JP2012140655A - Magnesium alloy sheet material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnesium alloy sheet material with excellent corrosion resistance.SOLUTION: The magnesium alloy sheet material comprises a magnesium alloy containing 7.3 mass%-16 mass% Al. When the Al content of the whole sheet material is x mass%, the area with the Al content of 0.8x mass%-1.2x mass% is 50 area% or more, the area with the Al content of 1.4x mass% or more is 17.5 area% or less and there is substantially no area with the Al content of 4.2 mass% or less. In the magnesium alloy sheet material, since the variation of Al concentration is low and there are few areas with extremely low Al content, the occurrence of localized corrosion and the progression of the corrosion are effectively prevented. Thus, the magnesium alloy sheet material has excellent corrosion resistance compared to a die cast material with the same overall Al content.

Description

  The present invention relates to a magnesium alloy plate material suitable for materials of various members such as casings for electric and electronic devices, automobile parts, and the like. In particular, the present invention relates to a magnesium alloy sheet having excellent corrosion resistance.

  Magnesium alloys containing various additive elements in magnesium have been used as constituent materials for various members such as casings of portable electric and electronic devices such as mobile phones and notebook personal computers and automobile parts.

  Main members made of magnesium alloys are die-cast materials and thixo-mold materials (ASTM standard AZ91 alloy). In recent years, a member obtained by pressing a plate made of a magnesium alloy for extension represented by ASTM standard AZ31 alloy is being used. Patent Document 1 proposes a magnesium alloy plate made of an alloy equivalent to the AZ91 alloy in the ASTM standard and having excellent press workability.

  Since magnesium is an active metal, corrosion resistance such as anodizing treatment or chemical conversion treatment is performed on the surface of the above-mentioned member or a magnesium alloy plate as a material thereof to enhance corrosion resistance.

JP 2007-098470 A

  Magnesium alloys containing Al such as the AZ31 alloy and AZ91 alloy described above tend to have better corrosion resistance as the Al content increases. For example, AZ91 alloy is said to be excellent in corrosion resistance among magnesium alloys. However, the above-described anticorrosion treatment is required even for members made of AZ91 alloy (mainly die-cast material or thixomold material). This is because even when a die-cast material made of AZ91 alloy or the like is not subjected to anticorrosion treatment, local corrosion may occur when a corrosion test is performed as described later. Therefore, further improvement in corrosion resistance is desired for the magnesium alloy plate material used as the material of the magnesium alloy member.

  Accordingly, an object of the present invention is to provide a magnesium alloy plate material having excellent corrosion resistance.

  As described above, the corrosion resistance can be improved as the Al content increases. Therefore, the present inventors made a plate material made of a magnesium alloy under various conditions and examined the corrosion resistance, targeting a magnesium alloy containing Al of 7.3% by mass or more. As a result, even when the Al content of the entire magnesium alloy sheet was the same, the corrosion resistance was superior. In order to elucidate the cause, first, the structure was examined. As a result, the magnesium alloy plate material with poor corrosion resistance was found to have coarse precipitates (based on additive elements in the alloy. Typically, at least one of Al and Mg was removed. In the magnesium alloy sheet having excellent corrosion resistance, fine precipitates are uniformly dispersed.

  Here, an additive element such as Al in the magnesium alloy exists mainly in at least one state of a precipitate (typically an intermetallic compound), a crystallized product, and a solid solution. When Al is used for precipitates or the like, the amount of Al in the matrix phase of the magnesium alloy constituting the precipitate and the region away from the surroundings decreases.

  The structure in which the coarse precipitate is present is a region where the Al concentration is higher than the surrounding area and the area of this high Al concentration portion is relatively large (mainly the region formed by the precipitate and the surrounding area). It can be said that this organization exists. In other words, it can be said that the region where the Al concentration is relatively low is a local and abundant tissue. And it is thought that corrosion tends to occur in each of the regions where the Al concentration is low, and local corrosion such as pitting corrosion occurs or proceeds.

  On the other hand, a structure in which fine precipitates exist uniformly can be said to be a structure in which the Al concentration is higher than that of the surroundings and a minute region consisting of this high Al concentration portion exists uniformly. In other words, it can be said that fine precipitates are uniformly dispersed and a large amount of Al remaining in the mother phase is uniformly dispersed. Since Al is uniformly dispersed, local corrosion as described above is unlikely to occur and progress, and the magnesium alloy sheet having such a structure is considered to be excellent in corrosion resistance.

  EPMA (Electron Probe Micro Analyzer) can be preferably used to analyze the Al concentration from the coarse area to the minute area as described above. Therefore, using the EPMA apparatus, as a result of analyzing the Al concentration for the magnesium alloy plate materials of the above various forms, as shown in the examples described later, the magnesium alloy plate material having excellent corrosion resistance is the Al content of the entire alloy plate material. When the amount is x mass%, the area of x mass% ± α occupies half of the area, where there are virtually no locations with very little Al content, and there are also places with very high Al content. We obtained knowledge that there was little target. That is, the present inventors have obtained the knowledge that it is possible to quantitatively specify that the corrosion resistance is excellent by using a parameter such as the area ratio of the Al concentration.

  The present invention is based on the above findings, and defines a magnesium alloy sheet having excellent corrosion resistance by the Al concentration and the area ratio.

The present invention relates to a magnesium alloy sheet made of a magnesium alloy containing Al in a range of 7.3 mass% to 16 mass%. This magnesium alloy sheet satisfies the following (1) to (3) when the content of Al in the entire magnesium alloy sheet is x mass%.
(1) Area where Al content is (x × 0.8) mass% or more and (x × 1.2) mass% or less is 50 area% or more
(2) Area where Al content is (x × 1.4) mass% or more is 17.5 area% or less
(3) There is substantially no region where the Al content is 4.2% by mass or less

  In the magnesium alloy sheet of the present invention, as described above, there is substantially no region inferior in corrosion resistance such as an Al content of 4.2% by mass or less, and a region having a high Al concentration (0.8x mass% to 1.2x mass%). In addition, more than half of the region) occupies more than half, and there are few regions with a very high Al concentration (regions of 1.4x mass% or more). That is, the magnesium alloy sheet material of the present invention can effectively prevent local corrosion because the region having a low Al concentration does not substantially exist. Further, the magnesium alloy sheet of the present invention has few or substantially no areas where the concentration of Al is very high (typically, precipitates containing Al etc. are fine and the total abundance is small (depending on the form) Is substantially not present)), and therefore, Al is sufficiently and widely dispersed in the magnesium alloy matrix itself. Thus, the magnesium alloy sheet of the present invention is in a state where the Al concentration is uniformly high over at least the entire surface side region. With this configuration, the magnesium alloy sheet of the present invention is excellent in corrosion resistance.

  As one embodiment of the present invention, a region where the Al content is (x × 0.8) mass% or more and (x × 1.2) mass% or less is 70 area% or more, and the Al content is (x × 1.4) mass% or more. In which the region is 5 area% or less.

  According to the above embodiment, a region with a high Al concentration (0.8x mass% to 1.2x mass% region) accounts for 70% or more, and a region with a very high Al concentration (region of 1.4x mass% or more) is very Therefore, Al is present more uniformly and is more excellent in corrosion resistance.

