warm and hot stamping of ultra high strength steel sheets using rapid resistance...

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K. Mori Toyohashi University of Technology, Japan

Warm and Hot Stamping of Ultra High Strength Steel Sheets Using Rapid Resistance Heating

Warm and Hot Stamping of Ultra High Strength Warm and Hot Stamping of Ultra High Strength Steel Sheets Using Rapid Resistance HeatingSteel Sheets Using Rapid Resistance Heating

Reduction in car weight Use of high strength steel sheets

Ultra high strengthsteel sheet > 1GPa

1) Ultra high strength steel sheets

2) Resistance heating

3) Warm and hot stamping

4) Quenching in hot stamping

5) Prevention of oxidation in hot stamping using oxidation preventive oil

6) Warm and hot shearing

7) Warm and hot spline forming

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

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

Pressquenching

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 −−×=ε s.&131020 −−×=ε s.&

1310291 −−×=ε s.&131020 −−×=ε s.&

131020 −−×=ε s.&

131010 −−×=ε s.&

131020 −−×=ε s.&

1310992 −−×=ε s.&

1310695 −−×=ε s.& 131020 −−×=ε s.&

1310451 −−×=ε s.&13109104 −−×=ε s.&

Flow stress curve of sheets at room temperatureFlow stress curve of sheets at room temperatureFlow stress curve of sheets at room temperatureLarge springback








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

Springback: largeFormability: small

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

5mm120mm

Copperelectrode

V

L130×W80×t1.2

Sheet holder

7.4MPa7.4MPa

Thermography

600m

m

DC, E=10VControl by input energy

Measurement of temperature distribution in sheet in resistance heating

Measurement of temperature distribution in sheet in resistance heating

Electrode

Resistance heatingResistance heating

SPFC980, thickness: 1.2mm, Power: 85kJ (10V, 2sec)

Distribution of temperature in resistance heating of rectangular sheet measured by

infrared thermography for SPFC980Y

Distribution of temperature in resistance heating of rectangular sheet measured by

infrared thermography for SPFC980Y

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

Relationship between measured heating temperature and input energy

Relationship between measured heating temperature and input energy

20 40 60 80

200

400

600

800

1000

0

SPFC980YSPFC780YSPFC590YSPFC440

Input energy /kJ

Hea

ting

tem

pera

ture

/ºC

Ene

rgy

effic

ienc

y /%


Energy efficiency of resistance heatingEnergy efficiency of resistance heating

200 400 600 800 1000

20

40

60

80

100

0

Inhomogeneous contact of sheet with electrode

Inhomogeneous contact of sheet with electrode

Distribution of temperature measured by infrared thermography for inhomogeneous

contact with electrode

Distribution of temperature measured by infrared thermography for inhomogeneous

contact with electrode








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

CNC Servo Press (80tonf）CNC Servo PressCNC Servo Press (80tonf(80tonf））

servo motor

conventional

time

Bottom dead centre

accuracy: 1µmspeed: 150mm/s

ball screw

Feedback control

Synchronising with resistance heating

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 /℃

0 200 400 600

10

10.5

11

11.5Holding time0.7s

7s

1.2

r


Die radius 10mmC

orne

r rad

ius

r /m

m

Relationship between corner radius of bent sheet and heating temperature for SPFC980Y

Relationship between corner radius of bent sheet and heating temperature for SPFC980Y

200 400 600 800 10000

10

5

7.5

2.5


Ben

ding

load

/kN

Relationship between bending loadand heating temperature

Relationship between bending loadand heating temperature

0 200 400 600 8000

20

40

60

80


Incr

ease

in w

eigh

t /m

g

980

Relationship between amount of oxidation of bent sheet and heating temperature for SPFC980Y

Relationship between amount of oxidation of bent sheet and heating temperature for SPFC980Y

