Formation and Evolution of Inclusions in High Chromium Steel

Abstract

Controlling inclusion content in high chromium steel is very important to prevent submerged entry nozzle from clogging in continuous casting and avoid the negative impacts of inclusions on steel properties. Therefore, effects of temperature and content of elements on phase stability diagram should be clarified in chromium bearing steel. However, the effect of chromium content on boundaries of MgO, MgO∙Al₂O₃ and Al₂O₃ in phase stability diagram are much different among the researchers. The direction of boundaries shift is affected by chromium content differently. Temperature dependencies of deoxidation equilibrium constants below 1873 K are also scattered. Calcium, which is used to avoid the negative effect of MgO∙Al₂O₃ inclusion, enlarges liquid region in phase stability diagram. However, the region replaced by liquid oxide is understood differently in low alloyed steel and high chromium steel. In TiO_X–Al₂O₃–MgO system inclusion, commercial thermochemical software predicts that boundaries of Ti₂O₃, Ti₃O₅, Al₂O₃ and TiOx–Al₂O₃ shift toward lower titanium content in high chromium steel. However, the calculated phase stability diagrams vary among studies even in liquid iron or low alloyed steel. Therefore, equilibrium experiments under various conditions and reliable technique of thermodynamic calculation with high accuracy are desired.

1. Introduction

MgO·Al₂O₃ spinel inclusions in high chromium steel are known to be detrimental to surface quality and mechanical properties such as formability and resistance to fatigue and fracture. The shortcomings intrinsically result from its high melting point and high hardness. There exist the other problems of plant operation from refining to casting. MgO·Al₂O₃ spinel inclusions are apt to adhere to the inner wall of the SEN (submerged entry nozzle) during casting. As a result, nozzle clogging decreases productivity and small debris flaked from the deposition results in surface defects.^1,2,3,4) The temperature of molten high chromium steel during casting is lower than that of liquid iron or low alloyed steel. Therefore, acquisition of thermodynamic data in lower temperature below 1873 K are also important, especially at as low as 1773 K. Some inclusions found in steel products have been reported to originate from refining slag entrapped during AOD (Argon Oxygen Decarburization) or ladle process. Alumina content in entrapped slag particle increases during the production process because of temperature drop from ladle to casting. Increase of alumina content in inclusion promotes precipitation of MgO∙Al₂O₃ phase. Presence of MgO∙Al₂O₃ in molten steel facilitates nozzle clogging due to the strong adherability as mentioned above. However, there are few thermodynamic data of high chromium steel under the condition of low temperature because many studies have mainly targeted low alloyed steel with high liquidus temperature.

On the other hand, spinel inclusions could facilitate the formation of equiaxed crystal in ferritic stainless steel containing titanium through promoting the formation of titanium nitride, TiN, which has good lattice consistency with delta ferrite.^5,6,7) Titanium is also added to high chromium steel in order to fix soluble carbon and nitrogen which result in grain-boundary corrosion by forming chromium deplete zone. Added titanium would partially oxidize and form some oxides, Ti₂O₃ and Ti₃O₄.⁸⁾ Moreover, Al₂O₃–TiOx–(MgO) oxide could be formed in Al killed steel.⁹⁾ The oxide deteriorates steel quality for the same reason as the spinel oxide does.

For the reasons stated above, it is very important to control inclusion composition in commercially operated steelmaking processes. In the past, many studies on controlling inclusion have been executed in liquid iron, low alloyed steel and high chromium steel. Park et al.¹⁰⁾ published comprehensive review targeted at spinel inclusion in stainless steel. Liu et al.¹¹⁾ and Deng et al.¹²⁾ also performed reviews on formation and evolution of inclusion for various grades of steel. However, the published values on thermodynamic calculation differ from one another to no small extent. Especially, the effects of chromium content on the behaviors of MgO·Al₂O₃ spinel and TiOx–Al₂O₃ inclusions are quite different among studies. Therefore, it is difficult to determine suitable deoxidation conditions for operation. In the present paper, the differences on thermodynamic estimation and the necessary future works for controlling inclusion composition are discussed.

