Rozprawy doktorskie na temat „Manganese TWIP/TRIP Steels”

O desenvolvimento e utilização de novos materiais, mais leves e com propriedades mecânicas superiores aos atuais, se mostram extremamente importantes devido à redução de peso e consequentemente redução na emissão de gases poluentes que poderiam gerar. As ligas de Fe-Mn-C com elevados teores de Mn (20-30%) representam um desenvolvimento muito recente de aços austeníticos, que, através dos seus mecanismos diferenciados de deformação reúnem elevada resistência mecânica com grande ductilidade. Essa nova classe de materiais estruturais possibilita uma efetiva redução de custos na produção através do reduzido tempo de processamento (sem a necessidade de tratamentos térmicos especiais e de processamentos termomecânicos controlados). A soldagem é, atualmente, o mais importante processo de união de metais usado no setor industrial. Dentro da variada gama de processos de soldagem existentes, a soldagem por fricção e mistura mecânica (SFMM, em inglês: Friction Stir Welding - FSW) se destaca por ser um processo de união no estado sólido que apresenta uma série de vantagens sobre as tecnologias convencionais de soldagem por fusão. Do ponto de vista metalúrgico, uma das suas principais vantagens se manifesta justamente na junção de materiais dissimilares, visto que o grau de mistura de composições e as transformações de fases entre materiais incompatíveis podem ser minimizados. Outra vantagem é que há um refino de grão no cordão de solda comparado com a microestrutura fundida que se forma nos processos convencionais. Este trabalho teve como objetivo produzir em escala laboratorial os aços de alta liga ao manganês com efeito TRIP, avaliar o impacto da velocidade de rotação da ferramenta na soldagem por fricção e mistura mecânica e avaliar a microestrutura e propriedades mecânicas das juntas soldadas. A microestrutura das juntas soldadas caracterizou-se pela presença apenas da zona de mistura e do metal base, além da formação de \'anéis de cebola\' na zona de mistura, esta não mostrou sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica para todas as velocidades de rotação investigadas com a formação de grãos refinados e com morfologia equiaxial. Os corpos de tração fraturaram todos nos metais de base, mostrando que as propriedades mecânicas da zona de mistura foram superiores à do metal base e que a variação de aporte térmico alcançada com a velocidade de rotação da ferramenta não comprometeu a qualidade das juntas soldadas.
The development and application of new light materials with superior mechanical properties is extremely important to weight reduction in vehicles and consequently reduction of greenhouse gases emission. The Fe-Mn-C steels with high Mn (20-30%) are a recent development of austenitic steels, which, due to their different mechanisms of deformation, possesses high strength and high ductility as well. In addition, this new type of structural steel allows an effective reduction of manufacturing costs due to its reduced processing time (it does not require special heat treatments and controlled thermo mechanical processing). Welding has been one of the most important processes for joining metals. Among the available welding processes, friction stir welding (FSW) is notable for being a solid state process with great advantages over the conventional welding methods. In the mettalurgical point of view, welding dissimilar materials is a significant advantage of FSW over the other process. The main reason is the reduction of mixture of material and phase transformations between the incompatible materials in the weld. Moreover, grain refinement is another advantage from the process. The present study aimed to produce laboratorial scale high Mn steels with TRIP effect, investigate the impact of tool speed ont the microstructure and mechanical properties of friction stir welded joints. The microstructure of the welded joints exhibited only the stirred zone (SZ) and the base material (BM), besides the presence of ´onion rings´ within the stirred zone. The SZ exhibited no signs of martensite suggesting that dynamic recrystallization have occurred for all the speed tested. Moreover, the grains in the SZ had equiaxial morphology and were significantly refined. The fracture of the tensile specimens occurred in the base material, bringing to light that the welding process was beneficial to the mechanical properties. Furthermore, the variation of heat input achieved with the speed did not compromise the quality of welded joints.

