2022 Volume 63 Issue 1 Pages 82-87
In the draw forming of ultrahigh-strength steel sheets into the L or T shape seen at the end of automobile parts, the top face easily wrinkles and the vertical wall easily breaks. For satisfactory forming, good elongation characteristics of the material are necessary. For this reason, it is difficult to form small-elongation ultrahigh-strength steel sheets into the L or T shape by draw forming. In this study, fundamental research has been carried out on the forming of an ultrahigh-strength steel sheet into the T model shape by finite element method (FEM) analyses and press experiments. The draw forming and the new bending method utilizing high-load pads “free bend sheet forming” were compared and the states and mechanisms of forming failure generation were analyzed. In free bend sheet forming, it was confirmed that an ultrahigh-strength steel sheet with a tensile strength of up to 1470 MPa can be formed into the T model shape by suppressing the generation of cracks and wrinkles.
This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 283–288.
The steel sheets for structural parts of automobile bodies have been continuously strengthened over time. Ultra-high-strength steel sheets with tensile strengths greater than 980 MPa have been increasingly applied in recent years to provide both a weight reduction and collision safety for a vehicle structure. The application scope is expanding from parts with a simple shape to main structural parts with more complex shapes. The main technical issues in the formation of high-strength steel sheets are the shape distortions of the formed products owing to springback during unloading, and poor formability owing to the low elongation of the sheets.1,2)
A conventional countermeasure against shape distortion is die shape anticipation, whereby the die shape is designed so as to compensate for the springback of the formed product. New approaches have been proposed to simplify the die shape anticipation by improving the springback estimation accuracy via finite-element method (FEM) analyses.3,4) Methods for reducing springback by changing product shapes and forming methods have also been developed.5)
To resolve forming failures such as cracks and wrinkles, new methods have been developed, such as a shearing technique to increase the elongation limit of the sheet edge and thus counter the cracking at the ends of the steel material (edge cracking),6,7) and forming methods for avoiding strain concentrations.8) However, few countermeasures are effective against cracking and wrinkling when forming a complex shape, where the cross-sectional line changes significantly from the forming.
Figure 1 shows examples of complex shapes that are difficult to form with high-strength steel sheets. The front and center pillars are the main structural parts of an automobile and have L and T shapes at the bottom, respectively, to connect to the side sill. The front pillar is an L shape with a curved vertical wall and flange, seen at the end of the part, for connecting to other parts. A T-shaped part can be considered as two overlapped L shapes. L and T shapes are difficult to form with high-strength steel sheets because cracks and wrinkles occur frequently in the deep drawing processes of these shapes.
Examples of L and T-shaped parts.
In this study, the crack and wrinkle occurrence processes in the deep drawing of a basic T shape with a curved vertical wall and flange were analyzed. A new method for avoiding cracking and wrinkling was examined and demonstrated by press experiments. In this method, wrinkling was overcome with a pad by restraining the flow of excess material into the portion under the pad, and cracking was controlled by bending to eliminate the material inflow resistance.
As a model shape of an actual automobile structural part, a T shape with a width of 310 mm, length of 285 mm, and height of 50 mm was selected. As shown in Fig. 2, a portion of the part was denoted as the upper side for the positive side of the y-axis of the part, the lower side was used for the negative side, and the side edges of the T shape at the lower portion of the part were in the x-directions. First, the deep drawing was studied using a FEM analysis, based on the tools and blank shape shown in Fig. 3. Steel sheets with a thickness of 1.2 mm and tensile strength ranging from 590 to 1470 MPa were used as the blank material; the representative mechanical properties are listed in Table 1. To investigate the effect of the inward material flow on the wrinkling and thickness decreases of the sheets, the blank holding force (BHF) was set to three levels: 490, 980, and 1470 kN. The die was pressed through the stroke end as the forming condition. A shell element (2 mm in size) with seven integration points in the thickness direction was applied to the blank. The die was set as a rigid body, and the friction coefficient between the blank and the die was 0.1. A universal dynamic explicit method solver (LS-DYNA ver. 971) was employed for the FEM analysis. The forming limit strains of the respective materials were measured and plotted in a forming limit diagram (FLD) to determine the crack occurrence. Wrinkling was judged based on the curvature distribution of the steel sheets when the die was 0.5 mm above its stroke end.
Model shape.
Die construction and blank shape in draw forming.
Figure 4 shows the results of the FEM analysis for Material B (980 MPa) under a BHF of 980 kN. Significant wrinkling occurs at the center of the top face. The maximum reduction in the thickness of the vertical wall is 15.5%. This value is almost the same as the elongation measured by the tension test, and thus may indicate a crack in the vertical wall. The wrinkle occurrence and thickness reduction show the same trends under all conditions. The analysis results under different conditions are presented in Table 2. The reduction in the thickness of material A (590 MPa) is within the forming limit. However, the reductions in thickness for materials with a tensile strength greater than 980 MPa exceed the forming limit, and thus, cracks are predicted. Moreover, significant wrinkling is predicted at the center of the top face under all conditions, regardless of the BHF.
