Fe-Mn ALLOY AND TIMEPIECE HAIRSPRING COMPOSED OF Fe-Mn ALLOY

Abstract

The purpose is to provide an Fe-Mn alloy that can restrain increases in production costs.　The Fe-Mn alloy contains, for the component composition in mass%, 19.7-25.0% manganese (Mn), 0.8-4.0% zirconium (Zr), 2.0-10.0% chromium (Cr), and 3.0-15.0% nickel (Ni), with the balance being iron (Fe) and unavoidable impurities.

Description

Fe-Mn alloy and watch hairspring made of Fe-Mn alloy

[0001] The present disclosure relates to a timepiece hairspring material made of an Fe—Mn alloy, and to a timepiece hairspring.

[0002] Because timepiece hairsprings can be affected by external magnetic fields such as those from magnets, which can affect the accuracy of the timepiece, they are preferably made from a material that is less susceptible to magnetic influences. One known example is an Fe-Mn alloy containing Ti (see, for example, JP 2020-501006 A).

[0003] However, when the Fe-Mn alloy described in JP 2020-501006 A is processed into a timepiece hair spring, if the alloy is subjected to a heat treatment process in a wound state, a Ti oxide film will form on the surface of adjacent hair springs, which may cause the hair springs to adhere to each other and become stuck together.

[0004] The present disclosure is intended to solve such problems and aims to provide an Fe—Mn alloy that can suppress an increase in manufacturing costs.

[0005] The Fe—Mn alloy according to the present disclosure has a composition containing, in mass%, 19.7% to 25.0% manganese (Mn), 0.8% to 4.0% zirconium (Zr), 2.0% to 10.0% chromium (Cr), and 3.0% to 15.0% nickel (Ni), with the remainder being iron (Fe) and unavoidable impurities.

[0006] The Fe—Mn alloy according to the present disclosure can suppress an increase in manufacturing costs.

[0007] 1 is an external view of a timepiece hair spring made of an Fe—Mn alloy according to an embodiment. It is a flowchart showing a method for manufacturing the hair spring shown in FIG. 1. (a) is a diagram showing the temperature dependence of Young's modulus of an Fe—Mn alloy when the Mn content is changed, and (b) is a diagram showing the temperature dependence of Young's modulus of an Fe—Mn alloy when the Zr content is changed.

[0008] FIG. 1 is an external view of a timepiece hairspring made of an Fe—Mn alloy according to an embodiment.

[0009] The hairspring 1 is formed by processing an Fe—Mn alloy according to an embodiment and is used in a speed-regulating mechanism of a mechanical timepiece. The Fe—Mn alloy is formed from a raw material having a composition, by mass, of 20.0% to 25.0% manganese (Mn), 1.0% to 4.0% zirconium (Zr), 2.0% to 10.0% chromium (Cr), and 3.0% to 15.0% nickel (Ni), with the balance being iron (Fe) and unavoidable impurities. The Fe—Mn alloy has a composition, by mass, of 19.7% to 25.0% manganese (Mn), 0.8% to 4.0% zirconium (Zr), 2.0% to 10.0% chromium (Cr), and 3.0% to 15.0% nickel (Ni), with the balance being iron (Fe) and unavoidable impurities.

[0010] Fe-Mn alloys have a crystalline structure consisting of an α-Fe phase and a γ-Fe or β-Mn phase. The α-Fe phase has a cubic crystalline structure and is easily magnetized. The α-Fe phase also exhibits brittle properties. The γ-Fe phase has a tetragonal crystalline structure. The γ-Fe phase also has good forgeability and extensibility. The β-Mn phase has a cubic crystalline structure. The γ-Fe and β-Mn phases are paramagnetic, meaning they are difficult to magnetize.

[0011] The Fe--Mn alloy has a low magnetic susceptibility due to its crystalline structure of γ-Fe phase or β-Mn phase.

