Effect of Ce–Mg–Fe Alloy Adding Timing on Formation and Evolution of Inclusions in SCr420H Gear Steel

Abstract

In the present study, the effect of different adding time of Ce–Mg–Fe alloy during the refining process was investigated by laboratory experiments and equilibrium thermodynamic calculation. In experiments, samples from different stages were analyzed by SEM-EDS and EPMA for revealing the evolution mechanism of inclusions. Combined with thermodynamic calculation, results indicated that different adding time can affect the yield of Ce and Mg in steel directly, and the difference of types of inclusions in as-cast samples was caused. CaO-containing inclusions as a main type of inclusions in the refining process can be modified by Ce after adding Ce–Mg–Fe alloy. A high Ce content can promote the formation of Ce–S and Ce–O–S in steel and suppress the precipitation of MnS. In consideration of microalloying effect, Ce–Mg–Fe alloy should be added at a later stage of steelmaking process for ensuring a certain content of Ce and Mg.

1. Introduction

SCr420H steel is widely used in automobile gearbox gears.^1,2) Gear steel should have excellent fatigue resistance to ensure the safety of automobile.³⁾ Improving cleanliness and refining microstructure can contribute to the improvement of fatigue resistance of gear steel. Moreover, machining performance is also a significant property of gear steel, and this is directly related to the processing cost of gears.⁴⁾ Some previous research indicated the control of “DS-type” inclusions (It is mainly CaO-containing inclusion whose size exceeds 13 μm, GB/T-10561-2005 Standard of China) and improving mechanical properties by microalloying effect are the focus of future research on gear steel.^5,6,7,8) Inclusions with a small size would be helpful to prevent the formation of voids and stress concentration at steel-inclusion interfaces. CaO-containing inclusions with large size can be easily formed in SCr420H steel, and these inclusions should be minimized as much as possible during production.

Ce and Mg are commonly used as a strong deoxidizer and inclusion modifier in steelmaking process.^{9,10,11,12,13,14,15,16,17)} With the consistent improvement of steelmaking technology in China, T.O can be decreased to a lower level compared with before and some research began to focus on the effect of Mg and Ce on the microstructure of steel.^{18,19,20,21,22,23)} Both Ce and Mg have excellent ability to refine the microstructure of low alloy steel and it is worthy to study the effect of Ce and Mg on gear steel. However, due to the chemical activity of Ce and Mg, no matter how it is added into steel, it will have a certain modification effect on inclusions, which is a problem that should be clear before studying its effect on microstructure and mechanical properties. A lot of work has been done to ascertain the effect of Ce and Mg on formation and evolution of inclusions.^{24,25,26,27,28,29,30,31,32,33,34)} As an example, Wang et al.²⁷⁾ revealed the modification process of the spinel inclusion with Ce addition in Al-killed steel. Huang et al.³⁴⁾ found that MgAl₂O₄ and CeAlO₃ can act as heterogeneous nucleation cores of multilayer carbonitrides in H13 steel, and Zhang et al.^30,31) discussed the effect of Mg content and addition order between Mg and FeS on the evolution of the inclusions. Lots of previous study showed Ce is an excellent inclusion modifier in steel. During refining process, Ce can be used to modify inclusions, which are especially CaO-containing inclusions. In addition, Ce is one of the sulfide forming elements, so it can be used to affect the precipitation of MnS in SCr420H steel. It is also known that Mg can form MgAl₂O₄ in Al-killed steel, which is a common inclusion with relatively small size and dispersion distribution characteristics.^30,31,33,34) The application of Ce–Mg–Fe alloy in steel has not been considered so far. So it is certainly worthwhile for industrial purposes if the fatigue life of SCr420H steel can be improved through the addition of Ce–Mg–Fe alloy. In the present study, Ce–Mg–Fe alloy was selected to be added in SCr420H steel.

