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Abstract: High-strength steels have been widely applied to automotive chassis parts. In order to form complex shapes, high hole expansion rates and high formability are required. Dual phase (DP) steel has a good formability, but a poor hole expansion rate. In this circumstance, another kind of steel which has a microstructure of ferrite-bainite, rather than ferrite-martensite, has been found to be an alternative solution. It is called FB steel. This steel with Si,C and Mn additions are applied in this study. A two-step cooling process is used to get the desired F + B microstructures. Continuous cooling transformation (CCT) diagrams are made with deformation and without deformation, and starting times and temperatures of the phase transformations of interest are obtained. It is shown that Si,C and Mn contents in the steel strongly affect the shapes and positions of the CCT diagrams, as well as the final microstructures of FB steel. An increase of the Si content can promote the formation of ferrite and move the CCT diagram toward the left. However, when Si content is too high, when comparing to carbon and manganese contents, the formation of bainite will be retarded because of the formation of more ferrite. It increases the amount of C in a solid solution in the untransformed austenite and promotes the formation of pearlite. C and Mn can inhibit the formation of ferrite and retard the accumulation of C in austenite. Therefore, the appropriate balance of C,Si and Mn contents in steels will be able to help in obtaining the desired microstructure.
Key words : FB steel ; microstructure ; CCT diagram
(ProQuest: ... denotes formulae omitted.)
1 Introduction
There have been increasing global concerns over environmental and safety issues with motor vehicles. Consumers require more economic vehicles with a higher standard of collision safety, which means that vehicles have to be lighter and stronger. In order to meeting this requirement, some new lightweight materials such as aluminum, magnesium and plastics are being used in vehicles. However, there are still some unsolved problems, such as formability, reliability, and recyclability-as well as cost-that need to be addressed. Therefore, it will be some time before alternative materials can be used to replace steels as structural components in vehicles.
In recent years, many...