KR20140136327A - Deep drawing method of stainless steel for preventing breakage - Google Patents

Abstract

According to the present invention, a deep drawing method for stainless steel sheet is characterized in that a flange part of the stainless steel sheet is divided into a low temperature area and a high temperature area and is heated to form a temperature gradient on the flange part; and the low temperature are is processed into a corner part in order to prevent a necking from being generated.

Description

Technical Field [0001] The present invention relates to a deep drawing method of a stainless steel sheet for preventing breakage,

The present invention relates to a deep drawing method of a stainless steel sheet for preventing breakage, and more particularly, to a deep drawing method of a stainless steel sheet with improved formability by preventing necking occurring at a corner portion having a large amount of molding.

As the necessity and importance of the plastic processing method, which is currently in the limelight due to mass production and economic means, are recognized, researches in this field have been actively carried out, and many studies have been conducted on the molding of thin plate metal.

Deep drawing in sheet metal press forming is carried out at room temperature but it requires stable process due to stable plastic deformation within the allowable stress range such as intermediate annealing due to work hardening characteristics during deformation. do.

As the demand has diversified along with the recent development of the technology, the shape of the product becomes complicated. In order to cope with the small quantity production of various kinds of products, a high value added product that improves the formability, shortens the process, improves the quality, Processing technology is required.

In order to solve such a problem, the conventional deep drawing technology utilizes a method of heating most of the metal to increase the moldability of the metal material.

Among austenitic stainless steels, austenitic steels are those with phase transformation depending on the pressing process. These steels have a phase transformation according to the pressing process and are easy to apply to warm forming. It is generally known that martensite induced by cold working has a bad influence on deep drawing workability because it increases work hardening degree.

In this case, if the temperature of the die and the blank holder or the like is raised, the press load is lowered, and the deep drawability is improved. That is, the deformation of the metal plate during drawing is concentrated at the flange portion, and this portion becomes the side wall of the cup by molding at a later time, and this portion hardly deforms. Consequently, in order to improve the moldability, the breaking stress of the side wall portion must be larger than the drawing resistance of the flange portion.

It is possible to expect an effect of improving the workability in press molding of a complicated shape such as an automobile fuel tank by designing a warm mold, but there is a problem that breakage may still occur at a corner portion having a larger molding amount.

It is an object of the present invention to provide a deep drawing forming method of a metal plate which prevents breakage due to necking of punch corner portions that may occur during deep drawing forming of stainless steel have.

According to an aspect of the present invention, there is provided a method of forming a deep drawing of a stainless steel sheet in which a flange portion of a stainless steel sheet is formed in a low temperature region and a high temperature region And the low temperature region is processed into a corner portion.

The high-temperature region and the low-temperature region of the flange portion may be formed by different amounts of heat supplied from a die or a blank holder provided between the flange portions of the stainless steel sheets.

By inserting a plurality of heaters into the die or the blank holder, the high temperature region and the low temperature region of the flange portion can be formed by changing the intervals.

A plurality of heaters are inserted into the die or the blank holder, and a high temperature region and a low temperature region of the flange portion can be formed by providing a heat insulating material between the heaters.

A plurality of heaters are inserted into the blank holder, and the blank holder is formed in a stepped shape, so that the high temperature region and the low temperature region of the flange portion can be formed depending on whether the stainless steel sheet is in contact with the blank holder.

The stainless steel plate may be stainless 304 steel.

Wherein the temperature of the high temperature region is controlled to be in the range of 150 to 200 ° C and the temperature of the low temperature region is controlled to be in the range of 100 to 125 ° C with the stroke speed of the punch being 1 m / 50-80 < 0 > C.

The deep drawing forming method of stainless steel according to the present invention has the following effects.

First, the occurrence of fracture can be reduced by preventing the necking of the corner portion of the stainless steel, so that the molding limit can be improved due to the increase of the limit drawing ratio.

