PDF Warm and Hot Stamping of Ultra High Strength Steel Sheets Using ... - TUT

(1)

Center pillar Side door beam

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 Heating Steel Sheets Using Rapid Resistance Heating

Reduction in car weight Use of high strength steel sheets

Ultra high strength steel 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 metals Specific strength for various sheet metals

115MPa 2.7

310MPa Aluminium alloy

A6061(T6)

44MPa 7.8

340MPa Mild steel SPCC

63-101MPa 490- 7.8

790MPa High strength

steel

126 - 188MPa 980 - 7.8

1470MPa Ultra high

strength steel

Strength-to- specific gravity

ratio Specific

gravity Tensile

strength Sheet

Strength of high strength steel sheets Strength of high strength steel sheets

1500 TRIP

DP

Martensite

Conventional

IF Recently developed Remarkable increase in strength

Ultra sheet Press quenching

0.05 0.1 0.15 0.2 0.25 0.3

200 400 600 800 1000 1200

0

Flow stress /MPa

SPCC SPFC440

A1050 SPFC980Y

SPFC780Y

A5083

Strain

1

103

5 64 × ⁻ ⁻

=

ε& . s

1

103

2 0 × ⁻ ⁻

=

ε . s

&

1

103

2 91 × ⁻ ⁻

=

ε& . s

1

103

2 0 × ⁻ ⁻

=

ε . s

&

1

103

2 0 × ⁻ ⁻

=

ε& . s

1

103

1 0 × ⁻ ⁻

=

ε& . s

1

103

2 0 × ⁻ ⁻

=

ε& . s

1

103

9 92 × ⁻ ⁻

=

ε& . s

1

103

6 95 × ⁻ ⁻

=

ε& . s 3 1

10 2 0 × ⁻ ⁻

=

ε& . s

1

103

4 51 × ⁻ ⁻

=

ε& . s

1

103

9 104 × ⁻ ⁻

=

ε . s

&

Flow stress curve of sheets at room temperature Flow stress curve of sheets at room temperature Flow stress curve of sheets at room temperature

Large springback

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

(2)

Thickness 1.2mm, strain-rate 0.4/s

Heating temperature T/ºC

Tensile strength /MPa Elongation /%

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: large Formability: small Warm and hot stamping

Forming of ultra high strength steel sheets Forming of ultra high strength steel sheets

Reduction in forming load Increase in formability Heating of sheet ?

Al, Mg, stainless steel: low temperature Steel: high temperature(>500ºC )

Heating Setting Forming

Furnace

zDecrease in temperature

zOxidation

0.2s 1.0s

3.0s 5.0s

200 400 600 800

20 40 60 80 100

Decrease in temperature /ºC 0 Heating temperature T/ºC

Interval between heating and forming

Conventional warm and hot stamping Conventional warm and hot stamping

5.0s Very big

Resistance heating and forming

Direct heating into dies

Prevention of drop in temperature Reduction in oxidation

Joule heat

Warm and hot stamping using rapid resistance heating

5mm

120mm

Copper electrode V

L130×W80×t1.2

Sheet holder

7.4MPa 7.4MPa

Thermography

600mm

DC, E=10V

Control by input energy

Measurement of temperature distribution in sheet in resistance heating Measurement of temperature distribution

in sheet in resistance heating

Electrode

Resistance heating Resistance heating

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

(3)

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

Input energy /kJ

Temperature

Input energy 892℃

87kJ

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

SPFC980Y SPFC780Y SPFC590Y SPFC440

Input energy /kJ

Heating temperature /ºC Energy efficiency /%

Heating temperature T/ºC Energy efficiency of resistance heating Energy efficiency of resistance heating

200 400 600 800 1000 20

40 60 80 100

0

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

(4)

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

Electrode

Holder

ダイポンチ Sheet

A

ON 〜

OFF

Transformer

200V 60Hz

A

ON 〜

OFF

Transformer

200V 60Hz

A

ON 〜

OFF

Transformer

200V 60Hz

A

ON 〜

OFF

Transformer

200V 60Hz

A

ON 〜

OFF

Transformer

200V 60Hz

A

ON 〜

OFF

Transformer

200V 60Hz

A

ON 〜

OFF

Transformer

200V 60Hz

Heating: 0.96s Forming

0.26s

Holding: 3.5s

Hat-shaped bending using resistance heating Hat-shaped bending using resistance heating

L130×W20×t1.2 Servo press

CNC Servo Press (80tonf） CNC Servo Press

CNC Servo Press(80tonf）(80tonf）

servo motor

conventional time Bottom

dead centre

accuracy: 1µm speed: 150mm/s

ball screw Feedback control

Synchronising with resistance heating

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 temperatures Products formed by hat-shaped bending of

SPFC980Y at different heating temperatures Relationships between springback and heating temperature in hat-shaped bending of various sheets

Relationships between springback and heating temperature in hat-shaped bending of various sheets

200 400 600 800 1000

2 4 6

0

SPFC980Y SPFC780Y SPFC590 SPFC440 SPCC

Springback ∆θ/deg.

