Origin and Control of Non-metallic Inclusions in Steel


Nonmetallic inclusions play an extremely important role in many properties of steel, and affect the processing and use of steel products.

This article shows the origin and classification of the latest non-metallic inclusions, and summarizes a lot of work done on this research in recent decades.

This article includes the dynamic conditions of inclusion formation and the influence of the current smelting conditions on the composition, quantity and size distribution of inclusions.

The research steel types of inclusion denaturation include radial steel wire, spring steel and bearing steel, so as to obtain the desired size and shape of inclusions.

At the same time, it is necessary to prevent the continuous casting nozzle from clogging by flocculent flow.

With the development of micro electron microscopy, the distribution characteristics of inclusions have been fully displayed.

The promising field of “oxide metallurgy” has also been discussed in inclusion engineering.

Finally, the improvement of inclusion characteristics and the difficulty of quantitative analysis are briefly described.

In the past few decades, the control of inclusions in steel has made remarkable progress, which is due to the deepening understanding of the interaction between thermodynamics, slag composition of molten steel, and steelmaking process, which enables the control of inclusions and process to optimize the characteristics of steel.

However, there are still some important problems that must be solved and the inclusion control and optimization process must be constantly improved.

1. Introduction

Until fifty years ago, people began to pay attention to the study of non-metallic inclusions (NMIs).

It was believed that the inclusions in steel came from the corrosion of refractory materials and the involvement of various mold fluxes and top slags.

Although the study of inclusions was an important subject at that time, the role of inclusions in steel has not been as popular as it is today.

Because inclusions in the steel matrix are non-metallic phases, many physical metallurgists do not focus on them.

Most of the research focuses on understanding the behavior of metals and phase transformation process.

When the steel requires challenging performance and more severe service conditions, the type, size and distribution of non-metallic inclusions in the steel matrix have a clear correlation with the performance composition.

Understanding these characteristics and behaviors of inclusions becomes extremely important, leading to in-depth research on the origin, characteristics and behaviors of inclusions in the smelting and processing of steel products.

The control and quantitative analysis of non-metallic inclusions have been greatly improved, and the influence of inclusions on the properties of metal materials has also been investigated.

Inclusion control has been carried out since the 1980s to improve the performance of steel, which has become the most important link in smelting.

The inclusion control engineering starts to do the desired inclusion characteristics, and then, through thermodynamics and appropriate process design and production of steel, the desired inclusion occupies the leading position.

In this article, the origin and control of inclusions are studied, the behavior of inclusions during machining is displayed and understood, the quantitative analysis and distribution characteristics of inclusions are briefly described, and the latest development of inclusion control engineering is also presented.

The influence of inclusions on the properties of steel is not discussed in depth here. The literature in this field is growing rapidly.

Readers can query according to specific topics and get the desired content at any time.

The classic books are written by Kiessling.

The conferences and papers of the International Clean Steel Organization are held every 3-5 years and sponsored by Hungary Mining and Metallurgical Association.

Interested readers can directly read these valuable knowledge.

2. Origin and classification of non-metallic inclusions

2.1 Classification of non-metallic inclusions

With the continuous development of modern steelmaking technology, it is noticed oxidation reactions and refining methods are used to remove harmful elements in steel.

These impurities harmful to steel, such as sulfur from coal and coke, enter liquid iron and steel, but the solubility in solid solution steel is very small.

During the solidification period, the molten steel moves from the crystallization front to liquid steel, and finally forms low melting point “FeO” and “FeS” compounds or eutectic containing both compounds.

This steel cannot be used for hot working rolling and forging.

The oxides, sulfides and alloy elements (such as Mn) in steel form a complex relationship. However, if high-quality steel is to be produced, the content of oxygen and sulfur dissolved in molten steel must be reduced.

Many elements can be used as alloy elements in steel, such as Mn, Al, Si, which have high affinity with oxygen and can be deoxidized in molten steel.

These deoxidized products become oxide non-metallic inclusions.

On the other hand, the steel contains sulfur, and the solubility of Ca and Mg in the steel is very small.

Their affinity with rare earth and S is high enough to form non-metallic inclusions of sulfide with low melting point.

As a result, most of the S in the steel enters the slag during refining and is removed, and the rest S precipitates sulfide inclusions during solidification.

These non-metallic inclusions can be divided into two categories according to their types: the chemical composition of inclusions (oxide and sulfide inclusions, etc.) and the stage of inclusion formation.

Solidification is the boundary point in the formation stage of inclusions.

The inclusions generated before solidification become primary inclusions, while the inclusions generated during and after solidification are called secondary inclusions.

In addition to these types, other commonly used classifications are also easy to confuse, involving the source of inclusions, and inclusions generated in the steelmaking process (such as oxide and sulfide inclusions) are classified as “endogenous” inclusions;

The inclusions from the outside are called “exogenic” inclusions (refractory chips, mold powder inclusion, etc.), which are classified as exogenic inclusions.

