Patent Publication Number: US-8991230-B2

Title: Roll forming method and shaped beam produced by using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2011-0126333 filed in the Korean Intellectual Property Office on Nov. 29, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a roll forming method and a shaped beam produced by using the same. More particularly, the present invention relates to a roll forming method and a shaped beam produced by using the same that prevents generation of non-uniform dents due to heel tap at an inward surface of a curvature when a shaped beam of a closed section produced through a roll forming process is formed to have the curvature. 
     (b) Description of the Related Art 
     Generally, a roll forming method is a method in which a coil is uncoiled and the uncoiled coil is bent to various shapes by passing through a roll forming unit. The roll former unit includes a plurality of roll formers that are sequentially disposed, and each roll former is provided with an upper forming roll and a lower forming roll. The roll forming method is used for manufacturing beams of linear type (particularly, a bumper beam for a vehicle) which is bent to the various shapes. 
       FIG. 1  is a schematic diagram of a typical roll forming system and steps of a roll forming method. 
     Referring to  FIG. 1 , the typical roll forming system and method thereof includes an uncoiler  1  at a front portion of a process line, and the uncoiler  1  performs an uncoil step S 1  at which a coil  10  is uncoiled. 
     A straightener  2  is provided at the rear of the uncoiler  1  in the process line and performs a straightening step S 2  at which the coil uncoiled from the uncoiler  1  is straightened to a panel  20  of plate shape. 
     A brake press  3  is disposed at the rear of the straightener  2  in the process line and performs a piercing step S 3  at which a plurality of holes for assembling is formed at the panel  20  supplied from the straightener  2 . 
     A roll forming unit  4  including at least seven roll formers R 1 -R 7  is disposed at the rear of the brake press  3  in the process line. The roll forming unit  4  performs a roll forming step S 4  at which the panel  20  passing through the uncoiler  1 , the straightener  2 , and the brake press  3  is sequentially bent such that a shaped beam  30  having a desired closed section is formed. 
     A laser welding device  5  is disposed at the rear of the roll forming unit  4  in the process line and performs a welding step S 5  at which a laser beam outputting from a laser oscillator  5   a  is irradiated to a welding portion of the shaped beam  30 . 
     In addition, a round bender  6  is provided at the rear of the laser welding device  5  in the process line and performs a bending step S 6 . At the bending step S 6 , the shaped beam  30  passes through a plurality of bending roll units disposed along a desired curvature such that the shaped beam  40  having the curvature is formed. 
       FIG. 2  is a side view of a typical round bender. 
     Referring to  FIG. 2 , the round bender  6  includes five bending roll units. 
     A first bending roll unit BR 1  includes a pair of bending rolls and is disposed at a front portion of a roll frame  6   a  in the process line. The first bending roll unit BR 1  guides the shaped beam  30  passing through the welding step S 5 . 
     A second bending roll unit BR 2  includes a pair of bending rolls and is disposed at the rear of the first bending roll unit BR 1  on the roll frame  6   a . The second bending roll unit BR 2  rolling-supports the shaped beam  30  along the curvature direction. 
     In addition, third, fourth, and fifth bending roll units BR 3 , BR 4 , and BR 5  respectively include a pair of bending rolls, and are sequentially disposed at the rear of the second bending roll unit BR 2  on the roll frame  6   a  along the curvature. The shaped beam  30  passes through the third, fourth, and fifth bending roll units BR 3 , BR 4 , and BR 5  sequentially such that the shaped beam  40  having the curvature is formed. 
     A cutting press  7  is disposed at the rear of the round bender  6  in the process line and performs a cutting step S 7  at which the shaped beam  40  is cut in a size of the finished product. 
       FIG. 3  is a perspective view of a shaped beam produced by a typical roll forming system and a method thereof. 
     Referring to  FIG. 3 , the shaped beam  50  produced by the typical roll forming system and the method thereof includes the closed sections at an upper portion and lower portion thereof, and is formed to have the curvature along a length direction thereof. In addition, widths of the upper closed section and the lower closed section are different. 
     The shaped beam  50  can be applied to components of a vehicle body or various industrial beam members, and particularly, to a bumper beam of a vehicle. 
     A plurality of non-uniform dents D due to heel tap can be generated at a surface of the lower closed section facing toward an inward of the curvature in the shaped beam  50  as shown in  FIG. 3 , when the shaped beam  50  is formed to have the curvature through the round bender  6 . The dent may deteriorate impact strength of the shaped beam  50 . 
     In a case that such a shaped beam  50  is applied to the bumper beam of the vehicle, a hole H is bored at one side of the shaped beam  50  and a towing hook pipe (not shown) is inserted in the hole H. At this state, the shaped beam  50  and the towing hook pipe are welded by using CO 2 . At this time, the non-uniform dent D on the shaped beam  50  causes non-uniform surfaces between the towing hook pipe and a welding portion of the shaped beam  50 , and bad welding may occur. 
