Publication: Desarrollo de nuevos aceros sinterizados de alta resistencia aleados mecánicamente con Nb
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2012-11
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2012-11-23
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Abstract
En aceros estructurales, la reducción del tamaño de grano es el único mecanismo que permite un aumento significativo tanto de la resistencia como de la tenacidad del material. Este criterio ha sido utilizado con éxito en los aceros microaleados, en los que la acción conjunta de tratamientos termomecánicos con la incorporación en la composición de pequeñas cantidades de elementos de microaleación conduce a una microestructura muy fina que proporciona una excelente combinación de propiedades. La posibilidad de microalear mediante técnicas pulvimetalúrgicas no se ha explorado completamente debido a ciertas dificultades técnicas, como la imposibilidad de realizar tratamientos termomecánicos durante un ciclo de sinterización convencional, la dificultad de distribuir uniformemente en la microestructura cantidades tan pequeñas de aleantes, la elevada afinidad por el oxígeno de los elementos de microaleación o la necesidad de que estos reaccionen durante el ciclo de sinterización para formar una distribución de carburos eficaz para controlar el crecimiento de grano. En este trabajo de investigación se estudia la posibilidad de desarrollar nuevos aceros sinterizados de alta resistencia aleados con niobio, aplicando el concepto de afino de grano desarrollado en los aceros microaleados. De entre los elementos de microaleación, el niobio es el que posee un mayor potencial para el control del crecimiento de grano en aceros sinterizados. En los aceros microaleados ha demostrado ser eficaz para inhibir el crecimiento de grano austenítico y retrasar la recristalización de la austenita en forma de carburos o carbonitruros. Además, estando en solución sólida, es uno de los elementos de aleación que produce una mayor reducción de la movilidad de los bordes de grano en aceros mediante solute drag. Mediante el empleo de la molienda mecánica como técnica de aleación se obtienen polvos prealeados nanoestructurados en los que el niobio se puede incorporar de forma elemental o directamente como carburos. De esta manera se posibilita la comparación de los dos mecanismos de control del crecimiento de grano planteados (particle pinning y solute drag) durante la sinterización. Así mismo, el uso de técnicas de sinterización asistidas por presión (hot pressing, SPS) permite la obtención de un material completamente denso utilizando menores tiempos y temperaturas de sinterización, lo cual conlleva un control adicional del tamaño de grano. Esta tesis doctoral abarca todo el proceso de diseño de un nuevo material, desde la elección de composiciones y fabricación del polvo prealeado mediante aleación mecánica hasta la evaluación y validación de los materiales sinterizados. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Grain refining is the only strengthening mechanism in structural steels that increases both yield strength and toughness. This concept have been applied successfully in microalloyed steels, in which the use of thermomechanical treatmens and the addition of small quantities of microalloying elements lead to a very fine microstructure that provides a great combination of properties. Microalloying in sintered steels have not been fully explored since it involves some difficulties, like the absence of thermomechanical treatments in a conventional sintering process, the high oxygen affinity of the microalloying elements or the need to ensure a good distribution of the microalloying elements in such small quantity. Besides, the sintering process itself, as a diffusive process, requires temperature and time enough to consolidate properly the material, leading to a quite large microstructure. This work studies the development of new high strength sintered steels alloyed with niobium, and applies the concept of grain refining implemented in microalloyed steels. Niobium is the microalloying element with a higher potential to control the grain growth of steels. In microalloyed steels, niobium have proven to inhibit the grain growth and the recrystallization of austenite in the form of carbides or carbonitrides by particle pinning. But also in solid solution is one of the alloying elements of steels that provides a stronger effect in the grain boundary mobility by solute or impurity drag. Mechanical alloying allows to distribute uniformly the alloying elements in a nanostructured iron powder. Through this technique niobium can be incorporated in elemental form or directly as niobium carbide, providing the possibility to compare both mechanisms to control grain growth (particle pinning and solute drag). In addition to that, the use of special consolidation techniques, like pressure assisted sintering (hot pressing, SPS), allows to reduce the sintering temperatures and times, which helps to moderate the grain growth during sintering. This thesis involves the whole process of design of a new material, from the choice of the composition and the production of the prealloyed powder by mechanical alloying, to the evaluation and validation of the sintered materials.
Grain refining is the only strengthening mechanism in structural steels that increases both yield strength and toughness. This concept have been applied successfully in microalloyed steels, in which the use of thermomechanical treatmens and the addition of small quantities of microalloying elements lead to a very fine microstructure that provides a great combination of properties. Microalloying in sintered steels have not been fully explored since it involves some difficulties, like the absence of thermomechanical treatments in a conventional sintering process, the high oxygen affinity of the microalloying elements or the need to ensure a good distribution of the microalloying elements in such small quantity. Besides, the sintering process itself, as a diffusive process, requires temperature and time enough to consolidate properly the material, leading to a quite large microstructure. This work studies the development of new high strength sintered steels alloyed with niobium, and applies the concept of grain refining implemented in microalloyed steels. Niobium is the microalloying element with a higher potential to control the grain growth of steels. In microalloyed steels, niobium have proven to inhibit the grain growth and the recrystallization of austenite in the form of carbides or carbonitrides by particle pinning. But also in solid solution is one of the alloying elements of steels that provides a stronger effect in the grain boundary mobility by solute or impurity drag. Mechanical alloying allows to distribute uniformly the alloying elements in a nanostructured iron powder. Through this technique niobium can be incorporated in elemental form or directly as niobium carbide, providing the possibility to compare both mechanisms to control grain growth (particle pinning and solute drag). In addition to that, the use of special consolidation techniques, like pressure assisted sintering (hot pressing, SPS), allows to reduce the sintering temperatures and times, which helps to moderate the grain growth during sintering. This thesis involves the whole process of design of a new material, from the choice of the composition and the production of the prealloyed powder by mechanical alloying, to the evaluation and validation of the sintered materials.
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Pulvimetalurgia, Aceros sinterizados, Microaleaciones, Niobio