Effect of Top Slag with Low Basicity on Transformation Control of Inclusions in Spring Steel Deoxidized by Si and Mn

Abstract

In order to study the effect of top slag with low basicity on transformation control of inclusions in spring steel deoxidized by Si and Mn, the optimum composition of CaO–SiO₂–Al₂O₃–MgO slag system with low basicity was firstly calculated by FactSage. Then the industrial experiments were carried out based on the results of calculation. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to determine the morphology, size and composition of inclusions in spring steel. It is proved that MnS, CaO–SiO₂–Al₂O₃–MgO and CaO–SiO₂–Al₂O₃–MgO–MnS system inclusions are three major nonmetallic inclusions in spring steel, and the transformation of inclusion compositions to plastic deformation can be achieved by controlling the binary basicity (CaO/SiO₂ by mass percent) at the range of 1.0 to 1.19 and C/A (C/A = CaO/Al₂O₃ by mass percent) above 9 in top slag. In addition, along with the refining process, the average oxygen content in molten steel and the alumina content in inclusions both decrease, and the number of inclusions greater than 5 um gradually decreases. Finally, the inclusions in spring wire rod are controlled effectively.

1. Introduction

Spring steel is used as a valve spring wire with an automobile engine, and their good mechanical properties, such as high strength, fatigue resistance and impact resistance, are demanded. It has been proven that nonmetallic inclusions as crack initiation have a great effect on the in-service life of spring wire.^1,2,3) As a result, inclusions with small size, little quantity, and good plasticity are preferred. Some researches indicate that the major factor influencing the plastic deformation of inclusions is the melting point, which has a close relationship with the chemical composition.^4,5,6) Therefore, inclusions with high melting point and high hardness, such as alumina, should be avoided, and silicon-manganese composite deoxidization should be used in the production of spring wire.

Top slag refining is a widely used technology to control inclusions in high-carbon spring steel production.^7,8) The basic principle of this method is to control the contents of strong deoxidization elements in liquid steel through the steel-slag reaction, such as acid-soluble aluminum ([Al]), magnesium ([Mg]) and calcium ([Ca]), which affect the composition of inclusions in turn through the chemical reaction of liquid steel-inclusions. Several thermodynamic calculations,^{9,10,11,12,13,14,15,16,17,18,19)} based on the activity of slag and interaction parameters of alloying elements (such as Mn, Si, Al and O) in steel, have been conducted in order to help control inclusions. Jiang et al.⁷⁾ discussed the evolution mechanisms of non-metallic inclusions in high strength alloyed steel refined by high basicity slag and deoxidized by Al in laboratory. Relations among inclusions, steel and top slag in spring steel were thermodynamically studied by Suito and Inoue.⁹⁾ Xue et al.¹⁰⁾ investigated the relationship between the activity of aluminum and calcium in molten steel and compositions of oxide inclusion precipitated at different temperatures spring steel. However, there is a shortage of report on the composition transform of inclusions in spring steel during industrial production. A systematic research is needed to elucidate the formation of inclusions in spring steel and establish the thermodynamic equilibrium between the steel and top slag. Furthermore, the studies of effect of CaO–SiO₂–Al₂O₃–MgO slag system on inclusions in spring steel deoxidized by Mn and Si are few. It has been found in industrial practice that the composition of spring steel can be controlled in the target range in many steel plants, but the problem is that the inclusions are often big in size and difficult to be transformed during the rolling process.

The study firstly chose the optimum composition of CaO–SiO₂–Al₂O₃–MgO slag system with FactSage 6.2 FT oxide Database and Phase Diagram Module. Then, the industrial experiments were carried out and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) was applied to obtain inclusion information. Finally, the slag-steel interaction, steel-inclusion interaction and transformation mechanism of inclusions were discussed.

2. Thermodynamic Description of Top Slag Refining

2.1. Steel-Slag Reactions

When top slag with low basicity and low alumina content is added during spring steel refining, the reaction takes place at the steel-slag interface as shown in reaction (1).   

$${\text{(M}}_1{\text{O}}_y)+{\text{[M}}_{\text{2}}\text{]}\to {\text{[M}}_{\text{1}}\text{]}+({\text{M}}_2{\text{O}}_y)$$(1)

Where [M] stands for components in liquid steel, and (M_xO_y) stands for that in slag. It can be seen that the [Ca], [Mg] and [Al] contents in steel would be controlled by the steel-slag reaction. As a result, a new equilibrium of steel-slag reaction should be built with the change of chemical composition of top slag.

