2021 Volume 62 Issue 12 Pages 1688-1694
We developed a hot stamping method for panel parts with a step-shaped wall. The die-set for the developed method divides the punch at a step-shaped wall into an outer punch and an inner punch. The outer punch is placed first. In addition, an opening is added to the blank of the developed method. The developed method makes it possible to change the main forming type from draw forming to cylindrical stretch flange forming in the middle of a forming stroke. The stretch flange forming can prevent a fall in temperature at a hole edge, because the hole edge does not come into contact with the die-set during forming and because the high ductility of high-temperature materials can be used, making it advantageous for hot stamping. If the preceding amount of the outer punch is large with the developed method, the effect of suppressing wrinkles is large but the sheet thickness reduction that can cause cracking rises. If the preceding amount of the outer punch is small, the rate of sheet thickness reduction ratio is greatly restrained, but the wrinkles become larger. By setting an appropriate preceding amount of the outer punch with the developed method, forming without cracks or wrinkles is possible.
This Paper was Originally Published in Japanese in J. JSTP 61 (2020) 75–80. Figure 1 is slightly modified. Table 2 is slightly modified. The caption of Fig. 9 is slightly modified.
Fig. 1 Schematics of the developed hot stamping method and a conventional cold stamping method.
As laws and regulations have been tightened worldwide for automobile crashworthiness and global environmental protection,1) automakers have been expanding the use of hot-stamped parts to improve the crashworthiness and fuel efficiency of automobiles.
Hot stamping is a method for heating steel blanks in the austenite region and forming the heated steel blanks with die-set while cooling them more than critical cooling speed to about 200°C (under martensitic transformation temperature) at the bottom dead center to secure a strength of the 1.5 GPa class and a small quantity of springbak.2–5) For these reasons, hot-stamped parts are mainly used as parts that must be crashworthy, such as frame parts and bumper beams.6–8) One of the disadvantages of hot stamping is a poor deep drawability. Bending is mainly applied to these parts.9) The poor deep drawability of hot stamping may be explained as follows. The flange of a deep-drawn part is held between the die-set throughout from the start of the forming stroke to the bottom dead center. The transformation of austenite to martensite or bainite in this process increases the deformation resistance of the flange. The resultant decrease in the material flow causes the fracture of the part at the punch shoulder or in the upper wall of a step-shaped wall. This poor deep drawability restricts the number of parts that can be formed by hot stamping. That is, hot stamping is not generally employed to produce parts that must be deep drawn like door inner panels. Especially, the door inner panels are deep drawn with step-shaped walls. When the door inner panel is cold stamped, the material in the step-shaped wall in the corner is not constrained, giving rise to wrinkling. As the step-shaped wall is formed, the stretch in the upper wall increases to increase the thickness reduction ratio. In this reason, the door inner panel is a difficult-to-form part.10) If hot stamping can be applied to such difficult-to-form parts, it will greatly contribute to the weight reduction of automobiles. One of the advantages of hot stamping is its high ductility at high temperatures. In cylindrical hole expansion, the part is formed so that the edge of the hole being expanded does not contact the die-set from the start of the forming stroke to the arrival at the bottom dead center. It is reported that cylindrical hole expansion by hot stamping prevent the cooling of the hole edge and increases the forming height.11) As a method for improving the deep drawability of hot-stamped parts, Ota et al.12) proposed a method whereby air is injected onto the vulnerable upper wall of the step-shaped wall in the part being formed in order to increase the fracture strength of the part in contact with the punch shoulder. Also, reports have been published about technologies for preventing the temperature drop of the flange of the part by providing a gap between the die and the holder.13,14) Furthermore, a method is proposed for bending and stretching the flange of the part to prevent wrinkling without holding the flange in hot stamping.15) Few forming methods are reported that make the most of the high ductility of hot blanks, which is one of the advantages of hot stamping. In this study, we developed a new hot stamping method that can form deep drawn and stepped parts without wrinkling and fracturing. In the early stage of the forming stroke, the part is drawn to the specified height. In the late stage of the forming stroke, the forming mode transitions to stretch flanging, which is another advantage of hot stamping. We clarified the effects of the forming mode transition timing on the wrinkling behavior and the thickness reduction.
