Vehicular center pillar

Abstract

To provide a vehicular center pillar having required strength, which can be suppressed from excessively deforming at the time of a side collision of a vehicle and which can be further reduced in weight.SOLUTION: In a vehicular center pillar, when a vehicle height direction is defined as an X axis, a vehicle longitudinal direction is defined as a Y-axis, a vehicle width direction is defined as a Z-axis, yield moment of a yz cross section of the center pillar is defined as My, full plastic moment is defined as Mp, bending moment at the time when the center pillar receives a load from a Z-axis direction is defined as M, an elapse time is defined as t, and m=(M-My) / (Mp-My) is defined, a relational expression of d / dt(|dm / dx|)≤0 is satisfied at a site in the center pillar where My<M and d2 m / dx2≤0 are satisfied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a vehicle center pillar. More specifically, the present invention relates to a vehicle center pillar that has the necessary strength, is able to suppress excessive deformation during a side collision of the vehicle, and is also lightweight. [Background technology]

[0002] Vehicle center pillars are one of the structural components of an automobile, primarily made from steel plates and positioned with their longitudinal direction facing the vertical direction of the vehicle. In recent years, there has been a demand for further improvements in safety for automobile structural components from the perspective of passenger protection, as well as for further weight reduction in order to comply with fuel economy regulations and achieve carbon neutrality.

[0003] In particular, center pillars are required to protect the passenger space while also cushioning the impact on the passengers when another vehicle collides with them from the side.

[0004] For example, because the upper part of the center pillar is close to the head and chest of the occupants, the upper part of the center pillar needs to be very strong to prevent deformation in a side collision.

[0005] On the other hand, the underside of the center pillar is relatively far from the occupants, so even if deformation occurs during a collision, it is unlikely to come into contact with the occupants. Therefore, the underside of the center pillar must have both adequate strength and ductility to absorb the impact during a collision.

[0006] To minimize the weight of a center pillar while still meeting collision safety standards, it is necessary to optimize factors such as the strength and thickness of the steel plate used, the shape of the center pillar, and the part of the center pillar that forms the boundary between the upper and lower parts of the vehicle body and where the strength and thickness change.

[0007] As a guideline for optimizing components, Patent Document 1 proposes a component for effectively absorbing collision energy based on the moment of inertia about an axis in the longitudinal direction of the vehicle body. Non-Patent Document 1 proposes technology for causing the lower part of a center pillar to buckle earlier than the upper part, based on the full plastic moment about an axis in the longitudinal direction of the vehicle body and a collapse moment determined from the full plastic moment, plate thickness, Young's modulus, yield stress, and component width in the longitudinal direction of the vehicle body. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-274590 [Non-patent literature]

[0009] [Non-Patent Document 1] Satoshi Hirose, Nippon Steel Technical Report, vol.412(2019)p.59 Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, various methods for optimizing members have been studied up to now. However, the method in Patent Document 1 is a design guideline based on the second moment of area, and does not optimize the strength of the steel plate.

[0011] Furthermore, the method of Non-Patent Document 1 is a design guideline based on the full plastic moment, and although the strength of the steel plate is optimized, the structure induces early buckling of the lower part of the center pillar, and depending on the specifications of the impact input during a collision, only the part that buckles early may undergo excessive deformation, which may result in the desired collision safety standards not being met.

[0012] Furthermore, in any of these methods, the optimization policy has the above-mentioned problems, and there is a problem that the center pillar cannot be made as light as possible.

[0013] The present invention has been made in view of the above-mentioned problems, and aims to provide a vehicle center pillar that has the necessary strength, is able to suppress excessive deformation at the portion that buckles early in the event of a side collision of the vehicle, and is also able to minimize weight. [Means for solving the problem]

[0014] (1) A vehicle center pillar according to one aspect of the present invention comprises: The vehicle height direction is the x-axis, The longitudinal direction of the vehicle is the y-axis, The vehicle width direction is the z-axis, The yield strength of the yz cross section perpendicular to the x axis is YP, The yield moment around the y-axis in the y-z cross section is defined as My as shown in the following formula 1: The total plastic moment around the y-axis in the y-z cross section is defined as Mp shown in the following formula 2: When the vehicle center pillar receives a load in the z-axis direction, The bending moment around the y-axis input to the vehicle center pillar is defined as M, The elapsed time when the load is applied is defined as t, When the relationship between My, Mp, and M is defined as m shown in the following formula 3, With the elapsed time t, My <Mかつd 2 m / dx 2 At the part where ≦0 d / dt(|dm / dx|)≦0 Meet the following. My=YP y I y / z y ...(Formula 1) Mp=∫ S z p YP p dS...(Formula 2) m = (M - My) / (Mp - My) (Equation 3) where: I y : Moment of inertia about the y-axis in the yz section z y : The distance from the neutral axis to the point at which the yz cross section yields earliest with the t YP y : The strength of the part of the yz cross section that yields earliest according to the t S: The yz cross section (cut surface of the center pillar) z p : The distance from the neutral axis of the infinitesimal element dS in the yz section YP p : Yield strength of the infinitesimal element dS of the yz cross section Neutral axis: The axis where compressive stress and tensile stress switch over in the yz cross section relative to M, and where the strain in the x-axis direction in the yz cross section is zero dm / dx: the first derivative of m in the x-axis direction d 2 m / dx 2 : The second derivative of m in the x-axis direction d / dt(|dm / dx|): Time derivative of the absolute value of dm / dx (2) In the vehicle center pillar described in (1) above, the yield moment My may vary discontinuously along the x-axis. (3) In the vehicle center pillar described in (1) or (2) above, the position at which the yield moment My changes discontinuously along the x-axis may be a laser welding position of the steel plate. (4) In the vehicle center pillar according to any one of (1) to (3) above, the total plastic moment Mp may vary discontinuously along the x-axis. (5) In the vehicle center pillar described in any one of (1) to (4) above, the position at which the full plastic moment Mp changes discontinuously along the x-axis may be a laser welding position of the steel plate. (6) In the vehicle center pillar according to any one of (1) to (5) above, the yield moment My may vary continuously along the x-axis. (7) In the vehicle center pillar according to any one of (1) to (6) above, the total plastic moment Mp may vary continuously along the x-axis. (8) In the vehicle center pillar according to any one of (1) to (7), Even when the bending moment M, which increases with the elapsed time t, reaches its maximum, My <Mかつd 2 m / dx 2 At the site where d / dt(|dm / dx|)≦0 may be satisfied. [Effects of the Invention]

