Generation Mechanism of TiN Inclusion for GCr15SiMn during Electroslag Remelting Process

Abstract

TiN inclusion with large size found in ESR ingot is of great harm to steel GCr15SiMn, it is significant to elucidate the possibility of this inclusion in consumable electrode retaining in the subsequent ingot and the effect of slag composition on the content of TiN inclusion in ingot after ESR refining. Based on three remeltings and with the help of ultrahigh-temperature confocal scanning violet laser microscope, it demonstrates that TiN inclusion in electrode will completely decompose in the solid-liquid coexistence region at the electrode tip, reflecting that TiN inclusion found in ESR ingot is regenerated. For different contents of Ti in slag, there is a corresponding equilibrium value of Ti content in steel, when the content of Ti in electrode is higher than this critical equilibrium value, it will decline in steel, otherwise Ti pick-up will occur. The effect of increasing SiO₂ content in slag on decreasing Ti content in steel is obvious due to the Si/Ti exchange reaction taking place, further leading to the low content of TiN in ingot.

1. Introduction

GCr15SiMn is one of the most important special steel, with higher demand for non-metallic inclusion. Electroslag remelting (ESR) has been widely used as an important technology to produce this steel in order to improve its cleanliness. However, in some special steel plants, TiN inclusion with large size is usually found in this steel’s consumable electrode and ESR ingot, which will limit its application, such as high speed train and large-scale equipment.

Many studies have focused on research of oxide behavior during ESR process, which have provided some understanding on minimizing oxide,^1,2,3) the evolution of oxide^4,5) and controlling the oxygen content.^6,7,8) As known, the effect of non-deformable TiN inclusion on bearing steel is worse than that of oxide in the same size.⁹⁾ However, now only a few studies have been reported TiN behavior during ESR process. Hua et al.¹⁰⁾ investigated the inclusion of Ni-22Cr-12Co before and after ESR refining, proposed that ESR process has almost no effect on removal of TiN inclusion, but this paper did not take the influence of ESR slag into consideration. S. F. Medina et al.¹¹⁾ studied the thermodynamic aspects in the manufacturing of microalloyed steels by ESR process, the result shows that the uniformity of chemical composition in the steels with Ti requires the presence of SiO₂ in slag to inhibit an Si/Ti exchange reaction.

In this paper, in order to clarify the generation mechanism of TiN in ingot for GCr15SiMn, it intends to make clear two aspects, first whether TiN inclusion in electrode will retain in the subsequent ingot, second the relationship between the slag composition and TiN content in ingot after ESR refining. The discussed results will provide some guidance for chemical composition design of electrode and ESR slag, further promote the cleanliness of ESR ingot.

2. Experimental Procedure

Three remeltings (Run E1, E2 and E3) were carried out in an electroslag furnace with a capacity to produce 6000 kg ingot. The electrode of GCr15SiMn was produced by the process of steelmaking, secondary refining and ingot casting, with radius 0.16 m. The size of ingot is 0.31 m in radius and 1.62 m in height, the weight is about 3500 kg.

The conditions under which the three remeltings were carried out are described in Tables 1 and 2. The melting rate of electrode is approximately 8 kg/min. After ESR process, chemical compositions of edge, 1/2 radius and center area in E1, E2 and E3 ingots were analysed.

Table 1. Conditions for three remeltings.

Run	Slag	Average chemical composition of electrode ( wt )
		C, %	Si, %	Mn, %	Cr, %	Al, %	Ti, ppm	N, ppm	O, ppm
E1	A	1.01	0.58	1.07	1.51	0.02	30	65	7
E2	A	1.01	0.5	1.05	1.54	0.025	48	56	7
E3	B	1.02	0.59	1.05	1.49	0.05	41	56	10

Table 2. Chemical composition of slag A and B (%, wt).

