Tube for hydroforming applications and method for hydroforming a tube

Description

[0001] The invention relates to a straight steel tube for hydroforming applications, especially for producing the A-pillar, B-pillar or C-pillar of an automobile. The invention also relates to a method for hydroforming a steel tube, especially for hydroforming the A-pillar, B-pillar or C-pillar of an automobile.

[0002] It is a well-known technology to use straight steel tubes (so without curves) for hydroforming applications. Often the tube is first bent before the tube is inserted in a hydroforming apparatus, where the tube is placed between two or more dies and the diameter of the tube is enlarged and/or the shape of the cross section of the tube is changed over at least part of the length of the tube by inserting a fluid under pressure in the tube while the ends of the tube are kept closed. However, it is not always necessary to bend the tube first.

[0003] A drawback of the hydroforming technology is that the amount of material in the tube is determined beforehand by choosing the diameter of the tube, while the circumference of the tube will often change during hydroforming. The result is that in such cases the thickness of the wall of the tube will be changed during hydroforming, often only locally, resulting in a weakened hydroformed part or a failure of the part during hydroforming.

[0004] Especially the parts of an automobile, notably the A-pillar, B-pillar and C-pillar should not bend easily and have to be strong and stiff. These requirements are a fortiori needed in the A-pillar of an open car, where the A-pillars should be able to withstand the forces during a roll-over of the car. Here also a slim part is required by the users of the car, so as not to hamper the sight.

[0005] It is an object of the invention to provide an improved tube for hydroforming applications, especially for automotive use.

[0006] It is another object of the invention to provide a tube for hydroforming applications with which the change in thickness during hydroforming can be reduced and/or failure of the tube during hydroforming can be prevented.

[0007] It is a further object of the invention to provide a tube for hydroforming applications with an improved strength and stiffness in relation to its circumference, using the same thickness of the wall of the tube.

[0008] According to the invention, one or more of these objects are reached by providing a straight steel tube for hydroforming applications, especially for producing the A-pillar, B-pillar or C-pillar of an automobile, wherein the steel has a tensile strength of more than 400 MPa, wherein the tube has a wall thickness of 1 - 4 mm, and wherein the tube has been provided with a longitudinal dimple over at least 25 % of the length of the tube, the tube having a mean diameter between 30 mm and 200 mm in the region of the dimple.

[0009] For the purpose of this invention a dimple is defined as a part of the circumference of a cross-section of the tube being curved inwardly, for instance approximately one third of the circumference being curved inwardly. Of course the transition between the normal semi-circular circumference of the cross-section and the inwardly curved part is smooth; no sharp angles are present. The mean diameter is the average of all the diameters of the cross section of the tube as if no dimple was present.

[0010] The invention makes it possible during the hydroforming operation to provide more material in that section of the tube where the cross-section of the tube is enlarged by the hydroforming, by supplying part of the material in the dimple to enlarge the cross-section of the tube. This is accomplished by the dimple becoming less deep. For the A-pillar and C-pillar of a car this is especially useful at the part of the pillar where the pillar protrudes above the plane of the bonnet.

[0011] Because the tube has to enlarge over part of its length, the dimple has to be present over at least 25 % of the length of the tube.

[0012] Of course it is known to use dimples in a tube, but those dimples are used to bend the tube, because the dimple gives a lower bending stiffness to the tube. Moreover, those known dimples are short in comparison to the length of the tube, because they are only present where the tube has to be bent.

[0013] It is also known to press dimples into a tube during the hydroforming process, so after the tube has been placed between the dies of a hydroforming apparatus, but in that case the tube is hydroformed in the same process, and a straight tube with a dimple according to the invention is never present as a separate part.

[0014] Preferably the dimple has been provided over at least 50 % of the length of the tube, more preferably over at least 75 % of the length of the tube. The longer the dimple is, the easier it is to provide a tube with such a dimple. The strength and stiffness of the tube will also be improved over a longer portion of the tube when the dimple is longer.

[0015] According to an improved embodiment, the dimple has been provided over the full length of the tube. Such tubes are relatively easy to produce, and usually the dimple is required over the full length, such as for A-, B- and C-pillars.

[0016] However, there are applications where is will be practical when the dimple is only present between the outer ends of the tube. This means that the end parts of the tube are not provided with the dimple, but that the dimple is present between the end parts of the tube. In such cases it will be more easy to connect the end part of the tube provided with the dimple to other parts.

[0017] The tube has preferably been made of high strength steel having a tensile strength of at least 500 MPa, preferably HSLA steel. HSLA steels having a tensile strength of at least 500 MPa are well suited for hydroforming applications are and are often used for automotive parts.

[0018] Preferably the tube has been made of high strength steel having a tensile strength of at least 600 MPa, preferably DP 600 or stronger, TRIP 600 or stronger, complex phase or TWIP steel. Most of these high strength steels are strong, but show a low plastic strain. For such steel types a dimple in the tube is required, because otherwise the circumference cannot be enlarged to more than a small extent without failure of the tube.

[0019] According to a preferred embodiment the tube is a roll formed tube, preferably a roll formed precision tube. Such tubes are easy to produce, and often used in hydroforming.

[0020] According to another preferred embodiment the tube is a tubular blank. Tubular blanks are produced from a planar sheet, and not as a continuous tube as is the case in roll formed tubes. Tubular blanks usually have a laser weld, which is stronger than a high frequency induction weld as in roll formed tubes.

[0021] In an embodiment the tube is a conical tube. Conical tubes without dimple are more expensive to produce than tubes having a constant diameter, because they cannot be produced continuous, but have to be produced as tubular blank.