[0015] According to the above aspect of the present invention, it is possible to provide a vehicle center pillar that has the necessary strength, is able to suppress excessive deformation at the location that buckles early in the event of a side collision of the vehicle, and is also able to achieve maximum weight reduction. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram illustrating a vehicle center pillar according to an embodiment of the present invention, showing the distribution of discontinuously changing full plastic moment Mp and yield moment My. [Figure 2] 1 is a diagram illustrating a vehicle center pillar according to the present embodiment, and is a diagram illustrating the distribution of the index m in the center pillar. FIG. [Figure 3] FIG. 1 is a diagram illustrating a conventional vehicle center pillar, showing the distribution of the index m in the center pillar. [Figure 4] 1 is a diagram illustrating an example of a vehicle center pillar according to the present embodiment, and is a diagram illustrating a distribution in which the total plastic moment Mp and the yield moment My change continuously. FIG. [Figure 5] 1 is a cross-sectional view illustrating an example of a vehicle center pillar according to an embodiment of the present invention. [Figure 6] FIG. 2 is a schematic diagram of a vehicle center pillar used in the analysis. [Figure 7] FIG. 1 is a diagram showing the analysis results of Example No. 1. [Figure 8] FIG. 1 is a diagram showing the analysis results of Example No. 1. [Figure 9]FIG. 10 is a diagram showing the analysis results of Example No. 2. [Figure 10] FIG. 10 is a diagram showing the analysis results of Example No. 3. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments of the present invention are described in detail below. However, the present invention is not limited to the configurations disclosed in the present embodiments, and various modifications are possible without departing from the spirit of the present invention. Furthermore, the drawings used in the following description may clearly show the features of the present embodiments, and the dimensional ratios of each component may not be the same as the actual ones. Furthermore, the numerical ranges described below include the lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" do not include the numerical range. "%" regarding the content of each element means "mass %."

[0018] The inventors of the present invention conducted extensive research into the strength and thickness distribution of each part of the center pillar, as well as the state of deformation of the center pillar when hit from the side of the vehicle, with the aim of obtaining a vehicle center pillar that has the necessary strength, is able to suppress excessive deformation in a side collision of the vehicle, and is also lightweight. As a result, they discovered that the desired center pillar can be obtained by suitably controlling the full plastic moment Mp and yield moment My at each part of the center pillar along the vehicle height direction (x-axis) in accordance with the bending moment M (moment M) about the vehicle fore-and-aft direction (y-axis) that is input in a side collision of the vehicle.

[0019] Specifically, the vehicle center pillar according to this embodiment is The vehicle height direction is the x-axis, The longitudinal direction of the vehicle is the y-axis, The vehicle width direction is the z-axis, Let YP be the strength of the yz section perpendicular to the x axis. The yield moment around the y-axis in the above yz cross section is defined as My as shown in the following formula 1: The total plastic moment around the y-axis in the yz cross section is defined as Mp shown in the following formula 2: When the vehicle center pillar receives a load in the z-axis direction, The bending moment around the y-axis input to the vehicle center pillar is defined as M, The elapsed time when the above load is applied is defined as t. When the relationship between the above My, the above Mp, and the above M is defined as m shown in the following formula 3, With the elapsed time t, My <Mかつd 2 m / dx 2 At the part where ≦0 d / dt(|dm / dx|)≦0 Meet the following. My=YP y I y / z y ...(Formula 1) Mp=∫ S z p YP p dS...(Formula 2) m = (M - My) / (Mp - My) (Equation 3) where: I y : Moment of inertia about the y-axis in the above yz section z y : Distance from the neutral axis to the point where the earliest yield occurs in the yz section at the above t YP y : The strength of the part of the above yz cross section that yields earliest according to the above t S: The above yz cross section (cut surface of the center pillar) z p : Distance from the neutral axis of the infinitesimal element dS in the above yz section YP p : Yield strength of the small element dS in the above yz cross section Neutral axis: The axis where compressive stress and tensile stress change over in the yz section relative to the above M, and where the strain in the x-axis direction in the yz section is zero. dm / dx: the first derivative of m in the x-axis direction d 2 m / dx 2: Second derivative of m in the x-axis direction as described above d / dt(|dm / dx|): Time derivative of the absolute value of dm / dx

[0020] In FIG. 1, as a center pillar for a vehicle according to this embodiment, an example is shown in which the full plastic moment Mp and the yield moment My are controlled according to the bending moment M around the y-axis input during a collision. In FIG. 1, as an example, a distribution in which the full plastic moment Mp and the yield moment My change discontinuously is shown.

[0021] In FIG. 1, the horizontal axis represents the position in the vehicle height direction, and the vertical axis represents the magnitude of the moment. Also, in FIG. 1, for the bending moment M around the y-axis input to the center pillar when the vehicle is collided from the side direction, the bending moment M after an arbitrary time has elapsed since the collision is shown as an example. Note that this bending moment M is a value that changes depending on the vehicle body structure and collision conditions. The details of the bending moment M will be described later.

[0022] The center pillar for a vehicle according to this embodiment preferably controls the full plastic moment Mp and the yield moment My at each part with respect to the assumed bending moment M. For example, in FIG. 1, from the lower part to the upper part in the vehicle height direction, the full plastic moment Mp and the yield moment My are increased in three steps.

[0023] The range where My < M in the center pillar as the collision deformation progresses is a part where buckling deformation is likely to occur during a side collision. The bending moment M input to the center pillar by the collision increases as the collision deformation progresses. However, when this bending moment M reaches the collapse moment, it becomes difficult to resist deformation at this part, and there is a possibility that the bending moment M will rapidly decrease. In this case, it becomes difficult to protect the passenger space as a center pillar, and it also becomes difficult to absorb the impact.

[0024] In the center pillar for a vehicle according to the present embodiment, concentration of collision deformation at a local part where My < M as described above is suppressed, and even if the collision deformation progresses, a rapid decrease in the bending moment M is suppressed.