2. Inclusion of MgO–Al₂O₃(−CaO) System

2.1. Phase Stability Diagram of MgO, MgO–Al₂O₃ and Al₂O₃ System

Park et al.¹⁰⁾ and Li et al.^13,14) calculated phase stability diagram of MgO, MgO·Al₂O₃ and Al₂O₃ inclusions using commercial thermochemical software and database (FactSage) at 1873 K in molten steel containing various chromium contents, respectively. For comparison, their calculated results are shown in Fig. 1. The figure shows that the boundaries of MgO/MgO·Al₂O₃ and MgO·Al₂O₃/Al₂O₃ shift toward higher magnesium content with increasing of chromium content when aluminum content is more than 0.003 mass%. On the other hand, the boundary of MgO/MgO·Al₂O₃ is less affected by chromium and aluminum contents. The boundary of MgO·Al₂O₃/Al₂O₃ shifts toward lower magnesium content with increasing of chromium content when aluminum content is less than 0.003 mass%.

Fig. 1.

Phase stability diagram of MgO, MgO·Al₂O₃ and Al₂O₃ inclusions at 1873 K by Lie et al. and Park et al. (Online version in color.)

In the above-mentioned thermochemical software, the calculation is based on thermodynamic model and database. The calculation procedures inside the programs are complicated. As a result, it is difficult to assess the contribution of various factors including equilibrium constants of deoxidation reactions, first and second order interaction parameters. The results by the commercial software with are compared with those calculated by conventional thermodynamic method. The adopted procedure and thermodynamic parameters are the same as those proposed by Itoh et al.¹⁵⁾ Reactions treated here are expressed by Eqs. (1), (2), (3) and the deoxidation constants of them are given by Eqs. (4), (5), (6) according to the earlier publications.^16,17,18) The activities of MgO and MgO·Al₂O₃ are unity and 0.8 at MgO saturation and those of Al₂O₃ and MgO·Al₂O₃ are unity and 0.47 at Al₂O₃ saturation, respectively.¹⁸⁾ Interaction parameters listed in Table 1 contains the parameters $e_{\text{Mg}}^{\text{Cr}}$, $e_{\text{Al}}^{\text{Cr}}$ and $e_{\text{O}}^{\text{Cr}}$ to take the effect of chromium content into consideration.   

$$\text{MgO(s)}=\underset{\_}{\text{Mg}}+\underset{\_}{\text{O}}$$(1)

  

$${\text{Al}}_2{\text{O}}_3(\text{s})=2\underset{\_}{\text{Al}}+3\underset{\_}{\text{O}}$$(2)

  

$$\text{MgO(s)}\cdot {\text{Al}}_2{\text{O}}_3(\text{s})={\text{Al}}_2{\text{O}}_3(\text{s})+\text{MgO(s)}$$(3)

  

$$\text{log}{K}_{\text{MgO}}=\text{log}\frac{a_{\text{Mg}}\cdot a_{\text{O}}}{a_{\text{MgO}}}=-4.280-\frac{4\mathrm{\hspace{0.17em} }700}{T}$$(4)

  

$$\text{log}\mathrm{\hspace{0.17em} }{K}_{{\text{Al}}_2{\text{O}}_3}=\text{log}\frac{a_{\text{Al}}{}^2\cdot a_{\text{O}}{}^3}{a_{{\text{Al}}_2{\text{O}}_3}}=11.620-\frac{45\mathrm{\hspace{0.17em} }300}{T}$$(5)

  

$$\text{log}\mathrm{\hspace{0.17em} }{K}_{\text{MgO}\cdot {\text{Al}}_2{\text{O}}_3}=\text{log}\frac{a_{\text{MgO}}\cdot a_{{\text{Al}}_2{\text{O}}_3}}{a_{\text{MgO}\cdot {\text{Al}}_2{\text{O}}_3}}=-0.820-\frac{1\mathrm{\hspace{0.17em} }085}{T}$$(6)

Table 1. First and second order interaction parameter, $e_{\text{i}}^{\text{j}}$, $r_{\text{i}}^{\text{j}}$ and $r_{\text{i}}^{\text{j},\text{k}}$ in molten steel used in this work.

