JP2007290582A - Manufacturing method for bumper reinforcing member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method for a bumper reinforcing member of high performance to be used for enhancing safety of an automobile by absorbing energy at the time of a collision of the automobile. <P>SOLUTION: This bumper reinforcing member 1 has a bending part 3 and a mounting part 4 on a vehicle body skeleton. By providing a strength lowering part in the member 1 by using means of hot stamping, laser welding and high frequency quenching, etc., a buckling form of the member 1 is controlled and the member 1 is made to have high absorbing energy. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a bumper reinforcement (bumper reinforcing member) that is used to improve safety of a vehicle by absorbing energy at the time of a vehicle collision.

  In recent years, in the automobile industry, the development of a vehicle body structure that can reduce injury to passengers during a collision has become an urgent issue. In order to solve this problem, various vehicle body structures have been studied. Among them, the bumper and the bumper reinforcing member have an important function of absorbing impact energy and transmitting the impact load appropriately to other members, particularly at the time of a frontal collision. Further, in a light collision at a relatively low speed, it is also an important function that the impact is absorbed only by the bumper and the bumper reinforcing member, thereby preventing deformation of other members and reducing the repair cost as a whole. On the other hand, weight reduction of the entire vehicle is important for improving fuel efficiency and driving performance, and the bumper reinforcing member is also required to be light.

  In order to reduce the weight of the bumper reinforcing member, a material replacement method is used. One of them is the use of an aluminum extruded material, and it is known that the balance between performance and weight is excellent (for example, Patent Document 1). However, the use of aluminum is generally known to be costly. In addition, the application of high-strength steel sheets has been actively studied (for example, Patent Document 2 and Patent Document 3). However, in general, the formability of high-strength steel sheets deteriorates as the strength increases, so there are concerns about cracks and wrinkles that are poorly formed, and it is difficult to obtain the required shape accuracy, and its application is relatively simple. It was limited.

  The hot stamping technology that has been developed in this situation is to form a heated steel sheet and quench it by removing heat from the mold. For example, as disclosed in Patent Document 4, a member having high strength and high shape accuracy can be obtained by using this technique. However, in general, when an impact load is applied to a member made of a material with a high yield point, the overall bending mode such as Euler buckling is likely to be superior, and the deformation load may decrease at a stretch beyond the collapse load. There is. In addition to the frontal collision, the bumper reinforcement member is required to stably absorb energy even under various load conditions of light collisions. It is necessary to stabilize the behavior, and a countermeasure against such a problem using the hot stamping technique has not been disclosed conventionally.

  For example, Patent Document 5 discloses that a change is made in the longitudinal direction of the bumper reinforcing member in order to cope with different load conditions. According to this, stability is increased by gradually changing the cross section at both ends. However, in general, the gradual change of the cross section tends to cause shape defects such as wrinkles and tends to increase costs. Furthermore, for example, as disclosed in Patent Document 6, a technique for manufacturing a bumper reinforcing member using a so-called tailored blank in which materials having different strengths and plate thicknesses are joined together is disclosed. Although this can change the longitudinal direction of the member, there is a problem in that molding is difficult due to deformation concentration of the joint portion, and the cost increases for joining.

Japanese Patent Laid-Open No. 5-278537 JP 2000-109951 A JP 2000-282175 A JP 2004-238640 A JP 2000-335333 A JP 2001-180398 A

  The present invention relates to a bumper reinforcing member manufacturing method used for enhancing the safety of an automobile by absorbing energy at the time of a car collision, and a bumper reinforcing member manufacturing method that achieves both high absorption energy and light weight. Is to provide.

  The present inventors have studied by paying attention to the effect of having a strength distribution in the bumper reinforcing member, and stably using the same material to arrange a low-strength portion locally to obtain a high absorbed energy stably. It has been found that the bumper reinforcement member shown can be obtained. The gist of the present invention is as follows.

  (1) In a bumper reinforcing member manufacturing method in which a metal plate is heated and then subjected to a quenching process while being molded using a mold, a reduced strength portion is formed by maintaining a gap between the mold and the metal plate. A method for manufacturing a bumper reinforcing member.

  (2) The method for manufacturing a bumper reinforcing member according to (1) above, wherein a gap between the mold and the metal plate is maintained by preforming the metal plate.