Slag	CaF2	Al2O3	CaO	SiO2	FeO	Ti	Others
A	50	30	13	5.3	0.45	0.005	1.2
B	58	30	10	0.1	0.12	0.005	1.8

At present, there are no measures taken to directly observe the evolution of inclusion in electrode during ESR refining, so in order to investigate the characteristics of TiN inclusion in electrode during the course of heating up, the ultrahigh-temperature confocal scanning violet laser microscope has been used to simulate this process. The heating rate is set as 2 K/s, the wave length of violet laser is 408 nm, and the protective atmosphere in experiment is argon.

Meanwhile, the content of TiN inclusion in electrode and ingots has been checked by Scanning Electron Microscope and Energy Dispersive Spectrometer (SEM-EDS). As to reduce the observed error, greater than 500 fields have been checked for every specimen of electrode and ingots.

3. Results and Discussion

3.1. Genetic Research of TiN Inclusion from Electrode to Ingot

The morphology and area scanning of TiN inclusion found in specimen cut from Run E2 electrode is shown in Fig. 1 by using SEM-EDS, which size is about 10 μm.

Fig. 1.

The morphology and area scanning of TiN in electrode specimen.

With increasing temperature of specimen, the evolution of TiN inclusion in it is shown in Fig. 2. In Fig. 2(a), the grain boundary of steel becomes clear at about 1433 K, and TiN inclusion is just on the boundary, which further illustrates the conclusion in the author’s previous work that TiN inclusion usually precipitates at later period of solidification of this steel.¹²⁾ In Fig. 2(b), small obvious pits begin to appear in this inclusion, and in Fig. 2(c), the interface between inclusion and steel becomes obscure at the solidus temperature of this steel about 1583 K, as calculated according to T_s=1811−ΣΔt_ix_i, where Δt_i and x_i are respectively the temperature coefficient and mass percentage of solute i.¹³⁾ From 1628 K, higher than the steel’s solidus temperature 45 K, the decomposition rate of TiN speeds up, and at about 1691 K, this inclusion almost disappears, as shown in Figs. 2(d)–2(j). The liquidus temperature of this steel is about 1733 K, greater than 1691 K, so it means that for 10 μm TiN inclusion, or even some larger, it will totally decompose in the solid-liquid coexistence region at the electrode tip.

Fig. 2.

The evolution of TiN during the process of heating up.

Actually, when the contents of Ti and N in steel are under the equilibrium value for TiN inclusion, TiN inclusion will begin to decompose in steel, the decomposition chemical reaction of TiN is given in Eq. (1).⁹⁾   

$$Ti{N}_{(s)}=[Ti]+[N]\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }lg{K}_{TiN,decomp.}=-\frac{15\mathrm{\hspace{0.17em} }218}{T}+5.64$$(1)

The contents of Ti and N on the surface of TiN inclusion (C_[Ti],Surf and C_[N],Surf) can be calculated through Eqs. (2), (3).¹⁴⁾   

$$\frac{A_{[N]}{D}_{[Ti]}^L}{A_{[Ti]}{D}_{[N]}^L}=\frac{\left(C_{[N],Surf}-C_{[N],L}\right)}{\left(C_{[Ti],Surf}-C_{[Ti],L}\right)}$$(2)

  

$$C_{[N],Surf}{C}_{[Ti],Surf}=\frac{K_{TiN,decomp.}}{f_{[N]}{f}_{[Ti]}}$$(3)

Where $D_{[Ti]}^L$ and $D_{[N]}^L$ represent diffusion coefficients of Ti and N in liquid steel. A_[Ti] and A_[N] are respectively the atomic mass of Ti and N. C_[Ti],L and C_[N],L are the contents of Ti and N in steel. f_[Ti] and f_[N] represent activity coefficients of Ti and N, respectively.

Based on Eqs. (1), (2), (3) and experimental data in Run E2, the contents of Ti and N on the surface of TiN inclusion are respectively 59 ppm and 200 ppm, greater than 48 ppm and 56 ppm in steel at the solidus temperature about 1583 K. However, due to difficult diffusion of Ti and N in steel, only the interface between inclusion and steel will become obscure at this time. With increasing temperature, the diffusion of Ti and N becomes easier, and the impetus of decomposition gets enhanced, further leading to the quick decomposition of TiN inclusion.

