Revolution and Control of Fe-Al-(Mg, Ti)-O Oxide Inclusions in IF Steel during 260t BOF-RH-CC Process

: The evolution of inclusions that contain Al, Mg, and Ti was studied through industrial-grade experiments. Field emission scanning electron microscopy, energy dispersive spectrometry, inductively coupled plasma atomic emission spectrometry, and FactSage software were used to analyze the evolution mechanisms of inclusions in Al-killed titanium alloyed interstitial free (IF) steel. The research found that the evolution of inclusions during the smelting process of IF steel is results in ‘large sphere-like SiO 2 -CaO-FeO-MgO-MnO’ and ‘small cluster spherical FeO-MnO’ change to cluster-like Al 2 O 3 and irregular MgO · Al 2 O 3 , then change to Al 2 O 3 · TiO x and Al 2 O 3 , and ﬁnally change to Al 2 O 3 . It is di ﬃ cult for Al 2 O 3 · TiO x to stably exist in the IF molten steel. It is the key to extend the holding time properly after Ruhrstahl Heraeus (RH) to ensure the removal of Al 2 O 3 inclusion. With the increase of Mg content, the change path of MgAl 2 O 4 inclusion in IF steel is that Al 2 O 3 changes to MgO · Al 2 O 3 , and ﬁnally changes to MgO. It is di ﬃ cult to suppress MgO · Al 2 O 3 spinel formation by controlling the oxygen in the steel, but Ca can modify part of the MgO · Al 2 O 3 spinel inclusions during RH reﬁning. In order to ensure the removal of 6–10 µ m inclusions, the holding time is suitable for 19–42 min.


Introduction
Ultra-low carbon interstitial-free (IF) steel is widely used in automobile plate production because of its excellent deep draw ability and uniform mechanical properties. With the increasing quality requirements of cold rolling sheet, the control requirements of inclusions in ultra-low carbon steel are increasingly strict [1,2]. In the smelting process of ultra-low carbon IF steel, a certain amount of titanium, niobium, and other elements should be added, and the interstitial atoms, such as carbon and nitrogen, in ultra-low carbon steel should be completely fixed as carbon nitrogen compounds, so as to obtain clean ferritic steel without interstitial atoms [3]. According to the difference in the chemical composition of the inclusions, the inclusions in ultra-low carbon steel can be divided into Al 2 O 3 inclusions, Al 2 O 3 -TiO x inclusions, and CaO-Al 2 O 3 -MgO inclusions [4][5][6][7]. Such inclusions can easily block immersion nozzles during continuous casting, and can also cause defects in the product. The study shows that Al 2 O 3 -TiO x inclusions are closely related to the clogging of the nozzle and the final product defect [8].
DC06 IF steel was produced by the 260 t BOF-RH-Holding-CC process at the Hanbao steel plant. During tapping, 600 kg lime was added into the BOF, and 680 kg aluminum slag deoxidant was added onto the top slag after tapping. RH vacuum treatment includes decarburization, followed by aluminum deoxidization, then 338 kg Ti-Fe containing 70% Ti alloying and keeping RH pure circulation time for 8-10 min. After RH treatment, molten steel remained unstirred in the ladle for 25-45 min before casting. The standard for judging the chemical composition of DC06 IF steel is shown in Table 1. In order to investigate the evolution of inclusions in the IF steel smelting process, samples were taken by samplers of Φ 30 mm × 10 mm during the industrial experiment steelmaking process, and the whole industrial experiments were carried out over 6 heats in total. A schematic diagram of charging and taking specimens during the steelmaking processes is shown in Figure 1. the whole industrial experiments were carried out over 6 heats in total. A schematic diagram of charging and taking specimens during the steelmaking processes is shown in Figure 1.

