Soft magnetic wire or soft magnetic steel bar

Abstract

To provide a soft magnetic steel that does not have a high content of alloy elements and achieves excellent magnetization characteristics, and a soft magnetic steel component that includes the soft magnetic steel and has excellent magnetization characteristics.SOLUTION: A soft magnetic steel contains specific levels of C, Si, Mn, P, S, Al, N, O, and Pb, with the balance being iron and unavoidable impurities. The area ratio of ferrite in the entire structure is 80% or more. The average value of ferrite crystal grain size is 100 μm or less. The standard deviation of ferrite crystal grain size is 31 μm or less. The KAM value is 0.218° or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a soft magnetic steel material and a soft magnetic steel part using the same. [Background technology]

[0002] In response to the trend toward energy conservation in automobiles, etc., power saving and precise control are required for many of the electrical components (especially electromagnetic components) in automobiles, etc. In particular, the steel materials that make up the magnetic circuits are required to have magnetic properties such as being easily magnetized in a weak external magnetic field and having a small coercive force.

[0003] The steel material used is typically soft magnetic steel, whose internal magnetic flux density is highly responsive to external magnetic fields. A specific example of the soft magnetic steel material is ultra-low carbon steel (pure iron-based soft magnetic material) with a C content of approximately 0.1% by mass or less. Soft magnetic steel parts used as the electromagnetic components described above are generally obtained by hot-rolling the steel material, followed by secondary processing steps such as pickling, lubrication, and wire drawing to obtain a steel wire, which is then subjected to part forming (forging, cutting), magnetic annealing, etc. (In some applications, the steel is rolled into a plate shape and then pressed into a part).

[0004] Patent Documents 1 to 3, for example, have proposed techniques for producing ultra-low carbon steels with excellent magnetic properties. Patent Document 1 describes a steel material for soft magnetic parts that has excellent cold workability and, after magnetic annealing, has excellent magnetic properties and high fatigue strength, and has a predetermined chemical composition, in which the average crystal grain size of ferrite grains in the steel material for soft magnetic parts is 5 to 200 μm, and further, the number of precipitates in the ferrite grains that have an equivalent circle diameter of 30 nm or more, Sv (particles / mm 2 ) is expressed by the formula (1): Sv≦10V×7.0×10 6(where V in formula (1) is substituted with the V content (mass %) in the steel material for soft magnetic parts) is shown. Patent Document 2 discloses soft magnetic wire rod or steel bar to which Ti or Ca has been added in an appropriate range, which has excellent hot ductility even when the Mn content of ultra-low carbon steel is reduced as much as possible and can be stably produced without cracking during the manufacturing process. Furthermore, Patent Document 3 discloses soft magnetic steel material with a predetermined composition and which satisfies parameter formula (1) related to elements such as C, as a soft magnetic steel material with excellent AC magnetic properties and high deformability. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 105698 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-180146 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-328462 Summary of the Invention [Problem to be solved by the invention]

[0006] All of Patent Documents 1 to 3 contain large amounts of alloying elements such as V, Cr, and Ti in consideration of the balance with properties other than magnetic properties, which raises concerns about increased manufacturing costs and deterioration of magnetic properties due to increased precipitates and dissolved elements.The present disclosure has been made in light of these circumstances, and aims to provide a soft magnetic steel material that does not contain large amounts of alloying elements and can achieve excellent magnetic properties (soft magnetic properties), and a soft magnetic steel part that uses this soft magnetic steel material and has excellent magnetic properties (soft magnetic properties). [Means for solving the problem]

[0007] Aspect 1 of the present invention is C: more than 0% by mass, 0.080% by mass or less, Si: 0 mass% or more, 0.50 mass% or less, Mn: 0.10% by mass or more, 1.00% by mass or less, P: more than 0% by mass, 0.100% by mass or less, S: More than 0% by mass, 0.100% by mass or less, Al: 0 mass% or more, 0.100 mass% or less, N: more than 0% by mass, 0.0200% by mass or less, O: more than 0 mass% and 0.0200 mass% or less, and Pb: 0 mass% or more, 0.100 mass% or less and the remainder being iron and unavoidable impurities, This soft magnetic steel has an area ratio of ferrite of 80% or more of the entire structure, an average ferrite grain size of 100 μm or less, a standard deviation of the ferrite grain size of 31 μm or less, and a KAM value of 0.218° or more.

