KR20110070884A - Formed product of magnesium alloy and magnesium alloy sheet - Google Patents

Abstract

Provided are a magnesium alloy molded body having excellent impact resistance and a magnesium alloy plate suitable for the material of the molded body. It is a molded object which press-molded the magnesium alloy plate containing 7 mass%-12 mass% of Al, and has a flat part which does not involve drawing deformation. In the flat part on the metal structure of a cross section cut in the plate thickness direction, the surface area is present an intermetallic compound more than coarse particles having a diameter of 5 ㎛ to the up to one-third of the plate thickness from the surface of the flat portion (Mg ₁₇ Al ₁₂ ) has five or fewer particles. This molded article has a structure in which coarse precipitates d ₁ are small and fine precipitates d ₀ are dispersed. Due to the dispersion strengthening of the fine precipitates and solid melt strengthening by sufficiently solid melting of Al, the molded article is hardly recessed even when subjected to impact.

Description

FORMED PRODUCT OF MAGNESIUM ALLOY AND MAGNESIUM ALLOY SHEET}

TECHNICAL FIELD This invention relates to the magnesium alloy plate suitable for raw materials, such as a case of a portable electric device, and the magnesium alloy molded object which press-molded this plate. In particular, it is related with the magnesium alloy molded object excellent in impact resistance characteristics.

As a case material of portable electric devices such as a mobile phone or a notebook PC, a resin such as ABS (acrylonitrile butadiene styrene copolymer) resin, a PC (polycarbonate) resin, or a metal such as aluminum alloy or stainless steel (SUS) is used. It is becoming.

In recent years, as said case material, the magnesium alloy which is lightweight and excellent in specific strength and specific rigidity is examined. As the case made of a magnesium alloy, a casting material obtained by a die casting method or a thixomolding method is mainly used, and a press-worked product is used on a plate made of a magnesium alloy for the whole body represented by the AZ31 alloy of the ASTM standard. Moreover, in patent document 1, performing press work to the AZ91 alloy of an ASTM standard is examined.

In recent years, a thin and lightweight case is required. Metals are generally more resistant to impact than resins, and are less likely to be broken, so they are thinner. However, the aluminum alloy is poor in plastic deformation resistance and deforms very easily due to impact such as falling. Stainless steel is heavy, although cracks and deformation do not easily occur.

Japanese Patent Publication No. 2007-098470

Magnesium alloys are superior in plastic deformation resistance to aluminum alloys, and are very lightweight compared to stainless steels. However, the casting material of magnesium alloy is inferior in strength compared with the press-molded body of magnesium alloy, and it is also difficult to thin. Press molded bodies of the AZ31 alloy also have insufficient strength.

When press molding is carried out to the rolled plate which consists of AZ91 alloy described in patent document 1, the molded object of higher strength than the press molded body which consists of AZ31 alloy can be obtained. However, as a result of investigation by the present inventors, when the content of Al increased to 7 mass% or more, the knowledge which the fluctuation | variation in the impact resistance characteristic may generate | occur | produce in the board used as a raw material or the press-molded body which shape | molded this board was obtained.

Therefore, one of the objectives of this invention is providing the magnesium alloy molded object excellent in impact resistance characteristics. Moreover, another object of this invention is to provide the magnesium alloy plate suitable for manufacture of the magnesium alloy molded object excellent in impact resistance characteristics.

MEANS TO SOLVE THE PROBLEM The present inventors produced the board which becomes a raw material by various manufacturing methods in the magnesium alloy containing 7 mass% or more of Al, produced the press molded object with the obtained board, and investigated the impact resistance (depression resistance), As a result, it dents well. be press molded article that is, Mg ₁₇ Al ₁₂ is small and the particles formed of the intermetallic compound (precipitate) such as to obtain the large particles is less knowledge. Therefore, as a result of examining the manufacturing method of controlling the maximum particle diameter and the number of the above-mentioned precipitates, that is, reducing the coarse precipitates, the total total time maintained in a specific temperature range mainly in the rolling process is compared with that of the conventional method. By shortening, the magnesium alloy plate with few coarse precipitates was obtained. Moreover, the press-molded object which press-molded this magnesium alloy plate was excellent in impact resistance characteristics. This invention is based on the said knowledge.

The magnesium alloy molded article of the present invention is a press-molded plate made of a magnesium alloy containing 7% by mass or more and 12% by mass or less of Al, and has a flat portion without drawing deformation, and the flat portion has a flat thickness direction. In the metal structure of the cross section cut | disconnected by the above, when the observation visual field prescribed | regulated below is taken, there are five or less particle | grains of the coarse intermetallic compound which exists in each observation visual field.

Moreover, the magnesium alloy plate of this invention is used for press molding, Comprising: The coarse intermetallic compound which consists of a magnesium alloy containing 7 mass% or more and 12 mass% or less of Al, and exists within each observation visual field prescribed | regulated below. There are five or less particles.

