Effect of Al 2 O 3 in Refining Slag on the Cleanliness and Fatigue Property of Ultra-low-carbon Automotive Steel

2021. The influence of Al 2 O 3 content in refining slag on the cleanliness and fatigue property of ultra-low-carbon (ULC) automotive steel were investigated based on the industrial experiments. The inclusion-adsorption capacity of the refining slag was calculated and the total [O] (T.O) content of ULC automotive steel was measured. The number density, size distribution and morphology of inclusions were analyzed and their effects on the cleanliness and fatigue property of ultra-low-carbon (ULC) automotive steel were investigated. The results showed that as the Al 2 O 3 content in refining slag increased from 19.92% to 39.73%, the inclusion-adsorption capacity of ULC automotive steel decreased from 210.30 down to 57.12, and the fatigue life from 1.4x10 4 down to 0.9x10 4 cycles, while the T.O content of steel increased from 12 up to18 ppm and the inclusion number density from 4 up to 9 per mm 2


Introduction
With the rapid development of the automotive industry, methods of improving the quality and performance of steel have attracted significant attention in recent years. The ULC steel, as important candidate steel for the automotive industry, requires high cleanliness to guarantee the appropriate mechanical properties for undergoing subsequent processing and utilization 1 .
It is now a clearly investigation that the non-metallic inclusions are usually the resources affecting the cleanliness and fatigue properties in ULC automotive steel. The microvoids and cracks at inclusion/steel interfaces produced by nonmetallic inclusions were found to be the resources of fatigue fractures, so the presence of non-metallic inclusions such as Al 2 O 3 could decrease the cleanliness and mechanical properties of the steel, especially of fatigue properties 2 . Therefore, how to control non-metallic inclusions at the ultralow level is in the pursuit of ULC automotive steel production 3,4 .
The investigation of the cleanliness of ULC automotive steel indicated that an appropriate refining slag caused the inclusions to be removed effectively from steel, resulting in the improvement of the cleanliness and the mechanical properties of the steel. Since the slag in the refining process could control and remove inclusions from the molten steel effectively 5 , slag systems and models were designed to investigate the influence of the mechanical properties [6][7][8] . The improved methods for controlling inclusions have thus been reported [9][10][11][12] . When the Al 2 O 3 content in the refining slag increased from 14.71% to 22.3%, the number density of inclusions increased from 5.2 to 6.5 and the largest size of inclusion raised from 7.3 to 8.3 μm in spring steel 13 .
Structural parts made of ULC automotive steel are prone to fatigue damage due to cyclic loading. Currently, the fatigue performance of bearing and spring steel has been reported largely [14][15][16] . Hu et al. reported that as the basicity of refining slag increased from 3.4 to 5.0, the fatigue life of spring steel increased from 3.5×10 4 to 7.8×10 6 cycles 17 . However, few reports focusing on the fatigue properties of ULC automotive steel have been available so far.
In this work, a plant trial of Al-killed ULC automotive steel was performed with five different Al 2 O 3 contents in refining slags, and the influence of Al 2 O 3 content in refining slag on cleanliness and fatigue property of ULC automotive steel have been investigated. The size, distribution, morphology and chemical compositions of inclusions were measured and analyzed. The inclusion-adsorption capacity of the refining slag has been calculated by using thermodynamic software. The fatigue life of ULC automotive steel was found from 1.4x10 4 down to 0.9x10 4 cycles at a stress amplitude of 400 MPa.

Experimental
The experiment was carried out in a domestic factory that producing ULC automotive steel. The production of ULC automotive steel undergoes via the process of basic oxygen furnace (BOF)→ladle furnace refining (LF)→Ruhrstahl Heraeus degassing (RH)→continuous casting (CC), and the volume of the experimental ladle is 110 t. Five groups of refining slag samples were analyzed and their adsorption capacity for inclusions were calculated. The extraction procedure of inclusions using electrolysis, anhydrous methanol was used as electrolyte. The polished sample was hung in a sleeve as the anode, and the outer cast iron sleeve was used as the *e-mail: duxy@lut.cn cathode. The samples were electrolyzed for 72 hours under a 6 A current, and a filter was placed in the middle to collect the anode slime. The anode slime was subjected to magnetic separation and washing to separate the inclusions, carbides, and iron oxides. After reduction and magnetic treatment, the inclusions were obtained. The schematic of the electrolysis devices and inclusion extraction process were given in Figure 1.The number, size, and morphology of inclusions were analyzed through the scanning electron microscopy (SEM) and INCASteel automatic inclusion analyzer.
In the production of Al-killed ULC automotive steel, accompanying the slag making, arc heating and argon stirring in the process of LF refining. The aims of the LF refining process are desulfuration, deoxygenation, arc heating and inclusion removal. The Al 2 O 3 contents in the slag were adjusted during slag-making process in LF refining. LF refining process and sampling time are given in Figure 2. In this process, the static blowing time is 8-10 min, and the rates of Ar gas about static blowing and strong stirring were controlled in the range of 40-60 Nm 3 /h and 800-1000 Nm 3 /h, respectively. After LF refining, the ladle was transported to the RH station for degassing with the vacuum of 2.0 mbar for 15-30 min. Finally, the ULC automotive steel was obtained after undergoing CC and rolling.
The slag samples were taken from the initial state and the end of LF process, the steel samples were taken from the end of LF process, as shown in Figure 2. The steel samples were extracted by pail samplers, racket samplers and acicular samplers at the position of 300 mm below the slag surface, as shown in Figure 3. The slag samples were obtained by  sampling stick, then, they were prepared in the form of tablet pressing. Slag samples were measured by Panaco Epsilon 3X EDXRF spectrometer, after calibration with the standard sample, the mass fractions of components in the slag samples were obtained. The pail samples were analyzed with the INCASteel automatic inclusions analysis system and the acicular samples were analyzed for the T.O content with the LECO-ON83 oxygen-nitrogen analyzer. The chemical components of steel samples were measured with the OBLF QSG750-II direct-reading spectrometer. The steel samples were subjected to test their fatigue properties at a rotating frequency of 500 Hz.

