Effects of electric current on IF steel

(1)

Effects of electric current on IF steel

구조재료 심화연구

Ju-Won Park 1 , Moon-Jo Kim 1 , Hye-Jin Jeong 1 , Hyun-Min Sung 1 , Sungwoo Jin 1 , Sung-Tae Hong 2 , Kyooyoung Lee 3 ,

Heung Nam Han 1

1 Seoul National University, 2 University of Ulsan, 3 POSCO,

(2)

Fuel economy I. Motivation

• Global comparison of passenger vehicle GHG emission standards

*GHG : greenhouse gas

http://www.transportpolicy.net/index.php?title=Global_Comparison:_Light-duty_Fuel_Economy_and_GHG

Fuel efficiency

Critical importance of need for energy reduction

2

Energy efficiency

&

Environmental protection policy

Automobile fuel economy regulations

(USA) behind 23.1km/l,

Imposed 137.5 penalty per 1km/l

(3)

3

Limit properties of high strength & lightweight metals

Body parts mostly consists of

high strength steel & lightweight metals

 Body parts components

Low cold formability High spring back

Drawback !!

High strength steel

Aluminum alloy

25

^o

C 100

^o

C

150

^o

C 200

^o

C

300

^o

C

www.formingandimpact.uwaterloo.ca/Research/metal

Ken-ichi Manabe, Osamu Shimomura, Tokyo Metropolitan Univ.

https://www.esi-group.com/sites/default/files/software-services/1557/springback http://openi.nlm.nih.gov/legacy/detailedresult.php?img=3132537

High strength & Lightweight V I. Motivation ehicle

(4)

Development of forming method I. Motivation 4

 Hot manufacturing process

Hot stamping Hot bending

Disadvantage  High cost, Thermal gradient, Die adhesion, Surface oxidation

 Incremental forming

Disadvantage  Local areas deformation, High cost

http://thumbs.dreamstime.com/z/automatic-hot-stamping-process

Theories, Methods and Numerical Technology of Sheet Metal Cold and Hot Forming Springer Series in Advanced Manufacturing (2013) 35-45.

single point incremental forming at university of Aveiro –SPIF-A project

http://www.fastenerdata.co.uk/ie6-redirect/

 Laser beam forming

(5)

Electrically-assisted forming (EAF) I. Motivation 5

Electrically-assisted forming

 Technics for improving formability by applying high current density during deformation

0.00 0.06 0.12 0.18 0.24 0.30 0.36 -50

0 50 100 150 200 250 300

No current Pulsed current

S tr e s s (M P a )

Strain Seoul Nat. Univ.

Elongation increase

No current Current

Reduction of Spring back Rapid heating

Characteristic change through electric current

(US 7516640 B2) (US 20120055217 A1) Double side incremental forming Die Punching

Tsing Hua Univ.

Elongation increase

Applicability of Real process (patent)

(6)

6

Accelerated spheroidization induced by high intensity electric pulse

Microstructure control by electric current

Electro pulse-induced cementite nanoparticle formation

in deformed pearlitic steels

Electric pulsing

Peak pulse width

=150μs

[9.61x10

³

A/mm

²

]

Ref) R. S. Qin, E. I. Samuel, A. Bhowmik, J. Mater. Sci. (2011) Fe

3

C nanoparticle

(~30nm)

Electro-treatment I. Motivation

(7)

Underlying mechanism I. Motivation 7

• Joule heating effect • Electromigration (Electron wind effect)

Electron

Anode (+)

Cathode (-)

Electric field e

^-

e

^-

M+

F total = F electron wind + F direct action of electric field

Metal

- Troitskii et al(1969), Conrad/Xu et al(1988) - Goldman et al (1981), Kilmov et al (1984),

Cao et al (2013)

P.D. Goldman et al, Scripta Matall. 15 353-356 (1981), D.G.Pierce, P.G.Brusius, Microelectronics Reliability (1997) S. Mahabunphachai et al, J. Eng. Mater. Technol. 135 041003 (2013) ,T. Kozlova et al, Nanotechnology, 24 505708 (2013)

Electric current density

= 10 5~6 A/mm 2 Pb

No current Current+air cooled

Current

 Joule heating vs Electron wind vs other..?!

