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Toyohashi University of Technology, Japan
Prof. Dr. Ken-ichiro Mori
Forming of High Strength Steel Sheets
1. High strength steel sheet2. Springback in bending3. Stretch flangeability4. Fracture5. Galling and seizure6. Wrinkling7. Shearing8. Tailored blank9. Plastic joining
Cold Stamping of High Strength Steel Sheets
Forming of lightweight automobile parts
Reduction in 100 kg weight: 1km/l
MaterialHigh strength steel sheet (7.8) Aluminium (2.7), Magnesium (1.8), Titanum (4.5)
Part Hollow parts One piece
Simulation Finite element method
Production of automobiles in countriesUnit: 10,000 cars
Thailand
Japan
China USA
Germany Canada
Korea
UK
France Italy
India
Russia
Spain
Brazil
Mexico
South AfricaBrazil
Toyota Prius
Toyota Prius PHV
Honda Fit-Jazz
Environmentally friendly automobiles: Hybrid
Nissan Leaf Mitsubishi i-MiEV
Tesla Roadster
Environmentally friendly automobiles: Electric
High tensile strength steel
CO2 emission for producing automobile materials
Mild steel
Aluminium
Magnesium
Magnesium
Carbon fibers
CO2 Emission per producing 1 kg material (kg)
自動車使用におけるCO2排出量
CO2 emission
Recycle
Production Running time Scrapping
Reduction in weight
Specific strength for various sheet metals
Sheet Tensile strength
Specific gravity
Strength-to-specific gravity
ratioUltra high
strength steel980 -
1470MPa 7.8 126 -188MPa
High strength steel
490-790MPa 7.8 63-101MPa
Mild steel SPCC 340MPa 7.8 44MPaAluminium alloy
A6061(T6) 310MPa 2.7 115MPa
High strength steel sheets
1500
IF steel:
Ultra sheet
Die quenching
TRIP steel
DP steel
Tensile strength /MPa
Elon
gatio
n/%
Martensite steel
DP steel TRIP steelFerrite
Retained austeniteBainite
FerriteMartensite
High strength steel sheet
Load
High Strength Steel Sheet, HSS, AHSS, Ultra HSSTensile strength: 440, 490, 590 780, 1180 MPa
Pmax/A0
Pmax
Displacement
A0: initial cross-sectional area
Mechanism of strengthening of high strength steel sheet
Mild steel
Solid solution
Precipitation
Transformation
Martensite
Bainite
Properties of high strength steel sheets
Elon
gatio
n (%
)
r-val
ueH
ole
expa
nsio
n ra
tio
(%)
Precipitation
Precipitation
Precipitation
Precipitation
Martensite
Martensite
Martensite
Bainite
Bainite
Bainite
Yield stress ratio
ULSAB-AVC (Ultra Light Steel Auto Body- Advanced Vehicle Concept)
Application of high strength steel sheets to automobile body
36% of total weight
Used steel sheets
Used steel sheets Results of ULSAB-AVC
Fuel consumption
Weight
Collision safety
Production cost
Gasoline GasolineDiesel Diesel
Mild steel46%
Alumimium4.8%
Ultra HSS12.8%
HSS36.4%
Material percentage
Toyota Crown, 45% of body panels
Subaru Legacy
Application of high strength steel sheets to automobile body
High strength steel sheets: from 40% to 60%
8% reduction in weight
Mazda, Demio
OldNew
Mazda, Atenza: from 42% to 49%
Mazda, Atenza, Roadstar
Mazda, Roadstar, 1500MPa: 12% (green)
(a) 2004 (b) 2014
Chang in percentage of use of HSS
(a) 2004 (b) 2014
Use of HSS in automobile body panels
PillarBumper
Inner panelOuter panel
PillarBumper
Inner panelOuter panel
Increase in strength and reduction in weight
500 1000 1500 2000 500 1000 1500 2000Tensile strength /MPa
20
40
60
80
0
20
40
0Incr