The steelmaking process of SCr420H steel followed the sequential steps through basic oxygen furnace (BOF) → ladle furnace (LF) → refining vacuum degassing (VD) → continuous casting (CC). Figure 1 illustrates the main manufacturing process. As a kind of resulfurized steel, sulfur wire is added in this steel during the late stage of VD process, and this will lead to a large change in the sulfur content of the molten steel. It is necessary to investigate the effect of different Ce–Mg–Fe alloy adding time on formation and evolution of inclusion in steel.

Fig. 1.

The main steelmaking process of SCr420H steel. (Online version in color.)

In the present research, the experiment of adding equal amount of Ce–Mg–Fe alloy at different time was carried out in laboratory. LF and VD processes were simulated on a time scale and adjusting slag is used as the boundary point between two stages. The destination is to reveal the formation and evolution mechanism of inclusion by analyzing inclusions in different samples, and provide reference for application of Ce–Mg–Fe alloy for SCr420H steel in plant.

2. Experimental Methods

2.1. Experimental Procedure

Experiments (Heat A and Heat B) were carried out in a MoSi₂ furnace, as shown in Fig. 2. Ce–Mg–Fe alloy was prepared by the following way. Metal powders of Ce, Mg and Fe were pressed into pieces by a mechanical powder compressing machine with 10 MPa for 10 min loaded to the powders. The whole process was under the protection of argon for preventing from oxidation. The addition of Ce–Mg–Fe alloy (5%Ce-20%Mg-75%Fe) was designed to be 0.5 pct. MgO crucible with 60 mm in inner diameter and 80 mm in depth was used in experiment for simulating the Mg-containing refractory in plant. Slag was prepared by chemical analytical reagents. Chemical powders were heated to 1073 K for 3 h to remove moisture. Then slag was weighed by cumulative method for experiments. For adjusting the basicity of slag, the weighed SiO₂ was put onto the surface of the molten slag by hopper during the refining process. The target composition of S is 0.02 pct. In each heat, about 700 g industrial pure iron, Cr, Mo, Cu and Ni were heated to 1600°C under highly pure Ar atmosphere, and this time was recorded as 0 min. Experiment ended and begin cooling at 73 min. Al deoxidation, alloying (adding C, Si and Mn), adding slag, adjusting slag and adding FeS were carried out at 5 min, 8 min, 13 min, 48 min and 53 min respectively. For heat A, Ce–Mg–Fe alloy was added in steel by quartz tube at 43 min. For heat B, Ce–Mg–Fe alloy was added in steel by quartz tube at 58 min. Table 1 lists the target steel composition. Table 2 lists sampling time. Table 3 lists the component of initial slag and adjusted slag. The addition of alloy and sampling procedures are showed in Fig. 3. The S1–S3 samples were taken by quarts tube sampler and quenched immediately in water. The S4 sample was cut after the ingot, which cooled down in the crucible after experiment.

Fig. 2.

Schematic diagram of experimental equipment (MoSi₂ furnace). (Online version in color.)

Table 1. The target steel composition (mass pct).

C	Si	Mn	Cr	Ni	Mo	Cu	S
0.21	0.30	0.87	1.20	0.15	0.025	0.17	0.02

Table 2. Sampling time in experiment.

	Heat A	Heat B
S1	Before adding Ce–Mg–Fe alloy	Before adjusting slag
S2	Before adjusting slag	Before adding Ce–Mg–Fe alloy
S3	5 min after adding FeS	5 min after adding Ce–Mg–Fe alloy
S4	As-cast	As-cast

Table 3. Composition of refining slag (mass pct).

	CaO	SiO2	Al2O3	MgO	Basicity
Initial slag	55	11	29	5	5
Adjusted slag	44	29	23	4	1.5

Fig. 3.

Addition of alloy and sampling procedure in heat A and B. (Online version in color.)

2.2. Analysis Methods for Steel and Inclusions

The inductively coupled plasma optical emission spectrometer (ICP-OES) was used to analyze the contents of Als, Ca, Ce and Mg. An ARL-4460 direct reading spectrometer was used to analyze the contents of Si, Mn, Cr, Mo, Cu. an infrared C/S analyzer was used to analyze the contents of C and S. A Leco TC500 N/O analyzer was used to analyze the T.O (Total oxygen).

