Post on 14-Mar-2020
Center pillarSide door beam
K. Mori Toyohashi University of Technology, Japan
Stamping of High Strength Steel Sheets Stamping of High Strength Steel Sheets Stamping of High Strength Steel Sheets
Reduction in car weight Use of high strength steel sheets
Ultra high strengthsteel sheet > 1GPa
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
Specific strength for various sheet metalsSpecific strength for various sheet metals
115MPa2.7310MPaAluminium alloyA6061(T6)
44MPa7.8340MPaMild steel SPCC
63-101MPa7.8490-790MPa
High strength steel
126 -188MPa7.8980 -
1470MPaUltra high
strength steel
Strength-to-specific gravity
ratio
Specific gravity
Tensile strengthSheet
Cheaper and higher strength
Strength of high strength steel sheetsStrength of high strength steel sheets
1500
TRIP
DP
Martensite
Conventional
IF Recently developed
Remarkable increase in strength
Ultra sheet
Diequenching
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
s/065.0=ε&s/0002.0s/091.0
s/093.0
s/096.0
s/051.0s/10.0
Flow stress curve of sheets at room temperatureFlow stress curve of sheets at room temperatureFlow stress curve of sheets at room temperature
s/0002.0
s/0002.0
s/0002.0
s/0002.0
s/0001.0
How are the sheets formed?How are the sheets formed?
High strength steel sheetsHigh strength: large forming load, large
springback, small tool lifeLow formability: fracture
1) Cold stamping: no heating2) Warm and hot stamping: heating
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
CNC Servo PressCNC Servo PressCNC Servo Press
servo motor
conventional
time
Bottom dead centre
accuracy: 1µmspeed: 150mm/s
ball screw
Feedback control
Direct driving typeDirect driving type
R1.2
90°
R1.2
9.6
90°
9.6
Reduction in springback in V-shaped bendingReduction in springback in VReduction in springback in V--shaped bendingshaped bending
Sheet
∆tf = t0
∆tt 0
Finishing reduction in thickness
(a) SPCC (b) SPFC980Y
v=24mm/s, f=0%, T=0.5s3 times slower
Deformation behaviour in V-shaped bendingDeformation behaviour in VDeformation behaviour in V--shaped bendingshaped bending
Forming speed /mms-10 10 20 30 40 50
-2
0
2
4
6
8
f=0%, T=0.1s
f=0.33%, T=0.1s
f=0.33%, T=0.5s
Sprin
gbac
k an
gle ∆θ
/º
Effects of finishing reduction in thickness and holding time at bottom dead centre for SPFC980Y
Effects of finishing reduction in thickness and holding time at bottom dead centre for SPFC980Y
-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
Distribution of stress in width direction just after unloading for finishing reduction in thickness
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy 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 sheetsFracture in stretch flanging of high strength steel sheets
PunchPunch
Die
Die
Holder
Blank
Sheet
PunchPunch
ブランクHolder Die
Die
Blank
Blank
(a) Shearing
(b) Stretch flanging
Shearing and stretch flangingShearing and stretch flanging
Fracture
(a) C=25% (b) C=15%
No fracture
Effect of clearance in shearing of 780MPa sheetEffect of clearance in shearing of 780MPa sheet
Quality of surface of sheared edge
5 10 15 20 25 300
20
40
60
Clearance ratio /%
Lim
iting
flan
ging
ratio
/%
JSC980
JSC780JAC590
Before bending
After bending
2mm
Limiting flanging ratio:
l1
l1 - l0
l0
l0
Relationship between limiting flanging ratioand clearance ratio
