Title of Invention: Press-Forming Method 5 Technical Field [0001] The present invention relates to a pressforming method which shapes high strength steel sheet to a final shaped article which has a bent part in a longitudinal direction. In particular, the present 10 invention relates to a press-forming method which suppresses warping and torsion of the final shaped article caused by residual stress. Background Art [0002] In recent years, from the viewpoint of 15 improving fuel economy and enhancing the collision safety of automobiles, high strength steel sheet or aluminum alloy with a high tensile strength has come to be used for frame parts in particular. A high tensile strength material can improve the collision performance without 20 increasing the sheet thickness of the material, so is useful for lightening the weight. [0003] However, due to the higher strength of materials, the warping and torsion of a final shaped article caused by residual stress at the time of press- 25 forming become larger and securing shape precision of the final shaped article becomes an issue. [0004] When shape precision of a final shaped article cannot be secured, a gap is formed with other parts when assembling the article in a vehicle. If the amount of the 30 gap is large, assembly problems result. Accordingly, strict shape precision is demanded from the final shaped article. Further, in the case of a part with a small curvature in a bent part of a final shaped article, that is, a radius of curvature of a bent part is 50 to 2000 35 mm, a high shape precision is particularly demanded. The shape of the bent part is an arc or a curve with a continuously changing curvature. If there are a plurality _ 2 - of such bent parts at the final shaped article, the warping and torsion in the longitudinal direction of the final shaped article which accompany planar stress of the final shaped article are large. For this reason, it is 5 further difficult to secure precision of the final shaped article. [0005] As a conventional general measure for countering poor shape precision, the method is adopted of using prototypes of the final shaped article or past 10 experience to predict the amount of springback and finishing the shape of the die to a shape different from the shape of the final shaped article so as to satisfy the predetermined dimensions. Further, in recent years, before making prototypes of the final shaped article, 15 springback and other aspects of the press-forming operation have been analyzed based on the final shape using the finite element method so as to make the die and thereby reduce the number of corrections to the die when making prototypes. 20 [0006] However, with designing a die based on trial and error, there was the problem that a long time is taken until devising a shape of a die which sufficiently reduces warping and torsion and until establishing shaping conditions. Further, since trial and error are 25 used to design the die, the cost of die correction soars and therefore there was the problem of reduction of cost of the final shaped article being obstructed. [0007] As a measure for improving the shape precision of the final shaped article, the art of imparting a bead 30 to the final shaped article so as to suppress warping and torsion of the final shaped article (PLT 1) has been disclosed. Further, the art of using the holding surfaces of a die and blank holder to locally press against a blank to form a bead at the blank and thereby increase 35 the tension of the vertical wall part so as to secure the shape precision of the final shaped article (PLT 2) has been disclosed. - 3 - [0008] The arts which are disclosed in PLT 1 and PLT 2 impart a bead to the final shaped article to improve the product shape to thereby suppress springback. Therefore, the shapes of the final shaped articles to which these 5 can be applied are limited. There is the problem that the arts are not universally applicable. [0009] PLT 3 discloses a press-forming method which improves the shape precision of a press-formed article which has a hat-shaped cross-section which comprises a 10 top sheet part, vertical wall parts, and flange parts. The press-forming method which is described in PLT 3 press-forms a metal sheet into an intermediate shaped