Joule heating

(8)

8

 Composition (wt.%)

 Specimen procedure

FH(냉연재), 초기조직 소둔재, 초기조직

TD

RD ND

Specimen Information II. Experimental procedure

C Mn P Al Nb B

0.001 0.2 0.045 0.04 0.02 0.0005

The effects of electric current on recrystallization of IF steel

Electric current

(9)

9

Pulsed DC Power supply

Pressure

Vacuum chamber Graphite die Graphite

punch

Specimen Electric

current Pulsed DC

Electric current

Pressure (~30MPa)

cu rr en t

time

DC pulse 20000 Hz

max : 4000A

 Sample dimension 10mm ~1 mm

Temp. ( o C)

Time (s) Heating rate

100

^o

C/min

Treatment time Target Temp.

Electro-Treatment

Air cooling

Instrumental Set-up II. Experimental procedure

(10)

10

 Current condition

0 100 200 300 400 500

0 6 12 18 24

610

^o

C-1min 690

^o

C-1min

C ur rent dens ity ( A /m m

2

)

Time (sec)

About 12~21A/mm

²

DC current was induced during test

Experimental condition II. Experimental procedure

 Case study

Temp. ( o C)

Time (s) 1min

Target Temp.

About temperature

Temp. ( o C)

Time (s) Treatment time

600

^o

C

About time

(11)

11

Temp. for full recrystallization E.T.  660 o C

H.T.  760 o C

 Vickers hardness analysis

Recrystallization occurs more rapidly in Electro-Treatment

580 620 660 700 740 780 820

80 120 160 200 240

Electro-Treatment Heat-Treatment

V ic ker s H ar dnes s ( H v 3 )

Annealing Temp. ( o C)

Treatment Time : 1min IF-FH

IF-O

※ At fixed time : 1min ※ At fixed temp. : 600 o C

Accelerated recrystallization

10 100 1000 10000 100000 1000000 80

120 160 200

240 Electro-Treatment

Heat-Treatment

V ic ker s H ar dnes s ( H v 3 )

Time (sec)

Annealing Temp. : 600

^o

C IF-FH

IF-O

Microstructure analysis

Accelerated recrystallization

Time for full recrystallization E.T.  ~10 min

H.T.  Not fully recrystallized after 1000min

Improvement of III. Results & discussion - 1 Recrystallization kinetics

(12)

12

Electro-Treatment 630℃-1min

Heat-Treatment 710℃-1min

Fully recrystallized Partially recrystallized

 OM & EBSD analysis

Electro-Treatment 660 ℃ -1min

Heat-Treatment 760 ℃ -1min

RD

ND TD

(13)

13

Activation energy under Electro-Treatment Is smaller than that under Heat-Treatment

Activation energy : ~420 KJ/mol Johnson Go, Univ. of British Columbia (1998)

Activation energy

1.00 1.03 1.06 1.09 1.12 1.15 1.18 1E-7

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10

Electro-Treatment Heat-Treatment

50% Recrystallized (Vickers Hardness)

k ( s -1 )

T -1 (10 -3 K -1 ) (k = t 50 -1 )

 Derived activation energy by Arrhenius equation

Electrical energy ↔ Activation energy for recrystallization

 Analysis of the effect of electric current on recrystallization Assumption

< 𝑸 E.T. = 366 ± 18.7 KJ/mol

(14)

14

 Electric current effect on decrease of activation energy

∆𝐺 = ∆𝐺 0 + ∆𝐺 𝑒

∆𝐺 𝑒 = 𝜇 0 𝑔𝜉(𝜎 1 , 𝜎 2 )𝑗 2 Δ𝑉

𝜇 0 : magnetic susceptibility g : positive geometric factor J : electric current density Δ𝑉 : volume of a nucleus

𝝃 𝝈 𝟏 , 𝝈 𝟐 = (𝝈 𝟐 − 𝝈 𝟏 )/(𝝈 𝟏 + 𝟐𝝈 𝟐 ) [ 𝜎 1 , 𝜎 2 : conductivity of nucleus, matrix]