ease
in b
endi
ng st
reng
th/%
Red
uctio
n in
wei
ght/
%
Tensile strength /MPa
1. High strength steel sheet2. Springback in bending3. Stretch flangeability4. Fracture5. Galling and seizure6. Wrinkling7. Shearing8. Tailored blank9. Plastic joining
Cold Stamping of High Strength Steel Sheets Springback in bending
ε
σ Mild steel
Ultra HSS
Elastic recovery
Springback: largeAccuracy of shape: low Tensile
CompressiveLoading
UnloadingCompressive
Tensile
Springback
Wall wrap
Twisting
Longitudinal wrap
Deterioration in shape accuracy of formed parts Springback in hat-shaped bending of HSS
Tensile strength
Springback in bending
：SpringbackM ：Bending moment (bt2/4)I ：Geometrical moment of inertia (bh3/12) ： Flow stressE ：Young's modulust ：Thickness ：Bend angleR ：Radius
M
E IR ３
~~R
tE
t
R
(a) Material (b) Thickness
5
4
3
2
1
0
曲げ角度 /º 曲げ角度 /º
スプ
リン
グバ
ック
角度
/º
Effects of material and thicknessin V-shaped bending
Sprin
gbac
k/º
Bending angle /º Bending angle /º
Punch corner radius / thickness
Effect of strength on springbackin hat-shaped bending
Effect of back pressure on springbackin U-shaped bending
Back pressure /MPa
Effect of strength on wrap in hat-shaped bending
ダイ肩R / 板厚
Die corner radius / thickness
Effect of blank holder force on wrapin hat-shaped bending
Blank holder force /kN
800kN CNC Servo Press
servo motor
conventional ti
me
Bottom dead centreaccuracy: 1mspeed: 150mm/s
ball screw
Feedback control
Direct driving type
90°
9.6
90°
9.6
Reduction in springback in V-shaped bending
Sheet
tf = t0
t 0
Reduction in thickness in bottoming
(a) SPCC (b) SPFC980Yv=24mm/s, f=0%, T=0.5s3 times slower
Deformation behaviour in V-shaped bending
0.05 0.1 0.15 0.2 0.25 0.3
200
400
600
800
1000
1200
0
Flow
stre
ss /M
Pa
SPCC
SPFC440
A1050
SPFC980Y
SPFC780Y
A5083
Strain
1310564 s.131020 s.
1310291 s.131020 s.
131020 s.
131010 s.
131020 s.
1310992 s.
1310695 s. 131020 s.
1310451 s.13109104 s.
Flow stress curve of sheets at room temperatureLarge springback
Relationship between springback and forming speed in V-shaped bending (f=0%, T=0s)
0 10 20 30 40 50-2
0
2
4
6
8SPFC980Y，Nominal
SPFC440
SPCCSprin
gbac
k
/º
Forming speed /mms-1
Sheet
SPFC980Y (DP), SPFC440 (IF), SPCC
Nominal thickness 1.2mm
SPCC
SPFC440
SPFC980Y（DP）
Scatter of initial thickness of sheets
nominal
1.18 1.19 1.2 1.21 1.220
10
20
30
40
Initial thickness t0 / mm
Num
ber o
f she
ets
Relationship between springback and forming speed in V-shaped bending (f=0%, T=0s)
0 10 20 30 40 50-2
0
2
4
6
8SPFC980Y, real
SPFC440
SPCCSprin
gbac
k
/º
Forming speed /mms-1
0
10
20
30
40
250 500 750 1000
Tensile strength /MPa
Spr
ingb
ack
/mm
270MPa
590MPa
980MPa
Required tolerance
±0.5mm
Relationship between springback and tensile strength
0 10 20 30 40 50-2
0
2
4
6
8
加工速度 v / mm・s-1
スプ
リン
グバ
ック
角度
Δθ
/ °
f=0%, T=0.1s
f=0.33%, T=0.1s
f=0.33%, T=0.5s
Effects of bottoming andholding time at bottom dead centre for SPFC980Y
Sprin
gbac
k an
gle
/º
Forming speed /mms-1
-2.0
1.5/GPa
1.0
0.5
0
-0.5
-1.0
-1.5
(a) f=0% (b) f=0.33%
Distribution of stress in width direction just after unloading for finishing reduction in thickness
Kaewtatip, P., Prasitkhetkhan, N., Khantachawana, A., Premanond, V., Hato, R., Sresomreong, B., Koga, N., Proc. 9th ICTP, (2008).
