Effect Of Microstructures On Warm Deformation Behavior Of Seveal Carbon Steels With Different Carbon Contents

In this work, the warm deformation of carbon steel with starting martensite (M) structure, full annealing structure and spheroidized annealing structure in Q235, 45, T8 and T12 steels were investigated by warm compression tests performed on a Gleeble3500 machine.The results show that the flow stresses of normal quenched M in the four steels increase at first and then decrease with the increase of carbon content in the same condition of the temperature between 600℃and 700℃, the strain with 50% and the strain rate from 1.0s-1 to 0.001s-1. The flow stresses of 45 steel are higher than the other steels. Among them, the flow stresses of Q235 steel are lower than that of 45 steel by 13 MPa～69 Mpa and the flow stresses of 45 are higher than that of T12 steel by 9 MPa～35 MPa. The explanation of the results is that due to fewer and uniform carbides precipitated during warm deformation, flow stresses of M in low carbon steel are lower than that of high and medium carbon steel. On the other hand, the reason that the flow stresses of M in medium carbon steel are higher than that of high carbon steel is that the activation energy of self-diffusion decreases with increasing of carbon content, and the rate of recovery and recrystallization increases as the flow stresses decrease with increasing of carbon content.When the temperature at 650℃and the strain at 50% are in constant and the strain rates are varied from 1.0s-1 to 0.001s-1and when the strain at 50% and the strain rate at 0.01s-1 are in constant and the temperatures are varied at 600℃and 700℃, the flow stresses of annealing structure in Q235 and 45 steels and spheroidized annealing structure in T8 and T12 steels increase at first and then decrease with the increase of carbon content. The flow stresses of Q235 steel are lower than that of 45 steel by 23 MPa～51 Mpa and the flow stresses of 45 steel are higher than that of T12 steel by 19 MPa～26 MPa. At the same time, there is a critical strain which makes the flow stresses of M structure lower than that of spheroidized annealing structure in T8 steel.In addition, the microstructures after warm deformation were investigated by using Transmission Electron Microscope (TEM) and Optical Microscope (OM). The results show that the particle sizes of carbides precipitated in M of 45 steel during warm deformation are relative small, about 60 nm in average. And the carbides are uniform, the mean space is about 200 nm; There are many undissolved carbides in cryptocrystalline M structure of T12 steel. The precipitated carbides make the carbides becoming coarse and uniform. The results show that their mean particle size is about 1.5μm in diameter and their mean space is about 2.5μm.