9th BrDDRG Congressin 42°SENAFOR

Hot stamping using resistance heating of
quenchable steel sheet and tube

Tomoyoshi MAENO
YOKOHAMA National Unibersity

Hot stamping using resistance heating of
quenchable steel sheet and tube

Hot stamping using resistance heating of
quenchable steel sheet and tube

• Hot stamping of quenchable steel sheet• Outline of direct resistance heating in hot stamping• Full hardening under rapid heating• Heating of non-rectangle blank

Tomoyoshi MAENO
YOKOHAMA National University(JAPAN)• Tailor tempering by ceramic tip• Hot stamping with hot blanking process• Hot stamping and gas forming of quenchable steel tube
• Miscellaneous technique in resistance heating

9th BrDDRG Congress in 42°SENAFOR
5th October 2023

Hot stamping of quenchable steel sheet
22MnB5 quenchable steel sheet(Bare, Al-Si and Z coated)

Stamping anddie-quenching

900°C
Temperature

Forming

Hot stamping of quenchable steel sheet

Laser

DieBlank heating
900°C,Austenitized

9th BrDDRG Congress in 42°SENAFOR
5th October 2023

Laser trimming

Ultra high-strength
products

Advantages• Tensile strength：1.5~1.8GPa• No-springback• Large elongation

die-quenching
Volkswagen Production - Press Shop , AP&T Group YouTube channel

Time
Red: hot-stamped panels

Hot stamping using resistance heating of
quenchable steel sheet and tube

Problems in hot stamping
Huge furnace

4Oxidation• Hot stamping of quenchable steel sheet
Al-Si coating
Zn coating• Outline of direct resistance heating in hot stamping• Full hardening under rapid heating

Heating time increased

20-40 mLow productivity

Trimming• Tailor tempering by ceramic tip

Laser trimming and hole making ⇨ high cost
Shearing ⇨ tool life, delayed fracture
Transferring
2sForming
3s• Hot stamping with hot blanking process• Hot stamping and gas forming of quenchable steel tube

Holding
10 ~20 s

Hot stamping： 2-3 spm
Cold stamping： 6-16 spm• Heating of non-rectangle blankHigh cost

• Miscellaneous technique in resistance heating
9th BrDDRG Congress in 42°SENAFOR
5th October 2023

1

9th BrDDRG Congressin 42°SENAFOR

Hot stamping using resistance heating ofquenchable steel sheet and tube

Advantages of direct resistance heating in hot stamping
Rapid heating

Tomoyoshi MAENO
YOKOHAMA National Unibersity

Hot stamping using resistance heating in Toyota Motor

High productivity with small in-progress productFurnace

Number in-progressproduct

Heating time

3 SPM: 13.5 = 270 s
20 s
Time per unit
production

Resistance heating

Heating synchronizedwith die quenching
6 SPM: 1

Small hot stamping line
Products size+Power source

100-200 m2<20 m2

Resistance Heating Die quenching
10s
10s

Flexible control of production
Furnace: heating and cooling time are a few days
24/7 running

Resistance heating

Furnace

Resistance heating: No start-up and shut-down time

Synchronization of heating and die quenching

Hot stamping, TOYOTA MOTOR CORPORATION YouTube channel

Video of synchronized hot stamping of heating and die quenching

New blank
Transferring
Current

Heating stage

Stamping stage(1) Transferring and feeding of blanks

HeatingHolding(2) Heating and stamping ＋die quenching

Limitation of direct resistance heating in hot stamping
Limited blank shape

Hot stamping using resistance heating of
quenchable steel sheet and tube

Melting of aluminum coating of sheet• Hot stamping of quenchable steel sheet
Hot spot• Outline of direct resistance heating in hot stamping• Full hardening under rapid heating• Heating of non-rectangle blank

Trapezium
Non heated edge
Blank

Lee, C.W. et al., Metall Mater Trans A 47, 2875–2884 (2016).

