JP4769065B2 - Zn-Al alloy having excellent elongation and method for producing the same - Google Patents

Description

  The present invention makes use of room temperature superplastic properties for building isolation and vibration control devices such as building dampers, shock absorbers for automobiles, spring materials with damping performance, precision molded parts, and seal members In addition, it relates to a Zn-Al alloy excellent in elongation that can be used for various uses such as a gasket material, a foil material that requires superplastic characteristics and deformation performance at room temperature, and a target material for forming a thin film, and a method for producing the same. is there.

  In recent years, Zn-Al alloys that can replace Pb that is toxic, show superplasticity at room temperature, and are not toxic as a metal for vibration control that can provide a small and lightweight device have attracted attention.

In relation to the Zn—Al alloy as described above, the nanocrystalline Zn-22% Al alloy has superplasticity that can follow deformation at a strain rate of 1 × 10 ⁻⁴ S ⁻¹ at 373 K (about 100 ° C.). (For example, refer nonpatent literature 1). However, since such superplasticity is not realized at room temperature, it cannot be practically used as a building-based seismic isolation device that requires elongation at room temperature.

  In addition, the Zn-22% Al-2% Cu alloy was water-cooled for soaking, and then cold worked to obtain a structure in which the β phase was precipitated inside the α phase, thereby exhibiting room temperature superplasticity. Is disclosed (see Non-Patent Document 2). The elongation shown here is 135%, indicating that a maximum elongation of 160% is obtained. However, this document does not show such elongation at room temperature when warm worked. Even in the case of cold working, as room temperature superplasticity (seismic isolation performance and seismic performance) is desired as an alternative to Pb dampers, further improvement in elongation (for example, 180%) (Elongation above) is necessary.

  Further, it has been reported that a Zn-22% Al alloy that exhibits superplasticity at room temperature was obtained using an experimental minute sample (see Non-Patent Document 3). Specifically, a cylindrical Zn-22% Al alloy having an initial grain size of 1 to 15 μm is subjected to torsional deformation (cold deformation) under a high pressure of 5 GPa, so that the final structure is It is disclosed that the thickness is 0.1 μm to 0.5 μm at the center which is a fine part.

  However, in the above method, even if the central part is a microstructure that may show superplasticity, the granular structure in the outer peripheral part away from the center is coarse and does not show superplasticity. Only different organizations can be obtained. In addition, since the size to which such torsional deformation can be applied is limited to a very small size of about 15 mm in diameter and 0.3 mm in thickness, a member that receives a heavy load such as a seismic isolation device is It is difficult to obtain the above-mentioned fine structure, and as a result, a material in which the entire member exhibits superplasticity cannot be obtained.

  In contrast, the present inventors have been further researching the above-described Zn-Al alloys from the viewpoint of improving the characteristics, and as part of the research, have a uniform and stable ultrafine structure. In addition, a Zn-Al alloy at a building member level capable of expressing elongation that can be said to be superplastic even at room temperature has been proposed (see Patent Document 1).

Development of this technology has realized a practical size Zn-Al alloy that exhibits excellent superplasticity at room temperature. However, the above-developed alloy has excellent deformability at low speed (hereinafter sometimes referred to as “static deformability”) at a strain rate of about 1 × 10 ⁻³ S ^−1, and is excellent at room temperature. Shows plasticity. However, there are cases in which a deformability at a relatively high speed (hereinafter, referred to as “dynamic deformability”) at a strain rate of about 10 ⁻¹ S ⁻¹ cannot be obtained stably. Moreover, such a phenomenon became more remarkable as the ingot became larger.

  On the other hand, in order to improve the dynamic deformability, in addition to the control of the ultrafine structure in the technology proposed as Patent Document 1, coarse Al inclusions and Al macrosegregation and microsegregation are reduced. It is proposed to plan (see Patent Document 2).

Furthermore, even if the control of the ultrafine structure is not strictly performed with an average crystal grain size of 0.05 μm or less, the average grain size of the pores in the alloy structure inevitably generated in the manufacturing process of this alloy is 0 It has also been proposed to obtain excellent dynamic deformability by preventing the presence of pores of 5 mm or more (see Patent Document 3).
Japanese Patent Laid-Open No. 11-222643 (full text) JP 2004-162103 A (full text) Japanese Patent Laying-Open No. 2005-194541 (full text) RSMishra et al., The observation of tensile superplasticity in nanocrystalline materials: Nanostruct Mater.Vol. 9, No. 1/8 p473-476 (1997) G. Toress-Villasenor et al., “A reinvestigation of the mechanical history on superplasticity of Zn-22Al-2Cu at roomtemperature” (Material. Science. Forum Vol. 243/245, P553 (1997)) M. Furukawa et al., "Fabrication of submicrometer-grained Zn-22% Al by torsion straining" J. Mater. Res. Vol. 11, No. 9 P2128 (1996)

  However, problems to be solved still remain in these Zn-Al alloys. That is, the Zn-Al alloy exhibiting excellent superplastic properties at room temperature also has a particularly low elongation at low temperatures, resulting in a decrease in superplastic properties. For this reason, the problem that it is unsuitable for the damper for construction used at low temperature, such as a cold region, arose newly.