In general, there are few large particle exogenous inclusions that are independent of the molten steel for a long time and do not react with the molten steel.

The change is very obvious. In the past, it was believed that the extra large inclusions came from refractory and casting mold.

However, in modern steel, such large particle inclusions have been significantly reduced.

However, when some students saw this topic, they finally believed that the most important non-metallic inclusion was “exogenous” inclusion.

This does not mean that the interaction between molten steel and refractory is irrelevant.

The existence of these inclusions in steel indicates that molten steel and refractory have an impact.

If the defined exogenous inclusions also include secondary oxidation and the involvement of mold powder, the scope of exogenous inclusions will be expanded, which is a more appropriate definition of exogenous inclusions.

However, these inclusions not only change in the smelting process, but also in some cases, there is a debate about what process is involved.

Therefore, whether “exogenous” or “endogenous” inclusion classification is still in dispute.

Finally, the common method of classification of non-metallic inclusions is related to the size of inclusions: that is, inclusions can be divided into macro inclusions and micro inclusions.

It should be known that there is a sensitive value on the size of inclusions. The classification proposed by Kiessling is used.

That is, if the steel products are damaged instantaneously due to the inclusion that is large enough during processing or use, this inclusion is called macro inclusion, as shown in Fig. 1.

In fact, we should realize that this classification is difficult to apply, and randomly divide inclusions into macro and micro according to size, which is difficult to define.

Fig. 1 Fracture of tire radial wire during drawing due to the existence of large particle hard phase.

The inclusion is indicated by the arrow, and the scanning electron microscope backscatter photo is taken.

2.2 Thermodynamic Basis

Due to the high steelmaking temperature, the inclusion formation reaction is close to the equilibrium state.

Therefore, thermodynamics has become an important tool to understand inclusions.

The thermodynamic basis of inclusion formation has been carried out as a research topic for a long time.

However, for example, the thermodynamic formation data related to inclusions in aluminum oxide are still inconsistent and contradictory, which is still worth further study.

As a general smelting steel, aluminum enters the steel as a deoxidizer, and the thermodynamic calculation process of inclusions generated is not a problem, but there is uncertainty in the smelting process of new-generation steel materials with high aluminum and high manganese, which is very important.

In the case of extremely limited dissolution of Mg and Ca in steel, the problem becomes complicated.

As these elements, it is still very common to study the related thermodynamic data in the steelmaking process.

In recent decades, thermodynamic calculation has been provided to solve complex problems.

It is still a little difficult to deal with these problems using conventional calculation methods.

Examples of applications in various aspects of iron and steel products and smelting in general have been discussed.

Examples of the application of related inclusion problems can be seen in many literatures.

3. Formation, removal and control of inclusions

It is beneficial to discuss the formation and removal of inclusions by classifying them into primary inclusions and secondary inclusions. It is feasible in principle to remove the primary inclusions in steel.

However, secondary inclusions occur during solidification and cannot be removed. They can only be modified to reduce their harm in steel as much as possible.

3.1 Primary inclusions

3.1.1 Nucleation and structure of inclusions

From the view that the primary inclusions formed in liquid steel are closely related to the thermodynamic process, two important aspects are considered: their nucleation and corresponding structures.

In general, when deoxidizer is added to molten steel, it will nucleate rapidly.

This is because high supersaturation is observed during deoxidizer addition and dissolution. Sigworth and Elliott carefully evaluated silicon nucleation conditions and observed that supersaturated dissolved oxygen is required.

However, Miyashita’s results and industrial observations did not show obvious supersaturation in silicon deoxidization during steelmaking.

Miyashita also observed the comparison between dissolved oxygen and total oxygen, indicating that the rate of total oxygen reduction is determined by the removal rate of deoxidized products, as shown in Fig. 2.

Origin and Control of Non-metallic Inclusions in Steel 1

Fig. 2 Total oxygen and dissolved oxygen in steel after silicon deoxidation in the molten bath are measured as a function of time.

In many deoxidation studies, the difference between total oxygen content and dissolved oxygen content in steel depends on the amount of oxide inclusions generated.

This highlight conclusion is shown in Fig. 3.

Origin and Control of Non-metallic Inclusions in Steel 2
Origin and Control of Non-metallic Inclusions in Steel 3
Origin and Control of Non-metallic Inclusions in Steel 4

Fig. 3

a is a simple example of total oxygen and dissolved oxygen depending on the amount of oxide inclusions.

In the example, deoxidation starts at the “a” point, and Al is added to the steel, starting with% Oi of dissolved oxygen.

In the absence of nucleation conditions at the nucleation boundary, aluminum oxide is generated at the point where the dissolved oxygen and aluminum content reach the “c” point.

The total aluminum content in the steel corresponds to the dissolved oxygen (% O) in the steel. The oxygen entering the aluminum oxide inclusion is still in the molten steel, corresponding to the “b” point.