Non heated edge

• Tailor tempering by ceramic tip• Hot stamping with hot blanking process

LDR is reduced• Hot stamping and gas forming of quenchable steel tube
• Miscellaneous technique in resistance heatingFurnace

Resistance heatng

9th BrDDRG Congress in 42°SENAFOR
5th October 2023

2

Hot stamping using resistance heating ofquenchable steel sheet and tube

Effect of heating rate on Vickers hardness and standard deviation

Ferrite fromunaltered ferite

3–5 min

Roller hearth furnace
3–10 sTemp.

3.2s

Current

Resistance heatingTime

100μm

Tomoyoshi MAENOYOKOHAMA National Unibersity

500

16300

122008Standard deviation

10000

Insufficient austenitisation

20Hardness

400Standard deviation in hardness[HV20]

Insufficient austenitisation in rapid resistance heating

Average Vickers hardness ofcenter portion [HV20]

9th BrDDRG Congressin 42°SENAFOR

450

100150200
Heating rate v [°C/s]

2500300

T. Maeno et al., J. Manuf. and Mater. Proc, 4-3(2020), 80.

Hot stamping using resistance heating of
quenchable steel sheet and tube

Effect of holding at austenitising temperature onVickers hardness and standard deviation for v = 275 °C/s

Average Vickers hardness ofcenter portion [HV20]400

Ferrite16

Full Martensite

300

1250μm50μm

20010002084

Standard deviation

02468Holding time at austenitising temperature t [s]10Large current
HeatingStandard deviation ofhardness [HV20]

Hardness

500• Hot stamping of quenchable steel sheet• Outline of direct resistance heating in hot stamping• Full hardening under rapid heating• Heating of non-rectangle blank• Tailor tempering by ceramic tip

Small current
Temperature holding intransferring0• Hot stamping with hot blanking process• Hot stamping and gas forming of quenchable steel tube
• Miscellaneous technique in resistance heating

Short temperature holding remarkably
improves quenchability .

9th BrDDRG Congress in 42°SENAFOR
5th October 2023

T. Maeno et al., J. Manuf. and Mater. Proc, 4-3(2020), 80.

Resistance heating of non-rectangle blank

FEM

FEM Hotspot

Parallelogram0

Trapezium
200

Uniform heating of non-rectangular sheet by divided circuit and bypass electrodes

400

600

800

Electricalcurrent tend toflow over shortdistance.
1000 ℃

Dividing of circuit

Divide the area and set an electrical circuit ineach area. And the current amount of eachpower supply are adjusted that the current
density is the same.

Separate bypass electrodes are installed inareas where the shape changes gradually.
The number of circuits can be reduced.
B.A. Behrens et al. Proc. 5th Int. Conf. Accuracy in Forming
Tech., Tronto (2015), pp. 157-173

3

9th BrDDRG Congressin 42°SENAFOR

Hot stamping using resistance heating of
quenchable steel sheet and tube

Hot stamping of non-rectangular steel sheets using resistance heating by local preheating
FEM
High-resistivity
Large Joule heating

PreheatingTomoyoshi MAENO
YOKOHAMA National Unibersity

Hot stamping using resistance heating ofquenchable steel sheet and tube
• Hot stamping of quenchable steel sheet• Outline of direct resistance heating in hot stamping• Full hardening under rapid heating

Experiment• Heating of non-rectangle blank

Copper block (400°C)• Tailor tempering by ceramic tip• Hot stamping with hot blanking process• Hot stamping and gas forming of quenchable steel tube
Just after
preheating

Just after
resistance heating• Miscellaneous technique in resistance heating
9th BrDDRG Congress in 42°SENAFOR
5th October 2023Y. Nakagawa et al., Procedia Manufacturing
Volume 50, 2020, Pages 298-302

Local cooling by contact with ceramics tool

Local prevention of hardening for punching of hot-stamped parts

Ceramic tipPunching

Ceramics tip

Low temperature(1) Resistance heating

Steel sheet

Non-hardened(2)Hot stamping(3) Cold punching

ElectrodeCeramics tip

K. Mori, Key Engineering Materials, 639 (2015), 205-212.

Temperature and hardness distribution of punching portion with local coolingTemperature /℃Vickers hardness /HV0.3

Ceramic tip

1000
800600Austenitization temp.

