JP2016017183A - Magnesium-based alloy malleable material and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Mg-based alloy malleable material having excellent room temperature processability by conducting hot and warm working with controlled temperature and area reduction ratio on a Mg-based alloy raw material to which only Mn, relatively cheap element, is added.SOLUTION: There is provided a Mg-based alloy malleable material containing 0.07 mass% to 2 mass% of Mn and the balance Mg with inevitable impurities, excellent in durability and having an average crystal particle diameter of a base material of 10 μm or less, a relationship between maximum load stress (σmax) and stress at fracture (σbk), (σmax-σbk)/σmax of 0.3 or more in a stress-strain curve figure and strain rate sensitivity index (m value) of 0.1 or more.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a fine-grained magnesium (Mg) -based alloy wrought 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 in critical shear stress (CRSS) between the bottom surface slip and the non-bottom surface slip represented by the column surface is extremely large near room temperature. Therefore, compared with other metal materials, such as aluminum (Al) and iron (Fe), ductility is scarce and room temperature secondary shaping and processing are difficult.

  In order to solve these problems, alloying by adding 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 plastic deformability. This is because the rare earth element has a function of lowering the non-bottom CRSS, that is, reducing the difference between the bottom and non-bottom CRSS and facilitating dislocation sliding movement of the non-bottom. On the other hand, since rare earth elements are used, there is a concern that the price of raw materials will rise, and from an economic point of view, improvements in ductility and formability by adding inexpensive general-purpose elements are required.

  On the other hand, it has also been pointed out that grain boundary compatibility stress acts in the vicinity of the Mg grain boundary, and non-bottom slip is active (Non-Patent Document 1). For this reason, it is considered that introducing a large amount of crystal grain boundaries (crystal grain refinement) is effective in improving ductility.

  Patent Document 3 discloses a fine grain Mg alloy containing a rare earth element or a general-purpose element in a small amount and having excellent strength characteristics. The strengthening of this alloy is mainly due to the segregation of these solute elements at the grain boundaries. On the other hand, in the fine-grain Mg alloy, the dislocation sliding motion on the non-bottom surface due to the effect of grain boundary compatibility stress is activated. However, regarding grain boundary sliding that has a function of complementing plastic deformation, in these alloys, since any additive element has a function of suppressing the expression of grain boundary sliding, the grain boundary sliding hardly contributes to deformation. Therefore, the ductility of these alloys is at the same level as conventional Mg alloys, and further improvements in ductility are required. That is, it is necessary to search for a solute element that does not suppress the occurrence of grain boundary slip while maintaining the microstructure of which grain boundary compatibility acts.

  On the other hand, the addition of Mn to Mg produces an intermetallic compound by a reaction with a heavy element such as Fe or nickel (Ni) that degrades the corrosion resistance of the Mg alloy, and reduces the material deterioration of the alloy due to these heavy elements. (Non-Patent Document 2), however, there is no description regarding the influence of Mn on the extension of crystal grains of Mg alloy stretched material.

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

  In Patent Document 5, 0.2 to 3 mass% of Mn is contained, and after annealing (pre-rolling annealing) under a predetermined condition that the average crystal grain size is 20 to 100 μm, accumulation of the bottom surface is performed by warm rolling. An Mg alloy material having a high degree of formability and excellent vibration damping properties with a random degree (texture) is disclosed. However, in order to maintain an excellent vibration damping characteristic, it is not desirable to introduce a large amount of crystal grain boundaries, and the crystal grain size of the Mg parent phase may have a recrystallized structure of 50 μm or less. Further, in order to achieve both improvement in formability and vibration damping properties, a dislocation motion of non-bottom slip is necessary, and randomizing the texture of the bottom surface is an essential structure control. Further, the moldability is evaluated by the Eriksen test.

  Patent Document 6 discloses a Mg alloy rolled material containing 0.01 to 20 mass% of Mn, having excellent strength characteristics and high workability. However, although Patent Document 6 discloses that the material has a yield stress of 250 MPa or more and 350 MPa or less, these mechanical properties cannot be achieved by adding Mn alone (for example, in the present specification, FIG. 7). It can be easily estimated that a plurality of solute elements are added compared with the mechanical properties.

