Soft-magnetic wire, soft-magnetic bar steel, and soft-magnetic part

Abstract

This soft-magnetic wire or bar steel satisfies a specific chemical composition and satisfies formulas (1) and (2).　Formula (1): F1 = 97.0 [C] + 10.9 [Si] + 4.2 [Mn] + 23.8 [P] +172.0 [S] +15.0 [Cu] - 0.03 [Ni] + 5.1 [Cr] + 8.6 [Al] + 34.0 [N] + 8.38 ≥ 19.0. Formula (2): F2 = -9.43 [C] - 0.0061 [Si] - 0.12 [Mn] + 0.054 [S] + 23.5 [N] + 1.398 ≥ 1.360.

Description

Soft magnetic wire and steel bars, and soft magnetic parts

[0001] The present disclosure relates to soft magnetic wire and steel bars, and soft magnetic components.

[0002] In response to the trend toward energy conservation in automobiles, etc., many electrical components (especially electromagnetic components) in automobiles, etc. are required to be more power-efficient and have improved responsiveness. Therefore, the steel materials that make up these electrical components are required to have sufficient electrical resistivity and, as magnetic properties, to have high magnetic flux density in a weak external magnetic field.

[0003] As the steel material, soft magnetic steel material is usually used, in which the magnetic flux density inside the steel material is easily responsive to an external magnetic field. Specific examples of the soft magnetic steel material include ultra-low carbon steel (pure iron-based soft magnetic material) with a C content of approximately 0.1% by mass or less. Generally, wire rods and steel bars are widely used as forms of soft magnetic steel material.

[0004] Patent Document 1 discloses a soft magnetic steel material that can achieve a high AC magnetic flux density even in a medium magnetic field region of about 800 A / m.

[0005] Japanese Patent Application Laid-Open No. 2008-228717

[0006] In recent years, as electrification has progressed more rapidly than ever before and demands for energy conservation have become more stringent, electrical components (e.g., hydraulic control solenoids) are being required to achieve further power savings and improved responsiveness. Therefore, soft magnetic steel materials used in electrical components are being required to achieve sufficient electrical resistivity and have high magnetic flux density (mainly DC magnetic flux density) even in low magnetic field regions (e.g., 400 A / m). From an economic standpoint, elimination of the magnetic annealing process, which requires precise control of the furnace atmosphere and temperature during heat treatment, has also been considered. However, Patent Document 1 does not consider improving the magnetic flux density in low magnetic field regions or elimination of the magnetic annealing process.

[0007] The present disclosure has been made in consideration of these circumstances, and aims to provide a soft magnetic wire or soft magnetic steel bar, as well as a soft magnetic component, that can achieve sufficient electrical resistivity and have sufficient magnetic flux density in a low magnetic field region even in a state where it has not been magnetically annealed.

[0008] A first aspect of the present invention is a soft magnetic wire or steel bar containing C: 0.060% by mass or less (excluding 0% by mass), Si: more than 0.30% by mass and less than 2.10% by mass, Mn: 0.20% by mass or more and less than 0.50% by mass, P: 0.012% by mass or less (including 0% by mass), S: 0.050% by mass or less (including 0% by mass), Cu: 0.30% by mass or less (including 0% by mass), Ni: 0.30% by mass or less (including 0% by mass), Cr: 0.30% by mass or less (including 0% by mass), Al: 0.100% by mass or less (including 0% by mass), and N: 0.0100% by mass or less (including 0% by mass), with the balance consisting of iron and inevitable impurities, and satisfying the following formulas (1) and (2): F1 = 97.0 [C] + 10.9 [Si] + 4.2 [Mn] + 23.8 [P] + 172.0 [S] + 15.0 [Cu] - 0.03 [Ni] + 5.1 [Cr] + 8.6 [Al] + 34.0 [N] + 8.38 ≧ 19.0 ... (1) F2 = - 9.43 [C] - 0.0061 [Si] - 0.12 [Mn] + 0.054 [S] + 23.5 [N] + 1.398 ≧ 1.360 ... (2) In the formulas (1) and (2), [C], [Si], [Mn], [P], [S], [Cu], [Ni], [Cr], [Al] and [N] respectively represent the contents of C, Si, Mn, P, S, Cu, Ni, Cr, Al and N expressed in mass%.

