Study On Microstructures And Mechanical Properties Of Low-Alloyed High-Carbon Steels By Low-Temperature Isothermal Treatment

The aim of the present dissertation is to investigate the transformation kinetics, microstructure, mechanical properties and the wear resistance of low temperature bainite in low-alloyed Si-containing high-carbon steels.Time-temperature-transformation (TTT) curves of 90-Mn and 90-Al steels were calculated by a software for phase diagram calculation and properties simulation of metal materials. At the same time the transformation points, Ac1, Accm and Ms were measured using thermal dilatometer and differential scanning calorimeter. Then different isothermal transformation treatments of the steels were carried out. The phase composition and microstructural morphology were studied by X-ray diffractometer, optical microscope and transmission electron microscope. At last, mechanical properties and the wear-resistance under the condition of dry sliding friction and wear were also measured.The results show that the low-tempeature bainitic transformation can be accelerated by adding Al. The martensitic transformation points Ms of 90-Mn and 90-Al steel are 91°C and 180°C, respectively. The microstructures of the steels subjected to isothermal transformation are composed of two phases, carbide-free bainitic ferrite and retained austenite, with lath-like and film-like morphologies, showing the features of low-temperature bainite. The volume fraction of retained austenite reaches to ~45.0% in 90-Mn steel isothermally treated at 180°C for 5 week after austenizing at 1000°C for 30 min and 1.5%~3.4% in 90-Al steel isothermally treated at 200°C and 220°C for different time after austenizing at 950°C and 1000°C for 30 min.As the transformation time increasing, the hardness of 90-Al steel decreases firstly, then increases at certain time, and decreases again. The impact energy of samples with low-temperature bainitic transformation is 8~11 J at the temperature from -40°C to ambient temperature (23°C). The room-temperature impact energies of isothermally treated samples are much more than that of quenched sample (4 J). The wear resistance of samples austenized at 1000°C is higher than that at 950°C, and relative wear-resistances are 1.19 and 0.67, respectively. In addition, the wear resistance of samples austenized at 1000°C is also higher than upper sample (high-speed steel).