An observation field is a metal structure of the flat part or the cross section which cut | disconnected the said magnesium alloy plate in the plate | board thickness direction WHEREIN: From the surface of the said flat part or the said plate surface to 1/3 of plate | board thickness in a plate | board thickness direction An area is made into a surface layer area | region, and it is set as the area | region of 100 micrometers x 100 micrometers selected from two arbitrary places of this surface layer area.

Coarse particle | grains are particle | grains of the intermetallic compound containing Al and Mg, and are particle | grains whose particle diameter is 5 micrometers or more.

A particle diameter is taken as the diameter of the circle which has an area equivalent to the area of the particle | grains in the said cross section.

In addition, the intermetallic compound present in the cross section can be determined by examining the composition and structure of the particles by, for example, EDS (Energy Dispersive X-ray Spectrometer) or X-ray diffraction. .

The alloy plate of this invention which has the said specific structure can be manufactured by the manufacturing method containing each following process, for example.

Preparation process: The cast plate which consists of magnesium alloy containing 7-12 mass% of Al, and manufactured by the continuous casting method is prepared.

Solvent step: The cast plate is subjected to a solution treatment at a temperature of 350 ° C. or higher.

Rolling process: It rolls to the said board | plate material processed by the said solution.

In particular, in the cooling process from the holding temperature of a solution treatment, the cooling rate from 350 degreeC to 250 degreeC is made into 0.1 degreeC / sec or more, and in a rolling process, the board | plate material which is a process is 250 degreeC or more and 350 degrees C or less. The total time maintained in the area shall be within 60 minutes.

As described above, in the cooling process (i.e., just before rolling) and the rolling process of the solution treatment, the time for which the precipitate is maintained in a specific temperature range (250 ° C to 350 ° C) that is likely to precipitate or grow and become coarse is possible. By making it shorter, coarse precipitates can be reduced, and as shown in Fig. 1 (1), a structure in which fine precipitates d ₀ are dispersed can be obtained.

Conventionally, rolling is performed a plurality of times (multiple passes) at an appropriate workability (rolling down rate) as shown in Fig. 2 (2) until the desired plate thickness is achieved (each pass is shown in Fig. 2). "Rolled n" (indicated by n = 1, 2, ...)). At this time, by heating a process object (cast plate or the rolled plate until the last rolling is performed) to 250 degreeC or more, plastic workability becomes high. Therefore, it is preferable to heat a process target and to warm-roll or hot-roll at least in the rolling initial stage (crude rolling) in a rolling process. However, especially in the case of magnesium alloy having a high Al content of 7% by mass or more, by heating to 250 ° C or more, precipitates such as intermetallic compounds are easily grown and coarse as described above. Moreover, even when it passes through the 250 degreeC-350 degreeC temperature range in the cooling process of a solution treatment process, a precipitate becomes easy to coarsen.

Conventionally, just before rolling and in the rolling process, it was not fully examined how much the total time which keeps a process object in the temperature range of 250 degreeC-350 degreeC. As a result of investigation by the present inventors, in the case of magnesium alloy whose content of Al is 7 mass%-12 mass%, when the total holding time of the said temperature range exceeds 1 hour at least in a rolling process, it shows in FIG. Likewise, a structure in which coarse precipitate (d ₁ ) having a particle diameter of 5 µm or more is present. On the other hand, in the rolling process, the coarse precipitate can be reduced by setting the total holding time of the temperature region to within 1 hour. Moreover, in addition to making the total holding time of the said temperature range into 1 hour only in a rolling process, coarse precipitate can be suppressed more effectively by making the cooling rate in a solution treatment process quicker. In particular, it is preferable to make the total time of the total holding time of the said temperature range in a rolling process, and the holding time of the said temperature range in the cooling process of a solution treatment process within 1 hour.

The alloy plate of the present invention has a structure in which coarse precipitates are small in the surface layer region, and very fine precipitates are dispersed and present (FIG. 1 (1)). In addition, the alloy plate of the present invention has less coarse precipitates, so that the amount of solid molten Al in the matrix Mg due to the large amount of coarse precipitates is small, and the decrease in solid melt strengthening due to the decrease in Al amount is small. I think it is. Therefore, the alloy plate of the present invention does not sink well even when subjected to an impact, and is excellent in impact resistance by maintaining the strength by improving the rigidity of the plate itself by strengthening the dispersion of precipitates and suppressing the decrease of the amount of solid molten Al. . Moreover, the alloy plate of this invention with few coarse precipitates is excellent also in plastic workability, and can press-process easily.

As mentioned above, the molded object of this invention can be manufactured mainly by performing press molding to the alloy plate of this invention obtained by controlling the holding time of a specific temperature range in a rolling process. In the case where the alloy plate of the present invention is used, the structure where the coarse precipitates constituting the alloy plate of the present invention is roughly retained at a location (flat portion) where the deformation of the molded article of the present invention is small due to press molding.

