Apr 24, 2008 - Q&P : Isothermal Transformation. Steel Composition (wt%). C. Mn. Ni. Etc. Averbach and Cohen (1949). 1.07. 1.00. 0.25. 0.37. -. 0.23Si.
Kinetics of Phase Transformations during Q&P Processing
S. J. Kim*, J. G. Speer**, Han S. Kim*, B. C. De Cooman* April 24th - 25th, 2008 * Materials Design Laboratory Graduate Institute of Ferrous Technology Pohang University of Science and Technology ** Advanced Steel Processing and Products Research Center Colorado School of Mines, Golden, Colorado, USA
• Q&P is a recently developed heat treatment concept for UHSS. Full or partial austenization Ae1/Ae3
Quenching MS
Two Step, TP>TQ TQ
Mf
Partitioning at TP
• Quenching properly annealed steels to an optimal temperature between MS and Mf to control the austenite fraction • Austenite stabilization by C partitioning
One Step, TP=TQ
time
• High strength over 1GPa with good formability, 0.1-0.2 total elongation
Q&P : Introduction • Athermal martensitic transformations under paraequilibrium conditions. • With TRIP composition, higher ultimate tensile
stress is obtained due to the martensitic matrix and the retained austenite results in increased toughness and ductility for Q&P processed steel. • Suppressed carbide formation by Si or Al addition.
Q&P : Introduction
wt %
C
Mn
Si
0.10
1.15
1.48
• SEM and TEM • Thin films of retained austenite between martensite laths.
Q&P : Isothermal Transformation • Isothermal Martensitic Transformation in 0.15C-1.5Mn-1.42Si steel during 2-step Q&P process 1000
0.005
o
950 C, 2 min
900 Isothermal Transformation
800 o
5 C/s
o
Temperature ( C)
0.004
700
ΔL/L0
600 o
-150 C/s
500
o
o
TQ=320 C
0.003 o
o
TQ=305 C
MS=393 C
400 300
MS=393 C
o
-150 C/s
o
o
Mf=260 C
200
TQ=313 C
0.002 o
Mf=260 C
100 0 0
50
100
150
200
250
300
Time (s)
Process Detail
350
400
450
0.001 100
200
300
400
Temperature ( C) o
Isothermal Expansion
500
600
Q&P : Isothermal Transformation • Isothermal transformation below MS was observed during the 2-step Q&P process in a 0.15C-1.5Mn-1.42Si steel. • Q&P process assumed an athermal martensitic transformation. The nature of this isothermal transformation needs to be clarified. • To examine the transformation kinetics, the dilatometry data were fitted to the JMAK equation and isothermal TTT curves were drawn.
Q&P : Isothermal Transformation • Isothermal martensitic transformation kinetics were examined in many alloy systems : Generally, high carbon, hyper-eutectoid or high Ni alloyed steels. • A relatively slow growth of the isothermal martensitic transformation in isolation at relatively low temperatures was observed in these compositions. • Recently, kinetics examination in hypo-eutectoid steels were reported.
Q&P : Isothermal Transformation • Isothermal Transformation • Isothermal transformation TTT curves were obtained from the dilatometry data.
0.006
TQ=350°C
0.005
ΔL/L0
0.004 0.003
MS~390°C
Isothermal Transformation
0.002
• For TQ below MS, the athermal martensitic transformation range was excluded from the data.
0.001 22% Athermal Martensite
0.000 -0.001 200
250
300
350
400 o
Temperature ( C)
Process Detail
450
500
• Isothermal transformation data was re-normalized.
Q&P : TTT curve • Faster phase transformation in higher Si steel • The transformation rate increased drastically in the early stage of the isothermal transformation. In the late stage of the isothermal transformation, the rate increased gradually. • A ‘Swing Back’ phenomenon was observed between MS and MS+30°C. • Swing back (Oka and Okatomo, 1988) : An accelerated transformation as temperature decreases – Mutual stimulation between martensite and lower bainite
Q&P : JMAK equation • JMAK equation, f (t, T ) = 1 − exp{−k (T )t n } o