[0008] Aspect 2 of the present invention is Furthermore, Cu: more than 0 mass%, 1.00 mass% or less, Ni: more than 0 mass%, 1.00 mass% or less, Cr: more than 0 mass% and 1.00 mass% or less, and The soft magnetic steel material according to aspect 1 further comprises one or more elements selected from the group consisting of more than 0 mass % and 1.00 mass % or less of Mo.

[0009] Aspect 3 of the present invention is Furthermore, Ti: more than 0 mass%, 0.100 mass% or less, V: more than 0 mass% and 0.100 mass% or less, and The soft magnetic steel material according to aspect 1 or 2, further comprising one or more elements selected from the group consisting of more than 0 mass % and 0.100 mass % or less of Nb.

[0010] A fourth aspect of the present invention is the soft magnetic steel material according to any one of the first to third aspects, further containing more than 0 mass % and 0.100 mass % or less of Sn.

[0011] A fifth aspect of the present invention is the soft magnetic steel material according to any one of the first to fourth aspects, further containing B: more than 0 mass % and 0.0050 mass % or less.

[0012] A sixth aspect of the present invention is a soft magnetic steel part using the soft magnetic steel material according to any one of the first to fifth aspects. [Effects of the Invention]

[0013] According to one embodiment of the present invention, it is possible to provide a soft magnetic steel material that does not contain a large amount of alloying elements and can achieve excellent magnetic properties (soft magnetic properties), and a soft magnetic steel part that uses the soft magnetic steel material and has excellent magnetic properties (soft magnetic properties). [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows a sample in the example before and after hot forging. [Figure 2] FIG. 2 shows the positions where samples were taken from hot forged product samples (samples after hot forging) in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted extensive research to solve the above problems. As a result, they have found that by appropriately adjusting the chemical composition, and further, in the metal structure, the area ratio of ferrite to the entire structure is 80% or more, the average ferrite grain size is 100 μm or less, the standard deviation of the ferrite grain size is 31 μm or less, and the KAM value is 0.218° or more, excellent magnetic properties (soft magnetic properties) can be achieved without adding large amounts of alloying elements. Each requirement specified in the embodiments of the present invention will be described in detail below.

[0016] 1.Chemical composition In an embodiment of the present invention, a soft magnetic steel material is used as a material for soft magnetic steel parts such as electromagnetic parts. The chemical composition of the soft magnetic steel material is described below. The chemical composition of the soft magnetic steel material disclosed herein does not require the alloy elements that have been used conventionally, and as will be described below, the content of elements is low, which allows for reduction in the manufacturing cost of the steel material.

[0017] [C: more than 0 mass%, 0.080 mass% or less] C is an element that controls the balance between the strength and ductility of steel materials; as the content decreases, the strength decreases and the ductility improves. From the viewpoint of efficiently producing steel materials, the lower limit is set to more than 0% by mass. The C content is preferably 0.001% by mass or more, more preferably 0.002% by mass or more. However, the magnetic properties are better as the ferromagnetic ferrite fraction increases. If the C content is excessive, the ferrite area ratio decreases and cementite precipitates, which impedes domain wall motion and deteriorates the magnetic properties. Therefore, the upper limit of the C content is set to 0.080% by mass. The C content is preferably 0.070% by mass or less, more preferably 0.060% by mass or less. It may even be 0.050% by mass or less.

[0018] [Si: 0 mass% or more, 0.50 mass% or less] If Si is contained in excess, the magnetic properties will deteriorate. Therefore, the upper limit of the Si content is set to 0.50 mass%. The Si content is preferably 0.40 mass% or less, more preferably 0.30 mass% or less, and may be 0 mass%.

[0019] [Mn: 0.10 mass% or more, 1.00 mass% or less] Mn effectively acts as a deoxidizer. Furthermore, Mn combines with S contained in the steel material to form finely dispersed MnS precipitates, which act as chip breakers for chips generated during cutting, contributing to improved machinability. To effectively exert this effect, the Mn content is set to 0.10% by mass or more. The Mn content is preferably 0.15% by mass or more, and more preferably 0.20% by mass or more. On the other hand, if the Mn content is too high, excessive MnS is formed, deteriorating magnetic properties, so the Mn content is set to 1.00% by mass or less. The Mn content is preferably 0.75% by mass or less, and more preferably 0.50% by mass or less.

[0020] [P: More than 0 mass%, 0.100 mass% or less] P (phosphorus) is an element that causes grain boundary segregation in steel and deteriorates magnetic properties. Therefore, the P content is limited to 0.100 mass% or less to improve magnetic properties. The P content is preferably 0.075 mass% or less, and more preferably 0.050 mass% or less. The lower the P content, the better, but it is usually about 0.001 mass%.

