JP7679186B2 - Ferritic Stainless Steel - Google Patents

Description

The present invention relates to ferritic stainless steel, and in particular to ferritic stainless steel used for motor cases and motor parts.

Ferritic stainless steels are used in a wide range of fields, including home appliances, electronic devices, and automobiles. In particular, in fields where materials are exposed to high temperatures, such as heating equipment, kitchen appliances, and the automotive field, the stainless steels used are required to have oxidation resistance and corrosion resistance.

Magnetic properties are also important for motor cases that house stepping motors and hysteresis motors, motor components such as motor cores, sensors such as electronic throttle sensors and EPS sensors, relays and solenoid valves, and their cores, yokes, connectors and housings. In particular, magnetic properties in an AC magnetic field are important for motor cases and motor components, as the positive and negative internal electrodes frequently switch.

The magnetic properties are determined from the values of saturation magnetic flux density (Bs), magnetic permeability (μ), residual magnetic flux density (Br), and coercive force (Hc). Saturation magnetic flux density Bs is an index showing the absolute value of the magnetic force of the material, and is the saturation magnetization that converges at a sufficiently large magnetic field H (A/m). The larger the saturation magnetic flux density Bs, the stronger the magnetic capacity to shield a strong magnetic field. Furthermore, magnetic permeability μ is an index of sensitivity to magnetic fields, and is calculated by the gradient (μ = B/H) of magnetization B (T) relative to the magnetic field H (A/m). The higher the magnetic permeability μ, the more sensitive the material is to magnetic fields and the easier it is to magnetize. Furthermore, residual magnetic flux density Br is the magnetic flux density remaining in the material when the magnetic field H is set to 0 from the state of saturation magnetic flux density Bs. Furthermore, coercive force Hc is the magnetic field value when the material is further demagnetized from this state and the magnetic flux density becomes 0. The smaller the residual magnetic flux density Br and the coercive force Hc, the easier it is to eliminate magnetization.

In addition, for the magnetic properties in an AC magnetic field, the maximum magnetic flux density (Bm) at each frequency and the iron loss (W) that affects this value are important. Increasing the content of Fe, Ni, and Co is effective for increasing the maximum magnetic flux density, and in stainless steel, the Fe content can be relatively increased by decreasing the Cr content. However, decreasing the Cr content decreases the corrosion resistance, which is the most important property of stainless steel.
Moreover, the cause of the increase in iron loss is eddy currents induced in the magnetic material. Increasing the electrical resistivity is effective in reducing the loss caused by eddy currents. For example, high Cr-containing ferritic stainless steel has a high Cr content, which increases the electrical resistivity and reduces the iron loss, so the magnetic flux density tends to be high in the high frequency range, but it is still difficult to prevent the magnetic flux density from decreasing in the frequency range of 1 kHz or more. Adding Al or Si is effective in further increasing the electrical resistivity, but adding Al or Si to high Cr-containing ferritic stainless steel reduces manufacturability and workability.

Patent Document 1 describes a ferritic stainless steel sheet with excellent magnetic properties, which contains, by weight, C≦0.01%, Si: 0.1-0.6%, Mn: 0.1-1.0%, S≦0.004%, Cr: 5-13%, Ti: 0.05-0.5%, O≦0.004%, N≦0.015%, C+N≦0.015%, and the balance is Fe and unavoidable impurities, and has a sum of (111) plane intensities in the surface layer and center layer of 10 or less and a maximum relative magnetic permeability of ≧4000. Compared to copper, copper alloys, or ceramics, it has excellent impact resistance and magnetic properties, but the content of Al and Si is low, and sufficient magnetic flux density cannot be ensured in the high frequency range.

Patent Document 2 describes a ferritic stainless steel sheet with excellent magnetic properties, which is produced by hot rolling a slab containing, by weight, C: 0.015% or less, N: 0.015% or less, Si: 1.5% or less, Mn: 1.0% or less, Cr: 10-14%, and Ti: 0.05-0.30%, into a hot-rolled sheet, which is then cold-rolled at a rolling reduction of 20-60%, and then annealed at 800-930°C. Compared to copper, copper alloys, or ceramics, this has excellent impact resistance and magnetic properties, but the Al and Si content is low, making it impossible to ensure sufficient magnetic flux density in the high frequency range.

Patent Document 3 discloses a highly corrosion-resistant magnetic stainless steel that contains C: 0.015 wt% or less, Si: 0.30 wt% or less, Mn: 0.30 wt% or less, Cr: 10.0-20.0 wt%, Mo: 0.5-2.0 wt%, Ti: 0.05-0.30 wt%, Cu: 0.3-1.5 wt%, and Al: 0.05-1.5 wt%, with the balance being essentially Fe, but corrosion resistance is ensured by the addition of Mo and Cu.