That is, the molded article of the present invention also has a structure in which coarse precipitates are small in the surface layer region and very fine precipitates are dispersed. Therefore, the molded article of the present invention is excellent in impact resistance and hardly swelled by dispersion strengthening by fine precipitates and solid melt strengthening by sufficient solid melting of Al as described above.

Hereinafter, the present invention will be described in more detail.

"Furtherance"

Examples of the magnesium alloy include those having various compositions in which Mg contains an additional element (residue: Mg and impurities). The plate and molded object of this invention shall consist of Mg-Al type alloy which contains 7 mass% or more and 12 mass% or less of Al in an additional element. Examples of additional elements other than Al include one or more elements selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare earth elements (except Y). When these elements are included, the content is 0.01 mass% or more and 10 mass% or less, Preferably 0.1 mass% or more and 5 mass% or less are mentioned. More specific Mg-Al-based alloys are, for example, AZ-based alloys (Mg-Al-Zn-based alloys, Zn: 0.2 to 1.5 mass%) in the ASTM standard, AM-based alloys (Mg-Al-Mn-based alloys, Mn: 0.15-0.5 mass%), Mg-Al-RE (rare earth element) type alloy, etc. are mentioned. In particular, Mg-Al-based alloys containing 8.3 to 9.5% by mass of Al and 0.5 to 1.5% by mass of Zn, typically AZ91 alloys, are more resistant to corrosion and strength than other Mg-Al alloys such as AZ31 alloys. Excellent mechanical properties such as plastic deformation.

<< thickness of the magnesium alloy plate >>

The alloy sheet of the present invention is subjected to press molding such as bending and drawing, and used for a material of a thin, lightweight member such as a case. In the case obtained by press molding, the thickness of the alloy plate of the present invention is 2.0 so that the thickness of a portion (flat portion in the molded article of the present invention) whose thickness does not substantially change due to deformation due to plastic working is reduced. Mm or less, especially 1.5 mm or less, 1 mm or less are more preferable. In the above range, the thicker the magnesium alloy plate is, the higher the strength is, and the thinner, the thinner and lighter case is suitable. What is necessary is just to select a board thickness according to a desired use.

<< mechanical characteristic >>

The alloy sheet of the present invention is not recessed well when subjected to an impact such as a drop. Specifically, when the following depression test was performed on a test piece of 30 mm x 30 mm and a thickness t _b cut out from the alloy plate of the present invention, the depression amount x _b of the test piece was x _b ≤ 0.47 x t _b ⁻ Satisfies ^1.25 In addition, in the molded article of the present invention, the flat portion which does not involve 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. For this reason, in the flat portion, the same test piece and the invention alloy sheet described above: when cut (thickness t _p) line the depressions examination of the following, depression amount x _p of the test specimen, x _p ≤0.47 × t _p Satisfies ^-1.25 In addition, the thickness t _p of the test piece cut out from the flat part of the molded object of this invention is substantially the same as the thickness t _{b of the} test piece cut out from the magnesium alloy plate used as the raw material before press molding, for example, the alloy plate of this invention (t _p = t _b ).

(Depression test)

A test piece is arrange | positioned so that this hole may be closed in the support which has a hole of 20 mm in diameter, and in this state, a cylindrical rod of 100 g in weight and a tip r = 5 mm is free-falled at the point of 200 mm in height from a test piece.

The depression amount x _b or x _p is taken as the distance from the straight line connecting both sides of the test piece after the depression test to the most recessed part.

<< shape of the molding >>

The molded object of this invention typically has the shape which has an upper plate part (bottom surface part) and the side wall part standing up from the peripheral edge of an upper plate part. More specifically, a rectangular plate-shaped upper plate portion, a shape having only a pair of side wall portions facing each other, a cross-sectional box having two sets of opposite side wall portions, or a cylindrical shape having a lid having a circular plate shape and a cylindrical side wall portion Upper body etc. are mentioned.

The form of the upper plate portion and the side wall portion is typically made of a flat surface, and the shape and size are not particularly limited. The upper plate portion and the side wall portion integrally form or join a boss or the like, have holes penetrating the front and back, grooves recessed in the thickness direction, have a stepped shape, or are formed locally by plastic working or cutting. It may have a part from which thickness differs suitably. In the molded article of the present invention, the flat portion without accompanying drawing processing is a portion having less warpage when the segment cut out in the region except the portion having the boss or the like is disposed on the horizontal plane. More specifically, in a surface facing each other with the horizontal plane in the segment arranged on the horizontal plane, the distance from the horizontal plane to the point furthest from the vertical direction is 1 mm or less. Since depression generally occurs in a flat part, in the alloy plate of the present invention and the molded article of the present invention, the evaluation of depression resistance is made as the flat part.

