Databases concerning basic physical properties of materials,such as molar volume,density,thermal expansion coefficient,elastic constants,thermal conductivity,etc.,are essential parts of the underlying knowledge for materials design.While thermodynamic databases provide chemical driving forces and phase equilibrium data,physical property databases provide essential physical parameters,such as volume,lattice constant,lattice misfit,elastic energy,interfacial energy,etc.,for phase transformation and microstructure simulations.Combined with thermodynamic databases,physical property databases established on the basis of the calculation of phase diagram(CALPHAD)method can be used to calculate physical properties together with phase equilibria,phase fractions,phase compositions,and thermodynamic properties for multi-component and multi-phase material systems and for the constituent phases.In this paper,we will discuss in detail various volume models based on the CALPHAD method which are capable of describing experimental data in a wide range from cryogenic temperatures to melting points,from the atmospheric pressure to high pressures for pure substances as well as multi-component and multiphase materials.
The influence of soaking temperature on microstructure of high temperature multi-pass compression deformation for two low carbon steels(steel A:wC=0.032%and wMn=0.25%;steel B:wC=0.165%and wMn=0.38%)is studied on the thermal-mechanical simulator in order to rationalize the hot-rolling schedule of low-carbon steel and to promote the low-temperature heating technology.The results show that the microstructures of steel A are almost not affected by reducing soaking temperature,but the acicular ferrite forms in steel B when the soaking temperature is reduced from 1 200 to 1170℃,due to its smaller initial austenite grain size according to recrystallization kinetics theory.
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.
在Gleeble-3500热/力模拟试验机上,采用双道次压缩试验研究了0.15C-0.14Si-0.2Mn-0.04Nb钢在高温压缩变形后的静态软化行为,分析了道次停留时间、变形温度、应变速率以及应变量对静态再结晶行为的影响。并通过2%应力补偿法结合流变应力曲线计算出了静态再结晶软化率。研究表明,在其他条件不变的情况下,再结晶软化率随着道次停留时间的延长、变形温度的升高、应变速率的增大以及变形量的增大而增大。还建立了试验钢的静态再结晶动力学模型,获得静态再结晶激活能为188.986 k J/mol。
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
