JP4849377B2 - Magnesium alloy screw manufacturing method and magnesium alloy screw - Google Patents

Description

  The present invention relates to a screw made of a magnesium alloy and a method for producing a magnesium alloy screw. In particular, it relates to a magnesium alloy screw excellent in heat resistance.

  Magnesium alloys are attracting attention as lightweight materials because of their excellent specific strength and low specific gravity, and are widely used in aircraft parts, automobile parts, bodies of various electrical products, and the like. In addition to the above components, it has been studied to use a magnesium alloy for a connecting member such as a screw (see Patent Documents 1 and 2).

  As described above, a high-strength and lightweight magnesium alloy is desired to be used not only in a room temperature environment but also in a higher temperature environment. Therefore, in order to improve the heat resistance of magnesium alloys, magnesium alloys to which rare earth elements (RE), Si, and Ca are added have been studied. A magnesium alloy product (casting material) having heat resistance can be obtained by casting a magnesium alloy containing an element such as RE.

JP 2001-269746 A JP-A-2005-48278

  However, the elements such as RE, Si, and Ca tend to lower the plastic workability of the magnesium alloy. Therefore, a magnesium alloy containing the above element is preferable for a casting material, and is not preferable for a material to be subjected to plastic working, particularly a screw material to be subjected to strong plastic processing such as head processing or rolling processing. In addition, since elements such as RE are generally expensive, when a screw is manufactured using a magnesium alloy containing such an element, the manufacturing cost increases.

  The present invention has been made in view of the above circumstances, and its main purpose is to produce a magnesium alloy screw excellent in heat resistance by reducing the addition of an element that inhibits plastic workability as much as possible. An object of the present invention is to provide a method for manufacturing a magnesium alloy screw that can be used. Another object of the present invention is to provide a magnesium alloy screw having excellent heat resistance.

As a result of various studies to produce a magnesium alloy screw having excellent heat resistance without adding an element that reduces plastic workability, the present inventors have performed screw processing (head processing and rolling processing). It was found that it is preferable to heat-treat the formed threaded body at a predetermined temperature. Moreover, as a result of examining the composition of the magnesium alloy, it was found that it is preferable to contain a specific amount of Al. Based on these findings, the manufacturing method of the magnesium alloy screw of the present invention is defined.
The manufacturing method of the magnesium alloy screw of the present invention includes a step of performing a first heat treatment on a screw molded body having a shaft portion on which a screw thread is formed and a head portion connected to the shaft portion. The screw molded body is made of a magnesium alloy containing 4.0 to 10.0% Al by mass%. In the first heat treatment, the heating temperature is set to 350 ° C. or higher.

In the magnesium alloy screw obtained by the above production method of the present invention, the peak intensity ratio of X-ray diffraction satisfies a specific value. Specifically, the magnesium alloy screw of the present invention is made of a magnesium alloy containing 4.0 to 10.0% Al by mass and the balance being Mg and impurities, and X-ray diffraction in a cross section parallel to the screw axis direction. the peak intensity ratio X _p satisfies the following equation.

  Further, the magnesium alloy screw obtained by the above production method of the present invention has an average crystal grain size satisfying a specific size. Specifically, the magnesium alloy screw of the present invention is composed of a magnesium alloy containing 4.0 to 10.0% Al by mass and the balance being Mg and impurities, and the average crystal grain size is 10 μm or more and 40 μm or less. .

  According to the manufacturing method of the present invention, the orientation of the crystal structure of the magnesium alloy constituting the screw formed body and the crystal grains of the magnesium alloy structure can be controlled within a specific range by performing a specific heat treatment on the thread formed body. . And the magnesium alloy screw which has a specific orientation and an average crystal grain diameter has improved high temperature creep characteristics and is excellent in heat resistance. Hereinafter, the present invention will be described in more detail.

  The manufacturing method of the present invention uses a screw molded body made of a magnesium alloy, particularly a screw molded body made of a magnesium alloy containing Al as an additive element. Specifically, a screw molded body made of a magnesium alloy containing 4.0% by mass or more of Al is used. A more specific composition of the magnesium alloy includes 4.0 to 10.0% by mass of Al, with the balance being Mg and impurities. Further, a thread molded body made of a magnesium alloy containing one or more elements selected from Zn, Mn, and Si in addition to Al as an additive element may be used. Specifically, the composition of the magnesium alloy is 4.0% to 10.0% Al by mass%, one or more elements selected from Zn: 2% or less, Mn: 2% or less, Si: 5% or less And the balance is Mg and impurities. In particular, a magnesium alloy containing Al and Mn, that is, a magnesium alloy containing 4.0 to 10.0% Al and 2% or less Mn by mass%, with the balance being Mg and impurities. preferable.

