[0001]The present invention relates to a manufacturing method of a member for forming a tubular portion from a metal material sheet, a manufacturing method of a member for a vehicle, and a die and punch.
Priority is claimed on Japanese Patent Application Nos. 2019-066238 and 2019-066239, filed March 29, 2019, the contents of which are incorporated herein by reference. [Related Art]
[0002]
As is well known, in the automobile industry, various vehicle suspensions have been put into practical use depending on the application.
For example, a torsion beam type suspension device has a configuration including a torsion beam Assy that rotatably supports left and right steel wheels with an arm and allows one end of a spring to be disposed in the vicinity of the left and right ends thereof, the spring that connects the torsion beam and the vehicle body, and an absorber.
[0003]
In the torsion beam Assy, for example, left and right trailing arms forming a pair that rotatably supports the left and right steel wheels are connected by the torsion beam, and spring receiving portions forming a pair are formed in the vicinity of the left
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and right ends of the torsion beam.
The torsion beam Assy is connected to the vehicle body via pivot shafts extending from the left and right sides of the vehicle body toward the center, so that the left and right steel wheels oscillate with respect to the vehicle body.
[0004]
One end side of the spring is disposed in the spring receiving portion, and the load received from the road surface is transmitted to the vehicle via the steel wheels, the trailing arms, and the springs. Therefore, for example, the trailing arm undergoes a large load, and is required to have high strength.
[0005]
As described above, a member for a vehicle (for example, a link member) such as the trailing arm is required to have high strength, and is also required to be lightweight. For this reason, a complex tubular shape is required. For example, there are cases where a member for a vehicle has a specific three-dimensional tubular portion in which at least any one of a circumferential length change rate changing portion in which the change rate of the circumferential length (the circumferential length of a cross section orthogonal to a centroid line) changes along the centroid line, a cross-sectional shape changing portion in which the shape of the cross section orthogonal to the centroid line changes along the centroid line, or a curved portion in which the centroid line has a curvature is formed. In the related art, in a case where a tubular member for a vehicle is manufactured from a metal material sheet, a joint portion is often welded after forming by multiple steps of press working including trimming therein, so that it cannot be easily said that a cost reduction is easy (for example, refer to Patent Documents 1 and 2).
[0006]
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On the other hand, in the manufacturing of a thick straight round pipe, such as a line pipe, a UO-forming method in which a steel sheet (metal material sheet) is subjected to cross-sectional U-shaped U-forming (for example, press forming or bending forming) and is then subjected to O-forming into a round pipe (tubular body) having a circular cross section has been used (for example, refer to Patent Document 3).
[0007]
In addition, regarding the circularity after an abutting portion forming step for the straight round pipe and the adhesion of a seam portion, the influence of the ratio a (= Du/Do, hereinafter referred to as the forming condition ratio a) based on the width Du of a U-forming punch and the recessed part width Do of an O-forming die (abutting portion forming die) has been sufficiently examined in terms of both aspects of analysis and experiment and has been technically established (for example, refer to Non-Patent Document 1).
[0008]
In recent years, in the above-mentioned UO-forming method, there has been a demand for a technique for efficiently manufacturing a member (member for a vehicle) having the above-mentioned specific three-dimensional tubular portion. [Prior Art Document] [Patent Document]
[0009]
[Patent Document 1] Japanese Patent No. 3114918
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2012-115905
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-141936
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[Non-Patent Document]
[0010]
[Non-Patent Document 1] "Analysis of Shape of Formed Article (Katsumi Kawada, Yasutoshi Tozawa)" (Plasticity and Processing, vol. 21, no. 230 (1980), P234 to P240
[Disclosure of the Invention] [Problems to be Solved by the Invention]
[0011]
However, for example, the technique described in Patent Document 3 targets a thick straight round pipe (API X60 or more, the ratio of thickness/exterior shape is 4% or more) and discloses that UO-forming is performed by setting a forming condition ratio a (= Du/Do) based on the width Du of a press forming punch and the recessed part width Do of an O-forming die to 0.7 or less (preferably 0.65 or less) in order to improve circularity, but is not suitable as, for example, a technique for causing abutting scheduled portions of the above-mentioned specific three-dimensional tubular portion to abut each other at high accuracy.
[0012]
Furthermore, it can be said that it is impossible to manufacture a member including a three-dimensional shape such as a member for a vehicle having the above-mentioned specific three-dimensional tubular portion by using, for example, ultrahigh tensile strength steel.
[0013]
In addition, in a thin tubular portion (the ratio of thickness/outer diameter is 10% or less) such as a member for a vehicle, when the above-mentioned forming condition ratio a (= Du/Do) is too small, the closed cross section after an abutting
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portion forming step has an elongated oval shape, resulting in a decrease in circularity. Therefore, there is a problem that application to a thin member is difficult.
[0014]
Moreover, research results described in Non-Patent Document 1 also targets a straight round pipe, and application to a tubular portion including a three-dimensional shape other than a round pipe having a straightly continuous circular closed cross section, for example, the above-mentioned specific three-dimensional tubular portion is difficult.
[0015]
In a case of manufacturing a member having the above-mentioned specific three-dimensional tubular portion by applying UO-forming, there are cases where a gap is generated between abutting portions (joining scheduled portions) due to springback. Therefore, in joining the abutting portions, it is necessary to perform the joining after minimizing the gap generated between the abutting portions by restraining the abutting portions. However, in this case, it is necessary to restrain the abutting portions during the joining, which causes problems that the workability is deteriorated and the productivity is lowered, such as jigs for restraining the pipe becoming complex.
[0016]
In particular, a high strength thin steel sheet, which is desirably applied to a member for a vehicle, has a large springback and causes a large gap between the abutting portions, so that it is extremely difficult to bring the three-dimensional shape of the tubular portion into close contact with a jig or the like. For this reason, there is a demand for a technique capable of efficiently manufacturing the above-mentioned member by bringing the abutting scheduled portions of a tubular portion having a three-dimensional shape or a deformed cross section, particularly the above-mentioned
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specific three-dimensional tubular portion, into close contact with each other at high accuracy.
[0017]
The present invention has been made in consideration of such circumstances, and an object thereof is to provide a manufacturing method of a member in which a member including the above-described specific three-dimensional tubular portion can be efficiently formed (manufactured) by processing a metal material sheet, a manufacturing method of a member for a vehicle, and a die and punch. [Means for Solving the Problem]
[0018]
In order to solve the above problems, this invention proposes the following means.
According to an aspect of the present invention, there is provided a manufacturing method of a member for manufacturing a member which includes a specific three-dimensional tubular portion including at least any one of a cross-sectional shape changing portion in which a cross-sectional shape of a tubular portion changes along a centroid line, a circumferential length change rate changing portion in which a circumferential length of the tubular portion changes along the centroid line and a change rate of the circumferential length changes, and a curved portion in which the centroid line of the tubular portion has a curvature, by processing a metal material sheet, the manufacturing method including: a U-forming step of performing U-forming on the metal material sheet using a U-forming die and punch including a U-forming punch to manufacture a U-formed article having a recessed cross-sectional shape; and an O-forming step of causing side end portions of the U-formed article to abut each other by an O-forming die to form abutting portions, in which a forming condition ratio a =
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Du/Do which is a ratio of a punch width Du of a portion of the U-forming punch corresponding to the specific three-dimensional tubular portion to a recessed part width Do of a portion of the O-forming die corresponding to the specific three-dimensional tubular portion is set to 0.85 to 0.95.
[0019]
According to another aspect of the present invention, there is provided a manufacturing method of a member for a vehicle, including: applying the manufacturing method of a member.
[0020]
According to another aspect of the present invention, there is provided a die and punch used in the manufacturing method of a member, including: the U-forming die and punch; and the O-forming die, in which the forming condition ratio a is set to 0.85 to 0.95.
[0021]
According to the manufacturing method of a member, the manufacturing method of a member for a vehicle, and the die and punch according to the aspects of the present invention, since the forming condition ratio a = Du/Do which is the ratio of the width Du of the press forming punch used when forming the U-formed article from the metal material sheet in the U-forming step to the recessed part width (the width of an abutting portion forming recessed part) Do of the O-forming die used in the O-forming step of causing the side end portions of the U-formed article to abut each other is set to 0.85 to 0.95, the springback can be appropriately suppressed, and the side end portions of the specific three-dimensional tubular portion formed in the O-forming step can be accurately and efficiently brought into close contact with each other or approach a target position. Here, in a case where the forming condition ratio a is less than 0.85, the
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springback of the specific three-dimensional tubular portion becomes excessive, and the closed cross section (cross section perpendicular to a longitudinal direction) of the specific three-dimensional tubular portion significantly deviates from the target shape. For example, in a case where the target shape is a true circle, the closed cross section becomes an elongated oval shape, resulting in a decrease in circularity. In a case where the forming condition ratio a exceeds 0.95, the side end portions of the specific three-dimensional tubular portion formed in the O-forming step are not sufficiently brought into close contact with each other.
[0022]
In addition, after the O-forming step, the end portions of abutting scheduled portions are brought into close contact with each other or disposed close to the target state and thus can be efficiently joined to each other without a complex jig or the like.
In addition, since the forming condition ratio a is set to less than 1.0, the U-formed article can be easily disposed in the O-forming die.
As a result, a tubular portion formed of a specific three-dimensional tubular portion or a tubular portion of which at least a portion is formed of a specific three-dimensional tubular portion can be efficiently formed. Furthermore, a reduction in the weight of the member and the member for a vehicle can be easily achieved and the manufacturing cost can be reduced.
[0023]
Here, the U-forming die and punch may have a press forming punch which is the U-forming punch, and a press forming recessed die in which a press forming recessed part corresponding to the press forming punch is formed, and the U-forming step may include press forming step of relatively moving the press forming punch in a direction toward the press forming recessed die to form the metal material sheet into a
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press-formed article having a recessed cross-sectional shape portion and an extending portion extending outward from both side end portions of the recessed cross-sectional shape portion, and a flangeless forming step of removing the extending portion from the press-formed article to manufacture the U-formed article.
[0024]
In this specification, press forming refers to forming a press-formed article by the press forming punch and the press forming recessed die in which the press forming recessed part is formed. More specifically, press forming refers to pressing a metal material sheet on the outer side (side portion) of the press forming recessed part by a metal material pressing tool (for example, a blank holder), relatively moving (advancing) the press forming punch in a direction toward the press forming recessed die, and while pressing an extending portion extending outward from the end portions of the press forming recessed part, forming a recessed cross-sectional shape portion in the metal material sheet.
[0025]
In addition, the flangeless forming step refers to forming a press-formed article without the extending portion (= flangeless press-formed article), and includes, for example, in addition to a step of removing a flange-shaped extending portion extending outward from both side end portions having a recessed cross-sectional shape after the press forming by trimming or the like, a step of pressing the extending portion temporarily formed during the press forming into the press forming recessed part to be formed as a portion of the recessed cross-sectional shape portion, a step of bending-forming the extending portion that has been temporarily formed to be formed as a portion of the recessed cross-sectional shape portion, and the like. The flangeless press-formed article is a form of the press-formed article, and is sometimes simply
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referred to as a press-formed article.
[0026]
In this specification, causing the abutting scheduled portions to abut each other includes, in addition to bringing the corresponding side end portions of the flangeless press-formed article into close contact with each other, for example, forming a gap (target interval) at which joining can be performed by welding or the like, forming a gap in a portion of the side end portions which are in close contact with each other, disposing the side end portions close to each other at a predetermined gap over the entire length.
In a case where a gap is formed, the interval of the gap may be formed so that a portion of a section in which the gap is formed is different from other portions. That is, the interval of the gap does not have to be constant over the entire section.
[0027]
In addition, the U-forming die and punch may have a bending forming punch which is the U-forming punch, and a bending forming recessed die in which a bending forming recessed part corresponding to the bending forming punch is formed, and in the U-forming step, the bending forming punch may be relatively moved in a direction toward the bending forming recessed die to manufacture the U-formed article.
[0028]
In this specification, bending forming refers to forming a U-formed portion by the bending forming punch and the bending forming recessed die, and more specifically, forming without pressing the metal material sheet by a blank holder or the like when pressing the metal material sheet by the bending forming punch. That is, the purpose of bending forming is not to form an extending portion (flange portion) extending outward from both side end portions of the metal material sheet having a
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recessed cross-sectional shape, or an excess thickness portion by drawing.
[0029]
In this specification, the longitudinal direction is a direction in which a long member (for example, a member having a specific three-dimensional tubular portion) extends. The centroid line refers to a line connecting the centroids of the minimum cross sections (that is, the cross sections orthogonal to the longitudinal direction) having the minimum cross-sectional area in each portion of the tubular portion in the longitudinal direction.
It is needless to say that the advancing and retreating directions of the press forming punch in the press forming step and the relative movement directions of a first recessed die and a second recessed die (dies constituting the O-forming die) in the O-forming step do not need to be directions orthogonal to the centroid line.
[0030]
The cross-sectional shape of the tubular portion is the shape of a cross section orthogonal to the centroid line (longitudinal direction) of the tubular portion. The circumferential length changing portion refers to a portion in which the circumferential length defined as being orthogonal to the centroid line in the tubular portion (that is, the length of an outer circumferential circle of a cross section orthogonal to the centroid line (longitudinal direction)) changes along the centroid line, and can be specified by a change in the circumferential length between any two points along the centroid line.
In addition, in the circumferential length changing portion, (the percentage of) a numerical value obtained by dividing the difference between the circumferential lengths at any two points set along the centroid line by the length between the two points along the centroid line is referred to as a circumferential length change rate.
The circumferential length change rate changing portion refers to a portion in
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which the circumferential length change rate changes along the centroid line.
[0031]
Here, the flangeless forming step may be performed by further advancing the press forming punch after forming the recessed cross-sectional shape portion and the extending portion in the press forming step.
[0032]
According to this aspect, since the flangeless forming step is performed by further advancing the press forming punch after forming the recessed cross-sectional shape portion and the extending portion in the press forming step, the extending portion necessary for pressing the metal material sheet can be formed into the flangeless press-formed article without taking the press-formed article out of the U-forming die and punch.
As a result, it is not necessary to provide a step of trimming the extending portion necessary for pressing the metal material sheet, and it is possible to efficiently form a flangeless formed article, thereby improving productivity.
[0033]
In addition, a counter of the press forming recessed die may be caused to advance and retreat along the advancing and retreating directions of the press forming punch with respect to the press forming recessed part.
[0034]
Here, the press forming recessed die may be provided with the counter.
[0035]
