Research Progress on Injection Technology in Converter Steelmaking Process

: During the converter steelmaking process, gas-slag-metal three-phase emulsiﬁcation is realized by injecting gas to complete metallurgical tasks such as slagging, dephosphorization, de-carburization, and heating. As green and intelligent development of the steel industry progresses, converter steelmaking injection technology is also constantly innovating. In this review, the types and applications of top blowing injection elements, bottom blowing injection elements, and injection the medium are reviewed. Three different types of combined blowing processes are compared. At the same time, the advantages and disadvantages of different bottom blowing elements and injection media are respectively discussed. Finally, based on the research and application status of converter injection technology, the development direction of converter steelmaking injection technology is discussed. Accelerating the innovation of converter steelmaking injection technology, especially the improvement and breakthrough of high efﬁciency, reductions the environmental burden, and long life technology, will play an important role in promoting the transformation and improvement of the steel industry.


Introduction
In the 1850s, Bessemer invented the bottom air-blowing acid steelmaking process by which air was injected into a bottom chamber of the converter and then entered into the molten steel throughporous bricks; this technology was not able to dephosphorize steel [1,2].In the 1870s, Thomas invented the bottom air-blowing basic steelmaking process, which could dephosphorize steel after decarburization [3,4].In the 1860s, Siemens and Martin invented the open-hearth steelmaking process [5], which achieved dephosphorization before decarburization; decarburization was achieved through the iron ore powders.By this technology, the slag was over the molten metal, the nitrogen in the final steel was low and steel scraps could be used.In 1949, Professor Durrer successfully performed top blowing oxygen steelmaking experiments and stimulated the Linz Works and Donawitz Works in Austria to start the industrial-scale practice of oxygen steelmaking in 1952, this process is thus called the LD process [6,7].Later, after some process modification, it became known as the BOF process.
The BOF process is the use of supersonic oxygen jets that impact the molten bath, promote gas-slag-metal three-phase emulsification to complete slagging, dephosphorization, decarbonization and heating [8][9][10].The interaction between top blowing oxygen jets and the molten bath has an important influence on the mass and heat transfer, reaction rate, splashing, and slagging [11][12][13].At the same time, the bottom blowing gas further strengthens the molten bath flow and promotes the transfer of reactive material and temperature.To further improve converter steelmaking efficiency, molten steel cleanliness, and product Metals 2022, 12,1918 3 of 16

Conventional Supersonic Oxygen Lance
The structure of a traditional supersonic oxygen lance is given in Figure 1, where the red lines in the top view indicate the circles formed by the distance of the inlet and outlet centers of the nozzle from the axis of the oxygen lance, respectively.When the highpressure oxygen passes through the constriction section of the Laval nozzle, the oxygen flow velocity increases and reaches sound velocity at the throat [19,20].A supersonic jet will be formed at the nozzle outlet when the ratio of outlet oxygen pressure to inlet oxygen pressure is P Outlet /P 0 < 0.528.During the forward process of the oxygen jet, the surrounding low-velocity gas is continuously suctioned, making the jet velocity decrease.Therefore, the oxygen stream can be divided into three sections: the potential core zone (each point velocity is equal to outlet velocity), the supersonic zone and the subsonic zone.The first two sections are collectively called the supersonic core zone.The length of the supersonic core zone is a key parameter when characterizing the jet characteristics of the supersonic oxygen lance.

Water Simulation
Impact dent size Study on the morphology of impact dents and molten bath splashes.

Mixing effect
Study on the mixing effect of the molten bath by mixing time.

Conventional Supersonic Oxygen Lance
The structure of a traditional supersonic oxygen lance is given in Figure 1, where the red lines in the top view indicate the circles formed by the distance of the inlet and outlet centers of the nozzle from the axis of the oxygen lance, respectively.When the high-pressure oxygen passes through the constriction section of the Laval nozzle, the oxygen flow velocity increases and reaches sound velocity at the throat [19,20].A supersonic jet will be formed at the nozzle outlet when the ratio of outlet oxygen pressure to inlet oxygen pressure is POutlet/P0  0.528.During the forward process of the oxygen jet, the surrounding low-velocity gas is continuously suctioned, making the jet velocity decrease.Therefore, the oxygen stream can be divided into three sections: the potential core zone (each point velocity is equal to outlet velocity), the supersonic zone and the subsonic zone.The first two sections are collectively called the supersonic core zone.The length of the supersonic core zone is a key parameter when characterizing the jet characteristics of the supersonic oxygen lance.To further understand the performance of the supersonic oxygen lance, the gas jet characteristics and molten bath stirring characteristics were investigated using CFD To further understand the performance of the supersonic oxygen lance, the gas jet characteristics and molten bath stirring characteristics were investigated using CFD models for different Mach numbers [21], operation pressures [22], ambient temperatures [23,24], nozzle hole numbers [25,26] and nozzle hole angles [27].It was found that, when the Mach number was 2.0-2.3, the fluctuation in operation pressure had the least effect on jet velocity, and the jet velocity, impact radius, and impact depth increased with the increase in operation pressure, ambient temperature, and nozzle hole numbers.The phenomenon of jet polymerization was suppressed at high ambient temperatures, and the attenuation of jet velocity was delayed at the same time.Oxygen lances with mesoporous holes improved the three-phase emulsification of the molten bath.The phenomenon of jet polymerization increased with the increase in nozzle hole numbers.However, the results of CFD models are based on certain assumptions and do not fully reflect the actual production of the converter.Therefore, the CFD model results need to be further validated [24].
Due to limitations relating to measurement techniques and experimental equipment, water simulation studies are not entirely effective when observing and resolving the complex phenomena in the converter.Therefore, researchers established a gas-slag-metal three-phase VOF (volume of fluid) model based on the VOF interface tracking method to simulate the impact phenomena of super-sonic oxygen lance jets on the molten bath [28].It was found that the impact dent and gas-slag-metal three-phase interface of the converter blowing process had obvious transient characteristics [29,30], and the wall shear stress near the gas-slag-metal interface was the largest [31].As the lance height decreased, the surface velocity of molten steel increased and the surface fluctuation was strengthened; however, the velocity distribution of molten steel was more uniform at higher lance heights [32].The above research on the multiphase flow behavior of converter molten baths reveals the change law of multiphase flow behavior, which can provide reference and guidance for the optimization of converters and the regulation of refining process parameters.
Table 3 lists the industrial tests performed by some researchers.The quality of the metallurgical effects could reflect the flow of molten steel.Oxygen lance jet behavior determines the molten bath flow state.Therefore, the jet stirring characteristics of the supersonic oxygen lance could be well verified by industry tests.