Mechanical Property Experiment
During the tensile experiment, the DC06 IF steel coil transverse sample was taken, and the steel plate was processed into a dumbbell-shaped sample by using the Zwick 2Z50 (Ennepeta, Nordrhein-Westfalen, Germany) sample preparation machine. The initial gauge lengths of the tensile samples l0 and b0 are 80 mm and 20 mm, respectively. Then the XKA714C (BYJC, Beijing, China) automatic numerical control machine was used to mill the edge of the sample to make it meet the standard GB/T228. . According to the inspection standard GB/T5213-2019, the Zwick automatic tensile testing machine (Model Z150robo Test L, Ennepeta, Nordrhein-Westfalen, Germany) was used to test the tensile test samples to obtain mechanical performance data, whereby 700 sets of tensile tests were carried out on DC06 IF steel, and the average value of mechanical properties was obtained.

Methods of Chemical Analysis
Each steel sample on the cross-sectional had been ground and polished by SiC papers and w1.5 diamond suspensions. The sample should be ground and polished in the way of transverse and longitudinal intersections, and inclusions of each steel sample were detected using FEI Nova NanoSEM400 (FEI, Hillsboro, OR, USA) scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS, FEI, Hillsboro, USA). The number, size, and chemical composition of inclusions were analyzed automatically using an ASPEX scanning electron microscope (ASPEX SEM, FEI, PA, USA). The concentration of the Al, Ti, and Ca in steel was determined by IRIS advantage radial inductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo Elemental, MA, USA). The [O] concentration in steel was measured online by a MSO-3690oxygen sensor (MINCO, Harland, WI, USA). Figure 2 shows the morphology and evolution of typical inclusions in molten steel. Sample 1 was taken from BOF before tapping. The morphology of inclusions before tapping is shown in Figure 2a. Inclusions in BOF steel are spherical. The inclusions in the BOF process are SiO2-CaO-FeO-MgO-MnO type multi-phase composite inclusions. Due to the deep decarburization by oxygen blowing in BOF, the molten steel has strong oxidizability. Therefore, there are many cluster spherical FeO-MnO inclusions below 5 μm in the molten steel, and other large sphere-like inclusions below 50 μm are SiO2-CaO-FeO-MgO-MnO.

Morphology and Evolution of Typical Non-Metallic Inclusions in Molten Steel
Sample 2 was taken from ladle before vacuum start. Figure 2b shows typical inclusions in molten steel before vacuum start during RH refining. The spherical and lump inclusions in the steel further grow up and float up, and many CaO-SiO2-FeO-Al2O3-MgO-MnO inclusions are in the 50 μm size. Due to the addition of Al slag modifier, large particles of Al2O3 above 50 μm appear in the steel.

Mechanical Property Experiment
During the tensile experiment, the DC06 IF steel coil transverse sample was taken, and the steel plate was processed into a dumbbell-shaped sample by using the Zwick 2Z50 (Ennepeta, Nordrhein-Westfalen, Germany) sample preparation machine. The initial gauge lengths of the tensile samples l 0 and b 0 are 80 mm and 20 mm, respectively. Then the XKA714C (BYJC, Beijing, China) automatic numerical control machine was used to mill the edge of the sample to make it meet the standard GB/T228.1-2010. According to the inspection standard GB/T5213-2019, the Zwick automatic tensile testing machine (Model Z150robo Test L, Ennepeta, Nordrhein-Westfalen, Germany) was used to test the tensile test samples to obtain mechanical performance data, whereby 700 sets of tensile tests were carried out on DC06 IF steel, and the average value of mechanical properties was obtained.

Methods of Chemical Analysis
Each steel sample on the cross-sectional had been ground and polished by SiC papers and w1.5 diamond suspensions. The sample should be ground and polished in the way of transverse and longitudinal intersections, and inclusions of each steel sample were detected using FEI Nova NanoSEM400 (FEI, Hillsboro, OR, USA) scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS, FEI, Hillsboro, USA). The number, size, and chemical composition of inclusions were analyzed automatically using an ASPEX scanning electron microscope (ASPEX SEM, FEI, Delmont, PA, USA). The concentration of the Al, Ti, and Ca in steel was determined by IRIS advantage radial inductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo Elemental, MA, USA). The [O] concentration in steel was measured online by a MSO-3690oxygen sensor (MINCO, Harland, WI, USA). Large inclusions have been partially removed, and the single Al 2 O 3 has been combined with other inclusions to form complex inclusions.

