Manufacturing method for Fe-Co alloy rod and Fe-Co alloy rod

Abstract

The present invention provides: an Fe-Co-based alloy rod which is capable of suppressing variation in the magnetic characteristics of a long rod; and a method for producing this Fe-Co-based alloy rod.　A method for producing an Fe-Co-based alloy rod, the method comprising a hot rolling step in which an Fe-Co-based alloy billet is subjected to hot rolling so as to obtain a hot rolled rod that has a length of 2 m or more, and a heating straightening step in which a tensile stress is applied to the hot rolled rod so that the area reduction ratio of the rod is 2.0% to 8.0%, while heating the hot rolled rod to 500°C to 900°C, without having a solution heat treatment between the hot rolling step and the heating straightening step; and an Fe-Co-based alloy rod.

Description

[Technical Field]

[0001] The present invention relates to a method for producing an Fe—Co alloy rod and an Fe—Co alloy rod. [Background technology]

[0002] Fe-Co alloy rods, such as permendur, are known to have excellent magnetic properties and are used in a variety of products, including sensors, cylindrical magnetic shields, solenoid valves, magnetic cores, etc. Patent Document 1, for example, describes a method for producing Fe-Co alloy rods, in which an ingot is heated to 1000°C to 1100°C, hot worked into a billet of about φ90 mm, surface scratches and the like are removed using a lathe, and the billet is heated to 1000°C to 1100°C and hot rolled into a billet of about φ6 to φ9 mm to produce a material (rod).

[0003] Furthermore, in Patent Document 2, the applicant of the present application has proposed an Fe-Co alloy bar having an area ratio of 20% or more of crystal grains with a GOS (Grain Orientation Spread) value of 0.5° or more, which makes it possible to stably obtain excellent magnetic properties, and a method for manufacturing the Fe-Co alloy bar, which includes a heating and straightening process in which tensile stress is applied while heating the hot-rolled material to a temperature of 500 to 900°C. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-166239 [Patent Document 2] International Publication No. 2021 / 182518 Summary of the Invention [Problem to be solved by the invention]

[0005] The Fe-Co alloy bar described in Patent Document 2 has excellent magnetic properties and is a very useful invention. However, the applicant has confirmed through his research that in the case of long bar materials, such as those of 2 m or more, the magnetic properties may vary along the length of the bar material. Patent Documents 1 and 2 do not mention how to obtain stable magnetic properties throughout such long bar materials, leaving room for further research. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an Fe-Co alloy bar that can suppress variations in magnetic properties in a long bar, and a method for producing the same. [Means for solving the problem]

[0006] The present invention has been made in view of the above-mentioned problems. That is, one aspect of the present invention is a method for producing an Fe-Co alloy bar, which includes a hot rolling step of hot rolling an Fe-Co alloy billet to obtain a hot rolled bar having a length of 2 m or more, and a heating and straightening step of applying tensile stress to the hot rolled bar while heating the hot rolled bar to 500 to 900°C so that the area reduction rate of the bar is 2.0 to 8.0%, without performing solution treatment between the hot rolling step and the heating and straightening step.

[0007] Another aspect of the present invention is an Fe—Co-based alloy bar having an average GOS (Grain Orientation Spread) of 0.3° or more and 1.5° or less, an average grain size number measured on an axial cross section of the bar of 8.0 or more and 12.0 or less, and a length of 2 m or more. [Effects of the Invention]

[0008] According to the present invention, it is possible to obtain an Fe—Co alloy bar having reduced variations in magnetic properties in a long bar. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 10 is a diagram showing the area reduction rate of a sample according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram showing the area reduction rate of a sample of a comparative example. [Figure 3] FIG. 10 is a diagram showing the coercive force distribution of the sample of the present invention. [Figure 4] FIG. 10 is a diagram showing the coercive force distribution of a sample of a comparative example. [Figure 5] FIG. 10 is a diagram showing the relationship between the area reduction rate and the coercive force in the samples of the invention and the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment of the present invention will be described below. First, a method for producing an Fe—Co alloy bar of the present invention will be described. The Fe—Co alloy bar of the present invention is a straight rod-shaped bar having a circular (including elliptical) or rectangular cross-sectional shape. Unless otherwise specified, the bar of this embodiment is a round bar having a circular cross-sectional shape. <Hot-rolled material composition> First, in this embodiment, a hot-rolled material of an Fe-Co alloy is prepared. The Fe-Co alloy in the present invention refers to an alloy material containing, by mass %, 95% or more of Fe and Co, and 25 to 60% of Co. This allows the material to exhibit high magnetic flux density.