The quenched samples were cut into cylinders (φ6 mm × 25 mm), The as cast samples (10 mm × 10 mm × 10 mm) were cut from the 0.5 h of the cast ingot to get a horizontal cross section. All samples were polished for inclusion analysis. An electron probe X-ray microanalysis (EPMA) and a scanning electron microscope (SEM, Zeiss Ultra Plus) with energy dispersive spectrometer (EDS) were applied to measure the morphology, size, number, and composition of non-metallic inclusions in steel.

3. Results and Discussions

3.1. Chemical Compositions of Steels

The measured contents of C, Si, Mn, Cr, Ni, Als, Ca, Mg, Ce, S and T.O are listed in Table 4. The results indicate that the main composition of steel in two heats was at the same level, but the contents of Ce, Mg and S were different. Heat A contains 0.0002 pct Mg, 0.003 pct Ce and 0.02 pct S, while heat B contains 0.0011 pct Mg, 0.01 pct Ce and 0.015 pct S.

Table 4. Measured contents of steels (mass pct).

Heat	C	Si	Mn	Cr	Ni	Als	Ca	Mg	Ce	S	T.O
A	0.209	0.332	0.862	1.174	0.148	0.023	0.0020	0.0002	0.003	0.020	0.0033
B	0.214	0.344	0.858	1.175	0.145	0.018	0.0025	0.0011	0.01	0.015	0.0037

3.2. Inclusions in Steels

Types of inclusions were detected in steel sample of two heats, and the details are summarized in Table 5. The morphology of inclusions and the elemental mappings of the typical composite inclusions are given in Tables 6, 7, Figs. 4 and 5. It should be pointed out that there was a change in content of SiO₂ in inclusion with the effect of adjusting slag, and it is not the focus of the present study.

Table 5. Types of inclusions at different stages.

Types of inclusions	Heat A				Heat B
	S1	S2	S3	S4	S1	S2	S3	S4
MgO	−	−	−	−	−	−	−	√
xCaO∙yAl2O3	√	−	−	−	√	√	−	−
Ca–Mg–Al–O	√√	−	−	−	√√	√√	−	−
MnS	−	−	√√	√√	−	√√	√√	√√
CaS	−	−	−	√	−	−	−	√
Ce–S/Ce–O–S	−	−	−	−	−	−	−	√
Ce–Ca–Mg–Al–O–(Si)	−	√√	√	√	−	−	√√	√
Ce–Ca–Al–O–(Si)	−	√	√	−	−	−	√	−
Ce–Mg–Al–O	−	−	√	√	−	−	√	√
MgAl2O4	√	√	√	√√	√	√	√√	√√
CeAlO3	−	√	√√	√√	−	−	√	√√

√√: main type of inclusions, √: a small amount of inclusions

Table 6. Types and morphology of inclusions at different stages in heat A.

Table 7. Types and morphology of inclusions at different stages in heat B.

Fig. 4.

The elemental mappings of the typical composite inclusions (Heat A, S4). (a) inclusion with a three layer structure (b) inclusion with a double layer structure. (Online version in color.)

Fig. 5.

The elemental mappings of the typical composite inclusions (Heat B, S4). (a) inclusion with a three layer structure (b) inclusion with a double layer structure. (Online version in color.)

For each heat, types of inclusions in S1 are common inclusions in Al-killed steel. Ca–Mg–Al–O inclusion is the main inclusion in S1 and MgAl₂O₄ can be detected in all samples. In S2 of heat A, calcium - aluminate inclusions were modified into Ce-containing aluminate inclusions and CeAlO₃ formed in steel. The morphology of these inclusions is circular and uniform. In S3 of heat A, after adding of FeS for 5 min, a large number of MnS were detected in samples, and the size was about 1–5 μm. In addition, Some CeAlO₃ and MgAl₂O₄ can also be found in this sample, and the amount of inclusion containing Ca decreased. In S4 of heat A, the main types inclusion are CeAlO₃, MgAl₂O₄ and MnS. Especially some MnS inclusions exceed 30 μm in size and (Ca,Mn)S were found, in addition, a little amount of Ca-containing inclusions can be still detected in this sample.

