In this work, the formability behavior of Interstitial-Free (IF) steel sheet, grade DC07 with 0.65 mm of nominal thickness, was evaluated by means of both linear and bi-linear strain-paths to define the Forming Limit Curve (FLC) at the onset of necking according to ASTM E22182 standard. In the first strain-path, flat-bottomed punch with 200 mm diameter and 10 mm corner die radius was adopted together with counter-blanks of an IF steel sheet grade DC07 with 0.80 mm nominal thickness in order to yield two equal amounts of plastic work under uniaxial tension and under equibiaxial stretching strain-paths. Afterwards, Nakajima’s 100 mm hemispherical punch stretching procedure and bulge tests were adopted to determine the FLC of both as-received and strained DC07 blanks with the help of an automated digital image correlation system to define the linear and bi-linear limit strains. Increasing the straining level (5 and 10%) of the first strain-path in uniaxial tension improved the limit strains of the DC07 steel sheet between the plane-strain intercept (FLC0) and the biaxial stretching region of the FLC. On the other hand, blanks which were firstly pre-strained in equibiaxial stretching mode (4.8 and 9%) provided better formability in the FLC drawing region and reduced limit strains in plane-strain and biaxial stretching regions.
Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C-Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200uC, the samples were cooled to 1100uC, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800uC for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800uC. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850uC.
The microstructure, crystallographic texture, strain hardening in the hole edge‐punched and stretch‐flangeability of hot‐rolled multiphase sheet steel is investigated by scanning electron microscopy, electron backscatter diffraction, and microhardness measurements. The results show that the strain hardening in the hole edge‐punched is not the main factor that influences the hole expansion ratio. The bainite fraction, average grain area, and kernel average misorientation distribution mean are microstructure factors that improve the hole expandability and reduce the effect of strain hardening in the hole edge‐punched on the hole expansion ratio. The crystallographic texture affects the hole expandability. The high intensity of {332}<113>α and {111}<112>α components that have higher normal anisotropy (rtrue¯) also reduces the effect of strain hardening at the hole edge‐punched. The cracks propagate through the interfaces following the banded microstructure direction at the hole edge‐punched during the punching process. The gamma distribution analysis of the kernel average misorientation distribution indicates that there are different energy transfer (β parameter) and dislocation densities due to different cooling rates.
equivalente a 80% da deformação do pico de tensão (ε p ), podendo ser determinada a partir da curva do gráfico da derivada dσ/dε (θ) pela tensão (Figura 1). (2) A transformação da austenita em ferrita induzida por deformação (TAFID) é um outro mecanismo restaurador, que pode ocorrer durante a deformação a quente dos aços.(4) Essa terminologia é adotada para enfatizar o papel da deformação no refino de grão e na aceleração da transformação de fase. Com o encruamento da austenita logo acima da temperatura de transformação de fase, em condição fora de equilíbrio termodinâmico (Ar 3 ), o potencial necessário para a transformação de fase aumenta. Esse aumento propicia uma elevação progressiva da temperatura INTRODUÇÃOQuando os materiais cristalinos são deformados em temperaturas elevadas, o acúmulo de discordâncias é continuamente aniquilado por dois fenômenos restauradores distintos de acordo com a energia de falha de empilhamento (EFE) do aço.(1) Nas fases que apresentam alta energia de falha de empilhamento (EFE), como é o caso da ferrita, o fenômeno atuante é a recuperação dinâmica. Já nas fases que apresentam baixa EFE, como é o caso da austenita, o fenômeno atuante é a recristalização dinâmica. Como a deformação a quente dos aços na maioria das vezes se processa na fase austenítica, o estudo da recristalização dinâmica, nesse caso, torna-se mais importante em relação à recuperação dinâmica. Para que a recristalização dinâmica da austenita (RDA) ocorra, é necessário que uma deformação crítica (ε c ) seja alcançada. Normalmente, essa deformação é ) e da intensidade de deformação verdadeira (1, 2 e 3) na ocorrência de mecanismos dinâmicos de refino de grão, em dois aços C-Mn sem e com adição de Nb (0,035%). Os aços estudados foram deformados por torção em simulador termomecânico Gleeble, sendo resfriados em água imediatamente após os ensaios. Os mecanismos dinâmicos de refino de grão atuantes variaram em função da temperatura de deformação aplicada e do aço estudado. De um modo geral, maiores temperaturas de deformação foram responsáveis pela ocorrência de recristalização dinâmica da austenita (RDA) e as menores, pela transformação austenita-ferrita induzida por deformação (TAFID). Palavras-chave: Recristalização dinâmica; Transformação; Deformação; Refino de grão. EFEITOS DE PARÂMETROS TERMOMECÂNICOS EM MECANISMOS DINÂMICOS DE REFINO DE GRÃO EM AÇOS C-MN-Nb E C-MN EFFECT OF THERMOMECHANICAL PARAMETERS ON DYNAMIC MECHANISMS OF GRAIN REFINEMENT IN C-MN-Nb AND C-MN STEELS AbstractIn this study, the effect of deformation temperature (800°C, 850°C and 900°C), strain rate (1 s -1 and 5 s -1) and amount of strain (1, 2 and 3) in the occurrence of dynamic mechanisms of grain refinement in two C-Mn steels without and with Nb addition (0.035 wt.%) was evaluated. The steels were deformed by torsion in a Gleeble thermomechanical simulator and water cooled immediately after deformations. Different dynamic mechanisms of ferritic grain refinement were obtained as a function of the deformation temperature. At higher deformation ...
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