article which has tapered parts between the vertical wall parts and flange parts, then again press-forms the 15 tapered parts and flange parts of the intermediate product to obtain the final shaped article. [0010] However, the press-forming method which is disclosed in PLT 3 raises the precision of the angles between the vertical wall parts and the flange parts at 20 the final shaped article and improves the flatness of the flange parts. It does not suppress warping or torsion of the final shaped article as a whole. [0011] PLT 4 discloses a press-forming method which improves the shape precision of a final shaped article 25 which comprises a top sheet part and vertical wall parts and which has a bent part. The press-forming method which is described in PLT 4 bends a metal sheet into an intermediate product which has bending angles of the top sheet part and vertical wall parts giving greater amounts 30 of bending than the final shaped article, then bends it back to the bending angles of the final shaped article. [0012] However, in the press-forming method of PLT 4, when the metal sheet is a soft steel sheet or other metal sheet with a tensile strength which is not that high, the 35 warping or torsion of the final shaped article could be suppressed, but when a high strength steel sheet or other metal sheet with a high tensile strength, warping or - 4 - torsion of the final shaped article cannot be suppressed. Further, when the final shaped article is provided with flange parts and has a cross-sectional shape of a hat shape, tensile stress easily remains at the flange part 5 at the inside of the bent part, so there is the problem that the warping and torsion of the final shaped article become further larger. Citations List Patent Literature 10 [0013] PLT 1. Japanese Patent Publication No. 2004- 25273A PLT 2. Japanese Patent Publication No. 11-290951A PLT 3. Japanese Patent Publication No. 2006-289480A PLT 4. Japanese Patent Publication No. 2004-195535A 15 Summary of Invention Technical Problem [0014] The present invention has as its object the provision of a press-forming method which can reduce the warping and torsion of a final shaped article which occur 20 due to the tensile stress which remains at the inside of a bent part when press-forming high strength steel sheet without formation of a bead at the final shaped article. Solution to Problem [0015] The inventors discovered that when press- 25 forming a high strength steel sheet to form a final shaped article which comprises a top sheet part, vertical wall parts, and flange parts and which has at least one bent part with a minimum radius of curvature of 50 to 2000 mm in the longitudinal direction, the following is 30 necessary to reduce the warping and torsion of the final shaped article. [0016] The present invention divides the press-forming operation into: 1) a first shaping process of bending a flange part at 35 an intersecting part until an angle of the flange part with a horizontal line becomes oti in a plane which includes a horizontal line which connects an intersecting - 5 - part of a vertical wall part and flange part and a center of curvature of the bent part and which is vertical to the high strength steel sheet and 2) a second shaping process of additionally bending the 5 flange part after the first shaping process at the intersecting part until the angle of the flange part with the horizontal line becomes ct2 in the plane. [0017] The fact that when, at this time, the additional bending angle P which is expressed by cti-a.2 is 10 in a predetermined range, warping and torsion of the final shaped article are reduced was discovered by the inventors. Further, the inventors discovered that even when using high strength steel sheet with a tensile strength of 440 to 4600 MPa where springback easily 15 occurs, by making the additional bending angle {3 a predetermined range, the amount of warping and the amount of torsion can be made the same extents as when using steel sheet with a tensile strength of less than 440 MPa. [0018] The present invention was made based on the 20 above discovery and has as its gist the following: [0019] (1) A press-forming method for press-forming a final shaped article comprising a top sheet part, vertical wall parts, and flange parts and having at least one bent part in a longitudinal direction, 25 the method comprising: a first shaping process in which high strength steel sheet with a tensile strength of 440 to 1600 MPa is used, a flange part is bent at an intersecting part until an angle of the flange part with a horizontal line 30 becomes a.