JP7186396B2 - Manufacturing method for high-strength rod-shaped magnesium alloy - Google Patents

Description

The present invention relates to a method for producing rod-shaped high-strength magnesium alloys.

Magnesium alloys are expected to be the next-generation lightweight structural materials, but the improvement of their strength is a challenge, and is one of the most important research subjects at present. A rare earth-added magnesium alloy, known as KUMADAI alloy, is an effective means for improving the strength of magnesium alloys. This rare-earth-added magnesium alloy develops a long-period stacking structure and improves strength by adding rare-earth elements such as gadolinium (Gd) and yttrium (Y) to the magnesium alloy (including magnesium metal). , Non-Patent Document 1 reported that a tensile strength of 300 MPa to 400 MPa was obtained. In addition, according to the latest data in Non-Patent Document 2, it is reported that a yield strength of 510 MPa and a maximum tensile strength of 580 MPa have been achieved in the Mg--Ni--Y alloy system. Furthermore, in research on rare earth-added magnesium alloys using powder metallurgy, Non-Patent Document 3 reports that a high tensile strength of 600 MPa was achieved.

These rare earth addition type magnesium alloys are very excellent from the viewpoint of high strength, but the addition rate of rare earths must be 5 to 7% by weight, and it is difficult to supply these rare earth materials stably. However, it is not easy to use it as a wide range of materials such as machine materials and structural materials because it is relatively expensive and its price is not stable.

As means for improving the strength of magnesium alloys, there is a grain refining method. This improves the strength by refining the crystal grains, but since the magnesium alloys that are generally available on the market are not easy to cold work, the crystal grains are refined by hot extrusion or the like. was However, since hot extruded materials are inferior in strength to aluminum alloys, they cannot be used as substitute materials for structural materials.

By the way, in Non-Patent Document 4, it is reported that crystal grains are refined (ultra-fine) by deformation twinning. It has been found that miniaturization can be promoted, thereby achieving high strength (650 MPa) (see Patent Document 1). However, in the above cold multiaxial forging method, the compressive strain during forging is set to about 10%, and the compressive strain is applied from three directions, so the product becomes a rectangular parallelepiped processed product. It has been difficult to apply this method to versatile materials such as long materials (round bars, etc.) because it cannot be rolled at a large reduction rate.

Japanese Unexamined Patent Application Publication No. 2011-121118 WO2013/002082 publication

Y.Kawamura, M.Yamasaki, Mater. Trans., Vol.48, pp.2986-2992 (2007) Yoshihito Kawamura, Kenji Higashida, "Strengthening Mechanism of High-Strength Magnesium Alloy with Long-Period Stacking Structure", Light Metal Scholarship Research Report, Light Metal Scholarship (2010) Y.Kawamura, K.Hayashi, A.Inoue, T.masumoto, Mater.Trans., Vol.42, pp.1172-1176 (2001) O. Stidikov, R. Kaibyshev, Mater. Trans., Vol.42, pp.1928 (2001)

In order to solve the above-mentioned problems, the inventors of the present application have found that it is possible to increase the strength of a material having a shape other than a rectangular parallelepiped by subjecting the material under restraint to a high-pressure press (Patent Reference 2). This high-strength press under restraint makes the plastic deformation very small (10% or less). Achieved altitude ascent. That is, it splits the initial grain structure by many deformation twinning activities, increases the dislocation density by sliding deformation of the structure, and further develops the texture by crystal rotation. The huge compressive stress, which causes fracture in normal plastic deformation, promotes the densification of deformation twins of multiple variants, and at the same time contributes to the increase in dislocation density. , a significant increase in strength can be obtained.