<< surface of molding >>

The molded object of this invention can be made into the form provided with the coating layer for the purpose of anticorrosion, protection, decoration, etc. on the surface of the plate which consists of magnesium alloys. The magnesium alloy which mainly comprises the molded object of this invention contains 7 mass% or more of Al, and is excellent in corrosion resistance compared with the alloy with little Al content, for example, AZ31 alloy. In addition, the corrosion resistance of the molded article of the present invention can be further improved by performing an anticorrosive treatment such as chemical conversion treatment or anodizing treatment on a plate made of the magnesium alloy to form an anticorrosive layer. In the formation process of coating layers, such as the said anticorrosion and coating, it does not substantially affect the magnitude | size or precipitation of a precipitate. Therefore, the molded article of the present invention has five coarse particles or less even if it is provided with a coating layer such as the above method, and satisfies x _p ≤ 0.47 x t _p ^-1.25 when the depression test is performed.

<< production method >>

[Preparation process]

As the cast plate, it is preferable to use a cast plate produced by a continuous casting method such as the twin roll method, in particular the casting method described in WO / 2006/003899. Since the continuous casting method enables rapid solidification, oxides, segregation, and the like can be reduced, and a cast plate excellent in rollability can be obtained. Although the size of a cast plate does not matter in particular, since it is easy to produce segregation when it is too thick, 10 mm or less, especially 5 mm or less are preferable.

Solvation Process

The cast plate is subjected to a solution treatment to achieve homogenization of the composition. The solution treatment makes holding temperature 350 degreeC or more. In particular, holding temperature: 380-420 degreeC, holding time: 60-2400 minutes are preferable, and it is preferable to lengthen holding time, so that Al content is high. Moreover, in order to manufacture the alloy plate of this invention, in the cooling process from the said holding temperature, the holding time of the temperature range of 350 degreeC-250 degreeC is controlled. 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 Is 0.5 ° C / sec or more (holding time: 3.3 minutes or less). Such a cooling rate can be achieved by water cooling, forced cooling such as an air blast, or the like. By making the holding time of the above temperature range as short as possible, precipitation of intermetallic compounds containing Al and Mg, even with high Al magnesium alloys, can be suppressed, and in particular, growth of coarse particles can be effectively suppressed. have.

[Rolling process]

In order to improve the plastic workability (rolling property) of the board | plate material to which the said solution treatment was performed, as mentioned above, in at least rough rolling, rolling to the board | plate material heated at the temperature of 200 degreeC or more, especially 250 degreeC or more is recommended. desirable. The higher the heating temperature, the higher the plastic workability of the plate can be. However, if it exceeds 350 ° C., there is a problem that baking occurs or the coarsened crystal grains decrease the mechanical properties of the plate after rolling. Preferably, more preferable heating temperature is 270 degreeC or more and 330 degrees C or less. By rolling a plurality of times (multiple passes), it can be set as desired plate | board thickness, and the average crystal grain diameter of a magnesium alloy can be made small, or press workability can be improved. You may use rolling by combining well-known conditions, for example, not only a board | plate material but a roll, or the control rolling etc. which are disclosed by patent document 1, and the like. Moreover, in the last path | pass and the path | route of the vicinity, you may make heating temperature of a board | plate material low (for example, room temperature), for the purpose of improving dimensional precision, etc.

In the said rolling process, the holding time of the temperature range of 250 degreeC-350 degreeC is controlled. Specifically, as shown in Fig. 2 (1), in each pass of the rolling step, in order to shorten the holding time of the temperature region, for example, the heating time for heating the processing object is shortened or rolled. Increase the speed (roll circumferential speed) or increase the cooling rate. And rolling conditions are controlled so that the total holding time of the 250 to 350 degreeC temperature range in a rolling process may be 60 minutes or less. Since the precipitate tends to precipitate as the amount of Al increases, the total sum of the holding time is preferably adjusted according to the content of Al. Moreover, the said total holding time is so preferable that it is short, and 45 minutes or less, especially 30 minutes or less are preferable. By performing such specific rolling, the alloy plate of the present invention having less coarse precipitates in the surface layer region and excellent in impact resistance as described above can be obtained.

Intermediate heat treatment is performed between the passes of rolling to remove and reduce distortions, residual stresses, textures, etc. introduced into the object to be processed by processing up to intermediate heat treatment, thereby preventing inadvertent cracks, distortions, and deformations in subsequent rolling. And rolling can be performed more smoothly. As for intermediate | middle heat processing, holding temperature: 250 degreeC-350 degreeC is preferable, In this temperature range, as mentioned above, a precipitate grows easily and becomes coarse. Therefore, in the case of performing the intermediate heat treatment, the treatment time is preferably included in the total holding time and controlled.

<< processing after rolling >>

(Final heat treatment (annealing))

The obtained rolled plate may be subjected to a final heat treatment of 300 ° C. or higher, for example, to remove work distortion due to rolling and completely recrystallize it. Even in this final heat treatment, precipitates tend to grow in the temperature range of 250 ° C to 350 ° C. Therefore, in the case of performing the final heat treatment after rolling, it is preferable to control the treatment time by including the total holding time. By controlling the final heat treatment time as described above, the present invention magnesium alloy plate with less coarse precipitates can be obtained.