[0025] Specifically, in the center pillar for a vehicle according to the present embodiment, at a part where My < M in the center pillar and where 2 dm / dx 2 ≤ 0, control is performed so as to satisfy d / dt(|dm / dx|) ≤ 0. If d / dt(|dm / dx|) ≤ 0, the collision deformation is dispersed without concentrating locally, and even if the collision deformation progresses, a rapid decrease in the bending moment M is suppressed. As a result, while protecting the passenger space as a center pillar, the impact at the time of collision can be mitigated. Details of each of the above parameters will be described later.

[0026] Note that at a part where My ≥ M in the center pillar, buckling deformation does not occur at the time of a collision from the side direction of the vehicle, and the passenger space can be protected at the time of a collision.

[0027] Also, in the center pillar for a vehicle according to the present embodiment, within the range where My < M and 2 dm / dx 2 ≤ 0, it is preferable to optimize the plate thickness of the steel plate or the like so that d / dt(|dm / dx|) ≤ 0. Similarly, within the range other than My < M, it is preferable to optimize the plate thickness of the steel plate or the like so that My ≥ M. As a result, weight reduction of the entire center pillar becomes possible.

[0028] Hereinafter, the center pillar for a vehicle according to the present embodiment will be described in detail.

[0029] The outline of the technical idea that led to the center pillar for a vehicle according to the present embodiment is described below.

[0030] (1) If the only requirement is to protect the passenger space during a collision, the entire center pillar should be controlled so that My ≥ M. However, as mentioned above, in addition to protecting the passenger space, there is now also a demand for impact absorption during a collision and weight reduction. Therefore, the shape of the center pillar and the thickness of the steel plate are optimally designed so that there is a point in the center pillar where the bending moment M exceeds the yield moment My.

[0031] (2) However, at locations where the bending moment M exceeds the yield moment My, the bending moment M drops sharply during collision deformation, increasing the risk of not meeting the desired collision safety standards. Therefore, at locations where the bending moment M exceeds the yield moment My, the relationship between M, My, and Mp is optimally controlled, including the full plastic moment Mp in addition to M and My.

[0032] The following describes specific features of the center pillar that satisfy the above technical concept.

[0033] < My=YP y I y / z y > My represents the yield moment of a vehicle center pillar. For a vehicle center pillar, the vehicle height direction is the x-axis, the vehicle front-to-rear direction is the y-axis, and the vehicle width direction is the z-axis. The following equation 1 is used to calculate the yield moment My, and more specifically, it is used to calculate the value of the bending moment required for a portion of the yz cross section (cross section perpendicular to the vehicle height direction) of the center pillar to reach a state of plastic deformation due to the bending moment M about the y-axis, which is the main input during a side collision of the vehicle. My=YP y I y / z y ...(Formula 1) where: YP y : The strength of the part that yields earliest due to the bending moment M that increases from moment to moment in the yz plane at coordinate x I y : Moment of inertia about the y-axis in the yz-plane at coordinate x z y : The z-coordinate (distance from the neutral axis) of the position at which the specimen will yield earliest due to an increasing bending moment M in the y-z plane at coordinate x. is. The neutral axis indicates the axis along which compressive stress and tensile stress switch in the yz cross section during bending deformation, and is the axis along which the strain in the direction perpendicular to the yz plane is zero.

[0034] < Mp=∫ S z p YP p dS > Mp represents the total plastic moment of the vehicle center pillar. The x-axis, y-axis, and z-axis are the same as above. The following equation 2 is used to calculate the total plastic moment Mp, and specifically, it is used to calculate the value of the bending moment required for the entire yz cross section (cross section perpendicular to the vehicle height direction) of the center pillar to reach a state of plastic deformation due to the bending moment M about the y-axis, which is the main input during a side collision of the vehicle. Mp=∫ S z p YP p dS...(Formula 2) where: S: The cross section of the center pillar appearing on the yz plane at the coordinate x, z p : the distance from the neutral axis of the infinitesimal element dS in the surface S, YP p : Yield strength of the small element dS in the above yz cross section is. The neutral axis indicates the axis along which compressive stress and tensile stress switch in the yz cross section during bending deformation, and is the axis along which the strain in the direction perpendicular to the yz plane is zero.

[0035] < m = (M-My) / (Mp-My) > m is an index used to control the relationship between the yield moment My and full plastic moment of the vehicle center pillar, and the bending moment M input to the center pillar during a collision, and represents the ratio of the bending moment M to the full plastic moment Mp, with the yield moment My as the base. Equation 3 below is used to calculate index m and is used to evaluate whether the bending moment M will suddenly decrease due to buckling that accompanies the progression of collision deformation. m=(M-My) / (Mp-My) (Formula 3)

[0036] < My <m> My<M indicates that the bending moment M about the y-axis (vehicle longitudinal direction) input to the center pillar during a side collision is greater than the yield moment My of the center pillar for the vehicle. In the part of the center pillar where My<M, the possibility of local concentration of deformation during a collision increases. However, the presence of a part where My<M in the center pillar leads to an improvement in impact absorption characteristics and a weight reduction of the center pillar. For example, the part of the center pillar where My<M is moderately lightweight, and My<M is one of the necessary conditions for buckling to occur due to the bending moment M. On the other hand, in the part of the center pillar where My≧M, buckling due to the bending moment M does not occur, and the weight is excessively large compared to the part where My<M.

[0037] < d 2 m / dx 2 ≦0 > d 2 m / dx 2 indicates the second-order differentiation of the index m with respect to the x-axis direction and represents the distribution form of the index m in the x-axis direction. As will be described later, the center pillar for the vehicle according to the present embodiment is characterized by controlling d / dt(|dm / dx|). This d / dt(|dm / dx|) represents the time change of the change rate (|dm / dx|) of the index m with respect to the position x. However, when the position x and the index m are functions of the third order or higher, at the inflection point where 2 m / dx 2 =0, the increasing and decreasing tendency of the change rate (|dm / dx|) of the index m with respect to the position x changes. Therefore, in order to correctly control d / dt(|dm / dx|), it is necessary to perform the evaluation under the condition that 2 m / dx 2 ≦0.

