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Springback behaviour in bending of ultra-high-strength steel sheets using CNC servo press
ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 47 (2007) 321–325 www.elsevier.com/locate/ijmactool
Springback behaviour in bending of ultra-high-strength steel sheets using CNC servo press K. Mori, K. Akita, Y. Abe Department of Production Systems Engineering, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Received 10 February 2006; received in revised form 21 March 2006; accepted 23 March 2006 Available online 18 May 2006
Abstract The springback behaviour of ultra-high-strength steel sheets in bending was investigated under controlled conditions using a CNC servo press. Although the ultra-high-strength steel sheets are attractive in reducing weight of cars, the amount of springback of the ultrahigh-strength steel sheets in the forming is very large due to high strength. The CNC servo press has the function of accurately controlling the motion by two servo motors. The effects of the material, the finishing reduction in thickness, the forming speed and the holding time at the bottom dead centre on the amount of springback in V-shaped bending were examined. The scatter of the springback for the ultrahigh-strength steel sheets was improved by using real thickness and not nominal thickness of each individual sheet in the control of the punch stroke. The amount of springback for the ultra-high-strength steel sheet in the V-shaped bending was much larger than that for the mild steel sheet, and the amount was decreased by the finishing reduction in thickness direction because of uniform stress distribution. The effects of the forming speed and the holding time at the bottom dead centre were small. The amount of springback for the steel sheets was almost proportional to the ratio of the tensile strength to the elastic modulus. r 2006 Elsevier Ltd. All rights reserved. Keywords: Springback; Ultra-high-strength steel sheet; Bending; CNC servo press; Finishing reduction; FEM simulation
1. Introduction To improve the fuel consumption of cars, reduction in weight is intensively required in automobile industry. Although the application of aluminium sheets to automobile parts is attractive for the reduction [1], high cost and small formability are crucial problems, and thus the industry still has a great interest in steel sheets. The strength of the high-strength steel sheets becomes increasingly high, and ultra-high-strength sheets having a tensile strength more than 1 GPa have been recently developed. The specific strength of the ultra-high-strength steel sheets is higher than that of aluminium alloy sheets as shown in Table 1. As known in the international co-operative ultralight steel auto body (ULSAB) project for the reduction in weight of cars, the use of high-strength steel sheets for automobile body panels rapidly increases. Most of the Corresponding author. Fax: +81 532 44 6690.
E-mail address: [email protected] (K. Mori). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.03.013
sheets used in present cars, however, are on a 590 MPa level or less, and the use of the ultra-high-strength sheets is still limited due to large springback and low formability. It is desirable in the automobile industry to develop forming processes suitable for ultra-high-strength steel sheets. Warm and hot stamping processes are attractive in reducing springback and in improving formability of ultrahigh-tensile-strength steel sheets. The warm and hot stamping processes are mainly applied to magnesium and aluminium alloy sheets [2–4]. The heating temperature in the warm and hot stamping of the steel sheets is much higher than that of the magnesium and aluminium sheets. Although the sheets are generally heated in a furnace, the decrease in temperature of the sheets taken out of the furnace until the forming is a problem, particularly in the steel sheets. In the warm and hot stamping using a furnace, the sheet is heated to a higher temperature because of the heat loss in transport. In addition, the sheets suffer much oxidation during the setting into the dies. The authors [5] have developed a warm and hot stamping
ARTICLE IN PRESS 322
K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325
Table 1 Strength–specific gravity ratios for various sheet metals Sheet
Tensile strength (MPa)
Specific gravity
Strength–specific gravity ratio (MPa)
Ultra-high-strength steel High-strength steel Mild steel SPCC Aluminium alloy A6061 (T6)
980–1470
7.8
126–188
490–790 340 310
7.8 7.8 2.7
63–101 44 115
