Influence of refining slag composition on cleanness and fatigue life of 60Si2MnA spring steel

Abstract

The influence of basicity and Al₂O₃ content of LF refining slag on T.[O] (total oxygen) as well as type, number and size of non-metallic inclusions in Al killed 60Si2MnA spring steel was investigated. The results showed that with the increase of slag basicity R(CaO/SiO₂) or the decrease of Al₂O₃ content in slag, the T.[O], number and size of non-metallic inclusions decreased significantly. On the one hand, as the slag basicity increased, inclusions in steel were transformed from Al₂O₃–SiO₂–CaO–MgO quaternary system to Al₂O₃–SiO₂–CaO–MgO–CaS quinary system, which made the formation of voids between inclusions and steel matrix to decrease. Furthermore, thermodynamic calculations showed that CaS could only form in steel (R ≥ 3.4). Al₂O₃–SiO₂–CaO–MgO came close to the compositions of the low melting point area, while Al₂O₃–SiO₂–CaO–MgO–CaS deviated from this. On the other hand, as the Al₂O₃ content in slag increased, Al₂O₃–SiO₂–CaO–MgO–CaS came close to the compositions of the low melting point area. In conclusion, the cleanness and fatigue life of 60Si2MnA spring steel had been improved by the increase of slag basicity or the decrease of Al₂O₃ content in slag.

Introduction

Spring steel plays an important part in mechanical equipment. It is now clearly established that fatigue fracture is the main form of spring steel failures and that non-metallic inclusions are usually the origins of fatigue cracks.¹ The T.[O] (total oxygen) of steel as well as type, number, size, morphology and distribution of inclusions in steel play essential roles in spring steel cleanness, and improvement of cleanliness can effectively prolong the fatigue life of the steel.^2–6 Furthermore, appropriate refining slag composition can reduce the T.[O] of spring steel, control inclusion composition and reduce the number and size of inclusions.^7–8

At present, silicon and aluminium both can be used as deoxidisers to produce spring steel, but the T.[O] of Al killed steel is lower in general. Al₂O₃ is the main deoxidation product in Al killed spring steel, which has the worst impact on fatigue life; however, there is a lack of theoretical description on how to control the composition, number, size, morphology and distribution of inclusions through adjusting LF refining slag composition in order to reduce the negative impact of Al₂O₃ inclusions. Jiyuan Iron and Steel Group Company (JISGC) uses a 100 t BOF to produce 60Si2MnA spring steel (0.6 C, 1.7 Si, 0.75 Mn), but it was difficult to control the T.[O] content to ≤ 10 ppm, which the ultra clean steel requires.⁹ Therefore, in order to investigate these problems, a plant trial of Al killed 60Si2MnA spring steel was performed with four different basicities and three different Al₂O₃ contents of refining slags respectively under clean process, and then the T.[O] of wire rods, the type, number, size, morphology and distribution of inclusions were analysed and compared.

Production process and experimental methodology

The process adopted to produce 60Si2MnA spring steel at JISGC is as follows: hot metal desulphurisation pretreatment → 100 t top and bottom combined blown BOF → LF refining furnace → tundish → continuous casting (165 mm × 165 mm) → high speed wire rod rolling mill. During tapping of BOF, the premelting slag and lime were added to the ladle for slag making, following raking off slag. At the same time, some aluminium was added for deoxidation. At the LF, refining slags with four different basicities (R = 2–3, R = 3–4, R = 4–4.5 and R>4.5) and three different Al₂O₃ contents (Al₂O₃ < 15%, Al₂O₃ = 15–20% and Al₂O₃ = 20–25%) were adjusted respectively, and CaF₂ was added to ensure the flowability of slag according to slag condition. Slag steel reaction time should be sufficiently long for reactions to proceed well. Ca–Fe wire was added after refining, followed by soft gas stirring so that inclusions could float and be adequately removed. Most of all, the addition of alloy such as Si–Fe, Si–Mn should be tried to carry out at the end of soft blowing, which could ensure the slag basicities and the Al₂O₃ contents in slags stable. Finally, the wire rods after smelting and high speed wire rod rolling were analysed and compared respectively.

