Microstructure And Mechanical Properties Of Low Carbon Silicon Manganese Steel Treated By Austempering And By Quenching-Partitioning Processes

The low carbon silicon manganese steel is regarded as a kind of steel with excellent properties.But the mechanical properties of this steel have not reach the expectation of the third generation of AHSS.Therefore,it is still a important project to improve comprehensive mechanical properties of the low carbon silicon manganese steel by changing heat treatment process recently.The low carbon silicon manganese steel has been treated by austempering and by quenching-partitioning process with different process parameters.The microstructures of the investigated steel after heat treatment treated by austempering and by quenching-partitioning have been examined by optical microscope,scanning electron microscope,transmission electron microscope and X-ray diffraction techniques.The effect of the heat treatments on mechanical properties of the steel has been studied in different degrees by means of impact,tension and microhardness tests.The microstructure of this steel treated by austempering is composed of bainitic ferrite and retained austenite,some martensite would exist if the steel is quenched below Ms.When austempering treatment increased,the toughness of steel increases and then decreases;hardness decreases and then increases;yield strength decreases continuously;tensile strength and elongation remains stable.A lot of blocky retained austenite,which is relatively unstable,will exist in the sample treated at high temperature.This kind of retained austenite will transform into martensite easily,which means that it can deteriorate impact toughness but enhance transformation induced plasticity effect by transform into martensite in the early stage of deformation.In the sample treated at low temperature,most of the retained austenite is film-like type which is helpful for impact toughness and useless for the strength and ductility because of its stability.The microstructure of this steel treated by quenching-partitioning process is composed of the martensite and retained austenite.Quenching temperature,partitioning time and partitioning temperature,these parameters will affect mechanical property and microstructure of the steel.When the partitioning temperature is 400 °C and partitioning time is 180 s,with rasing quench temperature form 165 °C to 210 °C,volume of retained austenite and elongation increase and then decrease;yield strength continuously decreases;hardness decreases and then increases.When the quenching temperature is 195 °C and partitioning temperature is 400 °C,with the extending of partitioning time,volume of retained austenite increases monotonously;elongation increases then decreases;yield strength continuously decreases;hardness decreases then increases.When the partitioning temperature is 300 °C,impact toughness increases continuously during partitioning from 30 s to 1 h.When the partitioning temperature is 400 °C,impact toughness increases then decreases.The best impact toughness acquired when the partitioning temperature is 300 °C is much better than the impact toughness when the partitioning temperature is 400 °C.