According to this aspect, since the press-formed article can be sandwiched between the counter of the press forming recessed die and the press forming punch, the press-formed article can be efficiently manufactured.
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[0036]
In addition, in the cross-sectional shape changing portion, a change in the length in a direction including the centroid line of the cross-sectional shape (the shape of a cross section orthogonal to the centroid line of the tubular portion) and an abutting portion (that is, the length of a line segment that passes through the centroid of the cross section and the abutting portion and intersects the outer circumference of the cross section) along the centroid line may be 10% to 50%.
[0037]
Here, the die and punch may be designed so that the cross-sectional shape changing portion has the above-mentioned properties.
[0038]
According to this aspect, the cross-sectional shape changing portion is a cross-sectional shape changing portion in which the change in the length in the direction (= press forming direction) including the centroid line of the cross-sectional shape and an abutting portion along the centroid line is 10% to 50%, and a member including a cross-sectional shape changing portion that is not easily formed can be efficiently manufactured.
[0039]
Here, the cross-sectional shape changing portion refers to a portion in which the cross-sectional shape orthogonal to the centroid line changes along the centroid line.
The change in the cross-sectional shape in the cross-sectional shape changing portion is represented by (the percentage of) a numerical value obtained by dividing the difference between the lengths in the direction including the centroid line of the cross-sectional shape and the abutting portion at any two points set along the centroid line by the length between the two points along the centroid line.
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[0040]
In addition, the change in the circumferential length change rate between a first end portion and a second end portion of the circumferential length change rate changing portion may be 0.035 mm"1 to 0.35 mm"1.
[0041]
Here, the die and punch may be designed so that the circumferential length change rate changing portion has the above-mentioned properties.
[0042]
According to this aspect, a member in which the change in the circumferential length change rate between the first end portion and the second end portion of the circumferential length change rate changing portion is set to 0.035 mm"1 to 0.35 mm"1 and thus forming is difficult can be efficiently manufactured.
[0043]
Here, the circumferential length change rate changing portion is a portion in which the circumferential length change rate of a tubular portion changes along the centroid line, and mathematically, refers to a portion in which the circumferential length change rate obtained by differentiating a circumferential length change amount along the centroid line changes along the centroid line.
In addition, the change (numerical value) in the circumferential length change rate between the first end portion and the second end portion of the circumferential length change rate changing portion is defined by a value (absolute value) obtained by dividing the difference in circumferential length change rate between the first end portion (start point) and the second end portion (end point) of the circumferential length change rate changing portion by the interval (length or dimension) along the centroid line between the first end portion and the second end portion.
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The circumferential length change rate can be calculated based on, for example, a curved surface shape measured by a shape measuring instrument or data measured by another measuring method. Other parameters related to the shape of the present invention can be measured by the same method.
[0044]
In addition, the curvature of the centroid line at the curved portion may be in a range of 0.002 mm"1 to 0.02 mm"1.
[0045]
Here, the die and punch may be designed so that the curved portion has the above-mentioned properties.
[0046]
According to this aspect, a member including a curved portion that includes a portion in which the curvature of the centroid line in the tubular portion is in a range of 0.002 mm"1 to 0.02 mm and thus cannot be easily formed can be efficiently manufactured.
[0047]
In addition, the forming condition ratio a may change along the centroid line of the specific three-dimensional tubular portion.
[0048]
Here, the die and punch may be designed so that the forming condition ratio a has the above-mentioned properties.
[0049]
According to this aspect, since the forming condition ratio a changes along the centroid line of the specific three-dimensional tubular portion, the side end portions can be accurately and efficiently brought into close contact with each other or approach a
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target position over the entire length of the abutting portions of the specific three-dimensional tubular portion.
[0050]
The manufacturing method of a member according to this aspect may further include: a forming condition ratio setting step performed in one or a plurality of cycles before the U-forming step, in which, in the forming condition ratio setting step, a finite element analysis is performed in consideration of conditions including the forming condition ratio a set in the forming condition ratio setting step in a previous cycle or an initial value of the forming condition ratio a, material properties of the member, a shape and a sheet thickness of the metal material sheet, forming conditions in the U-forming step, and forming conditions in the O-forming step to estimate a shape parameter including a strain amount generated in the U-forming step in a direction along the centroid line of the specific three-dimensional tubular portion, a strain amount generated in the O-forming step in the direction along the centroid line of the specific three-dimensional tubular portion, and relative positions of the side end portions, and the forming condition ratio setting step is repeated until the shape parameter satisfies desired conditions.
[0051]
Here, the die and punch may be designed based on the forming condition ratio a designed by the forming condition ratio setting step.
[0052]
According to this aspect, the finite element analysis is performed in consideration of the conditions including the forming condition ratio a, the material properties of the member, the shape and the sheet thickness of the metal material sheet, the forming conditions in the U-forming step, and the forming conditions in the O-
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forming step to estimate the shape parameter including the strain amount generated in the U-forming step in the direction along the centroid line of the specific three-dimensional tubular portion, the strain amount generated in the O-forming step in the direction along the centroid line of the specific three-dimensional tubular portion, and the relative positions of the side end portions. Since the forming condition ratio a is set based on this shape parameter, the accuracy of the forming condition ratio a can be further improved. Furthermore, the specific three-dimensional tubular portion can be formed more efficiently and stably.
[0053]
In this specification, the material properties of the member refer to the Young's modulus, the yield strength (proof stress), the relationship between stress and strain in a tensile test (stress-strain curve and the like), and the like of the material forming the member.
In addition, the shape and the sheet thickness of the metal material sheet refer to the shape of the metal material sheet and the sheet thickness of the metal material sheet formed to correspond to the member and the specific three-dimensional tubular portion.
The forming conditions in the U-forming step refer to, for example, the width Du of the U-forming punch (for example, the press forming punch or the bending forming punch), the shape of the forming recessed die (for example, the press forming recessed die or the bending forming recessed die), the forming load in the U-forming step or the displacement of the U-forming punch with respect to the forming recessed die in the U-forming step (relative positions of the forming recessed die and the U-forming punch), and the like.
In addition, the forming conditions in the O-forming step refer to the shape of
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the abutting portion forming recessed part of the O-forming die (including the recessed part width Do), and the forming load in the O-forming step, or the displacement of the second recessed die with respect to the first recessed die in the O-forming die (relative positions of the first recessed die and the second recessed die of the O-forming die).
Parameters that can be substituted for the above parameters may be used, or parameter other than the above parameters may be included. [Effects of the Invention]
[0054]
According to the manufacturing method of a member, the manufacturing method of a member for a vehicle, and the die and punch according to the above aspects of the present invention, it is possible to efficiently manufacture a member including a specific three-dimensional tubular portion including at least any one of a cross-sectional shape changing portion, a circumferential length change rate changing portion, or a curved portion in a tubular portion. [Brief Description of the Drawings]
[0055]
FIG. 1A is a view illustrating a first finding according to the present invention, and is a view showing an example of a first model according to the first finding.
FIG. IB is a view illustrating the first finding according to the present invention, and is a view showing an example of a metal material sheet according to the first model.
FIG. 1C is a view illustrating the first finding according to the present invention, and is a view showing an example of a press forming step according to the first model.
FIG. ID is a view illustrating the first finding according to the present
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invention, and is a conceptual view showing an example of a press-formed article having an extending portion during forming in the press forming step according to the first model.
FIG. IE is a view illustrating the first finding according to the present invention, and is a view of a cross section including a centroid line showing an example of the press forming step according to the first model.
FIG. IF is a view illustrating the first finding according to the present invention, and is a view showing an example of an abutting portion forming step according to the first model.
FIG. 1G is a view illustrating the first finding according to the present invention, and is a view of a cross section including a centroid line showing an example of an abutting portion formed article according to the first model.
FIG. 2 is a view conceptually showing an outline of the press forming step and the abutting portion forming step according to the present invention, and a press forming punch width Du and a recessed part width Do of an O-forming die (abutting portion forming die) constituting a forming condition ratio a.
FIG. 3A is a view illustrating a second finding according to the present invention, and is a view showing an example of a second model according to the second finding.
FIG. 3B is a view illustrating the second finding according to the present invention, and is a view showing an example of a metal material sheet according to the second model.
FIG. 3C is a view illustrating the second finding according to the present invention, and is a view showing an example of a press forming step according to the second model.
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FIG. 3D is a view illustrating the second finding according to the present invention, and is a view of a cross section including a centroid line showing an example of the press forming step according to the second model.
FIG. 3E is a view illustrating the second finding according to the present invention, and is a view showing an example of an abutting portion forming step according to the second model.
FIG. 3F is a view illustrating the second finding according to the present invention, and is a view of a cross section including a centroid line showing an example of an abutting portion formed article according to the second model.
FIG. 4A is a view illustrating a third finding according to the present invention, and is a view showing an example of the third model according to the third finding.
FIG. 4B is a view illustrating the third finding according to the present invention, and is a view showing an example of a metal material sheet according to the third model.
FIG. 4C is a view illustrating the third finding according to the present invention, and is a view showing an example of a press forming step according to the third model.
FIG. 4D is a view illustrating the third finding according to the present invention, and is a view of a cross section including a centroid line showing an example of the press forming step according to the third model.
FIG. 4E is a view illustrating the third finding according to the present invention, and is a view showing an example of an abutting portion forming step according to the third model.
FIG. 4F is a view illustrating the third finding according to the present invention, and is a view of a cross section including a centroid line showing an example
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of an abutting portion formed article according to the third model.
FIG. 5 is a view illustrating an example of a torsion beam Assy according to a first embodiment of the present invention.
FIG. 6 is a view illustrating a trailing arm according to the first embodiment of the present invention, and is a perspective view showing a railing arm body.
FIG. 7A is a view illustrating the trailing arm body according to the first embodiment, and is a view of the trailing arm body viewed from a punch side in a press forming direction.
FIG. 7B is a view illustrating the trailing arm body according to the first embodiment, and is a view of the trailing arm body viewed from a side orthogonal to the press forming direction.
FIG. 7C is a view illustrating the trailing arm body according to the first embodiment, and is a front side closed cross section of a trailing arm body 100 taken along the arrows VIIC-VIIC in FIG 7B.
FIG. 7D is a view illustrating the trailing arm body according to the first embodiment, and is a rear side closed cross section of the trailing arm body 100 taken along the arrows VIID-VIID in FIG 7B.
FIG. 8 is a flowchart illustrating an example of an outline of a manufacturing process of a member according to the present invention.
FIG. 9 is a flowchart illustrating a manufacturing process of the trailing arm body according to the first embodiment of the present invention.
FIG. 10 is a view illustrating a schematic configuration of a material steel sheet for manufacturing the trailing arm body according to the first embodiment of the present invention.
FIG. 11A is a view illustrating a press forming step in the manufacturing of the
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trailing arm body according to the first embodiment, and is a perspective view showing a schematic configuration of a press forming die and punch.
FIG. 1 IB is a view illustrating the press forming step in the manufacturing of the trailing arm body according to the first embodiment, and is a view of a press-formed article with an excess thickness portion formed in the press forming step viewed from a side opposite to a press forming punch in a press forming direction.
FIG. 11C is a view illustrating the press forming step in the manufacturing of the trailing arm body according to the first embodiment, and is a view of the press-formed article with an excess thickness portion formed in the press forming step viewed from a side orthogonal to the press forming direction.
FIG. 12A is a view illustrating an outline of a trimming step in the manufacturing of the trailing arm body according to the first embodiment, and is a perspective view showing a schematic configuration of a trimming die.
FIG. 12B is a view illustrating the outline of the trimming step in the manufacturing of the trailing arm body according to the first embodiment, and is a view of a flangeless press-formed article viewed from a side to the press forming punch in the press forming direction.
FIG. 12C is a view illustrating the outline of the trimming step in the manufacturing of the trailing arm body according to the first embodiment, and is a view of the flangeless press-formed article viewed from a side orthogonal to the press forming direction.
FIG. 13 A is a view illustrating an outline of an abutting portion forming step in the manufacturing of the trailing arm body according to the first embodiment, and is a perspective view showing a schematic configuration of an O-forming die.
FIG. 13B is a view illustrating the outline of the abutting portion forming step
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in the manufacturing of the trailing arm body according to the first embodiment, and is a view of an abutting portion formed article viewed from an abutting portion side.
FIG. 13C is a view illustrating the outline of the abutting portion forming step in the manufacturing of the trailing arm body according to the first embodiment, and is a view of the abutting portion formed article viewed from a side orthogonal to a forming direction during abutting portion forming.
FIG. 14 is a view illustrating an outline of a forming condition ratio a (= Du/Do) in a die and punch according to the first embodiment of the present invention.
FIG. 15 A is a view illustrating an outline of a manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of a state in which a material steel sheet is placed in a press forming die and punch, viewed from a front side of the trailing arm body.
FIG. 15B is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of a state in which the material steel sheet is pressed against a press forming recessed die by a press forming punch and press-formed, and a press-formed article with an excess thickness portion, viewed from the front side of the trailing arm body.
FIG. 15C is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of a state in which the press-formed article with an excess thickness portion is trimmed and a flangeless press-formed article after the trimming, viewed from the front side of the trailing arm body.
FIG. 15D is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a flangeless press-formed article.
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FIG. 15E is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view a state in which the flangeless press-formed article is formed into an abutting portion formed article by an O-forming die, viewed from the front side of the trailing arm body.
FIG. 15F is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of a state in which the abutting portion formed article is formed by the O-forming die and the formed abutting portion formed article, viewed from the front side of the trailing arm body.
FIG. 15G is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the abutting portion formed article viewed from the front side of the trailing arm body.
FIG. 16A is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the state in which the material steel sheet is placed in the press forming die and punch, viewed from a rear side of the trailing arm body.
FIG. 16B is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the state in which the material steel sheet is pressed against the press forming recessed die by the press forming punch and press-formed, and the press-formed article with an excess thickness portion, viewed from the rear side of the trailing arm body.
FIG. 16C is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a
- 24 -