Furthermore, the inventors of the present application have completed a dedicated press device and a dedicated mold for application to long materials (Japanese Patent Application No. 2018-034745). By using this special press machine and die, it is possible not only to refine the crystal grains by deformation twinning and to increase the strength by the synergistic effect of work hardening, but also to arrange the (0001) basal plane of the magnesium alloy on the forging surface. For example, by pressing in the longitudinal direction of a round bar, the (0001) bottom surface is arranged in the direction orthogonal to the longitudinal direction, and the (0001) bottom surface texture enables diameter reduction such as swaging. It was something. Furthermore, in the diameter reduction process, the (0001) bottom surface is aligned in the longitudinal direction to form a texture (the c-axis of the dense hexagonal crystal is parallel to the longitudinal direction), so that further work hardening results in higher strength. It made it possible to

However, when using this type of dedicated press machine and dedicated die, there is a limit to the length and diameter (in the case of a round bar) of a long object due to the shape of the die because it is a so-called batch type. rice field. In addition, since the mold must be rotated during processing, the work takes time, and high-speed and large-volume production is not easy. Therefore, there has been a strong demand for a manufacturing method that does not limit the length or the like and that can mass-produce high-strength elongated products at high speed.

The present invention has been made in view of the above points, and the object thereof is to eliminate length restrictions and mass-produce at as high speed as possible in manufacturing high-strength magnesium alloys. An object of the present invention is to provide a method for manufacturing a long object.

Accordingly, the present invention provides a method for producing a rod-shaped high-strength magnesium alloy, which is characterized by including a pressing step of radially pressing the rod-shaped magnesium alloy from its entire circumference with respect to the axial center thereof.

According to the above configuration, the rod-shaped magnesium alloy, which is a long material, is pressed radially toward the axis from the entire circumference, so that the rod-shaped magnesium alloy undergoes plastic deformation and work hardening due to the action of rolling. At the same time, a (0001) basal texture is formed radially from the axis.

The pressing step may be grooved roll rolling using a grooved roll having a semicircular cross-sectional shape. By arranging the semi-circular grooves of the grooved roll so as to face each other, the entire groove becomes circular, and by passing the rod-shaped magnesium alloy through this groove, it can be pressed radially from the entire circumference. As described above, since the cross section is reduced during this pressing (groove rolling), plastic deformation and work hardening can occur, and strengthening can be achieved by forming a (0001) basal texture.

When the rod-shaped magnesium alloy is a deformed rod having unevenness on the surface, the cross-sectional shape of the grooved roll is semicircular, and the unevenness formed on the surface of the rod-shaped magnesium alloy is engaged in the groove surface. In this case, grooved roll rolling can improve the strength of the deformed bar.

The present invention also provides a method for producing a rod-shaped high-strength magnesium alloy, comprising: a flattening step of pressing the rod-shaped magnesium alloy in a first direction perpendicular to the axis thereof to flatten it at a predetermined rate; and a restoring step of pressing in a second direction orthogonal to both of the first directions to restore the flattened state after the flattening step.

In the case of the above configuration, the rod-shaped magnesium alloy is flattened in the radial direction in the first step (flattening step), and restored to the flattened state in the second step (restoring step). Although it has a cross-sectional shape, the diameter is slightly reduced. Flattening and restoration can be performed by groove rolling, which causes plastic deformation and work hardening due to diameter reduction. In addition, both the flattening process and the restoring process cause plastic deformation and work hardening, but along with these, the (0001) basal texture develops in the flattened direction during the flattening process, and during the restoring process. A (0001) basal texture develops in the restoration direction (the direction perpendicular to the flattening direction), and the formation of the (0001) basal texture results in high strength.

In the above invention, the flattening step and the restoring step may be one processing step, and this processing step may be repeated a plurality of times. The pressing direction in the process may be different from the first direction in the first time.

By repeating the flattening process and the restoration process multiple times, work strain is introduced into the core of the bar material, causing further plastic deformation and work hardening, and also developing a (0001) basal texture to increase strength. will contribute to At that time, if the flattening direction and the restoring direction in the second and subsequent times are different from the flattening direction and the restoring direction in the first time, the direction in which the (0001) basal texture develops is not concentrated in the orthogonal direction. , it is formed radially from the axis, and it is possible to promote formation in a uniform state over the entire cross section.

In the invention relating to the manufacturing method including the flattening step and the restoring step, an aging treatment step may be included before and after the restoring step.

In the case of such a structure, the disappearance of deformation twins can be suppressed by aging treatment. This is because deformation twins are formed by the flattening process and the restoration process, but the deformation twins may reversibly disappear due to repeated high loads. That is, the aging treatment disperses the precipitates and suppresses the disappearance of reversible deformation twins.