Patent Document 4 discloses a highly cold forged magnetic stainless steel containing C: 0.02% or less, Si: 0.01-0.50%, Mn: 0.01-0.50%, Cr: 7.00-20.00%, Mo: 0.30-2.00%, Cu: 0.10-2.00%, Ti: 0.05-0.50%, Al: 0.05-3.00%, B: 0.0005-0.05%, N: 0.05% or less, with the balance being essentially Fe, and corrosion resistance is ensured by the combined addition of Ti, B, Mo, and Cu.

Patent Document 5 discloses a ferritic stainless steel sheet with excellent magnetic properties which contains, by mass%, C: 0.020% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.035% or less, S: 0.0030% or less, Cr: 10.0 to 18.0%, N: 0.020% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.10% or less, with the balance being Fe and impurities, and which has a texture on the sheet surface that satisfies the following (i) and (ii):
(i) The area ratio of {110}±15° orientation grains on the sheet surface, in which the angle difference between the normal direction to the steel sheet surface and the {110} plane orientation is within 15°, is more than 3.0% and less than 30%.
(ii) When the area ratio of grains oriented in {110}±15° on the plate surface is A and the area ratio of grains oriented in {111}±15° is B, 0.10<A/B<0.80.
Although the magnetic properties are ensured by controlling the crystal orientation, the Al content is low.

Patent Document 6 discloses a ferritic stainless steel with excellent magnetic properties, which contains, by mass%, C: 0.020% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.035% or less, S: 0.0030% or less, Cr: 10.0 to 18.0%, N: 0.020% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.10% or less, Sn: 0.001 to 0.5%, B: 0.005% or less, and the balance being Fe and impurities.
The addition of Sn, which is a grain boundary segregation element, suppresses the grain boundary segregation of P, S, etc., and improves the magnetic properties. However, the Al content is low, and sufficient magnetic flux density cannot be ensured in the high frequency range.

Patent No. 3629102 Japanese Patent Application Publication No. 11-61255 Japanese Unexamined Patent Publication No. 2-305944 Japanese Patent Application Publication No. 4-318153 JP 2020-63473 A JP 2020-63472 A

The present invention was made in consideration of the above circumstances, and aims to provide a ferritic stainless steel that has excellent corrosion resistance and workability, and is capable of exhibiting excellent magnetic properties in an AC magnetic field.

In order to solve the above problems, the present invention has the following configuration.
[1] In mass%,
C: 0.001-0.030%,
Si: 0.01-3.00%,
Mn: 0.01 to 2.00%,
P: 0.030% or less,
S: 0.0050% or less,
Ni: 0.01 to 3.00%,
Cr: 5.0-14.7 %,
Al: 0.001-5.000%,
V: 0.001-1.00%,
B: 0.0001 to 0.0100%,
N: 0.001-0.030%,
Nb: 0.001-0.30%
Contains
The following formula (1) is satisfied:
The balance is Fe and impurities,
The grain size number is 6.0 or more and 9.0 or less,
The average r value is 1.0 or more,
When magnetically annealed at 800 to 1000°C for 1 to 10 hours,
Electrical resistivity is 60 μΩcm or more,
A ferritic stainless steel characterized in that the maximum magnetic flux density is 0.80 T or more at a magnetic field peak value of 0.8 kA/m and a measurement frequency of 1.0 kHz .
Cr+15Al+20Si≧20.00… (1)
In the formula (1), Cr, Al, and Si are the mass percentages of the respective elements .
[2 ] Further, in place of a part of Fe,
Ti: 0.01 to 0.30%,
Mo: 0.01-3.00%,
Sn: 0.001 to 3.00%,
Cu: 0.01-3.00%,
W: 0.001-1.00%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001-0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001-0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001~0.100%
The ferritic stainless steel according to [1] , characterized in that it contains one or more of the following:
[ 3 ] The ferritic stainless steel according to [1] or [2] , which is applied to a motor case and a motor component.
[ 4 ] The ferritic stainless steel according to [ 3 ], characterized in that the motor case is a case for housing a stepping motor or a hysteresis motor.
[ 5 ] The ferritic stainless steel according to [ 3 ] or [ 4 ], characterized in that the motor part is a motor core.

According to the present invention, it is possible to provide a ferritic stainless steel that is excellent in corrosion resistance and workability, and is capable of exhibiting excellent magnetic properties in an AC magnetic field.
Furthermore, the ferritic stainless steel of the present invention can be suitably used for motor cases that house stepping motors, hysteresis motors, and the like, motor parts such as motor cores, sensors such as electronic throttle sensors and EPS sensors, relays and solenoid valves, as well as their cores, yokes, connectors, housings, and the like.