The size distribution of TiN inclusion in Run E2 electrode is shown in Fig. 3, which shows that most TiN inclusions in electrode are within 10 μm. According to above discussion, it concludes that for researched steel GCr15SiMn, all TiN inclusion in electrode will completely decompose during electrode remelting and solutes Ti and N will join in the following reaction between steel and slag. In other words, TiN inclusion found in ESR ingot is regenerated, not from the electrode.

Fig. 3.

Size distribution of TiN inclusion in electrode.

3.2. Relationship between Slag Composition and TiN Content in Ingot

Chemical compositions of edge, 1/2 radius and center area in Run E1, E2 and E3 ingots are listed in Table 3. Compared to the data in Table 1, it is obvious that for Run E1 and E2 using slag A, the contents of Ti in steel both decline, while Ti pick-up has occurred for Run E3 using slag B. Meanwhile, the content of N in Run E1 increases compared with that in electrode, while in Run E2 and E3, N contents are almost no change. This is possible due to the protective atmosphere used in Run E2 and E3 remeltings.

Table 3. Chemical compositions of ingots at different positions.

Run	Position	Chemical compositions of ingots (wt).
		C, %	Si, %	Mn, %	Cr, %	Al, %	Ti, ppm	N, ppm	O, ppm
E1	Edge	1	0.48	1.05	1.50	0.014	21	76	5
	1/2 radius	0.99	0.5	1.05	1.52	0.014	17/23	74	10
	Center	0.96	0.49	1.05	1.51	0.018	22	74	11
E2	Edge	1.1	0.49	1.05	1.55	0.02	28	53	9
	1/2 radius	1.08	0.52	1.05	1.55	0.019	27	51	9
	Center	1.07	0.51	1.05	1.54	0.02	27	52	9
E3	Edge	0.98	0.56	1.02	1.47	0.038	50	60	9
	1/2 radius	1	0.57	1.02	1.48	0.039	48	59	9
	Center	1	0.57	1.03	1.46	0.039	48	62	9

The content of TiN inclusion in center area for Run E1, E2 and E3 ingots is shown in Fig. 4, which illustrates that TiN content in Run E3, corresponding to the highest Ti content among three ingots, is greater than that in E1 and E2. In addition, TiN content in Run E1 is slightly less than that in E2, based on the data in Table 3, it concludes that the effect of Ti content on the content of TiN is greater than that of N content for this steel.

Fig. 4.

TiN content in center area of E1, E2 and E3 ingots.

Part of TiN inclusions found in ingot is shown in Fig. 5. It can be seen that this kind of inclusion usually has a regular shape, which will easily induce the microcrack between steel and inclusion, and further deteriorate the properties of steel.

Fig. 5.

Morphology of part TiN inclusions in ingot.

Based on the model proposed by J. H. Wei and A. Mitchell,¹⁵⁾ the relationship between slag composition and Ti content in steel has been theoretically calculated.

In ESR process, the thermodynamic equilibrium will be reached between steel and slag.¹¹⁾ At the interface between steel and slag, the oxidation reaction is given as Eq. (4). The exchange reaction between oxides and other solutes will be inferred through the oxidation reaction.   

$$x[M]+y[O]=(M_x{O}_y)$$(4)

In Eq. (4), M represents Fe, Si, Mn, Al and Ti, respectively. For the oxide of solute Ti, according to thermodynamic calculation, it knows that the oxide Ti₃O₅ is the most stable compared with other oxides of Ti under the same temperature.¹⁶⁾ So in this paper, it takes Ti₃O₅ as the oxide for solute Ti.

According to the mass conservation of M, Eq. (5) can be obtained.   

$$C_{[M]e}+x{C}_{(M_x{O}_y)e}=C_{[M]0}+x{C}_{(M_x{O}_y)0}$$(5)

Where C_(MxOy)0 and C_[M]0 represent the initial contents of M_xO_y in slag and M in steel, respectively. C_(MxOy)e and C_[M]e are respectively the equilibrium contents of M_xO_y and M at the interface between steel and slag.