  As one form of the present invention, a form in which the region where the Al content is (x × 0.9) mass% or more and (x × 1.2) mass% or less is 50 area% or more can be mentioned.

  According to the said form, since the area | region (area | region of 0.9x mass%-1.2x mass%) with higher Al concentration occupies half or more and there are many area | regions which are excellent in corrosion resistance, it is excellent in corrosion resistance.

  As one form of this invention, the form manufactured by rolling the said magnesium alloy board | plate material at least 1 pass or more is mentioned.

  According to the said form, it is excellent in mechanical characteristics, such as tensile strength, elongation, rigidity, because it is a fine structure or an internal defect is reduced.

  The magnesium alloy sheet of the present invention is excellent in corrosion resistance.

FIG. 1 is a composition mapping image of a magnesium alloy material by FE-EPMA. FIG. 1 (A) is Sample No. 1 and FIG. 1 (B) is Sample No. 100: Die-cast material. Fig. 2 is a bar graph showing the relationship between the Al concentration and the area ratio (%) of the magnesium alloy material. Fig. 2 (A) shows sample No. 1 and Fig. 2 (B) shows sample No. 100: die casting. It is a material. 3 is a micrograph (magnification 5000) of the magnesium alloy material, FIG. 3 (A) is Sample No. 1, and FIG. 3 (B) is Sample No. 100: Die-cast material. FIG. 4 is a graph showing the relationship between the Al concentration (mass%) by ICP emission spectroscopic analysis and the X-ray intensity by EPMA in magnesium alloy sheet materials having different Al contents.

Hereinafter, the present invention will be described in more detail.
[Magnesium alloy sheet]
(composition)
Examples of the magnesium alloy constituting the magnesium alloy sheet of the present invention include those having various compositions containing an additive element in Mg (remainder: Mg and impurities, Mg: 50% by mass or more). In particular, in the present invention, a high-concentration alloy containing 7.3% by mass or more of an additive element, particularly an Mg—Al alloy containing at least Al as an additive element is used. The higher the Al content, the better the corrosion resistance and the mechanical properties such as strength and hardness. Therefore, a high-concentration alloy having an Al content of 7.3% by mass or more as in the present invention is superior in corrosion resistance and mechanical properties as compared with an alloy having a low Al content. However, if the Al content exceeds 16% by mass, the plastic workability is lowered, so the upper limit is 16% by mass. The Al content is preferably 12% by mass or less, more preferably in terms of plastic workability, particularly 11% by mass or less, and more preferably 8.3% by mass to 9.5% by mass.

  Additive elements other than Al were selected from Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag, Sn, Li, Zr, Ce, Ni, Au, and rare earth elements (excluding Y and Ce) One or more elements are listed. When these elements are included, the content of each element is 0.01% by mass or more and 10% by mass or less, and preferably 0.1% by mass or more and 5% by mass or less. Of the above additive elements, at least one element selected from Si, Ca, Sn, Y, Ce, and rare earth elements (excluding Y and Ce) is 0.001% by mass or more in total, preferably 0.1% by mass in total. When it is contained in an amount of 5% by mass or less, heat resistance and flame retardancy are excellent. When the rare earth element is contained, the total content is preferably 0.1% by mass or more, and particularly when Y is contained, the content is preferably 0.5% by mass or more. Examples of the impurity include Fe.

  More specific compositions of Mg-Al alloys include, for example, AZ alloys (Mg-Al-Zn alloys, Zn: 0.2% to 1.5% by mass), AM alloys (Mg-Al-Mn) according to ASTM standards. Alloy, Mn: 0.15 mass% to 0.5 mass%), Mg-Al-RE (rare earth element) alloy, AX alloy (Mg-Al-Ca alloy, Ca: 0.2 mass% to 6.0 mass%), AJ Alloy (Mg—Al—Sr alloy, Sr: 0.2 mass% to 7.0 mass%) and the like. In particular, an Mg-Al alloy containing 8.3 mass% to 9.5 mass% Al and 0.5 mass% to 1.5 mass% Zn, typically AZ91 alloy, is preferable because of its excellent corrosion resistance and mechanical properties.

  In the present invention, the content of Al in the entire magnesium alloy sheet (hereinafter referred to as Al total average amount): x mass% is the presence state of Al in the magnesium alloy sheet (mainly precipitates, crystallized substances, and solid solutions). Regardless of (at least one), it means the total amount of Al contained in the magnesium alloy sheet. For the measurement of the total amount, typically, ICP emission spectroscopy (ICP-AES) can be suitably used.

(Al concentration and area ratio (area ratio))
The most characteristic feature of the magnesium alloy sheet of the present invention is the Al concentration distribution. Specifically, when the Al concentration is analyzed with respect to the surface of the alloy plate material, (1) the area where the Al content is the Al total average amount (x mass%) ± 20% occupies the majority (however, 7.3 ≦ x ≦ 16). The area below 0.8x mass% (minimum 5.84 mass%) is inferior in corrosion resistance, and the area above 1.2x mass% (maximum 19.2 mass%) has high corrosion resistance in this area itself. The presence of the concentrated concentration makes it relatively easy to have a region inferior in corrosion resistance. On the other hand, the region of 0.8x mass% to 1.2xmass% (hereinafter, this region is referred to as the central composition region) has a small difference in Al concentration, and the area where such Al concentration is uniform is 50 areas. % Or more, it is difficult for a location where the difference in Al concentration is large, that is, a region of less than 0.8x mass% and a region of more than 1.2x mass% as described above. Therefore, the magnesium alloy sheet material of the present invention has few or substantially no areas inferior in corrosion resistance, and at least the surface side area of the alloy sheet material is constituted by an area where the Al concentration is relatively high, thereby locally Corrosion hardly occurs and has excellent corrosion resistance. The higher the area ratio of the central composition region, the wider the region where the Al concentration is uniform as described above, and the Al concentration tends to be uniform. That is, the Al concentration distribution width tends to be narrow. Accordingly, the area ratio of the central composition region is preferably 55 area% or more, particularly 70 area% or more, and more preferably 90 area% or more. In addition, when the area where the Al concentration is higher, specifically, the area of 0.9x mass% to 1.2x mass% is 50 area% or more, the Al concentration is high and the high concentration area exists uniformly. Therefore, it is more excellent in corrosion resistance. Details of the Al concentration measurement method and the area ratio measurement method will be described later.

  The Al concentration can be measured at an arbitrary cross section along the thickness direction of the magnesium alloy sheet, and can be performed at an arbitrary position of the cross section. The region most involved in corrosion is the surface of the alloy sheet. Therefore, at least the surface of the magnesium alloy sheet of the present invention satisfies the Al concentration distribution defined above. In addition to the form in which the Al concentration distribution inside the magnesium alloy sheet (for example, the region exceeding 1/4 of the thickness in the thickness direction from the surface) is similar to the Al concentration distribution on the surface, in the present invention, the internal Al concentration The distribution allows a different form from the surface Al concentration distribution.