The inventors of the present invention have conducted extensive research with the aim of providing a ferritic stainless steel that is excellent in corrosion resistance and workability, and is capable of exhibiting excellent magnetic properties in an AC magnetic field. As a result, they have discovered the following:

First, it was found that in stainless steels with a relatively low Cr content of 5 to 18% (low Cr steels), the corrosion resistance improves as the Al and Si contents increase. This allows low Cr steels with excellent manufacturability and workability to contain large amounts of Al and Si, which also effectively improve electrical resistivity, and improves electrical resistivity and corrosion resistance, which are issues with low Cr steels, while ensuring a certain level of manufacturability and workability. In other words, by containing a predetermined amount of Al and Si, it is possible to provide a ferritic stainless steel that has better magnetic properties in an AC magnetic field and good corrosion resistance than conventional steels. Specifically, the ferritic stainless steel according to the present invention can be obtained by containing a certain amount of Cr, Al, and Si in total and making the electrical resistivity 60 μΩcm or more. More specifically, the threshold value of the amount of Cr, Al, and Si added is Cr+15Al+20Si≧20.00. However, in the formula, Cr, Al, and Si are the mass% of each element. More preferably, Cr+15Al+20Si≧30.00, and even more preferably, Cr+15Al+20Si≧40.00.

In addition, by keeping the C, P, S, and N contents low and making the crystal grains fine before magnetic annealing, the average r value can be improved to ensure good workability, and the magnetic properties can be improved by coarsening the crystal grain size during magnetic annealing. In other words, by making the crystal grains fine to a certain extent before magnetic annealing, the average r value can be improved to ensure workability, and then magnetic annealing promotes the growth of the crystal grains, resulting in improved magnetic properties. However, when the crystal grains grow, the crystal orientation that is advantageous for improving the r value grows preferentially, which can improve workability, but if the grains are made too fine, the r value will be low. Therefore, the grain size number of the steel before magnetic annealing is set to 9.0 or less. Also, if the grains are made too coarse, workability will decrease, so the grain size number is set to 6.0 or more. Then, by subjecting steel having such a grain size number to magnetic annealing, the grain size can be coarsened and the magnetic properties can be improved.

Regarding corrosion resistance, it is believed that Al dissolves as ions inside the early stages of pitting and then adsorbs to the surface, thereby inhibiting pitting growth and promoting repassivation. In addition, Si forms an oxide inside the pit, which is believed to inhibit pitting growth and promote repassivation.

Furthermore, during oxidation by magnetic annealing, the high Al and Si content makes it difficult for Fe-based oxides, which have low corrosion resistance, to form on the surface, which is thought to contribute to ensuring corrosion resistance.

This embodiment will be described below.
The ferritic stainless steel according to this embodiment has, in mass %, C: 0.001 to 0.030%, Si: 0.01 to 3.00%, Mn: 0.01 to 2.00%, P: 0.030% or less, S: 0.0050% or less, Ni: 0.01 to 3.00%, Cr: 5.0 to 18.0%, Al: 0.001 to 5.000%, V: 0.001 to 1.00 %, B: 0.0001-0.0100%, N: 0.001-0.030%, and further contains one or two of Ti: 0.01-0.30% and Nb: 0.001-0.30%, and satisfies the following formula (1), with the balance being Fe and impurities. The grain size number is 6.0 or more and 9.0 or less, and the average r-value is 1.0 or more.
Cr+15Al+20Si≧20.00… (1)
In the formula (1), Cr, Al, and Si are the mass percentages of the respective elements.
Furthermore, when the ferritic stainless steel according to this embodiment is magnetically annealed at 800 to 1000° C. for 1 to 10 hours, the electrical resistivity may be 60 μΩcm or more, and the maximum magnetic flux density may be 0.80 T or more at a magnetic field peak value of 0.8 kA/m and a measurement frequency of 1.0 kHz.
The chemical composition of the ferritic stainless steel of this embodiment will be described below. Note that the "%" for the content of each element means mass %.

C: 0.001-0.030%
C reduces magnetic properties, intergranular corrosion resistance, and workability, so its content must be kept low. Therefore, the C content is set to 0.030% or less. However, an excessively low C content increases refining costs, so the C content is set to 0.001% or more. The preferred range of C content is 0.002 to 0.020%, and the more preferred range is 0.003 to 0.010%.

Si: 0.01-3.00%
Silicon dramatically improves magnetic properties, electrical resistivity, oxidation resistance at low and medium temperatures (500-700°C) and oxidation resistance at high temperatures (700°C or higher). Silicon is also a very beneficial element that not only suppresses corrosion by concentrating on the surface, but also reduces the corrosion rate of the base material. Therefore, the silicon content is set to 0.01% or more. However, since excessive silicon content reduces manufacturability, workability, and weld penetration, the silicon content is set to 3.00% or less. The more preferable range of silicon content is 0.10-2.00%, the even more preferable range is 0.30-1.50%, and the even more preferable range is 0.80-1.20%.

Mn: 0.01-2.00%
Mn is useful as a deoxidizing element, but excessive Mn content deteriorates corrosion resistance. Therefore, the Mn content is set to 0.01 to 2.00%. The preferred range of the Mn content is 0.05 to 1.00%, and the more preferred range is 0.02 to 0.50%.

P: 0.030% or less P is an element that deteriorates magnetic properties, workability, and weldability, so its content must be limited. Therefore, the P content is set to 0.030% or less. The preferred range of the P content is 0.025% or less. However, excessively lowering the P content increases refining costs, so the P content may be set to 0.001% or more.