Morphology and Evolution of Typical Non-Metallic Inclusions in Molten Steel
Sample 4 was taken from ladle after adding aluminum. Typical inclusions in molten steel after adding aluminum for 2 min are shown in Figure 2d. After decarburization in the RH refining process, aluminum is added to the molten steel for deep deoxidization. Due to the reaction between aluminum and oxygen, a large number of cluster-like or coral-like Al 2 O 3 inclusions are generated by reaction (1). In addition, the inclusions in the steel aggregate and grow, and the lump MgO·Al 2 O 3 spinel inclusions are wrapped by compact coral-like Al 2 O 3 inclusions. Other Ca-based and Si-based inclusions are relatively rare after floating and removal.
Metals 2020, 10, x FOR PEER REVIEW 4 of 13 Sample 3 was taken from ladle after decarburization. After decarburization by RH vacuum refining, typical inclusions in molten steel are shown in Figure 2c. Irregular FeO-MnO-MgO and MgO-Al2O3 inclusions combine to form approximately 15 μm spherical inclusions or irregular FeO-MnO-MgO and CaO-Al2O3-FeO-SiO2 inclusions combine to form approximately 50 μm nearly spherical inclusions. Large inclusions have been partially removed, and the single Al2O3 has been combined with other inclusions to form complex inclusions.
Sample 4 was taken from ladle after adding aluminum. Typical inclusions in molten steel after adding aluminum for 2 min are shown in Figure 2d. After decarburization in the RH refining process, aluminum is added to the molten steel for deep deoxidization. Due to the reaction between aluminum and oxygen, a large number of cluster-like or coral-like Al2O3 inclusions are generated by reaction (1). In addition, the inclusions in the steel aggregate and grow, and the lump MgO·Al2O3 spinel inclusions are wrapped by compact coral-like Al2O3 inclusions. Other Ca-based and Si-based inclusions are relatively rare after floating and removal.
Sample 5 was taken from ladle after adding titanium alloy. Figure 2e shows typical inclusions in molten steel after RH refining and adding titanium alloy. After adding aluminum for deoxidization during RH refining, a large number of cluster-like Al2O3 inclusions are generated in the steel. With the floating of Al2O3 inclusions, the total oxygen content in the steel is greatly reduced. Subsequently, after adding Ti-Fe alloying, some [Ti] reacts with [Al] and [O] by reaction (2) in the steel to form Al2O3·TiOx inclusions below 10 μm, and a part of [Ti] reacts with [O] to generate TiOx, and the TiOx inclusion wraps around the outside of the Al2O3 inclusions to produce irregular lump Al2O3·TiOx inclusions by reaction (3).
Sample 6 was taken from ladle after RH treatment. After RH refining is completed, typical inclusions in molten steel are shown in Figure 2f. At this time, large-scale Al2O3·TiOx inclusions in the molten steel above 20 μm have floated and removed, but there are still newly generated Al2O3·TiOx inclusions below 20 μm in the molten steel, and Al2O3·TiOx grows with Al2O3 as the core and wraps Al2O3, and the inclusions are mainly Al2O3·TiOx, except for a few clusters of Al2O3 in the molten steel. [Ti] Sample 6 was taken from ladle after RH treatment. After RH refining is completed, typical inclusions in molten steel are shown in Figure 2f. At this time, large-scale Al 2 O 3 ·TiO x inclusions in the molten steel above 20 µm have floated and removed, but there are still newly generated Al 2 O 3 ·TiO x inclusions below 20 µm in the molten steel, and Al 2 O 3 ·TiO x grows with Al 2 O 3 as the core and wraps Al 2 O 3 , and the inclusions are mainly Al 2 O 3 ·TiO x , except for a few clusters of Al 2 O 3 in the molten steel.
Sample 7 was taken from tundish during continuous casting. Figure 2g shows typical inclusions in the molten steel of tundish. After the molten steel remains unstirred in the ladle for 25-45 min during the holding process, most of the large-particle Al 2 O 3 inclusions have been removed by floating. Since the molten steel is sufficiently homogenized, the Al 2 O 3 ·TiO x inclusions cannot stably exist in the steel, so the Al 2 O 3 ·TiO x inclusions have decomposed into stable Al 2 O 3 inclusions before entering the continuous casting mold, and Ti re-enters the molten steel. It can be proven from Figure 2g that the inclusions in the final molten steel are mainly Al 2 O 3 . Therefore, it is key to ensure the quality of IF steel to extend the holding time properly after RH to ensure the removal of Al 2 O 3 inclusions.