Figure 1 shows the conventional cold stamping method and the new hot stamping method for forming a panel parts with a step-shaped wall like a door inner panels. The die-set of the conventional cold stamping method are a punch, a die, and a holder. Since the step-shaped wall in the door inner panel is not constrained during forming, it is likely to wrinkle. As the step-shaped wall is formed near the bottom dead center, the stretch increases to increase the fracturing tendency of the upper wall. The new hot stamping method uses two punches, or an inner and an outer punch. As shown in Fig. 1(b), the outer punch is designed to move down to form the step-shaped wall of the door inner panel. The amount of this movement is hereinafter referred to as the amount δ. Also, the new hot stamping method is characterized by use of pre-holed blanks. Further, the new hot stamping method controls the gap between the die and the holder to more than the blank thickness by inserting a spacer between them.13) The forming mode of the new hot stamping method that uses these die-set and blanks is drawing in the early stage of the forming stroke and then transitions to stretch flanging when the material in the step-shaped wall is held between the outer punch and the die in the middle of the forming stroke. The new hot stamping method can prevent the wrinkling of the step-shaped wall because the material in the step-shaped wall influenced to wrinkling is held between the die and the outer punch before the blank reaches the bottom dead center. Because the blank has a hole, the material flows from the top of the inner punch to the upper wall and undergoes stretch flange deformation, which is a forming mode easy to occur in hot stamping. The fracturing of the upper wall is thus prevented.
Schematics of the developed hot stamping method and a conventional cold stamping method.
To investigate the effectiveness of the new hot stamping method and the effect of the amount δ on wrinkling and thickness reduction, we conducted experiments by using door inner panel forming die-set. The experiments used galvannealed steel blanks for hot stamping (0.2%C–0.2%Si–1.3%Mn–0.2%Cr–0.0015%B composition and 45/45 g coating weight). In addition, JFS A 2001 Grade JSC270D steel blanks generally used for door inner panels cold-stamped for comparison. The blanks were each 1.2 mm thick. The target tensile strength of the hot stamping blanks was 1,500 MPa. Table 1 shows the mechanical properties of the hot stamping steel at 750°C and the cold stamping steel at room temperature. As hot stamping conditions, the blank was heated in a gas-fired furnace at 900°C for 4 min, set in the die-set, and formed at 750°C. The forming speed was set to 40 mm/s. The holder force was set to 490 kN (contact pressure of 3.7 MPa) and the outer punch force was set to 294 kN (contact pressure of 2.6 MPa). The gap between the die and the holder was set to 1.3 mm. Figure 2 shows the appearance of the punches, the structure of the die-set, and the shape of the blank. As a experimental condition, the amount δ was set to 0, 5, 10, 15, and 20 mm. The amount δ of 0 mm is equivalent to that of the conventional die-set for comparison. Figure 3 shows the appearance of a door inner panel formed at δ = 15 mm as well as its front and bottom views and dimensions. The maximum forming height was set to 80 mm. The height of the vertical walls above and below the step-shaped wall at the maximum forming height was set to 40 mm for both the upper and lower walls. Holes were provided in the positions corresponding to the tops of the two punches to allow for stretch flange deformation in the middle of the forming stroke. The radius (R80) of curvature of the hole edge was set to form a concentric circle with the same center as that of the circle with the top view radius R200 of the inner punch. Three experiments were conducted under the same conditions.
Die-set and blank.
Formed panel.