(Warm correction processing)

Or you may perform the warm calibration process which adds a distortion with a roll leveler etc. in the state which heated the obtained rolled plate at 100-250 degreeC, without performing the said final heat processing after rolling. When press working is performed on the process board which performed the warm correction process, it can recrystallize at the time of press work, and the molded object of a fine crystal structure can be obtained. Compared with the case where the final heat treatment is performed, the crystal grains tend to be fine, and the fine precipitates tend to be more uniformly dispersed. Therefore, when the warm calibration process is performed, not only the coarse precipitates are small but also the fine structure, the magnesium alloy plate of the present invention having more excellent impact resistance characteristics can be obtained. In addition, in a warm calibration process, since the heating temperature of a rolled sheet is only 250 degreeC, it is thought that a precipitate does not become coarse.

[Press processing]

The molded article of the present invention can be produced by press molding the rolled sheet obtained by the rolling step or the processed sheet subjected to the final heat treatment or warm calibration treatment described above. It is preferable to perform press molding in the temperature range of 200 degreeC-300 degreeC so that the plastic deformation of the rolling board and a process board which may 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 of the temperature range of 250 ° C to 350 ° C is very short, so that coarsening of the precipitates as described above is performed. It is thought that there are few problems.

After press molding, heat treatment may be performed to eliminate distortion, residual stress, and mechanical properties, which are introduced by press working. As heat processing conditions, heating temperature: 100 degreeC-400 degreeC, heating time: 5 minutes-about 60 minutes are mentioned. Also in this heat treatment, it is preferable that the holding time of the temperature range of 250 ° C to 350 ° C is not long. Moreover, although the molded object obtained after a press does not need to be processed at all, as mentioned above, if the process of forming the coating layer for the purpose of an anticorrosion, protection, decoration, etc. is performed, corrosion resistance, commodity value, etc. can be improved further.

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

1: is a schematic diagram which shows the structure of a magnesium alloy plate, (1) The sample whose total holding time of the temperature range of 250 degreeC-350 degreeC is 60 minutes or less in the rolling process, (2) The total holding time is 60 minutes Excess sample.
2 is a graph mainly showing the relationship between the temperature of a rolling process and the holding time of the temperature in the manufacturing process of a magnesium alloy plate, (1) The total holding time of the temperature range of 250 degreeC-350 degreeC in a rolling process. When (total time) is 60 minutes or less, (2) shows the case where the total holding time (total time) is over 60 minutes.
3 is a schematic explanatory diagram for explaining a depression test.
4 is a graph showing the degree of depression of the magnesium alloy plate after the depression test, in which (1) shows sample a and (2) shows sample d.
5 is a graph showing the relationship between the thickness of the plate constituting the magnesium alloy molded body and the amount of depressions.

Hereinafter, embodiments of the present invention will be described.

[Test Example 1]

A plurality of plates made of a magnesium alloy and a press-formed product formed by press molding these magnesium alloy plates were fabricated to investigate metal structure and impact resistance characteristics.

A plurality of cast plates (4 mm thick) made of a magnesium alloy having a composition equivalent to the AZ91 alloy (Mg-9.0% Al-1.0% Zn (all mass%)) obtained by a twin roll continuous casting method were prepared. Each obtained casting plate was subjected to the solution treatment at 400 ° C for 24 hours. In the solution treatment, cooling was performed by air blast, and the k-angles from 350 ° C to 250 ° C were 0.1 ° C / sec or more. The plate material subjected to the solution treatment was subjected to rolling a plurality of times until the thickness became 0.6 mm under the following rolling conditions. The obtained rolled sheet was subjected to final heat treatment at 300 ° C. for 10 minutes to obtain a magnesium alloy sheet.

(Rolling condition)

Workability (rolling down rate): 5% / pass to 40% / pass

Heating temperature of plate: 200 ℃ ~ 400 ℃

Roll Temperature: 100 ℃ ~ 250 ℃

In this test, in each pass of a rolling process, by adjusting the heating time and rolling speed (roll circumferential speed) of a board | plate, the total time in which the board | plate material to be rolled is hold | maintained in the temperature range of 250 degreeC-350 degreeC is changed, Four types of samples were prepared in which the total time was 20 minutes (sample a), 35 minutes (sample b), 50 minutes (sample c), and 80 minutes (sample d).

A magnesium alloy sheet subjected to final heat treatment was subjected to a square cup deep-drawing process at a heating temperature of 250 ° C. to obtain a press-formed product. The press-formed body is a box-shape which has a rectangular upper board part of 48 mm x 98 mm, and the side wall part standing up from an upper board part.

For comparison, a commercially available AZ31 alloy material (thickness: 0.6 mm) and aluminum alloy material: A5052 material (thickness: 0.6 mm) were prepared, and the AZ31 alloy material was angularly drawn under the same conditions as the rolled plate made of the AZ91 alloy. The process was carried out, and the same angle drawing process was given to A5052 material at room temperature.

