K. Mori Toyohashi University of Technology, Japan Reduction in...

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





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


flanging4) Resistance heating5) Warm and hot stamping6) Warm and hot spline forming7) Prevention of oxidation in hot stamping



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





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 /%


Energy efficiency of resistance heatingEnergy efficiency of resistance heating

200 400 600 800 1000

20

40

60

80

100

0





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


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


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





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





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





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





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


Finite element simulation ofself piercing riveting of ultra high strength


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

K. Mori Toyohashi University of Technology, Japan Reduction in...

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