Shufeng Yang

Motion Behavior of Nonmetal Inclusions at the Interface of Steel and Slag. Part II: Model Application and Discussion

Based on a mathematical model for the dynamics of inclusions at the steel-slag interface, this study discusses the influences of interfacial tension, inclusion diameter, and slag viscosity on the movement and removal of inclusions at the interface. The results show that (1) the greater the interfacial tension, the smaller the rate of displacement that inclusions move and the longer the separations time needed; when inclusions fully permeate the slag phase, residence and separation times at the interface will lengthen as inclusion diameter increases. For very high interfacial tension, inclusions cannot enter the slag layer; instead, they oscillate across the interface between molten steel and slag phases. The greater the inclusion size, the more pronounced are the oscillations. (2) The interfacial tension shows no effect on the rebounding of inclusions, but only impacts the separation and removal of inclusions. (3) With increasing slag viscosity, the velocity with which inclusions enter the slag layer slows; as a result, the displacement in the slag layer shortens, until inclusions are able to separate and enter the slag layer. Under this condition, slag viscosity has no effect on the maximum displacement of inclusions in the slag layer.

Motion Behavior of Nonmetallic Inclusions at the Interface of Steel and Slag. Part I: Model Development, Validation, and Preliminary Analysis

The motion behavior of nonmetallic inclusions at the interface of molten steel and slag fundamentally affects the removal of inclusions. Therefore, from an analysis of forces, this study constructed a mathematical model of inclusion movement. Compared with other models that only consider the forces acting on nonmetallic inclusions at the interface, the proposed model considers not only cases in the inclusions which enter the slag interior and rebound into the molten steel, but also the effect of fluid flow containing the inclusions with different Re numbers on the drag force. The application of this established model has not taken Reynolds number of fluid flow into consideration. The model can predict the motion of inclusions at the interface and in nearby areas and provide a curve of inclusion displacement vs time. The mathematical model was verified with a physical model, with the curve of displacement vs time obtained from physical experiment being consistent with the calculated curve. The preliminary calculation results show that inclusions having liquid film at their surfaces are rebounded into the steel when they have size within a certain range but enter the slag phase directly when they are beyond that size range.

Addition of MgO Nanoparticles to Carbon Structural Steel and the Effect on Inclusion Characteristics and Microstructure

An innovative approach for pre-dispersing MgO nanoparticles with AlSi alloy nanoparticles was established, and the nanoparticles were dispersed well in carbon structural steel. After adding different mass fractions of MgO nanoparticles in steel, the majority of inclusions contained MgO·Al2O3 spinel and MgO-Al2O3-bearing hybrid inclusion, and these inclusions promoted acicular ferrite (AF) formation. With increasing amount of added nanoparticles, the average inclusion size increased from 0.90 to 1.50 μm and the inclusion size was considerably refined, but the ability of inclusions to induce AF was greatly declined. It was revealed that the inclusion size was the decisive factor influencing the inducing ability of inclusions for AF, which also got a solid support from the nucleation thermomechanical and dynamic analyses. When the mass fraction of MgO nanoparticles reached 0.05 pct, the proportion of AF in microstructure is relatively larger and the degree of interlocking of the AF within the microstructure was optimized. The ferrite grains also got refined and the average grain size decreased by more than 94 pct compared with that of the original steel.

Detection of Non-metallic Inclusions in Centrifugal Continuous Casting Steel Billets

In the current study, automated particle analysis was employed to detect non-metallic inclusions in steel during a centrifugal continuous casting process of a high-strength low alloy steel. The morphology, composition, size, area fraction, amount, and spatial distribution of inclusions in steel were obtained. Etching experiment was performed to reveal the dendrite structure of the billet and to discuss the effect of centrifugal force on the distribution of oxide inclusions in the final solidified steel by comparing the solidification velocity with the critical velocity reported in literature. It was found that the amount of inclusions was highest in samples from the tundish (~250 per mm2), followed by samples from the mold (~200 per mm2), and lowest in billet samples (~86 per mm2). In all samples, over 90 pct of the inclusions were smaller than 2μm. In steel billets, the content of oxides, dual-phase oxide–sulfides, and sulfides in inclusions were found to be 10, 30, and 60 pct, respectively. The dual-phase inclusions were oxides with sulfides precipitated on the outer surface. Oxide inclusions consisted of high Al2O3 and high MnO which were solid at the molten steel temperature, implying that the calcium treatment was insufficient. Small oxide inclusions very uniformly distributed on the cross section of the billet, while there were more sulfide inclusions showing a banded structure at the outside 25 mm layer of the billet. The calculated solidification velocity was higher than the upper limit at which inclusions were entrapped by the solidifying front, revealing that for oxide inclusions smaller than 8μm in this study, the centrifugal force had little influence on its final distribution in billets. Instead, oxide inclusions were rapidly entrapped by solidifying front.

Corrosion behaviors of the exposed side and underside of low alloy weathering steel in Qingdao and Wanning for 18 months

For 18 months, a newly developed low alloy weathering steel has been exposed in two coastal sites (Qingdao in the north China, Wanning in the south China). The different corrosion behaviors of the exposed side and the underside of the samples were characterized by X-ray diffraction (XRD), polarization curve, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and N2 adsorption approach. It was found that the samples exhibited higher corrosion rate in Wanning than that in Qingdao. The underside of the samples corrodes more seriously than the exposed side in Qingdao, whereas the result in Wanning is just the reverse. The protection performance of rust layers mainly depends on its compactness and the enrichment of Cu and Cr is a secondary causation. The different compactness of rust layers in the exposed side and the underside originates from different corrosion conditions of the two sides.