International application WO2013 / 180122 JP2008-214668 JP 2006-16658 A JP 2000-104136 A JP2013-129914A JP2012-41637

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

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

  The first of the present invention is an Mg-based alloy extending material containing 0.07 mass% or more and 2 mass% or less of Mn, with the balance being composed of Mg and inevitable components, and the average crystal grain size of the base material is 10 μm or less, In the stress-strain curve obtained by the tensile test of the stretched material, the relationship between the maximum load stress (σmax) and the stress at break (σbk), (σmax−σbk) / σmax is 0.3 or more. An excellent Mg-based alloy extension material is provided.

  A second aspect of the present invention is the Mg-based alloy extender of the first aspect, wherein the strain rate sensitivity index (m value), which is an index of the occurrence of grain boundary sliding, is 0.1 or more in room temperature tensile or compression tests. An excellent Mg-based alloy extension material is provided.

  In the third aspect of the invention, Mg—Mn intermetallic compounds having a diameter of 500 nm or less are dispersed at intervals of 1.95 μm or less in the Mg matrix and grain boundaries in the metal structure of the Mg-based alloy extension material of Invention 1 or 2. Provided is an Mg-based alloy extension material having excellent ductility.

  A fourth aspect of the present invention is a method for producing the Mg-based alloy extension material according to any one of the first to third aspects, wherein the Mg-based alloy cast material is subjected to a solution treatment at a temperature of 400 ° C. or higher and 650 ° C. or lower. As the first stage plastic strain application, hot plastic working with a cross-sectional reduction rate of 70% or higher is performed at a temperature of 100 ° C. or higher and 550 ° C. or lower, and then as the second stage plastic strain application, the cross section at a temperature of 100 ° C. or higher and 550 ° C. or lower is applied. Provided is a method for producing an Mg-based alloy extending material having excellent ductility, which is subjected to hot plastic working with a reduction rate of 70% or more and has a total sectional reduction rate of 70% or more.

  A fifth aspect of the invention is a ductility method according to the fourth aspect of the invention, wherein the first-stage plastic strain imparting method is any one of extrusion, forging, rolling, and drawing. The manufacturing method of the Mg-based alloy extension material excellent in is provided.

  A sixth aspect of the invention is a ductility method according to the fourth aspect of the invention, wherein the second stage plastic strain imparting method is any one of extrusion, forging, rolling, and drawing. The manufacturing method of the Mg-based alloy extension material excellent in is provided.

  7th of invention provides the manufacturing method of Mg base alloy extending | stretching material excellent in the ductility in which the plastic-strain imparting method consists only of the 1st step | paragraph in the manufacturing method of Mg base alloy extending | stretching material of invention 4. FIG.

Relationship between nominal stress, nominal strain curve, and stress drop. The photograph which observed the fine structure of the Mg-0.25 mass% Mn alloy extruded material with the optical microscope. (A) Example 7, (b) Comparative Example 1. The photograph which observed the fine structure of Example 1 with the optical microscope. The photograph which observed the fine structure of Example 2 by the scanning electron microscope / electron beam backscattering diffraction. The photograph which observed the fine structure of Example 5 by the scanning electron microscope / electron beam backscattering diffraction. FIG. 6 is a crystal orientation distribution diagram obtained by scanning electron microscope / electron beam backscatter diffraction of Example 2. The nominal stress-nominal strain curve obtained by the room temperature tensile test of the Example. The nominal stress-nominal strain curve obtained by the room temperature tensile test of the comparative example. The external appearance photograph after the room temperature tensile test of Example 7. FIG. The relationship between the flow stress and strain rate of 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 Mn content of 0.07 mass% (= 0.03 mol%) is the minimum addition amount of Mn, which is a solute element, affecting the deformation behavior. That is, when the content is 0.07 mass%, it can be estimated that the dissolved Mn atoms are present in the Mg crystal at intervals of 19.5 × 10 ⁻¹⁰ m. This distance corresponds to a magnitude about three times the Mg Burgers vector, and means that a lattice defect such as a dislocation is a limit value that causes an interaction in terms of atomic bonding. On the other hand, when the Mn content is 2 mass% or more, since the maximum solid solution amount of Mn in the Mg crystal is exceeded, coarse intermetallic compounds composed of Mg—Mn are dispersed in the crystal grains and in the 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 improving 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 even more preferably 5 μm or less. When the crystal grain size is larger than 10 μm, the grain boundary compatibility stress generated at the crystal grain boundary does not affect the entire region within the crystal grain. That is, it is difficult for non-bottom dislocation slip to be active throughout the crystal grains, and improvement in ductility cannot 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. Further, if the average crystal grain size can be maintained at 10 μm or less, heat treatment such as strain relief annealing may be performed after hot working.

