JP2015089721A - Structural member for vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a structural member for a vehicle capable of saving weight by using a super high tensile steel plate of 900 MPa or more, and suppressing generation of crack and breaking during vehicle collision.SOLUTION: A side surface where tensile stress is generated during collision is formed of a TRIP steel plate 1, and a side surface where compression stress is generated during collision is formed of a DP steel plate 2, thereby the structural member for the vehicle is provided. For securing intensity, the TRIP (transformation induction plasticity) steel plate and the DP (dual phase) steel plate are preferably coupled for forming a hollow cross sectional structure.

Description

  The present invention relates to an automotive structural member, and more particularly to an automotive structural member using a TRIP steel plate.

  In current automobiles, the strength of the vehicle body is required from the viewpoint of collision safety and occupant protection, and at the same time, the weight reduction of the vehicle body is required from the viewpoint of economic efficiency and environmental considerations. In order to realize these requirements, the adoption of high-strength steel plates called high tension has been promoted.

  As a conventional high-strength steel plate for automobiles, as disclosed in Patent Document 1, DP steel (composite structure steel) has been mainly used. DP steel is a steel in which a soft ferrite structure and a hard martensite structure are dispersed in a steel sheet, and is widely put into practical use as a steel material having both ductility and strength. However, there are many practical tensile strengths of less than 600 MPa, and recently, attempts have been made to further reduce the weight by using DP steel sheets of 900 MPa or more called super high tensile strength.

  However, such super high-tensile steel sheet has a smaller elongation than a DP steel sheet having a practical tensile strength of less than 600 MPa, which has been used in the past. There was a concern that it would break after it cracked, and there was a problem that it was difficult to use as a structural member for automobiles. Therefore, when using ultra high-tensile steel plates for structural members for automobiles, it has been devised to place a soft, low-strength steel plate around it and deform it so that the ultra-high tensile steel plate is not deformed as much as possible. This leads to an increase in the thickness of the structural member for automobiles, which has a contradictory effect on weight reduction.

  A TRIP steel sheet has also been developed as a super high-tensile steel sheet of 900 MPa or more. As shown in FIG. 1, before press forming, the yield stress is low and the elongation is large, so that the workability is good, and after forming, the material is work hardened to achieve high strength. However, when this is used for a structural member for automobiles, work hardening has occurred in a local press working part represented by a bead part or a ridge line, and therefore, when a vehicle collides, a conventional ultra high-tensile steel sheet (DP steel sheet of 900 MPa or more) and Similarly, there is a problem that cracks and breaks are easily generated starting from the work-hardened portion.

  In addition, depending on the complicated deformation progress state, such as at the time of a vehicle collision, secondary deformation may occur after temporary deformation. In this case as well, there is a problem that the TRIP steel sheet is likely to break. It was. If breakage occurs at the time of collision, it may not be possible to fully absorb the collision energy, and even if the collision energy can be absorbed, the deformation may be unstable from the viewpoint of robustness. Absent.

JP 2004-218066 A

  Accordingly, the object of the present invention is to solve the above-mentioned conventional problems, and can be reduced in weight by using a super high-tensile steel plate of 900 MPa or more, and further, the structure for an automobile that suppresses the occurrence of cracks and breaks at the time of a vehicle collision. It is to provide a member.

  As a result of repeated studies by the present inventors to solve the above problems, the TRIP steel sheet has the characteristics that the yield stress is low and the elongation is large before press forming, and the optimal arrangement of the structure and material is taken into consideration. This led to the idea that it could be used for the deformation performance in the event of a collision while avoiding the disadvantages of the TRIP steel sheet. In other words, this property has been used exclusively from the viewpoint of moldability so far, but if this property is used for absorbing deformation at the time of collision in consideration of the optimal arrangement of structure and material, it has an unprecedented high strength. It has been found that a structural member for automobile can be obtained.

  The present invention has been made on the basis of such knowledge, and the gist thereof is a structural member for an automobile made of a high-strength steel plate, and a side surface on which a tensile stress is generated at the time of collision is constituted by a TRIP steel plate. And, it is a structural member for an automobile characterized in that a side surface on which a compressive stress is generated at the time of a collision is made of a DP steel plate.

  As in claim 2, it is preferable that the DP steel sheet has a tensile strength of 900 MPa or more and / or a TRIP steel sheet has a tensile strength of 900 MPa or more. Moreover, it is preferable to set it as the structural member for motor vehicles which has the hollow cross-section structure which couple | bonded the TRIP steel plate and DP steel plate like Claim 3, and it is any one or both of a TRIP steel plate and DP steel plate like Claim 4. Is preferably a molded product having a hat-shaped cross section.

  Since the structural member for automobiles of the present invention is composed of a TRIP steel plate on the side where tensile stress is generated at the time of a collision, it can take advantage of the property that the TRIP steel plate has a large elongation, and to a place that could not be used conventionally. High-strength steel sheets can be used. In addition, compressive stress often occurs on the opposite side of the side where tensile stress is generated in the event of a collision. However, because this section uses DP steel with excellent compressive properties, the overall strength and weight is low, and the vehicle Generation of cracks and breaks at the time of collision can be suppressed.

