| With the improving of strength and toughness of pipeline steel, welding performance of high grade pipeline steel has become the criterion of the quality of pipeline steel. Since the conception of oxide metallurgy has been advanced by Nippon Steel, Ti-deoxidized products draw the attention and are studied in the metallurgy and material academic community. It has been accepted that IGF induced by the inclusions can maintain HAZ in a good condition of strength and toughness to improve the weldability of steel. But at present there are still controversies in the mechanism of oxide metallurgy and the phase-component of titanium oxides (mainly including TiO, TiO2, Ti2O3 and Ti3O5). Due to the technical monopoly from Nippon Steel and some other enterprises, the mechanism and technology development about oxides metallurgy are improved slowly. To promote this technology to apply in the industry as soon as possible, it is necessary to recognize the deoxidization process and products comprehensively, through which the smelting process can be controlled precisely in the base of mastering the forming law of IGF.Referring to a great deal of the literatures, this research has first calculated the thermodynamics and dynamics in deoxidization process controlling and precipitating products from high grade pipeline steel and then carried out this high grade pipeline steel smelting in laboratory. It has been made a conscientious study of the following: the deoxidization technology and oxygen potential controlling in high grade pipeline steel, the law of increasing oxygen in molten steel from decomposed refractory material in vacuum, the thermodynamical and dynamical factors of precipitating Ti2O3 which is favorable in forming IGF, the size, composition, shape and appearance, inner structure and phase composition of Ti-deoxidized products, the influence to coagulation structure from the Ti-deoxidized products, the mechanism of oxide metallurgy, research method of micron-sized inclusions, etc.By calculating the thermodynamics of precipitating Ti-oxidized products, it is known that Ti2O3 will always precipitate before other Ti-oxidized products and it is favorable to precipitate Ti2O3 in the form of particles with the oxygen content below 0.002%. And it has been determined to use Al-free deoxidization technology (with carbon-deoxidization first and Ti-deoxidization finally in vacuum) through analyzing the theory of competition in deoxidization between Ti and Al. Studying the precipitating competition between Ti2O3 and TiN in liquid-solid phase region at smelting temperature, it indicates that Ti2O3 is always precipitated preferentially than TiN in different Ti content level with the initial nitrogen content below 0.003%. By modeling the dynamics of precipitating Ti-deoxidization products, it also shows that getting tiny inclusions shall be in a high cooling rate first and let the oxygen content in a proper level.The theory of oxygen contamination from MgO and CaO crucible material to molten steel in smelting with carbon-deoxidization in vacuum indicates that if the practical oxygen content in molten steel is higher than the equilibrium oxygen content when the crucible material decomposing reaction reaches to equilibrium, crucible material will not supply oxygen to molten steel; and if the content is lower, it will supply to molten steel. Compared with these two crucible materials, the stability of CaO material is stronger. The carbon-deoxidization research has been carried out in vacuum in laboratory. The oxygen content in molten steel can reduce to 0.001%~0.002%, which can also meet the requirement of carbon content in high grade pipeline steel (0.04%~0.06%). By observing the outer shape and appearance of the Ti-deoxidization precipitating products extracted from metallographic and electrolyzation specimen, the main Ti-deoxidization products are 1~2μm spherical compound inclusions coming from Ti-O-Mn-S system. Following the reducing oxygen content the amount of the inclusions in steel reduces. But the average size first decreases then increases, which are showed by the statistic of the inclusion particle distribution. The observing the inner appearance, structure, phase composition of the superfine inclusions by SEM, EDS and EBSD have indicated that with the oxygen content controlled below 50ppm the inclusions are all composed by Ti2O3 which is favorable for nucleating IGF and also by MnS of which the nucleation core is Ti2O3 in cooling. The selected area electron diffraction under TEM has proved the analysis results from EBSD. The following are the new ideas of micron-sized inner structure and phase composition of inclusions. Ti-deoxidization steel is composed of IGF of which the microstructure refinement will be smaller with the increasing Ti content. The inclusions inducing IGF are Ti2O3, MnS and a few of compound inclusions containingAl2O3. Also tiny-sized MnS can promote to form IGF. The size of the inclusions inducing IGF is 1~3μm and the width and length of the IGF are 1~3μm and 5~15μm. From the above it is showed that the favorable condition of producing great deals of compound inclusions with Ti2O3 as the nucleation core which can induce to nucleate IGF are the Ti content in 0.015%~0.02% and oxygen content at 0.002%. Compared with the structure of air cooling and quenching, forming IGF in Ti-oxidization is related not only to the inclusion's natural property but also to the cooling rate. By analyzing the element contribution by STM from metallographic specimen and RTO it has been indicated that Ti and O exist mainly in the middle of the inclusions while S is distributed over the outer. Referring to the distribution of Mn, Mn exists both inner and outer. S exist the most part of the outer layer, which is to say the inclusions contain Mn-depleted zone (MDZ) in itself. Besides Ti2O3 and MnS by analyzing Raman Spectra The inclusions in Ti-deoxidization steel still contain MnTiO3. This research has been developed and completed the MDZ mechanism of Ti2O3 nucleating IGF by studying Ti-deoxidization products in depth.
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