  In the magnesium alloy sheet of the present invention, (2) there are few regions where the Al content is Al total average amount (x mass%) × 140% or more (provided that 7.3 ≦ x ≦ 16). In the region of 1.4x mass% (maximum 22.4 mass%) or more, although this region itself has high corrosion resistance, there is a region where Al concentration is relatively low and the corrosion resistance is inferior due to the concentration of Al in this region. It becomes easy to do. On the other hand, in the magnesium alloy sheet of the present invention, since the area of 1.4x% by mass or more (hereinafter, this area is referred to as an ultra-high concentration area) is as small as 17.5 area% or less, there is hardly any area inferior in corrosion resistance. Excellent corrosion resistance. As the area ratio of the ultra-high concentration region is lower, there are fewer regions with a relatively low Al concentration, and the region with poor corrosion resistance can be reduced. That is, the Al concentration distribution width tends to be narrow. Accordingly, the area ratio of the ultra-high concentration region is preferably 15 area% or less, more preferably 14 area% or less, and particularly preferably 5 area% or less.

  Further, in the magnesium alloy sheet of the present invention, (3) a region where the Al content is 4.2% by mass or less (hereinafter, this region is referred to as a low concentration region), that is, a region inferior in corrosion resistance as described above is substantially present. not exist. When there is a location where the Al content is relatively high, corrosion occurs preferentially at a relatively low location or the corrosion proceeds. On the other hand, the magnesium alloy sheet material of the present invention is excellent in corrosion resistance because there are substantially no places where the Al concentration is extremely low, that is, places where corrosion is likely to occur or where corrosion is likely to proceed. “Substantially absent” means that no point of 4.2 mass% or less is observed by EPMA measurement.

(Organization)
The magnesium alloy sheet of the present invention has Al ₁₂ Mg ₁₇ , and depending on the additive element, there are Al-rich precipitates represented by Al-rich intermetallic compounds such as Al ₂ Ca, Al ₄ Ca, Al ₃ Ni, etc. The intermetallic compound is small (average particle size: 3.0 μm or less, preferably 0.5 μm or less) and has a uniformly dispersed structure (total area ratio: 11% or less). The presence of an intermetallic compound uniformly can be expected to function as a barrier against corrosion. The magnesium alloy sheet of the present invention is excellent in corrosion resistance because the area ratio of the ultra-high concentration region satisfies a specific range and the low concentration region does not substantially exist even when the intermetallic compound is present. .

(Form)
As a form of the magnesium alloy sheet material of the present invention, a rolled material obtained by rolling an extruded material or a cast material, a straightened material obtained by further straightening the rolled material, and a heat treatment for further removing strain on the rolled material. Examples include heat-treated materials, rolled materials, straightening materials, and abrasives obtained by grinding heat-treated materials. Furthermore, a processed material obtained by subjecting a plate material in any form of a rolled material, a straightening material, a heat treatment material, and an abrasive material to machining such as cutting and punching.

  Rolled material has crystal grains refined by rolling.For example, the average crystal grain size is 10 μm or less, further 5 μm or less, and there are few, small, or substantially internal defects such as voids (nests). Or a structure that does not exist in the structure (a structure in which the density actually measured with respect to the theoretical density material calculated from the material composition is 99% or more). Having such a fine structure and a structure having a high density can be an index indicating that the material is a rolled material. In addition, since the rolled material has few or no internal defects as described above, and preferably is substantially non-existent, it has excellent mechanical properties such as tensile strength, elongation, and rigidity. It can be suitably used for these materials.

  For correction, for example, leveler roll processing or the like can be used. Depending on the degree of straightening, a straightening material by leveler roll processing may have a structure in which a clear grain boundary is difficult to observe even when microscopic observation is performed by introducing a shear band. In this case, since the monochromatic X-ray diffraction peak can be obtained, it is a non-amorphous structure, the monochromatic X-ray diffraction peak can be obtained, and it has a structure in which grain boundaries cannot be observed. It can be an index indicating that Correction materials, especially correction materials by leveler roll processing, tend to recrystallize during plastic processing such as press processing, and tend to be excellent in plastic workability. When the degree of correction is low, the appearance, structure, and mechanical properties may be similar to the rolled material.

  The abrasive has a smooth surface and excellent surface properties. Therefore, a small surface roughness (for example, a maximum height Rz of 20 μm or less) or the presence of polishing traces is an indicator of an abrasive.

  Although the heat treatment material depends on the heat treatment conditions, for example, (1) no shear band is observed inside the magnesium alloy sheet, (2) the proportion of particles having a crystal grain size of 0.1 μm or less in the cross section is 5 area% or less It can be considered that this can be an index indicating that it is a heat treatment material.

  The magnesium alloy sheet of the present invention is typically composed of parallel front and back surfaces that are substantially flat and side surfaces that connect the front and back surfaces, and the distance (= thickness) between the front and back surfaces over the entire surface. It is a substantially uniform shape. When viewed in plan, this plate material is typically rectangular, and by cutting or punching, various shapes, for example, circular, elliptical, polygonal, through holes (window Various planar shapes such as a form including a large one). In addition, by using a deformed roller or the like as will be described later, a form having a part having a different thickness, such as a form having a concave groove in part or a form having a rib (convex part) in part, is included.

  The thickness, width and length of the magnesium alloy sheet of the present invention can be appropriately selected. For example, a thickness of 5 mm or less, particularly 2 mm or less, further 1.5 mm or less, especially 1 mm or less, a width of 100 mm or more, further 200 mm or more, especially 250 mm or more. Such a thin-walled material or a wide-width material is suitable as a material for plastic working material as described above. In addition, a thin-walled material having a thickness of 2 mm or less can be suitably used as a material for a thin and lightweight plastic working material. However, the thickness is preferably 0.1 mm or more, and 0.3 mm to 1.2 mm is easy to use.

  Since the magnesium alloy sheet according to the present invention is excellent in corrosion resistance, it is expected that the magnesium alloy sheet can be sufficiently used depending on the corrosive environment even if it is not subjected to anticorrosion treatment such as chemical conversion treatment or anodizing treatment. In this case, the anticorrosion treatment process can be reduced, the productivity of the magnesium alloy sheet can be increased, and the waste can be reduced, so that it is expected that the environmental load can be reduced. Of course, the magnesium alloy sheet material of the present invention can be in a form in which an anticorrosion treatment such as a chemical conversion treatment or an anodizing treatment is performed, that is, a form having an anticorrosion layer. When the surface is provided with an anticorrosion layer, the Al concentration can be measured by removing the anticorrosion layer by polishing or cutting and exposing the surface of the base material made of a magnesium alloy without performing high-precision cross-sectional observation. When the coating layer is provided in addition to the anticorrosion layer, the corrosion resistance can be further improved, and the commercial value can be improved by coloring or applying a pattern. An anticorrosion layer and a coating layer are good to apply to a desired location.

