Ferritic Stainless Steel

Abstract

To provide a ferritic stainless steel that is excellent in corrosion resistance and workability and can express excellent magnetic properties in an AC magnetic field.SOLUTION: A ferritic stainless steel includes: 0.001 to 0.030% C, 0.01 to 3.00% Si, 0.01 to 2.00% Mn, 0.030% or less P, 0.0050% or less S, 0.01 to 3.00% Ni, 5.0 to 18.0% Cr, 0.001 to 5.000% Al, 0.001 to 1.00% V, 0.0001 to 0.0100% B, and 0.001 to 0.030% N, and further includes any one or two of 0.01 to 0.30% Ti and 0.001 to 0.30% Nb. The elements satisfy the following expression (1), and the balance is Fe and impurities. A crystal grain size number is 6.0 or larger and 9.0 or smaller, and the average R-value is 1.0 or larger. Cr+15Al+20Si≥20.00...(1)SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to ferritic stainless steel, and particularly to ferritic stainless steel used for motor cases and motor parts.

Background Art

[0002] Ferritic stainless steel is used in a wide range of fields such as home appliances, electronic devices, and automobiles. In particular, in fields where the material becomes hot, such as heating equipment, kitchen equipment, and the automotive field, the stainless steel applied is required to have oxidation resistance and corrosion resistance.

[0003] In addition, for motor cases that house stepping motors, hysteresis motors, etc., motor parts such as motor cores, sensors such as electronic throttle sensors and EPS sensors, relays, solenoid valves, and their cores, yokes, connectors, and housings, magnetic properties are important. Particularly in motor cases and motor parts, since the plus and minus of the internal electrodes frequently switch, the magnetic properties in an alternating magnetic field are important.

[0004] Specifically, magnetic properties are judged from the values of saturation magnetic flux density (Bs), permeability (μ), residual magnetic flux density (Br), and coercive force (Hc). The saturation magnetic flux density Bs is an index indicating the absolute value of the magnetic force of the material, and is the saturation magnetization that converges with a sufficiently large magnetic field H (A / m). The larger the saturation magnetic flux density Bs, the stronger the magnetic containment force and the stronger the shielding of the strong magnetic field. Also, the permeability μ is an index of sensitivity to the magnetic field, and is calculated by the gradient of magnetization B (T) with respect to the magnetic field H (A / m) (μ = B / H). The higher the permeability μ, the more sensitive the material is to the magnetic field and the easier it is to magnetize. Furthermore, the 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 the saturation magnetic flux density Bs. Moreover, the coercive force Hc is the magnetic field value when further demagnetization is performed from this state and the magnetic flux density becomes 0. The smaller both the residual magnetic flux density Br and the coercive force Hc are, the easier it is to demagnetize the magnetization.

[0005] In addition, regarding the magnetic properties in an alternating magnetic field, the maximum magnetic flux density (Bm) at each frequency and the iron loss (W) that affects this value are important. To increase the maximum magnetic flux density, it is effective to increase the content of Fe, Ni, and Co. In stainless steel, the relative content of Fe can be increased by reducing the Cr content. However, reducing the Cr content will lower the corrosion resistance, which is the most important property of stainless steel. Also, the cause of the increase in iron loss is the eddy current induced in the magnetic material. To reduce the loss due to eddy current, it is effective to increase the electrical resistivity. For example, high-Cr ferrite stainless steel has a high Cr content, so its electrical resistivity increases and the iron loss is reduced. Therefore, the magnetic flux density tends to be high in the high-frequency range. However, it is still difficult to prevent the decrease in magnetic flux density in the frequency range above 1 kHz. To further increase the electrical resistivity, adding Al or Si is effective. However, adding Al or Si to high-Cr ferrite stainless steel will deteriorate the manufacturability and workability.

[0006] Patent Document 1 describes a ferrite stainless steel sheet with excellent magnetic properties. By weight percentage, it contains 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%, and C + N≤0.015%. The balance consists of Fe and unavoidable impurities. The sum of the (111) plane intensities in the surface layer and the center layer is 10 or less, and the maximum relative permeability ≥4000. Compared with 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.

[0007] Patent Document 2 describes a ferrite stainless steel sheet with excellent magnetic properties. In terms of weight percentage, it contains 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%. A slab containing these elements is hot-rolled into a hot-rolled sheet, then cold-rolled with a reduction ratio of 20 - 60%, and finally annealed at 800 - 930°C to produce the ferrite stainless steel sheet. Compared with 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.