S: 0.0050% or less S is an element that deteriorates corrosion resistance, so its content must be limited. Therefore, the S content is set to 0.0050% or less. The preferred range of the S content is 0.0030% or less. However, excessively lowering the S content increases refining costs, so the S content may be set to 0.0001% or more.

Ni: 0.01-3.00%
Ni must be present in an amount of 0.01% or more to improve magnetic properties and corrosion resistance. However, a large amount of Ni increases the cost of the alloy, so the Ni content is set to 3.00% or less. The Ni content is preferably in the range of 0.05 to 1.00%, and more preferably in the range of 0.10 to 0.50%.

Cr:5.0~18.0%
Cr content must be 5.0% or more to ensure oxidation resistance and corrosion resistance in salt damage environments. As the Cr content increases, the oxidation resistance and corrosion resistance improve and the electrical resistivity also increases, but the Cr content decreases the weld penetration, thermal conductivity, workability, and manufacturability, so the Cr content is set to 18.0% or less. The preferred range of Cr content is 5.5 to 15.0%, and the more preferred range is 9.0 to 13.0%.

Al: 0.001-5.000%
Al is an important element in this embodiment. Al dramatically improves oxidation resistance, especially at high temperatures (700°C or higher). In addition, it is a very beneficial element that not only suppresses corrosion by concentrating on the steel surface, but also reduces the corrosion rate of the base material. Al also has the effect of increasing electrical resistivity. This effect is particularly noticeable in low Cr stainless steels. Therefore, the Al content is set to 0.001% or more. However, since excessive Al content causes a decrease in the toughness and elongation of the material and reduces manufacturability and workability, the Al content is set to 5.0% or less. The preferred range of the Al content is 0.800 to 3.000%, and the more preferred range is 1.000 to 2.000%.

V:0.001~1.00%
V must be present in an amount of 0.001% or more to improve corrosion resistance. However, a large content of V increases the alloy cost, so the V content is set to 1.00% or less. The preferred range of V content is 0.005 to 0.80%, and the more preferred range is 0.010 to 0.50%.

B: 0.0001-0.0100%
B is an element useful for improving secondary workability, and must be contained in an amount of 0.0100% or less. The lower limit of the B content is set to 0.0001% or more, at which a stable effect can be obtained. The preferred range of the B content is 0.0005 to 0.0050%, and the more preferred range is 0.0010 to 0.0030%.

N: 0.001-0.030%
N is a useful element for pitting corrosion resistance, but it reduces magnetic properties, intergranular corrosion resistance, and workability. Therefore, it is necessary to keep the N content low. Therefore, the N content is set to 0.030% or less. However, an excessively low N content increases refining costs, so the N content is set to 0.001% or more. The preferred range of the N content is 0.002 to 0.020%.

One or both of Ti: 0.01-0.30% and Nb: 0.001-0.30% In order to prevent sensitization of stainless steel, Ti and Nb must be contained at 0.01% or more for Ti and 0.001% or more for Nb. However, since a large content leads to an increase in alloy cost, a decrease in toughness, a decrease in corrosion resistance due to an increase in inclusions in the steel, and a decrease in manufacturability, the Ti content or Nb content is set to 0.30% or less. The preferred ranges of the Ti content and the Nb content are 0.03-0.25%, and the more preferred ranges are 0.04-0.20%, respectively. It is sufficient that either Ti or Nb is contained, and both Ti and Nb may be contained.

In particular, with regard to Ti, it is preferable to satisfy the condition 2Al+Si-10Ti≧0 (Al, Si, and Ti are the mass % of each element in ferritic stainless steel), as this significantly improves corrosion resistance.

The above is the basic chemical composition of the ferritic stainless steel of this embodiment, but in this embodiment, the following elements can also be added as necessary.

Mo, Sn, Cu, W, Sb, Co, Ca, Mg, Zr, Ga, Ta, and REM may be contained alone or in combination depending on the purpose. The lower limit of these elements is 0% or more, and preferably more than 0%.

Mo: 0.01~3.00%
Mo can be contained in an amount of 0.01% or more to improve corrosion resistance. However, excessive content of Mo deteriorates workability and is expensive, leading to increased costs. Therefore, the Mo content is set to 3.00% or less. The preferred range of Mo content is 0.05 to 2.00%, and the more preferred range is 0.05 to 1.00%.

Sn: 0.001-3.00%
Sn can be contained in an amount of 0.001% or more to improve corrosion resistance. However, excessive content leads to increased costs. Therefore, the Sn content is set to 3.00% or less. The preferred range of Sn content is 0.005 to 1.00%, and more preferably 0.010 to 1.00%.

Cu: 0.01~3.00%
Cu can be contained in an amount of 0.01% or more to improve corrosion resistance. However, excessive content leads to increased costs. Therefore, the Cu content is set to 3.00% or less. The preferred range of Cu content is 0.02 to 1.00%, and the more preferred range is 0.05 to 0.09%.

