Author: Li, Fangfang
Title: The relationship between microstructural evolution and mechanical behavior of boron steel 22MnB5 in hot stamping process / Li Fangfang.
Advisors: Fu, Mingwang (ME)
Degree: Ph.D.
Year: 2017
Subject: Boron steel.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: ix, 137 pages : color illustrations
Language: English
Abstract: With increasing requirements of vehicle safety and weight reduction, the performance customization of the mechanical properties of auto-body components becomes more and more important. It can be realized via the design of tailored mechanical properties, but it raises challenges from manufacturing and realization perspectives in tandem with this kind of design. It is well known that the mechanical properties of parts and components are determined by the chemical composition and microstructure of the materials of which the parts and components are made, and the microstructure of the deformed parts is further influenced by the forming process and its process parameters. In order to achieve the performance customization of hot stamped components and satisfactory realization of their desirable performance, the study on the relationship among microstructure and microstructural evolution, forming process and process parameters, the formability of the material and the mechanical and deformation behaviors of the generated design must thus be conducted; this has been well aware and recognized to be a non-trivial and tantalized issue in realization of performance customization. This is therefore chosen as the research focus in this PhD study.
In the first stage of this research, the kinetic equation of martensitic transformation of boron steel 22MnB5 considering deformation parameters and the kinetic equations of bainitic and ferritic transformations taking into account the start temperature of phase transformation are established through the experimental and theoretical analysis of microstructural evolution. With these developed equations, the different phase fractions in the deformed parts can be controlled by configuration of forming process and process parameters. In the second stage, the relationship among phase fraction and grain size after microstructural evolution, forming process and process parameters, and the formability of the drawn cupped parts in deep drawing process at elevated temperature is analyzed and established. The improvement method of formability of boron steel is obtained accordingly, and the constraints on the microstructure of boron steel based on its formability at elevated temperature are established. From the application perspective, the wide application of the performance customized components in industries is limited by the design capability as the present design methodology is not fully suitable for the design of the parts and components with tailored mechanical properties. In the last stage, the efficiency of B-pillar crash simulation model used to establish the relationship between microstructure and mechanical properties of B-pillar is validated by the crash experiment, and the design methods of B-pillar with the tailored mechanical properties are thus proposed and implemented based on the established relationship.
Rights: All rights reserved
Access: open access

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