The deoxidation process from the “a” to the “c” point reacts 2Al+3O=Al2O3 according to the chemical equation.

Note: Generally, the difference between full aluminum and acid soluble aluminum (% Als) is small and difficult to measure.

B is the relationship between the inclusion content and total oxygen content (% Ot) in steel determined by literature.

When the inclusion and total oxygen content are low, great care should be taken to obtain reliable measurement, as shown in Fig. c.

C is the total oxygen analysis from the density of oxide inclusions in the chemical analysis sample to the end of refining, and the number of oxide inclusions is automatically identified and counted by SEM.

Suitu and the collaborators discussed the formation of alumina inclusions in the laboratory study of supersaturated oxygen.

If Si is used as deoxidizer, it seems to be no problem in steelmaking.

In actual smelting operation, heterogeneous nucleation is very rich, and the conditions observed in the laboratory cannot repeat the industrial production site.

It is very meaningful to study the microstructure of aluminum oxide as inclusion core and growth in steel.

Therefore, aluminum killed steel is very important for large-scale industrial production.

Some authors have paid attention to the microstructure of aluminum oxide and its relationship with supersaturated oxygen in steel and smelting time in detail.

Fig. 4 shows the inclusion shape corresponding to the typical deoxidizer and oxygen activity studied by Steinmetz and his collaborators.

It can be seen from the figure that supersaturated dissolved oxygen plays an important role in the morphology of inclusion structure.

Recently, Tiekink and its collaborators tried to observe the functional relationship between aluminum oxide inclusion structure and supersaturated oxygen and aluminum composition, as shown in Fig. 5. This task is very complicated.

Origin and Control of Non-metallic Inclusions in Steel 5

Fig. 4 Functional relationship between regional oxygen activity, aluminum activity and oxide growth

Origin and Control of Non-metallic Inclusions in Steel 6

Fig. 5 Overview of morphology of alumina inclusions corresponding to different oxygen activities and Al content in steel

The structure of oxide inclusions has a great influence on the properties of the final product.

It is particularly noted that the inclusions formed and grown at the beginning in liquid steel have different structural morphology (see Figures 4 and 6), due to the impact of inclusions on each other (see Fig. 7).

If the refining time is long enough, the inclusion shape will change due to the surface energy.

Origin and Control of Non-metallic Inclusions in Steel 7
Origin and Control of Non-metallic Inclusions in Steel 8

Fig. 6 Structure of some alumina inclusions.

The inclusions extracted from the casting billet matrix are dissolved.

The aluminum oxide tree structure is in the dotted line a.

The fibrous filter element is used to keep the inclusions in the dissolution process, and also serves as the background background for the inclusions.

Origin and Control of Non-metallic Inclusions in Steel 9

Fig. 7 Alumina cluster sampled from ladle, deeply corroded with picric acid

3.1.2 Removal of inclusions

The floatation of non-metallic inclusions in static molten steel bath can be determined by a simple calculation method due to the limitation of Stokes law.

At the normal ladle depth, the floating rate of small particle inclusions is limited, and it takes a long time to reach the steel slag surface.

Such a long floating time is unacceptable, especially for aluminum oxide inclusions, but the impact polymerization between inclusions obviously helps them to float, so their clustering polymerization is very important.

The importance of this upward aggregation is observed online.

Emi and his collaborators observed the behavior of inclusions on the interface between steel and gas online.

They showed that under this condition, aluminum oxide clustering is very fast.

On the other hand, calcium aluminate inclusions are difficult to gather together with each other, and only complete collision occurs in the liquid.

Wikstrom and the collaborators expanded the work of online observation of steel slag surface and inclusions in slag, and they agreed with the results of Emi steel slag interface observation.

Emi and his collaborators observed that when the phenomenon occurs at the gas steel interface (such as on the surface of bubbles), it does not directly point out how the liquid steel gathers into clusters, which is particularly important for liquid inclusions.

Here, other forces may be relevant.

In any case, whether the inclusion is solid or liquid, it plays an important role in clustering polymerization.

For a long time, the basic understanding is that stirring promotes the agglomeration of inclusions, but the most important thing for inclusions is to submerge them in refining slag and refractory of ladle wall.

The work of Lindskog and his collaborators is to use radioactive tracer to test and trace this important inclusion into the refining slag and ladle wall.

Due to the limitations of current conditions, only BaO can be used as a suitable tracer to evaluate the final captured refining slag and mold flux in steel and their impact on steel cleanliness.

The use of BaO tracers is very effective in determining the effect of ladle refractory corrosion on the cleanliness of its heat number steel.

IRSID has developed the use of element lanthanum as a tracer for oxide inclusions.

Since La2O3 is very stable, alumina inclusions already exist when lanthanum is added to steel, and inclusions formed by re oxidation can be identified by La.