2.2. Chemical Reactions between Liquid Steel and Inclusions

The deoxidization reaction may take place in liquid steel, as shown in reaction (2), meaning that M_xO_y content in inclusions mainly depends on the deoxidization reaction. Reaction (3) shows that the inclusion composition was affected not only by M₁ but also by M₂.   

$$x\text{[M]}+y\text{[O]}={\text{M}}_x{\text{O}}_{\text{y(}inc\text{)}}$$(2)

  

$${\text{M}}_1{\text{O}}_{y(inc)}+{\text{[M}}_{\text{2}}\text{]}\to {\text{[M}}_{\text{1}}\text{]}+{\text{M}}_2{\text{O}}_{y(inc)}$$(3)

Where M_xO_y(inc) stands for components in inclusions.

3. Optimum Composition of CaO–SiO₂–Al₂O₃–MgO Slag System

In order to obtain the optimum contents of SiO₂, CaO, Al₂O₃ and MgO, the optimum CaO–SiO₂–Al₂O₃–MgO slag system must be fixed, which are determined by the size of liquid phase area projection of CaO–SiO₂–Al₂O₃–MgO slag system.

Figure 1 is the optimum area change of CaO–SiO₂–Al₂O₃–MgO slag system with different contents of MgO (MgO/(CaO+SiO₂+Al₂O₃) by mass percent, in calculation 3% MgO, 5% MgO, 7% MgO, 9% MgO, 12% MgO and 15% MgO is given respectively), which varies from 1400°C to 1600°C and is calculated by FactSage software. The area of less than 1400°C in the CaO–SiO₂–Al₂O₃–MgO slag system are defined as “low melting temperature area”. It can be seen from Fig. 1 that with the change of the mass percent of MgO, the low melting temperature area changes. Considering the control of oxygen, aluminum and sulfur contents in spring steel, the basicity R (R = CaO/SiO₂ by mass percent) should be well controlled above 1.0, C/A (C/A = CaO/Al₂O₃ by mass percent) above 9 in top slag. The low melting temperature area of the basicity R ≥ 1 and C/A ≥ 9 is defined as “optimum area”, and the size of the optimum area is regarded as the criterion of determining slag compositions. The range values coming from the optimum area are defined as “optimum composition”. The greater the size of the optimum area is, the lager the range of optimum composition is.

Fig. 1.

Change of optimum areas of CaO–SiO₂–Al₂O₃–MgO slag system with different MgO contents. (Online version in color.)

It can be seen from Fig. 1, with the increase of MgO content varying from 3% to 15%, the size of optimum area firstly increases and then reduces. When the mass percent of MgO is 7%, the size of optimum area reaches the maximum. When the contents of MgO are 12% and 15%, there is no optimum area. Therefore, for the CaO–SiO₂–Al₂O₃–MgO refining slag system, the mass percent of MgO should be controlled around 7%. From Fig. 1(c) the optimum composition of the CaO–SiO₂–Al₂O₃–MgO slag system with 7% MgO (hereafter called CaO–SiO₂–Al₂O₃ –7%MgO) can be obtained as showed in Table 1, and it can be deduced that in the optimum composition the basicity is at the range of 1.0 to 1.19 and C/A is above 9.0.

Table 1. Optimum composition in the CaO–SiO₂–Al₂O₃–7%MgO slag system (wt%).

