ultra fine grained steels

Microstructure Physics and Metal Forming R. Song

ULTRA FINE GRAIN IN PLAIN C-Mn

STEELS WITH 0.15-0.3% C

R. Song, D. Ponge, D. Raabe


?

Why do we study ultra fine grained steel?


The reason is

The development of industry needs a steel with advanced mechanical properties

Grain refinement is the only method to improve both

strength and toughness

That is because ......


Hall-Petch relationship

Ferrite Grain Size, d (µm)

40 10 5 3 1900

800

700

600

500

400

300

2002 6 10 14 18 22 26 30 34

d-1/2 (mm-1/2)

0

- 40

- 80

-120

-160

-200

-240

Normalizing TMCP Ultra fine grain

Yie

ld S

tren

gh (

MP

a) 50% FA

TT, (

0C)


How to get ultra fine grained steel?


High demands on the novel UF routes

Ar3

Ar1

Ac3

Ac1

heavy deformation

undercooling

I. II. III.

Niikura a t al.

I. Deform ation of undercooled II. Deform ation in the sta te o f m ultip le phasesIII. Inverse transform ation after heavy deform ation

- heavy deform ation per pass- rap id therm al cycling- narrow process w indows

Niikura et al.


Aims

To obtain UF grain in plain C-Mn steels

To determine the relationship between micro-structure and mechanical properties of UF grained steel

To consider the industrial applicability


Considerations

Lower cost elements

Easy recyclingPlain C-Mn steels

Industrially applicable process parameters Applicability in industry

Method of getting UF grain

Fine cementite dispersion in a ferritic matrix

Recrystallized ferrite microstructure


The effect of microstructure on strength

104 106 108 1010

Number of Grains in 1 mm3

Conventional Grain Size

ferrite + pearlite

Ultrafine Grain Size

Ferrite Grain Size, µm

20 10 5 2 1 0.5

800

700

600

500

400

300

Yei

ld S

tren

gh,

MP

a

+>300MPa

0.15C-0.3Si-1.5Mn Steel

ferrite + cementite

K. Nagai


The PonyMILL processing route

Conventional Hot Mill Line

Coiler

Coil Transfer

Un-CoilerPonyMILL Single High

Reduction Stand

Re-Coiler

Coil Handling

Run out table


Contents

Experiment and materials

Results and discussionOptimum hot rolling conditions

The effect of heavy deformation / coiling temperature on microstructure

The effect of heavy deformation strain on microstructure

Micro-hardness measurement

Summary


Materials

Chemical compositions (wt%),with calculated Tnr and Ae3

Tnr* : nonrecrystallization temperature. Mn has not been considered in the calculation Ae3 : calculated by Thermo-Calc

C Si Mn P S Al N Tnr* ℃ Ae3 ℃

0.15C .17 .22 0.76 .004 .004 .031 .001 899 834

0.2C .22 .21 0.74 .004 .003 .029 .001 925 820

0.2CM .23 .22 1.52 .004 .004 .030 .001 926 797

0.3C .31 .22 0.76 .003 .003 .030 .001 963 798


Experiment machine

The Hot Working Simulator

(WarmUMformSImulator)

“WUMSI”

W UM SI


Experiments from WUMSI

Cuboid SampleCuboid Sample Microstructure Investigation

Microstructure Investigation


Experimental routes

=0.3, =10s-1

holding compression 2min =4×0.4, =10s-1 air cooling simulated final coiling

5~12℃/sPF

A3

BS

50℃/s

Pearlite route Bainite routeⅠ Bainite route Ⅱ

hot deformation

heavy warm deformation

(conventional hot strip mill)

(PonyMILL)


Optimum austenite deformation temperature

Optimization of deformation temperature in austenite region (WUMSI)

Optimization of deformation temperature in austenite region (WUMSI) Water quenched microstructure after

deformation at 860 of 0.15%C steel℃Water quenched microstructure after deformation at 860 of 0.15%C steel℃

Tg=Ae3+100℃ for 3 min

air

Tde compression

=0.3, =10s-1

water


Selection of cooling rate to get desired initial microstructure (F+P or B)

Experiment schedule (deformation dilatometry)

Experiment schedule (deformation dilatometry)

Tg =Ae3+100℃ for 3 min

air compression

Ar3

cooling

64...2℃/s

Cooling rate, 0C/s

0 20 40 60 80 100 120

HV

10

100

200

300

400

500

MP

a

200

400

600

800

1000

1200

1400

1600

15C2C2CM3C

M+B+F

F+P+B

F+P

F+P +B +M

Changes in microstructure and hardness of experimental steels with different cooling rates Changes in microstructure and hardness of

experimental steels with different cooling rates

UT

S ,


DCCT diagram of the steels

DCCT diagram (ferrite + pearlite region) of 0.15%C, 0.2%C and 0.3%C steel

DCCT diagram (ferrite + pearlite region) of 0.15%C, 0.2%C and 0.3%C steel

1 10 100

500

600

700

800 15C2C

3C

F+P

Tem

pera

ture

, 0 C

Time, s

PR

0.1 1 10 100 10000

200

400

600

800

1000

2CM

64 32 16 8 4 20C/s

B

F+P

Tem

pera

ture

, 0 C

Time, s

BR I

BR II

DCCT diagram of 2CM steel DCCT diagram of 2CM steel


Effect of heavy deformation temperature on flow curves and temperature increase in 0.3%C steel

