JP5548578B2 - High strength magnesium alloy wire and manufacturing method thereof, high strength magnesium alloy component, and high strength magnesium alloy spring - Google Patents

Description

  The present invention relates to a high-strength magnesium alloy wire suitable for use in products mainly subjected to bending stress and / or torsional stress, a method for producing the same, a high-strength magnesium alloy component, and a high-strength magnesium alloy spring.

  Conventionally, in various fields such as aerospace, vehicles (automobiles, motorcycles, railways, etc.), medical equipment, welfare equipment, robots, etc., there is a strong demand for weight reduction of parts for the purpose of function enhancement, performance improvement, and operability improvement. In particular, in the field of vehicles including automobiles, in recent years, the demand for weight reduction for the purpose of reducing carbon dioxide emissions due to environmental problems, that is, improving fuel efficiency, has been increasing year by year.

  Research on weight reduction of parts has been active mainly in the vehicle field, and until now, research has been focused on improving the strength of steel by improving the composition, modifying the surface, or a combination thereof. For example, for springs that are representative of strength parts, high-tensile materials have already been used as the mainstream, and the fatigue strength is further improved by combining surface modification techniques such as nitriding and shot peening. As a result, the weight is reduced. However, with steel, the conventional increase in strength due to the composition dependence is approaching its limit, and it is not possible to expect a significant reduction in weight in the future.

  Thus, for further weight reduction, there are great expectations for light alloys with low specific gravity, such as titanium alloys, aluminum alloys, and magnesium alloys. Among them, the lightest magnesium alloy in practical use is about 1/4 of steel, about 1 / 2.5 of titanium alloy, and about 1 / 1.5 of aluminum alloy. In addition to this, early diffusion into the market is desired.

  However, conventional general magnesium alloys have limited product applications. The biggest cause is that the strength of a conventional general magnesium alloy is low, and in order to secure the strength as a part, it is inevitable that the part size is increased as compared with the conventional steel part. That is, the conventional magnesium alloy is not yet accepted in the market as a strength component because it is difficult to achieve both weight reduction and compactness.

  Against this background, research on high-strength magnesium alloys for the application of magnesium alloys to high-strength parts has been actively conducted. For example, Patent Document 1 discloses that a solid product is produced from a molten metal of Mg—Al—Zn—Mn—Ca—RE (rare earth element) by wheel casting, and the solid product is consolidated by drawing. The technology which can obtain the magnesium alloy member of 0.2% yield strength 565MPa is disclosed.

  Patent Document 2 discloses Mg-X-Ln (X is one or more of Cu, Ni, Sn, and Zn, and Ln is one or more of Y, La, Ce, Nd, and Sm). A technique is disclosed in which a magnesium alloy foil strip having a hardness of 200 HV or more can be obtained by rapidly solidifying an alloy from a molten metal to form an amorphous foil strip.

  Further, Patent Document 3 discloses a technique for obtaining a magnesium alloy wire having a tensile strength of 250 MPa or more and an elongation of 6% or more by drawing a cast material or an extruded material of an Mg—Al—Mn alloy.

Japanese Patent Laid-Open No. 3-90530 JP-A-3-10041 JP 2003-293069 A

  The means shown in these patent documents are effective for increasing the strength of the magnesium alloy. However, the magnesium alloy disclosed in Patent Document 1 does not have sufficient mechanical properties to satisfy market demand as a strength component. For example, assuming the application to a spring where bending stress and / or torsional stress mainly acts, the strength of the magnesium alloy wire that can be reduced in weight while maintaining the same size as the current steel spring is According to the inventors' calculations, 0.2% proof stress of 550 MPa or more is required inside the wire, and 0.2% proof stress of 650 MPa or more is required near the surface of the wire. Further, in order to form a coil spring or the like, at least 5% or more elongation is required inside the wire. However, the 0.2% proof stress 565 MPa invention having the highest strength disclosed in Patent Document 1 has poor ductility and only 1.6% elongation. On the other hand, the elongation of the invention having the most excellent ductility disclosed in Patent Document 1 is 4.7% and has an elongation close to the value desired in the present invention, but the strength is 0.8. The 2% proof stress is poor at 535 MPa, and the requirements cannot be satisfied.

  The magnesium alloy disclosed in Patent Document 2 has a hardness of 170 HV or higher. According to a trial calculation by the present inventors, this hardness is a hardness corresponding to a 0.2% proof stress of 650 MPa or more on the surface of the wire. However, Patent Document 2 does not disclose any characteristics indicating ductility. The magnesium alloy disclosed in Patent Document 2 contains a large amount of rare earth elements and is made of an amorphous phase of 50% or more, so its ductility is very poor, and the desired elongation cannot be obtained in the present invention. Is easily assumed. Further, the amorphous phase is thermally unstable and has a drawback that it easily crystallizes due to external factors such as environmental temperature. Amorphous phase and crystalline phase mixed phase alloys have different characteristics depending on the ratio of the phases, which makes it difficult to stably produce products with uniform characteristics in production, as well as quality assurance and security in the market. Because of this difficulty, it is inappropriate to apply it to industrial products.