     In order to secure welding quality of the shaped beam  50  and the towing hook pipe (not shown), all the welding portions should be inspected and additional welding should be performed at bad welding portions. However, theses may deteriorate productivity in automated processes. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide a roll forming method and a shaped beam produced by using the same having advantages of preventing occurrence of non-uniform dents and improving strength of the shaped beam as a consequence that a concave surface is formed at a surface of a closed section facing toward an inward of a curvature so as to absorb heel tap when the shaped beam of a closed section is formed to have the curvature. 
     In one or more exemplary embodiments of the present invention, a roll forming method including a roll forming step performed by a plurality of roll formers and a bending step performed by a round bender provided with a plurality of bending roll units are disclosed. 
     After a concave surface is formed at a surface of a shaped beam sequentially bent so as to have a closed section at the roll forming step, the concave surface is disposed so as to face toward an inward of a curvature of the shaped beam in the round bender and the shaped beam is formed to have the curvature through multiple steps at the bending step. 
     Formation of the concave surface begins at the rearmost roll former of the plurality of roll formers. 
     Formation of the shaped beam to have the curvature is performed by at least three bending roll units provided in the round bender. 
     The shaped beam includes an outer side surface adjacent to the concave surface, and the outer side surface is slanted toward the concave surface so as to have a slanted angle. 
     the slanted angle of the outer side surface is controlled at the roll forming step but is not controlled at the bending step. 
     The concave surface is formed as a predetermined curvature. 
     According to another aspect of the present invention, the shaped beam produced by using the roll forming method is disclosed. 
     The shaped beam is formed so as to have at least one closed section, and the concave surface is formed at a surface of the closed section facing toward the inward of the curvature of the shaped beam. 
     According to other aspect of the present invention, it is disclosed that a roll forming method including a roll forming step where a shaped beam having at least one closed section is roll-formed by sequentially bending a panel through a roll forming unit provided with a plurality of roll formers, and a bending step where the shaped beam is formed so as to have a curvature through a round bender provided with a plurality of bending roll units. 
     The roll forming method includes: a first step where a concave surface is formed at a surface of the shaped beam facing toward an inward of the curvature will be formed; a second step where the concave surface is formed to have a maximum allowable depth; a third step where the concave surface is formed to have a depth shallower than the maximum allowable depth such that residual stress of a formed portion is absorbed and plastic deformation is led; and a fourth step where the concave surface of the shaped beam is disposed so as to face toward the inward of the curvature such that heel tap due to formation of the curvature is absorbed, and the shaped beam is formed to have the curvature through multiple steps. 
     The first step is performed by the rearmost roll former of the roll forming unit. 
     The second step to the fourth step are performed by bending roll units of the round bender. 
     The shaped beam includes an outer side surface adjacent to the concave surface, and the outer side surface is formed to have a respective predetermined slanted angle at each step. 
     The slanted angle of the outer side surface is respectively controlled at the first step, the second step, and the third step. 
     The slanted angle of the outer side surface is not controlled at the fourth step such that a stress due to heel tap toward the concave surface occurring at formation of the curvature of the shaped beam is absorbed. 
     The fourth step is performed through at least three bending roll units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a typical roll forming system and steps of a roll forming method. 
         FIG. 2  is a side view of a typical round bender. 
         FIG. 3  is a perspective view of a shaped beam produced by a typical roll forming system and a method thereof. 
         FIG. 4  is a cross-sectional view of a shaped beam for explaining each step of a roll forming method according to an exemplary embodiment of the present invention. 
         FIG. 5  is a schematic diagram for explaining the first step of a roll forming method according to an exemplary embodiment of the present invention. 
         FIG. 6  is a schematic diagram for explaining the second step of a roll forming method according to an exemplary embodiment of the present invention. 
         FIG. 7  is a schematic diagram for explaining the third step of a roll forming method according to an exemplary embodiment of the present invention. 
         FIG. 8  is a schematic diagram for explaining the fourth step of a roll forming method according to an exemplary embodiment of the present invention. 
         FIG. 9  is a perspective view of a shaped beam produced by using a roll forming method according to an exemplary embodiment of the present invention. 
     