The inclusions in wire rods were observed and analysed by a scanning electron microscope (SEM, JSM-6480LV, × 800) and an energy dispersive X-ray spectrometer (EDS) attached to SEM, and the size of inclusions was measured directly by SEM. The longitudinal section of wire rod sample after polishing was eroded by the non-aqueous solution electrolysis method whereby the wire rod was employed as anode and copper piece was employed as cathode. Electrolytic solution was organic solution with pH 7, which was prepared by the mixture of dissolvent absolute methanol, solute tetramethyl ammonium chloride and dissolvent absolute acetylacetone,¹⁰ so that the combination morphology between inclusions and steel matrix could be observed obviously by SEM.

Two statistical parameters, the number density of inclusions and the contraction coefficient of inclusions, are defined as the following two equations to characterise non-metallic inclusions. (1) where ND is the number density of inclusions, per mm²; S _total is the total area scanned, mm²; and n is the number of detected inclusions on the area of S _total. (2) where CC is the contraction coefficient of inclusions; S _inclusions is the total area of detected inclusions, mm²; and S _voids is the total area of voids, mm².

In addition, the finished steel samples refined by different basicities or Al₂O₃ contents in slags were austenitised at 870°C for 30 min, and oil quenched, followed by the tempering at 440°C for 60 min. Then, the rotating bending fatigue test was carried out on a PQ6-02 fatigue machine following the standard HB5152-96, the maximum and minimum stresses of the test were set as ∼850 and − 850 MPa, the rotating frequency was 500 Hz, and then the fatigue life of every steel sample was measured respectively.

Results and discussion

Thirteen chemical compositions of slags and finished steel samples are listed in Tables 1 and 2 respectively.

Table 1

Chemical compositions of refining slags/%

Heat no.	R(CaO/SiO₂)	CaO	SiO₂	Al₂O₃	MgO	S	MnO	TFe
1#	2.3	48.96	20.86	23.48	4.92	0.30	0.10	0.39
2#	2.6	51.00	19.27	23.08	5.02	0.33	0.08	0.29
3#	3.1	52.63	17.15	22.84	5.78	0.36	0.07	0.25
4#	3.4	54.00	16.06	23.02	5.38	0.31	0.08	0.27
5#	3.4	55.25	16.25	21.47	5.07	0.52	0.05	0.15
6#	3.6	56.34	15.65	21.20	5.38	0.41	0.06	0.25
7#	4.0	55.18	13.80	22.30	5.62	0.29	0.06	0.22
8#	4.1	61.55	15.10	14.71	5.30	0.32	0.05	0.21
9#	4.2	58.16	13.76	18.65	5.26	0.28	0.07	0.32
10#	4.2	57.60	13.57	18.75	5.04	0.33	0.07	0.37
11#	4.4	56.18	12.80	22.80	5.22	0.27	0.05	0.32
12#	4.4	57.90	13.10	21.87	5.38	0.32	0.05	0.32
13#	4.8	59.21	12.22	21.85	5.38	0.35	0.07	0.39

Table 2

Chemical compositions of tested steels/%

Heat no.	C	Si	Mn	P	S	Cr	Ti	Als	Ca	T.[O]
1#	0.60	1.70	0.74	0.014	0.0023	0.15	0.0072	0.025	0.0016	0.0019
2#	0.59	1.74	0.75	0.014	0.0024	0.15	0.0075	0.027	0.0017	0.0017
3#	0.60	1.67	0.75	0.013	0.0019	0.15	0.0077	0.032	0.0021	0.0016
4#	0.59	1.66	0.74	0.013	0.0018	0.15	0.0076	0.031	0.0023	0.0015
5#	0.59	1.78	0.78	0.014	0.0020	0.16	0.0100	0.029	0.0015	0.0013
6#	0.58	1.75	0.77	0.015	0.0021	0.16	0.0100	0.028	0.0018	0.0016
7#	0.61	1.67	0.73	0.013	0.0018	0.14	0.0076	0.030	0.0025	0.0014
8#	0.60	1.72	0.78	0.013	0.0021	0.16	0.0100	0.037	0.0030	0.0010
9#	0.60	1.68	0.74	0.013	0.0017	0.15	0.0076	0.031	0.0026	0.0011
10#	0.60	1.74	0.75	0.014	0.0016	0.15	0.0076	0.036	0.0030	0.0012
11#	0.60	1.67	0.74	0.013	0.0016	0.15	0.0080	0.033	0.0026	0.0012
12#	0.61	1.69	0.75	0.015	0.0017	0.14	0.0080	0.034	0.0027	0.0010
13#	0.59	1.68	0.74	0.012	0.0016	0.15	0.0077	0.034	0.0028	0.0007