Secondly, it is possible to form products with complex shapes such as automobile fuel tanks, etc., and the number of processes can be shortened by improving the formability.

Third, a product having a relatively uniform thickness and strength can be obtained in the deep draw forming process.

1 is a schematic view of a deep drawing die of a stainless steel sheet according to an embodiment of the present invention.
2 is a perspective view of a blank holder according to an embodiment of the present invention.
3 is a cross-sectional view of a blank holder according to one embodiment of the present invention.
4 is a graph showing the surface temperature of a blank holder according to an embodiment of the present invention.
5 is a cross-sectional view of a blank holder according to another embodiment of the present invention.
6 is a cross-sectional view of a blank holder according to another embodiment of the present invention.
7 is a stress-strain graph of a stainless steel 304 steel according to an embodiment of the present invention.
8 is a view showing the temperature gradient of the blank holder during round cup deep drawing forming according to an embodiment of the present invention.
9 is a graph showing the temperature gradient of the blank holder during round cup deep drawing forming according to an embodiment of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified, and that other specific features, regions, integers, steps, operations, elements, components, and / And the like.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

Hereinafter, a method of forming a deep drawing of a metal plate according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

1 is a schematic view of a mold used in a deep drawing forming method of a stainless steel sheet according to an embodiment of the present invention. The stainless steel sheet P is placed between the die 10 and the blank holder 30 for deep drawing forming. The stainless steel sheet P is introduced into the space formed in the die 10 by pressurization by the punch 20 to be formed. At this time, the flange portion of the stainless steel sheet P between the die 10 and the blank holder 30 is subjected to a compressive stress in the circumferential direction and a tensile stress in the radial direction.

In such a deep drawing molding, most of the side wall is formed due to the inflow effect at the flange portion, rather than the tensile molding effect at the portion contacting the punch 20. Therefore, deformation may concentrate on the corner portion 40 of the stainless steel plate, resulting in necking, which may cause breakage of the final product.

In the embodiment of the present invention, in order to prevent the corner portion 40 of the stainless steel sheet P from being broken due to necking, the flange portion of the stainless steel sheet is divided into the low temperature region and the high temperature region, And is processed as the corner portion 40.

When the flange portion of the stainless steel sheet P is heated, the strength of the low-temperature region is higher than that of the high-temperature region, and the necking is dispersed in the high-temperature region.

At this time, the low temperature region and the high temperature region corresponding to the corner portions can be formed by different amounts of heat supplied from the die or blanking holder provided between the stainless steel plate members.

Further, in order to vary the amount of heat supplied to the flange portion of the stainless steel plate, a plurality of heaters are inserted into the die or the blanking holder, and the intervals between the heaters are varied, thereby forming the high temperature region and the low temperature region of the flange portion of the stainless steel plate.

2 is a schematic view of a blank holder according to one embodiment of the present invention. 3 is a cross-sectional view of a blank holder according to one embodiment of the present invention. As shown in FIGS. 2 and 3, five heaters 31 may be provided inside the blank holder 30, and the low temperature region of the flange portion corresponding to the corner portion 40 of the stainless steel sheet P The interval between the heater 31 inserted into the lower portion of the blank holder surface contacting the high temperature region of the flange portion is made narrower than the interval of the heater 31 inserted into the lower portion of the blank holder surface contacting the lower surface of the blank holder 30, The amount of heat transferred to the steel sheet P can be controlled.

The temperature of the surface of the blank holder 30, which is in contact with the stainless steel plate, can be adjusted by connecting the heater 31 to a separate temperature control device.

Further, the thickness of the blank holder 30 and the position of the heater 31 can be determined according to the heat transfer coefficient of the material of the blank holder.

4 shows the surface temperature of the blank holder according to the embodiment of the present invention by the position of the blank holder when heating the heater of the blank holder. It can be confirmed that the temperature gradually increases at each portion with respect to the corner portion of the stainless steel sheet depending on the position of the blank holder on the mold, thereby forming a temperature gradient.