When the cross-sectional shape of the rod-shaped magnesium alloy in the above invention is circular, the flattening step is performed by grooved roll rolling using grooved rolls having grooves whose cross-sectional shape is a horizontally long semi-elliptical shape or a horizontally long semi-elliptical shape. and the restoring step can be performed by groove roll rolling using groove rolls having grooves with a semicircular cross-sectional shape.

This is because by using a grooved roll having a horizontally long semi-elliptical or semi-elliptical cross section, the opposing grooved rolls form the horizontally long ellipse or horizontally long semi-ellipse groove shape, and this horizontally long ellipse Alternatively, the rod-shaped magnesium alloy can be easily flattened in the radial direction along the oblong semiellipse. In addition, in the restoration process, if a grooved roll having a semicircular cross section is used, the opposing grooved roll will form a circular groove shape, so a rod-shaped magnesium alloy with a circular cross section that matches this circular groove can be manufactured.

Further, when the cross-sectional shape of the rod-shaped magnesium alloy in the above invention is a square shape, the flattening step is by rolling with cylindrical rolls on two opposing surfaces, and the restoring step is the remaining two surfaces. Rolling with cylindrical rolls can be applied to the surface.

This is because, in the case of a rod-shaped magnesium alloy with a square cross section, the flattening process and the restoring process can be performed by cylindrical roll rolling without the need for groove roll rolling. In this case, the oblateness can be changed by adjusting the gap between the cylindrical rolls.

Furthermore, when the rod-shaped magnesium alloy is a deformed bar having unevenness on the surface, the flattening step includes forming the cross-sectional shape into a horizontally long semi-elliptical shape or a horizontally long semi-elliptical shape, and the surface that is engaged by the surface unevenness. The restoration step is performed by grooved roll rolling using a grooved roll having grooves formed with It can be by groove roll rolling using groove rolls.

This is to enable the flattening process and the restoring process while maintaining the unevenness formed on the surface of the magnesium alloy of the deformed bar. Depending on the unevenness of the surface of the deformed bar, for example, when the protrusions are slightly smaller than the recesses (protrusions are partially formed), the surface of the grooved roll is at least the deformed bar If a concave portion is formed in which the convex portion can be engaged, it is possible to perform the flattening process and the restoring process while maintaining the surface shape of the deformed bar.

Moreover, in the manufacturing method of each configuration described above, a cross-sectional reduction step may be provided after the pressing step or the restoring step. The cross-sectional reduction step may be, for example, swaging. In this case, the rod-shaped magnesium alloy can be subjected to cross-sectional reduction and straightening.

According to the present invention, by moving in the length direction and pressing in the direction orthogonal to the axis line by a method such as groove roll rolling, it is possible to roll a long rod-shaped object, eliminating the length limitation. A method for producing a high-strength magnesium alloy can be realized. Moreover, since it is based on a method such as groove roll rolling, it can be processed at high speed, and mass production is also possible.

1 is a schematic diagram illustrating an embodiment of the present invention; FIG. It is explanatory drawing of the grooved roll used for grooved roll rolling. It is explanatory drawing which shows the method of groove roll rolling. FIG. 4 is an SEM-EBSP image and an inverse pole figure showing the results of Experiment 1; FIG. 4 is an SEM-EBSP image and an inverse pole figure showing the results of Experiment 1; 4 is a graph showing the results of a tensile test in Experiment 1. FIG. FIG. 4 shows an SEM-EBSP image and an inverse pole figure showing the results of Experiment 2, a graph showing the Vickers hardness results, and a stress-strain curve showing the results of the tensile test.

<Embodiment>
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment in which a rod-shaped magnesium alloy with a circular cross section is subjected to a two-step rolling process. In addition, (a-1) is an outline of the initial material, (a-2) is a cross-sectional view thereof, (b-1) and (c-1) are an outline of the material after the flattening process, (b-2) and (c-2) is a cross-sectional view thereof, (d-1) is an outline of the material after the restoration process, and (d-2) is a cross-sectional view thereof.

