Thermodynamic study of non-metallic inclusion formation in SAE 1141 steel

The main purpose of this paper is the thermodynamic study of non-metallic inclusion formation in the CC tundish for SAE 1141 steel. The specific purposes are: 1) obtaining inclusions as function of steel composition and casting temperature. 2) establishing steel chemical composition to form less harmful inclusions to the SAE 1141 steel castability. Simulations using the commercial software FactSage and databases were carried out. Results showed both different solid oxides and liquid phase formation in inclusions by varying calcium content in the steel. Thus, it was possible: 1) to determine both the inclusion composition as a function of aluminum and calcium content of SAE 1141 steel. 2) to establish a range of calcium content in which inclusions are formed predominantly by liquid phase. 3) to calculate percentage of liquid and solid phases in inclusions, and oxides composition as well.


INTRODUCTION
Additions of calcium-based alloys are largely employed to control composition, distribution and morphology of remaining non-metallic inclusions in the steel.The greatest benefit of that practice is to minimize clogging occurrence during the continuous casting (CC), that is, to increase heats castability in the steel mill.
Another advantage of inclusions treatment with calcium is to have influence on the control of sulfides morphology.Sims' classification provides three specific types of sulfides which can be found in steels.Among them, type II MnS inclusions are found in steels with both low oxygen and Al content.Such type of inclusion precipitates along primary grain-boundaries during steel solidification.Thus, the primary goal of controlling the morphology of sulfides is to eliminate type II sulfide inclusions by forming (Ca, Mn)S, according to Kiessling [1].
Firstly, addition of Ca modifies oxide inclusions and later, it either forms CaS or converts MnS to (Ca, Mn)S.Inclusions of (Ca, Mn)S precipitate at higher temperatures than those of type II MnS (elongated MnS sulfides), according to Faulring [2].
Inclusions of high melting point, such as Al 2 O 3 and CaS, can work as nucleation sites for MnS making sulfides thinly dispersed in the steel matrix.Yaguchi [3] describes a study performed with a base steel (SAE 1030 -0.06%S) with large sulfides inclusions concentrated on the as cast product.Formation of sulfide lines in the matrix can be observed in the forged product.In the steel with addition of Ca and Mg, sulfide inclusions became both thin and globular, and thinly dispersed in the forged product, as well.
For the production of special steels (as those intended for automotive industry) quality requirements are above average, whether compared to the most part of commercial steels.According to Kirsch-Racine et al. [4], the present tendency is the production of both aluminum killed and resulfurized steels.The former, to answer clients demand, regarding both grain size and fatigue resistance, and the latter to improve machinability for the production of mechanical components.In this way, such steels are considered critical in relation to their production in steel mill, since contents of some of their elements (Al, S, O, Ca) must be controlled.
Thus, the general purpose of this work is the thermodynamic study of non-metallic inclusions formation in the CC tundish for SAE 1141 steel.Specific purposes are: 1) obtaining phases and compounds formed in inclusions due to chemical composition and CC temperature employed for SAE 1141 steel.2) establishing conditions of steel chemical composition for producing less harmful inclusions to the SAE 1141 steel castability.

Materials
SAE 1141 quality combines both good conformability and machinability in the same steel grade.It is widely employed in forging segments to produce components with both relative complexity and mechanical demand, such as forks and shaft ends for automotive industry, according to Capellari [5] SAE 1141 steel is classified by SAE-AISI as resulfurized carbon steel.The chemical composition used was obtained from a series of industrial heats, as it is showed in Table 1.

Methodology
Based on previous studies by Bielefeldt et al. [6], thermodynamic simulations were performed with the commercial software FactSage and databases.Simulations were carried out by using both global chemical composition and average temperature of SAE 1141 steel in the CC tundish.The result obtained is the steel composition and non-metallic inclusions (oxides and sulfides) at the temperature of 1,520 o C, during CC.
Thermodynamic study was performed with the commercial software FactSage 5.5.A general description of the software and databases was carried out by Bale et al [7,8], as well as current modifications.
The following databases were used, described according to FactSage Database Documentation [9] 2) By calculating CaS saturation lines: maximum of 5 and 10% of CaS.Furthermore, a more detailed analysis of liquid window was carried out, taking into account the following reasons: 1) There are few studies on oxide inclusions in resulfurized steels treated with calcium; 2) It could be interesting to establish percentages of both inclusions liquid and solid phases in liquid windows, what could increase knowledge not only on casting, but also on a wider control of phase formation in inclusions; 3) A major fraction of solids in oxide inclusions can help controlling the morphology of manganese sulfides (MnS), mainly to avoid forming elongated sulfides (type II), according to Wakoh [10]; 4) As stated by Fuhr et al. [11], according to industrial results, castability problems more evidently occur when solid inclusions proportion is higher than 60-70%.
Considering reasons presented above, a different approach was adopted in order to make possible calculating either the formation or not of solid phases.In this case, besides employing solutions databases, stoichiometric pure solids (FactSage FS50 databases) were used as well, what was not possible with only solutions databases.

RESULTS AND DISCUSSION
Figure 1 shows the junction of two approaches described on the methodology: 1) Liquid window calculated for SAE 1141 steel; 2) Curve points from the beginning of liquid phase formation, 40% of liquid phase and 100% of liquid phase.In order to understand better how points from Figure 1 were obtained, Table 2 shows a cut from Figure 1, highlighting curve points in 15 ppm of Al, as an example.Values of each selected point were obtained from Figure 2.Among 10 ppm and little above 20 ppm Al, there are two peaks forming 100% of liquid phase, according to Figures 2 and 3.
Figure 3 regards Al = 21.23 ppm, which is heats average collected in the plant.There are two zones of liquid phase, however, the first one (between 4 and 8 ppm of calcium) is smaller than the second one (initiating in 13 ppm of calcium).In the first zone, heats sampled in the plant are located, and the liquid phase coexists with inclusions of CaO. 2 Al 2 O 3 .Figure 5 shows both Ca and O contents dissolved in steel as function of total calcium content.Moreover, dissolved Ca contents are multiplied by 100, while oxygen ones, by 10.In Figure 8, there is not Gehlenite formation.Liquid phase starts with lower calcium contents.In Figure 9, for Al = 80 ppm, CaOAl 2 O 3 formation occurs; with a new increase of calcium content necessary to form liquid phase.