  The magnesium alloy disclosed in Patent Document 3 has a sufficient ductility of 6% or more. However, the maximum tensile strength was 479 MPa, and it was not possible to satisfy the 0.2% yield strength of 550 MPa or more inside the above-described wire.

  Thus, conventional magnesium alloys do not satisfy both the 0.2% proof stress and elongation required when assuming a strength component (for example, a spring) in which bending stress and / or torsional stress mainly acts. It was. Accordingly, the present invention satisfies both the properties having a trade-off relationship of 0.2% proof stress and elongation, thereby providing strength and formability (hereinafter, unless required otherwise, ductility necessary for bending processing, coiling processing, etc.). High strength magnesium alloy wire and high strength magnesium alloy parts suitable for use in products in which bending stress and / or torsional stress mainly acts, and a method for producing them. It is intended to provide.

The present invention is made of a magnesium alloy used for a member which is composed of Ni: 2 to 5%, Y: 2 to 5%, balance: Mg and unavoidable impurities, and mainly subjected to bending stress and / or torsional stress. The wire has a portion having a maximum hardness of 170 HV or more in the vicinity of the surface thereof, and the inside has a 0.2% proof stress of 550 MPa or more and an elongation of 5% or more.

  The vicinity of the surface of the wire means a range from the outermost surface of the wire to a depth of about d / 10 (d is the diameter of the wire), and has a maximum hardness of 170 HV or more in the vicinity of the surface. Furthermore, 0.2% proof stress of 650 MPa or more is satisfied near the surface of the wire. In the present invention, the strength (hardness) gradually decreases gradually from the vicinity of the surface toward the center portion, but the inner portion has a 0.2% proof stress of 550 MPa or more and an elongation of 5% or more. doing. That is, the product of the present invention is a magnesium alloy having strength and formability suitable for use in products in which bending stress and / or torsional stress mainly acts.

  As described above, the present invention has a high strength and high ductility region inside and a further high strength region in the vicinity of the surface. It can also be satisfied by having an appropriate distribution of mechanical properties for the part on which the torsional stress mainly acts. In this case, the outermost surface portion can be subjected to surface modification such as applying compressive residual stress by shot peening, for example. As a result, it is possible to further improve the fatigue resistance of parts to which bending stress and / or torsional stress mainly acts.

  Next, the method for producing a high-strength magnesium alloy wire according to the present invention comprises a step of producing a starting material in the form of a foil strip, a foil piece, or a thin wire made of a magnesium alloy by a rapid solidification method, The above-described wire is obtained by a sintering process in which a billet is formed by joining by sintering and a plastic working process in which the billet is plastically processed to be processed into a wire.

  In the present invention, it is desirable to use a starting material having a material composition to be described later, which is in the form of a foil strip, a foil piece, or a thin wire made of a magnesium alloy by a rapid solidification method. Thereby, when using a powder with a large specific surface area described as one means in Patent Document 1 or when using an alloy having a more active material composition, an instantaneous container after molding of the starting material Processes such as filling and canning are unnecessary.

  Moreover, as one means for producing the high-strength magnesium alloy wire of the present invention, a step of producing a fine wire made of a magnesium alloy by a molten metal extraction method, a sintering step of forming a billet by joining the thin wire by sintering, A manufacturing method can be adopted in which the billet is charged as it is into a container of a press and the wire is obtained by an extrusion process in which the billet is extruded.

  In the present invention, by directly extruding a billet that has not been canned, an internal high strength and high ductility region can be obtained, and a further high strength region can be obtained near the surface. Also, the internal high-strength and high-ductility region and the further high-strength region near the surface are gradually connected and do not have a clear boundary as a mechanical property. Particularly preferred. When both regions have clear boundaries, the possibility of the interface becoming the starting point of fracture increases due to the difference in hardness (or elastic strain), but the two regions gradually connect without having a clear boundary. This makes it possible to avoid the danger that the interface becomes a starting point of fracture. In the present invention, since the billet is charged as it is into the container of the press, the process is shortened compared to the case where canning is performed, and the billet can be manufactured at a low cost.

  The high strength magnesium alloy wire of the present invention has high surface strength and formability. Therefore, by using it for molded parts where bending stress and / or torsional stress mainly acts, it is possible to significantly reduce the weight without enlarging the part size when compared with conventional steel parts. . Specifically, for example, it can be applied to automobile parts such as seat frames that occupy a large weight and springs that require high strength (suspension springs, valve springs, clutch torsion springs, torsion bars, stabilizers), etc. Has excellent strength and formability.

