A Study On Macroscopic And Microscopic Mechanical Behavior Of Trip High Strength Steel For Vehicles

With the development of automotive industry, it is demanding to take measuresin light weight. On one side, reducing weight may alleviate environmental pollution;on the other side, safety must be ensured. As a kind of new steels, TRIP steel isfamous by the effect of transformation-induced plasticity and the virtues of highstrength and high ductility, which makes them the idealistic materials for auto bodylight weightin. Usually, TRIP steel has a multi-phase microstructure under roomtemperature, which consists of ferrite(F), bainite(B)and retained austenite(Ar). Theretained austenite is unstable, and will transform into martensite under certain plasticdeformation conditions. The new-born martenite has a better hardening propertycomparing to the former austenite, which increases the deformation capability andcontributes to plasticity property of the material. But, the transformation-inducedplasticity efect endows TRIP steels with both good mechanical properties and somedisadvantages, which affects and hinders their wide application to automotive. It issignificant to extend the use field and proportion and shorten the time of new productdesign if the mechanical properties and phase transformation are studied deeply.This thesis focused on mechanical properties of the TRIP steel produced byBaoshan Steel Company. Experiments were carried out based on both macroscopicand microscopic methods. The mechanical properties of TRIP steel were studiedthrough the study on tensile tests, microstructure observation with SEM, and XRDmeasurement. A material constitutive equation which based on Johnson-Cook modelwas developed and introduced into the numerical simulation. The thesis includesfollowing:1. The static uniaxial tensile experiment under three different tensile speeds(0.5mm/s,0.05mm/s,0.005mm/s) and three different pre-strain (5%,10%,15%) werecarried out, then the mechanical properties and stress-strain curves under variouscondition were obtained.2. The sheet forming property of TRIP steel were studied by using die, whichtake binder hold force、sheet shape、springback into account.3. Through the analysis on the microstructure with SEM, the Transform InducedPlasticity effect was investigated. Then, through the XRD experiment, the majorfactors which have impacts on the transformation of retained austenite were studied.The relationship between strain and volume of retained austenite was discussed. 4. Johnson-Cook model was employed to describethe behavior of the TRIP steel.Then, the constitutive model was verified by simulation of the uniaxial tensileexperiment.