[0038] At the part where My<M and in the part of the center pillar where 2 m / dx 2 ≦0, the function of the position x and the index m becomes a graph that is convex upward including the inflection point. In this part, the value of m gradually increases toward the maximum point of the graph. Therefore, 2 m / dx 2 That it is ≦0 is one of the necessary conditions for buckling to occur due to the bending moment M. On the other hand, for the part in the center pillar where d 2 m / dx 2 >0, as the distance from the above-mentioned maximum point increases, the value of m gradually decreases. Therefore, it becomes difficult for buckling to occur due to the bending moment M.

[0039] In the above, as an example, the case where the position x and the index m are functions of the third degree or higher has been described. However, in the present embodiment, the position x and the index m may be functions of the second degree or lower. For example, when the position x and the index m are quadratic functions, d 2 m / dx 2 only needs to be a negative constant value, and when the position x and the index m are linear functions, d 2 m / dx 2 only needs to have a value of zero.

[0040] < d / dt(|dm / dx|)≦0 > d / dt(|dm / dx|) indicates taking the time derivative of the absolute value of the first-order derivative dm / dx of the index m in the x-axis direction. That is, d / dt(|dm / dx|) represents the situation where the rate of change (|dm / dx|) of the index m with respect to the position x changes over time when the center pillar receives a load due to a collision.

[0041] In the center pillar for a vehicle according to the present embodiment, d / dt(|dm / dx|) is controlled to be 0 or less. At this time, even for a part in the center pillar where My < M, the collision deformation does not concentrate locally but disperses, and it is suppressed that the bending moment M rapidly decreases even as the collision deformation progresses.

[0042] FIG. 2 shows an example of the distribution situation of the index m in the center pillar for a vehicle according to the present embodiment. Further, FIG. 3 shows an example of the distribution situation of the index m in the center pillar for a vehicle of the prior art.

[0043] 2 and 3, the horizontal axis represents the vehicle height direction of the center pillar, and the vertical axis represents the size of the index m. 2 m / dx 2 2 and 3, the dashed line represents the distribution of the index m at elapsed time t1, and the solid line represents the distribution of the index m at elapsed time t2. <t2である。

[0044] As shown in FIG. 2, the rate of change (|dm / dx|) of index m at the same location at t1 and t2 is exemplified; if d / dt(|dm / dx|)<0, |dm / dx| decreases with time. On the other hand, as shown in FIG. 3, the rate of change (|dm / dx|) of index m at the same location at t1 and t2 is exemplified; if d / dt(|dm / dx|)>0, |dm / dx| increases with time. Note that the case where d / dt(|dm / dx|)=0 is not shown, but in this case, |dm / dx| does not change with time.

[0045] d 2 m / dx 2 ≦0 and d / dt(|dm / dx|)>0, the rate of change of the index m (|dm / dx|) increases over time. In other words, in the center pillar, the part where M>My and the value of m is larger than the surrounding area is d 2 m / dx 2 If d / dt(|dm / dx|) is less than or equal to 0 and d / dt(|dm / dx|) is greater than 0, the value of m will continue to increase over time. An increase in the value of index m corresponds to the bending moment M input to the center pillar approaching the collapse moment, so in this case, collision deformation tends to be concentrated in a localized area, making it difficult for the center pillar to protect the passenger space and cushion the impact.

[0046] On the other hand, d 2 m / dx 2 ≦0 and d / dt(|dm / dx|)≦0, the rate of change of the index m (|dm / dx|) decreases or remains unchanged over time. In other words, even in the center pillar where M>My and the value of m is larger than the surrounding area, d 2 m / dx 2 When ≦0 and d / dt(|dm / dx|) ≦0, with the passage of time, the increase in the value of m at the maximum point is suppressed. In this case, the collision deformation disperses without concentrating locally, protecting the passenger space as a center pillar and reducing the impact during a collision.

[0047] Note that the bending moment M around the y-axis, which is the main input during a side collision of the vehicle, is a value that changes with the elapsed time t when the center pillar receives a load. Therefore, the index m also becomes a value that changes moment by moment during a side collision of the vehicle. In the center pillar for a vehicle according to this embodiment, even at the moment when the bending moment M that increases with the elapsed time t reaches its maximum, within the center pillar for a vehicle, My < M and d 2 m / dx 2 It is preferable that at a site where ≦0, d / dt(|dm / dx|) ≦0 is satisfied.

[0048] For example, when receiving a collision from the side direction of the vehicle, the initial velocity at the time of collision of the collision object decreases with the passage of time t as energy is absorbed by the collision, but the bending moment M increases with the passage of time t. In the center pillar for a vehicle according to this embodiment, even when the velocity of the collision object approaches zero and the value of the bending moment M reaches its maximum, it is preferable to satisfy the above condition d / dt(|dm / dx|) ≦0.

[0049] < Bending moment M around the y-axis (vehicle longitudinal direction) input during a side collision > The bending moment M around the y-axis input to the center pillar during a side collision changes depending on the collision safety standard evaluation and the structure of the entire vehicle body that defines the restraint conditions of the center pillar.

[0050] In the center pillar for a vehicle according to this embodiment, this bending moment M is not particularly limited. For example, based on the bending moment M determined according to the vehicle body structure incorporating the center pillar and the collision safety standards required for that vehicle body, the site where My < M in the center pillar may be controlled as described above.

[0051] In addition, the portion where My < M in the center pillar is preferably included from the lower part to the middle part in the vehicle height direction of the center pillar. Specifically, it is preferable that the portion where My < M is included in the range of 70% from the lower end in the vehicle height direction with respect to the total length L of the center pillar. On the other hand, it is preferable that the range of 30% from the upper end in the vehicle height direction with respect to the total length of the center pillar is the portion where My ≥ M.

[0052] < There is a discontinuous change in Mp or My along the x-axis > In the center pillar for a vehicle according to this embodiment, a point where the full plastic moment Mp or the yield moment My changes discontinuously along the x-axis may be provided.

[0053] The arrangement of the full plastic moment Mp and the yield moment My in the center pillar may be set by any method. For example, a point where the full plastic moment Mp or the yield moment My changes discontinuously may be provided along the x-axis (vehicle height direction). In FIG. 1, the points where the change occurs from the lower part to the middle part and from the middle part to the upper part in the vehicle height direction correspond to the points where the full plastic moment Mp and the yield moment My change discontinuously.

