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INTRODUCTION
Conventional automotive mild and high strength steels are being
replaced by advanced high strength steels (AHSS) due to the
demands for vehicle weight reduction and safety performance
improvement. However, several local formability issues have been
raised in stamping processes such as edge cracking and shear fracture
in small radius stretch bending. It has been found that the edge
cracking issue is related to sheet metal shearing processes such as
blanking, trimming and piercing. Nakata et el. [1] has studied the
shear deformation properties and the damage behavior on both low
and high strength steels using the conventional shearing die. They
found that the thickness clearance is critical in trimming high strength
steels. The experimental comparison of the edge stretchability of
AHSS between standard punched hole, drilled hole and laser cutting
hole were conducted by Konieczny [2] and Karelova [3] et el. at
different punching clearances. Results showed a better edge
stretchability was directly associated with a better shearing process.
Golovashchenko [4] modied the conventional shearing process by
adding an elastic pad underneath the blank, which would reduce burrs
in a wide variety of clearances without deteriorating the total
elongation or edge stretchability. However, the value of the
elongation was lower than that observed from the conventional
shearing process. The shearing process was also modeled using
different Finite Element Analysis (FEA) models [1,5,6], but the
simulation results were in limited agreement with experiments.
Although some limited data or guidelines might be available on the
optimal shearing variables set up, they are often based on
conventional lower strength, higher ductility sheet steels. They may
not be applicable for shearing AHSS. To optimize the shearing
variables for AHSS, Shih et el. [7,8] developed a bevel shear hole
piercing process to improve the quality of the sheared edge based on
the stretchability or angeability of the sheared edge. The purpose of
this study is to further investigate the effects of punch congurations
on the AHSS sheared edge stretchability. Punches with different
geometries and surface treatments are fabricated to a production
piercing condition to study the as-pierced edge stretchability of AHSS
Effects of Punch Conguration on the AHSS Edge Stretchability
Hua-Chu Shih
United States Steel Corp.
Dajun Zhou
FCA US LLC
Bruce Konopinski
PCS Company
ABSTRACT
The hole piercing process is a simple but important task in manufacturing processes. The quality requirement of the pierced hole varies
between different applications. It can be either the size or the edge quality of the hole. Furthermore, the pierced hole is often subject to
a secondary forming process, in which the edge stretchability is of a main concern. The recently developed advanced high strength
steels (AHSS) and ultra high strength steels (UHSS) have been widely used for vehicle weight reduction and safety performance
improvements. Due to the higher strength nature of these specially developed sheet steels, the hole piercing conditions are more
extreme and challenging, and the quality of the pierced hole can be critical due to their relatively lower edge stretching limits than
those for the conventional low and medium strength steels. The stretchability of the as-sheared edge inside the hole can be inuenced
by the material property, die condition and processing parameters. Previous studies showed that the as-pierced edge stretchability can
be improved by implementing the bevel shape punch head with an optimal die clearance. In this study, production punches are
fabricated with different congurations and surface treatments to study the as-pierced edge stretchability of AHSS. The hole piercing
experiments are conducted on DP600, DP780 and DP980 steels using a computer controlled punching system. The hole expansion test
is used to evaluate the effect of pierced edge conditions on the edge stretchability. Results indicate that a selection of the 15% die
clearance (per side) and a conical shape punch results in a less damaged edge, which signicantly delays edge fracture in the forming
process and increases the edge stretchability of AHSS.
CITATION: Shih, H., Zhou, D., and Konopinski, B., "Effects of Punch Conguration on the AHSS Edge Stretchability," SAE Int. J.
Engines 10(4):2017, doi:10.4271/2017-01-1705.
Published 03/28/2017
Copyright © 2017 SAE International
doi:10.4271/2017-01-1705
saeeng.saejournals.org
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in this study. The computer controlled punch system was used for the
hole piercing test and hole expansion test was used to evaluate the
edge stretchability.
HOLE PIERCING TEST
A computer controlled hole piercing process was developed on a lab
hydraulic press and the setup is shown in Figure 1-1. A general-
purpose punch die set was inverted and mounted to the computer
controlled hydraulic press. A schematic diagram of the punch die set
is shown in Figure 1-2. The punch diameter (Pd) was 10 mm, and
various die with different inner diameters (Dd) were used to achieve
different die clearances (CL). The punch speed was kept at 1 mm/sec
and the die clearance per side was xed at 15% of metal thickness
(Shih et el. [7,8]) regardless of the test materials. For each material,
ve specimens were tested on each punch condition.
Figure 1. Schematic illustration of hole piercing die set.