As shown in these figures, for example, when the initial material A is a round bar with a circular cross section (see FIG. 1(a)), the first direction perpendicular to its axis (the diameter direction of the circular cross section, It is pressed in the vertical direction in the figure) X so as to flatten the cross-sectional shape, and is made into a generally elliptical cross section (hereinafter sometimes simply referred to as an "elliptical cross section") (Fig. 1(b)). reference). This is called a flattening process. The elliptical cross section of the material B after the flattening process has a short axis in the first direction X of the original circular cross section and a long axis in a direction (second direction) Y perpendicular thereto. Then, by pressing in the major axis direction (second direction) Y and compressing the major axis direction Y, the ratio between the major axis and the minor axis is balanced, and as a result, the circular cross section is restored. be. For example, by directing the long axis direction (second direction) Y in the vertical direction (see FIG. 1(c)) and pressing in the vertical direction in the same manner as in the flattening process, the long axis becomes equivalent to the short axis and has a circular cross section. becomes (see FIG. 1(d)). Circular cross-section This is called a restoration process.

In the flattening process, the initial material A is flattened at a predetermined rate, and in order to obtain an elliptical cross-section at a planned flattening rate, groove roll rolling is employed in this embodiment. In addition, in the restoring process, the material B after the flattening process was also subjected to groove roll rolling in order to restore the elliptical cross section having the major axis in the second direction Y into a circular shape. Grooved roll rolling uses two rollers (grooved rolls) 1 and 2 having grooves of a predetermined shape formed in the entire circumferential direction, as shown in FIG. The rotation shafts are kept parallel and pressed while facing each other. At this time, both grooved rolls 1 and 2 are in a state of being in contact with each other at the generatrix, and the grooves 11 and 21 formed in the circumferential direction are adjacent to each other so that a gap of a predetermined shape is formed by both grooves 11 and 21. It is formed. By passing the material through this gap, the material can be pressed and rolled while deforming the cross section into a predetermined shape.

Therefore, in the flattening process, as shown in FIG. 2(b), grooved rolls 1 and 2 having grooves 11 and 21 whose cross-sectional shapes are semielliptical or semielliptic are used. The grooved rolls 1 and 2 are arranged vertically, and the semi-elliptical or semi-elliptical shape of the grooves 11 and 21 is formed with the short axis directed in the radial direction of the grooved rolls 1 and 2 and the long axis directed in the direction of the rotation axis. By doing so, the long axis is horizontally elongated. As a result, the semi-elliptical or semi-elliptical grooves 11 and 21 formed on both the upper and lower sides face each other, so that the overall shape of the groove (gap) can be an elliptical cross section or a semi-elliptical cross section. Therefore, by rolling the initial material (round bar) of magnesium alloy with the grooved rolls 1 and 2, the circular cross section can be flattened in the first direction (vertical direction).

At this time, since the grooves 11 and 21 face each other to form an elliptical cross section or a semi-elliptical cross section, the flatness in the vertical direction is limited to the shape of the ellipse or ellipse. That is, the initial material is flattened in the short axis direction of the elliptical cross section or semi-elliptic cross section, and along with this, it stretches (or bulges) in the play direction in the long axis direction. After stretching (or bulging) until the directional play is eliminated, deformation becomes difficult. Groove rolling up to this limit enables flattening to a predetermined rate according to the initial plan. The cross-sectional area of the elliptical or oval gap formed by the grooves 11 and 21 of the grooved rolls 1 and 2 is slightly smaller than the cross-sectional area of the initial material, and the cross section is reduced (reduced in diameter) according to the difference. It will be done. At the same time, the surplus portion of the initial material due to the cross-sectional reduction is stretched (rolled) in the longitudinal direction (axial direction) of the rod-shaped member.

In the above flattening process, plastic deformation and work hardening are caused due to cross-sectional deformation and cross-sectional reduction. In addition, since a strong compressive force is applied to the initial material with a circular cross section in the flattening direction (the direction that becomes the short axis after flattening, the first direction), the direction in which the compressive force acts (first direction) (0001) basal texture develops. The (0001) basal texture at this time develops on the arc-shaped surface around the upper and lower ends (both ends in the first direction) of the initial material. On the other hand, the (0001) basal plane texture does not develop on the left and right sides (both tip sides in the second direction) of the initial material because no compressive load acts thereon. Therefore, rolling in the second direction is relatively easy.

