferrite grains having a particle size of 10 μm or less (hereinafter abbreviated as “Ρ”) was determined by image analysis after blackening crystal grains having a particle size of 10 μm or less in the structure photograph in each field of view. Furthermore, the size of the precipitate was evaluated by observation with an electron microscope, which will be described later.

  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, so the individual ferrite grain size was It changed from about 2 μm to about 100 μm.

  Further, regarding the size of the precipitate, the steel 3 having the component composition within the range of the present invention had a fineness of about 5 nm, but the steel 4 having the component composition outside the range of the present invention had a coarseness of 20 to 30 nm. It was.

FIG. 2 shows the relationship between the average crystal grain size of ferrite, the yield strength YS, the magnetic flux density B ₅₀ and the iron loss W _10/50 .
As can be seen from the figure, both the steel 3 whose component composition falls within the scope of the present invention and the steel 4 whose component composition falls outside the scope of the present invention both show that the yield strength YS tends to decrease as the ferrite grain size increases. In steel 3 with a fineness of 5 nm, a sufficiently high yield strength of 520 MPa can be obtained at least, whereas in steel 4 with a coarse precipitate of 20-30 nm, a yield strength of less than 500 MPa can be obtained at most.

The magnetic properties also vary depending on the ferrite grain size, but the grain size dependence differs greatly between Steel 3 and Steel 4. Steel 3 having a component composition within the range of the present invention and having a precipitate as fine as 5 nm exhibits excellent magnetic properties when the ferrite grain size is small and when the ferrite grain size is 10 μm or less. That is, when the ferrite grain size is 10% or less, the ferrite average grain size is 60 μm or more, the magnetic flux density B ₅₀ is 1.67 T or more, and the iron loss is 34 W / kg or less. On the other hand, if the ferrite grains having a grain size of 10 μm or less exceed 10%, the average magnetic grain diameter is 60 μm or more, the magnetic flux density is 1.66 T at the maximum, and the iron loss is 35 W / kg or more.

  On the other hand, in the steel 4 in which the component composition is outside the range of the present invention and the precipitate is 20 to 30 nm and coarse, the magnetic flux density tends to increase and the iron loss decreases as the ferrite grain size increases. 1.60T or less, iron loss is 40W / kg or more, showing only low magnetic properties.

  Similarly, the influence of the average grain size and the area ratio of ferrite grains having a grain size of 10 μm or less (hereinafter abbreviated as 同 様) on steels 5 and 6 having a higher Mn content than steels 3 and 4 The results of the investigation are shown in FIG. Here, the composition of steel 5 is within the range of the present invention, 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 being steel having a component composition of iron and unavoidable impurities, steel 6 having a component composition outside the scope of the present invention, C: 0.072%, Si: Contains 0.08%, Mn: 2.20%, Ti: 0.46%, Mo: 0.12%, P: 0.015%, S: 0.017%, Al: 0.031% and N: 0.0030%, the balance being the composition of iron and inevitable impurities It has steel.

FIG. 3 shows the relationship between the average crystal grain size of ferrite, the yield strength YS, the magnetic flux density B50, and the iron loss W10 / 50.
As can be seen from the figure, both the steel 5 whose component composition falls within the scope of the present invention and the steel 6 whose component composition departs from the scope of the present invention show a tendency that the yield strength YS tends to decrease as the ferrite grain size increases. In steel 5 with a fineness of 5 nm, a sufficiently high yield strength of at least 550 MPa is obtained, whereas in steel 6 with a coarse precipitate of 20-30 nm, a yield strength of less than 530 MPa is obtained.

The magnetic properties also vary depending on the ferrite grain size, but the grain size dependence differs greatly between Steel 5 and Steel 6. Steel 5 having a component composition within the range of the present invention and a fine precipitate of 5 nm exhibits excellent magnetic properties when the ferrite grain size is small and the area ratio of ferrite having a grain size of 10 μm or less is 10% or less. That is, if the area ratio of ferrite having a particle size of 10 μm or less is 10% or less, the ferrite average particle size is 60 μm or more, the magnetic flux density B ₅₀ is 1.66 T or more, and the iron loss is 35 W / kg or less. Show properties. On the other hand, when the ferrite area ratio exceeds 10%, even if the average ferrite grain size is 60 μm or more, the magnetic flux density is 1.66 T at the maximum, and the iron loss exceeds 35 W / kg.

