JP2011178179A - Structural member having superior impact absorbing characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a structural member improved in an impact absorbing characteristic by enhancing axial squeezing strength at low cost by contriving the cross-sectional shape of the structural member and a partial quenching method, even in the structural member made of a thin steel plate. <P>SOLUTION: In this structural member including a hat-shaped closed section structure in which a hat-shaped cross-sectional shape steel plate bent into a hat-shape and a flat steel plate are overlapped with each other and overlapped parts of a flange are molten and joined, a V-shaped recess is formed in a center part of a top side of the hat-shaped cross-sectional shape steel plate. A partial quenching hardening part by laser irradiation is formed over a longitudinal direction of the structural member in a ridge line region of a rounded corner round part at a bent portion. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an automobile structural member having excellent shock absorption characteristics such as a crash box and a front side frame.

From the aspect of collision safety, the vehicle body structure absorbs impact energy at the time of automobile collision by plastic deformation of structural members other than the passenger compartment (hereinafter referred to as cabin), and minimizes cabin deformation to provide a living space. A vehicle body structure that secures this is widely and generally adopted. In this case, it is important how to effectively absorb the impact energy by a structural member such as a crash box or a front side frame.
In general, automobile structural members are configured with a hat-shaped closed cross section, and are designed to absorb impact energy by buckling in a bellows shape when subjected to an impact load in the longitudinal direction (axial direction). . At this time, in order to improve the shock absorption characteristics, the bellows-like buckling deformation is regularly generated, and the average value of the load fluctuation that appears with periodicity after the buckling is shifted upward as a whole. It is important to increase

That is, in order to increase the stability and reproducibility of the shock absorption characteristics, a technique of providing a crushing bead at an appropriate position of the structural member has been proposed in order to regularly generate bellows-like buckling deformation. It is also very important to design such that the structural member can be reduced in weight by increasing the impact crushing energy (that is, the impact absorbing ability) per unit weight of the structural member by increasing the axial crushing strength.
Therefore, in order to increase the axial crushing strength of the entire structural member, for example, in Patent Document 1, a closed cross section formed by joining two U-shaped members with flanges (hereinafter referred to as hat-shaped cross-section steel plates). A structural member made of a thin steel plate in which a partition plate is arranged has been proposed.
Further, in Patent Document 2, a hat-shaped cross-sectional steel plate obtained by press-molding a specific ultra-low carbon steel plate is irradiated with high-density energy such as a laser to form a hardened and hardened portion to partially increase the strength of the steel plate, Thin steel plate structural members that have improved the axial crushing strength of the members have been proposed.

JP 2000-53023 A JP 7-26319 A

The structural member proposed in Patent Document 1 has the advantage of improving the maximum bending load and also being excellent in impact absorption characteristics at the time of axial crush deformation in the longitudinal direction. In addition, there is a drawback that an increase in the weight of the structural member due to an increase in the number of members is unavoidable.
In addition, the structural member in Patent Document 2 can suppress the thermal strain accompanying high-density energy irradiation to a small size by using an ultra-low carbon steel sheet with a small volume change of melt solidification / shrinkage and transformation strengthened structure, and is lightweight, While it has the advantage of being able to obtain structural members with excellent shock absorption characteristics, the axial crushing strength is increased by increasing the number of hardened and hardened parts by laser irradiation, but the parts that require strength are identified. Therefore, when the number of laser irradiations is increased unnecessarily, there is a drawback that the productivity may be greatly reduced.

  The present invention has been invented in view of these current situations, and even if it is a structural member made of a thin steel plate, the axial crushing strength is increased by devising the cross-sectional shape and the partial quenching method. An object of the present invention is to provide a structural member with improved impact absorption characteristics at low cost.

In order to achieve the purpose of the structural member having excellent shock absorption characteristics of the present invention, a hat-shaped cross-section steel plate bent into a hat shape and a planar steel plate are overlapped, and the overlapping portion of the flange is melt-bonded. A structural member having a hat-shaped closed cross-sectional structure, characterized in that a V-shaped depression is formed in the central portion of the top side of the hat-shaped cross-sectional steel plate.
It is preferable that a partially quenched and hardened portion by laser irradiation is formed in the longitudinal direction of the structural member in the ridge line region of the rounded corner R portion of the hat-shaped cross-sectional steel plate.

  According to the present invention, in a structural member having a hat-type closed cross-sectional structure, it is possible to greatly improve the shock absorption characteristics by only devising the cross-sectional shape of the structure and the partial quenching method. Therefore, the structural member excellent in performance can be provided at low cost according to the present invention.