In the restoration process, as shown in FIG. 2(c), grooved rolls 1 and 2 having grooves 11 and 21 with semicircular cross sections are used. These grooved rolls 1 and 2 can also apply compressive force in the vertical direction by arranging them vertically. Here, since the material to be rolled by the grooved rolls 1 and 2 is flattened as described above, its cross-sectional shape is elliptical or semi-elliptical. Therefore, by orienting the major axis direction (second direction) of the elliptical or semi-elliptical cross section vertically, the major axis is compressed, the major axis is shortened, and the minor axis is elongated. In the flattening process, the (0001) basal texture does not develop in the second direction, which facilitates rolling in this direction. The flattening and restoring strokes facilitate the introduction of work strain into the core of the bar, resulting in uniform work hardening throughout the bar.

Specifically, since the gap formed by the grooves 11 and 21 has a circular cross section, when the long axis of the elliptical cross section is arranged vertically, both end faces are rolled by the grooved rolls 1 and 2 on the long axis side. However, play is formed between the grooves 11 and 21 on both end surfaces on the short shaft side. Therefore, when the long axis side is pressed, the short axis side expands (or bulges), but the limit is reached when this play is eliminated. Since the limit is formed by the grooves 11 and 21 having a circular cross section, as a result, the rolled material can be restored to a circular cross section. Also in this restoration process, the cross-sectional area of the gap formed by the grooves 11 and 21 of the grooved rolls 1 and 2 is slightly smaller than the cross-sectional area of the material after flattening, and the cross-sectional area is reduced according to the difference. Stretching in the longitudinal direction is performed.

In this way, plastic deformation and work hardening associated with cross-sectional deformation and cross-sectional reduction can also occur in the restoration process. Also, a strong compressive force acts in the second direction, and the (0001) basal plane texture develops in that direction. The (0001) basal texture developed here is in a direction perpendicular to the basal texture developed by the flattening process. In contrast, the (0001) basal texture can be developed.

In the groove roll rolling using the groove rolls 1 and 2, as shown in FIG. 3, the material A (or B) before processing is placed from the opposite side of the groove rolls 1 and 2, By passing between the grooves 11 and 12 while rotating, the rolled material B (or C) is sent out to the other side according to the rotational speed of the grooved rolls 1 and 2. be.

Further, as described above, groove roll rolling is a processing method in which the cross section of the initial material is reduced and the material is stretched in the longitudinal direction (axial direction). This cross-sectional reduction causes plastic deformation and work hardening. At this time, the refinement of crystal grains is promoted by deformation twinning, and the strength is improved (higher strength) along with the refinement of the crystal grains. Therefore, in the present embodiment, a maximum cross-sectional reduction rate of about 10% is used as a guide in each of the flattening process and the restoring process. This is because common magnesium alloys tend to crack when cold-rolled by about 10% to 20%. The occurrence of cracks is due to the development of a sharp (0001) basal texture on the rolled surface, which makes further rolling difficult (Reference: Matsuo Ataka, Yuuki Sakurai, Kensuke Shinohara, Amada Foundation Grant Research Report 2006 Vol. 19 (2006)).

<Modification 1 of Embodiment>
Since the embodiment described above is based on the flattening process and the restoring process, the cross-sectional shape is generally the same as that of the initial material at the end of both processes. Therefore, as a modification of this embodiment, a method of repeating the flattening process and the restoring process can be exemplified. That is, the material restored to a circular cross section by the restoration process is flattened again and then restored. At this time as well, groove roll rolling can be used, but the shape of the grooves 11 and 21 (the size of the gap) must be gradually reduced.

As for the flattening direction in the second (further third) flattening step, the flattening direction may be the same as that in the first flattening, or a different direction may be selected. For example, when the above steps are repeated twice, the second flattening step is performed in the middle (45° direction) between the direction in the first flattening step (first direction) and the direction in the restoring step (second direction). In the case of repeating three times, the second time is in the direction inclined by 30°, and the third time is in the direction inclined by 60°. Furthermore, when repeating more than that, the angle can be changed more finely, or it can be repeated in the same direction.

