Impact Of Strain Rate On The Mechanical Properties Of Various Grade TRIP-aided Steels

When applied to manufacture car body parts, the steel is deformed at a rate inthe range of10-1～10s^-1.Nevertheless, during a car crash, the steel is deformed at arate up to102～103s^-1.Therefore, to clarify the mechanical behavior andmicrostructure evolution under various strain rates becomes a critical important topicduring the development of TRIP （TRansformation Induced Plasticity） steel. Withinthis topic, it is crucial to understand the mechanical response of each phase of thematerial and elucidate the strengthening and failure mechanism under various strainrates.Three grades of TRIP-aided steels,600MPa、700MPa and900MPa, wereselected in the current work. A Dual Phase steel （DP） is employed as a reference dueto its characteristic of no phase transformation during dynamic impact. Themechanical behaviors of samples were investigated by universal material testmachine, rotation disk bar-bar tensile impact apparatus as well as high-speedmaterial testing machine Zwick HTM5020equipped with an infrared radiationthermometer. The microstructure of the samples before and after tensile test wascharacterized by OM （optical microscopy）, SEM （scanning electron microscope） andTEM （transmission electron microscope）. Moreover, the phase transformationbehaviors of steels were investigated by thermodynamics and experiments via aninternal friction instrument plus an in-situ tensile test apparatus. The mainconclusions can be drawn as follows:1, The results of mechanical behaviors show that the yield strength and tensilestrength of TRIP-aided steels increase with the strain rate in the range of0.001～2000s^-1. The elongation to fracture of TRIP-aided steels firstly decrease with strainrate, then increase as the rate reached around600s^-1.600MPa grade TRIP-aided steelhas the biggest strain rate sensitivity because its strength and elongation are most fluctuant. Compared with the situation under static tensile load, the localization ofdeformation leads to even lower fracture elongation, while the adiabatic heating athigh strain rate leads to the increase of fracture elongation. Since the influence ofadiabatic heating is greater than that of deformation localization under the rate of2000s^-1, DP steel shows higher elongation to fracture compared with that under statictensile load. Because the “gradual transformation” of retained austenite is inhibitedby deformation localization, TRIP-aided steels show lower elongation than thatunder static tensile load.2, The mechanical properties of TRIP-aided steel mainly depends on the contentand stability of retained austenite. The in-situ tensile test results of three grades ofTRIP-aided steels show that the martensite transformation in600MPa gradeTRIP-aided steel only occurred after the yielding stage, and the retained austenite in700MPa and900MPa grade TRIP-aided steel transform to martensite at the appliedload502MPa（σ0.2=486MPa） and657.8MPa（σ0.2=538.8MPa）. It can be concludedthat this martensite transformation is caused by the strain. Furthermore, this workdetermined a relationship between the content of retained austenite and strain. Therelationship is based on experiment results on strain induced transformation behaviorof retained austenite and some other data from literatures. It shows that theprediction of this expression accords very well with the experiment results.3, The martensite transformation start temperature is of great importance forunderstanding the martensite transformation, designing new alloys and establishingthe heat treatment procedures. Based on the thermodynamics of martensitetransformation and sub-lattice model, this work worked out the expression of Gibbsenergy for martensite transformation of Fe-C-Mn-Si alloy. Combined withThermo-Calc and TQ interface, modeling with Compaq Visual FORTRAN6wascarried out to calculate the Ms temperature of Fe-C-Mn-Si alloy.4, Established the relationship between the real stress and real strain. And themodeling results agree well with the experiment. Combined with the relationship between retained austenite and strain as well as the Ms temperature model, it made aquantitative description about the energy of martensitic transformation of retainedaustenite.5, The investigation on the mechanism of fracture shows that TRIP-aided steelsshow a typical ductile fracture pattern including the nucleation, growth andcoalescence of micro-voids. The growth rate of micro-voids increases with the strainrate. And the nucleation and growth of micro-voids are retarded by the strain inducedtransformation of retained austenite. Moreover, the micro-voids are mainlydistributed in ferrite under dynamic tensile load. The critical strain for formation ofmicro-voids in TRIP-aided steels is higher than that in DP steel which possesses thesame content of ferrite with the former. For600MPa grade TRIP-aided steel, islandand film shaped retained austenite was detected in necking zone after static tensiletest, while only film shaped retained austenite was detected in necking zone afterdynamic tensile test.