  (3) The method for producing a bumper reinforcing member according to (1) or (2) above, wherein a local strength reduction portion is formed in a bent portion of the bumper reinforcing member.

  (4) The method for manufacturing a bumper reinforcing member according to any one of (1) to (3), wherein a local strength reduction portion is formed at a portion where the bumper reinforcing member is attached to the vehicle body skeleton.

  By applying the bumper reinforcing member according to the present invention to the vehicle body, the safety of the vehicle under various load conditions can be enhanced. In addition, it is possible to reduce the manufacturing cost of the bumper reinforcement member, and by changing the arrangement of the reduced strength portion, it is easy to make adjustments according to the size and load conditions of the vehicle, and the overall cost of vehicle development is also increased. Can be reduced.

  The inventors of the present invention first studied earnestly the characteristics required for the bumper reinforcing member. As a result, it was found that the bumper reinforcing member needs to have high deformation strength and that the strength does not extremely decrease with the progress of deformation. In order to increase the deformation strength, the thickness of the member is increased or the material to be used is increased in strength. However, the former is not preferable because it causes an increase in weight. Therefore, it is generally intended to use a material with high strength. However, a thin and high-strength member has a high deformation resistance until plastic deformation starts, but it rapidly starts when plastic deformation starts and cross-section collapses. It has the disadvantage that the deformation load is reduced. As a result of intensive studies, the present inventors have found that such a rapid load decrease can be avoided by intentionally providing a strength decreasing portion in the member.

  It is known that a hot stamping technique, which is a processing method in which hardening is performed using a mold after heating a metal plate, is a method capable of obtaining a high-strength member with good shape accuracy. Especially when steel plate is used, it is hardened while performing molding using a mold after heating to Ac3 temperature or higher in advance, and when sufficient cooling rate can be secured, it is hard martensite. Since a tissue is obtained, a high-strength member can be produced relatively easily. In this case, the important point is the state of contact with the mold. If sufficient contact is made with the mold, the strength becomes high, but if it is insufficient, the necessary strength may not be obtained. In particular, since the vertical wall portion is in close contact with the mold only after being sandwiched between the punch and the die near the bottom dead center of molding, depending on the contact state, there is a possibility that sufficient cooling is not performed and the strength is insufficient. On the contrary, the present inventors pay attention to this drawback, and by setting the clearance between the punch and the die at the strength-decreasing portion to a distance greater than or equal to the plate thickness, between the metal plate as the workpiece and the die. We have come up with a method of suppressing contact by keeping the gap and softening by slow cooling.

  Normally, the bumper reinforcing member has a structure having a curvature toward the end in the width direction of the vehicle due to a demand for design and aerodynamic performance. The present inventors have observed the deformation mode of the bumper reinforcing member used in the actual vehicle body structure, and studied the arrangement method of the reduced strength portion. In the prior art, the same material is used to reduce the cross-sectional height of the end to create a reduced strength part as a member depending on the shape, but this shape may cause wrinkles that are defective in shape in terms of production technology. In some cases, the material yield may be deteriorated and the number of processes may be increased. However, if the same cross-sectional area is made of the same material, the relative member rigidity of the portion with curvature increases, so that a large buckling occurs in a portion that is relatively close to a straight line corresponding to the center of the vehicle. It has been found that this causes a significant load drop. In order to avoid such a situation, it has been found that it is only necessary to dispose a reduced strength portion in a portion having a large curvature, that is, a bent portion. It was found that the bent portion buckled before buckling of the central portion when arranged at the bent portion, and the load reduction as a whole was small and sufficient load reaction force was exhibited even under large deformation.

  In addition to the bent portion, it is also effective to arrange the strength-reduced portion at a joint position with a mounting portion with the vehicle body skeleton, generally a member called a bumper stay. The bumper reinforcement member supports the load input from the front surface at the attachment part with the vehicle body skeleton located near both ends, but when this part precedes buckling, deformation also occurs at the bent part, buckling at the center part It was found that can be avoided.

  Further, as described above, in the manufacture of the member, the strength-reduced portion can be easily disposed on the vertical wall portion of the bumper reinforcing member, and the performance is sufficient. As a result of examining the clearance between the molds for slow cooling, it was found that the minimum value is desirably greater than 100%, preferably greater than 150% of the plate thickness. This is because if it is less than this value, it will not cool slowly due to contact with the mold. In order to achieve slow cooling, the larger the clearance, the better. However, particularly when trying to place a reduced strength portion in a bent portion having a large curvature, the restraint by the mold is reduced, and the required dimensional accuracy cannot be obtained. Therefore, the clearance is preferably less than 500% of the plate thickness.

