Effect of Deoxidizing and Alloying Routes on the Evolution of Non-Metallic Inclusions in 55SiCr Spring Steel

Abstract

Compared to the conventional deoxidation process with Al or Si-Mn for 55SiCr spring steel production, the possibility of using a Si-Ca-Ba compounded deoxidizer to control the behavior of non-metallic inclusions in spring steel was explored in this study. The effect of the addition sequence of deoxidizer and alloys into molten steel on the morphology, size, and composition of the inclusions was emphatically studied at 1873 K (1600 °C) using a high-temperature electric resistance furnace. The results indicate that adding alloy first can form less harmful inclusions in steel, which are roughly spherical and smaller than 3 μm in diameter, and its inclusion evolution path is MnO-SiO₂-Al₂O₃→CaO-SiO₂-Al₂O₃ and CaO-Al₂O₃-MgO. While adding deoxidizer first, the inclusions in steel are harmful due to its mostly irregular geometry and relatively large size over 5 μm. The inclusion evolution path is Fe-O→CaO-Al₂O₃(-SiO₂) and CaO-Al₂O₃-MgO. The formation and evolution mechanism of inclusions under different addition sequences were discussed. In addition, the solubility limits of MgO from refractory into steel were studied to inhibit its corrosion by molten steel.

1. Introduction

As one of the high-performance spring steels, steel 55SiCr serves for valve spring manufacture in most high-end automobiles with strict service conditions, and its fatigue life is required up to even 10⁷ cycles [1,2,3]. It has been shown that any brittle and spot-shaped nondeformable inclusions in steel will become the source of cracks during service, and may cause the fatigue failure of spring components [4,5,6]. The quantity, size, location, composition, property, and morphology of inclusions have a direct impact on the fatigue performance of steel [7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Klevebring et al. proposed a theory for determining the critical inclusion size concerning void formation during hot working operation, and found that inclusions above 2 to 3 μm in diameter can cause microcrack formation at the inclusion/matrix interface in steels [7]. Murakami et al. reviewed the effects of non-metallic inclusions on the fatigue strength of metals and proposed an inclusion effective projected area model, in which the fatigue properties of steels are estimated by the square root of the inclusion projected area perpendicular to the applied stress axis [8,9]. Krewerth et al. studied the influence of non-metallic inclusion features (type, size, chemical composition, morphology, and location) on fatigue life in high-cycle fatigue and/or super high-cycle fatigue regimes. They found that the inclusion depth away from the steel surface is one of the key parameters for fatigue life prediction [10]. Saberifar et al. found that most fatigue fractures were caused by the surface inclusions at the cycle regime less than 10⁶, whereas interior inclusions were the origins of the fatigue cracks in the high-cycle regime (10⁶ to 10⁷) for the investigated 30MnVS6 steels [11]. Beretta et al. proposed a method for the analysis of extreme defects and applied it to different steels containing multiple inclusions [12]. Que et al. found that fatigue cracks in 51CrV4 spring steel originated from ellipsoidal and low-plasticity alumina inclusions. The larger the inclusion size and the closer the inclusion position is from the surface, the shorter the fatigue life [13]. Meng et al. studied the size, distance from the surface, and composition of macro inclusions in the rotary bending fatigue fracture of 1950 MPa-class Si-killed 55SiCr suspension spring wire, and showed that the fatigue fracture was caused by exogenous large-sized inclusions [14]. Many other scholars [15,16,17,18,19,20] also studied the effects of inclusions on the fatigue life of steels, and obtained similar conclusions. Hence, if the size, morphology, and composition of inclusions can be reasonably controlled, the fatigue life of steel will be improved.