4002000-15-10-505x /mm

1015Hot stamping using resistance heating ofquenchable steel sheet and tube

• Outline of direct resistance heating in hot stamping

600500• Full hardening under rapid heating400• Heating of non-rectangle blank

300
200

As-recieved• Tailor tempering by ceramic tip

100
0-15

K. Mori, Key Engineering Materials, 639 (2015), 205-212.

• Hot stamping of quenchable steel sheet

Ceramic tip

700

Electrode-10-505x /mm

1015• Hot stamping with hot blanking process• Hot stamping and gas forming of quenchable steel tube
• Miscellaneous technique in resistance heating

K. Mori, Key Engineering Materials, 639 (2015), 205-212.

9th BrDDRG Congress in 42°SENAFOR
5th October 2023

4

9th BrDDRG Congressin 42°SENAFOR

Hot stamping using resistance heating ofquenchable steel sheet and tube

Large trimming of hot stamped panel in resistance heating

Hot stamping using blanking immediately after resistance heating

Hot stamping with furnace heating
Non-coatedsheet
180×150×1.0

Blanking

Heating

Stamping

Stamping,
die-quenching

Heating, blanking

Short trimming length

Tomoyoshi MAENO
YOKOHAMA National Unibersity

Punch

Dropped blanked sheet

Punch0 2 4

Laser trimming

Time [s]6 8 10 12 14 16 18

Electrode

Hot stamping with resistance heating
Rectangle
blankBlanking DieLong trimming length

Heating

RailDie(a) Heating

Hot blanking(b) Hot blanking

Transferring

Resistance heating

Laser trimming

Stamping

Stamping
5s

Holding

Fracture(c) Transferring
Limitation of blank shape(d) Stamping and die-quenching

Low formability due to Non-heating edge

T Maeno et al., Int J Adv Manuf Technol 97(2018), 3827–3837.

Video of hot stamping of reinforcements using
blanking immediately after resistance heating

Appearance of hot stamped reinforcement partsCamera

Fractured
ShearedRoll over

Shooting position

Hot blanked(a) Scrap

Cold(b) Blanked sheet(c) Formed panel(just before stamping：760℃)

T Maeno et al., Int J Adv Manuf Technol 97(2018), 3827–3837.

Reductions in thickness of flange portions of the hot-stamped
reinforcements using hot-blanked and rectangular sheets

Reduction in thickness [%]1Measuring position
234 5

2-stage progressive-die hot stamping of small products using resistance heating
Feeding

Uncoiled plateCurrent(a) Resistance heating

4
Rectangular(b) Blanking & punching3

1st stage(c) Inserting

Bending(d) Bending & die quenching

2nd stage

210

With hot blanking

With hot blanking
1234
Measuring position

5

RectangularFormability was improved byremoving the non-heating edge
T Maeno et al., Int J Adv Manuf Technol 97(2018), 3827–3837.

PlateElectrode

K Mori et al., Procedia Engineering 207 (2017) 681–686

59th BrDDRG Congressin 42°SENAFOR

Hot stamping using resistance heating of
quenchable steel sheet and tube

Hot stamping using resistance heating of
quenchable steel sheet and tube

Tomoyoshi MAENO
YOKOHAMA National Unibersity

Steel Tube Air Forming process (STAF) / Sumitomo Heavy Industries, Ltd.
Flange: for joining• Hot stamping of quenchable steel sheet
Tube• Outline of direct resistance heating in hot stampingPDie• Full hardening under rapid heating• Heating of non-rectangle blank