  In addition to this problem, with the increase in size of building dampers and the like, the Zn-Al alloy ingot is inevitably increased in size, but in this enlarged Zn-Al alloy, A new problem has arisen that the surface layer portion has non-uniform characteristics such as hardness.

  The present invention has been made to solve these problems, and an object thereof is to provide a Zn—Al alloy excellent in elongation and a method for producing the same.

  In order to achieve this object, the gist of the Zn-Al alloy excellent in elongation of the present invention is Zn-Al alloy containing Zn: 68 to 88% by mass, the balance being Al and inevitable impurities, and an average crystal The β phase is finely dispersed in the α phase or the α ′ phase having a particle size of 5 μm or less, and the Al macrosegregation is less than 3.0%, and the volume of the lamellar structure in the structure in the center The fraction is 30% or less, and the average hardness difference between the center portion and the surface layer portion is 15% or less.

In addition, in order to achieve the object, the gist of the Zn-Al alloy production method excellent in elongation of the present invention includes Zn: 68 to 88 mass%, and a molten Zn-Al alloy comprising the balance Al and inevitable impurities. When manufacturing by continuous casting after pouring into the mold, the temperature range of 425-375 ° C is 1.0 ° C / second or more in the casting process while shutting off the molten molten metal and the external atmosphere, and in the mold cooling process after casting. Cooling at an average cooling rate of 2 ° C./second or less , and cooling at a temperature range of 275 to 250 ° C. at an average cooling rate of 0.08 ° C./second or more and 0.9 ° C./second or less , 350 ° C. or more A reheating step of quenching with water at an average cooling rate of 0.5 ° C./second or more and 6 ° C./second or less, and a warm working step of 275 ° C. or less and an extrusion ratio of 4 or more after holding at 390 ° C. Is included.

  In order for a Zn-Al alloy to exhibit superplasticity, it must have a structure in which a β phase is dispersed and precipitated in an α phase or an α ′ phase (hereinafter also referred to as a β dispersed α phase). The β-dispersed α phase is completely different from the α phase in which β is not precipitated, and can exhibit an elongation of 200% or more by plastic deformation due to the movement of crystal grains.

  For this reason, the Zn—Al alloy ingot is usually reheated and rapidly cooled to obtain a β-dispersed α structure capable of exhibiting superplasticity with the β phase remaining precipitated in the α phase. During this rapid cooling, if the cooling rate is low, a lamellar structure in which β is diffused is generated, and if the processing rate in the processing is low, the α phase and β phase are not sufficiently refined, and at room temperature. Elongation of about 100 to 140%.

  The above is also disclosed in Patent Document 3 described above. However, although this Patent Document 3 recognizes the relationship between the formation of a lamellar structure and a decrease in elongation at room temperature, there is no recognition of the relationship between the formation of a lamellar tissue in the center and a decrease in elongation.

  In the present invention, the formation of a lamellar structure at the center, which causes a decrease in elongation at low temperatures, is suppressed, and a Zn—Al alloy excellent in elongation including low temperatures and room temperature is obtained. In addition, the suppression of the formation of the lamellar structure in the central part also leads to the solution of the problem that the characteristics of the central part and the surface part become non-uniform in the enlarged Zn-Al alloy. For this reason, the Zn-Al alloy excellent in uniformity can also be obtained.

(Organization as a premise)
First, the premise of the structure of the Zn—Al alloy of the present invention will be described. In the present invention, as described above and in the same manner as in Patent Documents 1 to 3, the Zn-Al alloy exhibits superplasticity. As a premise, the β phase is dispersed and precipitated in the α phase or the α ′ phase. Let it be a structure (β-dispersed α-phase). The α phase in which β is not precipitated does not exhibit superplasticity, but the β-dispersed α phase can exhibit an elongation of 200% or more at room temperature due to plastic deformation caused by the movement of crystal grains.

  Here, the α phase is a face centered cubic lattice crystal whose main component is Al, the α ′ phase is a face centered cubic lattice crystal whose main component is Zn, and the β phase is a hexagonal close-packed lattice whose main component is Zn. Defined as crystal.

  Note that the metal structure varies greatly depending on the Zn content. The metal structure exhibiting superplasticity is obtained by a Zn—Al alloy containing 68 to 88% by mass of Zn on the premise of a preferable manufacturing method described later. In the case of this Zn—Al alloy, it is an α single phase structure macroscopically, and has a structure in which the β phase is finely dispersed in each α phase or α ′ phase.