\ in a plane which includes a horizontal line which connects an intersecting part of a vertical wall part and a flange part and a center of curvature of the bent part and which is vertical to said high strength steel sheet when forming the top sheet part, vertical 35 wall parts, bent part, and flange parts, and a second shaping process in which the flange - 6 - part after the first shaping process is additionally bent at the intersecting part until the angle of the flange part with the horizontal line becomes a.2 in that plane, , and 5 wherein when the radius of curvature of the bent part in said plane is Ro (mm), the length of the flange parts is "b" (mm), the numerical value which shows the allowable value of strain is ecr, and the Young's modulus and tensile strength of said high strength steel sheet 10 are E (MPa) and cT (MPa), f o r cti and a2, the d i r e c t i o n of r o t a t i o n s t a r t i ng from s a i d h o r i z o n t a l l i n e in t h e d i r e c t i o n where the f l a n g e p a r t moves away from the top sheet p a r t is made p o s i t i v e , and 15 oti>0, ct2>0, ai-a2>0, Ro=50 to 2000 mm, and scr=0 to 0.023, oti-a2, t h a t i s , the a d d i t i o n a l bending angle p, is made t h e following r a n g e s: Mathematical Formula 1 20 When COS -1 bcos az - (-^—-T-+£ cr) R0 Jj b0-(M^+£Cr> ^ 9 0 cos" bcosa^ -(-—*-) Ro ,0. 5aT -azS(i 9 0 cos bcosa2-( ' p^lRo b l -{ 0.5aT. - ^ ^ ( 3 ^ 9 0 ° - ^ [0020] (2) The press-forming method according to (1) wherein the bent part is an arc or a curve with a curvature which continuously changes. 5 [0021] (3) The press-forming method according to (1) or (2) wherein at least at one of the first shaping process and the second shaping process, one of the facing dies is divided into a pad and a partial shaping die, the pad and the other of the facing dies press the steel ( 10 sheet, and the partial shaping die and the other of the facing dies are used to make the steel sheet plastically deform. Advantageous Effects of Invention [0022] According to the present invention, even when 15 using high strength steel sheet, it is possible to provide a final shaped article which comprises a top sheet part, vertical wall parts, and flange parts and which has at least one bent part with a radius of curvature of 50 to 2000 mm where the warping and torsion 20 are suppressed without providing the final shaped article with a bead etc. Brief Description of Drawings [0023] [FIG. 1] FIG. 1 is a view which shows one example of a final shaped article which has one bent 25 part. [FIG. 2] FIG. 2 shows the change in stress which is applied to the high strength steel sheet when applying. - 8 - tensile and compressive load to the high strength steel sheet. [FIG. 3] FIG. 3 is a view which shows a final shaped article which has two bent parts. 5 [FIG. 4] FIG. 4 is a schematic view which shows an outline of the cross-sectional shape of a part which forms a bent part in a die which is used in the first shaping process. [FIG. 5] FIG. 5 is a schematic view which shows an 10 outline of the cross-sectional shape of a part which forms a bent part in a die which is used in the first shaping process when forming a final shaped article with a width W of 15 to 30 mm. [FIG. 6] FIG. 6 is a schematic view which shows an 15 outline of the cross-sectional shape of a part which forms a bent part in a die which is used in the second shaping process when forming a final shaped article with a width W of 15 to 30 mm. [FIG. 7] FIG. 7 is a^ view which shows the shape of a i 20 final shaped article which has a portion of a bent part with a radius of curvature which continuously changes in the range of 700 to 1200 mm and has a straight part and which gently curves in the longitudinal direction when seen from a top view. 25 [FIG. 8] FIG. 8 is a view which shows a final shaped article which has a bent parts with radii of curvature of 1000 mm and 700 mm and has a straight part, which further