JP2019060026A - Magnesium-based alloy extension material and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Mg-based alloy material having excellent room-temperature workability by subjecting a Mg-based alloy material to which only Mn, which is a relatively inexpensive element, is added, to hot and warm working by controlling the temperature and the area reduction ratio. Provide extension material. SOLUTION: The Mg-based alloy stretched material of the present invention is a base alloy stretched material containing 0.07 mass% or more and 2 mass% or less of Mn, with the balance being Mg and an unavoidable component, and the average crystal of the base material. The stress obtained when a tensile test is performed on the stretched material having a particle size of 5 μm or less and a strain rate of 1 × 10 −5 / s or more and 1 × 10 −3 / s or less based on JIS standards. In the strain curve diagram, the relationship between the maximum load stress (σmax) and the stress at break (σbk), (σmax−σbk) / σmax is 0.3 or more, and has a rod-like shape. [Selection diagram] FIG.

Description

  The present invention relates to a fine grained magnesium (Mg) -based alloy extension material to which manganese (Mn) excellent in room temperature ductility is added, and a method for producing the same.

  Mg alloys are attracting attention as next-generation lightweight metal materials. However, since the crystal structure is a hexagonal crystal, the difference between the critical fracture shear stress (CRSS) of the non-bottom surface slide represented by the base surface slide and the cylindrical surface is extremely large around room temperature. Therefore, compared with other metal materials such as aluminum (Al) and iron (Fe), ductility is poor, and room temperature secondary forming and processing are difficult.

  In order to solve these problems, alloying by addition of rare earth elements is often used. For example, in Patent Documents 1 and 2, rare earth elements such as yttrium (Y), cerium (Ce), and lanthanum (La) are added to improve the plastic deformability. It is because the rare earth elements have a function to lower the non-bottom CRSS, that is, to reduce the difference between the bottom and non-bottom CRSS and to facilitate non-bottom dislocation sliding motion. On the other hand, since rare earth elements are used, there is a concern that the price of the material will rise, and from an economic point of view, improvement of ductility and formability by the addition of inexpensive general purpose elements is required.

  On the other hand, it has also been pointed out that grain boundary compatibility stress acts in the vicinity of Mg grain boundaries, and non-bottom sliding is activated (Non-Patent Document 1). Therefore, introducing a large amount of grain boundaries (grain refinement) is considered to be effective for ductility improvement.

  Patent Document 3 discloses a fine grained Mg alloy which contains a small amount of a rare earth element or a general-purpose element and is excellent in strength characteristics. The strengthening of this alloy is mainly attributed to the segregation of these solute elements at grain boundaries. On the other hand, in the case of a fine grained Mg alloy, nondislocation dislocation sliding motion is activated by the action of grain boundary compatibility stress. However, with regard to grain boundary sliding which has a function of complementing plastic deformation, in these alloys, any added element has the function of suppressing the occurrence of grain boundary sliding, so grain boundary sliding hardly contributes to deformation. Therefore, the ductility of these alloys is required to be further improved in ductility at the same level as that of conventional Mg alloys. That is, it is necessary to search for solute elements that do not suppress the occurrence of grain boundary sliding while maintaining the microstructure structure on which grain boundary compatibility works.

  On the other hand, the addition of Mn to Mg produces an intermetallic compound by reaction with heavy elements such as Fe and nickel (Ni) which degrades the corrosion resistance of the Mg alloy, and an effect of reducing material deterioration of the alloy by these heavy elements However, the effect of Mn on the grain refining of Mg alloy extension material is not described.

  Patent Document 4 discloses that the addition of 0.03 to 1% by mass of Mn has the function of reducing the crystal grain size of the Mg matrix at the time of casting by coexisting with boron (B). However, in the Mg alloy in this patent document, the grain size of the cast structure is a refinement effect from 100 μm to several tens of μm, and further refinement of the grain size is necessary to improve the ductility. is there.

  In patent document 5, after 0.2-3 mass% of Mn containing and annealing (pre-rolling annealing) on predetermined conditions used as average grain size becoming 20-100 micrometers, accumulation of the bottom is carried out by warm rolling processing. There is disclosed a Mg alloy material which is rich in formability and excellent in vibration damping properties, in which the degree (texture) is randomized. However, in order to maintain excellent damping properties, it is not desirable to introduce a large amount of grain boundaries, and it is said that the grain size of the Mg matrix has a recrystallized structure of 50 μm or less. In addition, in order to achieve both the formability and the improvement of the damping property, nondisplacement dislocation movement is required, and it is an essential structure control to make the texture of the bottom surface random. Also, the formability is evaluated by the Erichsen test.

