JP2002018531A - Hot press forming method of metal plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot press forming method of a material to be worked capable of improving formability of hot forming and a dimensional precision after forming in obtaining an ultra high strength press formed part. SOLUTION: Inside a die or outside/around the die, one or more of electrodes 4, 4' are mounted, by impressing an electric current between the electrodes, a metal plate is heated to a prescribed temperature by joule heat and then the hot 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, and more particularly to a method for directly heating a metal plate to be processed by electric heating and forming the metal plate at a temperature of 600 ° C. or higher.
The present invention relates to a hot press forming method in which quenching by rapid cooling is performed in a mold or outside the mold to obtain a molded product having high strength and good shape.

[0002]

2. Description of the Related Art Heretofore, hot forming to obtain a high-strength molded product, that is, die quenching, has been performed by performing hot forming and then quenching inside or outside a mold. The preheating method at that time has been performed using a heating furnace such as a hearth roller atmosphere heating furnace or a high frequency induction heating furnace.

[0003]

However, in the method of heating a workpiece by a heating furnace, a heating device and a transfer device are required, which increases the size of the device and increases the installation area of the device. In addition, the equipment cost is high.

[0004] When a hearth roller atmosphere heating furnace is used for hot working, for example, the conveying distance from the furnace to the inside of the mold after heating becomes long due to restrictions on equipment, so that the temperature of the heated metal plate decreases. There was a problem. In order to improve this, a method of increasing the furnace temperature has been adopted.However, if the temperature of the workpiece is increased, the yield point of the material is reduced, which makes handling difficult, In order to keep the temperature high, an excessive amount of heat is required, resulting in a problem of high cost.

[0005] On the other hand, when heating a rectangular blank during induction heating, the edge of the blank is preferentially heated, so that temperature unevenness is likely to occur, and an appropriate temperature depending on the shape of the blank is required. The shape of the heating coil must be studied and manufactured. In essence, when the material to be processed is a steel sheet, if the temperature exceeds the Curie point of the steel sheet, the heating efficiency decreases. Not suitable. Another problem is that the entire apparatus becomes large.

Meanwhile, in warm forming at 600 ° C. or lower, a method is generally used in which a mold is heated by various heaters and a work material is constrained by the mold during molding. However, since the temperature range handled in the present invention is an extremely high temperature range reaching 900 ° C. or more, such a heating means cannot be used from the viewpoint of heater capacity, mold life and the like.

[0007] It is an object of the present invention to improve the formability by hot forming and the dimensional accuracy after forming to obtain an ultra-high strength press-formed product. It is to provide a hot press forming method.

[0008]

In order to achieve the above object, in the present invention, as a method for heating a metal plate as a workpiece, one end of the metal plate to be heated is placed in or near a die. A hot press molding method is proposed in which a metal plate is heated to a predetermined processing temperature by Joule heat by sandwiching the electrodes between two or more electrodes and applying a DC or AC current between these electrodes.

That is, the present invention is as follows. (1) A press forming method for a metal plate, in which at least one electrode is attached to each end of the metal plate in a mold, and a current is applied between the electrodes, and the metal plate is subjected to Joule heat to predetermine the metal plate. A hot press forming method, wherein press forming is performed after heating to a processing temperature.

(2) A method for press-forming a metal plate, wherein one or more electrodes are attached to both ends of the metal plate in the vicinity of a die outside the die, and a current is applied between the electrodes to make a joule. A hot press forming method, wherein a metal plate is heated by heat, and then the metal plate is transferred into a mold and press-formed at a predetermined processing temperature.

(3) The electrode has a clamp mechanism, restrains the metal plate by the electrode, and applies a tension of 90% or less of a yield stress in a processing temperature range of the metal plate to the metal plate. The hot press forming method according to the above (1) or (2), wherein heating is performed while heating.