CaO	SiO2	Al2O3	MgO
47.4–50.2	41.9–45.6	0–2.79	6–8

4. Industrial Experiments

100 t EAF (Electric Arc Furnace) - LF (Ladle Furnace) - VD (Vacuum Degassing Furnace) - soft argon blowing - CC (Continuous Casting) process was used in the production of spring steel in a steel company. After oxygen blowing, active lime, CaF₂ and smokeless recarburization agent were added into EAF, low aluminum ferrosilicon and ferromanganese were added into steel ladle when molten steel was tapped out of EAF, a certain special refining slag shown in Table 1 was also added into the steel ladle to deoxidize and make the synthetic slag be washed. When the ladle was carried into LF station, the first alloy and slag charges including active lime, SiC and CaC₂ were added into the ladle. When electrical heating for about 10 min, the special refining slag was added into the ladle furnace, and then the second SiC and CaC₂ were added for the diffusion deoxidization after the new added slags melted. When top slag refining at LF was for about 50 min, the ladle was carried into VD station. Vacuum treatment at VD station was carried out for more than 15 min, while the vacuum below 300 Pa was conducted for at least 10 min. After vacuum treatment, soft argon blowing lasted for nearly 30 min, and the total steel-slag reaction time from LF to soft argon blowing reached 80 min. Special slag was added into the tundish as the cover agent, and the protective casting was employed during all the continuous casting process.

Concrete sampling points are at beginning of LF, middle stage of LF, end point of LF, after VD, after soft bottom-blown by argon bubbles, during the tundish at the stage of 30 tons of remaining molten steel in ladle, continuously cast bloom, wire rod abbreviated as LF1, LF2, LF3, VD1, VD2, ZB, ZP and CP in following discussion. Chemical compositions of steel and slag samples were analyzed at NACIS (National Analysis Center of Iron & Steel). Number and size distribution of inclusions were studied by Leica optical microscope. Morphology and composition of inclusions were investigated by SEM-EDS.

5. Results

5.1. Achievement of T.[O] (total oxygen) Control in Steel

Figure 2 presents the change of T.[O] through the whole process. As shown by Fig. 2, T.[O] rapidly decreases from LF1 to LF2 after adding low aluminum ferrosilicon, ferromanganese, SiC and CaC₂ because of precipitation deoxidization and diffusion deoxidization in top slag. The rate of deoxidization begins to be slower from LF2 to LF3 due to diffusion deoxidization playing a main role. After the vacuum treatment T.[O] further reduce by reason of deoxidization reactions such as carbon, aluminum, silicon, manganese and oxygen. It can also be seen from Fig. 2 that from VD2 to ZP, T.[O] stabilizes at the range of 0.0008%–0.001%, showing good shielded casting.

Fig. 2.

Change of T.[O] content through the whole process. (Online version in color.)

5.2. Composition, Morphology and Size of Typical Inclusions in Steel

Figure 3 shows three kinds of inclusions in casting billet. MnS (sulfide), CaO–SiO₂–Al₂O₃ –MgO (compound oxide) and CaO–SiO₂–Al₂O₃–MgO–MnS (compound oxide and sulfide) inclusions are three major nonmetallic inclusions in casting billet of spring steel. It is found from Fig. 3(c) that the shell of CaO–SiO₂–Al₂O₃–MgO–MnS inclusions is sulfide and the core is oxide. The morphology of CaO–SiO₂–Al₂O₃–MgO inclusions in steel at different stages is shown in Fig. 4. It is found that CaO–SiO₂–Al₂O₃–MgO inclusions are all spherical, the size of which firstly getting smaller from LF1 to LF3, and suddenly increase at the VD1 point, then became smaller again from VD1 to ZP. Finally, inclusions in the wire rod are mostly compressed and elongated along with the steel matrix.

Fig. 3.

Three kinds of typical inclusions in casting billet. a) MnS; b) CaO–SiO₂–Al₂O₃–MgO; c) CaO–SiO₂–Al₂O₃–MgO–MnS. (Online version in color.)

Fig. 4.

Morphology of CaO–SiO₂–Al₂O₃–MgO inclusions in steel through the whole process. (Online version in color.)

5.3. Number of Typical Inclusions in Steel

Figure 5 shows the change of the number of three typical inclusions greater than 5 um through the whole process. As shown in Fig. 5, the inclusion number is inclined to decrease generally, especially obvious from the LF2 to LF3 point. The average of the inclusion number in continuously cast bloom of four heats is lowered to 0.11/mm².

Fig. 5.

Change of inclusion number through the whole process.

5.4. Composition Transformation of Typical Inclusions in Steel

Because MnS is soft and easy to elongate with the elongation of the steel matrix, oxides surrounded by manganese sulfide are also easy to be deformed. As an independent monomer inclusion, the composition and plasticity of the manganese sulfide system inclusion are basically unchanged. Therefore, the composition transformation changes will be discussed only for oxide compound inclusions in this paper.