[0021] [S: More than 0 mass%, 0.100 mass% or less] S (sulfur) forms MnS, which is harmful to magnetic properties, and deteriorates the magnetic properties. If the S content is too high, the number of MnS, which is harmful to magnetic properties, will increase excessively, so the S content is set to 0.100 mass% or less. The S content is preferably 0.075 mass% or less, and more preferably 0.050 mass% or less. The lower the S content, the better, but it is usually about 0.001 mass%.

[0022] [Al: 0 mass% or more, 0.100 mass% or less] Al can combine with N in the steel to form AlN. The formed AlN acts as pinning particles that suppress grain growth in the annealing process described below, increasing the number of grain boundaries that hinder domain wall motion and degrading magnetic properties. Therefore, the Al content is set to 0.100 mass% or less. To achieve better magnetic properties, the Al content is preferably 0.075 mass% or less, and more preferably 0.050 mass% or less. The Al content may be 0 mass%.

[0023] [N: More than 0 mass%, 0.0200 mass% or less] If the N (nitrogen) content is high, nitrides are formed, which act as pinning particles that suppress grain growth during the annealing process, increasing the number of grain boundaries that hinder domain wall motion and degrading magnetic properties. Taking these factors into consideration, the upper limit of the N content is set to 0.0200 mass%. The N content is preferably 0.0150 mass% or less, and more preferably 0.0100 mass% or less. The lower the N content, the better, but it is usually 0.0001 mass% or more.

[0024] [O: more than 0 mass%, 0.0200 mass% or less] If the O (oxygen) content is high, oxides are generated, which hinder domain wall motion and degrade magnetic properties. Taking these factors into consideration, the upper limit of the O content is set to 0.0200 mass%. The O content is preferably 0.0150 mass% or less, and more preferably 0.0100 mass% or less. The lower the O content, the better, but it is usually 0.0001 mass% or more.

[0025] [Pb: 0 mass% or more, 0.100 mass% or less] Pb is a low-melting-point metal, and improves chip separability by melting and embrittlement due to heat generated during cutting. Pb may be 0% by mass, but may be added to achieve the above-mentioned effects. When Pb is included, the Pb content is preferably 0.001% by mass or more, more preferably 0.005% by mass or more. On the other hand, excessive Pb content reduces the magnetic moment of the Fe matrix and inhibits domain wall motion, resulting in reduced magnetic properties. Therefore, the Pb content is set to 0.100% by mass or less. The Pb content is preferably 0.075% by mass or less, more preferably 0.050% by mass or less.

[0026] [The balance is iron and unavoidable impurities] The balance is iron (Fe) and inevitable impurities. The inevitable impurities include trace elements (e.g., As, Sb, etc.) that are introduced depending on the conditions of raw materials, materials, manufacturing facilities, etc. For example, there are elements such as P and S, whose content is usually the lower the better, and therefore they are inevitable impurities, but whose composition ranges are separately specified as above. Therefore, in this specification, when referring to the "unavoidable impurities" that make up the balance, this concept excludes elements whose composition ranges are separately specified.

[0027] The soft magnetic steel material, soft magnetic steel part, and steel billet used in the production thereof according to this embodiment may contain the elements described above in their chemical compositions. The optional elements described below do not necessarily need to be included, but by including them as necessary together with the above elements, they contribute to imparting further properties such as corrosion resistance. The optional elements are described below.

[0028] [One or more selected from the group consisting of Cu: more than 0 mass% and 1.00 mass% or less, Ni: more than 0 mass% and 1.00 mass% or less, Cr: more than 0 mass% and 1.00 mass% or less, and Mo: more than 0 mass% and 1.00 mass% or less] Cu, Ni, Cr, and Mo are elements that improve corrosion resistance. The content of each of these elements is preferably greater than 0% by mass, more preferably 0.01% by mass or more. On the other hand, excessive Cu, Ni, Cr, and Mo decrease the magnetic moment of the Fe matrix phase, resulting in poor magnetic properties. The content of each of these elements is preferably 1.00% by mass or less, more preferably 0.75% by mass or less, and even more preferably 0.50% by mass or less.

