Papers by Author: Naoki Maruyama

Abstract: Subgrain growth in deformed ferrite and incomplete recrystallisation during intercritical annealing in low carbon (LC) steels was investigated by EBSD and FEGTEM/EDS. It was confirmed that fine dual phase (α+γ) microstructures could be obtained even without the addition of microalloying elements such as Nb and Ti, if the steels were heated above Ac1 temperature before the completion of primary recrystallisation and then intercritically annealed. The fine microstructure was found to be mainly due to the inhibition of primary recrystallisation, and also due to the inhibition of subgrain growth in deformed matrix by finely dispersed γ phase formed during heating. Mn segregation at α/γ interfaces seems to indicate that the kinetics of boundary migration in the existence of γ is controlled by the volume diffusion of substitutional alloying elements across the α/γ interfaces.

Abstract: Atomic-scale interface segregation behaviour of Nb and Mo during different stages of recrystallisation of a-Fe has been investigated using a three-dimensional atom probe (3DAP). Experimental procedures to analyse a specific region of interest in the specimens and to determine an orientation relationship between analysed contiguous grains are briefly described, and then analytical artefacts which may affect the measured solute distribution are discussed. Atom probe analysis reveals that Gibbs free energy of segregation of Nb is larger than that of Mo in a-Fe, implying that a stronger solute Nb-interface interaction can be a reason for the larger retardation effect of recrystallisation by Nb addition. The comparison of measured solute profiles at migrating recrystallisation interfaces with calculated solute profiles show that Cahn’s solute drag model gives a reasonable fit to solute profiles for migrating interfaces.

Abstract: In this paper we present results obtained in collaboration between NSC and Arcelor on the austenite of a multiphased steel and on a 70%Ni-30Fe alloy. The work concerns the formation of the crystallographic textures during the recrystallisation of austenite, since these textures have a strong influence, after the phase transformation, on the forming properties of the sheets. The microstructure and the textures of the austenite and the FeNi alloy were measured with X-Ray diffraction techniques and with EBSD on a high resolution FEG-SEM.