A review of research on MnS inclusions in high‐quality steel

MnS which has good plasticity is a non‐metallic inclusion commonly found in steel. For most steel types, the size, shape, and distribution of MnS have a significant influence on the properties of steel. The large‐sized MnS inclusions disrupt the continuity of the steel and cause the anisotropy in steel. The result is a decline of steel's overall performance. In contrast, the small‐sized MnS inclusions which in the shape of spherical or spindle in steel can diminish the incidence of thermal embrittlement and improve the machinability of steel. The morphology of MnS in steel is mainly affected by the ingredients of steel and heat treatment manner. MnS inclusions in steel are present in spherical, polyhedral, dendritic, and irregular shapes. The precipitation behavior is mainly affected by the steel ingredients, heat treatment system, and other factors. This paper summarizes the latest research results about the factors affecting MnS inclusions and controlling measures in high‐quality steel in recent years.

the size and morphology of MnS inclusions.In view of this, the factors affecting MnS inclusions and the modification treatment of MnS inclusions are discussed comprehensively in this paper.

Damage to steel by MnS inclusions
The precipitation of MnS, which occurs in most steels, has a significant impact on the high-temperature ductility of steel.
Because the high temperature ductility of MnS inclusions and the steel matrix is basically consistency.When steel rolling or forging process, it easy along the metal extension direction into long strips of MnS and leading to a serious decline in the transverse mechanical properties of steel.Yamamoto et al. 13 found that during hot rolling, MnS inclusions that extend into long strips which have a greater effect on the local ductility of the steel compared to the small-sized Al 2 O 3 inclusions.Hosseini et al. 14 directly observed the separation behavior of MnS inclusions from the matrix in the tensile process of medium carbon sulfur-containing steel samples.It was found that when the stress reached 20% of the tensile strength, the interfacial separation behavior of MnS inclusions from the matrix began to occur in the transverse samples as shown in Figure 4A.While the longitudinal specimens do not separate at the interface between MnS inclusion and matrix during the tensile process.MnS inclusion firstly breaks inside the MnS inclusion, and the stress value is 90% of the tensile strength, as shown in Figure 4B.In low-carbon steel, large and elongated MnS inclusions are the primary factors causing the anisotropy of tensile ductility and impact toughness of rolled plates. 15In high-strength steel, MnS inclusions significantly affect the microstructure and mechanical properties of the steel matrix.Refining MnS through oxide inclusions can enhance the strength of the steel and suppress its anisotropy. 16In 304 stainless steel, the dissolution of MnS inclusions is the first step.The second step involves intergranular corrosion caused by the dissolution of the Cr-depleted zone.Finally, passivation occurs in the intragranular region. 17

Benefits of MnS inclusions on steel
MnS inclusions have been used as effective inhibitors in oriented silicon steel for the first time.The dispersion of MnS inclusions in steel can effectively inhibit the growth of secondary grains in the process of recrystallization, promote the F I G U R E 4 Separation of MnS from matrix during tensile process. 14owth of Gaussian phase grains, and enhance the magnetic properties of oriented silicon steel. 18Yang et al. 19 proposed that in the solidification process of liquid steel, small size or nano size MnS can promote nucleation of intragranular ferrite and splitting austenite to refine the grain, which can improve the strength and toughness of steel.Wang et al. 20 found that MnS coated with oxides as cores formed composite inclusions, which could offset the harmful effects of oxides on the contact fatigue properties of steel.High speed heavy rail steel is highly sensitive to hydrogen white spots, but if it is difficult to completely remove hydrogen from steel, a large number of studies have found that the presence of MnS in steel can reduce the diffusion coefficient of hydrogen in steel. 4,21In non-tempered and sulfur-containing free-cutting steels, MnS inclusions exert a notch effect during cutting and machining, making the chips easily breakable and improving the machinability of the steel meanwhile reducing tool wear. 8,22An appropriate amount of MnS can serve as a grain boundary refiner, aiding in improving the processability of steel, including machining and forming processes, thereby enhancing production efficiency.A suitable quantity of MnS can result in the formation of uniformly distributed hard particles within the steel, contributing to improved wear resistance and prolonged service life.