It is sectional drawing which shows the metal fine wire manufacturing apparatus used by embodiment. It is a sectional side view which shows the extrusion apparatus used in embodiment. It is a graph which shows the relationship between the distance from the center in the cross section of the wire at each extrusion temperature in the Example of this invention, and hardness. It is a graph which shows the relationship from the distance from the center in the cross section of the wire in each material composition in the Example of this invention, and hardness. It is a graph which shows the relationship between the distance from the center in the cross section of the wire in each container internal diameter and extrusion ratio in the Example of this invention, and hardness.

1. Material Composition Conventionally, Zn has been mainly added as a first additive element for improving the strength and ductility of magnesium alloys. However, the addition of Zn is insufficient to achieve both the high strength and ductility desired in the present invention. Therefore, it is desirable to add Ni as the first additive element. Ni has a larger effect on high strength and high ductility than Zn.

  However, even with the addition of Ni that greatly contributes to high strength and high ductility, it is not easy to achieve the high strength aimed by the present invention. Therefore, it is desirable to add Y as the second additive element. Addition of Y forms a high-strength Mg—Ni—Y compound phase. Y has a high solubility in Mg, and is effective for solid solution strengthening in the α-Mg phase. In addition, as described later, it is possible to achieve higher strength by combining the starting material with the rapid solidification method. In addition, the magnesium alloy in this invention is not restricted to the composition from three elements, Mg, Ni, and Y. The main additive may be Mg, Ni, or Y, and a third additive element may be added for the purpose of refining crystal grains and improving corrosion resistance. In that case, for example, Zr or Al is effective.

Magnesium alloy of the present invention, in atomic%, Ni: 2~5%, Y : 2~5%, the balance: a composition consisting of Mg and inevitable impurities. When Ni is less than 2 atomic% and Y is less than 2 atomic%, the maximum hardness in the vicinity of the surface does not reach the desired hardness, and for a strength component on which bending stress and / or torsional stress mainly acts The strength is not enough. On the other hand, when Ni exceeds 5 atomic% and Y exceeds 5 atomic%, workability is remarkably deteriorated and breaks during extrusion. This is because the amount of the high hardness compound phase formed by the addition of Ni and Y is increased and the size thereof is coarsened. As a result, the deformation resistance is increased and the toughness is reduced, leading to fracture.

2. Preparation of starting material A starting material of a magnesium alloy having the above composition is prepared. As the method, a rapid solidification method such as a single roll method, a melt spinning method, or a molten metal extraction method is used, and a foil strip, a foil piece, or a thin wire is formed. Compared with a general casting method having a slow solidification rate, the amount of each additive element in the α-Mg phase in the foil strip, foil piece, or fine wire produced by the rapid solidification method is large. Therefore, even if the addition amount of each element is the same, the strength can be increased by solid solution strengthening. Further, in the rapid solidification method, the crystal grains become fine. Refinement of crystal grains contributes to improvement in strength and also improves ductility, and is effective in improving overall mechanical properties in combination with solid solution strengthening.

In addition, the rapidly solidified powder including the atomizing method generally used as a starting material consisting of rapid solidification is not suitable as the starting material in the present invention. Since Mg is active, when exposed to the atmosphere, an oxide film is easily formed although it is extremely thin on the surface. In the powder having a large specific surface area, the total area of the oxide film is very large as compared with the foil strip, foil piece, or fine wire in the present invention. Here, when considering a process in which the obtained powder is once exposed to the atmosphere and then sintered, the oxide layer formed on the surface hinders bonding at the contact surface between the powders. In addition, even when bonded, a large amount of oxide or oxygen decomposed by the oxide is taken into the inside. As described above, a powder having a large specific surface area is liable to cause poor bonding or embrittlement due to mixing of oxide or oxygen, and the characteristics are deteriorated as compared with the case of using a foil strip, a foil piece, or a fine wire. In order to avoid this problem, it requires instantaneous canning process after powder molding, as a result, to achieve high strength in the vicinity of the surface in the wire after the plastic working (eg extrusion) as described later It becomes difficult.

  Moreover, since there is a risk of dust explosion in the powder state, practically active magnesium alloy powder cannot be handled in the atmosphere. In other words, when powder is used, a powder made in a vacuum or an inert atmosphere is filled with a metal sheath such as copper in an apparatus having a continuous series of vacuum or inert atmosphere without being exposed to the atmosphere. In the case of the atmosphere, a canning step in a continuous vacuum process or an inert atmosphere process in which the metal sheath is further degassed and then sealed is essential. Equipment that can perform canning in a vacuum or in an inert atmosphere is greatly limited in the size of products that can be produced. In other words, powder is used for products to which the present invention is applied, such as automobile springs (suspension springs, valve springs, clutch torsion springs, torsion bars, stabilizers) and seat frames. It can be said that it is difficult to establish a series of continuous processes consisting of a vacuum process or an inert atmosphere process as an industrial mass production process.