Results
The chemical compositions of the slag samples before and after LF refining are listed in Table 1 and Table 2, the chemical compositions of steel samples are listed in Table 3, respectively. Table 1 contains six main components and alkalinity of the refining slag, which can be seen that TFe content is very high while the Al 2 O 3 content is low. The total compositions of slag in Table 1 are less than 100%, mainly because there are small amounts of  Moreover, the R 2 (CaO/ SiO 2 ) of the ULC automotive steel is between 3.07 and 3.72, which is not only beneficial to protect the furnace lining and prolong the service life of the furnace body, but also plays an important role in regulating the FeO content in steel slag. The Al 2 O 3 content was controlled by the pre-melted refining slag at different numerical value, the resulting compositions of the refining slag were detailed in Table 2, revealing slight variations in chemical compositions between the five samples except Al 2 O 3 and CaO contents. Table 4 details the T.O content of the five steel samples from the end of LF process, which can see that the T.O content is increasing. The inclusions were mainly counted that containing Al 2 O 3 in Table 5, with the increase of Al 2 O 3 content in refining slag, the maximum size of the inclusions increased. The schematic diagram of the Al 2 O 3 content in refining slag, the T.O content and number density of inclusions in molten steel were shown in Figure 4. The T.O content and number density of inclusions were observed to increase with the Al 2 O 3 content increasing as shown in Figure 5 and Figure 6. The information of non-metal inclusions in ULC steels were given in Table 5 and Table 6, the inclusions' size distribution was shown in Figure 7. Study has verified that when the number density of non-metal inclusions at 5.2-6.6 per mm 2 , the inclusions that containing Al 2 O 3 increased by about 5% at 5 -10 μm, it harmed the cleanliness and anti-fatigue properties of aluminum deoxidized steel 13 . The number density of non-metal inclusions were 5.22-9.06 per mm 2 in samples 2-5 in Table 5, therefore, it can be deduced that the proportion of inclusions greater than 10 μm increased by 5%, which seriously harmed the cleanliness and anti-fatigue performance of the ULC automotive steel.
The morphology and composition of the inclusions were analyzed by scanning electron microscope and energy spectrum in Figure 8. The inclusions are mainly yellow and white transparent inclusions with large particle size, yellow and black opaque inclusions, and some small spherical inclusions like Figure 8(a), Figure 8 (b) shows that the main ingredient of spherical morphologies inclusion was Al 2 O 3 observed by scanning electron microscopy-energy disperse spectroscopy (SEM-EDS). In addition, an analysis of inclusion clusters larger than 40 μm in Figure 8(c) found Al 2 O 3 to be a major constituent component.