[0012] The Fe—Mn alloy contains, by mass%, 19.7% to 25.0% Mn. Mn forms a solid solution with Fe, converting the Fe into a paramagnetic γ-Fe phase. This gives the Fe—Mn alloy a low magnetic susceptibility. When the Mn content in the Fe—Mn alloy is less than 19.7% or more than 25.0%, the rate of change in Young's modulus with temperature change increases. Specifically, when the Mn content in the Fe—Mn alloy is less than 19.7% or more than 25.0%, the rate of change in Young's modulus with temperature change from −20°C to +40°C, defined based on the Young's modulus at 25°C, is ±0.7% or more. Furthermore, when the Mn content in the Fe—Mn alloy is less than 20.0%, the proportion of the α-Fe phase increases, resulting in reduced workability.

[0013] The Fe—Mn alloy contains, by mass, 0.8% to 4.0% Zr. Zr functions as a hardening agent. When the Zr content in the Fe—Mn alloy is less than 0.8%, the alloy does not function as a hardening agent. Furthermore, when the Zr content in the Fe—Mn alloy exceeds 4.0%, the rate of change in Young's modulus with temperature change increases. Specifically, when the Mn content in the Fe—Mn alloy exceeds 4.0%, the rate of change in Young's modulus with temperature change from −20° C. to +40° C., defined based on the Young's modulus at 25° C., is ±0.7% or more.

[0014] The Fe—Mn alloy contains 2.0% to 10.0% Cr by mass. Cr forms an oxide film layer on the surface of the Fe—Mn alloy, contributing to improved corrosion resistance. It forms a solid solution with Fe, with a γ-Fe phase crystalline structure. If the Cr content in the Fe—Mn alloy is less than 2.0%, an oxide film layer is not sufficiently formed on the surface of the Fe—Mn alloy, resulting in reduced corrosion resistance. Furthermore, if the Cr content in the Fe—Mn alloy is more than 10.0%, the Fe—Mn alloy becomes excessively hard, impairing its workability.

[0015] The Fe—Mn alloy contains, by mass %, 3.0% to 15.0% Ni. Ni improves the forgeability of the Fe—Mn alloy in at least one of a hot working process and a cold working process. When the Cr content in the Fe—Mn alloy is less than 3.0%, the forgeability in the hot working process and the cold working process is reduced.

[0016] The remainder of the Fe-Mn alloy is Fe and unavoidable impurities. Inevitable impurities are elements that are inevitably mixed in from raw materials, etc., or elements that are unintentionally mixed in during the manufacturing process. Examples of unavoidable impurities include Si (silicon), P (phosphorus), and S (sulfur). By limiting the content of each unavoidable impurity to less than 0.1% by mass, the effect on the properties of the Fe-Mn alloy is negligible. Furthermore, it is preferable to limit the amount of each unavoidable impurity to less than 0.01% by mass so that even if the unavoidable impurities are concentrated in a portion of the alloy, they will not affect the properties of the Fe-Mn alloy.

[0017] FIG. 2 is a flowchart showing a method for manufacturing the hairspring 1.

[0018] The manufacturing method shown in FIG. 2 includes an ingot melting step (step S1), a hot working step (steps S2-S3), a cold working step (steps S4-S6), a plastic working step (step S7), and a hardening heat treatment step (step S8). In the ingot melting step, an ingot is melted. In the hot working step, the ingot is hot worked to produce a hot-worked product. In the cold working step, the hot-worked product is cold worked to produce a cold-rolled material having metal crystals with dislocations introduced. The cold-rolled material includes a γ-Fe phase and an α phase as its crystalline structure. In the hardening heat treatment step, the cold-rolled material is hardened to produce an Fe—Mn alloy. When dislocations are introduced into the metal crystals in the cold working step, a phase transformation from the γ-Fe phase to the β-Mn phase occurs in the hardening heat treatment step.