[0029] [One or more selected from the group consisting of Ti: more than 0 mass% and 0.100 mass% or less, V: more than 0 mass% and 0.100 mass% or less, and Nb: more than 0 mass% and 0.100 mass% or less] Since Ti, V, and Nb are carbide-forming elements, the reduction in the amount of solute C due to carbide formation can suppress strain aging and improve cold forgeability. The content of each of these elements is preferably greater than 0 mass%, more preferably 0.005 mass% or more, and even more preferably 0.010 mass% or more. On the other hand, if Ti, V, or Nb is contained in excess, the carbides will suppress the movement of domain walls, resulting in a deterioration in magnetic properties. The content of each of these elements is preferably 0.100 mass% or less, more preferably 0.075 mass% or less, and even more preferably 0.050 mass% or less.

[0030] [Sn: more than 0 mass%, 0.100 mass% or less] Sn is a low-melting-point metal, and improves chip separability by melting and embrittlement due to heat generated during cutting. To achieve this effect, the Sn content is preferably greater than 0% by mass, more preferably 0.001% by mass or more, and even more preferably 0.005% by mass or more. On the other hand, excessive Sn content reduces the magnetic moment of the Fe matrix and inhibits domain wall motion, resulting in reduced magnetic properties. Therefore, the Sn content is preferably 0.100% by mass or less, more preferably 0.075% by mass or less, and even more preferably 0.050% by mass or less.

[0031] [B: More than 0 mass%, 0.0050 mass% or less] B combines with N in the steel to form BN, reducing the amount of solute N, and is therefore effective in improving magnetic properties and cold forgeability by suppressing strain aging. The B content is preferably greater than 0% by mass, and more preferably 0.0005% by mass or more. Excessive B content causes compounds such as FeB to precipitate at grain boundaries, deteriorating magnetic properties. The B content is preferably 0.0050% by mass or less. The B content is more preferably 0.0040% by mass or less, and even more preferably 0.0030% by mass or less.

[0032] 2.Metal structure [Area ratio of ferrite is 80% or more] The metal structure must contain a large amount of ferritic structure, which is a ferromagnetic material, to increase the magnetic moment of the Fe matrix. Furthermore, if the proportion of ferritic structure is small, the cold forgeability of the soft magnetic steel material, for example, when it is molded into a part, deteriorates. For this reason, the area ratio of ferrite to the entire structure is set to 80.0% or more. The area ratio of ferrite is preferably 90.0% or more, more preferably 95.0% or more, and even more preferably 96.0% or more.

[0033] When a structure other than ferrite is contained, examples of such a structure include spheroidal cementite, pearlite, and bainite. Note that when pearlite is present, the layered ferrite in the pearlite is not included in the above-mentioned area ratio of ferrite.

[0034] [Average ferrite grain size is 100 μm or less] To sufficiently coarsen ferrite grains by magnetic annealing, it is important to control the ferrite grain size before magnetic annealing. If the ferrite grains before magnetic annealing are small, the grain boundaries (grain boundary energy) that drive grain growth increase, and magnetic annealing promotes grain growth and improves magnetic properties. Therefore, the average ferrite grain size (also referred to as the "average ferrite grain size") of the soft magnetic steel is set to 100 μm or less. The average ferrite grain size of the soft magnetic steel is preferably 96 μm or less. The soft magnetic steel according to this embodiment has a small ferrite grain size, which, for example, ensures good toughness and good crack resistance. From the viewpoint of enhancing the above-mentioned grain growth promotion effect, a smaller average ferrite grain size is preferable, but if it is too small, the grain growth promotion effect saturates. Therefore, the lower limit of the average ferrite grain size may be approximately 10 μm.

[0035] [Standard deviation (variation) of ferrite grain size is 31 μm or less] To uniformly coarsen ferrite grains through magnetic annealing, it is also important to minimize the variation in ferrite grain size before magnetic annealing. If the variation in ferrite grain size before magnetic annealing is large, grain growth will occur locally during magnetic annealing, resulting in fine grains remaining locally, resulting in a mixed grain structure and reduced magnetic properties. Furthermore, minimizing the variation in ferrite grain size eliminates non-uniformity in the material, thereby providing excellent crack resistance. From these perspectives, the upper limit of the standard deviation of ferrite grain size was set at 31 μm. While a smaller standard deviation is preferable, some variation in grain size will occur during manufacturing, and the lower limit of the standard deviation may be 1 μm, for example.

[0036] The area ratio of ferrite and the average value and standard deviation of the ferrite crystal grain size are determined under the conditions shown in the examples described later.

