Study On The Stamping Characteristics Of SUS304Metastable Austenitic Stainless Steel

SUS304metastable austenitic stainless steel belongs totransformation-induced plasticity（TRIP） steel，which has high tensilestrength and elongation. So it can satisfy the demand of crashworthinessand carbody lightening. However, the hardening exponent of SUS304isfar higher than other steels, which leads to serious work hardening.Wrinkle and pull cracks often happen during cold working for SUS304.The phase transformation of martensite to austenite during plasticdeformation makes the mechanical properties of SUS304unstable, whichresults in the complexity of forming behavior of SUS304. For the aboveshortcomings, under the supports from Doctoral Program Foundation ofInstitutions of Higher Education of China （20090073120058） and NaturalScience Foundation of China （51105248）, the study on the stampingcharacteristics of SUS304metastable austenitic stainless steel iscompleted. The main research content is as follows.The effect of different tensile strain rates on the mechanicalproperties of SUS304is studied. The effects of different tensile strain rateson the microstructure, martensite volume fraction and temperaturevariation are also analyzed. The results show that the temperature oftensile specimen could reach the highest at0.005s^-1. The martensitevolume fraction increases obviously when the strain rate is lower than0.005s^-1. However, when the value is higher than0.005s^-1, the martensitevolume fraction has few changes. Correspondingly, yield stress increasesslightly, the tensile stress and elongation decrease obviously withincreasing tensile strain rate.The fracture surfaces of tensile specimens are observed by scanning electron microscope（SEM）. The dimple fracture happens when the tensilerate is0.0005s^-1. The tensile fracture surface at0.005s^-1and0.05s^-1displayed a mix of dimple fracture and cleavage fracture. However,cleavage fracture is the main fracture characteristic at0.1s^-1. So when thetensile rate is0.0005s^-1, the best ductility for SUS304happens. The secondis at0.005s^-1and0.05s^-1.0.1s^-1is the worst.In order to depict the effect of different tensile strain rates on the truestress-strain curves correctly, two constitutive models—modifiedJohnson-cook and Swift are chose to fit stress-strain curves. Both modelsfit well. A new constitutive equation considering strain rate sensitivity isalso suggested, which fits well too.Based on LS-Dyna and Dynaform, the material model—Johnson-cook and Swift are used to simulate Cross-part drawing. Thepractical experiment of Cross-part drawing is also carried out. Thecomparison of simulation and experiment, from the aspects of thicknessvariation, load-displacement curve and major strain, shows that twosimulations predict the thickness variation of the bottom and wall of cupwell. The prediction of thinning at the punch radius and thickening at thedie radius and flange based on Johnson-Cook is more correct than Swiftwhich is more effective in predicting the load-displacement curve. Twoconstitutive equations also work well in predicting major straindistribution at the bottom and punch radius.The relationship between martensite volume fraction at differentlocations of Cross-part and deformation model is studied. The martensitevolume fractions reach the highest at the flange and die radius due to theserious deformation. The less is the wall and punch radius. The bottom hasthe least one. Correspondingly, the relationship between stress state andmartensite volume fraction is: the amount of martensite in twocompressive and a plus stress state is the highest followed by uniaxialtension and plane strain. Biaxial tension results in the least one.