Results of FEM analysis in draw forming (Material B: 980 MPa, BHF = 980 kN).
The main forming failures of the T-shaped model in drawing are the wrinkles at the center of the top face and cracks in the vertical wall. The details of this are discussed herein. As shown in Fig. 5, two 200-mm line segments are considered: a-a at the blank center, and b-b 150 mm away from the center in the y-direction. The shape of a-a does not change during forming, but the length of b-b in the y-direction shortens to form a vertical wall. Thus, a-a is pulled by b-b in the y-direction during forming, and a compressive force is induced, resulting in wrinkles at a-a. In contrast, b-b is stretched to b′-b′ while being restricted by a-a; thus, the inward flow is not sufficient, and results in the reduction of the thickness and cracking. Table 3 shows the change in the line length for material B under a BHF of 980 kN. a′-a′ is shorter than a-a by 6.7 mm and b′-b′ is longer than b-b by 18.0 mm. In the formation of the vertical wall, a noticeable material flow is necessary to realize the length change of line b-b, i.e., 16 mm to the lower direction from the upper side, and 4.5 mm to the upper direction from the lower portion. A similar difference in the material flow also occurs in the x-direction between the wall-forming zone and non-wall-forming zone. This localized difference in the material flow during the formation of the vertical wall is the reason for the wrinkles at the center of the top face and cracks in the vertical wall.
T shape models before and after forming.
During drawing, the material at the center of the top face should be stretched to suppress wrinkles. This stretching force tends to cause shortages in the formation of high-strength steel sheets, owing to the limits of the material elongation. To compensate for this limitation, a method for suppressing wrinkling was examined. The basic concept is to restrict the excess portion of the material with a tool to restrict the out-of-plane deformation in the initial forming stage, while controlling the flow resistance to reduce the elongation of the material.
A new method based on the basic concept is proposed for the T shape shown in Fig. 2. As shown in Fig. 6, it is a bending method, where the punch and pad clamp the top face from the initial forming stage to suppress wrinkling. Simultaneously, the expanded shape of the product is employed as the blank shape to remove the excess portion of the blank, and the blank holder is excluded from deep drawing. As a result, the flow resistance of the material is reduced, thereby suppressing cracking.9,10) In this paper, we refer to the proposed method as “free bend sheet forming”.
Die construction and blank shape in developed forming method.
The shape and materials in the free bend sheet forming analysis are identical to the conditions used in the drawing, i.e., as shown in Fig. 2 and Table 1. The FEM analysis conditions, such as the element and friction coefficient, analysis solver, and crack and wrinkle judgment methods, also remain the same as those used in the drawing simulation. As shown in Fig. 6, the tool layout is the same as that of the pad bending. The die is pushed through the stroke end in the analysis. The pad load is set to 147 kN to suppress wrinkles on the top face under all conditions. Cracks are predicted, owing to the strain concentrations on the edges when the expanded shape of the product is employed as the blank shape. Thus, the blank shape is slightly adjusted to allow for the strain distribution on the edges.
3.3 Results of FEM analyses of free bend sheet formingThe distributions of the reductions in thickness and curvature for materials A (590 MPa) and D (1470 MPa) are shown in Fig. 7. Furthermore, the analysis results for all conditions are summarized in Table 4. The reduction in thickness attains a maximum at the curved die shoulder under all conditions, but the value is 8.3% or less, and becomes smaller with increasing steel strength. Under all conditions, no cracks occur, and wrinkling can be suppressed, even on the top face; thus, satisfactory formation can be achieved. In addition, as shown in Fig. 8, wrinkles do not occur on the top face from the initial stage to the end of forming, because the out-of-plane deformation is restricted by the punch and pad. As all materials except for the top face are formed by bending, the material at the lower side of the T shape significantly flows into the curvature to form the vertical wall and flange. Whereas the material flows from the upper side to form the vertical wall in drawing, it flows from the lower side in free bend sheet forming.
Results of FEM analyses in developed forming method (rate of thickness decrease and curvature distribution).
Forming process in developed forming method.
Figure 9 shows the FLD of material B (980 MPa) in drawing, and Fig. 10 shows the FLD in free bend sheet forming. From comparing the strain states related to wrinkling, it can be seen that the strain occurring on the top face is widely distributed in the compressed area during drawing. In contrast, the strain level in free bend sheet forming is significantly reduced, and is hardly distributed in the compressed area. This phenomenon results from the fact that the high-load pad restricts the material from flowing into the top face throughout the forming process. From comparing the strain states relating to cracks, it can be seen that a strain exceeding the forming limit curve (FLC) occurs in the uniaxial tension area in drawing. A large strain occurs mainly in the vertical wall, where the sheet thickness is significantly reduced. In free bend sheet forming, the material from the side edges of the T shape flows into the curved portions; it is not stretched significantly during the formation of the vertical wall, and thus the maximum principal strain ε1 is significantly reduced. The strain occurring in free bend sheet forming is distributed in the same manner, regardless of the material strength. It is distributed approximately near the shear deformation area, except for in the uniaxial tension area occurring at the edges of the material. Figure 11 shows the strain state of the FLD in the free bend sheet forming of material B (980 MPa). The area near the ridge line of the punch shoulder at the curved portion is formed by shear deformation.
