Effect of Al addition on the solidification structure of 441 dual stabilised ferritic stainless steel

. Ferritic stainless steels (FSS) develop poor surface finish during processing and this is attributed to the large columnar grains formed during solidification. Addition of alloying elements such as Al, Ti and Nb used in dual stabilized FSS improve the solidification structures. In this work, two 441 dual stabilized FSS, with 0.023 and 0.068 %Al respectively were investigated in the as-cast condition. The columnar to equiaxed zone ratio was determined by the linear interception method. The results showed that the higher the Ti/Al ratio, the higher the equiaxed ratio. The inclusions reported from SEM showed that there were more spinel inclusions of Ti-Al-O as well as more TiN precipitates in the steel with 0.023 wt% Al.


Introduction
Despite the similarities in the properties and applications of ferritic stainless steels and most austenitic stainless steels in the field of automobile, sports, health etc., conventional, ferritic stainless steels have not shown formability anywhere close to that of austenitic stainless steels This is mainly due to their surface quality. For example, there are corrugations that are present on the surface of ferritic stainless steels after forming or drawing applications [1]. This phenomenon is generally referred to as ridging and it is known to occur in steels [2]. This has drawn much attention and research is still ongoing to find a sustainable solution to this problem because ferritic stainless steels, as compared to austenitic stainless steels, contain low Ni content and hence makes it cost effective [3,4,5]. In solidification, achieving more refined grains improves the equiaxed crystal structure and this improves the texture which in turn reduces the ridging problem [1,2].
To achieve both high strength and ductility, the grains of the final product are required to be fine. Grain refinement is achieved when there are effective heterogeneous nucleation sites. This occurs when there is also constitutional undercooling during solidification where nuclei is formed. Heterogeneous nucleation is effective when the nuclei formed is able to survive long enough throughout solidification for the nucleation of the desired phase. Nuclei that are mostly formed in these steels in the molten state are the oxides formed during deoxidification as well as the TiN [6]. These tend to serve as nuclei for heterogeneous nucleation of the desired phase; which in this study is the -ferrite [7,8]. The more there are heterogeneous nuclei present in the molten steel, the higher the equiaxed grain ratio, hence refining the grains.
Aluminium as a deoxidant, precipitates as Al2O3 at high temperatures and usually serves as heterogeneous nucleation sites for the TiN which in turn acts as a nucleation site for the -ferrite. This leads to increased equiaxed grain ratio of the solidifying cast structure [9]. TiN on its own is also stable in the molten metal and will also serve as heterogeneous nuclei [10]. According to Fujimura et. al., [11], oxides mass ratio of Mg/Al in the range of 0.3 to 0.5 which are covered by TiN produces more equiaxed grains in ferritic stainless steels [12,13]. Heintze also found that TiN forms on oxides of Al and Ti at higher temperatures in weld regions [14]. Hence with higher Ti/Al ratio, the TiN precipitated will be more.

Materials
The chemical compositions of the investigated steels with aluminium contents of 0.023 and 0.068 wt% are shown in Table 1. The steels were received as-cast from Columbus Stainless Pty. Ltd. For the purposes of this study, the steels are designated as FSS-0.023 and FSS-0.068 to depict steels with aluminium contents 0.023 and 0.068 wt% respectively.

Experimental methods
Thermo-Calc software was used to predict the stable phase present and precipitation behavior during solidification of the two steels. TC-Prisma software was also used to simulate the homogeneous nucleation of TiN in the liquid phase at 1475 C. The time for the simulation was set at 600 seconds, number of nucleation sites was taken as bulk, interfacial energy was calculated by the TC-Prisma model, molar volume was fixed at 7.0 × 10 −6 m 3 /mol for all the simulations done. The as-cast steels were characterized for the equiaxed grain ratio by measuring the grains with imageJ® software. Also, the inclusions and precipitates were characterised at sample positions of near surface, quarter thickness, center, bottom and quarter thickness from bottom. These positions are designated as S, QT1, C, B and QT2 respectively. Metallographic preparations consisted of SiC paper grinding from 400 grit to 1200 grit followed by diamond paste polishing from 3 μm to 1 μm and finally for SEM characterizations, etching with a solution of a mixture of three parts HCl and one part HNO3. A manual analysis of inclusions was done on images taken from JEOL Scanning Electron Microscope (SEM) of model JSM-IT300 equipped with an energy dispersive x-ray spectrometer. To analyze the inclusions, sizes, quantity and elemental compositions were acquired in selected areas. The SEM-EDS analysis was performed using an accelerating voltage of 15 kV.