[0054] By providing a point where the full plastic moment Mp or the yield moment My changes discontinuously along the x-axis, the value of d / dt(|dm / dx|) can be controlled within an appropriate range. As a method for providing a point where the full plastic moment Mp and the yield moment My change discontinuously, for example, changing the plate thickness, material properties, cross-sectional shape, etc. of the adjacent steel plates can be mentioned.

[0055] < The point where Mp or My changes discontinuously along the x-axis is the laser welding position of the steel plate > In the center pillar for a vehicle according to this embodiment, the point where the full plastic moment Mp or the yield moment My changes discontinuously along the x-axis may be the laser welding position of the steel plate.

[0056] The following method may be employed as a method for providing a point at which the full plastic moment Mp or the yield moment My changes discontinuously along the x-axis (vehicle height direction), and as a method for controlling the value of d / dt (|dm / dx|) within an appropriate range while also contributing to vehicle weight reduction. For example, a tailored blank may be employed, in which blanks of multiple materials that will form the center pillars are trimmed from a coil, joined by laser welding, and then pressed.

[0057] Here, by arranging adjacent steel plates with different plate thicknesses and material properties, it is possible to preferably discontinuously change the full plastic moment Mp or the yield moment My. Furthermore, with the tailored blank, adjacent steel plates are seamlessly fixed by laser welding at the points where Mp or My discontinuously change, which makes it easier for deformation to be dispersed between adjacent steel plates, and preferably enhances the impact absorption effect even for lighter components.

[0058] < Mp or My changes continuously along the x-axis > In the vehicle center pillar according to this embodiment, the full plastic moment Mp or the yield moment My may change continuously along the x-axis.

[0059] Fig. 4 shows the distribution of continuously changing full plastic moment Mp and yield moment My for the vehicle center pillar according to this embodiment. In Fig. 4, as in Fig. 1, the horizontal axis represents the distance in the vehicle height direction, and the vertical axis represents the magnitude of the moment, showing an example of bending moment M about the vehicle's fore-and-aft direction (y-axis) that is input during a side collision of the vehicle.

[0060] In FIG. 4, the total plastic moment Mp is continuously controlled from the bottom to the top in the vehicle height direction.

[0061] By continuously changing the full plastic moment Mp or the yield moment My along the x-axis, various characteristics can be continuously changed along the vehicle height direction of the vehicle center pillar. Therefore, when the center pillar is deformed during a collision from the side direction of the vehicle, buckling deformation is more dispersed, and excessive deformation can be preferably suppressed.

[0062] As a method for providing points where the full plastic moment Mp and the yield moment My continuously change, for example, a material can be provided such that the hardness of the member gradually changes before and after the points where the full plastic moment Mp and the yield moment My change.

[0063] Also, as a method for continuously changing the full plastic moment Mp and the yield moment My along the x-axis, for example, in the hot stamping forming process for manufacturing the vehicle center pillar, when heating the material to be formed, the heating conditions can be changed according to the part, when cooling the formed material after hot stamping, the cooling conditions can be changed according to the part, or when annealing the formed material after hot stamping, the annealing conditions can be changed according to the part.

[0064] < Other features >

[0065] Also, in the vehicle center pillar according to this embodiment, the steel structure is not particularly limited. In the vehicle center pillar according to this embodiment, as the steel structure, martensite, tempered martensite, bainite, retained austenite, ferrite, pearlite, etc. may be included.

[0066] However, since the vehicle center pillar according to this embodiment is premised on having high strength (hardness), the steel structure of the part where My≧M is preferably mainly composed of martensite. Also, in the vehicle center pillar according to this embodiment, the part where My<M may contain, in addition to martensite, retained austenite, bainite, tempered martensite, ferrite, pearlite, etc.

[0067] In addition, in the center pillar for vehicles according to the present embodiment, in order to preferably disperse plastic deformation at a site where My < M, it is preferable that the work hardening ability of the site where My < M is high. Specifically, it is preferable that the work hardening index n value is high and local deformation is suppressed. If a large amount of retained austenite is contained in the site where My < M, it is preferable because processing-induced transformation occurs and the work hardening ability increases.

[0068] In addition, in the center pillar for vehicles according to the present embodiment, a member that receives a load in a direction perpendicular to the longitudinal direction of the member is assumed. In order to suppress material fracture, it is preferable that the ultimate deformation ability is high. If there are few microvoids that are the starting points of fracture in the steel structure, it is preferable because the ultimate deformation ability increases. Further, if the steel structure does not contain ferrite or pearlite, it is preferable because the concentration of plastic deformation at one location is suppressed and the ultimate deformation ability increases.

[0069] In addition, in the center pillar for vehicles according to the present embodiment, the chemical composition of the steel is not particularly limited. The center pillar for vehicles according to the present embodiment, for example, as the steel composition, in mass%, C: 0.08 to 0.70%, Si: 0.100 to 3.000%, Mn: 0.100 to 3.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.1000% or less, Al: 3.0000% or less, B: 0.0 (0005) to 0.0200%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Co: 0 to 5.00%, Ni: 0 to 3.00%, Cu: 0 to 3.00%, V: 0 to 3.00%, W: 0 to 3.00%, Ca: 0 to 1.0000%, Mg: 0 to 1.0000%, REM: 0 to 1.0000%, Sb: 0 to 1.00% Zr: 0 to 1.00%, Sn: 0 to 1.00% As: 0 to 1.0000%, and the remainder is Fe and impurities.

[0070] In addition, in the vehicle center pillar according to this embodiment, the above-mentioned My <Mかつd 2 m / dx 2 The specific shape of the component is not particularly limited as long as it satisfies d / dt(|dm / dx|) ≦ 0 at the portion where ≦ 0. The component may have a shape that corresponds to the design of the automobile. For example, the vehicle center pillar according to this embodiment may have a cross-sectional shape of a component that is a single hat shape or a double hat shape when observed from the longitudinal direction. Figure 5(a) shows a cross-sectional schematic diagram of a center pillar whose cross-sectional shape, normal to the longitudinal direction, is a single hat shape, as an example, and Figure 5(b) shows a cross-sectional schematic diagram of a center pillar whose cross-sectional shape, normal to the longitudinal direction, is a double hat shape.