In S2 of heat B, lots of MnS were detected in samples after adding of FeS for 5 min. After the adding of Ce–Mg–Fe alloy, Ce-containing aluminate inclusions can be detected in S3 of heat B and calcium - aluminate inclusions without Ce can not be found. In S4 of heat B, the main types of inclusion are CeAlO₃, MgAl₂O₄, MnS. What’s more special than heat A is that MgO, Ce–S and Ce–O–S were detected in this sample.

Figure 4 displays element mapping of the typical composite inclusion in Heat A, S4. Inclusion (a) has three layers. The core is MgAl₂O₄, covered with Ce–Ca–Al–O inclusion (xCaO∙yAl₂O₃ and CeAlO₃), and the outermost layer is (Ca, Mn)S. Inclusion (b) has a double layer structure, whose core is MgAl₂O₄ and it is covered with MnS.

Figure 5 displays the typical composite inclusion in Heat B, S4. Inclusion (a) has three layers. The core is MgO and MgAl₂O₄, covered with a few Ce₂O₃ and Ca₂SiO₄, and the outermost layer contain MnS, CaS and Ce–S. Inclusion (b) has two layers. The core is MgO, and the outer layer is Ce–Ca–Al–O–S inclusion.

3.3. Equilibrium Thermodynamic Calculation

Thermodynamic calculation is a significant implement to analyze and predict the formation and evolution of inclusions in steel. In the present study, FactSage^TM 7.2 software was used to carry out the equilibrium thermodynamic calculation. The equilibrium products under this experimental system were obtained by phase diagram. Calculation database were FactPS and FSstel.

Figure 6 presents the stability diagram for Mn–O–Ca–S–Al–Ce–Mg–Fe system at 1873 K. This calculation was carried out in the phase diagram module, and calculation conditions are listed above the diagram. Due to limitation of calculation ability, some elements are neglected. Ce content and Mg content of heat A and heat B were placed in the diagram (The red solid circle represents heat A and the red star represents heat B). Equilibrium products corresponding to the composition of heat A at 1873 K are CeAlO₃, CaAl₂O₄ and CaS while products for heat B are CeAlO₃, MgO and CaS. Different adding time of Ce–Mg–Fe alloy caused the difference of the contents of Mg and Ce, and lead to the difference of equilibrium product directly. The destination of inclusion evolution in the molten steel between heat A and B are different theoretically.

Fig. 6.

Stability diagram for Mn–O–Ca–S–Al–Ce–Mg–Fe system at 1873 K. (Online version in color.)

Figure 7 shows equilibrium precipitation of inclusions for 0.20C-0.30Si-1.20Cr-0.87Mn-0.0035O-0.002Ca-0.02S-0.02Al-0.0030Ce-0.0002Mg-Fe system of heat A from 1673 K to 1873 K. This calculation was carried out in the equilib module. The liquidus and solidus temperature were presented by red dashed line in the diagram. Equilibrium products at different temperature under the composition of heat A were all plotted with different color in this diagram. Before solidification, CeAlO₃, Ca₂SiO₄ and Ca₂Al₂SiO₇ can precipitate in the molten steel. As cooling process promoted, MgAl₂O₄ and CaAl₄O₇ precipitated. During solidification, Ca₂Mg₂Al₂₈O₄₆, CaMg₂Al₁₆O₂₇, and a large amount of MnS can precipitate in steel. For heat A, the content of Ce is not enough to replace Ca in inclusions. The xCaO∙yAl₂O₃ inclusion is still stable in steel, and Ce and Mg can not affect formation of MnS and CaS.

Fig. 7.

Equilibrium precipitation of inclusions for C–Si–Cr–Mn–O–Ca–S–Al–Ce–Mg–Fe system from 1673 K to 1873 K (Heat A). (Online version in color.)