Transformation of average composition of the compound oxides during different stages is shown in Fig. 6. The compositions of the inclusions in steel are primarily CaO_(inc): 6.2%, SiO_2(inc): 80.7%, Al₂O_3(inc): 11.5% and MgO_(inc): 2.2% at LF1 sampling point. CaO_(inc) and MgO_(inc) contents in inclusions increase gradually, while the SiO_2(inc) content decreases gradually at LF2 point. At the LF3 point, the content of CaO_(inc) in inclusions is keeping increasing gradually, while the SiO_2(inc) content reaches the lowest level and Al₂O₃ and MgO a bit decreased. The compositions of inclusions are CaO_(inc): 43.1%, SiO_2(inc): 37.8%, Al₂O_3(inc): 8.2% and MgO_(inc): 9.8%, respectively. The CaO_(inc) content reaches the highest level and the SiO_2(inc) content increases a little at the VD vacuum treatment (VD1). The mass percents of CaO_(inc), SiO_2(inc), Al₂O_3(inc) and MgO_(inc) in inclusions are respectively 45.3%, 38.1%, 7.6% and 9%. After the soft argon blowing process (VD2), CaO_(inc) content slightly decreases while SiO₂ content increases gradually. The mass percents of CaO_(inc), SiO_2(inc), Al₂O_3(inc) and MgO_(inc) in inclusions are respectively 42.6%, 39.3%, 8% and 10.4%. The compositions of inclusions in steel are basically in the low melting temperature area.

Fig. 6.

Transformation of average composition of oxide compound inclusions during different stages.

6. Discussion

6.1. Slag-Steel Interaction

Figure 7 shows the change of (CaO), (Al₂O₃) and (MgO) contents in top slag. It is shown by Fig. 7 that (CaO) stabilizes at about 49% before LF2 process, and then declines to 43% after VD2. This is because active lime was added before LF2 process, then (CaO) in slag was consumed by the steel-slag reactions such as reaction (4) and (5). (MgO) keeps general rising trend in LF-VD process from 7% to 9%. (MgO) is supposed to decline if the reaction shown as reaction (6) is only considered, but actual data rises because MgO in furnace lining enters slag, and refining slag contains MgO. The (Al₂O₃) content presents slight rising trend in LF process, then rises suddenly to 3.4% after VD as a result of three factors: (i) Al₂O₃, the product of the reactions shown as reaction (4) and reaction (6) enters into the slag; (ii) inclusions are absorbed by slag.   

$$\text{3(CaO)}+2\text{[Al]}=3\text{[Ca]}+3({\text{Al}}_{\text{2}}{\text{O}}_3)$$(4)

  

$$\text{2(CaO)}+\text{[Si]}=2\text{[Ca]}+({\text{SiO}}_2)$$(5)

  

$$\text{3(MgO)}+2\text{[Al]}=3\text{[Mg]}+3({\text{Al}}_{\text{2}}{\text{O}}_3)$$(6)

Fig. 7.

Change of (CaO), (Al₂O₃) and (MgO) contents in the top slag. (Online version in color.)

Figure 8 shows the change of [Ca], [Mg], [Al] contents in steel. If the reactions shown as reaction (4) and reaction (6) are only considered, [Al] is supposed to decline. However, it is shown by Fig. 8 that the content of [Al] rises to 0.002% at LF3, then declines to 0.0016% at VD2, because the low aluminum ferrosilicon was added into slag for adjusting the composition of molten steel in early stage of LF process, leading to the increase of [Al] in steel, much more than the amount of [Al] consuming by steel-slag reactions and steel-inclusion reactions. The contents of [Mg] and [Ca] in steel rise to 0.00044% at LF3 and 0.00143% at LF2 respectively, because [Mg] and [Ca] generated by the steel-slag reactions enters into the molten steel. After that, [Mg], [Ca] in liquid steel, respectively, decline to 0.00038% and 0.00077% at VD1 because of steel-inclusion reactions and vaporization.

Fig. 8.

