The continuous development of hot forming tool steels has resulted in steels with improved mechanical properties. A change in alloying composition, primarily a decreased silicon content, makes them tougher and more wear resistant at elevated temperatures. However, it is at the expense of their machinability. The aim of this study is to explain the mechanisms behind this negative side effect. Hot work tool steels of H13 type with different Si content were characterised mechanically, and evaluated analytically and by dedicated machining tests. Machining tests verified that materials with low Si content displayed reduced machinability due to their stronger tendency to adhere to the cutting edge. Three hypotheses were tested. The first hypothesis, that the improved toughness of the low Si steels is the reason behind their relatively poor machinability, was rejected after machining tests with one low Si steel heat treated to the same relatively low toughness as conventional hot work tool steels. The second hypothesis, that a change in oxidation properties, also associated with the change in Si composition, lies behind the reduced machinability was investigated by dedicated tests and evaluations. It was found that the oxide thickness increased with reduced Si content and that there was an enrichment of Cr at the oxide/steel interface. The differences in oxide thickness and the possible differences in oxidation properties may influence the machinability of the materials through their different abilities to adhere to the cutting edge. The third hypothesis, that a high enough temperature to initiate phase transformation from ferrite to austenite is generated during machining of the tool steels, was also investigated. This may lead to a reduced machinability because higher austenite content is directly related to higher compressive stresses and higher cutting forces. This causes accelerated tool wear. This hypothesis was verified by ThermoCalc calculation of austenite content in the steels, which showed a good agreement with Gleeble compression tests and cutting force measurements. This thesis confirms that a reduced Si content in conventional H13 steel improves the toughness, reduces the oxidation resistance and lowers the ferrite-to-austenite transformation temperature. The reduction in austenite temperature is probably the most important factor behind the reduced machinability.

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This work is aimed to understand the mechanisms for evolution and reversed evolution of dislocation structure under variable strain amplitudes, using automotive-grade interstitial-free steels (IF steel) under strain ratio (R) = 0 condition. The microstructures were mainly examined by the SEM under BEI/ECCI mode and TEM were used for this study. Near the endurance limit, the dislocation cells smaller than 2£gm develop preferably along grain boundaries and triple junctions among the grains. Within grain interiors, it is hardly observed these small dislocation cells and cyclic hardening even at £`max =0.2%. When strain amplitudes were controlled at a range from £`max = 0.25% to 0.6%, a secondary cyclic hardening occurs prior to fatigue failure and less than 2um dislocation cells rapidly developed thoroughly. The secondary hardening rates were found to be directly proportional to the strain amplitudes. For high-low strain fatigue tests, while the maximum strain was decreased from 1.2% to 0.2% or 0.15%, dislocation cells were collapsed first and re-grouped into loop-patch structures due to the gliding behavior of dislocations changing from multiple-slips to single-slip. However, once the strain range is further reduced to 0.1%, dislocation cells would persist, showing no signs of collapse. Moreover, the reversal development of dislocation structures is independent of strain ratio. Furthermore newly developed loop patches are usually confined within dislocation domains with very condensed dislocation cell walls with high boundary misorientation.

