Os componentes automotivos em geral devem atender a especificações rigorosas de propriedades mecânicas, requisitos de limpeza inclusionária, entre outros. Enxofre e alumínio são elementos adicionados ao banho líquido de aço que desempenham um papel importante na obtenção destas propriedades. Porém, eles que dificultam o processo de produção por lingotamento contínuo por favorecerem a formação de partículas sólidas de alto ponto de fusão, que podem causar a obstrução das válvulas de lingotamento e dificultar o processo de desenvolvimento de aços com alta limpeza inclusionária. O objetivo do presente trabalho foi estudar a composição química do aço DIN 38MnSiVS6, focando nos elementos químicos que afetam a formação e remoção de inclusões, assim como a composição química da escória e outros parâmetros da aciaria como a temperatura e a lingotabilidade do processo de produção em escala industrial. Neste estudo, foi avaliada a hipótese de melhoria da lingotabilidade e da limpeza do aço através da alteração da etapa de adição de enxofre e também da quantidade de adição de cálcio Amostras de aço retiradas ao longo do processo foram analisadas através de microscopia ótica e eletrônica de varredura e também por ultrassom por imersão. Os dados de composição química foram sobrepostos a janelas de lingotabilidade e as características das escórias foram avaliadas por simulação termodinâmica. Foram avaliadas as correlações entre os resultados das diferentes técnicas de análise e com as referências de literatura. A hipótese de redução da quantidade de cálcio se mostrou satisfatória tanto para a melhoria da lingotabilidade quanto para a limpeza do aço. Porém, a alteração da etapa de adição de enxofre necessita de mais testes para comprovar sua eficiência. Dentre as técnicas empregadas, o uso do microscópio eletrônico de varredura acoplado a um sistema automatizado de análise de inclusões (ASPEX) se mostrou como uma ferramenta vantajosa por fornecer uma ampla gama de informações que podem auxiliar no entendimento da lingotabilidade e da limpeza inclusionária do aço. / Automotive components in general must meet stringent specifications of mechanical properties, inclusionary cleanliness requirements, among others. Sulfur and aluminum are elements added to the liquid steel bath which play an important role in obtaining these properties, but which hinder the production process by continuous casting by favoring the formation of solid particles of high melting point, that can cause the nozzle clogging and hamper the development process of steels with high inclusionary cleanliness. The aim of the present work is to study the chemical composition of the DIN 38MnSiVS6 steel, focusing on the chemical elements that affect the formation and removal of inclusions, as well as the chemical composition of the slag and other parameters such as temperature and castability in an industrial scale production process. In this study, the hypothesis steel cleanliness improvement was evaluated by changing the stage of sulfur addition as well as the amount of calcium addition. Steel samples collected throughout the process were analyzed by optical and scanning electron microscopy, and by immersion ultrasound. The chemical composition data were superimposed on liquid windows and the slag characteristics were evaluated by thermodynamic simulation. The correlations between the results of the different analysis techniques and the literature references were evaluated. The hypothesis of reduction in the amount of calcium was satisfactory both for the improvement of the castability and for the cleanliness of the steel, but the change of the sulfur addition stage requires more tests to prove its efficiency. The scanning electron microscope coupled with an automated inclusion analysis system (ASPEX) has proved to be an advantageous tool to provide a wide range of information that can help in the understanding of the castability and cleanliness of steel.