    
    
     
       
         
           
               
             
               
                   
               
               
                 &lt;Description of symbols&gt; 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1: uncoiler 
                 2: straightener 
               
               
                   
                 3: brake press 
                 4: roll forming unit 
               
               
                   
                 5: laser welding device 
                 6: round bender 
               
            
           
           
               
               
            
               
                   
                 7: cutting press 
               
               
                   
                 BR1, BR2, BR3, BR4, BR5: bending roll units 
               
            
           
           
               
               
               
            
               
                   
                 D: dent 
                 I: concave surface 
               
               
                   
                 F: outer side surface 
                 41: lower forming roll 
               
               
                   
                 42: upper forming roll 
                 61: first lower bending roll 
               
               
                   
                 62: first upper bending roll 
                 63: second lower bending roll 
               
            
           
           
               
               
            
               
                   
                 64: second upper bending roll 
               
               
                   
                 65: third, fourth, and fifth lower bending rolls 
               
               
                   
                 66: third, fourth, and fifth upper bending rolls 
               
               
                   
                 42a, 62a, 64a: concave surface forming portion 
               
               
                   
                 42b, 62b, 64b: slanted surface forming portion 
               
               
                   
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Size and thickness of components shown in the drawings may be differ from real size and real thickness of the components for better comprehension and ease of description, and thicknesses of some portions and regions are drawn with enlarged scale. 
     In addition, description of components which are not necessary for explaining the present invention will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification. 
       FIG. 1  is a schematic diagram of a typical roll forming system and steps of a roll forming method,  FIG. 2  is a side view of a typical round bender, and  FIG. 4  is a cross-sectional view of a shaped beam for explaining each step of a roll forming method according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a roll forming system to which a roll forming method according to an exemplary embodiment of the present invention is applied includes an uncoiler  1  disposed at a front portion of a process line and performing an uncoil step S 1  at which a coil  10  is uncoiled. 
     After completing the uncoil step S 1 , a straightening step S 2  at which the coil uncoiled from the uncoiler  1  is straightened to a panel  20  of plate shape through a straightener  2  is performed. 
     After completing the straightening step S 2 , a piercing step S 3  at which a plurality of holes for assembling is bored at the panel  20  supplied from the straightener  2  is performed. 
     After completing the piercing step S 3 , a roll forming step S 4  at which the panel  20  supplied from the brake press  3  is sequentially bent to roll form the shaped beam  30  having a closed section through a roll forming unit  4  including at least seven roll formers R 1 -R 7  is performed. 
     After completing the roll forming step S 4 , a welding step S 5  at which a laser beam from a laser oscillator  5   a  is irradiated to a welding portion of the shaped beam  30  having the closed section supplied from the roll forming unit  4 . 
     After that, a bending step S 6  is performed by a round bender  6  disposed at the rear of the laser welding device  5  in the process line. The round bender  6  includes a plurality of bending roll units BR 1 , BR 2 , BR 3 , BR 4 , and BR 5  disposed along the curvature radius and forms the shaped beam  30  having the closed section to have the curvature. 
     Referring to  FIG. 2 , the round bender  6  includes the plurality of bending roll units BR 1 , BR 2 , BR 3 , BR 4 , and BR 5 . 
     A first bending roll unit BR 1  includes a pair of bending rolls and is disposed at a front portion of a roll frame  6   a  in the process line. The first bending roll unit BR 1  guides the shaped beam  30  passing through the welding step S 5 . 
     A second bending roll unit BR 2  includes a pair of bending rolls and is disposed at the rear of the first bending roll unit BR 1  on the roll frame  6   a . The second bending roll unit BR 2  rolling-supports the shaped beam  30  along the curvature direction. 