Relationship between limiting flanging ratioand clearance ratio
Crack
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
Reduction in tensile stress at cornerusing gradual contact punch
Gradual contact punch
α
Crack No crack
Prediction of fracture at corner of 980MPa sheetusing gradual contact punch
Prediction of fracture at corner of 980MPa sheetusing gradual contact punch
(a) Flat punch, α=180º (b) Gradual contact punch,α=170º
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
Thickness 1.2mm, strain-rate 0.4/s
Heating temperature T/ºC
Tens
ile s
treng
th /M
Pa
Elo
ngat
ion
/%
Tensile strength
Elongation
200 400 600 800
200
400
600
800
1000
0
20
40
60
80
0
Variations in tensile strength and elongation with temperature for SPFC980Y
Variations in tensile strength and elongation with temperature for SPFC980Y
Large load, large springback, small formability
Warm and hot stamping
Forming of ultra high strength steel sheetsForming of ultra high strength steel sheets
Reduction in forming loadIncrease in formability
Heating of sheet ?Al, Mg, stainless steel: low temperatureSteel: high temperature(>500ºC )
Heating Setting Forming
Furnace
Decrease in temperatureOxidation
0.2s
1.0s
3.0s
5.0s
200 400 600 800
20
40
60
80
100
0Dec
reas
e in
tem
pera
ture
/ºC
Heating temperature T /ºC
Interval between heating and forming
Conventional warm and hot stampingConventional warm and hot stamping
5.0sVery big
Resistance heating and forming
Direct heating into dies
Prevention of drop in temperatureReduction in oxidation
Joule heat
Warm and hot stamping using rapid resistance heating
Warm and hot stamping using rapid resistance heating
Electrode
Resistance heatingResistance heating
SPFC980, thickness: 1.2mm, Power: 85kJ (10V, 2sec)
Inpu
t ene
rgy
/kJ
Temperature
Input energy
892℃
87kJ
Hea
ting
tem
pera
ture
T/ºC
Time /s
Variations in temperature and input energy in resistance heating of SPFC980
Variations in temperature and input energy in resistance heating of SPFC980
1 2 3
200
400
600
800
1000
0
20
40
60
80
100
0
Ene
rgy
effic
ienc
y /%
Heating temperature T /ºC
Energy efficiency of resistance heatingEnergy efficiency of resistance heating
200 400 600 800 1000
20
40
60
80
100
0
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
ElectrodeHolder
ダイ
ポンチ
Sheet
A
~ONOFF
Transformer
200V 60Hz
A
~ONOFF
Transformer
200V 60Hz
A
~ONOFF
Transformer
200V 60Hz
A
~ONOFF
Transformer
200V 60Hz
A
~ONOFF
Transformer
200V 60Hz
A
~ONOFF
Transformer
200V 60Hz
A
~ONOFF
Transformer
200V 60Hz
Heating: 0.96s
Forming
0.26s
Holding: 3.5s
Hat-shaped bending using resistance heatingHat-shaped bending using resistance heating
L130×W20×t1.2Servo press
Hat-shaped bending using resistance heating at 980 ºC
Hat-shaped bending using resistance heating at 980 ºC
Room temperature
550 ºC
740 ºC
980 ºC
Products formed by hat-shaped bending of SPFC980Y at different heating temperaturesProducts formed by hat-shaped bending of
SPFC980Y at different heating temperaturesRelationships between springback and heating
temperature in hat-shaped bending of various sheetsRelationships between springback and heating
temperature in hat-shaped bending of various sheets
200 400 600 800 1000
2
4
6
0
SPFC980YSPFC780YSPFC590SPFC440SPCC
Spr
ingb
ack ∆θ
/deg
.
