Twinning-induced plasticity (TWIP) steel represents a novel grade of advanced high strength and ductility with significant potential for automotive industry. However, high alloying in TWIP steel leads to the inhomogeneous solute distribution and anisotropic local deformation. It is well known that the refinement of solidification structure is an effective solution to the above defects. Much attention has been paid to heterogeneous nucleation by Ce particles, acting as nucleating sites in liquid steel. The present work focuses on how Ce content and casting parameters affect the refinement of solidification structure in Fe-22Mn-0.65C TWIP steel, aiming to provide an effective technology in high alloy steel production. The reaction products of Ce inoculation were predicted by thermodynamics software FactSage 7.0 and their effectiveness of heterogeneous nucleation was estimated by lattice misfit model. The solidification structure refinement by Ce inoculation under different conditions was experimentally studied by OM, SEM, EBSD and EPMA. The results show that, with increasing Ce content the reaction products transferred from Ce2O3 to Ce2O3+a small amount of Ce2O2S, and both kinds of particles can act as heterogeneous nucleation cores theoretically. For as-cast solidification structure, the ratio of equiaxed grain area increased from 25% to 72%, average equiaxed grain size decreased from 480 μm to 130 μm and the segregation ratio of Mn decreased from 1.61 to 1.41. Meanwhile, the tendency of particle agglomeration was weakened by lowering inoculation temperature, resulting in the improvement structure refinement. In this work, the recommended inoculation parameters are concluded as (0.02%~0.04%)Ce with superheat of 20 ℃.
Fig.1 Schematics of mould (a), casting ingot and processing method (b)
T
℃
Superheat
℃
Sample
No.
Mass fraction / %
Ce
Ce yield
1450
50
H0
0
1450
50
H1
0.010
13
1450
50
H2
0.034
16
1450
50
H3
0.064
25
1420
20
L0
0
1420
20
L1
0.013
14
1420
20
L2
0.022
10
1420
20
L3
0.062
23
Table 1 Compositions of twinning induced plasticity (TWIP) steel samples
Fig.2 Ce inoculation products in TWIP steel with superheat of 50 ℃ (L—liquid)
Phase
Crystal system
a / nm
c / nm
α / (10-6 ℃-1)
Case
δ / %
γ-Fe
fcc
0.3620
-
25
-
-
Ce2O3
hex
0.3891
0.6063
10
(0001)//(100)
6.2
(0001)//(100)
21.0
(0001)//(100)
19.5
Ce2O2S
hex
0.4001
0.6830
10
(0001)//(100)
5.2
(0001)//(100)
24.7
(0001)//(100)
21.1
Table 2 Planar disregistry between Ce inoculation products and the solidification phase of TWIP steel[24,25,26,27]
Fig.3 Crystallographic relationships between Ce inoculation products and TWIP steel solidified phase (a) (0001)//(100) (b) (0001)//(100)
Fig.4 Effects of Ce inoculation on as-cast solidification microstructure of TWIP steel (clumnar to equiaxed transition (CET) positions are marked by white lines) (a) H0 (b) H1 (c) H2 (d) H3 (e) L0 (f) L1 (g) L2 (h) L3
Fig.5 Effects of Ce content on the distance of CET position from surface (a) and the ratio of equiaxed grain area (b) of TWIP steel
Fig.6 EBSD images of equiaxed grain area of TWIP steel (Grain boundaries are marked by black lines) Color online (a) H0 (b) H1 (c) H2 (d) H3 (e) L0 (f) L1 (g) L2 (h) L3
Fig.7 Effects of Ce content on the average equiaxed grain size of TWIP steel
Fig.8 SEM images (a, c) and EDS analyses (b, d) of typical particles Ce2O3 (a, b) and Ce2O2S (c, d) in inoculated TWIP steel
Fig.9 3D size distributions of particles in inoculated TWIP steel in log-normal probability graph
Superheat
℃
Fitting result
A
B
R2
50
1.35×106
7.76
0.96
20
2.10×106
34.34
0.99
Table 3 Fitting results between the number density of particles and Ce content
Fig.10 Effects of Ce inoculation on Mn micro-segregation of TWIP steel Color online (a) H0 (b) H1 (c) H2 (d) H3 (e) L0 (f) L1 (g) L2 (h) L3
Fig.11 Effects of Ce content on Mn segregation ratio of TWIP steel
Fig.12 Standard Gibbs free energy (a) and density (b) of common particles in steel along with planar disregistry between particles and austenite phase[31,32,33,34,35,36,37]
Fig.13 Effects of Ce inoculation on TWIP steel solidification texture (X and Y correspond to reference directions of sample table) Color online (a) H0 (b) L0 (c) H3 (d) L3
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