The present invention relates to a press-formed product (hereinafter, also referred to simply as a “formed product”) which is shaped from a starting material of metal sheet by press working. Particularly, the present invention relates to a press-formed product including a flange section which is formed by stretch flange deformation, and a method for designing the formed product.
For automobile skeleton components (hereafter, also referred to simply as “skeleton components”) constituting a body of an automobile, efforts have been made to promote weight reduction and functional enhancement (for example, improvement of anti-collision performance). For that purpose, a tailored blank is used as the starting material for a skeleton component. The tailored blank is made up of a plurality of metal sheets integrated by being joined (for example, butt-welded) together, in which the plurality of metal sheets are different from each other in tensile strength, sheet thicknesses, and the like. Hereinafter, such a tailored blank is also referred to as a TWB. A press-formed product is obtained by press-working a TWB. A press-formed product is subjected, as needed, to trimming, restriking or the like, thereby being finished into a desired shape.
For example, a front pillar and a side sill are each a complex body of skeleton components. The front pillar is disposed on a fore side of a vehicle body, and extends vertically. The side sill is disposed in a lower portion of the vehicle body, and extends in a fore-to-aft direction. A lower end section of the front pillar and a fore end section of the side sill are coupled to each other. Here, some structures of the front pillar may adopt a structure which is divided into upper and lower sections. In this case, the upper section is called as a front pillar upper, and the lower section as a front pillar lower. A lower end section of the front pillar upper and an upper end section of the front pillar lower are coupled to each other.
The front pillar lower includes, as skeleton components, for example, a front pillar lower-outer (hereafter, also referred to simply as an “outer”), a front pillar lower-inner (hereafter, also referred to simply as an “inner”), and a front pillar lower-reinforcement (hereafter, also referred to simply as a “reinforcement”). The outer is disposed on the outer side in the vehicle width direction. The inner is disposed on the inner side in the vehicle width direction. The reinforcement is disposed between the outer and the inner. Among those, the outer is curved in an L-shape along the longitudinal direction, and has a hat-shaped cross section over the entire range in the longitudinal direction. Typically, the outer is a press-formed product.
As shown in
As shown in
It is possible to use a TWB for the production of such front pillar lower-outer 10. Regarding the method for shaping a press-formed product from the TWB, the following conventional techniques are available.
Japanese Patent Application Publication No. 2006-198672 (Patent Literature 1) discloses a technique to mitigate the load acting on the vicinity of a weld line of a TWB at the time of press working. In this technique, the TWB is provided with a cutout at a location slightly apart from the weld line. Patent Literature 1 describes that at the time of press working, strain which occurs in the vicinity of the weld line is dispersed by the cutout, thereby improving formability of the formed product.
Japanese Patent Application Publication No. 2001-1062 (Patent Literature 2) discloses a technique for applying press working on a TWB which is made up of two metal sheets each having a different tensile strength and a sheet thickness. In this technique, a weld line of the TWB is disposed on a portion where a gradient of strain would occur when a single metal sheet, which is not a TWB, is press worked. Then, a metal sheet having a higher strength is disposed on the side of larger strain, and a metal sheet having a lower strength is disposed on the side of smaller strain. As a result of this, strain will be reduced in press working such as deep drawing, bulging and the like. Patent Literature 2 describes that, as a result of that, cracking of the base metal which occurs in the metal sheet on the lower strength side is suppressed, thus improving the formability of formed product.
Japanese Patent Application Publication No. 2002-20854 (Patent Literature 3) discloses a technique to apply press working on a TWB which is made up of two metal sheets having similar levels of tensile strength and ductility. In this technique, a specific region in a formed product obtained by press working is subjected to a heat treatment such as nitriding, thereby strengthening the specific region. Patent Literature 3 describes that since deformation resistance of the metal sheet is uniform at the time of press working before the heat treatment, the formability of the formed product is improved.
When performing press-working, a portion of the blank (metal sheet) may undergo stretch flange deformation depending on the shape of the press-formed product. The stretch flange deformation refers to a deformation form in which as a working tool (press tooling) intrudes and moves into a blank, the blank stretches in a direction along the moving direction of the working tool as the working tool (press tooling) moves into the blank, and at the same time it stretches in a circumferential direction perpendicular to the moving direction.
