| TRIP steels are designed based on thermodynamic calculation using ThermoCalc software. Several TRIP steels are produced with designed heat treatment processes. For automotive application, miscellaneous researches are taken to stress at the revealing and improving of the steel behaviors. The front frame and cross member etc. in the personal car are manufactured successfully with TRIP steels stated above.The main part of the dissertation includes:1 Several TRIP steels 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.2 Forming limit curve (FLC) of low-carbon low-silicon TRIP steel SH is investigated. FLD0 value of 27.2% stand for the steel showing good formability respectively. Fracture morphology of FLD samples shows micro-void nucleation, growth and coalescence. Transformation induced plasticity relaxes the stress concentration at the crack tip and this results in the crack tip blunting. Thereafter, the stress increases again and the crack change their direction of motion. No cracks are found to propagate within the dispersed phase.3 High cycle fatigue test reveals thatσ0.1 of steel SH is 405MPa, and it follows the relationσ0.1=774-0.03981gN. Low cycle fatigue test shows n'=0.1887, K'=1547MPa, and (△εt)╱2=1265╱(2.05×105)(2Nf)-0.1122+0.3396·(2Nf)-0.5929. The critical value of crack propagation△Kth is 229MPa(mm)1╱2, and there exists equation (da)╱(dN)=4.130475×10-13(△K)2.772. All these show very good fatigue properties of the low-carbon low-silicon TRIP steel SH.4 780MPa-980MPa grades of low silicon C-Mn-Al-Si-P TRIP steels 1#, 2#, 3# and 4# with different carbon contents, with or without microalloy element Ti/V are developed. Tensile test and Erichsen test of the investigated steels exhibit excellent combination of strength and ductility with UTS of 780MPa-980MPa, TE around 20%, and dome height of 10mm in all cases. Regarding the laser welds of these steels, all are free of cracks and other imperfections. The UTS and hardness of the laser welds increase with the increase of overall carbon content while the total elongation decreases. Properties of the laser-welds become worse with the increase of carbon content. The formability in the regions influenced by laser welding is obviously deteriorated, however, this area is so narrow that the stretchability properties obtained using Erichsen test are acceptable.5 The welding ability of TRIP steels B and SH is studied using GMAW method. The weld joints exhibit satisfactory mechanical properties and impact toughness although a remarkable increase of the hardness in the welding zone has been observed. The welds of welding cracking test are all free of cracks and other imperfections for steel SH. In steel B, welding cracking tests are also free of cracks and other imperfections, but in the Y shape weld crack test, there exists a section plane crack rate of 3.4%. This value is still lower than the critical crack rate 10%. Test results reveal fairly good weldability of the two TRIP steels, and especially in steel SH where rather high weldability is observed.6 Weld simulation tests indicate the weldability of TRIP steels is crucially sensitive to CE (carban equivelent) of the steel. However the impact toughness of CGHAZ does not decrease drastically for TRIP steels microalloyed with Ti+V. 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.7 For automotive application, first of all, light weight potential is predicted for substituting traditional deep drawing steel with TRIP steel after structural analysis and CAE simulation. Secondly, several parts of a mini-bus are produced successfully with TRIP steel. The parts weight is reduced 19.6%-25%. CAE simulation technique greatly helps to cut down the development phase and the tentitive prodution expense. |
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