Deep drawing of SPFC980Y rectangular sheet for 800ºC

Deep 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








Steel blockElectrodeSheet

3030

115

115

Quenching test of ultra high tensile strength sheet without deformation

Quenching test of ultra high tensile strength sheet without deformation

L130×W20×t1.2

0.0040.012.231.000.14SPMnSiC

SPFC980YFrom austenite to martensite

Heating temperature and holding time

800 900 10000

100

200

300

400

500

600

t / s7.03.51.60.7


Vic

kers

har

dnes

s /H

V

Original

1100

Measurement

Relationship between Vickers hardness and heating temperaturefor different holding times at bottom dead centre

obtained from quenching test

Relationship between Vickers hardness and heating temperaturefor different holding times at bottom dead centre

obtained from quenching test

0.7 s

1.6 s

3.5 s7.0 s

2 4 6 8 10

200

400

600

800

1000

0Time from end of resistance heating /s

Tem

pera

ture

/ºC

End of heating Contact with tools Detachment

Cooling curve of sheet at T=980 ºC for different holding timesCooling curve of sheet at T=980 ºC for different holding times

(a) Original (c) t=7.0s(b) t=0.7s

10 µm

Measurement

Ferrite and fine pearlite Martensitic

Microstructures at T=980 ºC for two holding timesMicrostructures at T=980 ºC for two holding times

800 900 1000 11007000

100

200

300

400

500

600

Vic

kers

har

dnes

s /H

V

Furnace heating

Resistance heating, t=7.0 s

Original

Comparison between hardnesses obtained from resistance heating and furnace heating for t=7.0s

Comparison between hardnesses obtained from resistance heating and furnace heating for t=7.0s


Original

Original

t=7.0 s

t=0.7 s

t=0.7 s

800 900 1000 1100700

Tens

ile s

treng

th /M

Pa

1600

0

1200

800

400 Elo

ngat

ion

/%

05

10

15

20

25

30t=7.0 s

Variations in tensile strength and elongation with heating temperature

Variations in tensile strength and elongation with heating temperature









Sacrificial oxidationLiquefied filmOxidation

prevention

CCaSi, NaK, B, C, NaElement

DCBAOxidation preventive oil

Oxidation preventive oils used for hot stampingOxidation preventive oils used for hot stamping

840 ºC

Sheet Ethanol

Paper

Sponge rubber

Oxidation preventive oil

Testing

1) Ultrasonic cleaning (3 times) 2) Drying 3) Application

4) Drying5) Keeping

Coating of sheets with oxidation preventive oilCoating of sheets with oxidation preventive oil

Sheet Electrode

Insulator

Holder (1kN)

Resistance heating of sheet for evaluation of oxidation preventive oils

Resistance heating of sheet for evaluation of oxidation preventive oils

10mm

(a) Oil A (liquefied film)

(b) Oil C (sacrificial oxidation)T=740 ºC T=980 ºC

T=740 ºC T=980 ºC

Oxidation on surface of heated sheetfor oxidation preventive oils

Oxidation on surface of heated sheetfor oxidation preventive oils

Relationships between scale thickness and heating temperature for different oxidation preventive oilsRelationships between scale thickness and heating temperature for different oxidation preventive oils

800 900 1000 11000

1

2

3

4

5

6


Sca

leth

ickn

ess

/mm A

BCD

Non-coated

40 20R10

φ37.6

R1.8

Holder

ElectrodeSheet

Insulator

荷重（サーボプレス）

70kg 70kg

Heating

Forming

(0.26s)

Holding (3.5s)

(0.1s)

Hot hat-shaped bending using resistance heatingHot hat-shaped bending using resistance heating

(a) T=740℃ (b) T=840℃

(c) T=980℃ (d) T=1100℃

Hat-shaped bent sheet of oil-coated SPFC980Hat-shaped bent sheet of oil-coated SPFC980