EFFECT OF MNS MORPHOLOGY ON STEEL PROPERTIES
Some Japanese scholars 23 believe that: the shape control of MnS inclusions in steel is more important than reducing the S content in steel, because excessive pursuit of steel cleanliness is uneconomical and technically challenging to meet requirements.The performance of steel has an important connection with MnS inclusions.Different forms of MnS will produce different effects.Because chain inclusions and long inclusions can cause defects in the steel, it is important to try to control the MnS inclusions into fusiform or spherical shapes.In 1977 Segal et al. 24 investigated the effect of MnS size on its degree of deformation and showed that under the same deformation conditions, the small size MnS deformed less than the large size MnS.Baker et al. 25 came to the same conclusion through a large number of experiments.Academician Yin research 26 : the spindle-shaped inclusions (L/W ≤ 3) are not easy to deform during heat treatment.The transverse mechanical properties of steel are basically stable, and it is a preferable inclusion form.Because strip sulfide with aspect ratio more than 4:1 will not only destroy the continuity of steel matrix, but also cause cutting bonding, it is generally believed that spherical or spindle sulfide with uniform distribution and fine grain is more beneficial to improve the cutting performance of steel than long strip MnS inclusions. 27,28The results of Wang et al. 29 showed that after the inclusion denaturation, a large part of MnS changed from strip shape to spindle shape with the change of sulfide morphology, and the section shrinkage and anisotropy of steel decreased by about 10% and 34%, respectively.

Effect of elemental oxygen on MnS
Among the elements in steel, oxygen has the closest effect on MnS.Controlling oxygen content in steel is one of the main methods to change the shape of MnS in steel.Due to the dissolution of oxygen, (Mn, Fe) (S, O) inclusions are formed.

F I G U R E 6
Relationship between length-width ratio and area of MnS inclusions and oxygen content in steel. 33e presence of these inclusions, which often exhibit poor shapes, significantly diminishes the deformability of MnS.This control over the spherical shape of MnS after hot working is highly advantageous, particularly in enhancing the cutting performance of steel. 30In 1949, Sims et al. 9 found that when the oxygen activity in steel is high, MnS is easily modified to spherical shape, while when the oxygen activity decreases, MnS gradually transforms from spherical to chain and irregular shape.Eeghem et al. 31 demonstrated that in deoxidized steel, when the oxygen content (T[O]) exceeds 0.012%, the first type of MnS inclusions form.These inclusions typically exhibit a multi-phase structure and assume a spheroidal shape when subjected to high temperatures, thereby enhancing cutting performance.When T[O] ranges between 0.012% and 0.008%, the second type of MnS inclusions develop, often appearing in a net or chain formation along grain boundaries.Conversely, when T[O] is below 0.008%, the third type of MnS emerges.This type lacks oxide precipitation, is single-phase, easily deformable, but does not contribute positively to cutting performance.Xia et al. 22 showed that when oxygen content in steel is high, MnS formation changes from eutectic to monotectic, forming type I MnS; when oxygen content is low and most of the MnS precipitation eutectic solidification ends, type II MnS is formed.When oxygen activity in steel is high, type I MnS gradually change from I to III with the decrease of oxygen activity and the increase of sulfur and manganese activities, 32 as shown in Figure 5.In cast steel, as the total oxygen content rises, the aspect ratio of MnS inclusions decreases for a constant inclusion area.Conversely, the aspect ratio of MnS inclusions increases with the area.Moreover, the impact of oxygen content on MnS inclusion morphology intensifies with increasing inclusion area, 33 illustrated in Figure 6.
When the total oxygen content in steel is low, it is easy to form pure MnS inclusions with good plasticity.As shown in Figure 7A, this thin strip of MnS inclusions will be significantly extended in the rolling process, affecting the mechanical properties.When the total oxygen content in steel increases, the number density of inclusions containing Al 2 O 3 increases greatly.And the Al 2 O 3 core MnS heterogeneous nucleation complex inclusions increased in proportion, as shown in Figure 7B, the average size of this type of inclusions is smaller and harder than the MnS inclusions alone, and the possibility of deformation in the rolling process is less.

F I G U R E 8
Effect of different sulfur content on the morphology of MnS inclusions in steel. 37(A) Effect of different sulfur contents on MnS morphology; (B) Formation of dendritic MnS morphology at 0.63% sulfur content.