For example, as shown in
When shaping a press-formed product shown in
Conventionally, when producing a press-formed product by using a TWB, a weld line of the TWB has been disposed so as to avoid an area which undergoes stretch flange deformation (hereinafter, also referred to as a “stretch flange deformation field”). This is because if the weld line is disposed in a stretch flange deformation field, cracking occurs between the weld line and the base metal sheet due to the fact that deformation resistance is different between the welded metal and the base metal sheet.
Therefore, conventionally, the position to depose the weld line in the press-formed product shown in
Regarding such problems, in the technique of Patent Literature 1, a cutout provided in the TWB remains in the formed product after press-working. For that reason, it is inevitable to remove the cutout by trimming. In that case, it is difficult to reduce the production steps.
In the technique of Patent Literature 2, it is necessary to dispose a metal sheet having a higher strength on the side of larger strain, and a metal sheet having a lower strength on the side of smaller strain. Therefore, there is a risk that weight reduction and functional enhancement are hindered. Moreover, regarding the position to dispose the weld line of TWB, Patent Literature 2 only provides the following description. The weld line of TWB is disposed in a portion, 5 to 10 mm or more away, and within 200 mm or less, from a location where cracking occurs when press-working a single blank.
In the technique of Patent Literature 3, it is necessary to apply heat treatment such as nitriding to a formed product after press-working. Therefore, not only an excess amount of heat treatment cost is imposed, but also the number of the production steps will increase.
In short, any of the techniques of Patent Literatures 1 to 3 cannot readily realize improvement of the degree of freedom for designing a press-formed product.
The present invention has been made in view of the above described situations. It is an object of the present invention to provide a press-formed product having the following feature and a method for designing the same:
To improve the degree of freedom for designing a press-formed product which is shaped from a TWB.
A press-formed product according to one embodiment of the present invention comprises a tailored blank made up of a plurality of metal sheets butt-welded together. The press-formed product includes a flange section, and an arc-shaped area in which an inner peripheral edge is open in the area of the flange section. A weld line of the tailored blank intersects with the inner peripheral edge of the arc-shaped area, and an outer peripheral edge of the arc-shaped area. An angle formed by the weld line and a maximum principal strain direction is 17 to 84°.
The design method according to one embodiment of the present invention is a method for designing the above described press-formed product. In designing the press-formed product, the weld line is disposed such that during press-working, a relative difference between strain dεWLy′ in the direction along the weld line at the center in the width direction of the weld line, and strain dεBMy′ in the direction along the weld line in the vicinity of the weld line of the metal sheet is not more than 0.030.
A press-formed product of the present invention and a method for designing the same have the following prominent effect:
Effect of enabling to improve the degree of freedom for designing a press-formed product which is shaped from a TWB.
In order to achieve the above described objects, the present inventors have performed various tests, thereby conducting diligent investigation. As a result of that, they have obtained the following findings. When a press-formed product is produced from a TWB by press-working, if the weld line is simply disposed in a stretch flange deformation field, cracking occurs in the vicinity of the weld line, thereby deteriorating formability of the formed product. However, even when the weld line is disposed in the stretch flange deformation field, properly setting the position of the weld line makes it possible to suppress the occurrence of cracking, thus ensuring the formability of the formed product. As a result of that, it is possible to improve the degree of freedom for designing a press-formed product using a TWB.
The press-formed product of the present invention and the method for designing the same are completed based on the above described findings.
The press-formed product according to one embodiment of the present invention comprises a tailored blank made up of a plurality of metal sheets butt-welded together. The press-formed product includes a flange section, and an arc-shaped area in which an inner peripheral edge is open in the area of the flange section. The weld line of the tailored blank intersects with the inner peripheral edge of the arc-shaped area and an outer peripheral edge of the arc-shaped area. An angle formed by the weld line and a maximum principal strain direction is 17 to 84°. In a typical example, the press-formed product is shaped by press-working. At that moment, the arc-shaped area is formed by stretch flange deformation. The maximum principal strain direction is a maximum principal strain direction of the stretch flange deformation.