     In addition, third, fourth, and fifth bending roll units BR 3 , BR 4 , and BR 5  respectively include a pair of bending rolls, and are sequentially disposed at the rear of the second bending roll unit BR 2  on the roll frame  6   a  along the curvature. The shaped beam  30  passes through the third, fourth, and fifth bending roll units BR 3 , BR 4 , and BR 5  sequentially such that the shaped beam  40  having the curvature is formed. 
     After completing the bending step S 6 , a cutting step S 7  at which the shaped beam  40  is cut in a size of the finished product by a cutting press  7  is performed. 
     A roll forming method according to an exemplary embodiment of the present invention shown in  FIG. 4  to  FIG. 8  is performed between the roll forming step S 4  and the bending step S 6 . 
     After a concave surface is formed at a surface of the shaped beam  30  sequentially bent so as to have the closed section at the roll forming step S 4 , the concave surface of the shaped beam  30  is disposed so as to face toward an inward of the curvature of the shaped beam  30  in the round bender  6  and the shaped beam  30  is formed to have the curvature at the bending step S 6  in a roll forming method according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4  to  FIG. 8 , a roll forming method according to an exemplary embodiment of the present invention will hereinafter be described in detail. 
     Referring to  FIG. 4 , a roll forming method according to an exemplary embodiment of the present invention includes four steps ST 1 , ST 2 , ST 3 , and ST 4 . 
     A first step ST 1  is performed by the rearmost roll former R 7  of the roll forming unit  4  at the roll forming step S 4 . 
     In order to prevent occurrence of dents D at the surface of the shaped beam  30  facing toward the inward of the curvature due to heel tap when the shaped beam  30  having the closed section is formed to have the curvature by the round bender  6 , the concave surface I for leading the heel tap toward an inward cross-section of the shaped beam  30  is formed in advance at the first step ST 1 . 
     It is exemplified in this specification but is not limited that formation of the concave surface I at the first step ST 1  is performed by the rearmost roll former R 7  at the roll forming step S 4 . That is, the concave surface I can be formed at an additional forming step or by the first bending roll unit BR 1  of the bending step S 6  regardless of the roll forming step S 4 . 
     When the concave surface I is formed at the first step ST 1 , an outer side surface F adjacent to the concave surface I is formed slantedly toward the concave surface I by a predetermined angle in order to prevent occurrence of non-uniform deformation at neighboring side surface due to the concave surface I. 
     Referring to  FIG. 5 , formation of the concave surface I at the first step will be described in detail. 
     As shown in  FIG. 5 , the lower forming roll  41  of the rearmost roll former R 7  has the same size and shape as a lower surface of the shaped beam  30 . 
     In addition, a concave surface forming portion  42   a  is formed at an upper forming roll  42  so as to form the concave surface I at a portion of an upper surface of the shaped beam  30  (i.e., an upper surface of the closed section having wider width). 
     In addition, a slanted surface forming portion  42   b  connected to the concave surface forming portion  42   a  is formed at the upper forming roll  42  so as to form the outer side surface F adjacent to the concave surface I of the shaped beam  30  as a slanted surface. 
     The concave surface forming portion  42   a  is shaped as a curved surface having a predetermined curvature so as to form the concave surface I having a maximum depth d 1   a  of 3.7 mm, and the slanted surface forming portion  42   b  is shaped such that a slanted angle θ 1   a  between a vertical line and the outer side surface F is 8°. 
     Shape and size of the upper forming roll  42  are not limited to these and can be controlled as occasion demands. 
     After the concave surface I and the neighboring outer side surface F of the shaped beam  30  are formed by the concave surface forming portion  42   a  and the slanted surface forming portion  42   b  of the upper forming roll  42 , the maximum depth d 1  of the concave surface I is restored to 2 mm and the slanted angle θ 1  of the outer side surface F to the vertical line is restored to 6° due to spring back by structural resistance of the closed section. 
     In addition, a second step ST 2  is performed by the first bending roll unit BR 1  of the round bender  6  at the bending step S 6 . 
     The concave surface I formed at the surface of the shaped beam  30  facing toward the inward of the curvature by the rearmost roll former R 7  at the first step ST 1  is additionally formed such that the depth of the concave surface I is to be a maximum allowable depth d 2   a  at the second step ST 2 . 
     It is exemplified in this specification but is not limited that formation of the concave surface I to be the maximum allowable depth d 2   a  at the second step ST 2  is performed by the first bending roll unit BR 1  of the round bender  6  at the bending step S 6 . That is, the second step ST 2  can be performed at an additional forming step regardless of the roll forming step S 4  or the bending step S 6 . 