  On the other hand, in the steel 6 having a component composition outside the scope of the present invention and having a precipitate as coarse as 20 to 30 nm, the magnetic flux density tends to increase and the iron loss decreases as the ferrite grain size increases. 1.60 T or less and iron loss of 40 W / kg or more, showing only low magnetic properties.

Thus, when the precipitate is made as fine as 5 nm, high yield strength is obtained, and the influence of the precipitate, which has been considered harmful to magnetic properties, is reduced by increasing the average crystal grain size to 60 μm or more. Further, by setting the area ratio of ferrite having a particle size of 10 μm or less to 10% or less, further excellent magnetic properties can be obtained. And when such a study was conducted on steels with various changes in the size of the precipitates, when the size of the precipitates was less than 10 nm, the ferrite grain size should be 60 μm or more and the wrinkles should be 10% or less. in a high magnetic flux density B ₅₀ and low iron loss W _{lO / 50} was obtained.

Furthermore, in order to confirm this point, the same examination was performed on steels with various chemical compositions, precipitate diameters, and ferrite particle diameters. When the precipitate size was less than 10 nm, the average particle diameter of ferrite was determined. When the area ratio of ferrite having a grain size of 10 μm or less is set to 10 μm or less in addition to 60 μm or more, it has been clarified that high magnetic flux density B ₅₀ and low iron loss W _10/50 can be obtained. Therefore, from the viewpoint of magnetic properties, it is preferable that the area ratio of ferrite having an average grain size of ferrite of 60 μm or more and 10 μm or less is 10% 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 the magnetic properties is suppressed is not necessarily clear, but the relationship between the deterring force against domain wall motion and the crystal grain size is suggested. Is done.

  That is, in general, it is preferable that the domain wall movement is easy in terms of magnetic characteristics, and the precipitates have an adverse effect on the magnetic characteristics by preventing the domain wall movement. 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, when the domain wall length is sufficiently long and the precipitate is sufficiently fine, the influence of the precipitate on the domain wall movement is affected by the relative relationship between the domain wall movement deterring force by the precipitate and the driving force of the domain wall movement. It is assumed that it will be virtually 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 the large part of ferrite, but in the small part of ferrite, the fracture wall movement is hindered by the precipitates. It will be rate limited to some extent. 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. Therefore, 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 effect of precipitates on the magnetic properties will be lower than when the proportion of fine ferrite is not specifically considered it is conceivable that.

[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.

Area reduction ratio In the present invention, in order to obtain excellent magnetic properties, the average crystal grain size of ferrite needs to be 60 μm or more, and therefore the area reduction ratio in hot rolling is controlled. In that case, it is effective to reduce the area reduction rate in the final pass of the hot rolling. Specifically, since ferrite with a grain size of 60 μm or more can be obtained when this is 25% or less, the upper limit of the area reduction rate in the final pass of hot rolling is 25%.

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 the ferrite transformation and precipitation, it is necessary to lower the ferrite transformation start temperature, but when the finishing temperature in hot rolling is low, the strain introduced in the rolling increases the ferrite transformation start temperature, Impairs 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. In that case, if the cooling rate after hot rolling exceeds 1.0 ° C./s, a low-temperature transformation phase is generated, and 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. Further, 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 up to 500 degreeC, what is necessary is just to cool to 500 degreeC after hot rolling at a cooling rate of 1.0 degrees C / s or less.

