JP2015034350A - Magnesium alloy molding, magnesium alloy sheet, method of producing magnesium alloy molding and method of producing magnesium alloy sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnesium alloy molding excellent in impact resistance characteristics, a magnesium alloy sheet suitable as a material for the molding, a method of producing a magnesium alloy molding and a method of producing a magnesium alloy sheet.SOLUTION: A magnesium alloy molding is formed by press-molding a sheet composed of a magnesium alloy, and the magnesium alloy contains 7-12 mass% inclusive of Al. The molding has a flat part free of drawing deformation. In the metal structure of a cross section of the flat part cut in the sheet-thickness direction, with the region from the surface of the flat part to the depth of 1/3 of the sheet thickness in the sheet-thickness direction taken as the surface layer region, the 100 μm×100 μm100 region selected from two arbitrary parts of the surface layer region taken as observation visual fields and particles of an intermetallic compound containing Al and Mg and having particle sizes of 5 μm or greater taken as coarse particles, the number of coarse particles present in each observation visual field is 5 or smaller.

Description

  The present invention relates to a magnesium alloy plate suitable for a material such as a casing of a portable electric device, and a magnesium alloy molded body obtained by press-molding this plate. In particular, the present invention relates to a magnesium alloy molded body having excellent impact resistance characteristics.

  As housing materials for portable electrical devices such as mobile phones and notebook personal computers, ABS (acrylonitrile butadiene styrene copolymer) resin, PC (polycarbonate) resin, metals such as aluminum alloy and stainless steel (SUS) are used. Has been.

  Recently, magnesium alloys that are lightweight and have excellent specific strength and specific rigidity have been studied as the casing material. Cases made of magnesium alloy are mainly cast by die-casting or thixo-molding, and plates made of a magnesium alloy for expansion, represented by the ASTM standard AZ31 alloy, are being used. is there. Further, in Patent Document 1, it is considered to press the AZ91 alloy of ASTM standard.

  In recent years, a thin and lightweight housing has been desired. Metals are generally more resistant to impact than resins and are less likely to break, making them easier to thin. However, an aluminum alloy is inferior in plastic deformation resistance, and is very easily deformed by an impact such as dropping. Stainless steel is difficult to crack or deform but heavy.

JP 2007-098470 A

  Magnesium alloys are superior in plastic deformation resistance compared to aluminum alloys and are very light compared to stainless steel. However, a cast material of a magnesium alloy is inferior in strength to a magnesium alloy press-molded body, and it is difficult to reduce the thickness. The AZ31 alloy press-molded body also has insufficient strength.

  When press forming is performed on a rolled plate made of an AZ91 alloy as described in Patent Document 1, a formed body having higher strength than a press formed body made of an AZ31 alloy is obtained. However, as a result of investigations by the present inventors, when the Al content is increased to 7% by mass or more, variations in impact resistance characteristics may occur in the material plate and the press-molded body obtained by molding this plate. And gained knowledge.

  Then, one of the objectives of this invention is providing the magnesium alloy molded object which is excellent in an impact resistance characteristic. Another object of the present invention is to provide a magnesium alloy sheet suitable for the production of a magnesium alloy molded body having excellent impact resistance.

The inventors of the present invention have prepared a plate as a raw material by various manufacturing methods in a magnesium alloy containing 7 mass% or more of Al, and manufactured a press-molded body from the obtained plate, and have impact resistance characteristics (dent resistance) As a result, it was found that the press-molded body that was difficult to dent had small particles composed of an intermetallic compound (precipitate) such as Mg ₁₇ Al ₁₂ and few large particles. Therefore, when the production method for controlling the maximum particle size and the number of the precipitates, that is, reducing the coarse precipitates, was studied, the total time kept in a specific temperature range mainly in the rolling process was made larger than before. By shortening, a magnesium alloy sheet with few coarse precipitates could be obtained. Moreover, the press-molded body obtained by press-molding this magnesium alloy plate was excellent in impact resistance. The present invention is based on the above findings.

  The magnesium alloy molded body of the present invention is obtained by press-molding a plate made of a magnesium alloy containing 7 mass% or more and 12 mass% or less of Al, and has a flat portion not accompanied by drawing deformation. When the observation visual field defined below is taken in the metal structure of the cross section obtained by cutting the portion in the plate thickness direction, the number of coarse intermetallic compound particles present in each observation visual field is five or less.

  Further, the magnesium alloy plate of the present invention is used for press forming, and is made of a magnesium alloy containing 7 mass% or more and 12 mass% or less of Al, and is present in each coarse field of view defined below. The number of intermetallic compound particles is 5 or less.

The observation visual field means that the flat portion or the metal structure of a cross section obtained by cutting the magnesium alloy plate in the plate thickness direction is 1/3 of the plate thickness in the plate thickness direction from the surface of the flat portion or from the plate surface. The region up to is the surface layer region, and is a region of 100 μm × 100 μm selected from any two locations in the surface layer region.
Coarse particles are particles of an intermetallic compound containing Al and Mg and have a particle diameter of 5 μm or more.
The particle diameter is the diameter of a circle having an area equivalent to the area of the particles in the cross section.
In addition, the intermetallic compound which exists in the said cross section can be discriminate | determined by investigating the composition and structure of particle | grains by EDS (Energy dispersive X-ray analyzer: Energy Dispersive X-ray Spectrometer), X-ray diffraction etc., for example.

