Obtaining high levels of mechanical properties in steels is directly linked to the use of special mechanical forming processes and the addition of alloying elements during their manufacture. This work presents a study of a hot-rolled steel strip produced to achieve a yield strength above 600 MPa, using a niobium microalloyed HSLA steel with non-stoichiometric titanium (titanium/nitrogen ratio above 3.42), and rolled on a Steckel mill. A major challenge imposed by rolling on a Steckel mill is that the process is reversible, resulting in long interpass times, which facilitates recrystallization and grain growth kinetics. Rolling parameters whose aim was to obtain the maximum degree of microstructural refinement were determined by considering microstructural evolution simulations performed in MicroSim-SM® software and studying the alloy through physical simulations to obtain critical temperatures and determine the CCT diagram. Four ranges of coiling temperatures (525–550 °C/550–600 °C/600–650 ° C/650–700 °C) were applied to evaluate their impact on microstructure, precipitation hardening, and mechanical properties, with the results showing a very refined microstructure, with the highest yield strength observed at coiling temperatures of 600–650 °C. This scenario is explained by the maximum precipitation of titanium carbide observed at this temperature, leading to a greater contribution of precipitation hardening provided by the presence of a large volume of small-sized precipitates. This paper shows that the combination of optimized industrial parameters based on metallurgical mechanisms and advanced modeling techniques opens up new possibilities for a robust production of high-strength steels using a Steckel mill. The microstructural base for a stable production of high-strength hot-rolled products relies on a consistent grain size refinement provided mainly by the effect of Nb together with appropriate rolling parameters, and the fine precipitation of TiC during cooling provides the additional increase to reach the requested yield strength values.
NBR6656 LNE380, perictetic steels, with carbon content between 0,08% and 0,12% presented high Mold Level Fluctuation (MLF), thus generating great casting process instability beyond higher Break-Out occurrence probability and surface cracks on the slabs. The strategy that was chosen on this work is based on the modification of the Alloy Design in order to avoid the perictetic range. In previous works 1 the strategy was to increase the carbon content. For this work the strategy was to decrease the carbon content, due to the technical specifications restrictions, using other equations to the correct determination of C δ , C γ e C líq. In this paper will be discussed the countermeasures adopted to control the MLF, based on the alloy design, by the use of regression equations that take in account the effect of diverse alloying elements on the perictetic range. The impact of the new alloy on the mechanical properties will be evaluated and the statistical capability obtained for both the alloy design related to MLF.
ResumoEste trabalho tem como objetivo demonstrar a influência da laminação de desbaste durante o processo de laminação termomecânica de um aço microligado ao Nb, Ti e V, obtido em um laminador Steckel, nas suas propriedades mecânicas. Duas variações do plano de passe para realização da laminação de desbaste foram propostas. As variações consistiram na diferenciação da redução passe a passe de forma a obter diferentes refinamento de grão ao fim do processamento de desbaste. O processamento de um aço com composição química adequada à obtenção de propriedades mecânicas compatíveis com o grau LNE380 especificado na norma ABNT NBR 6656 é demonstrado. Foram alcançados limites de escoamento a 0,2% de deformação e a resistência à tração na condição 1 de 459MPa 563MPa e na condição 2 de 505MPa e 601MPa respectivamente, sendo que na condição 2 foram obtidos acréscimos de em torno de 40MPa em ambos LE e LR.
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