| Novel Quenching-partitioning-tempering (Q-P-T) steel is one of theadvanced high strength steels with the best product of strength andelongation. Although researchers domestic and overseas have done a lot ofresearches about Q-P-T steels, including precipitation hardeningmechanism, the mechanism of plasticity enhancement by retained austeniteand so on, most of these focus on the mechanical behavior andmicrostructure of Q-P-T steels, no researchers have yet studied itsformability in process.So far, forming limit diagram (FLD) is the most popular tool incharacterizing the formability of a material. Generally, there are two waysto determine FLD, one is through theoretical calculation, and another isthrough experiments. But the experiment costs a lot of time and the datafrom experiments can’t convince researchers because they are usually veryscattered, and thus, we usually use numerical methods to determine FLDbecause they are much more time-saving. Constitutive relationship（equation） of a material should be combined with instability theory innumerical methods. Because materials have defects and tiny thicknessinhomogeneities inevitably, in the M-K groove theory, they believe that allthe unstable failures start at groove defects and study FLD by taking thisgroove as the breach. However, numerical methods also have their ownshortcomings, and the most obvious one is that they can’t completelycharacterize the true nature of a material. Many micro-defects and heattreatment processes have influences on the constitutive relationship thatdirectly affects the strain limit prediction of a material. Besides, in the existing groove models for FLD study, researchers simulate the formabilitywith different groove angles, but the reasonable values of groove angleshave not been determined; in addition, although many studies haveconsidered the influence of initial thickness unevenness on the formability,no standard about the reasonable groove depth ratio for calculating stainlimit has been made. In this dissertation,Fe-0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb is treated by novelquenching-partitioning-tempering process and used as material forexperiments to explore the formability of Q-P-T steels.In this dissertation, theoretical analysis is made based onelastic-plastic behavior of single crystal, and the constitutive relationship iscarried out by using tensors. Then, combined with Swift’s instability theory,Hill’s instability theory and M-K instability theory respectively, themethods of predicting the strain limit for materials have been gotten. Afterthat, Q-P-T specimen with grooves are designed to determine the effects ofgroove on the Q-P-T steels, that is, the groove depth ratios are5%,10%,15%and25%; the groove angles are90°,60°and45°. The results ofuniaxial tension test indicates that when the groove depth ratio is fixed, thematerial’s strength and ductility get bigger as the groove angle becomessmaller; when the groove angle is fixed, the material’s strength andductility get smaller as the groove depth ratio becomes bigger; when thegroove depth ratio exceeds10%, the material’s strength markedlydecreases. In general, among all specimen with grooves (except the onewithout groove) in this experiment, specimen with groove depth ratio of5%and groove angle of45°has both the best strength and ductility. Afterthat, Ludwik model is determined to fit the constitutive relationship bycomparing different fitting models. Combining Ludwik model andSwift-Hill’s instability theory to calculate the strain limits for specimenthat the groove angle is fixed at45°with different groove depth ratiosunder uniaxial tension respectively, the conclusion is made that when thedepth ration is5%, the stain limit value meets the formability condition; when the depth ratio is much bigger than5%, the strain limit calculatedwould underestimate the formability of the material. For the case that thegroove depth ratio is fixed at5%, when the groove angle is bigger than45°,the strain limit calculated would also underestimate the formability of thematerial. Although the stain limits of specimen with no grooves are alsocloser to the NADDRG empirical data, they are bigger than NADDRGempirical data, which means this method overestimates the formability andsafety fator of a material, it is not applicable. To sum up, reasonablecalculation of the stain limit should be determined when the groove depthratio is around5%and the groove angle is equal or smaller than45°.Finally, by studying the microstructure fracture through scanning electronmicroscope (SEM)... |
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