The alloy plate of the present invention having the above specific structure can be manufactured by, for example, a manufacturing method including the following steps.
Preparation step: A cast plate made of a magnesium alloy containing 7 to 12% by mass of Al and manufactured by a continuous casting method is prepared.
Solution treatment step: Solution treatment is performed on the cast plate at a temperature of 350 ° C. or higher.
Rolling step: Rolling is applied to the solution-treated plate material.
In particular, in the cooling process from the holding temperature of the solution treatment, the cooling rate from 350 ° C. to 250 ° C. is set to 0.1 ° C./sec or more, and in the rolling process, the plate material to be processed is 250 ° C. to 350 ° C. The total time held in the temperature range is within 60 minutes.

As described above, in the cooling process of the solution treatment (that is, immediately before rolling) and in the rolling process, precipitates are deposited or maintained in a specific temperature range (250 ° C. to 350 ° C.) that tends to become coarse. By making the time to be as short as possible, coarse precipitates can be reduced, and a structure in which the structure in which fine precipitates d ₀ are dispersed is obtained as shown in FIG.

  Conventionally, rolling is performed a plurality of times (multi-pass) at an appropriate working degree (rolling rate) as shown in FIG. (Shown as n = 1, 2,...)). At this time, plastic workability is improved by heating the object to be processed (a cast plate or a rolled plate until final rolling is performed) to 250 ° C. or higher. Therefore, it is preferable to heat the object to be processed and perform warm rolling or hot rolling at least at the initial stage of rolling (rough rolling) in the rolling process. However, particularly in the case of a magnesium alloy having a high Al content of 7% by mass or more, precipitates such as intermetallic compounds are likely to grow and become coarse as described above by heating to 250 ° C. or higher. In addition, the precipitate tends to become coarse when passing through a temperature range of 250 ° C. to 350 ° C. in the cooling process of the solution treatment step.

Conventionally, it has not been sufficiently studied how much the total time for holding the object to be processed in the temperature range of 250 ° C. to 350 ° C. immediately before rolling and in the rolling process. As a result of investigation by the inventors, in the case of a magnesium alloy having an Al content of 7 to 12% by mass, when the total holding time in the temperature range exceeds 1 hour in at least the rolling process, it is shown in FIG. 1 (2). Thus, a structure in which coarse precipitates d ₁ having a particle diameter of 5 μm or more are present is obtained. On the other hand, the coarse precipitates can be reduced by setting the total holding time in the temperature range within one hour in the rolling process. In addition to making the total holding time of the above temperature range within 1 hour only in the rolling process, it is possible to more effectively suppress coarse precipitates by increasing the cooling rate in the solution treatment process. . In particular, it is preferable that the total time of the total holding time in the temperature range in the rolling process and the holding time in the temperature range in the cooling process of the solution treatment step be within 1 hour.

  The alloy plate of the present invention has a structure in which there are few coarse precipitates in the surface layer region and very fine precipitates are dispersed (FIG. 1 (1)). In addition, the present invention alloy plate, because there are few coarse precipitates, there is little decrease in the amount of solid solution Al in the matrix (Mg) due to the presence of many coarse precipitates, solid solution strengthening accompanying the decrease in Al amount It is thought that there is little decrease in Therefore, by improving the rigidity of the plate itself by dispersion strengthening of precipitates and maintaining the strength by suppressing the decrease in the amount of solute Al, the alloy plate of the present invention is difficult to dent even under impact and has excellent impact resistance characteristics. Furthermore, the alloy plate of the present invention with few coarse precipitates is excellent in plastic workability and can be easily pressed.

  As described above, the molded article of the present invention can be produced by subjecting the alloy sheet of the present invention obtained by controlling the holding time in a specific temperature range mainly in the rolling process to press molding. In the case where the alloy plate of the present invention is used, a structure with a small amount of coarse precipitates constituting the alloy plate of the present invention is generally maintained at a portion (flat portion) where deformation due to press forming is small in the formed body of the present invention.

  That is, the molded product of the present invention also has a structure in which the coarse precipitates are few in the surface layer region and very fine precipitates are dispersed. Accordingly, the molded article of the present invention is excellent in impact resistance and difficult to dent due to dispersion strengthening by fine precipitates and solid solution strengthening by sufficient solid solution of Al as described above.

