The Formation Mechanisms and Evolution of Multi-Phase Inclusions in Ti-Ca Deoxidized Offshore Structural Steel

Abstract

To understand and clarify the formation mechanisms and evolution of complex inclusions in Ti-Ca deoxidized offshore structural steel, inclusions in industrial steel were systematically investigated. The number density of total inclusions generally decreased from Ladle Furnace (LF), Vacuum Degassing (VD), Tundish to the final product except for Ti and Ca addition. The major inclusions during the refining process were CaO-Al2O3-SiO2-(MgO)-TiOx and CaO-Al2O3-SiO2. CaO-Al2O3-SiO2-(MgO)-TiOx inclusion initially originated from the combination of CaO-SiO2-(MgO) in refining slag or refractory and deoxidization product Al2O3 and TiO2. With the refining process proceeding and Ca addition, the Al2O3 concentration in the CaO-Al2O3-SiO2-(MgO)-TiOx inclusions gradually dropped while the CaO and TiO2 concentrations gradually increased. The CaO-Al2O3-SiO2 inclusions originally came from refining slag, existing as 2CaO∙ Al2O3∙ SiO2, and maintained a liquid state during the early stage of LF. After Ca treatment, it was gradually transferred to 2CaO∙ SiO2 due to Al2O3 continuously being reduced by Ca. The liquidus of 2CaO∙ SiO2 inclusion was higher than that of molten steel, so they presented as a solid-state during the refining process. After welding thermal simulation, CaO-Al2O3-SiO2-(MgO)-TiOx inclusions were proven effective for inducing intragranular acicular ferrite (IAF) while CaO-Al2O3-SiO2 was inert for IAF promotion. Additionally, Al2O3-MgO spinel in multiphase CaO-Al2O3-SiO2-(MgO)-TiOx inclusion has different formation mechanisms: (1) initial formation as individual Al2O3-MgO spinel as a solid-state in molten steel; (2) and it presented as a part of liquid inclusion CaO-Al2O3-SiO2-(MgO)-TiOx and firstly precipitated due to its low solubility.