By using a steel material having the above-described component composition and producing a steel bar according to the production conditions described above, a ferrite single-phase structure with an average crystal grain size of 60 μm or more is formed, and the ferrite has a grain size of less than 10 nm. A steel bar in which fine precipitates are dispersed can be obtained.
In the present invention, in addition, annealing may be performed after the hot rolling is completed in order to sufficiently precipitate fine precipitates and make the structure uniform so as to combine high strength, magnetic properties and machinability. . By making the annealing temperature appropriate, the area ratio of ferrite having a particle diameter of 10 μm or less can be made 10% or less as described above, and good magnetic properties can be obtained. Hereinafter, an appropriate range of the annealing temperature will be described.

Annealing temperature An experiment conducted for determining the annealing temperature range will be described in detail below.
Component composition is in the range of the present invention, C: 0.075%, Si: 0.09%, Mn: 1.62%, Ti: 0.24, Mo: 0.48%, P: 0.015%, S: 0.023%, Al: 0.025% and N : Steel A containing 0.0038% and the balance having a composition of iron and inevitable impurities was melted. After heating them to 1150 ° C, they were hot-rolled so that the area reduction rate in the hot rolling start pass was 32% and the area reduction rate in the final rolling pass was 15%, resulting in a steel bar with a length of 6m and a diameter of 120mm. . After rolling, the film was cooled to room temperature at an average cooling rate of up to 500 ° C at 0.165 ° C / s. Thereafter, the annealing temperature was varied in the range of 500 ° C to 850 ° C.

  Further, the component composition is within the scope of the present invention, C: 0.069%, Si: 0.05%, Mn: 2.23%, Ti: 0.26%, Mo: 0.52%, P: 0.010%, S: 0.022% and N: 0.0035 %, And the balance was made of steel B having the composition of iron and inevitable impurities. After heating them to 1180 ° C, they were hot-rolled to give a reduction in area of 35% in the hot rolling start pass and a reduction in area of 16% in the final rolling pass, resulting in a steel bar with a length of 6m and a diameter of 110mm. . After rolling, the film was cooled to room temperature at an average cooling rate of up to 500 ° C at 0.160 ° C / s. Thereafter, the annealing temperature was varied in the range of 500 ° C to 850 ° C.

The magnetic properties of the steel bar thus obtained were measured. As for magnetic properties, as shown in Fig. 4, a ring-shaped test piece with an inner diameter of 33 mm, an outer diameter of 45 mm and a thickness of 5 mm from a 1 / 4D position 1 m away from a cross section of 50 cm from the end face of the steel bar, and the ring plate surface of the bar Samples were taken so that they were parallel to the cross section, and were subjected to 100 primary windings and 100 secondary windings, and were excited to a magnetic flux density B ₅₀ at a DC excitation current of 5000 A / m and a magnetic flux density of 1.0 T at an AC 50 Hz. When the iron loss W _{lO / 50} was measured.

5 and 6 show the influence of the annealing temperature on the magnetic measurement results for steel A and steel B, respectively. According to FIG. 5, when steel A has an annealing temperature of 600 ° C. or more and 800 ° C. or less, there is little variation in magnetic properties, B ₅₀ is 1.67 T or more, and W _10/50 is 34 W / kg or less. In addition, as shown in FIG. 6, the steel B has an annealing temperature of 600 ° C. or more and 750 ° C. or less, and there is little variation in magnetic properties, B ₅₀ is 1.65 T or more, and W _10/50 is ₃₆ W / kg or less. It was. In contrast, non-tempered steel that does not undergo annealing after rolling, steel that has an annealing temperature of steel A and an annealing temperature outside the range of 600 ° C to 800 ° C, and steel B has an annealing temperature of 600 ° C to 750 ° C. Steels outside the range of ℃ or less showed a large variation in magnetic properties, and exhibited lower magnetic properties than when annealed in an appropriate temperature range.