     Herein, the maximum allowable depth d 2   a  is defined as the maximum depth of the concave surface I where breakage of a formed portion does not occur by forming the concave surface I at the shaped beam  30 . 
     At the second step ST 2 , the outer side surface F neighboring the concave surface I is formed to be slanted further toward the concave surface I in order to prevent occurrence of non-uniform deformation at the neighboring side surface due to the concave surface I. 
     Referring to  FIG. 6 , an additional formation of the concave surface I at the second step ST 2  will be described in detail. 
     As shown in  FIG. 6 , the first lower bending roll  61  of the first bending roll unit BR 1  has the same size and shape as the lower surface of the shaped beam  30 . 
     In addition, a concave surface forming portion  62   a  is formed at the first upper bending roll  62  just like the upper forming roll  42  so as to additionally form the concave surface I at the portion of the upper surface of the shaped beam  30  (i.e., an upper surface of the closed section having wider width). 
     In addition, a slanted surface forming portion  62   b  connected to the concave surface forming portion  62   a  is formed at the first upper bending roll  62  so as to form the outer side surface F adjacent to the concave surface I of the shaped beam  30  as the slanted surface. 
     The concave surface forming portion  62   a  of the first upper bending roll  62  is shaped as a curved surface having a predetermined curvature so as to form the concave surface I having the maximum allowable depth d 2   a  of 3.7 mm, and the slanted surface forming portion  62   b  is shaped such that a slanted angle θ 2   a  between the vertical line and the outer side surface F is 16°. 
     Shape and size of the first upper bending roll  62  are not limited to these and can be controlled as occasion demands. 
     After the concave surface I and the neighboring outer side surface F of the shaped beam  30  are formed by the concave surface forming portion  62   a  and the slanted surface forming portion  62   b  of the first upper bending roll  62 , the maximum depth d 2  of the concave surface I is restored to 3 mm and the slanted angle θ 2  of the outer side surface F to the vertical line is restored to 10° due to the spring back by structural resistance of the closed section. 
     After completing the second step ST 2 , a third step ST 3  is performed by the second bending roll unit BR 2  of the round bender  6  at the bending step S 6 . 
     The concave surface I additionally formed to have the maximum allowable depth d 2   a  by the first bending roll unit BR 1  is formed a depth shallower than the maximum allowable depth d 2   a  so as to absorb residual stress of the formed portion and lead plastic deformation at the third step ST 3 . 
     It is exemplified in this specification but is not limited that leading plastic deformation of the concave surface I at the third step ST 3  is performed by the second bending roll unit BR 2  of the round bender  6  at the bending step S 6 . That is, the third step ST 3  can be performed at an additional forming step regardless of the roll forming step S 4  or the bending step S 6 . 
     At the third step ST 3 , the outer side surface F neighboring the concave surface I is formed to be slanted further toward the concave surface I in order to prevent occurrence of non-uniform deformation at the neighboring side surface due to the concave surface I. 
     Referring to  FIG. 7 , formation of the concave surface I at the third step ST 3  will be described in detail. 
     As shown in  FIG. 7 , the second lower bending roll  63  of the second bending roll unit BR 2  has the same size and shape as the lower surface of the shaped beam  30 . 
     In addition, a concave surface forming portion  64   a  is formed at the second upper bending roll  64  just like the upper forming roll  42  so as to form the concave surface I at the portion of the upper surface of the shaped beam  30  (i.e., an upper surface of the closed section having wider width). 
     In addition, a slanted surface forming portion  64   b  connected to the concave surface forming portion  64   a  is formed at the second upper bending roll  64  so as to form the outer side surface F adjacent to the concave surface I of the shaped beam  30  as the slanted surface. 
     The concave surface forming portion  64   a  of the second upper bending roll  64  is shaped as a curved surface having a predetermined curvature so as to form the concave surface I having the maximum depth d 3   a  of 2.4 mm that is smaller than 3 mm being the maximum depth d 2  of the concave surface I restored after the second step ST 2  is completed by the first upper bending roll  62 . 
     In addition, the slanted surface forming portion  64   b  is shaped such that a slanted angle θ 3   a  is 8° that is smaller than 10° being the slanted angle θ 2  of the outer side surface F restored after the second step ST 2  is completed by the first upper bending roll  62 . 