To verify the effectiveness of the new hot stamping method in preventing wrinkling, we compared the shape of the formed panel and the outer punch in the step-shaped wall of the corner where wrinkling is likely to occur. The shape comparison method is shown in Fig. 4. The surface shape of the formed panel in the areas A and B in contact with the outer punch was measured with a noncontact three-dimensional shape measuring instrument COMET5-M2. The position of the panel was adjusted to minimize the difference in the shape data between the outer punch and the panel. Figure 5 compares the shape of the hot-stamped panel and the punch at δ = 0 mm for the conventional cold stamping method and of the hot-stamped panel and the outer punch at δ = 15 mm for the new hot stamping method. The larger the color change in Fig. 5, the larger the wrinkling. The wrinkling at δ = 15 mm for the new hot stamping method were smaller than those at δ = 0 mm for the conventional cold stamping method. This means that the new hot stamping method reduced the wrinkling tendency. Figure 6 shows the cross-sectional shape d of the hot-stamped and cold-stamped panels along the 150 mm long line C shown in Fig. 4. The effectiveness of the new hot stamping method in preventing wrinkling was also recognized with cold stamping. The average of the shape differences between the panels and the die-set along the line C in the out-of-plane direction is adopted as an index of the wrinkle size (or the average shape difference w). Figure 7 shows the effect of the amount δ on the average shape difference w. The wrinkling of the step-shaped wall tended to decrease as the amount δ increased for both the hot-stamped and cold-stamped panels. These results showed that the wrinkling of the step-shaped wall can be prevented by holding the step-shaped wall between the upper and lower die-set before the bottom dead center as aimed by the new hot stamping method. It was also found that increasing the amount δ increased the wrinkle preventing effect.
Measurement areas and lines.
Shape difference between punch and formed panel of HS.
Sectional shape difference between punch and forming panel.
Influence of preceding amount of outer punch δ on wrinkle.
Table 2 shows the relationship between the amount δ and fracturing. The open circles and x’s in the table indicate no fracturing and fracturing, respectively. According to the table, no fracture occurred until the amount δ reached 15 mm. At δ = 20 mm, the hot-stamped panel fractured in the upper wall on the line A shown in Fig. 4.
Figure 8 shows the distribution of the thickness reduction ratio et along the lines A and B at δ = 15 mm. The thickness reduction ratio et was derived from eq. (1). In eq. (1), t0 and t1 are the blank thicknesses before and after forming, respectively. The hot-stamped panel had a region on the line A where the thickness reduction ratio et was locally high in the upper wall. The cold-stamped panel had no regions where the thickness reduction ratio et was locally high. The line B was different from the line A in that the thickness reduction ratio et at the punch top was highest and uniformly distributed for both the hot-stamped and cold-stamped panels and that there were no regions where the thickness reduction ratio et was locally high. Figure 9 shows the effect of the amount δ on the maximum thickness reduction ratio etmax. The positions where the maximum thickness reduction ratio etmax was indicated were in the upper wall for both the hot-stamped and cold-stamped panels along the line A and were at the punch top for both the hot-stamped and cold-stamped panels along the line B. The thickness at 1 mm from the fractured edge was measured for the panels with step-shaped walls fractured at δ = 20 mm. With all the formed panels, the maximum thickness reduction ratio etmax increased as the amount δ increased. This is probably because the larger the amount δ, the more severely the material in the step-shaped wall is prevented from the early stage of the forming stroke and the smaller the material flow amount becomes. The effect of the amount δ on the maximum thickness reduction ratio etmax on the line A of the hot-stamped panels is larger than that under the other forming conditions. Consequently, the hot-stamped panels are considered to have fractured in the step-shaped wall at δ = 20 mm. To prevent fracturing in the new hot stamping method, it is important to reduce the effect of the amount δ on the maximum thickness reduction ratio etmax.
| \begin{equation} \text{e}_{\text{t}} = (\text{t}_{0} - \text{t}_{1})/\text{t}_{0} \times 100 \end{equation} | (1) |
Distribution of thickness reduction ratio et, δ = 15 mm.
Effect of the preceding amount of the outer punch δ on maximum thickness reduction ratio etmax.
The Vickers hardness was measured in one-quarter thickness positions at the center of each of the flange 1, the lower wall 2, the step-shaped wall 3, the upper wall 4, and the punch top 5 on the lines A and B at δ = 15 mm. These measurements were averaged. The results are shown in Fig. 10. The Vickers hardness at the punch top on the line B was lower than in the other regions. The other regions were equivalent in the Vickers hardness.
Vickers hardness, δ = 15 mm.