The formations of inclusions are closely related to the deoxidation process. There are two main processes for this steel currently [1,21], which are the cleanliness process using Al deoxidation and the inclusion plasticization process using Si-Mn deoxidation. The two processes have their respective advantages and disadvantages: the former can achieve a low total oxygen and fewer inclusions, but produces large-sized alumina inclusions unfavorable to the fatigue performance of steel, and the latter can produce deformable inclusions, but a relatively high total oxygen, which will lead to a large number of inclusions [22]. Accordingly, it is necessary to develop a new deoxidation process due to the shortcomings of the two conventional ones. The Ca and Ba elements both have strong deoxidation ability [23,24,25], and Ba acts as a diluent of Ca and decreases the vapor pressure of Ca in steel [26]. Their deoxidation products are generally small spherical inclusions that are less harmful to steel, with the aid of the Si element [27,28,29,30,31,32]. Moreover, Ca also plays an important role in desulfurization to reduce the harm of MnS inclusions [33]. Thus, the process of using a Si-Ca-Ba-compounded deoxidizer can combine the advantages of Si-Mn with Al deoxidation, and avoid their disadvantages. At present, a Si-Ca-Ba deoxidizer has been applied to some steels which are sensitive to the Al₂O₃ inclusions, such as wheel steel, heavy rail steel, austenitic stainless steel, and J55 oil well steel [24,25,34,35,36]. The process has obtained reliable results in high-end special steels, but has been reported in few spring steel applications. In this study, the Si-Ca-Ba-compounded alloy was adopted as a new deoxidizer of 55SiCr spring steel to explore an improved control of its inclusions.

It is well known that Si and Mn elements are almost completely oxidized during the primary smelting stage of steel in the converter, so adding them as alloying elements into the molten steel is an essential operation to meet the requirements of steel performance while tapping from the converter. Meanwhile, deoxidation is also indispensable because of the excess oxygen in molten steel. Of course, some alloys also play a deoxidation role in steel, such as Si and Mn. To distinguish a Si-Ca-Ba-compounded deoxidation agent from other alloys such as ferrosilicon, ferromanganese, and ferrochrome, the former is defined as deoxidizer and the latter as alloys in this study. If using Si-Ca-Ba as a novel compounded deoxidizer to 55SiCr spring steel, some questions about the addition sequence of the deoxidizer and alloys into molten steel need to be studied, since this will deeply affect the characteristics of the formed inclusions in steel and the subsequent steel performance. This is also a common concern in other steel research. To solve this question, the present study provides experimental and theoretical results for the application of a Si-Ca-Ba deoxidizer in 55SiCr spring steel. The study shows that adding the alloy first and then the deoxidizer can obtain less harmful inclusions in steel with a regular shape and small size compared with the opposite operation. The result is in good agreement with our expectation. This study will be a reference for the development and application to other steels as well.

2. Materials and Methods

The experiments were carried out in a vertical electric resistance furnace (Baotou Yunjie Electric Furnace Factory, Baotou, China) equipped with MoSi₂ heating bars and an alumina tube, as illustrated in Figure 1. A pure MgO crucible (Ø40 mm × 100 mm) was used to contain the raw steel materials. Pure iron, ferrosilicon (88.59 wt.% Si, 11.14 wt.% Fe), ferromanganese (80.90% Mn, 17.10% Fe, 0.02% Al), ferrochrome (60.41% Cr, 35.40% Fe, 1.29% Al), Fe₂O₃, and carbon powders were used as the raw materials to produce 55SiCr spring steel. The target steel compositions are presented in

Table 1. Target composition of spring steel and chemical composition of Si-Ca-Ba-compounded deoxidizer/wt.%.

Element	C	Si	Mn	Cr	P	S	Ca	Ba	Al	Fe
Spring steel	0.51~0.59	1.2~1.6	0.5~0.8	0.5~0.8	≤0.025	≤0.020	-	-	-	Other
Si-Ca-Ba deoxidizer	-	50.9	-	-	-	-	31.5	11.2	0.05	Other

. Scheme A added the Si-Ca-Ba-compounded deoxidizer first, and then added ferrosilicon, ferromanganese, and ferrochrome for alloying. Scheme B was on the contrary. The weight of the raw materials is provided in

Table 2. Weight of raw materials added in the experiment/g.

Scheme	Pure Iron	Ferrosilicon	Ferromanganese	Ferrochrome	Carbon Powder
A	400	6.9	3.6	5.5	2.3
B	392	6.9	3.5	5.4	2.2

. Both schemes added 0.15 g Si-Ca-Ba deoxidizer (its chemical composition is given in Table 1) into the steel. A schematic of the experimental procedure is shown in Figure 2.