[0037] [KAM value is 0.218° or more] The KAM value (Kernel Average Misorientation) is known as an index related to the amount of plastic strain when a material undergoes plastic deformation. Specifically, it refers to the average value of the crystal rotation (crystal orientation difference) between the target measurement point and surrounding measurement points. A larger value indicates greater strain in the crystal. Because strain energy also serves as a driving force for grain growth, a higher KAM value before magnetic annealing is preferable. Therefore, the lower limit of the KAM value was set to 0.218°. The KAM value is preferably 0.220° or more, more preferably 0.221° or more. If there is too much strain before magnetic annealing (too high dislocation density), many fine crystal grains will be generated during magnetic annealing, with dislocations acting as nuclei. Therefore, the KAM value is preferably 0.500° or less, more preferably 0.400° or less, and even more preferably 0.300° or less. The KAM value can be determined by the method described in the Examples below.

[0038] The soft magnetic steel material of the present disclosure is characterized in that, as described above, the material structure before magnetic annealing is controlled in order to achieve higher magnetic properties after magnetic annealing than conventional ones.

[0039] The soft magnetic steel material of the present disclosure is not particularly limited in shape and may be any of wire rod, steel bar, steel plate, etc., but is particularly preferably applicable to wire rod and steel bar. In a preferred embodiment, the "wire rod" and "steel bar" have a circular cross-section perpendicular to the longitudinal direction, but this is not limited thereto and may be a shape other than a circle, such as a square or a polygon including a regular hexagon. If the cross-sectional shape is not circular, the ratio of the longitudinal direction to the lateral direction within the cross-section is 2 or less. In the case of wire rod, its diameter (equivalent circle diameter if the cross-section is a shape other than a circle) is not particularly limited, but is, for example, 5.5 mm to 55 mm. In the case of steel bar, its diameter (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. According to this embodiment, the material contains few pure iron-based component elements, so it is very soft and can combine excellent magnetic properties with excellent cold forgeability (small deformation resistance, excellent crack resistance).As a soft magnetic steel material, wire rod or steel bar is ideally suited for manufacturing various electromagnetic parts for automobiles, trains, ships, etc., as described below.

[0040] 3. Manufacturing method Next, a method for manufacturing the soft magnetic steel material according to the present invention will be described.

[0041] First, for example, by a conventional method, steelmaking raw materials that satisfy the above-mentioned requirements for the chemical composition are melted and then cast to obtain a slab. The slab is heated and then subjected to hot working such as hot rolling, hot forging, and hot forging extension, thereby obtaining, for example, a steel bar or wire rod as the soft magnetic steel material. The hot working conditions are not important as long as the structure of the soft magnetic steel material according to the embodiment of the present invention can be obtained. From the viewpoint of easily obtaining the structure of the soft magnetic steel material according to the embodiment of the present invention, it is preferable that the hot working conditions be as follows.

[0042] Specifically, after heating to 950°C to 1250°C, hot working such as hot rolling, hot forging, and hot forging stretching is performed. By heating at these temperatures, i.e., in the ferrite single-phase region, it is possible to achieve a ferrite grain size and KAM value within the specified range. In hot working, the final hot working is preferably performed under the following conditions: 1000°C to 1250°C, hot working at a working rate of 65% or more at an average strain rate of 9.0 / s or less, or 1000°C to 1250°C, hot working at a working rate of less than 65%, at an average strain rate of 1.0 / s to 4.0 / s. The "working rate" refers to the total working rate in the final hot working, and can be the working rate of one pass or the total working rate of multiple passes. After heating to the above-mentioned temperature of 950°C to 1250°C, final hot working is carried out under the above-mentioned conditions, whereby the average value and standard deviation of the ferrite grain size and the KAM value can be controlled within the specified ranges.

[0043] The method for calculating the strain rate is not critical. For example, a tensile test or compression test is conducted in advance under conditions (working temperature, working speed, working ratio) that simulate actual hot working. Using the stress-strain relationship obtained from this test, FEM analysis is performed. The entire cross section of the test sample (e.g., a cylindrical sample) in the height direction, including the radius, is divided into 1 mm meshes. The strain rates of all elements are calculated, and the average value is taken as the average strain rate. Then, based on the correlation between the hot working conditions (working temperature, working speed, working ratio) and the average strain rate, the hot working conditions that will yield the desired average strain rate can be determined.

[0044] The above-mentioned "final hot working" refers to the final hot working when multiple types of hot working are performed, and when only one type of hot working is performed, it refers to that hot working itself. An example of performing multiple types of hot working is producing a steel bar with a larger diameter by performing hot forging after hot rolling, and in that case, controlling the hot forging conditions as described above.

