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Resumo O efeito do caminho de solidificação de um aço High Temperature Processing (HTP) API 5L X65 com 0,09 %Nb, 0,039%C e 0,54% Mn (percentagem em massa) na segregação, microestrutura e distribuição dos precipitados de Ti,Nb(C,N) foi estudado por microscopia ótica, eletrônica de varredura (MEV) e simulação computacional (ThermoCalc e DICTRA). Os resultados foram comparados com os de um aço microligado comercial com 0,04 %Nb, 0,09%C e 0,97 %Mn. Foi encontrado que o principal parâmetro que influencia no caminho de solidificação é o teor de C, o que está de acordo com a maior micro e macrossegregação observada no aço microligado comercial, em comparação ao aço HTP.

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The dynamic strain aging susceptibility of the fire resistant steel was studied and the mechanism responsible for this strengthening process were determined by mean of tensile tests performed at different temperatures and strain rates. The tests were done in temperatures between 25°C and 600ºC and strain rates of 10-1 , 10-2 e 10-3 s-1. The values of the activation energies related to the appearance of the Portevin-LeChatelier effect and to maxima on the tensile strength versus temperature curves were determined by the relation between strain rate and the temperature. Typical features of DSA was observed, such as the presence of plateau, maximum in yield strength, ultimate tensile strength and a minimum elongation versus temperature curves. It was also observed the serrated flow in the stress-strain curves (the Portevin-LeChatelier-PLC effect), for some conditions of strain rates and temperatures. The results indicate also that dynamic strain aging of the fire steel is probably associated to the dynamic interaction between dislocations and interstitials-substitutional dipole. This strengthening mechanism is an important contribution to the fire steel mechanical properties in high temperature.