JP2002066647A - Hydroform processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydroform processing method capable of positively preventing generation of cracks by introducing a design concept using a new indicator. SOLUTION: A contact position is changed by transforming a pre-shape of a tube stock 2 to lower a local distortion so that the local distortion within the scope surrounded by the point contact (line contact) between a mold 1 and the tube stock 2 does not exceed the limit distortion of the tube stock 2 and when the pipe expansion is a main forming process, a local pipe expansion rate is used instead of the local distortion and the contact position is changed by modifying the pre-shape of the tube stock 2 so as not to exceed the allowable pipe expansion rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydroforming method using a metal tube.

[0002]

2. Description of the Related Art There has been known a hydroforming method in which an end face of a material tube set in a mold is axially pushed while applying a liquid pressure to the inner surface. However, this processing method is governed by complicated phenomena, and the reality is that it has not yet come out of trial and error.

[0003] Failures in the hydroforming process are:
It can be roughly classified into cracking and buckling. Of these, cracks can be discussed depending on whether the degree of deformation of the material pipe exceeds the limit degree of deformation of the material pipe, and three-dimensional distortion should be originally adopted as the degree of deformation. However, since three-dimensional distortion is extremely difficult to measure and calculate, two-dimensional distortion is used for simplicity, ignoring the change in wall thickness of the material pipe. However, two-dimensional distortion is still difficult to measure, though not as much as three-dimensional distortion. In addition, a method for measuring critical two-dimensional strain in hydroform has not been established. For this reason, the limit two-dimensional strain obtained by the FLD theory in a press which has been established to some extent has been unavoidably substituted. That is, at present, there is anxiety about the measurement accuracy of the two-dimensional distortion to be compared, and there is anxiety that the limit two-dimensional distortion to be compared is not properly obtained.

[0004] Therefore, if it is more effective to discuss through the concept of expansion rate than to discuss under such anxiety, the expansion rate has been used in recent hydroform designs. I have. The pipe expansion ratio is a value defined as a ratio B / A of a peripheral length A of a material pipe in a certain cross section and a corresponding inner peripheral length B of a mold (that is, a product peripheral length). The critical expansion ratio is a value defined as an elongation ratio Bc / Ac until a raw material tube having a circumferential length Ac uniformly expands in the circumferential direction to reach a circumferential length Bc at which one of the circumferences breaks. . That is, recently, it has been designed so that B / A does not become larger than Bc / Ac. This critical expansion ratio is almost the same as “tensile breaking strain in a plane strain state” and is easily measured.

[0005] However, the critical expansion ratio is an overall concept indeed, and it is clear that it is impossible to design an optimal hydroform by itself, but there has been no guideline beyond this. For this reason, cracks are continually occurring even in hydroforms designed with a safe pipe expansion ratio, and as a result, either design in anticipation of a large undesired safety factor or increase the safety factor by a large amount of trial and error. Efforts have been made to make it smaller. In other words, it has been considered that the development period is prolonged and the development cost is unavoidably increased.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to introduce a design concept using an index that is more effective than the above-mentioned expansion ratio, thereby providing an excessive safety factor for preventing cracking and a large amount of trial and error. The object of the present invention is to provide a hydroform processing method which does not require the above.

[0007]

According to a first aspect of the present invention, there is provided an apparatus for axially pushing an end surface of a material tube set in a mold while applying a liquid pressure to the inner surface. In the hydroforming method, a preform designed so that all local strains are smaller than a critical strain of a material pipe is used. In this case, it is preferable to use a preform designed such that all local strains are the same or do not show a large difference.

Another object of the present invention is to provide a hydroforming method in which a raw material tube and a mold are brought into contact with each other during processing. It is characterized in that a preform designed so that all required local strains are smaller than the critical strain of the material pipe is used. In this case, it is preferable to use a pre-shape designed so that the local strain newly obtained by the newly generated contact portion is the same as all other local strains or does not show a large difference.

According to a fifth aspect of the present invention, there is provided a hydroforming method in which expansion is a main forming process, wherein all local expansion rates are smaller than a limit expansion rate of a raw material pipe, and the local expansion rates are the same. Or a preform designed so as not to show a large difference. In any of the inventions, the preform designed so that the contact portion between the material pipe and the mold expands at a gradual constant speed before the material pipe is constricted or at a speed close thereto at the progress of the processing. Preferably, it is used.