[0008] Patent Document 3 discloses a highly corrosion-resistant electromagnetic stainless steel containing 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 substantially Fe. Its corrosion resistance is ensured by adding Mo and Cu.

[0009] Patent Document 4 discloses a high cold-forging electromagnetic 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%, and N: 0.05% or less, with the balance being substantially Fe. Its corrosion resistance is ensured by the combined addition of Ti, B, Mo, and Cu.

[0010] Patent Document 5 discloses a ferrite stainless steel sheet with excellent magnetic properties, which contains, by mass percentage, 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 - 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 the grain structure on the sheet surface satisfies the following (i) and (ii). (i) The angular difference between the normal direction of the steel plate surface and the {110} plane orientation on the plate surface is within 15°. The area ratio of grains with a {110} ± 15° orientation is more than 3.0% and less than 30%. (ii) When the area ratio of grains with a {110} ± 15° orientation on the plate surface is A and the area ratio of grains with a {111} ± 15° orientation is B, 0.10 < A / B < 0.80. Magnetic properties are ensured by controlling the crystal orientation, but the Al content is low.

[0011] Patent Document 6 discloses a ferritic stainless steel excellent in 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 consists of Fe and impurities. By adding Sn, which is a grain boundary segregation element, the grain boundary segregation of P, S, etc. is suppressed to improve the magnetic properties, but the Al content is low, and sufficient magnetic flux density in the high-frequency range cannot be ensured.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0013] The present invention has been made in view of the above circumstances, and an object thereof is to provide a ferritic stainless steel that is excellent in corrosion resistance and workability and can exhibit excellent magnetic properties in an alternating magnetic field.

Means for Solving the Problems

[0014] To solve the above problems, the present invention has the following configuration. [1] By 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 14.7 %, Al: 0.001 to 5.000%, V: 0.001 to 1.00%, B: 0.0001 to 0.0100%, N: 0.001 to 0.030%, Nb: 0.001 to 0.30% is contained, satisfies the following formula (1), the balance is Fe and impurities, the crystal grain size number is 6.0 or more and 9.0 or less, the average r value is 1.0 or more , When magnetic annealing is performed at 800 to 1000 °C for 1 to 10 hours, the electrical resistivity is 60 μΩcm or more, the maximum magnetic flux density at a magnetic field peak value of 0.8 kA / m and a measurement frequency of 1.0 kHz becomes 0.80 T or more A ferritic stainless steel characterized by that. Cr + 15Al + 20Si ≧ 20.00 … (1) However, Cr, Al, and Si in formula (1) are the mass% of each element 。 [2 Further, instead of a part of Fe, by mass%, Ti: 0.01~0.30%, Mo: 0.01~3.00%, Sn: 0.001~3.00%, Cu: 0.01~3.00%, W: 0.001~1.00%, Sb: 0.001~0.100%, Co: 0.001~0.500%, Ca: 0.0001~0.0050%, Mg: 0.0001~0.0050%, Zr: 0.0001~0.0300%, Ga: 0.0001~0.0100%, Ta: 0.001~0.050%, REM: 0.001~0.100% characterized by containing one or more of the following: [1 to ferritic stainless steel as described. 3 characterized by being applicable to motor cases and motor parts [1] or [2] ferritic stainless steel as described. 4 characterized by the motor case being a case for housing a stepping motor or a hysteresis motor 3 ferritic stainless steel as described. 5 characterized by the motor part being a motor core 3 or 4 ferritic stainless steel as described.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a ferritic stainless steel that is excellent in corrosion resistance and workability and exhibits excellent magnetic properties in an alternating magnetic field. ​​​Furthermore, the ferritic stainless steel of the present invention can be suitably used for motor cases for housing stepping motors, hysteresis motors, etc., motor parts such as motor cores, sensors such as electronic throttle sensors and EPS sensors, relays, solenoid valves, and their cores, yokes, connectors, and housings.

Embodiments for Carrying Out the Invention

[0016] The inventors of the present invention have intensively studied with the aim of providing a ferritic stainless steel that is excellent in corrosion resistance and workability and can exhibit excellent magnetic properties in an alternating magnetic field. As a result, the following findings were obtained.

