JP2002143935A - Warm press forming method of metal plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a product having good formability and good dimensional precision by warm press forming a metal plate of high strength like a stainless steel plate and a high strength steel plate. SOLUTION: One or more electrodes are respectively mounted to both end parts of the metal plate inside a die or at a neighborhood of the die and outside the die, by impressing an electric current between the electrodes, the metal plate is heated to a prescribed working temperature with joule heat, successively, press forming is conducted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for press-forming a metal plate as a workpiece, and more particularly to a method for press-forming a metal plate directly or by hot-pressing by heating the metal plate by heating. The present invention relates to a press molding method that improves the properties and dimensional accuracy after molding.

[0002]

2. Description of the Related Art Conventionally, when press-forming a high-strength and difficult-to-form metal plate such as stainless steel (SUS), a metal plate (a material to be processed) is used in order to compensate for the poor formability of the metal plate. In the following, warm forming in which a workpiece is simply heated to 600 ° C. or lower, or hot forming in which it is heated to 600 ° C. or higher has been performed.

At this time, as a method of heating the material, there is a method of directly heating a mold to a predetermined processing temperature by a ring heater, a band heater, or the like as described in Press Technology Vol. 25, No. 9. Further, there is a method in which a workpiece is heated by a heating furnace and then transferred into a mold and pressed.

[0004]

However, in the method in which the entire mold is heated to a predetermined temperature by the heater, the heat capacity of the entire mold is increased when the dimensions of the mold are increased. In addition to difficulties in the handling of the mold, uniform heating of the mold surface becomes difficult.

[0005] When a workpiece is heated by a heated mold, when the thickness of the workpiece becomes large, it takes time for uniform heating, so that the production efficiency is low. Further, during production, the temperature decrease due to heat transfer to the workpiece and heat radiation to the atmosphere and the entire mold must be compensated for by constantly energizing the heater, which requires an electrical cost.

Further, there is a portion between the punch and the die or the plate holder where the workpiece and the die do not come into contact with each other by the sum of the punch shoulder radius, the die shoulder radius, and the clearance. Insufficient, and there is a possibility that the spring back of the punch shoulder may expand.

[0007] Next, in the method of heating a workpiece by a heating furnace, a heating device and a transport device are required, so that the device is enlarged, so that not only the installation area of the device is increased, but also the equipment cost is high. Becomes

In addition, regarding the induction heating, when heating a rectangular blank, the edge of the blank is preferentially heated, so that temperature unevenness is likely to occur, and an appropriate heating coil shape according to the blank shape is used. Must be considered and manufactured. Another problem is that the entire apparatus becomes large.

[0009] It is an object of the present invention to improve the formability and the dimensional accuracy after forming by press forming a high-strength metal plate such as a stainless steel plate. An object of the present invention is to provide a method for warm press forming a material.

[0010]

In order to achieve the above object, in the present invention, a method for heating a metal plate as a work material includes a method of heating a plurality of ends of a metal plate to be heated in or near a mold. Normally, both ends are sandwiched between one or more pairs of electrodes, and a DC or AC current is applied between the electrodes to heat the metal plate to a predetermined processing temperature with Joule heat and then press-mold. Suggest a method.

That is, the present invention is as follows. (1) In the press forming of the metal plate, inside the mold,
Attach electrodes at one or more places to multiple ends of the metal plate,
A warm press forming method characterized in that a metal plate is heated to 80 to 600 ° C. by Joule heat by applying a current between electrodes, and then press forming is performed.

(2) In the press forming of a metal plate, electrodes are attached at one or more positions to a plurality of ends of the metal plate in the vicinity of the die outside the die, and a current is applied between the electrodes, so that Joule heat is applied. A warm press forming method characterized in that a metal plate is heated and then conveyed into a mold and press-formed at 80 to 600 ° C.

(3) The metal plate is restrained by an electrode having a clamp mechanism, and heating is performed while applying a tension of 90% or less of the yield stress in the processing temperature range of the metal plate between the electrodes. The warm press forming method according to the above (1) or (2), which is characterized by the following.

(4) The tip is a spherical electrode having a radius of 20 mm or less,
Any one of the above (1) to (3), wherein at least one set of spherical electrodes is arranged at one or more locations at a plurality of ends of the metal plate to uniformly heat the entire surface of the metal plate. The warm press forming method as described in the above.

(5) The heating rate of the metal plate to be heated is 20 ° C./sec.
The warm press forming method according to any one of the above (1) to (4), characterized in that: (6) The warm press forming method according to any one of the above (1) to (5), wherein one step from the start to the end of the forming is performed in 6 seconds or less.

