Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.
LI Lin 1,GAO Yi 2,SHI Wen 1,LIU Ren-dong 2,HE Yan-lin 1,FU Ren-yu 1,ZHANG Mei 1,WANG Hua 1 (1.School of Materials Science and Engineering,Shanghai University,Shanghai 200072,China
The microstructural evolution of the gas atomized Fe-25Cr-3.2C powders was investigated by using optical microscope,scanning electron microscope,and X-ray diffraction. The experimental results showed that the atomized Fe-25Cr-3.2C powders were mainly composed of austenite and (Fe,Cr)7C3 carbide. Eutectic microstructure was developed in the larger particles,whereas dendritic microstructure was obtained in the particles with diameter less than 38 μm. The reason for microstructure change should be the difference of nucleation undercooling for particles.
YANG Min SONG Chang-jiang DAI Yong-xiang ZHU Liang LI Ke-feng ZHAI Qi-jie
By using a static and high-speed material testing machine,tensile deformation behaviors of two kinds of SiMn TRIP(transformation induced plasticity)steels and DP(dual phase)steel were studied in a large range of strain rates(0.001-2 000s-1).Temperature variation during adiabatic heating and the amount of retained austenite at fracture were measured by an infrared thermometer and an X-ray stress analyser,respectively.The microstructure of steels was observed by optical microscopy(OM)and scanning electron microscopy(SEM)before and after tensile test.It was found from the experimental results that the tensile strength of these steels increased,and the fracture elongation firstly decreased and subsequently increased,as the strain rate increased in the range of 0.1-2 000s-1.The temperature raised during adiabatic heating of TRIP steel was in the range of 100-300℃,while that of the DP steel was in the range of 100-220 ℃.The temperature rise of these steels increased with increasing the strain rate,as well as the amount of the transformed retained austenite in TRIP steels.It was confirmed that austenite to martensite transformation is not suppressed by adiabatic heating.