  Next, a manufacturing method for obtaining a fine structure will be described. The melted Mg—Mn alloy cast material is subjected to a solution treatment at a temperature of 400 ° C. or higher and 650 ° C. or lower. Here, when the solution treatment temperature is less than 400 ° C., it is necessary to maintain the temperature for a long time in order to dissolve Mn homogeneously, which is not preferable from an industrial viewpoint. On the other hand, when the temperature exceeds 650 ° C., the temperature is higher than the solid phase temperature, so that local dissolution starts, which is dangerous in operation. Of course, any casting method can be adopted as long as it is a method capable of producing the Mg-based alloy casting material of the present invention, such as gravity casting, sand casting, and die casting.

  After the solution treatment, hot strain is applied. The hot working temperature is preferably 100 ° C. or higher and 550 ° C. or lower. When the processing temperature is less than 100 ° C., since the processing temperature is low, dynamic recrystallization hardly occurs and a sound wrought material cannot be produced. When the processing temperature exceeds 550 ° C., recrystallization progresses during processing and the grain refinement is hindered, and further, the die life of the extrusion process is reduced. In addition, it is necessary to allow for an error of 10 ° C. in the upper and lower directions in the measurement of various temperatures, and from these facts, displaying various temperatures using 20 ° C. as a temperature zone is in line with the actual situation.

  The strain application during hot working is such that the total cross-section reduction rate is 70% or more, preferably 80% or more, more preferably 90% or more. When the total cross-section reduction rate is less than 70%, the strain is not sufficiently applied, so that the crystal grain size cannot be refined. Furthermore, it is conceivable that an intermetallic compound composed of Mg—Mn is generated in the parent phase and the grain boundaries before straining, 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 can be adopted as long as it is a plastic working method capable of imparting strain. However, the effect of the present invention cannot be obtained by performing solution treatment on the cast material without executing hot working, because the crystal grain size of the Mg matrix is coarse.

A new index for evaluating the ductility and formability of the Mg-based alloy wrought material, that is, the stress reduction degree and the strain rate sensitivity index (m value) will be described. Uniform elongation, total elongation, and the like are measured from the nominal stress and nominal strain curves in FIG. 1, and these values serve as indicators of the formability of the material. However, uniform elongation means elongation to uniform deformation and means uniaxial deformation. On the other hand, in actual molding, since there are many multiaxial states rather than uniaxial deformation, it is important to know the limit of plastic deformation of a material to determine whether it is excellent in formability. Here, the plastic deformation limit is associated with the process from the occurrence of ductile fracture to fracture, and the likelihood of deformation from the occurrence of microscopic defects (voids) to the growth / total fracture (= stress reduction degree) Determined by. That is, it can be said that the greater the degree of deformation reduction indicated by the double-headed arrow in FIG. On the other hand, when the degree of deformation reduction is small, stress concentration occurs and shear fracture occurs. In order to obtain excellent moldability, the value of the stress reduction degree of the formula (1) is

It is preferable that it is 0.3 or more.

The presence or absence of the occurrence of grain boundary slip can be predicted by using the m value. The m value of Equation (2) is

In relation to

Is the strain rate, A is a constant, and σ is the flow stress. The larger the m value, the greater the occurrence of grain boundary sliding and the greater the contribution to deformation. Under normal room temperature plastic deformation conditions for Mg alloys, the m value is 0.05 or less, and is preferably 0.1 or more.