(a) T=740ºC (c) T=840ºCBefore cleaning After cleaning

Scale on surface of bent sheetbefore and after cleaning

Scale on surface of bent sheetbefore and after cleaning

(c) T=980ºC (d) T=1100ºC

Before cleaning After cleaning

Before cleaning After cleaning Before cleaning After cleaning

10% phosphoric acid

Heating temperature T /ºC800 900 1000 1100

0

0.2

0.4

0.6

0.8

SheetBefore cleaning

After cleaning

Sur

face

roug

hnes

s /µ

mR

a

800 900 1000 11000

0.2

0.4

0.6

0.8

Sur

face

roug

hnes

s /µ

mR

aHeating temperature T /ºC

After cleaning

Before cleaningSheet

Surface roughness of bent sheet before and after cleaning

Surface roughness of bent sheet before and after cleaning

(a) Shoulder (b) Bottom








Large shearing load

Work-hardened cold formed part, press quenched part: very hard

Toll life reduction:wear, failure

Warm and hot shearing ofultra high strength steel sheetWarm and hot shearing of

ultra high strength steel sheet

Trimming, punching Chipping

・Sheet SPFC980, L130×W50×t2.0

・PunchSKH51 (TiCN coating), φ10mm

Conditions of warm and hot punching processConditions of warm and hot punching process

150Punching speed /mm･s-1

5, 10, 15Clearance ratio c / %650, 700, 830, 970, 1070Heating temperature T /ºC

ElectrodeSheetSheet holder

Servo press

Punch(φ10) Load cell

Warm and hot punchingWarm and hot punching

Die

Hot punching of SPFC980Y using resistance heatingHot punching of SPFC980Y using resistance heating

(a) Room temperature

(b) 650 ºC （c) 830 ºC (d) 1070 ºC

1mm

Surface and cross-section of punched edge of SPFC980Y

Surface and cross-section of punched edge of SPFC980Y

FractureBurnishing

Relationship between percentage of depths of burnished surface, fracture surface and rollover on sheared edge

and heating temperature

Relationship between percentage of depths of burnished surface, fracture surface and rollover on sheared edge

and heating temperature

200 400 600 800 1000

20

40

60

80

100

0


Dep

th p

erce

ntag

e /%

Fracture

Burnished

Rollover

200 400 600 800 1000

1

2

3

4

0

Rat

io o

f bur

r hei

ght t

o th

ickn

ess

/%

5%

10%

c=15%


Relationship between ratio of burr height on sheared edge to thickness and heating temperature for different

clearance ratios

Relationship between ratio of burr height on sheared edge to thickness and heating temperature for different

clearance ratios

200 400 600 800 1000

1

2

0Heating temperature T /ºC

Burnished

Fracture

Sur

face

roug

hnes

s /µ

mR

a

Relationship between roughness of burnished and fracture surfaces on sheared edge and heating

temperature for c=5%

Relationship between roughness of burnished and fracture surfaces on sheared edge and heating

temperature for c=5%


Max

imum

pun

chin

g lo

ad /k

N

200 400 600 800 1000

10

20

30

40

50

0

Relationship between maximum punching loadand heating temperature

Relationship between maximum punching loadand heating temperature

Furnace

Formed part

Shearing

1) Low heating efficiency2) Low dimensional accuracy3) Long heating time

Problem for heating in warm and hot shearingProblem for heating in warm and hot shearing

Whole heating

Resistance heating

Die

Knockout

Holder

Sheet

Punch

Local heating near shearing regionLocal heating near shearing region

1) High heating efficiency2) Compact apparatus3) Small oxidation

Electrifying between holder and knockout

Contact pin electrodes used for local heatingContact pin electrodes used for local heating

Contact pins in holder

Contact pins 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

50100200300400500600750

25℃

Knockout

Holder


(a) Diameter: 2.6mm, both 12 pins


(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

T=800℃, v=150mm/s

Punch

Local heating using contact pinsLocal heating using contact pins

(a) Room temperature

(b) T=500 ºC

(c) T=700 ºC (d) T=900 ºC

1mm

10mm

Punched sheet for c=10% and v=150mm/sPunched sheet for c=10% and v=150mm/s

T=800 ºC








Clutch drum

Spline forming

Spline forming of clutch drum Spline forming of clutch drum

High tensile strength steel

Cup Drum

Resistance 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

Formed drum

Die

Groove

Spline forming of clutch drum Spline forming of clutch drum 1) Ultra high strength steel sheets







warm and hot stamping of ultra high strength steel sheets using rapid resistance...

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