The present invention will be described in detail below.
"composition"
Examples of the magnesium alloy include those having various compositions containing an additive element in Mg (remainder: Mg and impurities). The plate and the molded body of the present invention are made of an Mg—Al alloy containing at least 7 mass% to 12 mass% of Al as an additive element. Examples of the additive element other than Al include one or more elements selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare earth elements (excluding Y). When these elements are contained, the content thereof is 0.01% by mass or more and 10% by mass or less, preferably 0.1% by mass or more and 5% by mass or less. More specific Mg-Al alloys include, for example, AZ alloys (Mg-Al-Zn alloys, Zn: 0.2 to 1.5 mass%) and AM alloys (Mg-Al-Mn alloys, Mn) according to ASTM standards. : 0.15-0.5 mass%), Mg-Al-RE (rare earth element) based alloys and the like. In particular, Mg-Al alloys containing 8.3 to 9.5% by mass of Al and 0.5 to 1.5% by mass of Zn, typically AZ91 alloy, are more resistant to corrosion than other Mg-Al alloys such as AZ31 alloy. Excellent mechanical properties such as strength and plastic deformation resistance.

<Thickness of magnesium alloy plate>
The alloy plate of the present invention is subjected to press forming such as bending and drawing, and is used as a material for a thin and lightweight member such as a casing. In the case obtained by press forming, the alloy of the present invention is formed so that the thickness of the portion where the thickness does not substantially change due to deformation accompanying plastic working (the flat portion in the formed product of the present invention) is reduced. The thickness of the plate is preferably 2.0 mm or less, particularly 1.5 mm or less, and more preferably 1 mm or less. In the above range, the thicker the magnesium alloy plate, the better the strength, and the thinner the magnesium alloy plate, the more suitable for a thinner and lighter casing. The plate thickness may be selected according to the desired application.

《Mechanical properties》
The alloy plate of the present invention is difficult to dent when subjected to impact such as dropping. Specifically, when the following dent test was performed on a test piece of 30 mm × 30 mm and thickness t _b cut out from the alloy plate of the present invention, the dent amount x _b of the test piece was x _b ≦ 0.47. ^Xt _b ^-1.25 is satisfied. Further, in the molded body of the present invention, the flat portion not accompanied by the drawing deformation has few coarse precipitates as described above, and substantially maintains the characteristics of the alloy plate of the present invention as described above. Therefore, when the test piece (thickness: t _p ) similar to the above-described alloy plate of the present invention is cut out from the flat portion and subjected to the following dent test, the dent amount x _p of the test piece is x _p ≦ 0.47 × t _p ^−1.25 is satisfied. The thickness t _p of the test piece cut out from the flat part of the present invention compacts the magnesium alloy sheet has a pre-press molding material, for example, the thickness t _b of the test piece cut out from the present invention alloy sheet Is substantially equal to (t _p = t _b ).
(Dent test)
Place a test piece on a support with a hole with a diameter of 20 mm so as to close the hole, and in this state, let a cylindrical rod with a weight of 100 g and a tip r = 5 mm fall freely from a point 200 mm high from the test piece. .
Dented x _b or x _p is the distance from the straight line connecting the both sides of the test piece after the dent test to the most recessed portion.

<Shape of molded product>
The molded body of the present invention typically has a shape having a top plate portion (bottom surface portion) and a side wall portion standing from the periphery of the top plate portion. More specifically, a rectangular plate-shaped top plate portion and only a pair of opposing side wall portions]], a cross section having two sets of opposing pair of side wall portions, or a top plate portion Examples thereof include a covered cylindrical body having a disk shape and a cylindrical side wall portion.

  The form of the top plate part and the side wall part is typically a flat surface, and the shape and size are not particularly limited. The top plate portion and the side wall portion are integrally formed or joined with bosses, etc., have holes penetrating front and back and grooves recessed in the thickness direction, have a stepped shape, plastic working, You may have the part from which thickness differs locally by cutting. In the molded body of the present invention, the flat portion without drawing is defined as a portion with less warping when a section cut out from a region excluding the portion having the boss or the like is arranged on a horizontal plane. More specifically, it is a portion where the distance from the horizontal plane to the farthest position in the vertical direction is within 1 mm on the surface facing the horizontal plane in the section arranged on the horizontal plane. In general, since the dent is likely to occur in a flat portion, in the alloy plate or the molded body of the present invention, the object of evaluation of the dent resistance is the flat portion.

<Surface of molded product>
The molded body of the present invention can be provided with a coating layer for the purpose of anticorrosion, protection, decoration and the like on the surface of a plate made of a magnesium alloy. The magnesium alloy that mainly constitutes the compact of the present invention contains 7% by mass or more of Al, so that it has excellent corrosion resistance compared to an alloy having a low Al content, for example, an AZ31 alloy. Furthermore, the corrosion resistance of the molded article of the present invention can be further improved by applying a corrosion prevention treatment such as chemical conversion treatment or anodizing treatment to the plate made of the magnesium alloy to provide a corrosion prevention layer. In addition, in the formation process of coating layers, such as said anti-corrosion and coating, the magnitude | size and precipitation of a deposit are not affected substantially. Therefore, even if the molded product of the present invention has a coating layer such as anticorrosion, the number of coarse particles is 5 or less, and when the dent test is performed, x _p ≦ 0.47 × t _p ^−1.25 is satisfied. .

"Production method"
[Preparation process]
As the cast plate, it is preferable to use a cast plate produced by a continuous casting method such as a twin-roll method, in particular, a casting method described in WO / 2006/003899. Since the continuous casting method can be rapidly solidified, it is possible to reduce oxides and segregation and to obtain a cast plate having excellent rolling properties. The size of the cast plate is not particularly limited, but segregation is likely to occur if it is too thick, and therefore it is preferably 10 mm or less, particularly 5 mm or less.

