Stamping of High Strength Steel Sheets

(1)

Center pillar Side 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 strength steel sheet > 1GPa

1) Ultra high strength steel sheets

2) Reduction in springback using servo press 3) Improvement of formability in stretch

flanging

4) Resistance heating 5) Warm and hot stamping 6) Warm and hot spline forming

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

8) Warm and hot shearing

9) Self pierce riveting of ultra high strength steel and aluminium alloy sheets

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

Cheaper and higher strength

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 Die 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 s / 065 .

=0 ε&

s / 0002 . 0

s / 091 . 0

s / 093 . 0

s / 096 . 0

s / 051 . 0

s / 10 . 0

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

s / 0002 . 0

s / 0002 . 0 s / 0001 . 0

How are the sheets formed?

High strength steel sheets High strength: large forming load, large

springback, small tool life Low formability: fracture

1) Cold stamping: no heating 2) Warm and hot stamping: heating

(2)

flanging

CNC Servo Press CNC Servo Press CNC Servo Press

servo motor

conventional time Bottom

dead centre

accuracy: 1µm speed: 150mm/s

ball screw Feedback control

Direct driving type Direct driving type

R1.2 90°

R1.2 9.6 90°

9.6

Reduction in springback in V-shaped bending Reduction in springback in V

Reduction in springback in V--shaped bendingshaped bending

Sheet

f = ∆t t0

∆t t0

Finishing reduction in thickness

(a) SPCC (b) SPFC980Y v=24mm/s, f=0%, T=0.5s

3 times slower

Deformation behaviour in V-shaped bending Deformation behaviour in V

Deformation behaviour in V--shaped bendingshaped bending

Forming speed /mms^-1

0 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

Springback angle ∆θ/º

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

(3)

flanging

780MPa high strength steel formed product

Fracture

Cold stamping Sheet

Shearing Stretch flanging

Tensile Sheared edge

Bending

Small formability Fracture in stretch flanging of high strength steel sheets Fracture in stretch flanging of high strength steel sheets

Punch Punch

Die Die

Holder

Blank Sheet

Punch Punch

ブランク Holder Die

Die

Blank

Blank (a) Shearing

(b) Stretch flanging Shearing and stretch flanging Shearing and stretch flanging

Fracture

(a) C＝25％ (b) C＝15％

No fracture Effect of clearance in shearing of 780MPa sheet Effect of clearance in shearing of 780MPa sheet

Quality of surface of sheared edge

5 10 15 20 25 30

0 20 40 60

Clearance ratio /%

Limiting flanging ratio /%

JSC980 JSC780 JAC590

Before bending

After bending 2mm

Limiting flanging ratio:

l₁

l₁-l₀ l₀

l₀

Relationship between limiting flanging ratio and 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 corner using gradual contact punch Reduction in tensile stress at corner

using gradual contact punch

Gradual contact punch

α

(4)

Crack No crack Prediction of fracture at corner of 980MPa sheet

using gradual contact punch

Prediction of fracture at corner of 980MPa sheet using gradual contact punch

(a) Flat punch, α=180º (b) Gradual contact punch, α=170º

flanging

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

Large load, large springback, small formability

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

z Decrease in temperature

z Oxidation

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

(5)

Electrode

Resistance heating Resistance heating

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

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

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

flanging

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

Hat-shaped bending using resistance heating at 980 ºC

(6)

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/℃

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

Relationship between hardness of bent sheets and heating temperature

800 900 1000 1100

0 100 200 300 400 500

Hardness /HV

As-received

Heating temperature T/ºC Die quenching

10µm 10µm 10µm

Martensite Ferrite

(a) T=740ºC (b) T=840ºC (c) T=980ºC Microstructures in bottom of bent sheets Microstructures in bottom of bent sheets

Deep drawing of SPFC980Y rectangular sheet for 800ºC

(7)

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

flanging

Clutch drum

Spline forming

Spline forming of clutch drum Spline forming of clutch drum

High strength steel

Cup Drum

Plate forging

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

(8)

(a) Before forming (b) After forming Spline-formed drum

Spline-formed drum

Cold forming After

forming

Cold forming

Target

Cross section of spline-formed drum Cross section of spline-formed drum

Warm forming Warm

forming

Before forming

flanging

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 oxidation Conventional prevention of oxidation