Os componentes automotivos em geral devem atender a especificações rigorosas de propriedades mecânicas, requisitos de limpeza inclusionária, entre outros. Enxofre e alumínio são elementos adicionados ao banho líquido de aço que desempenham um papel importante na obtenção destas propriedades. Porém, eles que dificultam o processo de produção por lingotamento contínuo por favorecerem a formação de partículas sólidas de alto ponto de fusão, que podem causar a obstrução das válvulas de lingotamento e dificultar o processo de desenvolvimento de aços com alta limpeza inclusionária. O objetivo do presente trabalho foi estudar a composição química do aço DIN 38MnSiVS6, focando nos elementos químicos que afetam a formação e remoção de inclusões, assim como a composição química da escória e outros parâmetros da aciaria como a temperatura e a lingotabilidade do processo de produção em escala industrial. Neste estudo, foi avaliada a hipótese de melhoria da lingotabilidade e da limpeza do aço através da alteração da etapa de adição de enxofre e também da quantidade de adição de cálcio Amostras de aço retiradas ao longo do processo foram analisadas através de microscopia ótica e eletrônica de varredura e também por ultrassom por imersão. Os dados de composição química foram sobrepostos a janelas de lingotabilidade e as características das escórias foram avaliadas por simulação termodinâmica. Foram avaliadas as correlações entre os resultados das diferentes técnicas de análise e com as referências de literatura. A hipótese de redução da quantidade de cálcio se mostrou satisfatória tanto para a melhoria da lingotabilidade quanto para a limpeza do aço. Porém, a alteração da etapa de adição de enxofre necessita de mais testes para comprovar sua eficiência. Dentre as técnicas empregadas, o uso do microscópio eletrônico de varredura acoplado a um sistema automatizado de análise de inclusões (ASPEX) se mostrou como uma ferramenta vantajosa por fornecer uma ampla gama de informações que podem auxiliar no entendimento da lingotabilidade e da limpeza inclusionária do aço. / Automotive components in general must meet stringent specifications of mechanical properties, inclusionary cleanliness requirements, among others. Sulfur and aluminum are elements added to the liquid steel bath which play an important role in obtaining these properties, but which hinder the production process by continuous casting by favoring the formation of solid particles of high melting point, that can cause the nozzle clogging and hamper the development process of steels with high inclusionary cleanliness. The aim of the present work is to study the chemical composition of the DIN 38MnSiVS6 steel, focusing on the chemical elements that affect the formation and removal of inclusions, as well as the chemical composition of the slag and other parameters such as temperature and castability in an industrial scale production process. In this study, the hypothesis steel cleanliness improvement was evaluated by changing the stage of sulfur addition as well as the amount of calcium addition. Steel samples collected throughout the process were analyzed by optical and scanning electron microscopy, and by immersion ultrasound. The chemical composition data were superimposed on liquid windows and the slag characteristics were evaluated by thermodynamic simulation. The correlations between the results of the different analysis techniques and the literature references were evaluated. The hypothesis of reduction in the amount of calcium was satisfactory both for the improvement of the castability and for the cleanliness of the steel, but the change of the sulfur addition stage requires more tests to prove its efficiency. The scanning electron microscope coupled with an automated inclusion analysis system (ASPEX) has proved to be an advantageous tool to provide a wide range of information that can help in the understanding of the castability and cleanliness of steel.

Os componentes automotivos em geral devem atender a especificações rigorosas de propriedades mecânicas, requisitos de limpeza inclusionária, entre outros. Enxofre e alumínio são elementos adicionados ao banho líquido de aço que desempenham um papel importante na obtenção destas propriedades. Porém, eles que dificultam o processo de produção por lingotamento contínuo por favorecerem a formação de partículas sólidas de alto ponto de fusão, que podem causar a obstrução das válvulas de lingotamento e dificultar o processo de desenvolvimento de aços com alta limpeza inclusionária. O objetivo do presente trabalho foi estudar a composição química do aço DIN 38MnSiVS6, focando nos elementos químicos que afetam a formação e remoção de inclusões, assim como a composição química da escória e outros parâmetros da aciaria como a temperatura e a lingotabilidade do processo de produção em escala industrial. Neste estudo, foi avaliada a hipótese de melhoria da lingotabilidade e da limpeza do aço através da alteração da etapa de adição de enxofre e também da quantidade de adição de cálcio Amostras de aço retiradas ao longo do processo foram analisadas através de microscopia ótica e eletrônica de varredura e também por ultrassom por imersão. Os dados de composição química foram sobrepostos a janelas de lingotabilidade e as características das escórias foram avaliadas por simulação termodinâmica. Foram avaliadas as correlações entre os resultados das diferentes técnicas de análise e com as referências de literatura. A hipótese de redução da quantidade de cálcio se mostrou satisfatória tanto para a melhoria da lingotabilidade quanto para a limpeza do aço. Porém, a alteração da etapa de adição de enxofre necessita de mais testes para comprovar sua eficiência. Dentre as técnicas empregadas, o uso do microscópio eletrônico de varredura acoplado a um sistema automatizado de análise de inclusões (ASPEX) se mostrou como uma ferramenta vantajosa por fornecer uma ampla gama de informações que podem auxiliar no entendimento da lingotabilidade e da limpeza inclusionária do aço. / Automotive components in general must meet stringent specifications of mechanical properties, inclusionary cleanliness requirements, among others. Sulfur and aluminum are elements added to the liquid steel bath which play an important role in obtaining these properties, but which hinder the production process by continuous casting by favoring the formation of solid particles of high melting point, that can cause the nozzle clogging and hamper the development process of steels with high inclusionary cleanliness. The aim of the present work is to study the chemical composition of the DIN 38MnSiVS6 steel, focusing on the chemical elements that affect the formation and removal of inclusions, as well as the chemical composition of the slag and other parameters such as temperature and castability in an industrial scale production process. In this study, the hypothesis steel cleanliness improvement was evaluated by changing the stage of sulfur addition as well as the amount of calcium addition. Steel samples collected throughout the process were analyzed by optical and scanning electron microscopy, and by immersion ultrasound. The chemical composition data were superimposed on liquid windows and the slag characteristics were evaluated by thermodynamic simulation. The correlations between the results of the different analysis techniques and the literature references were evaluated. The hypothesis of reduction in the amount of calcium was satisfactory both for the improvement of the castability and for the cleanliness of the steel, but the change of the sulfur addition stage requires more tests to prove its efficiency. The scanning electron microscope coupled with an automated inclusion analysis system (ASPEX) has proved to be an advantageous tool to provide a wide range of information that can help in the understanding of the castability and cleanliness of steel.

