1. Field of the Invention
The present invention relates to an automobile strength member high in bending repulsion at the beginning of impact and able to reduce production costs and lighten the weight.
2. Description of the Related Art
In recent years, standards for side impact of automobiles have become tougher. The structural design concepts of chasses have changed as well. Automobile doors are being provided with door guard bars. In the past, door guard bars have been considered members deforming at the time of side impact so as to absorb the impact energy and reduce the impact received by passengers.
In recent years, chassis structures have been designed to enable the impact energy at the time of a side impact to be absorbed by the door as a whole. In such a chassis structure, the door guard bar has been used as an energy transmitting member for transmitting the impact energy to other members around the door guard bar to have the other members absorb the impact energy by their own deformation. Therefore, it is important that the member used as the door guard bar keep any impact force received to within its elastic deformation range and not deform. In particular, a member having the property of a high bending repulsion at the beginning of impact has been demanded. For this, increase of the strength of the member may be considered, but a higher strength results in the problems of brittle fracture or hydrogen induced cracking. Just increasing the strength of the member is not preferable for an automobile member. Therefore, the shape was focused on and studied for improvement of the bending repulsion at the beginning of impact.
Up to now, members for door guard bars have been studied using static bending test from the viewpoint of the absorption energy. For example, it is known that a high absorption energy can be obtained by increasing the strength of a steel tube while its cross-section is round (for example, see Japanese Unexamined Patent Publication (Kokai) No. H4-63242). However, in bending deformation behavior of a round-section steel tube, the load concentrates right under the load, and the sectional shape easily flattens due to local buckling. The member is similarly weak in impact bending. Therefore, a high bending repulsion at the start of impact cannot be obtained.
Further, an automobile structural member having a closed sectional structure, a maximum dimension in the longitudinal direction, a maximum dimension in the direction perpendicular to that, an outer circumferential length, and a thickness defined in relationship by a tensile strength of at least 1400 MPa has been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2002-248941). With this member, it is true that the absorption energy and bending repulsion are improved compared with the past, but in each case the results were based on a static bending test. Whether a high bending repulsion at the beginning of impact is obtained even for an impact bending test is unclear.
On the other hand, as opposed to this, rectangular tubes superior in bending properties compared with round tubes are being developed. That is, there has been proposed a rectangular tube wherein the outsides of the corner parts are either not provided with an R or are providing with an R of a magnitude less than the thickness and the inner surfaces are provided with an R larger than the outside so as to make a load act directly on the two side surfaces parallel to the bending input direction and improve the bending repulsion (for example, see Japanese Unexamined Patent Publication (Kokai) No. H6-278458). The tensile strength of the member is the 1470 MPa class. With a member having this tensile strength, the possibility arises of brittle fracture or hydrogen induced cracking occurring due to the bending load applied. Further, when further working an electric resistance welded steel tube or other round tube into a rectangular tube, provision of a larger R at the inner surfaces of the corner parts than the outer surfaces is difficult in production, while when producing the above sectional shape, there is the problem that the cost ends up becoming remarkably higher.