It was confirmed that the surface temperature of the mold increases according to the three set temperatures. It was confirmed that the temperature does not show a large difference from the set temperature even when one or two of the five heat lines are not operated.

5 is a schematic view of a blank holder according to another embodiment of the present invention. As shown in FIG. 5, in the embodiment of the present invention, the blank holder 30 may have five heaters 31 therein, and the heaters 32 may be provided between the heaters. At least one heat insulating material 32 may be provided and the temperature gradient of the flange portion of the stainless steel sheet in the low temperature region and the high temperature region may be adjusted by adjusting the position of the heat insulating material 32 and the heating temperature of the heater 31.

5 is a schematic view of a blank holder according to another embodiment of the present invention. 5, the blank holder 30 may be provided with five heaters 31 at regular intervals, and the low temperature region of the flange portion of the stainless steel sheet, A step can be formed in the blank holder 30 so that the surface of the flange of the stainless steel sheet abutting the hot region of the flange is brought into contact with the surface. Accordingly, a temperature gradient can be formed in the flange portion of the metal plate to be formed due to the difference in the amount of heat transferred to the blank holder.

In one embodiment of the present invention, the stainless steel is a metastable austenitic stainless steel, which is a 304 steel containing 17.0 to 19.0% of Cr, 7.8 to 9.0% of Cr, 7.8 to 9.0% of Ni, 0.08 to 0.06 of N, %, And C: 0.02 to 0.06%. These stainless steels have a characteristic of being a metastable state at room temperature, and have a property of producing processed organic martensite under the change of phase stability upon plastic working. Therefore, the production of processed organic martensite is rapidly activated according to the amount of molding during drawing forming, and the material is strengthened. Therefore, the deformation resistance of the material caught around the mold bead sharply increases. In order to reduce the deformation resistance of the material applied to the mold bead, it is necessary to lower the work hardening sensitivity of the material, and it is necessary to lower the work hardening sensitivity of the material in the metastable austenitic stainless steel. In metastable austenitic stainless steels, the phase stability increases with increasing temperature, and thus the formation of the processed organic martensite phase degrades during material deformation.

7 is a stress-strain graph of a stainless steel 304 steel according to temperature. As shown in FIG. 7, the tensile properties of the stainless steel 304 steel are influenced by the temperature. In particular, the stress-strain curves at room temperature and 100 ° C show that as the temperature increases, the strength decreases and the strain increases somewhat. However, at higher temperatures above 200 ℃, the degree of the decrease in strength is small, but the strain is greatly reduced. As a result, it can be seen that the plastic deformation properties of the stainless steel 304 are influenced by the temperature and there is a favorable temperature condition for the processing.

In the embodiment of the present invention, when the deep draw forming is performed using the stainless steel 304, the temperature of the high temperature region of the flange portion of the stainless steel plate is 150 to 200 占 폚, the temperature of the low temperature region of the flange portion of the stainless steel plate is 100 to 125 占 폚 The stroke speed of the punch is within 1 m / s, and the temperature difference between the high-temperature region and the low-temperature region may be 50 to 80 ° C.

When the temperature of the flange portion of the stainless steel sheet is lower than 150 ° C, the improvement in drawability due to the reduction in deformation resistance becomes insignificant. When the temperature is higher than 200 ° C, the stress decrease due to temperature increase is insignificant, the strain is reduced and the effect of improving workability is not so large.

The temperature of the low-temperature region of the flange portion of the stainless steel sheet is set so as to cause a temperature gradient with respect to the high-temperature region. When the temperature is lower than 100 ° C, improvement in drawability due to reduction in deformation resistance becomes insignificant, The effect that the necking is dispersed to the peripheral portion can be small.

Therefore, when the temperature difference in the low-temperature region with respect to the high-temperature region is 50 to 80 ° C, the deep drawability of the stainless steel sheet can be improved and the deformation can be dispersed to minimize the thickness reduction of the material.

