JP2001219226A - High-strength steel tube for hydroforming - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel tube for hydroforming which can obtain a hydroformed component with a desired portion thereof strengthened without any weakening by the reduction in the wall thickness caused by the hydroforming at a low cost. SOLUTION: The high-strength steel tube for hydroforming has a multiple tubular structure comprising, for example, a base tube 1 and an add-on tube 2 on a part of the overall length. The multiple tubular structure is preferably formed by at least one kind of shrink-fit, cold-fit, and press-fit. A tube constituting the multiple tubular structure is preferably laser-welded.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a high-strength steel pipe for hydroforming. Hydroforming (hereinafter, appropriately referred to as HF) is a technique for manufacturing a hollow closed-section structural component by integrally forming a steel pipe by applying axial force and internal pressure.
Specifically, a steel pipe is mounted in a mold, a liquid is charged into the pipe, the pipe is extended in the circumferential direction, and the pipe is expanded into a predetermined shape. This technology is mainly used for the production of automobile parts.

[0002]

2. Description of the Related Art Hydroform processing materials include:
Usually, a single tube of an ERW steel pipe formed by ERW welding a low carbon steel plate is provided. A normal HF component manufactured by hydroforming this normal material can be applied to an independent member such as an engine cradle without any problem. However, structural members such as front side members and center pillars have both a low-strength part that absorbs impact energy by collapsing and a high-strength part that does not collapse to maintain space for occupant safety. However, it is difficult for ordinary HF parts to satisfy this requirement. In order to satisfy this requirement, it is considered effective to partially increase the expansion rate and increase the strength by work hardening, but it is unavoidable to reduce the thickness of the part where the expansion rate is increased, This is because there is naturally an upper limit to the expansion ratio that can be taken in terms of rigidity, and sufficient strength cannot be obtained after expansion.

[0003] For this reason, at present, automobile manufacturers and component manufacturers adopt tailored blanks or tailored tubes in which members having a plurality of strengths are joined as structural members such as front side members and center pillars. I have. However, in the tailored blank method (method of processing a tailored blank into parts),
After press-working a material (ie, a tailored blank) obtained by joining two or more types of steel plates, two or more parts are welded to form a closed cross-sectional structure. For this reason, the material requires a flange portion as a welding margin, and waste of material occurs as compared with HF processing. Further, in a flat portion where the tip of the punch hits at the time of press working, the deformation amount is small and an increase in strength due to work hardening cannot be expected. If the work hardening is increased by increasing the amount of processing as much as possible, the degree of non-uniformity of strain throughout the part is increased, and the thickness of the portion where the strain is large is excessively reduced, which may result in weakening.

In the tailored tube method (method of processing a tailored tube into a part), a plurality of thick or steel tubes are connected to each other by butt welding to form a tailored tube, which is processed by HF to form a part. Here, waste like the flange portion does not occur. However, there is a concern that the manufacturing cost is increased because the accuracy of the part where two or more kinds of tubes are butt-welded is severe.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention makes it possible to obtain an HF component in which a desired portion is strengthened without fear of weakening due to a decrease in wall thickness in HF processing. Moreover, an object of the present invention is to provide a high-strength steel pipe for hydroforming that can be manufactured at low cost.

[0006]

SUMMARY OF THE INVENTION The present invention is a high-strength steel pipe for hydroforming, which has a multiple pipe structure in a part of the entire length. In the present invention, a part of the total length means one or more partial length sections within the full length section. In the present invention, it is preferable that the multi-tube structure is formed by one or more of shrink fitting, cold fitting, and press fitting. In the present invention, it is preferable that the tubes constituting the multi-tube structure are laser-welded.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS When manufacturing a structural member by subjecting a pipe to HF processing, a process of prebend (bending the pipe) → prepress (pressing the bent pipe so as to enter a mold) → HF molding. Is taken.

[0008] For example, the front side member for an automobile is desirably relatively low in strength at the portion near the front of the vehicle body, because it is required to be crushed during a collision and absorb impact energy. In a portion close to the passenger compartment, it is desirable to have a relatively high strength in order to secure a space where the occupant is not crushed even in the event of a collision. However, in a single tube of an electric resistance welded steel pipe which is a conventional material for HF processing, the material strength of a specific portion can be increased to some extent by increasing the expansion ratio of a specific portion during HF forming.
When the expansion ratio is increased, the wall thickness is reduced accordingly, and it is difficult to secure the strength as a structural member.

On the other hand, in the present invention, since the steel pipe for hydroforming has a multi-tube structure in a part of the entire length, it is possible to reduce the wall thickness by increasing the pipe expansion rate with respect to the multi-tube structure. Deterioration in rigidity can be compensated for by strengthening with a multiple structure, and it is easy to secure strength as a structural member. The high-strength steel pipe for HF processing of the present invention can be manufactured by a method of fitting and fixing another pipe to a part of the entire length of one pipe. This manufacturing method is a tailored tube for butt-welding different (or different-thickness) steel pipes. Since the accuracy can be reduced as compared with the construction method, the manufacturing cost can be reduced.