Discussion
It is well established that reactions between slag and molten steel take place constantly, due to the huge contact area that provides good kinetic conditions. Thus, the slag, steel and inclusions can all react with each other, which enables inclusion removal and modification. To investigate the relationship between the three, the inclusions in steel were assumed to consist only of metal oxides during the refining process, and the resulting chemical reaction in molten steel is given as follows: ( ) Furthermore, inclusions are produced in steel during the refining process via the following chemical reaction: ( ) . As such, if the same activity standard is adopted to both the inclusions and slag, their standard Gibbs free energies should be equivalent 18 . Therefore, by combining equations (3) and (6), equations (7) and (8)   From the equation (8), it is evident that the inclusions have a similar composition to the slag, and the composition changes of inclusions are consistent with those of the slag in the refining process. Thus, as the Al 2 O 3 content in refining slag increased, the amount of Al 2 O 3 inclusions in the molten steel should also increase, which is consistent with the statistical results in Table 5.
At the preliminary stage of deoxidation, most of the inclusions were composed of Al 2 O 3 . However, as the reaction progresses, the [Mg] in the molten steel will react with Al 2 O 3 inclusions via the reaction in equation (9) 19 . The addition of aluminum results in a reaction with CaO and SiO 2 within the slag, which can lead to an increase of [Ca] and [Si] in the steel, as given by equation (10) and (11).
The increase of [Ca] and [Si] content in the steel converts the (MgO·Al 2 O 3 ) inclusions to CaO·MgO·SiO 2 ·Al 2 O 3 inclusions, given by the reaction in equation (12).
The conversion from Al 2 O 3 inclusion to CaO·MgO·SiO 2 ·Al 2 O 3 inclusion in the refining process has been investigated in previous work 20 . The CaO·MgO·SiO 2 ·Al 2 O 3 inclusions have a relatively low melting point and exist in liquid at steel-making temperature. Besides, the wettability of MgO inclusions in molten steel is poor, and agglomeration occurs hardly 21 , while Al 2 O 3 inclusions have a strong tendency to agglomerate 22 . As the refining process continues, the major components of inclusions are Al 2 O 3 in the later refining period, which leading Al 2 O 3 inclusions to form clusters 23 , and cluster inclusions contain a small amount of CaO and SiO 2 , as illustrated in Figure 8(c).
The inclusion-adsorption capacity of the refining slag can be given as follow 24 : Where slag ρ represents the density of slag, kg·m -3 , k B represents the Boltzmann constant, k B =1.3806×10 -23 J·K -1 , T represents the slag temperature, T=1873k, a 0 represents the radius of the diffusion unit of aluminum oxygen anion group, and it sets as 2.0×10 -10 m 24 . slag η represents Kinetic viscosity of slag, Pa·s, concentration in the slag system, which can be calculated by the balance module of FactSage thermodynamic calculation software. K Θ represents the inclusion-adsorption capacity of refining slag to Al 2 O 3 inclusion. The adsorption rate of Al 2 O 3 inclusions in molten steel by molten slag is different from that of alumina cylinder in molten slag under experimental conditions. However, under similar slag conditions, the value of K Θ can be used to represent the relative rate of alumina inclusion-adsorption by refining slag 24 .
From the equation (13), the increase of Al 2 O 3 content in the slag leading to a subsequent decrease in the adsorption capacity for Al 2 O 3 inclusions, curves of equal inclusion-adsorption capacity of refining slag for inclusions are shown in Figure 9. As illustrated by the thermodynamic computational analysis with FactSage in Figure 9, the inclusion-adsorption capacity of the refining slag for samples 6 to 10 in Table 2 Table 2, study has shown that when the CaO content in the refining slag was between 48-66%, it had little effect on the adsorption of Al 2 O 3 inclusions 25 . In Table 2, the CaO content in the refining slag was 45.58-57.05%, it was feasible to use K Θ to express the adsorption capacity of inclusions in the refining slag. With the increase of Al 2 O 3 content in the refining slag, the inclusion-adsorption capacity decreases sharply.
The analyses above are consistent with the results in Figure 6 and Table 5, where the number density of inclusions is observed increasing from 4 per mm 2 to 9 per mm 2 as the Al 2 O 3 content increased in the refining slag.
Fatigue behavior is an important mechanical property for many kinds of materials, especially in the automotive industry, and most of the fatigue fractures originate from internal or surface inclusions. Previous studies have proved that clusters of Al 2 O 3 particles cause poor fatigue properties in structural steel 26,27 , and oxide inclusions were most likely to cause fatigue fracture 28,29 . As fatigue life is an important mechanical performance index, the fatigue property of the five steel samples were investigated at 400 MPa stress amplitude. The shape and size of specimens are shown in Figure 10. These results are shown in Figure 11, indicating the fatigue life decreases with the increase of Al 2 O 3 content in the refining slag. By analyzing the fatigue fracture, it is found that there is Al 2 O 3 in the crack as shown in Figure 12.    When the Al 2 O 3 content increased from 19.92% to 39.73% in refining slag, the fatigue life was observed decreasing from 1.4x10 4 to 0.85x10 4 cycles as shown in Figure 11. The relationship between Al 2 O 3 content in the refining slag and fatigue life of steel is expressed in equation (14), it can help to predict the fatigue property of ULC automotive steel.
where Y represents the fatigue life and X represents the Al 2 O 3 content (%) in refining slag.

Conclusion
This study aimed at investigating the relationship between Al 2 O 3 content in refining slag and the cleanliness and mechanical properties of ULC automotive steel. By analyzing the fatigue property and statistically evaluating the inclusions in ULC automotive steel, the main conclusions may be drawn as follows: 1. With the increase of Al 2 O 3 content from 19.92% to 39.73% in refining slag, the T.O content and inclusion number density in steel increase from 12 to 18 ppm and 4 to 9 per mm 2 , respectively, the inclusion-adsorption capacity of refining slag from 210.30 down to 57.12.

The conversion of inclusion compositions from Al 2 O 3
to CaO·MgO·SiO 2 ·Al 2 O 3 in the refining process.
With the increase of Al 2 O 3 content in refining slag, the proportion of inclusions which sizes larger than 10μm rose from 0% to 7% due to the aggregation of inclusions in molten steel. 3. When the Al 2 O 3 content in refining slag increased from 19.92% to 39.73%, the fatigue life reduced from 1.4x10 4 to 0.9x10 4 cycles, and an expression for the fitted of this relationship was obtained to predict the fatigue property of ULC automotive steel.