Effect of bottoming
Nominal
Die surface
Afterforming
Springback
Expectation for springback
Prevention of springback by beads in side wall
Increase in stiffness
【Old shape】
【New shape】
Chamfer shape
Prevention of springback by chamfering Prevention of springback by control of blank holder force
(a) Draw bending (b) Crash forming without blank holder
Prevention of springback by crash forming Prevention of springback by overrun inducing punch
Tensile strength /MPa
452
597
838
1025
1211
1520
Prevention of springback by overrun inducing punch Prevention of springback by crash forming
Prevention of springback by crash forming
980MPa
(a) Drawing (b) Restriking
Prevention of springback and twistingby restriking
Drawing
Restriking
(a) Drawing (b) Slide locking
780MPa
Prevention of springback by slide locking
Chamfer
Push Back
590MPa, 1.0 mm
(b) Restriking(a) Bending
Prevention of springback and twistingby chamfered bending and push-back forming
FEM simulation
Difference from product
Adjustment
NG
OK
(a) Simulation(b) 1st
adjustment
Adjustment of die shape by finite element simulation
(c) 2nd adjustment
Effect of constitutive equations on calculated springback Effect of constitutive equations on calculated springback by finite element simulation
Strain
Stre
ss (M
Pa)
(a) Experimental result (980 MPa)
(b) Isotropic model (c) Yoshida-Uemori model
Isotropic modelYoshida-Uemori model
1. High strength steel sheet2. Springback in bending3. Stretch flangeability4. Fracture5. Galling and seizure6. Wrinkling7. Shearing8. Tailored blank9. Plastic joining
Cold Stamping of High Strength Steel Sheets
780MPa high strength steel formed product
Fracture
Cold stamping
SheetShearing Stretch flanging
TensileSheared edge
Bending
Small formability
Fracture in stretch flanging of high strength steel sheets
Properties of high strength steel sheets
Elon
gatio
n (%
)
r-val
ueH
ole
expa
nsio
n ra
tio
(%)
Precipitation
Precipitation
Precipitation
Precipitation
Martensite
Martensite
Martensite
Bainite
Bainite
Bainite
Yield stress ratio
Steel sheets used for punching and stretch flanging
Sheet Thickness [mm]
Yield stress [MPa]
Tensile strength [MPa]
Elongation [%]
n-value
JSC980Y 1.41 620 1027 18.7 0.12JSC780Y 1.41 558 823 19.0 0.12JSC590R 1.40 446 600 26.2 0.14JSC440W 1.41 320 455 33.8 0.18JSC390W 1.39 283 389 35.8 0.18JSC270C 1.39 223 333 41.2 0.19
Shearing conditions
200 1.4
Punch
Punch
Die
Die
Sheet holder
Velocity: 30mm/s
Clearance ratio
Blank
200
c=10, 15, 20, 25%
JSC980, JSC780
155o
Sheet holder
Blank
Stretch flanging conditions
1.4
L
R51.4
R5Punch
Punch
Die
Die
Flange length, 1 mm interval
1.4
Blank155o
2Burr side
Sheet holder Sheet
holder
Velocity: 30mm/s
Concave blank
Effect of clearance ratio of occurrence of fracture（JSC980Y，L=15mm）
(a) c =20%
Fracture
1mm
Surface Sheared edge Surface Sheared edge
0.1mm
(b) c =15%
1mm
Burnish
Fracture
0.1mm
Fracture
No fracture
No fracture 5 10 15 20 25 300
20
40
60
Clearance ratio /%
Lim
iting
flan
ging
ratio
/%
JSC980
JSC780
Before bending
After bending
2mm
Limiting flanging ratio:
l1
l1 - l0
l0
l0
Relationship between limiting flanging ratioand clearance ratio
Crack
(d) Punching
(a) Initial
s 1
(b) Shearing by upper punch
Stroke of upper punch
s 2
(c) Shearing by lower punch
Stroke of lower punch
Upper dieUpper punch
Load of lowerpunchLower punch
Lower die
Improvement of stretch flangeability by upper and Improvement of stretch flangeability by upper and lower punching
Upper and lower punching
Conventional(c=20%t)
600 800 10000
10
20
30
40
50
60
Lim
iting
exp
ansi
on ra
tio
/%
Tensile strength /MPa
Relationship between limiting expansion ratio and tensile strength
Smoothing
Sheet
Sheared edge
Shearing Stretch flanging
Tensile
DieConical punch
Sheet holderBlank
Compression Frac
ture
su
rfac
e
Smoothing of fracture surface on sheared edge with conical punch