[0011] Next, elements that may be contained in the Fe-Co alloy of the present invention will be described. To improve workability and magnetic properties, the Fe-Co alloy of the present invention may contain one or more of the following elements: V, Si, Mn, Al, Zr, B, Ni, Ta, Nb, W, Ti, Mo, and Cr, in a total amount of up to 5.0% by mass. Examples of other impurity elements that are inevitably contained include C, S, P, and O, and the upper limit of each of these elements is preferably set to 0.1%.

[0012] <Hot rolling process> In this embodiment, a columnar Fe-Co alloy billet approximately 90 mm in diameter obtained from an Fe-Co alloy steel ingot having the aforementioned composition is hot-rolled to obtain a hot-rolled bar as an intermediate material for the Fe-Co alloy bar. Since this intermediate material has an oxide layer formed due to hot rolling, a polishing process may be introduced to mechanically or chemically remove the oxide layer. The present invention also targets hot-rolled bars having a length of 2 m or more. The longer the hot-rolled bar, the higher the productivity in the heating and straightening process. However, the longer the bar, the more likely it is that the magnetic properties of the bar will vary along its length. According to the manufacturing method of the present invention, it is possible to suppress the variation in magnetic properties even for such long bars. Here, the hot-rolled bar preferably has a diameter of 5 to 20 mm, taking into account workability in subsequent processes. For bars other than round bars, the equivalent diameter of a circle of the cross section may be 5 to 20 mm.

[0013] In this embodiment, no solution treatment is performed between the hot rolling process and the heating and straightening process described below. If solution treatment is performed, grain growth progresses during the solution treatment, reducing the number of grain boundaries that serve as recrystallized grain nucleation sites, making it difficult to promote recrystallization during magnetic annealing. In areas with reduced area reductions of 2.5% or less, a duplex structure containing unrecrystallized grains results, resulting in extremely poor magnetic properties. Additionally, if solution treatment is performed, the rod may bend due to thermal contraction. If the bend is corrected by press straightening, described below, it is difficult to obtain a stable reduction in area due to work hardening caused by localized strain. If solution treatment is not performed, suppressing grain growth before the heating and straightening process promotes recrystallization during magnetic annealing. Even in areas with reduced area reductions of 2.0 to 2.5%, the grain size variation during magnetic annealing is reduced, resulting in stable magnetic properties. Here, the solution treatment is a process in which the hot-rolled bar material is heated to a temperature (e.g., 750 to 1050°C) above the order-disorder transformation point and then rapidly cooled, and the area reduction rate is the ratio obtained by dividing the difference in area before and after the heating and straightening process by the area before the heating and straightening process.

[0014] Before proceeding to the heating and straightening process described below, press straightening may be performed to adjust the shape of the bar stock, as long as it does not deviate from the area reduction range of the present invention. Press straightening allows the subsequent heating and straightening process to be carried out quickly. Furthermore, if solution treatment as in the present invention is not performed, excessive bending tends not to occur, so it is preferable not to perform press straightening with a high processing rate. By not performing press straightening, a stable area reduction rate can be obtained in the heating and straightening process.

