Patent Grant
8516868
The present invention relates to a die for shear drawing used for drawing a material such as a wire rod, a profile, or a rectangular bar, and more particularly, to a new die for shear drawing capable of performing ultra-fine grain refinement in a metal structure and improving mechanical properties by continuous drawing, and capable of performing continuous drawing and shear deformation simultaneously, which enables a decrease in a heat treatment temperature and a reduction of heat treatment time in carbon steels subjected to a spheroidizing heat treatment.
The present invention relates to the technical field pertaining to equal channel angular extrusion (ECAE, see References [1] and [2]), one of a range of severe plastic deformation technologies, and more specifically, to equal channel angular drawing (ECAD, see Reference [3]).
ECAE is a process imparting severe plasticity, due to shear deformation, to a metallic material by extruding the metallic material through a die in which two channels (inlet and outlet) having the same cross-sectional areas intersect with each other at an arbitrary angle. As a result, grain refinement and a reduction of spheroidizing time are achieved, and mechanical properties are improved (see Reference [4]). However, even though ECAE is a good severe plastic deformation technology, a continuous process is not possible because it is an extrusion process. Therefore, there is a limitation in the commercialization of ECAE.
Thereafter, ECAD capable of obtaining a material having similar characteristics to a material processed through ECAE and imparting severe plastic deformation as well as performing a continuous process was introduced. Although ECAD, as in the case of ECAE, uses an apparatus in which two channels having the same cross-sectional areas intersect to each other, ECAD is a method of drawing a workpiece instead of the extrusion thereof, as in ECAE. Therefore, ECAD was introduced as a processing technology capable of performing a continuous process, as well as imparting severe plastic deformation. However, since a material may not uniformly fill a die channel of the processing apparatus during a drawing process, i.e., a filling of a material is insufficient, there are limitations in that a cross section of the material is non-uniformly distributed in a length direction after the processing of the material, and necking is generated during the drawing of the material (see Reference [5]).
Although various apparatuses and methods capable of applying severe plastic deformation technology as well as performing a continuous process may be introduced in addition to the foregoing technology (see Reference [6]), materials having severe plastic deformation applied thereto are mainly sheets, and the apparatuses and methods do not suggest a concrete method for a material to be passed through an equal channel angle and to guarantee surface quality and cross-sectional uniformity of the sheets after processing.
An aspect of the present invention provides a die for shear drawing capable of performing continuous drawing and shear deformation simultaneously.
According to an aspect of the present invention, there is provided a die for shear drawing including: a material processing channel in which a material is sheared and drawn while passing therethrough, wherein the processing channel includes an inlet path positioned at a front end thereof, and an outlet path positioned at a rear end thereof, when viewed from a movement direction of a material, the inlet path and the outlet path are connected to intersect central axes thereof at a certain angle, and the processing channel includes a cross-section reduction segment allowing an outlet cross-sectional area of the outlet path to be smaller than an inlet cross-sectional area of the inlet path to thereby draw out a material from an exit of the outlet path with the material filled therein.
According to the present invention, continuous shear deformation is possible and a filling of a material in a die is good during shear drawing such that an almost constant value of an aspect ratio in a cross section of a material may be obtained along an entire length of the material after shear drawing. As a result, ultra-fine grain refinement may be performed and mechanical properties may be improved. With respect to carbon steels subjected to spheroidizing heat treatment, effects enabling a reduction of a heat treatment temperature and a time used therefor may be obtained.
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
a) and (b) are schematic views illustrating in
Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
According to the present invention, a die for shear drawing, performing shear deformation and drawing simultaneously is proposed, based on a typical drawing process, in order to solve limitations in an application of a continuous process which has been considered as the biggest obstacle to typical severe plastic deformation technology.
As illustrated in
In the present invention, an angle between the central axes of the inlet path and the outlet path is defined as an intersecting angle, and the technology for performing drawing and shear deformation at the same time is defined as shear drawing technology.
The intersecting angle according to the present invention may be in a range of 120°-160°.
The intersecting angle may not be more than about 160° for improving mechanical properties of a material by means of shear drawing.
The smaller the intersecting angle is, the more the increase in the amount of shear strain is. Accordingly, grain refinement is improved, but the filling of a material in a processing channel is decreased such that a material having a uniform cross-sectional area is difficult to be obtained after processing. Therefore, a lower limit of the intersecting angle may be 120°.
The intersecting angle, for example, may be in a range of 125°-140°.