Sectional drawing explaining the structure of a hat-type closed cross-section structural member Sectional drawing explaining the structure of the hat type closed cross-section structural member which has various cross-sectional shapes The figure explaining the buckling form of the structural member of the concave cross-sectional shape which provided the square cross-sectional shape and the V-shaped hollow. The figure explaining the case where the unstable buckling form was exhibited in the structural member of the concave cross-sectional shape which gave the U-shaped hollow. The figure explaining the case where the unstable buckling form is exhibited in the structural member which overlap | superposed and joined two hat-shaped cross-section steel plates of the concave cross-section shape which gave the V-shaped hollow. The figure which shows the hat type closed section structural member which used for drop weight test Conceptual diagram explaining the drop weight test method A diagram conceptually showing the load-displacement curve obtained in the drop weight test The figure which shows the performance evaluation result of the test body which is obtained with the drop weight test

The inventors of the present invention have made various studies on improving the performance of structural members that require high shock absorption characteristics. As a result, structural members with a hat-type closed cross-sectional shape can be obtained at low cost by improving the shock-absorbing characteristics by increasing the axial crushing strength by devising the cross-sectional shape and partial quenching method. all right.
Details will be described below.

As shown in FIG. 1, the structural member of the hat-shaped closed cross-sectional shape is formed by spot welding or laser welding a flange portion 3 in which a hat-shaped cross-sectional steel plate 1 and a flat steel plate 2 which are bent into a hat shape are overlapped. And a member having a closed cross-sectional structure with respect to a cross section perpendicular to the longitudinal direction.
In such a structural member with a hat-shaped closed cross section, four types of prototypes having different shapes as shown in FIGS. 2 (a), (b), (c), and (d) were produced, An evaluation test was conducted.

(A) Ordinary shape product without any ingenuity, (b) 12 hardened and hardened portions partially formed on a flat surface, (c) Top side of hat-shaped cross-section steel plate A V-shaped depression is formed at the center, and (d) six hardened and hardened portions are formed in a rounded ridgeline region at the corner R.
In addition, formation of a partial hardening hardening part is easily formed by irradiating a required location with a laser. That is, rapid heating is performed by continuous laser irradiation, rapid cooling is performed after laser irradiation, and the part is hardened in a quenched state.

  Details will be given in the description of the embodiment, but when a V-shaped depression is formed in the central part of the top side of the hat-shaped cross-section steel plate, the shock absorbing ability is remarkably improved, and the rounded corner of the bending portion is further improved. It was found that the impact absorbing ability was further improved when a partially hardened portion was formed by continuously performing partial heat treatment in the ridge line region of the R portion.

  As shown in FIG. 3 (b), if it has a concave cross-sectional shape that forms a V-shaped depression in the central part of the top side of the hat-shaped cross-sectional steel plate, compared to a simple square cross-section (FIG. 3 (a)) As a result, there are many curved surfaces, and the ridge line area greatly increases from four to seven. Further, the site that mainly absorbs the impact energy, that is, the site that generates a large plastic strain is more than the two-dimensional bending deformation site of the flat portion shown in the one-dot broken line ellipse in FIG. As indicated by an arrow in the figure, it is located on a curved surface where two flat portions intersect such as a so-called corner R portion that induces a complicated three-dimensional bending deformation. Furthermore, as an incidental effect, the plastic strain rate naturally increases in the portion where the plastic strain is high because the increase rate of the plastic strain per unit time is large. It is considered that these effects acted in combination to greatly increase the axial crushing strength and to obtain high impact absorption energy.

  As shown in FIG. 4, the recess formed in the central part of the top side of the hat-shaped cross-section steel plate is indicated by hatching in the figure when it is a concave cross-sectional shape having a U-shaped recess composed of three planes. Since the bending deformation in the two side wall portions intersecting with the flat bottom portion occurs under a tight condition, a part of the concave cross section tends to jump out to the front. In particular, in the case of a long structural member having a length L of the structural member / an average length D of the cross section of 4.0 or more (L / D in FIG. 4 is 450 / ((100 + 80) / 2) = 5.0). The tendency becomes more conspicuous, resulting in the loss of stable bellows-like buckling deformation. Therefore, as shown in FIG. 3B, the concave cross-sectional shape preferably forms a trapezoidal depression that is V-shaped or narrow in depth, which is composed of two planes.

  As a means to further increase the number of ridge line regions, when two hat-shaped cross-section steel plates are overlapped and joined as shown in FIG. There is a risk of discrepancies in the height level of the occurrence, leading to unstable buckling. As a steel plate shape to be bonded between the front and back, a structural member having a closed cross-section structure in which a hat-shaped cross-section steel plate having high rigidity and a planar steel plate having low rigidity are overlapped and joined as shown in FIG. 3B is desirable. . This is because the buckling on the low-rigidity steel plate side follows well with the buckling that occurs on the high-rigidity steel plate side, and interlocked buckling deformation occurs.

  As a means for further improving the shock absorption characteristics, it is effective to form a partially quenched and hardened portion in the ridge line region of the rounded corner R portion, instead of the above-mentioned means for increasing the ridge line region. The reason for this is that the material is hardened by quenching treatment at sites that have excellent physical effects such as plastic strain and plastic strain rate. This is because crushing strength and impact absorption energy can be obtained, but the stability of buckling deformation is not adversely affected.

