Unraveling the role of subcritical annealing time before intercritical annealing of a cold-worked Ni-Cr-Mo-V alloy steel: Phase transformations and mechanical properties

Abstract

This research implies dual-phase (DP) steel grades manufactured by applying a thermo-mechanical treatment on the Ni-Cr-Mo-V alloy steel. This steel with a ferritic-pearlitic structure was initially subjected to cold working via a rolling process (47 % rolling reduction). The subcritical annealing at 650 ℃ for 5, 30, and 60 min was subsequently implemented. Lastly, intercritical annealing was applied at 790 ℃ for 60 min to produce ferriticmartensitic DP5, DP30, and DP60 steels. The ferrite grains were refined by increasing the subcritical annealing time from 5 to 60 min. Meanwhile, highly refined ferrite grains were documented, and the martensite volume fraction was increased after intercritical annealing in order of DP5, DP30, and DP60 steels. The phase transformations, especially grain refinement, were well characterized by electron backscattered diffraction (EBSD). Transmission/scanning transmission electron microscopy (TEM/STEM) identified complex carbides like MC, M (C,N), and M2(C,N). TEM also showed a twinning structure at the ferrite and martensite phases. Among DP steels, DP60 steel attained suitable mechanical properties such as hardness of 577±6 HV, strength-ductility synergy of 19.7±1.0 GPa%, and tensile toughness of 188.2±7.0 J.cm􀀀 3. Hollomon analysis disclosed that DP60 steel had a better strain hardening capability than DP5 and DP30 steels. Damage mechanisms were interphase boundary decohesion of ferrite/martensite (responsible for the dimple creation) and martensite failure/cracking (responsible for the cleavage formation). Martensite softening and ferrite grain refinement played a part in the tensile toughness (toughening mechanisms), as described according to the principles of fracture mechanics.