categorised as an ultra-high-strength steel sheet and is made of dual-phase steel. The flow stress curves of the sheets at two strain rates are shown in Fig. 1. Although the strain-rate sensitivity for SPCC is comparatively large, the ultra-high-strength steel sheet SPFC980Y hardly exhibits the strain-rate sensitivity. As the strength increases, the strain-rate sensitivity decreases. This is due to very low strain-rate sensitivity of hardened microstructures in the ultra-high-strength steel sheet. 2.2. V-shaped bending The CNC servo press having a maximum load of 800 kN directly driven through ball screws by two servo motors shown in Fig. 2 was used for the bending. The position accuracy and the maximum speed of the ram were 1 mm 1200 1000
Flow stress / MPa
process for ultra-high-strength steel sheets using resistance heating. In this process, the decrease in temperature of the sheet before the forming is prevented by directly heating the sheets set into the dies by means of the electrical resistance, the so-called Joule heat. Since the heating time up to 800 1C is only 2 s, the resistance heating is rapid enough to synchronise with a press. Yanagimoto and Oyamada [6] have employed induction heating in warm and hot stamping. In addition, the strength of the formed sheets is heightened by die quenching in the hot stamping. CNC servo presses having high flexibility for the control of motion have been recently developed [7]. In these presses driven by servo motors, the motion of a punch is accurately controlled by real-time feedback of ram position measured with sensors like the conventional machine tools and thus, complicated motion is attainable. Otsu et al. [8] have reduced noise of the blanking by controlling the motion of the punch. Osakada et al. [9] have employed a double-axis servo press in extrusion with counter pressure. Although such complicated motion of the press may be effective in improving the dimensional accuracy of the formed products, the application of the servo press to stamping processes of high-strength steel sheets has not been reported yet. In the present study, the springback behaviour in bending of ultra-high-strength steel sheets was examined under controlled conditions using a CNC servo press. Knowledge of the springback behaviour of these newly developed sheets is useful for designing forming processes. The effects of the material, the forming speed and the finishing reduction in thickness on the springback behaviour in the bending were examined.
800
−3 −1 SPFC980Y ε = 64.5 ×10 s
ε = 0.2 × 10 −3 s −1 −3 −1 SPFC780Y ε = 91.2 × 10 s −3 −1 ε = 0.2 × 10 s −3 −1 SPFC440 ε = 92.9 × 10 s −3
600
ε = 0.2 × 10 s −1
−3 −1 SPCC ε = 95.6 × 10 s
ε = 0.2 × 10 −3 s −1
400 200 0
0.05
0.1
0.15
0.2
0.25
0.3
Strain Fig. 1. Flow stress curves of various sheets at two strain rates.
2. Method of experiment 2.1. Mechanical properties of sheets High-strength steel sheets SPFC440, 780Y, 980Y and a mild steel sheet SPCC were bent at room temperature. The SPFC denotes the high-strength steel sheet and the values are the nominal tensile strength. The nominal thicknesses of the steel sheets were 1.2 mm. The SPFC980Y is
Fig. 2. CNC servo press having maximum load of 800 kN directly driven through ball screws by the servo motors.
ARTICLE IN PRESS K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325 Overshooting t0
Punch 90º ∆t
9.6
102
Fig. 3. Tools used in V-shaped bending.
and 150 mm/s, respectively. Since the speed of the press decreases near the bottom dead centre, the average forming speed v is employed. The width and length of the bending specimen were 55 and 60 mm, respectively, and the bending direction is orthogonal to the rolling one. The tools used in the V-shaped bending are shown in Fig. 3. The sheets are bent by the punch and die having an angle of 901. The finishing reduction in thickness, f, is applied to reduce the amount of springback. Although the finishing reduction is hardly employed in the conventional sheet-metal-forming operations because of the increase in forming load, the finishing reduction is becoming common due to the emergence of controllable presses such as CNC servo presses. The finishing reduction in thickness is obtained from the thickness after the bending to eliminate elastic deformation of tools. 3. Results for bending
Lo ad ing
120
Unloading
100 80
(a)
98 96 94
Un lo
92
a di
90 0
1
2
3
4
(b)
Stroke /mm
88 3.2
3.6
3.4
3.8
ng 4
Stroke /mm
Fig. 4. Variation of bending angle with punch stroke measured by CCD video camera for SPFC 980Y, v ¼ 1 mm=s and f ¼ 0%; (a) whole stoke, and (b) vicinity of end of loading.
8
SPFC980Y 6
Real Nominal
4
SPFC780Y 2
SPFC440
SPCC
0 -2 0
10
20
30
40
50
Forming speed v /mm/s
3.1. Experimental results
Fig. 5. Relationship between difference from punch angle and forming speed in V-shaped bending for f ¼ 0%.