[0017] First, it was found that in stainless steels with a relatively low Cr content of 5 to 18% (low Cr-containing steels), the corrosion resistance improves as the Al and Si contents increase. By incorporating a large amount of Al and Si, which are also effective in improving the electrical resistivity, into low Cr-containing steels that are excellent in manufacturability and workability, it is possible to improve the electrical resistivity and corrosion resistance, which are problems of low Cr-containing steels, while ensuring a certain degree of manufacturability and workability. That is, by containing a predetermined amount of Al and Si, it is possible to provide a ferritic stainless steel that is superior in magnetic properties in an alternating magnetic field and has better corrosion resistance than conventional ones. Specifically, a ferritic stainless steel according to the present invention can be obtained by containing a total amount of Cr, Al, and Si of a certain amount or more and setting the electrical resistivity to 60 μΩcm or more. More specifically, the threshold values for the addition amounts of Cr, Al, and Si are Cr + 15Al + 20Si ≧ 20.00. However, Cr, Al, and Si in this formula are the mass percentages of the respective elements. More preferably, Cr + 15Al + 20Si ≧ 30.00, and even more preferably, Cr + 15Al + 20Si ≧ 40.00.

[0018] In addition, by keeping the contents of C, P, S, and N low and making the crystal grains before magnetic annealing fine, the average r-value can be improved to ensure good workability. Furthermore, by coarsening the crystal grain size during magnetic annealing, the magnetic properties can be improved. That is, by making the crystal grains fine to some extent before magnetic annealing, the average r-value can be improved to ensure workability, and then, by promoting the growth of the crystal grains by magnetic annealing, the magnetic properties can be improved as a result. However, when the crystal grains grow, the crystal orientations that are advantageous for improving the r-value grow preferentially and can improve the workability, but if the grains are made too fine, the r-value will decrease. Therefore, the crystal grain size number of the steel before magnetic annealing should be 9.0 or less. Also, if the grains are made too coarse, the workability will decrease, so the crystal grain size number should be 6.0 or more. By subjecting the steel having such a crystal grain size number to magnetic annealing, the crystal grain size can be coarsened and the magnetic properties can be improved.

[0019] Regarding corrosion resistance, it is considered that Al dissolves as ions inside the pitting corrosion at the initial stage of occurrence and then adsorbs on the surface, thereby suppressing the growth of pitting corrosion and promoting repassivation. Also, it is considered that Si forms oxides inside the pitting corrosion, suppressing the growth of pitting corrosion and promoting repassivation.

[0020] Furthermore, during oxidation by magnetic annealing, due to the high contents of Al and Si, it is difficult to form Fe-based oxides with low corrosion resistance on the surface, which is considered to contribute to ensuring corrosion resistance.

[0021] The following describes this embodiment. The ferritic stainless steel according to this embodiment contains, by 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 to 0.0100%, N: 0.001 to 0.030%, further contains either one or both of Ti: 0.01 to 0.30% and Nb: 0.001 to 0.30%, satisfies the following formula (1), the balance being Fe and impurities, has a crystal grain size number of 6.0 or more and 9.0 or less, and an average r value of 1.0 or more. Cr + 15Al + 20Si ≧ 20.00 … (1) However, Cr, Al, and Si in formula (1) are the mass% of each element. Further, when the ferritic stainless steel according to this embodiment is subjected to magnetic annealing 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 at a magnetic field peak value of 0.8 kA / m and a measurement frequency of 1.0 kHz may be 0.80 T or more. The chemical composition of the ferritic stainless steel of this embodiment will be described below. The “%” indication of the content of each element means mass%.

[0022] C: 0.001 to 0.030% Since C deteriorates magnetic properties, intergranular corrosion resistance, and workability, it is necessary to keep its content low. Therefore, the content of C is set to 0.030% or less. However, excessively lowering the C content increases the refining cost, so the C content is set to 0.001% or more. The preferable range of the C content is 0.002 to 0.020%, and the more preferable range is 0.003 to 0.010%.

[0023] Si: 0.01 to 3.00% Si can significantly improve magnetic properties, electrical resistivity, oxidation resistance at medium and low temperatures (500 - 700 °C), and oxidation resistance at high temperatures (above 700 °C). It not only concentrates on the surface to suppress corrosion occurrence but also reduces the corrosion rate of the base material, making it a very beneficial element. Therefore, the Si content is set to be 0.01% or more. However, excessive Si content will reduce manufacturability, workability, and weldability, so the Si content is set to be 3.00% or less. A more preferable range of the Si amount is 0.10 - 2.00%, an even more preferable range is 0.30 - 1.50%, and an even more preferable range is 0.80 - 1.20%.

