Effect of different strains on acicular ferrite and martensite-austenite islands in oxide metallurgy steel

Abstract

The microstructure transformation of oxide metallurgy steel was studied by deforming to strains of 0.1–1 at strain rates of 1 s−1 at 900 °C. The variation trends of AF laths and M-A islands size with increasing strain were investigated. The kernel average misorientation(KAM) data obtained by EBSD are used to calculate the geometrically necessary dislocation(GND) density of AF laths (ρAFGND) induced by inclusions and matrix (ρAveGND). The results showed that the large number of dispersed spherical inclusions in experimental steel can effectively induce the nucleation of AF under different strains, which is composed of Al–Ti–Mg–Zr–O and MnS. With the increase of strain, the deformation-induced ferrite transformation(DIFT) behavior dominates the microstructure evolution, the nucleation and growth of more ferrite led to the gradual increase of HAGBs and decrease of ρAveGND, and the microstructure transformed from granular bainite(GB) to granular structure(GS). The size and aspect ratio of AF laths and M-A islands gradually decreased, and the M-A islands gradually replaced by more carbides distributed inside the grains and on the grain boundaries. The growth of AF laths induced by inclusions is hindered by the influence of pre-nucleated proeutectoid ferrite, resulting the increasing of ρAFGND. Meanwhile, the AF laths nucleated on inclusions satisfies the K–S orientation relationship with parent austenite.