JPH038850B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dieless plate forming method using thermal stress.

In press forming of non-mass-produced large structures and small- to medium-sized mass-produced products, mold costs account for a large proportion of processing costs. It is clear that there would be great advantages if these products could be molded without a die. However, there are few research reports on dieless molding of such products.

One of them is the linear heating plate bending method (Shipbuilding Department, Ishikawajima Heavy Industries Co., Ltd.; Shipbuilding, Vol. 29, No. 12 (1956), pp. 1037 to 1044). This method uses an appropriately sized oxyacetylene burner as a heat source, moves this heat source over the surface of the steel plate in a straight line or in a curved line at an appropriate temperature, and rapidly cools the area around the heated area using a water spray device. In particular, it is intended to create a thermal strain difference in the thickness direction of the steel plate, and to bend the steel plate on both heating sides using the heating line as a broken line. For example, as shown in Fig. 1a, if the linear heating wire is sequentially moved at regular intervals as shown by the arrow 2 on the surface of the flat steel plate 1, as shown in Fig. 1b.
As shown in FIG. 2, the flat steel plate is curved slightly toward the heating surface side using the dotted chain line 3, which was the heating line, as a broken line, so that an arc-shaped steel plate 4 can be obtained. However, in order to increase the effectiveness of this method, it is said that it is preferable to apply a certain amount of load (a moment within the elastic range) to the steel plate beforehand before processing.

Another example is the transformation superplastic phenomenon, in which the metal material is press-formed under a low load while being subjected to a triangular waveform temperature cycle, with the phase transformation point that appears during heating and cooling of the metal material as an intermediate temperature. There is a press processing method for thin sheet metal materials that utilizes this method (see Japanese Patent Publication No. 56-44132). Furthermore, it is also stated that in some cases, this processing method can sufficiently perform such forming using only the weight of the material without requiring a load on the upper die.
In other words, this processing method applies a triangular waveform heating/cooling cycle with the phase transformation point at an intermediate temperature to the entire thin sheet metal material to be processed, and at the same time uses only the lower die and applies only the weight of the material to be processed. The method is to perform a molding process similar to dieless processing.

Unlike the conventional dieless forming method described above, the present invention utilizes only the thermal stress generated during heating of the front surface of the metal plate to be processed and cooling of the back surface immediately after heating is stopped, without applying any load in advance. The purpose of this invention is to provide a method for dieless plate forming.

In the process of conducting various experiments and studies on plastic working of metal plates, the present inventors discovered that the peripheral area of a metal material plate was designed to allow the metal plate to move freely without being restrained from the outside. While the metal plate is being supported, intermittent heating is applied to the surface of the metal plate in a dotted, linear or curved shape, and at the same time, a position corresponding to the surface heating position on the back side of the metal plate is heated during the intermittent heating. Heating and cooling during non-heating immediately after heating stops.
By repeating the cooling process a predetermined number of times and repeatedly applying a local temperature gradient between the front and back surfaces of the metal plate, the process can be performed without applying a certain amount of load to the metal plate as in the prior art. The present invention was achieved by discovering that the metal plate bends on both sides when heated around the surface heating portion.

Next, an outline of the deformation behavior of the metal plate to be processed during the heating/cooling process of the present invention will be explained with reference to FIG. Figure a shows the state before processing, 11 is a rectangular flat plate test piece, 14 is a high frequency heating coil installed in the width direction at the center of the surface of the test piece 11, and this coil 14 allows the test piece 11 to be heated in the width direction. It is heated linearly in a narrow width. Figure b shows the high frequency heating coil 14.
shows a state in which the center of the surface of test piece 11 was rapidly heated linearly, and if there was only thermal expansion, test piece 1
1 bends downward to the position shown by the broken line, but in reality, due to the thickness and rigidity of the test piece, the bend remains at the position shown by the solid line, and test piece 11 bends downward with the heating line as the center line. Show that. Figure c is specimen 11.
The figure shows the state during linear cooling on the back side of the test piece 11. When heating from the front side is stopped and immediately after that, the position corresponding to the surface heating line on the back side of the test piece 11 is cooled with water from the back side, the This is thought to be because the surface heating portion receives compressive stress in the longitudinal direction of the plate, but as shown in the figure, the test piece 11 is bent upward around the surface heating line. In the present invention, the bending angle α is assumed to be as shown in Figure c.