In the above described press-formed product, the angle formed by the weld line and a tangential line of the inner peripheral edge at an intersection point between the weld line and the inner peripheral edge is preferably 40 to 75°.
In the above described press-formed product, it is preferable that the number of the metal sheets for making up the tailored blank is two, and the two metal sheets are different from each other in at least one of tensile strength and sheet thickness.
In the case of this press-formed product, the following configuration may be adopted. The press-formed product is an automobile skeleton component which is curved in an L-shape along the longitudinal direction. The skeleton component has a hat-shaped cross-section over the entire range in the longitudinal direction. The skeleton component includes a curved region curved along its longitudinal direction, and a first region and a second region, respectively extending from both ends of the curved region. The skeleton component is a component which is supposed to be subjected to a collision load along an extended direction of the first region. The arc-shaped area is a flange section on the inner side of curve of the curved region. The sheet thickness of the metal sheet disposed on the side of the first region is larger than the sheet thickness of the metal sheet disposed on the side of the second region.
In the case of a press-formed product which has adopted such configurations, the following configuration can be adopted. The skeleton component is a front pillar lower-outer. The first region is coupled to a side sill, and the second region is coupled to a front pillar upper.
In a press-formed product which has adopted such a configuration, a multiplication value of a tensile strength and a sheet thickness of the metal sheet disposed on the side of the first region is substantially equal to a multiplication value of a tensile strength and a sheet thickness of the metal sheet disposed on the side of the second region. In a typical example, a difference between those multiplication values is not more than 600 mm·MPa.
The design method according to one embodiment of the present invention disposes the weld line so as to be in the following state, when designing the above described press-formed product. During press-working, a relative difference between a strain dεWLy′ in the direction along the weld line at the center in the width direction of the weld line, and strain dεBMy′ in the direction along the weld line in the vicinity of the weld line of the metal sheet is not more than 0.030. More preferably, the relative difference between strain dεWLy′ and strain dεBMy′ is 0 (zero).
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Here, as the press-formed product, a front pillar lower-outer among automobile skeleton components will be taken as an example.
[Press-Formed Product]
As shown in
The outer 10 of the present embodiment is shaped by press-working from a TWB 20 shown in
As shown in
In the outer 10 of the present embodiment, the sheet thickness on the side of the side sill (on the side of the first region 11) corresponds to that of the first metal sheet 21, and the sheet thickness of the side of the front pillar upper (on the side of the second region 12) corresponds to that of the second metal sheet 22. That is, the sheet thickness on the side of the side sill is larger than that of the side of the front pillar upper. Since the sheet thickness on the side of the first region 11 to be coupled to the side sill is large, axial collapse performance of the first region 11 will be improved. Thereby, it is possible to improve the anti-collision performance of the outer 10. On the other hand, since the sheet thickness on the side of the second region 12, which is to be coupled with the front pillar upper, is small, it is possible to realize weight reduction of the outer 10. Since the sheet thickness on the side of the second region 12 has a lower contribution to the axial collapse performance of the first region 11, there will be no hindrance to the anti-collision performance.
[Disposition of Weld Line]
If the weld line L of the TWB 20 is simply disposed in the arc-shaped area 14 of the outer 10, cracking will occur in the vicinity of the weld line L. This is because the arc-shaped area 14 becomes a stretch flange deformation field at the time of press-working. In the present embodiment, in the arc-shaped area 14 of the outer 10, an angle θ (hereinafter, also referred to as a “welding-line first angle”) formed by the weld line and a maximum principal strain direction of the stretch flange deformation is set to 17 to 84°. The maximum principal strain direction refers to a circumferential direction of a curved arc in a portion where a sheet-thickness reduction rate is maximum (hereinafter, also referred to as a “maximum sheet-thickness reduction section”) of the arc-shaped area 14 where the sheet thickness is reduced due to stretch flange deformation at the time of press working (see a dotted line arrow in
The maximum sheet-thickness reduction section appears in the vicinity of the weld line L on the side of the metal sheet which has a lower equivalent strength of the first and second metal sheets 21 and 22 joined to each other across the weld line L. The equivalent strength of the metal sheet refers to a multiplication value [mm·MPa] of tensile strength [MPa] and sheet thickness [mm] of the metal sheet. The vicinity of the weld line L means, for example, a range of 0.5 to 4 mm from a boundary between the weld line L and the metal sheet on the side of lower equivalent strength. When the sheet thickness of the metal sheet on the side of lower equivalent strength is t [mm], the vicinity of the weld line L may refer to a range of 0.5×t to 4×t [mm] from the boundary between the weld line L and the metal sheet on the side of lower equivalent strength. The maximum sheet-thickness reduction section refers to a region which exhibits a sheet thickness reduction up to a value of work hardening coefficient (n-value) of the metal sheet on the side of lower equivalent strength, or 0.8 times of the n-value.