[Solution process]
The cast plate is subjected to a solution treatment so as to homogenize the composition. In the solution treatment, the holding temperature is set to 350 ° C. or higher. In particular, a holding temperature: 380 to 420 ° C. and a holding time: 60 to 2400 minutes are preferable, and it is preferable to increase the holding time as the Al content increases. Moreover, in order to manufacture this invention alloy plate, the holding time of the temperature range of 350 to 250 degreeC is controlled in the cooling process from the said holding temperature. Specifically, as shown in FIG. 2 (1), in order to shorten the holding time of the temperature range, the cooling rate in this temperature range is 0.1 ° C. / sec or more (holding time: about 16.6 minutes or less), preferably , 0.5 ° C / sec or more (holding time: 3.3 minutes or less). Such a cooling rate can be achieved by forced cooling such as water cooling or blast. By shortening the holding time in the above temperature range as much as possible, precipitation of intermetallic compounds containing Al and Mg can be suppressed even in high Al magnesium alloys, and in particular, effectively suppressing the growth to coarse grains Can do.

[Rolling process]
In order to improve the plastic workability (rollability) of the plate material subjected to the solution treatment, at least in rough rolling, as described above, the plate material heated to a temperature of 200 ° C. or more, particularly 250 ° C. or more is subjected to rolling. Is preferred. The higher the heating temperature, the higher the plastic workability of the plate material. However, when it exceeds 350 ° C, there are problems such as seizure occurring and coarsening of crystal grains to deteriorate the mechanical properties of the plate material after rolling. Therefore, 350 ° C. or lower is preferable, and a more preferable heating temperature is 270 ° C. or higher and 330 ° C. or lower. By rolling a plurality of times (multi-pass), a desired plate thickness can be obtained, and the average crystal grain size of the magnesium alloy can be reduced, and press workability can be improved. The rolling may be performed by combining known conditions, for example, heating not only the plate material but also the roll, or the controlled rolling disclosed in Patent Document 1. Further, in the final pass and a pass in the vicinity thereof, the heating temperature of the plate material may be lowered (for example, room temperature) for the purpose of improving the dimensional accuracy.

  In the rolling step, the holding time in the temperature range of 250 ° C to 350 ° C is controlled. Specifically, as shown in FIG. 2 (1), in each pass of the rolling process, in order to shorten the holding time of the temperature range, for example, the heating time for heating the workpiece is shortened, the rolling speed Increase the (roll peripheral speed) or increase the cooling speed. And rolling conditions are controlled so that the total holding time of the temperature range of 250 degreeC-350 degreeC in a rolling process may be 60 minutes or less. As the amount of Al increases, precipitates are more likely to precipitate. Therefore, the total sum of the retention times is preferably adjusted according to the Al content. The total holding time is preferably as short as possible, preferably 45 minutes or less, particularly preferably 30 minutes or less. By performing such specific rolling, the alloy sheet of the present invention is obtained which has few coarse precipitates in the surface layer region and is excellent in impact resistance as described above.

  Perform intermediate heat treatment between rolling passes, and remove and reduce strain, residual stress, texture, etc. introduced into the plate material to be processed by processing up to intermediate heat treatment, and then inadvertent cracks and strain in subsequent rolling, Deformation is prevented and rolling can be performed more smoothly. The intermediate heat treatment is preferably performed at a holding temperature of 250 ° C. to 350 ° C., but in this temperature range, precipitates grow and become coarse as described above. Therefore, when performing the intermediate heat treatment, it is preferable to control the treatment time by including it in the total holding time.

<Processing after rolling>
(Final heat treatment (annealing))
The obtained rolled plate may be subjected to a final heat treatment at, for example, 300 ° C. or more to remove processing distortion due to rolling and be completely recrystallized. Even in this final heat treatment, precipitates easily grow in a temperature range of 250 ° C to 350 ° C. Therefore, when the final heat treatment is performed after rolling, it is preferable to control this processing time by including it in the total holding time. By controlling the final heat treatment time as described above, the magnesium alloy sheet of the present invention with few coarse precipitates can be obtained.

(Warm straightening treatment)
Or you may perform the warm correction process which provides distortion with a roll leveler etc. in the state heated to 100-250 degreeC, without performing the said final heat processing after rolling. When the processed plate subjected to the warm correction treatment is subjected to press working, it is recrystallized at the time of press working to obtain a compact having a fine crystal structure. Compared to the case where the final heat treatment is performed, the crystal grains are likely to be finer, and a structure in which fine precipitates are more uniformly dispersed is likely to be obtained. Therefore, when the warm straightening treatment is performed, the magnesium alloy sheet of the present invention, which has less coarse precipitates and is superior in impact resistance due to the fine structure, can be obtained. In the warm correction treatment, it is considered that the precipitates are not likely to become coarse by setting the heating temperature of the rolled sheet to 250 ° C. at the most.