(4) The electrode is a spherical electrode having a tip having a radius of 20 mm or less, and at least one set of spherical electrodes is arranged at one or more locations at both ends of the metal plate.
The above (1) to characterized in that the metal plate is heated
The hot press forming method according to any one of (3) and (4).

(5) The hot press forming method according to any one of the above (1) to (4), wherein a heating rate by applying a current to the metal plate is 50 ° C./sec or more. (6) The above-mentioned (1), wherein the time from the end of heating by applying an electric current to the start of press molding is 3 seconds or less.
The hot press forming method according to any one of (1) to (5).

(7) Assuming that the inflow amount at the position where the material inflow is the largest in the molded product is L [mm], the molding speed V [mm / se
c] The hot press forming method according to any one of the above (1) to (6), wherein press forming is performed at a forming speed such that 0.08 × L.

(8) The predetermined processing temperature of the metal plate is 600 ° C.
The hot press forming method according to any one of the above (1) to (8), wherein the temperature deviation of the same steel sheet is 30 ° C. or less.

According to the present invention, preheating can be performed in the mold or in the vicinity of the mold as described above. Even when the shape of a molded product changes, it is only necessary to change the shape of the electrode and its arrangement. The method according to the present invention can be said to be a molding method that is rich in equipment flexibility because no complicated equipment replacement is required. When conducting heating outside the mold, heating to a predetermined processing temperature and then stopping current application may be carried into the mold, or may be carried into the mold while continuing to apply current. The temperature may be set to a predetermined processing temperature when reaching the inside of the mold. However, in the latter case, care must be taken because the current may be short-circuited by contact with the mold.

The apparatus for carrying out the present invention is also compact in terms of equipment and can be easily attached to existing equipment, as compared with those in other heating methods. Moreover,
Since the transport distance is short, it is possible to prevent a decrease in temperature due to the transport of the metal plate, and it is possible to obtain stable quenching strength.

Furthermore, in the heating method according to the present invention,
Since it is sufficient to energize only when heating the workpiece, that is, intermittently, the power cost can be reduced as compared with other heating methods. When heating by constraining both ends of the workpiece, there is a problem that the central portion hangs down due to thermal expansion and a spark comes into contact with the mold, but in a preferred embodiment of the present invention, the heating electrode After the workpiece is constrained vertically or horizontally, for example, with a clamp mechanism, the workpiece is heated while applying a tension of 90% or less of the yield stress of the workpiece at the heating temperature, so that the central part is bent. Can be prevented.

In hot press forming, it is not always necessary to heat the entire surface of the blank material. However, in the case of improving the blank yield in order to rationalize the cost, or in the case of a molded product shape in which a large amount of material flows into the flange portion. If the entire surface of the blank is not heated, breakage may occur during press forming due to the difference in strength generated in the metal plate, or the low temperature part may damage the mold because the metal plate is not softened, May not be obtained, uneven quenching hardness may occur. In this case, if an electrode such as a roll used in rolling or the like is used, the length of the contact line with the roll becomes longer, so that the heat removal to the electrode is larger, and also in a region other than the region between the rolls. This is inconvenient in some cases because the region is hardly heated.

However, in a preferred embodiment of the present invention, by devising the shape of the electrodes, 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.

In the case of hot forming, the heating rate is 50 ° C./sec.
The above is preferred. If the heating rate is less than 50 ° C / sec,
The scale formed on the surface of the workpiece is thick, and a good chemical conversion film is not formed in a subsequent chemical conversion treatment in a practical shot blasting time considered from the tact time in mass production.

Further, 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. In hot forming, the temperature of the work material is rapidly lowered by the metal contact between the work material heated to a high temperature and the cold mold at the time of forming, and the strength of the molded product can be increased by quenching. In this case, if the molding speed is slower than the quenchable cooling rate of the work material, quenching is completed while the work material passes through the die face in the mold, and that part is Passing through the die shoulder may cause poor shape after molding or breakage. At this time, it was experimentally confirmed that the molding speed had to be maintained at a certain speed or more in consideration of the maximum inflow length of the molded product, the quenching possible cooling speed of the workpiece, and the like.