(7) When the inflow amount of the material at the position where the material inflow in the molded article is the largest is defined as L (mm), the molding speed V
The warm press forming method according to any one of the above (1) to (6), wherein press forming is performed at a forming speed such that (mm / sec)> 0.3 × L.

According to the present invention, since only the workpiece is preheated inside the mold or in the vicinity of the mold, it is easy to cope with a change in the shape of the molded product. It can be said that the molding method is highly flexible.

Also, the apparatus is compact in terms of equipment and easy to mount on existing equipment, as compared with those in other heating methods. Further, in the heating method according to the present invention, since the power needs to be intermittently applied only when the workpiece is heated, the power cost can be reduced as compared with other heating methods.

In the case where the workpiece is heated while being restrained at both ends as in the present invention, there is a problem that the central portion hangs down due to thermal expansion and a spark comes into contact with the mold. In the present invention, the heating electrode is provided with a clamp mechanism, for example, by constraining the workpiece from above and below, and heating while applying a tension of 90% of the yield stress at the heating temperature of the workpiece, so that the center portion sags. Can be prevented.

In the conventional direct current heating, due to the heating characteristics, the temperature rise around the electrode is easily hindered by heat removal to the electrode,
Further, when the material is sandwiched between roll-shaped electrodes as performed in a rolling facility or the like, it is difficult to uniformly heat the entire surface of the workpiece because no Joule heat is generated outside and between the rolls. When there is such heating unevenness, a difference in strength occurs in the workpiece, which may cause shape defects (warpage, twist) and the like after molding. In addition, there is a possibility that a mold may be broken during molding or a part having a low temperature may be damaged because the workpiece is not softened.

However, in the present invention, by devising the shape of the electrode, it is possible to reduce the temperature drop and the uneven heating due to heat removal. In the case where a workpiece is formed by heating a metal plate, particularly a steel plate, a scale film is formed on the surface when the workpiece is exposed to the atmosphere at a high temperature for a long time. This scale film not only causes a problem in appearance, but also causes a problem in a chemical conversion treatment in a subsequent step. Therefore, the shorter the exposure time to the atmosphere, the better. However, in the case of the method of the present invention, high-speed heating is possible, so that the scale generation can be controlled. However, in the case of warm forming, the heating rate is 20 ° C./sec or more.

Further, the work material is heated and softened,
On the other hand, when the mold is kept in a cold state, the mold is hardly abraded or damaged even in forming a workpiece such as a high-strength metal plate, and the life of the mold can be extended.

In addition, since it is only necessary to intermittently supply electricity only when the workpiece is heated, the electric cost can be reduced as compared with other heating methods.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to illustrated examples. FIG. 1 is a schematic explanatory view of a heating and forming apparatus for heat forming in the in-mold heating mode of the present invention.
FIG. 1A shows a state when the workpiece 5 is heated before molding, and FIG. 1B shows a state when the workpiece 5 is molded after heating.

As shown in FIGS. 1 (a) and 1 (b), the heating / forming apparatus used in the present invention comprises a punch 1, a die 2, a plate holder 3, heating electrodes 4, 4 ', a cushion pin 6,
It comprises a counter punch 7 and a temperature sensor 8.

The illustrated example is a schematic diagram of cushion forming by a single-acting press machine, but the present invention can be similarly implemented by a double-acting press. As shown in FIGS. 2 (a) and 2 (b), the product formed according to the present invention may be a product 10 having a closed shape such as a cylindrical draw, or a double end product such as a hat draw. The product 12 may have an open shape. In the case of the hat-shaped molding shown in FIG. 2B, it is desirable to perform the bending with less contact with the mold and less heat removal.

FIG. 3 is a schematic explanatory view showing a method of heating the workpiece 5 which is a molding material.
Is appropriately sandwiched between the current-carrying electrodes 4, 4 ', and is heated while applying a tension F in the direction of the white arrow in the figure.

Here, the molding method according to the present invention will be described with reference to FIGS. 1 and 3. First, as shown in FIG. 1 (a), the workpiece 5 is heated outside the mold by heating electrodes 4,4. The work material 5 is conveyed into the mold while the end portion 16 (see FIG. 3) of the work material 5 is restrained by the clamp constituted by '.

Next, as shown in FIG. 1 (b), the mold is heated by applying a current between the electrodes 4 and 4 'inside the mold. When the temperature reaches a predetermined temperature, the clamp is removed, the electrode is released to the outside of the mold, and the workpiece is dropped on the mold. Thereafter, a normal press is performed. At this time, it is desirable that the molding speed be high in order to prevent heat removal due to contact with the mold.

