TORSIONAL SEVERE PLASTIC DEFORMATION METHOD FOR METAL BAR, EMPLOYING SURFACE POLISHING TO IMPROVE MECHANICAL PROPERTIES OF METAL BAR

Abstract

The present invention relates to a torsional severe plastic deformation method for a metal bar to which surface polishing is applied to the metal bar to improve the mechanical properties of the metal bar. According to an embodiment of the present invention, there is provided a torsional severe plastic deformation method for a metal bar, which includes: applying torsion to a metal bar; and removing a surface defect on the surface of the metal bar, the surface defect being caused by the applying of torsion, wherein the removing of the surface defect is carried out in a continuous manner in which the removing of the surface defect is performed together with the applying of torsion or in a discontinuous manner in which the applying of torsion is temporarily stopped and then the applying of torsion is performed, and the removing of a surface defect increases the amount of torsional rotation or the shear strain applied to the metal bar.

Description

TECHNICAL FIELD

The present invention relates to a torsional severe plastic deformation method for a metal bar, to which surface polishing is applied, and more particularly, to a torsional severe plastic deformation method for a metal bar that is capable of improving the mechanical properties of a material by forming a gradient structure through shear deformation formed by applying a huge shear stress through torsion while substantially maintaining the shape of the metal bar, and by ultrafine crystallization or nano-crystallization of fine structures of a metal pipe material.

BACKGROUND ART

When plastic deformation is applied to a metal material, a dislocation cell structure having a small boundary angle is formed, and a phenomenon occurs in which the greater the amount of plastic deformation, the greater the crystal grain boundary angles of dislocation cell sub-grains, and the finer crystal grains gradually become. Using this, when a large plastic deformation is applied to a material and crystal grains thereof are thereby ultrafinely crystallized and nano-crystallized, the material has remarkably improved mechanical properties (strength, hardness, polishing resistance, superplasticity, etc.). Thus, there is growing importance and demand for a processing method for producing ultrafine/nano crystal materials, beyond a conventional material processing method which is mainly for shape formation.

Not only an amount of plastic deformation, such as compressive, tensile, and shear deformation, is important for the formation of ultrafine/nano crystal grains, but also the design of a die is important such that repeated processes capable of applying a large amount of plastic deformation may be performed and the shape of the material is substantially the same before and after the process.

Severe plastic deformation methods such as an equal channel angular pressing (ECAP) process, a high-pressure torsion (HPT) process, an accumulative roll bonding (ARB) process, and an equal channel angular rolling (ECAR) process, which meet the above requirements, have been developed to date.

However, when an ultrafine/nano crystal material is formed, the phenomenon occurs in which the strength and hardness of the material are improved, but ductility of the material decreases. A gradient structure of crystal grain sizes has been proposed as an alternative for solving the phenomenon of decreasing ductility. When a metallic material has a gradient structure of the crystal grain sizes thereof, ductility increases in a region formed of large crystal grains, and strength and hardness are improved by a region formed of ultrafine/nano crystal grains, and thus, opposed mechanical properties may both be achieved. Thus, for ultrafinely crystallized/nano-crystallized metal materials, a gradient structure has emerged as a solution for the ductility decrease problem.

A gradient structure may be formed through a high-pressure torsion (HPT) process among the existing sever plastic processing methods, but this processing method has a disadvantage that the size of the produced material is limited because of requiring high pressure. Thus, a simple torsion process is being required as a processing method for producing a bulk material having a gradient structure.

Such a torsional extreme-plastic process is stopped by a shear fracture phenomenon, but there is a case of being stopped while sufficient plastic deformation is not applied, and thus, a method for delaying a shear fracture phenomenon is being required. A torsional processing method has a characteristic that the greater the distance from a center axis, the greater the amount of deformation applied. Thus, a defect is caused on the surface to which the largest amount of deformation is applied, and a shear fracture phenomenon proceeds at the point at which the defect occurs while a process proceeds.

DISCLOSURE OF THE INVENTION

Technical Problem

An object of the present invention is to provide a torsional severe plastic deformation method for a metal bar with which larger deformation processing than existing torsional processing can be performed by adding a surface polishing process in a process of adding torsional severe plasticity to an existing metal bar, fine structures may be ultrafinely crystallized or nano-crystallized, and the mechanical properties of the metal bar may be improved by forming a gradient structure of crystal grain sizes.

Technical Solution

According to an embodiment of the present invention, there is provided a torsional severe plastic deformation method for a metal bar, which includes: applying torsion to a metal bar; and removing a surface defect on the surface of the metal bar, the surface defect being caused by the applying of torsion, wherein the removing of the surface defect is carried out in a continuous manner in which the removing of the surface defect is performed together with the applying of torsion or in a discontinuous manner in which the applying of torsion is temporarily stopped and then the applying of torsion is performed, and the removing of a surface defect increases the amount of torsional rotation or the shear strain applied to the metal bar.