Thus, according to the present invention, the maximum quenching hardness of the material can be realized only by press forming, and the heat treatment of a product having a complicated shape after forming can be omitted, which is of great practical significance.

[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 hot forming according to the present invention. FIG. 1 (a) shows a state before heating, and FIG. 1 (b) shows a state after forming after heating. Are respectively shown.

The heating / forming apparatus according to the present invention comprises a punch 1, a die 2, a plate holder 3, heating electrodes 4, 4 ', a cushion pin 6, a counter punch 7, and a temperature sensor 8, as shown in FIG. Be composed.

The illustrated example is a schematic diagram of press forming by a single-acting press machine, but the present mechanism can be similarly realized 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 rectangular drawing, or a product such as a hat drawing. The product 12 may have an open shape at both ends. In the case of the hat-shaped forming shown in FIG. 2B, draw forming in which the contact distance with the mold is large is desirable from the viewpoint of securing the cooling rate.

FIG. 3 is a schematic explanatory view showing a method of heating the workpiece 5 which is a molding material. Both ends 16 of the workpiece 5 are appropriately sandwiched between the current-carrying electrodes 4, 4 '. The electric heating is performed while applying the tension F in the direction of the outline arrow.

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

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 released, the electrodes 4 and 4 'are released to the outside of the mold, and the workpiece 5 is dropped onto the mold. Thereafter, normal press molding is performed.

Actually, assuming that the molding speed is V [mm / sec] and the inflow amount at the position where the material inflow is the largest in the molded product is L [mm], a value satisfying V> 0.08 × L is satisfied. In the case below this, the quenching may end when the material slides in the flange, so that breakage or poor shape may occur with the subsequent passage through the die shoulder.

Press working is performed in this manner, and after press forming, quenching is performed by holding the mold at the bottom dead center for an appropriate time. At this time, the press mold may be a mold for forming a normal high-strength steel sheet.

At this time, the material of the heating electrode is required to have heat resistance because the workpiece is heated. In order to increase the heating efficiency and to prevent the electrode from being damaged due to the heat generation and welding of the electrode to the workpiece, the smaller the electrical resistance, the better. Judging comprehensively, copper alloy or FC material is preferable.

In order to uniformly heat the workpiece,
As for the electrode shape, it is desirable that the tip portion is a spherical electrode having a spherical shape with a radius of 20 mm or less, and one or more spherical electrodes are arranged at both ends of a metal plate for heating a pair of upper and lower spherical electrodes. With such an electrode arrangement, the resistance around the electrodes is increased when the current flows around the electrodes, so that the electrodes are locally heated, and the outside between the electrodes is also heated, thereby suppressing temperature unevenness on the entire surface of the workpiece.
At this time, if the tip diameter of the electrode is larger than 20 mm, the effect of local heating does not appear, and the temperature deviation is rather large due to heat removal from the electrode, which is not good.

At this time, the temperature deviation of the workpiece is preferably 30 ° C. or less, more preferably 20 ° C. or less. If it is larger than this, the quenching hardness becomes insufficient or the shape after molding becomes poor.

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 preferably set to about 90% of the yield stress of the workpiece at a predetermined forming start temperature so that the workpiece does not undergo plastic deformation due to the tension.

In the case of hot forming, the heating rate is desirably 50 ° C./sec from the viewpoint of suppressing the formation of scale.
If it is slower than this, the scale thickness becomes large, which causes a problem in a removing step such as shot blasting or pickling. However, in general, it is preferable not to use the removal by pickling because it causes problems such as hydrogen embrittlement.

Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0038]

EXAMPLE 1 In this example, hat drawing was performed using the press apparatus shown in FIG. The punch shoulder radius and the die shoulder radius of the hat drawing were each 10 mm. The molding depth was 50 mm. At this time, the maximum inflow is about 50 mm. For lubrication, a graphite-based lubricant was used. In the work material,
A 440 MPa class cold-rolled steel sheet measuring 300 mm x 80 mm x 1.2 t was used.