The maximum principal strain direction can be easily recognized from the shape of the press-formed product (outer 10). Specifically, when concentric arcs centering on the arc center of the outer peripheral edge 14a of the arc-shaped area 14 is drawn, the direction along the tangential line to the arc in the maximum sheet-thickness reduction section becomes the maximum principal strain direction.
If the welding-line first angle θ is 17 to 84°, it is possible to reduce the sheet-thickness reduction rate in the maximum sheet-thickness reduction section, thereby allowing suppression of cracking. As a result of that, it is possible to ensure the formability of a formed product.
Moreover, if the weld line L of the TWB 20 is simply disposed on the arc-shaped area 14 of the outer 10, cracking is likely to occur in the vicinity of the intersection point between the weld line L and the inner peripheral edge 14b of the arc-shaped area 14. Such cracking occurs in the vicinity of the weld fine L on the side of the metal sheet having lower equivalent strength of the first and second metal sheets 21 and 22 joined to each other across the weld line L. Therefore, in the present embodiment, an angle γ (hereinafter, also referred to as a “welding-line second angle”) formed by the weld line L and the tangential line of the inner peripheral edge 14b at the intersection point between the weld line L and the inner peripheral edge 14b is set to 40 to 75°.
If the welding-line second angle γ is 40 to 75°, it is possible to suppress occurrence of cracking at the inner peripheral edge of the arc-shaped area. As a result of that, it is possible to ensure the formability of the formed product.
The mode of the press-forming for producing the outer 10 of the present embodiment may be appropriately selected according to the shape of the formed product. For example, not only flange forming, but also bending, drawing, bulging, bole expanding, and the like can be combined. As a press tooling, a die paired with a punch is used. Further, a blank holder, a pad, and the like for holding the blank may be used.
Moreover, in the outer 10 of the present embodiment, the weld line L is disposed in the curved region 13. This makes it possible to improve material yield compared with a case in which the weld line is disposed in a straight-shaped portion of the first region 11 (on the side of the side sill) or the second region 12 (on the side of the front pillar upper). Therefore, it is possible to reduce production cost of the formed product.
Further, the outer 10 of the present embodiment absorbs higher energy upon collision, thus improving anti-collision performance compared with a case in which the weld line is disposed in a straight-shaped portion on the side of the first region 11 to be coupled to the side sill. Moreover, the outer 10 of the present embodiment absorbs higher energy in view of unit volume upon collision compared with a case in which the weld line is disposed in a straight-shaped portion on the side of the second region 12 to be coupled with the front pillar upper. Therefore, it is possible to combine weight reduction and functional enhancement in a good balance.
As described above, the outer 10 of the present embodiment is shaped from a TWB 20 which is made up of the first metal sheet 21 and the second metal sheet 22. In this case, it is preferable that an equivalent strength of the first metal sheet 21 disposed on the side of the first region 11 is substantially equal to an equivalent strength of the second metal sheet 22 disposed on the side of the second region 12. This is because the deformation resistances of the first and second metal sheets 21 and 22 become equal at the time of press working, thus improving the formability of formed product. The statement “equivalent strength is substantially equal” permits the difference in equivalent strength up to 600 mm·MPa. That is, the difference between the equivalent strength of the first metal sheet 21 and the equivalent strength of the second metal sheet 22 is preferably not more than 600 mm·MPa. Such difference in the equivalent strength is preferably not more than 400 mm·MPa, and more preferably not more than 350 mm·MPa.