[0071] Next, a method for measuring the characteristics of the vehicle center pillar according to this embodiment will be described.

[0072] The above-mentioned full plastic moment Mp can be calculated using commercially available CAD (computer-aided design) software, by calculating ∫ S z p YP p dS can be calculated for multiple cross sections. This calculation should be performed at positions that divide the distance from x=0 to the total length L into at least 100 equal parts. Specifically, the shape of the center pillar is measured using a 3D measuring device or the like and converted into electronic data. Information necessary to calculate the full plastic moment Mp, such as proof stress, is obtained from the yz cross section of the center pillar, and then the full plastic moment Mp can be calculated using CAD as described above.

[0073] The yield moment My mentioned above can also be calculated using commercially available CAD (computer-aided design) software, using the formula My = YP y I y / z y is calculated for multiple cross sections. This calculation should be performed at positions that divide the distance from x=0 to the total length L into at least 100 equal parts. Specifically, the shape of the center pillar is measured using a 3D measuring device or the like and converted into electronic data, and the information necessary to calculate the yield moment My, such as the strength, is obtained from the yz cross section of the center pillar, and then the yield moment My is calculated using CAD as described above.

[0074] The following conditions may be used as an example of a method for determining the bending moment M about the y-axis. For example, calculation may be performed using a full-car crash analysis. Specifically, a test may be performed using the finite element method to simulate a collision in which a barrier weighing 1,400 kg is collided with a vehicle equipped with a center pillar from the side of the vehicle at an initial speed of 60 km / h in a direction parallel to the z-axis. Furthermore, if it is difficult to perform a full-car crash analysis because the overall vehicle structure is unknown, a partial structural crash analysis may be performed. Specifically, a test may be performed using the finite element method to simulate a collision in which the upper and lower ends of the center pillar are fully restrained and a barrier weighing 300 kg is collided with a position 100 mm from the bottom end of the center pillar at an initial speed of 20 km / h in a direction parallel to the z-axis from the side of the vehicle body. From the results of these crash analyses, the bending moment M may be calculated by calculating the constantly changing bending moment about the y-axis at any cross section within the center pillar normalized to the x-axis, and performing this calculation at multiple cross sections. This should be carried out at least at 100 equal positions from x=0 to the total length L. The shape of the center pillar can be measured using a 3D measuring device or similar and converted into electronic data. The information necessary to perform the finite element method, such as mechanical properties, can then be obtained from each part of the center pillar, after which the analysis can be carried out as described above. The analysis results should be output at intervals of Δt=0.0005 seconds.

[0075] Under the above analysis conditions, the indices m, Mp, and My are values ​​at discrete cross sections obtained by dividing the center pillar into 100 equal parts from x=0 to L (i.e., the distance Δx between adjacent cross sections is Δx=L / 100), and the above m is a value at a discrete time output every time Δt=0.0005 s. Therefore, from the difference values ​​of the indices m, Mp, and My between adjacent cross sections and between times, d 2 m / dx 2 and d / dt(|dm / dx|) may be calculated.

[0076] For example, for a cross section at position x among 100 cross sections obtained by dividing the center pillar into equal parts from x=0 to L, the values ​​of the indices m, Mp, and My at an arbitrary time t0 are defined as m(x, t0), Mp(x), and My(x), respectively. At this time, at position x and time t0, d 2 m / dx 2 ={m(x+Δx, t0)+m(x-Δx, t0)−2m(x, t0)} / (Δx) 2 d / dt(|dm / dx|)=(1 / Δt)(1 / Δx){|m(x+Δx, t0+Δt)−m(x, t0+Δt)|−|m(x+Δx, t0)−m(x, t0)|} Based on d 2 m / dx 2 and d / dt(|dm / dx|) may be calculated.

[0077] To measure the total longitudinal length L of the center pillar, measure the distance from the lowest point to the highest point of the center pillar when it is installed on the vehicle. If a vehicle is not available, measure the distance between any two points on the center pillar so that the length is the longest.

[0078] In the vehicle center pillar according to this embodiment, when measuring each feature, the units of each value may be as follows. Yield strength YP, YP y , Y.P. p :MPa Yield moment My: N m Fully plastic moment Mp: N m Bending moment M: N m Moment of inertia I y :mm 4 Elapsed time t: seconds distance x, z y , z p :mm yz cross section S:mm 2 Total length L:mm

[0079] The structural structure of the steel described above may be observed by the following method. The observation surface is a cross section perpendicular to the longitudinal direction, and this observation surface is polished and immersed in an acetylacetone-based electrolyte, electrolytically etched, and then observed at a magnification of 5000 times (for example, at least 625 μm) using a field emission scanning electron microscope equipped with a secondary electron detector. 2 By observing the secondary electron image in a field of view (field of view), it is possible to distinguish between ferrite and pearlite and structures other than ferrite and pearlite. A structure consisting of massive crystal grains that does not contain a substructure such as lath within the structure is considered to be ferrite. A structure consisting of layered plate-like ferrite and Fe-based carbides is considered to be pearlite.

[0080] Also, at the same observation point as above, a magnification of 10,000 times (for example, at least 150 μm 2 Tempered martensite and bainite can be distinguished by observing a secondary electron image in a field of view (field of view). A structure that is an aggregation of lath-shaped crystal grains and contains Fe-based carbides with a major axis of 20 nm or more and elongated in different directions is considered to be tempered martensite. A structure that is an aggregation of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more within the structure, in which Fe-based carbides have precipitated between the laths, and a structure in which Fe-based carbides have precipitated within the laths and the Fe-based carbides are elongated in the same direction is considered to be bainite.

[0081] In addition, at the same observation location as above, the observation surface is re-polished to a mirror finish, strain is removed, and crystal orientation information can be obtained by electron backscatter diffraction at measurement intervals of 0.4 μm. Regions with an fcc crystal structure are considered to be retained austenite.

[0082] Furthermore, the region other than ferrite, pearlite, tempered martensite, bainite, and retained austenite obtained by the above-mentioned method is considered to be martensite.