Punch Configuration
There are a total of seven different punch congurations included in
this study. The conventional at punch (FP), at punch with in-line
grinding (ILG), 4 micron ne polish (4M), at punch with radius on
the corner (Edge Hone, EH), edge hone punch with 4 micron ne
polish (4MEH), 6 degrees conical shear and 6 degrees bevel shear
(6D). All punches are made of M2 steel except for the at punch (FP)
and 6 degrees bevel shear (6D) are made of D2 steel with hardness of
60 HRC for all punches. The 6 degrees bevel shear punch was
developed from previous studies [7,8] for optimal piercing AHSS
with combining 15% die clearance and the shearing direction parallel
to the material rolling direction. The 6 degrees is also implemented to
make the conical shear punch, and the detail of each punch is
discussed below.
6 degrees Bevel Shear (6D)
The 6 degrees bevel shear punch was previously developed to
achieve optimal piercing condition for AHSS as shown in Figure 2.
Due to the beveled angle, the tip of the punch will rst contact and
pierce the material at one side of the hole, and the trailing edge will
eventually contact with the material and complete the piercing.
Figure 2. Schematic and photograph of bevel shear die set.
In the production piercing condition, it is a challenge to ensure the
material rolling direction is aligned parallel as the bevel shearing and
the horizontal force against the beveling shear face could deect the
punch tip when high speed piercing AHSS. This could lead to an
unbalanced die clearance condition. To prevent these situations, a 6
degrees fully symmetrical conical shear punch, as shown in Figure 3,
was fabricated for this study.
Conical Shear
The tip of the conical shear (Figure 3) can be used to locate the center
of the hole on the sample to ensure the even die clearance condition
throughout the piercing process regardless of the gauge and strength
of the material. The sheet metal within the punch hole area is pre-bent
and stretched before the edge of the punch begins to contact and
shear off the material, which is different from that of the bevel shear.
Figure 3. Conical shear punch.
Flat Punch (FP), In-Line Grinding Punch (ILG) and 4
Micron Polish Punch (4M)
In addition to the geometry and shear angle altering, different surface
polishing conditions were applied to the conventional at punch.
Figures 4-1 through 4-3 show the conventional at punch (FP) with
typical tangential direction polish, in-line (axial direction) grinding
and 4 micron 45 degrees ne polish (4M). The differences among
these three punches are the polish direction and surface smoothness.
Figure 4. 1) FP tangential direction polish 2) FP in-line (axial direction)
grinding 3) FP 4 micron 45 degrees fine polish.
Edge Hone (EH) and Edge Hone with 4 Micron Polish
(4MEH)
The sharpness of a fresh built punch would be slowly reduced after
production break-in. For the application of the AHSS, the pierced
hole edge quality could be affected by the reduced sharpness of the
punch. To address the concern, a 0.14 mm die corner radius was
applied to the at punch, usually called edge hone, and the
Shih et al / SAE Int. J. Engines / Volume 10, Issue 4 (October 2017)
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conguration is illustrated in Figure 5. Two edge hone punches with
conventional polish and 4 micron ne polish (EH and 4MEH), as
shown in the Figures 4-1 and 4-3 respectively, are compared in this
study.
Figure 5. Flat punch with die corner radius (Edge Hone).
Materials
Three commercially produced galvanneal sheet steels with different
thicknesses - DP600 1.5 mm, DP780 1.0 mm, and DP980 1.2 mm
and 2.0 mm - were included in this study. The mechanical properties
for those materials are given in Table 1.
Table 1. Mechanical properties of test materials.
Punch Force
The hole piercing test was carried out on a computer controlled
hydraulic press. The punch speed was kept at 1 mm/sec and the die
clearance per side was 15% of metal thickness (Shih et el. [7,8]). The
load displacement data among different punch conditions are
compared in Figure 6 for piercing DP780. It is observed that the edge
hone with 4 micron polish (4MEH) punch results in the highest peak
load and steeper load prole, indicating a higher friction and contact
pressure between punch and pierced edge of sample. The bevel shear
(6D) punch exhibits a much lower punch force due to less contact
area in piercing and two peak loads associated with the bevel shear
angle (leading punch tip and trailing edge) as discussed by Shih et al.
[7], which is as expected. For the conical shear punch, regardless
having a similar peak load as the conventional at punch (same
length of contact), the force prole is reduced and increases
progressively due to the angle shear and metal thinning associated
with pre-bent and stretching. Those punches with special surface
polish (4M, ILG) and die corner radius (EH) tend to have slightly
higher peak load due to higher friction in piercing.
Figure 6. Punch force comparison between different punch conditions.
The same trend was observed for piercing DP600 and DP980 steels,
where the peak load comparison are illustrated in Figures 7 and 8,
respectively. The 4MEH punch consistently shows the highest peak
load regardless of the test materials. This is associated with the
combining effect of the die corner radius and ne surface polishing.