Advanced High Strength Steel (AHSS) has become a popular steel grade among automakers to produce vehicle bodies. With improvements in strength and elongation, AHSS has evolved to its 2nd generation, including high manganese steel. Even though it has outstanding strength, the 2nd generation of AHSS faces some production problems due to its high alloying elements. With continual improvement, the 3rd generation of AHSS is currently in production. In this generation, the steel types still have a competitive strength and elongation like the 2nd generation of AHSS while having lower alloying element contents and production costs. One of the types of 3rd generation AHSS is medium manganese steel. Research related to the 2nd and 3rd generation of AHSS mainly focuses on their mechanical properties and microstructures. As there is a strong correlation between mechanical properties and inclusion characteristics, further investigation of the evolution of inclusions is still required. In this study, high-temperature experiments were conducted to investigate the effects of metal chemistry on the inclusion evolution in liquid steel. The concentrations of manganese, aluminum, and nitrogen were varied systematically. Two and three-dimensional analysis techniques were applied to study the number, composition, and size distribution of inclusions. Electrolysis extraction was used to identify the oxide, sulfide, and nitride inclusions, whereas an automated SEM with an ASPEX feature was used to detect a larger number of inclusions for better representation of the steel matrix. This work has established inclusion classification rules to distinguish nitride inclusions from oxide inclusions. To the best of the authors’ knowledge, this is the first discussion of this type of inclusion classification in the open literature. Based on the automated SEM (ASPEX Feature) analysis, the type of detected inclusions in medium and high manganese steels were Al2O3(pure), Al2O3-MnS, AlN(pure), AlN-MnS, AlON, AlON-MnS, and MnS inclusions. As the manganese content in the steel increased from 2% to 20%, the total amount of inclusions, especially AlN-contained inclusions, was raised. This phenomenon occurred due to the increase in nitrogen solubility with increased manganese content in the steel. The thermodynamic calculation also predicted that AlN inclusions would form when the steel was cooled or during the solidification. Moreover, AlN and MnS inclusions were observed to co-precipitate together. Similar to manganese, the increase in the aluminum content (Al = 0.5-6%) increased the total amount of inclusions in the steel, and the dominant inclusion type is AlN. AlN and Al2O3 inclusions can be heterogenous nucleation sites for MnS inclusions. Furthermore, Al2O3 inclusions also became heterogeneous nucleation sites for AlN inclusions. The experimental set-up was further modified to investigate the effect of nitrogen on the formation of inclusions in the medium manganese steels. The nitrogen was introduced by purging or injecting N2 gas into the steel system. Similar to the effect of manganese and aluminum, the increase in the nitrogen content also increased the total amount of inclusions. Once the nitrogen content in the steel exceeded the critical limit for the formation of AlN inclusions, AlN inclusions can be stable in the liquid steel. Moreover, regardless of the nitrogen content in the steel, AlN-MnS inclusions were formed in the slow-cooled steels. In terms of morphology, AlN inclusions can be formed of plate-like, needle, angular, agglomerate, or irregular shapes. Furthermore, a brief investigation on the addition of calcium and nitrogen to the medium manganese steels found that calcium led to the formation of other complex inclusions, such as CAx and CAS-Other inclusions. In the medium manganese steel composition in the present study, the number of CAS-Other inclusions was dominated by (Ca,Mn)S-Oxide inclusions after the addition of Ca. However, with time and after introducing N2 gas into the steel, the number of (Ca,Mn)S-Nitride inclusions also increased. The formation of (Ca,Mn)S-Nitride inclusions resulted from the co-precipitation of CaS, MnS, and AlN. The current work provides a better understanding of the formation mechanism of inclusions in medium manganese steels and high manganese steels. It presents complete information on the characteristics of inclusions, such as the number density, type, and morphology of inclusions. This knowledge can help steelmakers improve the steelmaking process to control the formation of inclusions, which can be problematic for the manufacture and performance of medium manganese steels and high manganese steels. / Dissertation / Doctor of Philosophy (PhD)