Patent Publication Number: US-2010126244-A1

Title: Method for forming high tensile strength metal sheet

Description:
TECHNICAL FIELD  
     The invention relates to metal sheet forming, more particularly to a technique of bending high tensile strength sheets. 
     BACKGROUND OF THE INVENTION  
     Because metals have good strength and flexibility, they are usually used as a structural element requiring strength. When a force applied on a unit area of metal exceeds its tensile strength, the metal will not split like ceramic materials, but will be deformed. That is to say, metals have a property of plasticity, i.e. plastic deformation. Usually, a power press machine is used for deforming a metal. There is an upper and lower dies in a power press machine. By means of stroke, the metal material being placed between the two dies can be deformed as the shape formed by the dies. 
       FIG. 1  illustrates a conventional technique of forming a high tensile strength metal sheet. The high tensile metal sheet  10  is placed between an upper die  20  and a lower die  22 . During the pressing process, a blank holder  24  is used for clamping the sheet  10 . However, after the pressing action, the ends  12  of the sheet  10  tend to form an undesired warp or curl (i.e. “recoil”) because of its inner stress against the pressing force. Thus, the quality of metal sheet forming will be adversely affected. As shown in  FIG. 2 , element  12  is the desired shape of sheet, but a really resultant shape is like element  12 ′. 
     Moreover, an additional post-production is required because the desired shape can not be accomplished at a time. Therefore, the conventional forming process with post-production includes the steps of: a)forming; b)cutting; c)trimming and d)re-trimming or, a)loading; b)forming; c)re-forming; d)trimming and e)re-trimming. Any of the above processes is lengthy and tardy. The most important point is that pressing veins or scratches will be left on the sheet after various steps. Quality of the finished products will be considerably low unless a finally additional polishing step is performed. 
     For example, in order to avoid warps or curls, and to increase accuracy of the products formed, the steps of forming and trimming must be repeatedly performed. It is very uneconomic. Furthermore, the forming process with multiple forming and trimming steps may also reduce lifetime of the dies. Meanwhile, the formed products will be hardened. It is disadvantageous to the latter process. 
     SUMMARY OF THE INVENTION  
     A primary object of the present invention is to provide a method for forming a high tensile strength sheet, which can avoid recoil after the HTSMS is formed. 
     An another object of the present invention is to provide a method for forming a high tensile strength sheet, which can shorten the forming process because of avoidance of recoil. 
     An another object of the present invention is to provide a method for forming a high tensile strength sheet, which can improve quality of finished products being formed by means of one-time forming. 
     To accomplish the above-mentioned objects, the present invention provides a method for forming a high tensile strength metal sheet (HTSMS), which is performed in a power press machine composed of a die, a punch and a blank holder. The HTSMS is placed between the die and punch with blank holder. A step portion appears at both an outer side of the punch and a pressing portion of the blank holder. The step portion is corresponding to a predetermined bent portion of the HTSMS. A step will be formed at an edge of the HTSMS by progressively pressing of the step portion to avoid recoil of the HTSMS after forming process. 
     By means of the above machine, the method of the present invention comprises the steps of: 
     a) setting the blank holder at a predetermined position which is coplanar with the punch, and the high tensile strength metal sheet (HTSMS) being placed on a plane formed by the punch and the blank holder; 
     b) driving the punch to ascend to a first position to make the HTSMS roughly deformed; 
     c) driving the punch to ascend to a second position to make the HTSMS sequentially deformed; and 
     d) driving the punch to ascend to a third position to make the HTSMS finally deformed with a step formed by the step portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
         FIG. 1  illustrates a conventional forming machine for high tensile strength metal sheet (HTSMS); 
         FIG. 2  illustrates the HTSMS formed by the machine shown in  FIG. 1 ; 
         FIG. 3  illustrates the power press machine of the present invention; 
         FIGS. 4-6  illustrate a series of pressing process using the machine shown in  FIG. 3 ; 
         FIG. 7  illustrates the HTSMS which is finally formed; and 
         FIG. 8  illustrates another embodiment of the power press machine according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     The high tensile strength metal sheets mentioned in the present invention can be high tensile strength steel sheets made of steel adding several metal elements, such as, but not limited to, niobium, copper, vanadium and titanium. The high tensile strength metal sheets increasingly become popular and important for car industry because they have better properties than traditional materials. 
