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Influence du micro-alliage sur la population inclusionnaire, le comportement élastoplastique et la résistance à la rupture ductile d'aciers martensitiques

Abstract : The present study focuses on the optimization of the chemical composition, of the microstructure, and of the control of coarse Ti (C,N) inclusions, on high strength martensitic steels for automotive applications.In order to vary the mechanical strength and inclusion cleanliness, laboratory steels were cast with various carbon and titanium contents, then heat treated to obtain different grain sizes and spatial distributions of iron carbides. From the obtained results, the chemical composition (mainly, the carbon content) is the first-order parameter influencing the mechanical strength as well as the local ductility at failure. Refining the parent austenite grains leads to an increase in yield strength, in the resistance to crack initiation and, to a lesser extent, in the ultimate tensile strength. An optimum temperature has been found, to increase the local ductility at fracture with only a small decrease in ultimate tensile strength. A thermodynamic analysis showed that the increase in the carbon content induces a titanium supersaturation in the inter-dendritic liquid at the solidification front. This depends on the solid phase at the interface, and thus on the chemical composition of the alloy, favouring the formation of Ti(C,N)inclusions during solidification into austenite. Increasing the titanium content in the first order, and carbon in the second order, increases the spatial density of inclusions. The addition of titanium also induces a refinement of parent austenite grains, and thus of the martensite microconstituents, beneficial on the resistance to fracture initiation. There is therefore a competition between the deleterious effect ofthe inclusions and the favourable effect of the refinement of the microstructure.The anisotropy in fracture initiation in bending is accentuated with the increase in the area fraction of inclusions, via the Ti(C,N) alignments due to rolling. The development damage was observed in tensile tests interrupted before failure on notched specimens. A more detailed analysis of this anisotropy was carried out using unit cell calculations. For this purpose, the Young's moduli of these inclusions were determined via nano-indentation tests.
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Submitted on : Tuesday, July 26, 2022 - 10:26:12 AM
Last modification on : Thursday, July 28, 2022 - 3:07:14 AM

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Adrien Milani. Influence du micro-alliage sur la population inclusionnaire, le comportement élastoplastique et la résistance à la rupture ductile d'aciers martensitiques. Matériaux. Université Paris sciences et lettres, 2022. Français. ⟨NNT : 2022UPSLM013⟩. ⟨tel-03738399⟩

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