  On the other hand, when the Zn content exceeds 88% by mass, the β phase is not finely dispersed in each α phase or α ′ phase, and it becomes a mixed structure of two phases of α + β macroscopically and does not show superplasticity. Become an organization. “Macro α phase” and “macro β phase” refer to a structure that can be recognized by microscopic observation at a magnification of about 1000 times. In contrast, the β phase finely precipitated in the β-dispersed α phase of the present invention is a structure that can be confirmed by microscopic observation of about 5000 times or more. For this reason, the β phase finely precipitated in the α phase of the present invention can be clearly distinguished from the macro β phase.

  In the case of a Zn—Al alloy having a Zn content exceeding 88 mass%, the macro β phase in the α + β two-phase mixed structure only exhibits a ductility of about 65% in the normal temperature recovery phenomenon. On the other hand, in the case of a Zn-Al alloy containing 68 to 88% by mass of Zn, the β-dispersed α phase exhibits an elongation of 200% or more, and it is possible to avoid stress concentration from occurring at the grain interface of the β phase. Shows an elongation of more than 160%.

  On the other hand, in a two-phase structure (α + β) of α phase and β phase in which there is no precipitation of β inside, such as a Zn-Al alloy whose Zn content exceeds 88 mass%, each of α phase and β phase Only ductility is expressed, and superplasticity cannot be expressed. In the macro β phase, a normal temperature recovery phenomenon (dislocation recovery) occurs and the deformation resistance is stabilized, but the elongation is about 65%. Therefore, in the two-phase structure (α + β) of α phase and β phase in which β is not precipitated, the overall elongation is only about 68%.

  In addition, it is preferable that the α phase in which β is not precipitated or the macro β phase is not present in the structure. However, if the structure has a β-dispersed α phase capable of exerting superplasticity, the superplasticity As long as it does not hinder, a macro β-phase may be mixed.

(Average crystal grain size)
The α phase or α ′ phase serving as the main phase and the β phase precipitated in the α phase or α ′ phase have an average crystal grain size of 5 μm or less, and particularly preferably 3.5 μm or less. As the α phase or α ′ phase and the β phase precipitated on the α phase or α ′ phase become finer, it becomes easier to exhibit superplasticity. On the other hand, when the average crystal grain size exceeds 5 μm, it becomes difficult to exhibit room temperature superplasticity (static deformability) with an elongation exceeding 160% at room temperature.

(Lamellar tissue)
In the present invention, in order to improve elongation at a low temperature or room temperature, the volume fraction of the lamellar tissue at the center is set to 30% or less to suppress the lamellar tissue at the center. When the Zn—Al alloy ingot is reheated and rapidly cooled, the center portion inevitably has a lower cooling rate than the surface layer portion. For this reason, the lamellar structure | tissue which (beta) diffused is easy to produce | generate. Conventionally, a lamellar structure is easily generated, and the elongation has been reduced including low temperature for this reason.

  In addition, the generation of a lamellar structure in the central part causes unevenness in the structure and characteristics of the central part and the surface layer part. For this reason, the characteristic of the center part and the surface part, especially the problem that hardness becomes non-uniform | heterogenous arises. For this reason, suppression to 30% or less of the lamellar structure | tissue of a center part improves the uniformity of thickness direction, such as hardness. Therefore, the average hardness difference between the center portion and the surface layer portion can be made uniform to 15% or less. On the other hand, if the volume fraction of the lamellar tissue at the center is suppressed to 30% or less on the surface layer side, the volume fraction of the lamellar tissue can inevitably be suppressed to 30% or less.

  Here, the center portion referred to in the present invention refers to a portion where the distance from the center position of the Zn—Al alloy is within 10% of the distance between the center position and the outermost portion. This center position refers to the center in the radial direction in the cross section if the alloy is, for example, a round bar. If the alloy is a square bar, it indicates the intersection of diagonal lines in the cross section, and if the alloy is in other shapes, it generally indicates the position of the center of gravity of the cross section. Moreover, when an alloy is a board | plate material, it is the board thickness center. Similarly, the surface layer portion referred to in the present invention refers to a portion whose distance from the surface layer is within 10% of the distance between the center position and the outermost portion.

  As described above, the lamellar structure can be observed as a structure having a layered structure of several to several tens of microns in the β-dispersed α phase by SEM (scanning electron microscope) observation of 3000 times or more. For this reason, it can be clearly distinguished from the granular α phase or α ′ phase. FIG. 1 shows the structure of Comparative Example 8 (the volume fraction of the lamellar tissue at the center is 25%) in Example Table 2 to be described later, using an SEM of 3000 times. In FIG. 1, a structure having a layered structure exemplified by circles is a lamellar structure. On the other hand, the surrounding white granular structure is the zinc β phase, and the black granular structure is the aluminum α phase or α ′ phase.