The die corner radius tends to compress the material before the
shearing process begins. This increases the contact pressure and
friction force locally around the punch corner, where the piercing
force was increased accordingly. The individual peak load
comparison between the edge hone (EH) and 4 micron (4M) punches
indicates that the die corner radius and ne surface polish have
similar effect in increasing peak load from the at punch. The 6
degrees bevel shear punch can easily reduce the punch force more
than 50% from the conventional at punch, which should be
considered when the load capacity of the piercing equipment is
limited and to extend the productive punch life and increase
production rates in between punch sharpening.
Figure 7. Variation of the peak load with different punch conditions (DP600).
Shih et al / SAE Int. J. Engines / Volume 10, Issue 4 (October 2017)
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Figure 8. Variation of the peak load with different punch conditions (DP980
1.2 mm).
Edge Stretchability Evaluation
The hole expansion test, as shown in Figure 9, is used to evaluate the
edge stretchability and angeability of sheet metal. A detailed
description of the hole expansion test can be found in publications
[2,9]. A conical punch was used in this study and a minimum of three
replicates were tested for each test condition. All of the specimens
were tested with the burr up condition [2] in the hole expansion test.
Figure 9. Schematic diagram of the hole expansion test.
The hole expansion ratio (HER) is calculated based on the initial and
nal diameters of the hole:
where D represents the diameter of the expanded hole and d
represents the initial diameter of the punched hole. Previous studies
[7,8] showed the effects of material blanking orientation on the edge
stretchability, which was accomplished by aligning samples both
parallel (L) and perpendicular (T) to the bevel shear direction when
piercing with 6D punch. Figures 10 and 11 show the HER value for
DP600 (1.6 mm) and DP980 (2.0 mm) under different punch
conditions. Results in both Figures indicate the conical shear punch
has the average highest and consistent HER values among all punch
conditions. It is due to the angle shear and proper pre-bent and
stretching effects in the piercing process. The pre-bent and stretching
concept was also proposed by Takahashi et al. [10] using a hump
bottom punch as shown in Figure 12. It was found that the specimen
hardness near the pierced edge from the hump bottom punch was
lower than that of the at punch. The same hardness reduction
behavior in the shear affect zone was discovered by Chiriac et al. [9]
using the bevel shear (6D) punch, as shown in Figure 13, which
resulted in a better edge stretchability. As expected, the HER value
for the bevel shear punch (6D) is comparable to the conical shear
punch, especially when it is pierced along the material rolling
direction. The at punch and edge hone punch generated the average
lowest HER value, while those punches applied with non-
conventional polish condition; ILG and 4M, tended to have better
HER value. Although higher friction forces and peak loads were
identied for these punches, the as-polish surface topography of the
punch serves as a micro surface grinder, which helps to remove some
imperfection on the pierced edge and results in the increase of the
HER value from the FP punch.
Figure 10. Variation of the HER with different punch conditions (DP600).
Figure 11. Variation of the HER with different punch conditions (DP980 2.0
mm).
Figure 12. Hump bottom punch (Takahashi et al. [10])
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Figure 13. Microhardness profile of the SAZ performed at the middle of the
burnish zone for DP780 [9].
The HER values for thin gauge DP780 and DP980 are compared in
Figure 14, where a similar trend as the thicker gauge was observed.
The results validate the advantage of the non-conventional polish
methodology.
Figure 14. Variation of the HER with different punch conditions (DP780 1.0
mm and DP980 1.2 mm).
Tool Wear
To examine the tool wear of the punch visually, a 20X macrograph
was taken from the worn edge on each punch after the experiments.
The photographs are displayed in order from the most severe to the
least severe wear condition, as shown from Figures 15 to 18. Each
punch was applied to pierce four different materials with ve
duplicates, which results in twenty piercing test. The only difference
is for the bevel shear (6D) punch that the piercing test number was
double due to bevel shearing in both material rolling and transverse
directions.
As illustrated in Figure 15, the D2 at punch has the most severe
wear among test punches including prominent abrasive wear, material
chip-out and tear conditions. The poor performance is partially due to
its relative lower alloy grade tool steel property of D2 comparing to
M2, and the at head geometry with conventional tangential direction
polish. During the piercing of the AHSS, the edge of the punch needs
to withstand a much higher cutting, bending and friction force
associated with the sheet steel property and the severer contact
condition. The corner of the edge serves as the stress concentration
point, where the tip tends to be worn out easier than other parts of the
punch. A decent heat treatment and better tool steel property are
critical for the longevity of the at punch.
Figure 15. Severe abrasive wear, chip out and tear for FP punch.
On the wear-severity ranking next to the at punch are the ILG and
4M punches as shown in Figures 16-1 and 2. Only the prominent
abrasive wear is observed for both punches. Although a higher
friction force induced by the axial and 45 degrees polishing was
identied in Figures 6 to 8 for these punches, the higher alloy content
of M2 steel prevents the punch from chipping out and tearing.