Total oxygen

Influence of basicity on T.[O]

The relationship between R and T.[O] of wire rod is shown in Fig. 1. When the Al₂O₃ content was ∼22%, the basicity of refining slag increased from 2.3 to 4.8, while the T.[O] of wire rod ranged from 19 to 7 ppm, which could be explained by the following aspects.

Effect of refining slag basicity on T.[O] in wire rod

The isoactivity lines of SiO₂ and Al₂O₃ and the liquid phase areas of 1600°C in CaO–SiO₂–Al₂O₃–5%MgO quaternary system were plotted; in addition, the positions of slags with four different basicties in the phase were also plotted, as shown in Figs. 2 and 3 (generated by the FactSage version 5.5).

Effect of basicity and CaO/ Al₂O₃ in slag on activity of SiO₂

Effect of basicity and CaO/ Al₂O₃ in slag on activity of Al₂O₃

When the basicity of slag increased, the activity of SiO₂ in slag decreased in Fig. 2, which could reduce the formation of Al₂O₃ inclusions as shown in equation (3)¹¹ (3) (4) As the slag composition came close to the saturated region of CaO in Fig. 3, the activity of Al₂O₃ in slag decreased¹²; therefore, the adsorbability for Al₂O₃ inclusions of slag was improved. In order to avoid the difficulty of 13# slag (R = 4.8, Al₂O₃ = 21.85%) melting, CaF₂ should be added into the ladle to ensure the flowability of slag in production.

The reactions between slag with high basicity and steel may occur as shown in equations (5) and (7), which urged the low melting point changes of inclusions to occur; the more inclusions with low melting points (C₁₂A₇, C₃A)¹³ (where C is CaO and A is Al₂O₃) could form, and these inclusions could float and be removed adequately. (5) (6) (7) As a result, the T.[O] of 60Si2MnA wire rods decreased significantly with the increase of slag basicity.

Influence of Al₂O₃ content on T.[O]

The relationship between Al₂O₃ content and T.[O] of wire rod is shown in Fig. 4. When the refining slag basicity was ∼4.1, the Al₂O₃ content of refining slag increased from 14.7 to 22.3%, since the activity of Al₂O₃ in slag rose and then the adsorbability for Al₂O₃ inclusions of slag went down, and the T.[O] of wire rod increased from 10 to 14 ppm.

Effect of the Al₂O₃ content in slag on T.[O] in wire rod

Inclusions

Influence of basicity on inclusion compositions

When the Al₂O₃ content of refining slag was ∼22%, the inclusions in 1#, 4#, 7# and 13# wire rods (R = 2.3, 3.4, 4.0 and 4.8) were detected respectively. The results showed that when the slag basicity R was 2.3, inclusions in wire rod were mainly Al₂O₃–SiO₂–CaO–MgO quaternary system, while with the increase of basicity (R = 3.4, 4.0 and 4.8), the [Ca] content of steel increased from 16 to 28 ppm (Table 2, CaS formed heavily in steel, and then Al₂O₃–SiO₂–CaO–MgO–CaS quinary inclusions could form largely.

A previous study¹⁴ showed that CaS could mainly be generated in the process of steelmaking by the following two reactions (equations (8) and (10)): (8) (9) (10) (11) During the calculation, the activity of CaS, whose standard state was taken as pure solid, was assumed to be unity. It was assumed that the calcium aluminate inclusions after Ca treatment had mainly changed to liguid 12CaO.7Al₂O₃ inclusions. Bjorkval et al. ¹⁵ calculated the activities of CaO and Al₂O₃ in 12CaO.7Al₂O₃ to be 0.372 and 0.0362 respectively, and the first order interaction coefficients of [Al], [S] and [Ca] in steel were used in the calculation by the studies.^16–17 The calculated stability diagrams of CaS in the case of equations (8) and (10) at 1873 K (refining temperature) and 1823 K (end temperature of refining) are shown in Figs. 5 and 6, indicating that CaS could not form directly by equations (8) and (10) in 1# steel (R = 2.3) at Ca–S and Al–S equilibrium states of refining process, while with the increase of slag basicity (R ≥ 3.4) and the decrease of molten steel temperature ( < 1823 K), CaS could be generated directly by equation (8), and then Al₂O₃–SiO₂–CaO–MgO–CaS quinary inclusions could form accordingly.¹⁸

