Soft magnetic wires, soft magnetic steel bars, and soft magnetic components

Abstract

To provide a soft magnetic wire material or steel bar, excellent in magnetic properties, cold forgeability and corrosion resistance, without adding a large amount of alloy elements, and also to provide a soft magnetic component using the same.SOLUTION: A soft magnetic wire material or steel bar includes 0.075 mass% or less C, 1.00 mass% or less Si, 0.10 to 1.00 mass% Mn, 0.100 mass% or less P, 0.100 mass% or less S, 1.00 mass% or less Cu, 1.00 mass% or less Ni, 1.00 mass% or less Cr, less than 0.030 mass% Al, 0.0200 mass% or less N, 0.002 to 0.050 mass% Sn, and the balance Fe with inevitable impurities, and includes ferrite by 80% or more in an area ratio, with a crystal grain size number of the ferrite being 5.0 or less, and Vickers hardness being HV 140 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to soft magnetic wire, soft magnetic bar steel, and soft magnetic parts.

Background Art

[0002] In response to energy conservation in automobiles and the like, many electrical components (especially electromagnetic components) in automobiles and the like are required to be power-saving and finely controlled. In particular, for the steel materials constituting the magnetic circuit, as magnetic properties, it is required that they can be easily magnetized by a weak external magnetic field and have a small coercive force.

[0003] Normally, as the above steel material, a soft magnetic steel material in which the magnetic flux density inside the steel material easily responds to an external magnetic field is used. Specifically, as the above soft magnetic steel material, for example, an extra-low carbon steel (pure iron-based soft magnetic material) having a C content of about 0.1 mass% or less is used. As forms of the soft magnetic steel material, plate materials (electromagnetic steel sheets), wire materials, and bar steels are generally widely used. Among these, plate materials are often used to obtain soft magnetic parts used as electromagnetic parts by performing relatively simple processing. On the other hand, when obtaining a soft magnetic part by processing a wire material or bar steel, after performing hot rolling on the wire material or bar steel, secondary processing steps, that is, steel wires obtained by performing pickling, lubrication treatment, drawing, etc., are often sequentially subjected to part forming (forging, cutting), magnetic annealing, etc. In recent years, from the viewpoint of reducing manufacturing costs, wire materials and bar steels are often formed by cold forging to obtain soft magnetic parts, and further complex shaping, high dimensional accuracy, and reduction of manufacturing costs during forging are required. For soft magnetic wire materials or bar steels, it is desired that the deformation resistance during cold forging is small.

[0004] Furthermore, electromagnetic components are required to have corrosion resistance depending on the use environment. Electromagnetic stainless steel is used in the parts where this corrosion resistance is required. Electromagnetic stainless steel is a special steel having both magnetic properties and corrosion resistance, and its applications include parts utilizing magnetic circuits such as sensors, actuators, motors, etc., and electromagnetic components used in corrosive environments. [[ID=二十一]] [[ID=二十二]]

[0005] [[ID=二十三]] Conventionally, 13Cr-based electromagnetic stainless steel has been used as the electromagnetic stainless steel mentioned above. For example, Patent Document 1 shows a method for improving the cold forgeability and machinability of 13Cr-based electromagnetic stainless steel.

[0006] On the other hand, for example, Patent Documents 2 and 3 disclose how to improve the strength and machinability of ultra-low carbon steel without reducing its magnetic properties by controlling the composition and the dispersion state of sulfides in the steel. Patent document 4 discloses a steel material that achieves both corrosion resistance and magnetic properties, and a method for manufacturing the same. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 06-228717 [Patent Document 2] Japanese Patent Publication No. 2010-235976 [Patent Document 3] Japanese Patent Publication No. 2007-46125 [Patent Document 4] Japanese Patent Publication No. 2014-198874 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the 13Cr-based electromagnetic stainless steel disclosed in Patent Document 1 is difficult to process, making it difficult to obtain excellent cold forgeability like that of ultra-low carbon steel. Furthermore, due to the high number of alloying elements, the material price is high, and when the price of alloying elements rises, the material price rises in conjunction with it, and there are also problems such as difficulty in supplying the material. Furthermore, the ultra-low carbon steels disclosed in Patent Documents 2 and 3 have not considered cases where corrosion resistance is required, and there is a risk that sufficient corrosion resistance cannot be obtained.

[0009] The steel material disclosed in Patent Document 4 achieves both excellent corrosion resistance and magnetic properties by forming an amorphous layer in the surface oxide film, but it requires the addition of 1% by mass or more of Si, which results in high deformation resistance during cold forging, i.e., poor cold forgeability.