It is a fact that hydroforming has been known through lubrication and the like that processing proceeds while the material tube contacts a mold, and that when the contact state changes, the workability also changes. However, the conventional design guideline of the expansion ratio cannot consider the contact state between the material pipe and the die, so it must be said that this point has a fatal defect. This is simply because it is difficult to take into account the contact situation. Therefore, in the present invention, instead of directly considering the contact situation,
The contact condition was determined by considering the width, length, or shape of the non-contact portion generated by the contact.

The local strain proposed in the present invention means that at a certain point, the length or area of a line or area surrounded by a point or a line at which the material pipe and the mold are in contact with each other is deformed, and the strain a accumulated there. Define. For example, FIG. 1 shows a situation in which a material pipe 2 contacts a mold 1 having the same shape as one corner of a cube, and a non-contact area 4 surrounded by a contact boundary 3 exists. Here, the shape 100 of the material pipe in the non-contact area before hydroforming
Becomes a shape 101 due to the progress of the hydroforming process.
0 is the local strain accumulated until the shape is transformed into the shape 101. Further, the critical strain is defined as a local strain b at which the material pipe is locally broken. That is, when the local strain a is smaller than the critical strain b, the material tube 2 is formed without cracking, and when the local strain a is large, the material tube 2 is formed by cracking or necking (it is extremely thinned even if not broken). Fail. Therefore, in the present invention, a pre-shape designed so that the local strain a does not exceed the limit strain b is used.

However, as described at the beginning, the measurement of distortion is not easy, and in many cases, the local distortion a and the limit distortion b are actually unclear. Therefore, it is a useful idea from an engineering point of view to replace it with the concept of area and to consider it. At a portion that is in a biaxial tensile strain state, it depends on the material characteristics such as the r value, but is divided when the equivalent local strain ε at that portion reaches the limit equivalent strain εcr of the material. Since the equivalent strain is a concept similar to the area change, the ratio between the original area dS and the final area dT is defined again as the local strain corresponding to the equivalent local strain ε, and the ratio corresponding to the limit equivalent strain εcr The critical value is redefined as critical strain. For example, if a crack occurs when the original area Sc becomes the area Tc, the limit value of the material is Tc / Sc. Specifically, the area of 100 in FIG.
If the area of 01 is dT, the local strain is considered as dT / dS, and if it exceeds its limit value Tc / Sc, it is considered that it will break.

The local expansion ratio, which is a concept newly proposed in the present invention, refers to a length dA on a non-contact circumference between points where a material pipe and a mold are in contact at a certain point in a cross section. And the corresponding length on the inner circumference of the mold (that is, the length of the non-contact portion to the product) dB. Therefore, in the present invention, a pre-shape designed so that dB / dA is not larger than Bc / Ac described above is used.

For example, FIG. 2 shows a mold 1 having the same cross section in the longitudinal direction.
2 shows a state in which the material pipes 2 having the same cross section are in contact with each other in the longitudinal direction, and the non-contact area 4 is generated by the contact boundary line 3. As described above, when the cross sections are almost the same in the longitudinal direction, it is possible to handle the whole two-dimensionally, and when the weakest point can be specified in a long shape, only the weakest point is used. Can be taken into account, and as a result, only the cross section needs to be considered, so that a two-dimensional study is possible. FIG.
In the above, the material pipe cross-sectional shape 102 in the non-contact area before the hydroforming becomes the shape 103 by the hydroforming. At this time, the local pipe expansion ratio in the non-contact area 4 is the ratio of the length of the shape 103 to the length of the shape 102. is there.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, each invention will be described in more detail. According to the first aspect of the present invention, in the hydroforming method in which the end face of the material pipe set inside the mold is axially pushed while applying hydraulic pressure to the inner surface, all local strains are smaller than the critical strain of the material pipe. Use the pre-shaped designed. The first aspect of the present invention considers an initial state in which a material tube is set inside a mold.

In the initial state, if the material tube and the mold come into contact with each other by clamping, even if the lubricant is used well, the frictional force at the contact position becomes considerably large, so that the contact position on the material tube moves during processing. It is reasonable to think not. Therefore, in the fixed point due to such contact or in the range surrounded by the fixed point, the deformation during processing is completed within that range, and the raw material pipe is supplied from outside the range, or the raw material pipe is placed in another area. There is no supply. Based on this finding, if the local strain in the range is not allowed to exceed the limit strain of the material pipe, cracks will not occur in the range. If there is a possibility that the local strain exceeds the critical strain of the material pipe, the preform of the material pipe is changed to change the contact position, thereby reducing the local strain. If this concept is applied to the entire surface of the material pipe and the preform is designed so that all local strains are smaller than the critical strain of the material pipe, the occurrence of cracks can be completely prevented.