0.0 0.5 1.0 1.5 2.0 2.50

100

200

300

400

500

600

700

Str

ess

, M

Pa

Strain

500 de℃

600 de℃

700 de℃

730 de℃

Starting temperature of heavy deformation

42℃

100℃114℃

170℃

0

30

60

90

120

150

180

500℃de 600℃de 700℃de 730℃de

Tem

p. in

crea

se d

urin

g he

avy

defo

rmat

ion,

℃


The effect of heavy deformation temperature on microstructure

5000C-coiling 5500C-coiling 6000C-coiling 7000C-coiling

5500C 6000C 6400C 7000C

bainite route I

bainite route II

ND


500 550 600 650 7001.0

1.5

2.0

2.5

3.0

3.5

bainite route I

Av.

gra

in s

ize,

m

Deformation temperature, 0C

26

28

30

32

34

Av.

mis

ori

enta

tio

n

ang

le,

0

1.92

1.98

2.04

2.10

2.16500 550 600 650 700

Asp

ect

rati

o

(a) grain size: 3.50µm

(b) grain size: 1.25µm

The effect of heavy deformation temperature on the microstructure in 0.3%C steel

85-95% are high angle

boundaries

7000C7000C

5000C5000C


Typical microstructure

0.3%C deformed at 6000C in BR II0.3%C deformed at 6000C in BR II

small grains equiaxed grains homogeneous cementite

distribution

small grains equiaxed grains homogeneous cementite

distribution

1m1m


The effect of strain on the microstructure

0

0.5

1

1.5

2

2.5

3

1 2 3

Ave

rage

fer

rite

grai

n si

ze,

µm

C-C

Q-C

Q-Q

C-Q

PR-5000C BR I-5000C

Effect of different local strain on grain size and aspect ratioEffect of different local strain on grain size and aspect ratio

Centre-Centre (C-C)

Quarter-Centre (Q-C)

Quarter-Quarter (Q-Q)

Centre-Quarter (C-Q)

Centre-Centre (C-C)

Quarter-Centre (Q-C)

Quarter-Quarter (Q-Q)

Centre-Quarter (C-Q)

aspect ratio

C-C 2.494

Q-C 2.188C-C 2.415

Q-C 1.946

C-Q

Y

ZX

C-C

Q-Q

Q-C

strainstrain

stra

inst

rain


Microstructure evolution during compression in PR

new ferrite grains

pro-eutectoid ferrite with subgrains

compression

compression

short pearlitic fragments

pearlitic ferrite

pro-eutectoid ferrite

pearlitic cementite lamella

pearlitic ferrite

1m1m 1m1m 2m2m


SEM micrographs of 0.3%C steel after bainite routeⅠ

Substructure in large grains

Heavy deformation at 500 and subsequent simulated coiling at 700℃ ℃

large grainlarge grain

subgrains


Low angle misorientation


* deformation temperature (PR and BR II) or simulated coiling temperature (BR I)

480 520 560 600 640 680 720140

160

180

200

220

240

260

pearlite route

bainite route I

3.5m2.86m

1.25m

1.28m

2.43m

2.23m

1.65m

2.04m

bainite route II

Har

dnes

s, H

V0.

1

Temperature*, 0C

Micro-hardness for different routes


Summary I

Optimum hot deformation temperatures have been determined to get fine and homogeneous austenite

Three new process routes for heavy warm deformation have been designed and employed to obtain UFG steel

Lower heavy deformation / coiling temperature: finer ferrite grains but higher aspect ratio


Summary II

The alignment of cementite particles affects ferrite grain shape (more elongated)

Pearlitic / bainitic ferrite grains: smaller, relatively equiaxed

Pro-eutectoid ferrite grains: larger, higher aspect ratio, composed of subgrains

UFG is effective to increase hardness


references

• R. Song, D. Ponge, R. Kaspar, D. Raabe: Z. Metallk. 95 (2004) 513517, Grain boundary characterization and grain size measurement in an ultrafine-grained steel

• L. Storojeva, D. Ponge, D. Raabe, R. Kaspar: Z. Metallkunde 95 (2004) 1108-1114, On the influence of heavy warm reduction on the microstructure and mechanical properties of a medium carbon ferritic-pearlitic steel

• R. Song, D. Ponge, D. Raabe, R. Kaspar: Acta Mater. 53 (2004) 845858, Microstructure and crystallographic texture of an ultrafine grained C-Mn steel and their evolution during warm deformation and annealing

• R. Song, D. Ponge, D. Raabe: Scripta Materialia 52 (2005) 1075-1080, Improvement of the work hardening rate of ultrafine grained steels through second phase particles

• R. Song, D. Ponge, D. Raabe: ISIJ International 45 (2005) 1721-1726, Influence of Mn Content on the Microstructure and Mechanical Properties of Ultrafine Grained C-Mn Steels

• R. Song, D. Ponge, D. Raabe: Acta Mater. 53 (2005) 4881-4892, Mechanical properties of an ultrafine grained C Mn steel processed by warm deformation and annealing

• R. Song, D. Ponge, D. Raabe, J.G. Speer, D.K. Matlock: Mater. Sc. Engin. A 441 , 2006) 1–17, Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels

ultra fine grained steels

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