[Production method]
As a manufacturing method of this invention magnesium alloy board | plate material, the manufacturing method provided with the following preparatory processes, an intermediate solution forming process, and a rolling process is mentioned, for example.
Preparation step: a step of preparing a cast material made of a magnesium alloy containing Al of 7.3 mass% to 16 mass% and manufactured by a continuous casting method.
Intermediate solution treatment step: A step of producing an intermediate solution material by subjecting the cast material to a solution treatment of holding temperature: not less than the minimum holding temperature below and holding time: not less than 1 hour and not more than 25 hours.
Minimum holding temperature: Temperature that is 10 ° C lower than the temperature at which Al dissolves in Mg (solidus temperature) in the Mg-Al binary phase diagram (mass%) Rolling process: More than 1 pass to the intermediate solution material A process of producing rolled material by performing warm rolling.
In particular, in the manufacturing process after the intermediate solution process, the total time for maintaining the material to be processed (typically rolled material) in a temperature range of 150 ° C. or higher and 300 ° C. or lower within 12 hours, and 300 The thermal history of the material is controlled so that it is not heated to a temperature higher than ° C.
And, in the rolling process, after the final warm rolling, the average cooling rate from the temperature of the material when starting cooling to the temperature of the material becomes 100 ° C or less is 0.8 ° C / min or more And

In the binary phase diagram (mass%) of the minimum holding temperature: Mg—Al, the temperature 10 ° C. lower than the solidus temperature is typically expressed as follows. Al total average amount in magnesium alloy: When x mass% is 5 mass% or more and 13 mass% or less, the solidus temperature is 283 ° C to 437 ° C, and the solidus temperature increases with the increase of Al total average amount The minimum holding temperature is expressed by the following linear expression.
(Formula) (Minimum holding temperature) = 20 × x + (182-10) = 20x + 172
On the other hand, when the Al total average amount is more than 13 mass% and 16 mass% or less, the minimum holding temperature is (437-10) ° C. = 427 ° C.

In the above production method, in particular, in the steps after the intermediate solution treatment, preferably until the final product, the total time for keeping the material in the temperature range of 150 ° C. to 300 ° C. is shortened to 12 hours or less. The temperature range of 150 ° C. to 300 ° C. is a temperature range in which an Al-rich intermetallic compound such as Al ₁₂ Mg ₁₇ is easily grown. By setting the holding time in this temperature range to a relatively short time as described above, particularly the growth of the intermetallic compound is suppressed, and the increase in the ultrahigh concentration region and the low concentration region is suppressed. And, when the cooling state is adjusted so that the specific cooling rate is at least 100 ° C. in the cooling step so that Al diffusion does not substantially occur after the final warm rolling, the intermetallic compound Growth can be suppressed, and an increase in the ultra-high concentration region and the low concentration region can be suppressed. Increasing the cooling rate is preferable because the increase in the ultrahigh concentration region and the low concentration region can be suppressed. Furthermore, the growth of the intermetallic compound is also suppressed by preventing the heating above 300 ° C.

  The rolled material manufactured by the above-described manufacturing method can be subjected to a final heat treatment for the purpose of removing distortion. That is, when the magnesium alloy sheet material of the present invention is used as a heat treatment material that has undergone a rolling process, it is manufactured by a manufacturing method that includes a final heat treatment process, which will be described later, in addition to the preparation process, intermediate solution forming process, and rolling process described above. it can.

  Alternatively, the rolled material manufactured by the above-described manufacturing method is subjected to correction (typically warm correction) for the purpose of increasing straightness and the like, or improved surface properties (oxidized layer, surface defect, rolling) For example, cleaning or polishing can be performed for the purpose of removal of the lubricant used in the above. That is, when the magnesium alloy plate material of the present invention is used as an orthodontic material or an abrasive, it can be produced by a production method including at least one of an orthodontic process and a polishing process described later in addition to a preparation process, an intermediate solution forming process, and a rolling process. .

  In the manufacturing method comprising the above preparation step, intermediate solution forming step, rolling step, and this manufacturing method further comprising at least one step selected from a final heat treatment step, a correction step, a polishing step and a washing step, In addition to a long plate material, a plate material having a predetermined length (a short plate material considered to be difficult to wind (for example, a length of 5 m or less, particularly 1 m or less), hereinafter referred to as a sheet material) is obtained.

  The sheet material is obtained, for example, by cutting the cast material into a predetermined length in the preparation step to obtain a cast material (cast plate) having a predetermined length, and using the cast material as a raw material through subsequent steps. Alternatively, a sheet material can be obtained by winding a long cast material in the preparation step to produce a cast coil material, producing a coil material in each step, and finally cutting it to a predetermined length. When using a coil material as a raw material, in each process after a preparatory process, coil | winding and winding-up of a coil material are mostly performed. When a coil material is used as a material, a large amount of material can be transferred or heated at a time, and processing of each process can be performed continuously. Therefore, the productivity of a magnesium alloy sheet is excellent. The magnesium alloy sheet of the present invention can be produced by using either a sheet material or a coil material as a material for each step.

In addition, when performing plastic working on the obtained magnesium alloy sheet material of the present invention, the following conditions are preferable.
Plastic processing step: Pre-heating the obtained plate material at a holding temperature: 350 ° C. or less (preferably 300 ° C. or less) and a holding time: 8 hours or less (preferably 0.5 hour or less), and plastic processing is performed on this heated plate material The process of applying.

Hereinafter, each process of a manufacturing method is demonstrated in detail.
≪Preparation process≫
The cast material preferably uses a continuous casting method. The continuous casting method can stably obtain a cast material of uniform quality in the longitudinal direction and can be rapidly solidified, so it can reduce oxides and segregation, and can also be the starting point of cracking during rolling. As a result, it is possible to suppress the formation of coarse crystal precipitates exceeding 10 μm that can be obtained, and a cast material excellent in plastic workability such as rolling can be obtained. In particular, the twin roll continuous casting method is easy to form a plate-shaped cast material with little segregation. The cross-sectional area, thickness, width, and length of the cast material are not particularly limited, but segregation is likely to occur if it is too thick. Therefore, the thickness is preferably 10 mm or less, more preferably 7 mm or less, and particularly preferably 5 mm or less. In addition, if a long cast material with a length of 30 m or more, 50 m or more, especially 100 m or more, or a wide cast material with a width of 100 mm or more, 250 mm or more, especially 600 mm or more is used as the material of the rolled material, the long Simple rolled plates and wide rolled plates can be produced. The cast material may be a cast coil material wound in a coil shape, or may be a cast sheet material cut into a predetermined length, and may be appropriately selected according to a desired form. When the inner diameter of the cast coil material is small when winding it in a coil shape, it can be wound up without cracking when it is heated to 150 ° C or more just before winding the cast material, and the cast coil material can be easily Can be made.

≪Intermediate solution process≫
The above cast material is subjected to an intermediate solution treatment to homogenize the composition, and by dissolving an element such as Al, the presence of coarse precipitates can be reduced, and a material excellent in plastic workability such as rolling can be obtained. be able to. The holding temperature of the intermediate solution treatment is typically 350 ° C. or higher and 450 ° C. or lower, particularly 380 ° C. or higher, and more preferably 390 ° C. or higher and 420 ° C. or lower. The holding time is 1 hour to 25 hours, particularly 10 hours to 25 hours. The holding time is preferably increased as the Al content increases. Further, in the cooling step from the holding time, if the cooling rate is increased by using forced cooling such as water cooling or blast, as in the cooling step after the final warm rolling described later (preferably 1 ° C./min or more) More preferably, it is 50 ° C./min or more), which is preferable because growth and precipitation of precipitates can be suppressed. In particular, when a cast sheet material is used, it is easy to control the cooling rate.

  The cast material may be subjected to an intermediate solution treatment as it is, but before the intermediate solution treatment is performed, rolling with a small reduction ratio (a reduction ratio of about 1% / 1 pass to 15% / 1 pass) Alternatively, surface grinding may be performed.