Effect of manganese and sulfur elements on MnS
When the concentration of Mn and S in the interdendrite liquid exceeds the thermodynamic equilibrium solubility of MnS in the steel, Mn and S in the steel tend to be separated and precipitated as MnS inclusions.Jianhua Chu et al. 35 determined the precipitation process of MnS through sulfur content.The content of S in the steel plays an important role in the precipitation time of MnS. 36Takada et al. 37 found that when sulfur content in steel is 0.01%-0.04%,the spherical MnS forms in the steel.When the sulfur content of steel is 0.05%, the elongated MnS forms in the steel.When the sulfur content in steel reaches 0.1% and continues to increase, the polyhedral-shaped MnS forms in the steel, as shown in Figure 8A.
When the sulfur content reaches 0.63% and the manganese content is close to 2.88%, large-sized dendritic or skeleton-like MnS will easily be formed in steel, as shown in Figure 8B.Oikawa et al. 11 found that as the sulfur content in steel increases, the precipitation temperature of MnS increases accordingly, and the more easily MnS precipitates.Wakoh et al. 38 showed that when sulfur content in steel ≤100 ppm, MnS selectively precipitated oxides with high sulfur capacity and low melting point as nucleation, and when sulfur content in steel >100 ppm, all oxides may be the core of MnS precipitation.The variation of MnS heterogeneous nucleation rate with sulfur content under different deoxidation conditions is shown in Figure 9.The figure indicates that silicomanganate and manganese titanate promote heterogeneous nucleation of MnS The influence of sulfur content on the nucleation rate of MnS under different deoxidation conditions. 38

F I G U R E 10
Relationship between ω(Mn)/ω(S) and MnS spindle rate and inclusion number in steel. 41der low sulfur conditions.Conversely, Al 2 O 3 does not facilitate MnS nucleation heterogeneously.Under high sulfur conditions, most deoxidation products effectively serve as cores for MnS formation.Kim et al. 39 found that the dotted and shell-shaped inclusions on the oxide surface in steels with high Mn content are MnS.The timing of MnS precipitation can be managed by adjusting the manganese and sulfur mass fractions in steel, along with the degree of undercooling.Specifically, decreasing the sulfur mass fraction and augmenting steel undercooling can postpone the precipitation of MnS inclusions. 40It was found that the manganese and sulfur content in steel has a great influence on the number and morphology of MnS inclusions in steel.It can be seen in Figure 10, with the increase of the manganese to sulfur ratio, the spindle-shaped proportions in steel increases meanwhile the overall number of inclusions in steel decreases constantly. 41n free-cutting steels, a suitable Mn, S mass fraction ratio not only controls the MnS shape, but also improves the cutting process by avoiding the formation of (Mn, Fe)S that is unfavorable for cutting. 42

Effect of calcium elements on MnS
Calcium in steel can change the shape, size, and distribution of MnS inclusions.This is because calcium has a stronger affinity for oxygen and sulfur than common elements, which will take away oxygen and sulfur in steel and generate CaO and CaS.The increase of calcium content can promote the transformation of MnS from strip shape to spindle shape.The shape and size of the inclusions are greatly affected by the ratio of the mass fraction of calcium and sulfur in steel.When the ratio is greater than 0.2, the MnS change into a spindle shape, and the average size of the inclusions decreases with the increase of the ratio. 43Elemental calcium stands as the most frequently employed modifier for MnS inclusions.

F I G U R E 12
Relationship between calcium content and MnS shape and aspect ratio. 45(A) Variation of calcium content on MnS shape; (B) Variation of calcium content on MnS aspect ratio.
Experimental evidence suggests that adding calcium alloys to steel can induce the transformation of MnS from elongated strips to spindle-shaped forms. 3According to relevant literature reports, 44 after the steel is treated with calcium, the surface of oxide inclusions (Al 2 O 3 , CaO) will be coated with MnS and CaS precipitated during the cooling process, as shown in Figure 11.The composite inclusions are in the shape of fusiform or spherical, which helps to reduce the influence of MnS on steel properties and improve the performance of steel.In 1997, Blais et al. 45 found that with the increase of ω(Ca)/ω(S) ratio, MnS changed from elongated shape to spindle shape, and the average length-diameter ratio of inclusions decreased.With the increase of Ca content in steel, the solid solution of uniform distribution of Ca in sulfide gradually increased, thus Ca played a role in solid solution strengthening.As shown in Figure 12A, it is this solution strengthening that increases the hardness of the inclusions and makes the MnS inclusions not easily deformed, changing from long strip shape to spindle shape.As shown in Figure 12B, spheroidization of MnS can be achieved when the calcium content of sulfide is between 0.7% and 2.0% (1 < L/W ≤ 3).The increase of calcium content in sulfide has no obvious effect on the formation of MnS.As shown in Figure 13A.The MnS without calcium treatment presents in an elongated shape.So it is appropriate to control the content of calcium in sulfide between 0.7% and 2.0%.Kanisawa et al. 46 added Ca into the steel, so that (Ca, Mn)S was mainly generated in the steel, and the high temperature plasticity of sulfide was significantly reduced, as shown in Figure 13B.Moreover, through calcium treatment, MnS inclusions in the steel were mainly in the shape of spindles rather than long strips, so that the influence of MnS inclusions in steel on steel performance was less.Qiao et al. 47 found that when the calcium content of sulfide exceeds 0.7%, the length-diameter ratio of sulfide is less than 3, inducing the transformation of MnS from elongated to fusiform, and showed a quantitative relationship between the calcium content of sulfide and the length-diameter ratio of MnS (L/W): L/W = 1.68 + 0.91/Ca.Yao et al. 3 found that MnS in untreated samples were mainly fine strips and clustered.Treatment with calcium reduced the number of fine sulfide strips, resulting in a higher proportion of spherical and evenly distributed MnS inclusions.Moreover, the proportion of MnS inclusions with a length-to-diameter ratio in the range of 1-2 significantly increased in billets treated with calcium.
The rolled steel with MnS inclusions is more concentrated in the interval of 1-3 in length-diameter ratio.