  After completely melting commercially available pure Mn (99.9%) and commercially available pure Mg (99.96%) in an Ar atmosphere using an iron crucible and holding at a melting temperature of 800 ° C. for 120 minutes or more. Then, it was cast into an iron mold to prepare an Mg—Mn master alloy (Mn = 4.8 mass%). Thereafter, the Mg-Mn master alloy and Mg are adjusted so that the target content of Mn is 0.1 mass%, 0.25 mass%, 0.7 mass%, and 1.35 mass%, and four types are used using an iron crucible. An Mg—Mn alloy casting was melted. The melting temperature was 700 ° C., the dissolution 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. The cast material was subjected to solution treatment at 500 ° C. for 2 hours, and then Mn and other elemental composition concentrations were analyzed and evaluated by ICP emission spectroscopic analysis. The results of the composition analysis are shown in Table 1.

  The cast materials 1 to 4 after the solution treatment were processed into cylindrical extruded billets having a diameter of 90 mm and a length of 80 mm by machining. The billet after processing is held in a container set at 300 to 400 ° C. for 30 minutes, then subjected to hot straining by extrusion at an extrusion ratio of 5: 1, and single-stage extrusion with a diameter of 40 mm and a length of 300 mm or more. A material was prepared. (Hereinafter referred to as the 1st extruded material.) This 1st extruded material was cut to a length of 40 mm and then held in a container set at the extrusion temperature shown in Table 2 for 30 minutes, with an extrusion ratio of 25: 1 (= reduced surface area). Rate: 94%), the second hot straining process was performed. (Hereinafter referred to as 2nd extruded material.)

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

  The 1st extrusion is intended to give a shape to a cylindrical billet having a diameter of 40 mm. 2nd extrusion and groove roll rolling are mainly aimed at crystal grain refinement utilizing dynamic recrystallization during strain imparting.

  The microstructure of the produced Mg-Mn alloy was observed using an optical microscope and a scanning electron microscope / electron beam backscattering diffraction apparatus. 2 to 5 show typical observation examples. In the figure, a region surrounded by a black line is one crystal grain. FIG. 2 shows examples of microstructure observation of Mg-0.25 mass% Mn alloy extruded materials produced at different processing temperatures. As the processing temperature is lowered, the crystal grain size becomes finer. Moreover, from the microstructure observation of FIGS. 3 to 5, it can be seen that any Mg—Mn alloy has a crystal grain size of 5 μm or less. The average crystal grain size of each Mg—Mn alloy was determined by the intercept method and summarized in Table 2. Here, in order to obtain the effect of the present invention, it is important that the average crystal grain size of the Mg—Mn alloy after hot working is 10 μm or less. Therefore, an Mg—Mn alloy having an average grain size exceeding 10 μm is used as a comparative example.

  FIG. 6 shows a crystal orientation distribution diagram obtained from a scanning electron microscope / electron beam backscatter diffraction. Each line represents a contour line, crystal orientations gather on the (0001) plane, that is, the bottom surface, and the maximum integration degree is 3.0 or more. On the other hand, generally, when it has a random texture, it is characterized in that contour lines are evenly distributed or there are no contour lines. Therefore, it turns out that the Mg-Mn alloy shown in an Example has a bottom face texture.

For 2nd extruded material and test pieces taken from the groove rolling was performed at room temperature tensile test at an initial strain rate of 1 × ¹⁰ -3 ^{s -1} and 1 × ¹⁰ -5 ^{s -1.} In the tensile test, a round bar test piece having a parallel part length of 15 mm and a parallel part diameter of 3 mm was used based on JIS standards. All specimens were taken from a direction parallel to the extrusion direction or the rolling direction. 7 and 8 show the nominal stress-nominal strain curves obtained by the room temperature tensile test. 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 a comparative example having an average crystal grain size of 10 μm or more (FIG. 8), it can be seen 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 “ruptured” (indicated as BK in the drawing), and the nominal strain at that time is summarized in Table 2 as elongation at break.

  Moreover, it turns out that the nominal stress and the nominal strain curve of the Example shown in FIG. 7 show the big stress reduction degree after the maximum load stress. For example, the value of (σmax−σbk) / σmax of Example 7 is 0.7, which indicates that the alloy of the present invention has a large plastic deformation limit and is excellent in formability. In FIG. 9, the external appearance photograph after the room temperature tensile test of Example 7 is shown. In the figure, in the vicinity of the fractured portion indicated by the arrow, it becomes a high-drawing type with large local shrinkage, confirming that the moldability is excellent. Moreover, it has also confirmed that another Example shows the same tendency.