FLD in draw forming.
FLD for developed forming method.
Shear deformation area in formed panel.
The hat shape of the upper side is a simple bent shape; therefore, it can be easily formed without the occurrence of a large strain. The challenge is in the formation of the vertical wall of the T shape on the lower side. In the case of drawing, the material flow from the lower side is also resisted, so the material primarily flows from the upper side in the formation of the vertical wall. Thus, the different amounts of material flowing into the center and edges cause the center of the top face to wrinkle, and the vertical wall to be significantly thinned. In contrast, in the case of free bend sheet forming, the out-of-plane deformation of the top face is restricted by the pad, and the material does not move significantly. Moreover, the material flows from the lower side to form the vertical wall without significantly stretching the material, as the material at the lower side is not restricted. The material flowing from the lower side into the curved portion stretches the material edge on the lower side, while a compressive force is applied to the area near the ridge line of the punch shoulder at the curved portion in the circumferential direction. The elongation of the lower side at this stage is not very large; therefore, the maximum strain can be reduced by optimizing the blank shape and distributing the strain to the level possible to form a high-strength steel sheet. The area near the ridge line of the punch shoulder at the curved portion tends to wrinkle because of the compressive force in the circumferential direction. However, the out-of-plane deformation of the nearby top face is suppressed by the pad, and the material of this portion is compressed in the circumferential direction to such a degree that wrinkles do not occur. The material is pushed out perpendicular to the ridge line and stretched, and as a result, it is deformed under the shear deformation mode. The basic points are the control of the out-of-plane deformation of the portion where the material tends to become excessive, and the reduction of the stretching strain by freely deforming the material. Therefore, free bend sheet forming is thought to significantly improve the formability of high-strength steel.
The above examination was conducted under a constant friction coefficient between the blank and die; however, a larger friction coefficient increases the maximum reduction in thickness, because the material flow from the side edges of the T shape is restricted. Thus, the effect of the friction coefficient on the material flow and reduction in thickness should be examined in detail in the future.
Experiments were conducted using a bending experimental die with a pad, as shown in Fig. 12, to confirm the effects of the formability improvement from free bend sheet forming. The die set consisted of a punch on the upper side and a die and pad on the lower side. The die cushion of the press machine was employed to apply a load to the pad. The product shape was almost the same as that in Fig. 2, but bead shapes were added at the flange portion, as shown in Fig. 13. Four types of steel sheets were used, in tensile strength classes of 590, 980, 1180, and 1470 MPa, respectively. The sheet thickness was 1.2 mm. The blank shape shown in Fig. 6 was used. As the forming conditions, the pad load and forming load were set to 147 kN and 1960 kN, respectively.
Experimental die set.
Model shape for experimental press.
Forming simulations were performed under the experimental conditions. The reduction in the thickness and curvature distributions of material C (1180 MPa) are shown in Fig. 14. The analysis results for all conditions are summarized in Table 5. The maximum reduction in thickness is slightly larger than that in Table 4, owing to a localized decrease in sheet thickness owing to the bead shapes of the flange. The maximum reduction in thickness is within the range of 8.2% to 9.3%, and the strain is within the FLC; therefore, it is predicted that cracking will not occur. The curvature distribution shows contact traces in the bead edge shape at the die shoulder on the curved portion of the vertical wall, but no wrinkles occur. The analysis results are almost identical to those in Fig. 7 and Table 4, except for the localized decrease in sheet thickness by the beads on the flange and the contact traces on the vertical wall.
Results of FEM analyses of experimental press (Material C: 1180 MPa).
Photographs of the panels subjected to free bend sheet forming are shown in Fig. 15. Forming is successful under all conditions, without necking, cracking, or wrinkles. As for the contact traces at the curved portion of the vertical wall, minor traces equivalent to those in the FEM analyses are recognized. The experimental results under all conditions are as predicted by the FEM analyses.
Prototypes fabricated with experimental press.
The L and T shapes observed at the ends of automobile structural parts are difficult to form with high-strength steel sheets. In this study, the mechanisms of forming failures in drawing were examined by using a T-shape as the model shape. A new forming method called “free bend sheet forming” and using a high-load pad as the countermeasure against forming failures was proposed. The following conclusions are derived as the results from the FEM analyses and experiments.