[0010] This disclosure has been made in view of the above circumstances and aims to provide soft magnetic wires or soft magnetic steel bars and soft magnetic components that have improved magnetic properties, cold forgeability, and corrosion resistance without adding large amounts of alloying elements. [Means for solving the problem]

[0011] One aspect of the present invention is: C: 0.075% by mass or less (including 0% by mass), Si: 1.00 mass% or less (including 0 mass%) Mn: 0.10% by mass or more, 1.00% by mass or less, P: 0.100% by mass or less (including 0% by mass), S: 0.100% by mass or less (including 0% by mass), Cu: 1.00% by mass or less (including 0% by mass), Ni: 1.00% by mass or less (including 0% by mass), Cr: 1.00% by mass or less (including 0% by mass), Al: Less than 0.030% by mass (including 0% by mass), N: 0.0200% by mass or less (including 0% by mass), and Sn: 0.002 mass% or more, 0.050 mass% or less, It contains, with the remainder consisting of iron and unavoidable impurities. It contains 80% or more ferrite by area, and the grain size number of the ferrite is 5.0 or less. It is a soft magnetic wire or steel bar with a Vickers hardness of HV140 or less.

[0012] Aspect 2 of the present invention is a soft magnetic wire or steel bar according to Aspect 1, wherein the Si content is 0.50% by mass or less (including 0% by mass).

[0013] Aspect 3 of the present invention is the soft magnetic wire or bar steel described in Aspect 1 or 2, further containing Mo: 1.00 mass% or less (excluding 0 mass%).

[0014] Aspect 4 of the present invention is the soft magnetic wire or bar steel described in any one of Aspects 1 to 3, further containing one or more selected from the group consisting of Ti: 0.100 mass% or less (excluding 0 mass%), V: 0.100 mass% or less (excluding 0 mass%), and Nb: 0.100 mass% or less (excluding 0 mass%).

[0015] Aspect 5 of the present invention is the soft magnetic wire or bar steel described in any one of Aspects 1 to 4, further containing B: 0.0050 mass% or less (excluding 0 mass%).

[0016] Aspect 6 of the present invention is the soft magnetic wire or bar steel described in any one of Aspects 1 to 5, containing ferrite with an area ratio of 90% or more.

[0017] Aspect 7 of the present invention is C: 0.075 mass% or less (including 0 mass%), Si: 1.00 mass% or less (including 0 mass%), Mn: 0.10 mass% or more and 1.00 mass% or less, P: 0.100 mass% or less (including 0 mass%), S: 0.100 mass% or less (including 0 mass%), Cu: 1.00 mass% or less (including 0 mass%), Ni: 1.00 mass% or less (including 0 mass%), Cr: 1.00 mass% or less (including 0 mass%), Al: less than 0.030 mass% (including 0 mass%), N: 0.0200 mass% or less (including 0 mass%), and Sn: 0.002 mass% or more and 0.050 mass% or less, containing the balance being iron and unavoidable impurities, containing ferrite with an area ratio of 80% or more, and the crystal grain size number of the ferrite being 5.0 or less, These are soft magnetic steel parts with a Vickers hardness of HV140 or less.

[0018] Embodiment 8 of the present invention is a soft magnetic wire steel component according to Embodiment 7, wherein the Si content is 0.50% by mass or less (including 0% by mass).

[0019] Aspect 9 of the present invention is a soft magnetic steel component according to aspect 7 or 8, further containing Mo: 1.00% by mass or less (excluding 0% by mass).

[0020] Embodiment 10 of the present invention is a soft magnetic steel component according to any of embodiments 7 to 9, further containing one or more selected from the group consisting of Ti: 0.100 mass% or less (excluding 0 mass%), V: 0.100 mass% or less (excluding 0 mass%), and Nb: 0.100 mass% or less (excluding 0 mass%).

[0021] Aspect 11 of the present invention is a soft magnetic steel component according to any one of aspects 7 to 10, further containing B: 0.0050 mass% or less (excluding 0 mass%).

[0022] Aspect 12 of the present invention is a soft magnetic steel component according to any one of aspects 7 to 11, which contains ferrite in an area ratio of 90% or more. [Effects of the Invention]

[0023] According to one embodiment of the present invention, it is possible to provide soft magnetic wires or soft magnetic steel bars and soft magnetic components that have improved magnetic properties (low coercivity), cold forgeability, and corrosion resistance without adding large amounts of alloying elements. [Modes for carrying out the invention]

[0024] The inventors diligently studied to solve the above problems. As a result, they found that by appropriately adjusting the chemical composition, and further setting the ferrite fraction in the metal structure to 80% or more by area, and the crystal grain size number of the ferrite to 5.0 or less, and the Vickers hardness to HV140 or less, it is possible to achieve excellent magnetic properties, excellent cold forgeability, and excellent corrosion resistance without adding large amounts of alloying elements. The requirements specified in the embodiments of the present invention will be described in detail below.

[0025] 1.Chemical composition The embodiments of the present invention focus on soft magnetic wires or soft magnetic steel bars and soft magnetic components (also referred to as "soft magnetic steel components"). The chemical composition is described below. As described below, the chemical composition of the wires, steel bars and soft magnetic components according to the embodiments of the present invention has a low content of additive elements, which can reduce manufacturing costs. In this specification, "wire rod" and "steel bar" have a circular cross-section perpendicular to the longitudinal direction in preferred embodiments, but are not limited to this and may have other shapes such as squares or polygons including regular hexagons. If the cross-sectional shape is not circular, the ratio of the longitudinal direction to the short direction within the cross-section is 2 or less. In the case of wire rod, its diameter (or equivalent diameter if the cross-section is not circular) is not particularly limited, but is for example 3.0 mm to 55 mm. Similarly, in the case of steel bar, its diameter (or equivalent diameter if the cross-section is not circular) is not particularly limited, but is for example 18 mm to 105 mm.