The test was performed according to the following procedure. First, a steel plate is clamped in a mold. At this time, the clamp mechanism whose tip is formed by the electrode is supported from outside the mold by the robot arm. Thereafter, when current is started to flow between the electrodes and a predetermined processing temperature is reached, the clamp is removed, the steel plate is dropped onto the mold, and the clamp is released outside the mold. Immediately after this, press forming is performed, and the bottom dead center is maintained for a time corresponding to the hardenability of the steel sheet.

In this test, the holding time was 30 seconds.
At this time, in order to prevent the material from sagging during heating, the tension by the clamp was set to 40 MPa. This value corresponds to about 80% of the yield stress of the workpiece at the predetermined processing temperature, that is, at the target heating temperature of 950 ° C.

FIG. 4A is an explanatory view of the shape and arrangement of the electrodes used in this example. The tip of the electrode 40 has a spherical shape of R20 (mm) or less, and is located at both ends of the workpiece 5. They are arranged symmetrically in pairs and are also provided vertically above and below the workpiece 5. FIG. 4B is a graph showing the heating temperature in the width direction of the workpiece at this time. In this case, 950
C was set as the target heating temperature.

As shown in the figure, in the present invention, a uniform temperature is obtained over the entire surface of the workpiece, and the temperature deviation is suppressed to 30 ° C. or less. As a comparative example, a preheating method using a soaking furnace was also studied. The distance from the heating furnace to the mold was 2 m. In this experiment, the furnace was taken out of the furnace and transported into the mold by hand.

At this time, according to the present invention, when heating at a predetermined position and dropping into the mold, positioning of the blank material is easy. In the case of preheating with a heating furnace,
The material must be conveyed in a greatly softened state, and it takes time to position the mold.

Here, the time from the end of heating to the start of pressing was taken as the handling time. In the present invention,
After finishing heating, remove the clamp, drop the workpiece on the mold, and handle the clamp until it retracts.
The shortest was 1.5 seconds. At this time, in order to see the influence of the handling time, the test was performed by changing the time for removing the clamp after the completion of the heating and adjusting the handling time to 5 seconds.
On the other hand, when the material was transferred from the heating furnace into the mold, it took 5 seconds from the removal from the furnace to the start of pressing.

At this time, as an evaluation, the shape was measured by measuring the width of the bottom of the side wall of the hat member, and the error was compared with the normal shape.
It was determined to be good if it was 0.5 mm or less. The hardness is measured by measuring the cross-sectional hardness of the side wall where the contact amount (surface pressure) with the mold is small during quenching of the mold in holding the bottom dead center. It was good.

Regarding the scale, shot blasting was performed. Alumina # 100 was used as the abrasive, and the blast pressure was 4 kg / cm ² . At this time, the case where the blast treatment was performed for a certain period of time and then the chemical conversion treatment was performed, and a case where a good chemical conversion crystal was obtained was regarded as good.

Table 1 shows the results. Compared to the case of heating in a heating furnace as a conventional example, in the example of the present invention, molding can be started in a handling time of 1.5 seconds, whereas in the conventional example, it is as long as 5 seconds. The large size caused problems in the subsequent chemical conversion treatment.
In addition, in this case, after the temperature fell below the Ar ₃ point during the transportation, quenching was started by cooling the mold. Therefore, a sufficient quenched structure was not obtained, and a predetermined hardness was not obtained. Also,
Looking at the influence of the handling time in the example of the present invention, 3
When the time is less than 2 seconds, a good molded product is obtained, but when the time is longer than 3 seconds, there is a problem in hardness and scale formation.