[Press working]
The molded product of the present invention can be produced by subjecting the rolled plate obtained by the rolling step or a processed plate obtained by subjecting the rolled plate to the final heat treatment and the warming treatment described above. The press forming is preferably performed in a temperature range of 200 ° C. to 300 ° C. so that the plastic deformability of the rolled plate or the processed plate to be processed can be improved. In addition, even if press molding is performed at a temperature overlapping with the temperature range of 250 ° C to 350 ° C, in press molding, the holding time in the temperature range of 250 ° C to 350 ° C is very short. There are few problems such as coarsening.

  Heat treatment may be performed after press forming to remove distortion and residual stress introduced by press working and improve mechanical properties. The heat treatment conditions include heating temperature: 100 ° C. to 400 ° C., heating time: about 5 minutes to 60 minutes. Also in this heat treatment, it is preferable not to increase the holding time in the temperature range of 250 ° C to 350 ° C. In addition, the molded body obtained after pressing may be left untreated, but if the treatment for forming a coating layer for the purpose of anticorrosion, protection, decoration, etc. is performed as described above, the corrosion resistance and commercial value are further increased. Enhanced.

  The magnesium alloy molded body of the present invention and the magnesium alloy sheet of the present invention are excellent in impact resistance.

It is a schematic diagram showing the structure of a magnesium alloy plate, (1) is a sample having a total holding time of 60 minutes or less in the temperature range of 250 ℃ to 350 ℃ in the rolling process, (2) is a total holding time of 60 minutes It is a super sample. In the manufacturing process of the magnesium alloy plate is a graph mainly showing the relationship between the temperature of the rolling process and the holding time of the temperature, (1) is the total holding time (total of 250 ° C ~ 350 ° C temperature range in the rolling process When (time) is 60 minutes or less, (2) shows the case where the total retention time (total time) is more than 60 minutes. It is a schematic explanatory drawing explaining a dent test. 4 is a graph showing the degree of dent of the magnesium alloy plate after the dent test, where (1) shows sample a and (2) shows sample d. It is a graph which shows the relationship between the thickness of the board which comprises a magnesium alloy molded object, and the amount of dents.

Embodiments of the present invention will be described below.
[Test Example 1]
A plurality of plates made of magnesium alloy and a press-formed body formed by press-molding these magnesium alloy plates were produced, and the metal structure and impact resistance characteristics were examined.

A plurality of cast plates (thickness 4 mm) made of a magnesium alloy having a composition equivalent to AZ91 alloy (Mg-9.0% Al-1.0% Zn (all by mass%)) and obtained by a twin roll continuous casting method were prepared. Each obtained cast plate was subjected to a solution treatment at 400 ° C. for 24 hours. In the solution treatment, cooling was performed by blast, so that the cooling rate from 350 ° C to 250 ° C was 0.1 ° C / sec or more. The solution-treated plate material was rolled a plurality of times under the following rolling conditions until the thickness became 0.6 mm. The obtained rolled sheet was subjected to a final heat treatment at 300 ° C. for 10 minutes to obtain a magnesium alloy sheet.
(Rolling conditions)
Degree of processing (rolling rate): 5% / pass to 40% / pass Heating temperature of plate: 200 ° C to 400 ° C
Roll temperature: 100 ℃ ~ 250 ℃

  In this test, in each pass of the rolling process, by adjusting the heating time and rolling speed (roll peripheral speed) of the plate, the plate material to be rolled is maintained in a temperature range of 250 ° C to 350 ° C. The total time was changed, and four types of samples with total time of 20 min (sample a), 35 min (sample b), 50 min (sample c), and 80 min (sample d) were prepared.

  The magnesium alloy plate subjected to the final heat treatment was subjected to corner drawing at a heating temperature of 250 ° C. to obtain a press-formed body. The press-molded body is a box shape having a rectangular top plate portion of 48 mm × 98 mm and a side wall portion standing from the top plate portion.

  For comparison, commercially available AZ31 alloy material (thickness: 0.6 mm), aluminum alloy material: A5052 material (thickness: 0.6 mm) are prepared, and the AZ31 alloy material has the same conditions as the rolled sheet made of the above AZ91 alloy. The A5052 material was subjected to the same corner drawing at room temperature.

  About the obtained magnesium alloy plate and the press-molded body, the metal structure was observed as follows, and the precipitate was examined. Moreover, the following dent test was done about the obtained magnesium alloy plate and the press-molded body, and the impact resistance characteristic was evaluated.

<Magnesium alloy plate>
<Precipitate>
Cutting the obtained magnesium alloy plate made of AZ91 alloy in the plate thickness direction, observing the cross section with an optical microscope (1000 times), from the surface layer region from the surface of the plate to 1/3 of the plate thickness in the cross section, Two regions of 100 μm × 100 μm are arbitrarily selected, and these regions are used as an observation field. Then, in each observation field, the particle diameter of the observed intermetallic compound containing Al and Mg was measured, and the number of particles having a particle diameter of 5 μm or more was counted.

