Effects of non-recrystallization zone reduction on microstructure and precipitation behavior of a ferrite-bainite dual phase steel

Abstract

A dual-phase pipeline steel with high deformability was processed by a two-stage controlled rolling and followed by relaxation process before accelerated cooling. The mechanical properties of the samples were studied, and the effects of different reductions in the second rolling stage on the microstructure and precipitation behavior of the steel were analyzed using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), positron annihilation technique (PAT) and physicochemical phase analysis. The samples mainly consisted of ferrite, bainite, and a small amount of martensite/austenite (M/A) islands. When the reduction increased to 80% in the second rolling stage, lower yield ratio and better impact toughness were obtained, and the microstructure with 49.6% (volume fraction) ferrite and higher dislocation density became more homogenous. The nominal chemical formulas of M(C, N) for sample A and B were (Nb0.735Ti0.260Cr0.005)(C0.420N0.580) and (Nb0.757Ti0.237Cr0.006)(C0.464N0.536), respectively and the phase structure of precipitates of both samples were almost the same. The strain induced more precipitation of M(C, N) in the non-recrystallization zone of austenite than in the intercritical zone. Also the fraction of M(C, N) and the precipitation strengthening effectiveness were promoted by the increase of reduction.