30
SPFC980Y
Nominal
SPFC440 Frequency /%
The variation of the bending angle with the punch stroke measured by a CCD video camera for SPFC980Y, the forming speed v ¼ 1 mm=s and the finishing reduction in thickness f ¼ 0% is shown in Fig. 4. The bending angle is an average of local angles in the bent sheet. The bending angle at the end of loading is not the 901 of the punch angle, i.e. the overshooting of the bending by considerably smaller length of the die than that of the sheet. Although the springback is strictly defined as the amount of elastic recovery, the difference between the angle of the bent sheet and the punch angle is used in practice, and moreover, it is not easy to measure the bending angle at the end of loading for high speed accurately. The difference from the punch angle is employed in the present study. The relationship between the difference from the punch angle and the forming speed in the V-shaped bending for f ¼ 0% is given in Fig. 5. The scatter of the results for SPFC980Y is improved by using real thickness of individual thickness (solid line) and not nominal thickness (dotted line) in the control of the punch stroke. The scatter of the thickness and the difference between the real and nominal thicknesses for SPFC980Y were larger than the other sheets as shown in Fig. 6. The servo press is applicable to accurate control the real thickness. The
Bending angle θ/°
Die
140
g
9.6
∆tt t0
in
Fini Finishing reduction in thickness f =
160
ad Lo
R1.2
100
Difference from punch angle ∆θ /º
90º
R1.2
Bending angle θ/°
180
Sheet
323
20 SPCC 10
0 98
99
100 Thickness /%
101
102
Fig. 6. Scatter of thickness for various sheets.
difference from the punch angle for SPFC980Y is much larger than that for SPCC, whereas the effect of the forming speed for SPFC980Y is small due to the small strain-rate sensitivity of the flow stress shown in Fig. 1. The relationship between the difference from the punch angle and the finishing reduction in thickness for v ¼
ARTICLE IN PRESS K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325
6
4
4 SPFC780Y
2 SPCC
0 SPFC440
-2 0
0.002
SPCC
SPFC440
0.004
0.006
Tensile strength / Young's modulus Fig. 9. Relationship between difference from punch angle and ratio of tensile strength to elastic modulus for f ¼ 0% and v ¼ 24 mm=s.
120 SPFC440 100 80
SPFC780Y
SPCC
60 SPFC980Y 40
0
SPFC780Y
0.2
0.4
0.6
Finishing reduction in thickness f /%
0 -2
SPFC980Y
6
20
SPFC980Y
2
8
Forming load /kN
Difference from punch angle ∆θ/°
24 mm=s is illustrated in Fig. 7. As the finishing reduction in thickness increases, the differences from the punch angle for SPFC980Y and 780Y decrease, while the finishing reductions for SPFC440 and SPCC are not affected. Since the amounts of springback for SPFC440 and SPCC are small, the differences for f ¼ 0% become negative due to the overshooting of the bending shown in Fig. 4. The finishing reduction in thickness for the ultra-high-strength steel sheet is effective in reducing the difference from the punch angle. The effect of the holding time T at the bottom dead centre on the difference from the punch angle for SPFC980Y and v ¼ 24 mm=s is illustrated in Fig. 8. Although the effect of finishing reduction on the difference from the punch angle is large, the effect of the holding time is small. The relationship between the difference from the punch angle and the ratio of the tensile strength to the elastic modulus for f ¼ 0% and v ¼ 24 mm=s is given in Fig. 9. As the ratio of the tensile strength increases, the difference from the punch angle increases. The relationship between the forming load and finishing reduction in thickness for v ¼ 24 mm=s is shown in Fig. 10.
Difference from punch angle ∆θ/°
324
Fig. 10. Relationship between forming load and finishing reduction in thickness for v ¼ 24 mm=s.
0
0.2 0.4 Finishing reduction in thickness f /%
0.6
Fig. 7. Relationship between difference from punch angle and finishing reduction in thickness for v ¼ 24 mm=s.
As the finishing reduction increases, the forming load sharply increases. The difference in the forming load between the sheets is small due to large constraint of deformation. Although the finishing reduction is effective in reducing the springback of the ultra-high-strength steel sheets, the increase in forming load is disadvantageous.
Difference from punch angle ∆θ/°
8
3.2. Calculated results 6
f = 0%, T = 0.1s
4
f = 0.33%, T = 0.1s f = 0.33%, T = 0.5s
2 0 -2 0
10
20
30
40
50
Forming speed v /mm/s Fig. 8. Effect of holding time at bottom dead centre on difference from punch angle for SPFC980Y and v ¼ 24 mm=s.