[0009] A second aspect of the present invention is a soft magnetic component containing C: 0.060 mass% or less (excluding 0 mass%), Si: more than 0.30 mass% and less than 2.10 mass%, Mn: 0.20 mass% or more and less than 0.50 mass%, P: 0.012 mass% or less (including 0 mass%), S: 0.050 mass% or less (including 0 mass%), Cu: 0.30 mass% or less (including 0 mass%), Ni: 0.30 mass% or less (including 0 mass%), Cr: 0.30 mass% or less (including 0 mass%), Al: 0.100 mass% or less (including 0 mass%), and N: 0.0100 mass% or less (including 0 mass%), with the remainder consisting of iron and inevitable impurities, and satisfying the following formulas (1) and (2). F1 = 97.0 [C] + 10.9 [Si] + 4.2 [Mn] + 23.8 [P] + 172.0 [S] + 15.0 [Cu] - 0.03 [Ni] + 5.1 [Cr] + 8.6 [Al] + 34.0 [N] + 8.38 ≧ 19.0 ... (1) F2 = - 9.43 [C] - 0.0061 [Si] - 0.12 [Mn] + 0.054 [S] + 23.5 [N] + 1.398 ≧ 1.360 ... (2) In the formulas (1) and (2), [C], [Si], [Mn], [P], [S], [Cu], [Ni], [Cr], [Al] and [N] respectively represent the contents of C, Si, Mn, P, S, Cu, Ni, Cr, Al and N expressed in mass%.

[0010] According to one embodiment of the present invention, it is possible to provide a soft magnetic wire or soft magnetic steel bar and a soft magnetic component that can achieve sufficient electrical resistivity and have sufficient magnetic flux density in a low magnetic field region even in a state where they are not magnetically annealed.

[0011] The present inventors have conducted extensive research to solve the above problems. As a result, the present inventors have found that by appropriately adjusting the chemical composition to satisfy formula (1) so as to achieve sufficient electrical resistivity and also satisfy formula (2), sufficient magnetic flux density can be ensured in a low magnetic field region even in a state that has not been magnetically annealed. Each requirement specified in the embodiments of the present invention will be described in detail below.

[0012] 1. Chemical Composition In embodiments of the present invention, soft magnetic wire or soft magnetic steel bar, as well as soft magnetic components, are the subject of the present invention. The chemical composition is described below. In this specification, "wire" and "steel bar" refer to a material having a circular cross section perpendicular to the longitudinal direction in a preferred embodiment. However, this is not limited to this and may be a non-circular shape, such as a square or a polygon, including a regular hexagon. If the cross section is not circular, the ratio of the longitudinal to lateral dimensions within the cross section is 2 or less. In the case of wire, the diameter (or equivalent circle diameter if the cross section is a shape other than a circle) is not particularly limited, but is, for example, 4 mm to 55 mm. In the case of steel bar, the diameter (or equivalent circle diameter if the cross section is a shape other than a circle) is not particularly limited, but is, for example, 18 mm to 105 mm.

[0013] [C: 0.060% by mass or less (excluding 0% by mass)] C is an element necessary for ensuring mechanical strength. A small amount of C can increase electrical resistance and suppress deterioration of magnetic properties due to eddy currents. However, C dissolves in steel, distorting the crystal lattice and reducing the magnetic moment. Therefore, excessive C content degrades magnetic properties. Therefore, the C content is set to 0.060% by mass or less, preferably 0.040% by mass or less, more preferably 0.020% by mass or less, and even more preferably 0.010% by mass or less. While there is no particular lower limit for the C content, the increase in magnetic moment due to reduced C content saturates at 0.001% by mass. Therefore, a lower limit of 0.001% by mass is preferred for the purpose of ensuring minimum strength.

[0014] In this specification, "not containing 0% by mass" means that the element is intentionally added, i.e., the element is contained in an amount exceeding the impurity level. On the other hand, in this specification, "containing 0% by mass" means that the element may be any added element and includes embodiments in which the element is not intentionally added, i.e., the content is at or below the unavoidable impurity level (it does not exclude cases in which the element is intentionally added).

[0015] [Si: More than 0.30 mass% and less than 2.10 mass%] Si is an element that acts as a deoxidizer during melting and refining. It also increases electrical resistance and can suppress deterioration of magnetic properties due to eddy currents. Therefore, the Si content is more than 0.30 mass%, preferably 0.50 mass% or more, more preferably 0.60 mass% or more, and even more preferably 1.0 mass% or more. However, excessive solid solution of Si reduces the magnetic moment in the steel and also deteriorates cold forgeability. Therefore, the Si content is less than 2.10 mass%. It is preferably 2.04 mass% or less, and even more preferably 2.00 mass% or less.