  Al has the effect of strengthening the magnesium alloy and improving the mechanical properties. However, in a screw molded body made of a magnesium alloy having an Al content of less than 4.0% by mass, a screw having sufficient heat resistance cannot be obtained even if a specific heat treatment is performed. Specifically, a screw formed by subjecting a screw molded body made of the low Al-containing magnesium alloy to a specific heat treatment is easily deformed in a high temperature environment of 100 ° C. or higher. On the other hand, considering the improvement in heat resistance, the Al content may be 4.0% by mass or more, but a magnesium alloy having an Al content exceeding 10.0% by mass has low plastic workability. Since it is difficult to perform strong processing such as screw processing, it is not preferable for screw materials. Therefore, considering both heat resistance and plastic workability, the Al content is 4.0 mass% or more and 10.0 mass% or less.

  The strength of the magnesium alloy can be further increased by containing at least one element selected from Zn, Mn, and Si in a specific range in addition to Al. As specific content, Zn: 0.1-2%, Mn: 0.1-2%, Si: 0.1-5% is mentioned by the mass%. You may use the well-known magnesium alloy containing such an element for a screw molded object. Examples of known magnesium alloys include AM alloys (for example, AM60, AM100), AZ alloys (for example, AZ61, AZ80, AZ91), and AS alloys (for example, AS41) in the ASTM standard. Si contributes to the improvement of heat resistance, but if added excessively, it inhibits plastic workability (deformability). Therefore, when Si is added significantly, its content is 5% by mass or less, preferably 2% by mass or less, more preferably 1% by mass or less. In consideration of plastic workability, an AZ-based alloy containing Al, Zn, and Mn as main additive elements is preferable. AZ61 is, for example, by mass: Al: 5.5-7.2%, Zn: 0.4-1.5%, Mn: 0.15-0.35%, Ni: 0.05% or less, Si : Magnesium alloy containing 0.1% or less. AZ91 is, for example, by mass%: Al: 8.1 to 9.7%, Zn: 0.35 to 1.0%, Mn: 0.13% or more, Cu: 0.1% or less, Ni: 0.00. It is a magnesium alloy containing 03% or less and Si: 0.5% or less. A magnesium alloy obtained by adding Si in the above range to an AZ-based alloy may be used for the screw formed body.

  In the production method of the present invention, a thread molded body made of a magnesium alloy having the above composition is prepared, and a specific heat treatment described later is performed on the thread molded body, thereby obtaining a magnesium alloy screw as a final product. The screw molded body is an intermediate product having a shaft portion having a thread on the outer periphery and a head portion connected to the shaft portion, and can be obtained as follows. A long linear material made of a magnesium alloy having the above composition is prepared, and the linear material is cut into a predetermined length to produce a short material. Or you may produce the short material of predetermined length by extrusion etc. Such a short material is subjected to a head forging process to produce a screw blank (an intermediate product in which a head portion having a desired shape is formed and a screw thread is not provided on a shaft portion). The screw blank can be formed by using a die for holding a short material and a punch for forming a head portion. A thread formed body is obtained by forming a screw thread by rolling the shaft portion of the obtained screw blank. The thread can be formed using a rolling die. When the screw blank and the thread are formed warm or hot, the plastic workability of the short material or the screw blank can be improved, and a screw molded body can be produced with high productivity. You may manufacture a screw molded object using a well-known manufacturing apparatus, for example, a head forging apparatus and a rolling apparatus as described in patent document 2. FIG.

  Examples of the long linear material include those produced by extrusion, rolling, and drawing. The drawn material can be obtained by drawing out an extruded material or a rolled material. Since the drawn material has a fine crystal structure as compared with the extruded material, it is excellent in plastic workability and also in dimensional accuracy. Therefore, when producing a screw molded object using a drawing material, a screw process can be performed stably and a screw molded object can be mass-produced. In the case of producing a screw molded body using an extruded material, the threading can be sufficiently performed by heating the short material or the screw blank to 250 ° C. or higher. When producing a screw molded body using a drawn material, even if the heating temperature of a short material or a screw blank is less than 250 ° C., screwing can be performed. However, it is preferable to heat the short material to 140 ° C. or higher and the screw blank to 100 ° C. or higher to perform head forging and rolling.