[0024] Mn: 0.01 - 2.00% Mn is useful as a deoxidizing element, but if it contains an excessive amount of Mn, the corrosion resistance will deteriorate. Therefore, the Mn content is set to be 0.01 - 2.00%. The preferable range of the Mn content is 0.05 - 1.00%, and a more preferable range is 0.02 - 0.50%.

[0025] P: 0.030% or less P is an element that deteriorates magnetic properties, workability, and weldability, so it is necessary to limit its content. Therefore, the P content is set to be 0.030% or less. The preferable range of the P content is 0.025% or less. However, excessively lowering the P content will increase the refining cost, so the P content may also be 0.001% or more.

[0026] S: 0.0050% or less S is an element that deteriorates corrosion resistance, so it is necessary to limit its content. Therefore, the S content is set to be 0.0050% or less. The preferable range of the S content is 0.0030% or less. However, excessively lowering the S content will increase the refining cost, so the S content may also be 0.0001% or more.

[0027] Ni: 0.01 - 3.00% Ni is required to be contained in an amount of 0.01% or more in order to improve magnetic properties and corrosion resistance. However, since a large amount of Ni increases the alloy cost, the Ni content is set to 3.00% or less. The preferable range of the Ni content is 0.05 - 1.00%, and the more preferable range is 0.10 - 0.50%.

[0028] Cr: 5.0 - 18.0% Cr is required to be contained in an amount of 5.0% or more in order to ensure oxidation resistance and corrosion resistance in a chloride environment. As the Cr content increases, the oxidation resistance and corrosion resistance improve, and the electrical resistivity also increases. However, since the weldability, thermal conductivity, workability, and manufacturability decrease, the Cr content is set to 18.0% or less. The preferable range of the Cr content is 5.5 - 15.0%, and the more preferable range is 9.0 - 13.0%.

[0029] Al: 0.001 - 5.000% Al is an important element in this embodiment. Al significantly improves the oxidation resistance, especially at high temperatures (700°C or higher). In addition, it not only concentrates on the steel surface to suppress the occurrence of corrosion but also reduces the corrosion rate of the base material. Al also has the effect of increasing the electrical resistivity. This effect is particularly remarkable in low-Cr stainless steels. Therefore, the Al content is set to 0.001% or more. However, excessive Al content causes a decrease in the toughness and elongation of the material, and reduces the manufacturability and workability. Thus, the Al content is set to 5.0% or less. The preferable range of the Al content is 0.800 - 3.000%, and the more preferable range is 1.000 - 2.000%.

[0030] V: 0.001 - 1.00% V is required to be contained in an amount of 0.001% or more in order to improve corrosion resistance. However, since a large amount of V increases the alloy cost, the V content is set to 1.00% or less. The preferable range of the V content is 0.005 - 0.80%, and the more preferable range is 0.010 - 0.50%.

[0031] B: 0.0001 - 0.0100% B is an element useful for improving secondary workability and its content needs to be 0.0100% or less. The lower limit of the B content is set to 0.0001% or more to obtain a stable effect. The preferable range of the B content is 0.0005 to 0.0050%, and the more preferable range is 0.0010 to 0.0030%.

[0032] N: 0.001 to 0.030% N is an element useful for pitting corrosion resistance, but it deteriorates magnetic properties, intergranular corrosion resistance, and workability. Therefore, it is necessary to keep the N content low. Thus, the N content is set to 0.030% or less. However, excessively lowering the N content increases the refining cost, so the N content is set to 0.001% or more. The preferable range of the N content is 0.002 to 0.020%.

[0033] One or two of Ti: 0.01 to 0.30% and Nb: 0.001 to 0.30% Ti and Nb need to contain 0.01% or more in the case of Ti and 0.001% or more in the case of Nb in order to prevent sensitization of stainless steel. However, containing a large amount 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, so the Ti amount or Nb content is set to 0.30% or less, respectively. The preferable ranges of the Ti content and Nb content are 0.03 to 0.25% and 0.04 to 0.20%, respectively. It is sufficient that either one of Ti and Nb is contained, and both Ti and Nb may be contained.

[0034] In particular, regarding Ti, it is preferable that 2Al + Si - 10Ti ≥ 0 (Al, Si, and Ti are the mass percentages of their respective elements in ferritic stainless steel) is satisfied because the corrosion resistance is significantly improved.

[0035] The above is the basic chemical composition of the ferritic stainless steel of the present embodiment. In the present embodiment, furthermore, the following elements can be contained as needed.

[0036] Mo, Sn, Cu, W, Sb, Co, Ca, Mg, Zr, Ga, Ta, REM may contain one or more of these according to the purpose. The lower limit of these elements is 0% or more, preferably more than 0%.