[0016] [Mn: 0.20% by mass or more, less than 0.50% by mass] Mn is an element that acts as a deoxidizer during melting and refining, and also binds with S in the steel to suppress embrittlement caused by S. Mn in the steel increases the electrical resistance of parts as a solid solution or precipitate, and can suppress deterioration of magnetic properties due to eddy currents. Therefore, the Mn content is set to 0.20% by mass or more. However, excessive Mn content reduces the magnetic moment in the steel, resulting in deterioration of magnetic properties. Therefore, the Mn content is set to less than 0.50% by mass, preferably 0.40% by mass or less, and more preferably 0.35% by mass or less.

[0017] [P: 0.012% by mass or less (including 0% by mass)] P has the effect of increasing electrical resistance, but is an element that is prone to segregation at grain boundaries and has a negative effect on hot workability and cold workability, so it is desirable to reduce the P content as much as possible. Therefore, the P content is set to 0.012% by mass or less, preferably 0.010% by mass or less.

[0018] [S: 0.050% by mass or less (including 0% by mass)] S increases the electrical resistance of parts and suppresses degradation of magnetic properties due to eddy currents. Furthermore, as described above, S combines with Mn to form MnS, so adding S within an appropriate range improves magnetic properties. However, excessive S content reduces magnetic properties and forgeability due to the formation of FeS. Taking these effects into consideration, the S content is set to 0.050% by mass or less. The lower limit of the S content is not particularly limited, but may be 0% by mass or more. For example, in the case of a steel material containing Mn, it is preferable to contain 0.002% by mass or more.

[0019] [Cu: 0.30% by mass or less (including 0% by mass)] Cu is an optional additive element. By dissolving in the ferrite phase, Cu improves strength and magnetic properties by increasing electrical resistivity. On the other hand, excessive Cu content deteriorates magnetic properties and cold forgeability, and also increases costs. Therefore, the Cu content is set to 0.30% by mass or less, preferably 0.20% by mass or less, and more preferably 0.10% by mass or less.

[0020] [Ni: 0.30% by mass or less (including 0% by mass)] Ni is an optional additive element. Ni improves strength, similar to Cu. On the other hand, excessive Ni content deteriorates magnetic properties and forgeability and increases costs. Therefore, the Ni content is set to 0.30% by mass or less, preferably 0.20% by mass or less, and more preferably 0.10% by mass or less.

[0021] [Cr: 0.30% by mass or less (including 0% by mass)] Cr is an optional additive element. Cr increases the electrical resistance of parts and suppresses deterioration of magnetic properties due to eddy currents. On the other hand, excessive Cr content deteriorates magnetic properties and forgeability, and also increases costs. Therefore, the Cr content is set to 0.30% by mass or less, preferably 0.20% by mass or less, and more preferably 0.10% by mass or less.

[0022] [Al: 0.100% by mass or less (including 0% by mass)] Al is an optional additive element. Al bonds with N to form AlN, which can suppress deterioration of magnetic properties due to a decrease in magnetic moment. On the other hand, excessive Al content causes excessive AlN formation, inhibiting grain growth and increasing grain boundaries that are harmful to magnetic properties. Furthermore, Al that does not bond with N dissolves in the steel, reducing the magnetic moment and degrading the magnetic properties. Therefore, when Al is intentionally added (when the Al content exceeds 0% by mass), the Al content is set to 0.100% by mass or less, preferably 0.070% by mass or less, more preferably less than 0.05% by mass, even more preferably 0.030% by mass or less, and even more preferably less than 0.002% by mass.

[0023] [N: 0.0100% by mass or less (including 0% by mass)] As mentioned above, N bonds with Al to form AlN, so adding N within an appropriate range improves magnetic properties. However, excessive N content dissolves in the steel, reducing the magnetic moment and degrading magnetic properties, and also causes deterioration of forgeability due to age hardening. Therefore, the N content is set to 0.0100% by mass or less, preferably 0.0070% by mass or less, more preferably 0.0050% by mass or less, and even more preferably 0.0030% by mass or less.

[0024] The basic components of the wire rod, steel bar, and soft magnetic component according to the embodiments of this specification are as described above, and in one preferred embodiment, the balance is iron and inevitable impurities. Elements (e.g., As, Sb, Sn, O, H, etc.) that are introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. are permitted as inevitable impurities. Note that, for example, elements such as P are generally preferable when their content is low, and therefore are inevitable impurities, but their composition ranges are separately specified as described above. Therefore, in this specification, the "unavoidable impurities" that make up the balance are a concept that excludes elements whose composition ranges are separately specified.

