Subcritical Spheroidizing Annealing Of SCM435 Cold Heading Steel

SCM435 steel, with the increase of chromium content, has the advantage of improving the hardenability in medium carbon alloy steel. It’s obtained a better impact resistance、low temperature impact toughness and high fatigue strength after quenching and tempering. 10.9-12.9 grade high strength or ultra high strength fasteners are mainly made from SCM435 steel wire rod by cold heading. Prior to cold deformation, the steel wire must be annealed to undergo considerable plastic deformation during the cold heading process. However, the spheroidization treatment is by far the most time and energy consuming phase of bolt manufacture. Therefore, there is an urgent need to simplify the spheroidization process and improve production efficiency for high-strength fasteners production.Subcritical spheroidizing annealing is one of the spheroidizing process, which involved heating to below the Acl for various times, However, it is considered time consuming to spherical of cementite when isothermal below the Acl, therefore, the subcritical spheroidizing annealing is used rarely in industrial applications. However, with the popularization of modern high speed wire rod production process, the fine microstructure of hot rolled wire attributed to the TMCP methods. Which was the prerequisite work for the applications of subcritical process. In view of this, the objective of the present paper was to investigate the spheroidizing annealing of SCM435 steel. Using the box-type resistance furnace to simulate different holding time, heating temperature etc. production conditions. The optical microscope and field-emission scanning electron microscopy was applied to monitor the microstructure changing during annealing. The mechanical properties were measured by tensile tests, upset tests and Vickers hardness. The following main conclusions were obtained:(1) The optimal isothermal temperature of subcritical spheroidizing annealing are 720℃, isothermal time are 5h.(2) Normalizing process has slight promoting effect on spheroidizing annealing of SCM435 steel. The cementite particles dispersed in the ferrite matrix of hot rolled steel after annealing, the presence of elongated ferrite grains is observed and most cementite particles are located at ferrite boundaries.(3) The using of a subcritical spheroidization process instead of an intercritical process carried out by an steel plant could achieve a higher aspect ratio and lower hardness, and shorten the time of annealing, simplify the annealing process.(4) The tensile fracture of hot rolled steel and annealed steel are ductile fracture, the fracture dimples from the subcritical annealing distributes much more uniform than the intercritical process and hot-rolled condition. With spheroidizaing annealing performed, impact fracture morphology is transformed from cleavage fracture into quasi-cleavage fracture from SEM observation.(5) With spheroidizaing annealing performed, the reduction of area and elongation percentage are increased, hardness and hardening exponent are decreased. split is observed in some upset sample from intercritical process, but no split is observed in upset sample from subcritical process yet. Moreover, with a lower hardening exponent and hardness, higher reduction of area, the subcritical annealed steel acquires lower work hardening sensitivity and better cold formability.