≪Rolling process≫
When rolling a magnesium alloy, if the temperature of the material is room temperature, it is difficult to increase the rolling reduction, resulting in a decrease in production efficiency. Therefore, considering productivity, at least one pass may be warm-rolled. preferable. Heating the material (intermediate solution material or rolled material during rolling) can improve the plastic workability such as rolling, and the higher the material temperature, the more the plastic workability can be improved. Precipitates such as intermetallic compounds are coarsened, leading to an increase in the ultra-high concentration region and the low concentration region, and a coarse precipitate causes a decrease in plastic workability. Accordingly, the temperature of the material is preferably 300 ° C. or lower, particularly 150 ° C. or higher and 280 ° C. or lower. The material can be heated using a heating means such as an atmospheric heating furnace by providing a preheating step. As the heating furnace, an appropriate one that can store a material can be used.

  In particular, when rolling the cast sheet material to produce a rolled material (sheet material), and the obtained rolled material is the magnesium alloy sheet material of the present invention having the specific Al concentration distribution, the holding temperature of the preheating step It is preferable to shorten the holding time. Here, as described above, when the material is controlled so as to be as short as possible (preferably within 12 hours) when the material is maintained in a specific temperature range: 150 ° C. to 300 ° C. at the time of rolling, precipitates ( In particular, the growth of the Al-rich intermetallic compound) can be effectively suppressed, and an increase in the ultra-high concentration region and the low concentration region can be prevented. As a method for shortening the preheating time, for example, an in-line heating device (typically a heating device using radiant heat, current heating, induction heating, etc.) is provided immediately before the rolling device to perform rapid heating. Is mentioned. By making it in-line, the time until rolling after heating can be shortened. In addition, as a method for shortening the time during which the material is maintained at 150 ° C. to 300 ° C., after passing through a rolling device (typically, a rolling roller), the rolled material is rapidly cooled by being immersed in a refrigerant or a lubricant. (Preferably, cooling rate: 1 ° C./sec or more). When both the rapid heating and the rapid cooling are performed, the time during which the material is held at 150 ° C. to 300 ° C. in the rolling process can be effectively shortened. In particular, the rapid heating and rapid cooling can be easily performed when the material to be rolled is a short material such as a cast sheet material. In addition, for example, even when preparing and stacking a plurality of materials and heating them at once, the time for heating each material to a uniform temperature is relatively short by providing an appropriate gap between the materials. it can. This technique can also be easily applied when the material to be rolled is a short material such as a cast sheet material. In producing the magnesium alloy sheet of the present invention, when performing warm rolling for 1 pass or more, the total holding time in the preheating before rolling may be 0.01 hours or more and 8 hours or less, particularly 0.01 hours or more and 0.3 hours or less. preferable. By controlling the preheating conditions in this manner, a magnesium alloy plate material that is substantially free of precipitates and has a narrower Al concentration distribution width, that is, a magnesium alloy plate material that is superior in corrosion resistance can be obtained.

  The rolling including the warm rolling may be performed in one pass or multiple passes. By rolling a plurality of passes, a rolled material with a small thickness is obtained, and the average crystal grain size of the structure constituting the rolled material is reduced (for example, 10 μm or less, preferably 5 μm or less), or press working. It is possible to improve the plastic workability. The number of passes, the reduction rate of each pass, and the total reduction rate can be appropriately selected so that a rolled material having a desired thickness can be obtained. In addition, known rolling conditions, for example, appropriate conditions such as heating not only the raw material but also the rolling roll may be used.

  In particular, in producing the magnesium alloy sheet of the present invention, in the cooling step after the final warm rolling, the average cooling from the temperature of the material at the start of cooling until the temperature of the material reaches at least 100 ° C. The speed is preferably 0.8 ° C./min or higher. By rapidly cooling the material after the final warm rolling, it is possible to effectively prevent the precipitate from growing during cooling and increasing the ultra-high concentration region and the low concentration region. The average cooling rate is, for example, the temperature of the raw material when starting cooling after the final warm rolling, and the time t (min) from the obtained measurement temperature: Tmp (° C.) to 100 ° C. Is set to a speed represented by (Tmp-100) / t (° C./min). The cooling state may be adjusted so that (Tmp-100) / t (° C./min)≧0.8 (° C./min). The material temperature may be measured using either a contact sensor such as a thermocouple or a non-contact sensor such as a thermography. It is recommended to prepare an extremely thin thermocouple and place it on the surface of the material.

  The cooling rate is preferably as high as possible, and is preferably 1 ° C./sec or more, and more preferably 5 ° C./sec or more. In the cooling step, any cooling means that can achieve the cooling rate can be used. In particular, if forced cooling is used, the cooling rate can be increased. There are various forced cooling means, such as those that use a gaseous medium such as a fan (air cooling) or blast (jet air cooling), those that use a liquid medium such as water cooling, and those that use a solid medium such as a cooling roll. Can be used. In particular, when air cooling such as blast is used, it is possible to obtain an effect that a step of removing the liquid refrigerant adhering to the material is unnecessary, and the surface properties are not deteriorated due to the adhesion of the liquid refrigerant. On the other hand, when the liquid refrigerant is used, it is easy to increase the cooling rate. It is preferable to use a liquid refrigerant that includes a cleaning agent (for example, a surfactant) that can remove the lubricant used in rolling or the like because cooling can be performed together with cooling. The forced cooling means may be arranged off-line, but if arranged in-line, a large contact area between the material surface and the cooling medium can be secured, so that the cooling efficiency can be improved. In the case of a coil material, after the final warm rolling, the cooling may be performed after winding. In the case of a coil material, the cooling may be performed in a wound state, but if it is performed in a rewinded state, the cooling rate can be easily increased. If the cooling rate can be achieved, natural cooling may be performed without using the forced cooling means.

  In addition, when performing rolling with a small rolling reduction by finish rolling etc., it can be set as cold work. In the cold working, the Al concentration is hardly changed, and the Al concentration distribution before the cold working is substantially maintained.

  When rolling a plurality of passes, an intermediate heat treatment can be performed between passes within a range in which the holding time in the temperature range of 150 ° C. to 300 ° C. described above is included in the total time. By the intermediate heat treatment, strain, residual stress, texture, etc. introduced into the material by plastic working (mainly rolling) up to the heat treatment can be removed and reduced. By carrying out like this, careless cracking, distortion, and a deformation | transformation can be prevented by the rolling after the said heat processing, and rolling can be performed more smoothly. The holding temperature of the intermediate heat treatment material is also preferably 300 ° C. or lower. The holding temperature is preferably 150 ° C. or higher, particularly 250 ° C. or higher and 280 ° C. or lower. The holding time is, for example, about 0.5 hour to 3 hours. Also in the cooling step after the intermediate heat treatment, it is preferable to increase the cooling rate (preferably 1 ° C./min or more, more preferably 50 ° C./min or more) because the growth of precipitates can be suppressed.

  As described above, the thickness, width, and length of the rolled material can be appropriately selected. Moreover, when the said rolling uses suitably a lubricant, the frictional resistance at the time of rolling can be reduced, the burning of a raw material etc. is prevented and it is easy to perform rolling. Furthermore, when a roll having a groove on the outer periphery of the roll is used as a rolling roll, a rolled material having a groove can be manufactured by using a rolled material having a rib, and a roller having a protrusion on the outer periphery of the roll. In addition, the obtained rolled material can be cut or ground to form a desired uneven shape or stepped shape, or a boss or a through hole can be formed.

