Hae-Geon Lee

Precipitation behavior of MnS on oxide inclusions in Si/Mn deoxidized steel

The precipitation behavior of the MnS phase of MnO-SiO2 oxides in Si/Mn deoxidized steels was examined. MnS phase formation in the oxide phase was clearly identified, and the Mn content in both phases increased with isothermal holding at 1,200°C. With increased cooling rate, both the size of inclusions and the precipitation ratio of the MnS phase in the oxides decreased. More than 90% of MnO-SiO2 oxides contain MnS phases and MnS precipitation is accompanied by an Mn-depleted zone around the oxides. This zone was created not just around the MnS, but around the whole oxide inclusion. One can tentatively conclude that the majority of the MnS islands after isothermal holding are formed by the diffusion of Mn and S from the matrix into the inclusions.

Nature of large (Ti, Nb)(C, N) particles precipitated during the solidification of Ti, Nb HSLA steel

To investigate the microsegregation phenomena and complex (Ti, Nb)(C, N) precipitation behavior during continuous casting, a unidirectional solidification unit was employed to simulate the solidification process. The samples of Ti, Nb-addition steels after unidirectional solidification were examined using field emission scanning electron microscope (FE-SEM) and electron probe X-ray microanalyzer (EPMA). In such specimens, dendrite structure and mushy zone can be detected along the solidification direction. It shows that the addition of titanium, niobium to high-strength low-alloyed (HSLA) steel results in undesirable (Ti, Nb)(C, N) precipitation because of microsegregation. The effect of cooling rate on (Ti, Nb)(C, N) precipitation was investigated. The composition of large precipitates was determined using FE-SEM with EDS. Large (Ti, Nb)(C, N) precipitates could be divided into three kinds according to the composition and morphology. With the cooling rate increasing, Ti-rich (Ti, Nb)(C, N) precipitates are transformed to Nb-rich (Ti, Nb)(C, N) precipitates.

Dissolution behavior of alumina in mold fluxes for steel continuous casting

Dissolution of alumina in various mold fluxes for steel continuous casting has been investigated by employing the rotating cylinder method. The weight loss of the rod, the dipping area and the immersed time were measured to determine dissolution rate of Al2O3. The dissolution rate increased with temperature of molten fluxes, the rotating speed of the rod and the addition of MgO, CaF2 and Na2O components in the mold flux. When the Na2O content exceeded 5%mass, the dissolution rate was found to decrease. Intermediate compounds such as CaO·6Al2O3, CaO·2Al2O3 and 2CaO·Al2O·SiO2 formed at the Al2O3/flux interface and formation of three compounds was found to play important roles in the dissolution rate. In conclusion, the dissolution of Al2O3 was controlled not only by the mass transfer in the molten flux also by the formation of intermediate compounds on the interface.

Effect of Oxide Scale Formation on the Behaviour of Cu in Steel during High Temperature Oxidation in O2‐N2 and H2O‐N2 atmospheres

Cu enrichment at the steel-scale interface and its migration from there was investigated during the heating of steel cast at 1200°C under various oxidizing conditions. The behaviour of Cu enrichment was found to be largely dependent on the morphology of oxide scale formed during oxidation. At the early stage of oxidation, Cu-rich phase formed and accumulated at the steel-scale interface in both O2-N2 and H2O-N2 atmosphere. However, as the oxidation proceeded, the enrichment was vastly different for each oxidizing atmosphere. In the case of O2-N2 oxidation, an oxide layer was formed initially at the steel surface, but a gap was developed soon after at the steel-scale interface and grew in size, which practically separated the scale from the steel substrate. The scale layer formed under this condition was porous. The Cu-rich phase initially formed at the interface seemed to migrate to the scale layer, leaving no Cu-rich phase at the interface. In the case of H2O-N2 oxidation, however, the scale layer formed was dense and tightly attached to the steel surface, and the Cu rich-phase continued to accumulate at the interface. Regarding the behaviour of the Cu-rich phase formed at the interface, it is proposed with experimental evidences that, when a gap forms at the steel-scale interface, it is the vaporization of Cu in the Cu-rich phase through the gap that brings Cu to the scale.

Formation and Thermal Stability of Large Precipitates and Oxides in Titanium and Niobium Microalloyed Steel

As-cast CC slabs of microalloyed steels are prone to surface and subsurface cracking. Precipitation phenomena initiated during solidification reduce ductility at high temperature. The unidirectional solidification unit is employed to simulate the solidification process during continuous casting. Precipitation behavior and thermal stability are systematically investigated. Samples of adding titanium and niobium to steels have been examined using field emission scanning electron microscope (FE-SEM), electron probe X-ray microanalyzer (EPMA), and transmission electron microscope (TEM). It has been found that the addition of titanium and niobium to high-strength low-alloyed (HSLA) steel resulted in undesirable large precipitation in the steels, i. e., precipitation of large precipitates with various morphologies. The composition of the large precipitates has been determined. The effect of cooling rate on (Ti, Nb), (C, N) precipitate formation is investigated. With increasing the cooling rate, titanium-rich (Ti, Nb) (C, N) precipitates are transformed to niobium-rich (Ti, Nb) (C,N) precipitates. The thermal stability of these large precipitates and oxides have been assessed by carrying out various heat treatments such as holding and quenching from temperature at 800 and 1 200 °C. It has been found that titanium-rich (Ti, Nb)(C, N) precipitate is stable at about 1 200 °C and niobium-rich (Ti, Nb)(C, N) precipitate is stable at about 800 %. After reheating at 1 200 °C for 1 h, (Ca, Mn)S and TiN are precipitated from Ca-Al oxide. However, during reheating at 800 °C for 1 h, Ca-Al-Ti oxide in specimens was stable. The thermodynamic calculation of simulating the thermal process is employed. The calculation results are in good agreement with the experimental results.