Effect of carbon element on MnS
Sims 48 and Eeghem 31 investigated the relationship between MnS morphology and acid-soluble aluminum in steel at different carbon contents, the main difference between the two is that Sims found that the region of presence of the elongated MnS widens with increasing carbon content, while the opposite results were obtained by Egehem.Fredriksson et al. 49 found that in the carbon-free Fe-Mn-S alloy, only the monoectic reaction existed, resulting in the formation of spherical and elongated MnS.In the alloy containing 0.2% carbon, the MnS weakly crystallized from the liquid under the condition of slow cooling, while in the alloy containing 1% carbon, the crystallization characteristics were strengthened when the carbon content increased.It crystallizes completely and forms only polyhedral-shaped MnS.Ao et al. 50studied that the upper limit of sulfur content in steel can be relaxed appropriately by adjusting the addition of alloying elements to ensure a certain carbon content and a suitable ω(Mn)/ω(C).Chen et al. 51 found that with the increase of carbon content in steel, the proportion of spherical MnS inclusions in manganese steel gradually increase, while the proportion of strip MnS inclusions gradually decreases, and the total area of inclusions gradually decreases, as shown in Figure 14.

Effect of aluminum element on MnS
Sims et al. 48found that in oxygen-containing steels, the MnS morphology changes from spherical to tiny strips and finally to polyhedral when Al varies from 0.005% to 0.15%, namely, the Al content in the steel has a strong influence on the MnS shape.Lei et al. 52 investigated nonmetallic inclusions in Al-deoxidized steel.They discovered that 98% of the inclusions Relationship between MnS aspect ratio and tellurium content in steel. 56 the steel were diffusely distributed, consisting of MnS and MnS-Al 2 O 3 composite inclusions.Morphologically, the MnS inclusions appeared angular and likely belonged to polyhedral structures.

Effect of tellurium element on MnS
Tellurium is a highly surface-active element that effectively reduces the shear resistance during steel deformation and produces denaturation and modification of MnS. 53When tellurium is added to steel, the dissolved tellurium elements dissolve into MnS inclusions.After reaching solid solution saturation, tellurium forms MnTe with Mn, and MnTe aggregates and wraps MnS.Lattice distortion occurs when MnS are wrapped, resulting in the increase of spherical and elliptic MnS and the decrease of strip and chain MnS in steel. 54Zhang et al. 55 discovered that adding tellurium to steel leads to the formation of a white Fe-Mn-Tellurium composite phase around MnS inclusions.This addition also induces spheroidization of MnS by facilitating solid solution formation between MnTe and MnS inclusions.Bai et al. 56 investigated the modification impact of MnS inclusions by adding high-purity tellurium powder to 20CrMnTi steel.Figure 15 demonstrates that the tellurium treatment reduced the aspect ratio of inclusions in the steel from 3.17 to 1.83, highlighting a more pronounced spheroidization effect.Projak et al. 57 found that the denaturation treatment of MnS morphology by tellurium elements is less reliant on the steel's oxygen content.Additionally, spheroidizing MnS inclusions through tellurium element addition enhances the steel's machinability and cutting performance.