About the obtained magnesium alloy plate and the press molding, the metal structure was observed and the precipitate was investigated as follows. Moreover, the following depression test was done about the obtained magnesium alloy plate and the press molded object, and the impact resistance characteristic was evaluated.

<Magnesium alloy plate>

<< precipitate >>

The magnesium alloy plate made of the obtained AZ91 alloy was cut in the plate thickness direction, and the cross section was observed with an optical microscope (1000 times), and in the cross section, 100 μm in the surface layer region from the surface of the plate to 1/3 of the plate thickness. An area of 100 µm is selected at random, and this area is set as the observation field of view. And in each observation visual field, the particle diameter of the intermetallic compound containing Al and Mg observed was measured, respectively, and the number of particle | grains whose particle diameter is 5 micrometers or more was counted.

<< shock characteristics >>

The magnesium alloy plate which consists of obtained AZ91 alloy, the prepared AZ31 alloy material, and A 5052 material (aluminum alloy material) were cut out to 30 mm x 30 mm, and the test piece was produced. In this test, as shown in Fig. 3, a support 20 having a circular hole 21 having a diameter d = 20 mm on a horizontal surface was prepared. The depth of the circular hole 21 was made into the magnitude | size which can fully insert the cylindrical rod 10 mentioned later. The test piece 1 is arrange | positioned so that this circular hole 21 may be closed, and in this state, the weight 100g, the tip r = 5mm, the cylindrical cylindrical rod 10 made from ceramics at the point of 200 mm in height from the test piece 1 The central axis and the central axis of the circular hole 21 were arranged so as to be the same axis. Then, after freely dropping the cylindrical rod 10 toward the test piece 1 at the arranged point, the amount of depression of the test piece 1 is measured. The depression amount (mm) measured the distance from the straight line which connects the opposite sides of the test piece 1 to the most recessed part using the point micrometer. In addition, about the samples a and d, in the test piece of 30 mm x 30 mm, the straight line which passes through the most recessed point as a straight line parallel to one side of 30 mm is selected, and the amount of depressions in these points on this straight line Was measured in the same manner as above. The result is shown in FIG.

<Press molded body>

<< precipitate >>

In the box-shaped press-formed body of the obtained AZ91 alloy, a flat portion without drawing deformation, specifically, the upper plate portion is cut in the plate thickness direction, the cross section is observed in the same manner as the magnesium alloy plate, and the observation field is taken. In the two observation fields, the number of particles of the intermetallic compound containing Al and Mg having a particle diameter of 5 µm or more was counted.

<< shock characteristics >>

In the box-shaped press-formed body of the obtained AZ91 alloy, the press-formed body of the AZ31 alloy produced separately, and the press-formed body of A5052, a test piece of 30 mm x 30 mm was cut out from the flat part, specifically, the upper plate part, without drawing deformation, The depression amount (mm) was measured using the jig shown in FIG. 3 similarly to a magnesium alloy plate.

"thickness"

In the box-shaped press-molded object of the obtained AZ91 alloy, thickness was measured about four arbitrary places of the said 30 mm x 30 mm test piece cut out from the upper plate part. As a result, all the points were equal to the thickness of the magnesium alloy plate (thickness of the test piece: 0.6 mm).

Table 1 shows the number of precipitates (pieces) and the amount of depression (mm). Moreover, about the sample a-d, the value of x = ^0.47xt-1.25 is shown in Table 1. The number of precipitates shows the smaller number in Table 2 in two observation visual fields.

It can be seen that a plate or a press-formed body made of a magnesium alloy having an Al content of 7% by mass or more is superior in impact resistance to a plate, a press-formed body made of an AZ31 alloy with a small Al content, or a plate or a press-formed body made of an aluminum alloy. .

As a result of observation of the metal structure, the samples a to d made of a magnesium alloy having an Al content of 7% by mass or more showed a large amount of precipitation of the intermetallic compound (Mg ₁₇ Al ₁₂ ) containing Al and Mg. However, as shown in Table 1, the samples a to c whose total time maintained at 250 ° C to 350 ° C in the rolling step is within 1 hour (60 min) are coarse with respect to both the magnesium alloy plate and the press-formed body. The intermetallic compound did not exist and had a structure in which the fine intermetallic compound was dispersed as shown in Fig. 1 (1). In addition, it is understood that the samples a to c having few such coarse precipitates have few depressions and are excellent in impact resistance characteristics. Even in the case of performing the final heat treatment after rolling, the sum of the holding time of the temperature range of 250 ° C to 350 ° C in the rolling step and the holding time of the temperature range of 250 ° C to 350 ° C in the final heat treatment after rolling By controlling it to be within 1 hour, it can be seen that the impact resistance is excellent.

[Test Example 2]

Magnesium alloy plates having different thicknesses and press molded bodies formed by press molding these magnesium alloy plates were fabricated, and metal structures and impact resistance characteristics were examined.