 𝜎 1 < 𝜎 2

 𝜉 𝜎 1 , 𝜎 2 < 0

 ∆𝐺 𝑒 < 0

Recrystallization is accelerated by electric current

 Spontaneous reaction process

matrix

nucleus

Low defect density High conductivity

high defect density low conductivity

[Ref] Yizhou Zhou et al., J. Mater. Res., Vol. 17, 2002 Yanbin Jiang et al., J. Mater. Res., Vol. 24, 2009 Guoliang Hu et al., Mat. Transac., Vol. 51, 2010

thermal athermal

Improvement of III. Results & discussion - 1 Recrystallization kinetics

(15)

15

 Carburization by the graphite mold

Graphite mold Conductive sheet Specimen

IF-O

800 840 880 920 960 1000

60 80 100 120 140

160 without conductive sheet

with conductive sheet

V ic ker s har dnes s ( H v

3

)

Time (sec)

Case 2 : with conductive sheet Case 1 : without conductive sheet

When not using conductive sheet, Hardness increase caused by C diffusion

Analysis of the electric current effect on carburization

Effect of electric current on Carburization III. Results & discussion - 2

(16)

16

 Experimental condition

Electro- Treatment

Heat- Treatment

Analysis of the electric current effect on the carburization

Temp. (℃)

Time (s) Heating rate

100℃ /min

Treatment time Target

temp.

Air cooling

Effect of electric current on Carburization III. Results & discussion - 2

(17)

17

 Hardness profile & EPMA analysis

Electro-Treatment : ~ 0.3mm Heat-Treatment. : ~ 0.08mm

※ Carburized depth

Carburization is accelerated under Electro-Treatment

Treatment condition : 800 ℃ -30min under 30MPa

0.0 0.2 0.4 0.6 0.8 1.0

0 100 200 300 400

Heat-Treatment

V ic ker s har dnes s ( H v

0.25

)

depth (mm)

~ 0.08mm

~ 0.3mm

0.0 0.1 0.2 0.3 0.4 0.5

0 40 80 120 160

Heat-Treatment

Int ens ity of c ar bon (c ount s)

depth (mm)

[Hardness profile] [EPMA analysis]

~ 0.08mm

~ 0.3mm

Effect of electric current on Carburization III. Results & discussion - 2

(18)

Summary 18

580 620 660 700 740 780 820

80 120 160 200 240

Electro-Treatment Heat-Treatment

V ic ker s H ar dnes s ( H v

3

)

Annealing Temp. (

^o

C)

Treatment Time : 1min

IF-FH

IF-O

Accelerated recrystallization

Heat-Treatment

710

^o

C-1min Electro-Treatment

660

^o

C-1min

^1.00 ^1.03 ^1.06 ^1.09 ^1.12 ^1.15 ^1.18

1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10

Electro-Treatment Heat-Treatment 50% Recrystallized (Vickers Hardness)

k ( s

-1

)

T

^-1

(10

^-3

K

^-1

) (k = t

₅₀^-1

)

𝑸

E.T.

= 366 KJ/mol

𝑸

H.T.

= 443 KJ/mol

비통전 대비 통전 시 전류인가에 따른 재결정 활성화 에너지 저감

0.0 0.2 0.4 0.6 0.8 1.0

0 100 200 300 400

500 Electro-Treatment

Heat-Treatment

V ic ker s har dnes s ( H v

0.25

)

depth (mm)

~ 0.07mm

~ 0.3mm

Hardness profile

0.0 0.1 0.2 0.3 0.4 0.5

0 40 80 120 160

200 Electro-Treatment

Heat-Treatment

Int ens ity of c ar bon (c ount s)

depth (mm)

EPMA analysis

통전처리 시 열처리에 비해 고상 침탄 활성화

(19)

19

Vacuum system furnace Dilatometer

Optical microscope Nano-indenter

A vailable Equipment in MMMPDL

Tensile tester Cutting & Polisher system

Hardness tester

(20)

A vailable Equipment in SNU 20

XRD

SEM & EBSD

(21)