Also, the processing channel is composed of an inlet path positioned at the front and an outlet path positioned at the rear when viewed from a movement direction of a material. To prevent the inferior filling of a material in the processing channel, a cross-section reduction segment has to be included, in which an outlet cross-sectional area of the outlet path is reduced in comparison to an inlet cross-sectional area of the inlet path in order for a material to be drawn out by at least filling an outlet portion of the outlet path. The cross-sectional area denotes a cross section perpendicular to a movement direction of a material, and a shape thereof may be varied to have a shape such as an oval or a polygon, in addition to a circle.
The processing channel may be formed to have a reduction ratio (RA)(where, RA=((AI−AO)/AI)×100) of 10-60% at an outlet of the outlet path of the processing channel reduced by means of the cross-section reduction segment. AO represents an outlet cross-sectional area and AI represents an inlet cross-sectional area of the processing channel, respectively.
When the reduction ratio (RA) is 10% or more, it is effective in preventing necking of a material. The more the increase in the reduction ratio is, the better the filling of a material in the processing channel becomes, such that the material may have a uniform cross section. However, when the reduction ratio is more than 60%, there is a limitation in that the material may break during processing due to an increase in a tensile load.
Also, the cross-section reduction segment may include a first cross-section reduction segment formed at one side of the processing channel and a second cross-section reduction segment formed at the other side of the processing channel.
The first cross-section reduction segment and the second cross-section reduction segment may include an overlapping segment overlapped each other when viewed from a direction perpendicular to a movement direction of a material, and a cross-section reduction of the processing channel is obtained at both sides of the processing channel in the overlapping segment.
Further, any one of the first cross-section reduction segment and the second cross-section reduction segment may have one or more cross-section reduction segments.
Any one of the first cross-section reduction segment and the second cross-section reduction segment may have one or more cross-section reduction segments, and the other cross-section reduction segment may have a curved portion having a constant radius of curvature R.
Also, the one or more cross-section reduction segments are formed at any one or both of the inlet path and the outlet path, and the other curved cross-section reduction segment may be formed between the inlet path and the outlet path.
The one or more cross-section reduction segments may have a slope in which a channel cross-section of a rear portion is smaller than that of a front portion when viewed from the movement direction of a material.
An inclined angle of the cross-section reduction segment may be in a range of 5-15°.
Hereinafter, a die for shear drawing according to the present invention is described in detail with reference to the following drawings.
Hereinafter, the die for shear drawing according to the present invention is described in detail with reference to
When a size of a processing channel L can be represented as a diameter as illustrated in
As illustrated in
The inlet path LI and the outlet path LO are combined to form a certain intersecting angle CA between respective central axes.
The processing channel L of the die for shear drawing according to the present invention includes diameter reduction segments A and B, in which the outlet diameter DO of the outlet path LO is reduced in comparison to the inlet diameter DI of the inlet path LI in order for a material to be drawn out by at least filling an outlet portion of the outlet path.
The diameter reduction segments A and B may include a first diameter reduction segment A formed at one side of the processing channel L and a second diameter reduction segment B formed at the other side.
Although only one second diameter reduction segment B is illustrated in
The first diameter reduction segment A and the second diameter reduction segment B may include an overlapping segment A+B overlapped each other when viewed from a direction perpendicular to the movement direction of a material, and a diameter reduction of the processing channel L is obtained at both sides of the processing channel L in the overlapping segment A+B.
The second diameter reduction segment B may have a slope at a certain angle AP in order that a channel diameter of a rear portion is smaller than that of a front portion when viewed from the movement direction of a material.
An inclined angle AP of the diameter reduction segment may be in a range of 5-15°.
The first diameter reduction segment A has a curved portion having a constant radius of curvature R between the inlet path and the outlet path.
The undescribed symbol RI in
Also, BL represents a bearing length connected to the outlet path of the present invention, wherein the bearing represents a segment determining the final diameter of a material after shear drawing deformation and is for improving dimensional accuracy.
Materials applied to the present invention may be nonferrous metals such as Al, Mg, or Cu in addition to carbon steels that require spheroidizing heat treatment. When a shear drawing method of the present invention is applied, mechanical properties may be improved by increasing effective strain by up to two times in comparison to a typical drawing process.
While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.
Hereinafter, Examples of the present invention is described in detail. However, the present invention is not limited to the following Examples.