The invention according to claim 2 is characterized in that a preform designed so that all local distortions are the same or do not show a large difference is used. That is, if the hydroform processing is analyzed by using the local strain introduced according to the present invention, cracks will be generated from locations where the local strain is maximum. Conversely, if all local strains are the same, or if a preform designed so as not to show a large difference is used, it is possible to deform the entire material tube more uniformly until it reaches a crack. . Thus, a design that brings the local strain as close as possible is an appropriate design.

In the above-mentioned inventions of the first and second aspects, the problem of local distortion in the mold clamping state has been a problem. In practice, however, processing often progresses and new contact often occurs. In this case, since the range in which one local distortion is considered is divided by a new contact, the concept of an individual local distortion is generated corresponding to each newly divided range. Therefore, according to the third aspect of the present invention, cracks during processing are prevented by using a preform designed so that all local strains newly obtained by newly generated contact portions are smaller than the limit strain of the material pipe. can do.

According to a fourth aspect of the present invention, the local distortion newly obtained by the newly generated contact portion is also the same as or larger than the local distortion (local distortion of the first aspect) considered from the beginning. It is characterized by using a pre-shape designed so as not to show a difference. Thereby, Claim 3
In the same manner as described above, the material pipe can be uniformly deformed until it is cracked.

According to the fifth aspect of the present invention, the concept of local expansion ratio is used instead of local distortion. That is, in a hydroforming method in which expansion is a main forming process rather than branch extension, the expansion ratio is more effective than distortion. Therefore, the invention of claim 5 uses a pre-shape designed so that all the local expansion rates are smaller than the limit expansion rate of the material pipe and the local expansion rates are the same or do not show a large difference. It is characterized by the following. As described above, the conventional pipe expansion ratio has a problem with the perimeter, whereas the local pipe expansion rate used here is intended for a range sandwiched between the contact points between the material pipe and the mold. Are completely different.

According to a sixth aspect of the present invention, the contact portion between the material pipe and the mold is designed to expand at a gentle constant speed or a speed close to the speed before the material tube is constricted with the progress of processing. It is characterized by using a preform. Claim 6 relates to the progress of the contact, that is, the movement of the contact boundary line (point).

As a result of the research, even if the local strain or the local expansion ratio is the same, the more uniform the progress speed of the contact, the more stable the elongation of the material pipe and the smaller the probability of occurrence of cracks (fact 1). It was also found that the slower the advancing speed, the more uniform the elongation of the material pipe extends over the entire range (fact 2).
The fact 2 is that, as the contact speed increases, the local strain and the local expansion ratio increase with the deformation, the risk of cracking increases (cause 1), and the non-contact portion is uniformly expanded and deformed. Instead, the reason is that the vicinity of the boundary between the contact portion and the non-contact portion is most locally deformed and reduced (cause 2). The cause 2 is governed by the same principle as that of providing a parallel portion at the center of the test piece in order to avoid breaking from the root of the grip portion in the shape of the tensile test piece. That is, it was clarified that the test piece was deformed by the gripping, and a shape change point such as a wall thickness was generated at the base of the gripping portion, and that the singular point became a fracture starting point.

In the hydroform, the material tube is pressed against the mold by the internal pressure, thereby being subjected to displacement restraint, and at the same time, the contact area is sandwiched between the liquid pressure and the mold to reduce the wall thickness. While the entire pipe is constrained by the frictional force, the internal pressure can move in the surface direction while the surface behavior of the inner and outer surfaces is the same at the non-contact part, but suddenly becomes the same near the contact part and the strength decreases. Therefore, it is easy to break. This rupture occurs when the contact progresses or is too slow, but it is optimal to enlarge the contact area at a slightly higher speed. Hereinafter, examples of the present invention will be described.

[0024]

(Embodiment 1) FIG. 3 shows an example of a system in which, after a blank material pipe 2 having a circular cross section is bent, a central portion is hydroformed into a medium-swelling square cross-sectional shape. In this case, by performing the bending process, the deformation at the center becomes a biaxial tensile strain state. The raw material tube 2 is made of carbon steel and has a diameter of 60 mm and a wall thickness of 2 mm.