The finite element method has been applied to simulate springback in bending of high-strength steel sheets [10]. To examine the effect of the finishing reduction in thickness on the springback, the plane-strain bending process was simulated by the implicit code in the commercial FEM software LS-Dyna. The distributions of stress in the length direction at the end of loading of plane-strain bending calculated from the finite element simulation for f ¼ 0:0%, 0.20% and 0.57%, and SPFC980Y are illustrated in Fig. 11. The thickness of the sheet is reduced with the inclined sides of the punch and die except for the vicinity of the tip of the punch, and thus the stress in the inclined region is changed by the finishing
ARTICLE IN PRESS K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325
0.0
0.0 0.0
Punch Tensile 0. 0 0.4 -0.4 0.0 -0.4 0.8 -1.2 -0.8 -0.8 -0.4 -1.2 0.4 -0.8 -1.6 -1.6 0.8 1.2 0.0 0.4 Die 1.2 0.8
(a)
f = 0.0 %
(b)
f = 0.20 %
Tensile 0.0 0.0 -0.4 0.4 -0.8 -0.4 -1.2 -1.6 0.0 0.4 1.2 0.8 /GPa
(c)
0.0
f = 0.57 %
Spring back angle /º
Fig. 11. Distributions of stress in length direction at end of loading of plane-strain bending calculated from finite element simulation for SPFC980Y; (a) f ¼ 0:0%, (b) f ¼ 0:20%, and (c) f ¼ 0:57%.
325
these sheets because of the high strength. It is desirable in automobile industry to develop forming processes suitable for ultra-high-strength steel sheets. For the development, the springback behaviour in bending of the ultra-highstrength steel sheet was investigated as follows: (1) The accurate control using real thickness of each individual sheet is necessary for preventing scattering of shapes of the formed products. (2) A finishing reduction in thickness is effective in reducing the springback. (3) The CNC servo press is useful for the control using real thickness. (4) The effects of the forming speed and holding time at the bottom dead centre on the springback are very small.
5 4
Acknowledgements
3 2 1 0
0.4 0.6 0.2 Finishing reduction in thickness f /%
0.8
The authors wish to thank Mr. K. Miyoshi of Komatsu Industries Co. for assistance. This work was supported in part by the Japanese Ministry of Education, Culture, Sports, Science and Technology, with Grant-in-aid for Scientific Research (B), no. 17360352.
Fig. 12. Real springback angle obtained from calculation for SPFC980Y.
References reduction in thickness. For the finishing reduction in thickness, a tensile stress appears around the upper surface in the inclined region, and the tensile stress has a function for reducing the springback because of the decrease in effect of the compressive stress around the centre of the upper surface. Since the difference from the punch angle is examined for practical use in the present paper, the difference is not the real springback due to the overshooting of the bending. Although the springback angle due to elastic recovery, the difference between the angle at the end of loading and the angle after the unloading, was obtained from the measurement using a video camera as shown in Fig. 4, the measurement for high speed is not easy. The real springback angle obtained from the calculation for SPFC980Y is given in Fig. 12. As the finishing reduction in thickness increases, the springback angle decreases; however, the decrease is saturated around f ¼ 0:2%; similar to the experimental angle for SPFC980Y shown in Fig. 7. 4. Conclusions Although the ultra-high-strength steel sheets are attractive in reducing the weight of cars, it is not easy to form
[1] M. Kleiner, M. Geiger, A. Klaus, Manufacturing of lightweight components by metal forming, Annals of the CIRP 52 (2) (2003) 521–542. [2] E. Doege, G. Kurz, Development of a formulation to describe the work softening behaviour of magnesium sheets for heated deep drawing processes, Annals of the CIRP 50 (1) (2001) 177–180. [3] P. Groche, R. Huber, J. Dorr, D. Schmoeckel, Hydromechanical deep-drawing of aluminium alloys at elevated temperatures, Annals of the CIRP 51 (1) (2002) 215–218. [4] K. Siegert, S. Jager, M. Vulcan, Pneumatic bulging of magnesium AZ 31 sheet metals at elevated temperatures, Annals of the CIRP 52 (1) (2003) 241–244. [5] K. Mori, S. Maki, Y. Tanaka, Warm and hot stamping of ultra high tensile strength steel sheets using resistance heating, Annals of the CIRP 54 (1) (2005) 209–212. [6] J. Yanagimoto, K. Oyamada, Springback of high-strength steel after hot and warm sheet formings, Annals of the CIRP 54 (1) (2005) 213–216. [7] C. Murata, T. Machida, S. Futamura, J. Endou, Development of direct drive digital servo press, Steel Grips 2 (Suppl.) (2004) 371–374. [8] M. Otsu, C. Yamagata, K. Osakada, Reduction of blanking noise by controlling press motion, Annals of the CIRP 52 (1) (2003) 245–248. [9] K. Osakada, R. Matsumoto, M. Otsu, S. Hanami, Precision extrusion methods with double axis servo-press using counter pressure, Annals of the CIRP 54 (1) (2005) 245–248. [10] C. Gomes, O. Onipedel, M. Lovell, Investigation of springback in high strength anisotropic steels, Journal of Materials Processing Technology 159 (1) (2005) 91–98.
