Fe-Co alloy rods

Abstract

As products become higher performance, materials themselves will need to have both high strength and favorable magnetic properties to match the uses of the products. Provided is an Fe-Co alloy bar stock that makes it possible to achieve both high strength and favorable magnetic properties.　The Fe-Co alloy bar stock is, by area ratio, 30%–80% crystal grains that have a grain orientation spread (GOS) value of at least 0.5°, and the Fe-Co alloy bar stock has an average crystal grain size number of above 8.5 but not above 12.0. The Fe-Co alloy bar stock also has a high yield stress of 0.2% after magnetic annealing to handle various high-strength uses.

Description

Technical Field

[0001] The present invention relates to Fe-Co alloy bars.

Background Art

[0002] Bars of Fe-Co alloys, typified by Permender (Permendur), known as alloys having excellent magnetic properties, are used in various products such as sensors, cylindrical magnetic shields, solenoid valves, and magnetic cores. As a manufacturing method of this Fe-Co alloy bar, for example, in Patent Document 1, after heating an ingot to 1000°C to 1100°C, hot working is performed into a billet of about φ90 mm, removal of surface scratches and the like is performed with a lathe, and after heating to 1000°C to 1100°C, a material (bar) hot-rolled to about φ6 to φ9 mm is produced.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] With the improvement in the performance of the above-described products, for example, products such as solenoid valves are becoming smaller, and compatibility between high strength and good magnetic properties is required. In the conventional manufacturing method as described in Patent Document 1, compatibility between the above-described strength and magnetic properties has not been considered, and there remains room for further study. Therefore, an object of the present invention is to provide an Fe-Co alloy bar capable of achieving compatibility between high strength and good magnetic properties.

Means for Solving the Problems

[0005] The present invention relates to an Fe-Co alloy rod having 30% to 80% of its area ratio of crystal grains exhibiting a GOS (Grain Orientation Spread) of 0.5° or higher, and an average crystal grain size number greater than 8.5 and less than or equal to 12.0. [Effects of the Invention]

[0006] According to the present invention, Fe-Co alloy rods suitable for applications requiring both high strength and good magnetic properties can be obtained. [Modes for carrying out the invention]

[0007] Embodiments of the present invention are described below. The Fe-Co alloy rod of the present invention is a straight rod with a cross-sectional shape including circular (including elliptical) and square shapes. When the Fe-Co alloy rod is a round rod, the diameter is 5 to 20 mm. For rods other than round rods, the equivalent diameter of the cross-section is 5 to 20 mm. Unless otherwise specified, the rod in this embodiment is a round rod with a circular cross-sectional shape. First, in this embodiment, a hot-rolled Fe-Co alloy is prepared. In this invention, the Fe-Co alloy refers to an alloy material in which Fe+Co accounts for 95% or more by mass, and Co is present at 25-60%. This allows for the expression of a high magnetic flux density.

[0008] Next, elements that may be included in the Fe-Co alloy of the present invention will be described. In order to improve workability and magnetic properties, the Fe-Co alloy of the present invention may contain one or more elements of 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. Other unavoidable impurity elements include, for example, C, S, P, and O, and it is preferable to set the upper limit of each of these to 0.1%.

[0009] The Fe-Co alloy rod of the present invention has grains with a GOS (Grain Orientation Spread) value of 0.5° or higher, accounting for 30% to 80% of the area ratio. This GOS value can be measured by the conventionally known "SEM-EBSD method (electron beam backscatter diffraction)" and can be derived by calculating the orientation difference of the points (pixels) that make up the grains. The crystal orientation difference obtained from the GOS value is an indicator of the strain imparted to the alloy by processing. When grains with a GOS value of 0.5° or higher account for 30% or more of the area ratio, the driving force for grain growth is introduced into the rod, which has the advantage of obtaining good magnetic properties. Another feature of the present invention is that the upper limit for grains with a GOS value of 0.5° or higher is set to 80% of the area ratio. This feature makes it possible to suppress excessive grain coarsening, thereby increasing the strength of the rod without degrading the magnetic properties. If the area ratio of grains with a GOS value of 0.5° or higher is less than 30%, the rod material lacks sufficient driving force for grain growth, and good magnetic properties cannot be obtained. The lower limit of the preferred area ratio is 35%, and more preferably 40%. Furthermore, if the area ratio of grains with a GOS value of 0.5° or higher exceeds 80%, the magnetic properties improve, but the strength tends to decrease. The upper limit of the preferred area ratio is 78%, and more preferably 75%. The above-mentioned grains with a GOS value of 0.5° or higher can be observed in a cross section perpendicular to the axis of the rod material. In addition, while the area ratio can be observed in both a cross section perpendicular to the axis and an axial cross section, it is preferable that the area ratio is between 30% and 80% in both the cross section perpendicular to the axis and the axial cross section of the rod material. This is because the effect of strain caused by rolling marks generated in the base material during the hot rolling process is easily observed in the axial cross section of the rod material, and the area ratio observed in the axial cross section may be smaller than the area ratio observed in the cross section perpendicular to the axis. Therefore, even in axial cross-sections, which tend to have a small area ratio, the effects of the present invention can be achieved more reliably if the above-mentioned area ratio values ​​are met.

