| Auto-body lightweight technology has been widely investigated by automotivecompanies to meet the demand of fuel economy and excellent service performancesimultaneously. The substitution of conventional high strength steels (HSS) withadvanced high strength steels (AHSS) can realize the auto-body lightweighteffectively while keeping or even improving its performance. The transformation ofretained austenite to martensite can improve local strain hardening, retard neckingphenomenon, making the TRIP steel be an ideal material for auto-body lightweight.During the whole service period, auto-body structure is subjected to repeated externalloads from road, engine and so on, and the resulting microscopic cyclic plasticity instress concentration area or weak region of structure can lead to fatigue damage to thematerials involved. Hence, fatigue is the main failure mode of auto-body structure,fatigue performance of TRIP steel and reasonable life prediction of TRIP steelauto-body component need to be studied. Up to now, there are few simultaneous andcomprehensive research fruits on the fatigue response of TRIP steel sheet, such asphase transformation kinetics of retained austenite in TRIP steel fatigue process,cyclic deformation behavior, cyclic stress-strain model and strain-life model.Furthermore, in the production processes, most auto-body components aremanufactured by plastic forming and undergo subsequent paint baking cycle. Thesemanufacture processes change fatigue performance of TRIP steel from that of theas-received state, making it more difficult to predict fatigue life of auto-body structurereasonably, lightweight design of the auto-body can’t be realized effectively. How toestablish the correct fatigue life prediction model considering the influence ofpre-strain and baking processes is the key point to the application of TRIP steel inauto-body structure.For the mentioned issue, this dissertation studied the effects of uniaxial tensionpre-strain and baking on the phase transformation kinetics of retained austenite incyclic case, static mechanical performance, cyclic deformation behavior, fatigue lifeof multiphase TRIP steel experimentally, established cyclic stress-strain prediction model and strain-life prediction model of TRIP steel sheet with pre-strain and bakingeffect for the local strain-life finite element method, reasonably analyzed fatigue lifeof a typical auto-body component, provided theoretical guidance for the application ofTRIP steel in auto-body lightweight. The main achievements are summarized asfollows:(1) Static mechanical performance of TRIP steelStatic mechanical performance is the basic mechanical performance of material,has significant influence on stiffness, strength and fatigue performance of auto-bodystructure. This dissertation studied the effect of pre-strain and baking on the staticmechanical properties of TRIP steel, provided the macroscopic analysis basis forfatigue performance research.(2) Phase transformation kinetics model of retained austenite in cyclic caseThe transformation of retained austenite to martensite affects the fatigueperformance of TRIP steel. In this dissertation, phase transformation kinetics ofretained austenite in the fatigue process of annealed, pre-strained, and baked TRIPsteel was investigated. The characteristic of phase transformation kinetics in cycliccase, the relation between phase transformation saturation value and strain amplitudewere analyzed. Based on TI model and Smaga model, a new kinetics model ofretained austenite phase transformation in cyclic case was proposed for annealed sheet.The variation rule of phase transformation model parameters with pre-strain degreewas investigated, and kinetics prediction model of retained austenite phasetransformation in cyclic case with manufacture effect was established, which providedthe microscopic analysis basis for TRIP steel fatigue performance research.(3) Cyclic deformation behavior and cyclic stress-strain prediction model ofTRIP steelFatigue life prediction of auto-body component by local strain-life finite elementmethod needs cyclic stress-strain model of material to calculate elastic-plastic stressand strain amplitude, so cyclic stress-strain model affects prediction precision offatigue life indirectly. This dissertation investigated the effect of pre-strain and bakingprocesses on stress amplitude response and mean stress response of TRIP steel. Theeffect rule of pre-strain and baking on the cyclic stress-strain curves of TRIP steel wasanalyzed, the functions between cyclic strength coefficient, cyclic strain hardeningexponent and pre-strain degree were developed for pre-strained and baked TRIP steel,cyclic stress-strain prediction model of TRIP steel sheet with pre-strain and baking effect was established, which ensured the calculation precision of elastic-plastic stressand strain amplitude for TRIP steel auto-body component.(4) Strain-life prediction model of TRIP steelThe strain-life model of material directly affects fatigue life prediction precisionof auto-body component. However, present research fruits cannot describe strain lifeof TRIP steel after any degree of pre-strain, without or with baking effect, cannotguide the fatigue life precise prediction of auto-body component effectively. In thisdissertation, the influence rule of pre-strain and baking on the strain life of TRIP steelwas analyzed, functions between fatigue strength coefficient, fatigue strengthexponent, fatigue ductility coefficient, fatigue ductility exponent and pre-strain degreewere developed for pre-strained and baked TRIP steel, strain-life prediction model ofTRIP steel sheet with pre-strain and baking effect was established, which improvedfatigue life prediction precision of TRIP steel auto-body component.(5) Fatigue endurance prediction of TRIP steel auto-body componentBased on the preceding cyclic stress-strain prediction model and strain-lifeprediction model with pre-strain and baking effect, joint simulation of manufactureprocess and fatigue life prediction of TRIP steel auto-body component wasimplemented in whole body environment. The effect of element thickness variationdue to press forming process, and local strain-life model variation due to pressforming and paint baking processes on prediction result of TRIP steel component’sfatigue life was investigated, providing technical guidance for the precise predictionof TRIP steel component’s fatigue life.The contents of this dissertation reveal the real fatigue performance of TRIP steelauto-body component, build the bridge between manufacture process and fatigue lifeprediction, and improve fatigue life prediction precision of TRIP steel auto-bodycomponent. In addition, the research finding of this dissertation can give usefulreferences for further optimization of TRIP steel metallurgy technology andcomponent fabrication technology. |
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