  FIG. 1 shows a schematic configuration of a fine metal wire manufacturing apparatus 100 (hereinafter referred to as “apparatus 100”), which is one means for producing a starting material, and FIG. 1A is a side sectional view of the schematic configuration of the entire apparatus 100. (B) is sectional drawing of the peripheral edge 141a of the rotating disc 141 used with the apparatus 100. FIG. FIG. 1B is a side cross-sectional view in the direction perpendicular to the paper surface of FIG.

  The apparatus 100 is an apparatus for producing fine metal wires using a molten metal extraction method. In the apparatus 100 using the molten metal extraction method, the upper end portion of the rod-shaped raw material M is melted, and the molten material Ma is brought into contact with the peripheral edge 141a of the rotating disk 141, whereby a part of the molten metal Ma is disc-shaped. A magnesium alloy fine wire F is formed by drawing out in a substantially tangential direction of the circumference and quenching. Here, Mg—Ni—Y based magnesium alloy is used as the raw material M, and for example, a magnesium alloy fine wire F having a wire diameter of 200 μm or less is manufactured. The wire diameter of the magnesium alloy fine wire F is not particularly limited, and can be appropriately selected from the viewpoints of productivity, handleability in the subsequent processes, and the like. A sufficient effect can be obtained by setting the wire diameter in the range of 200 μm or less with respect to the solid solution amount and the refinement of the structure.

  As shown in FIG. 1, the apparatus 100 includes a chamber 101 that can be sealed, and in the chamber 101, a raw material supply unit 110, a raw material holding unit 120, a heating unit 130, a thin metal wire forming unit 140, a temperature measuring unit 150, A high frequency generator 160 and a thin metal wire recovery unit 170 are provided.

  In the chamber 101, an inert gas such as an argon gas is used as the atmospheric gas in order to prevent oxygen, nitrogen, and the like from reacting with the molten material Ma from the atmosphere. The raw material supply unit 110 is provided at the bottom of the chamber 101, for example, and moves the raw material M toward the arrow B direction at a predetermined speed and supplies the raw material M to the raw material holding unit 120. The raw material holding unit 120 has a function of preventing the molten material Ma from moving in the radial direction and a guide function of guiding the raw material M to an appropriate position of the thin wire forming unit 140.

  The raw material holding unit 120 is a cylindrical member, and is provided below the disc 141 between the raw material supply unit 110 and the metal thin wire forming unit 140. The heating unit 130 is a high-frequency induction coil that generates a magnetic flux for forming the molten material Ma by melting the upper end portion of the raw material M. The material of the raw material holding unit 120 is preferably a material that does not react with the molten material Ma. As a practical material of the raw material holding unit 120, for example, graphite is suitable.

  The fine wire forming unit 140 forms the magnesium alloy fine wire F from the molten material Ma using the disk 141 rotating around the rotation shaft 142. The disc 141 is made of, for example, copper or copper alloy having high thermal conductivity. As shown in FIG. 1B, a V-shaped peripheral edge 141 a is formed on the outer periphery of the disc 141.

  The temperature measuring unit 150 measures the temperature of the molten material Ma. The high frequency generator 160 supplies a high frequency current to the heating unit 130. The output of the high frequency generator 160 is adjusted based on the temperature of the molten material Ma measured by the temperature measuring unit 150, and the temperature of the molten material Ma is kept constant. The fine metal wire collecting unit 170 accommodates the fine metal wire F formed by the fine metal wire forming unit 140.

  In the apparatus having the above configuration, first, the raw material supply unit 110 continuously moves the raw material M in the direction of arrow B and supplies the raw material M to the raw material holding unit 120. The heating unit 130 melts the upper end portion of the raw material M by induction heating to form the molten material Ma. Next, the molten material Ma is continuously fed toward the peripheral edge 141a of the disk 141 rotating in the direction of arrow A, and the molten material Ma contacts the peripheral edge 141a of the disk 141, and a part of the circular disk 141 141 is drawn in a substantially tangential direction of the circumference of the circumference, and is rapidly cooled to form a magnesium alloy fine wire F. The magnesium alloy fine wire F thus formed extends in a substantially tangential direction of the circumference of the disc 141 and is accommodated by the metal fine wire collecting portion 170 located at the tip thereof.

3. Sintering The produced starting material is formed into a billet for plastic working by sintering. As a sintering method, atmospheric sintering, vacuum sintering, discharge plasma sintering, or the like can be used, and the sintering can be performed by no pressure or pressure sintering. Further, the properties and quality of the billet after sintering affect the properties and quality of the product subjected to plastic working. Therefore, vacuum hot press (HP) that has a pressurization mechanism and can be sintered in a vacuum or inert gas atmosphere to form a dense billet with higher cleanliness, uniform structure and fewer pores Sintering by is preferred. By applying pressure while heating in a vacuum or an inert gas atmosphere, a billet having almost no pores can be obtained.