[0022] Preferably the dimple has a depth that is variable over the length of the dimple. When the depth of the dimple varies over the total length of the tube, it is possible to produce a conical tube starting from a tube having a constant diameter without dimple. The conical tube with dimple is thus less expensive than a standard conical tube, because it can be produced from a tube having a constant diameter.

[0023] According to a preferred embodiment the tube is a tailored tube. The tube thus can for example be formed from two different steel types, or can have two different thicknesses.

[0024] According to another preferred embodiment the tube has been made from a tailor rolled blank. The tube thus has portions having different thicknesses.

[0025] In an embodiment, the tube has been provided with two dimples on opposite sides of the tube. This can be useful to reduce the bending stiffness and strength of the tube at the location of the dimples.

[0026] In another embodiment, the tube has been provided with two or more dimples on the same side of the tube, preferably one dimple having been provided over at least 25 % of the length of the tube. In this way for instance only the ends of the tube are provided with a dimple.

[0027] Preferably, the tube has a mean diameter between 50 mm and 120 mm in the region of the dimple. Tubes to be hydroformed seldom have a diameter outside this range.

[0028] The invention also relates to a method for hydroforming a steel tube, especially for hydroforming the A-pillar, B-pillar or C-pillar of an automobile, wherein the steel has a tensile strength of more than 400 MPa, wherein the tube has a wall thickness of 1 - 4 mm, comprising the steps of:

• in a first step providing a straight steel tube with a longitudinal dimple over at least 25 % of the length of the tube, such that the mean diameter of the tube in the region of the dimple is between 30 mm and 200 mm, in accordance with the invention as described above;
• in a further step that is separate from the first step, hydroforming the tube which has been provided with a dimple.

[0029] This method describes the forming of the straight steel tube as described above and the forming of this tube by using hydroforming. In this way also the use of the tube with a dimple according to the invention is covered.

[0030] According to a preferred embodiment, between the first step and the further step during an intermediate step the straight tube provided with a dimple is bent. Often it will be necessary to bend the straight tube before it is hydroformed, since for instance an A-pillar is not straight.

[0031] According to a more preferred embodiment the intermediate step and the further step are combined into one step. The tube is thus bent in the hydroforming apparatus before the hydroforming itself takes place. In this way it is not necessary to use a separate bending apparatus. Of course this is only possible when the tube remains more or less straight.

[0032] As an example a tube for an A-pillar for an automobile is described. The tube has a length of approximately 2 meter and a wall thickness of 2 mm. Before the dimple is formed, the tube has a diameter of approximately 60 mm. A dimple is formed over the full length of the tube, the dimple having a variable depth such that at one end of the tube the dimple is deeper than at the other end. In this way a conical tube is formed, the tube having a smaller mean diameter at the end where the dimple is deepest. This end will form the top of the A-piller after the tube with dimple has been bent and hydroformed.

Claims

1. Straight steel tube for hydroforming applications, especially for producing the A-pillar, B-pillar or C-pillar of an automobile, wherein the steel has a tensile strength of more than 400 MPa, wherein the tube has a wall thickness of 1 - 4 mm, and wherein the tube has been provided with a longitudinal dimple over at least 25 % of the length of the tube, the tube having a mean diameter between 30 mm and 200 mm in the region of the dimple.

2. Tube according to claim 1, wherein the dimple has been provided over at least 50 % of the length of the tube, preferably over at least 75 % of the length of the tube.

3. Tube according to claim 1 or 2, wherein the dimple has been provided over the full length of the tube.

4. Tube according to claim 1 or 2, wherein the dimple is only present between the outer ends of the tube.

5. Tube according to any one of the preceding claims, wherein the tube has been made of high strength steel having a tensile strength of at least 500 MPa, preferably HSLA steel.

6. Tube according to any one of the preceding claims, wherein the tube has been made of high strength steel having a tensile strength of at least 600 MPa, preferably DP 600 or stronger, TRIP 600 or stronger, complex phase or TWIP steel.

7. Tube according to any one of the claims 1 - 6, wherein the tube is a roll formed tube, preferably a roll formed precision tube.

8. Tube according to any one of the claims 1 - 6, wherein the tube is a tubular blank.

9. Tube according to any one of the preceding claims, wherein the tube is a conical tube.

10. Tube according to any one of the preceding claims, wherein the dimple has a depth that is variable over the length of the dimple.

11. Tube according to any one of the preceding claims, wherein the tube is a tailored tube.

12. Tube according to any one of the preceding claims, wherein the tube has been made from a tailor rolled blank.

13. Tube according to any one of the preceding claims, wherein the tube has been provided with two dimples on opposite sides of the tube.

14. Tube according to any one of the preceding claims, wherein the tube has been provided with two or more dimples on the same side of the tube, preferably one dimple having been provided over at least 25 % of the length of the tube.

15. Tube according to any one of the preceding claims, wherein the tube has a mean diameter between 50 mm and 120 mm in the region of the dimple.

16. Method for hydroforming a steel tube, especially for hydroforming the A-pillar, B-pillar or C-pillar of an automobile, wherein the steel has a tensile strength of more than 400 MPa, wherein the tube has a wall thickness of 1 - 4 mm, comprising the steps of:

- in a first step providing a straight steel tube with a longitudinal dimple over at least 25 % of the length of the tube, such that the mean diameter of the tube in the region of the dimple is between 30 mm and 200 mm, in accordance with any one of the preceding claims;

- in a further step that is separate from the first step, hydroforming the tube which has been provided with a dimple.

17. Method according to claim 16, wherein between the first step and the further step during an intermediate step the straight tube provided with a dimple is bent.

18. Method according to claim 17, wherein the intermediate step and the further step are combined into one step.