[0019] First, an ingot is melted (Step S1). The ingot is produced by melting raw materials weighed to have a predetermined composition and pouring the melt into a mold. The raw materials are melted using a high-frequency vacuum melting apparatus having a mechanism for pouring and a heating section equipped with a room-temperature mold. The raw materials for the ingot contain, in mass %, 20.0% to 25.0% manganese (Mn), 1.0% to 4.0% zirconium (Zr), 2.0% to 10.0% chromium (Cr), and 3.0% to 15.0% nickel (Ni), with the remainder being iron (Fe) and unavoidable impurities.

[0020] In melting using a high-frequency vacuum melting device, first, a ceramic crucible containing weighed raw materials is loaded into the heating section of the device. -2A vacuum atmosphere of 1000 Pa or less is created, and an inert gas such as nitrogen or argon is filled. Under the inert gas atmosphere, the raw materials are high-frequency induction heated for 10 to 45 minutes, softening and melting them to a liquid molten state. The heated state is then maintained for 5 to 25 minutes so that the temperature of the molten raw materials is in the range of 1400°C to 2000°C. After being maintained in the heated state, the molten raw materials are poured into a mold at room temperature and rapidly cooled. The rapidly cooled raw materials are left to stand for 4 to 9 hours, cooling to room temperature, and forming a solid ingot. The interior of the high-frequency vacuum melting apparatus is then evacuated and opened to the atmosphere. The ingot is then removed from the mold. When the raw materials are in a liquid molten state, some elements in the composition may become gases and be released from the molten metal. Furthermore, some elements in the composition may form compounds with the ceramic crucible containing the raw materials, resulting in residual compounds remaining in the crucible. These factors can cause the composition of the ingot to vary from that of the raw materials. For example, the Mn content of the raw materials for the ingot is 20.0% to 25.0% by mass, but the Mn content of the ingot removed from the mold is 19.7% to 25.0% by mass. Furthermore, the Zr content of the raw materials for the ingot is 1.0% to 4.0% by mass, but the Cr content of the ingot removed from the mold is 0.8% to 4.0% by mass. By predicting the fluctuations in the content of these elements and taking measures to weigh the raw materials in advance in excess of the desired component ratio, the composition of the ingot removed from the mold can be made to match the desired composition of the raw materials for the ingot.

[0021] Next, the ingot is subjected to hot working including hot hammer forging (step S2) and hot groove rolling (step S3) to obtain a hot-worked product, which is a bar material. The hot working is performed at 1100°C or higher and 1250°C or lower. The hot-worked product obtained by the hot working is water-cooled.

[0022] The hot-worked product has the same elemental composition and area fraction of the crystal structure as the ingot. Preferably, the size of the metal crystal grains of the hot-worked product is 10 μm or less. By making the size of the metal crystal grains of the hot-worked product 10 μm or less, the final product, an Fe—Mn alloy, can have high hardness. Preferably, the working rate by hot working is 45% or more and 80% or less. The working rate is the rate of reduction of the cross-sectional area. By making the working rate by hot working 45% or more and 80% or less, the size of the metal crystal grains of the hot-worked product can be made 10 μm or less.

[0023] Next, the water-cooled hot-worked product is subjected to cold working including cold swaging forging (step S4), cold drawing wire drawing (step S5), and cold rolling (step S6), to obtain a cold-worked product, a cold-rolled material.

[0024] In cold swaging forging (step S4), the rod material, which is a hot-worked product, is cold forged and formed into a thin rod material with a reduced outer diameter. The thin rod material formed in cold swaging forging (step S4) is drawn using a diamond die in cold drawing wire drawing (step S5) to form a drawn wire material. The drawn wire material formed in cold drawing wire drawing (step S5) is rolled in cold rolling (step S6) so that the cross section of the drawn wire material changes from circular to rectangular, and is formed into a band-shaped ribbon material, which is a cold-rolled material.