Thermodynamics of Evolution and Control of Typical Oxide Inclusions during Refining
The predominance area diagram of Fe-Al-(Mg, Ti)-O system was calculated by using the thermodynamic software FactSage 7.0 (ThermFact Inc., Montreal, QC, Canada). The evolution and control of typical Fe-Al-(Mg, Ti)-O system inclusions in IF steel smelting process are analyzed by using these diagrams.  In the Figure 4, 'liquid oxide' represents CaO-Al2O3-MgO series liquid inclusions, and 'CxAy' represents (CaO)x(Al2O3)y. When 1 × 10 −6 [Ca] is added to the molten steel, the MgO·Al2O3 spinel area The steel samples were taken before RH, after RH, and in the tundish, and the samples were taken from six heats industrial experiment. The chemical compositions of Al, Ti, and Ca elements in the steel are analyzed, and the average compositions are shown in Table 2. The ladle slag is modified by Al slag modifier after tapping. Although the Al in the molten steel is about 0.363%, the oxygen in the molten steel is still high. When the content of Mg in the molten steel is 0.54 × 10 −

Typical Spinel Inclusions in Molten Steel before RH Refining
It can be seen from the phase diagram that Al2O3·TiOx may exist in steel in three situations: (1) −6 In the Figure 4, 'liquid oxide' represents CaO-Al 2 O 3 -MgO series liquid inclusions, and 'C x A y ' represents (CaO) x (Al 2 O 3 ) y . When 1 × 10 −6 [Ca] is added to the molten steel, the MgO·Al 2 O 3 spinel area is significantly reduced, a part of the MgO·Al 2 O 3 spinel area is replaced by the 'liquid oxide', and the original Al 2 O 3 phase area is also replaced by a part of the 'liquid oxide'.
For the IF steel containing 0.042% Al after RH refining in this experiment, when the content of [Mg] in the steel is less than 0.48 × 10 −6 , Mg is dissolved in the molten steel and the inclusions in the molten steel are Al 2 O 3 and C x A y type calcium aluminates. The overall area ratio is larger than that without Ca. When content of [Mg] in steel increases to 0.48 × 10 −6 , MgO·Al 2 O 3 spinel inclusions begin to generate in the molten steel and it also contains Al 2 O 3 and C x A y type calcium aluminates.  Figure 5 shows the predominance area diagram of Fe-Al-Ti-O system with different oxygen contents at 1600 °C. It can be seen from the figure that Al2O3 can remain stable in the molten steel. When the content of [O] in steel is less than 10 × 10 −6 , in addition to the [Al] and [Ti] dissolved in the molten steel, other trace Al and Ti in the steel are stable in the form of Al2O3 and Ti3O5, and Al2O3·TiOx will not be stable in the molten steel.