[0045] After hot working under the above conditions, the material may be cooled to 500°C or below at an average cooling rate of 0.1°C / min or above.

[0046] 4. Soft magnetic steel parts Soft magnetic steel parts using the above-mentioned soft magnetic steel material are also included in the present disclosure. Soft magnetic steel parts can satisfy the ranges of chemical composition and ferrite area ratio specified for soft magnetic steel materials. However, at least one of the chemical composition and ferrite area ratio of soft magnetic steel parts may not be completely identical to that of soft magnetic steel materials due to manufacturing conditions such as magnetic annealing and part molding performed using soft magnetic steel materials. In such cases, they may differ within the ranges specified in the present disclosure. The average ferrite grain size of soft magnetic steel parts is often larger than the average ferrite grain size of soft magnetic steel materials, but the average ferrite grain size of soft magnetic steel parts may also be smaller than the average ferrite grain size of soft magnetic steel materials.

[0047] Soft magnetic steel parts include various electromagnetic parts used in automobiles, trains, ships, etc., including iron core materials for electromagnetic valves, solenoids, relays, etc., magnetic shielding materials, actuator components, and motor / sensor components.

[0048] The following method can be mentioned as a method for obtaining soft magnetic steel parts using the soft magnetic steel material according to this embodiment. That is, soft magnetic steel parts can be obtained by subjecting the soft magnetic steel material according to this embodiment (for example, soft magnetic wire or soft magnetic steel bar) or a steel material obtained by drawing the soft magnetic steel material, to magnetic annealing, or processing and magnetic annealing. That is, the shape of the soft magnetic steel parts may be the same as the shape of the soft magnetic steel material, for example, soft magnetic wire or soft magnetic steel bar.

[0049] By subjecting soft magnetic steel material with the structure specified in this embodiment, i.e., a soft magnetic steel material with a structure satisfying a ferrite area ratio of 80% or more of the total structure, an average ferrite grain size of 100 μm or less, a standard deviation of 31 μm or less of the ferrite grain size, and a KAM value of 0.218° or more, and a strong driving force for grain growth, to magnetic annealing, grain growth is promoted, ferrite grains become coarse, and the area ratio of grain boundaries that deteriorate magnetic properties is reduced, thereby achieving excellent magnetic properties. In other words, even if soft magnetic steel material that does not satisfy the structure specified in this embodiment is subjected to magnetic annealing, it is difficult to ensure excellent magnetic properties. Furthermore, magnetic annealing can also remove strain that deteriorates magnetic properties.

[0050] The conditions for magnetic annealing are not particularly limited, and general conditions can be used. For example, a temperature of 700°C to 1000°C can be maintained for 1 hour to 5 hours. The cooling rate after maintaining the temperature is not particularly limited, but to promote grain growth, it is preferable to cool at an average cooling rate of 500°C / hr or less. The atmosphere for magnetic annealing is not important because it does not affect the ferrite grain size. For example, a reducing atmosphere using hydrogen, nitrogen, argon, etc., or an air atmosphere can be used.

[0051] To obtain soft magnetic steel parts of the desired part shape, the soft magnetic steel material according to this embodiment (e.g., soft magnetic wire or soft magnetic steel bar) or a steel material obtained by drawing the soft magnetic steel material according to this embodiment may be subjected to molding (parts processing). As the molding method, conventional high-processing methods for obtaining soft magnetic steel parts, such as cold forging, pressing, and cutting, can be used, as long as the material structure does not exceed the range specified in this invention. For example, when magnetic annealing is performed after processing, cutting can be used to process the parts, as this method maintains the ferrite grain size and KAM value to their pre-processing states better than other methods. The soft magnetic steel material according to this embodiment, particularly the soft magnetic wire or soft magnetic steel bar, exhibits excellent cold forgeability, making it suitable for producing soft magnetic steel parts.

[0052] Since the soft nitriding treatment and the plating treatment do not affect the crystal grain size after magnetic annealing, at least one of the soft nitriding treatment and the plating treatment may be carried out after magnetic annealing as necessary.

[0053] By subjecting soft magnetic steel material that satisfies the above chemical composition and metal structure to at least magnetic annealing, soft magnetic steel parts that exhibit good magnetic properties with a coercive force of 0.65 A / cm or less can be obtained. [Example]

[0054] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are included in the technical scope of the present invention.