Cold-rolled steel sheets having a thickness of 1.2 mm, 1.4 mm, and 1.8 mm, which are automotive workable cold-rolled high-tensile steel sheets having a tensile strength of 980 MPa as a material, were prepared.
Four types of hat-type closed cross-section structural members having the shape and size shown in FIG. 2 were produced using steel plates having a predetermined thickness. The length of the structural member was 450 mm as shown in FIG. Note that the flange portion of the press-formed hat-shaped cross-sectional steel plate and the planar steel plate were joined by spot welding at intervals of 35 mm.
As a means for creating a partially quenched state in the longitudinal direction along the ridge line region, the laser irradiation process was performed using a fiber laser processing apparatus, with an output of 2 kW and a traveling speed of 2 m / min.

An evaluation test related to shock absorption characteristics required for structural members is a test in which one end of a hat-type closed cross-section structural member is fixed, and a falling weight is dropped on the other end surface at a speed comparable to that in a car collision (this test method is described in this specification). In the book, the following “falling weight test”) was performed, and the appearance of the buckling form of the structural member was observed, and the impact absorption energy of the structural member was measured and evaluated.
As shown in FIG. 7, the drop weight test is a load acting on the hat-type closed cross-section member when the impact load is applied in the axial direction of the hat-type closed cross-section member and the shaft is crushed. This is a test method for investigating the relationship between and displacement.

  In the actual drop weight test, the hat-shaped closed cross-section structural member 61 is placed on a pedestal 63 incorporating the load cell 62 with the longitudinal direction thereof being vertical, and a 400 kg drop weight 64 is collided at a speed of 51 km / h to form a hat type. The closed cross-section structural member was crushed in the axial direction. The height of the stopper 65 was adjusted so that the falling weight stopped after crushing the hat-shaped closed cross-section structural member by 300 mm. The amount of movement (hereinafter referred to as displacement) after the falling weight collided with the structural member was continuously measured using a non-contact displacement meter (not shown) to obtain a load-displacement curve.

FIG. 8 is a graph of a load-displacement curve conceptually showing the transition when the impact load due to the falling weight acts in the axial direction of the structural member, measured in the drop weight test. In this case, the structural member is buckled at a plurality of locations in the longitudinal direction and crushed into a bellows shape. Corresponding to the occurrence of buckling, a plurality of load peak values appear periodically on the load-displacement curve. Here, it is obtained by integrating the load value for each arbitrary displacement, that is, when the area surrounded by the load-displacement curve in the figure is crushed to 300 mm, the structural member subjected to the drop weight test absorbs it. The amount of impact energy.
Table 1 shows the evaluation results obtained in the drop weight test.

The appearance of the specimen after the drop-weight test exhibited a relatively beautiful bellows-like buckling under all conditions.
As is clear from Table 1, regarding the shock absorption capacity, Test No. which is an example of the present invention. The impact absorption energy was significantly improved in the structural members 1 and 2.
From the viewpoint of the weight of the member, as shown in FIG. 9, the adoption of the present invention can reduce the weight and cost of the structural member.
That is, when the weights of the structural members of Test Nos. 1 and 2, which are examples of the present invention, are compared with the weights of the structural members having a square closed cross section (FIG. 2 (a)) having the same impact absorption energy, The weight can be reduced by 18% and 16%. In addition, the weight of the structural member having a square closed cross section having the same impact absorption energy as the structural members of Test Nos. 1 and 2 is obtained from the relationship of Test Nos. 3, 4, and 5 in FIG. Based on.
Shock absorption energy (kJ) = 0.463 × (weight of structural member (vs. base%)) − 19.6 (1 set)

  In addition, in the case of test No. 1 (Fig. 2 (d)), in which the rounded ridgeline area at the corner R is partially quenched, compared to test No. 2 (Fig. 2 (c)) The improvement rate of the impact absorption energy is 4.5% (= (37.0-35.4) × 100 ÷ 35.4), whereas test No. 6 in which the flat part was partially quenched (Fig. 2 (b)), the improvement rate in impact absorption energy accompanying partial quenching is less than 1.0% compared to test No. 4 (FIG. 2 (a)) (= (30.7-30.4) Since x100 ÷ 30.4), it can be seen that test No. 1 which is an example of the present invention is capable of partial quenching at an efficient site.

Claims (2)

  A structural member having a hat-shaped cross-sectional steel plate, wherein a hat-shaped cross-sectional steel plate is formed by superimposing a hat-shaped cross-sectional steel plate bent into a hat shape and a planar steel plate and melt-bonding the overlapping portion of the flange. A structural member having excellent shock absorption characteristics, wherein a V-shaped depression is formed in the central part of the top side.   The shock absorption according to claim 1, wherein a partially quenched and hardened portion by laser irradiation is formed in a longitudinal direction of the structural member in a ridge line region of a rounded corner R portion of the hat-shaped cross-sectional steel plate. Structural member with excellent characteristics.

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