Exogenous inclusions originate from mold flux, and alkaline oxides can be used as tracers.

Mold flux is usually only used in the continuous casting process and contains obvious alkaline oxides.

The vast majority of all inclusions removed in the ladle refining period are deoxidized products, including three stages: separate inclusion production/clustering;

The inclusion moves towards the refining slag or refractory wall of the ladle; Absorbed by refining slag and refractory.

The inclusion movement has two important factors: molten pool stirring, which is a major factor, and is dragged up by rising bubbles.

Most ladle refining shows that the size of argon stirring bubbles in the ladle is too large to remove inclusions and effectively reduce inclusions in the steel, unless the amount of argon blowing is especially large.

Zhang and Taniguchi calculations show that argon blowing stirring is effective in the case of high flow velocity of molten steel and small bubbles.

SEN submerged nozzle and ladle long nozzle of continuous casting have obvious effect on preventing secondary oxidation, and some advantages are also observed in RH vacuum treatment riser steel flow.

The ladle stirring promotes the inclusion to float up into the slag in clusters. Fig. 8 is one of the first batch of researchers.

The research clearly shows that increasing the stirring power (here it means that the ladle adopts electromagnetic stirring) means increasing the kinetic energy constant of removing the inclusion (by measuring the total oxygen content in the steel).

Origin and Control of Non-metallic Inclusions in Steel 10

Fig. 8 Total oxygen content in ASEA-SKF ladle refining furnace is a function of stirring current and processing time.

The final oxygen content depends on the residual aluminum content of each furnace.

The industrial observation seems to point out that the inclusion removal will reach a maximum value under a certain mixing energy.

The first report of this observation may be Suzuki and his collaborators.

Their results are expressed as a function of the special work of mixing, so they realize the importance of mixing energy: the reduction of the effect of refining to remove inclusion seems to be because the refractory of the ladle is added to the steel after corrosion, or the steel is wrapped with slag, because CaO and MgO type inclusions increase under strong stirring, their results are shown in Fig. 9.

Origin and Control of Non-metallic Inclusions in Steel 11

Fig. 9 The influence of mixing power on the degree of secondary oxidation. The circle point has reached below 20ppm total oxygen content

Later, Neifer and his collaborators, Ek and their collaborators used computational fluid dynamics and physical models to study the removal of oxide inclusions.

The flow rate of argon in the ladle and the removal of inclusions were treated as a functional relationship.

The results of the Neifer model pointed out that the efficiency of removing metal inclusions was improved through the optimization of gas flow.

They observed that increasing the gas flow rate had no effect on reducing the total oxygen content in the steel.

They attributed this phenomenon to the secondary oxidation of the molten steel in contact with the atmosphere.

These conclusions are consistent with the Suzuki team’s results.

Ek team showed that the influence of argon flow rate on inclusion removal is quite low.

The author suggested that it is better to use a lower argon flow rate to remove inclusions and clean molten steel.

Interestingly, the Neifer team’s industrial measurement indicated that the total oxygen content in steel decreased with the increase of gas flow.

They suggest using natural convection transport in industrial experiments to achieve optimal results.

Due to the limited measurement data in the industrial field ladle test, it is difficult to infer the observed results.

Interestingly, the Neifer team’s industrial measurement indicated that the total oxygen content in steel decreased with the increase of gas flow.

They suggest using natural convection transport in industrial experiments to achieve optimal results.

Due to the limited measurement data in the industrial field ladle test, it is difficult to infer the observed results.

Recently, Zhang and Thomas collected many kinetic constants for use in the functional relationship between oxide inclusion removal and stirring power, as shown in Fig. 10.

They obtained some measurement data and tried to find out the optimal mixing scheme.

They also carried out numerical simulation to reproduce the expected behavior data of the surrounding part in Fig. 10.

Origin and Control of Non-metallic Inclusions in Steel 12

Fig. 10 Oxygen removal constant is a function of stirring power in different secondary metallurgical reaction vessels in d% Ot/dt=- kt formula

Suzuki team pointed out that optimizing the mixing process may cause secondary oxidation, excessive mixing may cause slag opening on the top of the ladle, expose the molten steel to the atmosphere, and cause slag coating, which occurs at the edge of the opening.

Fig. 11 shows the change of chemical composition of non-metallic inclusion when the desulfurization process is strongly stirred.

The content of Ca and Mg in the inclusion proves that the slag has been emulsified.

Origin and Control of Non-metallic Inclusions in Steel 13

Fig. 11 The relationship between the average composition of all non-metallic inclusions and the stirring intensity was analyzed by sampling from the refining furnace, crystallizer and slab.

Kaushik’s team pointed out that strong stirring promoted the emulsification of slag during desulfurization.

The evidence was that the calcium content in inclusions was high.

In the case of excessive argon stirring and less top slag, it is observed that aluminum oxide inclusions are regenerated, so it is necessary to optimize the stirring power during the refining of clean molten steel to remove inclusions.