≪Final heat treatment≫
When the final heat treatment is performed after rolling, the holding temperature is preferably 300 ° C. or lower. More specific conditions include a holding temperature: 100 ° C. to 300 ° C. and a holding time: 5 minutes to 60 minutes. The time for holding the material (rolled material) in the temperature range of 150 ° C. or higher and 300 ° C. or lower in the final heat treatment step is preferably included in the total time, and the holding time is preferably less than 30 minutes. By having such a specific condition, a rolled material having a specific Al concentration distribution that satisfies the above conditions (1) to (3), and reduced or removed distortion during rolling, etc. can do.

≪Correction≫
By performing correction after rolling or after the final heat treatment, the flatness of the plate material can be improved. The correction can be carried out at room temperature or a temperature below room temperature, but if it is carried out warm, the flatness can be further improved. When performing warm correction, the holding temperature is preferably 300 ° C. or lower. More specific conditions include a holding temperature: 100 ° C. or higher and 300 ° C. or lower, preferably 150 ° C. to 280 ° C. It is preferable that the total time includes the time for holding the material (for example, rolled material) in the temperature range of 150 ° C. or more and 300 ° C. or less in the straightening process. Warm correction is, for example, a heating furnace capable of heating a material, and a roll unit in which a plurality of rolls are arranged in a staggered manner so as to bend (strain) the heated material continuously. A roll leveler device comprising: Even after warm straightening, if the average cooling rate from the temperature of the material at the start of cooling to the temperature of the material reaches 100 ° C or less is 0.8 ° C / min or more, it is extremely high due to the growth of precipitates. An increase in the amount of precipitates such as an increase in the concentration region and the low concentration region and an Al-rich intermetallic compound can be effectively suppressed. In order to achieve this cooling rate, the forced cooling means may be appropriately used as described above, or natural cooling may be used.

≪Plastic processing≫
When plastic working such as press working is applied to the magnesium alloy sheet of the present invention, the plastic workability can be improved by heating the material. The temperature of the material is preferably 350 ° C. or lower, more preferably 300 ° C. or lower, and particularly preferably 280 ° C. or lower. In particular, 150 ° C. or higher and 280 ° C. or lower, and more preferably 150 ° C. or higher and 220 ° C. or lower are preferable. And, when the material is preheated to the temperature, by keeping the holding time to 8 hours or less as described above, the growth of precipitates is suppressed, and the increase of ultra-high concentration region and low concentration region is effectively prevented. it can. If the material is heated to such an extent that desired plastic working is possible, the holding time is preferably as short as possible, 0.5 hours or less (30 minutes or less), and more preferably 0.3 hours or less. Although the time during plastic working such as press working itself depends on the shape, it is considered to be as short as several seconds to several minutes in press working, and it is considered that defects such as coarsening of precipitates do not substantially occur. By using the magnesium alloy sheet material of the present invention as a raw material and performing plastic working under the above-mentioned specific conditions, a plastic processed material having a specific Al concentration distribution that satisfies the above conditions (1) to (3) can be obtained.

  Heat treatment can be performed after the plastic working to remove strain and residual stress introduced by the plastic working and improve mechanical characteristics. The heat treatment conditions include a holding temperature: 100 ° C. to 300 ° C. and a holding time: about 5 minutes to 60 minutes. However, also in this heat treatment, it is preferable that the holding time in the temperature range of 150 ° C. to 300 ° C. is included in the total time.

≪Total time to keep the material in a specific temperature range≫
After the intermediate solution treatment as described above, preferably in the process up to the final product (rolling (including intermediate heat treatment), final heat treatment, straightening, preheating before plastic working, etc.), the material is 150 ° C ~ 300 It is preferable to control the total time kept in the temperature range of ° C to a relatively short time of 12 hours or less.

  In order to ensure a sufficient heating time for plastic working such as rolling, the total time of maintaining the temperature range of 150 ° C. to 300 ° C. is preferably 0.01 hours or more. More preferably, the temperature range: 150 ° C. or more and 280 ° C. or less, more preferably 150 ° C. or more and 220 ° C. or less, the total time: 8 hours or less, particularly 0.3 hours or less, the processing degree of each pass in the rolling process Control manufacturing conditions such as total processing degree, preheating conditions (such as preheating means and time), cooling process conditions (such as cooling means and time), and line speed. Moreover, since the Al-rich intermetallic compound described above tends to precipitate as the Al content increases, the total time is preferably adjusted according to the Al content.

  As described above, it is preferable not to heat to more than 300 ° C. after the intermediate solution treatment, but it should be a short time (preferably 8 hours or less, more preferably 1 hour or less) that does not cause coarsening of precipitates. If allowed.

  As a more specific manufacturing method, for example, casting → intermediate solution (preferably, cooling rate is controlled in the cooling step) → rolling → intermediate heat treatment (cooling rate is controlled in the cooling step depending on the holding temperature) → rolling → correction / A process of polishing / cleaning may be mentioned. According to this manufacturing method, by performing an intermediate solution treatment before rolling, precipitates can be made fine and minimal, and subsequent rolling can refine the structure and improve mechanical properties. .

≪Other processes≫
By providing a polishing step for polishing (preferably wet polishing) the rolled material, heat-treated material, and straightening material obtained by the above-described manufacturing method, a specific condition that satisfies the above-mentioned conditions (1) to (3) is satisfied. An abrasive having an Al concentration distribution (one form of the magnesium alloy sheet of the present invention) is obtained. In addition, the above-described manufacturing method further includes a step of performing an anticorrosion treatment such as a chemical conversion treatment or an anodizing treatment, or a step of forming a coating layer, thereby satisfying the above conditions (1) to (3). A magnesium alloy sheet material of the present invention comprising a base material having a specific Al concentration distribution and an anticorrosion layer or a coating layer formed on the base material is obtained. Known materials and conditions can be used for the material and forming conditions of the anticorrosion layer and the coating layer. In the anticorrosion treatment, pretreatment such as degreasing, acid etching, desmutting, and surface adjustment is preferably performed. In the case of performing plastic working, if the anticorrosion layer and the coating layer are formed after the plastic working, damage to the anticorrosion layer and the coating layer during the plastic working can be prevented.

Hereinafter, more specific embodiments of the present invention will be described.
[Test Example 1]
Magnesium alloy sheets containing Al were prepared, and the Al concentration distribution and corrosion resistance were investigated.

  In this test, a magnesium alloy plate material of sample No. 1 manufactured as follows and a commercially available die-cast material (AZ91 alloy, plate material having a thickness of 3 mm, a width of 75 mm, and a length of 150 mm) were prepared for comparison. This die-cast material was subjected to wet belt polishing under the same conditions as the polishing treatment applied to Sample No. 1 described later to produce a polishing plate. This polishing plate was designated as Sample No. 100.