  Patent Document 6 discloses a rolled Mg alloy material that contains 0.01 to 20 mass% of Mn, is excellent in strength characteristics, and is rich in workability. However, although Patent Document 6 discloses that it has a yield stress of 250 MPa or more and 350 MPa or less, these mechanical properties can not be achieved by the addition of Mn alone (for example, in the present specification, FIG. 7) It can easily be inferred that multiple solute elements are added (compared to mechanical properties).

International application WO2013 / 180122 JP 2008-214668 A JP 2006-16658 Japanese Patent Laid-Open No. 2000-104136 JP 2013-129914 A JP 2012-41637

J. Koike et al. , Acta Mater, 51 (2003) p2055. ASM specialty handbook, Magnesium and magnesium alloys, ASM (1999).

  The present invention is an Mg-based alloy having excellent room temperature processability by subjecting a Mg-based alloy material to which only relatively inexpensive elements, only Mn, are added to hot and warm processing with controlled temperature and reduction ratio. It is an issue to provide an alloy extension material. In addition to this, it is also an issue to define excellent room temperature processability by introducing a new index for evaluating the ductility and formability of the Mg-based alloy extension material.

The Mg-based alloy extension material excellent in ductility according to the present invention is a base alloy extension material containing 0.07 mass% or more and 2 mass% or less of Mn, with the balance being Mg and unavoidable components, and the average crystal of the base material Stress obtained when the particle diameter is 5 μm or less, and the extension material is subjected to a tensile test at a strain rate in the range of 1 × 10 ⁻⁵ / s or more and 1 × 10 ⁻³ / s or less based on the JIS standard In the strain curve diagram, the relationship between the maximum applied stress (σmax) and the stress at break (σbk), (σmax−σbk) / σmax is 0.3 or more, and has a rod-like shape.

  The average grain size of the base material may be less than 3 μm.

  The Mn content may be 0.11 mass% to 1.40 mass%.

  The method for producing the Mg-based alloy extension material according to the present invention comprises the cast material of Mg-based alloy containing 0.07 mass% or more and 2 mass% or less of Mn and the balance being Mg and unavoidable components is 400 ° C. or more and 650 ° C. or less After solution treatment at a temperature, hot straining is performed by any of the processing methods of extrusion processing, drawing processing, and grooved roll rolling until the total cross-section reduction rate becomes 70% or more at a temperature of 100 ° C to 550 ° C. It is characterized by

  The hot straining may be repeated.

  The hot straining may be a process of extrusion or grooved roll rolling.

  The hot straining may be performed until the total cross-sectional reduction rate becomes 90% or more.

Relationship between nominal stress, nominal strain curve and degree of stress drop. The photograph which observed the microstructure of Mg-0.25mass% Mn alloy extrusion material with an optical microscope. (A) Example 7, (b) Comparative Example 1. The photograph which observed the microstructure of Example 1 by the optical microscope. The photograph which observed the microstructure of Example 2 by scanning electron microscope / electron beam backscattering diffraction. The photograph which observed the microstructure of Example 5 by scanning electron microscope / electron beam backscattering diffraction. The crystal orientation distribution map obtained by the scanning electron microscope / electron beam backscattering diffraction of Example 2. FIG. The nominal stress-nominal strain curve obtained by the room temperature tension test of an example. The nominal stress-nominal strain curve obtained by the room temperature tension test of a comparative example. The external appearance photograph after the room temperature tension test of Example 7. FIG. Relationship between flow stress and strain rate in Example 2 and Comparative Example 2.

The content of Mn in the Mg-based alloy material for obtaining the effects of the present invention is 0.07 mass% or more and 2 mass% or less. The content of Mn of 0.07 mass% (= 0.03 mol%) is the minimum addition amount of Mn which is a solute element to affect the deformation behavior. That is, when the content is 0.07 mass%, it can be estimated that the solid solution Mn atoms are present in the Mg crystal at an interval of 19.5 × 10 ⁻¹⁰ m. This distance corresponds to about three times the size of the Burgers vector of Mg, which means that it is the limit value at which lattice defects such as dislocation cause atomic bonding interactions. On the other hand, when the Mn content is 2 mass% or more, the maximum solid solution amount of Mn in Mg crystal is exceeded, so a coarse intermetallic compound consisting of Mg-Mn is dispersed in crystal grains and in crystal grain boundaries. Dispersion of these coarse intermetallic compound particles becomes a starting point of fracture during plastic deformation, and is not preferable from the viewpoint of improvement of ductility. Here, the size of the Mg—Mn intermetallic compound particles is preferably 500 nm or less, more preferably 250 nm or less, and still more preferably 100 nm or less.