i	$e_{\mathrm{i}}^{\mathrm{A}\mathrm{l}}$	$e_{\mathrm{i}}^{\mathrm{M}\mathrm{g}}$	$e_{\mathrm{i}}^{\mathrm{O}}$	$e_{\mathrm{i}}^{\mathrm{C}\mathrm{r}}$
Al	0.04319)	−0.01920)	−1.9817)	0.009621)
Mg	−0.01720)	0	−43016)	0.04722)
O	−1.1717)	−28016)	−0.1719)	−0.03223)
$r_{\mathrm{A}\mathrm{l}}^{\mathrm{O}}$ = 40,17) $r_{\mathrm{A}\mathrm{l}}^{\mathrm{A}\mathrm{l},\mathrm{O}}$ = −0.028,17) $r_{\mathrm{A}\mathrm{l}}^{\mathrm{M}\mathrm{g},\mathrm{O}}$ =−26015)
$r_{\mathrm{M}\mathrm{g}}^{\mathrm{O}}$ = 350000,16) $r_{\mathrm{M}\mathrm{g}}^{\mathrm{A}\mathrm{l},\mathrm{O}}$ = −230,15) $r_{\mathrm{M}\mathrm{g}}^{\mathrm{M}\mathrm{g},\mathrm{O}}$ = −6100016)
$r_{\mathrm{O}}^{\mathrm{M}\mathrm{g}}$ = −20000,16) $r_{\mathrm{O}}^{\mathrm{A}\mathrm{l},\mathrm{O}}$ = 47.4,17) $r_{\mathrm{O}}^{\mathrm{M}\mathrm{g},\mathrm{O}}$ = 46200016)

As shown in Fig. 2, the calculated results indicate that both boundaries of MgO/MgO·Al₂O₃ and MgO·Al₂O₃/Al₂O₃ shift toward lower magnesium content with increasing of chromium content. The direction of shits derives from the positive value of $e_{\text{Mg}}^{\text{Cr}}$ (0.047). The sign of the parameter would be reliable because $e_{\text{Mg}}^{\text{Cr}}$ proposed by the other studies indicates positive values, 0.01( Nadif et al.²⁴⁾) and 0.022 (Jo et al.²⁵⁾). By comparing Fig. 2 with Fig. 1, it would be suggested that the phase stability diagram in Fig. 1 should be re-considered because chromium content has opposite effect on the shifting direction of phase boundaries.

Fig. 2.

Phase stability diagram of MgO, MgO·Al₂O₃ and Al₂O₃ inclusions at 1873 K. (Online version in color.)

In some experimental research with high chromium steel, the boundary of MgO/MgO·Al₂O₃ was identified. Liu et al.²⁶⁾ executed the laboratory-scale crucible experiment with molten steel containing 11 mass% chromium at 1873 K and Todoroki et al.⁴⁾ investigated the inclusions in molten steel samples containing 23.5 mass% chromium at commercially operated continuous casting. The results of them are marked with broken circle in Fig. 3. The thermodynamic calculation is carried out on the basis of the same condition as the experiments and the calculated results are also shown in the figure. By comparison of experimental and calculated results, the calculated boundary between MgO and MgO·Al₂O₃ is located at higher magnesium content excessively in high aluminum content (>0.3 mass%) and at lower magnesium content too much in low aluminum content (<0.1 mass%).

Fig. 3.

Phase stability diagram of MgO, MgO·Al₂O₃ and Al₂O₃ at 1873 K. (Online version in color.)

2.2. Influence of Equilibrium Constants on Phase Stability Diagram

Here, it is discussed whether the discrepancy mentioned above could be explained with selected combination of deoxidation equilibrium constants. Liu et al.¹¹⁾ pointed out that the boundaries between stable phases would shift depending on the combination of deoxidation equilibrium constants as shown in Fig. 4. The figure indicates that both boundaries of MgO/MgO·Al₂O₃ and MgO·Al₂O₃/Al₂O₃ shift toward lower magnesium content when lower equilibrium constant of K_MgO is adopted. In fact, there is greater variability among the equilibrium constants of K_MgO as listed in Table 2. Similarly, the boundaries shift depending on equilibrium constants of K_Al2O3 and K_MgO⋅Al2O3 which are listed in Tables 3 and 4. However, it is difficult to explain the experimental results locating at both sides of the calculated line because the selected combination of equilibrium constants could shift the boundary almost parallel. It suggests that the interaction parameters affect the position of boundaries.

