Softening In Carbon Cold Heading Steel

Typical manufacturing processing for high strength bolts includes spheroidizing annealing treatment to provide enough ductility for the cold heading. However, the spheroidization treatment is by far the most time and energy consuming stage in the processing of bolts. Hot compression tests using Gleeble simulator with different deformation parameters and subsequent controlled cooling were carried out to study the microstructure evolution and cementite spheroidization behavior of medium carbon steel 35K for cold heading. The purpose of this investigation is to explore the possibility of on-lining softening of medium carbon steel.The activation energy for hot deformation obtained from the true stress—true strain curves of the steel deformed at different temperatures of 650⁹50℃ and strain rates of 0.01 30s^-1 is about 409kJ/mol. The relationship of deformation stored energy △G_D with Zener-Hollomon parameter Z is: △ G_D = 295.4-16.2lnZ+0.22(ln²Z). It is obvious that deformation stored energy increases with the increase of Z or the decrease of deformation temperature and the increase of strain rate.Results show that for medium carbon steel, deformation induced ferrite (DIF) could be obtained through suitable deformation as deformation raises the transformation temperature from austenite to ferrite. The curve type for the volume fraction of DIF(V_F) via deformation temperatures looks like a mirror shape of S, that is to say, with the decreasing of deformation temperature above 800℃, but V_F increases slowly firstly, V_F increases fast with the decreasing of deformation temperature at the range of 800℃ and 750℃, when V_F exceeds the equilibrium content, V_F increasing will slow down with the decreasing of deformation temperature below 750℃ . The curve type for V_F via different amount of deformation at 700℃ looks like a S shape which called Arrami curve line. That is to say, Vf increases slow at small amount of deformation, but V_F increases fast at medium amount of deformation, when V_F exceeds the equilibrium content, V_F increases slow again with the increasing of amount of deformation.In the following isothermal holding below A₁ temperature just after deformation,supercooled austenite would decompose to different structures in three different ways according to the deformation temperature and strain: 1) When the deformation temperature is higher than Adi, conventional ferrite-lamellae pearlite structure was obtained. 2) When the deformation temperature is lower than Ad3 whereas higher than Ar3, DIF formed and retained austenite transformed to fefrire-pearlite-grain boundary cementite. The shape and amount of these phases would change with the deformation-stored energy, carbon concentration degree and the size of retained austenite which were significantly influenced by the deformation temperature. Small size of the retained austenites such as grain boundaries between DIF would directly transformed to globular or rod-like cementite and ferrite, large ones such as small islands or elongate shape prefers to transform to degenerated pearlite instead of lamellae pearlite. 3) When the deformation temperature decreased to just around Adi, a quite different structure of small cementite particle and ferrite instead of lamella pearJite was obtained.The microsiructure evolution and mechanical properties of the steel were studied after intercritieal and subcritical annealing cycle. It was found subcritical cycle could effectively save the hold time than intercritieal cycle and degenerated pearlite can easy soft than otheT microstrticture. In a word, the possibility of the on-lining soften of the 35K steel could achieve because deformation change the transformation model of austenite which can fast the spheroidization of cementite on the process of controlled cooling.