view of the state in which the press-formed article with an excess thickness portion is trimmed and the flangeless press-formed article after the trimming, viewed from the rear side of the trailing arm body.
FIG. 16D is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the state in which the flangeless press-formed article is disposed in the O-forming die, viewed from the rear side of the trailing arm body.
FIG. 16E is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the state in which the flangeless press-formed article is formed into the abutting portion formed article by the O-forming die, viewed from the rear side of the trailing arm body.
FIG. 16F is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the state in which the abutting portion formed article is formed by the O-forming die and the formed abutting portion formed article, viewed from the rear side of the trailing arm body.
FIG. 16G is a view illustrating the outline of the manufacturing method of a trailing arm body according to the first embodiment of the present invention, and is a view of the abutting portion formed article viewed from the rear side of the trailing arm body.
FIG. 17A is a view illustrating an outline of a manufacturing method of a trailing arm body according to a second embodiment of the present invention, and is a view of a state in which a material steel sheet is pressed by a steel sheet pressing member in a press forming die and punch, viewed from the rear side of the trailing arm
- 25 -

body.
FIG. 17B is a view illustrating the outline of the manufacturing method of a trailing arm body according to the second embodiment of the present invention, and is a view of a state in which the material steel sheet is pressed against a press forming recessed die by a press forming punch in cooperation with a counter in the press forming die and punch and press-formed, viewed from the rear side of the trailing arm body.
FIG. 17C is a view illustrating the outline of the manufacturing method of a trailing arm body according to the second embodiment of the present invention, and is a view of a state in which a flangeless press-formed article is formed by completing the press forming, viewed from the rear side of the trailing arm body.
FIG. 18A is a view illustrating the first finding according to the present invention, and is a view showing an example of a first model according to the first finding.
FIG. 18B is a view illustrating the first finding according to the present invention, and is a view showing an example of a metal material sheet according to the first model.
FIG. 18C is a view illustrating the first finding according to the present invention, and is a view showing an example of a bending forming step according to the first model.
FIG. 18D is a view illustrating the first finding according to the present invention, and is a view of a cross section including a centroid line showing an example of a bending-formed article according to the first model.
FIG. 18E is a view illustrating the first finding according to the present invention, and is a view showing an example of an abutting portion forming step
- 26 -