Ca–S equilibrium curves at different temperatures

Al–S equilibrium curves at different temperatures in C₁₂A₇ phase

Influence of basicity on melting point of inclusions

Fig. 7a and b (generated by FactSage version 5.5) shows the low melting point area ( < 1400°C) in the Al₂O₃–SiO₂–CaO–(10%) MgO quaternary phase of 1# and 4# wire rods (R = 2.3 and 3.4) respectively: when the basicity increased from 2.3 to 3.4, the Al₂O₃ content of Al₂O₃–SiO₂–CaO–MgO quaternary inclusions decreased, and then CaO/Al₂O₃ in inclusions went up. The melting point of most inclusions shifted from 1500 to 1600°C to the low melting area of 1400–1500°C.

a–e composition distribution of inclusions in wire rods refined by different slag basicities

As the basicity increased from 3.4 to 4.8, the SiO₂ content of inclusions decreased gradually; the average SiO₂ contents of inclusions were all < 10%, the MgO content of inclusions rose slightly, and the average MgO contents of inclusions were all ∼13%. Therefore, the SiO₂ and MgO contents were so low, which could hardly affect the melting point of inclusions. Figure 7c–e shows the low melting point area in the Al–S–Ca ternary phases¹⁹ of 4#, 7# and 13# wire rods (R = 3.4, 4.0 and 4.8) respectively. When the basicity went up, as the Al₂O₃ content of Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions went down gradually, CaO/Al₂O₃ in inclusions went up from 1.7 to 3.0, and the average CaS content increased from 27.37 to 48.66%, which made the inclusions deviate from the low melting point area of Al–S–Ca ternary phase gradually. These were caused by the following several aspects: (i) as the T.[O] in steel went down, the oxide inclusions decreased, and the relative percentage of CaS increased; (ii) on another hand, the reaction (equation (8)) could occur because of the high [Ca] content in steel with high slag basicity; (iii) what's more, the CaO/Al₂O₃ in inclusions went up gradually, more calcium aluminate inclusions with higher activity of CaO and lower activity of Al₂O₃(3CaO.Al₂O₃) could be generated, and the sulphur capacity of inclusions rose. Sulphur could be easier to regard 3CaO.Al₂O₃ inclusions as nucleation cores, which may make the reaction (equation (10)) occur possibly, causing the formation of more CaS.²⁰ Therefore, in order to make the Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions come close to the low melting point area in Al–S–Ca ternary phase, the appropriate [Ca] content in steel needs to be controlled strictly so as to ensure the complete degeneration of calcium aluminate inclusions and also to avoid generating too many CaS inclusions of high melting point.

Influence of basicity on void around inclusion

The longitudinal section of wire rod after polishing was eroded by the non-aqueous solution electrolysis method. Therefore, the combination morphology between inclusions and steel matrix refined by different basicities could be observed by SEM, and the inclusion contents were analysed by EDS. The Al₂O₃–SiO₂–MgO–CaO quaternary inclusions of 1# and 4# wire rods (R = 2.3 and 3.4) are shown in Fig. 8a and b ; the Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions of 4#, 7# and 13# wire rods (R = 3.4, 4.0and 4.8) are shown in Fig. 8c–e . The inclusions were all spherical or blocky. Voids were observed between inclusions and steel matrix in Fig. 8a and b , and the average CC of inclusions in the two wire rods were 36.09% and 22.27% respectively; however, there were no obvious voids between inclusions and steel matrix in Fig. 8c–e , and the average CC of inclusions in the three wire rods were 8.2%, 6.1% and 5.4% respectively, which were all < 10%.