When producing the outer 10 of the present embodiment, the width of the weld line L of the TWB 20 is preferably smaller. Because, in the present embodiment, focusing on the deformation in the weld line direction in an area including the weld line L and its vicinity, its deformation is investigated in line with actual situation. The deformation is based on the amount of strain in the weld line direction at the center in the width direction of the weld line L. As a welding method to form a narrow width weld line L, a laser welding may be adopted. Besides, a plasma welding may also be adopted.
[Design of Proper Disposition of Weld Line]
When the weld line of the TWB is disposed so as to intersect with the inner peripheral edge and the outer peripheral edge of the arc-shaped area, in the arc-shaped area which becomes a stretch flange deformation field of the press formed product, the deformation field (strain field) of an area including the weld line and its vicinity is strictly a deformation field of uniaxial tension, or a deformation field closer to plane strain. In particular, in the area other than the inner peripheral edge of the arc-shaped area, the deformation field becomes close to plane strain (hereinafter, also referred to as a “plane strain deformation field”). On the other hand, in the inner peripheral edge of the arc-shaped area, the deformation field becomes a uniaxial tensile deformation field. This is because the inner peripheral edge is open.
In this case, the radial strain dεy can be represented by the following Formula (1).
dεy=dεx×(−r)/(1+r) (1)
where, r represents an r-value.
Moreover, regarding strain components based on the circumferential strain dεx and the radial strain dεy which occur in the base metal sheets 21, 22 in the vicinity of the weld line, strain dεy′ in a direction along the weld line L (hereinafter, also referred to as a “weld line direction”) can be represented by the following Formula (2). Hereinafter, the strain dεy′ is also referred to as BM welding-line direction strain dεy′ (or “dεBMy′”). This Formula (2) is derived by coordinate transforming the circumferential strain dεx and the radial strain dεy by using the tensor coordinate transformation rule.
dεy′=dεx×(cos θ)2+dεy×(sin θ)2 (2)
Substituting Formula (1) into Formula (2), the BM welding-line direction strain dεy′ can also be represented by the following Formula (3).
dεy′=dεx×(cos θ)2+dεx×(−r)/(1+r)×(sin θ)2 (3)
Any of Formulas (1) to (3) is common to the uniaxial tensile deformation field and the plane strain deformation field. In such a stretch flange deformation field, the maximum sheet-thickness reduction section appears in the vicinity of the weld line on the side of the metal sheet having a lower equivalent strength of the two metal sheets 21 and 22 which are joined to each other across the weld line L. Here, regarding a portion of the weld line adjacent to the maximum sheet-thickness reduction section in the circumferential direction of the curved arc, let the strain in the weld line direction at the center in the width direction of the weld line be dεWLy′. Hereinafter, this strain dεWLy′ is also referred to as WL welding-line direction strain dεWLy′.
When the weld line L is disposed in the stretch flange deformation field, cracking that occurs in the vicinity of the weld line is caused by shear deformation which occurs between the weld line L and the base metal sheet (metal sheet 22 in
Then, in the present embodiment, when designing a press-formed product, the weld line is disposed such that relative difference between the WL welding line direction strain dεWLy′ and the BM welding-line direction strain dεy′ becomes small during press working. Specifically, according to actual situation, the weld line may be disposed such that relative difference between the WL welding-line direction strain dεWLy′ and the BM welding-line direction strain dεy′ becomes not more than 0.030. As relative difference between the WL welding-line direction strain dεWLy′ and the BM welding-line direction strain dεy′ decreases, the shear deformation which occurs between the weld line and the base metal sheet on the side of lower equivalent strength decreases. This will make it possible to suppress the occurrence of cracking, thus ensuring formability of the formed product. As a result, it is possible to improve the degree of freedom for designing a press-formed product using a TWB. In particular, disposing the weld line such that relative difference between the WL welding-line direction strain dεWLy′ and the BM welding-line direction strain dεy′ becomes 0, will make it possible to most effectively suppress the occurrence of cracking.