Testers Test Contents Test Results
Chen et al. [33] Optimized oxygen lance; increased nozzle hole angle and oxygen flow rate The average smelting time and the end-point Lv et al. [35] The effect of oxygen lance nozzle outlet wear angle on metallurgical effects was studied The phosphorus content, end-point [%C]• [%O], and T.Fe content increased from 0.029%, 0.0023, and 12.92% to 0.032%, 0.0028, and 14.58%, respectively.
Liu et al. [36] Studied the stirring ability and flow field characteristics of a conventional and nozzle-twisted oxygen lance It showed that the 8 • oxygen lance could stir molten better during the steelmaking process for the 120 t dephosphorization converter.These findings agree well with the experimental results of water experiments and numerical simulations.

Coherent Jet Oxygen Lance
To obtain greater impact depth and better molten bath stirring, the coherent jet technology developed in the EAF steelmaking process was introduced into the converter steelmaking process.Figure 2 displays a schematic diagram of coherent jet oxygen lance blowing.Adding an annular flame around the main oxygen jet provides an environment with a high temperature, high velocity, and low pressure inside the annulus.Delaying the decay of main jet velocity increases the impact force on the molten bath [37,38]. of a conventional and nozzle-twisted oxygen lance during the steelmaking process for the 120 t dephosphorization converter.These findings agree well with the experimental results of water experiments and numerical simulations.

Coherent Jet Oxygen Lance
To obtain greater impact depth and better molten bath stirring, the coherent jet technology developed in the EAF steelmaking process was introduced into the converter steelmaking process.Figure 2 displays a schematic diagram of coherent jet oxygen lance blowing.Adding an annular flame around the main oxygen jet provides an environment with a high temperature, high velocity, and low pressure inside the annulus.Delaying the decay of main jet velocity increases the impact force on the molten bath [37,38].Metallurgists had mainly studied the effects of nozzle structure parameters [39], ambient temperature [40], type [41], pressure [42], and the temperature of accompanying gas [43] on the jet characteristics of a coherent jet oxygen lance.It was found that using a coherent jet oxygen lance could effectively suppress the radial diffusion of jet energy, prolong the high-temperature zone's length of accompanying flow, and increase the length of the jet core zone.The jet characteristics differed for different accompanying gases, with alkane concomitant gas (CH4) being better than coal gas (CO).Compared to a traditional oxygen lance, the attenuation of a coherent jet oxygen lance is slow on the center axis, its impact depth is deep, the liquid region velocity is high in the center of the molten bath, and the flow ability of the liquid steel is good in the molten bath [44].
The authors [45] studied the jet characteristics of a coherent jet oxygen lance and conducted industry tests on a 35 t converter.It was found that the converter dephosphorization rate was significantly increased.Steel charge consumption was reduced by 3.4 kg/t, and oxygen utilization and metal yield were improved.

Nozzle-Twisted Oxygen Lance
To control jet motion and suppress splashing in the molten bath, metallurgists [46][47][48] designed the nozzle-twisted oxygen lance and applied it to converter steelmaking.The schematic diagram of a conventional oxygen lance and nozzle-twisted oxygen lance is given in Figure 3.The nozzle-twisted oxygen lance increases the twist angle (based on a conventional oxygen lance) so that it can give rise to large a tangential force while gener- Metallurgists had mainly studied the effects of nozzle structure parameters [39], ambient temperature [40], type [41], pressure [42], and the temperature of accompanying gas [43] on the jet characteristics of a coherent jet oxygen lance.It was found that using a coherent jet oxygen lance could effectively suppress the radial diffusion of jet energy, prolong the high-temperature zone's length of accompanying flow, and increase the length of the jet core zone.The jet characteristics differed for different accompanying gases, with alkane concomitant gas (CH4) being better than coal gas (CO).Compared to a traditional oxygen lance, the attenuation of a coherent jet oxygen lance is slow on the center axis, its impact depth is deep, the liquid region velocity is high in the center of the molten bath, and the flow ability of the liquid steel is good in the molten bath [44].