a–e morphology of inclusions in wire rods refined by different slag basicities

Considering the above phenomenon, since the expansion coefficient of calcium aluminate inclusions is smaller than that of steel, while the expansion coefficient of CaS inclusion is larger than that of steel, the residual stresses could appear around the Al₂O₃–SiO₂–CaO–MgO–CaS inclusions in the process of heat and solidification, which could prevent the formation of voids between inclusions and steel matrix effectively.²¹ Thus, the tendency of stress concentration around inclusions in spring steel under the periodic alternating stresses could be reduced, which could help to reduce the fatigue fracture of spring steel.²²

In conclusion, according to the results in Fig. 8 and Table 3, the average CC of the same type of inclusions decreased with the increase of CaO/Al₂O₃ and the CaS content of inclusions gradually.

Table 3

Chemical compositions of inclusions in Fig. 8

Inclusion no.	Al₂O₃/%	SiO₂/%	MgO/%	CaO/%	CaS/%
a	37.13	13.72	4.45	44.70	0
b	25.73	19.53	3.86	50.88	0
c	23.39	14.82	4.52	24.39	32.88
d	18.73	9.40	6.27	23.46	42.14
e	7.69	7.06	9.39	20.27	55.59

Influence of Al₂O₃ content on inclusion compositions

When basicity of refining slag was ∼4.1, the inclusions in 7#, 8# and 9# wire rods (the Al₂O₃ content of slags = 22.30%, 14.71% and 18.65%) were detected respectively, and the Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions of 7#, 8# and 9# wire rods are shown in Fig. 9a–c . When the Al₂O₃ content of slag increased, the average Al₂O₃ content of Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions increased from 11.31 to 16.91% and the CaO content also went up accordingly, while CaO/Al₂O₃ in inclusions was kept at ∼1.8 and the average CaS content decreased from 43.80 to 38.07%, thus the inclusion compositions came close to the low point melting area of Al–S–Ca ternary phase. Analysing the mechanism, when the Al₂O₃ content of slag increased, the activity of Al₂O₃ in slag rose and then the adsorbability for Al₂O₃ inclusions of slag went down, and the Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions with high Al₂O₃ content could not float and then remain in steel, which may make the reverse reaction of equation (10) to occur, resulting in the decrease of CaS content of inclusions.

a–c composition distribution of inclusions in wire rods refined by different Al₂O₃ contents of slags

Influence of basicity on number and size of inclusion

When the basicity of refining slag increased from 2.3 to 4.8, the ND of wire rods was reduced from 10.4 to 4.8, the average size of inclusions was reduced from 5.9 to 2.5 μm, and the largest size of inclusions was also reduced from 12.9 to 5.3 μm in Fig. 10a and b . The percentage of inclusions (1–3 μm) went up, and the percentage of large inclusions (>5 μm) went down gradually with the increase of slag basicity in Fig. 10c . Analysing the reasons, when the basicity of slags went up, since the CaO/Al₂O₃ in inclusions rose, the calcium aluminate inclusions could be degenerated completely, and then a lot of large calcium aluminate inclusions with low melting points could float and be removed adequately. Therefore, the inclusions remained mainly changed to Al₂O₃–SiO₂–CaO–MgO–CaS quintuple inclusions with a small and dispersed distribution.

a–c effect of refining slag basicity on number and size of inclusions

Influence of Al₂O₃ content on number and size of inclusion

When the Al₂O₃ content of refining slag increased from 14.71 to 22.30%, the ND of wire rods rose from 5.2 to 6.5, the average size of inclusions rose from 2.8 to 3.6 μm, and the largest size of inclusions also rose from 7.3 to 8.3 μm in Fig. 11a and b . The percentage of inclusions (1–3 μm) went down, and the percentage of large inclusions (>5 μm) went up gradually with the increase of the Al₂O₃ content in Fig. 11c . Analysing the reasons, when the Al₂O₃ content of slags went up, the Al₂O₃ content of inclusions rose accordingly, the degeneration percentage of calcium aluminate inclusions went down in the process of refining, and a small number of large inclusions without complete degeneration could not float and be removed, which made the number and size of inclusions remaining in steel to go up finally.