The manufacturing process of Sample No. 1 is shown below.
Sample No.1: Sheet material * No winding on the way
Casting (cutting after casting) → Intermediate solution → Rolling → Straightening → Polishing

<Sample No. 1>
Casting plate (thickness 5mm, width 300mm, length 500mm) made of a magnesium alloy with a composition equivalent to AZ91 alloy (Mg-8.75% Al-0.65% Zn (all mass%)) A plurality of cast sheet materials) were prepared. Each cast plate obtained was charged into a batch heating furnace and subjected to a solution treatment (intermediate solution treatment) at 400 ° C. (≧ 20 × 8.75 + 172 = 347 ° C.) × 24 hours. While cutting for adjustment, a plurality of passes were rolled under the following conditions to produce a plate material (rolled sheet material) having a thickness of 0.6 mm, a width of 270 mm, and a length of 500 m. Preheating was performed by a heating means capable of rapid heating before each rolling pass, and the material was heated to a predetermined temperature. The total holding time in the preheating is 3 hours.

(Rolling conditions)
Rolling rate: 5% / pass to 40% / pass Material temperature: 200 ° C to 280 ° C
Roll temperature: 100 ℃ ~ 250 ℃

  After the final warm rolling pass was applied to the material, the material was placed on a cooling steel plate (which can be controlled by circulating a heat medium) to adjust the cooling rate. Here, the average cooling rate until the temperature of the material (200 ° C to 280 ° C) reaches 100 ° C is 60 ° C / min, and the average cooling rate until it reaches room temperature (about 20 ° C) is 40 ° C / min. The steel sheet for cooling was cooled by adjusting the temperature and mounting time.

  The obtained plate material is warm-corrected with a roll leveler device to produce a straightened material (material temperature: 250 ° C), and wet belt type polishing is applied to this straightened material using a # 600 abrasive belt. The obtained polishing plate (sheet material) is designated as sample No. 1. In sample No. 1, in the manufacturing process from the intermediate solution treatment until the final abrasive is obtained, the total time for holding the material in the temperature range of 150 ° C. to 300 ° C. is within 12 hours, 300 No heating above ℃ was performed. Here, since warm correction is performed on the material thinned by rolling, the holding time of the temperature range can be set to about several minutes.

  The cooling rate can be easily adjusted by preparing the following correlation data in advance and referring to the correlation data. The temperature of the outermost surface of the object to be cooled with different thickness and length, or the temperature at a point 10 μm from the surface can be measured by a temperature sensor (for example, a groove is formed at the above point and the temperature sensor is embedded in the groove). When each parameter of forced cooling means such as cold air temperature, air volume, wind speed, etc. is appropriately changed, the cooling rate is obtained by measuring the time from the material temperature at the start of cooling to 100 ° C after the final rolling. Create correlation data between parameters and cooling rate.

  From the obtained sample No. 1 and comparative sample No. 100, a test piece for the total amount was cut out to measure the Al content of the whole sample (Al total average amount): x mass%. The total average amount of Al was determined by ICP emission spectroscopic analysis using x, and all samples were x = 8.75 mass%.

  From the obtained sample No. 1 and the comparative sample No. 100, a test piece for mapping was cut out, and the analysis and measurement of the element: Al on the surface of each test piece was performed using an FE (Field Emission) -EPMA device (JEOL JXA-8530F manufactured by Co., Ltd. was used. The measurement conditions are shown below.

(Measurement condition)
Acceleration voltage: 15kV
Irradiation current: 100nA
Sampling time: 50ms

  The content (mass%) of Al in the elemental analysis was determined by preparing the following calibration curve and converting the X-ray intensity of EPMA to the Al content (mass%) using this calibration curve.

(Create a calibration curve)
Commercially available AZ31 alloy materials, AZ61 alloy materials, and AZ91 alloy equivalent materials with different Al contents were subjected to solution treatment (400 ° C. × 120 hours) and homogenized to give samples. The AZ91 alloy equivalent material was used by cutting the coil material produced as follows. Using a twin-roll continuous casting method, a cast coil material (thickness 4 mm, width 300 mm) made of a magnesium alloy containing Mg-8.75% Al-0.65% Zn (all by mass%) was produced. The cast coil material is subjected to an intermediate solution treatment at 400 ° C. for 24 hours, and then the reduction rate: 5% / pass to 40% / pass, material temperature: 200 ° C. to 280 ° C., roll temperature: 100 ° C. to 250 ° C. A rolled coil material was produced by rolling a plurality of passes under the condition of ° C. The rolled coil material was warm-corrected with a roll leveler device (material temperature: 250 ° C.), and the correction material was subjected to wet polishing similar to Sample No. 1 to produce the coil material. Then, ICP emission spectroscopic analysis is performed on the surface of each sample, the Al content is measured, and elemental analysis is performed on FE-EPMA under the above measurement conditions, and the X-ray intensity (cps / μA) of Al is measured. To do.

Then, as shown in FIG. 4, the obtained X-ray intensity: y is expressed as a linear function of Al content: x, and an approximate expression of the linear function: y = 11977x + 1542.5 is used as a calibration curve. Note that this approximate expression is highly reliable with a correlation coefficient R ² of 0.9998.

  FIG. 1 is a mapping image (observation field: 24 μm × 18 μm) regarding the Al content obtained by analyzing the surface of each sample by FE-EPMA. FIG. 1 (A) shows the die-cast material of sample No. 1, and FIG. 1 (B) shows the sample No. 100. Although shown in gray scale in FIG. 1, in practice, black (Al concentration: 0% by mass here) in descending order of Al content, depending on the Al concentration, 紺 to blue to light blue to green to yellow to orange -Red-Pink-White (Al concentration: 8.75 x 1.4 = 12.25% by mass or more). The white granule in FIG. 1 (A) and the white variant in FIG. 1 (B) are Al-rich intermetallic compounds.

  As shown in FIG. 1B, it can be seen that the die-cast material of sample No. 100 has many regions with a very high Al concentration and regions with a very low Al concentration. On the other hand, as shown in Fig. 1 (A), sample No. 1 does not have a region where the Al concentration is very high, and there is substantially a region where the Al concentration is very low. I understand that I don't.

  Using this mapping image, in the observation field of each sample, the area ratio of the low concentration region where the Al concentration is 4.2% by mass or less, and the Al concentration is 0.8x (= 8.75 × 0.8 = 7) mass% or more 1.2x (= 8.75) × 1.2 = 10.5) Area ratio of the central composition region of mass% or less, Al concentration of 0.9x (= 8.75 × 0.9 = 7.875) mass area of 1.2% by mass or less, Al concentration of 1.4x (= (8.75 × 1.4 = 12.25) The area ratio of the ultra-high concentration region of mass% or more, the maximum value and the minimum value of the Al concentration were obtained. The results are shown in Table 1. The Al concentration distribution is shown in the graph of FIG.

FIG. 3 is a scanning electron microscope (SEM) micrograph of each sample No. 1,100 (x5000). The light gray granule in FIG. 3 (A) and the light gray variant in FIG. 3 (B) show precipitates. As shown in FIG. 3 (B), it can be seen that the die-cast material of sample No. 100 has large precipitates and an irregular shape. This is consistent with the fact that in the mapping image, the ultra-high concentration region having a very high Al concentration is large and has an irregular shape. On the other hand, as shown in FIG. 1 (A), it can be seen that Sample No. 1 has a structure in which particles with small precipitates are dispersed. This is consistent with the presence of small ultrahigh density regions in the mapping image. When the composition of the above-mentioned light gray particles and irregular shapes was examined with an EDS (Energy Dispersive X-ray Spectrometer), a metal containing Al or Mg such as Mg ₁₇ Al ₁₂ or Al (MnFe) It was an intermetallic compound. The presence of the intermetallic compound can also be determined by examining the composition and structure using X-ray diffraction or the like.