[0025] 2. F1 (Parameter Related to Electrical Resistivity) The present inventors came up with the idea of ​​adjusting the chemical composition to satisfy the following formula (1) in order to achieve electrical resistivity sufficient to improve the responsiveness of soft magnetic components, particularly to DC magnetic fields: F1 = 97.0 [C] + 10.9 [Si] + 4.2 [Mn] + 23.8 [P] + 172.0 [S] + 15.0 [Cu] - 0.03 [Ni] + 5.1 [Cr] + 8.6 [Al] + 34.0 [N] + 8.38 ≧ 19.0 (1) In formula (1), [C], [Si], [Mn], [P], [S], [Cu], [Ni], [Cr], [Al], and [N] represent the contents of C, Si, Mn, P, S, Cu, Ni, Cr, Al, and N, respectively, expressed in mass%.

[0026] The coefficients of F1 and the calculation procedures thereof are as disclosed in Patent Document 1. By adjusting the chemical composition so that F1 is 19.0 or more, a sufficient electrical resistivity (≈19.0 μΩcm or more) can be achieved.

[0027] 3. F2 (Parameter Related to Magnetic Flux Density) In order to improve the magnetic flux density of soft magnetic components in low magnetic field regions, even in a non-magnetically annealed state, it is important to increase the magnetic moment in the ferrite phase. Generally, increasing the content of alloying elements to increase electrical resistivity tends to decrease the magnetic moment. However, it is also known that when precipitates are formed by bonding between alloying elements, the magnetic moment in the ferrite phase can increase or decrease in response to changes in the amount of solid solution of the alloying elements and the phase fraction of the precipitates. Therefore, in order to express the relationship between the content of alloying elements and magnetic flux density with a single index, the inventors performed multiple regression analysis using the content (mass%) of each alloying element in the soft magnetic steel materials shown in the examples (Test Nos. 1 to 7 (Test Materials A to G)) described below as explanatory variables and the magnetic flux density B4 (unit: T) at 400 A / m of the soft magnetic component before magnetic annealing as the objective variable. As a result, the coefficient of determination R ２ = 0.9958. Based on this, a parameter F2 for designing the magnetic flux density B4 at 400 A / m was found, as shown in the following formula (2): F2 = -9.43 [C] - 0.0061 [Si] - 0.12 [Mn] + 0.054 [S] + 23.5 [N] + 1.398 ≧ 1.360 ... (2) In formula (2), [C], [Si], [Mn], [S], and [N] represent the contents of C, Si, Mn, S, and N, respectively, expressed in mass%.

[0028] As shown in the above formula (2), by adjusting the chemical composition so that F2 is 1.360 or more, a sufficient magnetic flux density B4 (≈1.36 T or more) can be obtained. By using a soft magnetic wire or steel bar having a sufficient magnetic flux density B4, as well as a soft magnetic part (e.g., an iron core of a solenoid), it is possible to realize, for example, an electrical part (e.g., a solenoid for hydraulic control) having sufficient attractive force.

[0029] 4. Manufacturing Method The soft magnetic wire or steel bar according to the embodiment of the present invention can be manufactured by a known method so as to satisfy the above-mentioned chemical composition, F1 and F2. For example, the soft magnetic wire or steel bar may be manufactured by melting and casting a steel raw material that satisfies the above-mentioned chemical composition, F1 and F2, and then hot rolling the melt.

[0030] In this specification, as described above, the wire rod or steel bar includes one having a circular cross section perpendicular to the longitudinal direction (although as described above, the cross section may be other than a circle). Such wire rod or steel bar can be obtained by the above-described hot rolling or hot forging and stretching, but in addition, one having a desired shape obtained by further performing cold working such as cold drawing after hot rolling or hot forging and stretching is also included in the "wire rod" or "steel bar" according to the embodiment of the present invention.

[0031] Unlike, for example, an electromagnetic steel sheet, the soft magnetic wire and steel bar according to the embodiment of the present invention can have improved magnetic properties by magnetic annealing. The soft magnetic wire and steel bar according to the embodiment of the present invention have sufficient magnetic flux density in a low magnetic field region even in a state where they are not magnetically annealed, but they may be subjected to magnetic annealing as necessary.