     Referring to  FIG. 3 , the power press machine applied in the invention includes a punch  32 , a die  30  and a blank holder  34 . The die  30  can also be called “female die”. Either the die  30  or the punch  32  is driven by a driving unit  40  to reciprocate upward and downward. The reciprocating motion of the die  30  or the punch  32  can be periodic or multi-staged by a program. The die  30  usually has a recess corresponding to the punch  32 . As shown in  FIG. 3 , the die  30  and punch  32  are correspondingly recessing and projecting, respectively. The blank holder  34  can provide a pressing force onto the high tensile strength metal sheet (hereinafter “HTSMS”) to clamp it when both the die  30  and punch  32  are in motion. During the forming process, the HTSMS  10  is placed and clamped between the blank holder  34  and die  30 , and then the punch  32  moves toward the HTSMS  10  and finally into the die  30 . 
     A feature of the invention is that a step portion S is formed at both the outer side of punch  32  and a pressing portion  34   a  of the blank holder  34 . The step portions S are arranged near the edge sections  12  which are the predetermined positions to be bent. The distance between one of the edge sections  12  and an adjacent edge is about 5 mm-20 mm. A step  14  is formed within the edge sections  12 . A gap C is retained between the punch  32  and the blank holder  34 . The blank holder  34  shown in the drawings is of a block type. The HTSMS  10  placed on the punch  32  and blank holder  34  is pressed by the die  30  and formed by the step portions S. Therefore, the step  14  is finally formed to avoid recoil of edge of the HTSMS  10 . 
     Some factors are not considered by the invention, such as the friction between the blank holder  34  and die  30  or punch  32 , and the pressing force from the blank holder  34 . Referring to  FIGS. 4-6 , the punch  32  connects to a driving mechanism  40 , for example an oil press, for driving punch  32  to reciprocate.  FIG. 4  illustrates the beginning of the forming process. The blank holder  34  is disposed at a predetermined height which is approximately coplanar with the punch  32 . The HTSMS  10  is placed on the plane formed by coplanar punch  32  and blank holder  34 .  FIG. 5  illustrates the first stage of forming. Blank holder  34  or its pressing portion  34   a  can provide a proper pressing force onto the HTSMS  10  to hold it when punch  32  and die  30  perform relative reciprocation. The driving mechanism  40  drives punch  32  to ascend to a first position so that the HTSMS  10  becomes roughly deformed. The deformed portion is driven by punch  32  to enter die  30 .  FIG. 6  illustrates the second stage of forming. Driving mechanism  40  drives punch  32  to further ascend to a second position so that the HTSMS  10  is sequentially deformed. The HTSMS  10  is protruded by ascending punch  32  to progressively reach die  30 . Step portion S initially appears.  FIG. 7  illustrates the third stage of forming. Driving mechanism  40  finally drives punch  32  to ascend to a third position so that the HTSMS  10  is completely deformed. Edge sections  12  of HTSMS  10  separately form the step  14  by step portions S constituted by punch  32  and blank holder  34 . Therefore, the HTSMS  10  can be sufficiently under pressure during the forming process so that the HTSMS  10  will generate a plastic deformation. After the HTSMS  10  is formed, the step  14  will overbear or offset the recoiling force from edge sections  12 . Accordingly, the formed products can be finished only after steps of cutting and trimming. 
     Additionally, the above-mentioned first, second and third positions are shown in separate drawings, but in fact, the ascension of punch  32  driven by driving mechanism  40  is linearly continuous. 
       FIG. 8  illustrates another preferred embodiment. An adjusting mechanism  36  is preferably disposed at an edge of die  30 . The adjusting mechanism  36  is composed of an adjustable screw rod  362  and a block  364 . The adjusting mechanism  36  can adjust step portions S between punch  32  and blank holder  34 . The positions of step portions S can be adjusted by rotating the screw rod  362  to make block  364  move inward or outward to a specific position. This can change a relative depth of step portions S formed by punch  32  and blank holder  34 . Thus, adjusting mechanism  36  can match requirements of different thickness, bending angles and/or properties of metal sheets so that the forming process can be preferably completed. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.