  In the case of HP, for example, in the case of HP, a heating chamber is arranged inside a vacuum vessel, and a mold is arranged inside the heating chamber, and a press ram protruding from a cylinder provided on the upper side of the vacuum vessel moves up and down in the heating chamber. The upper punch attached to the press ram is inserted into the mold. The HP mold thus configured is filled with a magnesium alloy fine wire F as a starting material, and the inside of the vacuum vessel is made a vacuum or an inert gas atmosphere and the temperature is raised to a predetermined sintering temperature. And the magnesium alloy fine wire F is pressurized and sintered by the upper punch inserted into the mold.

  In this sintering process, it is desirable to carry out at a heating temperature of 250 to 500 ° C., a heating time of 10 minutes or more, and a pressing force of 25 MPa or more. Under such conditions, the sintering at the contacts between the magnesium alloy fine wires is sufficiently performed. You can get an advanced billet. Furthermore, it is more desirable to carry out at a heating temperature of 350 to 500 ° C., a heating time of 30 minutes or more, and a pressing force of 40 MPa or more. Under such conditions, sintering at the contacts between the magnesium alloy fine wires sufficiently proceeds and the porosity is increased. A dense billet of less than 10% can be obtained. In addition, when heating temperature is less than 250 degreeC, sintering in the contact of thin wires does not fully progress, and many pores exist. Even if it reaches a product that has undergone a subsequent plastic working step, the contact between the thin wires that are insufficiently sintered and the unsintered interface between the fine wires remain, resulting in a decrease in strength. Is preferred. Further, when the heating temperature exceeds 500 ° C., the sintering at the contact between the thin wires proceeds sufficiently and there are almost no pores. However, when the heating temperature exceeds 500 ° C., the structure becomes coarse, and a desired fine structure cannot be obtained even when the product is subjected to a subsequent plastic working process. As a result, since it becomes difficult to obtain a magnesium alloy wire having the desired strength in the present invention, the heating temperature is preferably 500 ° C. or less.

  Here, when the starting material is powder, it is necessary to sinter before sealing in the canning process. However, as a series of apparatuses for providing a vacuum or an inert atmosphere becomes large, it is not easy to uniformly fill a mold or a metal sheath with powder in a closed apparatus. Production becomes difficult. In other words, when powder is used, canning is required before exposure to the atmosphere, and the sintered body in the metal sheath is insufficiently sintered between the powders, and there are many pores and density. It becomes a non-uniform sintered body. Furthermore, when the metal sheath is removed, since there are many pores communicating with the surface, it is inevitable that the interior is exposed to the atmosphere. Therefore, even in the billet state, the metal sheath cannot be removed, and in the next plastic working step, processing with canning is forced.

4). Plastic processing The processing from billet to wire is performed by plastic processing including drawing, rolling, extrusion, and forging as warm processing. Plastic processing at an appropriate temperature and degree of processing (cross-sectional reduction rate) is effective in increasing the strength of magnesium alloys because of the refinement of the structure and work hardening caused by dynamic recrystallization. Of these, drawing or extrusion is more preferable for a wire rod on which bending stress and / or torsional stress mainly acts. According to these plastic working methods, a uniform cross-sectional shape indispensable as a wire can be obtained, and larger strain can be introduced into the surface of the wire as compared with the inside. As a result, the structure in the vicinity of the surface of the wire becomes finer, and the strength on the surface can be further increased in addition to the characteristics inside the wire.

  Originally, strength and elongation are in a trade-off relationship. Various magnesium alloys that have been refined by means such as using powder and achieved high strength have been studied, but all have a high strength structure, but there are problems with ductility. There was an inconvenience that it was difficult to form the part shape due to the lack. And when using powder, since it processes in the state which covered the metal sheath, distortion in processing is preferentially introduced into the metal sheath used as the outermost layer. Therefore, the effect of increasing the strength near the surface as obtained in the present invention cannot be obtained.

  Further, when the billet is made of a casting, the strength cannot be increased even with a magnesium alloy having a composition equivalent to that of the present invention. This is because in the casting, the original α-Mg phase crystal grains are coarse, and the precipitated compound phase is also coarse. Therefore, a combination of a large deformation resistance and a large accumulation of strain leads to a desired microstructure. This is because shear failure occurs before reaching the point. In addition, since the amount of added element dissolved in the α-Mg phase is small, the effect of increasing the strength by solid solution strengthening of the α-Mg phase is also poor. On the other hand, in the case of a billet made of a foil strip, foil piece, or fine wire with a fine structure, by sintering at an appropriate heating temperature, the sintered structure is also fine, so that deformation resistance Is small. Therefore, because of its excellent deformability, it becomes possible to introduce a large strain at a lower temperature in plastic processing and to accumulate a large amount of internal energy as a driving force for recrystallization, so that a finer structure can be obtained. . In addition, since the amount of the additive element in the α-Mg phase is large, the effect of solid solution strengthening is great, and high strength is achieved in combination with the microstructure.

