Acicular Ferrite Formation on Ti-Rare Earth Metal-Zr Complex Oxides

Abstract

Acicular ferrite (AF) formation potency of Ti-Rare earth metal (REM)-Zr (TRZ) complex oxide has been investigated in the simulated heat affected zone of low carbon steel. The TRZ complex oxide shows higher AF formation potency than Ti and Al oxides. A TRZ oxide particle is composed of REM-rich and Zr-rich phases. AF crystals nucleate on the interface between austenite and the Zr-rich oxide phase, having a crystal structure that is face-centered cubic with lattice parameter of 0.44 nm. The Zr-rich phase and AF have an orientation relationship described by (011)_AF//(011)_Oxide, [100]_AF//[011]_Oxide (the AF-TRZ orientation relationship), which allows good lattice coherency. It is suggested that the formation of this orientation relationship promotes AF nucleation on TRZ complex oxides. The AF also satisfies the Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. It is considered that the coexistence of the AF-TRZ and K-S “three phase” orientation relationships is caused by variant selection of AF in addition to the formation of a rational orientation relationship between the Zr-rich oxide phase and the austenite matrix during the HAZ thermal cycle.