Fig. 4.

Calculated MgO/spinel/Al₂O₃ phase stability diagram at 1873 K.¹¹⁾

Table 2. Equilibrium constants of Mg–O system in liquid iron at 1873 K.

year	logK	Author	Ref.No.
1974	−7.86	G. K. Sigworth et al.	27
1974	−5.5	V. I. Yavoiskii et al.	28
1977	−5.12	E. B. Teplitskii et al.	29
1980	−9.24	A. P. Gorobetz	30
1985	−8.54	I. S. Kulikov	31
1986	−5.7	M. Nadif et al.	32
1991	−7.74	E. T. Turkdogan	33
1994	−7.8	R. Inoue et al.	34
1997	−7.86	H. Ohta et al.	35
1977	−6.03	Q. Han et al.	36
1997	−6.8	H. Itoh et al.	16
2000	−7.21	J. D. Seo et al.	37
2003	−7.24	W. G. Seo et al.	38
2009	−7.59	N. Satoh et al.	39
2011	−8.07	J. Gran et al.	40

Table 3. Equilibrium constants of Al–O system in liquid iron at 1873 K.

year	logK	Author	Ref.No.
1974	−13.34	G. K. Sigworth et al.	27
1980	−13.6	S. Gustafsson et al.	41
1988	−13.6	Steelmaking Data Sourcebook (JSPS)	19
1994	−13.35	H. Suito et al.	42
1994	−13.3	S. W. Cho et al.	43
1995	−14.01	S. Dimitrov et al.	44
1997	−12.57	H. Itoh et al.	17
1998	−12.99	J. D. Seo et al.	45
2009	−12.84	N. Satoh et al.	39
2003	−13.34	H. Ohta et al.	21

Table 4. Equilibrium constants of MgO·Al₂O₃ = MgO+Al₂O₃ at 1873 K.

year	logK	Author	Ref.No.
1965	−0.84	H. R. Rein et al.	46
1991	−1.18	O. Knacke et al.	47
2000	−1.4	K. Fujii et al.	18

2.3. Inclusion of Slag Entrainment Origin

So far it is assumed that the inclusions form by deoxidation reaction. However, some researchers have proposed that a part of spinel inclusions in molten steel might be caused by slag entrainment. Hojo et al.⁴⁸⁾ added strontium carbonate as a tracer at tapping from AOD to ladle or at tundish in stainless steel production plant to investigate the origin of inclusion formation as shown in Fig. 5. As a result, they found that MgO–Al₂O₃ spinel could precipitate in the suspended AOD slag droplets with decreasing of temperature in molten steel from refining to casting process through thermodynamic calculation. Similar findings have been published by other researchers.^49,50,51,52) Kim et al.⁴⁹⁾ indicated the variation in inclusion composition as shown in Fig. 6. Ehara et al.^51,52) showed that the change in alumina content in inclusion roughly corresponds to the calculated result considering temperature drop as depicted in Fig. 7. However, both values are not sufficiently in good agreement. One of the reasons might result from the selection of deoxidation equilibrium constant of K_Al2O3 because various values of the constant have been published by many researchers.^{17,19,27,39,45)} Figure 8 shows the relationship between logK_Al2O3 and reciprocal of temperature. The figure suggests that the variation among the proposed values are 1.0 (from −13.6 to −12.6) at 1873 K and 1.6 (from −15.5 to −13.9) at 1773 K. In other word, it means that the equilibrium constant varies more widely at lower temperature. It might result from few studies at the temperature as low as 1773 K, which is below melting point of liquid iron.

Fig. 5.

Experimental conditions.⁴⁸⁾

Fig. 6.

Composition of inclusions (SUS304).⁴⁹⁾

Fig. 7.

