ultra fine grained steels
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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
Microstructure Physics and Metal Forming R. Song
?
Why do we study ultra fine grained steel?
Microstructure Physics and Metal Forming R. Song
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 ......
Microstructure Physics and Metal Forming R. Song
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)
Microstructure Physics and Metal Forming R. Song
How to get ultra fine grained steel?
Microstructure Physics and Metal Forming R. Song
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.
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
The PonyMILL processing route
Conventional Hot Mill Line
Coiler
Coil Transfer
Un-CoilerPonyMILL Single High
Reduction Stand
Re-Coiler
Coil Handling
Run out table
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
Experiment machine
The Hot Working Simulator
(WarmUMformSImulator)
“WUMSI”
W UM SI
Microstructure Physics and Metal Forming R. Song
Experiments from WUMSI
Cuboid SampleCuboid Sample Microstructure Investigation
Microstructure Investigation
Microstructure Physics and Metal Forming R. Song
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)
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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 ,
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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,
℃
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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 Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
Low angle misorientation
Microstructure Physics and Metal Forming R. Song
* 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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
Microstructure Physics and Metal Forming R. Song
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
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