| The development trend of modern iron & steel materials is focused on how to improve the mechanical properties and raise the service life, based on the currently available material, by the ways of controlling the melting processes to reduce the content of harmful elements, controlling the processes of casting to reduce the segregation, and controlling the processes of hot-working to refine the microstructure. The pipeline steels with good match of strength and toughness for long distance oil and gas transportation is a key to ensure the security and reduce the operation cost, for which the X80 grade acicular ferrite pipeline steel will be extensively applied in the future. In this thesis, the discrimination method for acicular ferrite in pipeline steel, control and refinement acicular ferrite microstructure for clean steels that were developed based on the chemical composition of the commercial X60 grade pipeline steels bas been investigated by study on the phase transformation kinetics. The aim is to explore the key technology for improvement of mechanical properties of acicular ferrite pipeline steels.The continuous cooling phase transition kinetics and isothermal phase transition kinetics for low-carbon microalled steels have been constructed. There are three independent "C" curves in the TTT diagrams for low-carbon microallyed steel, polygonal ferrites/pearlite "C" curve, the mass ferrite "C" curve and the bainitic "C" curve, respectively. The mass ferritic "C" curve is overlapped with "C" curves of polygonal ferrite and bainite. The width of transformation region for mass ferrite is related with amount of carbon and alloying elements. The transformation region of mass ferrite increases with reduction of the amount of carbon in steels. Mo reduces the carbon activity, which results in increase of incubation period of mass ferrite.According to the phase transformation kinetics of low-carbon microallyed pipeline steels and the microstructure analysis of rolling plate, the acicular ferrite in pipeline steels has been defined as the mixted microstructure, under a continuous cooling condition, with mass ferrite, granular ferrite and bainite ferrite, in which some islands structure is contained. The microstructure characteristic of acicular ferrite in pipeline steels is the unregular and non equiaxed ferrite masses which are without clear ferrite grain boundary and some of which contain random arranged or regularly islands.The hot deformation behavior, static recrystallization and strain induced carbide precipitation for two test steels, steel B and steel E, have been studied by using Gleeble-2000 and Gleeble-3500 thermal simulation tests. The dynamic recrystallization activation energy and the static recrystallizational activation energy have been determined, respectively. The strain induced carbide precipitation curves (PTT curves) have also been determined.The dynamic phase transformation kinetics curves of the test steels on different deformation conditions have been constructed. The results show that the acicular ferrite phase transformation and polygonal ferrite phase transformation are accelerated observably, and the beginning point of phase transformation for acicular ferrite is raised notablely. The effect on acicular ferrite phase transformation point depends mainly on the amount of deformation and deformation temperature in the noncrystallization region. For deformation in the dual phase region deforming, the ferrite is observed at austenite grain boundary in steel B, which makes the acicular ferrite phase transformation point decrease. The influence of rolling parameters on microstructure and refinement has been studied by using Gleeble-3500 thermal simulation test. the results show that the best effect of controlling and refinement for acicular ferrite microstructure could be achieved by all controlling hot working processes, namely, accurate controlling the parameters of reheat temperature, rough rolling, finish rolling and cooling rate. The clean steel based on the chemical c... |
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