| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 107 === By alloying 1.5-2.0 wt% silicon in the steels and designing appropriate heat treatments, carbide-free bainite can be produced. This structure is consisted of plate-liked bainite and high carbon content retained austenite plus some blocky martensite. The steels with such structure include outstanding strength and toughness. However, as carbon content increases, the reaction rate reduces and the time for completion bainite transformation last longer. Therefore, alloying elements play an important role in affecting the properties of bainitic steels. In addition, by modifying the heat treatments can also reach the goal in decreasing the reaction time, such as two-step isothermal transformation. In this research, we are focusing on investigating the microstructure and mechanical properties in bainite by changing the amount of carbon and after undergoing two-step isothermal transformation. When the carbon content raises, overall reaction temperature will be lower, which causes the reaction time increase and the structure refinement. The quantity of retained austenite will also increase since the carbon rejected by bainite raised. The effects mention above lead to the mechanical properties’ enhancement. TEM and EBSD analysis indicate that only high carbon film-liked austenite will appear at ambient temperature. Moreover, by utilizing two-step isothermal transformation, which we put the specimen at a slightly higher temperature for a while to form some bainite, and then cool to lower temperature to complete the reaction can effectively shorten the time for nucleation and leads to the acceleration of low temperature bainite reaction. Furthermore, the contribution of this heat treatment will form different size of bainite and hence results in the reduction of strength and hardness.
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