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Fundamental Research on Precipitation of Ultrafine TiO_x Inclusions during Solidification of Steel

Author: LiHuiGai
Tutor: DiQiJie；ZhengShaoBo
School: Shanghai University
Course: Iron and steel metallurgy
Keywords: Fine inclusions Titanium oxide X80 pipeline steel Numerical simulation of coupled model Directional solidification
CLC: TG142.15
Type: PhD thesis
Year: 2008
Downloads: 500
Quote: 1
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Abstract


Nonmetallic inclusions often cause the steel surface and internal defects, but the current production process inclusions in steel can not be removed completely. Studies show that: when inclusion is less than 3μm, species appropriate and uniform distribution can improve the performance of the steel toughness. Titanium-based composite oxide of manganese and titanium nitride, sulfide inclusions are likely to cause a small intragranular ferrite (IGF), grain refinement, improved strength and toughness of steel. Hence, the control liquid steel titanium oxide precipitates to make it smaller than 3μm, and dispersed in the steel become the focus of research. As the molten steel during solidification inclusions precipitated relatively small size, the paper systematically studied during solidification of liquid steel ultrafine titanium oxide precipitates law, and through theoretical calculations and experimental validation of the directional solidification can be precipitated during solidification fine, dispersed titanium oxide inclusions. Firstly, from a thermodynamic calculation of the control of oxides of titanium precipitated in the solidification process and the thermodynamic conditions required to generate the type of titanium oxide. IGF titanium oxide easily become the core, in order to precipitate the titanium oxide of the steel must be controlled in the liquid steel titanium aluminum ratio. In the solidification temperature range, Ti oxides of Ti 3 O 5 is the most stable. When the molten steel temperature is 1873K, the Ti content in molten steel is 0.02%, the aluminum concentration should be less than 34ppm. Oxygen content before solidification is not the same, the liquid steel [Mn] may affect the type of titanium oxide inclusions. Equilibrium oxygen content is 30ppm, the inclusions of type manganese-titanium composite oxide; balance 20ppm oxygen content, the solidified pre-generated titanium oxide, manganese-titanium composite oxide post-build; oxygen content generated only titanium 10ppm oxide. Given titanium oxides Ti 3 O 5 thermodynamically most stable, the paper calculated theoretically using mathematical models of different oxygen content during solidification and precipitation under different cooling rates Ti 3 O 5 content and diameter. The steel solidification process, because the liquid and solid phases exist differences in solubility occurs micro segregation of solute elements, reduction of temperature during solidification, a large number of solute concentration in the solidification front, improved [Ti] and [O] of Calculated using the finite difference method to accomplish the computational domain is secondary dendrite arm spacing half of solute mass transfer controlled by diffusion only. Model in the calculation of the micro-segregation of solute elements taking into account Ti 3 O 5 of precipitation on the consumption of solute concentration, so that the microscopic solute segregation and Ti 3 O 5 more reasonable calculation of the amount of precipitation. By numerical simulation showed that: solidification of molten steel before the higher oxygen content in the resulting Ti 3 O 5 larger diameter: the greater the cooling rate, Ti 3 < / sub> O 5 diameter is smaller. When the cooling rate is 10K/min time, which corresponds to the actual production, the equivalent of molded cooling rate, precipitated during solidification of Ti 3 O 5 size is about 2.5 μm. When the cooling rate is 1000K/min (thin slab casting and cooling rate), Ti 3 O 5 size of 0.7μm, very small. Theoretically proved that the solidification process can form ultrafine titanium oxide. Based on theoretical analysis, this paper uses directional solidification experimental study of oxygen-bit, cooling rate during solidification inclusions precipitated number, type and size. Directional solidification experiments solute elements along the solidification direction one-dimensional diffusion, resulting in the enrichment of the solidification front solidification front inclusions in the liquid phase precipitation. Experiments were added ferromanganese, ferrosilicon, ferromanganese ilmenite or / ferrosilicon, ferromanganese / titanium oxide mixture of different precipitation during solidification law. Experimental results show that were added manganese, silicon, and silicon - manganese element, generated during solidification inclusions size of 1.3μm, 1.2μm, 1.1μm. Add manganese, the largest number of inclusions, when the least number of silicon added were 1.1 × 10 5 / mm 3 and 5.8 × 10 4 / mm 3 . Adding manganese, titanium element generated after the type of inclusions before solidification by the content of oxygen in the oxygen content 10ppm equilibrium, are precipitated during solidification of titanium oxide; 20ppm oxygen content of the resulting titanium oxide inclusions materials and manganese-titanium composite oxide; 30ppm oxygen content of manganese inclusions generated titanium composite oxide. Titanium oxide dimensions greatly influenced by the cooling rate. The cooling rate, the greater the average size is smaller. When the cooling rate is 9K/min, 89K/min and 143K/min, the average diameters of 2.0μm, 1.2μm and 1.0μm. The experimental procedure that can form fine solidification inclusions. For the inclusion of existing detection methods, we propose a method for detecting fine inclusions, extracted sample directional solidification experiments of ultrafine titanium oxide. That is, the acid separation; ultrafine pore size polycarbonate membrane filter extract inclusions; high resolution scanning electron microscopy and transmission electron microscopy detection. Obtain a three-dimensional ultra-fine inclusions morphology, size, chemical composition and structure of such a wealth of valuable information. This provides further analysis of its crystal growth habit, so as to control these inclusions in steel forms the foundation. Inclusions in steel by structural analysis found no manganese steels of the type titanium oxide Ti 3 O 5 : and when the manganese steel, First precipitated during solidification Ti 3 O 5 , then Ti 3 O 5 and steel [Mn] play a role in generating Ti 2 O 3 and MnO ultrafine composite oxide, manganese as MnS composite and depleted zones to create favorable conditions for the formation and Ti 2 O 3 Mn depleted zone is formed around the IGF favor nucleation. Improve the formation mechanism of the IOF.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Steel > Microstructure and properties of steel > Analysis of the steel test
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