     Shape and size of the second upper bending roll  64  are not limited to these and can be controlled as occasion demands. 
     After the concave surface I and the neighboring outer side surface F of the shaped beam  30  are formed by the concave surface forming portion  64   a  and the slanted surface forming portion  64   b  of the second upper bending roll  64 , residual stress remained at the concave surface I that is formed to the maximum allowable depth d 2   a  at the second step ST 2  and the outer side surface F is absorbed, and plastic deformation is led. Through the plastic deformation, the maximum depth d 3  of the concave surface I is maintained to be 3 mm and the slanted angle θ 3  of the outer side surface F to the vertical line is about 8°. 
     The fourth step ST 4  is performed by the third and fourth and the fifth bending roll units BR 3 , BR 4 , and BR 5  of the round bender  6  at the bending step S 6 . 
     After the concave surface I of the shaped beam  30  is disposed so as to face toward the inward of the curvature, the shaped beam  30  is formed to have the curvature through three steps at the fourth step ST 4 . 
     Referring to  FIG. 8 , deformations of the concave surface I and the slanted outer side surface F, when the shaped beam  30  is formed to have the curvature through the three steps at the fourth step ST 4 , will be described in detail. 
     As shown in  FIG. 8 , the third, fourth, and fifth lower bending rolls  65  of the third, fourth, and fifth bending roll units BR 3 , BR 4 , and BR 5  have the same size and shapes as the lower surface of the shaped beam  30 . 
     In addition, the concave surface forming portion  42   a  and the slanted surface forming portion  42   b  for forming the concave surface I and the slanted outer side surface F are not formed at the third, fourth, and fifth upper bending rolls  66 . 
     Accordingly, when the curvature is formed at the shaped beam  30  passing through the third, fourth, and fifth upper bending rolls  65 , the heel tap occurring at the surface of the shaped beam  30  facing toward the inward of the curvature is absorbed by the concave surface I. 
     At this time, the stress generated at the concave surface I by the heel tap is delivered to the outer side surface F adjacent to the concave surface I and is absorbed at the outer side surface F as the slanted angle becomes small. 
     As the shaped beam  30  passes through the third upper bending roll  65 , the maximum depth d 41  of the concave surface I is increased to 3.2 mm and the slanted angle θ 41  of the outer side surface F is reduced to 5° at the step ST 41 . 
     As the shaped beam  30  passes through the fourth upper bending roll  65 , the maximum depth d 42  of the concave surface I is increased to 3.4 mm and the slanted angle θ 42  of the outer side surface F is reduced to 2° at the step ST 42 . 
     As the shaped beam  30  passes through the fifth upper bending roll  65 , the maximum depth d 43  of the concave surface I is maintained to 3.4 mm but the slanted angle θ 43  of the outer side surface F is reduced to 0° at the step ST 43 . 
     As described above, after the concave surface I is formed at the surface of the shaped beam  30  sequentially bent to have at least one of closed section by the plurality of roll formers R 1 -R 7  at the roll forming step S 4 , the stress generated at the inward surface of the curvature of the shaped beam  30  by the heel tap is absorbed by the concave surface I at the bending step S 6  according to an exemplary embodiment of the present invention. Therefore, occurrence of the dent D is prevented. 
     When the concave surface I is formed at the inward surface of the curvature of the shaped beam  30 , the outer side surface F adjacent to the concave surface I is formed as the slanted surface so as to absorb irregular deformation of the neighboring side surface of the concave surface I. 
     At this time, the outer side surface F is formed to have predetermined slanted angles respectively at the first step ST 1 , the second step ST 2 , and the third step ST 3 . 
     On the contrary, the slanted angle of the outer side surface F is not regulated at the fourth step ST 4  so as to absorb the stress generated at the concave surface I by the heel tap when the shaped beam  30  is formed to have the curvature. 
     The shaped beam  40  formed to have the curvature by the roll forming method is formed to have two closed section, and is produced to be a bumper beam  60  for a vehicle shown in  FIG. 9  through the cutting step S 7 . 
     The shaped beam produced by the roll forming method prevents deterioration of impact strength as a consequence of preventing occurrence of the dents at the lower closed section due to the heel tap as shown in  FIG. 9 . 
     In a case that the shaped beam according to an exemplary embodiment of the present invention is applied to the bumper beam  60  for the vehicle, curved surface is not formed around inner and outer holes H to which the towing hook pipe (not shown) is assembled. Therefore, towing hook pipe (not shown) can be easily welded. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.