In the experiments, industrial pure iron and 0.1 g Fe₂O₃ powder were placed into a MgO crucible and transferred to the center of an Al₂O₃ protective crucible in the uniform temperature zone of the MoSi₂ furnace. The melt was heated to 1600 °C under argon atmosphere at a flow rate of 200 L·h⁻¹. In scheme A, after the steel had melted for 5 and 7 min, the deoxidizer and pre-weighed alloys were added, respectively, and samples A-S1, A-S2, A-S3, and A-S4 were taken, respectively, as shown in Figure 2a. In scheme B, the pre-weighed alloys were added immediately upon the steel melting, and then the deoxidizer was added after 5 min. Samples B-S1, B-S2, B-S3, and B-S4 were taken, respectively, as shown in Figure 2b. The above samples were all sucked by a Ø6 mm quartz tube and then quenched in water immediately. Their weight was about 10~15 g for each one. For samples A-S4 and B-S4, the C content in them was determined by combustion analysis (EMIA-820V, Horiba, Kyoto, Japan), the Si, Mn, and Cr contents were measured using an X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Tokyo, Japan), and the oxygen was measured using the fusion and infrared absorption method (TCH 600, LECO, St. Joseph, MI, USA). The morphology, chemical composition, and size of the inclusions in all samples were analyzed by scanning electron microscope equipped with energy-dispersive spectroscopy (SEM-EDS, Phenom ProX, Phenom, Eindhoven, The Netherlands).

3. Results

The measured chemical compositions of samples A-S4 and B-S4 are listed in

Table 3. Measured chemical composition of S4 samples in different schemes/wt.%.

Composition	C	Si	Mn	Cr	O
A-S4	0.54	1.24	0.68	0.78	0.0020
B-S4	0.54	1.30	0.69	0.79	0.0019

. It is seen that they are both within the target range of 55SiCr spring steel.

The morphology and size of typical inclusions in different samples of the two schemes were observed first and then classified according to their chemical composition.

3.1. Evolution of Inclusions in Scheme A

In scheme A, the Si-Ca-Ba-compounded deoxidizer was added first, followed by alloying. The morphology of typical inclusions is shown in Figure 3, and the composition distribution of the inclusions is displayed in Figure 4. The shaded parts in Figure 4 represent the liquid-phase region of the inclusion system at 1600 °C.

Sample A-S1 was taken from molten liquid after the pure iron and Fe₂O₃ powder melted. Its total oxygen content is 101 ppm, indicating that the purpose of adding Fe₂O₃ powder to increase oxygen was achieved. The inclusions in this sample are mainly Fe-O type, and most of them are spherical with a diameter of smaller than 2 μm. Since pure iron contains a little Si and Mn, some inclusion peripheries are adhered by crescent-shaped silicon oxide or manganese oxide. The morphology of typical inclusions is shown in Figure 3a,b.

Sample A-S2 was taken one minute after the addition of the Si-Ca-Ba-compounded deoxidizer. The inclusions in it are mainly CaO-Al₂O₃ and CaO-Al₂O₃-MgO types. Some of them contain CaS. Though the Ba element is included in the deoxidizer, it has not been detected in these inclusions. This is because Ba has a high atomic weight, and its generated mass fraction is very small. Other scholars [28,33,37] also found a similar phenomenon. The composition distribution of the inclusions is shown in Figure 4a,b. It is obvious that the CaO content is over 50 wt.% in most inclusions, and it mainly originates from the deoxidation products of Ca in the Si-Ca-Ba deoxidizer. Most inclusions do not contain SiO₂, indicating that the Si in Si-Ca-Ba mainly plays an alloying role rather than a deoxidizing one. There are some Al₂O₃ in inclusions, which originate from the deoxidation product of residual Al in the deoxidizer. The generation reactions of these inclusions are shown as Equations (1)–(4) [38,39]. However, the distribution of CaO-Al₂O₃-MgO-type inclusions is relatively dispersed with different MgO contents. The MgO is possibly formed by Ca or Al reducing the crucible refractory unevenly or by the crucible peeling, and also by [Mg] and [O] recombination in steel. The related reactions are presented as Equations (5)–(7) [39,40]. The complex CaO-Al₂O₃-MgO inclusions are formed as expressed by Equations (8) and (9). Most inclusions are spherical or ellipsoidal with 1–3 μm in diameter. The morphology of typical inclusions is shown in Figure 3c,d.

$$\left[\mathrm{Ca}\right]+\left[\mathrm{O}\right]=\mathrm{CaO}\left(\mathrm{s}\right)$$