  As a result of further studies, it has been found that hot stamping may be performed after preforming a portion where the strength is desired to be reduced in advance in order to achieve both the shape accuracy and the setting of the strength-decreasing portion due to slow cooling. The above-mentioned decrease in shape accuracy is due to a decrease in restraint from the mold on the metal plate as the clearance increases. It was found that the shape accuracy could be secured even if the restraint decreased. The same effect can be achieved by forming the final shape including the uneven shape in the strength-decreasing portion in the cold, and then heating the molded product, followed by quenching with a mold having an appropriate clearance in the strength-decreasing portion. Obtainable. It is also possible to preform in an intermediate shape up to the final shape. However, although the performance is almost the same regardless of which steps are produced, in general, the larger the degree of preforming, the higher the production cost of the preforming mold and the handling of the preformed parts. Since the heat treatment becomes difficult, it is more preferable to process only preforming close to a flat plate.

  The type of metal plate used in the present invention is not particularly limited as long as it can give a difference in strength depending on the cooling rate. However, the material is preferably a steel plate in consideration of the controllability of the strength by the cost and the cooling rate. Since the steel plate component has a sufficient quenching strength, the C amount needs to be 0.05 mass% to 0.35 mass%, and is selected according to the required strength. When the tensile strength is about 1200 MPa, it is necessary to add 0.16% by mass or more and 0.25% by mass or more at 1500 MPa. Also, Mn, Si, Cr, etc. are added in a timely manner for the purpose of controlling the hardenability in order to create a strength reduced portion by the cooling rate. Further, the balance between performance and weight is good when the plate thickness is between 0.8 mm and 2.6 mm.

  The presence or absence of the surface treatment is not particularly relevant, but a surface-treated steel sheet can be used for productivity and simplification of post-treatment. In this case, it is particularly preferable to use an aluminized steel sheet.

[Example 1]
The technical contents of the present invention will be described below with examples. Table 1 shows the steel plate components used in this study. Moreover, the temperature (Ac3 temperature) which becomes an austenite single phase area | region with respect to these steel is also shown collectively. In the case of using the hot stamping technique, it is necessary to heat to the Ac3 temperature or higher before forming to form an austenite single phase. The strength after quenching is mainly governed by the C content. Components a, b, and d are adjusted to be about 1200 MPa, 1500 MPa, and 1800 MPa, respectively, after quenching. Further, c has a strength that is almost the same as b, but the hardenability is improved by increasing the amount of Mn added.

  Actual bumper reinforcement members are diverse, including their shapes and attachment methods to the vehicle body. However, basically, the longitudinal direction of the member is perpendicular to the load input direction and is deformed by so-called bending to absorb energy. Further, the end of the vehicle body is usually curved from the viewpoint of design and aerodynamic characteristics. In other words, it is common that both ends bend with respect to a relatively straight portion at the center, and that the attachment portion to the vehicle body structure (bumper stay) is located between the bent end portions. . Therefore, in this study, a bumper reinforcing member schematically shown in FIG. 1 was used as a model member. This member has a straight central portion 2 and a bent portion 3 in the bumper reinforcing member main body 1, and further comprises a connecting portion (bumper stay) 5 to the vehicle body structure and an attaching portion 4 to the main body 1. The cross-section of the bumper reinforcing member was constant and had a so-called hat shape as shown in FIG. The strength-decreasing portion is the vertical wall portion 8 (portion approximately parallel to the load direction) shown in FIG. 2, and the upper side portion 7 is cooled and hardened by close contact with the mold. The total length of the member is 1150 mm, the bent portion is located between 150 mm and 225 mm from the end, and the length of the central straight portion is 600 mm. The outer diameter R of the bent portion was 400 mm. In the cross section, the length of the upper side portion was 50 mm, the height of the vertical wall portion was 30 mm, and the corners R were all 7 mm. The plate thickness was all 1.6 mm in this study. For this member, a 1.6 mm 590 MPa class steel plate contact plate 9 was joined by spot welding with an interval of 30 mm to make a closed cross section, which was used in the experiment. The position in the longitudinal direction of the strength-decreasing portion was either the bent portion or the attachment portion to the vehicle body skeleton, or both.