Figure 16. Abrasive wear for ILG and 4M punches.
When a radius was implemented on the punch corner (edge hone), the
stress concentration condition in piercing was reduced and the edge
of the punch can be preserved more with no chipping and less
abrasive wear as identied in Figures 17-1 and 2 for both EH and
4MEH punches. The light reection area illustrated a larger surface
grinding area (Figure 17-1) for EH as compared to 4MEH punch,
while the 4MEH punch has a slightly more abrasive wear area due to
its higher punch and friction force (Figures 7 and 8).
Figure 17. Limited abrasive wear for EH and 4MEH punches.
Figure 18 shows relatively no wear for the 6D bevel shear punch and
a very minor wear for conical shear punch. This is as expected since
the geometry of angle shear results in much lower punch and friction
force in piercing. The conical shear tends to have slightly more wear
than 6D bevel shear, which is associated with its higher punch and
friction force.
Shih et al / SAE Int. J. Engines / Volume 10, Issue 4 (October 2017)
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Figure 18. Minor wear for 6D bevel shear and conical shear punches.
CONCLUSION
• The conical shear punch produced the pierced edge with the
most consistent and best edge stretchability among all test
punches. It is recommended for production piercing AHSS when
the better edge stretchability is required.
• The bevel shear punch can reduce the punch force more
than 50% from the conventional at punch, which should be
considered when the load capacity of the piercing equipment
is limited and to extend the productive punch life and increase
production rates in between punch sharpening.
• The punch force has no direct correlation with pierced edge
stretchability.
• The new polish methodologies (ILG and 4M) can improve the
pierced edge stretchability, but not the tool wear reduction due
to higher friction and punch force in piercing.
• The angle shear punches (conical shear and bevel shear) resulted
in the least tool wear, and the punches with die corner radius
(edge hone) were also found to be effective in reducing the tool
wear.
REFERENCES
1. Nakata, M, Uematsu, K. and Koseki, S., "Shear Deformation Properties
of Ultra High Strength Steel Sheet," IDDRG, pp. 527-534, 2006.
2. Konieczny, A. and Henderson, T., "On Formability Limitations in
Stamping Involving Sheared Edge Stretching," SAE Technical Paper
2007-01-0340, 2007, doi:10.4271/2007-01-0340.
3. Karelova, A. and Krempaszky, C., "Influence of the Edge Conditions on
the Hole Expansion Property of Dual-Phase and Complex-Phase Steels,"
MS&T, pp. 159-169, 2007.
4. Golovashchenko, S. F., and Ilinich, A. M., "Trimming of Advanced High
Strength Steels," IMECE 2005-79983, 2005.
5. Dalloz, A., Gourgues, A-F., Pineau, A. and Besson, J., "Influence of
the Shear Cutting Process on Damage in Laboratory Dual Phase Steels
Developed for Automotive Application," MS&T, pp. 171-181, 2007.
6. Widenmann, R., Sartkulvanich, P. and Altan, T., "Finite Element
Analysis on the Effect of Sheared Edge Quality in Blanking Upon Hole
Expansion of Advanced High Strength Steel," IDDRG, pp. 559-570,
2009.
7. Shih, H-C, Chiriac, C. and Shi, M., "The Effects of AHSS Shear Edge
Conditions on Edge Fracture," MSEC2010-34062, 2010.
8. Shih, H-C and Shi, M., "An Innovative Shearing Process for AHSS Edge
Stretchability Improvements," JMSAE-061018, 2011.
9. Chiriac, C., and Shih, H-C., "Investigations of Shear Edge Image of
Dual Phase 780 Steel," MS&T 2011.
10. Takahashi, Y., Kawano, O., Horioka, S., and Ushioda, K., "Improvement
of Stretch Flangeability of High-Tensile-Strength Steel Sheets by
Piercing under Tension Using Humped Bottom Punch," SAE Technical
Paper 2013-01-0609, 2013, doi:10.4271/2013-01-0609.
CONTACT INFORMATION
Hua-Chu Shih
hshih@uss.com
Dajun Zhou
dj.zhou@fcagroup.com
Bruce Konopinski
bkonopinski@pcs-company.com
DISCLAIMER
The material in this paper is intended for general information only.
Any use of this material in relation to any specic application should
be based on independent examination and verication of its
unrestricted availability for such use and a determination of
suitability for the application by professionally qualied personnel.
No license under any patents or other proprietary interests is implied
by the publication of this paper. Those making use of or relying upon
the material assume all risks and liability arising from such use or
reliance
ACKNOWLEDGMENTS
The author would like to thank the Dayton Lamina of Misumi for
fabricating the punches and United States Steel Corporation for
permission to publish this paper.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or
otherwise, without the prior written permission of SAE International.
Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper.
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