International Journal of Machine Tools & Manufacture 47 (2007) 321–325 www.elsevier.com/locate/ijmactool
Springback behaviour in bending of ultra-high-strength steel sheets using CNC servo press K. Mori, K. Akita, Y. Abe Department of Production Systems Engineering, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Received 10 February 2006; received in revised form 21 March 2006; accepted 23 March 2006 Available online 18 May 2006
Abstract The springback behaviour of ultra-high-strength steel sheets in bending was investigated under controlled conditions using a CNC servo press. Although the ultra-high-strength steel sheets are attractive in reducing weight of cars, the amount of springback of the ultrahigh-strength steel sheets in the forming is very large due to high strength. The CNC servo press has the function of accurately controlling the motion by two servo motors. The effects of the material, the finishing reduction in thickness, the forming speed and the holding time at the bottom dead centre on the amount of springback in V-shaped bending were examined. The scatter of the springback for the ultrahigh-strength steel sheets was improved by using real thickness and not nominal thickness of each individual sheet in the control of the punch stroke. The amount of springback for the ultra-high-strength steel sheet in the V-shaped bending was much larger than that for the mild steel sheet, and the amount was decreased by the finishing reduction in thickness direction because of uniform stress distribution. The effects of the forming speed and the holding time at the bottom dead centre were small. The amount of springback for the steel sheets was almost proportional to the ratio of the tensile strength to the elastic modulus. r 2006 Elsevier Ltd. All rights reserved. Keywords: Springback; Ultra-high-strength steel sheet; Bending; CNC servo press; Finishing reduction; FEM simulation
1. Introduction To improve the fuel consumption of cars, reduction in weight is intensively required in automobile industry. Although the application of aluminium sheets to automobile parts is attractive for the reduction [1], high cost and small formability are crucial problems, and thus the industry still has a great interest in steel sheets. The strength of the high-strength steel sheets becomes increasingly high, and ultra-high-strength sheets having a tensile strength more than 1 GPa have been recently developed. The specific strength of the ultra-high-strength steel sheets is higher than that of aluminium alloy sheets as shown in Table 1. As known in the international co-operative ultralight steel auto body (ULSAB) project for the reduction in weight of cars, the use of high-strength steel sheets for automobile body panels rapidly increases. Most of the Corresponding author. Fax: +81 532 44 6690.