Change of [Ca], [Al] and [Mg] contents in the molten steel. (Online version in color.)

6.2. Transformation Mechanism of Inclusions

Figure 9 is the composition transformation of oxide compound inclusions during different stages in ternary phase diagram, in which “black hollow symbols” are compositions of each inclusion during each stage in ternary phase diagram and “red solid symbols” are average compositions of total inclusions during each stage.

Fig. 9.

Composition transformation of oxide compound inclusions during different stages in ternary phase diagram. (Online version in color.)

(1) LF1 sampling point

It is seen from Fig. 9(a) that at LF1 sampling point the main component of inclusions is SiO_2(inc) with a small amount of Al₂O_3(inc), CaO_(inc) and MgO_(inc). The average contents of inclusions are SiO_2(inc): 80.7%, Al₂O_3(inc): 11.5%, CaO_(inc): 6.2% and MgO_(inc): 2.2%. This is mainly because inclusions generated by composite deoxidization transformed under the action of the steel-slag reaction and steel-inclusion reaction. The specific processes are as follows:

When molten steel was tapped out of EAF, low aluminum ferrosilicon, ferromanganese, active lime and refining slag were added into steel ladle. During this process, the chemical reactions in liquid steel are mainly deoxidization reactions, as shown in reaction (7)–(9), which generate a large amount of SiO_2(inc) and MnO_(inc) inclusions and a little of Al₂O_3(inc) inclusions.   

$$\text{[Si]+2[O]}={\text{SiO}}_{\text{2(}inc\text{)}}$$(7)

  

$$\text{[Mn]+[O]}={\text{MnO}}_{\text{(}inc\text{)}}$$(8)

  

$$\text{2[Al]+3[O]}={\text{Al}}_{\text{2}}{\text{O}}_{\text{3(}inc\text{)}}$$(9)

As the steel-slag reactions progress, [Mg] would be reduced from slag by chemical reaction (6). With a very low level of Fe_tO content in slag, [Ca] could be reduced from slag by reaction (4) and (5). With the steel-inclusion reactions progress, MnO_(inc), SiO_2(inc) and Al₂O_3(inc) in inclusions react with [Si], [Al], [Ca] and [Mg] in molten steel, as shown in reaction (10)–(14), the MnO_(inc) component in inclusions drops to a very low level, while the CaO_(inc) and MgO_(inc) components in inclusions increase gradually. Because the [Ca] and [Mg] contents are a little, the contents of CaO_(inc) and MgO_(inc) in inclusions are a small quantity. Therefore, at LF1 sampling point, there are SiO_2(inc), Al₂O_3(inc), CaO_(inc) and MgO_(inc) components in inclusions, in which contain a large amount of SiO_2(inc) and a little of Al₂O_3(inc), CaO_(inc) and MgO_(inc).   

$$2{\text{MnO}}_{\text{(}inc)}+\text{[Si]}={\text{SiO}}_{\text{2}(inc)}+2\text{[Mn]}$$(10)

  

$${\text{3SiO}}_{\text{2}(inc)}+4\text{[Al]}=2{\text{Al}}_{\text{2}}{\text{O}}_{3(inc)}+3\text{[Si]}$$(11)

  

$${\text{SiO}}_{\text{2}(inc)}+2\text{[Ca]}=2{\text{CaO}}_{(inc)}+\text{[Si]}$$(12)

  

$${\text{Al}}_{\text{2}}{\text{O}}_{3(inc)}+3\text{[Ca]}=3{\text{CaO}}_{(inc)}+2\text{[Al]}$$(13)

  

$${\text{Al}}_{\text{2}}{\text{O}}_{3(inc)}+3\text{[Mg]}=3{\text{MgO}}_{(inc)}+2\text{[Al]}$$(14)

(2) LF2 sampling point

When the ladle is carried into LF station, the alloy and slag charges including active lime, SiC and CaC₂ are added into the ladle again. The steel-slag reactions (4)–(6) continue toward the positive direction. In addition, a portion of [Mg] comes from the reaction between refractory and liquid steel. With the time gone the [Ca] and [Mg] contents in molten steel increase, which results in the reactions (12)–(14) continuing. Therefore, the contents of CaO_(inc) and MgO_(inc) in inclusions increase rapidly while the SiO_2(inc) decreases quickly. It can be found from Fig. 9(b) that the CaO_(inc) and MgO_(inc) contents in inclusions increase obviously at LF2 sampling point, the average contents of inclusions are CaO_(inc): 34.1%, SiO_2(inc): 39.7% and MgO_(inc): 14.4%.

