K. Kasai, J. Inoue, T. Koseki, The University of Tokyo, Tokyo, Japan; H. Kato, Nippon Steel Corporation, Chiba, Japan
Intragranular ferrite (IGF) nucleating on oxide inclusions is known to improve toughness of steel weld metals and heat affected zones. However, oxide inclusions existing in steels often consist of multiple phases with a variety of chemistries, and thus, questions still remain in terms of what kind of oxides are effective and by which mechanism inclusions nucleate IGF. In this study, in order to clarify the effects of lattice matching and chemical composition of oxide on IGF nucleation, single phased oxide and steel interfaces are employed. By electron-beam deposition, chemically pure oxide thin film was deposited on steel. Microstructure of steel at the interface was observed by optical microscope and SEM. Orientation relationship between ferrite and oxide on the interface was investigated by EBSP and XRD. At the same time, an ab-initio study was carried out for the bcc Fe and oxides interfaces with various chemical compositions and orientation relationships.
At MgO-steel interface, ferrite grains preferred to nucleate on (100) orientated MgO films. On the other hand, fewer ferrite grains nucleated on (111) orientated MgO films. The ferrite grains which did not hold Bain relation with austenite tended to hold (100) plane parallel to the interface in the former case, while did not show particular trends in the later case. In the case of TiO, which has the same crystal structure with MgO, rich ferrite nucleated regardless of TiO crystal orientation. From ab-initio calculations, it was indicated that the formation of chemical bonds between Ti and Fe atoms cause a decrease in interface energy between bcc Fe and MgO, some Mg atoms of which were replaced with Ti atoms.
These results demonstrate IGF nucleation is influenced not only by lattice matching but also by chemical composition of oxide.
Summary: In order to clarify the effects of lattice matching and chemical composition of oxide on intragranular ferrite nucleation, single phased oxide and steel interfaces were employed. Transformation behaviors of the steel from the interfaces were observed and factors affecting the ferrite formation from the interfaces were investigated.