  The average particle size (μm) and the ratio (area%) of the total area of the intermetallic compound of each sample No. 1,100 were measured. The results are also shown in Table 1. The average particle diameter and area ratio can be easily calculated by image processing the above micrograph using a commercially available image processing apparatus.

  The average particle size of the intermetallic compound was measured as follows. For each sample, five cross sections are taken in the thickness direction, and three fields of view (here, one field of view: 22.7 μm × 17 μm region) are arbitrarily taken from the observation image of each section. Here, the field of view was selected from the surface side region up to about 100 μm in the thickness direction from the surface of each sample. For each observation field, the equivalent circle diameter of each intermetallic compound existing in one observation field (the diameter of the equivalent area circle of the area of each intermetallic compound) is obtained, and the sum of the equivalent circle diameters is obtained as one observation field. A value obtained by dividing by the number of intermetallic compounds present therein: (total of equivalent circle diameter) / (total number) is defined as the average particle diameter of the observation visual field. Table 1 shows the average of the average particle diameters of 15 observation fields for each sample.

The ratio of the total area of the intermetallic compound was measured as follows. Take the observation field as described above, and for each observation field, calculate the total area by examining the area of all intermetallic compounds present in one observation field, this total area is the area of one observation field ( Here, the value divided by 385.9 μm ² ): (total area) / (area of observation field) is defined as the area ratio of the observation field. Table 1 shows the average of the area ratios of the 15 observation fields for each sample.

Each sample No. 1,100 was subjected to a salt water corrosion test to measure the corrosion weight loss (μg / cm ² ) and the Mg elution amount (μg / cm ² ). The results are shown in Table 1.

Corrosion weight loss was measured as follows by performing a salt spray test in accordance with JIS H 8502 (1999) as a salt water corrosion test. After preparing a test piece for corrosion from sample No. 1,100 and measuring the mass (initial value) of this test piece for corrosion, the test surface of the size set in advance is exposed on the test piece for corrosion. Mask unnecessary parts of the test piece. The masked corrosion test piece is inserted into the corrosion test apparatus and is placed so as to be inclined at a predetermined angle with respect to the apparatus bottom face (here, the angle formed by the apparatus bottom face and the test piece: 70 ° to 80 °). The test solution (5 mass% NaCl aqueous solution, temperature: 35 ± 2 ° C.) is sprayed on the corrosion test piece for a predetermined time (here 96 hours). After a predetermined time has elapsed, the corrosion test piece was taken out of the corrosion test apparatus, masking was removed, and the corrosion test piece was generated in accordance with the method described in Reference Table 1 of JIS Z 2371 (2000). Corrosion products are removed by chromic acid dissolution. The mass of the test piece for corrosion after removing the corrosion product is measured, and the value obtained by dividing the difference between the mass and the initial value by the area of the test surface of the test piece for corrosion is weight loss (μg / cm ² ). And

The amount of Mg elution was measured as follows by performing a salt water immersion test as a salt water corrosion test under the following conditions. A corrosion test piece is prepared from sample No. 1,100, and unnecessary portions of the corrosion test piece are masked so that a test surface having a predetermined size is exposed in the corrosion test piece. Make the masked corrosion test piece into the test solution (5 mass% NaCl aqueous solution, liquid amount: (A) x 2 ml when the test piece area (exposed area) is (A) cm ² ) Hold for a predetermined time in a completely immersed state (here, hold for 96 hours at room temperature (25 ± 2 ° C.) under air conditioning). After a predetermined time has elapsed, the test solution is collected, the amount of Mg ions in the test solution is quantified with ICP-AES, and the value obtained by dividing the Mg ion amount by the area of the test surface of the corrosion test piece is the Mg elution amount ( μg / cm ² ).

  As shown in Table 1, the plate material of sample No. 1 has a central composition region with an Al concentration of 0.8x mass% to 1.2x mass% (here x = 8.75) in at least the surface side region of 50 area% or more. Occupied (70 area% or more here) and there is no low concentration region with Al concentration of 4.2 mass% or less, and ultrahigh concentration region with Al concentration of 1.4x mass% or more is 17.5 area% or less (here It can be seen that is less than 5 area%. In particular, Sample No. 1 has an area where the Al concentration is 0.9 x mass% to 1.2 x mass% is 50 area% or more. That is, sample No. 1 has a small variation in Al concentration. This can also be seen from the graph in FIG. As shown in FIG. 2, it can be seen that Sample No. 1 has an Al concentration distribution in which the Al total average amount is 8.75 mass% and has a peak in the vicinity thereof. Further, it can be seen that Sample No. 1 does not have a portion where the Al concentration is extremely low. And, as shown in Table 1, Sample No. 1 with such a small variation in Al concentration has low corrosion weight loss and Mg elution amount, and is excellent in corrosion resistance.

  On the other hand, the die-cast material of Sample No. 100 has a low concentration region where the Al concentration is 0.8x mass% to 1.2x mass% and the central composition region is small, and the Al concentration is 4.2 mass% or less. In particular, the minimum value of Al is equivalent to AZ31 alloy. From this, it can be considered that the sample No. 100 has a relatively inferior corrosion resistance, resulting in inferior corrosion resistance.

[Test Example 2]
After pressing the plate material of Sample No. 1 produced in Test Example 1, the Al concentration was measured in the same manner. The prepared plate material was preheated to 250 ° C. and subjected to pressing in this heated state. The preheating holding time and the total time during pressing are 2 minutes (0.1 hour or less).

  All of the obtained press-worked materials (plastic work materials) had the same Al concentration distribution as Sample No. 1. From this, it is expected that these pressed materials are also excellent in corrosion resistance.

  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 (particularly the Al content), the shape of the magnesium alloy sheet, the specifications (thickness, width, length), production conditions, etc. can be changed as appropriate.

  The magnesium alloy sheet of the present invention is a component member of various electric / electronic devices, in particular, a portable or small-sized electric / electronic device casing, members of various fields where high strength is desired, for example, It can be suitably used for materials such as automobile parts.

Claims (4)

A magnesium alloy sheet made of a magnesium alloy containing Al in a range of 7.3 mass% to 16 mass%,
When the content of Al of the entire magnesium alloy sheet is x mass%,
The area where the Al content is (x × 0.8) mass% or more and (x × 1.2) mass% or less is 50 area% or more,
The area where the Al content is (x × 1.4) mass% or more is 17.5 area% or less,
A magnesium alloy sheet characterized by substantially not including an Al content of 4.2% by mass or less.   2. The magnesium alloy sheet according to claim 1, wherein a region having an Al content of (x × 0.9) mass% or more and (x × 1.2) mass% or less is 50 area% or more. The area where the Al content is (x × 0.8) mass% or more and (x × 1.2) mass% or less is 70 area% or more,
3. The magnesium alloy sheet according to claim 1, wherein a region having an Al content of (x × 1.4) mass% or more is 5 area% or less.   4. The magnesium alloy sheet according to claim 1, wherein the magnesium alloy sheet is manufactured by rolling at least one pass.