Effect of rare earth Ce on MnS inclusions
Since the affinity between sulfur and rare earth is greater than Mn, adding an appropriate amount of rare earth to steel will form Re x S y .Because of its unique electronic structure of 4f layer and strong chemical activity, rare earth Ce is widely used in the fields of inclusion reforming, steel purification and solidification structure control. 58As MnS inclusions precipitate out during solidification, previous studies have shown that adding Ce can refine MnS inclusions in steel.The mechanism of Ce action is the heterogeneous nucleation effect of high melting point rare earth inclusions on primary phase.The results of Li et al. 59 showed that after adding the appropriate amount of rare earth Ce, the precipitation of MnS in the steel was effectively inhibited and played a well role in the metamorphic inclusions.Wang et al. 60 showed that the MnS size of high-strength IF steel without rare earth Ce was significantly extended during the rolling process, about 10 μm.The addition of rare earth forms S-O-Ce type inclusions with spherical morphology and size of 2-5 μm and independent dispersion distribution, which has little effect on the continuity of steel microstructure and is beneficial to the improvement of steel performance.Wang et al. 61 studied the addition of rare earth Ce in EH40 ship plate steel, and found that with the increase of Ce content in steel, the MnS inclusions precipitates with Ti-Ce oxide as the core are spherical, and the refining effect of Ce on inclusions is obvious.The size of inclusions larger than 4 μm decreases significantly, and the percentage of submicron inclusions increases.Li et al. 61 found that the thermodynamic order of the possible inclusions generated in steel after the addition of rare earth Ce elements was CeAl 2 O 3 > Al 2 O 3 > Ce 2 O 3 > MnS.The rare earth element Ce affects the morphology and size of sulfur and oxygen inclusions, leading to diffuse spheroidization within the steel matrix.The size of most inclusions is controlled within 2 ∼ 3 μm, which improves the performance of the steel.An et al. 62 discovered that elevating the rare earth Ce content in steel decreased CeAlO 3 -like inclusions while increasing the number of Ce 2 O 2 S-type inclusions.Additionally, the average inclusion size decreased from 2.83 to 2.66 μm with a rise in Ce content from 0.015% to 0.028%.

Effect of temperature and heating rate on MnS
After heat treatment, the size and morphology of MnS inclusions in solid steel vary with temperature, and MnS inclusions of suitable morphology (spindle-shaped and nearly spherical) can be obtained to reduce their ductility during rolling process. 5,63In 1981, Macfarland et al. 64 found that rolled steel samples held at 925 • C for varying 0-30 h were found to have long strips of MnS gradually splitting into spherical or granular shapes, as shown in Figure 16.In 1996, Lou et al. 65 deformed the MnS inclusions at 1000-1050 • C by heat treatment, which was able to reduce the aspect ratio of MnS and thus improve the transverse properties of the steel.In 2011, Wang et al. 66 found that the number of MnS inclusions larger than 5 μm decreased significantly when the heat treatment temperature was held at 900-1300 • C for 2 h.It was found that the large-size MnS split into multiple particles at a heating rate <2 K/S.When the heating rate increased to 6 K/S and 10 K/S, the large-size elongated MnS no longer split, the low heating rate favors the splitting of large-sized MnS into spheres.In 2016, Zhang et al. 67 studied the influence of different heating temperatures and holding time at 1200-1400 • C on MnS inclusions in heavy rail steel.The test results showed that large inclusions with long MnS in rail were divided into particles with smaller size and thickness.The number of MnS inclusions larger than 80 μm decreased, and the number of MnS inclusions smaller than 5 μm increased.Murty et al. 68 studied the morphological changes of polyhedral-shaped MnS, and after holding at 1310 • C for different times, the MnS inclusions changed from long blade-like to short thick rods shape, ellipsoidal and spherical shapes in order, as shown in Figure 17.The inclusions in steel before heat treatment are generally flocculent and long irregular morphology, and after heating to 1200 • C MnS transforms into ellipsoidal inclusions and small particle size inclusions near spherical shape, as shown in Figure 18.High temperature heating can significantly promote the transformation of inclusions morphology to spherical shape in steel, and it has a significant effect on reducing the number of large size MnS inclusions in steel. 33Shao et al. 66 found that MnS inclusions in steel increased as the heat treatment temperature continued to rise.At the same time, with the increase of the temperature in the steel, the large size and slender MnS inclusions continue to split, and it is also found that the lower heating rate (≤2 K/S) is conducive to the modified MnS to be fusiform or nearly spherical, as shown in Figure 19.Park et al. 69 also concluded by their study that low heating rate is favorable for the splitting of large size long strips of MnS.Qi et al. 70   1300 • C, the size of MnS becomes large and spherical; when it is close to 1400 • C, there is a large amount of liquid phase around MnS, and MnS begins to redissolve in the steel matrix.Following heat treatment, the size and morphology of MnS inclusions in solid steel vary with temperature.High-temperature heating significantly promotes the transformation of inclusion morphology towards a spherical shape, effectively reducing the quantity of large-sized MnS inclusions in the steel.Low heating rates (≤2 K/S) are conducive to altering the morphology of MnS inclusions.The evolution of MnS inclusion morphology progresses from elongated, to elliptical, and eventually to spherical shapes.MnS inclusions in steel form elliptical shapes at around 1200 • C, with small particle sizes approaching spherical shapes.However, at higher temperatures, MnS inclusions increase in size and gradually dissolve.

