The effect of the application of constant temperatures (isothermal plateau) and variables (cooling ramp) during the overaging (OA) stage of the continuous annealing cycle in the microstructural characteristics and mechanical properties of a TRIP steel of the class of 780 MPa was investigated. It was simulated six overaging conditions in the equipment Gleeble. The microstructures presented variable fractions of ferrite, bainite, retained austenite and a small martensite portion. All the conditions achieved the minimum strength of 780 MPa it is possible to highlight the conditions for initial OA temperature of 440 °C where presented the best results of mechanical properties and typical TRIP steels behaviors.

Stabilization of ferritic stainless steel ASTM 409, for use in automobile exhaust systems, is increasingly in demand, due to higher exhaust gas temperatures. Ti is one of the most commonly used elements. However, high concentrations cause problems during steel casting; however, stabilization with Ti and Nb represents a good alternative. The present study aims to investigate the effect of adding Nb on the microstructure, texture, and forming properties of Ti-stabilized ferritic steels. The results demonstrated that Nb-addition leads to a more deformed microstructure after hot rolling with the strongest {110}〈001〉 Goss component. In the deformed state, the steels displayed the α-fiber, but 11CrTi+Nb steel showed a more intense γ-fiber. After annealing, Nb-addition led to an increase in the γ-fiber and in average normal coefficient of anisotropy (R). Thus, it was demonstrated that the dual stabilization (Ti+Nb) of the ferritic steel was responsible for improving the recrystallization texture and forming proprieties.

A comparison of the influence of microstructure and texture on mechanical behavior between the niobium-stabilized ferritic stainless steel type ASTM 430, 430Nb, and the niobium-titanium-stabilized ferritic stainless steel type ASTM 439 was performed. The two steels were supplied as cold rolled thin sheets and the annealing was interrupted in different temperatures aiming the characterization of the microstructure and texture in different stages of recrystallization using optical microscopy, Vickers microhardness and Electron Backscatter Diffraction. The annealed samples were tensile tested to determine the mechanical properties and undergone to Swift test to evaluate the drawability. The steel 430Nb showed smaller grain size and greater yield stress. The steel ASTM 439 presented higher normal anisotropy coefficient, R, and higher Limit Drawing Ratio due to greater proportion of γ fiber. These results are presented and discussed in terms of precipitates and crystallographic texture developed in the recrystallization of both steels.

This study seeks to evaluate the effect of intermediate annealing on the microstructure, texture, and formability of Nb-stabilized ferritic stainless steel. Two routes - direct cold rolling and cold rolling with an intermediate annealing were performed. The total reduction was 80%, 3-0.6 mm and 3-1.2-0.6 mm. The characterization of the samples was conducted using X-ray diffraction, electron backscatter diffraction, and tensile tests to evaluate the formability by the average normal anisotropy coefficient, r value. The results showed that intermediate annealing promoted the strongest γ-fiber with peak at {111}〈112〉 and weak α-fiber after cold rolling. After the final annealing, it was observed that intermediate annealing reduces the banded microstructure and θ-fiber, and improves the γ-fiber uniformity and the r value. It is worth noting that the γ-fiber fraction was the same for direct reduction. The increase in r value was related to the reduction in θ-fiber and the uniformity of texture recrystallization.

O envelhecimento na martensita no aço inoxidável austenítico AISI 304 pré-deformado em tração de 25% a -5ºC foi estudado com o objetivo de se determinarem os aspectos fenomenológicos e os mecanismos relacionados ao envelhecimento. As condições de pré-deformação utilizadas, de 25% a -5ºC, resultaram em uma fração volumétrica de martensita igual a 60%. Os tratamentos de envelhecimento foram realizados na faixa de temperatura de 80 a 160ºC, em intervalos de tempo variando de 1 a 400 minutos. Observou-se que o limite de escoamento aumenta com o aumento do tempo e da temperatura de envelhecimento,saturando em torno de 20 minutos a 160ºC. Encontrou-se um valor igual a ½ para o expoente do tempo na equação que descreve a lei cinética do envelhecimento. Esse valor foi associado à formação de precipitado épsilon. O valor da energia de ativação obtido, 65kJ/mol, sugere que o processo de envelhecimento é controlado pela difusão do carbono na martensita.