  The crystal grain size of the Mg matrix after hot working is preferably 10 μm or less, more preferably 7 μm or less, and still more preferably 5 μm or less. When the grain size is larger than 10 μm, the grain boundary compatibility stress generated at the grain boundary does not affect the entire area within the grain. That is, it is difficult for non-bottom dislocation sliding to be active in the entire region of the crystal grain, and improvement in ductility can not be expected. Of course, if the crystal grain size is 10 μm or less, an Mg—Mn intermetallic compound of 500 nm or less may be dispersed in the Mg crystal grains and in the crystal grain boundaries. In addition, heat treatment such as strain relief annealing may be performed after hot working as long as the average grain size can be maintained at 10 μm or less.

  Next, a manufacturing method for obtaining a fine structure will be described. The molten Mg—Mn alloy cast material is subjected to solution treatment at a temperature of 400 ° C. or more and 650 ° C. or less. Here, when the solution treatment temperature is less than 400 ° C., it is necessary to maintain the temperature for a long time to form a solid solution of Mn, which is not preferable from an industrial viewpoint. On the other hand, if the temperature exceeds 650 ° C., local melting starts because the temperature is above the solid phase temperature, which is dangerous for operation. Of course, as the casting method, any method such as gravity casting, sand casting, die casting and the like can be adopted as long as it can produce the Mg-based alloy cast material of the present invention.

  After the solution treatment, hot straining is performed. The temperature for hot working is preferably 100 ° C. or more and 550 ° C. or less. If the processing temperature is less than 100 ° C., the processing temperature is low, so dynamic recrystallization is unlikely to occur, and a healthy extension material can not be produced. If the processing temperature exceeds 550 ° C., recrystallization proceeds during processing to inhibit grain refinement, which further causes a decrease in die life of extrusion processing. In addition, it is necessary to allow at least an error of 10 ° C. at the minimum for measurement of various temperatures, and from these facts, displaying various temperatures with 20 ° C. as a temperature zone is in line with the actual situation.

  The straining at the time of hot working is performed such that the total cross-section reduction rate is 70% or more, preferably 80% or more, and more preferably 90% or more. If the total cross-section reduction rate is less than 70%, the straining is insufficient, and thus the grain size can not be reduced. Furthermore, it is conceivable that intermetallic compounds consisting of Mg—Mn are formed in the matrix and grain boundaries before strain application, that is, during holding in a furnace or container heated to a predetermined temperature. In such a case, it is difficult to finely disperse these intermetallic compounds unless sufficient strain is applied. The hot working method is typically extrusion, forging, rolling, drawing or the like, but any working method may be adopted as long as it is a plastic working method that can impart strain. However, the effect of the present invention can not be obtained because the crystal grain size of the Mg matrix is coarse only by solution treatment of the cast material without executing hot working.

A new index for evaluating the ductility and formability of Mg-based alloy extensions, that is, the stress reduction and strain rate sensitivity index (m value) will be described. From the nominal stress and the nominal strain curve in FIG. 1, uniform elongation, total elongation, etc. are measured, and these values are indicators of the formability of the material. However, uniform elongation means uniaxial deformation since it is elongation to uniform deformation. On the other hand, it is important to know the limit of plastic deformation of the material whether or not it is excellent in formability, because in actual forming, there are many uniaxial states instead of uniaxial deformation. Here, the plastic deformation limit corresponds to the process from the occurrence of ductile fracture of a material to the fracture, and the likelihood of deformation from the occurrence of microscopic defects (voids) to growth / total fracture (= the degree of stress reduction) Determined by That is, as the degree of deformation reduction indicated by the double arrow in FIG. 1 is larger, it is more likely that a necking occurs, and the deformation is excellent in a multiaxial state. On the other hand, when the degree of deformation reduction is small, stress concentration occurs and shear failure occurs. In order to obtain excellent formability, the value of the degree of stress reduction of equation (1) is
(Σmax−σbk) / σmax (1)
It is preferable that it is 0.3 or more.

Further, the presence or absence of grain boundary sliding can be predicted by using the m value. The m value of equation (2) is
In the relationship of
Is the strain rate, A is a constant, and σ is the flow stress. As the m value is larger, the occurrence of grain boundary sliding is larger and the contribution to deformation is larger. Under the room temperature plastic deformation conditions of a general Mg alloy, the m value is preferably 0.05 or less, and is preferably 0.1 or more.