[0015] <Heating straight process> In this embodiment, the hot-rolled material is subjected to a heating and straightening process in which tensile stress is applied while being heated. If the hot-rolled material is in the shape of a rod, the rod is pulled in the longitudinal direction to apply the tensile stress. This process allows the hot-rolled material to have excellent magnetic properties and straightness while imparting residual strain. The heating temperature is set to 500 to 900°C. If the temperature is lower than 500°C, workability is reduced, and the rod may break when tensile stress is applied. On the other hand, if the heating temperature is higher than 900°C, it is difficult to impart desirable residual strain to the hot-rolled material. The lower limit of the heating temperature in the heating and straightening process is preferably 600°C, more preferably 700°C. The upper limit of the heating temperature is preferably 850°C, more preferably 830°C, and even more preferably 800°C. When the above-mentioned solution treatment step is omitted, the lower limit of the heating temperature is preferably 700°C, more preferably 730°C, and even more preferably 740°C. This embodiment is characterized in that the area reduction rate of the bar during the heating and straightening process is adjusted to 2.0 to 8.0%. This imparts strain to the bar, which serves as a driving force for obtaining coarse crystal grains, resulting in a bar with less variation in magnetic properties. This strain can be expressed by the average GOS value, which will be described later. A preferred lower limit for the area reduction rate is 2.2%, more preferably 2.5%, and a preferred upper limit is 7.5%. The area reduction rate of the present invention is calculated by measuring the diameter before and after the heating and straightening process with a micrometer, and these diameters are measured at multiple equally spaced locations along the axial direction of the bar to determine the area reduction rate distribution within one bar. In this embodiment, measurements are taken at 28 equally spaced locations along the axial direction of the bar, and the area reduction rate distribution within one bar is determined. While heating methods such as electrical heating and induction heating can be used in this heating and straightening process, electrical heating is preferred because it has the advantage of rapidly (e.g., within one minute) and uniformly heating the material to the target temperature while facilitating the alignment of the easy axes of magnetization of the crystal grains in the hot-rolled material. Furthermore, the tensile load applied to the bar during the heating and straightening process is preferably adjusted to 4 to 90 kN to more reliably obtain the desired residual strain. Furthermore, the elongation is preferably adjusted to 3 to 10% of the total length before the heating and straightening process. The Fe—Co alloy bar obtained by the manufacturing method of the present invention can have an area reduction of 2.0 to 8.0% by appropriately adjusting these values ​​while omitting the solution treatment. In the heating and straightening process using electrical heating applied in the present invention, the chucking portion is at a constant voltage and no current flows, so the temperature does not rise in the chucking portion. In addition, since the bar is mechanically restrained, there is almost no elongation in the chucking portion, which is the portion that will be cut and removed in a later process. Therefore, in the present invention, the area reduction rate in the range from the end of the bar to 250 mm, where the influence of the chucking portion in the heating and straightening process is significant, is not taken into consideration. Alternatively, in the manufacturing method of the Fe-Co alloy bar of the present invention, it is sufficient that the above-mentioned area reduction rate of 2.0 to 8.0% is achieved within a continuous length of at least 2 m of the bar.

[0016] In this embodiment, the bar stock that has undergone the heating and straightening process may be subjected to centerless grinding using, for example, a centerless grinder. This removes the black scale from the surface of the bar stock, thereby improving the roundness and tolerance accuracy of the shape. In the present invention, since the straightness of the bar stock is improved by the heating and straightening process, even long bar stock that is 2 m or longer can be subjected to centerless grinding without cutting.

[0017] Next, we will explain the Fe—Co alloy rod of the present invention, which can be obtained by the above-mentioned manufacturing method of the present invention. The Fe—Co alloy rod of the present invention has an average GOS (Grain Orientation Spread) value of 0.3° to 1.5°. The preferred lower limit of the average GOS value is 0.5°, and the preferred upper limit is 1.2°. The GOS value can be measured by the conventionally known "SEM-EBSD method (Electron Backscatter Diffraction)." That is, the GOS value of a crystal grain can be calculated by calculating the average value of the orientation differences between one point (pixel) constituting a crystal grain and all other points within the crystal grain, performing this operation at all points within the crystal grain, and then calculating the average value. Here, since the GOS value is determined for each crystal grain, the average value can be considered as either a number average or an area average. However, the GOS average referred to in the present invention refers to the area average. The area average is a value calculated by adding the GOS of crystal grains present in the measurement field of view, weighted by the area ratio of each crystal grain to the field of view. The GOS average can be used as an indicator of the strain imparted to the alloy by processing. By setting the GOS average to 0.3° to 1.5°, strain that serves as the driving force for recrystallization is introduced into the bar, which is appropriate for obtaining coarse crystal grains, resulting in good magnetic properties. If the GOS average is less than 0.3°, the driving force for recrystallization is insufficient, resulting in unrecrystallized grain regions that adversely affect the magnetic properties, making it difficult to obtain good magnetic properties. If the GOS average exceeds 1.5°, excessive tensile stress is applied to the bar, which can easily cause localized area reduction and result in insufficient magnetic properties. The GOS average can be observed at either a transverse or axial cross section, but can also be observed at the axial cross section of the bar. It is preferable that the GOS average be 0.3° to 1.5° when observed at both a transverse and axial cross section of the bar.