(1)

$$\left[\mathrm{Ca}\right]+\left[\mathrm{S}\right]=\mathrm{CaS}\left(\mathrm{s}\right)$$

(2)

$${2\left[\mathrm{Al}\right]+3\left[\mathrm{O}\right]=\mathrm{Al}}_2{\mathrm{O}}_3 \left(\mathrm{s}\right)$$

(3)

$${\mathrm{xCaO}\left(\mathrm{inc}\right)+\mathrm{yAl}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right)=\mathrm{xCaO}\cdot {\mathrm{yAl}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right)$$

(4)

$$\left[\mathrm{Ca}\right]+\mathrm{MgO}\left(\mathrm{s}\right)=\mathrm{CaO}\left(\mathrm{s}\right)+\left[\mathrm{Mg}\right]$$

(5)

$${2\left[\mathrm{Al}\right]+3\mathrm{MgO}\left(\mathrm{s}\right)=\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+3\left[\mathrm{Mg}\right]$$

(6)

$$\left[\mathrm{Mg}\right]+\left[\mathrm{O}\right]=\mathrm{MgO}\left(\mathrm{s}\right)$$

(7)

$$\mathrm{x}\left[\mathrm{Ca}\right]+\mathrm{y}\left[\mathrm{Al}\right]+\mathrm{z}\left[\mathrm{Mg}\right]+(\mathrm{x}+\frac{3}{2}\mathrm{y}+\mathrm{z}\left)\right[\mathrm{O}]=\mathrm{xCaO}\cdot \frac{\mathrm{y}}{2}{\mathrm{Al}}_2{\mathrm{O}}_3\cdot \mathrm{zMgO}\left(\mathrm{inc}\right)$$

(8)

$${\mathrm{xCaO}\left(\mathrm{inc}\right)+\mathrm{yAl}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right)+\mathrm{zMgO}\left(\mathrm{inc}\right)=\mathrm{xCaO}\cdot {\mathrm{yAl}}_2{\mathrm{O}}_3\cdot \mathrm{zMgO}\left(\mathrm{inc}\right)$$

(9)

The composition distribution of the inclusions in A-S3 is shown in Figure 4c,d. It is seen that although the inclusion types are similar to that in the sample A-S2, the SiO₂ and Al₂O₃ contents in the inclusions are higher because the added alloys (ferrosilicon, ferromanganese, and ferrochrome) contain Si and Al. However, a high Al content is unfavorable for controlling Al₂O₃ inclusions in the steel. Therefore, using low Al alloys is preferable, or a change to the addition sequence of the alloys. In this sample, most CaO-SiO₂-Al₂O₃-type inclusions are spherical with a diameter less than 2 μm, while CaO-Al₂O₃-MgO-type inclusions are spherical or of irregular geometry with the size of 1–5 μm. The morphology of typical inclusions is shown in Figure 3e–g.

Sample A-S4 was taken eleven minutes after the addition of the deoxidizer. The composition distribution of the inclusions is shown in Figure 4e,f. Compared with sample A-S3, the SiO₂ content in CaO-SiO₂-Al₂O₃-type inclusions slightly increases. This is caused by the addition of ferrosilicon. The distribution of the CaO-Al₂O₃-MgO-type inclusions is relatively dispersed, and the MgO content in it increases compared with sample A-S3 because the corrosion of the crucible is aggravated by the molten steel with the smelting time extension. The size of the inclusions in A-S4 is larger than in sample A-S3; most of them are in 2–3 μm, with some even greater than 5 μm. The morphology of the typical inclusions is shown in Figure 3h–j.

It can also be seen from Figure 4 that most inclusions are out of the liquid-phase region of 1600 °C, indicating that their melting points are relatively high, which might be unfavorable to the subsequent rolling and the use of steel products, especially when large-sized inclusions occur.

3.2. Evolution of Inclusions in Scheme B

In scheme B, alloying was performed after the pure iron and Fe₂O₃ powder melting, and then the Si-Ca-Ba deoxidizer was added. Figure 5 shows the composition distribution of the inclusions. Compared with scheme A in Figure 4, the distribution of inclusions in scheme B is more concentrated. The CaO-SiO₂-Al₂O₃-type inclusions are mostly located in the liquid-phase region of 1600 °C, indicating that these inclusions will be easy to deform when rolling, which will further lower the fatigue damage on spring products. Though the CaO-Al₂O₃-MgO-type inclusions occur in the liquid-phase region of 1600 °C, their harm can be ignored because their sizes are smaller than 2 μm. The morphology of typical inclusions is shown in Figure 6.