[0032] 5. Soft Magnetic Components Soft magnetic components can be obtained by processing the wire rods and steel bars according to the embodiments of the present invention. However, this is not limited to this. As long as the wire rods and steel bars according to the embodiments of the present invention have the above-described chemical composition, they can be obtained using other steel materials, particularly other wire rods or steel bars. Soft magnetic components obtained in this manner are also within the technical scope of the embodiments of the present invention. Soft magnetic components obtained using wire rods or steel bars often have a circular outer periphery or a shape with a partially deformed circle in a cross section perpendicular to the axial direction (e.g., in one or more cross sections when multiple cross sections are observed). However, this is not a characteristic of all soft magnetic components obtained using wire rods or steel bars, and some do not have this characteristic.

[0033] Soft magnetic parts include various electromagnetic parts for automobiles, trains, ships, etc., including iron core materials for electromagnetic valves, solenoids, relays, etc., magnetic shielding materials, actuator members, and motor sensor members.

[0034] To process soft magnetic wire or steel bar into soft magnetic parts, for example, hot forging may be performed, followed by cutting as necessary. Forging as a forming process may be either hot forging or cold forging, but hot forging can reduce the influence of processing strain. To further improve magnetic properties, hot forging is preferably performed at 1100°C or higher.

[0035] The magnetic properties of the soft magnetic component according to the embodiment of the present invention can be improved by magnetic annealing. The soft magnetic component according to the embodiment of the present invention has sufficient magnetic flux density in a low magnetic field region even in a state where it is not magnetically annealed. However, magnetic annealing may be performed as necessary. For example, magnetic annealing may be performed on a soft magnetic component after hot forging and / or cutting.

[0036] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0037] Test materials Nos. A to G having the chemical compositions shown in Table 1 were produced by button melting to obtain cast materials. The resulting cast materials were heated to 1000 to 1100°C, hot forged, and then normalized at 950°C for 1 hour to produce wire rods with a diameter of 10 mm. Test materials Nos. H to I having the chemical compositions shown in Table 1 were produced by vacuum melting to obtain cast materials. The resulting cast materials were heated to 1000 to 1100°C, hot forged, and then normalized at 900°C for 1 hour to produce wire rods with a diameter of 10 mm. Note that the melting method for test materials Nos. H to I was different from that for test materials Nos. A to G, but the subsequent normalization was performed, so it is believed that the effects of the different melting methods were eliminated.

[0038]

[0039] The obtained wire rod was wire-cut to obtain a cylindrical test piece with a diameter of 8 mm and a length of 12 mm. At this time, the center of the cylinder before and after processing was aligned. The cylindrical test piece was hot-worked using a hot processing reproduction test device (Fuji Electric Power Machinery Co., Ltd. THERMECMASTOR_Z). Specifically, the temperature was first raised to 1200 ° C at 10 ° C / s and held for 300 seconds after the temperature increase. Then, it was cooled to 1100 ° C at 5 ° C / s and compressed at a reduction rate of 60% and a strain rate of 1.0 / s. After completion of compression, it was cooled to 500 ° C at 5 ° C / s and then cooled to room temperature by gas blowing.

[0040] The compressed cylindrical test pieces were processed into ring shapes with an outer diameter of 10 mm, an inner diameter of 6 mm, and a thickness of 3 mm to obtain ring-shaped test pieces (soft magnetic parts) of Test Nos. 1 to 7 and 9 to 10 (see Table 2 for the test materials used). At this time, the center of the cylinder before processing and the center of the ring after processing were aligned.

[0041] An insulating-coated conductor was wound uniformly around the circumference of the ring-shaped test specimen as a magnetic field application coil and a magnetic flux detection coil. After demagnetization by applying an AC magnetic field, a B-H curve was measured at room temperature using a DC current under a maximum magnetic field of 400 A / m, and the magnetic flux density B4 at 400 A / m was determined. After measuring the magnetic flux density B4, the ring-shaped test specimen of Test No. 1 was unwound and subjected to magnetic annealing in a vacuum furnace at 850°C for 3 hours (referred to as Test No. 8). The magnetic flux density B4 was then measured again using the procedure described above. The results are summarized in Table 2. Note that "F1" and "F2" in Table 2 are values ​​calculated using formulas (1), (2), and the chemical composition shown in Table 1.