  A first heat treatment is applied to the thread compact. The first heat treatment is a heat treatment corresponding to so-called annealing, which removes strain associated with screw machining and promotes recrystallization of the magnesium alloy constituting the screw molded body. In Patent Document 2, heat treatment at 100 ° C. or higher and lower than 350 ° C. is performed on a thread-formed product after the rolling process to remove the strain and recrystallize, and to refine the crystal structure and improve the strength. Is disclosed. On the other hand, the present inventors set the heating temperature of the first heat treatment to 350 ° C. or higher with respect to the screw molded body containing a specific amount of Al. Thus, a screw having a specific orientation in a magnesium alloy structure or a screw having a specific average crystal grain size was obtained, and it was found that such a screw was excellent in heat resistance. Therefore, in the manufacturing method of the present invention, the heating temperature of the first heat treatment is set to 350 ° C. or higher. If the heating temperature is less than 350 ° C., the screw after heat treatment does not satisfy a specific peak intensity ratio, or the average crystal grain size of the screw after heat treatment does not satisfy a specific range, and heat resistance cannot be improved. A preferable heating temperature is 350 to 400 ° C. The upper limit of the heating temperature is less than the temperature at which the magnesium alloy melts or burns. The melting or burning temperature varies depending on the composition of the magnesium alloy. The heating time (holding time) may be a time sufficient for recrystallization and distortion removal to be performed, and is suitably from 10 minutes to 2 hours. A particularly preferable holding time is about 30 to 60 minutes.

After the first heat treatment, the screw molded body may be further subjected to a second heat treatment. The second heat treatment is a heat treatment corresponding to so-called artificial aging, in which a precipitate composed of Mg and Al (for example, Mg ₁₇ Al ₁₂ ) is precipitated. If precipitates are generated when using a magnesium alloy screw in a high temperature environment, the screw is likely to be deformed. Therefore, the deformation can be suppressed by performing the second heat treatment to precipitate the precipitate in advance and reducing the precipitation of the precipitate during use. That is, the heat resistance of the screw can be improved. In order to obtain such an effect, the heating temperature of the second heat treatment is preferably 150 ° C. or higher and 250 ° C. or lower. A more preferable heating temperature varies depending on the composition of the magnesium alloy, but about 200 ° C. is appropriate. The lower the heating temperature, the slower the precipitation, but finer precipitates are easily dispersed and precipitated in the alloy structure. The heating time (holding time) may be a time during which the precipitation of the precipitate is sufficiently performed, and is preferably 1 hour or longer, and appropriately about 24 hours or shorter. As the Al content increases, it is preferable to sufficiently perform the second heat treatment to sufficiently precipitate the precipitate.

Magnesium alloy screw obtained by the present invention production process, the peak intensity of X-ray diffraction was measured in a cross section parallel to the axial direction, the following peak intensity ratio represented by Equation 1 X _p is 0.55 or less Fulfill. The magnesium alloy screws meet the peak intensity ratio X _p ≦ 0.55 is excellent in heat resistance. When the heating temperature of the first heat treatment is lower than 350 ° C., the screw after the heat treatment has a peak intensity ratio X _p > 0.55 and does not have sufficient heat resistance. The peak intensity ratio tends to decrease as the heating temperature of the first heat treatment increases, and the peak intensity ratio tends to be superior in heat resistance as the peak intensity ratio decreases. Therefore, the lower limit of the peak intensity ratio X _p is not particularly specified.

The peak intensity ratio _Xp is determined as follows. A magnesium alloy screw is cut in parallel to the axial direction to prepare a sample having a longitudinal section, and X-ray diffraction in the axial direction (longitudinal direction) of the screw is performed on this section. Then, the peak intensity of the four surfaces is measured, and the ratio of the (0002) surface to the four surfaces is obtained. When a screw is manufactured using a drawn material or an extruded material, the axial direction of the drawn material and the axial direction of the screw are manufactured in the same direction. Therefore, the obtained screw forms a texture in which the (0002) plane of the hcp structure is aligned in parallel to the axial direction of the drawn material, that is, the axial direction of the screw. Therefore, when finding the peak intensity ratio X _p, it measures the peak intensity in a cross section parallel to the axial direction of the screw.