Sample B-S1 was taken three minutes after the addition of the alloys. The inclusions in it are mainly SiO₂-MnO-Al₂O₃-type and some of them are surrounded by MnS. The composition distribution of the inclusions is shown in Figure 5a. Because Si and Mn are not only the alloying elements, but also the deoxidizers, they can react with [O] in molten steel to generate SiO₂ and MnO, as expressed by Equations (10) and (11) [39]. The Al from the alloys will also be oxidized by the oxygen in the molten steel to form Al₂O₃, as shown in Equation (3). These oxides will combine to form complex inclusions, as expressed by Equation (12). The MnS is precipitated in the cooling process of the sample, as shown in Equation (13) [41], and wraps around the periphery of these inclusions. Most inclusions are spherical or ellipsoidal with a size smaller than 2 μm. The morphology of typical inclusions is shown in Figure 6a–c.

$$\left[\mathrm{Mn}\right]+\left[\mathrm{O}\right]=\mathrm{MnO}\left(\mathrm{s}\right)$$

(10)

$${\left[\mathrm{Si}\right]+2\left[\mathrm{O}\right]=\mathrm{SiO}}_2\left(\mathrm{s}\right)$$

(11)

$${\mathrm{xSiO}}_2{\left(\mathrm{inc}\right)+\mathrm{yMnO}\left(\mathrm{inc}\right)+\mathrm{zAl}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right){=\mathrm{xSiO}}_2\cdot \mathrm{yMnO}\cdot {\mathrm{zAl}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right)$$

(12)

$$\left[\mathrm{Mn}\right]+\left[\mathrm{S}\right]=\mathrm{MnS}(\mathrm{s})$$

(13)

Sample B-S2 was taken one minute after the addition of the Si-Ca-Ba deoxidizer. The inclusions are mainly CaO-SiO₂-Al₂O₃ and CaO-Al₂O₃-MgO types, and their composition distribution is shown in Figure 5b,c. Some of the CaO-SiO₂-Al₂O₃ inclusions are from the chemical reaction between [Ca] and dissolved [O], [Al], and [Si] in the molten steel after the Si-Ca-Ba deoxidizer was added, as shown in Equation (14). The others are from the modification of MnO-SiO₂-Al₂O₃-type inclusions by [Ca], as shown in Equations (15) and (16) [39], respectively. The SiO₂ content in these inclusions is relatively high in this sample.

$$\mathrm{x}\left[\mathrm{Ca}\right]+\mathrm{y}\left[\mathrm{Al}\right]+\mathrm{z}\left[\mathrm{Si}\right]+(\mathrm{x}+\frac{3}{2}\mathrm{y}+2\mathrm{z}\left)\right[\mathrm{O}]=\mathrm{xCaO}\cdot \frac{\mathrm{y}}{2}{\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{zSiO}}_2\left(\mathrm{inc}\right)$$

(14)

$$\left[\mathrm{Ca}\right]+\mathrm{MnO}\left(\mathrm{inc}\right)=\left[\mathrm{Mn}\right]+\mathrm{CaO}(\mathrm{inc})$$

(15)

$$\left[\mathrm{Ca}\right]+\frac{1}{2}{\mathrm{SiO}}_2\left(\mathrm{inc}\right)=\frac{1}{2}[\mathrm{Si}]+\mathrm{CaO}\left(\mathrm{inc}\right)$$

(16)

For CaO-Al₂O₃-MgO-type inclusions, the MgO in them has two possible origins. One is that the [Ca] and [Al] in the molten steel react with MgO particles peeled from the MgO crucible to form [Mg], according to Equations (5) and (6), which will further reduce MnO(inc.) and SiO₂(inc.) to form complex inclusions, as shown in Equations (17) and (18) [39]. In addition, the [Al] in the molten steel will possibly react with MnO(inc) and SiO₂(inc) in the inclusions, according to Equations (19) and (20) [39]. Because CaO-Al₂O₃-MgO-type inclusions were detected after the deoxidizer was added for only one minute, this suggests that the erosion is very quick. The inclusions in sample B-S2 are mostly spherical with sizes of 1–2 μm. The morphology of typical inclusions is shown in Figure 6d,e.

$$\left[\mathrm{Mg}\right]+\mathrm{MnO}\left(\mathrm{inc}\right)=\left[\mathrm{Mn}\right]+\mathrm{MgO}(\mathrm{inc})$$

(17)

$$\left[\mathrm{Mg}\right]+\frac{1}{2}{\mathrm{SiO}}_2\left(\mathrm{inc}\right)=\frac{1}{2}\left[\mathrm{Si}\right]+\mathrm{MgO}\left(\mathrm{inc}\right)$$

(18)

$$\frac{2}{3}\left[\mathrm{Al}\right]+\mathrm{MnO}\left(\mathrm{inc}\right)=\left[\mathrm{Mn}\right]+\frac{1}{3}{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right)$$

(19)

$$\frac{2}{3}\left[\mathrm{Al}\right]+\frac{1}{2}{\mathrm{SiO}}_2\left(\mathrm{inc}\right)=\frac{1}{2}\left[\mathrm{Si}\right]+\frac{1}{3}{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{inc}\right)$$

(20)

Sample B-S3 was taken six minutes after the deoxidizer had been added. The types of inclusions are similar to those in B-S2, and their composition distribution is shown in Figure 5d,e. Comparing Figure 5e with Figure 5c, the distribution of the inclusions in the former is more concentrated, and the MgO content has an increased tendency, indicating that the erosion of the crucible is aggravated as the refining time is extended. The inclusions are spherical or nearly spherical. Their size is slightly larger than those in B-S2, suggesting that the inclusions have grown in size with the extension of the refining process, but they are generally less than 3 μm, as displayed in Figure 6f,g.

Sample B-S4 was taken eleven minutes after the deoxidizer had been added. The composition distribution of the inclusions is shown in Figure 5f,g. It can be seen from Figure 5f that the CaO-SiO₂-Al₂O₃-type inclusions are mainly concentrated in the low melting point region of 1400–1500 °C. They will deform easily with steel rolling, which will alleviate the harmful effect on the fatigue properties of the subsequent rolling steels and finished products. Compared with Figure 5d and Figure 5b, the SiO₂ content in the inclusions in Figure 5f decreases and the Al₂O₃ content slightly increases. It indicates that the [Ca] and [Al] in molten steel will continuously modify some SiO₂ in inclusions into CaO and Al₂O₃ as the refining time extends, as expressed by Equations (16) and (20). Comparing Figure 5g with Figure 5e, the MgO content of the former is slightly lower. That is because the [Ca] and [Al] in the molten steel will continuously have a reduction and modification effect on the MgO in inclusions for some time after the Si-Ca-Ba deoxidizer is added, as expressed by Equations (5) and (6). The size of the inclusions in this sample is slightly larger than those in samples B-S2 and B-S3, but they are all less than 3 μm. The morphology of typical inclusions is shown in Figure 6h,i.

4. Discussion

Our previous research [22,30] also shows that the inclusions from the Si-Ca-Ba deoxidation process are smaller in size than those from Al or Si-Mn deoxidation, which will be beneficial to controlling the size of the inclusions in spring steel using this deoxidizer.

Comparing the evolution of inclusions in schemes A with B, the evolution path in the former is Fe-O→CaO-Al₂O₃(-SiO₂) and CaO-Al₂O₃-MgO, and in the latter is MnO-SiO₂-Al₂O₃→CaO-SiO₂-Al₂O₃ and CaO-Al₂O₃-MgO.

With the observations and analysis of the morphology, composition, and size of the inclusions in these samples taken from different refining stages under two schemes, it is found that in scheme A with the Si-Ca-Ba deoxidizer added first, the active Ca in the deoxidizer does not only combine with the oxygen in the molten steel, but also reduces MgO refractory to generate large-sized and irregular-shaped inclusions; in scheme B, with the alloys added first, the silicon–manganese alloys will combine with the oxygen in the molten steel to generate relatively small-sized inclusions containing SiO₂ and MnO. After the Si-Ca-Ba deoxidizer is added, it will not only continue to deoxidize, but will also modify the MnO-SiO₂-Al₂O₃-type inclusions into the low melting point of the CaO-SiO₂-Al₂O₃ type. In addition, it will also modify the MgO·Al₂O₃ spinel into the spherical CaO-Al₂O₃-MgO type. Therefore, for 55SiCr spring steel, the process of alloying first and then adding Si-Ca-Ba deoxidizer will be favorable for controlling the morphology, size, and other properties of the inclusions, which will further reduce the harmful effect on the subsequent products.

4.1. Evolution Mechanism of Inclusions

In summary, the evolution mechanism of the inclusions in the two schemes is clearly illustrated in Figure 7 and Figure 8. In scheme A, the inclusions in the molten steel are mainly Fe-O-type when pure iron and Fe₂O₃ powder are melted. Since pure iron may contain a little Si and Mn, they will combine with [O] in the molten steel to generate SiO₂ and MnO. These oxides adhere to the outer layer of Fe-O-type inclusions to form Fe-O-Si-Mn inclusions, as illustrated in Figure 7a. After the Si-Ca-Ba deoxidizer is added to molten steel, the dissolved [Ca] and [Al] from the deoxidizer will react with the MgO peeled off the crucible or will combine with [O] and [Mg] in the molten steel to generate Ca-Al-Mg-O-type inclusions, as illustrated in Figure 7b. When adding the ferrosilicon, ferromanganese, and ferrochrome alloys in sequence to adjust the composition of spring steel, the [Si] and [Mn] in them mainly play a role in alloying. However, the dissolved [Al] from these alloys will transform Ca-Mg-O-type inclusions into Ca-Al-Mg-O-type. Meanwhile, those previously formed Ca-Al-Mg-O-type inclusions will grow into bigger inclusions, as illustrated in Figure 7c. In addition, the new inclusions will also constantly generate, grow, and be modified with the refining process time extension, as illustrated in Figure 7d.

In scheme B, when the ferrosilicon, ferromanganese, and ferrochrome alloys are added into the molten steel system composed of molten pure iron and Fe₂O₃ powder, the [Si], [Mn], and [Al] from alloys will firstly combine with the [O] in the liquid iron to generate MnO-SiO₂-Al₂O₃ (Mn-Si-Al-O)-type inclusions, as illustrated in Figure 8a. After adding the Si-Ca-Ba deoxidizer, the [Ca], [Si], and residual [Al] will react with [O] in the system, or modify the previously formed MnO-SiO₂-Al₂O₃-type inclusions into CaO-SiO₂-Al₂O₃ (Ca-Si-Al-O)-type. Ca is a more active element than Mg, so it will also reduce MgO in the crucible refractory together with Al, or combine with dissolved [Mg], [Al], and [O] in the molten steel to generate CaO-Al₂O₃-MgO (Ca-Al-Mg-O)-type inclusions, as illustrated in Figure 8b. As the refining extends, the two types of inclusions will generate and grow constantly, as shown in Figure 8c,d.

4.2. Solubility Limits of MgO in Molten Steel

The above experimental results show that lots of inclusions in molten steel contain MgO, which is usually related to refractory corrosion during refining. The industrial practices [22,42,43] also showed that the steel samples from basic oxygen furnace (BOF) or electric arc furnace (EAF) tapping did not contain MgO inclusions, but after a period of refining, many large-sized MgO·Al₂O₃ spinel inclusions were observed. These inclusions are harmful to the subsequent rolling of steels, especially when they distribute on the surface of rolling products. The formation mechanism of MgO·Al₂O₃ spinel inclusions has been studied by many scholars [38,44,45,46,47], but little attention has been paid to the corrosion limit of the refractory. Understanding the corrosion limit will be helpful to control the formation of such inclusions.

The solubility limit of MgO from refractory into molten steel was studied through thermodynamic calculations in the present work. The Equilib module in Factsage 8.1 (codeveloped by Thermfact/CRCT, Montreal, QC, Canada, and GTT-Technologies, Herzogenrath, Germany) and the FT msic and FT oxid databases were adopted for this calculation. The basic chemical composition of the 55SiCr steel selected the measured A-S4 in Table 3. The Ca content in the steel was set as 20 ppm, Al as 0, 50, and 100 ppm, and MgO as 0–50 ppm. The calculation result is shown in Figure 9, in which the abscissa represents that the MgO content was corroded into molten steel from refractory. It reflects the corrosion extent of refractory. The red line represents the change of Mg content in molten steel (i.e., [Mg]) with the refractory corrosion, the black line represents MgO content in the generated liquid inclusions in the steel (MgO(inc.)), and the blue line is the solid MgO content in the steel, which is saturated precipitation (MgO(s)).

It can be seen that as the MgO content from refractory corrosion increases, the [Mg] firstly increases and then remains unchanged. It shows that the MgO from refractory is reduced to [Mg] because the molten steel contains the strong deoxidization element Ca, as expressed by Equation (5). The [Mg] content remains unchanged until the reaction limit is reached. The change tendency of MgO(inc.) is similar to that of [Mg]. When the contents of MgO(inc.) and [Mg] remain unchanged, the MgO(s) begin to precipitate. It means that as the refractory corrosion aggravates, MgO will no longer exist in the form of spinel or the composite liquid inclusions, but will reach saturation and then precipitate as calcite in the steel. This result is also in agreement with the observed inclusions, which contain different contents of MgO in the experimental samples.

From Figure 9a, when the molten steel does not contain [Al], the maximum value of [Mg] is 9 ppm, MgO(inc.) is less than 2 ppm, and the MgO(s) will precipitate when the MgO in molten steel from refractory corrosion reaches to 19 ppm. When the [Al] content is 50 ppm (Figure 9b), the maximum value of [Mg] and MgO(inc.) is 9.6 and 5 ppm, respectively, and the MgO(s) will precipitate as the MgO in the steel increases to 22 ppm. In Figure 9c, the [Al] content is 100 ppm, the maximum values of [Mg] and MgO(inc.) are 10 and 6 ppm, respectively, and the critical precipitation point of MgO(s) increases to 23 ppm. These results indicate that increasing the [Al] content in steel has little effect on the dissolution of [Mg]. Usually, excess [Al] will reduce the MgO in steel and then lead to increased [Mg] content, as expressed by Equation (6). However, the more active [Ca] in steel restrained the reduction reaction of [Al] in the present study. Additionally, an increase of [Al] will possibly produce acidic Al₂O₃, which combines with alkaline MgO from refractory, leading to an increment of MgO(inc.) in the inclusions. Accordingly, for controlling MgO·Al₂O₃ spinel and MgO(inc.), the [Al] content in steel should be limited to a low level.

As noted, the calculation above is based on the corrosion of the crucible refractory, and the X axis in Figure 9 can also be regarded as MgO content from slag into the molten steel. Therefore, adding an appropriate amount of MgO into slag will inhibit the corrosion of refractory by making [Al] and [Ca] in molten steel react with the MgO in slag instead of the MgO in refractory.

5. Conclusions

(1)

The process of adding a Si-Ca-Ba deoxidizer after alloying (scheme B) is favorable for controlling the morphology, size, and other properties of inclusions in spring steel.

(2)

In scheme A with the addition of deoxidizer before alloying, the observed inclusions in the steel are irregular and large in size, with some even over 5 μm in length. In scheme B with alloying first, the inclusions are spherical and relatively small in size, mostly smaller than 3 μm in diameter.

(3)

The evolution path of the inclusions during the steel refining process in scheme A is Fe-O→CaO-Al₂O₃(-SiO₂) and CaO-Al₂O₃-MgO, which is much different from MnO-SiO₂-Al₂O₃→CaO-SiO₂-Al₂O₃ and CaO-Al₂O₃-MgO, as observed in scheme B. In addition, the CaO-SiO₂-Al₂O₃-type inclusions from scheme B are in a low melting point and are deformable during hot rolling, which will be beneficial to an improved fatigue resistance for the final spring products.

(4)

The observed Al₂O₃ in the inclusions under the two schemes are all related to the residual Al from alloys and deoxidizers. To avoid harmful inclusions with high Al₂O₃ or MgO content, it is essential to use alloys and deoxidizers with a minimum Al content in advance, and/or to select scheme B when refining the steel.

(5)

The MgO in inclusions originate from refractory. The maximum solubility limit of Mg in molten steel is around 10 ppm. The maximum solubility limit of MgO in liquid inclusions is 6 ppm. When beyond this value, it will exist in the steel in the form of calcite.

Thus, it can be seen that the choices of a deoxidizer and the refining route show great potential for the control of harmful inclusions in steel production.