combines a shape with a radius of curvature which continuously changes in 1200 to 2000 mm in range, and 30 which gently curves in the longitudinal direction when seen from a top view. [FIG. 9] FIG. 9 is a view which shows a final shaped article which has bent parts with radii of curvature of 1000 mm and 700 mm and has a straight part, which further 35 combines a shape with a radius of curvature which continuously changes in 1200 to 2000 mm in range, and which gently curves in the longitudinal direction when - 9 - seen from a top view. Note that, the range of additional bending is part of the inside flange. [FIG. 10] FIG. 10 is a view which shows a final shaped article which has a bent part with a radius of curvature 5 of 1000 mm and has a straight part, which further a bent part with a radius of curvature of 3000 mm and a straight part in the direction seen from the side surface, and which gently curves in the longitudinal direction when seen from a top view. 10 [FIG. 11] FIG. 11 is a view which shows one example of a final shaped article which has one bent part. [FIG. 12] FIG. 12 is a view which shows the effect of the radius of curvature Ro (mm) of the bent part 10 and the ci which is applied to the final shaped article on the 15 warping, torsion, and wrinkles of the final shaped article. [FIG. 13] FIG. 13 is a view which explains the positive and negative directions of oti and CC2. [FIG. 14] FIG. 14 shows the cross-section of a final 20 shaped article along the line I-I in FIG. 1(a) when ct2+P exceeds 90°. Description of Embodiments [0024] FIG. 1 is a view which shows one example of a final shaped article which comprises a top sheet part, 25 vertical wall parts, and flange parts and which has one bent part with a radius of curvature of 50 to 2000 mm in the longitudinal direction. FIG. 1(a) is a perspective view, while FIG. 1(b) is a cross-sectional view along the line I-I which is shown in FIG. 1(a). In (a) of the 30 figure, reference numeral 1 shows the final shaped article. [0025] The final shaped article 1 comprises a top sheet part 2, vertical wall parts 3a, 3b, and flange parts 4a, 4b. The vertical wall part 3a and the flange 35 part 4a are at the inside of the bent part 10, while the vertical wall part 3b and the flange part 4b are at the - 10 - outside of the bent part 10. The vertical wall part 3a and the flange part 4a intersect at an intersecting part 5a. The vertical wall part 3b and the flange part 4b intersect at an intersecting part 5b. 5 [0026] FIG. 1(b) shows a cross-sectional view along the line I-I in FIG. 1(a). The cross-section which is shown by the solid lines is a cross-section after the second shaping process, that is, of the final shaped article 1. The position of the flange part 4a after the 10 second shaping process is indicated as L3. Further, the cross-section which is shown by the broken lines is a cross-section of the flange part 4a after the first shaping process. The position of the flange part 4a after the first shaping process is indicated as L2. 15 [0027] For one position "r" of the bent part on the intersecting part 5a between the vertical wall part 3a and the flange part 4a, the center of curvature O with respect to the position "r" of the bent part and the line segment Ll which connects the center of curvature o' and i 20 the position "r" are defined as in FIG. 1(b). [0028] For the center of curvature O, consider the small range AG about the center axis of curvature L0 of the position "r" of the bent part. The small plane Si which passes through the line segment Ll and includes the 25 small range A6 is defined. The small plane Si forms part of the horizontal surface which includes the line segment Ll and the axis LO' vertical to the center axis of curvature L0. Note that, this horizontal plane is for convenience made horizontal as the reference plane. These 30 explanations will be given by the cross-section along the line I-I in FIG. 1(a), that is, the cross-section which is shown in FIG. 1(b). The cross-section which is shown by FIG. 1(b) is a plane which includes a horizontal line H which connects the intersecting part 5a of the vertical 35 wall part 3a and the flange part 4a and the center of curvature O of the bent part 10 and which is vertical to the steel sheet