Figure 8 presents equilibrium precipitation of inclusions for 0.20C-0.30Si-1.20Cr-0.87Mn-0.0035O-0.002Ca-0.015S-0.02Al-0.01Ce-0.0011Mg-Fe system of heat B from 1673 K to 1873 K. Before solidification begins, CeAlO₃, CaS and MgO can precipitate in the molten steel. Before solidification, Ce₂O₂S precipitated in steel. During solidification, Ce₂S₃, MgAl₂O₄, and MnS can precipitate in steel. For heat B, xCaO∙yAl₂O₃ inclusion can not precipitate in steel theoretically. Under the influence of high yield of Ce and Mg, the precipitation quantity of CeAlO₃ increased markedly than heat A, which leads to the formation of Ce₂S₃ and Ce₂O₂S in steel and suppress the precipitation of MnS.

Fig. 8.

Equilibrium precipitation of inclusions for C–Si–Cr–Mn–O–Ca–S–Al–Ce–Mg–Fe system from 1673 K to 1873 K (Heat B). (Online version in color.)

Combined Tables 6 and 7 with Figs. 6, 7, 8 for comparison, the consistency and difference were summarized as follows: (i) In experiment, Ce can be detected in most oxide inclusions after adding Ce–Mg–Fe alloy in steel, and CeAlO₃ was found in S4 sample of two heats. Figure 6 shows that CeAlO₃ is one of the equilibrium products for two heats. (ii) In experiment, MgAl₂O₄ can be found in all samples, and Fig. 6 shows that it is not an equilibrium product in 1873 K, but can only precipitate during cooling. This is consistent with the calculation of Huang et al.:³⁴⁾ with the increase of Ce content in the molten steel, MgAl₂O₄ in the equilibrium product in 1873 K is replaced by CeAlO₃. (iii) According to Figs. 6 and 8, under equilibrium conditions, the existence of Ce prevents the formation of CaO-containing inclusions in heat B. However, CaO-containing inclusions still existed in S4 sample of heat B as shown in Table 7. (iv) The formation of Ce–S inclusion in S4 sample of heat B as shown in the Table 7 corresponds to the precipitation of Ce₂S₃ during solidification as presented in Fig. 8.

3.4. Formation and Evolution Mechanism of Inclusions

Based on the experimental results and thermodynamic analysis, the evolution mechanism of inclusions with different adding time was illustrated in Fig. 9. The yield of Ce and Mg of heat A is much smaller than that of heat B. For heat A, Ce–Mg–Fe alloy was added before FeS addition, and the addition time is earlier than heat B. At this time, the cleanliness of the molten steel is worse than that of heat B, and inclusions containing Mg and Ce have enough time to float into slag. In addition, the solubility of Mg in the molten steel at 1873 K is very small, and Mg can form bubbles to escape the molten steel due to the difference of vapor pressure.^9,31,35) These factors lead to the difference of yield rate and further cause different inclusions in steel.

Fig. 9.

The evolution mechanism of main inclusions in the present study. (Online version in color.)

According to the equilibrium calculation (Figs. 6, 7, 8), the difference of Ce and Mg content between heat A and heat B plays an important role on types of inclusions. This is in general agreement with the experimental results. Sulfur-containing Ce inclusions and MgO were found in heat B, but not in heat A. A high Ce content in steel can lead to the reaction between Ce and S during cooling process, this phenomenon indicates Ce can suppress the formation of MnS. In particular, the mutual solution formed between MnS and Ce–S presented the possibility of Ce control of the morphology of MnS.

After adding Ce–Mg–Fe alloy, regardless of the difference in adding time, a remarkable phenomenon is that Ce can be detected in CaO-containing inclusions, and lots of CeAlO₃ inclusions formed in steel. CaO-containing inclusions as a main type of inclusions in steel before adding Ce–Mg–Fe alloy were modified by Ce, and CeAlO₃ was precipitated in inclusions. The modification reaction was showed as Eq. (1), and this inclusion can also form directly by the reaction of solute element in the molten steel as showed in Eq. (2). In the molten steel, CeAlO₃ form directly in steel by reaction between Ce, Al and O, as shown in Eq. (3).   