(3) LF3 sampling point

After the LF2 sampling point, a large amount of refining slags are added into liquid steel, the activity of (CaO) in top slag in steel increases, and [Ca] content in steel increases through the steel-slag reactions (4) and (5), which results in the steel-inclusion reaction (12) and (13) continuing toward the positive direction. So it can be seen from Fig. 9(c) that at the LF3 point the CaO_(inc) content in inclusions gets a certain degree of rise and reaches to 43.1%.

(4) VD1 sampling point

During VD vacuum treatment, the slag-steel reactions and the steel-inclusion reactions are promoted to keep toward the positive direction further. Therefore, as shown in Fig. 9(d), the contents of CaO_(inc) and SiO_2(inc) in inclusions rise to 45.3% and 38% respectively while the Al₂O_3(inc) and MgO_(inc) contents in inclusions slightly drop to 7.7% and 9% respectively.

(5) VD2 sampling point

Compared with VD vacuum treatment, the process of soft argon blowing is not only to promote the inclusions floatation but also to promote the slag-steel-inclusion reaction. A lot of alumina inclusions in liquid steel collide together and grow up, which are caught by the slag layer at the steel-slag interface, then the (Al₂O₃) content in slag increases, the activity of (CaO) in slag drops. As a result, the [Ca] content in steel decreases. In addition, the [Mg] content in steel increases due to the eroded refractory during VD vacuum treatment. So the reaction (12) and (13) present toward the inverse direction while the reaction (14) is toward the positive direction. It can be seen from Fig. 9(e) that the CaO_(inc) content in inclusions decreases and the MgO_(inc) content in inclusions slightly increases. At last, the slag-steel-inclusion reactions basically reach a balance. The composition of inclusions are more concentrated and the mass percent of CaO_(inc), SiO_2(inc), Al₂O_3(inc) and MgO_(inc) in inclusions are 42.6%, 39.3%, 8% and 10.4%, respectively.

From the above, it can be concluded that the compositions of inclusions are well controlled by low basicity and low alumina refining slag in industrial production. Finally, most of inclusions in the wire rods are of excellent plasticity.

7. Conclusions

Effect of top slag with low basicity on transformation control of inclusions in spring steel deoxidized by Si and Mn in production was studied combining with thermodynamic theories and industry experiments. The following conclusions were obtained:

(1) According to thermodynamics calculation results, for the CaO–SiO₂–Al₂O₃–MgO slag system, the content of MgO should be controlled around 7%. With the basicity R (R = CaO/SiO₂ by mass percent) at the range of 1.0 to 1.19 and C/A (C/A = CaO/Al₂O₃ by mass percent) above 9.0 in top slag, T.[O] content in steel can be controlled at low level, and inclusions in spring steel can be controlled in the plastic area of the phase diagram.

(2) In the industrial experiments, the average T.[O] content in blooms achieve 0.00089%, symbolizing high cleanness of final product. MnS, CaO–SiO₂–Al₂O₃–MgO and CaO–SiO₂–Al₂O₃–MgO–MnS system inclusions are three major nonmetallic inclusions in casting billet of spring steel. Oxide compound inclusions in steel are mainly CaO–SiO₂–Al₂O₃–MgO system with the size of about 2 μm. The number of three typical inclusions greater than 5 um through the whole process is inclined to decrease generally.

(3) With the decrease of alumina content in slag, [Al] content in steel decreases. Correspondingly, alumina content in inclusions decreases. Along with the refining process, the alumina content in inclusions decreases gradually, the compositions of inclusions transform to plastic deformation.

(4) The morphology and composition of inclusions in steel can be controlled effectively by silicon-manganese composite deoxidization and refining with low basicity and low alumina slag as well as controlling the [Al] content in steel.

Acknowledgement

The authors would like to acknowledge the financial support from Hangzhou Iron and Steel Group Company, Hangzhou, Zhejiang, China.

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