material. - 11 - [0029] The final shaped article 1 is formed as follows: First, for the steel sheet material, the flange part 4a is bent at the intersecting part 5a until the angle of the flange part 4a with respect to the 5 horizontal line H becomes cci. This bending operation is referred to as the "first shaping process". Next, the flange part 4a after the first shaping process is additionally bent at the intersecting part 5a until the angle of the flange part with respect to the horizontal 10 line H becomes a.2. This additional bending operation is referred to as the "second shaping process". That is, in the first shaping process, the steel sheet material is formed into the intermediate product, then in the second shaping process, the flange part 4a of the intermediate 15 product is further additionally bent to obtain the final shaped article 1. [0030] After the end of the first shaping process, tensile stress remains at the vertical wall part 3a and the flange part 4a at the inside of the bent part 10. 20 This tensile residual stress becomes a cause of springback. Therefore, after the first shaping process, an additional bending operation (second shaping process) is used to plastically deform the intersecting part 5a of the vertical wall part 3a and the flange part 4a by 25 compression. As a result, the tensile residual stress at the time of the end of the first shaping process is reduced and warping and torsion of the final shaped article 1 can be suppressed. [0031] In the cross-section which is shown in FIG. 30 1(b), the radius of curvature R0 (mm) of the bent part 10 is defined at the intersecting part 5a of the vertical wall part 3a and the flange part 4a in the cross-section. Here, the radius of curvature of the front end of the flange part 4a at the time of the end of the first 35 shaping process is indicated as Ri (mm) . At the time of the end of the second shaping operation, that is, at the final shaped article, the radius of curvature of the - 12 - front end of the flange part 4a is indicated as R2 (mm) . Further, the length of the flange part 4a is indicated as "b" (mm). In this case, Ri=Ro-bcosa.i 5 R2=Ro~bcosot2 Note that, Ro, Ri, and R2 are made the radii of curvature at the small range A0. Therefore, the bent part 10 can be made a free curved surface where the curvature continuously changes. 10 [0032] At this time, the strain £1 which is given to the front end part of the flange 4a is expressed by the following: £i= (R1-R2) /Ri=b (cosa2-cosai) / (R0-bcoscti) [0033] From the above £1, the angle cti which is formed 15 by the vertical wall part 3a and the flange part 4a which are formed in the first shaping process becomes: ai=cos-1{ (bcosa2-EiR0)/b(l-Si) } [0034] Therefore, the additional bending angle p for changing cti to a2 becomes: 20 p=ai-a2=cos-1{ (bcosct2-£iRo) / (b(l-ei) }~a2... (A) [0035] Here, the strain ,Z\ which is given to the front end part of the flange 4a is £i=oT/E (where, aT is the tensile strength (MPa) of steel sheet, and E is the Young's modulus (MPa) of steel sheet) if steel sheet with 25 a tensile strength of less than 440 MPa (for example, soft steel sheet etc.) [0036] Howeve r, when the ten s i .1. e st rength o V. the stee 1 sheet which is used as the material for press-forming is 440 to 1600 MPa, that is, in the case of high strength 30 steel sheet (high tensile strength steel sheet), there is the phenomenon of £1 becoming smaller than aT/E. [0037] This phenomenon will be explained. FIG. 2 shows the change in stress which is applied to high strength steel sheet when high strength steel sheet with a tensile 35 strength of 440 to 1600 MPa is given a tensile load - 13 - right before break and then is given a compressive load. [0038] High strength steel sheet with a tensile strength of 440 to 1600 MPa, due to the Bauschinger effect, suffers from an early yield phenomenon where at 5 the time of stress reversal, the stress Aa which is required for the high strength steel sheet to second yield decreases from the usual yield stress. Accordingly, Si also decreases. [0039] Here, Ci is the compressive strain which is 10 given for reducing the tensile stress which remains at the inside of the bent part 10 and causes springback. The lower limit of compressive strain is given by ci=0.5aT/E. On the other hand, the upper limit of compressive strain is given by Si=0. 