JP2012077320A - Magnesium alloy sheet material for bending and method for producing the same, and magnesium alloy pipe and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a magnesium alloy sheet material for bending and a method for producing the magnesium alloy sheet material; and a magnesium alloy pipe and a method for producing the magnesium alloy pipe.SOLUTION: A magnesium alloy molten metal, which has a composition containing, by mass%, 2 to 11% Al, 0.2 to 0.5% Mn, 0.008 to 0.2% Si, and if necessary, 0.2 to 2% of one or more selected from Zn and Ca, with the balance of Mg and unavoidable impurities, is subjected to continuous casting and rolling at a cooling rate of 300 K/sec by means of a twin-roll process to be formed in a plate-shape, further is subjected to rolling to be reduced in thickness and then is subjected to cold working. Thus, a magnesium alloy pipe or the like, high in strength and excellent in damping, can be obtained by good workability.

Description

  The present invention relates to a magnesium alloy sheet for bending and a manufacturing method thereof, and a magnesium alloy pipe and a manufacturing method thereof.

Magnesium alloys are the lightest among practical alloys and can be recycled. Therefore, the use of magnesium alloys is expected to be expanded to various products in order to reduce the environmental impact. Many products made of this magnesium alloy are manufactured by a casting method such as die casting or thixo molding. In that case, in the casting using the injection molding, it is possible to obtain a component having a complicated shape by one injection molding. However, since it is a casting, there is a difficulty in increasing the strength. On the other hand, when a wrought material is used, it is possible to obtain a strength higher than that of a casting due to the influence of refinement of the microstructure and work hardening.
By the way, the structural member using a magnesium alloy can be expected to be developed for various uses because of its high specific strength. For example, a round pipe for an automobile frame, a metal bat, a round pipe for reinforcing a metal bat, and the like can be given. In consideration of productivity and the like, an extruded material produced by extrusion or a drawn material produced by drawing is suitable for producing these round pipes. A technique for manufacturing a pipe product using a magnesium alloy by extrusion or drawing has also been proposed (see, for example, Patent Document 1).

JP 2007-268553 A

However, since the workability decreases as the aluminum content in the magnesium alloy increases, the required extrusion pressure must be increased, for example, during extrusion. Extruding large billets at the level is very difficult. The same applies to the drawing. For this reason, it is difficult to obtain a high-strength magnesium alloy round pipe or the like by extrusion or drawing with current equipment.
Further, as a required characteristic as a structural member, in addition to strength, vibration damping is also mentioned, but sufficient vibration damping is not obtained with a conventional magnesium structural member.

  The present invention has been made against the background of the above circumstances, and it is an object of the present invention to provide a magnesium alloy plate material for bending with excellent strength, a manufacturing method thereof, a magnesium alloy pipe, and a manufacturing method thereof.

  That is, among the present invention, the magnesium alloy sheet for bending according to the first present invention is in mass%, Al: 2 to 11%, Mn: 0.2 to 0.5%, Si: 0.008 to 0 .2%, with the balance being composed of Mg and inevitable impurities.

  The magnesium alloy sheet material for bending according to the second aspect of the present invention includes, by mass%, Al: 2 to 11%, Mn: 0.2 to 0.5%, Si: 0.008 to 0.2%, Further, it is characterized in that it contains 0.2 to 2% of one or two selected from Zn and Ca, and the balance is composed of Mg and inevitable impurities.

  The magnesium alloy sheet for bending according to the third aspect of the present invention is characterized in that, in the first or second aspect of the present invention, the maximum size of the Al—Mn compound contained in the structure is less than 2.5 μm.

  According to a fourth aspect of the present invention, there is provided a method for producing a magnesium alloy sheet for bending, comprising casting and rolling a molten magnesium alloy having the composition according to any one of the first to third aspects at a cooling rate of 300 K / second or more. It is characterized by a plate shape.

  According to a fifth aspect of the present invention, there is provided a method for producing a magnesium alloy pipe for bending by casting and rolling a molten magnesium alloy having the composition according to any one of the first to third aspects of the invention at a cooling rate of 300 K / second or more. It is characterized in that it is made into a plate shape and further thinned by warm rolling and then processed into a pipe shape by cold working.

  The manufacturing method of the magnesium alloy pipe for bending according to the sixth aspect of the present invention is the method according to the fifth aspect, wherein the cold working is a cold three-point bending by a roll bender or a cold press. It is characterized by.

  According to a seventh aspect of the present invention, there is provided a method of manufacturing a magnesium alloy pipe for bending according to the fifth or sixth aspect of the present invention, wherein after processing into a pipe shape, the axial joint is welded to form a pipe shape. To do.

