The effect of precipitation and microstructure on hot ductility in high Al, Nb containing TRIP steels

Tuling, A. (2010). The effect of precipitation and microstructure on hot ductility in high Al, Nb containing TRIP steels. (Unpublished Doctoral thesis, City, University of London)

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Abstract

Transformation induced plasticity assisted steels (TRIP steels) are alloyed with Al, Mn and Nb, among other elements, to provide a combination of high strength and ductility. While these additions are necessary for the TRIP e ect, their in uence on the steel's castability must be addressed. Al and Nb are known to be detrimental for hot ductility and casting. Yet the combination of very high Al contents with Nb is unique. Therefore, the ductility loss mechanisms during casting high Al-Nb TRIP steels were investigated. Using a simple hot ductility simulation, 0.15C-2.5Mn-0.025Nb TRIP steels had similar ductility trough shapes at low (<0.05%) and high (1.53%) Al levels, but at the 1.05%Al steel there was an extended trough. The ductility loss in the 0.05%Al and 1.5%Al steel was shown to be dependant on the Nb(C,N) precipitation size and the austenite grain size. The 0.05%Al steel had a poorer ductility than the 1.5%Al steel at the same volume fraction and size Nb precipitation, since it had a larger grain size resulting in a lower inter-particle distance. Intergranular failure occurred as the Nb(C,N) precipitates pinned the grain boundaries and facilitated cavity/crack link up. The Al additions radically altered the phase stabilities of these steels, leading to steels that vary from hypo-peritectic to hyper-peritectic compositions. At 1%Al level, not only was the steel peritectic, leading to a large austenite grain size, but dendritic AlN precipitated at the austenite grain boundaries. This resulted in intergranular rock candy fracture along the AlN dendrites at the immobile grain boundaries. Therefore, the 1%Al steel the ductility trough was wider than the Ae3 { Ar3, due to the high density of AlN precipitation along the grain boundaries at all temperatures. While this precipitation has been observed before as a hexagonal close packed crystal, in the current analysis it was identi ed as a face centered cubic structure. With further con rmatory investigation it is recommended that 1.5%Al levels be used over the 1%Al levels in these steels.

(Keywords: High Al TRIP steels, Nb additions, hot ductility, peritectic, dendritic AlN precipitation)

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering
Doctoral Theses > School of Mathematics, Computer Science and Engineering
URI: http://openaccess.city.ac.uk/id/eprint/19660

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