[0025] Dislocations are introduced into the metal crystals of the thin rod material obtained by cold swaging forging (step S4), the drawn wire material obtained by cold drawing (step S5), and the ribbon material obtained by cold rolling (step S6). Preferably, the reduction rate by cold working is 20% to 90%, more preferably 40% to 80%. This introduces an appropriate amount of dislocations into the metal crystals, facilitating a phase transformation from the γ-Fe phase to the β-Mn phase in the crystal structure, thereby imparting a desired hardness to the final product, the hair spring 1. Note that, because the β-Mn phase has a higher hardness than the γ-Fe phase, it is preferable that the area fraction of the β-Mn phase in the Fe—Mn alloy be greater than the area fraction of the γ-Fe phase. This allows the hair spring 1 to have a desired hardness.

[0026] The cold-rolled material has the same elemental composition and area fraction of the crystal structure as the ingot. Preferably, the size of the metal crystal grains of the cold-rolled material is 10 μm or less. By making the size of the metal crystal grains of the cold-rolled material 10 μm or less, the hair spring 1 can have a desired hardness.

[0027] Next, in the plastic working step (step S7), the ribbon material, which is a cold-rolled material, is cut to a predetermined length and then held in a spiral shape using a jig or the like. In this way, the ribbon material is formed into the shape of the hair spring 1.

[0028] Then, in a hardening heat treatment step (step S8), the formed cold-rolled material is subjected to hardening heat treatment to obtain the hair spring 1. The hardening heat treatment causes a phase transformation from the γ-Fe phase to the β-Mn phase.

[0029] In the hardening heat treatment, the hair spring 1 is hardened while being wound, and then released from the wound state after the hardening heat treatment. Since the composition of the Fe—Mn alloy of this embodiment does not contain Ti, there is no risk of adjacent ribbon materials being adhered to each other by a Ti oxide film during the hardening heat treatment, and the hair spring 1 can be released from the wound state and have a natural length with the inner periphery and outer periphery spaced apart.

[0030] The hardening heat treatment is performed at a temperature of 550°C or higher and 800°C or lower. Preferably, the hardening heat treatment is performed at a temperature of 600°C or higher and 700°C or lower. The hardening heat treatment can impart a desired hardness to the final product, the hair spring 1. If the hardening heat treatment temperature is too high, the hardness of the hair spring 1 may be reduced. Furthermore, the hardening heat treatment is performed for 10 minutes or higher and 12 hours or lower. By performing the hardening heat treatment for 10 minutes or higher and 12 hours or lower, the area fraction of the β-Mn phase in the Fe—Mn alloy becomes 50% or higher, and the hair spring 1 can be imparted with low magnetic susceptibility and the desired hardness. If the hardening heat treatment time exceeds 12 hours, the hardness of the hair spring 1 may be reduced. The hair spring 1 obtained by the hardening heat treatment is air-cooled.

[0031] In the method for manufacturing the hair spring 1, a homogenization heat treatment step of homogenizing the ingot may be performed before the hot working step. The homogenization heat treatment is performed, for example, at a temperature of 1000°C to 1200°C for 0.5 hours to 3 hours. This makes the metal crystals of the ingot uniform.

[0032] In the method for manufacturing the hair spring 1, an annealing step of annealing the hot-worked product obtained in the hot-working step may be performed between the hot-working step and the cold-working step. The annealing is performed, for example, at a temperature of 1000°C to 1200°C for 0.5 hours to 3 hours. This makes the metal crystals of the hot-worked product uniform.

[0033] The method for manufacturing the hair spring 1 is not limited to the above-described example. The hair spring 1 may be manufactured by a manufacturing method different from the above-described manufacturing method.

[0034] The Fe-Mn alloy according to the embodiment does not contain Ti, so there is no risk of a decrease in production yield due to the problem of timepiece hairsprings sticking together caused by the formation of a Ti oxide film on the surface, and it is possible to suppress an increase in production costs.

[0035] The Fe—Mn alloy according to the embodiment can suppress the rate of change in Young's modulus with temperature to ±0.7% or more, thereby suppressing the temperature-dependent fluctuation in Young's modulus of a member formed using the Fe—Mn alloy according to the embodiment. For example, a timepiece hairspring formed using the Fe—Mn alloy according to the embodiment has suppressed temperature-dependent fluctuation in Young's modulus, suppressing rate fluctuations due to temperature changes in the cycle of a speed regulator in which the timepiece hairspring is mounted, thereby improving the timekeeping accuracy of the timepiece.