Typical Al-Ti Inclusions in Molten Steel after RH Refining
When the [O] content in steel increases to 10 × 10 −6 , the stable region of Al2O3 increases. However, Ti exists as Ti2O3 instead of Ti3O5. As the dissolved [O] in steel increases above 15 × 10 −6 , Al2O3 may react with Ti or TiOx by reaction (3) or (4) to form Al2O3·TiOx.
It can be seen from the phase diagram that Al2O3·TiOx may exist in steel in three situations: (1) when the dissolved [O] in the molten steel is up to 15 × 10 −6 . (2) When the local Ti concentration in the molten steel is up to 0.49% during the titanium alloying adjusting process. (3) When the local Al concentration in the molten steel is as low as 0.0079%. As the dissolved oxygen in ultra-low carbon steel is low, the Al-Ti inclusions in the ultra-low carbon steel liquid cannot exist stably. When the composition of Al and Ti in the steel liquid is uniform, the Al-Ti inclusions will react with the Al in the steel continuously to form Al2O3 inclusions. [Ti] It can be seen from the phase diagram that Al 2 O 3 ·TiO x may exist in steel in three situations: (1) when the dissolved [O] in the molten steel is up to 15 × 10 −6 . (2) When the local Ti concentration in the molten steel is up to 0.49% during the titanium alloying adjusting process. (3) When the local Al concentration in the molten steel is as low as 0.0079%. As the dissolved oxygen in ultra-low carbon steel is low, the Al-Ti inclusions in the ultra-low carbon steel liquid cannot exist stably. When the composition of Al and Ti in the steel liquid is uniform, the Al-Ti inclusions will react with the Al in the steel continuously to form Al 2 O 3 inclusions.

Typical Al-Mg Inclusions in Tundish Molten Steel
The average [O] content in the six experimental heats steel dropped to 3.9 × 10 −6 before Ti adding, but the [O] content increases to 10 × 10 −6 after RH refining. In order to understand the transformation process of MgO·Al 2 O 3 spinel, the predominance area diagram of inclusions in Fe-Al-Mg-O system with [O] = 10 ppm and [Ca] = 1 ppm at 1600 • C is shown in Figure 6.

Typical Al-Mg Inclusions in Tundish Molten Steel
The average [O] content in the six experimental heats steel dropped to 3.9 × 10 −6 before Ti adding, but the [O] content increases to 10 × 10 −6 after RH refining. In order to understand the transformation process of MgO·Al2O3 spinel, the predominance area diagram of inclusions in Fe-Al-Mg-O system with [O] = 10 ppm and [Ca] = 1 ppm at 1600 °C is shown in Figure 6. As can be seen from Figure 6, in the Fe-Al-Mg-O system, when the content of [O] and [Al]s in the steel is 10 × 10 −6 and 0.033%, respectively, MgO and MgO·Al2O3 and Al2O3 inclusions will be precipitated separately with the difference in Mg content. If [Mg] content is less than 0.48 × 10 −6 in the steel, Al2O3, CA2, and CA6 inclusions will be precipitated in the molten steel. If [Mg] content increases to 0.48 × 10 −6 , that is, the critical line of the Al2O3 and MgO·Al2O3 phases, MgO·Al2O3 spinel begins to precipitate in the molten steel. At this time, [Mg] and Al2O3 inclusions in the molten steel react to form MgO·Al2O3 spinel at the boundary between the Al2O3 phase and the MgO·Al2O3 phase. As the Mg content increase to 0.99 × 10 −6 , Al2O3 will be transformed to MgO·Al2O3, and the CA6 phase will disappear. When [Mg] content reaches to 3.87 × 10 −6 , Al2O3 will be completely transformed into MgO·Al2O3 spinel, and the CA2 phase will disappear. When [Mg] content continues to increase to 10.73 × 10 −6 , MgO begins to precipitate in the molten steel. When the content Mg increases to 13.66 × 10 −6 , the spinel is completely transformed to MgO. With the increase of Mg content, the change path of inclusions is that Al2O3 change to MgO·Al2O3, and finally change to MgO.