[0026] [C: 0.075% by mass or less (including 0% by mass)] Carbon (C) is the element that governs the balance between strength and ductility in steel; reducing the amount of C reduces strength and improves ductility. Vacuum degassing treatments are performed to reduce the C content, but it is difficult to completely eliminate C in the normal steel manufacturing process, and it is usually present as an impurity of about 0.001 to 0.010 mass%. Magnetic properties are better when the ferrite fraction, which is a ferromagnetic material, is high. If the amount of C is excessive, the ferrite grain size becomes small, and the grain boundaries hinder magnetic domain wall movement, thus deteriorating the magnetic properties. If the amount of C is even more excessive (for example, 0.100 mass% or more), the ferrite area ratio decreases significantly and cementite precipitation is promoted, and cementite hinders magnetic domain wall movement, thus deteriorating the magnetic properties. In addition, if the amount of C is too high, the precipitation of cementite, which is the initiation point of cracks, becomes excessive, and cold forgeability decreases. Furthermore, since cementite acts as a local cell in corrosive environments, if the amount of C is excessive and the amount of cementite increases too much, corrosion resistance deteriorates. Therefore, the upper limit for the amount of carbon (C) was set at 0.075% by mass. The amount of carbon is preferably 0.060% by mass or less, and more preferably 0.050% by mass or less. As long as the amount of carbon is 0.075% by mass or less, carbon may be intentionally added.

[0027] In this specification, "0% by mass" means that the element has been intentionally added, i.e., that it is present in an amount exceeding the impurity level. On the other hand, in this specification, "0% by mass" means that embodiments in which the element is not intentionally added, i.e., that the content is at or below the unavoidable impurity level (this does not exclude cases in which the element is intentionally added).

[0028] [Si: 1.00% by mass or less (including 0% by mass)] Si has the effect of improving magnetic properties. Si may be added to effectively exert the above effect (i.e., it may not contain 0 mass%). However, Si is not an essential element and may not be intentionally added if the required magnetic properties can be satisfied (i.e., it may contain 0 mass%). Si is sometimes used as a deoxidizer during melting. In the normal steel manufacturing process, it is difficult to completely eliminate the amount of Si, and it is usually present as an impurity of about 0.005 to 0.01 mass%. If Si is present in excess, the magnetic properties and cold forgeability will decrease. For this reason, the upper limit of the Si amount is set at 1.00 mass%. The Si amount is preferably 0.75 mass% or less, more preferably 0.50 mass% or less, and even more preferably 0.30 mass% or less.

[0029] [Mn: 0.10 mass% or more, 1.00 mass% or less] Mn acts effectively as a deoxidizing agent. Furthermore, Mn combines with S contained in the steel material to form MnS precipitates, which are finely dispersed and act as chip breakers for the chips generated during cutting, contributing to improved machinability. To effectively exert these effects, the amount of Mn was set to 0.10 mass% or more. Preferably, the amount of Mn is 0.15 mass% or more, and more preferably 0.20 mass% or more. If the amount of Mn is too high, the magnetic properties and cold forgeability deteriorate, so the amount of Mn was set to 1.00 mass% or less. Preferably, the amount of Mn is 0.75 mass% or less, and more preferably 0.50 mass% or less.

[0030] [P: 0.100% by mass or less (including 0% by mass)] P is an unavoidable impurity in steel, as it causes grain boundary segregation, degrading magnetic properties and cold forgeability. Therefore, the amount of P is kept below 0.100 mass% to improve magnetic properties. The amount of P is preferably 0.075 mass% or less, and more preferably 0.050 mass% or less. The lower the amount of P, the better, but it is usually present at around 0.005 mass%.

[0031] [S: 0.100% by mass or less (including 0% by mass)] S is an unavoidable impurity in steel, as it causes grain boundary segregation, degrading magnetic properties and cold forgeability. Therefore, the amount of S is kept below 0.100 mass% to improve magnetic properties. The amount of S is preferably 0.075 mass% or less, and more preferably 0.050 mass% or less. The less S there is, the better, but it is usually present at around 0.005 to 0.010 mass%.

[0032] [Cu: 1.00% by mass or less (including 0% by mass)] Cu is an element that improves corrosion resistance. Cu may be added to effectively achieve the above effect, and since intentional addition is not required, it can be present at 0 mass%. In other words, the lower limit is 0 mass%. When Cu is intentionally added, the Cu content is preferably 0.03 mass% or more, more preferably 0.05 mass% or more. However, if Cu is present in excess, the magnetic moment of the Fe matrix decreases, and sufficient magnetic properties cannot be obtained, so the amount of Cu should be 1.00 mass% or less. The amount of Cu is preferably 0.50 mass% or less, more preferably 0.30 mass% or less, and even more preferably 0.10 mass% or less. Even when no addition is made, Cu is usually present at an impurity level of about 0.01 mass%.