A shape of a die for shear drawing according to the present invention and a shape of an ECAD die having the same channel diameter as the die for shear drawing were prepared. In order to compare degrees of filling of a material between two dies, experiments were carried out by using a finite element analysis program and fabricating the dies.
Finite element analyses were performed on the typical ECAD die having the same channel diameter and the die for shear drawing of the present invention in
In
As illustrated in
On the other hand, as illustrated in
Also,
As shown in
It may be understood that the filling of a material is improved according to an application of design factors of the present invention, and optimum design factor values appropriate for working conditions and workpieces may be also applied.
Experiments for the optimization of design factor values were performed in order to design a die having the best filling of a material as in the foregoing Example 1.
Design factors in the present Example are defined based on the drawing shown in
A finite element analysis program was used as in Example 1 to obtain conditions for the design factors having the maximum effective strain, and simulation conditions are as follows. A material diameter at an inlet of 10.0 mm, a material diameter at an outlet of 8.5 mm (a reduction ratio of 28%), an intersecting angle of 135°, a drawing speed of 100 mm/min, and a friction coefficient of 0.13 were used, and a test material used was medium carbon steel (0.45 wt % of C).
First, flow stress diagrams were analyzed by performing compression tests, and then, final finite element analyses were performed after obtaining effective stresses under a large strain of 1.1 or more of effective stain.
Design factor values in certain ranges as in Table 1 were applied.
After processing according to each design factor value through the finite element analyses, an average long/short-axis diameter of a final cross section of a material, i.e., a filling of a material was obtained.
In order to obtain design factor values exhibiting the maximum filling of a material, Experimental Examples of 2, 5 and 19, in which final diameters of materials calculated by each condition were close to an outlet path diameter of 8.5 mm, were selected. Drawings of dies for shear drawing designed according to the three selected conditions were presented in
Herein, it may be understood that a material subjected to shear drawing has an effective strain of 1.2-2.2 times greater than a typical material subjected to the same reduction ratio.
Particularly, it may be understood that a condition in Experimental Example 19 among the three selected conditions exhibits an excellent effective strain as well as the filling of a material.
As shown in the Example 2, it is important to apply optimum design factor values according to an intersecting angle and a reduction ratio. Final mechanical properties of a material are improved according to an improvement in the filling of a material, and particularly, grain refinement and spheroidization may be promoted by an increase in effective strain.
An optimized die was fabricated in Example 2, and various materials were deformed by shear drawing. Drawing conditions were the same as those of Example 2, and medium carbon steel (0.45 wt % of C), subjected to a spheroidizing heat treatment, was used as a workpiece.
The spheroidizing heat treatment is a process applied to a material mainly subjected to a cold forging process, and is a heat treatment process facilitating cold forging by a softening of a material. That is, the spheroidizing heat treatment is a process of transforming lamellar cementites having a hard microstructure into spherical shapes.
Although heat treatment conditions differ according to steel types or heat treatment facilities, a material during typical spheroidizing heat treatment is heated above an A1 temperature point and maintained just below the A1 temperature point for a certain period of time, and then, is stepwise cooled in a furnace. A total process may require a lengthy period of time, i.e., about 20-40 hours.
A portion of cementites having a lamellar structure is finely segmented due to the diffusion of carbon during heat treatment and a portion is redissolved in a matrix. Thus, spheroidization of the finely segmented cementites occurs at the same time.
Therefore, when the lamellar cementites are deformed by processing, spheroidization is promoted because end portions of the cementites are energetically unstable in comparison to the surroundings. This is similar to a phenomenon in which spheroidization is promoted in a material subjected to a drawing process.
It may be understood that spheroidization is better performed for the material subjected to shear drawing according to the present invention, and as a result, spheroidizing heat treatment time may be greatly reduced. Even considering sizes between a furnace in a laboratory and a furnace for a real process, spheroidizing heat treatment time in the real process may be reduced by half or more.
Therefore, the shear drawing process of the present invention may provide a promoting effect on spheroidization when the same reduction ratio is applied by substituting a die used in a typical drawing process. Also, it is considered that when shear drawing is applied to non-ferrous metals such as Al, Mg, or Cu, effective strain may be increased to be about two times as that of a typical drawing process based on the results of the Example 2 so that mechanical properties may be improved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2008-0134872 | Dec 2008 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR2009/007399 | 12/10/2009 | WO | 00 | 6/21/2011 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2010/074438 | 7/1/2010 | WO | A |
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| Number | Date | Country | |
|---|---|---|---|
| 20110247388 A1 | Oct 2011 | US |
