Solidification Structure Refinement in TWIP Steel by Ce Inoculation

doi:10.11900/0412.1961.2019.00288

Acta Metall Sin

2020, Vol. 56

Issue (5): 704-714 DOI: 10.11900/0412.1961.2019.00288

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Solidification Structure Refinement in TWIP Steel by Ce Inoculation

LI Gen, LAN Peng(

), ZHANG Jiaquan

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China

Cite this article:

LI Gen, LAN Peng, ZHANG Jiaquan. Solidification Structure Refinement in TWIP Steel by Ce Inoculation. Acta Metall Sin, 2020, 56(5): 704-714.

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Abstract

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 ℃.

Key words: Ce TWIP steel inoculation solidification structure refinement micro-segregation

Received: 02 September 2019

ZTFLH:

TF771.1

Fund: National Natural Science Foundation of China(51604021);Foundamental Research Funds for the Central Universities(FRF-TP-19-017A3)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00288 OR https://www.ams.org.cn/EN/Y2020/V56/I5/704

Fig.1 Schematics of mould (a), casting ingot and processing method (b)

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) $C e 2 O 3$ //(100) $γ - F e$	6.2
					(0001) $C e 2 O 3$ //(100) $γ - F e$	21.0
					(0001) $C e 2 O 3$ //(100) $γ - F e$	21.0
					(0001) $C e 2 O 3$ //(100) $γ - F e$	19.5
Ce₂O₂S	hex	0.4001	0.6830	10	(0001) $C e 2 O 2 S$ //(100) $γ - F e$	5.2
					(0001) $C e 2 O 2 S$ //(100) $γ - F e$	24.7
					(0001) $C e 2 O 2 S$ //(100) $γ - F e$	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)

C e 2 O 3

//(100)

γ - F e

(b) (0001)

C e 2 O 2 S

//(100)

γ - F e

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

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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