<Impact resistance>
The obtained magnesium alloy plate made of AZ91 alloy and the prepared AZ31 alloy material and A5052 material (aluminum alloy material) were cut out to 30 mm × 30 mm to prepare test pieces. In this test, as shown in FIG. 3, a support base 20 having a circular hole 21 with a diameter d = 20 mm on a horizontal surface was prepared. The depth of the circular hole 21 was set such that a cylindrical rod 10 described later can be sufficiently inserted. The test piece 1 is arranged so as to close the circular hole 21, and in this state, a weight of 100 g, a tip r = 5 mm, a ceramic cylindrical rod 10 is placed at a point 200 mm from the test piece 1 with its central axis. The circular hole 21 was arranged so as to be coaxial with the central axis. Then, the cylindrical rod 10 is freely dropped from the arranged point toward the test piece 1, and then the dent amount of the test piece 1 is measured. The amount of dent (mm) was measured by using a point micrometer to measure the distance from the straight line connecting opposite sides of the test piece 1 to the most recessed portion. For samples a and d, in a 30mm x 30mm test piece, select a straight line that is parallel to one side of 30mm and passes through the most concave part, and the amount of dents at multiple points on this line Was measured as described above. The results are shown in FIG.

<Press-molded body>
<Precipitate>
In the obtained box-shaped press-formed body of AZ91 alloy, a flat portion without drawing deformation, specifically, the top plate portion is cut in the plate thickness direction, and the cross section thereof is made the same as that of the magnesium alloy plate. In addition, the number of intermetallic compound particles containing Al and Mg having a particle diameter of 5 μm or more in the two observation fields was counted.

<Impact resistance>
In the obtained AZ91 alloy box-shaped press-molded body, separately manufactured AZ31 alloy press-molded body, and A5052 press-molded body, a flat portion without drawing deformation, specifically, 30 mm from the top plate portion. A test piece of × 30 mm was cut out, and the dent amount (mm) was measured using the jig shown in FIG. 3 in the same manner as the magnesium alloy plate.

"thickness"
In the obtained box-shaped press-formed body of the AZ91 alloy, the thickness was measured at any four locations on the 30 mm × 30 mm test piece cut out from the top plate. As a result, every portion was equal to the thickness of the magnesium alloy plate (test piece thickness: 0.6 mm).

Table 1 shows the number of precipitates (pieces) and the amount of dents (mm). Table 1 shows the values of x = 0.47 × t ^−1.25 for the samples a to d. Table 1 shows the smaller number of precipitates in the two observation visual fields.

  Plates and press-molded bodies made of magnesium alloy with an Al content of 7% by mass or more are more resistant than plates and press-formed bodies made of AZ31 alloy and aluminum alloy with less Al content. It can be seen that the impact characteristics are excellent.

As a result of observation of the metal structure, in the samples a to d made of a magnesium alloy having an Al amount of 7% by mass or more, many precipitations of intermetallic compounds (Mg ₁₇ Al ₁₂ ) containing Al and Mg were observed. However, as shown in Table 1, the samples a to c in which the total time kept at 250 ° C. to 350 ° C. in the rolling process is within 1 hour (60 min) are coarse for both the magnesium alloy plate and the press-formed body. No intermetallic compound was present, and a fine intermetallic compound was dispersed as shown in FIG. 1 (1). And it can be seen that the samples a to c with few coarse precipitates have few dents and are excellent in impact resistance. In addition, even when the final heat treatment is performed after rolling, the holding time in the temperature range of 250 ° C to 350 ° C in the rolling process and the holding time in the temperature range of 250 ° C to 350 ° C in the final heat treatment after rolling It can be seen that the impact resistance is excellent by controlling the total time within 1 hour.

[Test Example 2]
Magnesium alloy plates with different thicknesses and press-molded bodies formed by press-molding these magnesium alloy plates were produced, and the metal structure and impact resistance characteristics were examined.

  Prepare multiple cast plates (composition equivalent to AZ91 alloy, thickness 4mm) similar to Test Example 1, and under the same conditions as in Test Example 1, solution treatment (400 ° C x 24 hours, 350 ° C to 250 ° C) (Cooling rate: 0.1 ° C / sec or more), multiple rolling (rolling rate: 5% / pass to 40% / pass, plate heating temperature: 200 ° C to 400 ° C, roll temperature: 100 ° C to 250 ° C) A rolled plate was obtained. In this test, as in Test Example 1, the total time during which the plate material was maintained in the temperature range of 250 ° C. to 350 ° C. in the rolling process was changed. In this test, the thickness of the rolled plate was varied by adjusting the rolling reduction, and the total time was set to 35 min or 80 min by adjusting the heating time and rolling speed of the plate. In this test, samples having a total time of 45 min (sample α) and 90 min (sample β) including the time of the final heat treatment after rolling were prepared.

  The obtained rolled plate was subjected to a final heat treatment at 300 ° C. for 10 minutes, and then subjected to a corner drawing at a heating temperature of 250 ° C. to obtain a box-shaped press-formed body similar to Test Example 1.

  For the magnesium alloy plate and press-molded body obtained by the final heat treatment, the number of precipitates was measured by observing the structure of the cross section in the same manner as in Test Example 1. In addition, a test piece was prepared in the same manner as in Test Example 1, a dent test was performed, and the dent amount was measured. These results are shown in Table 2. In Table 2, a sample having a thickness of 0.6 mm (0.6 mm) shows the result of Test Example 1.