W: 0.001-1.00%
W can be contained in an amount of 1.00% or less in order to improve corrosion resistance. In order to obtain a stable effect, the W content is set to 0.001% or more. The preferred range of the W content is 0.005 to 0.80%.

Sb: 0.001-0.100%
Sb can be contained in an amount of 0.100% or less in order to improve general corrosion resistance. In order to obtain a stable effect, the Sb content is set to 0.001% or more. The preferred range of the Sb content is 0.010 to 0.080%.

Co:0.001~0.500%
Co can be contained in an amount of 0.500% or less in order to improve secondary workability and toughness. In order to obtain a stable effect, the Co content is set to 0.001% or more. The preferred range of the Co content is 0.010 to 0.300%.

Ca: 0.0001-0.0050%
Ca is contained for desulfurization, but if contained in excess, water-soluble inclusions CaS are formed, which reduces corrosion resistance. Therefore, Ca can be contained in the range of 0.0001 to 0.0050%. The preferred range of Ca content is 0.0005 to 0.0030%.

Mg: 0.0001-0.0050%
Mg is useful for refining the structure and improving workability and toughness. Therefore, Mg can be contained in a range of 0.0050% or less. To obtain a stable effect, the Mg content is set to 0.0001% or more. The preferred range of Mg content is 0.0005 to 0.0030%.

Zr: 0.0001-0.0300%
Zr can be contained up to 0.0300% in order to improve corrosion resistance. In order to obtain a stable effect, the Zr content is set to 0.0001% or more. The preferred range of the Zr content is 0.0010 to 0.0100%.

Ga: 0.0001-0.0100%
Ga can be contained in an amount of 0.0100% or less in order to improve corrosion resistance and hydrogen embrittlement resistance. In order to obtain a stable effect, the Ga content is set to 0.0001% or more. The preferred range of the Ga content is 0.0005 to 0.0050%.

Ta: 0.001-0.050%
Ta can be contained in an amount of 0.050% or less in order to improve corrosion resistance. In order to obtain a stable effect, the Ta content is set to 0.001% or more. The preferred range of the Ta content is 0.005 to 0.030%.

REM: 0.001~0.100%
REM has a deoxidizing effect and is useful in refining, so it can be contained in an amount of 0.100% or less. In order to obtain a stable effect, the REM content is set to 0.001% or more. The preferred range of the REM content is 0.003 to 0.050%.
Here, REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to the general definition. REM is one or more elements selected from these rare earth elements, and the content of REM is the total amount of rare earth elements.

The ferritic stainless steel of this embodiment is composed of Fe and impurities (including unavoidable impurities) in addition to the elements described above. In addition to the elements described above, other elements may be contained within a range that does not impair the effects of the present invention. In this embodiment, for example, Bi, Pb, Se, H, etc. may be contained, but in this case, it is preferable to reduce the amount as much as possible. On the other hand, the content ratio of these elements is controlled to the extent that the problem of the present invention is solved, and Bi may be contained at 0.01% or less, Pb at 0.01% or less, Se at 0.01% or less, and H at 0.01% or less, as necessary.

The grain size number of the ferritic stainless steel of this embodiment is 6.0 or more and 9.0 or less. In order to increase the average r-value and improve workability, it is important to make the grains fine with a grain size number of 6.0 or more. To maximize this effect, it is preferable to make the grain size number 7.0 or more. On the other hand, when the grains grow, the crystal orientation that is advantageous for improving the r-value grows preferentially, which can improve workability, but if the grains are made too fine, the r-value will be low. Therefore, the grain size number is 9.0 or less. Preferably, it is 8.5 or less.

The grain size number is determined by cutting a test piece 30 mm long and 20 mm wide from the steel plate, embedding it in resin so that the cross-sectional structure in the rolling direction can be observed, and then mirror polishing and etching it. The grain size number of the cross-sectional structure in the rolling direction is then measured in accordance with JIS G 0551:2013. Measurements are performed from the center of the plate thickness for five tests, and the average value is used.

The ferritic stainless steel of this embodiment has an average r-value (Lankford value) of 1.0 or more. By making the average r-value 1.0 or more, the workability of the ferritic stainless steel is improved and more severe processing can be performed. The average r-value is preferably 1.1 or more, and more preferably 1.2 or more.

The average r value can be measured using the plastic strain ratio test method of JIS Z 2254 (2008) and can be calculated using the following formula (A).

Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (A)
In the formula (A), _r0 represents the r value in the rolling direction, _r90 represents the r value in the direction perpendicular to the rolling direction, and _r45 represents the r value in the 45 degree direction to the rolling direction.