  Although it is an experimental method, it is difficult to take out and evaluate only the characteristics of the bumper reinforcing member in a state where it is attached to the vehicle body. Therefore, a drop weight test simulating the bumper reinforcing member was performed as shown in FIG. A falling weight 10 having a width of 850 mm, an angle R of 30 mm, and a mass of 200 kg was used. The bumper reinforcing member 1 was rigidly fixed to the support jig 11 using bolts. The input energy in the drop weight test was adjusted by changing the drop height so that the amount of deformation was almost the same for members made of materials of different strengths. The values are also shown in Table 2. As an index of the impact absorbing performance of the bumper reinforcing member, the back displacement of the central portion 2 of the bumper reinforcing member after 10 msec from the start of deformation was used. The smaller this value, the better the shock absorbing ability and the better the characteristics as a bumper reinforcing member.

  Table 2 shows the production conditions and experimental results of the bumper reinforcing member. The presence or absence of the strength-decreasing portion was determined by setting the punch and die clearance. When the punch-to-die clearance is 100% or less of the plate thickness, the material sufficiently adheres to the mold at the bottom dead center and is removed from heat, so that martensitic transformation occurs and hardening occurs. The clearance at the bottom dead center was set to 100% so that sufficient curing occurred in areas other than the strength-decreasing portion. Further, in Example 1, a mold was manufactured with a clearance of 95% so that sufficient contact occurred at the bent portion. The material used was b in Table 1 and was heated under the condition that the temperature at the start of press molding did not fall below 850 ° C after heating to 900 ° C. In addition, the decision was made for 10 seconds at the bottom dead center.

  The molded member was measured at 5 points for the Vickers hardness of each part. The average value is shown in Table 2. In addition, the hardness of a site | part with a reduced strength is a measurement result of a vertical wall part. In Example 1 in which the clearance is sufficiently small, the central portion and the bent portion have substantially the same hardness by being cooled by the mold, and the strength is substantially equal in the longitudinal direction. On the other hand, in Examples 2 to 5, the clearance of the bent portion is increased. As the clearance increases, the heat conduction is hindered by not being in close contact with the mold, the martensite transformation does not occur, and it is confirmed that a relatively soft ferrite + pearlite structure or bainite structure can be produced so that a reduced strength part can be produced. . However, it has been found that when the clearance exceeds 500%, the restraint from the mold is reduced, so that fine wrinkles are seen in the bent portion, and the shape accuracy is deteriorated. Further, in Example 6, the mold clearance of the attachment portion to the vehicle body skeleton which is both ends of the bumper reinforcing member is set to 300%, and the strength is reduced. In this case as well, a reduced strength portion was obtained, but the restraint from the mold was further reduced in a state where the restraint was originally small at the end of the member, so that a slight twist occurred. Furthermore, Example 7 is a case where the clearances of both the bent portion and the attachment portion to the vehicle body skeleton are increased. In this case as well, a reduced strength portion was obtained, but fine wrinkles and slight twisting were observed. However, in any case, the deterioration of the shape accuracy was such that there was no problem in the subsequent joining of the contact plates by spot welding.

  Table 2 shows the results of performing drop weight tests on these members and measuring the back displacement. In Example 1, since the material strength was almost the same in the longitudinal direction, buckling occurred in the central portion having relatively low rigidity compared to the bent portion. As a result, the back displacement increased and the performance as a bumper reinforcing member was not excellent. On the other hand, in Example 2 to Example 4 in which the bending portion is provided with a reduced strength portion, the bending portion buckled before the central portion, and since the buckling of the central portion was avoided, the back displacement was reduced and the performance was excellent. It was. Example 6 is a case where a reduced strength portion is arranged at the attachment portion to the vehicle body skeleton. Although the characteristics were slightly inferior to the arrangement at the bent portion, the characteristics were excellent as compared with Example 1. Further, in Example 7, strength reduction portions are provided in both the bent portion and the attachment portion to the vehicle body skeleton. In this case, the same excellent characteristics as the arrangement in the bent portion were exhibited. In this way, the placement method and performance of the strength reduction part are closely correlated, and it is necessary to consider the placement position appropriately according to the vehicle body, but basically it is installed in the part with a large curvature at the initial shape or deformation It is desirable to do.