Ce travail est centré sur la modélisation de l’évolution de microstructures induite par le procédé SMAT pour des aciers TWIP/TRIP. Les caractéristiques de base d’un acier TWIP/TRIP sont mises en évidence par MEB, DRX et nanoindentation. La nanoindentation est appliquée sur différentes couches pour l’étude des propriétés mécaniques du matériau à gradient de microstructure. Un modèle basé sur la densité de dislocations tenant compte de l’influence du maclage et de la transformation de phase martensitique dans un acier TWIP/TRIP est proposé pour étudier l’influence des paramètres induits par SMAT. Ensuite, la densité de dislocations dans l’austénite et celle dans la martensite ainsi que les fractions volumiques de macles et de martensite au cours d’un impact sont évaluées numériquement avec un modèle d’éléments finis. Par ailleurs, afin d’étudier l’évolution de l’endommagement pendant le processus d’impact, l’endommagement a été introduit dans le modèle de la densité de dislocations. Enfin, un modèle visco-élastoplastique basé sur la densité de dislocations considérant l’effet de la taille de grain, le maclage et le taux de déformation a été proposé afin de mieux comprendre les effets du SMAT sur les propriétés mécaniques d’un acier TWIP/TRIP avec gradient de microstructure. Ainsi, pour étudier la réponse globale du matériau SMATé, la loi de mélange est utilisée en considérant le gradient du matériau comme une structure composée de différentes couches avec des microstructures et des propriétés mécaniques différentes
This work focuses on modeling the microstructure evolution induced by SMAT in TWIP and/or TRIP steels. The features of the generated gradient microstructure of a 304L TWIP/TRIP steel are characterized by SEM, XRD, and nanoindentation. Nanoindentation is applied on different layers for the investigation of the mechanical properties of the gradient microstructure. Based on the experimental results, a dislocation density model considering the influence of twinning and martensitic transformation of TWIP/TRIP steel is proposed to investigate the effect of SMAT controlling parameters. Then the dislocation density of the austenite and that of the martensite as well as the volume fraction of twinning and martensitic transformation during impact loading is numerically evaluated using a full finite element model. Afterwards, to study the evolution of the damage during the SMAT process, the damage was introduced in the dislocation density model. Finally, a dislocation density based visco-elastoplastic model considering the effect of grain size, dislocation density, twin, and strain rate was proposed to further understand the effect of impact loadings on the mechanical properties of TWIP/TRIP steel with gradient microstructure based on the results of nanoindentation tests. Then to study the overall elastoplastic response of the SMATed material, the rule of mixtures is used by considering the gradient material as a gradient structure consisting of different layers with distinct microstructures and mechanical properties

ThermoMechanical Processing (TMP) is a tool whereby a controlled microstructure and mechanical properties are achieved by a combination of controlled deformation and heat treatment steps. However, because there exists a trade off between strength and ductility, steels destined for both high formability and high strength applications must further undergo a sequence of energy intensive heat treatment steps. Alternatively, TRIP (TRansformation Induced Plasticity) steels have been acknowledged to possess excellent combinations of both strength and ductility. The timely strain or stress induced transformation of Retained Austenite (RA) to martensite locally strengthens these steels at the point of plastic instability, causing failure by necking to be postponed and shifted elsewhere along the steel. This phenomenon repeated over and over again allows increased levels of deformation, prior to fracture.
In the current TRIP grades, Si is a key elemental addition. However, in levels exceeding 1.0 wt.%, it is unpopular as it is responsible for a tenacious oxide layer. In this work, it is suggested that Mo may potentially replace Si in part.
Multi pass rolling simulations were also performed using torsion. (Abstract shortened by UMI.)

Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) steels possess excellent combination of high strength and high ductility. Deformation twinning and strain induced martensitic phase transformation are the dominant deformation mechanisms in TWIP and TRIP steels, respectively. The TWIP and TRIP effects are strongly dependent on stacking fault energy (SFE), which essentially decides the operating deformation mechanisms. In TWIP steels, which are characterized by SFE in the range 18 - 45 mJ/m2, deformation takes place primarily by twinning, whereas in TRIP steels where the SFE below 18 mJ/m2, deformation is by martensitic transformation. TWIP steels contain very high amount of manganese (Mn) for the stabilization of austenitic phase and to keep the SFE within the prescribed range. In TRIP steels, the Mn content is rather low. The present thesis primarily deals with the evolution of the microstructure, texture and mechanical properties in TWIP/TRIP steels for a range of Mn content. Chapter 1 of the thesis presents the general introduction of the TWIP/TRIP steels and an extended review of published literature on these materials. The experimental procedures and the methodology of data analysis are presented in chapter 2. Chapter 3 deals with the effect of deformation on the evolution of microstructure and texture in a high Mn steel. The evolution of microstructure and texture has been examined for deformation up to very large strains. In chapter 4, two medium Mn steels with 18 and 12 wt. pct. Mn have been studied about evolution of microstructure and texture, hence the associated the deformation mechanisms have been explored. For both the materials deformation texture has been characterized by the evolution of Bs-type texture. Deformation twinning has been identified as main mechanisms in the early stages of deformation and shear banding at intermediate to large deformation. Thermal stability of the deformed microstructure has been examined and recrystallization mechanisms have been identified. Chapter 5 deals with the effect of Al addition on deformation mechanisms, texture evolution and mechanical properties in medium Mn TWIP steels. The effect of Al addition has been studied more comprehensively. It was found that different Al containing alloy led to different mechanical response in terms of yield strength and strain hardening, which has been attributed to different propensity of twinning. In chapter 6, the basic principles of alloy design, deformation mechanisms and mechanical properties have been investigated for low Mn steels (<10 wt. pct. Mn), exhibiting TWIP/TRIP effects. It has been found that in these materials, microstructural development takes place on elemental partitioning during inter-critical annealing. The phase fraction of austenite in the microstructure depends on the inter-critical annealing temperature, annealing time, chemical composition and rolling conditions before annealing. An elaborate study on possible alloying additions and their consequences have also been discussed. Overall conclusions pertaining to the investigations carried out in the entire thesis have been summarized in chapter 7 along with the suggestions for the future work.