5o*T/E+ecr. Here, ecr is the allowable value 15 of strain where the flange part 4a of the final shaped article 1 does not wrinkle. The range of scr is found by experiments and is 0 to 0.023. That is, in the final shaped article 1, the flange part 4a does not wrinkle when si is in the range of 0 . 5aT/E to (0 . 5aT/E)+ecr. The 20 same is true in the case of using the first shaping process to obtain the intermediate product. [0040] If converting the range of ei to the range of the additional bending angle p based on the above formula (A), the result becomes the 2 5 Mathematical Formula 3 bcosat -(—---!-•)-£ cr)R0 bjl-(——-L + ecrH [0041] FIG. 12 is a view, prepared based on the above inequality, which shows the effect of the radius of 30 curvature RQ (mm) and compressive strain 8i of the bent . ,0. 5cJMn cos" b I-{ 0. 5aT -a2 ^/3^cos" _ 14 - part 10 on the warping, torsion, and wrinkles of the final shaped article. In FIG. 12, Curve 1 is the curve which shows Mathematical Formula 4 bco$<^-(^^-+ecr)Rfl«0 b 5 when the tensile strength aT of the steel sheet which is used as a material is 390, 490, 590, 710, 980, and 1200 MPa. [0042] In FIG. 12, the range of £i and the vertical 10 direction of the Curve 1 can be divided into the region A to region D. The regions A and B are regions where ecr is 0 to 0.023 in range, that is, regions where Ei is a value of 0.5crT/E plus the allowable value ecr of strain. That is, the value of the upper limit of Ei at the regions A and B 15 changes depending on the aT of the material. FIG. 12 shows as typical examples the values of 8i when ecr=0.023 at the values of aT=390 MPa and 1200 MPa by two lines. The value of Si of a steel material with a aT of 390 to 1200 MPa may be considered to be substantially between these two 20 lines. Therefore, in the region A and the region B, the intermediate product and the final shaped article are formed without causing wrinkling. On the other hand, in the region C and the region D, Cj is over 0.023, so even if formed, the intermediate product and the final shaped 25 article are wrinkled. [0043] Here, to obtain a final shaped article with small warping and torsion without causing wrinkling, in the region A and the region B where Gi is ecr, the additional bending angle p which is defined by oii-a2 has 30 to be made a predetermined range. Below, the range of the additional bending angle p will be explained divided into - 15 - 10 the region A and the region B. Note that/ for oti and 0.2, as shown in FIG. 13(a), the direction of rotation starting from the position of the horizontal line H in the direction where the flange part 4a moves away from the top sheet part 2 is defined as "positive". Conversely, the direction of rotation starting from the position of the horizontal line H in the direction where the flange part 4a moves toward from the top sheet part 2 is defined as "negative". [0044] In FIG. 12, region A, when making cti>0, a2>0, oti-ci2>0, and Ro=50 to 2000 mm, ai-a2, that is, the additional bending angle p, has to be made the range of Mathematical Formula 5 When COS -I bcosa^ -( ' T +ecr)Ra . E Wl-<^!*+6cr) ^ 9 0 15 COS -1 bcosofc-M^Ito b l-( 0. 5 aT. -az^j3^cos bcos a2 fQ.5aT ' E +ecr)Rr W l_(01 | o L + g c r ) - « 2 [0045] Here, as shown in FIG. 12, if Ro becomes larger or Ei becomes larger, the value of Mathematical Formula 6 bcosa; -( T +ecr)R0 20 sometimes becomes a negative value. The value for calculating the arc cosine from this value is, as explained above, cti, so this value becoming negative means - 16 - the value of cti is over 90°. If the value of al is over 90°, as shown in FIG. 14, the angle which the flange part 4a forms with the vertical wall part 3a becomes 180° or less. If considering a die such as in FIG. 4, the die 5 cannot be pulled out and the shaped article cannot be produced. Therefore, the region A Mathematical Formula 7 bcosa2-( ' T +£cr)R0 being positive is a required condition. Under this 10 condition, the value of ai minus ct2, that is, the value of P, can be found. The value of the upper limit of p can be found as 0.023 of the value of the upper limit scr where no wrinkles occur. Further, theoretically, 8cr may also be zero.. In