[0010] Furthermore, the Fe-Co alloy rod of the present invention preferably has an average grain size number greater than 8.5 and less than or equal to 12.0. This tends to allow for the stable acquisition of high-strength alloy rods while exhibiting good magnetic properties after magnetic annealing. A more preferable lower limit for the average grain size number is 9.0 or higher, and a more preferable upper limit is 11.5 or lower. An even more preferable upper limit is 11.0 or lower. The average grain size number can be measured according to JIS G 0551. It can be measured in a cross section perpendicular to the axis or in the axial direction of the rod. The strength of the Fe-Co alloy rod of the present invention can be evaluated by the 0.2% yield strength measured in a room-temperature tensile test. To accommodate various high-strength applications, the rod of the present invention preferably has a 0.2% yield strength of 200 MPa or higher after magnetic annealing. A more preferable 0.2% yield strength is 210 MPa or higher. This 0.2% yield strength can be measured according to the tensile testing method for metallic materials specified in JIS Z 2241.

[0011] Next, an example of a manufacturing method for obtaining the Fe-Co alloy rod of the present invention is shown. In this embodiment, a billet obtained from an Fe-Co alloy steel ingot having the above-mentioned components is subjected to hot rolling to obtain a hot-rolled material as an intermediate material for the Fe-Co alloy rod. Since an oxide layer is formed on this intermediate material due to hot rolling, a polishing process to remove the oxide layer mechanically or chemically may be introduced. This hot-rolled material has the shape of a "hot-rolled rod" corresponding to the Fe-Co alloy rod, for example. Furthermore, considering the processability in subsequent processes, the diameter may be 5 to 20 mm. For rods other than round bars, the equivalent diameter of the cross-section may be 5 to 20 mm. Here, in order to satisfy the area ratio of crystal grains for which the GOS value of the present invention is 0.5° or more, it is preferable not to perform solution treatment on the hot-rolled rod. Solution treatment is a process in which the hot-rolled rod is heated to, for example, 800 to 1050°C and then rapidly cooled. Furthermore, it is preferable to perform the heating and straightening process described later without performing the solution treatment.

[0012] <Heating straight process> In this embodiment, a heating and straightening process is performed on the hot-rolled material described above, in which tensile stress is applied while heating. At this time, if the hot-rolled material is in the shape of a "bar", the tensile stress is applied by pulling it in the longitudinal direction of the hot-rolled bar. Through this process, a bar with very good magnetic properties and straightness can be obtained while applying residual strain to the hot-rolled material. The heating temperature at this time is set to 500 to 900°C. If it is lower than 500°C, the workability will decrease, and there is a risk that the bar will break when tensile stress is applied. On the other hand, if the heating temperature exceeds 900°C, it is not possible to apply the desired residual strain to the hot-rolled material. The lower limit of the preferred heating temperature in the heating and straightening process is 600°C, more preferably 700°C. The upper limit of the preferred heating temperature is 850°C, more preferably 830°C, and even more preferably 800°C. If the solution treatment step described above is omitted, the preferred lower limit of the heating temperature is 700°C, more preferably 730°C, and even more preferably 740°C. In this heating and straightening process, heating methods such as electrotherapy, which involves directly passing an electric current through a conductive object to be heated and heating it by Joule heating due to the object's internal resistance, or induction heating can be used. However, electrotherapy is preferred because it has the advantage of making it easier to align the easy magnetization axes of the crystal grains in the hot-rolled material in a certain direction, and it can heat the material to the target temperature rapidly (e.g., within 1 minute) and uniformly. Furthermore, the tension during the heating and straightening process is preferably adjusted to 1 to 4 MPa in order to more reliably obtain the desired residual strain. It is also preferable to adjust the elongation to 3 to 10% of the total length before the heating and straightening process.