according to the first model.
FIG. 18F is a view illustrating the first finding according to the present invention, and is a view of a cross section including a centroid line showing an example of an abutting portion formed article according to the first model.
FIG. 19 is a view conceptually illustrating an outline of the bending forming step and the abutting portion forming step according to the present invention, and a bending forming punch width Du and a recessed part width Do of an O-forming die constituting a forming condition ratio a.
FIG. 20A is a view illustrating the second finding according to the present invention, and is a view showing an example of a second model according to the second finding.
FIG. 20B is a view illustrating the second finding according to the present invention, and is a view showing an example of a metal material sheet according to the second model.
FIG. 20C is a view illustrating the second finding according to the present invention, and is a view showing an example of a bending forming step according to the second model.
FIG. 20D is a view illustrating the second finding according to the present invention, and is a view of a cross section including a centroid line showing an example of a bending-formed article according to the second model.
FIG. 20E is a view illustrating the second finding according to the present invention, and is a view showing an example of an abutting portion forming step according to the second model.
FIG. 20F is a view illustrating the second finding according to the present invention, and is a view of a cross section including a centroid line showing an example
- 27 -

of an abutting portion formed article according to the second model.
FIG. 21A is a view illustrating the third finding according to the present invention, and is a view showing an example of a third model according to the third finding.
FIG. 21B is a view illustrating the third finding according to the present invention, and is a view showing an example of a metal material sheet according to the third model.
FIG. 21C is a view illustrating the third finding according to the present invention, and is a view showing an example of a bending forming step according to the third model.
FIG. 21D is a view illustrating the third finding according to the present invention, and is a view of a cross section including a centroid line showing an example of a bending-formed article according to the third model.
FIG. 21E is a view illustrating the third finding according to the present invention, and is a view showing an example of an abutting portion forming step according to the third model.
FIG. 21F is a view illustrating the third finding according to the present invention, and is a view of a cross section including a centroid line showing an example of an abutting portion formed article according to the third model.
FIG. 22 is a flowchart illustrating an example of an outline of a manufacturing process of a member according to the present invention.
FIG. 23 is a flowchart illustrating a manufacturing process of a trailing arm body according to an embodiment of the present invention.
FIG. 24 is a view illustrating a schematic configuration of a material steel sheet for manufacturing the trailing arm body according to the embodiment of the present
- 28 -

invention.
FIG. 25 A is a view illustrating a bending forming step in the manufacturing of the trailing arm body according to the embodiment, and is a perspective view showing a schematic configuration of a bending forming die and punch.
FIG. 25B is a view illustrating an outline of a bending-formed article formed in the bending forming step according to the embodiment, and is a view of the bending-formed article viewed from a side opposite to a bending forming punch in a bending forming direction.
FIG. 25C is a view illustrating an outline of the bending-formed article formed in the bending forming step according to the embodiment, and is a view of the bending-formed article viewed from a side orthogonal to the bending forming direction.
FIG. 26A is a view illustrating an outline of an abutting portion forming step in the manufacturing of the trailing arm body according to the embodiment, and is a perspective view showing a schematic configuration of an O-forming die.
FIG. 26B is a view illustrating an outline of an abutting portion formed article connected in the abutting portion forming step according to the embodiment, and is a view of the abutting portion formed article viewed from an abutting portion side.
FIG. 26C is a view illustrating an outline of the abutting portion formed article subjected to abutting in the abutting portion forming step according to the embodiment, and is a view of the abutting portion formed article viewed from a side orthogonal to a forming direction during abutting portion forming.
FIG. 27 is a view illustrating an outline of a forming condition ratio a (= Du/Do) in a die and punch according to the embodiment of the present invention.
FIG. 28A is a view illustrating an outline of a manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view
- 29 -

of a state in which the material steel sheet is placed in the bending forming die and punch, viewed from the front side of the trailing arm body.
FIG. 28B is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of a state in which the material steel sheet is pressed against a bending forming recessed die by a bending forming punch and bending-formed, and a bending-formed article, viewed from the front side of the trailing arm body.
FIG. 28C is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of a state in which the bending-formed article is disposed in the O-forming die, viewed from the front side of the trailing arm body.
FIG. 28D is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of a state in which the bending-formed article is formed into an abutting portion formed article having a closed cross section by the O-forming die, viewed from the front side of the trailing arm body.
FIG. 28E is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of a state in which the abutting portion formed article is formed by the O-forming die and the formed abutting portion formed article, viewed from the front side of the trailing arm body.
FIG. 28F is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of the trailing arm body formed by joining abutting portions of the abutting portion formed article, viewed from the front side.
- 30 -

FIG. 29 A is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of the state in which the material steel sheet is placed in the bending forming die and punch, viewed from the rear side of the trailing arm body.
FIG. 29B is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of the state in which the material steel sheet is pressed against the bending forming recessed die by the bending forming punch and bending-formed, and the bending-formed article, viewed from the rear side of the trailing arm body.
FIG. 29C is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of the state in which the bending-formed article is disposed in the O-forming die, viewed from the rear side of the trailing arm body.
FIG. 29D is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of the state in which the bending-formed article is formed into the abutting portion formed article having a closed cross section by the O-forming die, viewed from the rear side of the trailing arm body.
FIG. 29E is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view of the state in which the abutting portion formed article is formed by the O-forming die and the formed abutting portion formed article, viewed from the rear side of the trailing arm body.
FIG. 29F is a view illustrating the outline of the manufacturing method of a trailing arm body according to the embodiment of the present invention, and is a view
- 31 -

of the trailing arm body formed by joining the abutting portions of the abutting portion formed article, viewed from the rear side. [Embodiments of the Invention]
[0056]
The inventors of the present invention intensively studied a technique of efficiently manufacturing a member applicable to a member for a vehicle or the like having a tubular portion as shown in (1) to (3) below by applying a tubular portion forming method for forming a tubular portion from a metal material sheet. (1) A circumferential length change rate changing portion in which the change rate of the peripheral length changes in a circumferential length changing portion in which the circumferential length of a cross section orthogonal to a centroid line (cross section orthogonal to a longitudinal direction) changes along the centroid line, (2) a cross-sectional shape changing portion in which the shape of the cross section orthogonal to the centroid line changes along the centroid line, and (3) a curved portion in which the centroid line has a curvature.
As a result, the following first to sixth findings were obtained. In the present embodiment, the numerical range represented using "to" means the range including the numerical values before and after "to" as the lower limit and the upper limit.
[0057]
[First Finding]
The first finding is a finding regarding the circumferential length change rate changing portion.
Hereinafter, the first finding of the present invention will be described with reference to FIGS. 1A to IF and FIG. 2. FIGS. 1A to IF and FIG. 2 are views illustrating the first finding according to the present invention. In FIGS. ID, IE, and
- 32 -