[Disposition of Weld Line in Plane Strain Deformation Field: Welding-Line First Angle θ]
As shown in
As a blank for shaping the press-formed product 15, a TWB 25 made up of two metal sheets A and B was adopted as shown in
Press working was performed by using a die 26, a punch 27 and a pad 28 as shown in
As shown in Table 1, the sheet-thickness reduction rate was lowest when the welding-line first angle θ was 40°. Therefore, in the present embodiment, based on conditions actually used in press working, the welding-line first angle θ is preferably 17 to 84°. This is because the sheet-thickness reduction rate can be kept low, and thus the occurrence of cracking in the vicinity of the weld line can be suppressed. The welding-line first angle θ is preferably 17 to 71°, more preferably 19 to 71°, and further preferably 25 to 71°.
The relative difference (|dεy′−dεWLy′|) between the WL welding-line direction strain dεWLy′ and the BM welding-line direction strain dεy′ is preferably as small as possible. Therefore, the relative difference is preferably not more than 0.030, more preferably not more than 0.025, and further preferably 0.
[Disposition of Weld Line in Uniaxial Tensile Deformation Field: Welding-Line Second Angle γ]
The hole expansion test is a test to thrust a punch into a blank formed with a circular hole, thereby expanding the hole in a concentric manner. As shown in
In the stretch flange deformation field in the hole expansion test, as the working tool (punch) enters and advances, the blank stretches in a direction along the moving direction of the working tool. This direction is a radial direction of the hole 30a as shown by a solid-line arrow in
Since the hole 30a and the outer peripheral edge of the circular area 31 are concentric circles in the press-formed product 30 by the hole expansion test, θ can be replaced by γ in Formula (3) described above. In this case, supposing dεx to be 1, the following Formula (4) will be derived. As shown in Formula (4), BM welding-line direction strain dεy′ varies depending on the angle γ of the weld line (that is, the welding-line second angle), and the r-value of the base metal sheet.
dεy′=(cos γ)2(−r)/(1+r)×(sin γ)2 (4)
To suppress the occurrence of cracking in the vicinity of the intersection point between the hole of the formed product by the hole expansion test (that is, the inner peripheral edge of the arc-shaped area of the press-formed product) and the weld line, it is necessary to arrange that the BM welding-line direction strain dεy′ is −0.2 to 0.2. Here, a common metal sheet (examples: hot-rolled steel sheet, cold-rolled steel sheet, plated steel sheet, Al alloy sheet, and Ti alloy sheet) has an r-value of 0.5 to 3.0. The r-value is that of the base metal sheet on the side of lower equivalent strength in which cracking is more likely to occur. From what has been described so far, the welding-line second angle γ is preferably 42 to 72°.
In the present embodiment, the welding-line second angle γ may be defined to be 40 to 75°, slightly wider than 42 to 72°. This is because, considering the amount of deformation of an area which softens due to welding heat in the vicinity of weld line, a slight extension of the angle γ can be permitted.
The BM welding-line direction strain dεy′ is preferably as small as possible. Therefore, the BM welding-line direction strain dεy′ is preferably −0.1 to 0.1, more preferably −0.025 to 0.025, and further preferably 0. Accordingly, from
When shaping an outer as a press-formed product of the present embodiment, steel sheet having a tensile strength of not lower than 440 MPa, Al alloy sheet, and Ti alloy sheet, are used as a metal sheet. The r-values of these metal sheets are 0.5 to 3.0. Therefore, in this case, the welding-line second angle γ is preferably 45 to 72°.
Besides, the present invention will not be limited to the above described embodiments, and can be subjected to various modifications within a scope not departing from the spirit of the present invention. For example, the press-formed product will not be particularly limited as long as it includes a flange section formed by stretch flange deformation. Moreover, an automobile skeleton component as a press-formed product will not be limited to a front pillar lower-outer as long as it is a component which is curved in an L-shape along the longitudinal direction, and is supposed to be subjected to a collision load along an extended direction of the first region, and may be a rear side outer, etc.
Moreover, the TWB will not be particularly limited, as long as it is made up of a plurality of metal sheets butt-welded together. For example, when the TWB is made up of two metal sheets, it is only necessary that the metal sheets are different from each other in at least one of tensile strength and sheet thickness. The TWB may be made up of three or more metal sheets.
[Hole Expansion Test]
A hole expansion test was conducted by using a TWB to investigate the relationship between the welding-line second angle γ and the formability.
