| Deformation twins，martensite phase transformation and mechanical propertieshad been investigated. For steels with higher stacking fault energy，twinning is themain deformation mechanism. TWIP（Twinning Induced Plasticity）steels exhibit hightensile strength (650～1100MPa), extremely large elongation rate(19%~90%). Themicrostructures, mechanical properties and fracture behaviors of nine differentcomposition TWIP steels were investigated. Tensile tests were carried out at differentstrain rates, and the tensile strain behaviors under various strain rates of nine differentcomposition TWIP steels were investigated. The dynamic tensile tests were carriedout on the pneumatic indirect bar-bar tensile impact tester. The formation of twins andmartensite were analyzed by optical microscopy, XRD， SEM and TEM. Thecontribution of the dissertation lies in providing theoretical basis for lighting weightand anti-crashing of automobiles.The primary coverage of the dissertation includes:(1) The components of five kinds of low carbon TWIP steel and four kinds ofmedium carbon TWIP steel are determined based the research results of Fe-Mn phasediagram and the thermodynamic calculation. The microstructures of nine differentMn-content TWIP steels before and after deformation are investigated bymetallography, XRD and TEM. The effects of heat treatment and alloy elements onthe morphology and volume fraction of each phase are analyzed. The results showthat the microstructure of steel1#is composed of austenite and second-phase. Withthe increasing of manganese content, the microstructure of the TWIP steels changes toaustenite totally and deformation twins occurs during deformation.(2) The tensile tests are carried out on nine TWIP steels in the strain rate range of10-5～103s-1. The results indicate that the yield strength increases and tensile strengthof steel1#and steel2#decreases, while those of steels3#~5#, z1#~z4#change little,and the uniform elongation rate and total elongation rate of steel1#increase, while these of other steels decrease with the strain rate. During dynamic tensile stage, thetensile strength, yield strength and total elongation increase, while the uniformelongation rate hardly changes with the strain rate, showing that the TWIP steels aresensitive to strain rate.(3) The volume fraction of austenite after deformation as a function of strain andstrain rate is investigated by XRD. The results show that martensite transformationoccurred in steel1#and steel2#, but not in steel3#. The volume fraction oftransformed austenite decreases with the strain rate, while it increases with the strain.(4) The strain-hardening exponent n as a function of strain and strain rate isdetermined. It shows that the n value varied during tensile deformation. The n valueof steel1#obeys a parabolic relation: n=aε2+bε+c. While the n value of fullaustenite TWIP steels obeys a logarithmic relation: n=alnε+b. In the strain rangeof0.4%～4%, the full austenite TWIP steels are strengthened by dislocation piling,but in the range of10%～50%, these steels are strengthened by twinning. In these twostages, the n value keeps constant. However, the n value increases continuously in thestrain range of4%～10%. Strain induced martensite transformation and formation oftwins during deformation significantly influence the strain-hardening behavior of theTWIP steels.(5) The microstructure of the TWIP steels after dynamic tension test wascharacterized by SEM. The fractograph of the TWIP steels exhibits a typical ductilefracture pattern. Due to the presence of second phases, nucleation of micro-voids insteel1#occurred at the weak spot in the material, i.e. the austenite/second phaseinterface. However, due to the interaction of dislocations and twins, the nucleation ofmicro-voids in the full austenite steel5#occurred at boundaries of deformation twinsas well as the interface between deformation twins and dislocations. No matter whatthe mechanism is, the nucleation is followed by the growth and coalescence of themicro-voids in all samples.(6) In present work, the dominating deformation mechanism of the TWIP steels is twinning, also accompanied by dislocation mechanism. For the twinningmechanism, the content of twins increases with strain, and Cu{112}<111> orientationrotates its orientation to the twinned position CuT {552}<115>. Meanwhile, underthe continuous action of the dislocation mechanism, the CuT {552}<115> rotates toG{011}<100> orientation, and the G{011}<100> rotates to B{011}<211>. After therotation, the intensities of textures vary with the strain. |
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