[0018] The Fe-Co alloy rod of the present invention preferably has an average grain size number of 8.0 or more and 12.0 or less. This increases the number of grain boundaries and triple points that serve as recrystallization nucleation sites during magnetic annealing, making recrystallization more likely to occur and resulting in stable magnetic properties. A more preferred lower limit of the average grain size number is 8.5, and a more preferred upper limit of the average grain size number is 11.5. An even more preferred lower limit of the average grain size number is 9.0, and an even more preferred upper limit of the average grain size number is 11.0. The average grain size number can be measured in accordance with JIS G 0551. It can be measured on an axial cross section of the rod. Alternatively, it can be measured on a cross section perpendicular to the axis or on an axial cross section of the rod. In the Fe-Co alloy bar of the present invention, it is preferable that the above-mentioned GOS average and average grain size number are satisfied over the entire length of 2 m or more, but it can also be said that it is sufficient that they are satisfied over a continuous length of at least 2 m. [Example]

[0019] Example 1 An Fe-Co alloy steel ingot having the composition shown in Table 1 was bloomed and then hot rolled to prepare a hot rolled bar having a diameter of 11.8 mm and a length of 2900 mm. <Samples No. 1 to 3> The hot-rolled bar material described above was not subjected to solution treatment, but was subjected to a heating and straightening process in which the hot-rolled bar material was pulled in its length direction under a tensile load of 27 kN while being heated to a temperature of about 750°C, thereby producing Fe-Co alloy bars of Samples Nos. 1 to 3, which are examples of the present invention, each having a length of 3,050 mm. <Samples No. 4-6> The hot-rolled bar was heated to 850°C for 30 minutes, and then rapidly cooled for solution treatment. Then, a heating and straightening process was carried out to produce Fe-Co alloy bars having a length of 3050 mm as comparative examples, as samples Nos. 4 to 6. The heating and straightening process conditions were the same as those for samples Nos. 1 to 3.

[0020] [Table 1]

[0021] The area reduction distribution was examined for the prepared samples of the present invention and comparative examples. The area reduction was calculated by measuring the diameter before and after the heating and straightening process with a micrometer. These diameter measurements were performed at 28 equally spaced locations along the rod axis to determine the distribution within a single rod. The area reduction distribution within the rod is shown in Figures 1 and 2. Note that in the heating and straightening process using electrical heating, the chucking section is at a constant voltage and no current flows, so the temperature does not rise in the chucking section. In addition, because the chucking section is mechanically restrained, there is almost no elongation in the chucking section, which will be cut and removed in a later process. Therefore, the area reduction within the range of 250 mm from the end of the rod, where the chucking section has a significant effect during the heating and straightening process, was not considered. Table 2 shows the maximum, minimum, average, and standard deviation of the area reduction obtained from Figures 1 and 2. Here, the maximum, minimum, average and standard deviation of the area reduction rate of the invention examples were derived from samples Nos. 1 to 3, and the maximum, minimum, average and standard deviation of the area reduction rate of the comparative examples were derived from samples Nos. 4 to 6. From Table 2, it can be seen that although the average value of the area reduction rate of the invention examples is almost the same as the average value of the comparative examples, the difference between the maximum and minimum values ​​of the area reduction rate of the invention examples is small, and the standard deviation is also small, so it was confirmed that the variation in the area reduction rate of the invention examples is more suppressed than that of the comparative examples. [Table 2]