IG, the arrows facing each other indicate compressive strain, and the arrows pointing to the opposite sides indicate tensile strain.
The first finding is, for example, as shown in FIG. 1 A, an example of a member model (hereinafter, referred to as a first model) Ml00 having a closed cross section that is circular when viewed in an axis (centroid line) direction and including a conical shape portion Ml01 in which the circumferential length of the closed cross section gradually changes at a constant change rate along the centroid line, a straight portion M102 having a circular cross section connected to the small-diameter side of the conical shape portion Ml01, and a circumferential length change rate changing portion Ml03 that is formed at the connection portion between the conical shape portion Ml01 and the straight portion Ml02 and changes in circumferential length change rate.
[0058]
The first model Ml00 is formed by forming a metal material sheet W100 as shown in FIG. IB in order of a press forming step, a flangeless forming step, and an O-forming step (abutting portion forming step). In the O-forming step, for example, a flangeless press-formed article (U-formed article) having a side end portion formed in the press forming step is used. Here, an example in which the first model Ml00 is formed by the press forming (drawing) will be described, but the same finding is established even in a case where the first model Ml00 is formed by bending forming. The bending forming will be described later.
The metal material sheet W100 includes a fan-shaped portion W101 corresponding to the conical shape portion Ml01, a rectangular portion W102 corresponding to the straight portion Ml02, and a connection portion W103 corresponding to the circumferential length change rate changing portion Ml03.
[0059]
- 33 -

In the press forming step, as shown in FIG. 1C, a press-formed article is formed using a press forming die and punch (U-forming die and punch) DUO including a press forming recessed die Dl 11, a press forming punch Dl 12, a metal material sheet pressing tool Dl 13. The left figure of FIG. 1C is a side view of the press forming die and punch DUO and the metal material sheet W100 set in the press forming die and punch DUO, and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
[0060]
In the press forming recessed die Dl 11, a press forming recessed part Dl 11A having a lower forming shape portion corresponding to the final shape of the first model Ml00 in a range of approximately 180° (lower half) with respect to the centroid line when viewed in the centroid line direction of the first model Ml00 and an upper forming shape portion that is connected to the lower forming shape portion and is formed to extend upward from the upper end of the lower forming shape portion is formed.
In addition, in the press forming punch Dl 12, a press forming protrusion Dl 12A configured to correspond to the inner circumferential surface of the press-formed article at a predetermined interval from the press forming recessed part Dl 11A is formed.
[0061]
Regarding the metal material sheet pressing tool Dl 13, the metal material sheet W100 disposed across the press forming recessed part Dl 11A is pressed against the press forming recessed die Dl 11 on both outer sides of the press forming recessed part Dl 11A by the metal material sheet pressing tool Dl 13 during press forming.
[0062]
- 34 -

Next, the press forming punch Dl 12 advances (is lowered) to insert the metal material sheet W100 into the press forming recessed die Dill. At this time, in the portion of the metal material sheet W100 pressed by the metal material sheet pressing tool Dl 13, a tensile force is applied to the formed portion.
[0063]
As a result, as shown in FIG. ID, a member (an intermediate article of the press-formed article) Ml 10A having a recessed shape portion (recessed cross-sectional shape portion) formed to be open to the press forming punch Dl 12 side and be recessed toward the press forming recessed die Dl 11 side and having an extending portion Ml 10B extending outward from the press forming recessed part Dl 11A is formed in the press forming recessed die Dill. The left figure of FIG. ID is a side view of the member Ml 10A, and the right figure is a right side view of the member Ml 10A (a figure of the structure of the left figure viewed from the right side).
[0064]
Next, the press forming punch Dl 12 further advances (is lowered) (the flangeless forming step) to insert the extending portion Mil0B into the press forming recessed die Dill, whereby a flangeless press-formed article Ml 10 having no extending portion as shown in FIG. IE is formed. The flangeless press-formed article Ml 10 has a portion Ml 11 corresponding to the conical shape portion Ml01, a portion Ml 12 corresponding to the straight portion M102, and a portion Ml 13 corresponding to the circumferential length change rate changing portion Ml03. In this example, the flangeless forming step is a part of the press forming step.
In the press forming step, as shown in FIGS. ID and IE, compressive strain is generated in the portion Ml 13 corresponding to the circumferential length change rate changing portion Ml03. In the flangeless forming step, the extending portion may be
- 35 -

inserted into the press forming recessed die described above, or the extending portion may be trimmed as shown in a first embodiment, which will be described later.
[0065]
In the O-forming step, as shown in FIG. IF, the O-forming step is performed using an O-forming die D120. Specifically, by a lower die (first recessed die) D121 in which a lower die recessed part D121A is formed and an upper die (second recessed die) D122 in which an upper die recessed part D122A that side end portions Ml 10E of abutting scheduled portions are to follow is formed, the respective side end portions Ml 10E of the abutting scheduled portions of the flangeless press-formed article Ml 10 are caused to abut each other to form abutting portions M100C, whereby the first model Ml00 is formed. The left figure of FIG. IF is a side view of the lower die D121, the upper die D122, and the flangeless press-formed article MHO disposed therebetween, and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
In the abutting portion forming step, as shown in FIG. 1G, tensile strain is generated in the abutting portions M100C of the circumferential length change rate changing portion Ml03.
[0066]
Here, the change in circumferential length change rate between a first end portion and a second end portion of the circumferential length change rate changing portion M103 may be 0.035 to 0.35. Here, the first end portion may be one end portion of the circumferential length change rate changing portion Ml03 in the longitudinal direction, the second end portion may be the other end portion. The change (numerical value) in circumferential length change rate between the first end portion and the second end portion of the circumferential length change rate changing
- 36 -

portion Ml03 is defined by a value (absolute value) obtained by dividing the difference in circumferential length change rate between the first end portion (start point) and the second end portion (end point) of the circumferential length change rate changing portion by the interval (length or dimension) along the centroid line between the first end portion and the second end portion. The circumferential length change rate can be calculated based on, for example, a curved surface shape measured by a shape measuring instrument or data measured by another measuring method.
[0067]
Next, the concept of a forming condition ratio a and a method for calculating the forming condition ratio a will be described with reference to FIG. 2.
FIG. 2 is a view conceptually showing an outline of the press forming step according to the present invention, an outline of the O-forming step, and a press forming punch width Du and a recessed part width (the width of an abutting portion forming recessed part) Do of the O-forming die constituting the forming condition ratio a.
[0068]
As shown in FIG. 2, regarding the formation of the tubular portion according to the present finding, in the press forming step, the metal material sheet is pressed against the press forming recessed die by the metal material pressing tool and is press-formed into the press-formed article (press-formed portion) by the press forming punch. Thereafter, the press-formed article is trimmed to obtain the flangeless press-formed article. Then, in the O-forming step, the side end portions of the flangeless press-formed article are formed to follow the recessed part of the O-forming die (the first recessed die and the second recessed die), whereby the side end portions (both end portions in a cross section, that is, abutting scheduled portions) of the flangeless press-
- 37 -

formed article (side end portion forming press-formed portion) abut each other.
[0069]
As shown in FIG. 2, the forming condition ratio a is a numerical value defined by the ratio (Du/Do) of the width Du of the press forming protrusion of the press forming punch that presses the metal material against the press forming recessed die in the press forming die and punch in which press forming is performed in the press forming step to the recessed part width Do of the abutting portion forming recessed part of the O-forming die (the first recessed die and the second recessed die) used in the O-forming step.
[0070]
In this case, the forming condition ratio a is set to 0.85 to 0.95 (fourth finding). By setting the forming condition ratio a to a value within such a range, the abutting scheduled portions of the circumferential length change rate changing portion Ml03 can be brought into close contact with each other at high accuracy during the O-forming step, and furthermore, the first model Ml00 having the circumferential length change rate changing portion Ml03 can be efficiently manufactured. The forming condition ratio a of the die and punch corresponding to at least the specific three-dimensional tubular portion (here, the circumferential length change rate changing portion Ml03) may be 0.85 to 0.95. The forming condition ratio a may be 0.85 to 0.95 over the entire area of the die and punch. The same applies to other findings and embodiments described later.
[0071]
In a case where the forming condition ratio a is smaller than 0.85, the strain generated in the press forming step and the O-forming step becomes excessive, and the springback of the circumferential length change rate changing portion Ml03 becomes
- 38 -

excessive. Therefore, the closed cross section (cross section orthogonal to the longitudinal direction) of the circumferential length change rate changing portion Ml03 has an elongated oval shape that significantly deviates from the target shape (in this case, a true circle), resulting in a decrease in circularity. In a case where the forming condition ratio a exceeds 0.95, the side end portions of the circumferential length change rate changing portion Ml03 formed in the O-forming step are not sufficiently brought into close contact with each other or do not approach each other.
[0072]
The first model Ml00 including the circumferential length change rate changing portion Ml03 can take various shapes. Therefore, depending on the shape of the first model Ml00, there is a probability that the suitable range of the forming condition ratio a may exist within the range of 0.85 to 0.95. In order to find such a suitable forming condition ratio a, the following processing may be performed prior to the U-forming step (here, the press forming step).
[0073]
That is, prior to the U-forming step, a forming condition ratio setting step is performed in one or a plurality of cycles to set a suitable forming condition ratio a. Here, in the forming condition ratio setting step, a finite element analysis is performed in consideration of conditions including the forming condition ratio a (initial value of the preset forming condition ratio a in the case of the first cycle) set in the forming condition ratio setting step of the previous cycle, the material properties of the member, the shape and sheet thickness of the metal material sheet, the forming conditions in the U-forming step, and the forming conditions in the O-forming step. Accordingly, shape parameters including the amount of strain in the direction along the centroid line of the specific three-dimensional tubular portion (here, the circumferential length change rate
- 39 -

changing portion Ml03) generated by the U-forming step, the amount of strain in the direction along the centroid line of the specific three-dimensional tubular portion (here, the circumferential length change rate changing portion Ml03) generated by the O-forming step, and the relative positions of the side end portions (the abutting scheduled portions) are estimated.
[0074]
Here, the material properties of the member refer to the Young's modulus, the yield strength (proof stress), the relationship between stress and strain in a tensile test (stress-strain curve and the like), and the like of the material forming the member.
In addition, the shape and sheet thickness of the metal material sheet refer to the shape of the metal material sheet and the sheet thickness of the metal material sheet formed to correspond to the member and the specific three-dimensional tubular portion (here, the circumferential length change rate changing portion Ml03).
The forming conditions in the U-forming step (here, the press forming step) refer to, for example, the width Du of the U-forming punch (here, the press forming punch), the shape of the forming recessed die (here, the press forming recessed die), the forming load in the U-forming step or the displacement of the U-forming punch with respect to the forming recessed die in the U-forming step (relative positions of the press forming recessed die and the press forming punch), and the like.
In addition, the forming conditions in the O-forming step refer to the shape of the abutting portion forming recessed part of the O-forming die (including the recessed part width Do), and the forming load in the O-forming step, or the displacement of the second recessed die with respect to the first recessed die in the O-forming die (relative positions of the first recessed die and the second recessed die of the O-forming die). As a matter of course, parameters other than the above may be considered.
- 40 -

