NONMETALLIC INCLUSIONS IN AUSTENITIC STAINLESS STEEL AISI 303 MICROALLOYED WITH ZIRCONIUM AND TELLURIUM

The control of formation of nonmetallic inclusion and the characterization present the basis of improvement of steel product properties and lead to sustainable development in design of new steel grades. In order to produce steels with better machinability, such as AISI 303 grades, a modification of inclusions with carefully designed chemical composition is presented. Sulphur by creating sulphide inclusions reduces friction and cutting resistance, and increases the brittleness of the chip.Considering its harmful effect in steel, as well as the fact that non-metallic inclusions have been insufficiently tested for this type of high-alloyed steel, the aim of this research is to determine effects of microalloying on the possibility of modification of nonmetallic inclusions. Modification with zirconium favorably affects the ductile properties of steel, and a step forward in this study is a modification of inclusions with

The control of formation of nonmetallic inclusion and the characterization present the basis of improvement of steel product properties and lead to sustainable development in design of new steel grades. In order to produce steels with better machinability, such as AISI 303 grades, a modification of inclusions with carefully designed chemical composition is presented. Sulphur by creating sulphide inclusions reduces friction and cutting resistance, and increases the brittleness of the chip.Considering its harmful effect in steel, as well as the fact that non-metallic inclusions have been insufficiently tested for this type of high-alloyed steel, the aim of this research is to determine effects of microalloying on the possibility of modification of nonmetallic inclusions. Modification with zirconium favorably affects the ductile properties of steel, and a step forward in this study is a modification of inclusions with tellurium.It is of particular importance to determine the behavior of non-metallic inclusions in the process of production of the structural part and in subsequent exploitation. Therefore, plastic processing of austenitic stainless steel was also carried out, forging and rolling with two different level of processing.
Since 1950, stainless steels have seen the greatest increase in consumption, with the most frequent, austenitic [2]. In addition to alloying with at least 10.5% chromium, for stainless steel to be corrosion-resistant (passive), another condition must be fulfilled, namely the existence of a homogeneous single-phase ferrite, austenitic or martensitic microstructure The use of stainless steels is small compared to carbon steels, but shows steady growth, Figure 1. [4]. In Figure 2.
[4], which shows the annual growth rate of major metals from 1980 to 2018, it is easy to see that the growth rate of stainless steels is by far the highest.  Stainless steel is the most recycled material in the world and it is estimated that 82% of stainless steel used is recycled into new steel. When recycled, melted recycled steel has as good qualities and properties as the original steel.
Today, approximately 60% of the raw materials used for the production stainless steels are recycled steels [5].

Influence Of Alloying Elements In Steel:
Manganese is most commonly used as a deoxidizer and desulphuriser during steel production. Due to its high affinity for sulphur, manganese produces MnSsulphide, thus preventing the negative effect of FeSsulphide [3].
High sulphur content has a positive effect on machinability characteristics. Tool wear is reduced, and chip separation is more favorable [6].
Zirconium addition causes sulphide inclusions to be spherical (globular) rather than elongated, which improves the strength and ductility of microalloyed cast steel [7].

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The presence of tellurium in steel leads to the formation of globular sulphide inclusions, which at the same time favorably affect the machinability of steel, since its presence in steel reduces the energy required to separate the material in the shear zone during cutting [8].
Tellurium forms manganese telluride (MnTe) inclusions and is apparently more effective than the sulphur for machinability of austenitic stainless steels. It also promotes globularization and expansion of sulphide inclusions [6,7].

Experimental Research And Test Results:-
The melting and casting of austenitic stainless steel X8CrNiS18-9 was carried out in a vacuum induction furnace, and is located at the Department for melting and metal casting of the Institute "Kemal Kapetanović".
Four meltings were done. The first melt is austenitic stainless steel X8CrNiS18-9 without alloying elements. Subsequently, in the following three melt, the composition with the corresponding contents of zirconium and tellurium was modified so that each of the above elements was added independently, and then in combinations with both alloying elements. Chemical analyzes of all melt variants are given in Table 1.  All ingots are subjected to heat treatment: solution annealing -heating to 1050 °C, followed by rapid cooling in water. After the heat treatment, samples were taken next to the ingot head for metallographic testing of the cast state ( Figure 3 -after grinding and polishing).
Subsequently, an analysis of the content, size and distribution of the nonmetallic inclusions in the unetched state was performed, and the results of the tests are given in Table 2. The imaging of samples under a specific magnification (x50) was performed on an Olympus PMG3 type optical microscope, and one image was given for each sample (Figure 4). The figures show inclusions of average size, while Table 2 also lists individual inclusions that are significantly larger than average. 906 Table 2:-Results of metallographic testing of casted samples [9].

Melt variants Size of sulphide inclusions (m)
The  Figure 4:-Microstructure of all melt variants for the casted state [9].

Metallographic testing of forged samples:
After solution annealing, the specimens were hot deformed, namely forging on a hydraulic press and a hammer, which are located at the Department for Plastic Processing of Metals of the Institute "Kemal Kapetanović", up to a dimension of  50 mm. The samples, after the completion of forging process and rough machining, are shown in 907 Figure 5. Upon completion of the forging process, samples were taken to perform metallographic testing for the forging condition. Figure 5:-The samples after the completion of forging process and rough machining [9].
As with the cast samples, an analysis of the content, size and distribution of the nonmetallic inclusions in the unetched state was performed, and the test results are given in Table 3. Samples were also imaging on an OLYMPUS PMG3 type optical microscope (x50), and one image was taken for each sample ( Figure 6). Table 3:-Results of metallographic testing of forged samples [9].  Figure 6:-Microstructure of all melt variants for forged state [9].

Metallographic testing of rolled samples:
The rolling was performed on the SKET rolling mill, with the first section being reduced to 18 mm, while the second one reached a final sample size of 14 x 50 mm. The rolling speed was 400 rpm. After completion of rolling, all samples were quenched in water in order to avoid the effect of sensitisation. All samples are of different lengths depending on the amount of material in each variant. Figure 7 shows all the samples after the rolling process has been carried out. Upon completion of the second stage of deformation (rolling to dimensions 14 x 50 mm), samples were taken to perform metallographic testing for the rolling condition. Figure 7:-The samples after the rolling process was performed [9].
As with the previous samples, the content, size and distribution of nonmetallic inclusions in the unetched state were analyzed, and the test results are given in Table 4. The test for the rolling condition was performed in accordance with ASTM E45-11 -Standard Test Methods for Determining the Contents of Inclusions in Steel. BAS EN 10088-1 does not specify limit values for the content of nonmetallic inclusions. Sample imaging under a certain magnification (x50) was performed on an OLYMPUS PMG3 type optical microscope, and one image was given for each sample (Figure 8).  Many small sulphide inclusions with thickness less than 2µm have been observed. A complex oxysulfide inclusion of 500 µm size was also observed. Te  3  3 Many small sulphide inclusions with thickness less than 2µm have been observed. Complex inclusions of size 600, 500, 300, 200 µm were also observed.

alloyed with
alloyed with Zr i Te 1,5 3 Many small sulphide inclusions with thickness less than 2µm have been observed. Complex inclusions of size 600, 150, 60 µm were also observed.