In the initial state immediately after the mold clamping, the material tube 2 is arranged in the mold 1 so that the axis of the mold shape coincides with the axis, and the material tube 2 is positioned along lines α1 to α4 in FIG. In contact with the upper, lower, left and right sides of the mold equally. Material pipe 2 by processing
Expands, and a non-contact portion 4 and the like surrounded by a line β1 in FIG. Until the final stage of processing, it is generally possible to perform processing without any problems.
It is conventionally considered that it is very difficult to make the members contact each other.

In the initially set target shape, each dT / dS corresponding to each non-contact portion is 18%, 11
%, 6% and 2%. When the hydroforming was continued as it was, a corner having a dT / dS of 18% was cracked. According to the measurement, the critical strain of this material was 12%. dT / dS
Are scattered as described above, and were broken because the deformation was concentrated on the corner having the largest value. Therefore, as shown in FIG. 5, when the peripheral length was changed to a shape such that all dT / dS at the end of processing were substantially the same, each dT / dS corresponding to each non-contact portion was about 6.8%. It was able to be processed favorably without generating cracks.

(Embodiment 2) FIG. 6 shows a body system in which after a blank material pipe 2 having a circular cross section is bent, a central portion is hydroformed into a medium swelling square cross section. In this case, by performing bending, the deformation at the center becomes a biaxial tensile strain state. The material pipe 2 is made of 35 kgf carbon steel, 60 mm in diameter,
The wall thickness is 2 mm, and the size of the central part is a shape in which corners of a square cross section of 64 mm on a side are cut off.

Initially, the blank tube 2 is arranged in the mold 1 so that the axis of the mold shape coincides with the axis, and does not contact the mold 1 at the center. However, when the processing is started and the material tube expands, the material tube is moved along the line γ1 in FIG.
At the end of processing, a non-contact portion 4 surrounded by a line δ1 in FIG. 8 remains. Here, the respective dT / dS corresponding to each non-contact portion 4 was 25%, 11%, 8%, and 6%. Continue hydroforming 25%
Corner 1 was broken. Here, as shown in FIG.
By shifting the amount of dT / dS toward .about.3 by an amount corresponding to the magnitude of dT / dS, all the dT / dS at the end of machining are about 9.5.
%, And could be processed well.

(Embodiment 3) Next, an embodiment corresponding to claim 5 which is very effective for hydroforming long members will be described. FIG. 10 is a cross-sectional view showing hydroforming of a long structural material. The inner surface shape of the mold 1 is a shape whose basic shape is a square with a side of 25 mm and whose four corners are rounded off with R5. The raw material tube 2 is a general carbon steel that has not been subjected to any particular surface treatment. A steel tube having an original outer diameter of 26 mm and a wall thickness of 1.5 mm is deformed by pre-processing before clamping, and is placed in the mold 1 as shown in FIG. Is set.

At the time of completion of the mold clamping, four contact points 31 to 34 and non-contact portions 11 to 14 and 21 to 21 at four corners are provided.
24 are present. Since the mold 1 has a symmetrical shape in the vertical and horizontal directions, the length of the non-contact portions 11 to 14 is the same 22.85 mm by setting each of the contact points 31 to 34 at the center of each side. Similarly, if the material tube 2 is set so as to be symmetrical in the vertical and horizontal directions, the non-contact portions 21 to 21
24 is the same length of 20.42 mm. As a result,
The local expansion ratio of each non-contact part is 22.85 / 20.42 =
1.12, which is 1.2 or less, which is a value generally recognized as the critical expansion ratio of general carbon steel. According to the present embodiment, good hydroforming is achieved without generating cracks.

Here, since it is not always easy to set the material tube 2 having an original outer diameter of 26 mm in a mold having a smaller side length of 25 mm, the material tube 2 having an original outer diameter of 24 mm is easily set. When a pipe is used, the length of each non-contact portion 11 to 14 is 18.85 mm, the local expansion ratio is 22.85 / 18.85 = 1.21, and the critical expansion ratio of general carbon steel is 1 Over .2. In this case, the risk of cracking could be estimated, and as a result of the experiment, it was confirmed that cracking actually occurred. This example shows that a study on a hydroform design which could not be sufficiently performed conventionally can be simply and accurately performed.