Oxidation preventive oils

Quenchable steel sheet for hot stamping

K,B,C,Na,P,Ca K,B,C,Na

Element

Liquefied film Liquefied film

Oxidation prevention

Titanium warm stamping Stainless steel warm

stamping Lubricant

B A

Oil

0.0014 B 0.015

P 1.2 Mn 0.25

Si 0.21

C

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

Target: 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 oil Coating of sheets with oxidation preventive oil

(9)

Heating temperature：800, 825，850, 875，900，925ºC Dimension: L130mm, W20mm, t1.2mm

Heating Cooling by air

Evaluation test of oils under natural cooling Evaluation test of oils under natural cooling

Heating temperature：800, 825，850, 875，900，925ºC Dimension: L130mm, W20mm, t1.2mm

Heating Cooling by air

Evaluation test of oils under natural cooling Evaluation 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

n=１

n=3

1 2 3 4

0.05 0.1 0.15 0.2 0.25

0 0.005 0.010 0.015

0

Number of coating n

Improvement of oxidation prevention by repeating of coating (T=900ºC) Improvement of oxidation prevention by

repeating of coating (T=900ºC)

Increase in weight /g Amount of coating /mg･mm2

Servo press

Sheet Guide

40 20

R10 φ37.6

R1.8

5s Forming 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

(10)

(a) n=１ (b) n＝３ T=850ºC

T=900ºC T=800ºC

Hat-shaped bent sheets Hat-shaped bent sheets

×

800 850 900 950

0 100 200 300 400 500 600

Hardness /HV

t=5s 3.5s 2s

825 875 925

Heating temperature T/ºC As-received

Measuring point 0

100 200 300 400 500 600

Hardness /HV

800 825 850 875 900 925 950

Heating temperature T/ºC As-received

×

Measuring point t=5s

3.5s 2s

Hardness of bent sheets Hardness 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

(a) n=１ (b) n=３

Before After

10mm

Bottom centre

Scale on surface of sheet bent at T=900ºC and before and after cleaning

Die corner

Before After

flanging

Large shearing load Work-hardened cold

formed part, press quenched part: very hard

Toll life reduction:

wear, failure

Shearing of ultra high strength steel sheet Shearing of ultra high strength steel sheet

Trimming, punching Chipping

Laser cutting: low productivity and high cost

(11)

Resistance heating

Die Knockout

Holder

Sheet

Punch

Warm and hot shearing using local resistance heating near shearing region

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

Electrifying between holder and knockout Whole heating: low efficiency and accuracy

Contact pin electrodes used for local heating Contact 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

Holder

Knockout Load cell

12 contact pins, φ2.6

Local heating using contact pins Local heating using contact pins

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

(a) Diameter: 2.6mm, both 12 pins

FEM simulation of temperature distribution in local heating FEM 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 ºC Local heating using contact pins for T=800 ºC

(12)

10mm

Punched sheet for local heating Punched sheet for local heating

SPFC440，t=2.6mm SPFC980，t=1.4mm

T=25ºC 600ºC 700ºC 800ºC Rollover

Fracture Burnishing

0.5mm 1mm

Rollover Burnishing

Surface of punched edge for local heating Surface of punched edge for local heating

Fracture

T=25ºC 600ºC 700ºC 800ºC

flanging

Die Holder

Rivet (hard steel) Punch

C-frame Joining point

Cold plastic joining

No pre-drilling hole

Short joining time

Hydraulic unit

Self pierce riveting of sheets Self 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 joining

zDriving though upper sheet

zSpreading in lower sheet

0.5

φ10φ2 1.5 φ2

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

(13)

Finite element simulation of

self piercing riveting of ultra high strength steel and aluminium alloy sheets

Finite element simulation of

self piercing riveting of ultra high strength steel and aluminium alloy sheets

Upper: SPFC980, 1.4mm Lower: A5052-H34, 1.5mm

Conventional die Optimised die

SPFC980, 1.4mm

A5052 1.5mm

Self pierce riveting of ultra high strength steel and aluminium alloy sheet using optimised die Self pierce riveting of ultra high strength steel and aluminium alloy sheet using optimised die

Cold stamping

Reduction in springback using servo press Improvement of formability in stretch flanging Self pierce riveting of high strength steel and

aluminium alloy sheets Warm and hot stamping Warm and hot stamping Warm and hot spline forming

Prevention of oxidation in hot stamping using oxidation preventive oil

Warm and hot shearing