博士 / 國立臺灣大學 / 材料科學與工程學研究所 / 104 / Ultra high strength offshore steel is one of high strength low alloy steel (HSLA) which possessed high Y.S (≥ 690 MPa) by TMCP process. According to Norsok S690Q standard, the average of low temperature (-40 °C) impact energy is higher than 100 J and one of other is must higher than 70 J. There are four chemical compositions and two rolled ratio (OS1 & OS2 are 67 %, OS3 & OS4 are 80 %) in this study. The investigation of low temperature impact toughness used tempered offshore steels which were manufactured by TMCP and then carried on temper heat treatment. The result showed that offshore steel with 80 % thickness reduction possessed excellent low temperature impact toughness. In addition, the structure of surface (tempered martensite) and center (tempered martensite and bainite) were studied. It is tempered martensite that revealed good low temperature toughness and lower ductile to brittle transition temperature (DBTT). EBSD crystal orientation map of tempered martensite was examined which showed its grain boundaries and sub-structure were high-angle boundaries.
Two 80 % thickness reduction specimens were conducted by austempered heat treatment (430 °C and 490 °C ) to investigate low temperature impact toughness of upper and lower bainite which could be provided for welding process. The impact energy of austempered treatment showed that the toughness of lower bainite is higher than upper bainite. Because of the increasing amount of lower-angle boundaries and uncompleted transformation of retained austenite which formed MA constituent, toughness of upper bainite was degenerated which was examined by EBSD crystal orientation map. MA constituent located at grain boundaries and interface of sub-structure due to its hard and brittle which degenerated low temperature toughness of upper bainite. Fractagraphy of upper bainite revealed brittle quasi-cleavage. However, it results that adding Mo and V in steel promoted formation of MA constituent. According to microscopic examination, there is MA constituent in OS4 specimen which was carried out 430 °C austempered treatment. After 490 °C austempered treatment, the amount of MA constituent were increased. Due to element effect which promoted formation of MA constituent, the impact toughness of OS4 specimens were lower than OS3 specimens for identical austempered treatment.
In addition to experiment, the cooling process of TMCP was simulated by Simufact software. Simufact software which calculated metallurgical properties according to database of JMatPro simulated the ratio of phase and retained stress. The parameters of simulation were divided into ideal state and nonideal state that is the cooling system of top and bottom whether identity or not. The result of simulation showed non-ideal state caused steel deformation by TMCP.
| Identifer | oai:union.ndltd.org:TW/104NTU05159019 |
| Date | January 2016 |
| Creators | Chieh Yu, 于傑 |
| Contributors | 薛人愷 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 100 |
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