[0022] Next, the average grain size, average GOS, and DC magnetic properties were determined for Sample No. 3 (invention example) and Sample No. 6 (comparison example). The average grain size was determined using an Olympus optical microscope in accordance with the comparative method of JIS G 0551. GOS measurements were performed using a ZEISS field-emission scanning electron microscope and a TSL EBSD measurement and analysis system, OIM (Orientation-Imaging-Micrograph). Observations were performed with a measurement field of view of 600 μm × 600 μm, a step distance between adjacent pixels of 1.5 μm, and a grain size discrimination condition of a misorientation between adjacent pixels of 2° or more. The average GOS values ​​were calculated from the resulting GOS map. The average grain size and average GOS values ​​were measured at the central axis position on a longitudinal cross section (axial cross section passing through the central axis). For DC magnetic properties, samples were taken from the obtained bar material, subjected to magnetic annealing at 850°C for 3 hours, and the maximum magnetic permeability and coercivity were measured using a DC magnetization specific testing device. These measurement results, along with the area reduction rates at the sample collection positions, are shown in Table 3. For the invention example and comparative example, samples were collected at four or three locations, respectively, where the area reduction rate was at its maximum, minimum, and center values. The results in Table 3 confirm that the invention example had a larger average grain size number than the comparative example (smaller grain size than the comparative example), and that stable DC magnetic properties were obtained despite fluctuations in the area reduction rate and the GOS average value (distortion index), and that variation was suppressed.

[0023] [Table 3]

[0024] Next, the coercive force distribution was measured for Sample No. 2 of the invention example and Sample No. 5 of the comparative example. After extracting magnetic property samples from the bar material, they were subjected to magnetic annealing at 850°C for 3 hours and then measured using a DC magnetization specific tester. The coercive force distribution and the area reduction ratio distribution are shown together in Figures 3 and 4. However, in Figures 3 and 4, the measurement ranges of 0 to 250 mm and 2800 to 3050 mm (shaded areas in Figures 3 and 4) were not examined due to the significant influence of chucking. In the comparative example, the coercive force was extremely large (deteriorated) in areas with small area reduction ratios, whereas in the invention example, the deterioration of coercive force in areas with small area reduction ratios was suppressed, resulting in stable coercive force and reduced variation.

[0025] Next, the relationship between the area reduction rate and the coercive force for the invention example and the comparative example, derived from the results of Table 3, Figures 3 and 4, is shown in Figure 5. When the area reduction rate is 2.5% or more, the comparative example exhibits excellent coercive force. The invention example also exhibits excellent coercive force, although not as good as the comparative example. When the area reduction rate is 2.5% or less, the comparative example exhibits a rapid deterioration in coercive force and a very unstable coercive force. It was confirmed that the invention example obtained a stable coercive force even when the area reduction rate was in the range of 2.0 to 2.5%, and variation was suppressed.

Claims

1. A hot rolling process in which an Fe-Co alloy billet containing 95% or more Fe+Co by mass and 25 to 60% Co is hot rolled to obtain a hot rolled bar material having a length of 2 m or more; a heating and straightening step of applying a tensile stress to the hot-rolled bar while heating the bar to 500 to 900°C so that the area reduction rate of the bar in a continuous length range of at least 2 m is 2.0 to 8.0%; No solution treatment is performed between the hot rolling step and the heating and straightening step, The method for producing an Fe—Co-based alloy bar provides an Fe—Co-based alloy bar having an average GOS (Grain Orientation Spread) of 0.3° or more and 1.5° or less over a continuous length of at least 2 m, and an average grain size number measured on an axial cross section of the bar of 8.0 or more and 12.0 or less.

2. In mass%, Fe + Co is 95% or more, and Co is contained in an amount of 25 to 60%, An Fe—Co-based alloy bar having a length of 2 m or more, wherein the average value of GOS (Grain Orientation Spread) in a continuous length range of at least 2 m is 0.3° or more and 1.5° or less, the average grain size number measured on an axial cross section of the bar is 8.0 or more and 12.0 or less.

Images