Load
Conical punch
Blank (c=20%)
Sheet holder
Dir\e
Guide φ12
Punching load PFracture surface
side
Die
φ10.72
Blank (c=20%)30°
Smoothing of fracture surface on sheared edge with conical punch in punching
Conical punch
Sheet holder
(a) P=0 kN (b) P=6 kN (c) P=12 kN
(f) P=22 kN(e) P=20 kN(d) P=16 kN
Smoothing of fracture surface on sheared edge ofpunched sheet (JSC980Y)
Smoothed sheared edge (JSC980Y)
0 m
30 m
(a) P=0kN (Punched edge)
(b) P=22kN
0.3mm
(i) Surface (ii) Height (i) Surface (ii) Height
5 10 15 20 25
30
60
90
120
150
180
210
0Sommthing load P /kN
Lim
iting
exp
ansi
on ra
tio/%
JSC390W
JSC980Y
JSC390W,electrical discharge machining
JSC980Y,electrical discharge machining
Improvement of limiting expansion ratio by smoothing (JSC980Y)
Punch
Sheet holder
Die
Sheet holder
Smoothing
BlankConical punch
Burr side
Die
Blankholder
15° 2.5
Conical punch φ45
P
Blank
Concave sheetPunch
Die
Smoothing of fracture surface on sheared edge with conical punch in punching
(c) P=9kN
BurnishedFracture
Rollover
Smoothed(b) P=2.5kN(a) P=0kN
1mm
0
20
40
60
80
100
Dep
th p
erce
ntag
e/%
Load P / kN
Rollover
Burnished
Frature
Smoothed
0 4 6 82
Sheared edge after smoothing (JSC780, c=20%) Improvement of limiting stretch flanging ratio (JSC780, c=20%)
0
20
40
60
80
0
0.5
1
1.5
荷重P /kN
Lim
iting
stre
tch
flang
ing
ratio
/%
Aver
age
surf
ace
roug
hnes
s of
frac
ture
surf
ace
Ra
/m
590MPa, no smoothing
4 6 82
Sheared edge before and after smoothing(JSC980Y, c=15%）
0.5mm
(a) P = 0 kN (b) P = 1 kN (c) P = 2 kN
BurnishedSmoothedFracture
Fractre
Smoothing load P /kN0.5 1 1.5 2
0
102030405060
Lim
iting
flan
ging
ratio
/%
590MPa, no smoothing
Improvement of limiting stretch flanging ratio (JSC980Y, c=15%)
ダイス
凹形状ブランク
引張り小
(a) Before forming (b) During forming (c) After forming
ダイスダイス
Die
Blank
Gradual contact punch
Reduction in tensile stress
Die
Reduction in tensile stress at cornerusing gradual contact punch
Gradual contact punch
10o
45o
(a) Flat punch
(b) Gradual contact punch
Reduction in tensile stress
Tensile stress
Reduction in tensile stress at cornerusing gradual contact punch
Shearing conditions
Sheet Thick-ness /mm
Yield stress /MPa
Tensile strength /MPa
Elonga-tion /%
Reduction in area /%
n-value
JSC780Y 1.4 395 847 18.8 66 0.15JSC980Y 1.4 660 1014 16.4 45 0.15JSC1180Y 1.2 864 1209 10.8 46 0.14
Punch
Die
Blank
Punch
Die
Blank holder
Clearance ratio
200
c =10, 15, 20, 25%
R50
W0=200Blank holder
W/W0 = 0.1-1 = 0-85o
L = 0-27mm
Burr side
t
R5R5Punch
ダイ
W0=200
Sheet holder
Die
Punch
1. 4
=155,130,120o
Blank
2L
Stretch flanging conditions
Die
Punch
W Blank
Sheet holder
Sheet holder
Software LS-DYNASymmetry 1/2 model
Sheet Elastic-plastic shell elements
Tools RigidCoefficient of friction 0.15
Punch speed 100mm/s
Punch
Blank holder
Blank
Calculated conditions of stretch flanging
Die
Distribution of longitudinal strain by flat punch（JSC780, =0o ， L=18mm）
Flat punch
Die
Longitudinal strain00.5 0.4 0.3 0.2 0.1
Gradual contact punch
Die
Longitudinal strain00.5 0.4 0.3 0.2 0.1
Distribution of longitudinal strain by gradual contact punch（JSC780, =10o , W/W0=1.0， L=18mm)
(b) Gradual contact punch（=10o, W/W0=1.0)
(a) Flat punch (=0o)
00.5 0.4 0.3 0.2 0.1Longitudinal strain
Distribution of longitudinal strain by gradual contact punch（JSC780, L=17mm)
(b) Gradual contact punch（=10o, W/W0=1.0) No fracture
(a) Flat punch (=0o) Fracture205.9
20.9 22
.5
205.2
20.7 22
.8
Prediction of fracture at corner of 780MPa sheetusing gradual contact punch
Crack No crack
Prediction of fracture at corner of 980MPa sheetusing gradual contact punch
(a) Flat punch, =180º (b) Gradual contact punch,=170º
17.2
Flat punch
W/W0=0.2, =45o
13.7
18.0
(a) JSC980Y
(b) JSC1180Y
W/W0=0.2, =45o
21.9
Improvement of limiting flange height by gradual contact punch
Flat punch