[0037] Mo: 0.01 - 3.00% Mo can be contained in an amount of 0.01% or more in order to improve corrosion resistance. However, excessive content will deteriorate workability and increase cost because it is expensive. Therefore, the Mo content is set to 3.00% or less. The preferable range of the Mo content is 0.05 - 2.00%, and the more preferable range is 0.05 - 1.00%.

[0038] Sn: 0.001 - 3.00% Sn can be contained in an amount of 0.001% or more in order to improve corrosion resistance. However, excessive content will lead to an increase in cost. Therefore, the Sn content is set to 3.00% or less. The preferable range of the Sn content is 0.005 - 1.00%, and more preferably 0.010 - 1.00%.

[0039] Cu: 0.01 - 3.00% Cu can be contained in an amount of 0.01% or more in order to improve corrosion resistance. However, excessive content will lead to an increase in cost. Therefore, the Cu content is set to 3.00% or less. The preferable range of the Cu content is 0.02 - 1.00%, and the more desirable range is 0.05 - 0.09%.

[0040] 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 preferable range of the W content is 0.005 - 0.80%.

[0041] Sb: 0.001 - 0.100% Sb can be contained at 0.100% or less in order to improve the overall corrosion resistance. In order to obtain a stable effect, the Sb content should be 0.001% or more. The preferred range of the Sb content is 0.010 - 0.080%.

[0042] Co: 0.001 - 0.500% Co can be contained at 0.500% or less in order to improve the secondary workability and toughness. In order to obtain a stable effect, the Co content should be 0.001% or more. The preferred range of the Co content is 0.010 - 0.300%.

[0043] Ca: 0.0001 - 0.0050% Ca is contained for desulfurization, but if it is contained in excess, water-soluble inclusion CaS will be generated and the corrosion resistance will be reduced. Therefore, Ca can be contained in the range of 0.0001 - 0.0050%. The preferred range of the Ca content is 0.0005 - 0.0030%.

[0044] Mg: 0.0001 - 0.0050% Mg refines the structure and is also useful for improving the workability and toughness. Therefore, Mg can be contained in the range of 0.0050% or less. In order to obtain a stable effect, the Mg content should be 0.0001% or more. The preferred range of the Mg content is 0.0005 - 0.0030%.

[0045] Zr: 0.0001 - 0.0300% Zr can be contained at 0.0300% or less in order to improve the corrosion resistance. In order to obtain a stable effect, the Zr content should be 0.0001% or more. The preferred range of the Zr content is 0.0010 - 0.0100%.

[0046] 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 should be 0.0001% or more. The preferred range of the Ga content is 0.0005 - 0.0050%.

[0047] 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 should be 0.001% or more. The preferred range of the Ta content is 0.005 - 0.030%.

[0048] REM: 0.001 - 0.100% Since REM has effects such as deoxidation effect and is a useful element in refining, it can be contained in an amount of 0.100% or less. In order to obtain a stable effect, the REM amount should be 0.001% or more. The preferred range of the REM content is 0.003 - 0.050%. Here, REM (rare earth element) refers to the 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 selected from these rare earth elements, and the content of REM is the total amount of rare earth elements.

[0049] The ferritic stainless steel of the present embodiment consists of Fe and impurities (including inevitable impurities) other than the elements described above. In addition to the elements described above, they can be contained within a range that does not impair the effects of the present invention. In the present embodiment, for example, Bi, Pb, Se, H, etc. may be contained, but in that case, it is preferable to reduce them as much as possible. On the other hand, within the limit of solving the problems of the present invention, the content ratio of these elements is controlled, and if necessary, Bi may be contained in an amount of 0.01% or less, Pb may be contained in an amount of 0.01% or less, Se may be contained in an amount of 0.01% or less, and H may be contained in an amount of 0.01% or less.

[0050] The crystal 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 the workability, it is important to make the crystal grain size number 6.0 or more to obtain fine grains. In order to further exert this effect, the crystal grain size number is preferably 7.0 or more. On the other hand, when the crystal grains grow, the crystal orientation that is advantageous for improving the r-value preferentially grows and can improve the workability. However, if the grains are made too fine, the r-value will decrease. Therefore, the crystal grain size number is 9.0 or less. Preferably, it is 8.5 or less.