Next, looking at the influence of the molding speed in FIG. 5, when the molding height is 50 mm, which is the present test condition, the molding speed is 1 to 1.
When changed to 10 mm / sec, the shape was good when the molding speed was 4 mm / sec or more, but when the speed was lower than that, the shape deteriorated. This shape degradation results from the increase in hardness as the material passes through the die face. Therefore, it was considered that the conditions for obtaining a good shape could be determined by the material inflow amount and the molding speed that determine the die face passage time, and a passage test in which the molding height was changed was performed. As a result, as shown in FIG. 5, it was experimentally found that the molding speed V needs to be higher than (0.08 × molding height Lmm).

FIG. 6 shows that the target heating temperature in this embodiment is 950.
4 is a graph showing a correlation between a scale thickness and a heating rate when the temperature is set to ° C. As can be seen from the illustrated results, the scale thickness can be reduced by setting the heating rate to 50 ° C./sec or more.

[0050]

[Table 1] This example shows an example of heating inside the mold as an example, but heating outside the mold also requires only transfer to the inside of the mold,
This can be performed in the same manner as described above. In this case, if there is no problem of short-circuit of current due to contact with the peripheral device and the mold, energization heating can be performed even during transfer into the mold.

[0051]

As described above, according to the present invention, as a method of heating a workpiece in hot forming, a hot heating is performed directly in a mold or outside a 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 function and effect of this, it is possible to improve the formability of a high-strength metal sheet and improve the dimensional accuracy after press forming. For example, it is possible to apply a high-strength steel sheet 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.

The present apparatus is characterized in that the apparatus itself is relatively inexpensive and the installation area is small because the heating apparatus itself is compact.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an apparatus for performing hot 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 view of an electric heating method according to the present invention.

FIG. 4A shows an electrode shape in the embodiment, and FIG.
(b) is a diagram showing temperature uniformity, respectively.

FIG. 5 is a graph showing the relationship between moldability and molding speed in Examples.

FIG. 6 is a graph showing a correlation between a heating rate and a scale thickness in Examples.

[Explanation of symbols]

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

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshiyuki Kaseda 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Inside Sumitomo Metal Industries, Ltd. (72) Yozo Hirose 4-chome, Kitahama, Chuo-ku, Osaka-shi, Osaka No. 5 33 Sumitomo Metal Industries, Ltd. F term (reference) 4E087 AA10 BA19 CB01 DB15 DB22 EA11 EC11 GA07 GA09

Claims (8)

[Claims] 1. A method for press-forming a metal plate, comprising: mounting at least one electrode at each end of a metal plate in a mold; applying a current between the electrodes; Press working after heating to a predetermined processing temperature. 2. A press forming method for a metal plate, wherein one or more electrodes are attached to both ends of the metal plate in the vicinity of a die outside the die, and an electric current is applied between the electrodes so that Joule heat is applied. A hot press forming method, wherein the metal plate is heated by a heating method, and then the metal plate is transferred into a mold and press-formed at a predetermined processing temperature. 3. The electrode has a clamp mechanism, restrains the metal plate by the electrode, and applies a tension of 90% or less of a yield stress in a processing temperature range of the metal plate to the metal plate. The hot press forming method according to claim 1, wherein heating is performed while heating. 4. The method according to claim 1, wherein the electrode is a spherical electrode having a tip having a radius of 20 mm or less, and at least one set of spherical electrodes is disposed at one or more locations at both ends of the metal plate to heat the metal plate. The hot press forming method according to any one of claims 1 to 3, wherein 5. The hot press forming method according to claim 1, wherein a heating rate by applying a current to the metal plate is 50 ° C./sec or more. 6. The hot press forming method according to claim 1, wherein the time from the end of heating by applying a current to the start of press forming is 3 seconds or less. 7. A molding speed V [mm / sec] when an inflow amount at a position where a material inflow is the largest in a molded article is defined as L [mm].
7. The hot press forming method according to claim 1, wherein the press forming is performed at a forming speed such that> 0.08 × L. 8. The hot press forming according to claim 1, wherein a predetermined working temperature of the metal plate is 600 ° C. or more, and a temperature deviation of the same steel plate is 30 ° C. or less. Method.