  The volume fraction of the lamellar structure in the central part is observed in the structure of 10 visual fields each having a size of about 35 μm × about 25 μm in a 3000-fold backscattered electron image by SEM in the central part of the Zn—Al alloy. Then, the lamellar tissue clearly identified as described above is traced or image-processed, the volume fraction of the lamellar tissue in each visual field is measured, and averaged over 10 visual fields.

(Al macrosegregation)
Macro segregation of Al is segregation that occurs in the top and bottom of the Zn-Al alloy ingot and in the thickness direction. In the present invention, in order to improve the dynamic deformability of the Zn—Al alloy, the Al macrosegregation is less than 3.0%. This macro segregation of Al cuts the top part and the bottom part at the time of ingot in the obtained Zn-Al alloy warm work material, respectively, and the center part and surface layer part in these cut surfaces (cross section) Test specimens are collected from a total of four locations, and the Al content (concentration) at each site is measured. And the thing with the largest deviation from Al content (average) of Zn-Al alloy is made macro-segregation, and the difference from Al content (average) of Zn-Al alloy (Al content maximum value-Al content ) As macro segregation of Al. When the macrosegregation of Al is 3.0% or more, the dynamic deformability at room temperature and low temperature decreases.

(Al inclusions)
In order to improve the dynamic deformability of the Zn—Al alloy, it is preferable to reduce coarse inclusions in the Zn—Al alloy. The coarse inclusions are mainly Al inclusions such as Al ₂ O ₃ , but the coarse inclusions are not limited to this and restrict all coarse inclusions. These coarse inclusions become the starting point of fracture and reduce not only the dynamic deformability but also the static deformability. Therefore, in order not to deteriorate these characteristics, it is preferable that the maximum diameter of inclusions in the tissue is 50 μm or less in terms of the equivalent circle diameter.

(Pore)
In order to improve the dynamic deformability of the Zn—Al alloy, it is preferable that pores (voids) having an equivalent circle diameter of 0.5 mm or more do not exist. This pore also serves as a starting point for destruction and reduces not only the dynamic deformability but also the static deformability. Therefore, in order not to deteriorate these characteristics, it is preferable that the pores in the tissue have a circle equivalent diameter of less than 0.5 mm.

(Chemical composition)
The chemical component composition of the Zn—Al alloy of the present invention will be described. The composition of the Zn—Al alloy of the present invention is such that the Zn content is 68 to 88% by mass, and the balance is Al and inevitable impurities. As described above, the metal structure of the alloy varies greatly depending on the Zn content. The metal structure exhibiting superplasticity is obtained by a Zn—Al alloy having the above composition on the premise of a preferable production method described later. In the case of this Zn—Al alloy, it is an α single phase structure macroscopically, and has a structure in which the β phase is finely dispersed in each α phase or α ′ phase. This β-dispersed α-phase exhibits an elongation of 200% or more, and it is possible to avoid stress concentration at the β-phase grain interface, so that an overall elongation of more than 160% is shown.

  On the other hand, when the Zn content exceeds 88% by mass, as described above, the β phase is not finely dispersed in each α phase or α ′ phase, and in a macroscopic manner, a mixed structure of two phases of α + β (α + β) It becomes. The macro β phase in this α + β two-phase mixed structure undergoes a normal temperature recovery phenomenon (recovery of dislocations) and the deformation resistance is stabilized, but only exhibits a ductility of about 65% in the normal temperature recovery phenomenon. is there. Therefore, this α + β two-phase mixed structure is such that only the ductility of each of the α phase and β phase is manifested, and the overall elongation is only about 68% and does not exhibit superplasticity.

  On the other hand, in the above range, as the Zn content decreases, the amount of β precipitation decreases, and even if plastic deformation occurs due to the movement of crystal grains, the elongation tends to decrease. And if content of Zn is less than 68 mass%, even if it processes on the conditions of this invention, the elongation exceeding 100% cannot be expressed.

  As long as the Zn-Al alloy of the present invention satisfies the above requirements, the strengthening elements such as Cu, Si, Mn, Mg may be contained. Moreover, Zr, Ti, and B effective for crystal refinement may be contained in order to improve elongation.

(Production method)
A preferable production method for efficiently obtaining the Zn-Al alloy of the present invention satisfying the above requirements and excellent in elongation will be described below.

  A preferred production method is that when a molten Zn-Al alloy having the above composition is poured into a mold, the casting process is performed while shutting off the molten molten metal and the external atmosphere, the casting mold cooling process after casting, the reheating process, Consists of the main processing steps.