Next, an embodiment of the method for forming a dieless plate using thermal stress according to the present invention will be described. First, both ends of a test piece 11 made of an S10C pressed steel plate with a thickness of 2 mm, a width of 5 mm, and a length of 140 mm were placed on a heat-resistant glass round bar. Next, the water-cooled high-frequency heating coil 14 heats the center of the surface of the test piece 11 in a straight line. In this case, the heating rate (Hr) is 40deg/sec, the heating/cooling treatment upper limit holding temperature (Th), that is, the maximum temperature at which the surface of the heated part of the test piece 11 should be held is 950°C, and the holding time (HT) That is, the time period during which the temperature of the surface of the heated portion of the test piece 11 should be maintained at the heating/cooling treatment upper limit holding temperature (Th) is set to 5 seconds.
That is, the test piece 11 is heated by the high frequency heating coil 14.
By heating for about 23 seconds at a heating rate (Hr) of 40 deg/sec, the temperature of the heated part of test piece 11 was brought to 950°C, and then the temperature of the heated part of test piece 11 was increased for 5 seconds.
Hold at 950℃. Next, high frequency heating coil 1
At the same time, the position corresponding to the surface heating position on the back side of the test piece 11 was cooled with water for 25 seconds. Next, leave it for 5 seconds. Such heating and cooling treatments are repeated a predetermined number of times.

Furthermore, the present invention will be explained in detail using experimental examples.

(1) Test materials The test materials were S10C rolled polished steel plates with thicknesses t of 2 and 3 mm, and their chemical compositions are shown in a table in Figure 3. Test pieces were cut out from these materials and machined to a width of 5 mm and a length of 140 mm.

(2) Experimental method Figure 4 is a schematic diagram of the experimental apparatus used in this experiment, with figure a being a front view and figure b being a side view.

In the figure, 11 is a test piece, and its both ends are placed on a heat-resistant glass round rod 13 placed on a support stand 12 with a lifter equipped with a heat-resistant brick stand so that it can move freely without being restrained from the outside. Supported in line contact.

14 is located close to the test piece 11 at right angles to the longitudinal direction at the center of the surface of the test piece 11 (with a spacing of approximately 0.7 mm).
The test piece 11 is a U-shaped water-cooled high-frequency heating coil (tube diameter 6 mm, center spacing 6.2 mm), and is connected to a high-frequency heating power source (output 5 kW, frequency 430 kHz) 15. 14, in a straight line in the width direction (heating width approximately 2
~3 mm).

Reference numeral 16 denotes a water cooling tube, and the water 17 ejected from the tube cools the back surface of the test piece 11 at a position corresponding to the surface linear heating wire.

The temperature control of the test piece 11 is carried out by feeding back the thermoelectromotive force of a CA thermocouple 18 spot-welded to the center of the back surface of the test piece 11 to a heating/cooling waveform control device 19 connected to the thermocouple 18, and at the same time controlling this control device 19. The opening and closing of the valve 20 of the water cooling pipe 16 is also controlled by the
The heating/cooling process was repeated a predetermined number of times by heating to a predetermined temperature at an arbitrary heating rate of 90 deg/sec, holding it for a predetermined time, then stopping the heating and cooling immediately. It should be noted that the number of repeated heating/cooling treatments (N) was 20 times in all cases except for the one in which the influence of the number of heating/cooling treatments was investigated.

(3) Experimental results (3.1) Effect of the number of heating/cooling treatments Figure 5 shows the relationship between the bending angle (α) and the number of repeated heating/cooling treatments (N). The figure shows the heating rate (Hr), upper limit temperature holding time (HT), plate This figure shows the relationship between the bending angle and the number of heating and cooling treatments when each parameter of thickness (t) is different. From the figure, a proportional relationship is established between the bending angle and the number of heating and cooling treatments. I know that there is. That is, under any conditions, the bending angle increases as the number of heating/cooling treatments increases.

(3.2) Effect of heating rate Heating and cooling conditions are thought to have a significant effect on the bending angle. FIG. 6 shows the relationship between the bending angle α and the heating rate (Hr) under conditions of a constant holding time (HT) using the heating/cooling treatment upper limit holding temperature (Th) as a parameter. From the figure, it can be seen that the higher the heating/cooling treatment upper limit holding temperature (Th) is, the better the bending becomes. Furthermore, compared to the case where the heating/cooling treatment upper limit holding temperature (Th) is 750°C, a clear maximum value appears when Th is 950°C. That is,
It can be seen that optimal molding conditions exist.

(3.3) Effect of holding time The rate of phase transformation is roughly proportional to the heating rate, and the holding time also affects the progress of transformation. Therefore, we believe that there should be a close relationship between the two, and we investigated this point. I looked into it.