  FIG. 2 is a view showing an extrusion apparatus 200 used when an extrusion process is adopted as the plastic process. In FIG. 2, reference numeral 205 denotes an outer mold, and reference numeral 210 denotes a container accommodated in the outer mold 205. The container 210 has a cylindrical shape, and a lower mold 220 is coaxially disposed on one end face side thereof. A die 230 is disposed between the container 210 and the lower mold 220. A punch 240 is slidably inserted into the container 210. Furthermore, a heater 260 is disposed on the outer periphery of the container 210.

  In the extrusion apparatus 200 having the above-described configuration, when the billet B that has been heated in advance is placed in the container 210, the punch 240 is lowered and the billet B is compressed. The compressed billet B is extruded into the space in the lower mold 220 while being reduced in diameter by the die 230 to form a wire.

  Extrusion by the above-described extrusion apparatus is desirably performed at a heating temperature of billet B: 315 to 335 ° C., an extrusion ratio: 5 to 13, and a forward speed of punch 240: 0.01 to 2.5 mm / second. Under these conditions, refinement of the structure by induction of dynamic recrystallization and work hardening by introducing strain are appropriate, forming a high-strength magnesium alloy wire with high strength and high ductility inside and higher strength near the surface. Is done. That is, the maximum hardness near the surface is 170 HV or more, the inside has a 0.2% proof stress of 550 MPa or more and an elongation of 5% or more, and is suitable for strength parts to which bending stress and / or torsional stress mainly acts. A high strength magnesium alloy wire can be obtained.

  When the heating temperature of the billet B is less than 315 ° C., the deformation resistance is large, and thus extrusion processing becomes difficult, and breakage during the extrusion processing, rough skin on the surface of the wire, and generation of cracks are caused. In addition, when the strength of the wire material that can be processed is increased, the ductility is impaired and the elongation of 5% or more necessary for formability cannot be obtained. On the other hand, when the heating temperature exceeds 335 ° C., the structure is hardened by dynamic recrystallization and work hardening by introducing strain is not sufficient. As a result, the desired hardness in the vicinity of the surface cannot be obtained, and it cannot be used for application to a strength component in which bending stress and / or torsional stress mainly acts.

  Here, the conditions in the extrusion process are not limited to the values in the above-mentioned range and the examples described later, but mainly to ensure high strength and high ductility inside and further increase in strength near the surface. It should be set within an appropriate range. In other words, the introduction of strain and the induction of dynamic recrystallization in plastic processing are influenced by complicated relationships such as material composition, processing rate, processing temperature, etc., and are guided by setting conditions appropriately by theory, experience, and experiment. Is.

  The average crystal grain size of the α-Mg phase measured by the EBSD method in the portion having the highest hardness near the surface of the high-strength magnesium alloy wire produced as described above is desirably 1 μm or less. It is well known that refinement of crystal grains, such as the Hall Petch rule, contributes greatly to increasing strength, and it is also important to suppress the occurrence of initial cracks on the surface of fatigue parts subjected to repeated stress. Is effective. In the examples of the present invention to be described later, in the production examples where the maximum strength near the surface is 170 HV or more, the average crystal grain size of the α-Mg phase in the vicinity of the surface is very fine as 1 μm or less, and the static strength is Of course, it is also suitable for fatigue resistance.

Hereinafter, the present invention will be described in more detail with reference to specific examples.
First, each element raw material for casting production was weighed so as to obtain a desired magnesium alloy component in accordance with a predetermined casting size, and a casting was produced by vacuum melting using each weighed element raw material. Table 1 shows the components of the casting. In vacuum melting, a graphite crucible and a copper alloy mold were used. Next, using the produced casting as a raw material, a thin wire was formed by a molten metal extraction method using the apparatus 100 shown in FIG. In the fine wire forming by the molten metal extraction method, a thin wire having an average wire diameter of 60 μm was formed in an inert atmosphere by Ar gas replacement using a graphite raw material holding part and a copper alloy disc.

  The formed thin wire was filled in a graphite sintering mold as it was without canning, and sintered by HP to produce a billet having a diameter of 15 mm and a length of 50 mm, and a billet having a diameter of 33 mm and a length of 50 mm. . In addition, sintering by HP was performed by the sintering temperature of 300-525 degreeC, and the press pressure of 50 MPa under the inert atmosphere (atmospheric pressure 0.08MPa) by Ar gas substitution.