[0083] The chemical composition of the steel may be measured by a general steel analysis method. For example, the chemical composition may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. [Example]

[0084] The effects of one aspect of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved. The present invention will be explained in more detail below by illustrating examples and comparative examples.

[0085] The steel composition, in mass%, is: C: 0.08-0.70%, Si: 0.100-3.000%, Mn: 0.100-3.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.1000% or less, Al: 3.0000% or less, B: 0.0005-0.0200%, Nb: 0-0.200%, Ti: 0-0.200%, Cr: 0-1.00%, Mo: 0-1.00%, Co: 0-5.00%, Ni: 0-3.00%, Cu: The deformation state of a vehicle center pillar made from a steel plate containing the following elements: 0-3.00%, V: 0-3.00%, W: 0-3.00%, Ca: 0-1.0000%, Mg: 0-1.0000%, REM: 0-1.0000%, Sb: 0-1.00%, Zr: 0-1.00%, Sn: 0-1.00%, As: 0-1.0000%, with the remainder being Fe and impurities was analyzed using the finite element method (FEM) when the vehicle center pillar was subjected to a load in the z-axis direction.

[0086] Figure 6 shows the shape of the vehicle center pillar 1 used in the analysis. The bending moment M around the y-axis input to the center pillar during a collision was calculated using the finite element method under the following conditions: the center pillar is fully restrained at the top and bottom ends, and a barrier weighing 300 kg is collided with the vehicle from the side in a direction parallel to the z-axis at an initial speed of 20 km / h at a position 570 mm from the bottom end of the center pillar. In the finite element method analysis, the elements were treated as 2 mm square shells, and the R part of the member was divided into seven elements. The analysis results were output at intervals of Δt = 0.0005 seconds.

[0087] Additionally, the total plastic moment Mp and yield moment My of the vehicle center pillar were controlled by laser welding materials with different material properties together and then pressing, or by partially tempering the center pillar after pressing. These total plastic moments Mp and yield moments My were calculated using CAD (computer-aided design) software. In the CAD calculations, the center pillar was divided into 100 equal parts from x = 0 to its full length L, and Mp and My were calculated for each yz cross section.

[0088] The above analysis results are summarized in Table 1. In Table 1, the analysis results are shown for 0.0005 seconds (elapsed time t4, described later) before the time when the speed of the barrier in the z-axis direction gradually decreases after the barrier collides with the center pillar and eventually becomes 0. <Mかつd 2 m / dx 2 ≦0, and "Condition b" indicates whether there is a site where My <Mかつd 2 m / dx 2 "Condition c" indicates whether d / dt(|dm / dx|) ≦ 0 is satisfied at the point where d / dt ≦ 0 is satisfied. Condition c indicates whether the full plastic moment Mp and yield moment My change continuously, discontinuously, or mixed (continuous + discontinuous) along the x-axis of the center pillar. In Table 1, a "good" "deformation state" indicates that excessive deformation at the point where early buckling occurs during a side collision of the vehicle was suppressed, while a "poor" "deformation state" indicates that excessive deformation could not be suppressed. In Table 1, a "good" "weight reduction" indicates that the overall weight of the center pillar was reduced by optimizing the steel plate thickness, while a "poor" "weight reduction" indicates that the overall weight of the center pillar was not reduced.

[0089] The "deformation state" and "weight reduction" were evaluated as follows. For example, the "deformation state" was evaluated as follows: When the load in the z-axis direction applied to the barrier is measured over time, the load gradually increases as the collision progresses. However, if the collision deformation is concentrated in a localized area, the load decreases at a certain point. If the load decreases during the barrier collision, it can be said that the collision deformation is concentrated in a localized area, and the "deformation state" was judged to be "poor." If there is no decrease in load, the "deformation state" was judged to be "good." Furthermore, the "weight reduction" was evaluated based on the presence or absence of plastic deformation. Specifically, it was judged based on whether there is a point in the center pillar where the constantly changing bending moment M exceeds the center pillar's yield moment My. If there is no point in the center pillar where the bending moment M exceeds the yield moment My, it can be said that there is still room for weight reduction in the center pillar, and the "weight reduction" was judged to be "poor." If there is a point in the center pillar where the bending moment M exceeds the yield moment My, the "weight reduction" is judged to be "good."

[0090] Note that under the above analysis conditions, the above m, Mp, and My are values ​​at discrete cross sections obtained by dividing the center pillar into 100 equal parts from x=0 to L (i.e., the distance Δx between adjacent cross sections is Δx=L / 100), and the above m is also a value at a discrete time output every time Δt=0.0005 s. Therefore, whether or not the above conditions a and b are satisfied was evaluated from the difference values ​​of m, Mp, and My between adjacent cross sections and between times.

[0091] Specifically, for a cross section at position x among 100 cross sections obtained by dividing the center pillar into equal parts from x=0 to L, the values ​​of m, Mp, and My at an arbitrary time t0 are defined as m(x, t0), Mp(x), and My(x), respectively. At this time, at position x and time t0, d 2 m / dx 2 ={m(x+Δx, t0)+m(x-Δx, t0)−2m(x, t0)} / (Δx) 2 d / dt(|dm / dx|)=(1 / Δt)(1 / Δx){|m(x+Δx, t0+Δt)−m(x, t0+Δt)|−|m(x+Δx, t0)−m(x, t0)|} It is required as follows.

[0092] As mentioned above, under the above analysis conditions, the values ​​of Mp and My are values ​​at discrete cross sections. It is also possible to evaluate whether or not these Mp and My satisfy condition c by increasing the number of cross sections into which the center pillar is divided and minimizing the value of the distance Δx between the cross sections. However, condition c shown in Table 1 shows the results of evaluation based on the manufacturing method of the component. In other words, the changes in Mp and My were considered "discontinuous" at the boundary between sections where materials with different material properties were joined by laser welding and then press-formed. Furthermore, the changes in Mp and My were considered "continuous" at the boundary between sections that were partially tempered after press-formed and sections that were not tempered.