A plurality of casting plates (composition equivalent to AZ91 alloy, thickness 4mm) were prepared as in Test Example 1, and solution treatment (400 ° C for 24 hours and cooling rate to 350 ° C to 250 ° C) under the same conditions as in Test Example 1. : 0.1 degreeC / sec or more), several times rolling (rolling reduction rate: 5% / pass-40% / pass, heating temperature of board | plate: 200 degreeC-400 degreeC, roll temperature: 100 degreeC-250 degreeC), and it rolls Got a plate. This test also changed the total time which the board | plate material hold | maintains in the temperature range of 250 degreeC-350 degreeC in rolling process similarly to the test example 1. Moreover, in this test, the thickness of the rolled sheet was changed by adjusting the reduction ratio, and the total time was 35 minutes or 80 minutes by adjusting the heating time and rolling speed of the plate. In this test, samples for 45 minutes (sample α) and 90 minutes (sample β) were prepared including the time of the final heat treatment after rolling as the total time held in the temperature range.

After the final heat treatment of the obtained rolled sheet at 300 ° C. for 10 minutes, the angular drawing processing was performed at a heating temperature of 250 ° C. to obtain a box-shaped press molded body similar to that of Test Example 1.

About the magnesium alloy plate and press-formed body obtained by performing the said final heat processing, it carried out similarly to Test Example 1, and measured the number of precipitates by the structure observation of a cross section. Moreover, the test piece was produced like Example 1, the depression test was done, and the depression amount was measured. These results are shown in Table 2. In Table 2, the sample (0.6 mm) of thickness 0.6mm shows the result of the test example 1.

As shown in Table 2, although the amount of depression differs depending on the plate thickness of the magnesium alloy plate and the press-formed body (upper plate part), the sample α having a total holding time in the temperature range of 250 ° C to 350 ° C within 60 minutes in the rolling step is Regardless of the plate thickness, coarse intermetallic compounds having a particle diameter of 5 µm or more do not exist in the surface layer region (0 pieces), indicating that the amount of depression is smaller than that of the sample β having the same thickness.

In this manner was examined, a press molded product of plate thickness t _p between the depression amount x of (top plate) on a press formed article excellent impact resistance properties. The result is shown in FIG. From the graph shown in Figure 5, indicates the relationship between the thickness t _p and the depression amount x in the sample simply the α, x = k × t _p ^- can be considered a ¹ (k is a coefficient). The coefficient k for distinguishing between the sample α and the sample β is obtained by substituting a value of 0 to 1 in units of 0.01 in the range of the plate thickness t _p = 0.5 to 0.8, and it is considered that k = 0.5 and its vicinity are preferable. However, since the coefficient k tends to change slightly depending on the plate thickness, considering the case where the plate thickness t _p is less than 0.5 mm and more than 0.8 mm, the relational expression when k = 0.5 is x = 0.5 × t ^−1. The relationship which distinguishes from the sample (beta) in the range which the sample (alpha) does not deviate as much as possible from was reviewed. Specifically, it fixed at k = 0.5, substituted the exponent of plate | board thickness t _p by 0.01 unit, calculated | required the preferable curve, and obtained the coefficient k similarly to the above, As a result, x = 0.47 x t ^-1.25. Was saved. Therefore, this x <= 0.47 * t _p ^-1.25 is used as an index which shows the molded object of this invention. Similarly, as a result of investigating the magnesium alloy sheet, the magnesium alloy sheet also had x≤0.47 x t _b ^-1.25 (t _b). : Plate thickness) can be applied, and x <= 0.47 * t _b ^-1.25 is used as an index which shows the magnesium alloy plate of this invention.

[Test Example 3]

The magnesium alloy plate produced by changing the process performed after rolling was prepared, the press molding which press-molded on this magnesium alloy plate was produced, and the metal structure and impact resistance characteristic were investigated.

In this test, a plurality of cast plates (composition equivalent to AZ91 alloy, thickness 4 mm) were prepared in the same manner as in Test Example 1, and solution treatment (400 ° C for 24 hours and 350 ° C to 250 ° C) was performed under the same conditions as in Test Example 1. Cooling rate: 0.1 ° C / sec or more). The rolled plate was subjected to a plurality of rollings (rolling down rate: 5% / pass to 40% / pass, heating temperature of plate: 200 ° C to 280 ° C, roll temperature: 100 ° C to 250 ° C) to obtain a rolled plate. . In this test, the total time for which the plate was held in the temperature range of 250 ° C to 350 ° C in the rolling step was 45 minutes.

Warm correction process was performed to the obtained rolled sheet. Here, a warm calibration process is performed using the roll leveler apparatus provided with the heating furnace which can heat a rolled plate, and the roll part which has a some roll which continuously provides bending (distortion) to a heated rolled plate. The roll part includes a plurality of rolls arranged in a zigzag shape facing up and down.

By the said roll leveler apparatus, a rolling plate is sent to the said roll part, heating in the said furnace, and every time it passes between the rolls of the upper and lower sides of a roll part, bending is provided by these rolls sequentially. Here, the calibration was carried out in a temperature range of 220 to 250 ° C, and the conveyance speed and the like during the calibration were adjusted so that the total time for which the rolled plate was maintained at a temperature of 250 to 350 ° C was within 60 minutes (min).