After the calculation for C_(MxOy)e and C_[M]e, the diffusion of M in steel and M_xO_y in slag will be considered through penetration theory.

Assume that the position x=0 is the interface between steel and slag, and the slag is on the x>0 direction, the steel is on the x<0 direction. For Ti+Ti₃O₅ system, Eq. (6) can be get.   

$$\begin{array}{l}\frac{\partial C_{[Ti]}}{\partial t}=D_{[Ti]}^L\frac{{\partial }^2{C}_{[Ti]}}{\partial x^2}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }x\le 0\\ \frac{\partial C_{(T{i}_3{O}_5)}}{\partial t}=D_{(T{i}_3{O}_5)}\frac{{\partial }^2{C}_{(T{i}_3{O}_5)}}{\partial x^2}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }x\ge 0\end{array}$$(6)

Where C_[Ti] and C_(Ti3O5) represent the contents of Ti in steel and Ti₃O₅ in slag, respectively. D_(Ti3O5) is the diffusion coefficient of Ti₃O₅ in slag.

According to Eq. (6) and its initial and boundary conditions, the content distribution of solute Ti in steel at different positions and time can be calculated, as shown in Eq. (7).   

$$C_{[Ti]}=C_{[Ti]0}\left\{1-\frac{3C_{(T{i}_3{O}_5)e}-3C_{(T{i}_3{O}_5)0}}{3C_{(T{i}_3{O}_5)e}-3C_{(T{i}_3{O}_5)0}+C_{[Ti]e}}\left[1+erf\left(\frac{x}{2\sqrt{(t{D}_{[Ti]}^L})}\right)\right]\right\}$$(7)

The average residence time t_e and the thickness δ of the liquid metal film at the electrode tip are given in Eqs. (8) and (9).¹⁷⁾   

$$t_e=3.35{\left(\frac{2\pi cos\theta }{3W_m}\right)}^{\frac{2}{3}}{\left(\frac{{\mu }_m}{g{\rho }_{m}sin\theta }\right)}^{\frac{1}{3}}{\left(\frac{R}{cos\theta }\right)}^{\frac{5}{3}}$$(8)

  

$$\delta ={\left[\frac{3{\mu }_m{W}_m}{2\pi \left({\rho }_m-{\rho }_s\right)gsin\theta cos\theta }\left(1-\frac{l^{2}co{s}^2\theta }{R^2}\right)\right]}^{\frac{1}{3}}$$(9)

Where θ is the cone angle of electrode tip, g represents acceleration due to gravity. W_m and R are the volumetric melt rate and radius of electrode, respectively. μ_m is the viscosity of metal, and l is the length of cone edge in contact with the slag. ρ_m and ρ_s represent the densities of steel and slag, respectively.

The equilibrium constants of Eq. (4) and the diffusion coefficients of Ti, Si, Mn and O in steel are shown in Tables 4^13,16) and 5.^13,15,18)

Table 4. The equilibrium constants of Eq. (4).

lgKFeO	6317/T−2.73	lgKSiO2	30110/T−11.40
lgKTi3O5	92075/T−29.84	lgKMnO	15050/T−6.75
lgKAl2O3	64900/T−20.63

Table 5. Diffusion coefficients of Ti, Si, Mn and O in steel.