Strain aging of martensite in austenitic stainless steel AISI 304, 25% pre-deformed at -5ºC, was studied to determine phenomenological aspects and mechanisms of strain aging. Under these conditions, the result was a martensite volume fraction of 60%. Ageing treatments were performed from 80 to 160ºC during 1 to 400 minutes. Yield strength increases with the increase of aging time and temperature, saturation being reached in about 20 minutes at 160ºC. A time exponent of ½ was found for the equation that describes the kinetics of the process. This value was associated to the formation of e precipitates. The value of the activation energy (65kJ/mol) was related to the diffusion of carbon in martensite.

O principal objetivo desse trabalho foi estudar o fenômeno de Delayed Cracking, em aços inoxidáveis austeníticos, quando foram, severamente, deformados em operações de estampagem. Durante a conformação, a austenita desses aços se transformou em martensita e a fração volumétrica dessa fase apresentou variação em relação à altura da parede do copo e à composição química dos aços. O cobre é um elemento de liga que foi capaz de tornar o aço menos sensível ao fenômeno de Delayed Cracking pela inibição da transformação martensítica. Gradientes de martensita ao longo da parede dos copos embutido e reembutido foram determinados para um aço inoxidável austenítico convencional e um com maior quantidade de cobre em sua composição química. Os efeitos desse gradiente sobre o fenômeno de Delayed Cracking também foram analisados.

The main objective of this work was to study the Delayed Cracking Phenomenon in metastable austenitic stainless steels when they were significantly deformed by deep drawing. During the forming operations, the austenite of these steels has transformed substantially to martensite. The volume fraction of this phase has changed with the cup height and chemical composition. Copper is an alloying element able to turn the steels less prone to the cracking by inhibiting the martensitic transformation. Martensite gradients throughout the drawn and redrawn cups wall were determined for one conventional and a copper-added austenitic stainless steel and their effects on the Delayed Cracking Phenomenon were evaluated.

A influência da adição de cromo no envelhecimento após deformação foi estudada em dois aços perlíticos stelmorizados e trefilados para uma redução de área de 86%, contendo um dos aços, 0,25% de cromo em sua composição química. Foram observados dois estágios de envelhecimento no intervalo de temperatura de tratamento considerado para cada aço. O mecanismo responsável pelo envelhecimento no primeiro estágio foi o ancoramento de deslocações por átomos de nitrogênio e carbono em solução sólida na ferrita. O segundo estágio foi associado ao ancoramento de deslocações em células nas interfaces ferrita-cementita. O efeito do cromo foi a diminuição no espaçamento interlamelar da perlita, o que intensificou os efeitos do envelhecimento nas propriedades mecânicas. Não foram observadas diferenças significativas nos valores da energia de ativação entre os aços no segundo estágio de envelhecimento, significando que pelo menos para o teor investigado, 0,25%, o cromo não afetou a estabilidade da cementita.

The influence of addition of chromium in the strain aging were studied in two stelmorized pearlitic steels, after drawing to an area reduction of 86%, contains one of the steels, 0,25% of chromium in the chemical composition. Two stages of strain aging were observed for both steels in the treatment temperature interval considered. The mechanism responsible for the strain aging in the first stage was the locking of dislocations by nitrogen and carbon atoms in solid solution in the ferrite. The second stage of aging was associated with dislocation locking in the ferrite/cementite interfaces. The effect of chromium was the reduction of the pearlitic interlamelar spacing, increasing the intensity of the strain aging effects in the mechanical properties. No significant differences were observed in the activation energy for the second of aging, meaning that at least for the investigated amount, 0,25%, chromium does not affect the stability of cementite.