As shown in
The metal sheet C was made of 980 MPa class High Tensile Strength Steel, and its sheet thickness was 1.6 mm. The metal sheet D was made of 780 MPa class High Tensile Strength Steel, and its sheet thickness was 1.4 mm. That is, the equivalent strength of the metal sheet C was higher than that of the metal sheet D.
On the metal sheet D on the side of lower equivalent strength, an average r-value (average plastic strain ratio) at an additional strain amount of 10% was calculated in conformity with JIS Z 2254 (1996), and found to be 0.712. When the r-value was 0.712, supposing the angle γ be 57.2°, the BM welding-line direction strain dεy′ in Formula (4) described above will become 0 (zero).
As shown in
λ=(d2−d1)/d1×100 (5)
It was confirmed that if the weld line was disposed in the stretch flange deformation field as shown in
The hole expansion rate in Table 2 indicates an average value at each level. The hole expansion rate became most favorable when the welding-line second angle γ was 59°. That is, it was revealed that disposing the weld line such that the BM welding-line direction strain du′ defined by the Formula (4) described above decreases will enable improvement of formability while suppressing the occurrence of cracking.
[Collision Test]
A front pillar lower-outer was adopted as a press-formed product of the present embodiment and, on this outer, a test to confirm anti collision performance upon frontal collision was performed by an FEM analysis.
At that time, the energy that the outer 10 absorbed as the impactor 51 intruded into the outer 10 was determined. By dividing the absorbed energy of the outer 10 by the volume of the outer 10, absorbed energy per unit volume was calculated.
In any of Inventive Example 1 of the present invention and Comparative Examples 1 and 2, a metal sheet E was used as the metal sheet on the side of the second region 12 (on the side of the front pillar upper) with respect to the weld line L, and a metal sheet F was used as the metal sheet on the side of the first region 11 (on the side of the side sill) with respect to the weld line L. The metal sheet E was made of 980 MPa class High Tensile Strength Steel, and its sheet thickness was 1.2 mm. The metal sheet F was made of 780 MPa class High Tensile Strength Steel, and its sheet thickness was 1.5 mm. The metal sheet E has a characteristic that it is more subject to cracking compared with the metal sheet F, and the r-value of the metal sheet E was 0.790.
As shown in
Here, the absorbed energy at the time of collision test varies depending on the sheet thickness. As the area where the sheet thickness is large increases, absorbed energy tends to increase. For that reason, the absorbed energy of Comparative Example 2 which had a larger area of the metal sheet F with a larger sheet thickness was slightly more excellent than the absorbed energy of Inventive Example 1 of the present invention.
On the other hand, as shown in
[Material Yield]
A front pillar lower-outer was adopted as the press-formed product of the present embodiment, and material yield was investigated on a case in which the outer was fabricated from a metal sheet.
As shown in
[Simple method for setting welding-line first angle θ (second angle γ)] As described so far, disposing the welded line such that the relative difference between the WL welding-line direction strain dεWLy′ and the BM welding-line direction strain dεy′ (dεBMy′) is not more than 0.030 will make it possible to suppress the occurrence of cracking. Therefore, an optimum condition for suppressing cracking is that the relative difference between dεWLy′ and dεy′ is 0. That is, dεWLy′ is equal to dεy′. Substituting this condition (dεWLy′=dεy′) into Formula (2) described above, and further dividing both sides of Formula (2) described above by the circumferential direction strain dεx in the base metal sheet in the vicinity of the weld line will lead to the following Formula (6).
dε
WL
y′/dεx=(cos θ)2+dεy/dεx×(sin θ)2 (6)
In Formula (6), since the term “dεy/dεx” in the right-hand side is strain ratio β, substituting the term “dεWLy′/dεx” by χ will lead to the following Formula (7).
λ=(cos θ)2+β×(sin θ)2 (7)
From Formula (7), for each welding-line first angle θ, the relationship between a proportion χ of WL welding-line direction strain dεWLy′ with respect to maximum principal strain dεx in the base metal sheet in the vicinity of the weld line, and a strain ratio β, is determined.
The present invention is usable for automobile skeleton components and production thereof.
Number | Date | Country | Kind |
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2015-104700 | May 2015 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2016/063867 | 5/10/2016 | WO | 00 |