Main oxygen nozzle
The authors [45] studied the jet characteristics of a coherent jet oxygen lance and conducted industry tests on a 35 t converter.It was found that the converter dephosphorization rate was significantly increased.Steel charge consumption was reduced by 3.4 kg/t, and oxygen utilization and metal yield were improved.

Nozzle-Twisted Oxygen Lance
To control jet motion and suppress splashing in the molten bath, metallurgists [46][47][48] designed the nozzle-twisted oxygen lance and applied it to converter steelmaking.The schematic diagram of a conventional oxygen lance and nozzle-twisted oxygen lance is given in Figure 3.The nozzle-twisted oxygen lance increases the twist angle (based on a conventional oxygen lance) so that it can give rise to large a tangential force while generating axial and radial forces.The tangential force causes the molten bath to rotate horizontally, improving stirring within the molten bath [49][50][51].At present, research on nozzle-twisted oxygen lance mainly focuses on jet dynamics, molten bath stirring, splashing rate, gas-slag-metal interaction and metallurgical effect.With the increased twist angle, axial jet attenuation accelerates, the impact depth decreases, the impact area increases, and jet polymerization weakens [52].Compared to the traditional supersonic oxygen lance, using a nozzle-twisted oxygen lance can promote steel surface flow and shorten the molten bath mixing time [53].In addition, using a nozzle-twisted oxygen lance can reduce the splash rate and splash height [54].
Yin et al. [55] studied the slagging rate and gas-slag-metal three-phase emulsification phenomenon in the blowing process of nozzle-twisted oxygen lances.It was found that the efficiency of kinetic energy transfer at the gas-liquid interface was a key factor affecting slagging reaction; nozzle-twisted oxygen lances could accelerate the slagging reaction.Lv et al. [56] studied the jet characteristics and gas-slag-metal three-phase flow of nozzletwisted oxygen lances.It was found that the larger the twist angle, the smaller the length of the supersonic core zone and the weaker the polymerization between the oxygen streams; slag coverage area, splash volume, and splash height were also reduced.
Due to the effective dynamic stirring effect of nozzle-twisted oxygen lances, researchers have tried to use them in converter dephosphorization, vanadium extraction, and other processes.Liu et al. [36] applied an 8° twist oxygen lance to a 120 t dephosphorization converter.The dephosphorization rate increased by 7.7% and the the T.Fe content in the slag decreased by 0.7%.The authors also applied a 10° nozzle-twisted oxygen lance to a 150 t vanadium extraction converter.The vanadium mass fraction of semi-steel was reduced by 17.8%, the converter vanadium extraction rate was increased by 5.1%, and the V2O5 yield of vanadium slag was increased by 0.93.

Secondary Combustion Oxygen Lance
A schematic diagram of a secondary combustion oxygen lance is shown in Figure 4.The secondary combustion oxygen lance enables CO to be burned twice in the furnace, allowing for reasonable use of CO energy and improving the scrap ratio, while promoting slagging [57,58].The secondary combustion oxygen lance can be divided into single-channel oxygen lances and double-channel oxygen lances according to the position of primary and auxiliary holes [59].The primary and auxiliary holes of single-channel oxygen lances share a common oxygen supply pipeline.The transformation process of single-channel oxygen lances is easy, with only the change of the nozzle required before it can be put into use [60].However, the auxiliary oxygen of a single-channel oxygen lance cannot be controlled separately during the smelting process, and combustion efficiency is low.The double-channel oxygen lance adopts an independent oxygen supply pipeline, which can precisely control the flow ratio of primary and auxiliary oxygen holes and improve the combustion ratio.However, the lance structure of double-channel oxygen lances is complicated to manufacture [61].At present, research on nozzle-twisted oxygen lance mainly focuses on jet dynamics, molten bath stirring, splashing rate, gas-slag-metal interaction and metallurgical effect.With the increased twist angle, axial jet attenuation accelerates, the impact depth decreases, the impact area increases, and jet polymerization weakens [52].Compared to the traditional supersonic oxygen lance, using a nozzle-twisted oxygen lance can promote steel surface flow and shorten the molten bath mixing time [53].In addition, using a nozzle-twisted oxygen lance can reduce the splash rate and splash height [54].
Yin et al. [55] studied the slagging rate and gas-slag-metal three-phase emulsification phenomenon in the blowing process of nozzle-twisted oxygen lances.It was found that the efficiency of kinetic energy transfer at the gas-liquid interface was a key factor affecting slagging reaction; nozzle-twisted oxygen lances could accelerate the slagging reaction.Lv et al. [56] studied the jet characteristics and gas-slag-metal three-phase flow of nozzletwisted oxygen lances.It was found that the larger the twist angle, the smaller the length of the supersonic core zone and the weaker the polymerization between the oxygen streams; slag coverage area, splash volume, and splash height were also reduced.
Due to the effective dynamic stirring effect of nozzle-twisted oxygen lances, researchers have tried to use them in converter dephosphorization, vanadium extraction, and other processes.Liu et al. [36] applied an 8 • twist oxygen lance to a 120 t dephosphorization converter.The dephosphorization rate increased by 7.7% and the the T.Fe content in the slag decreased by 0.7%.The authors also applied a 10 • nozzle-twisted oxygen lance to a 150 t vanadium extraction converter.The vanadium mass fraction of semi-steel was reduced by 17.8%, the converter vanadium extraction rate was increased by 5.1%, and the V2O5 yield of vanadium slag was increased by 0.93.