According to the present invention, the molding speed is set to 10 spm (stroke).
per minute: the number of strokes per minute, that is, the number of presses). The heating temperature of the work material can be freely controlled by the applied current or the energizing time by direct energizing heating, but if the heating temperature is low, the softening of the metal plate does not proceed, and the effect of the heat forming is not exhibited, which is preferable. Absent. In addition, it is difficult to adjust the variation between the heating and the heat removal to the atmosphere, and it is difficult to control the temperature. Therefore, in warm forming, the lower limit is practically 80 ° C.

Conversely, when the heating temperature is high, the softening of the material is accelerated and there are many advantages from the viewpoint of forming. However, the structure of the metal plate changes due to recrystallized grains and the like, and the tensile strength after forming becomes insufficient. It is not preferable because the scale adheres to the surface of the workpiece.

In the case of warm forming, if the heating temperature is high, not only the heating time is required but also the electrodes are welded to the metal plate,
In addition to reducing the appearance quality of the molded product, the life of the electrode is shortened. Therefore, during warm forming, the heating temperature is 600 ° C.
It is desirable to make the following.

The material for the heating electrode is required to have heat resistance because the workpiece is heated. Also, in order to increase the heating efficiency, the smaller the electric resistance, the better. Judging comprehensively from the above, the electrode material is preferably a copper-aluminum alloy for forming a stainless steel plate (SUS) or the like, and is preferably an FC material for a plain steel plate. Considering the heat removal by the mold, a copper alloy or FC material is preferable.

In order to uniformly heat the workpiece, the electrode is formed into a spherical electrode having a tip of a radius of 20 mm or less, and a plurality of ends, usually both ends, of a metal plate for heating a pair of upper and lower spherical electrodes. It is desirable to arrange more than two places. With such an electrode arrangement, the periphery of the electrode is locally heated, so that the outside between the electrodes is also heated, and uneven heating on the entire surface of the workpiece is suppressed.

In order to prevent the central portion of the workpiece from sagging, it is desirable to apply an appropriate tension between the electrodes. The tension at this time is 90% of the yield stress at the processing temperature so that the workpiece does not undergo plastic deformation due to the tension.
It is preferable to keep the degree.

Next, the molding speed in the present invention is determined by setting the inflow amount of the material at the position where the inflow of the material is the largest in the molded product to L (m
When m), the molding speed is preferably such that the molding speed V (mm / sec)> 0.3 × L. If the speed is lower than this, the heat removal from the mold during molding becomes large, the material temperature decreases, and the moldability of the material may deteriorate. Further, the material strength during molding increases, the elastic recovery after molding increases, and the dimensional accuracy of the product deteriorates. In particular, when the method of the present invention is used for bending, the effect on dimensional accuracy is remarkable, and a product with good dimensional accuracy is obtained when the forming speed is equal to or higher than a specified value.

In addition, comparing the method of the present invention with the conventional method of heating the mold within the same production time (10 spm or more), the conventional method cannot sufficiently form the metal plate due to insufficient heating (breakage occurs). Or, even if a product is obtained, the dimensional accuracy is extremely deteriorated.

Next, in order to clarify the effects of the present invention,
An embodiment will be described.

[0039]

EXAMPLE 1 In this example, hat drawing and bending were performed using a press device shown in FIGS. 1 (a) and 1 (b). FIGS. 4 (a) and 4 (b) show the state of bending and hat drawing.

In the case of the bending shown in FIG. 4A, a die 22 is placed on a blank 22 which is a workpiece placed on a punch 20.
Press forming is performed using the blank 24. In this case, the peripheral portion 26 of the blank 22 is set as a free end.

On the other hand, in the case of the draw forming shown in FIG. 4B, the peripheral portion 26 of the blank 22, which is the workpiece, is pressed by the die 24 and the blank holder during the press forming using the die 24.
28 and fixed.

The white arrow in each figure indicates the direction in which the die 24 presses. Hat type punch width is 80mm,
The punch shoulder radius and the die shoulder radius were each 5 mm. Nippon Machine Oil G-2576 was used for lubrication.

At this time, if the thickness of the lubricating film is set to about 2 g / m ² , the lubricating property is satisfied and the influence on the electric heating is small. The material of the workpiece was a 590 MPa class cold-rolled high-strength steel sheet with a thickness of 1.2 mm, and the material of the heating electrode was FC250.

As a comparative example of the heating method, a method of forming a die and a plate holder by heating with a pipe heater was also carried out. The blank size was 80 × 180 mm. FIG. 5 is a graph showing the heating temperature in the longitudinal direction of the workpiece at this time. In the drawing, the solid line is a temperature measurement graph of the example of the present invention, and the broken line is that of the comparative example. In this case, 450 ° C. was set as the heating target temperature.