  The magnesium alloy pipe according to the eighth aspect of the present invention is characterized by using the magnesium alloy sheet according to any one of the first to third aspects of the present invention.

  The composition, production method and the like specified in the present invention will be described below. In addition, the component amount shown below is shown by the mass%.

Al: 2 to 11%
Al is added for the purpose of improving mechanical properties such as castability and strength, and corrosion resistance. However, if the Al content is less than 2%, sufficient castability, strength and corrosion resistance cannot be obtained. On the other hand, when the Al content exceeds 11%, the strength increase is saturated. On the other hand, if the Al content exceeds 6%, the workability in the rolling process gradually decreases, and if it exceeds 11%, rolling becomes difficult. For these reasons, the content range of Al is defined above. For the purpose of increasing strength, the lower limit is desirably 3%, and the upper limit is desirably 9% from the viewpoint of workability.

Mn: 0.2 to 0.5%
Since Mn has an effect of mitigating the influence of elements that lower the corrosion resistance, it is positively added. That is, by adding Mn, the influence of Fe, which is an impurity element that lowers the corrosion resistance, can be mitigated. By containing 0.2% or more, this effect can be effectively obtained. However, if the content exceeds 0.5%, a coarse intermetallic compound is produced during production and the rollability deteriorates, so the Mn content is set to 0.2 to 0.5%.

Si: 0.008 to 0.2%
Si has a crystal grain refinement effect and is added in a small amount. That is, Si forms magnesium and a Mg ₂ Si compound, crystallizes on the grain boundary, and contributes to improvement of mechanical properties. This effect can be obtained when the content is 0.008% or more. If the content exceeds 0.2%, the material becomes brittle during production. Therefore, the Si content is set to 0.008 to 0.2%. If it is less than 0.008%, the effect disappears.

One or two of Zn and Ca: 0.2 to 2% each
Zn, like Al, contributes to improvement of mechanical properties such as castability and strength, and is thus contained as desired. Ca has a high affinity with Al, forms a compound, is effective in improving the yield strength, and contributes to flame retardancy. Those effects are insufficient at less than 0.2%. On the other hand, if it exceeds 2%, the density of the compound increases and the elongation decreases remarkably. For this reason, workability deteriorates. This effect can be obtained by containing 0.2 to 2% of each element.

  The magnesium alloy sheet for bending according to the present invention contains inevitable impurities in addition to the above components. The type of inevitable impurities is not particularly limited and may include Fe, Cu, Ni, Cl, but these components form a local battery with magnesium, and when the critical concentration is exceeded, the substrate is significantly dissolved, Deteriorates corrosion resistance. Therefore, when these components are included as inevitable impurities, it is desirable to suppress Fe to less than 40 ppm and Cu, Ni, and Cl to each less than 20 ppm.

Al-Mn compound: Maximum size less than 2.5 μm If an Al—Mn compound with an equivalent circle diameter of 2.5 μm or more is formed in the microstructure, it becomes the starting point of breakage during cold working, and the material in the cold working The bendability is lowered.
In a conventional material obtained by rolling a slab of AZ or AM-based magnesium alloy, coarse Al-Mn based intermetallic compounds having a size exceeding 15 μm are scattered in the microstructure. It becomes a starting point of breakage during processing, and its forming becomes difficult. In the present invention, the maximum size in the equivalent circle diameter of the Al—Mn compound is regulated to be less than 2.5 μm, thereby facilitating cold working of the magnesium alloy rolled plate that is difficult to process. Note that the size of the Al—Mn compound can be evaluated by observing the microstructure with an optical microscope.

By cooling the cast and rolled magnesium alloy melt very rapidly at a cooling rate of 300 K / second or more, crystallization of a coarse Al—Mn compound can be suppressed. For example, the twin roll method is a casting and rolling method with a very fast cooling rate. When the cooling rate is less than 300 K / sec, an Al—Mn compound of 2.5 μm or more comes to crystallize in the microstructure, and the workability of the material in cold working is lowered. The cooling rate can be calculated from the secondary dendrite arm interval (DAS) of the cross-sectional portion in the thickness direction of the plate material produced by continuous casting and rolling. The relationship between the secondary dendrite arm interval DAS (μm) and the estimated cooling rate V (K / second) is expressed by the following equation (1).
DAS = 35.5 × V− ^0.31 (1)
See D. Dube: Int. J. CastMetals Res. 11 (1998) 139-144.