[0051] The crystal grain size number is obtained by cutting out a test piece with a length of 30 mm and a width of 20 mm from the steel sheet, embedding it in resin so that the cross-sectional structure in the rolling direction can be observed, and performing mirror polishing and etching. Then, in accordance with JIS G 0551:2013, the grain size number of the cross-sectional structure in the rolling direction is measured. The measurement is carried out 5 times from the center of the plate thickness, and the average value is adopted.

[0052] The ferritic stainless steel of this embodiment has an average r-value (Lankford value) of 1.0 or more. By setting the average r-value to 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.

[0053] The average r-value can be measured by the plastic strain ratio test method of JIS Z 2254 (2008) and can be obtained by the following formula (A).

[0054] Average r-value = (r 0 + 2r 45 + r 90 ) / 4 ···(A) However, r 0 in formula (A) is the r-value in the rolling direction, r 90 is the r-value in the direction perpendicular to the rolling direction, and r 45 is the r-value in the 45-degree direction of the rolling.

[0055] Here, the characteristics when magnetic annealing is performed on the ferritic stainless steel according to this embodiment described above will be described. When the ferritic stainless steel of this embodiment is subjected to magnetic annealing at 800 to 1000 °C for 1 to 10 hours, it 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. 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 annealing condition range. Regarding the magnetic properties in an alternating magnetic field, the maximum magnetic flux density (Bm) at each frequency and the iron loss (W) that affects this value are important. The cause of the increase in iron loss is the eddy current induced in the magnetic material. To reduce the loss due to eddy current, it is effective to increase the electrical resistivity. Therefore, in this embodiment, by controlling to have the above-described chemical composition and controlling the crystal grain size, sufficient electrical resistivity and maximum magnetic flux density can be ensured. Specifically, by performing magnetic annealing using the ferritic stainless steel of this embodiment, a ferritic stainless steel capable of exhibiting 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 can be obtained. Further, thereby, it can be suitably used for motor cases that house stepping motors, hysteresis motors, etc., motor parts such as motor cores, sensors such as electronic throttle sensors and EPS sensors, relays, solenoid valves, and further their cores, yokes, connectors, and housings. Note that in this embodiment, the electrical resistivity can be measured by using an electrical resistance measuring device and by the so-called four-terminal method.

[0056] Further, the form of the ferritic stainless steel of this embodiment is not particularly limited, but it is preferably a steel sheet.

[0057] Next, the manufacturing method of the ferritic stainless steel of this embodiment will be described by taking the case where the form is a "steel sheet" as an example.

[0058] The manufacturing method of the ferritic stainless steel of the present embodiment consists of the steps of steelmaking - hot rolling - annealing and pickling of hot-rolled sheet - cold rolling - annealing of cold-rolled sheet. Regarding the manufacturing conditions of each step, they may be appropriately determined within the range that does not impair the effects of the present invention. However, from the viewpoint of controlling the crystal grain size number and the average r value, it is necessary to appropriately control the conditions of annealing of hot-rolled sheet, the reduction ratio of cold rolling, and annealing of cold-rolled sheet respectively. Hereinafter, each step and conditions of the manufacturing method will be described in detail.

[0059] In steelmaking, a method of melting steel containing the above essential components and components added as required in a converter and then performing secondary refining is suitable. The melted molten steel is made into a slab by casting (continuous casting). The slab is heated to a predetermined temperature and hot-rolled by continuous rolling to a predetermined plate thickness. Considering the crystal grain size and crystal orientation of the final product, the slab heating temperature is preferably 1190 °C or higher and 1300 °C or lower, and the slab thickness is preferably 3.0 mm or more and 300.0 mm or less.

[0060] The annealing step after hot rolling (hot-rolled sheet annealing step) is an important step for achieving appropriate 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 should be 850 °C or higher. Also, if the soaking temperature in hot-rolled sheet annealing exceeds 1000 °C, there is a risk of reduced toughness due to coarsening of crystal grains, so the soaking temperature should be 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 10 seconds to 120 seconds from the viewpoint of completion of recrystallization.

[0061] After hot-rolled sheet annealing, pickling and cold rolling are sequentially performed. At this time, the reduction ratio of cold rolling is preferably 40% or more. If the reduction ratio is less than 40%, there is a risk of poor recrystallization during subsequent annealing of cold-rolled sheet, so the reduction ratio is preferably 40% or more, more preferably 50% or more. On the other hand, if the reduction ratio becomes too high, there is a risk of deterioration in productivity, so the reduction ratio is preferably 95% or less.