E-mail address: [email protected] (K. Mori). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.03.013
sheets used in present cars, however, are on a 590 MPa level or less, and the use of the ultra-high-strength sheets is still limited due to large springback and low formability. It is desirable in the automobile industry to develop forming processes suitable for ultra-high-strength steel sheets. Warm and hot stamping processes are attractive in reducing springback and in improving formability of ultrahigh-tensile-strength steel sheets. The warm and hot stamping processes are mainly applied to magnesium and aluminium alloy sheets [2–4]. The heating temperature in the warm and hot stamping of the steel sheets is much higher than that of the magnesium and aluminium sheets. Although the sheets are generally heated in a furnace, the decrease in temperature of the sheets taken out of the furnace until the forming is a problem, particularly in the steel sheets. In the warm and hot stamping using a furnace, the sheet is heated to a higher temperature because of the heat loss in transport. In addition, the sheets suffer much oxidation during the setting into the dies. The authors [5] have developed a warm and hot stamping
ARTICLE IN PRESS 322
K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325
Table 1 Strength–specific gravity ratios for various sheet metals Sheet
Tensile strength (MPa)
Specific gravity
Strength–specific gravity ratio (MPa)
Ultra-high-strength steel High-strength steel Mild steel SPCC Aluminium alloy A6061 (T6)
980–1470
7.8
126–188
490–790 340 310
7.8 7.8 2.7
63–101 44 115
categorised as an ultra-high-strength steel sheet and is made of dual-phase steel. The flow stress curves of the sheets at two strain rates are shown in Fig. 1. Although the strain-rate sensitivity for SPCC is comparatively large, the ultra-high-strength steel sheet SPFC980Y hardly exhibits the strain-rate sensitivity. As the strength increases, the strain-rate sensitivity decreases. This is due to very low strain-rate sensitivity of hardened microstructures in the ultra-high-strength steel sheet. 2.2. V-shaped bending The CNC servo press having a maximum load of 800 kN directly driven through ball screws by two servo motors shown in Fig. 2 was used for the bending. The position accuracy and the maximum speed of the ram were 1 mm 1200 1000
Flow stress / MPa
process for ultra-high-strength steel sheets using resistance heating. In this process, the decrease in temperature of the sheet before the forming is prevented by directly heating the sheets set into the dies by means of the electrical resistance, the so-called Joule heat. Since the heating time up to 800 1C is only 2 s, the resistance heating is rapid enough to synchronise with a press. Yanagimoto and Oyamada [6] have employed induction heating in warm and hot stamping. In addition, the strength of the formed sheets is heightened by die quenching in the hot stamping. CNC servo presses having high flexibility for the control of motion have been recently developed [7]. In these presses driven by servo motors, the motion of a punch is accurately controlled by real-time feedback of ram position measured with sensors like the conventional machine tools and thus, complicated motion is attainable. Otsu et al. [8] have reduced noise of the blanking by controlling the motion of the punch. Osakada et al. [9] have employed a double-axis servo press in extrusion with counter pressure. Although such complicated motion of the press may be effective in improving the dimensional accuracy of the formed products, the application of the servo press to stamping processes of high-strength steel sheets has not been reported yet. In the present study, the springback behaviour in bending of ultra-high-strength steel sheets was examined under controlled conditions using a CNC servo press. Knowledge of the springback behaviour of these newly developed sheets is useful for designing forming processes. The effects of the material, the forming speed and the finishing reduction in thickness on the springback behaviour in the bending were examined.
800
−3 −1 SPFC980Y ε = 64.5 ×10 s
ε = 0.2 × 10 −3 s −1 −3 −1 SPFC780Y ε = 91.2 × 10 s −3 −1 ε = 0.2 × 10 s −3 −1 SPFC440 ε = 92.9 × 10 s −3
600
ε = 0.2 × 10 s −1
−3 −1 SPCC ε = 95.6 × 10 s
ε = 0.2 × 10 −3 s −1
400 200 0
0.05
0.1
0.15
0.2
0.25
0.3
Strain Fig. 1. Flow stress curves of various sheets at two strain rates.
2. Method of experiment 2.1. Mechanical properties of sheets High-strength steel sheets SPFC440, 780Y, 980Y and a mild steel sheet SPCC were bent at room temperature. The SPFC denotes the high-strength steel sheet and the values are the nominal tensile strength. The nominal thicknesses of the steel sheets were 1.2 mm. The SPFC980Y is
Fig. 2. CNC servo press having maximum load of 800 kN directly driven through ball screws by the servo motors.
ARTICLE IN PRESS K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325 Overshooting t0
Punch 90º ∆t
9.6
102
Fig. 3. Tools used in V-shaped bending.
and 150 mm/s, respectively. Since the speed of the press decreases near the bottom dead centre, the average forming speed v is employed. The width and length of the bending specimen were 55 and 60 mm, respectively, and the bending direction is orthogonal to the rolling one. The tools used in the V-shaped bending are shown in Fig. 3. The sheets are bent by the punch and die having an angle of 901. The finishing reduction in thickness, f, is applied to reduce the amount of springback. Although the finishing reduction is hardly employed in the conventional sheet-metal-forming operations because of the increase in forming load, the finishing reduction is becoming common due to the emergence of controllable presses such as CNC servo presses. The finishing reduction in thickness is obtained from the thickness after the bending to eliminate elastic deformation of tools. 3. Results for bending
Lo ad ing
120
Unloading
100 80
(a)
98 96 94
Un lo
92
a di
90 0
1
2
3
4
(b)
Stroke /mm
88 3.2
3.6
3.4
3.8
ng 4
Stroke /mm
Fig. 4. Variation of bending angle with punch stroke measured by CCD video camera for SPFC 980Y, v ¼ 1 mm=s and f ¼ 0%; (a) whole stoke, and (b) vicinity of end of loading.