this case, the value of 8i is made 0.5aT/E. 15 Accordingly, as the range of p, ci changes from aT/E in the range of the value which is calculated in the range of 0.5o-T/K+scr. [0046] The processing method of the present invention provides a shaping method which first bends the material 20 by a small amount, then further bends it in the same direction, so cti<0 never stands. Further, large bending from the start is not preferable since the material easily wrinkles. Further, a2<0 is not preferable since deformation of the flange parts causes the flange part to 25 easily wrinkle. Further, if a,i~az<0, the present invention provides a shaping method which first bends the material by a small amount, then further bends it in the same direction, so a\-a2^0 never stands. Further, cti-a^O is not preferable since the material is worked in the reverse 30 direction and easily wrinkles at the time of the first shaping operation. Therefore, ai>0, a2^0, and cti-a2>0 are - 17 - set. [0047] Further, if R0 is less than 50 mm, at the time of the end of the first shaping process, the tensile stress which remains at the vertical wall part 3a and the 5 flange part 4a at the inside of the bent part 10 becomes extremely large. Therefore, even if making p the range of the above inequality, it is not possible to relieve the residual tensile stress at the second shaping process. As a result, the warping and torsion of the final shaped 10 article 1 become larger. On the other hand, if R0 exceeds 2000 mm, the final shaped article 1 becomes straight in shape in the longitudinal direction, so at the time of end of the first shaping process, the tensile stress which remains at the vertical wall part 3a and the flange 15 part 4a at the inside of the bent part 10 becomes smaller. Accordingly, even if not applying the present invention, the warping and torsion of the final shaped article 1 are small. Furthermore, when the final shaped article has a plurality of curvatures, in the present 20 invention, the minimum radius of curvature is made R0. [0048] Further, when Mathematical Formula 8 COS" b c o s a . - t ^ 1 1 +ecr)Rr r0. 5crT + e cr) > 9 0 a2H"P, that is, cti, exceeds 90° starting from the 25 horizontal line. FIG. 14 shows the cross-section of the final shaped article at the line I-I in FIG. 1(a) when cc2+p, that is, cti, exceeds 90°. As shown in FIG. 14, the flange part 4a becomes inclined in reverse with respect to the direction of advance of the die. It is clear that 30 it is not possible to use the die to form the final shaped article 1. - 18 - [0049] Further, when the range of the additional bending angle P does not satisfy the Mathematical Formula 9 COS" bcosa2-( ' T)R0 . D W~i 0. 5crT -a2^£^cos -i kos a2 - ( ' 7 +s cr) R0 bi f0. 5 aT +scr) -a, 10 while the intermediate product and final shaped article 1 can be formed without causing wrinkling, the warping and torsion of the final shaped article 1 are large. [0050] Next, in the region B of FIG. 12, when making cti>0, a2>0, ai-a2>0, and R0=50 to 2000 mm, the range of cti- CC2, that is, the additional bending angle p, has to be made the Mathematical Formula 10 When , cos -l bcos 02 ~ ( ' r ° x +£ cr) RD b i-{ 0.5ar +£ cr) >90 cos -I bcosofr-C-' T)R0 bl r0. 5aT •a,^i3^90°-a, 15 [0051] The reasons for making cti>0, a2>0, ai-a2>0, and Ro=50 to 2000 mm are similar to those of the case of region A. [0052] Further, when not satisfying Mathematical Formula 11 - 19 - bcos «2 -( ' T +6 cr)R0 cos"1 • T——J* =— > 9 0 ° [ b(l-(M.ft+6cr)} j as explained above, ct2+P, that is, cti, exceeds the 90° starting from the horizontal line and the flange part 4a becomes inversely inclined with respect to the direction 5 of advance of the die, so it is not possible to use the die for shaping. Therefore, the upper limit of the additional bending angle p was made 90°-ct2. Here, 0, a2>0, cti-a2>0, R0=50 to 2000 mm, and ecr=0 to 0.023, 35 CC1-CX2, that is, the additional bending angle p, is made the following ranges: i - 43 Mathematical Formula 1 When COS bcos a2 - (-L-=^I-+£ cr) R0 w , _ ( 0 1 | o L + e c r ) ^ 9 0 cos -1 bcos Oj -( ' T)R( . is b l-( 0. 5 a r -a2^/3^cos bcosctt-{ ' p gT+£cr)R0 c bU-(M2L+6Cr) a2 and Mathematical Formula 2 When jCOS -I bcos a, - ( ' T 1 E + £ c r ) RF Wl_(i|oL + ecrJ >9 0 cos bcos