a–c effect of Al₂O₃ content on number and size of inclusions

Fatigue life

As seen in Figs. 12 and 13, as the basicity of refining slag increased from 2.3 to 3.4, 4.0 and 4.8, the fatigue life of spring steel increased from 5.2 × 10⁴ to 9.8 × 10⁴, 1.7 × 10⁶ and 9.1 × 10⁶ cycles. As the Al₂O₃ content of the refining slag increased from 14.71 to 18.65 and 22.30%, the fatigue life decreased from 8.8 × 10⁶ to 7.5 × 10⁶ and 1.7 × 10⁶ cycles. Therefore the fatigue life of 60Si2MnA spring steel could be improved by the increase of slag basicity or the decrease of Al₂O₃ content in slag, that was to say, reducing the T.[O], the number and size of non-metallic inclusions in steel had a significant impact on improving the fatigue life of spring steel.

Effect of refining slag basicity on fatigue life of spring steel

Effect of Al₂O₃ content in slag on fatigue life of spring steel

Conclusions

(i)

When the Al₂O₃ content of refining slag was ∼22%, the basicity of slag increased from 2.3 to 4.8, the T.[O] of spring steel decreased from 19 to 7ppm; and when the basicity of slag was ∼4.1, the Al₂O₃ content of slag increased from 14.71 to 22.30%, the T.[O] of spring steel increased from 10 to 14ppm.

(ii)

When the basicity of refining slag increased, the [Ca] content in steel went up and the inclusions in steel were transformed from Al₂O₃–SiO₂–CaO–MgO quaternary system to Al₂O₃–SiO₂–CaO–MgO–CaS quinary system, which could reduce the formation of voids between inclusions and steel matrix. Furthermore, thermodynamic calculation results showed that CaS could only form in steel refined by the high slag basicity (R ≥ 3.4).

(iii)

When the basicity of refining slag increased, CaO/Al₂O₃ in inclusions also went up, and Al₂O₃–SiO₂–CaO–MgO quaternary inclusions came close to the compositions of low melting point area, while Al₂O₃–SiO₂–CaO–MgO–CaS quinary inclusions with the increase of CaS content deviated from the compositions of low melting point area.

(iv)

When the Al₂O₃ content of refining slag increased, the Al₂O₃ and CaO content of inclusions both also went up, while CaO/Al₂O₃ in inclusions was still the same, the CaS content went down, and the inclusions came close to the compositions of low melting point area.

(v)

(vi)

(vii)

When the basicity of refining slag increased from 2.3 to 3.4, 4.0 and 4.8, the fatigue life of spring steel increased from 5.2 × 10⁴ to 9.8 × 10⁴, 1.7 × 10⁶ and 9.1 × 10⁶ cycles. While as the Al₂O₃ content of refining slag increased from 14.71% to 18.65% and 22.30%, the fatigue life decreased from 8.8 × 10⁶ to 7.5 × 10⁶ and 1.7 × 10⁶ cycles.

In summary, as the slag basicity increased or the Al₂O₃ content in slag decreased, the T.[O] as well as the number and size of non-metallic inclusions in steel went down significantly. Inclusions were transformed from Al₂O₃–SiO₂–CaO–MgO quaternary system to Al₂O₃–SiO₂–CaO–MgO–CaS quinary system, and the formation of voids between inclusions and steel matrix was reduced. As a result, the cleanness and fatigue life of 60Si2MnA spring steel had been improved by the increase of slag basicity or the decrease of Al₂O₃ content in slag.

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Influence of refining slag composition on cleanness and fatigue life of 60Si2MnA spring steel

Abstract

Introduction

Production process and experimental methodology

Results and discussion

Chemical compositions of refining slags/%

Chemical compositions of tested steels/%

Total oxygen

Influence of basicity on T.[O]

Influence of Al2O3 content on T.[O]

Inclusions

Influence of basicity on inclusion compositions

Influence of basicity on melting point of inclusions

Influence of basicity on void around inclusion

Chemical compositions of inclusions in Fig. 8

Influence of Al2O3 content on inclusion compositions

Influence of basicity on number and size of inclusion

Influence of Al2O3 content on number and size of inclusion

Fatigue life

Conclusions

References

Influence of Al₂O₃ content on T.[O]

Influence of Al₂O₃ content on inclusion compositions

Influence of Al₂O₃ content on number and size of inclusion