[0075]
Then, the forming condition ratio setting step is repeated until the shape parameters estimated above satisfy the desired conditions. Here, the desired conditions can be variously set according to the properties (strength, dimensional accuracy, and the like) required for the specific three-dimensional tubular portion. In any case, in a case of manufacturing a member having a specific three-dimensional tubular portion, by setting the forming condition ratio a within the range of 0.85 to 0.95, the abutting scheduled portions of the specific three-dimensional tubular portion can be brought into close contact with each other at high accuracy, and furthermore, the member having the specific three-dimensional tubular portion can be efficiently manufactured.
[0076]
The first finding is also applied to bending forming. Hereinafter, an example in which the first finding of the present invention is applied to bending forming will be described with reference to FIGS. 18A to 18F and FIG. 19. FIGS. 18A to 18F and FIG. 19 are views illustrating the example in which the first finding according to the present invention is applied to bending forming. In FIGS. 18D and 18F, the arrows facing each other indicate compressive strain, and the arrows pointing to the opposite sides indicate tensile strain.
The first finding is, for example, as shown in FIG. 18A, an example of a member model (hereinafter, referred to as a first model) Ml00' having a closed cross section that is circular when viewed in an axis (centroid line) direction and including a conical shape portion Ml01' in which the circumferential length of the closed cross section gradually changes at a constant change rate along the centroid line, a straight portion M102' having a circular cross section connected to the small-diameter side of
- 41 -

the conical shape portion M101', and a circumferential length change rate changing portion Ml03' that is formed at the connection portion between the conical shape portion Ml01' and the straight portion Ml02' and changes in circumferential length change rate.
[0077]
The first model Ml00' is formed by forming a metal material sheet W100 as shown in FIG. 18B in order of a bending forming step and an O-forming step (abutting portion forming step).
The metal material sheet W100' includes a fan-shaped portion W101' corresponding to the conical shape portion M101', a rectangular portion W102' corresponding to the straight portion Ml02', and a connection portion W103' corresponding to the circumferential length change rate changing portion Ml03'.
[0078]
In the bending forming step, as shown in FIG. 18C, a bending-formed article Ml 10' is formed using a bending forming die and punch (U-forming die and punch) DUO' including a bending forming recessed die Dl 11' and a bending forming punch Dl 12'. The left figure of FIG. 18C is a side view of the bending forming die and punch DUO' and the metal material sheet W100' set in the bending forming die and punch DUO', and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
[0079]
In the bending forming recessed die Dl 11', a bending forming recessed part Dl 11 A' having a lower forming shape portion corresponding to the final shape of the first model M100' in a range of approximately 180° (lower half) with respect to the centroid line when viewed in the centroid line direction of the first model Ml00' and an
- 42 -

upper forming shape portion that is connected to the lower forming shape portion and is formed to extend upward from the upper end of the lower forming shape portion is formed.
In addition, in the bending forming punch Dl 12', a bending forming protrusion Dl 12A' configured to correspond to the inner circumferential surface of the bending-formed article Ml 10' at a predetermined interval from the bending forming recessed part Dl 11 A' is formed.
[0080]
Next, the bending forming punch Dl 12' advances (is lowered) to insert the metal material sheet W100' placed on the bending forming recessed part Dl 11 A' into the bending forming recessed die Dill'.
As a result, as shown in FIG. 18D, a member (bending-formed article) Ml 10' having a recessed shape portion formed to be open to the bending forming punch Dl 12' side and be recessed toward the bending forming recessed die Dl 11' side is formed.
In the bending forming step, compressive strain as shown in FIG. 18D is generated on the recessed shape portion of a portion Ml 13' corresponding to the circumferential length change rate changing portion Ml03'. The bending-formed article MHO' has a portion Mill' corresponding to the conical shape portion M101', a portion Ml 12' corresponding to the straight portion M102', and a portion Ml 13' corresponding to the circumferential length change rate changing portion Ml03'.
[0081]
In the O-forming step, as shown in FIG. 18E, the O-forming step is performed using an O-forming die D120'. Specifically, by a lower die (first recessed die) D121' in which a lower die recessed part D121 A' is formed and an upper die (second recessed die) D122' in which an upper die recessed part D122A' that side end portions Ml 10E'
- 43 -

of abutting scheduled portions are to follow is formed, the respective side end portions M110E' of the abutting scheduled portions of the bending-formed article MHO' are caused to abut each other to form abutting portions M100C, whereby the first model Ml00' is formed. The left figure of FIG. 18E is a side view of the lower die D121', the upper die D122', and the bending-formed article Ml 10' disposed therebetween, and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
In the O-forming step, tensile strain as shown in FIG. 18F is generated in the abutting portions M100C of the circumferential length change rate changing portion M103.
[0082]
Next, the concept of a forming condition ratio a and a method for calculating the forming condition ratio a will be described with reference to FIG. 19A.
FIG. 19 is a view conceptually showing an outline of the bending forming step according to the present invention, an outline of the O-forming step, and a bending forming punch width Du and a recessed part width (the width of an abutting portion forming recessed part) Do of the O-forming die constituting the forming condition ratio a.
[0083]
As shown in FIG. 19, regarding the formation of the tubular portion according to the present finding, the bending-formed article (bending-formed portion) that has been bending-formed in the bending forming step is formed to follow the recessed parts of the O-forming die (the first recessed die and the second recessed die) in the O-forming step. Accordingly, the side end portions (both end portions in a cross section) of the bending-formed article (bending-formed portion) abut each other.
- 44 -

As shown in FIG. 19, the forming condition ratio a is a numerical value defined by the ratio (Du/Do) of the width Du of the bending forming protrusion of the bending forming punch that presses the metal material against the bending forming recessed die in the bending forming die and punch in which bending forming is performed in the bending forming step to the width Do of the abutting portion forming recessed part of the O-forming die (the first recessed die and the second recessed die) used in the O-forming step.
[0084]
As in the case of the press forming, the forming condition ratio a is set to 0.85 to 0.95 (fourth finding). By setting the forming condition ratio a to a value within such a range, the abutting scheduled portions of the circumferential length change rate changing portion Ml03' can be brought into close contact with each other at high accuracy during the O-forming step, and furthermore, the first model Ml00' having the circumferential length change rate changing portion Ml03' can be efficiently manufactured. The forming condition ratio a of the die and punch corresponding to the overall length or a part of at least the specific three-dimensional tubular portion (here, the circumferential length change rate changing portion Ml03') may be 0.85 to 0.95. The forming condition ratio a may be 0.85 to 0.95 over the entire area of the die and punch.
[0085]
In a case where the forming condition ratio a is smaller than 0.85, the strain generated in the bending forming step and the O-forming step becomes excessive, and the springback of the circumferential length change rate changing portion Ml03' becomes excessive. Therefore, the closed cross section (cross section orthogonal to the longitudinal direction) of the circumferential length change rate changing portion
- 45 -

Ml03' has an elongated oval shape that significantly deviates from the target shape (in this case, a true circle), resulting in a decrease in circularity. In a case where the forming condition ratio a exceeds 0.95, the side end portions of the circumferential length change rate changing portion Ml03' formed in the O-forming step are not sufficiently brought into close contact with each other or do not approach each other.
[0086]
The first model Ml00' including the circumferential length change rate changing portion Ml03' can take various shapes. Therefore, depending on the shape of the first model Ml00', there is a probability that the suitable range of the forming condition ratio a may exist within the range of 0.85 to 0.95. Here, as described above, a suitable forming condition ratio a may be found by the finite element analysis. The specific processing method is as described above.
[0087]
Here, the parameters to be considered include, for example, the material properties of the member, the shape and sheet thickness of the metal material sheet, the forming conditions in the bending forming step, the forming conditions in the O-forming step, and the like. Particularly, the material properties of the member, the shape and sheet thickness of the metal material sheet, and the forming conditions in the O-forming step are as described above. The forming conditions in the bending forming step refer to, for example, the width Du of the bending forming punch, the shape of the bending forming recessed die, the forming load in the bending forming step or the displacement of the bending forming punch with respect to the bending forming recessed die in the bending forming step (relative positions between the bending forming recessed die and the bending forming punch), and the like.
[0088]
- 46 -

