The resulting chemical composition of the samples is given in Table 2
. The calculated zirconium yield is roughly 60%. There was an increase in oxygen when Zr was added, which is due to reactions between Zr and oxygen in the protective atmosphere. There is also a minor increase of nitrogen, as Zr increases nitrogen solubility in the steel melt [24
The microstructural analysis revealed that all samples consisted of tempered martensite, and the main difference was in the grain size. The Zr additions had refined the martensitic microstructure as shown in Figure 1
. The light optical microscope analysis revealed that the samples with 0 and 300 ppm Zr have visible oxide inclusions, while the 700 ppm has only visible nitride inclusions.
The SEM EBSD and EDS analysis of the representative non-metallic inclusions are presented in Figure 2
, Figure 3
and Figure 4
. The sample without Zr contains typical Al-killed alumina inclusions with the occasional silicate and manganese sulphide (Figure 2
). Fine zirconium oxide (ZrO2
) inclusions are dominant in the sample, with 300 ppm Zr (Figure 3
). The sample with 700 ppm Zr (Figure 4
) has a slight increase in the size of the inclusions, compared to the sample with 300 ppm Zr, but this is mainly due to the formation of zirconium nitrides (ZrN) on the ZrO2
nucleus. Although the oxides are still roughly the same size, the ZrN formation increases the total non-metallic inclusion area. The non-metallic inclusions were also analyzed by the EBSD technique that provides Kikuchi patterns. They were used for determining the inclusion crystal structure type. Figure 2
, Figure 3
and Figure 4
also show the Kikuchi patterns and crystal structure types for the three typical types of inclusions (ZrN, ZrO2
) found in the samples. The ZrN has a cubic face-centered crystal structure, while ZrO2
has a monoclinic structure and Al2
has a hexagonal close-packed structure.
The inclusions were set into five categories—Al2
, MnS, SiO2
, ZrN and ZrO2
—based on their composition. The SEM automatic inclusion analysis showed that most of the inclusions are alumina-based in the non-modified sample. After the Zr addition, most of the inclusions become ZrO2
type. When additional Zr is added, ZrN appear in larger numbers, and the relative size also increases, as ZrN frequently form on previous ZrO2
inclusions (as shown in Figure 5
). Complex inclusions such as ZrN formed on ZrO2
, and were classified as ZrN, as they presented the bulk of the non-metallic inclusion, as well as the matrix/inclusion phase boundary.
The SEM analysis revealed that zirconium drastically decreased the surface area of all non-metallic inclusion, as shown in Figure 6
(results of the Inca feature analysis). The dark inclusions are alumina and zirconium oxides, while the zirconium nitrides are seen as white inclusions.
The Zr additions promote the formation of numerous small homogeneously dispersed non-metallic inclusions. These results are in agreement with our previous work, where Zr was added to non-Al-killed steel [21
]. One explanation for the formation of smaller ZrO2
non-metallic inclusions is that zirconium oxides require a lower critical radius for nucleation, as the result of low free Gibbs formation energy and low surface tension energy (better steel melt wettability), in comparison to alumina inclusions [25
]. Another reason for the homogeneous distribution is the fact that during solidification, the ZrO2
particles are engulfed by the solidification front, while the alumina particles are pushed into the segregation bands by the solidification front [26
The histogram of inclusions showed that the total area reduction is the result of a significant drop in the size of the inclusions. Figure 7
clearly shows that the largest share of inclusions modified by Zr is smaller than 1 µm. In the case of 300 ppm Zr, over 80% of inclusions are smaller than 1 µm. This is in good agreement with the research of Karasov and Suito, where the majority of the Zr inclusions were less than 1 µm in diameter [27
The results of the tensile test are presented in Table 3
(room temperature) and Table 4
(500 °C). Surprisingly, there was no significant improvement in the mechanical properties after Zr additions. There was a slight improvement at 300 ppm Zr, but tensile properties decreased at 700 ppm Zr. This trend continues when the samples are tensile tested at 500 °C.
The slight increase in mechanical properties can be attributed to the decrease in grain size. The subsequent fall in tensile strength in the 700 ppm Zr sample and the minimal increase in the 300 ppm Zr sample can be attributed to the loss of the precipitation hardening effect caused by small submicron nitride precipitates such as V(C, N) and the formation of micron-sized ZrN during solidification, preventing their formation during heat treatment. The precipitation hardening effect is more effective when the precipitates are a couple of nanometers in size [29
]. The multitude of Zr-based sharp edged inclusions, both oxide and nitride, probably decrease the mechanical properties due to the notch effect and subsequent void formation. Therefore, it can be concluded that the zirconium additions should be lower than 0.07% to prevent extensive nitride formation, and the weakening of the precipitation hardening effect during tempering.
The diagram with Gibbs free energy for the formation of Al2
, ZrN and ZrO2
at 1600 °C in liquid X11CrNiMo12 steel for various zirconium contents is shown in Figure 8
. The reason behind the modification effect of zirconium is the low Gibbs free energy for the formation of ZrO2
. When adding elements like Zr, the basic thermodynamic reactions must be considered, the yield strongly depends on reactions with oxygen. Zirconium has a high affinity to oxygen, and at around 50 ppm of Zr it starts to be more reactive than the 200 ppm Al in steel (Figure 8
). The high affinity to oxygen is one of the reasons why so much zirconium is present in non-metallic inclusions. The exact thermodynamic values were taken from Outotec HSC 8.0 Chemistry Software at 1600 °C, and the interaction coefficients in dilute Fe(l) and the Gibbs free energy changes due to the solution in liquid iron in wt.% were taken from Sigworth et al. [30
]. Furthermore, Figure 8
clearly shows that ZrN nitride non-metallic inclusions become more stable as the Zr content increases. This is why the ZrN content is low in the sample with 300 ppm Zr and high in the sample with 700 ppm Zr, and in addition, the active oxygen content is lowered due to oxide formation, which further promotes nitride formation.