Here, the characteristics of the ferritic stainless steel according to the present embodiment described above when it is subjected to magnetic annealing will be described.
The ferritic stainless steel of this embodiment exhibits an electrical resistivity of 60 μΩcm or more, a magnetic field peak value of 0.8 kA/m, and a maximum magnetic flux density of 0.8 T (tesla) or more at a measurement frequency of 1.0 kHz when magnetically annealed at 800 to 1000° C. for 1 to 10 hours. Note that the steel according to this embodiment exhibits an electrical resistivity of 60 μΩcm or more, a magnetic field peak value of 0.8 kA/m, and a maximum magnetic flux density of 0.8 T or more at a measurement frequency of 1.0 kHz under any conditions within the above-mentioned range of annealing conditions.
Regarding the magnetic properties in an AC magnetic field, the maximum magnetic flux density (Bm) at each frequency and the iron loss (W) that affects the value are important. The cause of the increase in iron loss is the eddy current induced in the magnetic material. Increasing the electrical resistivity is effective in reducing the loss due to eddy current. Therefore, in this embodiment, sufficient electrical resistivity and maximum magnetic flux density can be ensured by controlling the chemical composition as described above and controlling the crystal grain size. Specifically, by performing magnetic annealing using the ferritic stainless steel of this embodiment, a ferritic stainless steel can be obtained that can exhibit an electrical resistivity of 60 μΩcm or more, a magnetic field peak value of 0.8 kA/m, and a maximum magnetic flux density of 0.8 T (tesla) or more at a measurement frequency of 1.0 kHz. In addition, this can be suitably used for motor cases that house stepping motors, hysteresis motors, and the like, motor parts such as motor cores, sensors such as electronic throttle sensors and EPS sensors, relays and solenoid valves, and further their cores, yokes, connectors, and housings. In this embodiment, the electrical resistivity can be measured by a so-called four-terminal method using an electrical resistance measuring device.

The form of the ferritic stainless steel in this embodiment is not particularly limited, but it is preferably a steel plate.

Next, the manufacturing method of the ferritic stainless steel of this embodiment will be explained using an example in which the form is "steel plate."

The method for producing a ferritic stainless steel of this embodiment comprises the steps of steelmaking-hot rolling-hot-rolled sheet annealing/pickling-cold rolling-cold-rolled sheet annealing, and the production conditions for each step may be appropriately determined within a range that does not impair the effects of the present invention. However, from the viewpoint of controlling the grain size number and the average r-value, it is necessary to appropriately control the conditions for the hot-rolled sheet annealing, the cold rolling reduction, and the cold-rolled sheet annealing.
Each step and the conditions of the production method will be described in detail below.

In steelmaking, a suitable method is to melt steel containing the above essential components and components added as necessary in a converter, followed by secondary refining. The molten steel is cast (continuous casting) into slabs. The slabs are heated to a specified temperature and hot rolled to a specified plate thickness by continuous rolling. Considering the crystal grain size and crystal orientation of the final product, the slab heating temperature is preferably 1190°C to 1300°C, and the slab thickness is preferably 3.0 mm to 300.0 mm.

The annealing process after hot rolling (hot-rolled sheet annealing process) is an important process for optimizing the crystal grain size and obtaining a structure with excellent magnetic properties. Specifically, the soaking temperature in hot-rolled sheet annealing is set to 850°C to 1000°C. If the soaking temperature in hot-rolled sheet annealing is less than 850°C, there is a risk of poor recrystallization, so the soaking temperature is set to 850°C or higher. If the soaking temperature in hot-rolled sheet annealing is more than 1000°C, there is a risk of reduced toughness due to grain coarsening, so the soaking temperature is set to 1000°C or lower. Preferably, the soaking temperature in hot-rolled sheet annealing is 880°C to 980°C. The soaking time (holding time) in hot-rolled sheet annealing is not particularly limited, but it is preferably set to 10 seconds to 120 seconds from the viewpoint of completing recrystallization.

After the hot-rolled sheet annealing, pickling and cold rolling are carried out in sequence. At this time, the reduction ratio of the cold rolling is preferably 40% or more. If the reduction ratio is less than 40%, there is a risk of poor recrystallization during the subsequent cold-rolled sheet annealing, so the reduction ratio is preferably 40% or more, and more preferably 50% or more. On the other hand, if the reduction ratio is too high, there is a risk of productivity deteriorating, so the reduction ratio is preferably 95% or less.

Cold rolling after pickling can be done using either a normal Sendzimir mill or a tandem mill, but tandem mill rolling is more preferable when considering the workability of the steel sheet, and Sendzimir mill rolling is more preferable for obtaining a structure with excellent magnetic properties. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected within normal ranges. Intermediate annealing may be performed during cold rolling.

The final annealing after cold rolling (cold-rolled sheet annealing) is important from the viewpoint of controlling the grain size and average r-value. In particular, the soaking temperature must be 880°C to 1000°C. If the soaking temperature for cold-rolled sheet annealing is less than 880°C, there is a risk of poor recrystallization, so the soaking temperature must be 880°C or higher. If the soaking temperature for cold-rolled sheet annealing is more than 1000°C, there is a risk of grain coarsening, so the soaking temperature must be 1000°C or higher. Preferably, the soaking temperature for cold-rolled sheet annealing is 900°C to 950°C. The soaking time (holding time) for cold-rolled sheet annealing is not particularly limited, but it is preferably 10 seconds to 120 seconds from the viewpoint of promoting recrystallization.