Effect of alumina content and solid phase in molten flux on dissolution of alumina

A dissolution experiment of alumina in mold flux for the continuoss casting of steel was conducted using the rotating cylinder method at 1550 °C. The weight loss of the rod, the initial dipping area, and the immersion time were measured to determine the dissolution rate of Al2O3. It was concluded that the dissolution rate decreased with increasing Al2O3 content, but increased to a great extent when the solid phase of 2CaO·SiO2 existed together with the liquid in the molten flux. The dissolution rate was found to be linearly proportional to the concentration driving force and the viscosity, [(mass%Al2O3)s-(mass%Al2O3)b]η−0.988. The physical erosion of the rod surface by the solid 2CaO·SiO2 dispersed in the liquid was attributed to fast alumina dissolution. An intermediate compound of CaO·2Al2O3 was observed on the interface of Al2O3 rods after the dissolution experiment. In addition, the dissolution of alumina in industrial mold fluxes has been examined.

Reactions in the Tuyere Zone of Ironmaking Blast Furnace

A series of slags can be formed in the lower part of the ironmaking blast furnace that play important roles in smooth furnace operation, and in determining iron quality and productivity. The final slag tapped from the BF has been investigated extensively as it can be collected directly. Unfortunately, difficulties in accessing the interiors of the blast furnace limit the full understanding of other slags such as primary and bosh slags. In this study, different types of samples directly obtained from the tuyere zone of the blast furnace have been systematically analyzed and characterized using scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA), and X-ray fluorescence (XRF), with focus on the characteristics of slags formed in the tuyere level. The samples were identified into three groups according to their morphological, mineralogical, and chemical properties: (1) tuyere slags originating from the reactions between ash and dripping slags; (2) bosh slags in the CaO-SiO2-Al2O3-MgO-FeO system, with a CaO/SiO2 weight ratio of around 1.50, and Al2O3 and MgO concentrations close to those of final slags; and (3) coke ash that did not react with bosh slags. These findings will provide useful information on the evaluation of slags inside the blast furnace and the reactions in the tuyere zone.

In-situ X-ray fluoroscopic observation for motion of bubbles in liquid iron for correction of drag coefficient used in numerical simulation

Rising of Ar bubble in C-saturated liquid iron was investigated in-situ by employing a high power X-Ray fluoroscope (maximum power of 450 kV and 4.5 mA) coupled with a high speed camera. This analysis enabled to track the actual motion of rising bubble in the liquid iron. After post-processing of X-Ray images, size, shape, velocity, and trajectory of the bubble were obtained. The bubbles were found to be not spherical, but ellipsoidal. Their average size could be estimated about 1.1×10-2 m. The bubbles wobbled during rising and arrived at their terminal velocities within 0.1 sec. The obtained terminal velocities revealed that the governing forces acting on the motion of ellipsoidal bubble were inertia and surface force. This was quite different from that of spherical bubble which was widely used in the assumption for the numerical analysis. As a result, widely applied equation for the drag coefficient (CD = 24 (1 + 0.15Re0.687) / Re) is seen to be applicable only for low Re regime, and this was also confirmed by the drag coefficient derived from the present experimental observation. Therefore, it is suggested to use the following equation for the drag coefficient CD = max [24 (1 + 0.15Re0.687) / Re, 8Eo / 3(Eo + 4)]. Numerical simulation for the Ar bubble behavior in the three dimensional (3D) continuous casting mold was conducted in order to evaluate the effect of the drag coefficient for the behavior of spherical and ellipsoidal bubbles. The numerical results showed that the increased CD based on ellipsoidal regime affected the entire fluid in the mold.

Erratum to: Thermodynamics of MnO-SiO2-Al2O3-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

YE-JIN KIM, Graduate Student, HENRI GAYE, formerly Professor, HAE-GEON LEE, Professor, and YOUN-BAE KANG, Assistant Professor, are with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology, Pohang, Republic of Korea. Contact e-mail: ybkang@postech.ac.kr DAE-HEE WOO, formerly Graduate Student, Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology, Pohang, Republic of Korea, is now with Technical Research Laboratories, POSCO Ltd, Kwangyang, Cheonnam, Republic of Korea. The online version of the original article can be found under doi: 10.1007/s11663-011-9500-y. Article published online October 16, 2013.