The magnesium alloy plate obtained by performing the said warm calibration process was angular drawing process at 250 degreeC of heating temperature, and the box-shaped press molding body similar to the test example 1 was obtained.

About the obtained magnesium alloy plate and the press molded object, it carried out similarly to the test example 1, and measured the number of precipitates by the observation of the structure of a cross section. Moreover, the test piece was produced like Example 1, the depression test was done, and the depression amount was measured. These results are shown in Table 3.

As shown in Table 3, it turns out that all the samples have little depression amount and are excellent in impact resistance. In particular, Sample No. 3-1 using a magnesium alloy plate subjected to warm calibration after rolling was compared with Sample No. 2-1 (0.6 mm t-α of Test Example 2) subjected to final heat treatment after rolling. It can be seen that the amount of depression 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-formed body, and the like can be appropriately changed.

The magnesium alloy molded body of the present invention can be suitably used for parts of various electric devices, in particular, cases of portable electric devices. The magnesium alloy plate of this invention can be used suitably for the raw material of the magnesium alloy molded object of the said invention.

1 test piece 10 cylindrical rod
20: support 21: circular hole
d ₀ , d ₁ : intermetallic compound (precipitate)

Claims (9)

A magnesium alloy molded body formed by press molding a plate made of magnesium alloy,
The magnesium alloy contains 7% by mass or more and 12% by mass or less of Al,
The molded body has a flat portion that does not involve drawing deformation,
In the metal structure of the cross section which cut | disconnected the said flat part in plate | board thickness direction, the area | region from the surface of a flat part to 1/3 of plate | board thickness in plate | board thickness direction is used as surface layer area | region, and in arbitrary two places of the said surface layer area | region When the selected area of 100 μm × 100 μm is used as the observation field, and the particles having a particle diameter of 5 μm or more as coarse particles as particles of an intermetallic compound containing Al and Mg,
Magnesium alloy molded object characterized by the number of the said coarse particle which exists in each said observation visual field 5 or less. 2. The method of claim 1, wherein when performing the test of the recessed below with respect to the flat portion 30 ㎜ × 30 ㎜, test pieces having a thickness of t _p in the cut-out, depression amount x _p of the test piece is x _p ≤0.47 × t _p ^-1.25 Magnesium alloy molded body, characterized in that to satisfy.
(Depression test)
A test piece is arrange | positioned so that this hole may be closed in the support which has a hole of 20 mm in diameter, and in this state, a cylindrical rod of 100 g in weight and a tip r = 5 mm is free-falled at the point of 200 mm in height from a test piece.
The depression amount x _p is taken as the distance from the straight line connecting both sides of the test piece after the depression test to the most recessed part. 3. The magnesium alloy of claim 1, wherein the magnesium alloy includes at least one element selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare earth elements (excluding Y). Magnesium alloy molded body. The magnesium alloy molded body according to claim 3, wherein the magnesium alloy contains 8.3% or more and 9.5% or less of A1, and 0.5% or more and 1.5% or less of Zn in mass%. The magnesium alloy molded body according to claim 4, wherein an anticorrosive layer formed by chemical conversion treatment is provided on a surface of the plate made of the magnesium alloy. As a magnesium alloy plate used for press molding,
The magnesium alloy contains 7% by mass or more and 12% by mass or less of Al,
In the metal structure of the cross section which cut | disconnected the said alloy plate in the plate | board thickness direction, the area | region up to 1/3 of plate | board thickness from a plate surface to a plate | board thickness direction was made into the surface layer area | region, and 100 selected from arbitrary two places of the said surface layer area | region When a region of 100 µm x 100 µm is used as the observation field of view, and when particles having a particle diameter of 5 µm or more are used as coarse particles as particles of an intermetallic compound containing Al and Mg,
Magnesium alloy plate, characterized in that 5 or less of the coarse particles present in each observation field. 7. The method of claim 6, 30 ㎜ × 30 ㎜ cut from the alloy sheet, when performing the following dent test of a test piece having a thickness of t _b, depression amount x _b of the test piece is x _b ≤0.47 × t _b ^-1.25 Magnesium alloy plate, characterized in that to satisfy.
(Depression test)
A test piece is arrange | positioned so that this hole may be closed in the support which has a hole of 20 mm in diameter, and in this state, a cylindrical rod of 100 g in weight and a tip r = 5 mm is free-falled at the point of 200 mm in height from a test piece.
The depression amount x _b is taken as the distance from the straight line which connects both sides of the test piece after a depression test to the most recessed part. 8. The magnesium alloy of claim 6, wherein the magnesium alloy includes at least one element selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare earth elements (excluding Y). Magnesium alloy plate made. The magnesium alloy plate according to claim 8, wherein the magnesium alloy contains 8.3% or more and 9.5% or less of A1, and 0.5% or more and 1.5% or less of Zn in mass%.

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