The importance of secondary oxidation to the cleanliness of steel cannot be overemphasized.

Nadif team reported the importance of secondary oxidation and measurement control.

In recent decades, the steelmaking plant has taken many measures to control the source of secondary oxidation after refining;

The submerged nozzle SEN and ladle nozzle are regularly used to isolate the atmosphere in slab continuous casting.

Especially in the production of long products, the tundish and crystallizer are protected by inert gas, which has become the standard configuration for the production of high-quality steel grades.

Special attention shall be paid to the design of inert gas valve to avoid air intrusion caused by negative pressure in the valve system.

The surface tension contribution of inclusions attached to refining slag is the largest, followed by the ability of slag to dissolve inclusions.

Regardless of the composition of the slag, most refining slag and inclusions are wet.

This is due to the surface energy difference between inclusions and molten steel and between inclusions and refining slag.

This phenomenon has been discussed for a long time, at least from the literature, and others are summarized by Olette.

The liquid fraction in the refining slag promotes the removal of non-metallic inclusions, which was clearly known in very early literature and confirmed by gifted experiments.

However, there are still some contradictions in slag viscosity.

Nakajima and Okamura put forward a model to describe the process of inclusions passing through the steel slag interface.

Later, many studies discussed and recently studied the subject of inclusion absorption by slag in detail.

Nakajima and Okamura proposed that under certain conditions, inclusions enter slag from steel, which may include metal film from the interface as a channel, while in other examples, especially in solid inclusions, such metal film does not exist, as shown in Fig. 12.

Origin and Control of Non-metallic Inclusions in Steel 14
Origin and Control of Non-metallic Inclusions in Steel 15

Fig. 12 shows two types of inclusions crossing the steel slag interface, introduced from Nakajima. The Reynolds number of inclusions reaching the interface determines their behavior

Their conclusion is that slag viscosity and corresponding surface energy are important parameters to determine the inclusion passing through the interface, and reduce the risk of inclusion returning to the molten steel again.

This phenomenon is summarized by a condition, namely, the Reynolds number when the inclusion is close to the interface.

Recently, the Sridhar team has observed the channel of this film online, which is a common phenomenon.

In most cases, the path of inclusion entering the slag is extended.

Once the inclusion leaves the molten steel, the liquid inclusion will be dissolved in the slag immediately.

With online observation, the thermodynamics of solid inclusion dissolution can be observed experimentally.

In some cases, the thermodynamics of dissolution is controlled by transport (diffusion in the boundary layer).

In other cases, such as MgO inclusion, the formation of the intermediate layer depends on the chemical composition of the slag, which may hinder the dissolution of the inclusion at various chemical stages in refining.

This is confirmed by the results obtained from the previous common technical methods.

Recently, Yan team estimated the dissolution of MgO in the slag, pointing out that all the data obtained are controlled by the quality transmission.

Holappa team investigated and studied the activity of tundish covering agent. It is a key characteristic that the tundish covering agent absorbs inclusions.

They observed that there is a complex interaction between the chemical composition, thermodynamic conditions, surface tension and viscosity of the slag when solid non-metallic inclusions are dissolved in the given slag.

The Holappa team concluded that “further systematic research is needed to obtain more knowledge in this field,… the residue of optimization needs to be developed”.

It is a good thing that non-metallic inclusions are adsorbed on the refractory surface of the ladle, but such inclusions adsorbed on the refractory surface of the ladle may also be the source of inclusions in the next furnace, which often depends on the composition of the ladle slag.

If inclusions are adsorbed in the molten steel pipeline channel, it will cause very troublesome accidents, such as long treatment time and high cost due to nozzle blockage in the continuous casting process.

This nozzle clogging phenomenon is well described in the references, which will be discussed later.

It is noted that the flocculent flow at the nozzle is caused by the adhesion and accumulation of aluminum oxide inclusions, and the FeO and inclusions that may be formed in the secondary oxidation gather together.

The references clearly describe this phenomenon.

There are a large number of publications describing the absorption of primary inclusions by mold powder during continuous casting and ingot casting.

The literature points out that this is possible, but the main points of view are the same as those discussed above.

The mold flux for continuous casting and mold casting (similar to the tundish covering agent) must have multiple functions and fluidity.

It is subject to various constraints in the mold, including avoiding the inclusion of mold flux into the surface of the primary green shell, which to some extent limits the movement of inclusions and remains in the mold flux, optimizes the flow of molten steel in the tundish and mold, so that its molten steel can reach the slag interface with inclusions, However, it conflicts with other relevant metallurgical objectives.

Removal of inclusions caused by convection is the best thing for ladle refining, and it is also necessary to prevent secondary oxidation from causing new inclusions, which is an important part of production of clean steel.

Another problem is the movement process of the primary inclusions in the continuous casting slab.

It has been well recognized that the inclusions are asymmetrically distributed on the cross section due to the arc continuous casting.