[Second Finding]
The second finding is a finding regarding the cross-sectional shape changing portion.
Hereinafter, the second finding of the present invention will be described with reference to FIGS. 3A to 3D. FIGS. 3A to 3D are views illustrating the second finding according to the present invention. In FIGS. 3D and 3F, the arrows facing each other indicate compressive strain, and the arrows pointing to the opposite sides indicate tensile strain.
The second finding is, for example, as shown in FIG. 3A, an example of a member model (hereinafter, referred to as a second model) M200 including a rectangular closed cross section M201 that is rectangular when viewed in an axis (centroid line) direction, a circular closed cross section M202 that is circular, and a cross-sectional shape changing portion M203 that is formed between the circular closed cross section and the rectangular closed cross section and gradually changes in shape along the centroid line.
[0089]
The second model M200 is formed by forming a rectangular metal material sheet W200 as shown in FIG. 3B in order of a press forming step, a flangeless forming step, and an O-forming step (abutting portion forming step). Here, an example in which the second model M200 is formed by the press forming will be described, but the same finding is established even in a case where the second model M200 is formed by bending forming. The bending forming will be described later.
[0090]
In the press forming step, as shown in FIG. 3C, a press-formed article is formed using a press forming die and punch (U-forming die and punch) D210 including
- 47 -

a press forming recessed die D211, a press forming punch D212, a metal material sheet pressing tool D213. The left figure of FIG. 3C is a side view of the press forming die and punch D210 and the metal material sheet W200 set in the press forming die and punch D210, and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
[0091]
In the press forming recessed die D211, a press forming recessed part D211A having a lower forming shape portion corresponding to the final shape of the second model M200 in a range of approximately 180° (lower half) with respect to the centroid line when viewed in the centroid line direction of the second model M200 and an upper forming shape portion that is connected to the lower forming shape portion and is formed to extend upward from the upper end of the lower forming shape portion is formed. In addition, in the press forming punch D212, a press forming protrusion D212A configured to correspond to the inner circumferential surface of the press-formed article at a predetermined interval from the press forming recessed part D211A is formed.
[0092]
The metal material sheet pressing tool D213 presses the metal material sheet W200 against the press forming recessed die D211 on both sides across the press forming recessed part D211A during press forming.
[0093]
Then, the metal material sheet W200 is press-formed by the press forming punch D212 and the press forming recessed die D211, whereby a press-formed article that is open to the press forming punch D212 side and has a recessed shape portion on the press forming recessed die D211 side is formed.
- 48 -

In the press forming step, compressive and tensile strains as shown in FIG. 3D are generated on the recessed shape portion side of a portion M213 corresponding to the cross-sectional shape changing portion M203. Reference numeral M210 denotes a flangeless press-formed article. The flangeless press-formed article M210 includes a portion M211 corresponding to the rectangular closed cross section M201, a portion M212 corresponding to the circular closed cross section M202, and the portion M213 corresponding to the cross-sectional shape changing portion M203.
[0094]
In the O-forming step, as shown in FIG. 3E, the O-forming step is performed using an O-forming die D220. Specifically, by a lower die (first recessed die) D221 in which a lower die recessed part D221A is formed and an upper die (second recessed die) D222 in which an upper die recessed part D222A that side end portions M210E of abutting scheduled portions are to follow is formed, the respective side end portions M210E of the abutting scheduled portions of the flangeless press-formed article M210 are caused to abut each other to form abutting portions M200C, whereby the second model M200 is formed. The left figure of FIG. 3E is a side view of the lower die D221, the upper die D222, and the flangeless press-formed article M210 disposed therebetween, and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
In the O-forming step, compressive and tensile strains as shown in FIG. 3F are generated on the abutting portion M200C side of the cross-sectional shape changing portion M203.
[0095]
Here, a change in the length in the direction including the centroid line of the cross-sectional shape of the cross-sectional shape changing portion M203 (the shape of
- 49 -

the cross section orthogonal to the centroid line of the cross-sectional shape changing portion M203) and the abutting portion M200C (that is, the length of a line segment that passes through the centroid of the cross section and the abutting portion M200C and intersects the outer circumference of the cross section) along the centroid line may be 10% to 50%. The change in the cross-sectional shape in the cross-sectional shape changing portion M203 is represented by (the percentage of) a numerical value obtained by dividing the difference between the lengths in the direction including the centroid line of the cross-sectional shape and the abutting portion M200C at any two points set along the centroid line by the length between the two points along the centroid line.
[0096]
Even in such press forming, the forming condition ratio a is set to 0.85 to 0.95 (fourth finding). By setting the forming condition ratio a to a value within such a range, the abutting scheduled portions of the cross-sectional shape changing portion M203 can be brought into close contact with each other at high accuracy during the O-forming step, and furthermore, the second model M200 having the cross-sectional shape changing portion M203 can be efficiently manufactured. By performing the above-mentioned forming condition ratio setting step in one or a plurality of cycles, a preferable range of the forming condition ratio a may be found.
[0097]
The second finding is also applied to bending forming. Hereinafter, an example in which the second finding of the present invention is applied to bending forming will be described with reference to FIGS. 20A to 20D. FIGS. 20A to 20D are views illustrating the example in which the second finding according to the present invention is applied to bending forming. In FIGS. 20D and 20F, the arrows facing each other indicate compressive strain, and the arrows pointing to the opposite sides
- 50 -

indicate tensile strain.
The second finding is, for example, as shown in FIG. 20A, an example of a member model (hereinafter, referred to as a second model) M200' including a rectangular closed cross section M201' that is rectangular when viewed in an axis (centroid line) direction, a circular closed cross section M202' that is circular, and a cross-sectional shape changing portion M203' that is formed between the circular closed cross section and the rectangular closed cross section and gradually changes in shape along the centroid line.
[0098]
The second model M200' is formed by forming a rectangular metal material sheet W200' as shown in FIG. 20B in order of a bending forming step and an O-forming step (abutting portion forming step).
[0099]
In the bending forming step, as shown in FIG. 20C, a bending-formed article M210' is formed using a bending forming die and punch (U-forming die and punch) D210' including a bending forming recessed die D211' and a bending forming punch D212'. The left figure of FIG. 20C is a side view of the bending forming die and punch D210' and the metal material sheet W200' set in the bending forming die and punch D210', and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
[0100]
In the bending forming recessed die D211', a bending forming recessed part D211 A' having a lower forming shape portion corresponding to the final shape of the second model M200' in a range of approximately 180° (lower half) with respect to the centroid line when viewed in the centroid line direction of the second model M200' and
- 51 -

an upper forming shape portion that is connected to the lower forming shape portion and is formed to extend upward from the upper end of the lower forming shape portion is formed. In addition, in the bending forming punch D212', a bending forming protrusion D212A' configured to correspond to the inner circumferential surface of the bending-formed article M210' at a predetermined interval from the bending forming recessed part D211A' is formed.
[0101]
Then, the metal material sheet W200' is bending-formed by the bending forming punch D212' and the bending forming recessed die D211', whereby the bending-formed article M210' that is open to the bending forming punch D212' side and has a recessed shape portion on the bending forming recessed die D211' side is formed.
In the bending forming step, compressive and tensile strains as shown in FIG. 20D are generated on the recessed shape portion side of a portion M213' corresponding to the cross-sectional shape changing portion M203'. The flangeless press-formed article M210' includes a portion M211' corresponding to the rectangular closed cross section M201', a portion M212' corresponding to the circular closed cross section M202', and the portion M213' corresponding to the cross-sectional shape changing portion M203'.
[0102]
In the O-forming step, as shown in FIG. 20E, the O-forming step is performed using an O-forming die D220'. Specifically, by a lower die (first recessed die) D221 in which a lower die recessed part D221 A' is formed and an upper die (second recessed die) D222' in which an upper die recessed part D222A' that side end portions M210E' of abutting scheduled portions are to follow is formed, the respective side end portions
- 52 -

M210E' of the abutting scheduled portions of the bending-formed article M210' are caused to abut each other to form abutting portions M200C, whereby the second model M200' is formed. The left figure of FIG. 20E is a side view of the lower die D221', the upper die D222', and the bending-formed article M210' disposed therebetween, and the right figure is a right side view of these members (a figure of the structure of the left figure viewed from the right side surface).
In the O-forming step, compressive and tensile strains as shown in FIG. 20F are generated on the abutting portion M200C side of the cross-sectional shape changing portion M203'.
[0103]
Here, a change in the length in the direction including the centroid line of the cross-sectional shape of the cross-sectional shape changing portion M203' (the shape of the cross section orthogonal to the centroid line of the cross-sectional shape changing portion M203') and the abutting portion M200C (that is, the length of a line segment that passes through the centroid of the cross section and the abutting portion M200C and intersects the outer circumference of the cross section) along the centroid line may be 10% to 50%. The change in the cross-sectional shape in the cross-sectional shape changing portion M203' is represented by (the percentage of) a numerical value obtained by dividing the difference between the lengths in the direction including the centroid line of the cross-sectional shape and the abutting portion M200C at any two points set along the centroid line by the length between the two points along the centroid line.
[0104]
Even in such bending forming, the forming condition ratio a is set to 0.85 to 0.95 (fourth finding). By setting the forming condition ratio a to a value within such a
- 53 -

range, the abutting scheduled portions of the cross-sectional shape changing portion M203' can be brought into close contact with each other at high accuracy during the O-forming step, and furthermore, the second model M200' having the cross-sectional shape changing portion M203' can be efficiently manufactured. By performing the above-mentioned forming condition ratio setting step in one or a plurality of cycles, a preferable range of the forming condition ratio a may be found.
[0105]
[Third Finding]
The third finding is a finding regarding the curved portion.
Hereinafter, the third finding of the present invention will be described with reference to FIGS. 4A to 4D. FIGS. 4A to 4D are views illustrating the third finding according to the present invention. In FIGS. 4D and 4F, the arrows facing each other indicate compressive strain, and the arrows pointing to the opposite sides indicate tensile strain.
The third finding is, for example, as shown in FIG. 4A, an example of a member model (hereinafter, referred to as a third model) M300 including, for example, a curved portion having a circular closed cross section that is bent in a curved shape in the forming direction.
[0106]
The third model M300 is formed by forming a rectangular metal material sheet W300 as shown in FIG. 4B in order of a press forming step, a flangeless forming step, and an O-forming step (abutting portion forming step). Here, an example in which the third model M300 is formed by the press forming will be described, but the same finding is established even in a case where the third model M300 is formed by bending forming. The bending forming will be described later.
- 54 -

