• Olasolo, Maider

Abstract

This thesis is the result of several research projects whose main objective was to optimize the production of low carbon Nb microalloyed steel plates, by means of thin slab casting and direct rolling technology, used for the fabrication of high strength line pipes for oil and gas transportation (API). In order to maintain the confidentiality demanded by the industrial partner only the work done in the laboratory has been published.<br/><br/>Therefore, the most relevant results obtained from the cooling cycles and coiling simulation tests performed at the laboratory are presented here. These simulations are based on two of the main controlled rolling processes: Recrystallization Controlled Rolling (RCR) and Conventional Controlled Rolling (CCR), that is, with recrystallized and deformed prior austenite grains before transformation, respectively.<br/><br/>The transformation products obtained from continuous cooling and coiling simulations of a Nb-V and a Nb- Mo microalloyed low carbon steels have been analysed. These transformation products were analyzed using different techniques: optical microscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), and analyzing the most representative microstructures using EBSD / OIMTM technique (Electron backscattered Diffaction / Orientation Imaging MicroscopyTM). All samples have been characterized mechanically by hardness tests and tensile test in some selected samples.<br/><br/>Regarding the continuous cooling cycle simulations, the effect of test conditions on the transformation products has been studied, as well as the final mechanical properties. Specifically, the parameters analyzed are: the cooling rate, the accumulated deformation and the microalloying. As discussed in the text of this thesis, all these parameters affect the microstructure (grain size, phase fraction, dislocation density, misorientation angle, fraction and type of grain boundaries, etc..), which in turn directly affects the final hardness.<br/><br/>It should be emphasized that the accumulated strain in the prior austenite affects both the reconstructive and displace transformations. The accumulation of strain raises the number of nucleation sites, and this increase in the nucleation sites has its influence on the phase balance and grain size of the transformation products.<br/><br/>Regarding the coiling simulations, the effect of different parameters, such as, the cooling rate prior to coiling, the coiling temperature and the accumulation of strain on different aspects of the microstructure, microalloying elements precipitation and final mechanical properties has been studied.

Topics

• density
• impedance spectroscopy
• Carbon
• grain
• grain size
• phase
• scanning electron microscopy
• simulation
• strength
• steel
• hardness
• transmission electron microscopy
• dislocation
• precipitation
• casting
• electron backscatter diffraction
• optical microscopy
• recrystallization