On the other hand, the speed of the stroke of the punch is preferably 1 m / s or less so as to prevent breakage.

8 and 9 are views showing the temperature gradients of the blank holder portion during round cup deep drawing forming according to an embodiment of the present invention.

In one embodiment of the present invention, a finite element analysis was performed using a round cup to derive the optimal conditions of the temperature distribution between the mold and the material within the temperature range favorable for molding. As shown in FIGS. 8 and 9, the deep drawability according to the temperature gradient of the blank holder was examined, and it was examined whether the thickness reduction can be minimized while maximizing the deep drawability with a temperature gradient.

The smaller the maximum thickness reduction after molding according to the temperature gradient of the blank holder, the more the heating effect of the flange portion of the stainless steel was maximized in the deep drawing forming, and the results of analysis are shown in Table 1 below.

NO LDR The low temperature region (Ti) (占 폚) High temperature region (To) (占 폚) Whether it is molded Minimum thickness (mm) One 2.3 25 150 Fracture - 2 50 150 O 0.426 3 75 150 O 0.451 4 100 150 O 0.47 5 150 150 O 0.475 6 25 200 O 0.379 7 50 200 O 0.465 8 75 200 O 0.471 9 100 200 O 0.475 10 150 200 O 0.475 11 200 200 O 0.474 12 2.35 25 150 Fracture - 13 50 150 Fracture - 14 75 150 O 0.417 15 100 150 O 0.449 16 150 150 Fracture 0.456 17 25 200 Fracture 18 75 200 O 19 100 200 O 20 125 200 O 21 150 200 Fracture 22 200 200 Fracture

As shown in Table 1, when the temperature of the blank holder in the high-temperature region is in the range of 150 to 200 占 폚 when the limit drawing ratio (LDR) value is 2.3 (the diameter of the blank is 115 mm), the temperature of the blank holder in the low- Lt; 0 > C or more, thickness reduction is minimized and moldability is improved.

When the temperature of the blank holder in the high-temperature region is 200 ° C when the temperature of the blank holder in the low-temperature region is 75 to 100 ° C when the temperature of the blank holder in the high-temperature region is 150 ° C when the LDR value is 2.35, It was confirmed that the moldability was improved when the temperature of the blank holder was 75 to 125 ° C.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

P: Stainless steel plate 10: Die
20: punch 30: blank holder
31: heater 40:

Claims (7)

Wherein the stainless steel sheet is heated in such a manner that a flange portion of the stainless steel sheet is divided into a low-temperature region and a high-temperature region so as to form a temperature gradient, and the low-temperature region is processed into a corner portion so as to prevent occurrence of necking at a corner portion Drawing molding method. The method according to claim 1,
Wherein the high temperature region and the low temperature region of the flange portion are formed by different amounts of heat supplied from a die or a blanking holder provided with a flange portion of the stainless steel plate interposed therebetween. The method of claim 2,
Wherein a plurality of heaters are inserted into the die or the blank holder, and the high-temperature region and the low-temperature region of the flange portion are formed by varying the intervals. The method of claim 2,
Wherein a plurality of heaters are inserted into the die or the blank holder, and a high-temperature region and a low-temperature region of the flange portion are formed by providing a heat insulating material between the heaters. Claim 2
Wherein a plurality of heaters are inserted into the blanking holder so that the blank holder is formed in a stepped shape to form a high temperature region and a low temperature region of the flange portion depending on whether the stainless steel plate is in contact with the blank holder . The method according to any one of claims 1 to 5,
Wherein the stainless steel sheet is a stainless steel 304 steel. The method of claim 7,
Wherein the temperature of the high temperature zone is in the range of 150 to 200 ° C, the temperature of the low temperature zone is in the range of 100 to 125 ° C, the stroke speed of the punch is within 1 m / s, Is in the range of 50 to 80 占 폚.

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