  As shown in Table 2, although the amount of dents varies depending on the thickness of the magnesium alloy plate or press-formed body (top plate part), the total holding time in the temperature range of 250 ° C to 350 ° C in the rolling process is within 60 minutes. In the sample α, the coarse intermetallic compound having a particle size of 5 μm or more does not exist in the surface region regardless of the plate thickness (0), and the amount of dents is small compared to the sample β having the same thickness. I understand.

Thus the pressed bodies having excellent impact resistance, were examined and the plate thickness t _p of the pressed bodies (top plate), the relationship between the dented x. The results are shown in FIG. From the graph shown in FIG. 5, the simplest representative of the relationship between the thickness t _p and dented x in sample _{α, x = k × t p} -1 (k is a coefficient) is considered. The coefficient k for distinguishing between the sample α and the sample β was determined by substituting the numerical value of 0 to 1 in increments of 0.01 in the range of the plate thickness t _p = 0.5 to 0.8. Conceivable. However, the coefficient k is, because there tends to vary slightly depending the thickness less than the thickness t _p is 0.5 mm, considering the case of 0.8mm greater than the relational expression when a k = 0.5 x = 0.5 × t ^-1 to reexamine the relational expression that distinguishes sample β from sample β within the range where sample α does not deviate as much as possible. When Specifically, fixed to k = 0.5, the index of thickness t _p by substituting at -1 in 0.01 increments, seek preferred curve, again, to determine the coefficient k in the same manner as described above, x = 0.47 × t ^-1.25 was obtained. Therefore, x ≦ 0.47 × t _p ^−1.25 is used as an index representing the molded body of the present invention. We also examined similarly for the magnesium alloy plate, magnesium alloy plate for even ^{_{x ≦ 0.47 × t b -1.25 (}} t b: plate thickness) can be applied, the present invention x ≦ 0.47 × t _b ^-1.25 Used as an index to indicate a magnesium alloy plate.

[Test Example 3]
A magnesium alloy plate produced by changing the treatment applied after rolling was prepared, and a press-molded body obtained by press-molding the magnesium alloy plate was produced, and the metal structure and impact resistance characteristics were examined.

  In this test, a plurality of cast plates similar to Test Example 1 (composition equivalent to AZ91 alloy, thickness 4 mm) were prepared, and solution treatment (400 ° C x 24 hours, 350 ° C ~ (Cooling rate to 250 ° C .: 0.1 ° C./sec or more). The solution-treated plate material is subjected to multiple rolling (rolling rate: 5% / pass to 40% / pass, plate heating temperature: 200 ° C to 280 ° C, roll temperature: 100 ° C to 250 ° C), and rolled. I got a plate. In this test, the total time for which the plate material was maintained in the temperature range of 250 ° C. to 350 ° C. in the rolling process was 45 min.

  The obtained rolled plate was subjected to warm correction treatment. Here, the warm straightening process uses a roll leveler apparatus including a heating furnace capable of heating a rolled plate and a roll unit having a plurality of rolls that continuously bend (strain) the heated rolled plate. Do it. The roll section includes a plurality of rolls arranged in a staggered manner facing each other in the vertical direction.

  With the roll leveler device, the rolled plate is fed to the roll part while being heated in the heating furnace, and each time it passes between the upper and lower rolls of the roll part, the roll is sequentially bent. Here, warm correction was performed in the temperature range of 220 to 250 ° C., and the conveyance speed at the time of correction was adjusted so that the total time during which the rolled plate was maintained at a temperature of 250 to 350 ° C. was within 60 minutes.

  The magnesium alloy plate obtained by performing the warm correction treatment was subjected to corner drawing at a heating temperature of 250 ° C. to obtain a box-shaped press-molded body similar to Test Example 1.

  For the obtained magnesium alloy plate and press-formed body, the number of precipitates was measured by observing the structure of the cross section in the same manner as in Test Example 1. In addition, a test piece was prepared in the same manner as in Test Example 1, a dent test was performed, and the dent amount was measured. These results are shown in Table 3.

  As shown in Table 3, it can be seen that all the samples have a small amount of dents and are excellent in impact resistance. In particular, after rolling, sample No. 3-1 using a magnesium alloy plate subjected to warm straightening treatment was sample No. 2-1 (0.6 mmt-α of Test Example 2) subjected to final heat treatment after rolling. In comparison, it can be seen that the dent amount is small and the impact resistance is excellent.

  The above-described embodiment can be appropriately changed without departing from the gist of the present invention, and is not limited to the above-described configuration. For example, the composition of the magnesium alloy, the thickness of the magnesium alloy plate, the shape of the press-molded body, and the like can be changed as appropriate.

  The magnesium alloy molded body of the present invention can be suitably used for various types of electrical equipment parts, particularly for portable electrical equipment cases. The magnesium alloy sheet of the present invention can be suitably used as a material for the magnesium alloy molded body of the present invention.

