| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 97 === To support the recent construction and transportation industries, a high strength steel plate associated with the high heat input welding technique has been developed. By alloy design, controlling austenite grain size and desired microstructure, the toughness in heat affected zone has been largely improved.
The high heat input welded steel plate is produced by China Steel Corporation.In one aspect, the purpose of this study is to understand the relationship among microstructure of welded zone, microstructure of heat affected zone and the excellent mechanical properties after welding. In the other aspect, this study aims to elucidate the transformation mechanism of intragranular ferrite(IGF) in the heat affected zone.
The microstructure of welded and heat affected zone have been investigated by optical microscope and transmission electron microscope. The acicular ferrite in welded zone and intragranular ferrite in heat affected zone both have high dislocation density. By addition of Ti, Nb, B, the prior austenite grain size can be refined to 500μm. Besides, the amount of widmenstatten ferrite can be reduced and further increase the amount of Intragranular ferrite by the present alloy design. Such a fine microstructure in the heated affected zone achieves excellent toughness superior to the toughness in welded zone.
In order to study the transformation mechanism of intragranular ferrite, Gleeble machine was used to simulate the practical welding condition and the microstructure was studied by step quenching method using OM and TEM. During the cooling process, the complex carbonitrides (Ti,Nb)(C,N) precipitated from austenite grain with the cube-to-cube orientation relationship. These carbonitrides act as the nucleation sites for intragranular ferrite. The intragranular ferrite precipitating on the carbonitrides with the Baker-Nutting orientation relationship is called primary IGF. The following Intragranular ferrite called secondary IGF can transform by sympathetic nucleation and adopt K-S orientation relationship with the prior austenite grain.
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