[0036] The Fe—Mn alloy according to the embodiment is preferably formed so that its Young's modulus increases with increasing temperature. By forming the Fe—Mn alloy according to the embodiment so that its Young's modulus increases with increasing temperature, it can be made an even more suitable material for a timepiece hair spring. A speed regulator equipped with a timepiece hair spring has a balance formed by a timepiece hair spring and a weight called a balance. The balance is affected by thermal expansion and thermal contraction and has a temperature characteristic of losing rate as the temperature increases. By forming the Fe—Mn alloy according to the embodiment so that its Young's modulus increases with increasing temperature, the timepiece hair spring formed from the Fe—Mn alloy according to the embodiment has a temperature characteristic of gaining rate as the temperature increases. Because the timepiece hair spring formed from the Fe—Mn alloy according to the embodiment has a temperature characteristic of gaining rate as the temperature increases, the temperature characteristic of the balance wheel of losing rate as the temperature increases can be offset. The Fe—Mn alloy according to the embodiment is formed so that its Young's modulus increases with increasing temperature, thereby making it possible to suppress fluctuations in rate caused by temperature changes in a governor equipped with a timepiece hairspring formed from the Fe—Mn alloy according to the embodiment.

[0037] The rate of change in Young's modulus was measured while changing the applied temperature for each of the Fe—Mn alloys according to Examples 1 to 5 and the Fe—Mn alloys according to Comparative Examples 1 to 4. The temperatures applied to the Fe—Mn alloys according to Examples 1 to 5 and the Fe—Mn alloys according to Comparative Examples 1 to 4 were −20° C., 0° C., 10° C., 25° C., 35° C., 50° C., and 70° C. The rate of change in Young's modulus was defined with the Young's modulus at 25° C. as the reference (0%).

[0038] The Fe—Mn alloys according to Examples 1 to 5 were formed into rods with a diameter of 13 mm. They were then formed and heat-treated into the shape of timepiece hairsprings. The Fe—Mn alloy according to Example 1 was formed from a raw material containing 61.00% Fe, 20.00% Mn, 2.00% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Example 2 contained 56.00% Fe, 25.00% Mn, 2.00% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Example 3 is formed from a raw material containing 61.95% Fe, 20.00% Mn, 1.05% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Example 4 is formed from a raw material containing 60.00% Fe, 20.00% Mn, 3.00% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Example 5 is formed from a raw material containing 59.00% Fe, 20.00% Mn, 4.00% Zr, 7.00% Cr, and 10.00% Ni.

[0039] The Fe—Mn alloys according to Comparative Examples 1 to 4 were formed into rods having a diameter of 13 mm, similar to the Fe—Mn alloys according to Examples 1 to 5. They were then molded and heat-treated into the shape of a timepiece hairspring. The Fe—Mn alloy according to Comparative Example 1 was formed from a raw material containing 71.00% Fe, 10.00% Mn, 2.00% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Comparative Example 2 was formed from a raw material containing 66.00% Fe, 15.00% Mn, 2.00% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Comparative Example 3 is formed from a raw material containing 51.00% Fe, 30.00% Mn, 2.00% Zr, 7.00% Cr, and 10.00% Ni. The Fe—Mn alloy according to Comparative Example 4 is formed from a raw material containing 58.00% Fe, 20.00% Mn, 5.00% Zr, 7.00% Cr, and 10.00% Ni.