[0033] [Ni: 1.00% by mass or less (including 0% by mass)] Ni is an element that improves corrosion resistance. Ni may be added to effectively achieve the above effect, or it may be included at 0 mass% as intentional addition is not required. In other words, the lower limit is 0 mass%. When Ni is intentionally added, the Ni content is preferably 0.03 mass% or more. More preferably 0.05 mass% or more. However, if Ni is present in excess, the magnetic moment of the Fe matrix decreases and sufficient magnetic properties cannot be obtained, so the amount of Ni should be 1.00 mass% or less. The amount of Ni is preferably 0.50 mass% or less, more preferably 0.30 mass% or less, and even more preferably 0.10 mass% or less. Even when Ni is not added, the impurity level is usually around 0.01 mass%.

[0034] [Cr: 1.00% by mass or less (including 0% by mass)] Cr is an element that improves corrosion resistance. To effectively achieve the above effect, Cr may be added, or it may not be necessary to intentionally add it, so it can be present at 0 mass%. In other words, the lower limit is 0 mass%. When Cr is intentionally added, the Cr content is preferably 0.03 mass% or more. More preferably 0.05 mass% or more. However, if there is an excess of Cr, the magnetic moment of the Fe matrix decreases and sufficient magnetic properties cannot be obtained, so the amount of Cr should be 1.00 mass% or less. The amount of Cr is preferably 0.50 mass% or less, more preferably 0.30 mass% or less, and even more preferably 0.10 mass% or less. Even when no Cr is added, the impurity level is usually around 0.01 mass%.

[0035] [Al: Less than 0.030% by mass (including 0% by mass)] Al is an element that reduces the magnetic moment of the Fe matrix and thus degrades its magnetic properties. Furthermore, Al is an unavoidable impurity that can combine with N in the steel to form AlN. The formed AlN acts as a pinning particle that suppresses grain growth during the annealing process, increasing the grain boundaries that hinder Al domain wall movement and thus degrading magnetic properties. In addition, the refinement of ferrite grains due to the suppression of grain growth also worsens cold forgeability. Therefore, the amount of Al is set to less than 0.030 mass%. To exhibit better magnetic properties, the amount of Al is preferably 0.025 mass% or less, and more preferably 0.020 mass% or less. The less Al there is, the better, but it is usually present at around 0.001 mass%.

[0036] [N: 0.0200% by mass or less (including 0% by mass)] Nitrogen (N) is an unavoidable impurity that dissolves in steel, causing strain aging and degrading cold forgeability. Furthermore, high N content leads to the formation of nitrides, which act as pinning particles during the annealing process, inhibiting grain growth and increasing grain boundaries that hinder domain wall movement, thus reducing magnetic properties. Considering these factors, the upper limit for N content is set at 0.0200 mass%. Preferably, N content is 0.0150 mass% or less, and more preferably 0.0100 mass% or less. While lower N content is preferable, it is typically present at around 0.0010 mass%.

[0037] [Sn: 0.002 mass% or more, 0.050 mass% or less] Sn is a particularly important element in the embodiments of the present invention. In pure iron-based component systems with low component content, such as wire rods, steel bars, and soft magnetic components according to the embodiments of the present invention, elemental diffusion is easy, and even a small amount of Sn can form a Sn-based oxide film on the surface, exhibiting a significant improvement in corrosion resistance. However, if the amount of Sn is too low, the formation of the Sn-based oxide film is insufficient, and sufficient corrosion resistance cannot be obtained. For this reason, the amount of Sn was set to 0.002 mass% or more. Preferably, the amount of Sn is 0.004 mass% or more, more preferably 0.006 mass% or more, and even more preferably 0.010 mass% or more. Also, if the amount of Sn is too high, the cold forgeability decreases. Taking these factors into consideration, the upper limit of the amount of Sn was set to 0.050 mass%. Preferably, the amount of Sn is 0.045 mass% or less, and more preferably 0.040 mass% or less.

[0038] The basic components of the wire rods, steel bars, and soft magnetic components according to the embodiments of this specification are as described above, and in one preferred embodiment, the remainder is iron and unavoidable impurities. As unavoidable impurities, the inclusion of elements (e.g., As, Sb, Ca, O, H, etc.) introduced depending on the conditions of the raw materials, materials, manufacturing equipment, etc., is permissible. Furthermore, for example, elements such as P and S are generally preferable in lower amounts and are therefore unavoidable impurities, but their composition range is specified separately as described above. For this reason, in this specification, the "unavoidable impurities" that constitute the remainder are a concept that excludes elements whose composition range is specified separately.

[0039] Other selective elements Furthermore, in another preferred embodiment of the present invention, elements other than those described above may be included as needed, provided that the effects of the embodiment of the present invention are not impaired. Examples of such selective elements are shown below. Depending on the components included, the properties of the steel may be further improved.

