To examine the e®ect of processing parameters on microstructural evolution and to obtain the excellent combination of strength and toughness, simulation of thermo-mechanical processing was conducted using the Gleeble machine. Trial production was then conducted under the conditions obtained by Gleeble tests.Based on the results of microstructure analysis and mechanical property evaluation, the relationship between microstruc-tural features and mechanical properties was elucidated. The result shows that the volume fraction of constituted phases can be controlled through adjusting the cooling rate and ¯nish cooling temperature in order to get di®erent strength levels. As cooling rate increases, the volume fraction of upper bainite increases, which leads to the increase of strength. The upper shelf energy (USE) increases with increasing volume fraction of acicular ferrite in bainite base because of the small e®ective acicular ferrite grain size.Ductile-brittle transition temperature (DBTT) decreases with increasing acicular ferrite volume fraction. High reduction in the rough stage has great in°uence on grain re¯nement.
1. Introduction
Linepipe steel is used to transport natural gas and oil from remotely located sources. Until now a primary interest has been in obtaining higher strength in order to improve the transportation effciency. However, as the application of linepipe steel expanded to frontier reserve areas such as arctic regions or the deep sea, improvement of toughness without decreasing strength became more important. The ultra high strength linepipe steel consists mostly of bainite or martensite, which has poor toughness. Some researchers have proposed that lower bainite might be helpful to improve the toughness of high strength linepipe steels. However, it is very diffcult to obtain such microstructure in practical rolling processes because of the extremely limited phase-eld in the continuous cooling transformation(CCT) diagram. In current study, improvement in toughness by the formation of acicular ferrite withinbainitic structure is considered. To examine the e®ect of processing parameters on microstructural evolution and to obtain adequate rolling conditions for the optimum microstructure with the excellent combination of strength and toughness, simulation of thermomechanical processing was conducted using a Gleeble machine.
Thermomechanical treatment (i:e:, rolling)was also conducted in pilot plant scale under the con-ditions obtained by Gleeble tests. Based on the results of microstructure analysis and property evaluation, the relationship between microstructural features and mechanical properties was elucidated in the present study.
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