Effect of cooling method on MnS inclusions
The limiting factors for the formation of MnS inclusions are the Mn and S concentration products and temperature.Under the condition of constant Mn content, the only way to reduce the aggregation of Mn and S at the solidification front and thus further reduce the size and the number of precipitated MnS inclusions is to reduce the S content and use strong cooling. 71Kinetic analysis shows that increasing the cooling rate and improving the cooling intensity during the solidification of the molten steel can control the timing and morphology of MnS precipitation and reduce its size, thus reducing its harmful effects of the steel performance. 67Imagumbai et al. 72 found that slow cooling favors the formation of spherical MnS inclusions and generates elongated sulfides when the cooling rate is faster.Shao et al. 8 investigated the effect of different cooling methods on the morphology of MnS inclusions after heat holding.MnS was linear under furnace cooling conditions, chain-like under air cooling conditions, and ellipsoidal under water cooling conditions.Some authors 73 analyzed the MnS formation and concluded that increasing the cooling rate can reduce the average radius of MnS, as shown in Figure 22.Tian et al. 74 observed the morphology of MnS in water-cooled and air-cooled samples by scanning electron microscopy, and took the average diameter of inclusions.The statistical results are shown in Figure 23.It can be seen that the size of MnS in air-cooled samples is larger than that in water-cooled samples.

Effect of high melting point oxides on MnS
Oxide metallurgy is a technique that alters the shape of MnS in steel, refines steel grain, and enhances steel toughness.It achieves this by utilizing high melting point, finely dispersed oxide inclusions formed within the steel as nucleation cores for MnS precipitates, thereby promoting ferrite formation in steel. 75,76With the introduction of the concept of oxide metallurgy, the use of fine diffusely distributed oxides in steel to refine MnS formed during solidification has become a hot spot for research.Figure 24 shows the schematic diagram of this method of refining MnS.The left side is a schematic diagram of MnS extending along the rolling or forging direction in general sulfur-containing steel, with large size and high aspect ratio.The right side is a schematic diagram of refining MnS using the oxide metallurgy method, with MnS with small size, high number, uniform distribution, low aspect ratio, and the existence of oxide particles in most of the core. 77It was found that different oxides (Mn-Si-O, Ti-Mn-O-N, ZrO 2 , Al 2 O 3 ) have an effect on the precipitation of MnS, which precipitates embedded on high melting point oxides, while the controlled cooling regime has an important effect on the precipitation of MnS. 78In 1997, OiKawa et al. 5 demonstrated that the addition of Ti to steel considerably decreased the size of MnS inclusions.This reduction occurred because of the liquid state of (Ti, Mn) O, high sulfur content, and nucleation growth along with MnS inclusions at the solidification front.In the same year, Gregg et al. 79 concluded that TiO exhibited a more potent nucleation effect.In 2001 Hasegawa et al. 80 found that ZrO 2 has a heterogeneous nucleation effect on MnS in Fe-Si alloys and most of the MnS inclusions have ZrO 2 inside.In 2003 Kim et al. 81 found that SiO 2 -MnO has a high sulfur capacity due to the high sulfur content in the SiO 2 -MnO oxide.In 2006, Liu et al. 82 showed that titanium-rich inclusions can promote the generation of ferrite in steel, which in turn contributes to the easier precipitation of MnS as a heterogeneous core of oxide during the solidification of the molten steel.In 2007, Zhanbing Yang et al. 83 discovered that adding approximately 0.02% titanium to medium-carbon untempered steel leads to the formation of composite inclusions containing titanium.These inclusions induce ferrite formation through heterogeneous nucleation.Guo et al. 84 found that ZrO 2 has a favorable heterogeneous nucleation effect on MnS because it has similar lattice parameters as MnS.Sarma et al. 85 found that Al 2 O 3 inclusions tend to form clusters in steel and cannot be distributed diffusely, so they cannot be used as an effective nucleation core for MnS.