(Casting process)
When the molten Zn—Al alloy is injected into the mold and manufactured, it is preferable to block the injected molten metal from the external atmosphere. By casting while blocking the injected molten metal and the external atmosphere, the bond with oxygen can be suppressed, and as a result, coarsening of oxide inclusions such as Al2 O3 can be suppressed. As a result, the maximum diameter of inclusions such as Al can be reduced to a circle equivalent diameter of 50 μm or less. As a specific blocking method, it is effective to set the ambient atmosphere during casting to a vacuum atmosphere or an Ar gas atmosphere (Ar seal), or to immerse the injection nozzle in the molten metal (nozzle immersion).

(Mold cooling process 1 after casting)
In the mold cooling process after casting, first, the temperature range of 425 to 375 ° C. is cooled at an average cooling rate of 1.0 ° C./second or more. Coarse solidification that causes macro segregation by cooling the temperature range of 425-375 ° C corresponding to the solid-liquid two-phase region at an average cooling rate of 1.0 ° C / second or more in the mold cooling process after casting. Tissue generation can be suppressed. That is, by cooling the above temperature range at a relatively high cooling rate, the Al crystallization is coarsened, thereby suppressing the formation of a coarse solidified structure.

(Mold cooling process 2 after casting)
In the mold cooling process after casting, the temperature range of 275 to 250 ° C. is then cooled at an average cooling rate of 0.08 ° C./second or more. In the mold cooling process after casting, by cooling the temperature range of 275 to 250 ° C. corresponding to the α + β two-phase region at an average cooling rate of 0.08 ° C./second or more, precipitation of the coarse β phase can be suppressed, Microsegregation caused mainly by the coarse β phase in the α phase can be suppressed. That is, by cooling the above temperature range at a relatively fast rate, the coarsening of Zn and Al precipitates is suppressed, thereby suppressing the formation of coarse β phase and obtaining a finely dispersed β phase. Can do.

(Reheating process)
In the reheating step, the ingot cooled to room temperature or a temperature of 250 ° C. or lower by the above mold cooling is reheated, heated and held at 350 ° C. or higher, and then rapidly cooled. As described above, by increasing the cooling rate in the mold cooling process, the formation of coarsely solidified structure can be suppressed to some extent, but in order to further suppress the formation of coarsely solidified structure, reheating is performed after the mold is cooled and homogeneous It is effective to make it easier.

  In order to achieve sufficient homogenization, the soaking temperature is preferably 350 ° C. or higher. However, if the temperature is 390 ° C. or higher, the ingot may be dissolved.

  Further, the holding time at the above-mentioned temperature at the time of reheating is sufficiently homogenized in about 1 hour in the case of a small ingot of 50 kg or less, for example, when a large ingot of 150 kg class or more, for example, It takes a long time of 8 hours or more to bring the entire alloy to 350 ° C. or higher. This is because when the β particles are re-dissolved in the α matrix, the endotherm is too large and the ingot absorbs heat from the outside. In the case of a large ingot, the atmosphere heating must be prolonged. No longer get. For these reasons, high-frequency heating can be considered. Since high-frequency heating is forcibly heated, it may not be necessary to heat for a long time. However, in the case of a large ingot, it is an industrial cost increase factor.

  After the heating and holding at 350 ° C. or higher, rapid cooling is performed at an average cooling rate of 0.5 ° C./second or higher. By maintaining the temperature at 350 ° C. or higher during reheating, the β phase is confined in the α phase to prevent microsegregation, and a β-dispersed α structure alloy that can exhibit superplasticity after cooling is obtained after reheating. It is necessary to rapidly cool (after soaking) at an average cooling rate of 0.5 ° C./second or more. The rapid cooling may be performed to room temperature or may be performed to a warm processing temperature described later.

  By this rapid cooling, the transition from α ′ to stable α can be suppressed, and β can be prevented from diffusing to the extent of two-phase separation at the macro level. As a result, the β phase can be kept precipitated in the α phase, and a β-dispersed α structure capable of exhibiting superplasticity can be obtained. In order to obtain rapid cooling at an average cooling rate of 0.5 ° C./second or higher, water cooling is preferable. In furnace cooling or air cooling, it is difficult to obtain an average cooling rate of 0.5 ° C./second or more, and β diffuses to form a lamellar structure. When a lamellar structure is formed at this stage, if the processing rate in the following processing is low, the α phase and β phase are likely to be insufficiently refined, and the elongation at room temperature is only about 100 to 140%. An elongation of over 160% cannot be reliably achieved.

(Bundling process)
The lump processing is selectively performed at a temperature of 275 ° C. or less after the reheating (soaking) and rapid cooling. In the production method of the present invention, a method of performing warm processing without performing lump processing is recommended in terms of process efficiency. Moreover, the structure control performed by the conventional block processing can be achieved by warm processing without performing the block processing.