Heating temperature T/℃

(5)

0 200 400 600 10

10.5 11

11.5 Holding time

0.7s 7s

1.2

r

Heating temperature T/ºC Die radius

Corner radius r /mm 10mm

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 1000 0

10

5 7.5

2.5

Bending load /kN

Relationship between bending load and heating temperature Relationship between bending load

and heating temperature

0 200 400 600 800 0

20 40 60 80

Increase in weight /mg

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

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 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

(6)

Steel block Electrode

Sheet

3030

115

Quenching test of ultra high tensile strength sheet without deformation

L130×W20×t1.2 0.004 0.01 2.23 1.00 0.14

S P Mn Si C

SPFC980Y

From austenite to martensite

Heating temperature and holding time

800 900 1000

0 100 200 300 400 500 600

t / s 7.0 3.5 1.6 0.7

Vickers hardness /HV

Original

1100 Measurement

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

obtained from quenching test

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

obtained from quenching test

0.7 s 1.6 s

3.5 s

7.0 s

2 4 6 8 10

200 400 600 800 1000

0

Time from end of resistance heating /s

Temperature /ºC

End of heating Contact with tools Detachment

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

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

10 µm

Measurement

Ferrite and fine pearlite Martensitic

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

800 900 1000 1100

0 700 100 200 300 400 500 600

Vickers hardness /HV Furnace heating

Resistance heating, t=7.0 s Original

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 1100

700

Tensile strength /MPa

1600

0 1200 800

400 Elongation /%

0 5 10 15 20 25 t=7.0 s 30

Variations in tensile strength and elongation with heating temperature

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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

Sacrificial oxidation Liquefied film

Oxidation prevention

C Ca Si, Na K, B, C, Na

Element

D C B Oxidation A

preventive oil

Oxidation preventive oils used for hot stamping Oxidation 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) Drying 5) Keeping

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

Sheet Electrode

Insulator

Holder (1kN)

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 sheet for oxidation preventive oils Oxidation on surface of heated sheet

for oxidation preventive oils

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

800 900 1000 1100 0

1 2 3 4 5 6

Heating temperature T /ºC

Scalethickness /mm

A B C D

Non-coated

(8)

40 20 R10

φ37.6 R1.8

Holder Electrode Sheet 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 heating Hot 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 SPFC980 Hat-shaped bent sheet of oil-coated SPFC980

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

Scale on surface of bent sheet before and after cleaning Scale on surface of bent sheet

before and after cleaning

(c) T=980ºC (d) T=1100ºC Before cleaning After cleaning

Before cleaning After cleaningBefore cleaning After cleaning 10% phosphoric acid

Heating temperature T800 900 1000 /ºC1100 0

0.2 0.4 0.6 0.8

Sheet Before cleaning

After cleaning

Surface roughness /µmRa

800 900 1000 1100 0

0.2 0.4 0.6 0.8

Heating temperature T/ºC After cleaning

Before cleaning Sheet

Surface roughness of bent sheet before and after cleaning

(a) Shoulder (b) Bottom

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

Large shearing load Work-hardened cold

formed part, press quenched part: very hard

Toll life reduction:

wear, failure

Warm and hot shearing of ultra high strength steel sheet

Trimming, punching Chipping

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・Sheet

SPFC980, L130×W50×t2.0

・Punch

SKH51 (TiCN coating), φ10mm

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

Punching speed /mm･s^-1 150

5, 10, 15 Clearance ratio c/ %

650, 700, 830, 970, 1070 Heating temperature T /ºC

Electrode Sheet

Sheet holder Servo press

Punch(φ10) Load cell

Warm and hot punching Warm and hot punching

Die

Hot punching of SPFC980Y using resistance heating Hot 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

Fracture Burnishing

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

200 400 600 800 1000 20

40 60 80 100

0

Depth percentage /%

Fracture

Burnished Rollover

200 400 600 800 1000 1

2 3 4

0

Ratio of burr height to thickness /%

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

(10)

200 400 600 800 1000 1

2

0

Heating temperature T/ºC Burnished Fracture

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

temperature forc=5%

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

temperature forc=5%

Maximum punching load /kN

200 400 600 800 1000 10

20 30 40 50

0

Relationship between maximum punching load and heating temperature

Furnace

Formed part

Shearing

1) Low heating efficiency 2) Low dimensional accuracy 3) Long heating time

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

Whole heating

Resistance heating

Die

Knockout Holder

Sheet

Punch

Local heating near shearing region Local heating near shearing region

1) High heating efficiency 2) Compact apparatus 3) Small oxidation

Electrifying between holder and knockout

Contact pin electrodes used for local heating Contact 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

Holder

Knockout Load cell

12 contact pins, φ2.6

Local heating using contact pins Local heating using contact pins

(11)

50 100 200300 400 500 600 750

℃25 Punch

Die Holder

Knockout Sheet

FEM simulation of temperature distribution in local heating FEM simulation of temperature distribution in local heating

50 100 200300 400 500 600 750 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 pins Local 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/s Punched sheet for c=10% and v=150mm/s

T=800 ºC

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

(12)

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

Electrify electrode

Uniform resistance heating of cup Uniform resistance heating of cup

Cup

Cupper electrode

Cup

1000 800 600 400 200 900 700 500 300 Temperature

[℃]

Resistance heating of cup Resistance heating of cup

Bolster Die

Punch 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