Heating temperature T /℃
200 400 600 800 10000
10
5
7.5
2.5
Heating temperature T /ºC
Ben
ding
load
/kN
Relationship between bending loadand heating temperature
Relationship between bending loadand heating temperature
Relationship between hardness of bent sheetsand heating temperature
Relationship between hardness of bent sheetsand heating temperature
800 900 1000 11000
100
200
300
400
500H
ardn
ess
/HV
As-received
Heating temperature T /ºC
Die quenching
10µm10µm 10µm
MartensiteFerrite
(a) T=740ºC (b) T=840ºC (c) T=980ºC
Microstructures in bottom of bent sheetsMicrostructures in bottom of bent sheetsDeep drawing of SPFC980Y rectangular
sheet for 800ºCDeep drawing of SPFC980Y rectangular
sheet for 800ºC
Formed products in deep drawing of SPFC980Y rectangular sheet
Formed products in deep drawing of SPFC980Y rectangular sheet
Fracture
(a) T=600ºC
(b) T=800ºC
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
Clutch drum
Spline forming
Spline forming of clutch drum Spline forming of clutch drum
High strength steel
Cup Drum
Plate forgingResistance heating
Rectangular blank
Trapezoidal
electrode 1000
900
800
700
600
950
850
750
650
Temperature [℃]
Side wall: uniform cross-section
Electrifyelectrode
Uniform resistance heating of cupUniform resistance heating of cup
Cup
Cupper electrode
Cup
1000
800
600
400
200
900
700
500
300
Temperature [℃]
Resistance heating of cupResistance heating of cup
Bolster
DiePunch
Upper electrode Cup
Lower electrode Air cylinder
Slide
Spline forming of clutch drum Spline forming of clutch drum
(a) Before forming (b) After forming
Spline-formed drum Spline-formed drum
Cold forming
Afterforming
Cold forming
Target
Target
Cross section of spline-formed drum Cross section of spline-formed drum
Warm forming
Warm forming
Beforeforming
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
Shot blasting
Heating Stamping Shot blasting
Coated sheets Heating Stamping
Increase in processes, accuracy reduction
Aluminium and zinc coatings
Oxidation
No oxidation
High cost
Conventional prevention of oxidationConventional prevention of oxidation
Oxidation preventive oils
Quenchable steel sheet for hot stamping
K,B,C,Na,P,CaK,B,C,NaElementLiquefied filmLiquefied filmOxidation
prevention
Titanium warm stamping
Stainless steel warm stampingLubricant
BAOil
0.0014B
0.015P
1.2Mn
0.25Si
0.21C
Oxidation preventive oils used for hot stampingOxidation preventive oils used for hot stampingTarget: 900ºC, 10s
Sheet Ethanol Sponge rubber
Oxidation preventive oil
Testing
1) Ultrasonic cleaning (3 times) 2) Drying 3) Application
4) Drying
Coating of sheets with oxidation preventive oilCoating of sheets with oxidation preventive oil
Heating temperature:800, 825,850, 875,900,925ºCDimension: L130mm, W20mm, t1.2mm
Heating Cooling by air
Evaluation test of oils under natural coolingEvaluation test of oils under natural cooling
Heating temperature:800, 825,850, 875,900,925ºCDimension: L130mm, W20mm, t1.2mm
Heating Cooling by air
Evaluation test of oils under natural coolingEvaluation test of oils under natural cooling
T=850ºC
10mm
T=800ºC
(a) Non-coated (b) Oil A (b) Oil B
T=900ºC
Oxidation on surface of heated sheet for different coatings
Oxidation on surface of heated sheet for different coatings
n=1
n=31 2 3 4
0.05
0.1
0.15
0.2
0.25
0
0.005
0.010
0.015
0Number of coating n
Improvement of oxidation prevention by repeating of coating (T=900ºC)
Improvement of oxidation prevention by repeating of coating (T=900ºC)In
crea
se in
wei
ght
/g
Am
ount
of c
oatin
g /m
g・m
m2
Servo press
SheetGuide
40 20
R10
φ37.6
R1.8
5sForming
Die quenching(Holding time 3.5s)
Heating
Hat-shaped bending for evaluating oxidation preventive oil
Hat-shaped bending for evaluating oxidation preventive oil Hat-shaped bending of quenchable steel sheetHat-shaped bending of quenchable steel sheet
(a) n=1 (b) n=3
T=850ºC
T=900ºC
T=800ºC
Hat-shaped bent sheetsHat-shaped bent sheets
×
800 850 900 9500
100
200
300
400
500
600
Har
dnes
s /H
V
t=5s3.5s
2s
825 875 925Heating temperature T /ºC
As-received
Measuring point
0
100
200
300
400
500
600
Har
dnes
s /H
V
800 850 900 950825 875 925Heating temperature T /ºC
As-received
×Measuring point
t=5s3.5s
2s
Hardness of bent sheetsHardness of bent sheets
(a) Bottom centre
(b) Die cornere