8
SPFC980Y 6
Real Nominal
4
SPFC780Y 2
SPFC440
SPCC
0 -2 0
10
20
30
40
50
Forming speed v /mm/s
3.1. Experimental results
Fig. 5. Relationship between difference from punch angle and forming speed in V-shaped bending for f ¼ 0%.
30
SPFC980Y
Nominal
SPFC440 Frequency /%
The variation of the bending angle with the punch stroke measured by a CCD video camera for SPFC980Y, the forming speed v ¼ 1 mm=s and the finishing reduction in thickness f ¼ 0% is shown in Fig. 4. The bending angle is an average of local angles in the bent sheet. The bending angle at the end of loading is not the 901 of the punch angle, i.e. the overshooting of the bending by considerably smaller length of the die than that of the sheet. Although the springback is strictly defined as the amount of elastic recovery, the difference between the angle of the bent sheet and the punch angle is used in practice, and moreover, it is not easy to measure the bending angle at the end of loading for high speed accurately. The difference from the punch angle is employed in the present study. The relationship between the difference from the punch angle and the forming speed in the V-shaped bending for f ¼ 0% is given in Fig. 5. The scatter of the results for SPFC980Y is improved by using real thickness of individual thickness (solid line) and not nominal thickness (dotted line) in the control of the punch stroke. The scatter of the thickness and the difference between the real and nominal thicknesses for SPFC980Y were larger than the other sheets as shown in Fig. 6. The servo press is applicable to accurate control the real thickness. The
Bending angle θ/°
Die
140
g
9.6
∆tt t0
in
Fini Finishing reduction in thickness f =
160
ad Lo
R1.2
100
Difference from punch angle ∆θ /º
90º
R1.2
Bending angle θ/°
180
Sheet
323
20 SPCC 10
0 98
99
100 Thickness /%
101
102
Fig. 6. Scatter of thickness for various sheets.
difference from the punch angle for SPFC980Y is much larger than that for SPCC, whereas the effect of the forming speed for SPFC980Y is small due to the small strain-rate sensitivity of the flow stress shown in Fig. 1. The relationship between the difference from the punch angle and the finishing reduction in thickness for v ¼
ARTICLE IN PRESS K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325
6
4
4 SPFC780Y
2 SPCC
0 SPFC440
-2 0
0.002
SPCC
SPFC440
0.004
0.006
Tensile strength / Young's modulus Fig. 9. Relationship between difference from punch angle and ratio of tensile strength to elastic modulus for f ¼ 0% and v ¼ 24 mm=s.
120 SPFC440 100 80
SPFC780Y
SPCC
60 SPFC980Y 40
0
SPFC780Y
0.2
0.4
0.6
Finishing reduction in thickness f /%
0 -2
SPFC980Y
6
20
SPFC980Y
2
8
Forming load /kN
Difference from punch angle ∆θ/°
24 mm=s is illustrated in Fig. 7. As the finishing reduction in thickness increases, the differences from the punch angle for SPFC980Y and 780Y decrease, while the finishing reductions for SPFC440 and SPCC are not affected. Since the amounts of springback for SPFC440 and SPCC are small, the differences for f ¼ 0% become negative due to the overshooting of the bending shown in Fig. 4. The finishing reduction in thickness for the ultra-high-strength steel sheet is effective in reducing the difference from the punch angle. The effect of the holding time T at the bottom dead centre on the difference from the punch angle for SPFC980Y and v ¼ 24 mm=s is illustrated in Fig. 8. Although the effect of finishing reduction on the difference from the punch angle is large, the effect of the holding time is small. The relationship between the difference from the punch angle and the ratio of the tensile strength to the elastic modulus for f ¼ 0% and v ¼ 24 mm=s is given in Fig. 9. As the ratio of the tensile strength increases, the difference from the punch angle increases. The relationship between the forming load and finishing reduction in thickness for v ¼ 24 mm=s is shown in Fig. 10.