[0062] The cold rolling after pickling may be carried out using either a normal Sendzimir mill or a tandem mill. However, considering the workability of the steel sheet, tandem mill rolling is more desirable. From the perspective of obtaining a structure with excellent magnetic properties, Sendzimir mill rolling is more desirable. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected within a general range. Intermediate annealing may be carried out during cold rolling.

[0063] The final annealing (cold rolled sheet annealing) after cold rolling is important from the perspective of controlling the crystal grain size and the average r value. In particular, the soaking temperature needs to be 880°C to 1000°C. If the soaking temperature of the cold rolled sheet annealing is less than 880°C, there is a risk of poor recrystallization, so the soaking temperature should be 880°C or higher. Also, if the soaking temperature of the cold rolled sheet annealing exceeds 1000°C, there is a risk of grain coarsening, so the soaking temperature should be 1000°C or lower. Preferably, the soaking temperature of the cold rolled sheet annealing is 900°C to 950°C. Note that the soaking time (holding time) in cold rolled sheet annealing is not particularly limited, but it is preferably 10 seconds to 120 seconds from the perspective of promoting recrystallization.

[0064] Note that the intermediate annealing carried out during cold rolling and the final annealing after cold rolling may be either batch annealing or continuous annealing. Also, the atmosphere for each annealing may be bright annealing in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas if necessary, or annealing may be carried out in the atmosphere. After the final annealing, salt treatment, pickling, electrolytic pickling, etc. may be carried out.

[0065] The ferritic stainless steel according to this embodiment can be manufactured by the manufacturing method described above. However, for the processes other than the above, they may be appropriately carried out within a range that does not impair the effects of the present invention. For example, after the final annealing, a tension leveling process for shape correction may be carried out, or the sheet may be passed through.

[0066] Note that the ferritic stainless steel according to this 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 the parts. The conditions for magnetic annealing may be appropriately determined according to the applied product, application, etc. For example, in a vacuum of 1×10 -2 ~9×10 -2 Pa, it is desirable to perform the process under the conditions of a heating rate of 1~100 °C / min, a soaking temperature of 800~1000 °C, and a soaking time of 1~10 hours, preferably 1~3 hours. After such heat treatment, cooling with Ar gas or the like may be performed, or air cooling or furnace cooling may also be used. By performing such magnetic annealing, removal of processing strain and coarsening of crystal grains occur, resulting in good magnetic properties and an improvement in magnetic flux density. 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 alternating magnetic field can be improved.

[0067] As described above, the ferritic stainless steel according to this embodiment and its preferred manufacturing method have been described. 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 exhibits excellent magnetic properties in an alternating magnetic field. Furthermore, by performing optimal magnetic annealing on the ferritic stainless steel, it is possible to provide a ferritic stainless steel that is suitable for motor parts such as motor cases and motor cores of stepping motors and hysteresis motors, and is excellent in corrosion resistance and magnetic properties in an alternating magnetic field.

Examples

[0068] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to the conditions used in the following examples. Note that the underlines in the table indicate those outside the scope of the present invention.

[0069] Steel with the compositions shown in Table 1A and Table 1B was melted, and the 200-mm-thick slab was hot-rolled until it reached a thickness of 3 mm. Subsequently, the hot-rolled steel sheet was subjected to heat treatment (hot-rolled sheet annealing) at the temperatures shown in Table 2 for 60 seconds, and then shot pickling was performed. Thereafter, cold rolling (reduction ratio: 73%) was carried out until the sheet thickness reached 0.8 mm, heat treatment (cold-rolled sheet annealing) was performed at the temperatures shown in Table 2 for 1 minute, and then salt treatment and pickling were carried out. Pickling was electrolytic pickling in a solution with a nitric acid concentration of 150 g / L. In this way, a ferritic stainless steel sheet was manufactured.

[0070] Also, a test piece with a length of 30 mm and a width of 20 mm was cut out from near the center in the width direction of the manufactured steel sheet, embedded in resin so that the cross-sectional structure in the rolling direction could be observed, and mirror polishing and etching were performed. Thereafter, in accordance with JIS G 0551:2013, the grain size number of the cross-sectional structure in the rolling direction was measured by the line segment method. The measurement was carried out 5 times from the center of the sheet thickness, and the average value was adopted.

[0071] Also, a tensile test piece No. 13B described in Appendix B of JIS Z 2241:2011 was made from near the center in the width direction of the manufactured steel sheet, and the dimensions and gauge length of the test piece were measured. Thereafter, in accordance with the plastic strain ratio test method of JIS Z 2254:2008, after applying a strain of 14.4% by a tensile test, the dimensions and gauge length of the test piece were measured, and the average r value was calculated from both measurement results. When calculating, the above formula (A) was used. Note that as a criterion for workability, steel grades with an average r value of 1.0 or more were considered qualified.