$$\left[\mathrm{C}\mathrm{e}\right]+\mathrm{x}\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{y}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3=\mathrm{C}\mathrm{e}\mathrm{A}\mathrm{l}{\mathrm{O}}_3\cdot \mathrm{z}\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{h}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+\left[\mathrm{C}\mathrm{a}\right]$$(1)

  

$$\left[\mathrm{C}\mathrm{e}\right]+\mathrm{x}\left[\mathrm{C}\mathrm{a}\right]+\mathrm{y}\left[\mathrm{A}\mathrm{l}\right]+\mathrm{z}\left[\mathrm{O}\right]=\mathrm{C}\mathrm{e}\mathrm{A}\mathrm{l}{\mathrm{O}}_3\cdot \mathrm{h}\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{l}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$$(2)

  

$$\left[\mathrm{C}\mathrm{e}\right]+\left[\mathrm{A}\mathrm{l}\right]+3\left[\mathrm{O}\right]=\mathrm{C}\mathrm{e}\mathrm{A}\mathrm{l}{\mathrm{O}}_3$$(3)

Significantly, based on the results of the equilibrium calculation (Figs. 6 and 8), CaO-containing inclusions is not an equilibrium product in heat B. However, this types of inclusion is also detected in as cast sample. This phenomenon indicates that Ce can not completely replace Ca in inclusions. Despite a high yield of Ce–Mg–Fe alloy acquired in heat B, the reaction time left to inclusion modification after the addition of Ce–Mg–Fe alloy is limited, which limits the promotion of modification reaction, and preserve the composition of Ca in inclusions.

In the present experiment, Mg-containing inclusion is mainly MgO and MgAl₂O₄. Refractory, slag and Ce–Mg–Fe alloy are the origin of Mg. Before the addition of Ce–Mg–Fe alloy, under the effect of refractory and the MgO in slag, MgAl₂O₄ formed in the molten steel. After the addition of Ce–Mg–Fe alloy, [Ce] and [Mg] increase suddenly in steel. According to the results of thermodynamic calculation, CeAlO₃ was considered to be more stable than MgAl₂O₄ in 1873 K, but MgAl₂O₄ can precipitate during cooling process, and in the experiment, the transfer of Mg from refractory and slag persisted during the later stage, so in as cast samples, a lot of inclusions containing CeAlO₃ and MgAl₂O₄ can be discovered in large quantities as shown in Fig. 10.

Fig. 10.

A type of inclusion containing CeAlO₃ and MgAl₂O₄ (Online version in color.)

4. Conclusions

(1) Different adding time of Ce–Mg–Fe alloy can affect directly the yield of Ce and Mg in steel. This is related to the cleanliness of molten steel, the escape of magnesium vapor, and the floating of inclusions. Ce–Mg–Fe alloy should be added at a later stage for ensuring a certain content of Ce and Mg in steel.

(2) CaO-containing inclusions as a main type of inclusions in steel can be modified by Ce after adding Ce–Mg–Fe alloy. A high Ce content can promote the formation of Ce–S and Ce–O–S in steel and suppress the precipitation of MnS. The effect of Ce–Mg–Fe alloy on morphology control of MnS inclusions has potential research value in the future.

(3) Under this experimental condition, a later adding time of Ce–Mg–Fe alloy can limit the modification of inclusion, and cause the difference between equilibrium thermodynamics and experimental results.

5. Future Work

It should be noted that much work needs to be done in future. The Ce content in the molten steel has a great influence on the type of inclusions. Based on the results of the present study, a higher amount of Ce–Mg–Fe alloy addition in steel should be further investigated. Significantly, when Ce content in steel is too high, excessive formation of inclusions would occur, and the performances of steels would be deteriorated. The optimum addition of Ce–Mg–Fe alloy should be determined so as to make CaO-containing inclusions just be modified completely. Further, an optimum addition prediction model should be established by coupling experimental research and thermodynamics to guide industrial application.

Acknowledgements

This work was supported by National Key Research and Development Program of China (Grant Nos. 2016YFB0300105), the Transformation Project of Major Scientific and Technological Achievements in Shenyang (Grant Number. Z17-5-003), and the Fundamental Research Funds for the Central Universities (N172507002).

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