(Comparative Example 1) FIG. 11 shows a system which can be examined two-dimensionally. The mold 1 has a rectangular shape (15 mm in width and 5 mm in length) having an inner surface cross-sectional length of 40 mm, and a material tube 2 has an outer peripheral length of 36 m.
It is an example in which the shape is m square. This material tube 2 is a stainless steel tube having a wall thickness of 0.8 mm. When the material tube 2 is set in the mold 1, it is crushed in the vertical direction as shown in FIG.

[0033] In this hydroform, the contact portion is considerably long at 8 mm on each of the upper side and the lower side.
The conventional expansion ratio is 40/36 = 1.11, but the local expansion ratio is as large as ((40/2) -8) / ((36 / 2-8) = 1.20. Although the critical expansion ratio of the stainless steel pipe is less than 1.25, the contact point 35 moves toward the final contact point 36 at a very high speed. This is a typical example in which the local expansion rate is sharply increased due to localized deformation, and exceeds 1.25 in the middle of processing.

(Embodiment 4) FIG. 12 shows an embodiment having the same local expansion ratio as that of FIG. 11, but having a different shape. This is also a system that can be examined two-dimensionally, in which the mold 1 has an 8-character shape with an inner surface cross-sectional circumference of 40 mm (width 12 mm, height 5 m
m), the material pipe 2 has a perfect circular shape with an outer peripheral length of 38 mm.
However, the material tube 2 is crushed in the vertical direction when the mold is clamped and set. This material tube 2 is a stainless steel tube having a wall thickness of 0.8 mm. In this hydroform, the contact portion exists at a considerably long length of 8 mm on each of the upper side and the lower side, and the conventional expansion ratio is 1.11 and the local expansion ratio proposed by the present invention is 1.20.

However, the shape of the mold 1 is an optimum shape corresponding to the expanded shape of the material pipe 2, and the contact boundary point 3
5 gradually moves toward the final contact point 36 with the progress of the expansion, starting from the current position as a starting point. Since the contact point moves at an appropriate speed in this manner, the material pipe 2 is not constricted, and the entire non-contact portion of the material pipe can contribute to the expansion evenly, so that the hydroforming is actually completed. be able to. This example shows that if the local pipe expansion rate is equal to or less than the limit value, cracks can be suppressed by appropriate shape design.

(Comparative Example 2) FIGS. 13 and 12 show FIGS.
This is a comparative example having the same local expansion ratio as that of, but having a different shape. This example is also a system that can be examined two-dimensionally,
The mold 1 has a figure-eight shape with an inner surface cross-sectional circumference of 40 mm (width 11
mm, the vertical width is 6 mm), and the material pipe 2 is a perfect circle having an outer peripheral length of 38 mm. However, the material tube 2 is crushed in the vertical direction when the mold is clamped and set. This material tube 2 is 0.8m thick
m stainless steel pipe. In this hydroform, the contact portion exists at a considerably long length of 8 mm on each of the upper side and the lower side, and the conventional expansion ratio is 1.11 and the local expansion ratio proposed by the present invention is 1.20.

However, the shape of the mold 1 in this example is a more extreme figure-eight shape, and the contact point 35 almost stops even after the start of processing. If the contact point is stationary or its moving speed is too slow, cracks occur near the contact point. When the local pipe expansion rate is relatively close to the limit value, stopping the contact point leads to constriction in the vicinity thereof, and it cannot be said that the optimum design was made.

(Embodiment 5) FIG. 14 is an enlarged view of one corner of the cross section shown in FIG. Initially, the target angle R was set to 3 mm. However, since the shape 40 could not be hydroformed with a predetermined material pipe, the shape was changed to R5 in FIG. The reason why the shape 40 could not be formed was that the material pipe 2 was constricted when the tube was expanded outside the shape 41, and cracks occurred before the shape 40 was reached. This is because the movement of the contact point is too slow as in the example shown in FIG.

Therefore, in this embodiment, in order to achieve R3 of the shape 40, the sections on both sides leading to the corner R are slightly cut to form the shape 42. This shaved section can be said to be a run-up section for increasing the moving speed to the contact point up to the corner, and thereby the corner of R3 can be formed while preventing cracking.