Cold working By cold working a material that has been cast and rolled at a cooling rate of 300 K / sec or more, not only can the strength be improved by work hardening, but it is also possible to generate many deformation twins in the alloy and to control vibration. Improves. The generated twin interface absorbs vibration applied to the material and converts it into heat, so that the vibration damping property is improved.
When the processing is performed at a temperature higher than that of the warm, the crystal structure is homogenized and sufficient strength cannot be obtained. Further, since the crystal structure is homogenized, the generation of deformation twins can be suppressed and sufficient vibration damping cannot be obtained.

Examples of cold working include three-point bending using a roll bender, or a press molding method using a die that has been prepared to have a predetermined R in advance. By cold working without heating the material, not only can the strength be increased by work hardening, but many deformation twins can be generated in the microstructure.
Magnesium alloy has an hcp structure, and only the bottom surface (0001) slip contributes to deformation in the room temperature region. In order to compensate for this, twins are easily formed compared to other aluminum. When a deformation twin is formed in the microstructure, the twin interface plays a role of absorbing vibration of the material and converting it into heat, so that it is considered that the damping property of the material is improved.

  As described above, the magnesium alloy plate material for bending according to the present invention has a mass% of Al: 2 to 11%, Mn: 0.2 to 0.5%, Si: 0.008 to 0.2%. In addition, if desired, it contains one or two of Zn and Ca in an amount of 0.2 to 2%, and the balance is composed of other additive elements, Mg and unavoidable impurities. The fracture | rupture at the time of the cold work which becomes can be suppressed, and while being excellent in workability, it is excellent in the strength after a process. Further, by cold working the magnesium alloy sheet, deformation twins can be generated in the magnesium alloy, and the formed magnesium alloy structural member has excellent strength and vibration damping properties. .

It is the microscope picture which observed the structure | tissue in the Example of this invention. Similarly, it is a figure which shows the evaluation result of damping property.

Hereinafter, an embodiment of the present invention will be described.
In mass%, Al: 2 to 11%, Mn: 0.2 to 0.5%, Si: 0.008 to 0.2%, and further, if desired, one or more selected from Zn and Ca Prepare a magnesium alloy containing 0.2 to 2%, the balance being Mg and inevitable impurities, melting the magnesium alloy, preferably by continuous casting and rolling at a cooling rate of 300 K / sec or more using a twin roll method Manufacture magnesium alloy sheets. Since the twin roll method is a casting and rolling method with a very fast cooling rate, growth of the Al—Mn compound can be suppressed.

Next, the magnesium alloy cast plate produced above is rolled into a rolled plate material having a product plate thickness. As a product plate thickness, when thinning to 2.0 mm or less, it is desirable to perform hot rolling up to 2.0 mm thickness, and to perform further thinning by warm rolling.
Homogenization heat treatment can be performed before or after the rolling process or in the middle of the rolling process. Homogenization heat processing can be performed by heating at 380-400 degreeC for 2 to 16 hours, for example. By this homogenization heat treatment, segregation at a high concentration at the dendrite cell boundary and the center of the plate thickness of the solute element in the rapidly solidified magnesium alloy plate can be eliminated.

The rolling process can be performed by hot rolling or a process in which hot rolling and warm rolling are combined. Hot rolling can be performed, for example, by heating the material temperature to 280 to 350 ° C. and heating the roll temperature to 150 to 250 ° C. Warm rolling can be performed, for example, by heating the material temperature to 180 to 260 ° C. and heating the roll temperature to less than 100 ° C.
In the rolling step, an intermediate heat treatment can be interposed. The intermediate heat treatment can be performed between the hot rolling process and the warm rolling process, or can be performed during the warm rolling process. The intermediate heat treatment can be performed, for example, by heating at 200 to 350 ° C. for 0.5 to 2 hours. In addition, when the said homogenization heat processing is performed before hot rolling, it is desirable to perform the said intermediate heat processing again after hot rolling. When the homogenization heat treatment is performed after hot rolling, the intermediate heat treatment may not be performed.

The plate material thinned to the above product plate thickness is subjected to cold processing to be processed into a desired shape such as a pipe shape. Examples of the processing method include three-point bending using a roll bender, or press molding using a die that is prepared in advance so as to have a predetermined bending R.
In the case of forming a pipe, it is possible to obtain a pipe shape by joining the joint portion by TIG welding or the like after processing into a pipe shape by cold working.