Secondary Combustion Oxygen Lance
A schematic diagram of a secondary combustion oxygen lance is shown in Figure 4.The secondary combustion oxygen lance enables CO to be burned twice in the furnace, allowing for reasonable use of CO energy and improving the scrap ratio, while promoting slagging [57,58].The secondary combustion oxygen lance can be divided into singlechannel oxygen lances and double-channel oxygen lances according to the position of primary and auxiliary holes [59].The primary and auxiliary holes of single-channel oxygen lances share a common oxygen supply pipeline.The transformation process of singlechannel oxygen lances is easy, with only the change of the nozzle required before it can be put into use [60].However, the auxiliary oxygen of a single-channel oxygen lance cannot be controlled separately during the smelting process, and combustion efficiency is low.The double-channel oxygen lance adopts an independent oxygen supply pipeline, which can precisely control the flow ratio of primary and auxiliary oxygen holes and improve the combustion ratio.However, the lance structure of double-channel oxygen lances is complicated to manufacture [61].The main indicators when evaluating the performance of a secondary combustion oxygen lance are secondary combustion rate (CO2/(CO+CO2)), secondary combustion thermal efficiency, and scrap ratio.The factors that affect the performance of secondary combustion oxygen lance are mainly primary and auxiliary oxygen hole spacing, auxiliary oxygen jet angle, and primary and auxiliary oxygen hole flow rate [62,63].As primary and auxiliary oxygen hole spacing increases, the amount and rate of secondary combustion increases.Increasing the auxiliary oxygen jet angle aids secondary combustion in the furnace.The Sumitomo Corporation of Japan finds that combustion effect is better when the angle is 30°.When it is less than 30°, the auxiliary oxygen jet is mainly involved in the decarbonization reaction, and the combustion effect becomes worse.The primary and auxiliary oxygen hole flow rates have a direct effect on the secondary combustion amount and scrap ratio.As the proportion of auxiliary oxygen hole flow to total flow increases, the secondary combustion amount and scrap ratio increase, and when the proportion exceeds 20%, the changing trend slows down significantly.

Double-Parameter Oxygen Lance
The double-parameter oxygen lance divides the nozzle holes into two groups, and each group is designed with different nozzle hole angles, throat diameters, and Mach numbers [64,65].A schematic diagram of a double-parameter oxygen lance is shown in Figure 5.The double-parameter oxygen lance can reduce cross-interference between the oxygen jets and splashes, and increase the impact area of the molten bath [66,67].The fusion distance of double-parameter oxygen lance jets increases and then decreases with the increase in nozzle hole angle.A double-parameter oxygen lance with nozzle hole angle of 11°/11.5°has the longest fusion distance.The diagonal nozzle hole  The main indicators when evaluating the performance of a secondary combustion oxygen lance are secondary combustion rate (CO 2 /(CO+CO 2 )), secondary combustion thermal efficiency, and scrap ratio.The factors that affect the performance of secondary combustion oxygen lance are mainly primary and auxiliary oxygen hole spacing, auxiliary oxygen jet angle, and primary and auxiliary oxygen hole flow rate [62,63].As primary and auxiliary oxygen hole spacing increases, the amount and rate of secondary combustion increases.Increasing the auxiliary oxygen jet angle aids secondary combustion in the furnace.The Sumitomo Corporation of Japan finds that combustion effect is better when the angle is 30 • .When it is less than 30 • , the auxiliary oxygen jet is mainly involved in the decarbonization reaction, and the combustion effect becomes worse.The primary and auxiliary oxygen hole flow rates have a direct effect on the secondary combustion amount and scrap ratio.As the proportion of auxiliary oxygen hole flow to total flow increases, the secondary combustion amount and scrap ratio increase, and when the proportion exceeds 20%, the changing trend slows down significantly.

Double-Parameter Oxygen Lance
The double-parameter oxygen lance divides the nozzle holes into two groups, and each group is designed with different nozzle hole angles, throat diameters, and Mach numbers [64,65].A schematic diagram of a double-parameter oxygen lance is shown in Figure 5.The double-parameter oxygen lance can reduce cross-interference between the oxygen jets and splashes, and increase the impact area of the molten bath [66,67].The main indicators when evaluating the performance of a secondary combustion oxygen lance are secondary combustion rate (CO2/(CO+CO2)), secondary combustion thermal efficiency, and scrap ratio.The factors that affect the performance of secondary combustion oxygen lance are mainly primary and auxiliary oxygen hole spacing, auxiliary oxygen jet angle, and primary and auxiliary oxygen hole flow rate [62,63].As primary and auxiliary oxygen hole spacing increases, the amount and rate of secondary combustion increases.Increasing the auxiliary oxygen jet angle aids secondary combustion in the furnace.The Sumitomo Corporation of Japan finds that combustion effect is better when the angle is 30°.When it is less than 30°, the auxiliary oxygen jet is mainly involved in the decarbonization reaction, and the combustion effect becomes worse.The primary and auxiliary oxygen hole flow rates have a direct effect on the secondary combustion amount and scrap ratio.As the proportion of auxiliary oxygen hole flow to total flow increases, the secondary combustion amount and scrap ratio increase, and when the proportion exceeds 20%, the changing trend slows down significantly.