[0042]

[0043] The following can be considered from Tables 1 and 2. Test Nos. 1, 5, 9, and 10 are examples that satisfy all of the requirements defined in the embodiments of the present invention, in which F1 was adjusted to a predetermined value or greater so as to achieve sufficient electrical resistivity (i.e., F1≧19.0), and even without magnetic annealing, they had sufficient magnetic flux density in the low magnetic field region (i.e., magnetic flux density B4≧1.36). On the other hand, Test Nos. 2 to 4 and 6 to 7 are examples that do not satisfy the requirements defined in the embodiments of the present invention, in which F1 was less than 19.0 or the magnetic flux density B4 was insufficient.

[0044] In Test No. 2, F2 was less than 1.360, and the magnetic flux density B4 was insufficient.

[0045] In Test No. 3, the C content was excessive, F2 was less than 1.360, and the magnetic flux density B4 was insufficient.

[0046] In Test No. 4, the Si content was insufficient, F1 was less than 19.0, and based on Patent Document 1, it is presumed that the electrical resistivity was insufficient.

[0047] In Test No. 6, the Si content was excessive, F2 was less than 1.360, and the magnetic flux density B4 was insufficient.

[0048] In Test No. 7, the Mn content was excessive, F2 was less than 1.360, and the magnetic flux density B4 was insufficient.

[0049] Test No. 8 is an example in which magnetic annealing was performed, unlike Test Nos. 1 to 7, 9, and 10. Test No. 8 exhibited a high magnetic flux density B4 by performing magnetic annealing.

[0050] This application claims priority from Japanese Patent Application No. 2024-025688, filed February 22, 2024, and Japanese Patent Application No. 2024-191982, filed October 31, 2024. Japanese Patent Application Nos. 2024-025688 and 2024-191982 are incorporated herein by reference.

Claims

1. A soft magnetic wire or steel bar containing C: 0.060 mass% or less (excluding 0 mass%), Si: more than 0.30 mass% and less than 2.10 mass%, Mn: 0.20 mass% or more and less than 0.50 mass%, P: 0.012 mass% or less (including 0 mass%), S: 0.050 mass% or less (including 0 mass%), Cu: 0.30 mass% or less (including 0 mass%), Ni: 0.30 mass% or less (including 0 mass%), Cr: 0.30 mass% or less (including 0 mass%), Al: 0.100 mass% or less (including 0 mass%), and N: 0.0100 mass% or less (including 0 mass%), with the remainder consisting of iron and inevitable impurities, and satisfying the following formulas (1) and (2). F1 = 97.0 [C] + 10.9 [Si] + 4.2 [Mn] + 23.8 [P] + 172.0 [S] + 15.0 [Cu] - 0.03 [Ni] + 5.1 [Cr] + 8.6 [Al] + 34.0 [N] + 8.38 ≧ 19.0 ... (1) F2 = - 9.43 [C] - 0.0061 [Si] - 0.12 [Mn] + 0.054 [S] + 23.5 [N] + 1.398 ≧ 1.360 ... (2) In the formulas (1) and (2), [C], [Si], [Mn], [P], [S], [Cu], [Ni], [Cr], [Al] and [N] respectively represent the contents of C, Si, Mn, P, S, Cu, Ni, Cr, Al and N expressed in mass%.

2. A soft magnetic part containing C: 0.060 mass% or less (excluding 0 mass%), Si: more than 0.30 mass% and less than 2.10 mass%, Mn: 0.20 mass% or more and less than 0.50 mass%, P: 0.012 mass% or less (including 0 mass%), S: 0.050 mass% or less (including 0 mass%), Cu: 0.30 mass% or less (including 0 mass%), Ni: 0.30 mass% or less (including 0 mass%), Cr: 0.30 mass% or less (including 0 mass%), Al: 0.100 mass% or less (including 0 mass%), and N: 0.0100 mass% or less (including 0 mass%), with the remainder consisting of iron and inevitable impurities, and satisfying the following formulas (1) and (2). F1 = 97.0 [C] + 10.9 [Si] + 4.2 [Mn] + 23.8 [P] + 172.0 [S] + 15.0 [Cu] - 0.03 [Ni] + 5.1 [Cr] + 8.6 [Al] + 34.0 [N] + 8.38 ≧ 19.0 ... (1) F2 = - 9.43 [C] - 0.0061 [Si] - 0.12 [Mn] + 0.054 [S] + 23.5 [N] + 1.398 ≧ 1.360 ... (2) In the formulas (1) and (2), [C], [Si], [Mn], [P], [S], [Cu], [Ni], [Cr], [Al] and [N] respectively represent the contents of C, Si, Mn, P, S, Cu, Ni, Cr, Al and N expressed in mass%.

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