[0040]

As described above, the present invention introduces a new design concept of local strain or local expansion ratio instead of the conventional expansion ratio, so that the material tube and the metal tube which were not considered at all in the past are considered. It proposes a hydroform design that incorporates the state of contact with the mold. According to the present invention, if the preform of the material pipe is designed such that the local strain or the local expansion rate does not exceed the critical strain or the critical expansion rate, it is possible to prevent cracks even in a shape which was conventionally impossible to form, while preventing cracking. Hydroform molding becomes possible. According to the present invention, cracks can be reliably prevented, so that an excessive safety factor and a large amount of trial and error for preventing cracks as in the related art can be eliminated, and the practical effect is extremely large.

[Brief description of the drawings]

FIG. 1A is a perspective view showing a non-contact portion of a material tube at one corner of a cubic mold, and FIG. 1B is a perspective view showing a change in the shape of the non-contact portion as the processing proceeds.

FIG. 2A is a perspective view showing a non-contact portion between a mold and a material pipe in a system which can be examined two-dimensionally, and FIG. 2B is a cross-section showing a change in shape of the non-contact portion as the processing proceeds. FIG.

3A and 3B are explanatory views of a processing system according to the first embodiment, wherein FIG. 3A is a plan view and FIG. 3B is a cross-sectional view.

4A and 4B are explanatory views showing a problem at the end of processing in this processing system, wherein FIG. 4A is a cross-sectional view, FIG. 4B is a perspective view, and FIG.

FIGS. 5A and 5B are explanatory views showing a state in which good working is performed according to the first embodiment, wherein FIG. 5A is a cross-sectional view and FIG. 5B is a perspective view.

FIG. 6 is an explanatory diagram of a processing system according to a second embodiment.

FIG. 7 is an explanatory diagram showing an initial state of machining in this machining system.

8A and 8B are explanatory views showing a problem at the end of processing in this processing system, wherein FIG. 8A is a cross-sectional view and FIG. 8B is a perspective view.

FIGS. 9A and 9B are explanatory views showing a state in which favorable processing is performed according to the second embodiment, in which FIG. 9A is a cross-sectional view and FIG. 9B is a perspective view.

FIG. 10 is a sectional view showing a third embodiment.

FIG. 11 is a sectional view showing Comparative Example 1.

FIG. 12 is a sectional view showing a fourth embodiment.

FIG. 13 is a cross-sectional view showing Comparative Example 2.

FIG. 14 is a sectional view showing a fifth embodiment.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 mold 2 material pipe 3 contact boundary line between mold and material pipe 4 non-contact portion surrounded by contact boundary line 11 to 14 non-contact portion on mold side in FIG. 10 21 to 24 material tube side in FIG. 10 Non-contact part 31-34 Contact point in FIG. 10 35 Contact boundary point in FIGS. 11, 12, 13 36 Final contact point in FIGS. 11, 12 40 Shape showing target angle R in FIG. 14 41 Corrected target angle R in FIG. 42 Shape with run-in section 100 Shape of non-contact portion before hydroforming in FIG. 1 101 Shape of non-contact portion after hydroforming in FIG. 1 102 Shape of non-contact portion before hydroforming in FIG. 103 103 Shape of uncontacted part after hydroforming in Fig. 2

Claims (6)

[Claims] In a hydroforming method in which an end face of a material pipe set inside a mold is axially pushed while applying a liquid pressure to an inner surface, all local strains are set to be smaller than a critical strain of the material pipe. A hydroforming method characterized by using a designed preform. 2. The hydroforming method according to claim 1, wherein a preform designed such that all local strains are the same or do not show a large difference is used. 3. In a hydroforming method in which a material pipe and a mold are newly brought into contact with each other during processing, all local strains newly obtained by a newly generated contact portion are smaller than the critical strain of the material pipe. 3. The hydroforming method according to claim 1, wherein a preform designed as described above is used. 4. Whether the local distortion newly obtained by the newly generated contact portion is the same as all other local distortions,
4. The hydroforming method according to claim 3, wherein a preform designed so as not to show a large difference is used. 5. In a hydroforming method in which expansion is a main forming process, all local expansion rates are smaller than a critical expansion rate of a raw material pipe, and the local expansion rates are the same or show large differences. A hydroforming method characterized by using a pre-shape designed so as not to exist. 6. A pre-shape designed so that a contact portion between a material tube and a mold expands at a gentle constant speed or a speed close thereto before the material tube is constricted with the progress of processing. The hydroform processing method according to claim 3.