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SWISS STEEL, filiale du groupe SCHMOLZ + BICKENBACH, finance des travaux de thèse menés au Centre de Recherches d’UGITECH, en collaboration avec le laboratoire SIMaP de Grenoble. Le but de ce programme est de trouver des alternatives au plomb (Pb) dans les aciers bas carbone de décolletage. Les objectifs sont de conserver le niveau d’usinabilité des aciers de décolletage au plomb, sans leurs défauts (toxicité, recyclage des aciers au plomb). La production d’acier au plomb est stratégique pour SWISS STEEL, puisqu’elle représente environ 30% de son activité. Parmi les différentes alternatives signalées dans la littérature, l’addition d’étain, et la graphitisation des aciers, sont les concepts les plus intéressants. En effet, il est suggéré que l’étain présente un comportement similaire à celui du plomb lors de l’usinage. Par ailleurs, Le graphite est connu pour être un lubrifiant utilisé dans des applications industrielles, ce qui peut améliorer l’usinabilité. Une caractérisation complète d’aciers industriels au plomb, et sans plomb, en provenance de SWISS STEEL (11SMn30 et 11SMnPb30), a été menée : analyse chimique, observations métallographiques et microscopiques (MEB), propriétés mécaniques, etc. Ensuite, nous avons essayé de comprendre les mécanismes par lesquels le plomb améliore l’usinabilité des aciers de décolletage aux basses vitesses de coupes (Vc < 100 m/min), grâce à des essais d’usinage spécifiques. Des calculs thermodynamiques ont été menés sur le procédé de graphitisation, et des analyses d’aciers ont été déterminées. Ce qui nous a conduits à élaborer des aciers à l’étain et des aciers graphitiques à l’échelle du laboratoire. La caractérisation de ces coulées de laboratoire, avec la même procédure que celle utilisée par les aciers industriels, a permis de vérifier la fiabilité de ces concepts, et leur bienfaits sur l’usinabilité. Des résultats encourageants ont été enregistrés. Pour une étude plus approfondie, il a été proposé de produire, à l’échelle industrielle, une analyse d’acier particulière, susceptible de remplacer le plomb dans les aciers de décolletage au plomb. / SWISS STEEL, a steel company part of SCHMOLZ + BICKENBACH group, has supported a PhD program, lead in UGITECH’s Research Center, in collaboration with the SIMaP laboratory of Grenoble. The aim of this program is to find some alternatives to lead (Pb) in low carbon free-cutting steels. The objectives are to keep the machinability level of leaded free-cutting grades without their drawbacks (toxicity, recycling of Pb steels). The production of leaded free-cutting steels is strategic for SWISS STEEL, since it represents about 30% of its activity. Among the different alternatives pointed out by the bibliography, tin addition, and graphitisation of steels, were the most interesting concepts. Indeed, it is suggested that tin presents a similar behaviour to that of lead during machining. Moreover, graphite is a well known lubricant in industrial applications, which could improve machinability. A complete characterisation on leaded, and non-leaded, grades form SWISS STEEL has been carried out (11SMn30 and 11SMnPb30): chemical analysis, metallographic and SEM observations, mechanical properties, etc. Then we tried to understand, thanks to specific machining tests, the mechanisms by which lead improves the machinability of free-cutting steel, at low cutting speed (Vc < 100 m/min). Thermodynamic calculations have been performed to study the graphitisation process, and conducted to the determination of steels analyses. This lead us to develop tin-added-, and graphitic steels at the laboratory scale. The characterisation of these laboratory grades, with the same procedure, than that of industrial steels, allowed to check the reliability of these concepts, and their benefits on machinability. Promising results have been recorded. For deeper exploration, it has been proposed to produce, at the industrial scale, a particular steel analysis, which could replace leaded free-cutting steels.

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Orientador: Tomaz Manabu Hashimoto / Banca: Marcelo dos Santos Pereira / Banca: Alfeu Saraiva Ramos / Banca: Jorge Otubo / Banca: Rosinei Batista Ribeiro / Resumo: Os aços multifásicos constituídos de ferrita, bainita, austenita retida e martensita apresentam combinações muito atrativas de resistência e tenacidade. Há ainda um potencial adicional de melhorias de propriedades mecânicas quando a fração de austenita retida for alta, conferindo alta conformabilidade pelo efeito TRIP. Neste contexto, é fundamental a análise qualitativa e quantitativa das fases, especialmente de austenita retida e sua transformação em martensita induzida por deformação. Este trabalho enfoca a transformação da austenita retida em martensita por deformação em tração monotônica em um aço AISI 4340 com estrutura multifásica. Os resultados confirmam a transformação da austenita retida em martensita atingindo cerca de 80% de transformação. As frações volumétricas de austenita retida antes e após a deformação foram estimadas por duas técnicas. A primeira foi feita por análise de imagens em microscopia óptica e a segunda por magnetização de saturação, tendo em vista o caráter paramagnético desta fase. As frações estimadas pelas duas técnicas foram comparadas, concluindo-se que o método magnético deve ser reavaliado, tendo sido proposto um fator de correção na equação básica deste processo e presenta na literatura. / Abstract: The multiphase steels made of ferrite, bainite, retained austenite and martensite present very attractive combinations and toughness. There is still an additional potential of improvement of the mechanical properties when the fraction of retained austenite is high allowing high conformability by the TRIP effect. In this context the qualitative and quantitative analysis of the phases is essentual specially from retained austenite and its transformation in martensite induced by deformation. This work focus on the transformation of retained austenite in martensite by the deformation in monotonic traction in one steel AISI 4340 with multiphasic structure. The results confirm the transformation of retained in martensite reaching about 80% of transformation. The volumetric of retained austenite before and after the deformation were estimated by two technique: the first was made by the anllysis of images in optic microscopy and the second by magnetization of saturation taking into consideration the paramagnetica character of this phase. The estimated fraction by the two technique were compared leading to the conclusion that the magnetic method must be evaluated again using a proposed factor of correction in the basic equation of this process and present in the literature. / Doutor

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