Double-Parameter Oxygen Lance
The double-parameter oxygen lance divides the nozzle holes into two groups, and each group is designed with different nozzle hole angles, throat diameters, and Mach numbers [64,65].A schematic diagram of a double-parameter oxygen lance is shown in Figure 5.The double-parameter oxygen lance can reduce cross-interference between the oxygen jets and splashes, and increase the impact area of the molten bath [66,67].The fusion distance of double-parameter oxygen lance jets increases and then decreases with the increase in nozzle hole angle.A double-parameter oxygen lance with nozzle hole angle of 11 • /11.5 • has the longest fusion distance.The diagonal nozzle hole angle difference of 0.5 • is conducive to increasing the fusion distance.At this time, the proportion of the weak flow zone in the molten bath is high [68].The inclination of the small nozzle has the greatest influence on the impact diameter, and the flow rate of a large nozzle has the greatest influence on impact depth [69].The effective impact area of the jet on the molten bath decreases gradually with the decrease in nozzle spacing.The larger the nozzle spacing, the greater the fluctuation in jet velocity.Reducing nozzle spacing can stabilize the jet velocity faster [70].
Liu et al. [71] designed a double-parameter oxygen lance, with nozzle hole inclination angles of 17 • and 12 • and a 60% large hole flow rate, and tested it on a 260 t converter.It was found that the average oxygen blowing time was shortened by 1 min, steel material consumption was reduced by 20.3 kg per ton of steel, and oxygen supply intensity was increased by 0.23 m3/(t•min).To improve the oxygen supply flow rate of a 300 t converter, Jiang et al. [72] designed a six-hole double-parameter oxygen lance.Through an industrial test, it was found that the average oxygen blowing time was shortened by 2.0 min, the average dephosphorization rate was 87.5%, and the average life of an oxygen lance nozzle was 190 heats.

Combined Blowing Process
The combined blowing process is developed based on bottom blowing and top blowing.The bottom blowing gas-flow is used to strengthen molten bath stirring, so that the reaction in the converter is close to equilibrium.At the same time, the combined blowing process also retains the characteristics of the top blowing process to easily control slagging, and thus has better economic and technical indexes [73,74].Table 4 lists the three different types of combined blowing processes.The purpose of which is to strengthen molten bath stirring.This process refers to the simultaneous blowing of oxygen at the top and bottom, which is an intensive refining type of combined blowing process.
This technology refers to top blowing oxygen and bottom blowing or side-blowing oxygen, while bottom blowing into the fuel, which is used to increase scrap types in the combined blowing process.

Bottom Blowing Element Types
The bottom blowing element is one of the cores of the combined blowing process, mainly including nozzle-type elements and brick type-elements.Table 5 lists the structural characteristics and performance of the bottom blowing elements [5].Requirements for the bottom blowing system include gas-flow dispersion, uniformity and stability, and a large range of gas volume adjustments [75].Therefore, metallurgists have developed a variety of different structural forms for bottom blowing elements.

Nozzle Type Element
Nozzle type elements can be divided into the single tube type, sleeve type, and circular seam type.Among them, the circular seam type is divided into a single circular seam type and double circular seam type [76].Different types of nozzle element terminal faces are shown in Figure 6.The early use of bottom blowing elements mainly involved the single tube type.When gas-flow velocity is lower than sound velocity, the gas-flow will be interrupted during the gas supply process, resulting in the nozzle being blocked.At the same time, due to the cooling effect of gas, the molten steel is cooled, forming an umbrella which causes the nozzle to be bonded.Nozzle type elements can be divided into the single tube type, sleeve type, and circular seam type.Among them, the circular seam type is divided into a single circular seam type and double circular seam type [76].Different types of nozzle element terminal faces are shown in Figure 6.The early use of bottom blowing elements mainly involved the single tube type.When gas-flow velocity is lower than sound velocity, the gas-flow will be interrupted during the gas supply process, resulting in the nozzle being blocked.At the same time, due to the cooling effect of gas, the molten steel is cooled, forming an umbrella which causes the nozzle to be bonded.The sleeve type nozzles have been designed to increase nozzle life.Blockage is prevented by blowing oxygen through the inner tube and protective gas through the circular seam.However, it has the problem of a small range of gas volume adjustment, which cannot meet the bottom blowing gas supply requirements when refining medium and high carbon steel.The steelmaking converter mainly adopts the mode of weak bottomblow agitation, and the bottom blowing gas is mainly inert gas with low bottom-blow intensity.Therefore, the sleeve bottom blowing element is mainly used in GOR converter refining of stainless steel and converter bottom blowing powder injection processes [77].
To solve the problem of gas volume adjustment, the Iron and Steel Research Institute has developed a circular seam nozzle.It has the advantages of a large gas volume adjustment range, good stirring effect, and small nozzle erosion and has been widely used in the steelmaking production process.The key to the circular seam type nozzle is to maintain the concentricity of double casing during processing, ensuring the stability of bottom blowing gas-flow [78].It is easy to generate a slag-metal mushroom head after adopting the circular seam bottom blowing element, which effectively protects the bottom blowing The sleeve type nozzles have been designed to increase nozzle life.Blockage is prevented by blowing oxygen through the inner tube and protective gas through the circular seam.However, it has the problem of a small range of gas volume adjustment, which cannot meet the bottom blowing gas supply requirements when refining medium and high carbon steel.The steelmaking converter mainly adopts the mode of weak bottomblow agitation, and the bottom blowing gas is mainly inert gas with low bottom-blow intensity.Therefore, the sleeve bottom blowing element is mainly used in GOR converter refining of stainless steel and converter bottom blowing powder injection processes [77].
To solve the problem of gas volume adjustment, the Iron and Steel Research Institute has developed a circular seam nozzle.It has the advantages of a large gas volume adjustment range, good stirring effect, and small nozzle erosion and has been widely used in the steelmaking production process.The key to the circular seam type nozzle is to maintain the concentricity of double casing during processing, ensuring the stability of bottom blowing gas-flow [78].It is easy to generate a slag-metal mushroom head after adopting the circular seam bottom blowing element, which effectively protects the bottom blowing element.After Anshan Iron and Steel adopted the circular seam bottom blowing element, the end-point [%C]• [%O] was reduced from 0.00316 to 0.00273, and the T.Fe content of slag was reduced by 1.69% [79].After WISCO adopted the circular seam bottom blowing element, the end-point [%C]• [%O] was reduced by 0.0017, the Mn content of molten steel was increased by 0.02%, the Metals 2022, 12, 1918 9 of 16 average T.Fe content was reduced by 2.25%, the slag consumption was reduced by 4.00 kg/t, and the refining cycle was shortened by 42 s [80].