  The magnesium alloy screw obtained by the above-described production method of the present invention has a relatively high crystal grain size by growing the recrystallized grains of the magnesium alloy by relatively increasing the heating temperature of the first heat treatment to 350 ° C. or higher. Become. Specifically, the average crystal grain size is 10 μm to 40 μm. Here, in a high-temperature environment, grain boundary sliding becomes a major factor causing deformation (creep), and therefore, grain boundary sliding can be prevented, that is, heat resistance can be improved by reducing the number of grain boundaries. Therefore, the magnesium alloy screw of the present invention having a relatively coarse crystal structure with an average crystal grain size of 10 μm to 40 μm has few grain boundaries and is excellent in heat resistance. When the heating temperature of the first heat treatment is less than 350 ° C., the screw after the heat treatment has a fine grain structure with an average crystal grain size of less than 10 μm and does not have sufficient heat resistance. The average crystal grain size tends to increase as the heating temperature of the first heat treatment increases, and the average crystal grain size tends to be excellent in heat resistance as the average crystal grain size increases. The average crystal grain size is basically independent of the Al content and the heating temperature of the second heat treatment.

  The average crystal grain size is measured as follows. In the cross section of the magnesium alloy screw (longitudinal section or transverse section (plane orthogonal to the axial direction)), a region having a depth of 100 μm from the surface toward the center is the surface layer portion, and the distance from the surface to the center is r. When the region at the position of r / 2 is the central part and the vicinity of the center is the central part, the structure is observed using an optical microscope or the like at any one or more positions in each part (magnification: 200 to 1000 times) and specified. The grain size of crystal grains existing within an area (for example, 100 to 300 μm × 100 to 300 μm) is measured. Measurement of the particle size may be performed according to a cutting method (see JIS H 0501). Such measurement is performed on two or more cross sections. And let the average of the particle diameter of all the obtained crystal grains be an average crystal grain diameter. The crystal grain size may be measured using a transmission electron microscope (TEM) or a backscattered electron diffraction method (EBSP).

  According to the production method of the present invention, a screw molded body made of a magnesium alloy containing a specific amount of Al and subjected to threading is subjected to a heat treatment at a specific heating temperature, thereby satisfying a specific orientation and an average. A magnesium alloy screw having a coarse crystal structure with a relatively large crystal grain size can be produced. In particular, in the manufacturing method of the present invention, as an additive element of the magnesium alloy, an element that causes a decrease in plastic workability is not added as much as possible. Therefore, a screw molded body can be easily manufactured and excellent in heat resistance. A magnesium alloy screw can be obtained with high productivity. Further, since the magnesium alloy screw of the present invention is excellent in heat resistance, it can be sufficiently used not only in a room temperature environment but also in a high temperature environment of 100 ° C. or higher, and is expected to be used in various high temperature environments.

Embodiments of the present invention will be described below.
(Test Example 1)
A drawn material (φ 7.1 mm, average crystal grain size 4 to 6 μm) made of a magnesium alloy having the composition shown in Table 1 (unit of content of additive element is mass%) was prepared. After the drawn material of each composition was cut into a predetermined length, the obtained short material was subjected to + head screw head processing to produce a screw blank. Subsequently, the thread blank was subjected to a rolling process to produce a thread molded body (M8 equivalent material) having a shaft portion in which a thread was formed and a head portion connected to the shaft portion. The head processing and rolling processing were performed by heating the short material and the screw blank to 250 ° C., respectively. Among the obtained thread molded bodies, some of the thread molded bodies were subjected to heat treatment A (annealing) under the conditions of 250 to 400 ° C. × 30 min. The heating temperature (annealing temperature) of the heat treatment A was appropriately selected from 250 to 400 ° C. In addition, some of the thread molded bodies subjected to the heat treatment A were further subjected to heat treatment B (aging) under the conditions of 170 ° C. × 16 hours or 200 ° C. × 16 hours. Si of compositions I to IV is an inevitable element.

The X-ray diffraction peak intensity ratio X _p and the average crystal grain size of the screw subjected to the heat treatment A and the screws subjected to the heat treatments A and B were measured. Moreover, the creep test was done to these screws and the minimum creep speed was measured. The results are shown in Table 2.

Peak intensity ratio X _p was determined as follows. The screw after the heat treatment was cut in parallel to the axial direction, and X-ray diffraction was performed in the axial direction (longitudinal direction) of the screw in the cross section (longitudinal section) to determine the peak intensity of each surface. The resulting peak intensity was used to calculate the peak intensity ratio X _p shown in the equation (1).

  The average crystal grain size was determined as follows. The heat-treated screw is cut and the cross section thereof is observed with an optical microscope (magnification: 400 times). Crystal grains existing in a specific area (200 μm × 150 μm) in the same cross section are in accordance with the cutting method (see JIS H 0501). To measure. This measurement was performed on three cross sections, and the average of the particle diameters of all the obtained crystal grains was calculated.