  The produced billet was processed into the wire using the extrusion apparatus 200 shown in FIG. Specifically, using graphite-based lubricant (manufactured by Nippon Atchison, OILDAG-E), the extrusion ratio is 3 to 15, the extrusion speed (advance speed of the punch 240) is 0.01 to 5 mm / min. The extrusion temperature is in the range of 300 to 425 ° C., a container 210 having an inner diameter of 16 mm and a die 230 having a hole diameter of 5 mm are used for a billet having a diameter of 15 mm (extrusion ratio 10), and a container 210 having an inner diameter of 35 mm is used for a billet having a diameter of 33 mm. And a hole diameter of 20 mm (extrusion ratio of 3), a hole diameter of 15.5 mm (extrusion ratio of 5), a hole diameter of 11 mm (extrusion ratio of 10), a hole diameter of 9.7 mm (extrusion ratio of 13), and a hole diameter of 9 mm (extrusion ratio of 15). Each of the dice 230 was used to produce a wire. For comparison, a cast billet was also extruded to produce a wire.

  A tensile test was performed on the wire prepared as described above. In the tensile test, a test piece with a parallel part diameter of 1.6 mm was produced by machining from a wire with a diameter of 5 mm, and a test piece with a parallel part diameter of 3 mm was produced by machining from a wire with a diameter of 9 mm or more. Then, a universal material testing machine (manufactured by Instron, model number 5586) was used for each test piece, and a tensile test was performed at a test speed of 0.5 mm / min at room temperature. Table 2 shows the results of the tensile test.

  In Table 1, the “billet form” represents the manufacturing method up to the billet before extrusion, the “fine wire sintered body” is a billet produced by sintering fine wires, and the “cast” is the raw material casting. Show billets as they are. Table 1 also shows the extrusion results. In Table 1, “x” indicates that the wire was not obtained after being broken during extrusion, and “△” indicates that the wire was obtained, but surface roughness and cracks were visually confirmed on the surface layer. “O” indicates that a good wire material without rough skin and cracks was obtained. Tensile tests were carried out for extrusion results of “Δ” and “◯”.

  Hardness was measured for the wires whose extrusion results were “Δ” and “◯”. The test piece for hardness measurement is mirror-finished by mechanical polishing after resin embedding so that the cross section of the extruded wire is exposed, and a Vickers hardness tester (Futuretech, FM-600) is used as the hardness tester. The radial distribution in the cross section of the extruded material was measured at a test load of 25 gf. The results of the hardness measurement are shown in Table 2 and FIGS.

  In Table 2 and FIGS. 3 to 5, the maximum hardness in the vicinity of the surface of the wire is 170 HV or more, the internal 0.2% proof stress measured by a tensile test is 550 MPa or more, and the elongation is 5.0% or more. It is an Example (manufacture example 4-8, 14, 15, 18-20, 22, 25, 26, 28-30) of invention. Compared with Comparative Examples 2 and 3 produced from a cast billet, the strength in Examples is remarkably high. The inside of the wire has a high strength and high ductility region with a 0.2% proof stress of 563 MPa or more and an elongation of 5% or more. Moreover, in these Examples, since the maximum hardness is 170 HV or more near the surface of the wire, it has a further high strength region that satisfies 0.2% proof stress of 650 MPa or more. The internal high-strength and high-ductility region and the further high-strength region near the surface are gradually connected and do not have clear boundaries, and the entire wire has excellent strength and toughness and has sufficient formability. ing.

  As shown in Table 1, in Production Examples 1 and 2, since the extrusion temperature (billet heating temperature) was low, the deformation resistance was large, and the wire rod was not obtained by breaking during extrusion. Further, in Production Example 3, although the wire was obtained, rough surface and cracks were observed on the surface layer, and the ductility was impaired as the strength of the wire increased, and required as moldability. An elongation of 5% or more is not obtained.

  On the other hand, in Production Examples 9 to 12 and 23, since the extrusion temperature exceeds 335 ° C., work refinement due to refinement of the structure by dynamic recrystallization and introduction of strain is not sufficient. As a result, the maximum hardness in the vicinity of the surface is less than 170 HV, and the hardness in the vicinity of the surface is insufficient for application to a strength component in which bending stress and / or torsional stress mainly acts. Further, in Production Example 13, since the contents of Ni and Y are as low as 1.0 atomic%, the solid solution strengthening in the α-Mg phase and the precipitation amount of the high-strength Mg—Ni—Y compound phase are scarce. As a result, the maximum hardness of 170 HV near the surface was not obtained. On the other hand, in Production Example 16, since the contents of Ni and Y are as large as 6.0 atomic%, the amount of precipitation of the high-strength Mg—Ni—Y compound phase formed by the addition of Ni and Y is large and coarse. As a result, the deformation resistance increased and the toughness decreased, leading to breakage during extrusion.