Tube: uniform cross section

Resistance heating 1st Blowing (Bulging)• Tailor tempering by ceramic tip

2nd Blowing (calibration)
die closing• Hot stamping with hot blanking process

Melted coating is not moved• Hot stamping and gas forming of quenchable steel tube

Al-Si coated steel pipe
Available• Miscellaneous technique in resistance heating
9th BrDDRG Congress in 42°SENAFOR
5th October 2023

Steel Tube Air Forming process (STAF) by SUMITOMO heavy indust

Hollow products with flange

R. Funada, Presented at Global Automotive
Lightweight Materials, Detroit, 2019.

：Lorentz force

K. Nogiwa, Plastos, 5-55(2022), 448-452

Hot stamping of steel tube using with fiber reinforced ice mandrel
Pure Ice orplants fiber reinforced ice

Buckling

Pure ice

With plant fiber(Toilet paper)

GapResistance heating

Compression of tube
• Low shape accuracy
due to buckling• Insufficient quenching
due to gap

20 mm

Vickrs hardness [HV10]

Without mandrel

Water quenched• Outline of direct resistance heating in hot stamping
300300• Full hardening under rapid heating

200200• Heating of non-rectangle blank

As-received

100
10000

1010

• Tailor tempering by ceramic tip

Plants fiber
Pure ice
Without mandrel

xPlants fiber

Ice mandrel
Ice mandrel• Generate reaction force inside tube
• Direct cooling with ice• No-control of internal pressure• Safe medium (non compressive)• Hot stamping of quenchable steel sheet

40040000
Pure ice

Quenched productHot stamping using resistance heating ofquenchable steel sheet and tube

Effect of ice mandrel on cross sectional shape andquenched hardness
500
500

Stamping, quenching20203030x [mm]40405050• Hot stamping with hot blanking process
6060

Use of fiber reinforced ice mandrel remarkably effective toimprove shape accuracy and quenching• Hot stamping and gas forming of quenchable steel tube
• Miscellaneous technique in resistance heating
9th BrDDRG Congress in 42°SENAFOR
5th October 2023

6

9th BrDDRG Congressin 42°SENAFOR

Hot stamping using resistance heating ofquenchable steel sheet and tube

Improvement of fitting of electrode with flat braided copper wire
Steel sheet

Tomoyoshi MAENO
YOKOHAMA National Unibersity

Reduction of non- heated edge with toothed electrodes

Sheet holder
Toothed electrodes system
ElectrodeHardness measurement

Flat braided copper wire1000 ℃(a) Without braided copper wire(b) With braided copper wire

100 ℃

• Width of Non heated edge is reduced, and hardened area is increased.
• Material yield will be increased.
B.A. Behrens et al., Patent application, PCT/EP2015/053382.

Temperature measurement in rapid resistance heating with
Two-color thermometer (Ratio thermometer, Multi‐wavelength thermometer)
Small oxidation:
degree of oxidation isunstableTwo-color thermometer:
temperature is determined from the ratio ofbrightness at two different wavelengths, theinfluence of emissivity is eliminated.

Emissivity changing during
rapid heating
Radiation thermometers andthermography are not suitable
for rapid heating.
Black bodyε = 0.95

Camera 30,000-40,000 US$•

High productivity with small in-progress productLess material waste in case of trouble•Flexible control of production line
Suitable for medium volume production•Combination with forming tool such as hot blanking, trimming and punching
Hot stamping can be applied to small parts
Limited production of hot stamping in the cold stamping line•

Limitation of blank shape is partially resolved•

Tube material is suitable for resistance heating due to uniform heating and coating•

Non-heated edge is unavoidable
Material yield becomes small•

Al-Si coating is not available
Removing of thin oxide layer is required

Spot pyrometer 2,500 US$

Stainless
ε = 0.3~0.5

IR thermography

Conclusion

Two-colorthermography7