Thermal Stability of Non-Equilibrius Microstructure in Microalloyed Steel During Reheating

Cooled in water after isothermal relaxation of deformed austenite for different times, an Nb-bearing microalloyed steel always exhibits synthetic microstructures, in which bainitic ferrite dominates. Dislocation configurations and distributions of strain induced precipitates inside bainitic ferrite of samples relaxed for different times were distinct. When compared with the austenite model steel, which maintained fcc structure even at room temperature, the strain induced precipitates were not found in the sample without relaxation whereas these were distributed outside dislocations in sample relaxed for 1000s. Most of the strain induced precipitates distribute along dislocations and pin dislocations in sample relaxed for appropriate time. After bainitic transformation, the dislocations formed in deformed austenite remain to be pinned by the precipitates. When these samples were reheated to and held at 650 or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones. The sample relaxed for 60s displayed the highest thermo-stability, whereas microstructure evolution was the quickest in the sample relaxed for 1000s even though it was the softest prior to reheating. Dislocations inside laths got rid of pinning of precipitates, and their polygonization became the precursor to the evolution of microstructures during reheated and held, followed by gradual disappearance of lath boundaries caused by dislocation climbing. Finally, recrystallization occurred and polygonal ferrite appeared. By hardness measurement, it was found that softening is not a single process occurring during reheated, in which hardness fluctuates with time. There were two peaks in the hardness-time curve of each sample having undergone relaxation, while single peak occured in the curve of the sample not being relaxed. These results indicated that the thermo-stability of microstructures was determined by their history of formation to a considerable degree.

Effect of Mg Addition on the Refinement and Homogenized Distribution of Inclusions in Steel with Different Al Contents

To investigate the effect of Mg addition on the refinement and homogenized distribution of inclusions, deoxidized experiments with different amounts of aluminum and magnesium addition were carried out at 1873 K (1600 °C) under the condition of no fluid flow. The size distribution of three-dimensional inclusions obtained by applying the modified Schwartz–Saltykov transformation from the observed planar size distribution, and degree of homogeneity in inclusion dispersion quantified by measuring the inter-surface distance of inclusions, were studied as a function of the amount of Mg addition and holding time. The nucleation and growth of inclusions based on homogeneous nucleation theory and Ostwald ripening were discussed with the consideration of supersaturation degree and interfacial energy between molten steel and inclusions. The average attractive force acted on inclusions in experimental steels was estimated according to Paunov’s theory. The results showed that in addition to increasing the Mg addition, increasing the oxygen activity at an early stage of deoxidation and lowering the dissolved oxygen content are conductive to the increase of nucleation rate as well as to the refinement of inclusions Moreover, it was found that the degree of homogeneity in inclusion dispersion decreases with an increase of the attractive force acted on inclusions, which is largely dependent on the inclusion composition and volume fraction of inclusions.

Fatigue Life Improving of Drill Rod by Inclusion Control

Abstract Large and hard inclusions often deteriorate the service performance and reduce the fatigue lifetime of drill rods. In this paper, the main reasons of the rupture of drill rods were analyzed by the examination of their fracture and it is found that the large inclusions were the main reason of breakage of rod drill. The inclusions were high of Ca content or Al2O3 rich. Smaller and better deformability inclusions were obtained by the optimization of refining slag, calcium treatment process and the flow control devices of tundish. Results of industrial experiment after optimization show that total oxygen content of drill rods decreased by more than 50%, macro-inclusions weight fraction decreased from about 4 mg/10 kg to about 0.3 mg/10 kg and the micro-inclusions average size decreased from 6 to 3.6 μm. The average using times of drill rods after optimization were increased by about 60%.

Effects of Different Hot Working Techniques on Inclusions in GH4738 Superalloy Produced by VIM and VAR

Hot working is a key process in the production of superalloys; however, it may result in the formation of inclusions that affect the superalloy performance. Therefore, the effects of hot working on inclusions in a superalloy must be studied. GH4738 superalloy was manufactured, herein, by vacuum induction melting and vacuum arc remelting. Hot working was performed by unidirectional drawing, upsetting and drawing, and upsetting/drawing with radial forging. The types and distributions of inclusions after these three hot working processes and those in an original ingot were analyzed using scanning electron microscopy, energy dispersive spectroscopy, and Image-Pro Plus software. The results showed that the melting technology essentially determined the inclusion types in GH4738. Four types of inclusions were found in the experiments: TiC–TiN–Mo–S composite, TiC–TiN composite, Ce–Mo–S composite, and SiC inclusions. In the case of hot working by unidirectional drawing, the average inclusion size first decreased, and then increased from the center to the edge. In the case of upsetting and drawing, and upsetting/drawing with radial forging, the average inclusion size decreased from the center to the edge.

Inclusion characteristics and microstructure properties under different cooling conditions in steel with nanoparticles addition

ABSTRACT In the present study, the experimental steel containing nanoparticles was manufactured and then cooled to room temperature under different cooling rates. The differences on the inclusion characteristics and microstructure morphologies between the original steel and experimental steel were compared. The results revealed that the majority of reaction products in the experimental steel were Al2O3–MgO-bearing hybrid inclusions. These inclusions under brine quenching, and water quenching had large potential for inducing intra-granular acicular ferrites (IAF). Hence the microstructure in the experimental steel mainly consisted of fine IAF. For the original steel, SiO2–MnO–MnS-based inclusions could hardly induce IAF, and the microstructure consisted of relatively larger intra-granular banded ferrites, and intra-granular polygonal ferrites. The differences on microstructure morphologies between the two steels had a correlation with the inclusion composition, and size. These were the primary factors to determine whether a particular type of inclusion could induce IAF effectively or not.