Asymmetrical inclusions are often related to the clogging of floc flow at the nozzle.

Sichen recently demonstrated the effect of the secondary refining model, especially the refining furnace process.

The model attempts to give the interface reaction of steel slag, the opening of the transition stirring slag layer, the generation of inclusions, nucleation, growth and separation and floatation removal, using most of the available technologies.

However, Sichen noted that the main variables in the ladle refining process, such as mass transfer efficiency, inclusion floating removal rate, over stirring slag layer opening, and argon flow rate, are difficult to simulate due to uncertainties in industrial production, such as ladle vent plug and gas pipeline leakage.

It is difficult to control and detect argon flow velocity in industrial ladle refining.

Camera and image analyzer can be used to control the opening of ladle slag layer, and vibration measurement can be used to control the argon flow. These technologies have been adopted in some steel plants.

3.2 Secondary inclusions

During solidification, the driving force of secondary inclusion precipitation increases the segregation of solute elements, and the solubility of oxides and sulfides in steel decreases with the decrease of temperature.

The precipitation of inclusions due to the change of solubility in steel has been discussed for a long time.

Since the 1960s, the names of primary and secondary inclusions have been determined, and the relationship between segregation and inclusion precipitation has been established.

At that time, the first model describing this phenomenon was established.

Turkdogan and Flemings have made an important contribution to understanding the comprehensive effect of the change of solubility with decreasing temperature on the segregation of secondary inclusions.

During the 1980s and 1990s, Nippon Steel and IRSID developed sophisticated models.

The same technology has also been successfully applied to the precipitation of nitride in HSLA microalloyed steel during solidification.

These models open a way to study inclusion engineering.

Today, we know that coupling the thermodynamic database and the kinetic database, solidification simulation and calculation of inclusion formation.

These calculations start with the required chemical composition of steel, observe the precipitation of inclusions, and design the refining slag composition during ladle refining, so as to meet the requirements of smelting clean steel.

The interaction between liquid steel between dendrites and inclusions formed at the front of crystallization solidification is an important topic.

On line observation shows that the solidification conditions play an important role in the formation of inclusions pushing to the liquid phase at the front of the interface and engulfed inclusions.

In theory, these results can be calculated and adjusted to describe them, taking into account the effects of additional surface tension and density.

The theoretical research is mostly focused on the composition of metal matrix, and the results of non-metallic inclusions in steel are also more consistent with the actual situation.

The results show that the critical growth velocity V can be expressed by V=k/R as the interface velocity of inclusions with R radius of engulfment and repulsion, where k depends on the type of inclusions.

The structure of secondary inclusions is particularly affected by the reaction that takes place during precipitation, and the precipitation of carbide is about the best example.

Since Sims observed the effect of re-oxidation on sulfide structure in 1930, he later proposed three typical types of sulfide, which were described in detail by many authors.

Recently, Ishida team pointed out that, in addition to the reaction type accompanying sulfide formation, the surface tension also has an important influence on sulfide structure.

4. Inclusion denaturation treatment

Gaye team has made the best conceptual comprehensive exposition in the thermodynamic application of inclusion engineering in steel.

Fig. 13 gives a brief explanation in two adiabatic ternary phase diagrams.

Once the required inclusions are determined, the chemical composition of the steel producing these inclusions can be calculated.

Then, the composition of the refining slag used for refining can be calculated according to the chemical composition of the steel through the steel slag balance.

As the saying goes, “Steel can only be made after smelting slag” is reasonable.

This idea has been successfully applied in the production of various steels.

In steel, it is necessary to avoid aluminum oxide composite inclusions (such as spinel) of hard phase.

For example, in bearing steel, when the steel undergoes phase transformation during cooling, the inclusion is used as the nucleation core.

Calcium treatment turns inclusions into liquid inclusions, and together with Ca, sulfide is modified to avoid nozzle plugging.

Although it may seem simple in the process of inclusion modification, it is also discussed in this chapter.

Origin and Control of Non-metallic Inclusions in Steel 16

Fig. 13 shows the transformation process of inclusions. From the thermodynamic point of view, low melting point Al2O3 inclusions are expected to be obtained in Si Mn killed steel.

The expected inclusions are shown in the simplified phase diagram of MnO-SiO2-Al2O3 ternary system.

The left side of the diagram shows that the system is on the 1470 ℃ isotherm.

This area is 100% liquid phase limited in the thin line, as shown in Figure.

In this system, the aluminum content in molten steel with liquid inclusion equilibrium is given by the thick and solid isopleth at the selected temperature, and the dotted line indicates the chemical composition of the inclusion in 0.35% Si, 1% Mn steel at a certain temperature (as a function of the aluminum content of the steel grade).

From this figure, if you want liquid inclusions, the aluminum content of steel cannot be up to the gray circle (8ppm).

