Quantitative effects of phase transition on solute partition coefficient, inclusion precipitation, and microsegregation for high-sulfur steel solidification

doi:10.1016/j.jmst.2019.05.058

Abstract

Abstract:

Segregation and inclusion precipitation are the common behaviours of steel solidification, which are resulted from the redistribution and diffusion of the solute elements at the solid-liquid interface. The effect of the phase transition of high-sulfur free-cutting steel is quantified in the present work for the solute partition coefficient (k_i), inclusion precipitation, and microsegregation by establishing a coupling model of microsegregation and inclusion precipitation, wherein the quantified dependencies of k_i in terms of temperature, phase and carbon (C) content were applied. Results showed that the solidification temperature range and phase transition of high-sulfur steel that under different solidification paths and C contents were quite different, leading to differences in k_i and eventually in microsegregation. k_C, k_P, and k_S were mainly affected by phase composition and k_Si was primarily by temperature, while k_Mn depended on both phase composition and temperature during solidification. Increasing the C content within the interval 0.07-0.48 wt%, the ‘proportion of the δ phase maintained temperature region during solidification’ (P_δ), kPave and kSave (kiave, the average value of the k_i across the whole stages of solidification) decreased monotonically, whereas kCave increased linearly. The peritectic reaction impacted on the phase composition and k_i, leading to the change in microsegregation. Such effect of the peritectic reaction was more significant at the last stage of solidification. When the P_δ was between 75% and 100% (corresponding to 0.07-0.16 wt% C), the solidification path resulted in a greater effect on the microsegregation of solutes C, P, and S because of the peritectic reaction. The microsegregation of solutes Mn and S were comprehensively influenced by k_Mn, k_S and MnS precipitation as well. The studies would help reveal the solute redistribution at the solid-liquid interface, and improve the segregation of high-sulfur steel by controlling the solidification and precipitation in practice.

Key words: Phase transition, Microsegregation, Solute partition coefficient, Inclusion precipitation, High-sulfur steel

Lintao Gui, Mujun Long, Shixin Wu, Hua Zheng, Zhihua Dong, Jianguo Li, Dengfu Chen, Yunwei Huang, Yunwei Huang, Huamei Duan, Levente Vitos. Quantitative effects of phase transition on solute partition coefficient, inclusion precipitation, and microsegregation for high-sulfur steel solidification[J]. J. Mater. Sci. Technol., 2019, 35(10): 2383-2395.

Figures/Tables 16

Table 1 Solute diffusion coefficients in the δ and γ phases of steel [15,21,28].

Elements	$D_{i}^{\delta}$ (cm²/s)	$D_{i}^{\gamma}$ (cm²/s)
C	0.0127exp(-19450/(RT))	0.0761exp(-32160/(RT))
Si	8.0exp(-59500/(RT))	0.3exp(-60100/(RT))
Mn	0.76exp(-53640/(RT))	0.055exp (-59600/(RT))
P	2.9exp(-55000/(RT))	0.01exp (-43700/(RT))
S	4.56exp(-51300/(RT))	2.4exp(-53400/(RT))

Fig. 1. Flow chart of the current coupling model of microsegregation, inclusion precipitation, and changed solute partition coefficient and diffusion coefficient.

Fig. 2. Verification of the solute partition coefficient thermodynamic model.

Table 2 Chemical compositions of the steels (wt%).

Steels	C	Si	Mn	P	S	Refs.
S1	1.00	0.27	0.36	0.019	0.018	[28]
S2	0.13	0.35	1.52	0.016	0.002	[15]
S3	0.21	0.04	1.50	0.0036	0.0021	[41]

Fig. 3. Verification of the current coupling model.

Fig. 4. The phase diagram of high-sulfur steel solidification.

Table 3 Phase transition temperature, coexisting phase, and temperature range of each coexisting phase for high-sulfur steel under different solidification paths.

Path	C (wt%)	L+δ range (K)		ΔT_L+δ	L+δ + γ range (K)		ΔT_L+δ+γ	L+γ range (K)		ΔT_L+γ	T_L-T_S
Path	C (wt%)	$T_{start}^{\delta+L}$	$T_{end}^{\delta+L}$	ΔT_L+δ	$T_{start}^{\delta+\gamma+L}$	$T_{end}^{\delta+\gamma+L}$	ΔT_L+δ+γ	$T_{start}^{\gamma+L}$	$T_{end}^{\gamma+L}$	ΔT_L+γ	T_L-T_S
I	0.02	1793	1760	33	-	-	-	-	-	-	33
	0.04	1791	1751	40	-	-	-	-	-	-	40
	0.07	1789	1739	50	-	-	-	-	-	-	50
II	0.11	1785	1742	43	1742	1735	7	-	-	-	50
II	0.135	1783	1743	40	1743	1732	11	-	-	-	51
III	0.16	1781	1743	38	1743	1737	6	1737	1728	9	53
	0.25	1774	1747	27	1747	1744	3	1744	1712	32	62
	0.35	1765	1751	14	1751	1750	1	1750	1694	56	71
IV	0.5	-	-	-	-	-	-	1752	1668	84	84

Fig. 5. The evolution of the mass fractions of liquid, δ and γ phases for high-sulfur steel under different solidification paths. (a) liquid phase, (b) δ phase, and (c) γ phase.

Fig. 6. The proportion of phase maintained temperature region for δ and γ phases (Pδ and Pγ) in high-sulfur steel under different solidification paths. (a) Sketch map of Pδ and Pγ, and (b) Pδ and Pγ with C content.

Fig. 7. The variation of solute partition coefficients for high-sulfur steel under different solidification paths: (a) kC, (b) kSi, (c) kP and kS, and (d) kMn.

Fig. 8. The average partition coefficient of solutes (kiave) for high-sulfur steel that under different solidification paths. (a) C, (b) Si, (c) P and S, and (d) Mn.

Fig. 9. The precipitation of MnS inclusion in high-sulfur steel under different solidification paths.

Fig. 10. The precipitation starting time and final precipitation amount of MnS inclusion in high-sulfur steel under different solidification paths.

Fig. 11. The predicted solute concentration profiles for high-sulfur steel solidification by current coupling model (0.16 wt% C, path III). (a) C, (b) S, (c) Si, and (d) Mn.

Fig. 12. The variation of solute concentrations under different solidification paths of high-sulfur steel. (a) P, and (b) Mn.

Fig. 13. The solute microsegregation ratio under different solidification paths of high-sulfur steel. (a) C, (b) S, (c) P, and (d) Mn.

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