[0040] Table 1 shows the composition ratios of the Fe—Mn alloys according to Examples 1 to 5 and Comparative Examples 1 to 4, as well as the temperature dependence of Young's modulus, workability, and magnetic properties. In Table 1, the composition ratios of Examples 1 to 3 and 5 are analytical values ​​obtained by actually measuring the composition ratios of the Fe—Mn alloys. Meanwhile, the composition ratios of Example 4 and Comparative Examples 1 to 4 are design values ​​indicating the composition ratios contained in the raw materials of the ingots. Measurements of the temperature dependence of Young's modulus and magnetic properties were conducted using specimens cut from a rod having a diameter of 13 mm. The temperature dependence of Young's modulus was determined based on whether the rate of change in Young's modulus was less than ±0.7% over a temperature range of −20°C to 40°C. Workability was determined based on whether or not processing defects such as breakage occurred during the processing steps from ingot formation to rod formation, and whether or not defects such as adhesion occurred on the surface of the timepiece hairspring during the processing and heat treatment steps from the rod to the timepiece hairspring. The magnetization properties were determined based on whether or not the material was attracted to a permanent magnet.

[0041]

[0042] FIG. 3( a) is a graph showing the temperature dependence of Young's modulus of an Fe—Mn alloy when the Mn content is changed, and FIG. 3( b) is a graph showing the temperature dependence of Young's modulus of an Fe—Mn alloy when the Zr content is changed. In FIGS. 3( a) and 3(b), the horizontal axis represents the applied temperature, and the vertical axis represents the rate of change of Young's modulus. In FIG. 3(a), waveform W101 represents the rate of change of Young's modulus in Example 1, and waveform W102 represents the rate of change of Young's modulus in Example 2. Waveform W103 represents the rate of change of Young's modulus in Comparative Example 1, waveform W104 represents the rate of change of Young's modulus in Comparative Example 2, and waveform W105 represents the rate of change of Young's modulus in Comparative Example 3. In FIG. 3(b), waveform W201 represents the rate of change of Young's modulus in Example 1, and waveform W202 represents the rate of change of Young's modulus in Example 3. Waveform W203 shows the rate of change in Young's modulus for Example 4, waveform W204 shows the rate of change in Young's modulus for Example 5, and waveform W205 shows the rate of change in Young's modulus for Comparative Example 4.

[0043] In Example 1, the rate of change of Young's modulus is within the range of -0.3% to +0.3% in the temperature range of -20°C to 40°C, and the change in response to temperature changes is small, resulting in good temperature dependency of Young's modulus. Furthermore, in Example 1, the Young's modulus increases with increasing temperature in the temperature range of -20°C to 0°C, so the temperature characteristic of the balance wheel, which causes rate to slow with increasing temperature, can be offset in the temperature range of -20°C to 0°C, making it suitable as a material for timepiece hairsprings.

[0044] In Example 2, the rate of change of Young's modulus is within the range of -0.5% to 0.0% in the temperature range of -20°C to 40°C, and similarly to Example 1, the change in response to temperature changes is small, and the temperature dependency of Young's modulus is favorable. In Example 1, the Young's modulus increases with increasing temperature in the temperature range of 0°C to 25°C, and therefore the temperature characteristic of the balance wheel, which causes the rate to slow with increasing temperature, can be offset in the temperature range of 0°C to 25°C, making it suitable as a material for timepiece hairsprings.

[0045] In Comparative Example 1, the rate of change of Young's modulus in the temperature range of -20°C to 40°C is within the range of -0.5% to +1.1%, and the change in response to temperature is large. In Comparative Example 2, the rate of change of Young's modulus in the temperature range of -20°C to 40°C is within the range of -0.3% to +0.7%, and the change in response to temperature is large. In Comparative Example 3, the rate of change of Young's modulus in the temperature range of -20°C to 40°C is within the range of -0.3% to +1.2%, and the change in response to temperature is large. Comparative Examples 1 to 3 have good temperature dependence of Young's modulus from room temperature to high temperatures, but the temperature dependence of Young's modulus deteriorates from room temperature to low temperatures, as shown by the rate of change of Young's modulus at -20°C of +1.1%, +0.7%, and +1.2%, respectively.

[0046] In Example 3, the rate of change in Young's modulus is within the range of -0.3% to +0.3% in the temperature range of -20°C to 40°C, and the change in response to temperature change is small, resulting in good temperature dependency of Young's modulus.