Brick Type Element
Brick type elements can be mainly divided into dispersion type breathable brick, gap type combined brick, straight hole type breathable brick, and porous plug brick (MHP).The structure of different brick elements is given in Figure 7. Dispersion type breathable brick is composed of dispersed micropores, and bottom blowing gas enters the molten bath from the micropores.However, its erosion resistance is poorer and service life is low.Subsequently, gap type combination brick was developed, which consists of multiple refractory bricks put together in different forms where the bottom blowing gas enters the molten bath through the brick gap.Gap type combination bricks are denser and resistant to erosion but have problems with cracking and unstable gas supply.The straight hole type breathable brick has numerous fusible metal wires embedded in the brick when it is being made.The metal wire melts to form a straight hole during the firing process, and the gas-flow enters the molten bath through the straight holes.Porous plug brick (MHP) is manufactured by burying a number of 10-150 fine metal tubes with an inner diameter of 0.1-3.0mm in the parent refractory material.Its gas volume adjustment range is large, while the gas supply is uniform and stable.Therefore, it has been widely used in the converter production process.After continuous improvement, MHP-D porous plug brick was developed, which adds an air supply box to the outer metal tube of the brick [81].
Metals 2022, 12, x FOR PEER REVIEW 10 of 17 element.After Anshan Iron and Steel adopted the circular seam bottom blowing element, the end-point [%C]• [%O] was reduced from 0.00316 to 0.00273, and the T.Fe content of slag was reduced by 1.69% [79].After WISCO adopted the circular seam bottom blowing element, the end-point [%C]• [%O] was reduced by 0.0017, the Mn content of molten steel was increased by 0.02%, the average T.Fe content was reduced by 2.25%, the slag consumption was reduced by 4.00 kg/t, and the refining cycle was shortened by 42 s [80].

Brick Type Element
Brick type elements can be mainly divided into dispersion type breathable brick, gap type combined brick, straight hole type breathable brick, and porous plug brick (MHP).The structure of different brick elements is given in Figure 7. Dispersion type breathable brick is composed of dispersed micropores, and bottom blowing gas enters the molten bath from the micropores.However, its erosion resistance is poorer and service life is low.Subsequently, gap type combination brick was developed, which consists of multiple refractory bricks put together in different forms where the bottom blowing gas enters the molten bath through the brick gap.Gap type combination bricks are denser and resistant to erosion but have problems with cracking and unstable gas supply.The straight hole type breathable brick has numerous fusible metal wires embedded in the brick when it is being made.The metal wire melts to form a straight hole during the firing process, and the gas-flow enters the molten bath through the straight holes.Porous plug brick (MHP) is manufactured by burying a number of 10-150 fine metal tubes with an inner diameter of 0.1-3.0mm in the parent refractory material.Its gas volume adjustment range is large, while the gas supply is uniform and stable.Therefore, it has been widely used in the converter production process.After continuous improvement, MHP-D porous plug brick was developed, which adds an air supply box to the outer metal tube of the brick [81].

Injection Medium Types and Applications
The purpose of a bottom gas supply is to strengthen molten bath stirring; therefore, there is a wide range of injection media available.Bottom blowing injection media mainly includes N2, Ar, CO, O2, CO2, etc., of which N2 and Ar are the most widely used.In addition, some powders are also used as an injection medium.Table 6 shows the characteristics of different bottom blowing gases.

Injection Medium Types and Applications
The purpose of a bottom gas supply is to strengthen molten bath stirring; therefore, there is a wide range of injection media available.Bottom blowing injection media mainly includes N 2 , Ar, CO, O 2 , CO 2 , etc., of which N 2 and Ar are the most widely used.In addition, some powders are also used as an injection medium.Table 6 shows the characteristics of different bottom blowing gases.