[0055] 1. Sample Preparation Test materials (Nos. 1 to 4) with the chemical compositions shown in Table 1 were melted in a converter, and the resulting cast billets were hot-rolled to obtain steel bars with diameters of 40 mm to 60 mm. In Table 1, all of steel Nos. 1 to 4 contain Cu, Ni, and Mo in the range of 0.01 to 0.03 mass%, respectively, and steel No. 3 contains 0.001 mass% Sn, but these are unavoidable impurities and not intentionally added. Next, to prepare multiple samples with various average and standard deviation ferrite grain sizes and KAM values, the steel bars were cut to lengths suitable for forging (the resulting hot-forging samples were 50 (D) × 75 (H) mmL (H / D = 1.5)). As shown in Table 2, the steel bars (round bars) were hot-forged from the axial direction under different conditions of hot-forging temperature, compression ratio, and strain rate, as indicated by the black arrows in Figure 1(a). The hot-forged samples shown in Figure 1(b) were obtained. After hot forging, the bars were allowed to cool to room temperature (average cooling rate: approximately 0.5°C / min to 5°C / min). The compression ratios in Table 2 were calculated using the following formula: Compression ratio (%) = [(Hh) / H] x 100 (where H is the height of the hot forging sample, and h is the height of the hot forged product sample)

[0056] In order to obtain multiple samples manufactured at various average strain rates, in this example, a single hot forging sample was hot forged so that the strain rate varied significantly depending on the region, resulting in hot forged samples with various strain rates depending on the region. Test specimens were then taken from multiple regions of this hot forged sample with different strain rates. That is, test specimens were taken from each of the resulting hot forged samples, A to E, as shown in Figure 2. In Figure 2, the thick solid line indicates the frame of the height-direction cross section including the radius of the sample shown by the thick line in Figure 1(b), and the sample line L in Figure 2 indicates approximately the central axis of the hot forged sample. In addition, auxiliary lines (dashed lines) are shown to indicate the positions of A to E. The strain rate was calculated using the finite element method (FEM) at the same predetermined compression ratio as in the experiment. The load-stroke relationship obtained from the hot forging experiment was converted to a stress-strain relationship, and the strain rate was calculated using the finite element method (FEM). Any analysis software can be used, but as used in this example, for example, FORGE, a plastic processing simulation software made by Transvalor, can be used. When performing deformation analysis using FORGE, the effects of friction with the die and heat transfer to the die were ignored because they do not significantly affect the analysis results. The calculated strain rates are also shown in Table 2.

[0057] [Table 1]

[0058] [Table 2]

[0059] The hot forged sample in Fig. 1(b) was cut so that its vertical cross section, including the radius indicated by the bold line in Fig. 1(b), could be observed. Then, as described in detail below, structure observation and coercivity evaluation were performed at each of positions A to E in Fig. 2.

[0060] For evaluation of the structure, specifically evaluation of the ferrite crystal grain size, standard deviation, and KAM value, samples that had been hot forged but not magnetically annealed were used, and for evaluation of the magnetic properties (soft magnetic properties), i.e., measurement of the coercive force, samples that had been hot forged and further magnetically annealed were used, as shown below.

[0061] [Organizational Evaluation] (Method of calculating the average and standard deviation of ferrite grain size and KAM value) The hot-forged samples were cut so that their longitudinal cross sections (cross sections parallel to the axis) could be observed, and then hot-molded in resin. They were then mirror-polished to obtain samples for microstructural observation. The microstructural observation positions are shown in Figure 2, A–E. An EBSP analyzer and FE-SEM were used to measure the average bcc-Fe grain size, standard deviation of grain size, and KAM value of the microstructural observation samples. The observation (measurement) area was 2 mm × 2 mm. The OIM software from TSL Solutions, Inc. was used as the analysis tool. A "grain" was defined as a boundary with a crystal orientation misorientation (also called the "oblique angle") exceeding 15°, i.e., a high-angle grain boundary. The diameter of the bcc-Fe grain when its area was converted into a circle was calculated as the grain size, and its average value (average circle-equivalent grain size) and standard deviation were calculated.

[0062] The KAM value was determined using an EBSP analyzer as the average value of the crystal orientation difference between the measurement point and all of its neighboring measurement points. The measurement area was 2 mm x 2 mm, as described above, with measurement steps of 1 to 5 μm. Measurement points with a confidence index of 0.1 or less, which indicates the reliability of the measurement orientation, were removed (i.e., excluded) from the analysis.

