Study On Constitutive Model With Trip Behavior Of High Strength Auto Sheet And Its Hardening Properties

Along with the prosperity of automobile industry, the conflicts between the automobiles and the environment and the energy concern the entire world. In this circumstance, lightening-weight products become vital goal in the future development of automobile. At the same time, it is also very important to keep the secure property of automobiles in the lightening-weight process. Therefore, further development of high strength sheet steels in automobile has been a trend. As a kind of new steel, TRIP steel is characterized by high strength and high ductility because of the TRIP effect brought by transformation process during which retained austenite transforms into martensite with better mechanical properties. However, another practical problem is how to apply the new steels to the automobile body widely. At present, studies on the TRIP steel at home and abroad are mainly conducted in the material science domain. The extant research focuses on TRIP steels material preparation, the influence of alloy element on its mechanics and its motivation performance of microscopic movement, and so on. Whereas, few studies are conducted on the transformation-induced plasticity behaviors in the sheet forming process. Especially, extant literatures had not yet aimed at the study on influence of punching speed and forming step on the transformation-induced plasticity behaviors in a punching process.This thesis aims at studying the transformation-induced plasticity behaviors and the following application of TRIP steel to the sheet forming process. Considering forming speed and pre-strain effect, a novel martensite transformation models with punching behaviors are established in a sheet forming process. Based on TRIP steel's multiphase behaviors, a simple and valid law of multiphase mixed hardening is given for describing stress-strain relationship of TRIP steels. Considering the martensite's volume change, an anisotropic yield equation is constructed. Based on the above studies, the further work is to establish a new constitutive equation of TRIP steels to describe the inner relationship of a punching process by the expansion of classical theory of elastic-plastic mechanics. With the support of the constitutive equation of TRIP steels, effects of martensite transformation on the material behaviors of TRIP steels are investigated. And the research effectively supports the following study on the material behaviors of TRIP steels in a punching and forming process. Finally it is arranged for exploring the influence of punching speed and loading step on forming behaviors of TRIP steels and establishing a controlling of deformation in forming process.The main content of this thesis is divided into three parts:(1)TRIP steel constitutive equation considering strain rate & pre-strain The retained austenite transformation in TRIP steels is sensitive to deformation state. So the potential of TRIP effect is opened in the specific or combined deformation states. Although some authors studied the behaviors of TRIP steels, they worked mainly on the simple experimental models which derived chiefly from experiential expressions. So it is necessary to establish a transformation-induced plasticity novel model in a sheet forming process for the engineering application.On the basis of three classical models of strain-induced transformation, here establishes novel constitutive equations of TRIP steels sheet forming process with considering the forming speed and muti-step forming. Specifically, considering temperature increasing resulting from strain rate, martensite transformation models for TRIP steels are established under the condition of different forming speed. On the other hand, considering pre-strain, martensite transformation models for TRIP steels are established under the condition of multi-step forming. At the same time, based on the existing mixed hardening laws of austenite steels with only two phases (martensite, retained austenite), a new multi-transformation mixed hardening laws are built here to describe stress-strain relationship of TRIP steels with four phases (martensite, retained austenite, ferrite and bainite) under the condition of strain rate and pre-strain respectively. Considering the volume change of TRIP steels during the transformation process, new constitutive equation is characterized by the embeddedness of the Miller yield law with volume effect and the new anisotropic yield law. By comparison our experimental results to the extant literatures'results, it is found that the new constitutive equation of TRIP steels proposed here describe the TRIP sheet forming process much better than the existing models. This result provides a rigid theoretical basis for the further studies on TRIP steel sheet forming.(2)Study on hardening behaviors and diffuse instability of TRIP steelsAs one of the parameters with describing the hardening behavior of materials, hardening rate, the slope of stress-strain curve, represents the strain strength level. Strain hardening exponent denotes the capability of uniformity deformation before necking of material. The value of strain hardening exponent not only reflects the plastic working range but also signifies correct selection of the material. Fracture is one of the key quality issues in the sheet forming process. In essence, the fracture is the result of plastic instability. Controlling plastic instability of the material is prerequisite to manufacture qualified products. Based on the constitutive equations in this thesis, the material properties of the hardening rate of TRIP steels, strain hardening exponent and plastic instability are explored under the usual condition, strain rate condition and pre-strain condition respectively.In the TRIP steels'plastic deformation process, the hardening rate and strain hardening exponent change with the different share of the retained austenite volume and the martensite volume which affected by the martensite transformation, namely, by strain rate and pre-strain. Therefore, the hardening rate and strain hardening exponent of TRIP steels are distinguished from those defined in traditional manner. Moreover, because of the volume change deriving from the martensite transformation in plastic deformation of TRIP steels, it is necessary to consider the impact of the volume change on the instability point while happening diffuse instability of TRIP steels. The instability point changes farther under the strain rate condition and the pre-strain condition respectively.(3)The influence of different forming condition on the formability of TRIP steels and deformation controlling in forming processIn order to make the effective use of TRIP steels in the engineering practice, this part aims at studying the effects of forming velocity loading paths and initial stamping depth in multi-step forming process on the minimum thickness of cupulate-part and springback angle of U-part on the basis of the constitutive relations established in the thesis while considering the mechanics properties in the TRIP steels'sheet forming process. Also, this part explores the effects of forming velocity loading paths and initial stamping depth in multi-step forming process on the sheet formability by hardening rate. Different position (such as punch radius, side, and die radius) in the forming sheet is found to have different distribution to the formability of TRIP steels. New methods are given here to control the formability better in TRIP steels sheet forming process.