CONCLUSIONS
For SAE 1141 steel conditions, ranging from 10 ppm to around 45 ppm of Al, there is a great difference between formation curves of 35% of CaO, and the points of liquid fraction.However, from 45 ppm of Al, they coincide.
There are two peaks of 100% liquid phase formation, from 10 ppm and little above 20 ppm of Al.
Considering Al = 21.23 ppm, O total = 24.5 ppm and average total calcium content of 5.7 ppm, the following values obtained from dissolved elements in steels were: O = 11.5 ppm, Ca = 0.085 ppm and Al = 10.6 ppm.
For Al = 50 ppm, there is not Gehlenite formation, and the liquid phase starts with lower calcium content.
However, for Al = 80 ppm, formation of CaO.Al 2 O 3 occurs, backing to increase the calcium content necessary to form the liquid phase.
Both conditions of chemical composition for obtaining window of liquid inclusions and the influence of aluminum content on forming inclusions for SAE 1141 steel were satisfactorily evaluated.

Figure 1 :
Figure 1: Liquid window for SAE 1141 steel and liquid phase formation.O total = 24.5 ppm and T = 1,520 ºC.

Figure 2 :
Figure 2: Mass percentage (wt %) of oxides and CaS in inclusions x SAE 1141 steel calcium content for Al = 15 ppm and O total 24.5 ppm.

Table 2 : 9 First
Points of liquid fraction for SAE 1141 steel with 15 ppm Al.Points in line of 15 ppm Al Ca content in ppm Beginning of liquid A 0.

Figure 3 :
Figure 3: Mass percentage (wt %) of oxides and CaS in inclusions x SAE 1141 steel calcium content for Al = 21.23 ppm and O total 24.5 ppm.

Figure 4
Figure 4 shows a liquid phase composition in mass percentage %.Considering both content of Al = 21.23 ppm and O total = 24.5 ppm, and other conditions for SAE 1141 steel.If the first zone is compared to the second one, both an increase of CaO content and a decrease of Al 2 O 3 occur, with increasing of calcium content (from 17.7 ppm of Ca).SiO 2 content remains practically constant.Figure5shows both Ca and O contents dissolved in steel as function of total calcium content.Moreover, dissolved Ca contents are multiplied by 100, while oxygen ones, by 10.Figure6shows dissolved Al in steel as function of total calcium content.In figures 5 and 6 both Al = 21.23 ppm and O total = 24.5 ppm contents and other conditions for SAE 1141 steel were taken into consideration.these figures -for average total calcium content of 5.7 ppm -values obtained were: O = 11.5 ppm, Ca = 0.085 ppm and Al = 10.6 ppm.

Figure 6
Figure 4 shows a liquid phase composition in mass percentage %.Considering both content of Al = 21.23 ppm and O total = 24.5 ppm, and other conditions for SAE 1141 steel.If the first zone is compared to the second one, both an increase of CaO content and a decrease of Al 2 O 3 occur, with increasing of calcium content (from 17.7 ppm of Ca).SiO 2 content remains practically constant.Figure5shows both Ca and O contents dissolved in steel as function of total calcium content.Moreover, dissolved Ca contents are multiplied by 100, while oxygen ones, by 10.Figure6shows dissolved Al in steel as function of total calcium content.In figures 5 and 6 both Al = 21.23 ppm and O total = 24.5 ppm contents and other conditions for SAE 1141 steel were taken into consideration.these figures -for average total calcium content of 5.7 ppm -values obtained were: O = 11.5 ppm, Ca = 0.085 ppm and Al = 10.6 ppm.

Figure 5 :
Figure 5: Dissolved Ca and O content depending on Ca content for SAE 1141 steel, Al = 21.23 ppm and O total = 24.5 ppm.

Figure 7 (
Figure 7 (Al = 30 ppm) shows the only formation of a liquid phase zone (from 14.1 ppm of Ca), coexisting with calcium-silicate inclusions 2CaOAl 2 O 3 SiO 2 (Gehlenite).In Figure8, there is not Gehlenite formation.Liquid phase starts with lower calcium contents.In Figure9, for Al = 80 ppm, CaOAl 2 O 3 formation occurs; with a new increase of calcium content necessary to form liquid phase.

Figure 6 :
Figure 6: Dissolved Al content depending on Ca content for SAE 1141 steel Al = 21.23 ppm and O total = 24.5 ppm.

Figure 7 :
Figure 7: Mass percentage (wt %) of oxides and CaS in inclusions x calcium content in SAE 1141 steel for Al = 30 ppm and O total = 24.5 ppm.

Figure 8 :
Figure 8: Mass percentage (wt %) of oxides and CaS in inclusions x calcium content in SAE 1141 steel for Al = 50 ppm and O total = 24.5 ppm.

Figure 9 :
Figure 9: Mass percentage (wt %) of oxides and CaS in inclusions x calcium content in SAE 1141 steel for Al = 80 ppm and O total = 24.5 ppm.

Table 1 :
Chemical composition of SAE 1141 steel in mass percentage (wt %).
*Al, Ca and O total (total oxygen) in ppm.