  In the case of performing block processing, when the processing temperature exceeds 275 ° C., the structure is transformed, and the formed β-dispersed α phase may become a two-phase structure of α or α ′ phase and β again. In this respect, it is preferable to perform the lump processing at 200 ° C. or less. On the other hand, if the processing temperature is too low, there is a possibility that processing cracks will occur, so it is recommended to perform the lump processing at 100 ° C. or higher. Moreover, it is preferable to cool at a cooling rate of about 3 ° C./second or more after the chunk processing. The reason is to fix the β-dispersed α-phase obtained in the same manner as the cooling after reheating, and specifically, it is preferable to cool with water.

(Warm processing process)
Warm processing is performed after the reheating (soaking) and rapid cooling without the lump processing, or after the lump processing. Examples of the warm working means include forging, extrusion, wire drawing, and the like, but extrusion is also preferred in terms of production efficiency.

  Extrusion conditions are preferably warm processing at a temperature of 275 ° C. or lower and an extrusion ratio of 4 or higher. When the warm processing temperature exceeds 275 ° C., the structure is transformed, and the formed β-dispersed α phase may become a two-phase structure of α or α ′ phase and β again. In this respect, it is preferable to perform warm working at 200 ° C. or lower. On the other hand, if the warm working temperature is too low, processing cracks may occur, so it is recommended to perform warm working at 100 ° C. or higher.

  The extrusion ratio in the warm working is set to 4 or more in order to give a physical external force and develop an elongation that can be said to be superplastic at room temperature. That is, in the case of no lump processing, the β-dispersed α phase is obtained at the stage of rapid cooling after the soaking, the α phase and the α ′ phase are about 10 to 2 μm, the α phase and the α ′ phase are within the phase. The β phase is about 0.05 to 0.1 μm. Such a structure exhibits an elongation of 180% or more, which can be said to be superplastic, at a high temperature range of about 100 to 150 ° C., but does not exhibit such an elongation at room temperature.

  For this reason, in order to develop elongation that can be said to be superplastic at room temperature, after applying the above-mentioned soaking and quenching, a physical external force (amount of strain) is given warmly, α or α ′ crystal grains, and further in α Alternatively, it is necessary to refine the β phase present in α ′ and to crush the pores. Therefore, after the soaking and quenching, warm working is performed at an extrusion ratio of 4 or more. When the extrusion ratio in warm processing is less than 4, the amount of strain is too small and, like the above-described structure that has been rapidly cooled after soaking, it exhibits elongation that can be said to be superplastic at high temperatures, but it can be said to be superplastic at room temperature. Not expressed.

  What is necessary is just to cool to room temperature at about 3 degree-C / sec or more after warm processing. Specifically, it is preferable to perform water cooling. This is to fix the obtained β-dispersed α-phase in the same manner as the cooling after the reheating. When the cooling rate at this time is slow, the β-dispersed α-phase becomes coarse, and the superplasticity at room temperature is It will not develop.

  In the present invention, since the microstructure can be refined by this warm working, further cold working may be performed, but it is not necessary to perform further.

(Joining)
The Zn-Al alloy of the present invention has a hardness almost equal to or slightly softer than that of mild steel, so it can be used for general joining techniques such as bolting and riveting, and can be easily joined to building structures. It can be carried out. However, when joining by applying heat like soldering, it is necessary to suppress the heating temperature to 250 ° C. or lower, preferably 100 ° C. or lower. As described above, the structure may be transformed at 250 ° C. or higher, and if it is not rapidly cooled after heating to 100 ° C. or higher, the fine structure obtained is coarsened and the elongation exceeding 160% is secured at room temperature. This may be difficult.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. Included in the range.

(Production of Zn-Al alloy)
Continuous casting of a Zn-Al alloy melt having the composition shown in Table 1 (all of which Fe, Cu, Si, Mn, Mg, Zr, Ti, and B have a total impurity content of 0.5 mass% or less) with a diameter of φ142. This was cast at a cooling rate shown in Table 1 to obtain a Zn-Al alloy ingot (ingot size: 180 kg).

  The cooling rate of the ingot was determined by measuring a change over time (cooling curve) of the ingot internal temperature by installing a thermocouple at a position 300 mm (cross-sectional center position) from the bottom of each ingot. The average cooling rate (average cooling rate 1) in the solid-liquid two-phase region (425 to 375 ° C.) and the average cooling rate (average cooling rate 2) of the β precipitation start point temperature (275 to 250 ° C.) are Calculated from the curve.

  In addition, the ingot seal (interruption from the external atmosphere) was previously sealed with Ar inside the mold and the tou at the time of ingot charging. Further, during continuous casting, sealing was performed by immersing the nozzle in the molten metal.