The state of the warm press forming in this example is shown in FIG.
FIGS. 6A and 6B schematically show the results, and FIG. 6C collectively shows the results. That is, a blank, which is a workpiece having a width of 100 mm and a length of 300 mm, was heated at 450 ° C.
Free BHF molding was performed using a press die with the specifications shown in (a). The warpage of the side wall at this time was measured in the manner shown in FIG. 6B, and the result is shown in a graph in FIG. 6C.

According to the present invention, the molding speed is set to 10 spm (stroke).
Per minute: the number of strokes per minute, that is, the number of presses) It can be seen that the amount of warpage of the side wall portion is small, that is, press molding excellent in molding and excellent in dimensional accuracy can be performed.

In the case of the comparative example, it takes 15 seconds to raise the temperature of the workpiece to a predetermined temperature, and the productivity is extremely deteriorated. In addition, since the end in the longitudinal direction did not reach the predetermined temperature, when the treatment of the cut surface of the end was poor, minute cracks occurred.

FIG. 7 is a graph showing the correlation between the scale thickness and the heating rate when the set temperature is 450 ° C. in this example. By setting the heating rate to 20 ° C./sec or more, the scale thickness can be reduced.

In this example, the processing time required from the attachment of the workpiece to the electrode to the removal of the press-formed product is 8
~ 25 seconds, averaging 12 seconds.

[0050]

As described above, according to the present invention, as a method of heating a workpiece by heat molding, direct current heating is performed in the mold or in the vicinity of the outside of the mold, thereby taking advantage of the feature of high-speed temperature rise. In addition, the processing time can be reduced, the power cost can be reduced, and the surface scale can be suppressed.

As an overall operational effect, since the formability of a high-strength metal sheet can be improved and the dimensional accuracy after press forming can be improved, for example, a high-strength steel sheet can be applied to automobile parts. As a result, it is possible to contribute to improving the safety of automobiles and reducing fuel consumption by the effect of reducing the body weight.

It should be noted that the apparatus for practicing the present invention has advantages that the apparatus itself is relatively inexpensive and the installation area is small because the heating apparatus itself can be made compact.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an apparatus for performing warm press forming according to the present invention. FIG. 1 (a) shows a state before forming, and FIG. 1 (b) shows a state at the time of forming.

FIGS. 2 (a) and 2 (b) are schematic diagrams of the shape of a molded product.

FIG. 3 is an explanatory diagram of an electric heating method according to the present invention.

FIGS. 4 (a) and 4 (b) are schematic illustrations of a molding method in each of the examples.

FIG. 5 is a graph showing a comparison between electric heating according to an example of the present invention and heating in a comparative example.

6 (a) and 6 (b) are schematic illustrations of warm press forming in Examples, and FIG. 6 (c) is a graph showing the results.

FIG. 7 is a graph showing the correlation between the heating rate and the scale thickness when the set heating temperature in Example 1 is 450 ° C.

[Explanation of symbols]

 1: Punch 2: Die 3: Plate holder 4, 4 ': Heating electrode 5: Work material 6: Cushion pin 7: Counter punch 8: Temperature sensor

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiyuki Kaseda 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4E063 AA01 AA20 BA01 CA06 KA03 KA06 KA09

Claims (7)

[Claims] In a press forming of a metal plate, an electrode is attached at one or more positions to a plurality of ends of the metal plate inside a metal mold, and a current is applied between the electrodes. A warm press forming method characterized by performing press forming after heating to up to 600 ° C. 2. In the press forming of a metal plate, an electrode is attached to one or more positions on each of a plurality of ends of the metal plate in the vicinity of a die outside the die, and a current is applied between the electrodes.
A warm press forming method characterized in that a metal plate is heated by Joule heat, then transferred into a mold and press-formed at 80 to 600 ° C. 3. The method according to claim 1, wherein the metal plate is restrained by an electrode having a clamp mechanism, and the metal plate is heated while giving a tension of 90% or less of a yield stress in a processing temperature range of the metal plate between the electrodes. The warm press forming method according to claim 1 or 2, wherein A spherical electrode having a radius of not more than 20 mm at the tip and at least one set of spherical electrodes disposed at one or more positions on each of a plurality of ends of the metal plate to uniformly heat the entire surface of the metal plate. The warm press forming method according to any one of claims 1 to 3, wherein: 5. The warm press forming method according to claim 1, wherein a heating rate of the metal plate to be heated is 20 ° C./sec or more. 6. The method according to claim 1, wherein one step from the start to the end of the molding is performed in 6 seconds or less (10 spm or more).
6. The warm press forming method according to any one of items 1 to 5. 7. A molding speed V (m) when a flow rate of a material at a position where a material flow is largest in a molded product is defined as L (mm).
The warm press forming method according to any one of claims 1 to 6, wherein press forming is performed at a forming speed such that m / sec)> 0.3 × L.