The figure on the right shows the simplified ladle refining slag of CaO-SiO2-Al2O3 system.

The 1520 ℃ isotherm corresponds to the conditions of the ladle refining furnace, indicating that the 100% liquid inclusion area is limited to the fine solid line.

At the selected temperature, the aluminum content of the steel in the equilibrium state in this slag system is given by the aluminum content line of the coarse solid line.

The grey line gives the corresponding oxygen content in the steel under study.

If liquid inclusions are required (on the left side of the figure), the refining slag composition must be selected from these according to the instructions of the drawing, so as to ensure that the aluminum content in the steel is less than 8ppm.

4.1 Wire and spring steel of radial tire

The tire radial steel wire is high carbon steel, which is deoxidized by silicon manganese.

Many automotive spring steels are also manufactured by similar methods.

Brittle non-metallic inclusions (usually aluminum oxide inclusions or other inclusions with high aluminum oxide content) are very harmful to the drawing performance of the steel wire, and also have a very important impact on the spring steel.

The composition of steel must be adjusted to prevent the formation of aluminum oxide inclusions or aluminum oxide rich inclusions, while silicon oxide inclusions with low melting point and good plasticity are relatively advantageous to aluminum oxide, which requires strict control of aluminum oxide content in slag, control of raw and auxiliary materials to prevent aluminum from entering steel, and use of low alkalinity binary slag system.

When this problem is solved for the first time, it is contradictory to the prevailing refining operation at that time.

There are many excellent examples and articles on inclusion treatment control thermodynamics of tire meridian and spring steel.

4.2 Bearing steel

The influence of single type of inclusions on the fatigue life of bearing steel is still controversial. Now it is clear that the size and quantity of inclusions in steel have an important influence on the fatigue life of bearing steel.

On the whole, it is recognized that calcium aluminate inclusions and spinel inclusions deteriorate the performance of bearing steel.

Therefore, there is also a view that the production of bearing steel requires very low total oxygen content and very low sulfur and aluminum content. 

Thus, the non-metallic inclusions can be kept at a low volume fraction.

Furthermore, magnesium brought from slag will lead to spinel inclusion, which must be avoided.

Different process methods are adopted in different steel plants according to different conditions to achieve the goal of producing high-quality bearing steel.

However, controlling the chemical composition of refining slag is always one of the key points to control non-metallic inclusions in bearing steel.

Fig. 14 shows the influence of Al, O and Ag contents of bearing steel 100Cr6 (AISI52100) on the composition of slag.

The calculated and measured aluminum and oxygen contents are compared in steel.

Origin and Control of Non-metallic Inclusions in Steel 17
Origin and Control of Non-metallic Inclusions in Steel 18

Fig. 14

a. Under the equilibrium state of bearing steel, the refining slag% Al2O3=5%,% CaO=48% remain unchanged, and the influence of MgO on Al, O and Mg is calculated using Thermo calc and SLAG2 databases at 1540 ℃.

b. Compare the calculated value and measured value of bearing steel after finishing the refining of furnace 3, and Thermo calc® and SLAG2 database is used for calculation.  

4.3 Prevention of nozzle clogging by calcium treatment and alumina inclusion

Calcium treatment is used to denature sulfide inclusions and control the anisotropy of hot rolled materials or forgings.

It can also be used to control the workability of inclusions through calcium treatment.

The use of calcium treatment to denature aluminum oxide inclusions into liquid composite inclusions to prevent clogging of the nozzle flocs has been widely used in recent decades, but it is controversial.

The treatment of calcium is complex, involving the solubility of calcium, the yield of calcium, and the high vapor pressure caused by oxidation when adding calcium, which have been extensively studied.

The mechanism of inclusion denaturation and the correct amount of calcium required to achieve the desired effect have also been extensively studied.

The formation of inclusions is complex. The outer layer, often with oxides as the core, is covered with a sulfur rich compound coating.

This phenomenon and the distribution of individual elements are shown in the example in Fig. 15.

Origin and Control of Non-metallic Inclusions in Steel 19

Fig. 15

Calcium treatment is used to improve castability. The compound calcium aluminate large particle inclusions found in the slab, sulfide and AgO are dissolved in the liquid inclusions during continuous casting.

Mg is reduced from slag into steel (see Section 4.2)

In the process of treatment, a considerable part of inclusions become liquid phase, and will not flocculate through the nozzle.

However, if the temperature of molten steel is too low, casting will be difficult.

The further reaction of non-metallic inclusions formed during solidification is complicated, as shown in Fig. 16.

Origin and Control of Non-metallic Inclusions in Steel 20

Fig. 16

Broken large calcium aluminate inclusions with complex phases in the slab sample, and dendritic solidification structure can be seen in the inclusion shell

The calcium content required for the modification of calcium oxide inclusions depends on the total oxygen content in the steel.

There is no method to determine the total oxygen content in the steel in time, so it is difficult to determine the amount of calcium added to the steel.