  Based on the results of each tensile test, the value of the nominal stress when the nominal strain is 0.05 is defined as the flow stress, and FIG. 10 shows the relationship between the flow stress and the strain rate. In the figure, the slope of the straight line corresponds to the m value, and Table 2 shows values obtained by the mean square method. The m value of the Mg—Mn alloy in the example is 0.1 or more, and high ductility is achieved by the occurrence of grain boundary sliding. On the other hand, the m-value of the Mg—Mn alloy of the comparative example is small, and in order to obtain the effects of the present invention, it is essential that the crystal grain size is fine.

  In the examples of the present invention, examples of extrusion processing as the first stage plastic strain applying method and examples of extrusion processing and groove rolling roll processing as the second stage plastic strain applying method are shown. As a strain imparting method, a combination of any one of extrusion processing, forging processing, rolling processing, and drawing processing may be adopted. Of course, if the cross-section reduction rate within the appropriate range can be taken, the second-stage plastic strain applying method can be omitted, and if the cross-section reduction rate is less than a predetermined value, the third step or more is further reduced. It is also possible to apply plastic strain.

  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 such as a plate shape. In addition, since breakage does not occur even when a large strain is applied, it can be applied as an impact absorbing material such as an automobile or a structural material. In addition, since grain boundary sliding occurs, it can be applied to a part that is excellent in internal friction characteristics and has problems of vibration and noise. Furthermore, since no rare earth element is used, the price of the material can be reduced as compared with a conventional rare earth-added Mg alloy.

σmax Maximum load stress σbk Stress at break BK Nominal strain value when stress is reduced by 20% or more FS Nominal stress value when nominal strain is 0.05, also known as flow stress m Strain rate sensitivity index ED Parallel to extrusion Direction RD Parallel to rolling process TD Vertical to extrusion or rolling process

Claims (7)

An Mg-based alloy extension material containing 0.07 mass% or more and 2 mass% or less of Mn, with the balance being Mg and inevitable components, and the average crystal grain size of the base material is 10 μm or less, and the extension material In the stress-strain curve obtained by the test, the relationship between the maximum load stress (σmax) and the stress at break (σbk), (σmax−σbk) / σmax is 0.3 or more, and has excellent ductility Mg based alloy extender.   The Mg-based alloy extension material according to claim 1, wherein the strain-rate sensitivity index (m value) is 0.1 or more in a room temperature tensile or compression test. Wood. The Mg-Mn intermetallic compound having a diameter of 500 nm or less is dispersed at intervals of 1.95 μm or less in the Mg matrix and crystal grain boundaries in the metal structure of the Mg-based alloy extender according to claim 1 or 2. Mg based alloy extender with excellent ductility characterized by   4. The method for producing the Mg-based alloy extension material according to claim 1, wherein the Mg-based alloy cast material is subjected to a solution treatment at a temperature of 400 ° C. or more and 650 ° C. or less, and then the first treatment is performed. As step plastic straining, hot plastic working with a cross section reduction rate of 70% or more is performed at a temperature of 100 ° C. or higher and 550 ° C. or lower, and then as step 2 plastic strain, the cross section decreases at a temperature of 100 ° C. or higher and 550 ° C. or lower. A method for producing a Mg-based alloy extension material having excellent ductility, characterized by performing hot plastic working at a rate of 70% or more and having a total cross-section reduction rate of 70% or more.   The method for producing an Mg-based alloy stretch material according to claim 4, wherein the first-stage plastic strain imparting method is any one of an extrusion process, a forging process, a rolling process, and a drawing process. A method for producing an Mg-based alloy extension material having excellent ductility.   The method for producing an Mg-based alloy stretch material according to claim 4, wherein the second-stage plastic strain imparting method is any one of extrusion, forging, rolling, and drawing. A method for producing an Mg-based alloy extension material having excellent ductility.   5. The method for producing an Mg-based alloy extension material excellent in ductility according to claim 4, wherein the plastic strain imparting method comprises only the first stage.