Effect of deoxidation method on MnS
In recent years, a large number of oxide particles have been shown to have a good heterogeneous nucleation effect on MnS.Many scholars both domestically and abroad have conducted various studies on the effect of different deoxidation methods on MnS in steel.Figure 25 shows the effect of different oxides on the heterogeneous nucleation of MnS in low sulfur steel.Figure 25 illustrates that various deoxidation methods yield different oxides, each with distinct effects on MnS nucleation in steel.The composite deoxidation products formed by the composite deoxidizer result in over 80% heterogeneous nucleation of MnS compared to single deoxidizers. 86In 1996, Wakoh et al. 38 added Ti alone, Zr alone, and composite Ti-Zr to low sulfur steel and found that Ti-Zr composite deoxidation had the best effect for increasing the proportion of MnS nucleation.In 1999, Oikawa et al. 87 investigated the effect of Al and Ti deoxidation products on MnS morphology and quantity.They found that Ti oxide concentration was easily trapped at the solidification front, promoting MnS nucleation at this location.In 2010, Yang et al. 88 studied the effect of Ti and Al composite deoxidation on precipitation of MnS inclusions and found that MnS morphology was mainly mosaic and encapsulated.In 2013, Li et al. 89 investigated the effect of Al and Ti deoxidation on precipitation behavior of MnS in non-tempered steel.They concluded that Ti's deoxidation products were more favorable for heterogeneous nucleation of MnS while high Al content in steel was not favorable for refinement of MnS inclusions.In 2015, Zheng et al. 76 investigated the heterogeneous nucleation effect of Ti-Mg deoxygenating products on MnS inclusions under different Ti/Mg ratios.They concluded that an optimal molar ratio for Ti/Mg was between 0.05-0.2.In 2016, Wang et al. 90 investigated the effects of aluminum, calcium, and magnesium deoxidation processes on oxide nucleated MnS.As shown in Figure 26, the magnesium deoxidation process was found to be the most effective for nucleating MnS inclusions.

PRECIPITATION OF MNS INCLUSIONS IN SEVERAL TYPICAL STEEL FRADES
The results of the study 91 showed that in sulfur-containing free-cutting steels, the formation temperature of MnS increases with an increase in the Mn and S elements in the steel.MnS forms in the molten steel when the content of Mn and S elements is too high.Additionally, the content variation of C and Cr elements has almost no effect on the formation temperature of MnS.As can be seen from the phase diagram calculated in Figure 27, the MnS formation temperature decreases slightly as the C content increases, and the effect of the change in elemental C content on the MnS formation temperature is slight.

F I G U R E 28
Relationship between mass fraction of Mn and S and driving force of MnS nucleation chemistry in oriented silicon steel. 92I G U R E 29 Change of precipitation amount of MnS inclusions with temperature during solidification. 40 manganese steel, the degree of influence on the formation temperature of MnS inclusions, in descending order, is Mn, C, Si, P, S, Ni, while the influence of Cr, Mo, Cu can be ignored.In the actual production of 20Mn steel, implementing lower limit control on the C and Si content can effectively inhibit the precipitation of MnS inclusions. 51un et al. 92 found that the formation of MnS particles in oriented silicon steel is mainly influenced by the chemical driving force.The chemical driving force for MnS precipitation varies for different Mn and S contents, as shown in Figure 28 With an increase in the mass fraction of S content, the chemical driving force also increases.Compared with CaS, MgO, and other inclusions, MnS formation temperature is the lowest, but the extent of MnS precipitation increases with the increase of S content of the molten steel.
Zhang et al. 40 used the Factsage 6.4 thermodynamic calculation software, as shown in Figure 29, to calculate the formation temperature of MnS inclusions in U75V steel as 1631 K, which is 63 K different from the equilibrium thermodynamic parameters calculated as 1694 K.This method can accurately predict the precipitation behavior of MnS inclusions and reduce the difficulty of thermodynamic analysis of MnS precipitation.When the formation temperature of MnS is significantly lower than the steel's phase transition temperature during solidification, MnS will mainly precipitate in the supersaturated austenite.This can effectively reduce the size of MnS and control its morphology.

