Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire

Abstract

Hypereutectoid steel wires with 6.35GPa tensile strength after a cold-drawing true strain of 6.02 were annealed between 300 and 723K. The ultrahigh strength remained upon annealing for 30min up to a temperature of 423K but dramatically decreased with further increasing temperature. The reduction of tensile strength mainly occurred within the first 2–3min of annealing. Atom probe tomography and transmission electron microscopy reveal that the lamellar structure remains up to 523K. After annealing at 673K for 30min, coarse hexagonal ferrite (sub)grains with spheroidized cementite, preferentially located at triple junctions, were observed in transverse cross-sections. C and Si segregated at the (sub)grain boundaries, while Mn and Cr enriched at the ferrite/cementite phase boundaries due to their low mobility in cementite. No evidence of recrystallization was found even after annealing at 723K for 30min. The stability of the tensile strength for low-temperature annealing (<473K) and its dramatic drop upon high-temperature annealing (>473K) are discussed based on the nanostructural observations.