  The obtained Zn-Al alloy ingot was reheated (soaked) in an atmospheric furnace to the temperature shown in Table 1, and held at this temperature for 8 hours. This holding time is the time after the thermocouple is brought into contact with the ingot surface in an atmospheric furnace and the ingot temperature reaches a predetermined heating temperature.

  After this reheating, immediately after taking out the ingot from the furnace, it was water-cooled at an average cooling rate shown in Table 1 up to the isothermal extrusion temperature which is warm working. Thereafter, isothermal extrusion was performed at the warm working temperature shown in Table 1, and then water-cooled at an average cooling rate shown in Table 1 to obtain a rod of φ60 or φ36.

  The structure, room temperature characteristics and low temperature characteristics of the thus obtained Zn—Al alloy bar were evaluated by the following methods. The results are shown in Table 2. Here, as described above, the center portion of the Zn—Al alloy rod means a portion where the distance from the center position of the rod is within 10% of the distance between the center position and the outermost portion. This center position refers to the center in the radial direction in the round bar cross section. Further, as described above, the surface layer portion of the bar refers to a portion whose distance from the bar surface is within 10% of the distance between the center position and the outermost part.

(Structure discrimination, α average crystal grain size)
With respect to the obtained Zn-Al alloy bar, the metal structure is observed with an electron microscope to determine whether it is a β-dispersed α-phase capable of exhibiting superplasticity, and the average of α (including α ′) The crystal grain size was measured. A sample is taken from the center, and an SEM (scanning electron microscope) observation of the buffed alloy sample is performed at a magnification of 5000 times, and three micrographs are taken to determine whether it is a β-dispersed α phase, The equivalent circle diameter of the α phase was examined, and this was used as the crystal grain size to determine the average value in three fields of view.

(Lamellar tissue volume fraction)
The volume fraction of the lamellar structure in the central part is observed in the structure of 10 visual fields each having a size of about 35 μm × about 25 μm in a 3000-fold backscattered electron image by SEM in the central part of the Zn—Al alloy. The identified lamellar tissue is then traced. As image processing software, Image-ProPlus manufactured by MEDIACYBERNETICS was used to measure the volume fraction of lamellar tissue in each visual field and averaged over 10 visual fields.

(Inclusions)
The maximum diameter of inclusions such as Al was 7 μm or less in each example. The maximum diameter of the inclusions was observed at a position of 100 mm in the extrusion direction from the top of the obtained Zn-Al alloy rod (the surface after the buffing) with an optical microscope with a magnification of 1000 times. Three photographs were taken, and the inclusions with the largest particle size (equivalent circle diameter) among the observed (identified) inclusions were judged as the maximum particle size of the inclusions.

(Macro-segregation)
The top part and the bottom part at the time of ingot of the obtained Zn-Al extruded material are cut respectively, and test pieces are collected from a total of four places including the center part and the surface layer part in these cut surfaces (cross sections). Then, the Al content (concentration) in each part is measured. And the thing with the largest deviation from Al content (average) of Zn-Al alloy is made macro-segregation, and the difference from Al content (average) of Zn-Al alloy (Al content maximum value-Al content ) Was determined as macrosegregation of Al.

(Pore)
There was no pore having a diameter of 0.5 mm or more in each example. For the measurement of the pores, the collected sample (50 mm square) was reheated for 10 hours and then subjected to HIP treatment to completely remove the pores. After this, prepare a drill hole with a diameter of 0.5 mm in the center of the standard sample piece, and perform UT inspection (ultrasonic flaw detection) on this sample to detect this 0.5 mm hole. I checked the level. Then, pores having a diameter of 0.5 mm or more exist for those in which noise of the above noise level or more is generated.

(Tensile test)
Room temperature tensile properties and low temperature tensile properties of the obtained Zn-Al alloy bar were evaluated. A JIS No. 4 round bar test piece having a parallel part diameter of 14 mm was taken from the Zn-Al bar, the gauge length was 50 mm, and the crosshead speed was 50 mm / min (distortion speed 1.67 × 10 ⁻² / s). : Dynamic deformability) tensile test, tensile strength TS, elongation at break (breaking elongation) and squeezing are measured, and each alloy's dynamic characteristics (tensile strength TS and breaking elongation at high speed deformation) ) Was evaluated. This was performed at a room temperature of 25 ° C. and a low temperature of −10 ° C. The squeezing was determined from the cross-sectional area of the fracture portion of the test piece after fracture.