If the annealing temperature is less than 600 ° C., fine precipitates cannot be precipitated, so that high strength cannot be achieved. In addition, the area ratio of ferrite having a particle size of 10 μm or less cannot be reduced to 10% or less, and the magnetic properties and machinability cannot be improved with respect to steel that is not annealed. Therefore, the annealing temperature is 600 ° C. or higher. On the other hand, if the annealing temperature is too high, fine precipitates are coarsened, and the second phase is generated 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 as follows:
When Mn content (% by mass) is 0.5 to 1.7%: 600 ° C or higher and 800 ° C or lower
When Mn content (mass%) is more than 1.7% to 3.0%: 600 ° C. or more and 750 ° C. or less.

[Example 1]
Steels having the compositions shown in Table 1 were melted, and these were hot-rolled into bar steels having predetermined dimensions under the conditions shown 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 rate was changed by changing the rolling finish dimensions and air cooling after rolling.

The steel bar thus obtained was subjected to a structure observation and a tensile test, and the magnetic properties were measured.
For structure observation, specimens for structure observation were collected from 20 locations in total, and the structure was identified, and the average cross-sectional area of the crystal grains was determined for each specimen by the cutting method of JIS G O552. From this, the crystal grain size of each test piece was calculated as the diameter of the equivalent circle, and the average value of a total of 20 locations was determined to determine the average crystal grain size of the entire steel bar.

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. The precipitate was identified. In the tensile test, a test piece having a parallel part diameter of 6 mm and a parallel part length of 40 mm was taken from the 1 / 4D position of the steel bar in the longitudinal direction of the steel bar and subjected to measurement. 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 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 and secondary. The iron loss W _10/50 was measured when 100 windings were applied, and the magnetic flux density B ₅₀ at a direct current excitation current of 5000 A / m and the magnetic flux density 1.0 T at an alternating current of 50 Hz were measured.

The results of the above-described structure observation, tensile test and magnetic measurement are shown in Tables 2 and 3.
No. in the table is for classifying individual results, and a number is assigned based on the combination of steel number and hot rolling conditions so that the combination of the test steel and hot rolling conditions is clearly indicated (for example, When the steel No. 1 was hot-rolled under the condition A, it was 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 the volume fraction exceeds 5% or more. For the precipitate, the average particle size is described. 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 effect of the steel composition with the hot rolling conditions as the scope of the present invention. As is apparent from the table, the present invention examples satisfying the scope of the present invention in both the steel composition and the hot rolling conditions. The yield strength of 500 MPa or more is obtained, and the magnetic properties of the magnetic flux density B ₅₀ are 1.60 T or more and the iron loss W _10/50 is 40 w / kg or less.

  On the other hand, in the comparative example in which the steel composition is out of the scope of the present invention, the yield strength is less than 500 MPa and the magnetic properties are inferior.

  No. 17-A has a low amount of C, a short amount of fine precipitates, and a low yield strength.

No. 18-A has a high C content, coarse precipitates, and low yield strength. When the precipitate is coarse, the precipitate adversely affects the magnetic properties as described in detail, so the magnetic flux density B ₅₀ is 1.59 T and the iron loss W _10/50 is about 40.4 W / kg. Inferior to magnetic properties.

In No. 19-A, since the Mn is low, ferrite transformation and precipitation do not sufficiently compete with each other, and the precipitate is coarsely precipitated, resulting in low yield strength. In addition, the iron loss W _10/50 is as high as 43.8 W / kg due to deterioration of magnetic properties due to coarsening of precipitates.

In No. 20-A having a high Mn, a low-temperature transformation phase is generated and precipitation strengthening due to fine precipitates is insufficient, so that the yield strength is low. Although seems to be due to formation of the low-temperature transformation phase, the magnetic flux density B ₅₀ is 1.55T and low.

No. 21-A has a low yield strength because Ti is low, resulting in an insufficient amount of fine precipitates. On the other hand, in No. 22-A with high Ti, the precipitates were coarsened, the yield strength was low, and both the magnetic flux density B ₅₀ and the iron loss W _10/50 were inferior.

No. 23-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. 24-A having a high Mo, a low-temperature transformation phase is generated and the yield strength is low because precipitation strengthening due to fine precipitates is insufficient. In addition, the magnetic flux density B ₅₀ is as low as 1.56 T, similarly to No. 20-A in which Mn is high and the low temperature transformation phase is generated.