We numerically analyzed why the amount δ had a larger effect on the maximum thickness reduction ratio etmax on the line A of the hot-stamped panel than on the line B and why the hardness at the punch top was lower than in the other regions on the line B. Heat-forming coupled analysis was conducted by using the general-purpose FEM code LS-DYNA Version 971. Thermo-elastoplastic shell elements were used for the blank and rigid shell elements were used for the die-set by considering thermal conduction. The mesh size of the blank was set to 2 mm, the friction coefficient was set to 0.4,16) the forming start temperature was set to 750°C, and the forming speed was set to 400 mm/s. As physical properties by temperature, the density, thermal conductivity, Young’s modulus, specific heat, and stress-strain curves were measured at intervals of 100°C from room temperature to 700°C and thereafter at 750 and 900°C. Since the numerical analysis and the experimentation differed in the forming speed, the thermal conductivity values were scaled 10 times.
4.4.2 Numerical analysis resultsFigure 11 shows the distribution of the thickness reduction ratio at the bottom dead center at δ = 15 mm. Figure 12 compares the experimental and calculated distributions of the thickness reduction ratio et along the line A. From these figures, it can be confirmed that the calculated distribution of the thickness reduction ratio et is locally large in the upper wall along the line A in the same way as indicated by the experimental distribution. Also, at the punch top near the line B, there is a wide region where the thickness reduction ratio et is high as indicated by the experimental results. Figure 13 shows the temperature distribution of a hot-stamped panel near the line A at 1 mm above the bottom dead center at δ = 15 mm. This can confirm that the region where the thickness reduction ratio is locally high in the upper wall is at a high temperature. If the blank being hot stamped has a temperature distribution, a high-temperature region with small deformation resistance preferentially deforms. If the high-temperature region is narrow, therefore, the thickness reduction ratio in the high-temperature region becomes locally high. After the timing when the material in the step-shaped wall was held between the die and the outer punch, the material flow from the flange to the upper wall disappeared. The larger the amount δ, the earlier this timing becomes and the more the material must flow from the punch top to the upper wall. As shown in Fig. 14, the line A is different from the line B in that there is no hole above the punch top. This prevents the material flow from the punch top to the upper wall after the timing when the material in the step-shaped wall is held between the die and the outer punch. As a result, it is considered that the larger the amount δ, the larger the deformation of the upper wall became and the larger the thickness reduction ratio became in the high-temperature region. On the line B, the high-temperature region in the upper wall is narrow. Because there is a hole above the punch top, however, the material flows from the punch top to the upper wall and causes stretch flange deformation after the material is held between the die and the outer punch. The stretch flange deformation is considered to have prevented the maximum thickness reduction ratio etmax from increasing in the upper wall. The stretch flange deformation is a forming mode easy to occur in hot stamping. Because the thickness reduction region at the punch top is wide, the effect of the amount δ on the maximum thickness reduction ratio etmax at the punch top is considered to have become smaller than in the upper wall on the line A. From the above, the reason why the effect of the amount δ on the maximum thickness reduction ratio etmax is larger along the line A of the hot-stamped panel than along the line B may be considered as follows: The forming mode could not transition from drawing to stretch flanging along the line A. Consequently, the strain was concentrated in the high-temperature region in the upper wall as the amount δ increased. With the new hot stamping method, it is important to appropriately locate a hole above the punch top and to transition from drawing to stretch flanging when the material in the step-shaped wall is held between the die and the outer punch.
Distribution of thickness reduction ratio, δ = 15 mm.
Distribution of thickness reduction ratio et of line A, δ = 15 mm.
Temperature distribution of 1 mm just above bottom dead center, δ = 15 mm.
Cross-sectional shapes of lines A and B.
Lastly, the hardness was lower at the punch top than in other regions along the line B. The above-mentioned stretch flange deformation is considered to have widened the thickness reduction region at the punch top, created a gap between the top of the inner punch and the blank at the bottom dead center, and reduced the cooling speed of the blank. This problem may be countered by designing the die-set in consideration of the thickness reduction at the punch top so that no gap occurs between the blank and the top of the inner punch at the bottom dead center.
To form deep-drawn parts with step-shaped walls without wrinkling and fracturing, we developed a hot stamping method that makes the most of the high ductility of hot blanks. The high ductility of hot blanks is one of the advantages of the hot stamping method. The new hot stamping method we developed is as follows:
