1 Introduction

Non-quenched and tempered steel has been widely used in automobile engine crankshaft, connecting rod and other components due to its advantages such as energy and material savings, environmental friendliness, and excellent performance [1,2,3]. To improve the machinability of non-quenched and tempered steel, a certain amount of sulfur content (0.035–0.075 wt.%) is typically added to the steel to form MnS, which acts as a lubricant during cutting processes, reducing tool wear and improving the machinability of the steel and components [4, 5]. However, MnS belongs to plastic inclusions and is prone to deformation into elongated shapes during rolling, leading to a reduction in the transverse mechanical properties of the material and causing anisotropy in the mechanical properties of the steel [6,7,8]. Studies have shown that elongated MnS inclusions have a significant effect on the plasticity, fatigue, impact and tensile properties of steel [9,10,11,12,13]. On the other hand, spherical and spindle-shaped sulfides (with an aspect ratio less than 3) have a less detrimental effect on the steel properties [14, 15]. Therefore, controlling the morphology of sulfides is crucial for improving the performance of non-quenched and tempered steel [16, 17].

Currently, the control of sulfide morphology is mainly achieved through modification treatment. This involves the reaction of active metals with the already formed MnS in the molten steel to obtain products with higher hardness than MnS, reducing the subsequent deformation of sulfides [18]. Calcium treatment is the most common method for MnS modification. CaS can dissolve with MnS to form smaller and less deformable (Mn, Ca)S inclusions, increasing the hardness of MnS inclusions and limiting their deformation during rolling [19, 20]. For non-quenched and tempered steel, calcium treatment increases the calcium content in the steel, allowing oxides to become nucleation cores for MnS, forming composite inclusions, and effectively changing the morphology and quantity of sulfide inclusions [21]. Studies have shown that the proportion of pure MnS inclusions decreases and the proportion of composite inclusions increases, while the aspect ratio of sulfides decreases after calcium treatment. Ahmad et al. [22] found that the proportion of pure MnS inclusions decreased and the proportion of composite inclusions increased in the steel after calcium treatment. In the industrial experiments conducted by Li et al. [23], it was found that after undergoing calcium treatment, non-quenched and tempered steel showed an increase in the proportion of spindle-shaped MnS inclusions and composite inclusions in the rolled products.

Magnesium treatment is another commonly used method for MnS modification. Compared with calcium treatment, magnesium promotes the formation of fine oxides (MgO–Al2O3 and MgO), which can act as nucleation cores for MnS to refine the size. Furthermore, magnesium treatment improves the cleanliness of the steel [24]. Fu et al. [25] studied the effect of Mg treatment on inclusions in 35CrNi3MoV steel, and found that the addition of magnesium transforms a large number of elongated MnS inclusions into elliptical composite MgS–MgO and MgS–MnS–MgO. This transformation was also found in the study by Zhang et al. [26]. This transition demonstrates the effectiveness of magnesium treatment in improving sulfide morphology. Additionally, magnesium treatment enhances the refinement of the steel microstructure and reduces the number of elongated MnS inclusions. The formation of more composite sulfides with Mg–Al–O as the core further confirms the improvement in sulfide morphology due to magnesium treatment [27].

Based on the advantages of Ca treatment and Mg treatment in the process of inclusion modification, many scholars have studied the effect of Mg–Ca composite treatment on sulfide. Liu et al. [28] found that Mg–Ca composite treatment transformed MnS in 20CrMnTi steel into MnS–CaS(–MgS)–MgO(–Al2O3–CaO), which effectively reduced the size of inclusions and improved the cutting performance of steel. He et al. [29] discovered that Mg–Ca treatment reduced the proportion of sulfides larger than 10 μm in the ingots of 38MnVS steel, effectively controlling sulfide morphology. Xie et al. [30] examined the effects of Mg–Ca treatment on sulfides in gear steel and observed a tendency for CaS in the molten steel to transform into (Ca, Mn)S during the solidification process. Shen and Fu [31] compared the effects of Ca treatment and Mg–Ca composite treatment on sulfides in 49MnVS3 steel, revealing that Mg–Ca treatment resulted in a higher proportion of spindle-shaped sulfides with smaller sizes compared to Ca treatment alone. Tian et al. [32, 33] used Mg–Ca composite treatment to control the morphology of MnS in steel and increased the proportion of spindle-shaped sulfides. Su et al. [34] investigated the proportions of spindle-shaped sulfides in non-quenched and tempered steel treated with Ca, Mg, and Mg–Ca, and the proportion was 23.31%, 19.39% and 43.24% in turn.

Previous studies on Ca–Mg composite treatment have mainly focused on the influence of Ca and Mg on the composition of sulfide and changes in the proportion of spindle-shaped sulfides, and the research was mainly carried out under laboratory conditions. In this study, an industrial experiment was carried out in a domestic steel plant to compare and analyze the effects of Ca treatment and Mg–Ca composite treatment on inclusions in the steel, with a particular emphasis on understanding the formation mechanism of MnS with oxide cores. Additionally, the influence of sulfide morphology on the mechanical properties of steel was analyzed. The research findings can serve as a reference for controlling sulfide morphology in steel and improving the properties of sulfur-containing non-quenched and tempered steel.

2 Experimental

The smelting process of non-quenched and tempered 45MnVS steel in a domestic steel mill is as follows: electric arc furnace (EAF) → ladle furnace (LF) → continuous casting (CC) → rolling (rod: ϕ60 mm). Two experiments were conducted, with A furnace subjected to Ca treatment and B furnace subjected to Mg–Ca treatment. Calcium wire was used for Ca treatment, and magnesia-calcium cored wire was used for Mg–Ca treatment. The main chemical composition of Mg–Ca cored wire is Ca 13.5, Mg 13.5, Si 40.5 and Fe 32.5, in wt.%.

Table 1 shows the main chemical composition of the experimental steel. The content of C and S in steel was determined by high frequency infrared carbon sulfur analyzer (CS-230). The content of total O in steel was determined by oxygen nitrogen hydrogen analyzer (ONH836). The content of Ca and Mg was determined by inductively coupled plasma mass spectrometer (ICP-MS). The content of other elements was analyzed by optical emission spectrometer. The magnesium content of the rolled material after calcium and magnesium–calcium composite treatment was 0.0002 and 0.007 wt.% and Ca content was 0.0011 and 0.0006 wt.%, respectively. The contents of other alloying elements, such as C, Si, Mn, S, and Al, were similar.

Table 1 Chemical composition of steel (mass%)
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Figure 1 shows that the metallographic sample is taken from the 1/2 radius position of the round bar (the observation surface is parallel to the rolling direction). Each metallographic sample was polished and then observed and analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) to examine the morphology and composition of inclusions. The Aspex was used for quantitative analysis of the type and aspect ratio of inclusions. The Image Pro-Plus 6.0 software was used to measure the oxide core (denoted as Aoxide) and the entire area (denoted as Asulfide) of each sulfide inclusion, and to calculate the area ratio between the two. Figure 2 shows the measurement diagrams of aspect ratio and area ratio.

Fig. 1
figure 1

Schematic diagram of sample dimensions and sampling locations

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Fig. 2
figure 2

Schematic diagrams for measuring aspect ratio (a) and area ratio (b) of sulfides

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The thermodynamic software FactSage 8.0 was used to calculate and analyze the thermodynamic conditions for the formation of inclusions.

The samples are then subjected to the impact toughness test using a JB300B pendulum impact testing machine to determine their impact toughness. The microstructure of the impact fracture is observed and analyzed using SEM.

Figure 1 also shows a cylindrical position where a rod is cut by wire cutting, and the impact samples (10 mm × 10 mm × 55 mm, V-notch) are cut in both the longitudinal (parallel to the rolling direction) and transverse (perpendicular to the rolling direction) directions. The samples are then subjected to impact toughness test using a JB300B pendulum impact testing machine to determine their impact toughness. The micro-morphology of the impact fracture is observed and analyzed using SEM.

3 Results

3.1 Type, size and aspect ratio of inclusions

The Aspex analysis was performed to determine the type, size, and aspect ratio of inclusions in steel samples before and after Ca and Mg–Ca treatment, and Fig. 3 shows the results. It can be seen from Fig. 3a that three major types of inclusions were identified: pure MnS, MnS–oxide (referring to MnS with oxide cores), and other types. In Ca-treated steel, the quantity percentage of pure MnS was found to be 62.04%, while MnS–oxide accounted for 22.66% of the total inclusions. In Mg–Ca-treated steel, the percentage of pure MnS decreased to 53.62%, and the percentage of MnS–oxide increased to 30.66%. This indicates that Mg–Ca treatment leads to the formation of more MnS–oxide inclusions.

Fig. 3
figure 3

Type (a), size (b) and aspect ratio (c) of inclusions in rolled samples

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Figure 3b shows that Ca-treated steel had a lower percentage of small-sized inclusions (0–3 μm) of 7.21%, whereas Mg–Ca-treated steel exhibited a significant increase in the percentage of small-sized inclusions (15.85%).

Figure 3c reveals that Ca-treated steel had 19.99% spindle-shaped sulfide inclusions, whereas Mg–Ca-treated steel showed an increased proportion of 35.38%. This suggests that Mg–Ca treatment is more effective in improving the morphology of sulfide inclusions, resulting in more spindle-shaped sulfides.

3.2 Morphology and composition of inclusions

The morphology and composition of typical inclusions in the rolled steel samples were determined by SEM–EDS. Figure 4 shows the results, where Fig. 4a–c depicts the morphology of inclusions in Ca-treated steel samples. In Ca-treated rolled samples, elongated sulfides with sizes exceeding 20 μm were observed (Fig. 4a). These sulfides have a significant effect on the impact toughness, fatigue and tensile properties of steel [35]. The rolled samples also contained a small amount of spindle-shaped and spherical sulfide inclusions with oxide cores (Fig. 4c). The core of these inclusions was composed of Ca–Al–O oxide, while the sulfide coating consisted of composite sulfides (Mn, Ca)S with dissolution of Ca. Figure 4d, e displays the morphology of sulfide inclusions in Mg–Ca-treated rolled samples. In this sample, elongated sulfide inclusions were less abundant, and most of them exhibited spindle-shaped or spherical morphologies with oxide cores. The size of the inclusions was around 5 μm. The core of the sulfides was composed of Mg–Al–O–Ca (in small amounts) oxides, while the sulfide coating was composed of composite sulfides (Mn, Ca, Mg)S with dissolution of Ca and a small amount of Mg.

Fig. 4
figure 4

Morphology of typical inclusions in rolled samples. a–c Ca treatment; d–f Mg–Ca treatment

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Compared to Ca-treated steel, Mg–Ca-treated rolled samples showed a decrease in the size of inclusions and a lower quantity of elongated sulfides. Instead, most of the inclusions exhibited spindle-shaped morphology with oxide cores, which is consistent with the statistical results mentioned in Sect. 3.1 using Aspex analysis.

Figure 5 shows the element distribution of typical composite inclusions in Ca-treated and Mg–Ca-treated steels. It can be seen from Fig. 5a that after Ca treatment, the sulfide (Ca, Mn)S precipitates with Al2O3 as the core in the sample. After Mg–Ca treatment, the sulfide (Ca, Mg, Mn)S precipitates with MgO·Al2O3 spinel as the core in the sample, as shown in Fig. 5b.

Fig. 5
figure 5

SEM mapping images of typical inclusions in rolled products. a Ca treatment; b Mg–Ca treatment

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3.3 Impact energy

Three samples were taken from the transverse and longitudinal directions of each rod, and the sampling diagram is shown in Fig. 1. The impact energy of the samples was measured by a pendulum impact tester. Table 2 shows the longitudinal and transverse impact energy of Ca-treated and Mg–Ca-treated steels. The average values of longitudinal impact energy of Ca-treated steel and Mg–Ca-treated steel are 40.577 and 45.449 J, and the average values of transverse impact energy are 25.785 and 32.119 J, respectively. The ratio of the average transverse impact energy to the average longitudinal impact energy of Ca-treated steel is 0.635, and the value of Mg–Ca-treated steel is 0.707. It can be seen that Mg–Ca treatment improves the transverse impact toughness of the steel and reduces the anisotropy of its impact toughness.

Table 2 Impact energy of experimental steel (J)
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4 Discussion

4.1 Influencing factors of sulfide morphology

Aspect ratio is one of the important parameters to describe the morphology of inclusions, and its magnitude has an important influence on the properties of steel. Typically, the morphology of MnS with aspect ratio less than 3 is called spindle-shaped, while the morphology with aspect ratio greater than 3 is called elongated-shaped. Elongated MnS significantly reduces the transverse mechanical properties of steel, such as transverse impact toughness, fatigue life and tensile properties [9,10,11,12]. Therefore, in order to reduce the adverse effects of MnS to steel properties, it is necessary to increase the proportion of spindle-shaped MnS in the steel as much as possible [15, 16]. In this section, the relationship between the area, composition and area ratio of inclusions in steel and the aspect ratio of sulfides was studied in order to explore the factors affecting the morphology of sulfides.

4.1.1 Area of sulfide inclusions

The areas and aspect ratios of two types of sulfides, MnS and MnS–oxide, in Ca- and Mg–Ca-treated steels were analyzed using Aspex detection, and Fig. 6 shows the results. Figure 6a and c depicts the relationship between the aspect ratio of MnS and its area in Ca- and Mg–Ca-treated steel samples. It can be seen from Fig. 6 that there is no apparent correlation between the aspect ratio and area of MnS in the steel. Figure 6b and d illustrates the relationship between the aspect ratio and area of MnS–oxide in Ca- and Mg–Ca-treated steel samples. Similarly, there is no significant correspondence between the aspect ratio and area of MnS–oxide, which contains an oxide core. Therefore, under the same deformation conditions, the area (which can be considered as the size) of the sulfides does not affect their aspect ratio. However, studies conducted by Segal and Charles [36] demonstrated that under similar deformation conditions, smaller-sized MnS in steel undergoes less deformation compared to larger-sized MnS.

Fig. 6
figure 6

Relationship between aspect ratio of inclusions and area. a, b Ca treatment; c, d Mg–Ca treatment

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4.1.2 Composition of sulfide inclusions

The morphological and compositional analysis of sulfides with oxide cores (MnS–oxide) present in rod samples was conducted using SEM–EDS. The Image Pro-Plus 6.0 software was employed to measure the aspect ratio and area of the sulfides (as illustrated in Fig. 2a). Ca content in the sulfide coating of the oxide after Ca treatment and Mg + Ca content in the sulfide coating of the oxide after Mg–Ca treatment were individually quantified. The relationship between Ca and Mg + Ca contents and sulfide aspect ratio was analyzed, as shown in Fig. 7. From Fig. 7a, it can be observed that, after Ca treatment, the sulfide aspect ratio decreases with increasing Ca content in the sulfide coating. Spindle-shaped sulfides dominate when Ca content in the sulfide coating exceeds 5 wt.%. From Fig. 7b, it can be seen that, after Mg–Ca treatment, the sulfide aspect ratio decreases with increasing Mg + Ca element content in the sulfide coating. When Mg + Ca content in the sulfide coating exceeds 5 wt.%, the sulfides mainly exhibit spindle-shaped morphology.

Fig. 7
figure 7

Relationship between aspect ratio of inclusions and Ca (a) and Ca + Mg (b) contents in sulfide coating

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The aspect ratio of MnS–oxide is approximately inversely proportional to the content of Ca and Mg in the oxide core. The study of Li et al. [37] showed that the higher the CaO content in the oxide core, the higher the Ca content in the composite sulfide (Mn, Ca)S of sulfide coating. Consequently, the sulfides are more likely to exhibit a spindle-shaped morphology after rolling. Therefore, it is necessary to analyze the composition of the oxide core before the formation of the composite sulfide. The composition of the oxide core before the formation of the composite sulfide can be represented by Ca and Mg content in the sulfide coating. The calculation formula is as follows:

$$W_{\text{o} - {i}} = w_{\text{o}} + w_{\text{s}}$$
(1)

where Wo−i represents the alloying element content in the oxide core before the formation of complex sulfide; i represents the alloying elements; wo represents the content of alloying elements in the oxide core; and ws represents the content of alloying elements in the sulfide coating. According to the above formula, Ca and Mg contents of the oxide core before the formation of the composite sulfide are calculated, and Fig. 8 shows the results.

Fig. 8
figure 8

Relationship between aspect ratio of inclusions and element content in oxide core. a Ca treatment; b Mg–Ca treatment

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By comparing the relationship between the content of Ca and Mg + Ca in the oxide core and aspect ratio of inclusion, it can be concluded that in order to form more spindle-shaped sulfides, the appropriate sum of the content of Ca and Mg elements in the oxide core should be greater than 10 wt.%. The higher the content of Ca and Mg in the sulfide coating, the less likely the sulfides to deform. According to the literature [38], MnS and CaS have similar crystal structures and lattice constants, allowing them to form nearly ideal solid solutions. With the increase in CaS content, the micro-hardness of sulfides increases, while their relative plasticity decreases [39].

4.1.3 Aspect ratio of sulfide inclusions

Image Pro-Plus 6.0 software was used to measure Aoxide and Asulfide in the sulfide of the rod sample. The area ratio, Asulfide/Aoxide, was calculated based on the measurements (as shown in Fig. 2b). Additionally, the aspect ratio of the entire inclusion was measured. The statistical analysis of the relationship between the area ratio and the aspect ratio of sulfides is presented in Fig. 9.

Fig. 9
figure 9

Relationship between aspect ratio and area ratio of inclusions. R2—Determination coefficient

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It can be seen from Fig. 9 that when the area ratio is 0–5, the sulfide is spindle-shaped, and when the area ratio is larger than 5, the sulfides appear as elongated bars. With the increase in the area ratio, the aspect ratio of the sulfide also increases accordingly. Similar conclusions were obtained in the research by Kano and Hanyuda [40]. Wu et al. [41] found that the deformation of inclusions was mainly affected by the thickness of MnS coating on the surface of oxide core.

According to the data in Fig. 9, a relationship equation can be fitted to describe the aspect ratio of sulfides in terms of the area ratio, as follows:

$$ A_{\text{s}} = - 1.03181 + 1.99192 \times A_{\text{r}} $$
(2)

where As represents the aspect ratio of sulfide; and Ar represents the area ratio Asulfide/Aoxide.

It can be concluded from Eq. (2) that the area ratio should be controlled between 0 and 5 in order to make the sulfide into a spindle.

According to the previous analysis, the morphology (i.e., aspect ratio) of MnS–oxide sulfides is mainly affected by Ca + Mg content in the oxide core before the formation of composite sulfides, as well as the area ratio (Asulfide/Aoxide), rather than the size (equivalent to area) of the sulfides. Figure 10 shows the relationship between the aspect ratio of sulfide and Ca + Mg content in the oxide core, the area ratio (Asulfide/Aoxide) and the size of sulfide (represented by area). When Ca + Mg content in the oxide core is higher and the area is lower, the sulfide after rolling is more likely to be spindle-shaped. According to the literature research [42], the dissolution of Ca in MnS will hinder its growth and reduce the area ratio. According to the research findings, controlling Ca + Mg content in the oxide core at or above 10 wt.% and the area ratio below 5 can facilitate the formation of spindle-shaped MnS–oxide sulfides after rolling.

Fig. 10
figure 10

Relationship among aspect ratio of sulfides, content of Ca and Mg in oxide core, and area ratio (Asulfide/Aoxide)

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4.2 Formation of composite sulfide with an oxide core

4.2.1 Effect of oxide activity

50% liquid phase region of CaO–MgO–Al2O3 ternary phase diagram at steelmaking temperature (1600 °C) was calculated by Factsage software, and the FactPS and FToxid databases were selected for this purpose. The composition of the oxide in MnS–oxide core of the rolled sample and the composition of the oxide not wrapped by the sulfide are marked, and the results are shown in Fig. 11. As shown in Fig. 11a, the composition of the oxide that is not wrapped by sulfides is randomly distributed within the phase diagram. According to Fig. 11b, the composition of the oxides of MnS–oxide core of Ca-treated steel is mainly concentrated in the lower right corner of the phase diagram, which corresponds to the regions of MgO·Al2O3, Al2O3, CaO·Al2O3, CaO·2Al2O3, and CaO·2MgO·8Al2O3. In addition to these regions, the composition of the oxides of the sulfide core of Mg–Ca-treated steel is also distributed within 50% liquid phase region of the phase diagram. Comparing Fig. 11a and b, it can be observed that the content of CaO in the oxide decreases after the oxide is wrapped by sulfide.

Fig. 11
figure 11

Composition distribution of oxides in core of ternary phase diagram. a Oxide components not wrapped by sulfides; b oxide components in MnS–oxide sulfide core. C—CaO; A—Al2O3; Sp.—spinel

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According to the chemical composition of the steel in Table 1, the equilibrium cooling model of the Equilibe module of Factsage software was used to calculate the precipitation of oxides and sulfides during the solidification of the test steel. The FactPS, FSstel and FToxid databases were selected for this calculation. Figure 12 shows the calculation results. It should be noted that CaS, MgS and MnS in Fig. 12 refer to the corresponding sulfides in the composite sulfides. According to the variation of the content of each substance in Fig. 12, for Ca-treated steel, the reactions of inclusions in steel at different temperatures can be inferred, as shown in Table 3 for Ca-treated steel and in Table 4 for Mg–Ca-treated steel. From Tables 3 and 4, it can be seen that Ca and Mg in the sulfide coating of MnS–oxide mainly come from its oxide core. As the reaction progresses, Ca and Mg in the oxide react with S, resulting in an increase in Ca + Mg content in the sulfides, forming complex sulfides such as (Mn, Ca)S, (Mn, Mg)S, and (Mn, Ca, Mg)S. Ca + Mg content in the oxide decreases, which is consistent with the distribution of oxidation components in CaO–MgO–Al2O3 ternary phase diagram (Fig. 11a), where the components of oxide without sulfide coating are randomly distributed. However, Ca + Mg content of the oxides with sulfide coating is reduced (Fig. 11b), with the components mainly distributed in the lower right corner of the phase diagram near Al2O3. Similar conclusions have also been reported in Refs. [43, 44]. It should be noted that Fig. 12 represents the calculation results when the reactions reach equilibrium, and it is difficult to achieve complete equilibrium in actual production or experimental process. However, the calculation results can still represent the trend of relevant chemical reactions and can be used to predict the variation trends of inclusion composition.

Fig. 12
figure 12

Equilibrium precipitation process of inclusions of steel during solidification. a Ca treatment; b Mg–Ca treatment

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Table 3 Reaction of inclusions in Ca-treated steel at different temperatures
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Table 4 Inclusion reaction in Mg–Ca-treated steel at different temperatures
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From the previous analysis, Ca and Mg in the composite sulfides are mainly derived from oxide cores, which can be expressed in the following Eqs. (3) and (4). Therefore, the activity of oxides has an important influence on the formation of composite sulfides.

$$ \left( {{\text{CaO}}} \right)_{{{\text{incl}}}} + \left( {\text{S}} \right)_{{{\text{sulf}}}} = \left( {{\text{CaS}}} \right)_{{{\text{sulf}}}} + \left( {\text{O}} \right)_{{{\text{incl}}}} $$
(3)
$$ \left( {{\text{MgO}}} \right)_{{{\text{incl}}}} + \left( {\text{S}} \right)_{{{\text{sulf}}}} = \left( {{\text{MgS}}} \right)_{{{\text{sulf}}}} + \left( {\text{O}} \right)_{{{\text{incl}}}} $$
(4)

where the subscript incl represents composite oxides; and the subscript sulf represents composite sulfides.

Ca + Mg contents in sulfide coating and oxide core of MnS–oxide are artistically analyzed, and the results are shown in Fig. 13. From Fig. 13, it can be seen that the content of Ca in the sulfide coating is higher than that in the oxide core, while the content of Mg in the sulfide coating is lower than that in the oxide core. Similar conclusions were also reported in a study conducted under laboratory conditions [33].

Fig. 13
figure 13

Ca and Mg contents in sulfide coating and oxide cores. a Ca treatment; b Mg–Ca treatment

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The isoactivity curves of CaO and MgO in CaO–MgO–Al2O3 system at 1300–1600 °C were calculated by Factsage, as shown in Fig. 14. It can be seen from Fig. 14a that in CaO–MgO–Al2O3 ternary phase diagram, the higher the CaO content in the composite oxide, the greater the activity of CaO, which is favorable for the progression of Eq. (3) towards the right, leading to the formation of complex sulfides and increasing Ca content in the complex sulfides. Previous studies have suggested that increasing the activity of CaO in inclusions promotes the formation of complex sulfides like (Mn, Ca)S and increases Ca content in the sulfides [40]. Combined with Fig. 11b, it can be inferred that, for Ca-treated steel, the content of CaO in the oxide core is relatively low, which is mainly due to the occurrence of the reaction in Eq. (3), so that the content of CaO in the oxide after the reaction is reduced, and its composition is distributed near the area of Al2O3, which is consistent with the reaction in Table 3. The composition of the oxide after the reaction is mainly CaO·6Al2O3 and Al2O3. It can be seen from Figs. 11b and 14b that the activity of MgO in the oxide core is in a low area, resulting in the fact that Eq. (4) is not easy to proceed. This leads to minimal changes in MgO content in the oxides, resulting in relatively low Mg content in the sulfide coating.

Fig. 14
figure 14

Isoactivity diagrams of CaO and MgO in CaO–MgO–Al2O3 system at 1300–1600 °C. a CaO isoactivity line; b MgO isoactivity line

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4.2.2 Effect of sulfur capacity

Studies have shown that the sulfur capacity of inclusions has an important influence on the formation of composite sulfides [45,46,47]. According to Ref. [48], inclusions with high sulfur capacity are more likely to react with sulfur, leading to the form of composite sulfides with oxide cores. The higher the sulfur capacity of calcium aluminate inclusions, the more likely they are to form composite sulfide (Mn, Ca)S. The sulfur capacity CS of oxides can be expressed by Eq. (6) according to Eq. (5) [49]:

$$ \frac{1}{2}{\text{S}}_{2} ({\text{g}}) + {\text{O}}^{2 - } ({\text{slag}}) = \frac{1}{2}{\text{O}}_{2} ({\text{g}}) + {\text{S}}^{2 - } ({\text{slag}}) $$
(5)
$$ C_{{\text{S}}} = K\frac{{a_{{{\text{O}}^{2 - } }} }}{{f_{{{\text{S}}^{2 - } }} }} = m_{{\text{S}}} \sqrt {\frac{{p_{{{\text{O}}_{2} }} }}{{p_{{{\text{S}}_{2} }} }}} $$
(6)

where K represents the reaction equilibrium constant of Eq. (5); \(a_{{{\text{O}}^{2 - } }}\) represents the activity of oxygen ions in oxides; \(f_{{{\text{S}}^{2 - } }}\) represents the activity coefficient of sulfur ions in oxides; mS represents the sulfur content (mass fraction) in oxides; and \(p_{{{\text{O}}_{2} }}\) and \(p_{{{\text{S}}_{2} }}\) represents the partial pressure of O2 and S2 in the gas phase, respectively. In this paper, the KTH model (a model developed at the Department of Metallurgy, KTH) is used to calculate the sulfur capacity of oxides, as shown in the following [50, 51]:

$$ C_{{\text{S}}} = \exp \left( {\frac{{ - \Delta G^{ \circ } }}{RT}} \right)\left( {\frac{{a_{{{\text{O}}^{2 - } }} }}{{f_{{{\text{S}}^{2 - } }} }}} \right) = \exp \left( {\frac{{ - \Delta G^{ \circ } }}{RT}} \right)\exp \left( {\frac{\xi }{RT}} \right) $$
(7)

where \(\Delta G^{ \circ }\) represents the Gibbs free energy change, \(\Delta G^{ \circ } = 111,535 - 58.82T\;({\text{J/mol}})\) of Eq. (5); T represents the temperature, K; R represents the gas constant 8.314 J/(mol K); and \(\xi\) is described as a function of both temperature and composition, and it can be calculated by the following formula:

$$ \xi = \sum {X_{i} \xi_{i} } + \xi_{{{\text{mix}}}} $$
(8)

where Xi represents the mole fraction of component i in the multicomponent system, and is the linear coefficient of temperature to component i; and \(\xi_{{{\text{mix}}}}\) represents the interaction of different components in the inclusion. The value of \(\xi_{i}\) and \(\xi_{{{\text{mix}}}}\) can be obtained by referring to the literature [50].

Figure 15 shows the sulfur capacity calculation results of inclusions with different compositions in CaO–Al2O3–MgO system at 1873 K. It can be seen from Fig. 15 that when the ratio of CaO/Al2O3 (mass%) in the inclusions is relatively high (> 1.86), increasing MgO content will decrease the sulfur capacity of the inclusions. On the other hand, when the value of CaO/Al2O3 is relatively low (< 1.86), increasing MgO content will increase the sulfur capacity of the inclusions. The statistical values of CaO/Al2O3 for the oxide cores before the formation of complex sulfides are all below 1.5. Therefore, under the experimental conditions, the inclusion with a certain amount of MgO can enhance its sulfur capacity, which is beneficial to the formation of the composite sulfide.

Fig. 15
figure 15

Sulfur capacity of inclusions with different compositions in CaO–Al2O3–MgO system

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4.2.3 Influence of lattice mismatch

The nucleation and precipitation of sulfides on oxides can be regarded as heterogeneous nucleation. The difficulty of heterogeneous nucleation depends on the lattice mismatch between the matrix phase and the nucleation phase [52]. The lattice mismatch (δ) between MnS and CaO–MgO–Al2O3 inclusions can be calculated according to Eq. (9) proposed by Bramfitt [53].

$$\delta_{{\left({hkl} \right)_{{\text{n}}} }}^{{\left( {hkl} \right)_\text{s} }} = \sum\limits_{x = 1}^{3} {\frac{{\frac{{\left| {\left( {{d}\left[ {uvw} \right]_{{\text{s}}}^{x} \cos \theta } \right) - {d}\left[ {uvw} \right]_{{\text{n}}}^{x} } \right|}}{{d}{\left[ {uvw} \right]_{{\text{n}}}^{x} }}}}{3}} \times 100\% $$
(9)

where (hkl)s is a low-index crystal plane of the base phase; [uvw]s is a low-index direction on the crystal plane (hkl)s; (hkl)n is a low-index crystal plane of the nucleation phase; [uvw]n is a low-index direction on the crystal plane (hkl)n; d[uvw]s is the atomic spacing along [uvw]s direction; d[uvw]n is the atomic spacing along [uvw]n direction; and θ is the angle between [uvw]s and [uvw]n.

Table 5 shows the parameters used in the calculation, and the data are sourced from the Materials Projects database. The calculation results are displayed in Fig. 16. Studies have shown that nucleation is most effective when the lattice misfit δ is less than 6% [53]. When 6% < δ < 12%, nucleation is moderately effective. When δ is greater than 12%, heterogeneous nucleation is not likely to occur. From the diagram, it can be seen that as MgO content in the inclusion system increases, the lattice mismatch between the inclusions and MnS decreases, which facilitates the nucleation and precipitation of MnS with this type of oxide as the core. Therefore, compared with Ca-treated steel, Mg–Ca-treated steel has a greater quantity of composite sulfides with oxide cores (as shown in Fig. 3a).

Table 5 Types and lattice parameters of inclusions
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Fig. 16
figure 16

Minimum lattice mismatch between MnS and different CaO–MgO–Al2O3 inclusions

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In summary, the formation of MnS–oxide is related to the composition of the oxide core, where Ca is more prone to reacting with MnS to form composite sulfide, while Mg is more conducive to the formation of an oxide core that facilitates MnS nucleation.

4.3 Effect of sulfides on impact performance

The morphology of sulfide has an important influence on the mechanical properties of steel. The mechanical properties of free-cutting steel were studied in the literature. Compared with the longitudinal samples, the tensile strength, yield strength and elongation of the transverse samples decreased by 23.8%, 24.6% and 77.0%, respectively [16]. In the sulfur-containing non-quenched and tempered steel, the reduction in transverse tensile strength, yield strength, elongation, and reduction of area was 6.66%, 7.57%, 44.39%, and 56.19%, respectively [42]. In the hot rolled steel plate containing sulfur (0.013 wt.%), the transverse impact energy was reduced by 47%–57% compared with the longitudinal impact energy [54]. It can be seen that the effect of sulfide on the mechanical properties of steel is mainly caused by the anisotropy of mechanical properties. In this experiment, the ratio of transverse to longitudinal impact energy of Ca-treated steel is 0.635, and that of Mg–Ca-treated steel is 0.707. The degree of anisotropy in mechanical properties of Mg–Ca-treated steel is lower.

Figure 17 illustrates the fracture morphology of the longitudinal impact sample. It can be seen from Fig. 17 that MnS inclusions exist at the dimple regions of the fracture in both Ca-treated steel and Mg–Ca-treated steel samples, and there is a noticeable separation between MnS and steel matrix. Compared with Ca-treated steel, the dimples in Mg–Ca-treated steel are deeper and larger. The quantity and depth of dimples to some extent reflect the toughness and brittleness of the material. The higher quantity and deeper dimples indicate that the formation of cracks will undergo greater local plastic deformation, indicating better plasticity and toughness of the material. Therefore, Mg–Ca-treated steel exhibits better impact performance as demonstrated in Table 3.

Fig. 17
figure 17

Typical fracture morphology of impact sample of longitudinal sample. a Ca-treated steel; b Mg–Ca-treated steel

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Figures 18 and 19 display the fracture morphology of the transverse impact sample. It can be seen from Figs. 18 and 19 that both steel samples exhibit elongated MnS in the fracture surfaces. MnS in the fracture of Ca-treated steel samples is predominantly elongated-shaped (Fig. 18), while MnS in the fracture of Mg–Ca-treated steel samples is mostly spindle-shaped (Fig. 19). The types of sulfides in the fracture surfaces of three transverse impact samples are analyzed and the results are presented in Fig. 20. Based on Fig. 20, it can be observed that the proportion of spindle-shaped sulfides is higher in the fracture surfaces of Mg–Ca-treated steel samples. Therefore, it can be concluded that the shape of sulfides has a significant influence on the transverse impact toughness of the material.

Fig. 18
figure 18

Micro-morphology and elemental mappings of impact fracture of transverse sample of Ca-treated steel

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Fig. 19
figure 19

Micro-morphology and elemental mappings of impact fracture of transverse sample of Mg–Ca-treated steel

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Fig. 20
figure 20

Number density and percentage of sulfide in impact fracture of transverse samples

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The research [55] shows that the stress of MnS during the tensile process of the sample is illustrated in Fig. 21. During the impact testing process, the stress state of MnS is similar to that during the tensile deformation. In the longitudinal samples, MnS mainly experiences compressive stress from the matrix (Fig. 21a), with the stress direction perpendicular to the length of MnS. Thus, MnS primarily elongates and is less prone to separating from the matrix. Simultaneously, the movement of dislocations and crack propagation in contact with MnS can cause fracture of MnS under pressure. Therefore, the fractured MnS located at the dimple is observed at the longitudinal fracture, as shown in Fig. 17. In the transverse sample, MnS mainly experiences stretching stress from the matrix (Fig. 21b), with stress directly acting on the interface between MnS and the steel matrix. This leads to the separation of MnS from the matrix, forming microcracks as a source of cracks. It also facilitates crack propagation along the length direction of MnS, greatly reducing the transverse impact performance. As a result, the transverse impact toughness is significantly lower than that of the longitudinal samples, as shown in Table 3, leading to material anisotropy.

Fig. 21
figure 21

Schematic diagram of tensile force during tensile tests. a Transverse tensile test; b longitudinal tensile test. F—Stress

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Ductile fracture typically occurs through the formation of voids on non-metallic inclusions under low plastic strain, followed by strain hardening which causes the voids to grow. Eventually, the voids connect and fracture occurs through local shear mechanisms. In McClintock’s ductile fracture model, the conditions for localized shear around inclusions can be expressed as follows [56, 57]:

$$ \frac{1}{\sigma } \cdot \frac{{{\text{d}}\sigma }}{{{\text{d}}\varepsilon }} < kF^{2} \left( {\frac{f}{1 - f}} \right)^{2} \sqrt {\left( {\lambda^{2} + 1} \right)} $$
(10)

where σ is the true stress; ε is the true strain; dσ/dε is the work hardening rate; k is a constant; F is a hole growth factor; f is the volume fraction of inclusions; and λ the inclusion aspect ratio.

Under the same conditions, reducing the aspect ratio of inclusions is unfavorable for the formation of localized shear bands, resulting in improved toughness of the steel material.

The stress concentration area is usually the initiation sites of cracks, and the theoretical stress concentration factor \(k_{t}\) at the inclusion tip can be calculated by the following [58]:

$$ k_{t} = 1 + 2\frac{a}{b} = 1 + 2 \times A_{\text{s}} $$
(11)

where a represents the length of the inclusion; As represents aspect ratio; and b represents the width of the inclusion. It is evident that with a larger aspect ratio of inclusions, there is a greater concentration of stress at the location of the inclusion, making it more susceptible to crack formation. Consequently, the impact performance of the material tends to be poorer.

The relationship between fracture strain and inclusion size was studied in the literature, as shown in the following [54]:

$$ \varepsilon_{{\text{f}}} = \varepsilon_{{\text{m}}} - 0.87\left( {\frac{{V_{{\text{v}}} }}{d}} \right) $$
(12)

where \(\varepsilon_{{\text{f}}}\) is fracture strain; \(\varepsilon_{{\text{m}}}\) is matrix fracture strain; Vv is volume fraction of inclusions; and d is the inclusion dimension perpendicular to the fracture plane. d indirectly reflects the influence of the aspect ratio of inclusions on the fracture strain. As d increases, which indicates a smaller aspect ratio, the fracture strain also increases, leading to better toughness of the material.

In summary, the aspect ratio of sulfide has an important influence on the impact performance of steel. A higher aspect ratio is more favorable for the formation of localized shear bands and stress concentration, resulting in lower fracture strain and poorer impact performance in the transverse samples.

Comparing the two tested steels, Mg–Ca-treated steel has a relatively higher proportion of spindle-shaped sulfides, with a percentage of 35.38% (as shown in Fig. 3c). Ca-treated steel, on the other hand, has a percentage of 19.99% for spindle-shaped sulfides (as shown in Fig. 3c). Therefore, Mg–Ca-treated steel exhibits better transverse impact toughness.

To improve the transverse impact performance and reduce the anisotropy of mechanical properties of sulfur-containing steel, it is favorable to decrease the aspect ratio of sulfides and increase the number of spindle-shaped sulfides. Among the various methods, Mg–Ca treatment shows better effectiveness compared to traditional Ca treatment.

Machinability is one of the important properties of non-quenched and tempered steel. Therefore, cutting tests were conducted on those two types of steel, Ca-treated steel and Mg–Ca-treated steel. Figure 22 is the photo of lathe for cutting test. The cutting velocity was set at 575 r/min, with feed rate of 2 mm/r. Figure 23 shows the photo of the hot-rolled bar after the cutting test and typical chips observed in cutting process. The chips of Ca-treated steel are mainly C-type chip with part of the spiral chip, and that of Mg–Ca-treated steel are all C-type chip. Based on the analysis of chip formation during the cutting process, it can be concluded that the steel chips from Ca–Mg-treated steel are shorter, indicating easier chip breaking and better cutting performance. In the cutting process, the size and shape of chips affect the cutting performance and the surface quality of the steel. The easier the chips break, the easier they are to be removed and the better machinability the steel [59]. Literature suggests that the greater the quantity of spindle-shaped sulfides in the steel, the better the cutting performance [60]. From the previous analysis, it is evident that the proportion of spindle-shaped sulfides is higher in Mg–Ca-treated steel, thus resulting in better cutting performance.

Fig. 22
figure 22

Photo of lathe for cutting test

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Fig. 23
figure 23

Morphology of cutting samples and typical chips observed in cutting process

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5 Conclusions

  1. 1.

    There are mainly three types of inclusions in the steel: MnS, MnS–oxide and other types. Compared with Ca-treated steel, there are more MnS–oxide inclusions in Mg–Ca-treated steel, and the number of small-sized inclusions increases significantly. The percentage of spindle-shaped sulfides in Ca-treated steel is 19.99%, while it increases to 35.38% in Mg–Ca-treated steel.

  2. 2.

    Under the same deformation conditions, the area of sulfide (which can be equivalent to the size of sulfide) does not affect its aspect ratio.

  3. 3.

    For the sulfide MnS–oxide with an oxide core, controlling the content of Ca and Mg in the oxide core at or above 10 wt.% and the area ratio at or below 5 helps them assume a spindle-shaped morphology after rolling. The relationship between the aspect ratio and the area ratio of this type of sulfide can be expressed as: As =  − 1.03181 + 1.99192 × Ar.

  4. 4.

    When CaO/Al2O3 (mass%) > 1.86, increasing MgO content will reduce the sulfur capacity of inclusions. When CaO/Al2O3 < 1.86, increasing MgO content will increase the sulfur capacity of inclusions. Under the conditions of this experiment, the inclusion with a certain amount of MgO can improve its sulfur capacity, facilitating the formation of composite sulfides.

  5. 5.

    With the increase in MgO content in the inclusion system, there is a tendency for a reduction in lattice mismatch between inclusions and MnS, which favors the nucleation and precipitation of MnS with oxide cores. Therefore, compared to Ca-treated steel, Mg–Ca-treated steel contains more composite sulfides with oxide cores.

  6. 6.

    The transverse impact energy of Ca-treated steel is 25.785 J, and that of Mg–Ca-treated steel is 32.119 J. For sulfur-containing steel, Mg–Ca treatment can increase the number of spindle-shaped sulfides in the steel, thereby improving its transverse impact toughness compared to traditional Ca treatment methods.