Solutes	$D_{[i]}^L$ (cm2s−1)	Solutes	$D_{[i]}^L$ (cm2s−1)
Ti	$3.1\times {10}^{-3}exp\left(-\frac{11\mathrm{\hspace{0.17em} }500}{RT}\right)$	Si	$6.65\times {10}^{12}exp\left(-\frac{71\mathrm{\hspace{0.17em} }123}{T}\right)$
Mn	$5.97\times {10}^{10}exp\left(-\frac{62\mathrm{\hspace{0.17em} }535}{T}\right)$	O	$3.34\times {10}^{-3}exp\left(-\frac{12\mathrm{\hspace{0.17em} }000}{RT}\right)$

At present, there is no reliable data for the diffusion coefficient of Al in steel, in this paper, it takes $D_{[Al]}^L$ = $D_{[Si]}^L$ for calculation.¹⁵⁾

When taking CaF₂ as an inert component, the main difference between slag A and B is the content of SiO₂, based on above Eqs. (4), (5), (7), the effect of SiO₂ in slag on the content of Ti in steel has been investigated, as shown in Fig. 6. The data of Ti content corresponding to Run E1, E2 and E3 in Table 3 is also marked in Fig. 6, which shows that the theoretically expected is in agreement with the experimentally proved.

Fig. 6.

Effect of SiO₂ content in slag on Ti content in ESR ingot.

It is obvious that, from Fig. 6, the content of SiO₂ in slag has a significant influence on Ti content in steel. With increasing SiO₂ content in slag, the content of Ti in steel will decline. When the content of SiO₂ in slag is lower than a certain value, Ti pick-up in steel will occur in ESR process, like Run E3. According to Refs. 19), 20), the Si/Ti exchange reaction has occurred in Run E3 following the expression given in Eq. (10), as a consequence of the low activity of SiO₂ in slag (about 6.21×10⁻⁵ through Eq. (11)¹⁵⁾ and data in Table 2) and high activity of Si in steel (about 1.06 by using the data in Table 1 and Ref. 13) through the model of Wagner).   

$$5[Si]+2(T{i}_3{O}_5)=6[Ti]+5(Si{O}_2)$$(10)

  

$$\begin{array}{l}lg{\gamma }_{Si{O}_2}=lg{\gamma }_{FeO}-\frac{11\mathrm{\hspace{0.17em} }800}{T}(N_{CaO}+N_{MgO})\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.17em}-\frac{4\mathrm{\hspace{0.17em} }916}{T}{N}_{MnO}-\frac{3\mathrm{\hspace{0.17em} }562}{T}{N}_{A{l}_2{O}_3}\end{array}$$(11)

Where N_i is the molar percentage of slag component, and γ_i represents the activity coefficient of Raoult for slag component.

Once there is a certain amount of SiO₂ in slag, the Si/Ti exchange reaction towards the formation of SiO₂ will be prevented, and with increasing content of SiO₂ in slag, the reaction will be reversed, leading to a decline in Ti of steel, like Run E1 and E2.

Under using slag A in Run E1 and E2, Fig. 7 demonstrates that for different contents of Ti in slag, there is a corresponding equilibrium value of Ti content in steel, with about 12 ppm and 25 ppm of Ti content in steel respectively corresponding to that of 0.005% and 0.05% in slag. When the content of Ti in electrode is greater than this critical equilibrium value, Ti will be oxidized and the oxidation degree will rise with higher Ti content in electrode, while under the opposite case, Ti content in steel will increase.

Fig. 7.

Relationship between Ti content in electrode, slag and ingot.

4. Conclusions

In this paper, the generation mechanism of TiN inclusion has been investigated for clean GCr15SiMn ESR ingot production. The conclusions in the present study are summarized as follows.

(1) During the course of consumable electrode remelting, TiN inclusion in it will completely decompose in the solid-liquid coexistence region at the electrode tip. TiN inclusion found in ESR ingot is regenerated during solidification of steel, not the one from electrode.

(2) The content of SiO₂ in slag has an obvious effect on Ti content in steel due to the Si/Ti exchange reaction taking place. With the content of SiO₂ in slag increasing, Ti content in steel will decline, further lead to the low TiN content in ESR ingot.

(3) When the content of Ti in consumable electrode is higher than the critical equilibrium value in steel corresponding to the Ti content in slag, it will decline in steel, and the decrease degree will rise with higher Ti content in electrode.

Acknowledgements

The authors wish to express their thanks to the workers for this production and the financial support provided by 863 Project in China (No.2012AA03A503).

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