Grain structure of Fe-0.3mass%C-9mass%Ni steel processed through α→γ→α′ transformation caused by spontaneous reverse transformation

Abstract

α→γ→α′ (ferrite→ austenite→martensite) transformation caused by spontaneous reverse transformation (SRT) in Fe-0.3C-9Ni mass% steel has been demonstrated using laboratory rolling mill, and grain structure of the plate has been investigated. SRT is a new process to obtain an ultra refined austenite, and resulting ultra refined transformation products, which occurs due to adiabatic heating caused by deformation at temperatures in ferrite phase field. SRT occurred all through the plate thickness by intensive rolling with 90% reduction and final microstructure was ultra-refined martensite. Mean lineal intercepts of the martensite grains were 0.90 μm at mid thickness and 0.83 μm at quarter thickness. Ultra-refined grain structure attributes to enhancement of austenite nucleation due to the intensive rolling before α→γ→α ′ transformation. The martensite texture after the α→γ →α′ transformation caused by SRT composed of RD//〈110〉α and ND//〈111〉α fibers. However, these were not sharp, thus crystallographic grain orientation was quite randomised in spite of the intensive rolling. Two consecutive transformations weakened the initial ferrite rolling texture remarkably. Randomization of the grain orientation was more prominent at the quarter-thickness than at the mid-thickness due to a shear strain. © 2005 ISIJ.