Change of Al₂O₃ content in inclusions with temperature.⁵²⁾

Fig. 8.

Comparison of temperature dependence of log K_Al2O3 in liquid iron. (Online version in color.)

2.4. Effect of Calcium Content on Inclusion Content

Many researchers have proposed utilization of calcium to render detrimental spinel inclusions harmless.⁵³⁾ Calcium in molten steel forms liquid oxide and transforms MgO·Al₂O₃ spinel inclusion into MgO–Al₂O₃–CaO system inclusion. Yang et al.⁵⁴⁾ performed the thermodynamic calculation using the same method as Itoh et al.¹⁵⁾ and evaluated the effect of calcium treatment on phase stability diagram of inclusion in low carbon Al killed steel without chromium as shown in Fig. 9. The figure shows that the width of MgO·Al₂O₃ region between MgO and Al₂O₃ would not change whereas the region of liquid oxide 12CaO·7 Al₂O₃ increases by replacing spinel region. On the other hand, Li et al.^13,14) calculated phase stability diagram in 11 mass%Cr and 18 mass%Cr steel with thermochemical software. As shown in Fig. 10, the calculated results indicate that the width of MgO·Al₂O₃ region between MgO and Al₂O₃ becomes narrower with increasing of calcium content, which disagrees with those by Yang et al.⁵⁴⁾ It is absolutely unclear why the influence of calcium content on shrinkage behavior of MgO·Al₂O₃ region varies depending on the presence of chromium in steel. For the reasons stated above, it is necessary to re-evaluate the thermodynamic data including equilibrium constants and interaction parameters in high chromium steel in order to understand stability phase diagram of MgO–Al₂O₃–CaO system and to control the inclusion content.

Fig. 9.

Phase stability diagram of MgO, MgO·Al₂O₃ and CaO·2Al₂O₃. inclusion formation in liquid iron at 1873 K ([Ca] = 2 ppm).⁵⁴⁾

Fig. 10.

Phase stability diagram of inclusion in 18%Cr molten steel at 1873 K. (Online version in color.)

3. Inclusion of TiO_x–Al₂O₃–MgO System

3.1. Phase Stability Diagram in Inclusion of TiOx–Al₂O₃ System

The studies aimed at Ti bearing Al killed steel have been carried out on a basis of thermochemical software by Kang et al.⁵⁵⁾ and Li et al.,^13,14) respectively. The reported results are shown in Fig. 11. The figure indicates that the phase boundaries slightly shift toward lower magnesium content with increasing of chromium content. However, the phase stability diagram in Fe–Al–Ti–O system is controversial even in liquid iron without chromium although the topic has been studied by many researchers.^{55,56,57,58,59)} Ruby-Meyer et al.⁵⁶⁾ executed thermochemical software calculation on phase stability diagram and indicated that liquid phase exists between Ti₂O₃ and Al₂O₃ region at 1793 K. On the other hand, the other researchers^55,57,58,59) could not find liquid phase in the range of [Al]>0.001 mass% even at 1873 K. The calculated results except that by Ruby-Meyer et al. are shown in Fig. 12. The figure indicates that the published values differ from one another. Therefore, it is much anticipated that the thermodynamic calculations are verified with equilibrium experiments in both liquid iron and high chromium steel.

Fig. 11.

Phase stability diagram of inclusion in 0%, 11% and 18%Cr molten steel at 1873 K. (Online version in color.)

Fig. 12.

Phase stability diagram of inclusion in liquid iron at 1873 K. (Online version in color.)

3.2. Effects of Slag on Inclusion Behavior

Kim et al.⁴⁹⁾ studied inclusions consisting of CaO–SiO₂–Al₂O₃–MgO–TiO₂ in samples of 18Cr-8Ni stainless steel product. They found that inclusion of slag origin is entrapped from slag into molten steel, partly is deoxidized by added titanium and increases the content of TiO₂ in inclusion due to temperature drop from AOD to mold as shown in Fig. 13. As a result, MgO·Al₂O₃ and TiO₂ inclusions with high melting point precipitate as shown in Fig. 14.

Fig. 13.