Change and Control of Inclusions during Refining and Holding Time
After the RH deoxidization is completed, the histogram of the change in the number of inclusions in the molten steel at different times is shown in Figure 7. The number, size, and chemical composition of inclusions were analyzed automatically by using ASPEX SEM. As can be seen from Figure 7, the number of inclusions reaches a maximum after 4 min of aluminum deoxidization, which indicates that within 4 min, the oxygen in aluminum and steel reacts quickly to form Al2O3 inclusions. When aluminum is added for 4-8 min, the removal rate of inclusions floating up is greater than the rate of generation, and the total number of inclusions in molten steel decreases, especially the number of inclusions larger than 10 μm. Therefore, in order to ensure that the inclusions generated by deoxidization are fully floated and removed, the pure circulation time after alloying should be greater than 8 min.

Change and Control of Inclusions during Refining and Holding Time
After the RH deoxidization is completed, the histogram of the change in the number of inclusions in the molten steel at different times is shown in Figure 7. The number, size, and chemical composition of inclusions were analyzed automatically by using ASPEX SEM. As can be seen from Figure 7, the number of inclusions reaches a maximum after 4 min of aluminum deoxidization, which indicates that within 4 min, the oxygen in aluminum and steel reacts quickly to form Al 2 O 3 inclusions. When aluminum is added for 4-8 min, the removal rate of inclusions floating up is greater than the rate of generation, and the total number of inclusions in molten steel decreases, especially the number of inclusions larger than 10 µm. Therefore, in order to ensure that the inclusions generated by deoxidization are fully floated and removed, the pure circulation time after alloying should be greater than 8 min.
After RH refining, the change of the number of inclusions in the molten steel with holding time is shown in Figure 8. It can be seen that the number of inclusions in the molten steel decreases with the extension of the holding time within 20 min, especially the number of large particle inclusions above 10 µm decreases to 1 mm −2 , and Al 2 O 3 ·TiO x also decreases to 1 mm −2 .
Metals 2020, 10, x FOR PEER REVIEW 10 of 13 After RH refining, the change of the number of inclusions in the molten steel with holding time is shown in Figure 8. It can be seen that the number of inclusions in the molten steel decreases with the extension of the holding time within 20 min, especially the number of large particle inclusions above 10 μm decreases to 1 mm −2 , and Al2O3·TiOx also decreases to 1 mm −2 . Inclusions float in the molten steel in the way of Stokes to the surface of the ladle. If the time required to reach the top of the molten steel is less than the average residence time of the molten steel, the inclusions can be floated to the top of the molten steel and removed from the molten steel [17].
The floating velocity of inclusions in the still molten steel can be calculated using the Stokes settlement formula (5).
where Vs is the floating velocity of the inclusions, cm/s; g is the acceleration of gravity, m/s 2 ; d is the diameter of the inclusions, mm; ρ1 is the density of the molten steel, ρ1 = 7.0 g/cm 3 ; ρ2 is the density of the inclusions, ρ2 = 3.5 g/cm 3 ; η is molten steel viscosity, η = 0.05 g/cm·s at 1600 °C. It can be seen from the above formula that the floating speed of inclusions is proportional to the square of the diameter of the inclusions, so large inclusions are easy to float. Substituting the data gives: After RH refining, the change of the number of inclusions in the molten steel with holding time is shown in Figure 8. It can be seen that the number of inclusions in the molten steel decreases with the extension of the holding time within 20 min, especially the number of large particle inclusions above 10 μm decreases to 1 mm −2 , and Al2O3·TiOx also decreases to 1 mm −2 .  Inclusions float in the molten steel in the way of Stokes to the surface of the ladle. If the time required to reach the top of the molten steel is less than the average residence time of the molten steel, the inclusions can be floated to the top of the molten steel and removed from the molten steel [17].
The floating velocity of inclusions in the still molten steel can be calculated using the Stokes settlement formula (5).
where Vs is the floating velocity of the inclusions, cm/s; g is the acceleration of gravity, m/s 2 ; d is the diameter of the inclusions, mm; ρ1 is the density of the molten steel, ρ1 = 7.0 g/cm 3 ; ρ2 is the density of the inclusions, ρ2 = 3.5 g/cm 3 ; η is molten steel viscosity, η = 0.05 g/cm·s at 1600 °C. It can be seen from the above formula that the floating speed of inclusions is proportional to the square of the diameter of the inclusions, so large inclusions are easy to float. Substituting the data gives: Inclusions float in the molten steel in the way of Stokes to the surface of the ladle. If the time required to reach the top of the molten steel is less than the average residence time of the molten steel, the inclusions can be floated to the top of the molten steel and removed from the molten steel [17].
The floating velocity of inclusions in the still molten steel can be calculated using the Stokes settlement Formula (5).
where V s is the floating velocity of the inclusions, cm/s; g is the acceleration of gravity, m/s 2 ; d is the diameter of the inclusions, mm; ρ 1 is the density of the molten steel, ρ 1 = 7.0 g/cm 3 ; ρ 2 is the density of the inclusions, ρ 2 = 3.5 g/cm 3 ; η is molten steel viscosity, η = 0.05 g/cm·s at 1600 • C. It can be seen from the above formula that the floating speed of inclusions is proportional to the square of the diameter of the inclusions, so large inclusions are easy to float. Substituting the data gives: The depth of molten steel in this ladle is about 3.5 m, and the floating time of inclusions can be calculated by Equation (7), and the result is shown in Figure 9. 0 d . V s   (6) The depth of molten steel in this ladle is about 3.5 m, and the floating time of inclusions can be calculated by Equation (7), and the result is shown in Figure 9.  It can be seen from Figure 9 that inclusions with particle size less than 6 μm need about 42 min to float up. When smaller inclusions aggregate into inclusions with a particle size greater than 9 μm, they can float up and remove within about 19 min, so the holding time should be 19-42 min.