Hereinafter, the present invention will be described more specifically by taking a double pipe as an example. In the double tube, the longer tube is called a base tube, and the shorter tube is called an add-on tube. FIG. 1 shows a fitting type (referred to as an IA type) 2 having an add-on tube 2 as an inner tube and a base tube 1 as an outer tube.
It is sectional drawing which shows an example of a heavy pipe, (a) is a fitting,
(B) is obtained by further performing laser welding on (a).

As the fitting method, any of shrink fitting, cold fitting, and press fitting can be preferably used as necessary. These may be used in combination. It is also preferable to fix the position of the double tube structure (the mutual position of the inner tube and the outer tube) by laser welding the inner tube and the outer tube fitted by any method. The welding location may be appropriately selected as needed. There are various welding methods other than laser welding, but laser welding is optimal from the viewpoint of welding reliability.

In the case of the IA type, when laser welding is not performed after fitting, it is necessary to pay attention to the gap 3 between the inner pipe and the outer pipe. In the case of HF molding (hydraulic type HF) in which a liquid is inserted into a pipe and an internal pressure is applied, if the gap 3 is not well-sealed, the pressurized liquid enters the gap at the start of HF molding and hydraulic pressure acts on the gap. As a result, the inner pipe does not expand and only the outer pipe expands, and the desired effect cannot be obtained. To prevent this, the fitting specification may be designed so that the fitting pressure is higher than the liquid pressure at the start of HF molding.

However, in the case of using IA type, when laser welding is used, as shown in FIG. 1B, both ends of the add-on tube 2 are welded all around the base tube 1 to form a gap 3.
Can be prevented by infiltration of the pressurized liquid by sealing the welding line 4 with the welding line 4.
The determination may be made irrespective of the fluid pressure at the start of F-forming. FIG.
It is sectional drawing which shows an example of the fitting type | mold (referred to as OA type) partial double pipe which made the add-on tube 2 an outer tube, and made the base tube 1 an inner tube.

As the fitting method, any of shrink fitting, cold fitting, and press fitting can be preferably used as necessary. These may be used in combination. It is likewise preferable to fix the position of the double pipe structure by laser welding the inner pipe and the outer pipe fitted by any method. The welding location may be appropriately selected as needed. In the OA type, the pressurized liquid does not come into contact with the outer tube (does not enter the gap 3) due to its structure, so that it is not necessary to pay special attention to the sealed state of the gap 3.

Further, since the multiple pipe section does not expand unless a higher internal pressure is applied than the single pipe section, the multiple pipe section is multiplexed into the high expansion ratio processing chamber 6 of the HF mold 5 as shown in FIG. Pipe section 20
When the internal pressure is applied, the pipe section 10A with the high expansion ratio of the single pipe section expands preferentially and bursts there when the internal pressure is applied, which is likely to cause HF molding failure. Is high. Therefore, when forming the multi-tube structure according to the present invention, as shown in FIG. 3B, a mounting mode in which only the multi-tube portion 20 enters the high expansion ratio processing chamber 6 of the HF mold 5 (applicable). It is important to perform the alignment so that the example is obtained.

In the case of a triple or multiple tube structure,
It can be formed according to a double tube structure.

[0017]

EXAMPLES (Examples 1 to 8) A steel plate equivalent to STKM13A having a yield strength of 350 MPa was formed into a tube by bending, and the seam (bent end) was subjected to electric resistance welding to vary the wall thickness and outer diameter. A multi-tube structure is formed on a part of the entire length of the tube by the method shown in Table 1, and an add-on tube is fitted inside and outside the IA type or OA type double tube and the base tube. A triple tube was manufactured as an example of the present invention. Here, the formation range of the multi-tube structure was a range from the longitudinal center of the tube to one end.

[0018]

[Table 1]

These double pipes and triple pipes (materials for processing) are prepressed, and a corner bending radius: 3 mm and a cross section: 60
It is placed in a processing chamber in a mold (HF mold) having a processing chamber with a square shape of mm square and a length of 300 mm, and is subjected to HF molding in which a pipe end is fixed and liquid pressure is applied inside. It was a prismatic tube having the dimensions shown in FIG. Here, the wall thickness of the single pipe section 10 and the multiple pipe section 20 is targeted to be 1.6 mm in the circumferential average in the single pipe section 10 and 2.6 mm in the circumferential average of the total inner pipe and outer pipe in the multiple pipe section 20. We met this goal by trial and error.

