Study On Hardness Prediction Model Of Low Carbon Steel In Cold Forming Process

Hardness is an important index of material mechanical properties. It reflects the ability of resistance to plastic deformation, cutting, scratches and wear. It is also a measure of forge-ability of a material undergoing a cold forging process. The present studies had focused on the relationship between hardness and equivalent strain during cold forming processes. While material hardness depends not only on the material deformation degree, processing condition, such as room temperature, high temperature, but also on the microstructures, metal crystal lattice type, lattice parameters and so on. Therefore, it is of important theoretical significance and practical value to study the relationship between hardness and microstructures, and establish the hardness prediction model based on the microstructures of material during cold forming.In this article, the relationship between microstructure and hardness of single and dual phase materials, during cold forming process, was studied through experiment combined with finite element simulation. Also hardness prediction models based on microstructure parameters for single and dual phase materials were established respectively. Based on this model, the forming process of flange was optimized, and the hardness distribution of the flange was predicted.Annealing treatments were performed at different temperatures and holding time to obtain single and dual materials with different microstructures. And the influence of annealing temperature and holding time on the grain size and volume fraction of phases was analyzed. The results show that as the temperature and holding time increases, the ferrite grain size increases. And when the ferrite grain grows to a certain extent, it no longer changes with the holding time. The volume fraction of pearlite phase presents a decreasing trend with the increase of temperature and holding time.The compression tests and finite element simulations were conducted to study the influence of grain size and volume fraction of phase on the hardness of materials. And a hardness predicted model considering ferrite grain size effect was established. According to the rule of mixtures, hardness predicted model for ferrite-pearlite dual phase material were established, based on the single phase hardness prediction model. It is found that, for ferrite single phase material, initial hardness HV0, hardening coefficient K and hardening exponent n are influenced by grain size, and all decrease with the increase of grain size. While for ferrite-pearlite dual phase material, HV0, K and n were influenced not only by grain size, but also by volume fraction of phase. And they all decreased with the increase of grain size and the decrease of volume fraction of pearlite. Both of the models were verified by experiments and can be used to predict the hardness distribution in the cold forgings with different initial microstructures.According to the characters of flange, a forming process compounding upsetting and extrusion was proposed. And based on the hardness prediction model, the reasonable range of blank diameter was determined, and hardness distribution was predicted. The results show that predicted hardness match well with the measured values, which further verified the accuracy of the model.