[0063] (Calculation method of ferrite fraction) The hot-forged sample was cut so that its longitudinal cross section could be observed, mirror-polished, and then nital etched to reveal the structure. Each position in Figure 2 was photographed using an optical microscope at 50-100x magnification. Ten equally spaced vertical lines and ten equally spaced horizontal lines were drawn on the photographs (each photograph measuring 950-1200 μm vertically and 1900-2400 μm horizontally) to form a grid. This resulted in 100 intersections between the vertical and horizontal lines. The number of intersections located on ferrite (the number of ferrite points) was counted. The same procedure was repeated for each of the photographs, and the average of the total area ratio (%) was calculated.

[0064] Table 3 shows the average value and standard deviation of the ferrite grain size and the measurement results of the KAM value.

[0065] [Evaluation of magnetic properties (coercive force)] Samples measuring 1 mm in diameter and 3 mm in length were taken from each location in Figure 2 where photographs were taken for the above-mentioned structural evaluation, with the longitudinal direction parallel to the photographing direction. Each sample was then subjected to magnetic annealing by holding it at 850°C for 3 hours in a reducing atmosphere, followed by cooling to 400°C at a rate of 100°C / hour. These magnetic annealing conditions were typical. The coercivity of each sample (corresponding to a soft magnetic steel part) obtained after magnetic annealing was then measured using an automatic coercivity meter (Hc meter, K-HC1000, manufactured by Tohoku Special Steel Co., Ltd.). Each measurement was performed three times, and the average value was calculated. A sample with a coercivity of 0.65 A / cm or less after magnetic annealing was deemed to have good magnetic properties. The coercivity measurement results are shown in Table 3.

[0066] In the examples of this specification, the structure observation and coercivity measurement are performed on the cross section of the sample after hot forging, but in the case of a round bar sample or a part-shaped sample after hot rolling, for example, the structure observation and coercivity measurement may be performed at any position in the region 0.5 mm or more from the surface. In the case of the round bar sample, for example, evaluation may be performed from D / 4, where D is the diameter.

[0067] [Table 3]

[0068] Tables 1 to 3 reveal the following: Samples d, k, l, and n satisfy all the requirements of this embodiment and exhibit low coercivity, i.e., excellent magnetic properties. In contrast, the other samples do not satisfy at least one of the requirements of this embodiment, resulting in high coercivity and poor magnetic properties.

[0069] That is, although Samples Nos. a-c, e-j, m, and o used steel materials satisfying the chemical composition according to this embodiment, they did not satisfy the preferred manufacturing conditions. As a result, at least one of the average ferrite grain size, standard deviation, and KAM value was outside the range specified in this embodiment. Grain growth during magnetic annealing was insufficient, resulting in high coercivity after magnetic annealing. More specifically, Samples Nos. a-c and e-i were manufactured with a hot-working reduction rate of less than 65% and a strain rate outside the range of 1.0 to 4.0 s, resulting in at least one of the average ferrite grain size, standard deviation, and KAM value being outside the range specified in this embodiment. Furthermore, Samples Nos. j, m, and o were manufactured with a hot-working reduction rate of 65% or more and a strain rate exceeding 9.0 s, resulting in at least the standard deviation of the ferrite grain size being outside the range specified in this embodiment.

[0070] On the other hand, in Samples Nos. p to bb, the steel materials used did not satisfy the chemical composition according to this embodiment, and therefore the coercive force after magnetic annealing was high.

Claims

1. C: more than 0 mass%, 0.050 mass% or less, Si: 0 mass% or more, 0.30 mass% or less, Mn: 0.20 mass% or more, 0.50 mass% or less, P: more than 0 mass%, 0.050 mass% or less, S: more than 0 mass%, 0.050 mass% or less, Al: 0% by mass or more, 0.050% by mass or less, N: more than 0 mass% and 0.0100 mass% or less, and O: More than 0% by mass, 0.0150% by mass or less and the remainder being iron and unavoidable impurities, A soft magnetic wire or soft magnetic steel bar in which the area ratio of ferrite to the entire structure is 80% or more, the average value of the ferrite crystal grain size is 100 μm or less, the standard deviation of the ferrite crystal grain size is 31 μm or less, and further the KAM value is 0.218° or more and 0.270° or less.

2. Furthermore, Cr: more than 0 mass%, 0.50 mass% or less The soft magnetic wire or soft magnetic steel bar according to claim 1, comprising:

3. A soft magnetic steel part using the soft magnetic wire or soft magnetic steel bar according to claim 1 or 2.

Images