Abstract The crystallographic aspects after air-quenching treatment and the pitting corrosion resistance after single tempering and double tempering treatments of AISI 420 martensitic stainless steel were studied using optical microscopy, Electron Backscatter Diffraction and potentiodynamic polarization tests. The results showed that the observed average orientation relationship (OR) was closer to the Greninger-Troiano (G-T) OR model with a minimum average deviation of 2.66°. The average OR of AISI 420 steel with respect to austenite matrix was φ1 = 3.6°, Φ = 46.0°, φ2 = 6.2° in the area mapped. Blocks showed variant pairs belonging to the same Bain group forming sub-blocks with misorientation 5.51° < 6 ¯ 0 11> (V1/V4 pair), consequently, the AISI 420 martensitic stainless steel showed higher intervariant boundary density for the same Bain group and close-packed plane group. In the same air-quenching condition, the different sizes of prior austenite grain in the microstructure did not influence the variant selection. The air-quenching and single tempering treatments did not impair the pitting corrosion resistance and did not cause significant loss of hardness in relation to the air-quenched treatment. Therefore, the double tempering should be dispensed saving production costs.

RESUMO Estudou-se o efeito da taxa de resfriamento sobre a cinética de formação da martensita e sua evolução microestrutural em um aço inoxidável ferrítico com 0,011 %C - 11,1 %Cr - 0,013 N, usando a técnica de dilatometria e a microscopia ótica. Amostras foram austenitizadas a 1000°C por um minuto, seguida de resfriamento sob várias taxas até a temperatura ambiente. A microestrutura obtida é composta, predominantemente, por martensita temperada com uma pequena fração volumétrica de ferrita, com valores de microdureza Vickers variando de 319±4 HV0.2 a 338±4 HV0.2, quando as taxas de resfriamento aumentam de 0,5 para 100 °C/s. Os resultados mostram que as temperaturas de início e fim de formação da martensita diminuem com o aumento da taxa de resfriamento. A taxa de transformação martensítica cresce abruptamente alcançando o valor máximo quando a fração de martensita atinge aproximadamente 42 %, e então, essa taxa diminui até zero quando a transformação da martensita chega ao fim. A taxa máxima da transformação martensítica cresce com o aumento da taxa de resfriamento, sendo 0,01 s-1 a 0,5 °C/s e 2,54 s-1 para 100 °C/s.

ABSTRACT The effect of the cooling rate on martensite formation kinetic and its microstructural evolution in a ferritic stainless steel with 0.011 %C - 11.1 %Cr - 0.013 N was studied, using the dilatometric technique and optical microscopy. Samples were austenitized at 1000°C during one minute, followed by cooling under various rates to room temperature. The microstructure consists predominantly composed of martensite with a small volumetric fraction of ferrite, and the values of Vickers microhardness vary from 319±4 HV0.2 to 338±4 HV0.2 when the cooling rates increase from 0.5 to 100 °C/s. The results show that the martensite start and finish temperatures decrease when the cooling rate increases. The martensitic transformation rate grows abruptly reaching the maximum value when the martensitic fraction is about 42 % and then it decreases to zero when the martensitic transformation comes to the end. The maximum rate of martensitic transformation grows with increase of cooling rate, being 0.01 s-1 to 0.5 °C/s and 2,54 s-1 to 100 °C/s.

Samples of duplex stainless steel 32205 (standard duplex) and 32304 (lean duplex) were investigated in the as-received state, cold rolling (with thickness reduction of 64 % and 62 % respectively) and after annealing at 1055ºC for 20 s soaking. Crystallographic microtexture was obtained via Electron BackScatter Diffraction (EBSD) technique. The results showed that the crystallographic microtexture of ferrite phase in the as-received, cold rolled and annealed condition were more intense in alpha fiber=<011> // RD and tetha fiber. The austenite microtexture was shown to be, for all conditions, mainly composed by Brass component.

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 recozimento de materiais com grãos grosseiros permite observar importantes efeitos de orientação na recristalização. Amostras do aço inoxidável ferrítico AISI 430 estabilizado ao nióbio e com estrutura colunar de grãos foram deformadas até reduções de 50 e 80% e recozidas em temperaturas variando de 300 a 1100°C para avaliação do amolecimento e da cinética de recristalização. As amostras compreendem a seção longitudinal de uma placa obtida via lingotamento contínuo. A laminação foi realizada a morno (<FONT FACE=Symbol>»</FONT> 480°C), numa configuração na qual os grãos colunares permaneceram alinhados com a direção normal (contornos de grão paralelos à DN). A caracterização microestrutural das amostras utilizou microscopia eletrônica de varredura (MEV) no modo de elétrons retroespalhados. Os resultados mostram que o tamanho de grão recristalizado varia localmente de região para região, indicando efeitos de orientação importantes.

Annealing of coarse-grained materials allows the observation of important orientation effects during recrystallization. Samples of the niobium-stabilized ferritic stainless steel 430 with columnar grain structure have been deformed up to reductions of 50% and 80% and annealed at temperatures ranging from 300°C up to 1100°C to evaluate its softening behavior and the recrystallization kinetics. Samples comprise the longitudinal section of a continuous-cast slab. Warm rolling has been performed (<FONT FACE=Symbol>»</FONT> 480°C) in such a configuration that the columnar grains are aligned with the normal direction (grain boundaries parallel to the ND). Microstructural characterization has been performed using scanning electron microscopy (SEM) in the backscattered electrons mode. Results show that the recrystallized grain size varies locally from one region to another indicating significant orientation effects.