[0047] In Example 4, the rate of change of Young's modulus is within the range of -0.2% to 0.3% in the temperature range of -20°C to 40°C, and similarly to Example 3, the change in response to temperature changes is small, and the temperature dependency of Young's modulus is favorable. Furthermore, in Example 4, the Young's modulus increases with increasing temperature in the temperature range of -20°C to 0°C, and therefore the temperature characteristic of the balance wheel, which causes the rate to slow with increasing temperature, can be offset in the temperature range of -20°C to 0°C, making it suitable as a material for timepiece hair springs.

[0048] In Example 5, the rate of change of Young's modulus is within the range of -0.3% to 0.3% in the temperature range of -20°C to 40°C, and similarly to Example 3, the change in response to temperature changes is small, and the temperature dependency of Young's modulus is favorable. Furthermore, in Example 5, the Young's modulus increases with increasing temperature in the temperature range of -20°C to 0°C, and therefore the temperature characteristic of the balance wheel, which causes the rate to slow with increasing temperature, can be offset in the temperature range of -20°C to 0°C, making it suitable as a material for timepiece hair springs.

[0049] In Comparative Example 4, the rate of change in Young's modulus is within the range of -1.1% to +0.4% in the temperature range of -20°C to 40°C, and the change in response to temperature is large. Comparative Example 4 has good temperature dependency of Young's modulus from room temperature to high temperatures, but the temperature dependency of Young's modulus deteriorates from room temperature to low temperatures, as shown by the rate of change in Young's modulus at -20°C being -1.1%.

[0050] The Fe—Mn alloys according to Examples 1 to 5 and the Fe—Mn alloys according to Comparative Examples 1 to 4 were found to have good workability, with no breakage or other processing defects occurring in the processing steps from ingot formation to bar material, and the wound ribbon material could be formed into hair springs without sticking together in the hardening heat treatment step for timepiece hair springs.

[0051] The Fe—Mn alloys according to Examples 1 to 5 and Comparative Examples 3 and 4 have a very low magnetic susceptibility of 1.00E-02 or less, are not attracted to permanent magnets, and do not function as magnetic bodies, making them suitable materials for timepiece hair springs. The Fe—Mn alloy according to Comparative Example 1 has a higher Fe content and a lower Mn content than Examples 1 to 5, and a high magnetic susceptibility of 1.12E-01 or less, so it functions as a magnetic body attracted to a permanent magnet. When Comparative Example 1 is used as a timepiece hair spring, it is easily magnetized by an external magnetic field, and the rate accuracy of the governor is likely to decrease. The Fe—Mn alloy according to Comparative Example 2 has a higher Fe content and a lower Mn content than Examples 1 to 5, and a high magnetic susceptibility of 9.30E-02 or less, so it functions as a magnetic body attracted to a permanent magnet. When Comparative Example 2 is used as a timepiece hair spring, it is easily magnetized by an external magnetic field.

Claims

1. An Fe-Mn alloy characterized in that its composition contains, by mass, 19.7% to 25.0% manganese (Mn), 0.8% to 4.0% zirconium (Zr), 2.0% to 10.0% chromium (Cr), and 3.0% to 15.0% nickel (Ni), with the remainder being iron (Fe) and unavoidable impurities.

2. The Fe-Mn alloy according to claim 1, wherein the rate of change of Young's modulus is less than ±0.7% in the temperature range of -20°C to 40°C.

3. The Fe-Mn alloy according to claim 1, having a magnetic susceptibility of 1.00E-02 or less.

4. A timepiece hairspring formed from the Fe-Mn alloy according to claim 1.

5. The Fe-Mn alloy according to claim 1, formed from raw materials having a composition, in mass%, of 20.0% to 25.0% manganese (Mn), 1.0% to 4.0% zirconium (Zr), 2.0% to 10.0% chromium (Cr), and 3.0% to 15.0% nickel (Ni), with the remainder being iron (Fe) and unavoidable impurities.

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