It is a stress strain diagram of TRIP steel and DP steel. In one embodiment of the present invention, a TRIP steel plate and a DP steel plate are both schematic views showing a structural member for automobiles formed of a molded product having a hat-shaped cross section. It is explanatory drawing which shows the usable position of the structural member for motor vehicles of this invention. It is the graph and state explanatory drawing which show the relationship between the load including a fracture | rupture determination at the time of applying this invention to B pillar.

Preferred embodiments of the present invention are shown below.
FIG. 2 is a schematic view showing an automotive structural member according to an embodiment of the present invention, in which two steel plates 1 and 2 processed into a hat-shaped cross section are joined together by a flange portion 3 and have a hollow cross-sectional shape. Is configured. The steel plate 1 is a TRIP steel plate, and the steel plate 2 is a DP steel plate.

  The TRIP steel plate is arranged as a side surface that generates tensile stress at the time of collision, that is, a side surface that is the inside of the vehicle body, and the DP steel plate is arranged as a side surface that generates compressive stress at the time of the collision, that is, a side surface that is outside the vehicle body. is there. These TRIP steel plates and DP steel plates preferably have a tensile strength of 900 MPa or more.

  As described above, the DP steel sheet is a steel in which a soft ferrite structure and a hard martensite structure are dispersed in the steel sheet, and the DP steel sheet having a strength of less than 600 MPa has a large elongation as shown in FIG. However, the DP steel sheet of 900 MPa or more has a small extension and may be broken when used on the tension side. However, if used on the compression side, there is no risk of breakage, and high strength characteristics can be sufficiently exhibited.

  On the other hand, as shown in FIG. 1, the TRIP steel sheet has a large elongation until it breaks. Therefore, when the tensile strength is 900 MPa or more, it is difficult to break when used on the tension side, and also exhibits high strength characteristics sufficiently. can do. In order to take advantage of this characteristic, the TRIP steel sheet 1 needs to avoid local pressing as much as possible in the forming stage and suppress work hardening. Moreover, it is preferable to use it for the site | part which a secondary deformation does not occur in order to prevent the fracture | rupture by a secondary deformation | transformation from occurring after the generation | occurrence | production of the local hardening by a primary deformation. Furthermore, it is not preferable to provide a locally processed portion, such as a bent bead, in a direction perpendicular to the tensile direction in which elongation deformation is expected, in order to avoid work hardening.

  As described above, the structural member for automobiles of the present invention has a tensile strength TRIP steel plate 1 (preferably a tensile strength of 900 MPa or more) on the side surface where tensile stress is generated at the time of collision, and a DP steel plate (if necessary) on the opposite side. Preferably, the tensile strength is 900 MPa or more), so that both the front and back surfaces can be made of super high-tensile steel plates, and the weight can be reduced. Moreover, the characteristic that the elongation of the TRIP steel plate is large can be utilized, and the occurrence of cracks and breaks at the time of a vehicle collision can be suppressed.

  The structural member for automobile of the present invention can be used for a center pillar (B pillar) inner, a side sill inner and the like against a vehicle side collision. It can also be used for dash cross members, dash lowers, front bumper reinforcements, etc. for vehicle front collisions. It can also be used for rear end cross member inners, rear bumper reinforcements, etc. against vehicle rear collisions. These use sites are shown in FIG. In most cases, it can be predicted whether a side surface is a side where tensile stress is generated or a side surface where compressive stress is generated depending on the use site.

  In the embodiment of FIG. 2 described above, in order to ensure the strength, both the TRIP steel plate and the DP steel plate have a hollow cross-sectional structure in which two steel plates 1 and 2 processed into a hat-shaped cross section are joined. The present invention is not limited to this embodiment. For example, two steel plates 1 and 2 having one plate shape or one or both of which have semicircular cross sections may be joined. The cross-sectional shape can be changed continuously or discontinuously in the longitudinal direction of the structural member for automobile. Further, it goes without saying that various attachment members can be joined and integrated by welding or other methods, or can be drilled.

  FIG. 4 shows the relationship between load and displacement including fracture determination when the present invention is applied to a B-pillar. When the TRIP steel plate was arranged on the inner side where tensile stress was generated, no fracture occurred, but when the DP steel plate was arranged on the inner side, the fracture occurred.

  As described above, the structural member for automobiles of the present invention can be reduced in weight by combining two types of high-strength steel plates having different properties, and also suppresses the occurrence of cracks and breaks during a vehicle collision. There are advantages that can be made.

1 Steel plate (TRIP steel plate)
2 Steel plate (DP steel plate)
3 Flange

Claims (4)

  An automotive structural member made of a high-strength steel plate, wherein the side surface where tensile stress is generated at the time of collision is composed of a TRIP steel plate, and the side surface where compressive stress is generated at the time of collision is composed of a DP steel plate Element.   The structural member for automobile according to claim 1, wherein the tensile strength of the DP steel plate is 900 MPa or more and / or the tensile strength of the TRIP steel plate is 900 MPa or more.   The automotive structural member according to claim 1 or 2, wherein the structural member has a hollow cross-sectional structure in which a TRIP steel plate and a DP steel plate are combined.   4. The automotive structural member according to claim 3, wherein one or both of the TRIP steel plate and the DP steel plate are formed with a hat-shaped cross section.