This raises an important problem for industrial production.

It is a process route to use thermodynamics to understand the clogging of floc flow in nozzle and define the castable window of continuous casting.

Dissolved oxygen can be measured, and the data of dissolved oxygen can also be used to control the calcium treatment efficiency, as shown in Fig. 17.

Origin and Control of Non-metallic Inclusions in Steel 21

Fig. 17

0.025% Al, 0.01% S and various total oxygen contents (20, 25 and 30 ppm, from left to right) in steel at 1540 ℃. The above figure shows the functional relationship between Ca content and dissolved oxygen calculation.

The presence of non-metallic phase is expressed in each composition interval.

Thermo-Calc® and SLAG3 databases are used for calculation.

Each point is the experimental measurement value of dissolved oxygen content in steel, which is obtained without nozzle blockage.

4.4 Oxide metallurgy

The final casting involves a problem that requires the control of the microstructure and cannot completely rely on the hot rolling process.

It has been proved that the ferrite nucleation of the weld metal is beneficial to non-metallic inclusions.

Based on the theory of inclusion nucleation, inclusion formation will deplete Mn in surrounding matrix, which seems to be effective.

Nonmetallic oxide inclusions are the nucleation core of MnS inclusions, and their applications have achieved the desired results.

Furthermore, the secondary oxidation of titanium in silicon manganese steel changes the inclusion into titanium oxide, or the oxide and nitride have high nucleation efficiency in ferrite, which has been confirmed.

Koseki, lnoue, Suito and Park proved that titanium nitride can be a powerful nucleating agent, which promotes the appearance of large equiaxed grains in continuous casting stainless steel and welding process.

Park and Kang recently demonstrated the development in this field.

Thermodynamic calculation and model calculation prove that alloy design and process design in oxide metallurgy can be very beneficial.

5. Quantitative analysis of inclusions

In recent decades, the practice clearly shows that the classification and quantification of non-metallic inclusions by means of comparison charts and images is not satisfactory in the iron and steel industry.

In order to improve the quantitative analysis of inclusions (information on the size, volume fraction and composition of inclusions), some new methods and practices have emerged.

It is obvious that in many cases, several methods should be used at the same time to understand the nature and process of non-metallic inclusions.

It has been learned that some characteristics depend on the distribution of inclusions and other characteristics.

For example, the fatigue performance of steel depends on the size of the largest inclusion.

Except for low-end products, the range of cleanliness of steel products varies greatly.

The total oxygen content of low carbon aluminum killed steel (LCAK) is about 40ppm.

The total oxygen content of typical bearing steel is about 5ppm, and the volume fraction of oxide inclusions are very different, but sulfide inclusions are not mentioned.

Extreme value statistics and its application are the most important in fatigue. These methods are almost not included in the general literature.

As a literature review, these methods are included. Readers can read the references.

The grade method of inclusion evaluation using extreme value statistics is proposed by Murakami in the program.

It has been widely used in the field of fatigue and has obtained very good results.

It must be mentioned here that with this method, fatigue does not consider the limitation of the maximum inclusion size.

In fact, the volume fraction of inclusions caused by large particle inclusions is increased.

Fatigue does not consider the inclusion of the largest particle, which is somewhat inconsistent with the expectation of steelmakers.

6. Conclusion

The classic adage “Making good slag is making good steel” has been deeply rooted in steelmaking operations.

In the past decades, understanding the impact of non-metallic inclusions on the properties of steel has led the steelmaking process from preventing “inevitable” inclusion pollution to optimizing the composition, quantity and distribution of inclusions in steel.

This transformation process affects all processes of the steel smelting plant, raw material selection (such as avoiding aluminum pollution), slag composition design, optimization of secondary refining conditions (such as refining process time and hydrodynamic conditions), and careful control of tundish and mold operation.

It has become the standard condition for smelting variety steel to control secondary oxidation carefully in all processes.

In the research and development of the influence of inclusions in steel, thermodynamics plays a decisive role.

Understanding thermodynamics, the chemical composition of steel and refining slag, and the interaction of steelmaking process conditions have become very popular.

The improvement of modeling tools is very obvious. The important knowledge of the control of inclusions in steel can be based on science.

These technologies have been widely used and continuously developed in the process of nonmetallic inclusion modification.

It is still necessary to continuously improve refining slag and correctly understand the role of nonmetallic inclusion in steel.

Inclusion modification and oxide metallurgy engineering are widely used in steel plants.

The cleanliness of steel is at least one order of magnitude higher than that of decades ago. Continuous development challenges the qualitative and quantitative analysis of non-metallic inclusions.

Quantitative analysis of all inclusions on the properties and behavior of the steel is a basic requirement, and there is huge room for discussion in future publications.

Therefore, although all advanced technologies and views are summarized in this review, in the coming decades, the important problem is still to continuously improve various technologies to improve the quality of steel.

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