Oxygen Injection
Oxygen is usually used as a top blowing medium, that is, the combined blowing process of top blowing oxygen and bottom blowing inert gas.Representative processes are LBE, LD-KG, and LD-OTB.The oxygen passes through the Laval nozzle to form supersonic jets, which impact the surface of the molten bath to form impact dents, promote the emulsification of the gas-slag-metal three-phase, and oxidize with elements such as C and P. Therefore, reasonable oxygen supply is of great significance for improving converter refining efficiency, reducing production costs and improving molten steel cleanliness.At present, converter oxygen supply intensity is usually 2.5-4.5 Nm 3 /(t•min) in China, while the converter oxygen supply intensity can reach more than 5.0 Nm 3 /(t•min) in Japan.The oxygen supply intensity of different capacity converters is shown in Table 7 [82].Oxygen can also be used as the bottom blowing injection medium, mainly included as part of BSC-BAP, LD-OB, K-BOP and other processes.The top oxygen supply ratio is 60-95% and the bottom oxygen supply ratio is 40-50%.At this time, the bottom gas supply element is a sleeve type nozzle, and the center tube supplies oxygen which reacts with carbon in the molten bath to produce carbon dioxide that enhances the molten bath's stirring effect.The ring seam supplies natural gas, LPG, etc., as the coolant.However, when oxygen is used as the bottom blowing injection medium, the life of the bottom blowing element is relatively shorter.

Inert Gas Injection
The inert gas injected in the converter steelmaking process is mainly N 2 or Ar.N 2 is a by-product of oxygen production that is cheap and does not react with molten elements.It is one of the widely used bottom blowing gas sources at present.However, blowing N 2 will increase the nitrogen content of steel and affect steel quality [83].Bottom blowing Ar can not only ensure the stirring of the molten bath, but also does not have any adverse effect on the quality of molten steel.It is the most ideal bottom blowing stirring gas source at present.However, Ar is expensive and the refining cost is high.
Refining of plain carbon steel and other steel refining usually involves injection of N 2 throughout the whole process; steel refining of pipeline steel and automobile plates injects Ar, and alloy structural steels use N 2 -Ar switching injection mode.Sun et al. [84] studied the effect of the N 2 -Ar switching injection mode on the quality of molten steel and found that with the extension of the N 2 -Ar switching time node, the amount of nitrogen added to molten steel gradually increased.When the oxygen blowing ratio was within 56%, the nitrogen absorption rate was very small at this time, and the change in the N 2 -Ar switching node had less effect on the nitrogen content of molten steel.

Carbon Dioxide Injection
CO 2 is not only easy to obtain but also has a good physical cooling effect and chemical cooling effect.In addition, carbon is oxidized when CO 2 enters the molten bath, which doubles the volume of the gas molecules and facilitates molten bath stirring [85].
At the early stage of refining, the temperature is low, and silicon and manganese are preferentially oxidized when CO 2 is blown; in the middle of refining, where the decarburization rate is the highest, CO 2 is mainly involved in the decarburization reaction of the steel, and the decarburization rate of the molten bath is greater when CO 2 is blown than other inert gases; at the end of refining, CO 2 mainly reacts with Fe [86].A mixed injection of CO 2 -O 2 to a basic oxygen furnace was applied to enhance dephosphorization, and promising results were reported [87].Feng et al. [88] studied the change in the molten bath flow field with a CO 2 -O 2 mixed injection and found that the jet convergence point advanced and the jet impact area increased with increasing CO 2 and O 2 flow rates.Han et al. [89] found that CO 2 injection could reduce dust generation, increase the CO concentration of the converter gas, improve dephosphorization, and decrease nitrogen content.Zhu et al. [85] found that, in the CO 2 -O 2 mixed injection process, CO 2 conversion of up to 80% or more could be obtained by controlling the CO 2 ratio, with significant energy savings.
The authors conducted industrial tests with CO 2 injection on a 30 t converter and found that the T.Fe content and phosphorus content distribution decreased by 5.94% and 0.007%.In addition, the author proposed a new dephosphorization process, in which CO 2 was mixed in an oxygen jet, and conducted industry tests in a 300 t dephosphorization converter [90].It was found that using CO 2 as the dephosphorization oxidant could control the molten bath temperature and obtain a better dephosphorization effect.

Powder Injection
Injecting powder can significantly improve the converter's slag-metal interface area and impurity element removal rate, which can accelerate lime melting.Injected powders are mainly lime powder and iron ore powder.The powder is usually injected into the molten bath, with bottom blowing inert gas as a carrier or through the top blowing oxygen lance [91].
In 1952 and 1953, oxygen top blowing converters were built and put into production at the Linz and Donawitz plants in Austria, which became known as the LD process.Compared to the Bessemer process with acidic or alkaline bottom blowing air, the LD converter could refine hot metal with high manganese and medium to high phosphorus content, and the nitrogen content of steel was much lower [92].Therefore, the ductility of steel was significantly improved, and the products could be widely used in the rapid development of the automobile industry at that time.Isaide developed the LD-OLP process and the LD-AC process was developed by the French Metallurgical Research Center, both of which used an oxygen lance to inject lime powder into the converter to smelt high-phosphorus hot metal [93].In the 1980s, metallurgists conducted industrial tests of dephosphorization from converter powder injecting.The results [94,95] showed that the blowing process was stable and that the metal yield was high when injecting powder for dephosphorization, and the dephosphorization rate of molten steel increased with the increase in powder injection.Tang et al. [96,97] studied molten bath flow characteristics in the process of top blowing limestone particles and bottom blowing lime powder respectively.They concluded that the lance height should be reduced when top blowing powder, and the powder penetration ratio increased with the increase in solid-gas ratio and powder particle size.Liang et al. [98] conducted a water simulation experiment of bottom blowing oxygen and lime powder and found that powder injecting could promote molten bath stirring; the powder diffusion rate increased with the increase in bottom blowing carrier gas.Zhou et al. [99,100] found that bottom blowing O 2 -CaO improved reaction efficiency; the amount of slag per ton was reduced by 11.2 kg, and the phosphorus content was decreased by 0.005%.The authors [101] conducted experiments with a 150 t converter on the subject of vanadium extraction through powder injection and studied the stirring effect and impact characteristics of powder injecting.It was found that powder injecting improved the kinetic and thermodynamic conditions of the vanadium extraction reaction and promoted the vanadium extraction reaction.