Examples of the present invention will be described below.
A magnesium alloy having the composition shown in Table 1 (remainder Mg and other inevitable impurities) is dissolved, and continuous casting and rolling is performed by a twin roll method at a cooling rate shown in Table 1 to obtain a cast and rolled plate having a thickness of 6.0 mm. Obtained. This rolled plate was hot-rolled to obtain a plate material having a thickness of 2.0 mm. The conditions for hot rolling were a roll surface temperature of 175 ° C. and a material heating temperature of 300 ° C. After the hot rolling, the plate material was heated at 400 ° C. for 2 hours and subjected to homogenization heat treatment to obtain a magnesium alloy plate material for bending. The surface of this magnesium alloy sheet for bending was observed with a microstructure using an optical microscope, and the maximum size of the Al—Mn compound was measured.
Thereafter, the magnesium alloy sheet for bending was cut into a size of 162.3 × 300 mm so that the rolling direction (RD direction) was the short side direction. The plate material after cutting was subjected to three-point bending using a roll bender to form a round pipe shape in which the rolling direction was the circumferential direction. Next, the axial joint was TIG welded using an AZ31 filler material to obtain a round pipe having an outer diameter of 53.7 mm, a length of 300 mm, and a plate thickness of 2.0 mm.

A sample cut out from the round pipe and a rolled plate sample before bending were prepared as test materials. About each test material, tensile strength (TS), yield strength (YS), and elongation were measured by the tensile test. The tensile test was performed in 0 ° direction and 90 ° direction with respect to the RD direction.
Further, the internal friction coefficient was measured by a free resonance method according to JIS R1642-2. The measurement device was a room temperature elastic modulus measurement device (JG-RT manufactured by Nippon Techno Plus Co., Ltd.), and was carried out in a room temperature atmosphere. The damping property was evaluated by an internal friction coefficient, and the internal friction coefficient was obtained from the equation (2) from the resonance frequency (f ₀ ) measured by torsional vibration and its half width (f ₁ -f ₂ ).
Q ⁻¹ = (1 / √3) × ((f ₁ −f ₂ ) / f ₀ ) (2)
In the equation, f ₀ is a resonance frequency, and f ₁ and f ₂ are frequencies indicating a detection output value that is half the detection output value (a peak value) indicated by the resonance frequency (f ₂ <f ₀ <f ₁ ).
These measurement results are shown in Table 2.

At the stage before bending, the strength of the material is low due to the influence of the homogenization treatment, and the anisotropy is also strong due to the influence of the texture. When compared with the RD direction at 90 ° with respect to the one after bending, the tensile strength increased to about 60 MPa and the proof stress to about 50 MPa, while the elongation decreased to about half. This difference in characteristics is considered to be due to the effect of work hardening at the time of processing into a round pipe in about 8 passes using a roll bender.
On the other hand, in the case of an extruded material that is a conventional material, the final shape can be obtained by a single processing by the manufacturing method, and therefore it is not possible to expect an improvement in strength by work hardening.

Next, the test material No. cut out from the round pipe was used. 1 was observed by a micrograph (FIG. 1). As a result of the observation, in the round pipe manufactured by bending, a striped pattern is observed in the grains, and it is understood that the deformation twins are remarkable.
Furthermore, the above test material No. 2 shows the internal friction coefficient before and after bending. In the test material after bending, the internal friction coefficient was improved about 4 times compared to the test material before bending, and a significant improvement in vibration damping was recognized.

Claims (8)

  It is characterized by containing Al: 2 to 11%, Mn: 0.2 to 0.5%, Si: 0.008 to 0.2%, and the balance being composed of Mg and inevitable impurities. Magnesium alloy sheet for bending.   In mass%, Al: 2 to 11%, Mn: 0.2 to 0.5%, Si: 0.008 to 0.2%, and one or two selected from Zn and Ca A magnesium alloy sheet for bending, characterized by comprising 0.2 to 2% of each, and the balance being composed of Mg and inevitable impurities.   The magnesium alloy sheet for bending according to claim 1 or 2, wherein the maximum size of the Al-Mn compound contained in the structure is less than 2.5 m.   A method for producing a magnesium alloy sheet for bending, characterized in that the molten magnesium alloy having the composition according to any one of claims 1 to 3 is cast and rolled at a cooling rate of 300 K / sec or more into a plate shape.   The molten magnesium alloy having the composition according to any one of claims 1 to 3 is cast and rolled at a cooling rate of 300 K / second or more to form a plate, and further thinned by warm rolling, and then formed into a pipe by cold working. The manufacturing method of the magnesium alloy pipe characterized by processing.   6. The method of manufacturing a magnesium alloy pipe according to claim 5, wherein the cold working is a cold three-point bending by a roll bender or a cold press.   The method of manufacturing a magnesium alloy pipe according to claim 5 or 6, wherein after processing into the pipe shape, the axial joint portion is welded to form a pipe shape.   The magnesium alloy pipe which uses the magnesium alloy plate material in any one of Claims 1-3.