Changes in composition of inclusions and molten steel temperatures at stainless steelmaking process.⁴⁹⁾

Fig. 14.

SEM micrograph of a typical inclusion causing defects on coil surface and EDS of each phase.⁴⁹⁾

Phase stability diagrams of TiO_X–MgO–Al₂O₃ system were calculated using thermochemical software at 1873 K P_O2=10⁻¹³ atm by Park et al.⁹⁾ and at 1873 K P_O2=10⁻¹² atm by Li et al.,⁶⁰⁾ respectively. The former assessed the effect of slag basicity and indicated that the inclusions change from liquid to spinel + liquid as basicity increases. The latter evaluated the influence of CaF₂ content in slag and indicated that the inclusions change from TiOx–Al₂O₃(Sp) to MgO–Al₂O₃–TiO_X(TiSp)+liquid, MgO+MgO−Al₂O₃–TiO_X(TiSp)+liquid with increasing of CaF₂ content in slag as shown in Fig. 15 (Sample A: premelted before slag/metal reaction, Sample A3: after reaction with slag containing 10.07 mass%CaF₂). Li et al.⁶⁰⁾ suggested that increase in MgO content in inclusion could be explained by the effect of CaF₂ on magnesium content in molten steel on a basis of thermodyanamic calculation. As seen above, the influence of slag is not negiligible in both slag entrainment and control of minor constituent.

Fig. 15.

Composition distributions of inclusions in MgO–Al₂O₃–TiOx phase stability diagram (1873 K, P_O2 = 10⁻¹² atm). (Online version in color.)⁶⁰⁾

4. Thermodynamic Analysis

Wagner’s⁶¹⁾ equation has been widely used to calculate thermodynamic relationship. Additionally, Darken’s quadratic formalism^62,63) and Redlich-Kister type polynomial^64,65) have been proposed. Especially, the latter has been arduously adopted for calculation targeted at high alloyed steel.^{8,39,66,67,68)} Recently, it is clarified that deoxidation equilibrium could be predicted with a high degree of accuracy by taking into consideration the third order interaction parameter in Fe–Ni alloy.⁶⁹⁾ More application of Redlich-Kister type polynomial to various high alloyed steel could be expected in the future.

5. Conclusions

Controlling inclusion content in high chromium steel is very important to prevent submerged entry nozzle from clogging in continuous casting and avoid the negative impacts of inclusions on steel properties including surface defects. Therefore, reliable thermodynamic data is necessary. However, the published boundaries of MgO, MgO∙Al₂O₃ and Al₂O₃ in phase stability diagram of chromium bearing steel are much different among the researchers. The boundaries shift toward lower magnesium content according to the calculation on a basis of interaction parameters and deoxidation equilibrium constants. On the other hand, whereas thermochemical software calculates quite different results. Research on temperature dependency of deoxidation equilibrium constants below 1873 K is also important for controlling inclusions. However, the deoxidation equilibrium constants at low temperature vary among the studies. Calcium has been used to avoid the negative effect of MgO∙Al₂O₃ inclusion, but the region replaced by liquid oxide in phase stability diagram is understood differently between low alloyed steel and high chromium steel.

In TiOx–Al₂O₃–MgO system inclusion, thermochemical software suggests that the boundaries of Ti₂O₃, Ti₃O₅, Al₂O₃ and TiOx–Al₂O₃ shift toward the lower titanium content on the whole, but there exists a problem that calculated phase stability diagrams vary among studies even in liquid iron or low alloyed steel. Moreover, it is reported that basicity and CaF₂ content of slag have a strong influence on content of TiOx–Al₂O₃–MgO system inclusion, but the behaviors are not fully clarified.

As noted above, equilibrium experiments at various conditions are much expected to establish reliable thermodynamic data. For the purpose, new thermodynamic calculation technique with high accuracy, for example, Redlich-Kister type polynomial, should also be targeted at high chromium steel.

Acknowledgments

This study was carried out with the support and advice of the Iron and Steel Institute of Japan, “Clean Cr steel production by slag, inclusion control” research group. We would like to express our deepest gratitude to the Iron and Steel Institute of Japan and its study group.

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