FIG. 7 shows the relationship between the bending angle (α) and the holding time (HT) under the condition that the heating/cooling treatment upper limit holding temperature (Th) is constant, using the heating rate (Hr) as a parameter. The maximum bending angle is obtained at holding time (HT) = 5 seconds. This tendency is similar for both heating rates of Hr=40 and 80 deg/sec, but a larger bending angle can be obtained at a faster heating rate than at a slower heating rate. The reason why the bending angle decreases as the holding time increases is thought to be because the heating effect on deformation decreases based on the broadening of the heating line width of the specimen and the increase in the average temperature of the specimen.

(3.4) Influence of plate thickness Figures 8 and 9 show the influence of changes in plate thickness, which is one of the test specimen dimensions, on dieless bending. Figure 8 shows heating/cooling treatment upper limit holding temperature (Th) = 950℃, holding time (HT) =
The relationship between the bending angle (α) and the heating rate (Hr) under a constant condition of 5 seconds is shown using the plate thickness (t) as a parameter. In addition, Fig. 9 shows that the upper limit holding temperature (Th) of heating and cooling treatment is 950℃,
The relationship between bending angle (α) and holding time (HT) under constant conditions of heating rate (Hr) = 40 deg/sec is shown using plate thickness (t) as a parameter. From both figures, it can be seen that the thinner the plate, the better it bends. However, it has become clear that the behavior is almost the same even if the plate thickness is different.

Figure 10 shows the bending angle (α) under the condition of holding time (HT) = 5 seconds when the plate thickness is 3 mm.
The relationship between and heating rate (Hr) is shown using the heating/cooling treatment upper limit holding temperature (Th) as a parameter. Figure 11 shows that when the plate thickness is 3 mm, the heating/cooling treatment upper limit holding temperature (Th) =
The relationship between bending angle (α) and holding time (HT) under the condition of 950°C is shown using heating rate (Hr) as a parameter. From both figures,
It can be seen that a similar trend is observed when the plate thickness is 2 mm.

(3.5) Effect of upper limit holding temperature for heating/cooling treatment Figure 12 shows the bending angle (α) and upper limit holding temperature for heating/cooling treatment (Th )
This relationship is shown using the heating rate (Hr) as a parameter. As is clear from the figure, for each curve, the higher the upper limit holding temperature and the slower the heating rate, the larger the bending angle.
In each curve, almost no bending occurred at Th=400°C even after repeated heating and cooling. However, in the case of 750℃ and 950℃, which correspond to the transformation temperature range, that is, heating and cooling are _A1 transformation point,
A: In heating and cooling treatments that pass through ₃ transformation points,
There is a bend in the curve near the transformation point, which is thought to be due to the effect of the transformation superplastic phenomenon, but this bend is not so large, and in the heating/cooling process of the present invention, the influence of the transformation plasticity on the bending angle can be seen. shows that it is not that big.

(3.6) Properties of bent part The same mild steel plate as the test piece was bent in a V shape (α = 20°)
FIG. 13 shows the cross-sectional structure of the bent portion (corroded with 5% nital solution) of the dieless plate bent according to the present invention. Figure d is the material S10C steel plate, Figures a and b
Figure c shows the structure of the specimen with N = 20 cycles and Th = 950°C and 750°C, respectively, and Figure c shows the structure of the specimen subjected to normal V-shaped bending. Figure c shows the structure of the product processed by V-shaped bending (α = 20°).
No significant changes in the structure were observed compared to the structure before processing shown in Figure d. Figure a shows the structure after dieless plate bending (Th = 950°C), and compared to the one before processing shown in Figure d, there is a remarkable tendency for crystal refinement. From this organization, the perlite section
_A1 metamorphosis followed by austenitization to ferrite is observed. Note that this refinement was not observed in the structure of the dieless plate bending molded (Th=750°C) shown in Figure b.

FIG. 14 is a diagram showing the distribution of microhardness (micro-hardness Hv=200 g) in the bent cross section. As can be seen from the figure, dieless plate bending causes an increase in hardness due to microstructural refinement that is equal to or greater than the increase in hardness due to work hardening in V-shaped bending. In other words, the bending portion can be expected to be strengthened.

In the above, the case where the present invention is applied to a mild steel plate having a phase transformation point has been explained.
The present invention provides similar effects to austenitic stainless steel sheets that do not have a phase transformation point. An example is shown in FIG. The figure shows the heating rate (Hr) for a SUS304 stainless steel specimen with a plate thickness of 2 mm.
These are the results of determining the relationship between the bending angle α and the heating/cooling treatment upper limit holding temperature (Th) under the conditions of 40 deg/sec and holding time (HT) = 5 seconds. The figure shows that the results are almost the same as in the case of the mild steel plate shown in Fig. 12, but SUS, which has poor thermal conductivity, bends better than mild steel.