Advantageous Effects

According to a torsional severe plastic deformation method of the present invention, a gradient structure of crystal grain sizes may be formed by applying shear deformation while maintaining the shape of a metal bar, and fine structures may be ultrafinely crystallized, and thus, the mechanical properties of the material may be improved. In addition, the torsional severe plastic deformation method of the present invention may improve the degree of gradient and the degree of fine crystallization of fine structures by applying a more amount of deformation than existing torsional deformation through surface polishing. In addition, the torsional severe plastic deformation method of the present invention may adjust torsional deformation and mechanical properties by adjusting a rotation speed.

In addition, the torsional severe plastic deformation method of the present invention is capable of freely adjusting the amount of deformation applied to a material by adjusting the rotation speed of a die, and thus is easy to reinforce the physical properties of metal bars and adjust fine structures of the metal bars.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a torsional severe plastic deformation method and processing equipment according to an embodiment of the present invention.

FIG. 2 is a view illustrating a cross-sectional surface of a specimen used in an embodiment of the present invention.

FIG. 3 is a view illustrating each of metal bars before and after a torsional severe plastic deformation according to an embodiment of the present invention.

FIG. 4 is a result of comparing the hardness of a metal bar according to an embodiment of the present invention with that of an existing metal bar.

FIG. 5 is a schematic view illustrating a region for which an analysis has been performed on a metal bar processed according to an embodiment of the present invention.

FIG. 6 is a result of electron back-scatter diffraction (EBSD) analysis on a metal bar on which simple torsional processing is completed.

FIG. 7 is a result of electron back-scatter diffraction (EBSD) analysis on a metal bar according to an embodiment of the present invention.

FIG. 8 is a table comparing simple torsion processing on a metal bar and an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail on the basis of preferred embodiments of the invention. However, the following embodiments are merely examples for helping understanding of the invention, and the scope of the invention is not reduced or limited by the embodiments.

In the present invention, it is proposed that a process of continuously removing surface defects is added to an existing torsion process to delay a shear fracture phenomenon, and processes for forming ultrafine crystal grains or nanocrystal grains are improved by applying a sufficient amount of plastic deformation to a material.

When applying the surface treatment of the present invention, much greater deformation by further delaying surface fracture than existing torsional severe plastic deformation methods, and the degree of ultrafine crystallization or nano crystallization may be reinforced to be finer.

FIG. 1 is a view illustrating a torsional severe plastic deformation method and processing equipment according to an embodiment of the present invention.

FIG. 2 is a view illustrating a cross-sectional surface of a specimen used in an embodiment of the present invention.

Referring to FIGS. 1 and 2, a torsional severe plastic deformation method for metal bars according to an embodiment of the present invention includes: a process of applying torsion to a metal bar; and a process of removing surface defects caused on the surface of the metal bar by the process of applying torsion, wherein the process of removing the surface defects is carried out in a continuous manner of being performed along with the process of applying torsion or in a discontinuous manner of being performed while temporarily stopping the process of applying torsion, and a torsional rotation amount and a shear strain which are applied to the metal bar may be increased.

Specifically, the process of applying torsion may include: installing a metal bar between a pair of dies; and performing torsion of the metal bar by rotating at least one of the pair of dies.

For example, dies conforming to the shape of the metal bar are attached on both upper and lower sides of the metal bar, a surface polishing operation is performed while applying torsion to the metal bar, and a gradient structure of crystal grain sizes may be formed while ultrafinely crystallizing or nano-crystallizing the fine structures of the metal bar.

Here, in the performing of torsion of the metal bar, the gradient structure of crystal grain sizes may be formed in the metal bar by using shear deformation formed by shear stress.

The process of removing surface defects may include a polishing process.

Here, the polishing process may be performed by using a silicon carbide (SiC) polishing paper or by using a polishing material in which the greater a torsional rotation amount or a shear strain, the smaller the surface roughness.

In addition, in the surface polishing process, the metal bar is separated from the dies, and then the surface of the metal bar may be polished in a discontinuous manner or the surface of the metal bar fixed by the dies may be polished in a continuous manner.

At this point, in the surface polishing process, a silicon carbide (SiC) grinding paper having the size of ×400, ×600, ×800 and ×1200 may be used. The surface roughness of each silicon carbide grinding paper is 22 μm, 15 μm, 10 μm or 5 μm, and when surface polishing is performed with a specific silicon carbide polishing paper, the surface roughness of the specimen (metal bar) is formed to be the corresponding roughness or below.

In addition, when discontinuously performing the surface polishing process, the surface roughness of the polished surface formed during the torsion process may be reduced through polishing by at most approximately 5 μm per one-cycle surface polishing process.

If a procedure proceeds to a next process without polishing, in the surface polishing process, the surface defects formed by torsion to have a roughness of at most approximately 5 μm, there occurs a problem in that a shear fracture phenomenon proceeds from the remaining defects and the torsion process is stopped. In addition, when the surface polishing is excessively performed even though the surface defects have been removed, there may occur a problem in that the cross-sectional area of the specimen decreases.