  As is apparent from Tables 1 and 2, Invention Examples 1 to 6 have the chemical component composition defined in the present invention, and are manufactured within a preferable manufacturing condition range. For this reason, the β phase is finely dispersed in the α phase or α ′ phase having an average crystal grain size of 5 μm or less, and the macro segregation of Al is caused by the difference between the maximum Al content in the alloy and the Al content in the alloy. It has a tissue that is less than 3.0%, and the volume fraction of the lamellar tissue in the tissue at the center is 30% or less. For this reason, it is excellent not only at room temperature but also at low temperatures (superplastic characteristics and deformation performance), and the average hardness difference between the center portion and the surface layer portion is 15% or less, and the uniformity is also excellent.

  On the other hand, in Comparative Examples 7 to 11, either the chemical component composition or the preferable production conditions are out of the range. For this reason, the structure of the β phase dispersed α phase, the average crystal grain size of the α phase, the macro segregation of Al, and the volume fraction of the lamellar structure in the structure at the center do not satisfy the provisions of the present invention. For this reason, any of the elongation at room temperature, the elongation at low temperature, and the hardness difference between the center portion and the surface layer portion is remarkably inferior to the inventive examples.

  Comparative Example 7 has too much Zn content. For this reason, despite being manufactured within the range of preferable manufacturing conditions, the elongation at room temperature and the elongation at low temperature are remarkably inferior to those of the inventive examples.

  In Comparative Example 8, both the temperature range average cooling rate of 425 to 375 ° C. and the average cooling rate of the temperature range of 275 to 250 ° C. in the mold cooling process after casting are both too small. For this reason, the volume fraction of the lamellar structure | tissue in a center part becomes large, and elongation at room temperature and elongation at low temperature are remarkably inferior compared with the invention example.

  In Comparative Example 9, the reheating temperature is too low. For this reason, the volume fraction of the lamellar structure | tissue in a center part becomes large, and the elongation at room temperature, the elongation at low temperature, and the hardness difference of a center part and a surface layer part are remarkably inferior compared with the invention example.

  In Comparative Example 10, the average cooling rate after reheating is too small. For this reason, the volume fraction of the lamellar structure | tissue in a center part becomes large, and the elongation at room temperature, the elongation at low temperature, and the hardness difference of a center part and a surface layer part are remarkably inferior compared with the invention example.

  In Comparative Example 11, the extrusion ratio (distortion amount) at the time of warm working is too small. For this reason, the elongation at room temperature and the elongation at low temperature are remarkably inferior to those of the inventive examples.

  The above results support the significance of the chemical component composition, the structure definition, and the preferred production conditions for obtaining this structure for the elongation and uniformity at low temperature as well as room temperature.

  As described above, according to the present invention, a Zn—Al alloy excellent in elongation and a method for producing the same can be provided. As a result, the Zn-Al alloy of the present invention utilizes the superplastic properties at room temperature and low temperature, is used for seismic isolation / seismic devices such as building dampers, shock absorbing members for automobiles, etc., and spring materials with damping performance It can be used for various applications such as precision molded parts, seal members, gasket members, foil materials that require superplastic properties and deformation performance at room temperature, and target materials for thin film formation.

It is a drawing substitute photograph which shows the structure | tissue of the comparative example Zn-Al alloy.

Claims (4)

  Zn: Zn—Al alloy containing 68 to 88% by mass, the balance being Al and unavoidable impurities, the β phase being finely dispersed in the α phase or α ′ phase having an average crystal grain size of 5 μm or less, and Al And the volume fraction of the lamellar structure in the structure at the center is 30% or less, and the average hardness difference between the center and the surface layer is 15%. % Zn-Al alloy excellent in elongation characterized by being not more than%.   The Zn-Al alloy excellent in elongation according to claim 1, wherein the interval between the lamellar structures is 1000 nm or less.   The Zn-Al alloy excellent in elongation according to claim 1 or 2, wherein the maximum diameter of inclusions in the structure is not more than 50 µm in terms of equivalent circle diameter and there is no pore having an equivalent circle diameter of 0.5 mm or more. . Zn: a step of casting while containing molten aluminum and unavoidable impurities in a mold and injecting a molten Zn-Al alloy into the mold by continuous casting , while shutting off the molten molten metal and the external atmosphere, In the mold cooling process after casting, the temperature range of 425 to 375 ° C. is cooled at an average cooling rate of 1.0 ° C./second or more and 2 ° C./second or less , and the temperature range of 275 to 250 ° C. is 0.08 ° C. Cooling at an average cooling rate of not less than 0.9 ° C / second and not more than 0.9 ° C / second , after holding at 350 ° C to less than 390 ° C , and then having an average cooling rate of not less than 0.5 ° C / second and not more than 6 ° C / second A process for producing a Zn—Al alloy excellent in elongation, comprising a reheating step of water- cooling and quenching at 275 ° C. and a warm working step of 275 ° C. or less and an extrusion ratio of 4 or more.

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