Table 3 shows the results of hot pressing steel No. 3 which is the steel of the present invention under various conditions. As is clear from the table, steel No. 3 which is the steel of the present invention is a condition within the scope of the present invention. In the example of the present invention that was hot-rolled at a high yield strength of 570 MPa or higher, the magnetic properties were excellent with a magnetic flux density B ₅₀ of 1.60 T or more and an iron loss W _10/50 of 40 W / kg or less. The value is shown.

  Here, paying attention to the area reduction rate in the final pass of hot rolling, it can be seen that the ferrite grain size changes depending on the area reduction rate, and this affects the magnetic properties.

That is, when the area reduction rate is 25% or less (No. 3-D to No. 3-G), the ferrite particle size is 60 μm or more. When the precipitate is as fine as less than 10 nm, when the ferrite grain size is 60 μm or more, the adverse effect of the precipitate on the magnetic properties disappears, so the area reduction rate is 25% or less No. 3-D to No. 3-G Shows excellent magnetic properties. However, when the area reduction rate exceeds 25%, the ferrite is refined to a grain size of less than 60 μm (No. 3-H and No. 3-I), so even if the precipitate is less than 1 Onm, Precipitates have an adverse effect on magnetic properties, and magnetic flux density B
₅₀ has deteriorated to 1.58T and iron loss W _10/50 to 42 W / kg. As described above, in order to prevent the adverse effect of the precipitates on the magnetic properties and obtain high magnetic properties, it is necessary to reduce the area reduction rate in the final pass of hot rolling to 25% or less.

  In addition to the area reduction ratio in the final pass, 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.

  No.3-K has a low heating temperature, and the precipitate deposited on the slab before hot rolling does not sufficiently dissolve in the heating furnace. The strength is low, and the magnetic properties do not meet the target value. 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.3-N, the finishing temperature is low, and the strain introduced by rolling increases the starting temperature of the ferrite transformation and prevents the competition between the ferrite transformation and the precipitation. As a result, the precipitate becomes coarse and the yield strength decreases. Magnetic properties are degraded.

  No. 3-O 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 during cooling, and fine precipitates are not sufficiently precipitated, resulting in a decrease in yield strength.

[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 the magnetic properties were measured. For tissue observation, specimens for tissue observation were collected from arbitrary positions of the dedicated steel, a total of 20 locations, and the structure was identified. Select 100 grains for each test piece, obtain the cross-sectional area of these by image processing, and individually set the diameter of a total of 2000 grains as the diameter of the equivalent circle with the equivalent cross-sectional area. While calculating, the average grain size of the whole steel bar was calculated | required by calculating | requiring these average values.
Further, the area ratio of the ferrite grain diameter of 10 μm or less was determined by image analysis after blackening crystal grains of 10 μm or less in the structure photograph in each field of view.

Furthermore, the size of the precipitate was evaluated by observation with an electron microscope.
Furthermore, a thin film sample is prepared by electropolishing, and the particle size of the precipitate is measured by observing with a transmission electron microscope (EDX) according to the above-described method, and an energy dispersive X-ray spectrometer (EDX). In combination, precipitates were identified.
In the tensile test, a specimen having a parallel part diameter of 6 mm and a parallel part length of 40 mm was taken from the 1 / 4D position of the steel bar in the longitudinal direction of the steel bar and used for measurement.
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 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 and secondary. Iron loss W ₁₀ when 100 windings are applied and magnetic flux density B ₁₀ at DC exciting current 1000 A / m, magnetic flux density B ₅₀ at exciting current 5000 A / m, and magnetic flux density 1.0 T at 50 Hz AC are excited. _{/ 50} was measured.