[0072] Furthermore, the following tests were conducted to evaluate magnetic properties and corrosion resistance. First, the temperature was raised at a rate of 1 to 100 °C / min in a vacuum of 1.3×10 -2 Pa, magnetic annealing was performed at the magnetic annealing temperature shown in Table 2 for 2 hours, and then air cooling was carried out.

[0073] A test piece measuring 30 mm in length and 20 mm in width was cut out from near the center in the width direction 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. Thereafter, in accordance with JIS G 0551:2013, the grain size number of the cross-sectional structure in the rolling direction was measured by the line segment method. The measurement was carried out 5 times from the center of the plate thickness, and the average value was adopted.

[0074] A test piece measuring 75 mm in width and 150 mm in length was cut out from near the center in the width direction of the steel sheet after magnetic annealing and used as a test piece for the JASO-CCT test. The JASO-CCT test was carried out for 12 cycles in accordance with JASO M 610-92. As the judgment criterion for the JASO-CCT test, the rating number was judged by a method conforming to JIS G 0595:2004, and "3" was used as the boundary value. Steel grades with a rating number 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 grades with a rating number of 0 to 3 were judged to have poor corrosion resistance and are indicated by the symbol "×" in Table 2.

[0075] Also, a test piece measuring 4 mm × 60 mm was cut out from near the center in the width direction 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 type manufactured by ULVAC-RIKO, Inc.). The measurement current was 0.4 A, the voltage drop distance was 40 mm, the measurement temperature was room temperature, and it was judged as qualified when it was 60 μΩcm or more.

[0076] Also, 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 in the width direction of the steel after magnetic annealing. This ring sample was cured with Kapton film, wound with 100 turns of the secondary winding, stored in an acrylic case, and further wound with 200 turns of the primary winding to obtain a ring-shaped test sample for AC magnetic measurement. Using this ring-shaped test sample, AC magnetic measurement was carried out with an AC magnetic measurement device (B-H analyzer SY-8258 manufactured by Iwatsu Instruments Co., Ltd.). The measurement conditions were a magnetic field peak value of 0.8 kA / m and a measurement frequency of 1.0 kHz. It was judged as qualified when the maximum magnetic flux density was 0.80 T or more. When the maximum magnetic flux density was less than 0.80 T, it was judged as unqualified.

[0077] The results are shown in Table 2. When the composition and crystal grain size meet the present invention and Cr + 15Al + 20Si ≥ 20 (where Cr, Al, and Si represent the mass% concentrations of the respective elements), it can be seen that the average r value (workability), electrical resistivity, magnetic flux density in alternating current, and the JASO - CCT test results are qualified (「○」). However, No.A2, 6, 7, 16, 20, 21, 24 and 28 were used as reference examples.

[0078] In Comparative Examples B1 to B9, Cr + 15Al + 20Si ≥ 20 (where Cr, Al, and Si represent the mass% concentrations of the respective elements) was not satisfied, and the evaluation results for both magnetic properties and corrosion resistance were 「×」. In Comparative Examples B10 to B12, since the annealing temperature of the cold - rolled sheet annealing was low, the crystal grains before magnetic annealing became excessively fine (not meeting the crystal grain size number), and the average r value (workability) was inferior. In Comparative Examples B13 to B15, since the annealing temperature of the cold - rolled sheet annealing was high, the crystal grains before magnetic annealing became coarse, and as a result, the average r value (workability) was inferior.

[0079]

Table 1A

[0080]

Table 1B

[0081]

Table 2

Industrial Applicability

[0082] The ferritic stainless steel of the present invention is excellent in workability and can achieve both corrosion resistance and magnetic properties in an alternating magnetic field. Therefore, it can be suitably used for motor parts such as motor cases and motor cores of stepping motors, hysteresis motors, etc., sensors such as electronic throttle sensors and EPS sensors, relays, solenoid valves, and further their cores, yokes, connectors, housings, etc. That is, the present invention is extremely industrially beneficial.

Claims

1. 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 to 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, 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: 0.0001-0.0100%, 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. 3. The ferritic stainless steel according to claim 1, which is used for a motor case and a motor part.

4. 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.

5. The ferritic stainless steel according to claim 3 or 4, characterized in that the motor part is a motor core.

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