  After completely dissolving commercially available pure Mn (99.9%) and commercially available pure Mg (99.96%) in an Ar atmosphere using an iron crucible, and maintaining the solution temperature at 800 ° C. for 120 minutes or more The cast iron was cast in a mold made of iron to prepare a Mg-Mn master alloy (Mn = 4.8 mass%). After that, the Mg-Mn mother alloy and Mg are adjusted so that the target Mn content becomes 0.1 mass%, 0.25 mass%, 0.7 mass%, and 1.35 mass%, and four types of iron crucibles are used. The Mg-Mn alloy casting material was melted. The melting temperature was 700 ° C., and the melting and holding time was 5 minutes, and casting was performed using an iron mold having a diameter of 90 mm and a height of 200 mm. After the cast material was subjected to solution treatment at 500 ° C. for 2 hours, the concentrations of elemental compositions of Mn and other elements were analyzed and evaluated by ICP emission spectrometry. The results of composition analysis are shown in Table 1.

  Casting materials 1 to 4 after solution treatment were machined into cylindrical extruded billets of 90 mm in diameter and 80 mm in length. After holding the processed billet for 30 minutes in a container set at 300 to 400 ° C., hot strain application processing by extrusion is performed at an extrusion ratio of 5: 1, and one-step extrusion with a diameter of 40 mm and a length of 300 mm or more The material was made. After cutting this 1st extruded material to a length of 40 mm, it is held for 30 minutes in a container set to the extrusion temperature shown in Table 2 and an extrusion ratio of 25: 1 (= surface reduction) The second hot straining process was performed at a rate of 94%. (Hereafter, it is called 2nd extrusion material.)

  Next, the grooved roll material rolling process will be described. A grooved roll material of Mg-Mn alloy was manufactured in the following procedure. From the 1st extruded material (diameter 40 mm), a cylindrical billet having a length of 80 mm was cut out. The grooved roll rolling cylindrical billet was held for 60 minutes in a muffle furnace set to the processing temperature shown in Table 2, and grooved roll rolling was performed. Here, the roll surface temperature was room temperature, the cross-sectional area reduction due to grooved roll rolling was 18% per pass, and rolling was repeated 15 times such that the total reduction in area was 92%.

  In addition, 1st extrusion aims at shape provision to the cylindrical billet which consists of diameter 40 mm. The 2nd extrusion and grooved roll rolling mainly aim at grain refinement using dynamic recrystallization during straining processing.

  The microstructure of the prepared Mg-Mn alloy was observed using an optical microscope and a scanning electron microscope / electron beam backscattering diffractometer. Typical observation examples are shown in FIGS. In the figure, a region surrounded by a black line is one crystal grain. FIG. 2 is an example of observing the microstructure of an extruded material of Mg-0.25 mass% Mn alloy manufactured at different processing temperatures. As the processing temperature decreases, the grain size becomes finer. Further, from the microstructure observation in FIGS. 3 to 5, it is understood that the crystal grain size is 5 μm or less in any of the Mg—Mn alloys. In addition, the average grain size of each Mg-Mn alloy is calculated | required by the segment method, and is put together in Table 2. Here, in order to obtain the effects of the present invention, it is important that the average grain size of the Mg-Mn alloy after hot working is 10 μm or less. Therefore, Mg-Mn alloys having an average grain size of more than 10 μm are used as comparative examples.

  FIG. 6 shows a crystal orientation distribution map obtained from scanning electron microscope / electron backscattering diffraction. Each line represents a contour line, and crystal orientations gather on the (0001) plane, ie, the bottom, and the maximum integration degree is 3.0 or more. On the other hand, generally, in the case of having a random texture, it is characterized in that contour lines are evenly distributed or contour lines do not exist. Therefore, it turns out that Mg-Mn alloy shown in an example has bottom face texture.

A room temperature tensile test was performed on test pieces collected from the 2nd extruded material and grooved roll rolling at an initial strain rate of 1 × 10 ⁻³ s ⁻¹ and 1 × 10 ⁻⁵ s ⁻¹ . The tension test used the round bar test piece which consists of parallel part length 15 mm and parallel part diameter 3 mm based on JIS specification. All test pieces were taken from the parallel direction to the extrusion direction or the rolling direction. 7 and 8 show nominal stress-nominal strain curves obtained by room temperature tensile tests. It can be confirmed that the Mg-Mn alloy of the example (FIG. 7) exhibits excellent ductility. On the other hand, in the case of the comparative example having an average crystal grain size of 10 μm or more (FIG. 8), it is understood that the ductility is reduced as compared with the example. Here, the case where the stress is reduced by 20% or more is defined as "broken" (indicated as BK in the figure), and the nominal strain at that time is summarized in Table 2 as a breaking elongation.