Mechanical Properties of IF Steel
The results of the DC06 IF steel mechanical properties test and requirements (GB/T 5213-2019) are shown in Table 3. According to Table 3

1
The IF steel inclusions in the BOF process are large sphere-like SiO2-CaO-FeO-MgO-MnO multi-phase composite inclusions below 50 μm and cluster spherical FeO-MnO inclusions below 5 μm. With the addition of Al-slag modifier and Al deoxidizer, a large amount of cluster-like or coral-like Al2O3 inclusions are formed. Moreover, MgO·Al2O3 spinel inclusions reduce gradually. Al2O3·TiOx inclusions begin to form after Ti addition, and Al2O3-type inclusions increase slightly. During the holding process, the inclusions are removed and the number of Al2O3 inclusions and Al2O3·TiOx inclusions in the steel is greatly reduced. In addition, Al2O3·TiOx inclusions are larger in size compared to Al2O3 inclusions. After the molten steel It can be seen from Figure 9 that inclusions with particle size less than 6 µm need about 42 min to float up. When smaller inclusions aggregate into inclusions with a particle size greater than 9 µm, they can float up and remove within about 19 min, so the holding time should be 19-42 min.

Mechanical Properties of IF Steel
The results of the DC06 IF steel mechanical properties test and requirements (GB/T 5213-2019) are shown in Table 3. According to Table 3, the yield strength R p0.2 , tensile strength R m , elongation after fracture A 80 , tensile strain hardening index N 90 , and plastic strain ratio R 90 of DC06 IF steel are 138.7 MPa, 304.7 MPa, 45.3%, 0.24, and 2.91, respectively. Compared with standard values of GB/T5213-2019, the mechanical properties of DC06 IF steel meet the requirements.