Phosphoric acid(10%,70ºC)
Product
Heater
2min
Cleaning of layer remaining on surface of formed product by phosphoric acid
Cleaning of layer remaining on surface of formed product by phosphoric acid
(a) n=1 (b) n=3Before After
10mm
Bottom centre
Scale on surface of sheet bent at T=900ºC and before and after cleaning
Scale on surface of sheet bent at T=900ºC and before and after cleaning
Die corner
Before After
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
Large shearing load
Work-hardened cold formed part, press quenched part: very hard
Toll life reduction:wear, failure
Shearing of ultra high strength steel sheetShearing of ultra high strength steel sheet
Trimming, punching Chipping Laser cutting: low productivity and high cost
Resistance heating
Die
Knockout
Holder
Sheet
Punch
Warm and hot shearing using local resistance heating near shearing region
Warm and hot shearing using local resistance heating near shearing region
1) High heating efficiency2) Compact apparatus3) Small oxidation
Electrifying between holder and knockout
Whole heating: low efficiency and accuracy Contact pin electrodes used for local heatingContact pin electrodes used for local heating
Contact pins with spring in holder
Contact pins with spring in knockout
Sheet
Uniform heating
V
Servo press
Die
Punch, φ19.8 Sheet,
SPFC980,50x50xt2.0
Spring
Spring
Holder
Knockout
Load cell
12 contact pins, φ2.6
Local heating using contact pinsLocal heating using contact pins
50100200300400500600750
25℃
Punch
Die
Holder
Knockout
Sheet
FEM simulation of temperature distribution in local heatingFEM simulation of temperature distribution in local heating
(a) Diameter: 2.6mm, both 12 pins
FEM simulation of temperature distribution in local heatingFEM simulation of temperature distribution in local heating
(b) Diameter: 1.6mm, both 12 pins
(c) Diameter: 2.6mm, both 8 pins (d) Diameter: 2.6mm, inside: 12 pins, outside: 24 pins
Sheet
Contact pins
Punch
Local heating using contact pins for T=800 ºCLocal heating using contact pins for T=800 ºC
10mm
Punched sheet for local heating Punched sheet for local heating
SPFC440,t=2.6mmSPFC980,t=1.4mm
T=25ºC 600ºC 700ºC 800ºC
Rollover
Fracture
Burnishing
0.5mm 1mm
Rollover
Burnishing
Surface of punched edge for local heatingSurface of punched edge for local heating
Fracture
T=25ºC 600ºC 700ºC 800ºC
1) Ultra high strength steel sheets2) Reduction in springback using servo press3) Improvement of formability in stretch
flanging 4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping
using oxidation preventive oil8) Warm and hot shearing9) Self pierce riveting of ultra high strength
steel and aluminium alloy sheets
Die
Holder
Rivet (hard steel)
Punch
C-frame
Joining point
Cold plastic joiningNo pre-drilling hole Short joining time
Hydraulic unit
Self pierce riveting of sheetsSelf pierce riveting of sheets
mild steel, 1.2mm
A5052,2.0mm
Self pierce riveting of mild steel and aluminium alloy sheets
Self pierce riveting of mild steel and aluminium alloy sheets
Requisites for joiningDriving though upper sheetSpreading in lower sheet
0.5
φ10φ2 φ21.
5 φ13
1.5
2.5
Optimised die for joining of ultra high strength steel and aluminium alloy sheets obtained from
inite element simulation
Optimised die for joining of ultra high strength steel and aluminium alloy sheets obtained from
inite element simulation
(a) Conventional die (b) Optimised die
Diameter of cavity, depth of projection
Finite element simulation ofself piercing riveting of ultra high strength
steel and aluminium alloy sheets
Finite element simulation ofself piercing riveting of ultra high strength
steel and aluminium alloy sheets
Upper: SPFC980, 1.4mmLower: A5052-H34, 1.5mm
Conventional die Optimised die
SPFC980, 1.4mmA50521.5mm
Self pierce riveting of ultra high strength steel and aluminium alloy sheet using optimised dieSelf pierce riveting of ultra high strength steel and aluminium alloy sheet using optimised die
Cold stampingReduction in springback using servo pressImprovement of formability in stretch flangingSelf pierce riveting of high strength steel and
aluminium alloy sheets
Warm and hot stamping Warm and hot stampingWarm and hot spline formingPrevention of oxidation in hot stamping using
oxidation preventive oilWarm and hot shearing