Tables 5 and 6 show the results of the above-described structure observation, tensile test, and magnetic measurement.
No. in the table. Is used to classify the individual results, and the steel number and the hot rolling conditions were combined in order to clearly indicate the combination of the test steel and the hot rolling conditions (for example, the steel number 1 is the condition A). In case of hot rolling at 1-A, start with 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 the 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 the influence of the steel composition with the hot rolling conditions as the scope of the present invention. As is apparent from the table, the present invention examples satisfying the scope of the present invention in both the steel composition and the hot rolling conditions. , Yield strength of 500 MPa or more is obtained, and the magnetic properties are excellent with a fracture density B ₅₀ of 1.65 T or more and an iron loss W _10/50 of 34 W / kg or less.

On the other hand, in the comparative example in which the steel composition is out of the range of the present invention, the yield strength is low and less than 500 MPa, and the magnetic properties are inferior. No. 14-A has a low C, the precipitation amount of fine precipitates is insufficient, and the yield strength is low.
No. 15-A has high C, precipitates are coarsened, and yield strength is low. When the precipitate is coarse, the magnetic property is inferior because the precipitate adversely affects the magnetic property as described above.
In No. 16-A, since Mn is low, ferrite transformation and precipitation do not sufficiently compete with each other, and as a result of coarse precipitation, the magnetic properties are deteriorated.

In No. 17-A having a high Mn, a low-temperature transformation phase is formed, and the yield strength is low because precipitation strengthening due to fine precipitates is insufficient. Further, due to the generation of the low temperature transformation phase, the magnetic flux density B ₁₀ is as low as 0.93 T and B ₅₀ is as low as 1.59 T.
No. 18-A has a low yield strength due to a low amount of fine precipitates because Ti is low. On the other hand, in No. 19-A having a high Ti, the precipitates are coarsened, the yield strength is low, and both the magnetic flux density B ₅₀ and the iron loss W _10/50 are low.
No. 20-A is low in Mo, so the amount of fine precipitates is insufficient and yield strength is low. On the other hand, in No. 21-A having a 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. Similar to No. 17- A, which has a high Mn and also produced a low-temperature transformation phase, the magnetic flux density B ₅₀ is low.

Table 6 shows the results of annealing the steel No. 4 which is the steel of the present invention by hot rolling under various conditions. As is clear from the table, in the inventive examples, the yield strength is as high as 500 MPa or more. As for the magnetic characteristics, the magnetic flux density B ₁₀ is 1.00 T or more, B ₅₀ is 1.65 T or more, and the iron loss W _10/50 is 34 W / kg or less.

Since No. 4-G steel has a high annealing temperature, the precipitates are dissolved, and the second phase is precipitated during cooling. Therefore, the magnetic properties are inferior compared with the case where the annealing temperature is appropriate.
Since No. 4-H steel is not annealed, the area ratio of ferrite having a particle size of 10 μm or less is high, and the magnetic flux density B ₁₀ is lower than when the annealing temperature is appropriate.
No. 4-I steel and No. 4-N steel have a high area reduction ratio in the final rolling pass in hot rolling, the average crystal grain size of ferrite is below the lower limit of 60 μm of the present invention, and Since the area ratio of ferrite of 10 μm or less is high, the magnetic properties are low.

In addition to the area reduction ratio in hot rolling, it is necessary to optimize the heating temperature finishing temperature and the cooling rate. In comparative examples where these manufacturing conditions are outside the scope of the present invention, the yield strength is low.
That is, because the heating temperature of No. 4-O steel is low, the precipitate on the slab 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.
No.4-S steel has a low finishing temperature, and the strain introduced by rolling raises the starting temperature of the ferrite transformation and inhibits the competition between the ferrite transformation and precipitation. As a result, the precipitate becomes coarse and the yield strength decreases. In addition, the magnetic properties deteriorate.
The No. 4-T 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 the cooling rate is high, a low temperature transformation phase is precipitated, and even if annealing is performed, fine precipitates are not sufficiently precipitated, so that the yield strength is lowered.