[0040] [Mo: 1.00% by mass or less (excluding 0% by mass)] Mo is an element that improves corrosion resistance. Mo may be added to effectively exhibit this effect. That is, the amount of Mo does not include 0 mass%, or in other words, the lower limit may be greater than 0 mass%. The amount of Mo is preferably 0.01 mass% or more. However, if there is an excess of Mo, the magnetic moment of the Fe matrix decreases and the magnetic properties deteriorate, so the amount of Mo may be 1.00 mass% or less. The amount of Mo is preferably 0.50 mass% or less, more preferably 0.30 mass% or less, and even more preferably 0.10 mass% or less.

[0041] [One or more elements selected from the group consisting of Ti: 0.100 mass% or less (excluding 0 mass%), V: 0.100 mass% or less (excluding 0 mass%), and Nb: 0.100 mass% or less (excluding 0 mass%)] Ti, V, and Nb are carbide-forming elements that generate carbides and reduce solid-solution carbon, thus improving magnetic properties and cold forgeability by suppressing strain aging. For this reason, one or more elements selected from the group consisting of Ti, V, and Nb may be added. That is, one or more elements selected from the group consisting of Ti, V, and Nb may not be present in 0 mass%, or in other words, the lower limit may be greater than 0 mass%. When each of the elements Ti, V, and Nb is added, its content is preferably 0.005 mass% or more. If each of Ti, V, and Nb is present in excess, grain growth will be inhibited due to the pinning effect of carbides, and magnetic properties will deteriorate. Therefore, when each of the elements Ti, V, and Nb is added, its content is 0.100 mass% or less, preferably 0.075 mass% or less, and more preferably 0.050 mass% or less.

[0042] [B: 0.0050% by mass or less (excluding 0% by mass)] B is an element that can improve magnetic properties and cold forgeability by suppressing strain aging, by combining with N in steel to form BN and reducing solid solution N. B may be added to effectively exert this effect. That is, the amount of B does not include 0 mass%, or in other words, the lower limit may be greater than 0 mass%. The amount of B is preferably 0.0005 mass% or more. If B is present in excess, compounds such as Fe2B will precipitate at the grain boundaries, degrading the magnetic properties. For this reason, when B is added, the amount of B should be 0.0050 mass% or less. The amount of B is preferably 0.0040 mass% or less, and more preferably 0.0030 mass% or less. Note that B is usually present as an impurity at a concentration of about 0.0003 mass%.

[0043] 2.Metal structure [Ferrite area ratio of 80% or more] To increase the magnetic moment of the Fe matrix, it is necessary to include a large amount of ferrite, which is a ferromagnetic material. Furthermore, if the proportion of ferrite is small, the cold forgeability deteriorates. Therefore, in the metal structure of the wire rods, steel bars, and soft magnetic components according to the embodiments of the present invention, the proportion of ferrite (ferrite fraction) is 80.0% or more in terms of area. The area fraction of the ferrite is preferably 90.0% or more, more preferably 95.0% or more, and even more preferably 96.0% or more.

[0044] Furthermore, if structures other than ferrite are present, examples of such structures include spheroidal cementite, pearlite, and bainite. For clarification, if pearlite is present, the layered ferrite within the pearlite is not included in the above-mentioned ferrite area ratio.

[0045] [Ferrite grain size number is 5.0 or less] If the grain size of wire rods, steel bars, and soft magnetic components is too small, the influence of grain boundaries on the movement of magnetic domain walls becomes greater, leading to a decrease in magnetic properties. Therefore, it is necessary to increase the grain size and reduce the density of grain boundaries. For this reason, the wire rods, steel bars, and soft magnetic components according to the embodiment of the present invention have a ferrite grain size number of 5.0 or less. Preferably, the ferrite grain size number is 4.5 or less. From the viewpoint of achieving higher magnetic properties, a larger grain size is better, but obtaining a very large grain size is difficult in industrial production. Furthermore, if the grains become extremely coarse, ductility and toughness decrease and cold forgeability deteriorates. Therefore, the ferrite grain size number is preferably -3.0 or higher, more preferably -1.0 or higher, and even more preferably 0.0 or higher. The grain size number can be determined by measurement in accordance with Japanese Industrial Standard G0511 (JIS G0511). For clarification, if pearlite is present, the layered ferrite within the pearlite is not included in the measurement of the ferrite grain size number as described above.

[0046] 3. Vickers hardness Processing strains imposed by hot and cold working degrade magnetic properties. The inventors have found that excellent magnetic properties can be obtained by controlling the Vickers hardness as a property corresponding to the amount of processing strain. Specifically, in the component system of the embodiment according to the present invention, excellent magnetic properties can be obtained by setting the Vickers hardness to HV140 or less. If the Vickers hardness exceeds HV140, the magnetic properties deteriorate in accordance with the amount of processing strain. The Vickers hardness is preferably HV130 or less, more preferably HV120 or less, and even more preferably HV115 or less.

[0047] Vickers hardness is measured at the D / 4 position (a position located one-quarter of the diameter D from the surface toward the center; if the cross-sectional shape is not circular, D is the equivalent diameter of a circle), which is a representative position of the properties of the wire or bar. In accordance with JIS Z2224, the Vickers hardness is calculated by averaging three points measured at a distance of 3d (d: diagonal length of the indentation) or more from adjacent indentations. The load used is 1 kgf (9.81 N).