  In the creep test, a heat-treated screw is cut and processed into a rounded cross-section test piece (parallel part diameter: 3.0 mm, parallel part length: 15 mm), stress 50 MPa, temperature 100 ° C., 125 ° C., 150 ° C. Performed under conditions. The minimum creep rate was determined using the creep strain-time relationship data obtained from the test.

As shown in Table 2, it can be seen that the minimum creep rate can be further reduced by performing heat treatment A at 350 ° C. or higher on a screw molded body made of a magnesium alloy containing 4.0 to 10.0% by mass of Al. That is, it can be seen that the heat resistance of the magnesium alloy screw can be improved. Further, since the Al from Table 2 from 4.0-10.0 wt% containing the magnesium alloy, screws peak intensity ratio _{X p} of X-ray diffraction satisfies 0.55 or less, that the minimum creep rate is smaller Recognize. Furthermore, it can be seen from Table 2 that a screw made of a magnesium alloy containing 4.0 to 10.0% by mass of Al and having an average crystal grain size of 10 to 40 μm has a smaller minimum creep rate. Further, from Table 2, as the peak intensity ratio X _p of X-ray diffraction is small, or the average grain size becomes larger, read tends to improve heat resistance.

  In addition, it can be seen from Table 2 that the heat resistance can be further improved by performing the heat treatment B in addition to the heat treatment A. This is considered to be due to the fact that precipitates were deposited in addition to the larger crystal grains.

  Moreover, the screw was able to be manufactured continuously by using the drawn material. Therefore, it is suitable to use a drawn material for mass production of magnesium alloy screws.

(Test Example 2)
An extrusion material (φ7.1 mm, average crystal grain size 20 to 30 μm) made of a magnesium alloy having compositions II and III shown in Table 1 was prepared, and this extrusion material was cut into a predetermined length for head processing and rolling processing. (A processing temperature of 290 ° C. was applied), and a screw molded body corresponding to M8 was produced. For the screw molded body having the composition II, the sample No. Heat treatments A and B were performed under the same conditions as in Sample 3 (Sample No. 3 ′). For the screw molded body having the composition III, the sample No. Heat treatments A and B were performed under the same conditions as in Sample 6 (Sample No. 6 ′). For the heat-treated screws (Sample Nos. 3 ′ and 6 ′), the X-ray diffraction peak intensity ratio X _p , the average crystal grain size, and the minimum creep rate were measured in the same manner as in Test Example 1. The results are shown in Table 3.

  As shown in Table 3, the minimum creep rate is further reduced by heat treatment at 350 ° C. or higher on a screw molded body made of a magnesium alloy containing 4.0 to 10.0% by mass of Al, thereby reducing the magnesium alloy screw. It can be seen that the heat resistance of can be improved.

  The manufacturing method of the magnesium alloy screw of the present invention is optimal for obtaining a magnesium alloy screw having excellent heat resistance. In particular, since the manufacturing method of the present invention does not add as much an element as possible that hinders plastic processing such as screw processing, a screw molded body can be manufactured with high productivity and a magnesium alloy screw can be manufactured with high productivity. Further, the obtained magnesium alloy screw can be used in a high-temperature environment where heat resistance such as automobile parts is required.

Claims (5)

A step of performing a first heat treatment on a screw molded body having a shaft portion on which a thread is formed and a head portion connected to the shaft portion;
The screw molded body is made of a magnesium alloy containing 4.0 to 10.0% Al by mass% and Mn: 2% or less, with the balance being Mg and inevitable impurities ,
The method of manufacturing a magnesium alloy screw, wherein the first heat treatment is performed at a heating temperature of 350 ° C. or higher. Furthermore, the method includes a step of performing a second heat treatment on the thread formed body subjected to the first heat treatment,
The method for producing a magnesium alloy screw according to claim 1, wherein the second heat treatment is performed at a heating temperature of 150 ° C. or more and 250 ° C. or less. 3. The magnesium alloy screw according to claim 1, wherein the screw molded body further contains at least one element selected from Zn: 2% or less and Si: 5% or less by mass%. Production method. It is composed of a magnesium alloy containing 4.0 to 10.0% Al by mass% and Mn: 2% or less, with the balance being Mg and inevitable impurities,
The magnesium alloy has an average crystal grain size of 10 μm or more and 40 μm or less,
Magnesium alloy screws peak intensity ratio X _p of X-ray diffraction at a cross section parallel to the screw axis direction and satisfies the following expression.

The magnesium alloy screw according to claim 4 , further comprising at least one element selected from Zn: 2% or less and Si: 5% or less in mass%.