1 Specimen 10 Cylindrical bar 20 Support base 21 Circular hole d ₀ , d ₁ Intermetallic compound (precipitate)

Claims (11)

A magnesium alloy molded body obtained by press-molding a plate made of a magnesium alloy,
The magnesium alloy contains 7 mass% or more and 12 mass% or less of Al,
The molded body has a flat portion without drawing deformation,
In the metal structure of the cross section obtained by cutting the flat portion in the plate thickness direction, a region from the surface of the flat portion to 1/3 of the plate thickness in the plate thickness direction is defined as a surface layer region, and from any two locations in the surface layer region When the selected region of 100 μm × 100 μm is an observation field and the particles of the intermetallic compound containing Al and Mg and the particles having a particle diameter of 5 μm or more are coarse particles,
The magnesium alloy molded body, wherein the coarse particles present in each observation field are 5 or less. 30 mm × 30 mm cut out from the flat portion, with respect to the test piece having a thickness of _{t p,} when subjected to the following dent test, dented _{x p} of the test piece, _{x p} ≦ 0.47 × _{t p} The magnesium alloy compact according to claim 1, which satisfies ^−1.25 .
(Dent test)
A test piece is placed on a support base having a hole with a diameter of 20 mm so as to close the hole, and in this state, a cylindrical bar having a weight of 100 g and a tip r = 5 mm is freely dropped from a point 200 mm high from the test piece. .
The concave depth x _p is the distance from the straight line connecting the both sides of the test piece after the dent test to the most recessed portion.   The magnesium alloy contains one or more elements selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare earth elements (excluding Y). 2. Magnesium alloy molded product according to 1.   4. The magnesium alloy compact according to claim 3, wherein the magnesium alloy contains, by mass%, Al in a range of 8.3% to 9.5% and Zn in a range of 0.5% to 1.5%. .   The magnesium alloy molded body according to claim 4, further comprising an anticorrosive layer formed by chemical conversion treatment on a surface of the plate made of the magnesium alloy. A magnesium alloy plate used for press forming,
The magnesium alloy contains 7 mass% or more and 12 mass% or less of Al,
In the metal structure of the cross section obtained by cutting the plate in the plate thickness direction, the region from the plate surface to the plate thickness direction to 1/3 of the plate thickness is set as the surface layer region, and is selected from any two locations in the surface layer region. When the region of the observation field is an observation visual field and the particles of an intermetallic compound containing Al and Mg, and the particles having a particle diameter of 5 μm or more are coarse particles,
The magnesium alloy plate, wherein the number of coarse particles present in each observation field is 5 or less. When the following dent test was performed on a test piece of 30 mm × 30 mm and thickness t _b cut out from the plate, the dent amount x _b of the test piece was x _b ≦ 0.47 × t _b ^−1. The magnesium alloy sheet according to claim 6, satisfying ^.25 .
(Dent test)
A test piece is placed on a support base having a hole with a diameter of 20 mm so as to close the hole, and in this state, a cylindrical bar having a weight of 100 g and a tip r = 5 mm is freely dropped from a point 200 mm high from the test piece. .
The dent amount _xb is the distance from the straight line connecting both sides of the test piece after the dent test to the most recessed part.   The magnesium alloy includes one or more elements selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare earth elements (excluding Y). 2. Magnesium alloy plate described in 1.   The magnesium alloy sheet according to claim 8, wherein the magnesium alloy contains, by mass%, Al in a range of 8.3% to 9.5% and Zn in a range of 0.5% to 1.5%. A method for producing a magnesium alloy molded body, in which a molded body is produced by subjecting a plate made of a magnesium alloy to press molding,
A preparation step of preparing a cast plate made of a magnesium alloy containing 7 to 12% by mass of Al and manufactured by a continuous casting method;
A solution treatment step for subjecting the cast plate to a solution treatment at a temperature of 350 ° C. or higher;
A rolling step of rolling the solution-treated plate material;
A press working step for press forming the rolled plate obtained by the rolling step,
In the solution treatment step, in the cooling step from the holding temperature of the solution treatment, the cooling rate from 350 ° C. to 250 ° C. is 0.1 ° C./sec or more,
In the rolling step, the total time for which the plate material to be processed is maintained in a temperature range of 250 ° C. or higher and 350 ° C. or lower is set to 60 minutes or less,
In the said press work process, press molding is performed in the temperature range of 200 to 300 degreeC, The manufacturing method of the magnesium alloy molded object characterized by the above-mentioned. A magnesium alloy plate manufacturing method for manufacturing a plate made of a magnesium alloy and used for press molding,
A preparation step of preparing a cast plate made of a magnesium alloy containing 7 to 12% by mass of Al and manufactured by a continuous casting method;
A solution treatment step for subjecting the cast plate to a solution treatment at a temperature of 350 ° C. or higher;
A rolling process for rolling the solution-treated plate material,
In the solution treatment step, in the cooling step from the holding temperature of the solution treatment, the cooling rate from 350 ° C. to 250 ° C. is 0.1 ° C./sec or more,
In the rolling process, the total time for which the plate material to be processed is maintained in a temperature range of 250 ° C. or higher and 350 ° C. or lower is set to 60 minutes or less.

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