[0048] 4. Manufacturing method The soft magnetic wire or steel bar according to the embodiment of the present invention can be manufactured by performing a predetermined hot rolling or hot forging in a predetermined temperature range, followed by cooling under predetermined conditions, as described below.

[0049] First, molten steel obtained by melting steelmaking raw materials to satisfy the above-mentioned component composition is cast to obtain a casting material. The method for obtaining the casting material may be the usual method used in the manufacture of wire rods and steel bars. Casting may be carried out in batches to obtain ingots, or by continuous casting. Furthermore, the casting material may be subjected to processing such as surface machining as necessary. Next, the obtained cast material is heated to 950°C to 1250°C, then hot-rolled or hot-forged at 950°C or higher to obtain the desired shape, and then cooled to 500°C at an average cooling rate of 0.1°C / sec to 10°C / sec. Cooling in the temperature range below 500°C may be carried out at any rate. This makes it possible to obtain a ferrite structure with a predetermined area ratio and grain size number, as well as a predetermined Vickers hardness.

[0050] In this specification, wire rods or steel bars include those having a circular cross-sectional shape perpendicular to the longitudinal direction (as described above, other cross-sectional shapes may also be used). Such wire rods or steel bars can be obtained by the hot rolling or hot forging described above, but in addition, those obtained by further cold working, such as cold drawing, after hot rolling or hot forging are also included in the "steel wire" or "steel bar" of the present invention. However, excessive cold working increases the ferrite grain size number and Vickers hardness, so a cold working ratio (e.g., cold drawing ratio) of 20% or less can be exemplified as a preferred processing condition. However, it should be noted that even with the same cold working ratio, the amount of strain introduced differs depending on the processing conditions such as processing speed and processing temperature, so it may be possible to obtain the desired ferrite grain size number and Vickers hardness even if the cold working ratio exceeds 20%.

[0051] If the desired ferrite grain size number and Vickers hardness cannot be obtained, magnetic annealing may be performed as needed to obtain the desired ferrite grain size number and Vickers hardness. Examples of wire rods and bars after magnetic annealing include those that are hot-rolled or hot-forged and then magnetically annealed, and those that are hot-rolled or hot-forged, then cold-drawn and then magnetically annealed. Magnetic annealing is preferably performed under the conditions described in "4. Soft Magnetic Steel Parts" below. If the final wire rods and bars obtain the desired ferrite area ratio, desired ferrite grain size number, and Vickers hardness, intermediate annealing may be performed during cold drawing.

[0052] Furthermore, as the diameter of the wire rod and steel bar decreases, cold drawing becomes necessary and the cold drawing rate increases, thus necessitating more reliable magnetic annealing and intermediate annealing. In particular, when the diameter is less than 3.0 mm, the number of annealing cycles (total number of magnetic and intermediate annealing cycles) increases. For this reason, it is preferable that the wire rod and steel bar according to the embodiment of the present invention have a diameter or equivalent circle diameter of 3.0 mm or more.

[0053] 5. Soft magnetic steel parts Soft magnetic steel parts can be obtained by using wire rods and steel bars according to embodiments of the present invention and performing either processing or magnetic annealing, or both. However, the invention is not limited thereto. As long as they have the chemical composition, ferrite area ratio, ferrite grain size number, and Vickers hardness HV specified for the wire rods and steel bars according to embodiments of the present invention, they can be obtained using other steel materials, particularly other wire rods or steel bars. Soft magnetic steel parts obtained in this manner are also included within the technical scope of the present invention. Soft magnetic steel parts obtained using wire rods or steel bars often have a circular outer circumference or a shape in which part of the circle is deformed, in a cross section perpendicular to the axial direction (for example, in one or more of the cross sections when multiple cross-sections are observed). However, this is not a characteristic of all soft magnetic steel parts obtained using wire rods or steel bars, and some do not have this characteristic.

[0054] Examples of soft magnetic steel components include various electromagnetic components for automobiles, trains, and ships, which include iron core materials for solenoid valves, solenoids, and relays, magnetic shielding materials, actuator members, and motor / sensor members.

[0055] To obtain soft magnetic steel parts, when forming a desired part shape using a soft magnetic wire or soft magnetic steel bar according to the embodiment of the present invention, the soft magnetic steel parts may be obtained by cold forging and, if necessary, performing magnetic annealing after cold forging. Alternatively, when using a wire or steel bar that differs from the soft magnetic wire and soft magnetic steel bar according to the embodiment of the present invention but satisfies the chemical composition requirements, the soft magnetic steel parts according to the embodiment of the present invention may be obtained by cold forging and magnetic annealing after cold forging. Since a larger cold forging ratio (processing rate for cold forging) increases the ferrite grain size number and Vickers hardness, it is preferable to keep the cold forging ratio at 20% or less. If the desired ferrite grain size number and Vickers hardness cannot be obtained after cold forging, magnetic annealing may be performed under the conditions described below. Furthermore, if the final obtained soft magnetic part has the desired ferrite area ratio, desired ferrite grain size number, and Vickers hardness, intermediate annealing may be performed during the cold forging process.