  In Production Example 27, since the extrusion ratio exceeded 13, the toughness decreased with the increase in strength of the wire and broke during extrusion. In Production Example 21, since the sintering temperature exceeded 500 ° C., the phase effective for increasing the strength during the sintering and the coarsening of the crystal grains were caused during the sintering, and the hardness in the vicinity of the surface was less than 170 HV. In Production Example 17, since the sintering temperature was less than 350 ° C., it was difficult to produce a dense billet. The billet has many unbonded interfaces between thin wires that become defects as a wire after extrusion that is difficult to disappear even after the plastic processing step, which is the next step, and the degree of bonding at the contact points between magnesium alloy thin wires Since the hardness was improved, the improvement in hardness was observed, but the desired sufficient characteristics for 0.2% proof stress and elongation were not obtained. In Production Example 31, since the extrusion speed exceeded 2.5 mm / second, the surface of the wire surface was rough due to insufficient lubrication. Such roughening of the skin released the processing strain, and while a 0.2% yield strength of 600 MPa and an elongation of 5.1% were secured inside, the hardness in the vicinity of the surface was less than 170 HV. In Comparative Examples 1 and 2, since it was a casting, the α-Mg phase was coarse and the precipitated compound phase was also coarse. Therefore, the deformation resistance was large and the accumulation of strain was large. In Comparative Example 1, the film was broken during the extrusion process, and in Comparative Example 2, the surface of the wire was roughened and cracked by the extrusion process. In Comparative Example 3, the extrusion temperature was high, so that no breakage occurred during the extrusion process, but the desired characteristics were not obtained.

  Next, the relationship between the average crystal grain size of the α-Mg phase in the vicinity of the surface and the hardness of the wire rods of Examples of the present invention and Comparative Example 3 was investigated. The results are shown in Table 3. The average crystal grain size of the α-Mg phase was measured by using the test piece used for the hardness test as it was, and the EBSD method using FE-SEM (field emission scanning electron microscope, JEOL: JSM-7000F) ( In the position where the maximum hardness was obtained in the vicinity of the surface in the cross section of the wire extruded by an electron beam backscattering diffractometer (manufactured by TSL), the analysis magnification was 10,000 times for the example and the comparative example 3 was The measurement was performed at an analysis magnification of 2,000. For the hardness, the maximum hardness near the surface was used.

  As shown in Table 3, in the examples of the present invention, the average crystal grain size of the α-Mg phase is 0.19 to 0.76 μm, which is very fine as compared with 6.76 μm in Comparative Example 3. It is clear that these fine crystal grains contribute to the improvement of the hardness near the surface.

  The high-strength magnesium alloy wire of the present invention is suitable for high-strength parts to which bending stress and / or torsional stress mainly acts. By using the high-strength magnesium alloy wire of the present invention, the weight can be significantly reduced without substantially increasing the size of the component when compared with conventional steel components. For example, as an automobile part, the weight reduction effect is great in a seat frame that occupies a large weight ratio or a spring (suspension spring, valve spring, clutch torsion spring, torsion bar, stabilizer) that requires high strength.

Claims (8)

It is a magnesium alloy wire made of a material consisting of Ni: 2 to 5%, Y: 2 to 5%, balance: Mg and inevitable impurities, and used mainly for bending stress and / or torsional stress. The wire has a portion with a maximum hardness of 170 HV or more in the vicinity of the surface thereof, and has a 0.2% proof stress of 550 MPa or more and an elongation of 5% or more in the interior. . 2. The high-strength magnesium alloy wire according to claim 1 , wherein an average crystal grain size measured by an EBSD method of the highest hardness portion of the surface layer portion is 1 μm or less. Producing a starting material in the form of a foil strip, foil piece or fine wire made of magnesium alloy by rapid solidification method;
A sintering step in which the starting materials are joined by sintering and pressed to form a billet;
A method for producing a high-strength magnesium alloy wire, characterized in that the wire according to claim 1 or 2 is obtained by subjecting the billet to plastic working to form a wire. Producing a thin wire made of a magnesium alloy by a molten metal extraction method;
A sintering process in which the fine wires are joined by sintering and pressed to form a billet;
The billet as it is charged into a press container of a high strength magnesium production method of alloy wire, characterized in that to obtain a wire rod according to claim 1 or 2 by the extrusion step of performing extrusion on said billet. 5. The method for producing a high-strength magnesium alloy wire according to claim 3 , wherein the sintering step is performed at a heating temperature of 350 to 500 ° C., a heating time of 10 minutes or more, and a pressing force of 25 MPa or more. 5. The high strength magnesium alloy wire according to claim 4 , wherein the extrusion step is performed at a heating temperature of 315 to 335 ° C., an extrusion ratio of 5 to 13 and a press ram advance rate of 2.5 mm / second or less. Production method. High-strength magnesium alloy part, characterized in that using a high-strength magnesium alloy wire according to claim 1 or 2. High strength magnesium alloy spring characterized by using a high-strength magnesium alloy wire according to claim 1 or 2.

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