Difference from punch angle ∆θ/°
324
Fig. 10. Relationship between forming load and finishing reduction in thickness for v ¼ 24 mm=s.
0
0.2 0.4 Finishing reduction in thickness f /%
0.6
Fig. 7. Relationship between difference from punch angle and finishing reduction in thickness for v ¼ 24 mm=s.
As the finishing reduction increases, the forming load sharply increases. The difference in the forming load between the sheets is small due to large constraint of deformation. Although the finishing reduction is effective in reducing the springback of the ultra-high-strength steel sheets, the increase in forming load is disadvantageous.
Difference from punch angle ∆θ/°
8
3.2. Calculated results 6
f = 0%, T = 0.1s
4
f = 0.33%, T = 0.1s f = 0.33%, T = 0.5s
2 0 -2 0
10
20
30
40
50
Forming speed v /mm/s Fig. 8. Effect of holding time at bottom dead centre on difference from punch angle for SPFC980Y and v ¼ 24 mm=s.
The finite element method has been applied to simulate springback in bending of high-strength steel sheets [10]. To examine the effect of the finishing reduction in thickness on the springback, the plane-strain bending process was simulated by the implicit code in the commercial FEM software LS-Dyna. The distributions of stress in the length direction at the end of loading of plane-strain bending calculated from the finite element simulation for f ¼ 0:0%, 0.20% and 0.57%, and SPFC980Y are illustrated in Fig. 11. The thickness of the sheet is reduced with the inclined sides of the punch and die except for the vicinity of the tip of the punch, and thus the stress in the inclined region is changed by the finishing
ARTICLE IN PRESS K. Mori et al. / International Journal of Machine Tools & Manufacture 47 (2007) 321–325
0.0
0.0 0.0
Punch Tensile 0. 0 0.4 -0.4 0.0 -0.4 0.8 -1.2 -0.8 -0.8 -0.4 -1.2 0.4 -0.8 -1.6 -1.6 0.8 1.2 0.0 0.4 Die 1.2 0.8
(a)
f = 0.0 %
(b)
f = 0.20 %
Tensile 0.0 0.0 -0.4 0.4 -0.8 -0.4 -1.2 -1.6 0.0 0.4 1.2 0.8 /GPa
(c)
0.0
f = 0.57 %
Spring back angle /º
Fig. 11. Distributions of stress in length direction at end of loading of plane-strain bending calculated from finite element simulation for SPFC980Y; (a) f ¼ 0:0%, (b) f ¼ 0:20%, and (c) f ¼ 0:57%.
325
these sheets because of the high strength. It is desirable in automobile industry to develop forming processes suitable for ultra-high-strength steel sheets. For the development, the springback behaviour in bending of the ultra-highstrength steel sheet was investigated as follows: (1) The accurate control using real thickness of each individual sheet is necessary for preventing scattering of shapes of the formed products. (2) A finishing reduction in thickness is effective in reducing the springback. (3) The CNC servo press is useful for the control using real thickness. (4) The effects of the forming speed and holding time at the bottom dead centre on the springback are very small.
5 4
Acknowledgements
3 2 1 0
0.4 0.6 0.2 Finishing reduction in thickness f /%
0.8
The authors wish to thank Mr. K. Miyoshi of Komatsu Industries Co. for assistance. This work was supported in part by the Japanese Ministry of Education, Culture, Sports, Science and Technology, with Grant-in-aid for Scientific Research (B), no. 17360352.
Fig. 12. Real springback angle obtained from calculation for SPFC980Y.
References reduction in thickness. For the finishing reduction in thickness, a tensile stress appears around the upper surface in the inclined region, and the tensile stress has a function for reducing the springback because of the decrease in effect of the compressive stress around the centre of the upper surface. Since the difference from the punch angle is examined for practical use in the present paper, the difference is not the real springback due to the overshooting of the bending. Although the springback angle due to elastic recovery, the difference between the angle at the end of loading and the angle after the unloading, was obtained from the measurement using a video camera as shown in Fig. 4, the measurement for high speed is not easy. The real springback angle obtained from the calculation for SPFC980Y is given in Fig. 12. As the finishing reduction in thickness increases, the springback angle decreases; however, the decrease is saturated around f ¼ 0:2%; similar to the experimental angle for SPFC980Y shown in Fig. 7. 4. Conclusions Although the ultra-high-strength steel sheets are attractive in reducing the weight of cars, it is not easy to form
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