Mechanical properties of continuous cooled steels for long products applications: bainite vs. acicular ferrite nucleated on V(C,N) precipitates

摘要

The effect of chemical composition and processing parameters on the formation of acicular ferrite and its application for grain refinement of heavy hot rolled products has been investigated. It was concluded that vanadium effectively promotes the formation of acicular ferrite. In terms of thermal path, continuous cooling with two-stages cooling at 50 and 2 deg C/s with temperature change (TC) at 630 is especially suitable to favour acicular microstructures homogeneously distributed through thickness of a heavy plate. Results reported in this work allow us to conclude that acicular ferrite can only be achieved alloying with vanadium and nitrogen, meanwhile bainite is promoted in steels with a low level of nitrogen. It is concluded that higher strength values are obtained in acicular ferrite than in bainitic steel. In spite of the a priori better behaviour of the interlocked acicular ferrite microstructure, similar brittle -ductile transition temperature (BDT), and lower values of impact absorbed energy (KV) has been recorded in nitrogen-rich steel, which indicate that alloying the steel with vanadium and nitrogen to improve mechanical properties is not a successful strategy.