  In this embodiment, the fact that the thermal conductivity differs by 10 times or more depending on the state of close contact with the mold is used, and a reduced strength portion is provided by setting the mold clearance. However, when the shape accuracy is more severe, the thermal conductivity is low. It is possible to embed ceramics in a part of the mold so that the steel plate is not quenched, and the effect remains the same. However, when the embedded area is large, it is disadvantageous from the viewpoint of an increase in mold cost and stable productivity.

[Example 2]
The effect of preforming was studied to improve the deterioration of shape accuracy due to the increase in clearance. Although the final member shape is shown in FIG. 4, the bead 12 is disposed at the bent portion. This bead was prepared by preforming at a corresponding position on the base plate. The width of the bead is 30 mm, the height is 30 mm, the depth at the die shoulder R is 5 mm, and the punch shoulder R is not wedged, and the three beads are bent with an interval of 15 mm. Placed in the department. After performing this preliminary molding, a bumper reinforcing member was obtained by performing the main molding with a mold clearance of 5 mm at the bent portion (the clearance is 310% because the plate thickness is 1.6 mm).

  In Examples 8, 10, 12, and 14, after preforming using materials from a to d in Table 1, the main molding was performed in a cold state, followed by heating to 900 ° C. and water quenching as it was. It is a comparative example. In Examples 9, 11, 13, and 15, after preforming using materials from a to d, heating to 900 ° C. was performed under conditions such that the temperature at the start of press molding does not fall below 850 ° C. Went. In Examples 8, 10, 12, and 14, since the whole was water-quenched, the strength became uniform. On the other hand, after pre-molding and placing the beads, hot stamping was performed with the mold clearance at the bent portion, so that the vertical wall portion of the portion became a reduced strength portion. By improving the local rigidity by preforming as in this example, the shape accuracy of the member was excellent in any case. Table 3 shows the results obtained by measuring Vickers hardness at 5 points in each part in the same manner as in Example 1 and calculating the average.

  The drop weight test was performed in the same manner as in Example 1 (FIG. 5). However, the input energy was changed depending on the falling height of the falling weight so that the deformation amount was almost the same even for the members having different material strengths. Table 3 also shows the drop height of each member. In Examples 8, 10, 12, and 14 that do not include the strength-reduced portion, buckling occurs from the center, and thus it was found that the back surface displacement was large and the performance was poor. On the other hand, in Examples 9, 11, 13, and 15, buckling occurs from the bent portion whose strength is reduced in the vertical wall portion, and the deformation is diffused throughout, so that the back surface displacement is reduced and excellent characteristics as a bumper reinforcing member are exhibited. I understood that.

It is explanatory drawing of a bumper reinforcement member. It is sectional drawing of a bumper reinforcement member. It is an arrangement plan of a drop weight test. It is a figure which shows the bending part shape of the bumper reinforcement member using preforming. It is a schematic diagram of the drop weight test of the bumper reinforcement member using preforming.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bumper reinforcement member 2 Center part of bumper reinforcement member 3 Bending part of bumper reinforcement member 4 Attachment part of bumper reinforcement member to body frame 5 Joint part with body frame (bumper stay)
6 Cross-sectional view of bumper reinforcing member 7 Upper side portion of bumper reinforcing member 8 Vertical wall portion of bumper reinforcing member 9 Bump plate 10 to bumper reinforcing member Dropping weight 11 Support jig 12 Bead 13 made by preforming Pre-molding was used Bumper reinforcement

Claims (4)

  In the method of manufacturing a bumper reinforcing member that performs quenching while forming using a mold after heating the metal plate, forming a reduced strength portion by maintaining a gap between the mold and the metal plate. A method for manufacturing a bumper reinforcing member.   The method for manufacturing a bumper reinforcing member according to claim 1, wherein a gap between the mold and the metal plate is maintained by preforming the metal plate.   The method for manufacturing a bumper reinforcing member according to claim 1, wherein a local strength reduction portion is formed in a bent portion of the bumper reinforcing member.   The method for manufacturing a bumper reinforcing member according to any one of claims 1 to 3, wherein a locally reduced strength portion is formed in an attachment portion of the bumper reinforcing member to the vehicle body skeleton.

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