Metal materials with good thermal conductivity, such as copper plates, can also be bent by appropriately selecting heating and cooling treatments.

Note that the heating method may be infrared heating, laser heating, etc. in addition to high-frequency induction heating, and the cooling method may also be air cooling, oil cooling, etc. in addition to water cooling.

As is clear from the above explanation,
Dieless plate forming is possible by applying local heating and cooling treatments to the front and back sides of a metal plate.The effect of this method on the bending angle is that a linear law holds true for the number of heating and cooling treatments. In order to obtain a desired bending angle, the number of heating/cooling treatments, the upper limit temperature of the heating/cooling treatments, the heating rate, and the holding time may be appropriately selected depending on the type and thickness of the metal plate material to be processed.

Furthermore, it is clear that more complex curved surface processing is possible with the dieless forming of the present invention by applying localized heating and cooling treatments in the form of points, straight lines, or curves to a plurality of predetermined locations on the plate material. be.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the conventional linear heating plate bending method.
Figure 2 is a schematic explanatory diagram of the deformation behavior of the test piece in the present invention, Figure 3 is a table showing the chemical composition of the S10C rolled polished steel plate used in the experiment of the present invention, and Figure 4 is a diagram showing the chemical composition of the S10C rolled polished steel plate used in the experiment of the present invention. Figure 5 is a schematic diagram showing the experimental equipment using the high-frequency induction heating method. ) is a diagram showing the relationship between the bending angle (α) and the number of heating/cooling treatments (N), and Figure 6 shows the relationship between the holding time (H.
Figure 7 shows the relationship between the bending angle (α) and the heating rate (Hr) when the upper limit holding temperature (Th) of the heating/cooling process is varied while holding T.) constant at 5 seconds. Figure 8 shows the relationship between bending angle (α) and holding time (HT) when the upper limit holding temperature (Th) is kept constant at 950°C and the heating rate (Hr) is varied.
The figure shows heating/cooling treatment upper limit holding temperature (Th) = 950℃,
Figure 9 shows the relationship between bending angle (α) and heating rate (Hr) when the holding time is constant at 5 seconds.
Figure 10 shows the relationship between bending angle (α) and holding time (HT) when the heating rate (Hr) is constant at 40 deg/sec and the plate thickness (t) is changed.
= 3 mm, holding time (HT) = 5 sec, and a diagram showing the relationship between bending angle (α) and heating rate (Hr) when heating/cooling treatment upper limit holding temperature (Th) is changed.
Figure 11 is a diagram showing the relationship between bending angle (α) and holding time (HT) when the heating rate (Hr) is changed, with the plate thickness = 3 mm and the heating/cooling treatment upper limit holding temperature (Th) = 950°C. , Figure 12 is a diagram showing the relationship between the bending angle (α) and the heating/cooling treatment upper limit holding temperature (Th) when the plate thickness = 2 mm and the holding time (HT) = 10 seconds, and the heating rate (Hr) is changed. , Fig. 13 is a diagram showing the microscopic structure of the material subjected to the dieless bending process of the present invention (Figs. a and b), the item subjected to the conventional V-shaped bending process (Fig. c), and the material (Fig. d); Fig. 14 Figure 15 shows the microhardness distribution of the bending cross section of dieless plate bent material and V-shaped bent material.
Heating rate in SUS304 stainless steel plate (Hr)
It is a figure showing the relationship between the bending angle (α) and the heating/cooling treatment upper limit holding temperature (Th) when = 40 deg/sec and holding time (HT) = 5 seconds. In the figure, 11... test piece, 12... support stand with lifter, 13... heat resistant glass rod, 14... high frequency heating coil, 15... high frequency power supply, 16... water cooling pipe, 17... jetting water, 18...CA thermocouple,
19... Heating/cooling waveform control device, 20... Valve.

Claims (1)

[Claims] 1 Applying point, linear or curved intermittent heating to the surface of a metal plate supported so that it can move freely without being restrained from the outside, and at the same time applying the surface heating position on the back side of the metal plate. A heating/cooling process is repeated for a predetermined number of times to cool the position corresponding to the above during non-heating immediately after the heating stops during the intermittent heating, thereby repeatedly creating a local temperature gradient between the front and back surfaces of the metal plate. A dieless plate forming method using thermal stress, characterized in that the metal plate is bent by a predetermined angle toward the front surface of the metal plate around the surface heating position.