Even when such processes are repeated, the direction of compression in the restoration process is the direction perpendicular to the direction of compression in the flattening direction. It enables tissue development. In particular, when the flattening direction in the second (and third) rolling is different from that in the first rolling, the direction in which the (0001) basal plane texture by the first rolling does not develop, and a sufficiently rollable state is obtained. Become.

<Modification 2 of Embodiment>
As in Modification 1 above, for example, when groove rolling is repeated multiple times while changing the angle, the (0001) bottom surface texture is not limited to the orthogonal direction, and the (0001) bottom texture in different directions develops, generally from the axis. It becomes a form that develops radially to the surroundings. Therefore, for example, if a compressive force can be applied from the entire circumference in one groove roll rolling, a similar (0001) bottom texture can be developed. Therefore, as a modification of the above-described embodiment, it is conceivable to perform groove roll rolling by groove rolls 1 and 2 having a groove shape (groove diameter) capable of obtaining an appropriate cross-sectional reduction ratio.

At this time, for example, when the initial material is a rod-shaped member with a circular cross section, the cross sections of the grooves 11 and 21 are semicircular, and are opposed to form a circular gap. By setting the circular gap at this time to be smaller than the cross-sectional area of the circular cross-section of the initial material, it becomes possible to press the initial material from the periphery. By pressing from the periphery, the (0001) basal texture develops radially from the axial center, and the strength can be improved by the action of plastic deformation and processing effects.

<Modification 3 of Embodiment>
Further, as an application of the above modification 1, as in the case where the angle of the pressing direction is changed and rolling is repeated multiple times, the (0001) basal texture is developed and the direction is not developed. If utilized, for example, a high-strength rod-shaped magnesium alloy can be produced by repeating rolling a plurality of times for a rod-shaped member having a quadrilateral cross section or other polygonal cross sections. At this time, when the pressed portions are flat surfaces facing each other, rolling can be performed using flat rolls (simple cylindrical or columnar rollers) without using grooved rolls.

<Modification 4 of Embodiment>
Furthermore, in the case of a material having a complicated shape such as a deformed steel bar, that is, a material having unevenness on the surface, the pressing should be performed in such a manner as to avoid the protrusions formed on the surface. In other words, although the convex portion may move in accordance with the deformation of the concave portion, the shape of the convex portion is not deformed, and the concave portion is restored to restore the original deformed shape. Even in this case, when the concave portion is flattened and restored, plastic deformation and work hardening occur, and the (0001) basal texture also develops, so it is possible to manufacture a deformed bar with high strength as a whole. can be done.

<Modification 5 of Embodiment>
In the above embodiment or each modified example, a step of performing aging treatment may be added after performing deformation processing by groove rolling or roll rolling. In particular, when the two steps of the flattening step and the restoring step are used, the aging treatment step may be performed before and after the restoring step. Since the flattening process is completed before the restoration process, there are cases where aging treatment is performed at that stage, and cases where aging treatment is performed after all deformation processing that has completed the restoration process are completed. It is conceivable that the aging process is performed at each end of the period. The aging treatment includes, for example, annealing. In this case, the treatment can be performed at a temperature ranging from 373K to 473K for several minutes to several hours. The aging treatment may be performed under other conditions.

In either case, aging treatment can suppress the disappearance of deformation twins. In other words, when increasing strength by plastic deformation, grain refinement by deformation twinning is used, but precipitates are dispersed by aging treatment, which suppresses reversible disappearance of deformation twins. be. For example, by repeating the flattening process and the restoring process, or by repeating these processes, the deformation twins may reversibly disappear due to repeated high loads, but this is suppressed by the dispersion of precipitates due to aging treatment. I do. In addition, since the β phase is precipitated by the aging treatment, it can have the effect of improving the heat resistance.

<Others>
Although representative embodiments and modifications of the present invention are as described above, these embodiments and modifications show examples of the present invention and are not intended to limit the present invention. Therefore, it is possible to modify the configurations in the above-described embodiment and modifications, or add other elements.