When proceeding the torsion process at a constant rotation speed, the shear fracture phenomenon occurs according to the inherent property of a material and the maximum torsional rotation amount or the maximum shear strain at which deformation is stopped may be different.

In the present invention, in order to increase the maximum torsional rotation amount or the maximum shear strain of each material by delaying the shear fracture phenomenon through the surface polishing, the surface polishing operation may be performed in the order of silicon carbide (SiC) polishing papers of ×400, ×600, ×800 and ×1200 in the process of applying torsion. Here, as approaching the maximum torsional rotation amount or the maximum shear strain of a metal bar, surface defects at the point to which stress (stress) is concentrated are removed by increasing the frequency or the number of surface polishing operations, and thus, the efficiency of increasing the maximum torsional rotation amount or the maximum shear strain may be maximized.

The torsional severe plastic deformation method of the present invention applies more plastic deformation by increasing the maximum torsional rotation amount or the maximum shear strain than existing metal bars, and achieves improved ultrafine crystallization or nano-crystallization and the formation of a gradient structure of crystal grain size, whereby it is easy to reinforce the property of the metal bar and to adjust the fine structures of the metal bar. In addition, the torsional severe plastic deformation method of the present invention may improve the strength and hardness by applying more plastic shear deformation than the existing simple torsion process through surface polishing.

FIG. 3 is a view illustrating each of metal bars before and after a process of applying torsion according to an embodiment of the present invention.

FIG. 4 is a result of comparing the hardness of a metal bar according to an embodiment of the present invention with that of an existing metal bar. Referring to FIG. 4, it may be confirmed that a metal bar according to an embodiment of the present invention receives a more deformation amount by surface polishing and has an improved hardness.

FIG. 5 is a schematic view illustrating a region for which an analysis has been performed on a metal bar processed according to an embodiment of the present invention.

FIG. 6 is a result of electron back-scatter diffraction (EBSD) analysis on a metal bar on which simple torsion processing is completed. Here, an analysis has been performed on the basis of the position in FIG. 5.

FIG. 7 is a result of electron back-scatter diffraction (EBSD) analysis on a metal bar according to an embodiment of the present invention. Here, an analysis has been performed on the basis of the position in FIG. 5.

FIG. 8 is a table showing results of comparison between simple torsion processing on a metal bar and an embodiment of the present invention. Referring to FIG. 8, the degree of ultrafine crystallization or nano-crystallization on a metal bar according to an embodiment of the present invention may be confirmed.

According to a torsional severe plastic deformation method of the present invention, a gradient structure of the crystal grain sizes can be formed by applying shear deformation on a material while maintaining the shape of the metal bar, and ultrafine crystallization of fine structures may be achieved, and thus, the mechanical properties of the material may be improved.

In addition, the torsional severe plastic deformation method of the present invention may improve the degree of gradient and the degree of fine crystallization of fine structures by applying a more amount of deformation than existing deformation through surface polishing.

In addition, the torsional severe plastic deformation method of the present invention may adjust torsional deformation and mechanical properties by adjusting a rotation speed.

In addition, the torsional severe plastic deformation method of the present invention may freely adjust the amount of deformation applied to a material by adjusting the rotation speed of a die, thereby easily reinforcing the physical properties of metal bars and adjusting fine structures.

So far, the technical idea of the present invention has been described with reference to the accompanying drawings, but this is for merely illustrating a preferred embodiment of the present invention rather than limiting the present invention. In addition, it is obvious that any one skilled in the art could carry out various modification and imitation without departing from the scope of the technical idea of the present invention.

Claims

1. A torsional severe plastic deformation method for a metal bar, comprising:

applying torsion to a metal bar; and

removing a surface defect on the surface of the metal bar, the surface defect being caused by the applying of torsion, wherein

the removing of the surface defect is carried out in a continuous manner in which the removing of the surface defect is performed together with the applying of torsion or in a discontinuous manner in which the applying of torsion is temporarily stopped and then the applying of torsion is performed, and the removing of a surface defect increases the amount of torsional rotation or the shear strain applied to the metal bar.

2. The torsional severe plastic deformation method for a metal bar of claim 1, wherein the removing of a surface defect comprises a polishing process.

3. The torsional severe plastic deformation method for a metal bar of claim 2, wherein a silicon carbide (SiC) grinding paper is used for the polishing process.

4. The torsional severe plastic deformation method for metal bars of claim 2, wherein the polishing process is performed by using a polishing material that can reduce the surface roughness of the metal bar according to the increase in the amount of torsional rotation or shear strain.

5. The torsional severe plastic deformation method for a metal bar of claim 4, wherein when the polishing process is performed in a discontinuous manner, the polishing process is performed so that the surface roughness of the metal rod is 5 um or less per torsion process.