Development Direction of Injecting Technology
Based on the traditional supersonic oxygen lance, metallurgists have designed different types of oxygen lance to meet different refining requirements, and achieved good results.However, it is necessary to further optimize the nozzle structure and strengthen nozzle cooling, as well as apply the newly designed oxygen lance in industrial tests to check the performance of the oxygen lance.Optimizing the bottom blowing element's arrangement and increasing the bottom blowing element's life are the current development directions of bottom blowing technology.In terms of injection medium, in addition to using conventional gases such as N 2 and Ar, it is necessary to further find a suitable bottom blowing source.Using CO 2 as a carrier gas to inject powder into converters may be a good bottom blowing process.

High Efficiency
Increasing gas injection intensity and strengthening molten bath stirring have become the main measures used to promote the efficiency of converter steelmaking.At present, the large capacity of the converter has become inevitable.To meet the efficient production need for the large converters, metallurgists will further optimize the oxygen lance nozzle structure and reasonably arrange bottom blowing elements to promote the realization of high-flow oxygen supply technology and high-intensity bottom blowing technology [102,103].On the one hand, improving the oxygen supply intensity to above 4.5 Nm 3 /(t•min) could shorten the refining time and improve production efficiency.On the other hand, increasing the bottom blowing intensity to above 0.2 Nm 3 /(t•min) could further strengthen molten bath stirring, and reduce the dead zone area of the molten bath.

Reduce Environmental Burden
With the implementation of environmental protection guidelines and policies, the steel industry must also move towards green development while producing efficiently.The greening of converter injection technology should not only achieve clean production, but also reflect the idea of ecological industry and the circular economy, that is, "reduce, reuse, and recycle".The CO 2 emitted during the steel production process is recovered and used as a source of top and bottom blowing gas [89,104], reducing CO 2 emissions while saving energy.In addition, the amount of flue gas generated during the converter blowing process increases with the application of converters with high-flow oxygen supply technology.Therefore, dust removal efficiency should be further improved to realize the green production of converter steelmaking.

Long Life
The long life of injection elements ensures the high efficiency of converter injection technology.To realize the long life of oxygen lance nozzles, casting nozzles with a short life have been gradually replaced by a forging nozzle, and the lifespan can currently reach more than 500 heats.On the one hand, the processing method, material, internal structure, and water-cooling measures of the oxygen lance nozzle need to be further optimized to improve the life of converter oxygen lance nozzles [105].On the other hand, the life of the bottom blowing element should be prolonged by improving the material of the bottom blowing element, adjusting the nozzle structure, and reasonably arranging the bottom blowing element [106].

Conclusions and Outlook
The steel industry is an essential pillar of national industrialization and plays an important supporting role for related industries.Nowadays, under the new situation of structural adjustments in the steel industry, it will become the key development direction of converter steelmaking in the future as the industry seeks to further build a clean steel production platform that is highly efficiency, green, and intelligent.Based on this, accelerating the technological innovation of converter steelmaking injection, especially improvements and breakthroughs related to efficiency, green production methods and long life technology, will play an important role in promoting the transformation and improvement of the steel industry towards green and intelligent technology.

Figure 1 .
Figure 1.Schematic diagram of a traditional supersonic oxygen lance.

Figure 1 .
Figure 1.Schematic diagram of a traditional supersonic oxygen lance.

Figure 2 .
Figure 2. Schematic diagram of coherent jet oxygen lance blowing.

Figure 2 .
Figure 2. Schematic diagram of coherent jet oxygen lance blowing.

Figure 5 .
Figure 5. Schematic diagram of a double-parameter oxygen lance.

Figure 4 .
Figure 4. Schematic diagram of a secondary combustion oxygen lance.

Figure 5 .Figure 5 .
Figure 5. Schematic diagram of a double-parameter oxygen lance.The fusion distance of double-parameter oxygen lance jets increases and then decreases with the increase in nozzle hole angle.A double-parameter oxygen lance with nozzle hole angle of 11°/11.5°has the longest fusion distance.The diagonal nozzle hole

Figure 6 .
Figure 6.Diagram of different types of nozzle end face: (a) single tube type, (b) sleeve type, (c) single circular seam type, and (d) double circular seam type.

Figure 6 .
Figure 6.Diagram of different types of nozzle end face: (a) single tube type, (b) sleeve type, (c) single circular seam type, and (d) double circular seam type.

Table 3 .
The industrial tests carried out by some researchers.

Table 4 .
Three different types of combined blowing processes.

Table 5 .
Comparison of the structural characteristics and performance of bottom blowing elements.

Table 5 .
Comparison of the structural characteristics and performance of bottom blowing elements.

Table 6 .
Different bottom blowing gas characteristics.

Table 6 .
Different bottom blowing gas characteristics.

Table 7 .
Oxygen supply intensity of different capacity converters.