For example, a step of reducing the cross section by swaging or the like may be added to the rod-shaped member having the increased strength as described above. The reason for adding the cross-sectional reduction step is to contribute to further increase in strength and to correct the straight state in the axial direction by groove roll rolling. Grooved roll rolling is performed by rotating two opposing rolls, and if there is a difference in rotational speed, bending may occur, so swaging or the like is introduced as a finishing process. In addition to swaging, the cross-sectional reduction process may include wire drawing, extrusion, rolling or rolling. In addition, in the case of processing by rolling, it is also possible to process a deformed rod-shaped material having unevenness on the surface by the rolling.

<Experiment 1>
An experiment was conducted to observe the change in the structure of a general magnesium alloy by performing a pressing process. Specifically, a commercially available hot-extruded round bar (20 mm in diameter) of AZ80 magnesium alloy was used, flattened by grooved roll rolling, and then restored by grooved roll rolling. The maximum cross-sectional reduction rate in the flattening process and the restoring process was set to about 10% each, and about 20% in total. The flattening process and the restoring process were performed once each.

4 and 5 show a projection image (hereinafter referred to as "SEM-EBSP image") and an inverse pole figure using an EBSP (Electron Back Scatter Diffraction Patterns) method with a scanning electron microscope (SEM) in the above experiment. In addition, (a) in FIG. 4 is an SEM-EBSP image and an inverse pole figure of the initial material (hot extrusion), and (b-1) and (b-2) in FIG. 4 are both after the flattening process. SEM-EBSP image and inverse pole figure. However, (b-1) is observed from the minor axis direction of the elliptical cross section, and (b-2) is observed from the major axis direction. Also, (a-1) and (a-2) of FIG. 5 are the SEM-EBSP image and the inverse pole figure after the restoration process. (a-1) is an observation from the minor axis direction (original minor axis direction) in the elliptical cross section before restoration, and (a-2) is an observation from the major axis direction.

As a result of the above experiment, the following items were found from FIGS. 4 and 5. FIG. That is, from the state of the SEM-EBSP image, refinement of crystal grains can be confirmed, and improvement in strength by the crystal grain refinement method can be confirmed. At the same time, it is also possible to read an increase in work strain, ie, work hardening, due to a decrease in tissue image quality. In addition, the (0001) basal texture indicated by inverse pole figure and MAX is also sharpened. These are improved from the initial material by the flattening process and further improved by the subsequent restoration process. The strength of the texture is slightly different depending on the observation surface, such as the short axis direction and the long axis direction in the flattening process and the restoration process. It is shown that the strength improvement to Also, the selective development of the (0001) basal texture against this pressing surface enables the continuous flattening and restoring processes.

In the course of the above experiment, tensile tests were performed on the initial material, after the flattening process, and after the restoring process, and the results are shown in FIG. In the figure, 1 indicates the tensile test result of the initial material, 2 indicates the tensile test result of the material after the flattening process, and 3 indicates the tensile test result of the material after the restoration process.

As is clear from this figure, compared to the initial material, the yield strength and ultimate tensile strength increased after the flattening process, and further increased after the restoring process. Although it is difficult to read the details of the numerical values in the graph in the figure, the measured values of the material after the restoration process showed a yield strength of 370 MPa and a maximum tensile strength of 397 MPa. Although lower than recent data for rare earth-added alloys, it is comparable to the ultimate tensile strengths comparable to conventional rare earth-added alloys, which could be obtained by simple groove rolling.

<Experiment 2>
Further, the above material was subjected to cross-sectional reduction processing by swaging following the restoration process. SEM-EBSP images and inverse pole points at this time are shown in FIGS. 7(a-1) and (a-2), and the Vickers hardness distribution is shown in FIG. 7(b). Also, the stress-strain curve obtained by the tensile test is shown in FIG. 7(c). In addition, (a-1) in FIG. 7 is the observation result near the center of the material, (a-2) is the observation result near the surface of the material, and (b) is the measurement of the material with a diameter of 20 mm and a length of 500 mm. Five points were selected in the longitudinal direction, and measurements were taken at points 1 to 5 marked with circles. In the graph, "near the surface" means around 1 mm from the surface, "near the edge" means around 3 mm from the surface, and "near the center" means around 10 mm from the surface.

