NLMK Group, Moscow, Russia
A. I. Zhitenev, Cand. Eng., Head of Technological Projects, e-mail: zhitenev.ai1991@gmail.com

Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russia

A. S. Rovbo, Engineer of the Scientific and Technological Complex “New Technologies and Materials”, e-mail: harchenko.annna@yandex.ru
D. V. Nechaev, Research Engineer of the Scientific and Technological Complex “New Technologies and Materials”, e-mail: nechaev_dv@spbstu.ru
S. V. Ryaboshuk, Senior Lecturer, e-mail: ryaboshuk_sv@spbstu.ru

Bismuth is one of the most promising elements, allowing us to largely eliminate the use of sulfur and lead added to modern free-cutting steels. Thermodynamic modeling was used to determine the optimal amount of this element and to develop an effective production technology. The modeling results were verified by comparison with experimental studies of inclusions in experimental ingots and rolled a free-cutting steel 0,4C-Cr-Bi with different bismuth and sulfur contents. The distribution of inclusions of bismuth and manganese sulfides over the cross section of experimental ingots has been studied, and the patterns of changes in the number, size and composition of inclusions depending on the characteristics of the cast structure have been studied. It has been shown that there are critical concentrations of bismuth that determine the technology for its addition to liquid steel. Thus, adding bismuth in amounts above 0.15 % is not effective, since its solubility limit in liquid steel is exceeded. Adding bismuth to concentrations of 0.10 % and higher is also not effective, since in this case its primary and secondary inclusions are formed, randomly distributed throughout the volume of the ingot, forming segregations. It has been studied that during hot rolling and heat treatment the number and size of inclusions of bismuth and manganese sulfide do not change. During experimental cutting, it was shown that the addition of bismuth in an amount of 0.08 %, and the formation of 0.06 % vol. There are enough inclusions to significantly increase the machinability of free-cut steels. Based on experimental and computational studies, the optimal concentrations of bismuth and sulfur necessary for the formation of the required number of inclusions were determined, which are 0.05-0.09 % for bismuth and <0.02 % for sulfur.
The research is funded by the Ministry of Science and Higher Education of the Russian Federation within the framework of the World-Class Research Center: Advanced Digital Technologies program (agreement No. 075-15-2020-311 of 04/20/2022).

1. Xie J., Fan T., Zeng Z., Sun H. et al. Bi-sulfide existence in 0Cr18Ni9 steel: Correlation with machinability and mechanical properties. Journal of Materials Research and Technology. 2020. Vol. 9. pp. 9142–9152.
2. Ryabov A. V., Chumanov I. V. On the possibility of obtaining a new easily machined corrosionresistant steel. Elektrometallurgiya. 2012. No. 2. pp. 33–35.
3. Kurka V., Kubon Z., Kander L., Jonsta P. et al. The effect of bismuth on technological and material characteristics of low-alloyed automotive steels with a good machinability. Metals. 2022. Vol. 12. No. 2. pp. 301–315.
4. Hu S., Li Z., Fan T., Fu J. Effect of bismuth on sulfide in high sulfur free-cutting steel. IOP Conference Series: Materials Science and Engineering. 2019. Vol. 611, Iss. 1. 012017.
5. Ryabov A. V. Solubility of bismuth and lead in liquid and solid iron. Vestnik Yuzhno-Uralskogo gosudarstvennogo universiteta. Seriya: Metallurgiya. 2013. Vol. 13. No. 2. pp. 27–32.
6. Ryabov A. V., Povolotsky D. Ya., Ryabov V. V. Recovery of bismuth during alloying of automatic steel during uphill casting. Izvestiya Chelyabinskogo nauchnogo tsentra. 2001. No. 1. pp. 81–90.
7. Ryabov A. V. Solubility of fusible, easily oxidized and easily evaporated elements in iron-based alloys. Vestnik Yuzhno-Uralskogo gosudarstvennogo universiteta. Seriya: Metallurgiya. 2014. Vol. 14. No. 3. pp. 19–24.
8. Ryabov A. V., Trofimov E. A. The influence of bismuth on phase equilibria realized in “iron – alloying element” systems. Vestnik Yuzhno-Uralskogo gosudarstvennogo universiteta. Seriya: Metallurgiya. 2015. Vol. 15. No. 4. pp. 142–146.
9. Liu H., Chen W., Li W., Yu Y. Solubility of bismuth in liquid Bi – S based free cutting steel. High Temperature Materials and Processes. 2014. Vol. 33. pp. 187–191.
10. FactSage.Com. Available at: https://www.factsage.com/ (accessed: 26.06.2023).
11. Kazakov A. A., Zhitenev A. I. Assessment and interpretation of nonmetallic inclusions in steel. CIS Iron and Steel Review. 2018. Vol. 16. pp. 33–38.
12. Zhitenev A., Salynova M., Shamshurin A., Ryaboshuk S. et al. Database clustering after automatic feature analysis of nonmetallic inclusions in steel. Metals. 2021. Vol.11. 1650.
13. Physico-chemical foundations of metallurgical processes (FKHOMP 2022): collection of works of the International Scientific Conference named after Academician A. M. Samarin, dedicated to the 120^th anniversary of the birth of the outstanding metallurgist, Academician of the USSR Academy of Sciences A. M. Samarin, the 265^th anniversary of the founding of the Vyksa Metallurgical Plant and the 20th anniversary of the Vyksa branch of NUST "MISiS", Vyksa, 2022. 549 p.
14. Biswas D. K., Venkatraman M., Narendranath C. S., Chatterjee U. K. Influence of sulfide inclusion on ductility and fracture behavior of resulfurized HY-80 steel. Metall. Mater. Trans. A. 1992. Vol. 23. pp. 1479–1492.
15. Kazakov A., Kiselev D. Industrial application of thixomet image analyzer for quantitative description of steel and alloy’s microstructure. Metallogr. Microstruct. Anal. 2016. Vol. 5. pp. 294–301.
16. Kazakov A. A., Zhitenev A., Kovalev P. Distribution pattern of nonmetallic inclusions on a cross section of continuous-cast steel billets for rails. Microscopy and Microanalysis. 2015. Vol. 21 (S3). pp. 1751, 1752.
17. Luo S., Wang B., Wang Z., Jiang D. et al. Morphology of solidification structure and MnS inclusion in high carbon steel continuously cast bloom. ISIJ International. 2017. Vol. 57. pp. 2000–2009.
18. Shu Q., Visuri V.-V., Alatarvas T., Fabritius T. Model for inclusion precipitation kinetics during solidification of steel applications in MnS and TiN inclusions. Metall. Mater. Trans. B. 2020. Vol. 51. pp. 2905–2916.
19. Gu J. P., Beckermann C. Simulation of convection and macrosegregation in a large steel ingot. Metall. Mater. Trans. 1999. Vol. 30. pp. 1357–1366.
20. Kazakov A. A., Zhitenev A. I., Salynova M. A., Kolpichon E. Yu. Nonmetallic inclusions quantitative assessment for forgings made from super large steel ingot. Chernye Metally. 2018. No. 12. pp. 50–56.
21. Koji W., Tatsuya I., Toshiharu A. Development of lead-free free-cutting steel and cutting technology. Nippon steel & Sumitomo Metal Technical Report. 2017. Vol. 116. pp. 32–37.
22. Maciejewski J. The Effects of sulfide inclusions on mechanical properties and failures of steel components. J. Fail. Anal. and Preven. 2015. Vol.15. pp. 169–178.