[0056] As an example of magnetic annealing conditions, holding the material at a temperature of 700°C to 1000°C for 1 to 5 hours is used. Under these conditions, strain that degrades magnetic properties can also be removed. The cooling rate after holding is not particularly limited, but it is preferable to cool to 400°C at an average cooling rate of 500°C / hour or less to promote grain growth and remove strain (reduce Vickers hardness). In this case, the cooling rate in the temperature range below 400°C is not particularly limited as it does not substantially affect grain growth and thermal strain associated with cooling, but air cooling or rapid cooling is preferred from the viewpoint of productivity. The atmosphere is not particularly limited, but it is preferable to process in an inert gas atmosphere such as nitrogen, argon, or hydrogen.

[0057] Even if surface treatments such as soft nitriding or plating are performed after magnetic annealing, the ferrite area ratio and ferrite grain size number do not change. Therefore, these treatments may be performed as needed, as long as the desired Vickers hardness is satisfied.

[0058] For soft magnetic steel parts, the ferrite area fraction, ferrite grain size number, and Vickers hardness should be measured at the D' / 4 position (where D' is the length of the longest transverse line in the cross-section) of the longest transverse line perpendicular to the part surface, moving from the part surface toward the interior of the part. [Examples]

[0059] Test materials having the chemical composition shown in Table 1 were melted using a conventional melting method to obtain cast materials. The obtained cast materials were heated to 1100°C, then hot-forged at 1100°C, followed by cooling to 500°C at an average cooling rate of 0.9°C / second for 10 minutes. Samples No. 1 to 10 were produced as 10 mm diameter wires, and samples No. 11 and 12 were produced as 12 mm diameter wires. For samples No. 11 and 12, cold drawing was performed after hot forging to produce 10 mm diameter wire samples in one pass (drawing rate: approximately 30%). For sample No. 6, magnetic annealing was performed by heating to 850°C and holding for 3 hours, followed by cooling to 400°C at an average cooling rate of 100°C / hour. For sample No. 12, magnetic annealing was performed by heating to 550°C and holding for 30 minutes, followed by rapid cooling with nitrogen gas. Although samples No. 4, No. 6, and No. 12 have the same composition, they differ in the presence or absence of magnetic annealing and the conditions under which magnetic annealing was performed, as shown in Table 2. Regarding the amount of Si, samples No. 7 and 8 had Si intentionally added, while the other samples have Si at impurity levels. Regarding the amounts of each element Cu, Ni, Cr, Mo, V, and Nb, sample No. 9 had Si intentionally added, while the other samples have Si at impurity levels. Regarding the amounts of each element Ti and B, samples No. 9 and 10 had Si intentionally added, while the other samples have Si at impurity levels.

[0060] [Table 1]

[0061] For each sample, the following tests were performed under the conditions described below: measurement of ferrite area fraction (ferrite fraction) and ferrite grain size, measurement of Vickers hardness, measurement of coercivity, corrosion resistance evaluation test, and cold forgeability evaluation test.

[0062] (Measurement of ferrite area ratio) After mirror-polishing the cross-section (a section perpendicular to the axis) of each sample, the metallic structure was revealed by Nital etching. Three fields of view (each field of view measuring 950-1200 μm vertically and 1900-2400 μm horizontally) were photographed at position D / 4 (D: diameter of the wire sample) of the cross-section using an optical microscope at 50-100x magnification. Ten equally spaced vertical lines and ten equally spaced horizontal lines were drawn on the photographs to form a grid. This created 100 intersections of vertical and horizontal lines. The number of intersections located on ferrite (number of ferrite points) was measured, and the ferrite area ratio was calculated from the occupancy rate of the intersections by ferrite. The same procedure was performed for each of the three photographs (three fields of view), and the average of the ferrite area ratios (%) in each field of view was taken as the ferrite area ratio of that sample.

[0063] (Ferrite grain size number) For each of the above samples, the grain size number was determined for each of the three field-of-view photographs in accordance with Japanese Industrial Standard G0511 (JIS G0511), and the average value was taken as the ferrite grain size number for that sample.

[0064] (Vickers hardness measurement) Measurements were taken at the D / 4 position of each sample (2.5 mm from the surface, given that the diameter D is 10 mm). In accordance with JIS Z2224, a load of 1 kgf (9.81 N) was applied, and three measurements were taken at three points such that the distance between adjacent indentations was 3d (d: diagonal length of the indentation), and the average of these three values ​​was taken as the Vickers hardness.

[0065] (Coercivity measurement) As part of the magnetic property evaluation, the coercivity of each sample was measured. The measurements were performed using an automatic coercivity meter (Hc meter, manufactured by Tohoku Special Steel Co., Ltd., model K-HC1000). Two measurement samples measuring φ8.0 mm × 40.0 mm were prepared from each sample by machining (machined to φ8.0 mm so that the center line of the original φ10 mm wire sample coincided). Each measurement sample was measured three times, and the average value of the measurement results was calculated to determine the coercivity of each sample. When measuring the coercivity, a magnetic field was applied so that the axial direction of the cylindrical measurement sample was parallel to the magnetization direction. A coercivity of less than 100 A / m was judged to indicate good magnetic properties.