  Also, it can be seen that the nominal stress and nominal strain curves of the embodiment shown in FIG. 7 show a large degree of stress drop after the maximum applied stress. For example, the value of (σmax−σbk) / σmax in Example 7 is 0.7, which indicates that the plastic deformation limit of the alloy of the present invention is large, and the formability is excellent. The external appearance photograph after the room temperature tension test of Example 7 is shown in FIG. In the figure, in the vicinity of the broken part shown by the arrow, it becomes a high drawing type with a large local contraction, and it supports that it is excellent in moldability. It is also confirmed that the other examples show the same tendency.

  Based on the results of each tensile test, the nominal stress value at a nominal strain of 0.05 is taken as the flow stress, and the relationship between the flow stress and the strain rate is shown in FIG. In the figure, the slope of the straight line corresponds to the m value, and the value obtained by the mean square method is shown in Table 2. The m value of the Mg-Mn alloy in the examples is 0.1 or more, and the occurrence of grain boundary sliding brings about high ductility. On the other hand, the m value of the Mg-Mn alloy of the comparative example is small, and in order to obtain the effect of the present invention, it is essential that the grain size is fine.

  In the embodiment of the present invention, although an example of extrusion processing as the first stage plastic strain application method and extrusion and groove rolling roll processing as the second stage plastic strain application method is shown, the first stage and second stage plasticity are described. A combination of any one processing method among extrusion, forging, rolling and drawing may be adopted as a strain application method. Of course, the second stage plastic strain application method can be omitted if the section reduction rate within the appropriate range can be obtained, and if the section reduction rate is smaller than a predetermined value, the third stage or more Plastic strain can also be applied.

  Since the Mg-Mn alloy of the present invention exhibits excellent ductility, it is rich in secondary workability and can be easily formed into a complex shape including a plate shape. In addition, since breakage does not occur even if a large strain is applied, it can be said that it can be applied as an impact absorbing material or structural material including automobiles and the like. In addition, since grain boundary sliding appears, it is considered that the internal friction characteristics are excellent, and adaptation to a portion where vibration or noise is a problem is considered. Furthermore, since the rare earth element is not used, it is possible to reduce the cost of the material compared to the conventional rare earth-doped Mg alloy.

σmax Maximum load stress σbk Stress at break 20% or more of nominal strain value at which stress is reduced by 20% or more FS nominal stress value at nominal strain 0.05, alias: flow stress m strain rate sensitivity index ED parallel to extrusion Direction RD parallel to the rolling process TD perpendicular to the extrusion or rolling process

Claims (7)

A Mg-based alloy extender containing 0.07 mass% or more and 2 mass% or less of Mn, with the balance being Mg and unavoidable components, wherein the base material has an average crystal grain size of 5 μm or less, and the extender, In the stress-strain curve diagram obtained when the tensile test is performed at a strain rate in the range of 1 × 10 ⁻⁵ / s or more and 1 × 10 ⁻³ / s or less based on the JIS standard, the maximum applied stress (σ max) and A ductile excellent Mg-based alloy extended material characterized by a stress at break (σbk), (σmax-σbk) / σmax of 0.3 or more, and having a rod-like shape.   The Mg-based alloy extender according to claim 1, wherein the base material has an average crystal grain size of less than 3 m, and the ductility is excellent.   It is Mg base alloy extension material of Claim 1 or 2, Comprising: Mn is contained 0.11 mass% or more and 1.40 mass% or less, Mg base alloy extension material excellent in ductility characterized by the above-mentioned.   A method for producing the Mg-based alloy extender according to any one of claims 1 to 3, wherein the Mg-based alloy contains 0.07 mass% or more and 2 mass% or less of Mn, and the balance contains Mg and an unavoidable component. After solution treatment of the cast material at a temperature of 400 ° C. or more and 650 ° C. or less, any one of extrusion, drawing, and grooved roll rolling until the total cross-section reduction rate becomes 70% or more at 100 ° C. or more and 550 ° C. or less A method for producing a ductile excellent Mg base alloy extension material characterized by performing hot straining by the processing method of   The method according to claim 4, wherein the hot strain application is repeated, and the ductility is excellent.   In the method of manufacturing the Mg-based alloy extension material according to claim 4 or 5, the hot strain application is a processing method of extrusion processing or grooved rolling, and the Mg-based alloy extension excellent in ductility characterized in that Material manufacturing method.   The method of manufacturing a Mg-based alloy extender according to any one of claims 3 to 6, wherein the hot straining is performed until the total cross-section reduction rate becomes 90% or more. Method of manufacturing extension material.