The intermediate annealing performed during cold rolling and the final annealing after cold rolling can be either batch annealing or continuous annealing. The atmosphere for each annealing can be bright annealing in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, if necessary, or annealing in air. After the final annealing, salt treatment, pickling, electrolytic pickling, etc. can be performed.

The ferritic stainless steel according to this embodiment can be manufactured by the manufacturing method described above, but other processes may also be carried out as appropriate as long as they do not impair the effects of the present invention. For example, after final annealing, a tension leveler process may be carried out to correct the shape, and the sheet may also be threaded.

The ferritic stainless steel according to the present embodiment can be suitably used for motor cases and motor parts. In that case, it is possible to obtain a product with excellent magnetic properties by performing magnetic annealing after processing into parts. The conditions of magnetic annealing may be appropriately determined according to the applied product, use, etc., but it is desirable to perform the annealing under the conditions of, for example, a temperature rise rate ^of 1 to 100°C/min, a soaking temperature of 800 ^to 1000°C, and a soaking time of 1 to 10 hours, preferably 1 to 3 hours, in a vacuum of 1 x 10 -2 to 9 x 10 -2 Pa. After such heat treatment, cooling with Ar gas or the like may be performed, or air cooling or furnace cooling may be performed. By performing such magnetic annealing, the removal of processing strain and the coarsening of crystal grains occur, the magnetic properties become good, and the magnetic flux density can be improved. Specifically, by performing such magnetic annealing, a ferritic stainless steel having coarse crystal grains with a crystal grain size number of 4.0 or less can be obtained, and as a result, the magnetic properties in an AC magnetic field can be improved.

The above describes the ferritic stainless steel according to this embodiment and a suitable manufacturing method thereof. According to the ferritic stainless steel of this embodiment, it is possible to provide a ferritic stainless steel that is excellent in corrosion resistance and workability, and is capable of exhibiting excellent magnetic properties in an AC magnetic field. Furthermore, by subjecting the ferritic stainless steel to an optimal magnetic annealing, it is possible to provide a ferritic stainless steel that is excellent in corrosion resistance and magnetic properties in an AC magnetic field, and is suitable for motor parts such as motor cases and motor cores for stepping motors and hysteresis motors.

The present invention will be described in more detail below based on examples, but the present invention is not limited to the conditions used in the following examples. Note that underlines in the table indicate conditions outside the scope of the present invention.

Steel with the composition shown in Tables 1A and 1B was melted and hot rolled into a 200 mm thick slab until the sheet thickness reached 3 mm. The hot rolled steel sheet was then heat treated for 60 seconds at the temperature shown in Table 2 (hot rolled sheet annealing), and then shot and pickled. It was then cold rolled (reduction rate: 73%) until the sheet thickness reached 0.8 mm, and then heat treated for 1 minute at the temperature shown in Table 2 (cold rolled sheet annealing), followed by salt treatment and pickling. Pickling was performed by electrolytic pickling in a solution with a nitric acid concentration of 150 g/L. In this way, ferritic stainless steel sheets were produced.

A test piece measuring 30 mm in length and 20 mm in width was cut from near the center of the width of the prepared steel plate, embedded in resin so that the cross-sectional structure in the rolling direction could be observed, and mirror-polished and etched. The grain size number of the cross-sectional structure in the rolling direction was then measured using the line segment method in accordance with JIS G 0551:2013. Measurements were performed from the center of the plate thickness for five tests, and the average value was used.

A tensile test piece No. 13B described in Appendix B of JIS Z 2241:2011 was also prepared from near the center of the width direction of the prepared steel plate, and the dimensions and gauge length of the test piece were measured. Then, in accordance with the plastic strain ratio test method of JIS Z 2254:2008, the dimensions and gauge length of the test piece were measured after applying a strain of 14.4% by tensile testing, and the average r value was calculated from both measurement results. The above formula (A) was used for the calculation. Note that as a criterion for machinability, steel types with an average r value of 1.0 or more were deemed to pass.

Furthermore, the following tests were carried out to evaluate the magnetic properties and corrosion resistance.
First, the temperature was increased at a rate of 1 to 100° C./min in a vacuum of 1.3×10 ⁻² Pa, and magnetic annealing was performed for 2 hours at the magnetic annealing temperature shown in Table 2, followed by air cooling.

A test piece 30 mm long and 20 mm wide was cut from near the center of the width of the steel sheet after magnetic annealing, embedded in resin so that the cross-sectional structure in the rolling direction could be observed, and mirror polished and etched. The grain size number of the cross-sectional structure in the rolling direction was then measured using the line segment method in accordance with JIS G 0551:2013. Measurements were performed from the center of the sheet thickness for five tests, and the average value was used.