[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 the magnetic properties were measured.
For tissue observation, specimens for tissue observation were collected from arbitrary positions of the steel bar, a total of 20 locations, and the structure was identified. Select 100 grains for each test piece, obtain the cross-sectional area of these by image processing, and individually set the diameter of a total of 2000 grains as the diameter of the equivalent circle with the equivalent cross-sectional area. While calculating, the average grain size was calculated | required by calculating | requiring these average values.
Further, the area ratio of the ferrite grain diameter of 10 μm or less was determined by image analysis after blackening crystal grains of 10 μm or less in the structure photograph in each field of view.

Furthermore, the size of the precipitate was evaluated by observation with an electron microscope.
Furthermore, a thin film sample is prepared by electropolishing, and the particle size of the precipitate is measured by observing with a transmission electron microscope (EDX) according to the above-described method, and an energy dispersive X-ray spectrometer (EDX). In combination, precipitates were identified.
In the tensile test, a test piece having a parallel part diameter of 6 mmφ and a parallel part length of 40 mm was taken from the 1 / 4D position of the steel bar in the longitudinal direction of the steel bar and used for measurement.
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 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 and secondary. Iron loss W ₁₀ when 100 windings are applied, and magnetic flux density B ₁₀ at DC excitation current 1000 A / m, magnetic flux density B ₅₀ at excitation current 5000 A / m, and magnetic flux density 1.0 T at 50 Hz AC _{/ 50} was measured.

Tables 8 and 9 show the results of the above-described structure observation, tensile test, and magnetic measurement.
No. in the table. Is used to classify the individual results, and the steel number and the hot rolling conditions were combined in order to clearly indicate the combination of the test steel and the hot rolling conditions (for example, the steel number 1 is the condition A). In case of hot rolling at 1-A, start with 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 the 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 of 500 MPa or more has been obtained, and the magnetic properties are excellent with magnetic flux density B ₅₀ of 1.63 T or more and iron loss W _10/50 of 35 W / kg or less.

On the other hand, in the comparative example in which the steel composition is out of the scope of the present invention, the yield strength is less than 500 MPa, and the magnetic properties are inferior.
No. 14-A has a low C, the precipitation amount of fine precipitates is insufficient, and the yield strength is low.
No. 15-A has high C, precipitates are coarsened, and yield strength is low. When the precipitate is coarse, the magnetic property is inferior because the precipitate adversely affects the magnetic property as described above.

In No. 16-A, since Mn is low, ferrite transformation and precipitation do not compete sufficiently, and as a result of precipitation of the precipitate coarsely, the strength is low and the magnetic properties are deteriorated.
No. with high Mn. In 17-A, a low-temperature transformation phase is generated, and the yield strength is low because precipitation strengthening due to fine precipitates is insufficient. Further, due to the formation of the low-temperature transformation phase, the magnetic flux density B ₁₀ is 0.94T, B ₅₀ is 1.55T and low.
In No. 19-A with high Ti , the precipitates are coarsened, the yield strength is low, and the magnetic flux density B ₅₀ and the iron loss W _10/50 are low.
No. 20-A has low Mo, so the amount of fine precipitates is insufficient and yield strength is low. On the other hand, No. 21-A having a high Mo generates a low-temperature transformation phase and lacks precipitation strengthening due to fine precipitates, resulting in low yield strength and poor magnetic properties. The magnetic flux density is low as in No. 17- A, which has a high Mn and also generates a low-temperature transformation phase.

Table 9 shows the result of hot rolling and annealing steel No. 2 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 1.00 T or more, B ₅₀ is 1.63 T or more, and the iron loss W _10/50 is 35 W / kg or less.
Since No.2-G steel has a high annealing temperature, the precipitate is dissolved and the second phase is precipitated during cooling. As a result, the magnetic properties are inferior compared to the case where the annealing temperature is appropriate.