[0107]
In the press forming step, as shown in FIG. 4C, a press-formed article is formed using a press forming die and punch D (U-forming die and punch) 310 including a press forming recessed die D311, a press forming punch D312, a metal material sheet pressing tool D313. The left figure of FIG. 4C is a side view of the press forming die and punch D310 and the metal material sheet W300 set in the press forming die and punch D310, and the right figure is a cross-sectional view perpendicular to the longitudinal direction of these members.
[0108]
In the press forming recessed die D311, a press forming recessed part D311A having a lower forming shape portion corresponding to the final shape of the third model M300 in a range of approximately 180° (lower half) with respect to the centroid line when viewed in the centroid line direction of the third model M300 and an upper forming shape portion that is connected to the lower forming shape portion and is formed to extend upward from the upper end of the lower forming shape portion is formed. In addition, in the press forming punch D312, a press forming protrusion D312A configured to correspond to the inner circumferential surface of the press-formed article at a predetermined interval from the press forming recessed part D311A is formed.
[0109]
The metal material sheet pressing tool D313 presses the metal material sheet W300 against the press forming recessed die D311 on both sides across the press forming recessed part D311A during press forming.
[0110]
Then, the metal material sheet W300 is press-formed by the press forming
- 55 -

punch D312 and the press forming recessed die D311, whereby a press-formed article that is open to the press forming punch D312 side and has a recessed shape portion on the press forming recessed die D311 side is formed.
In the press forming step, compressive strain as shown in FIG. 4D is generated on the recessed shape portion side of a press-formed article M310. Reference numeral M310 denotes a flangeless press-formed article.
[0111]
In the O-forming step, as shown in FIG. 4E, the O-forming step is performed using an O-forming die D320. Specifically, by a lower die (first recessed die) D321 in which a lower die recessed part D321A is formed and an upper die (second recessed die) D322 in which an upper die recessed part D322A that side end portions M310E of abutting scheduled portions are to follow is formed, the respective side end portions M310E of the abutting scheduled portions of the flangeless press-formed article M310 are caused to abut each other to form abutting portions M300C, whereby the third model M300 is formed. The left figure of FIG. 4E is a side view of the lower die D321, the upper die D322, and the flangeless press-formed article M310 disposed therebetween, and the right figure is a cross-sectional view perpendicular to the longitudinal direction of these members.
In the O-forming step, compressive strain as shown in FIG. 4F is generated on the abutting portion M300C side.
[0112]
Here, the curvature of the centroid line at the curved portion may be in a range of 0.002 mm"1 to 0.02 mm"1.
[0113]
Even in such press forming, the forming condition ratio a is set to 0.85 to 0.95
- 56 -

(fourth finding). By setting the forming condition ratio a to a value within such a range, the abutting scheduled portions of the curved portion can be brought into close contact with each other at high accuracy during the O-forming step, and furthermore, the third model M300 having the curved portion can be efficiently manufactured. By performing the above-mentioned forming condition ratio setting step in one or a plurality of cycles, a preferable range of the forming condition ratio a may be found.
[0114]
The third finding is also applied to bending forming. Hereinafter, an example in which the third finding of the present invention is applied to bending forming will be described with reference to FIGS. 21A to 2ID. FIGS. 21A to 21D are views illustrating the example in which the third finding according to the present invention is applied to bending forming. In FIGS. 21D and 2IF, the arrows facing each other indicate compressive strain, and the arrows pointing to the opposite sides indicate tensile strain.
The third finding is, for example, as shown in FIG. 21 A, an example of a member model (hereinafter, referred to as a third model) M300' including, for example, a curved portion having a circular closed cross section that is bent in a curved shape in the forming direction.
[0115]
The third model M300' is formed by forming a rectangular metal material sheet W300' as shown in FIG. 21B in order of a bending forming step and an O-forming step (abutting portion forming step).
[0116]
In the bending forming step, as shown in FIG. 21C, a bending-formed article M310' is formed using a bending forming die and punch (U-forming die and punch)
- 57 -

D310' including a bending forming recessed die D311' and a bending forming punch D312'. The left figure of FIG. 21C is a side view of the bending forming die and punch D310' and the metal material sheet W300' set in the bending forming die and punch D310', and the right figure is a cross-sectional view perpendicular to the longitudinal direction of these members.
[0117]
In the bending forming recessed die D311', a bending forming recessed part D311A' having a lower forming shape portion corresponding to the final shape of the third model M300' in a range of approximately 180° (lower half) with respect to the centroid line when viewed in the centroid line direction of the third model M300' and an upper forming shape portion that is connected to the lower forming shape portion and is formed to extend upward from the upper end of the lower forming shape portion is formed. In addition, in the bending forming punch D312', a bending forming protrusion D312A' configured to correspond to the inner circumferential surface of the bending-formed article M310' at a predetermined interval from the bending forming recessed part D311A' is formed.
[0118]
Then, the metal material sheet W300' is bending-formed by the bending forming punch D312' and the bending forming recessed die D311', whereby the bending-formed article M310' that is open to the bending forming punch D312' side and has a recessed shape portion on the bending forming recessed die D311' side is formed.
In the bending forming step, compressive strain as shown in FIG. 21D is generated on the recessed shape portion side of the bending-formed article M310'.
[0119]
- 58 -

In the O-forming step, as shown in FIG. 21E, the O-forming step is performed using an O-forming die D320'. Specifically, by a lower die (first recessed die) D321' in which a lower die recessed part D321 A' is formed and an upper die (second recessed die) D322' in which an upper die recessed part D322A' that side end portions M310E' of abutting scheduled portions are to follow is formed, the respective side end portions M310E' of the abutting scheduled portions of the bending-formed article M310' are caused to abut each other to form abutting portions M300C, whereby the third model M300' is formed. The left figure of FIG. 21E is a side view of the lower die D321', the upper die D322', and the bending-formed article M310' disposed therebetween, and the right figure is a cross-sectional view perpendicular to the longitudinal direction of these members.
In the O-forming step, tensile and compressive strains as shown in FIG. 21F are generated on the abutting portion M300C side.
[0120]
Here, the curvature of the centroid line at the curved portion may be in a range of 0.002 mm"1 to 0.02 mm"1.
[0121]
Even in such bending forming, the forming condition ratio a is set to 0.85 to 0.95 (fourth finding). By setting the forming condition ratio a to a value within such a range, the abutting scheduled portions of the curved portion can be brought into close contact with each other at high accuracy during the O-forming step, and furthermore, the third model M300' having the curved portion can be efficiently manufactured. By performing the above-mentioned forming condition ratio setting step in one or a plurality of cycles, a preferable range of the forming condition ratio a may be found.
[0122]
- 59 -

[Fifth Finding]
The metal material (metal sheet) to which each of the above findings (and each embodiment described later) can be applied is not particularly limited, but may be, for example, a steel sheet. Examples of the steel sheet include a thin material (sheet thickness/equivalent diameter (diameter of a cross section perpendicular to the longitudinal direction of the tubular portion) is 10% or less) and a high tensile material (tensile strength (TS) is 300 MPa or more, and more preferably 400 MPa or more). In a case of using such a steel sheet, the springback increases. However, by setting the forming condition ratio a to 0.85 to 0.95, the springback can be appropriately suppressed. Examples of other kinds of metal sheet include an Al sheet. The thickness of the metal sheet is not particularly limited, but may be, for example, 1.0 to 2.9 mm.
[0123]
[Sixth Finding]
For the evaluation of the strain amount of the abutting scheduled portion, the calculation based on the geometrical relationship is effective, and for example, the analysis using a finite element method is particularly effective.
[0124]
As is clear from the above findings and examples described later, for example, in a case where all of the following conditions are satisfied when a steel sheet having a tensile strength of 300 to 600 MPa and a sheet thickness of 1.5 to 3.0 mm is subjected to UO forming into a specific three-dimensional tubular portion, it is possible to efficiently form (a member having) the specific three-dimensional tubular portion. • At least one or more of conditions that the change Rh in the length in the direction including the centroid line of the cross-sectional shape of the cross-sectional shape
- 60 -

changing portion and the abutting portion along the centroid line is 10% to 50%, the change Re in the circumferential length change rate of the circumferential length change rate changing portion is 0.035 mm"1 to 0.35 mm"1, and the curvature Rl of the curved portion is 0.002 mm"1 to 0.02 mm"1 is satisfied. • The forming condition ratio a is 0.85 to 0.95.
[0125]