[0013] In this embodiment, centerless grinding may be performed on the rod material after the heating and straightening process, for example, using a centerless grinder. This removes the mill scale from the surface of the rod material, further improving the roundness and tolerance accuracy of the shape. In this invention, since the straightness of the rod material is improved by the heating and straightening process, centerless grinding can be performed on long rod materials of 1000 mm or more in length without cutting them. [Examples]

[0014] (Example 1) After dividing the Fe-Co alloy steel ingots having the compositions shown in Table 1, hot rolling was performed to prepare hot-rolled bars with a diameter of Φ11.5 mm. <Sample No. 1, Sample No. 2> Fe-Co alloy rods, samples No. 1 and 2, which are examples of the present invention, were fabricated by performing a heating and straightening process on the aforementioned hot-rolled rod, in which the hot-rolled rod was stretched along its length while being heated to a temperature of 750°C under a tension of 2.7 MPa. <Sample No. 3> The aforementioned hot-rolled bar material was subjected to a solution treatment by heating it to 850°C and then rapidly cooling it. Following this, a heating and straightening process was carried out to produce the comparative example, Sample No. 3, an Fe-Co alloy bar material. The conditions for the heating and straightening process were the same as those for Samples No. 1 and No. 2. <Sample No. 4> A comparative example, Fe-Co alloy rod, sample No. 4, was also prepared by performing a solution treatment on the aforementioned hot-rolled rod under the same conditions as sample No. 3, but without the heating and straightening process, and using the same other processes as the present invention example.

[0015] [Table 1]

[0016] Subsequently, the average crystal grain size, GOS value, and DC magnetic properties of the samples of the present invention example and the comparative example were confirmed. For the average crystal grain size, in the cross-section (section perpendicular to the axis), using an Olympus optical microscope, 10 fields of view of 500 μm × 350 μm were observed, and in accordance with JIS G 0551, the grain size number was determined using the crystal grain size standard chart plate I. For the GOS value, it was carried out using a field emission scanning electron microscope manufactured by ZEISS and an EBSD measurement / analysis system OIM (Orientation-Imaging-Micrograph) manufactured by TSL. Regarding sample No. 4, the cross-section (section perpendicular to the axis) was observed, and for samples No. 1, No. 2, and No. 3, in addition to the cross-sections of the above-mentioned samples, the longitudinal section (section in the axial direction passing through the central axis) was also observed. The measurement field of view was 100 μm × 100 μm, and the step distance between adjacent pixels was 0.2 μm. Also, observations were made under the condition of discriminating the boundary where the azimuth difference between adjacent pixels is 5° or more as the crystal grain boundary, and from the obtained GOS value map, the area ratio of the crystal grains with a GOS value of 0.5° or more to the entire observed field of view was determined. Regarding the DC magnetic properties, after samples were collected from the obtained bar materials, magnetic annealing was performed at 850 °C for 3 hours, and the maximum permeability and coercive force were measured using a DC magnetization specific test device. The observation results are shown in Table 2.

[0017] [Table 2]

[0018] From Table 2, for samples No. 1 and No. 2 which are examples of the present invention, the average crystal grain size number is larger than that of the comparative example (the crystal grain diameter is smaller than that of the comparative example), and it was confirmed that for the area ratio of the crystal grains with a GOS value of 0.5° or more, the present invention example has a smaller value than the comparative example. Regarding the magnetic properties, samples No. 1 to No. 3 had higher permeability and lower coercive force than the conventional example. From this, it was confirmed that samples No. 1 and No. 2 of the present invention example and sample No. 3 of the comparative example have magnetic properties superior to those of the conventional example.

[0019] (Example 2) The 0.2% yield strength at room temperature was measured for rod materials No. 1 to No. 3 that had undergone magnetic annealing at 850°C for 3 hours. The test specimens used for the measurement were half-scale versions of JIS No. 4 test specimens as defined in JIS Z 2241, and the 0.2% yield strength was measured according to the tensile test method for metallic materials specified in JIS Z 2241. The results are shown in Table 3. From the results in Table 3, it was confirmed that the present invention example, in which the area ratio of crystal grains with a GOS value of 0.5° or higher is 30-80%, has a superior 0.2% yield strength compared to the comparative example, in which the area ratio of crystal grains with a GOS value of 0.5° or higher is more than 80%. From this, it can be seen that the Fe-Co rod material of the present invention combines good magnetic properties with high mechanical strength and is suitable for various product applications such as sensors, cylindrical magnetic shields, solenoid valves, and magnetic cores.

[0020] [Table 3]

Claims

[Claim 1] Fe-Co alloy rod material having 30% to 80% of its area ratio of crystal grains exhibiting a GOS (Grain Orientation Spread) of 0.5° or higher, and an average crystal grain size number greater than 8.5 and less than or equal to 12.0.

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