Since No.2-H and No.2-M steels are not annealed, the area ratio of ferrite having a particle size of 10 μm or less is high, and the magnetic flux density B ₁₀ is lower than that in the case of annealing.
No. 2-I steel and No. 2-N steel have 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 60 μm of the present invention, and 10 μm or less. Since the area ratio of ferrite 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 comparative examples where these manufacturing conditions are outside the scope of the present invention, the yield strength is low.
Since the heating temperature of No. 2-O steel is low, the precipitate of the slab 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.
No.2-S steel has a low finishing temperature, and the strain introduced by rolling raises the start 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. In addition, magnetic properties are deteriorated due to coarse precipitates.
No.2-T 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 the cooling rate is fast, a low temperature transformation phase is precipitated, and even if annealing is performed, fine precipitates are not sufficiently precipitated, so that the yield strength is lowered.

The figure which shows the relationship between the average crystal grain diameter of a ferrite, a magnetic characteristic (magnetic flux density _B50 , iron loss _{W10 / 50} ), and yield strength. It is a figure which shows the influence which the average crystal grain diameter of a ferrite has on various characteristics, (a) The relationship between the average crystal grain diameter of ferrite and yield strength, (b) is the average crystal grain diameter of ferrite and iron loss W _{10 / 50} the relationship between, (c) shows the relationship between the average crystal grain size and the magnetic flux density B ₅₀ of ferrite. It is a figure which shows the influence which the average crystal grain diameter of a ferrite has on various characteristics regarding other steel, (a) Relation between the average crystal grain diameter of ferrite and yield strength, (b) is the average crystal grain diameter of ferrite and the relationship between the iron loss W _10/50, (c) shows the relationship between the average crystal grain size and the magnetic flux density B ₅₀ of ferrite. It is a figure which shows a sample collection position. It is a figure which shows the relationship between an annealing temperature and a magnetic characteristic. It is a figure which shows the relationship between annealing temperature and a magnetic characteristic regarding another steel.

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%
Includes having a component composition of the balance Fe and unavoidable impurities, consists of the average crystal grain size 60μm or more ferrite area ratio is 95% or more of tissue, fine precipitates having a size of less than 10nm in the ferrite is dispersed An electromagnetic steel bar characterized by   The electromagnetic bar steel according to claim 1, wherein the structure has an area ratio of ferrite having a particle size of 10 μm or less of 10% or less. The electromagnetic bar steel according to claim 1 or 2, 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 any one of claims 1 to 3, wherein the fine precipitate is a carbide of Ti and Mo. As the above component composition,
Nb: 0.08% or less,
The electromagnetic bar steel according to claim 1 or 2, comprising one or more of V: 0.15% or less and W: 1.5% or less. The electromagnetic bar steel according to claim 5, wherein the component composition satisfies the following expression (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 5 or 6, wherein the fine precipitate is a carbide containing Ti and Mo and at least one of Nb, V and W. As said component composition, it is further mass% S: 0.01-0.1%
And including
Pb: 0.2% or less,
Ca: 0.005% or less,
The electromagnetic bar steel according to any one of claims 1 to 7, comprising one or more of Bi: 0.1% or less and B: 0.02% 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%
Steel material with the remaining Fe and unavoidable impurity composition is heated to 1100 ° C or higher, then hot rolled under conditions of area reduction of 25% or less in the final pass and finishing temperature: 880 ° C or higher And then cooling at a cooling rate of 1.0 ° C./s or less. The method for manufacturing an electromagnetic steel bar according to claim 9, further comprising annealing after the cooling in the following temperature range.
Record
When Mn content (% by mass) is 0.5 to 1.7%: 600 ° C or higher and 800 ° C or lower
When Mn content (% by mass) is over 1.7% to 3.0%: 600 ° C or higher and 750 ° C or lower The said component composition satisfy | fills following (1) Formula, The manufacturing method of the electromagnetic bar steel of Claim 9 or 10 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. As the above component composition,
Nb: 0.08% or less,
The method for producing an electromagnetic steel bar according to claim 9 or 10, comprising one or more of V: 0.15% or less and W: 1.5% or less. The method for producing an electromagnetic steel bar according to claim 12, wherein the component composition satisfies the following expression (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. As said component composition, it is further mass% S: 0.01-0.1%
And including
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 13, comprising one or more of Bi: 0.1% or less and B: 0.02% or less.

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