Research On Inelastic Recovery Behavior Of The Third Generation Automobile Sheet Steels Presenting With Ultrahigh Strength And High Ductility

With increasing requirements of the automotive lightweight and crash safety,automotive steel develop to present with high strength and high ductility, and RmxA hasbecome an important measure of the performance of automotive steel. As the first-generationautomotive high-strength steel, RmxA of DP steel and TRIP steel are less than10~20GPa·%.However, due to the high strength and low ductility, structural parts and safety parts appearcracking, springback and die wear and other issues during stamping process; As the secondgeneration automotive high-strength steel, RmxA of austenitic steel and TWIP steel are up to50~70GPa·%. The second generation automotive high-strength steels have high ductility, butthe high cost, poor technology and metallurgical production difficulties are the disadvantagesof the second generation automotive high-strength steels. Accordingly, the above twogenerations steels can not be widely used in the automotive industry now. Formability andcost of the third generation high-strength automotive steel are between that of the firstgeneration high-strength automotive steel and second generation high-strength automotivesteel, so the third generation high-strength automotive steel in automotive industry isincreasingly popular. Microstructures of the third generation high-strength automotive steelmainly consist of martensite, retained austenite and ferrite (not fully austenitic). Its ductilitymechanism is achieved by TRIP effect, i.e. residual austenite phase to martensite, whichdirectly affect the inelastic recovery behavior of automotive sheet during unloading,resulting in the unloading modulus changing with plastic strain. In the finite elementsimulation of metal forming, as an important simulation parameters, elastic modulus hasbeen considered a constant. Due to deviation in the elastic modulus, stamping springback simulation value is much smaller than actual value. Accuracy of finite element simulation istoo low to predict springback of the third generation high-strength automotive steel.Accordingly, it is necessary to carry out related research.In this paper, in macro using uniaxial tension test and tension-unloading-reloading cycletension test, mechanical properties and non-elastic recovery behavior of the third generationautomotive Q&P980steels were studied. Explore the anisotropy and rate sensitivity ofQ&P980steels. Analyze the characteristics of the inelastic recovery behavior, and itsoccurrence, development and changes. Determine the composition of the springback and theratio of the inelastic recovery. At last, mathematical models of unloading modulus withplastic strain were set up. In micro, using optical microscopy (OM), scanning electronmicroscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM)method study the microstructure of Q&P980steels, analyze the changes of microstructureand dislocation, and reveal the microscopic mechanism of inelastic recovery of Q&P980steels.The results of tension test show: Q&P980steels exhibit high tensile strength and highductility; Q&P980steels have a high work hardening ability; Q&P980steels exhibit nosignificant anisotropy before fracture; Q&P980steels show no significant rate sensitivity inthe limited range between10-4s-1and10-2s-1; The stress-strain curve of unloading andreloading form an unloading loop, also called the Bauschinger effect loop; Bauschingereffect loop width and Bauschinger effect loop area are presented the power relationship withplastic strain; Unloading total recovery strain is divided into two components, elastic andinelastic recovery strain; Unloading modulus decreases with increasing strain, however,when the plastic strain increases to a certain extent, unloading modulus saturates;Mathematical models of unloading modulus are as follow, chord modulus unloadingmathematical model: EC177.331.8exp(42.9p); Equivalent unloading modulusmathematical model: E175.433.8exp(59.3p).The results of microscopic test show: Hard phase martensite leads to a large number ofdislocations piled during deformation; The phase transformation of retained austenite tomartensite in Q&P980steels during deformation causes an increase in the dislocation density, dislocation tangles and other microscopic changes; And hard martensite phase generatedfrom phase transformation in the micro is equivalent to an increase of dislocation pinningnodes promoting dislocation pile-up during deformation. Both the density of movingdislocations and the curved extent of dislocation lines significantly affect unloading modulus.In sum, both the high density of moving dislocations and the large curved extent ofdislocation line are microscopic mechanisms of Q&P980steels inelastic recovery behavior.