CONCLUSION
1. Manganese and sulfur have a strong affinity for non-calcium-treated steel, the addition of manganese to the steel will preferentially form MnS inclusions, in the form of spherical shape, polyhedral shape, irregular shape and other forms existence in steel.MnS inclusions are generally low hardness, appearing in almost steel grades, which has great impact for high temperature ductility of steel, because the high temperature ductility of MnS inclusions and steel matrix is basically the same, in the steel rolling or forging process, easy along the metal extension direction into long strips of MnS, resulting in a serious decline in the transverse mechanical properties of steel.2. The use of high melting point, fine diffuse oxide inclusions as the nucleation core for MnS precipitates in steel can promote the formation of ferrite and alter the shape of MnS in steel.This process can split austenite, refine the grain, and improve the strength and toughness of the steel.In contrast, strip sulfide with an aspect ratio of more than 4:1 can damage the continuity of the steel matrix and cause bonding during cutting.It is generally believed that the distribution of uniformly sized, finer particles of spherical or spindle-shaped sulfide is more conducive to improving the performance of steel compared to long strip MnS inclusions.3. The precipitation behavior of MnS inclusions in steel is primarily determined by factors such as steel composition and heat treatment regime.When the appropriate amounts of rare earths are added to the steel, the affinity between sulfur and rare earths exceeds that of sulfur and manganese, resulting in the formation of Re x S y .4. Rare earth elements are widely utilized in reformation of inclusions, purification of steel, and control of solidification organization due to their unique 4f-layer electronic structure and strong chemical reactivity.The size and morphology of MnS inclusions in solid steel vary with temperature.By applying appropriate heat treatment, the desired morphology of MnS inclusions (spindle and nearly spherical shape) can be obtained to decrease their ductility during the rolling process.

F I G U R E 5
Relationship between MnS inclusion morphology and oxygen content in steel.32

F I G U R E 7
MnS inclusions and MnS-Al 2 O 3 composite inclusions in steel. 34(A) MnS inclusions; (B) MnS-Al 2 O 3 composite inclusions.

F I G U R E 11
Formation of spherical composite inclusions by sulfide encrusting oxides.44

F I G U R E 13
Morphology of MnS in steel before and after calcium treatment.46(A) Morphology of MnS before calcium treatment; (B) Morphology of MnS after calcium treatment.F I G U R E 14 Effect of carbon content on morphology and area of MnS inclusions. 51(A) The effect of carbon content on the morphology of MnS; (B) The effect of carbon content on the area of MnS.
found that the morphology of MnS inclusions changed with increasing temperature, When <600 • C, MnS inclusions did not change significantly; when 600-870 • C, spherical transformation occurred; when >870 • C, the area of MnS inclusions began to increase; as shown in Figure 20.When >1100 • C, MnS inclusions did not increase significantly.When the temperature in the steel is close to 1200 • C, the MnS inclusions grow up and become round obviously,as shown in Figure 21; when it is close to F I G U R E 16 Changes of MnS morphology at 925 • C for different time. 64(A) The morphology of MnS of the original rolled sample; (B) The morphology of MnS after holding at 925 • C for 5 h; (C) The morphology of MnS after holding at 925 • C for 10 h; (D) The morphology of MnS after holding at 925 • C for 20 h; (E) The morphology of MnS after holding at 925 • C for 30 h.

F I G U R E 17
Morphology changes of MnS inclusions in steel at 1310 • C for different holding times. 68(A) The morphology of the original rolled material MnS; (B) The morphology of MnS after holding at 1310 • C for 0.5 h; (C) The morphology of MnS after holding at 1310 • C for 10 h; (D) The morphology of MnS after holding at 925 • C for 100 h.F I G U R E 18 Morphological changes of MnS before and after heat treatment at 1200 • C. 66 (A) Morphology of MnS before heat treatment; (B) Morphology of MnS after heat treatment.

F I G U R E 19 66 F I G U R E 20
Effects of different heat treatment temperatures and heating rates on the morphology of MnS inclusions.Morphology changes of MnS inclusions during continuous heating.70 (A) morphology of MnS at 15 • C; (B) morphology of MnS at 817 • C; (C) morphology of MnS at 1075 • C; (D) The morphology of MnS at 1177 • C; (E) The morphology of MnS at 1271 • C; (F) The morphology of MnS at 1377 • C.

F I G U R E 21 70 F 73 F I G U R E 23
Solid solution of MnS inclusions at 1175 • C. I G U R E 22 Relationship between cooling rate of steel and average radius of MnS.Distribution of MnS diameter in samples with different cooling modes.74

F I G U R E 24
Schematic diagram of MnS morphology using oxide nucleation.77(A) morphology of MnS before oxide nucleation; (B) morphology of MnS after oxide nucleation.

F I G U R E 25 86 F
Effect of different deoxidation modes on MnS heterogeneous nucleation.I G U R E 26 Different effects of Al, Ca, and Mg deoxidation processes on nucleating MnS. 90(A) The morphology of MnS after Al deoxidation process; (B) The morphology of MnS after Ca deoxidation process; (C) The morphology of MnS after Mg deoxidation process.