[0093] [Table 1]

[0094] The analysis results of Test No. 1 shown in Table 1 are shown in Figures 7(a) to 7(d) and Figures 8(a) to 8(d). In Figures 7(a) to 7(d), the horizontal axis indicates the position in the vehicle height direction (x-axis direction), the vertical axis indicates the magnitude of the moment, and the figures show the bending moment M about the y-axis, which changes with elapsed time (t1 to t4). As shown in Figures 7(a) to 7(d), the bending moment M increases with elapsed time. In Figures 8(a) to 8(d), the horizontal axis indicates the position in the vehicle height direction (x-axis direction), and the vertical axis indicates the magnitude of the moment at elapsed time t4, the indices m and d, respectively. 2 m / dx 2 , and d / dt (|dm / dx|). As shown in Figure 8(a) to (d), Test No. 1 shows the distribution of My <Mかつd 2 m / dx 2 The part where it is ≤0 satisfied d / dt(|dm / dx|) ≤0. Therefore, excessive deformation of the part that buckles early during a collision could be suppressed, and furthermore, weight reduction to the maximum extent was also possible. In addition, Test No. 1 had a part where My ≥ M in the center pillar for vehicles, so it also had the necessary strength.

[0095] The analysis results of Test No. 2 shown in Table 1 are shown in FIGS. 9(a) to 9(d). FIGS. 9(a) to (d) show the position in the vehicle height direction (x-axis direction) on the horizontal axis, and the vertical axis shows the magnitude of the moment, the index m, d 2 m / dx 2 , and the distribution of d / dt(|dm / dx|). As shown in FIGS. 9(a) to (d), Test No. 2 did not satisfy d / dt(|dm / dx|) ≤0 at the part where My < M and d 2 m / dx 2 ≤0. Therefore, excessive deformation of the part that buckles early during a collision could not be suppressed.

[0096] The analysis results of Test No. 3 shown in Table are shown in FIGS. 10(a) and 10(b). FIGS. 10(a) and (b) show the position in the vehicle height direction (x-axis direction) on the horizontal axis, and the vertical axis shows the distribution of the magnitude of the moment and the index m at the elapsed time t4. As shown in FIGS. 10(a) and (b), Test No. 3 did not have a part where My < M even at the elapsed time t4, and the value of the index m became 0 or less. Therefore, the impact absorption characteristics were not satisfied, and weight reduction of the center pillar could not be achieved.

[0097] Regarding the analysis results of Tests No. 4 to 9 shown in Table 1, although not shown in the figures, among Tests No. 4 to 9, the inventive examples satisfied d / dt(|dm / dx|) ≤0 at the part where My < M and d 2 m / dx 2 ≤0. Therefore, excessive deformation of the part that buckles early during a collision could be suppressed, and furthermore, weight reduction to the maximum extent was also possible. In addition, these inventive examples also had the necessary strength. On the other hand, among Tests No. 4 to 9, the comparative examples had My < M and d 2 m / dx 2 There was no portion where d / dt(|dm / dx|) ≦ 0 was not satisfied at a portion where it becomes ≦ 0, or there was no portion where My < M.

Industrial Applicability

[0098] According to the above aspect of the present invention, it is possible to provide a vehicle center pillar that can suppress excessive deformation of a portion that buckles early during a collision from the side direction of the vehicle while having the required strength, and can further achieve weight reduction to the maximum extent. Therefore, the industrial applicability is high.

Explanation of Signs

[0099] 1 Vehicle center pillar Mp Fully plastic moment My Yield moment M Bending moment around the longitudinal direction of the vehicle input during a collision from the side direction of the vehicle< / m>

Claims

1. In vehicle center pillars, The vehicle height direction is the x-axis, The longitudinal direction of the vehicle is the y-axis, The vehicle width direction is the z axis, The yield strength of the yz cross section perpendicular to the x axis is YP, The yield moment around the y-axis in the y-z cross section is My shown in the following formula 1: The total plastic moment around the y-axis in the y-z cross section is defined as Mp shown in the following formula 2: When the vehicle center pillar receives a load in the z-axis direction, The bending moment around the y-axis input to the vehicle center pillar is defined as M, The elapsed time when the load is applied is defined as t, When the relationship between My, Mp, and M is defined as m shown in the following formula 3, With the elapsed time t, My<M and d ２ m / dx ２ At the part where ≦0 d / dt(|dm / dx|)≦0 fulfill, A vehicle center pillar. My = YP ｙ · I ｙ / z ｙ ··· (Equation 1) Mp=∫ Ｓ z ｐ •YP ｐ dS ・・・(Equation 2) m=(M-My) / (Mp-My)...(Formula 3) where: I ｙ : second moment of area around the y-axis in the y-z cross section z ｙ : Distance from the neutral axis of the point at which the yield occurs earliest in the yz cross section according to the t YP ｙ : Proof strength of the point at which the yz cross section yields earliest with the t S: the yz cross section (cut surface of the center pillar) z ｐ : Distance from the neutral axis of the infinitesimal element dS in the yz section YP ｐ : proof stress at the infinitesimal element dS of the yz cross section Neutral axis: An axis along which compressive stress and tensile stress switch in the yz cross section relative to M, and on which the strain in the x-axis direction becomes zero in the yz cross section dm / dx: first derivative of m in the x-axis direction d ２ m / dx ２ : second derivative of m in the x-axis direction d / dt(|dm / dx|): time derivative of the absolute value of dm / dx

2. 2. The vehicle center pillar according to claim 1, wherein the yield moment My varies discontinuously along the x-axis.

3. 3. The vehicle center pillar according to claim 2, wherein the position where the yield moment My discontinuously changes along the x-axis is a laser welding position of the steel plate.

4. 3. The vehicle center pillar according to claim 2, wherein the total plastic moment Mp varies discontinuously along the x-axis.

5. 5. The vehicle center pillar according to claim 4, wherein the position where the full plastic moment Mp changes discontinuously along the x-axis is a laser welding position of the steel plate.

6. 2. The vehicle center pillar according to claim 1, wherein the yield moment My varies continuously along the x-axis.

7. 7. The vehicle center pillar according to claim 6, wherein the total plastic moment Mp varies continuously along the x-axis.

8. Even when the bending moment M, which increases with the elapsed time t, reaches a maximum, My<M and d ２ m / dx ２ At the site where d / dt(|dm / dx|)≦0 fulfill, 8. The vehicle center pillar according to claim 1, wherein the center pillar is a center pillar for a vehicle.

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