Two kinds of low-carbon low-silicon TRIP (Transformation Induced Plasticity) steels containing vanadium are designed using ThermoCalc software in the light of both thermodynamics and kinetics.TRIP heat treatment process of different steels is determined according to the calculation results respectively.Weld HAZ (Heat-Affected Zone) simulation tests indicate the weldability of TRIP steels is crucially sensitive to CE (carban equivelent) of the steel.However the impact toughness of CGHAZ (Coarse Grain Heat-Affected Zone) does not decrease drastically for TRIP steels microalloyed with Ti+V.The steel containing both of vanadium and titanium shows smaller grain size compared with that containing vanadium solely.This is because the precipitation of Ti/V carbonitride slows down the grain boundary motion speed and then retards the grain size coalescence in CGHAZ.
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
Thermodynamics of martensite transformation in low alloy TRIP steel was evaluated.It follows from the calculation that ε phase might be formed at above room temperature in the low Mn steel.Tested steel was therefore prepared according to the estimation result.High volume fraction of retained austenite(above 20%) containing 1.37%C was obtained in the steel after continuous annealing treatment.Tensile test showed that the ultimate strength of the steel was 1248 MPa,elongation rate 25%,the product of strength and elongation rate 31200 MPa%,reaching the grade of the third generation automobile steel.XRD detection indicated that after tensile test there existed ' martensite while TEM revealed that ε martensite also existed in the steel.Strengthening-toughening of the steel was thus raised due to the effect of transformation induced plasticity of two kinds of martensite.In addition,the phase stability of carbonitrates containing V was estimated and the complex micro-alloying effect was applied to the steel to control the size of austenite(about 1 μm) and enhance the stability of austenite.
LI LinGAO YiZHU NaQiongHE YanLinLIU RenDongHE ZhongPingSHI WenZHANG Mei
＜正＞Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation a...
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
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。