Thermo-Calc predictions
The amount of various precipitates as predicted by Thermo-Calc for FSS-0.023 and FSS-0.068 steels are shown in Fig. 1 (a) and (b) respectively. As may be seen, the Laves phase dissolves around 910 C while (Nb, Ti)C at 1150 C. In FSS-0.023, (Ti,Al)O forms in the molten steel and mostly transforms to complex oxides of Al, Ca, Ti. In other words, the stable phases in FSS-0.023 are Laves phase, MnS, TiN, (Nb, Ti) C, complex oxides, (Ti, Al) O and −ferrite. Likewise in FSS-0.068, the stable phases are Laves phase, TiN, (Nb, Ti) C, complex oxides, MnS, (Ti,O), and − . With FSS-0.023 having Ti/Al ratio of 7.83 and that of FSS-0.068 being 2.38, it is expected that the precipitates to be predicted in FSS-0.023 would be more Ti-rich than those in FSS-0.068. In FSS-0.023 steel, Thermo-Calc predicted more formation of Ti-Al complex oxides. These complex oxides are known to be good nucleation sites for the heterogeneous nucleation of -ferrite as well as TiN, which improves the formation of equiaxed grains [15,16,17]. Therefore, with TiN as a stable phase in both steels, more equiaxed grains are expected to form heterogeneously in the steel with more precipitates that reach the critical size for the nucleation of -ferrite. In other words, when the radius of TiN is less than the critical size, it will not serve as a nucleation site [18,19]. According to Mu, the critical radius for nucleation of ferrite was found to be 0.178 [18]. This is confirmed by simulations done in TC-Prisma (Fig. 2) where the predictions for nucleation rate, volume fractions, number density, mean radius and size distribution of TiN was compared for the two steels. The predictions show that the nucleation rate (Fig. 2a) of TiN in FSS-0.023 is higher than in FSS-0.068. Hence, it is expected that the volume fraction and number density of TiN would be more in FSS-0.023 and this is confirmed in Fig. 2 (b) and (c). The mean radius (Fig. 2d) of TiN in FSS-0.023 is predicted to be 0. 21 which is bigger than the critical radius reported by Mu. However, that of FSS-0.068 is predicted to be 0. 16 which is slightly less than the critical radius.  respectively. This means that there were more Ti-based particles (inclusions and precipitates) in FSS-0.023 than in FSS-0.068. The equiaxed grain ratio increased with an increase in the Ti/Al ratio i.e. it was found to be 29 and 73% for the FSS-068 and FSS-023 respectively. The equiaxed grain ratio (EGR) of FSS-0.023 sampled from surface (S), quarter thickness (QT1), center (C), bottom (B) and the whole slab is shown in Table 2 whereas that for FSS-0.068 is shown in Table 3. At lower Ti/Al ratio, oxygen competes with the nitrogen for the formation of nitrides [12] and this means less TiN as well as Ti-based inclusions will be formed and nucleation sites for equiaxed grains are limited hence the decrease in equiaxed grain ratio [20]. This is confirmed in Fig. 4 and Fig. 5 where the number of inclusions in FSS- 370, 03003 (2022) https://doi.org/10.1051/matecconf/202237003003 MATEC Web of Conferences 2022 RAPDASA-RobMech-PRASA-CoSAAMI Conference 0.023 is more than in FSS-0.068 (Fig. 4). The size distribution was found to be more in FSS-0.023 than FSS-0.068 (Fig. 5) which favoured more heterogeneous nucleation of theferrite.  Fig. 4 shows that the inclusions and precipitates in FSS-0.068 are more in the quarter thickness (QT1) than in any other region of the steel. This is why the quarter region is more columnar. The size of inclusions and precipitates in this region are small hence the surface area for nucleation of the − rrite is small and equiaxed grain ratio is not improved. If TiN formation is limited, probably equiaxed grain formation could also limited  Figure 6 illustrates that particles with bigger particle radius provides more surface area for nucleation of − [21]. Hence, the size distribution of TIN in FSS-0.023 (Fig. 5) shows that there are more inclusions that are greater than the critical radius which provides more surface area for the heterogeneous nucleation of the − rrite and in turn improving the equiaxed grain ratio.  Fig. 7 and 8 showed that the inclusions were complex oxides of Ti, Al for FSS-0.023Al and for FSS-0.068Al, the inclusions were complex oxides of Al, Ca. Fig. 8 shows the SEM micrographs of precipitates in the matrix and on grain boundaries. As may be seen, the inclusion analysis indicates that there are more inclusions in FSS-0.023 than in FSS-0.068. The TiN particles heterogeneously nucleate around the spinel oxides, hence more precipitates in FSS-0.023 than in FSS-0.068 in the liquid phase.  Generally, the precipitates that were characterised in both steels were cuboidal TiN particles surrounding oxide inclusions of Al, Ti, Mg, Ca. After solidification, these were found on both the grain boundaries as well as in the matrix. Nb-rich carbides, (Nb, Ti) C, only formed during the solid state phase transformation and therefore would only contribute to grain refinement through Zener pinning effect.

Conclusion
The effect of Al addition on the evolution of inclusions, precipitates and the solidification structure of 441 dual-stabilized ferritic stainless steel was investigated and the following conclusions were drawn: • The increase in Al content, led to decrease in the Ti/Al ratio which resulted in the decrease in volume fractions of TiN during solidification and the decrease in equiaxed grain ratio. • The formation of complex oxides of Ti, Al in FSS-0.023 served as cores for TiN precipitation. These were found to be exhibit a wider size distribution than in FSS-068.