Test pieces with a width of 75 mm and a length of 150 mm were cut from near the center of the width direction of the steel plate after magnetic annealing to prepare test pieces for the JASO-CCT test. The JASO-CCT test was performed for 12 cycles in accordance with JASO M 610-92. As the judgment standard for the JASO-CCT test, the rating number was determined using a method in accordance with JIS G 0595:2004, with "3" being the boundary value. Steel types with rating numbers of 4 to 9 were judged to have excellent corrosion resistance, and are indicated by the symbol "○" in Table 2. On the other hand, steel types with rating numbers of 0 to 3 were judged to have poor corrosion resistance, and are indicated by the symbol "X" in Table 2.

In addition, a 4 mm x 60 mm test piece was cut from near the center of the width of the steel after magnetic annealing, and the electrical resistivity was measured by the four-terminal method using an electrical resistance measuring device (TER-2000RH model, manufactured by ULVAC-RIKO Inc.). The measurement current was 0.4 A, the voltage drop distance was 40 mm, and the measurement temperature was room temperature, and a value of 60 μΩcm or more was judged to be acceptable.

After magnetic annealing, a ring sample with an outer diameter of 45 mm, an inner diameter of 33 mm, and a height of 0.8 mm was cut out by electrical discharge machining from near the center of the width of the steel. This ring sample was covered with Kapton film, and after 100 turns of secondary winding were wound around it, it was stored in an acrylic case, and then 200 turns of primary winding were wound around it to create a ring-shaped test sample for AC magnetic measurement. Using this ring-shaped test sample, AC magnetic measurement was performed using an AC magnetic measurement device (Iwatsu Measurement Co., Ltd., B-H Analyzer SY-8258). The measurement conditions were a magnetic field peak value of 0.8 kA/m and a measurement frequency of 1.0 kHz. A maximum magnetic flux density of 0.80 T or more was judged to be a pass. A maximum magnetic flux density of less than 0.80 T was judged to be a fail.

The results are shown in Table 2. It can be seen that when the composition and grain size of the present invention are satisfied and Cr+15Al+20Si≧20 (Cr, Al, and Si indicate the mass% concentration of each element) is satisfied, the average r value (workability), electrical resistivity, AC magnetic flux density, and JASO-CCT test results are passed ("○"). However, Nos. A2, 6, 7, 16, 20, 21, 24, and 28 are used as reference examples.

Comparative Examples B1 to B9 did not satisfy Cr+15Al+20Si≧20 (where Cr, Al, and Si indicate the mass percent concentration of each element), and the evaluation results for both the magnetic properties and corrosion resistance were “x”.
In Comparative Examples B10 to B12, since the annealing temperature of the cold-rolled sheet annealing was low, the crystal grains before magnetic annealing were excessively fine (not satisfying the crystal grain size number), and the average r value (workability) was poor.
In Comparative Examples B13 to B15, the annealing temperature of the cold-rolled sheet annealing was high, so that the crystal grains before the magnetic annealing became coarse, and as a result, the average r value (workability) was poor.

The ferritic stainless steel of the present invention has excellent workability and is able to combine corrosion resistance with magnetic properties in an AC magnetic field, and can therefore be suitably used for motor parts such as motor cases and motor cores for stepping motors and hysteresis motors, sensors such as electronic throttle sensors and EPS sensors, relays and solenoid valves, as well as their cores, yokes, connectors and housings. In other words, the present invention is extremely useful industrially.

Claims (5)

In mass percent,
C: 0.001-0.030%,
Si: 0.01-3.00%,
Mn: 0.01 to 2.00%,
P: 0.030% or less,
S: 0.0050% or less,
Ni: 0.01 to 3.00%,
Cr: 5.0-14.7 %,
Al: 0.001-5.000%,
V: 0.001-1.00%,
B: 0.0001 to 0.0100%,
N: 0.001-0.030%,
Nb: 0.001-0.30%
Contains
The following formula (1) is satisfied:
The balance is Fe and impurities.
The grain size number is 6.0 or more and 9.0 or less,
The average r value is 1.0 or more,
When magnetically annealed at 800 to 1000°C for 1 to 10 hours,
Electrical resistivity is 60 μΩcm or more,
A ferritic stainless steel characterized in that the maximum magnetic flux density is 0.80 T or more at a magnetic field peak value of 0.8 kA/m and a measurement frequency of 1.0 kHz .
Cr+15Al+20Si≧20.00… (1)
In the formula (1), Cr, Al, and Si are the mass percentages of the respective elements. Further, replacing a part of Fe, by mass%,
Ti: 0.01 to 0.30%,
Mo: 0.01-3.00%,
Sn: 0.001 to 3.00%,
Cu: 0.01-3.00%,
W: 0.001-1.00%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001-0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001~0.100%
2. The ferritic stainless steel according to claim 1 , further comprising one or more of the following: 3. The ferritic stainless steel according to claim 1, which is used for a motor case and a motor part. The ferritic stainless steel according to claim 3 , characterized in that the motor case is a case for housing a stepping motor or a hysteresis motor. The ferritic stainless steel according to claim 3 or 4 , characterized in that the motor part is a motor core.