[0066] (Corrosion resistance evaluation test) From each sample, a φ5.0 mm × 20.0 mm sample for corrosion resistance evaluation testing was prepared by machining (cutting to φ5.0 mm so that the center line coincided with the original φ10 mm wire sample). These corrosion resistance evaluation test samples were immersed in a 1% H2SO4 aqueous solution in a beaker test, stirring the solution at room temperature for 24 hours (Hr). After the test, corrosion loss was measured. The "corrosion loss" was determined by dividing the change in mass of the test piece before and after immersion by the initial surface area of ​​the test piece. Corrosion loss: 70g / m 2 The following were determined to indicate good corrosion resistance.

[0067] (Cold forging performance evaluation test) From each sample, a cold forging test sample measuring φ8.0 mm × 12.0 mm was prepared by machining (machined to φ8.0 mm so that the center line coincided with the original φ10 mm wire sample). Two cold forging tests were performed on these cold forging test samples using a forging press at room temperature with a strain rate of 5 / sec to 10 / sec and a processing rate of 80%. To explain the 80% processing rate cold forging test in more detail, a cylindrical cold forging sample with a height of 12.0 mm was compressed in a direction parallel to the axial direction of the cylinder until its height was 2.4 mm. In each cold forging test, the deformation resistance at a processing rate of 40% was measured. The average value of the obtained deformation resistances was taken as the deformation resistance of the sample. If the deformation resistance of the sample was 460 MPa or less, it was judged to have good cold forging properties. Table 2 shows the grain size number, ferrite area fraction, Vickers hardness, coercivity, corrosion loss, and deformation resistance measured by the above method.

[0068] [Table 2]

[0069] Samples No. 3, 4, and 6-9 all satisfied the component composition, ferrite area ratio, ferrite grain size number, and Vickers hardness specified in the embodiments of the present invention, and exhibited good magnetic properties, corrosion resistance, and cold forgeability. Among these, Sample No. 6 is a sample that has undergone magnetic annealing. Even after magnetic annealing, it satisfies all requirements regarding component composition, ferrite area ratio, ferrite grain size number, and Vickers hardness, and possesses excellent magnetic properties, corrosion resistance, and cold forgeability. Sample No. 1 has poor corrosion resistance because its Sn content is too low. Sample No. 2 has an excessive amount of carbon and an excessively large grain size number, resulting in inferior magnetic properties, cold forgeability, and corrosion resistance. Sample No. 5 has poor cold forging properties because it contains an excessive amount of Sn. Sample No. 10 lacks added Sn and has an excessive Al content, resulting in poor corrosion resistance and magnetic properties. Sample No. 11 has an excessive cold working rate and has not undergone magnetic annealing, resulting in an excessively high Vickers hardness and inferior magnetic properties. Sample No. 12 exhibits excessive Vickers hardness and poor magnetic properties due to an excessive cold working rate and an insufficient magnetic annealing temperature.

Claims

1. C: 0.075% by mass or less (including 0% by mass), Si: 1.00% by mass or less (including 0% by mass), Mn: 0.10% by mass or more, 1.00% by mass or less, P: 0.100% by mass or less (including 0% by mass), S: 0.100% by mass or less (including 0% by mass), Cu: 1.00% by mass or less (including 0% by mass), Ni: 1.00% by mass or less (including 0% by mass), Cr: 1.00% by mass or less (including 0% by mass), Al: Less than 0.030% by mass (including 0% by mass) N: 0.0200% by mass or less (including 0% by mass), and Sn: 0.004 mass% or more, 0.050 mass% or less, It contains, with the remainder consisting of iron and unavoidable impurities. It contains 80% or more ferrite by area, and the grain size number of the ferrite is 5.0 or less. Soft magnetic wire or steel bar with a Vickers hardness of HV140 or less.

2. A soft magnetic wire or steel bar according to claim 1, satisfying one or more of the following (a) to (d). (a) Si content of 0.50% by mass or less (including 0% by mass) (b) Mo: Further containing 1.00% by mass or less (excluding 0% by mass) (c) Further containing one or more elements selected from the group consisting of Ti: 0.100% by mass or less (excluding 0% by mass), V: 0.100% by mass or less (excluding 0% by mass), and Nb: 0.100% by mass or less (excluding 0% by mass). (d) B: Further containing 0.0050% by mass or less (excluding 0% by mass)

3. A soft magnetic wire or steel bar according to claim 1 or 2, containing ferrite in an area ratio of 90% or more.

4. A soft magnetic steel component using the soft magnetic wire or steel bar described in Claim 1.

5. A soft magnetic steel component using the soft magnetic wire or steel bar described in Claim 2.

6. A soft magnetic steel component using the soft magnetic wire or steel bar described in Claim 3.

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