(Comparative Example 1) From the same steel strip used in the example, a 1.6 mm thick plate for a thin portion and 2.6 mm for a thick portion
A thick plate was selected, and a tailored blank was prepared by the method shown in Table 1 to obtain Comparative Example 1. This tailored blank (material for processing) is pressed and further welded, and the corner bending radius: 3 mm, cross section: rectangular shape of 60 mm square, length: 300 mm (thin part length 150 mm, thick part length 150 mm), flange width: Ten
mm prism tube was manufactured. This is an example of the so-called tailored blank method.

(Comparative Example 2) From the same tubes used in the examples, a tube to be thinned and a tube to be thickened were selected.
Comparative Example 2 without tailored tube by the method shown in Table 1.
And This tailored tube (material for processing) was subjected to the same pre-pressing and HF forming as in the example to obtain a prismatic tube having the same dimensions as in FIG. The thin and thick portions of the prismatic tube correspond to the single tube portion 10 and the multiple tube portion 20 in FIG. 4, respectively. In addition, thickness of each part (thick part 1.6mm, thick part 2.6mm)
Was adjusted to the target value by trial and error as in the example.

With respect to the prismatic tubes (parts equivalent to structural members for automobiles) manufactured from the examples 1 to 8 and the comparative examples 1 and 2 as described above, the strains (true strains) at the portions A to F shown in FIG. It was measured. Table 2 shows the results.

[0023]

[Table 2]

From Table 2, the true strains of Examples 1 to 8 are larger than Comparative Example 1 and are comparable to Comparative Example 2. That is, in the HF part made of a steel pipe having a multiple pipe structure according to the present invention, the same work hardening strength as that of the HF part made of a tailored tube can be obtained. Both have higher strength than the parts (non-HF parts) by the tailored blank method.

Further, one end in the longitudinal direction of these parts (FIG. 5)
A 10kg weight is subjected to a frontal collision at 50km / h (the collision direction is parallel to the long axis direction of the parts) at the end of the X section of the above), and a stopper is set so as to stop the weight when the one end is displaced 150mm. load a load of between collisions start to weight stopped continuously measured - calculated displacement curves were calculated absorption energy E _s by integrating the load displacement based on this. Table 2 shows the results. Note that the absorbed energy per unit weight e _s
Are also shown.

[0026] From Table 2, Examples 1-8 were comparable to Comparative Example (multiple pipe steel having a structure) 2 (tailored tubes), Comparative Example 1 absorbed energy E _s and parts unit weight larger than (tailored blanks) Energy _s per unit is obtained. Note that in Comparative Example 1, the parts weight is larger than in Examples 1 to 8 and Comparative Example 2 by the amount of the need for the flange portion (5.3 / 4.2 = 1.26 kg of Comparative Example 1, for example, Examples 3 and 5). 5.7 / 4.9 = 1.16 kg in Comparative Example 2).

Further, when the man-hours for manufacturing the materials in Examples 1 to 8 and Comparative Example 2 were compared, the man-hours required for forming the multi-tube structure in Examples 1 to 8 were approximately the same as the man-hours for manufacturing the tailored tube in Comparative Example 2. It was 70%, which confirmed that the present invention was significantly more advantageous in terms of manufacturing cost than the tailored tube.

[0028]

As described above, according to the present invention, it is possible to obtain an HF part having a shock absorbing performance comparable to that of using a tailored tube as a material. Since it can be manufactured at a low cost, the material for HF processing that contributes to the structural members for automobiles can be supplied more economically, which has a remarkable effect on industrial contribution.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an IA type double tube of the present invention.

FIG. 2 is a cross-sectional view showing an example of the OA type double pipe of the present invention.

FIGS. 3A and 3B are cross-sectional views showing a non-conforming example (a) and a conforming example (b) of a mounting mode of a partially double pipe to an HF mold.

FIG. 4 is a dimensional view of an HF component made of the embodiment.

FIG. 5 is an explanatory view showing a strain measurement site of a prismatic tube.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Base tube 2 Add-on tube 3 Gap 4 Welding line 5 HF mold 6 High expansion rate processing chamber 10 Single pipe section 10A Single pipe section high expansion rate processing chamber section 20 Multiple pipe section

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Akira Ito 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term in the Technical Research Institute, Kawasaki Steel Corporation (reference) 3H111 AA01 BA03 CB03 CB30 DA26 DB19 EA12 EA16 4E068 AA02 BG00 DA15 DB01

Claims (3)

[Claims] 1. A high-strength steel pipe for hydroforming having a multiple pipe structure in a part of the entire length. 2. The high-strength steel pipe for hydroforming according to claim 1, wherein the multiple pipe structure is formed by one or more of shrink fitting, cold fitting and press fitting. 3. The high-strength steel pipe for hydroforming according to claim 1, wherein the pipe constituting the multi-pipe structure is laser-welded.