As shown in FIGS. 7(a-1) and (a-2), the observation of the vicinity of the surface after swaging clearly shows that the twin density is high. Also, the density of (0001) basal texture is reduced both near the center and near the surface. Moreover, from the results of the Vickers hardness test shown in FIG. 7(b), the hardness near the surface is higher than near the edges and near the center. Therefore, it can be seen that the swaging process causes work hardening of the surface. Furthermore, from FIG. 7(c), it was found that the tensile strength slightly exceeds 400 MPa, which is equal to or higher than that of the rare earth-added alloy. However, in the tensile test, a dumbbell-shaped tensile test piece prepared by cutting a high-hardness surface obtained by swaging was used, so the increase in strength was limited. As can be seen from the hardness distribution in FIG. 7(b), it can be easily understood that the strength of the sample including the surface is even higher.

<Experiment 3>
The AZ70 magnesium alloy was also subjected to the flattening process and the restoring process with similar grooved rolls in the same manner as described above. The degree of accumulation of (0001) basal texture was confirmed for the final product after the restoration process. The results are shown in Table 1 together with the AZ80 magnesium alloy.

As a result of the above, anisotropy is observed in the distribution of (0001) basal texture in the processed material. Further, from a comparison of more accurate pole figure data, it was found that the processed material after the restoration process had the highest degree of accumulation when observed in the original minor axis direction. Furthermore, when compared with the observation results in the long axis direction and the original long axis direction in the processed material after the flattening process and the restoration process, the accumulation degree is slightly increased in the original long axis direction in the processed material after the restoration process. became. This is because under the experimental conditions of this time, the long axis direction was also slightly pressed during the restoration process.

<Experiment 4>
In the same manner as described above, the AZ31 magnesium alloy was also subjected to the flattening process and the restoring process using similar grooved rolls. Table 2 shows the results of observing the distribution of deformation twins at that time. Similar observations have been made on the AZ70 and AZ80 magnesium alloys that have already been tested, and are also shown in Tables 2 to 4.

From the above experimental results, most of the deformation twins introduced by the flattening process and the restoration process were (10-12) tension twins. In addition, it was found that more tensile twins were introduced in the long axis direction than in the short axis direction in the processed material after the flattening process, and that the distribution of deformation twins was also anisotropic. In addition, it was found that the tensile twins introduced by the flattening process are reduced depending on the processed material after the restoration process.

1, 2 grooved rolls 11, 21 grooves A initial material B processed material C after flattening process processed material X after restoration process first direction (diameter direction of circular cross section, flattening direction)
Y second direction (direction perpendicular to first direction, direction to restore)

Claims (5)

A method for producing a rod-shaped high-strength magnesium alloy, comprising:
A flattening step of pressing in a first direction orthogonal to the axis of the rod-shaped magnesium alloy to flatten it at a predetermined rate, and the flattening step of pressing in a second direction orthogonal to both the axis and the first direction. and a restoration step for restoring the flattened state afterward,
Both the flattening step and the restoring step involve cold rolling with a cross-sectional reduction rate of 10% or less. A method for producing a high-strength rod-shaped magnesium alloy characterized by: The rod-shaped magnesium alloy has a circular cross-sectional shape, and the flattening step is performed by grooved roll rolling using a grooved roll having grooves with a cross-sectional shape of a horizontally long semi-elliptical shape or a horizontally long semi-elliptical shape, The restoration step is by groove roll rolling using a groove roll having a groove with a semicircular cross-sectional shape,
Both the grooved rolls used in the flattening process and the grooved rolls used in the restoring process are such that the cross-sectional area of the gap formed by the grooves of these grooved rolls is slightly smaller than the cross-sectional area of the material before processing. , and the cross-section is reduced and stretched in the longitudinal direction according to the difference. A method for producing a high-strength rod-shaped magnesium alloy according to claim 1. 3. The method according to claim 1 or 2, wherein said flattening step and said restoring step are set as one processing step, and this processing step is repeated multiple times. A method for producing a high-strength rod-shaped magnesium alloy. 4. The method according to claim 3, wherein the pressing direction of the flattening step in the second and subsequent processing steps is different from the first direction in the first step. A method for producing a high-strength rod-shaped magnesium alloy. The manufacturing method according to any one of claims 1 to 4, characterized in that an aging treatment step is included before and after the restoration step. A method for producing a high-strength rod-shaped magnesium alloy.

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