Efeito de diferentes tratamentos térmicos na configuração microestrutural em aço TRIP (transformação induzida por deformação)
Nenhuma Miniatura disponível
Data
2025-09-29
Autores
Martins, João Batista Ribeiro
Título da Revista
ISSN da Revista
Título de Volume
Editor
Universidade Federal do Espírito Santo
Resumo
This thesis investigated the development of advanced high-strength steels (AHSS), produced in a vacuum electric furnace, with controlled variations in chemical composition and thermal parameters. The resulting ingots showed satisfactory metallurgical quality and compositions aligned with the study's objectives. The phase transformation curves under continuous cooling conditions, also known as CCT (Continuous Cooling Transformation), were essential to define three kinetic ranges: (i) 0.5–1.0 °C/s with ferrite and pearlite; (ii) 3–9 °C/s with bainite and retained austenite; and (iii) 25–100 °C/s with lath martensite. The steel with the lowest carbon and manganese content (steel C) showed lower hardenability, expanding the bainitic field. Optical microscopy (OM) identified the main constituents (bainite, martensite, and retained austenite), while scanning electron microscopy (SEM) confirmed the refined morphology of the phases under rapid cooling. Characterization by electron backscatter diffraction (EBSD) revealed the progressive replacement of ferrite by bainite and martensite, and showed the predominance of displacive phases at high cooling rates. The novel application of the gamma distribution to misorientation data up to 5° of the substructures, also known as KAM (Kernel Average Map), enabled quantification of microstructural homogeneity (parameter α) and the average amplitude of misorientations (β), highlighting greater regularity in the steel with higher carbon and manganese content (steel A). The trend inversion (α↓/β↑) at 100 °C/s was interpreted as martensitic saturation, with reduced internal variability. Additionally, an in-situ diffraction experiment at the Brazilian Synchrotron Light Laboratory (LNLS), under 100 °C/s, confirmed the sequence austenite → epsilon martensite → body centered tetragonal martensite (γ → ε → α′), evidencing the transient formation of epsilon martensite, consistent with the double-step martensitic transformation theory. The results demonstrate that the compositions of steels with higher carbon and manganese (steel A) and lower carbon and manganese (steel C) have high potential as third-generation AHSS. Steel A, with greater sensitivity to cooling rate, allows microstructural adjustments for high strength, while steel C shows greater bainite stability. Thermal modulation proved to be an efficient strategy to balance strength and ductility without the need to alter the chemical composition.
Descrição
Palavras-chave
Aços avançados de alta resistência (AHSS , Transformações de fase , Martensita épsilon , Difração in situ (LNLS) , Mapa de Kernel