Jingshe Li

Thermodynamic models for predicting dephosphorisation ability and potential of CaO–FeO–Fe2O3–Al2O3–P2O5 slags during secondary refining process of molten steel based on ion and molecule coexistence theory

Thermodynamic models for predicting phosphorus distribution ratio LP and phosphate capacity of CaO–FeO–Fe2O3–Al2O3–P2O5 slags during secondary refining process of molten steel, according to the ion and molecule coexistence theory (IMCT), i.e. IMCT–LP and IMCT– models, have been developed by coupling with the developed thermodynamic model for calculating the mass action concentrations Ni of structural units or ion couples in the slags, i.e. the IMCT–Ni model, based on the IMCT. The developed IMCT–LP and IMCT– models have been verified with the experimental results from the literature and the predicted results by the summarised five LP models and three models. Besides the total dephosphorisation ability and potential of the slags as LP and , the respective phosphorus distribution ratio LP,i and the respective phosphate capacity of six dephosphorisation products as P2O5, 3FeO·P2O5, 4FeO·P2O5, 2CaO·P2O5, 3CaO·P2O5 and 4CaO·P2O5 can also reliably be predicted by the developed IMCT–LP and IMCT– models. 3CaO·P2O5 accounts for 95.00% of dephosphorisation products com- pared with 5% for 4CaO·P2O5. The comprehensive effect of CaO + FetO, which can be described by the mass action concentration ratio , controls dephosphorisation ability and potential of the slags. An exponentially growing relationship of lg LP against slag oxidisation ability and slag basicity as the simplified complex basicity (%CaO)/[(%P2O5) þ (%Al2O3)] or optical basicity can be established for the slags in a temperature range from 1811 to 1927 K. However, a linearly increasing relationship of lg against slag oxidisation ability and the aforementioned two kinds of slag basicity can be correlated for the slags in the lower temperature range from 1811 to 1828 K and the middle temperature range from 1870 to 1876 K; an inversely asymmetrical U type relationship of lg against the aforementioned parameters can be observed for the slags in the higher temperature range from 1918 to 1927 K. The influences of the aforementioned parameters on dephosphorisation ability are significantly different from those on dephosphorisation potential of the slags.

Prediction model of sulphur distribution ratio between CaO–FeO–Fe2O3–Al2O3–P2O5 slags and liquid iron over large variation range of oxygen potential during secondary refining process of molten steel based on ion and molecule coexistence theory

A thermodynamic model for predicting sulphur distribution ratio LS between CaO–FeO–Fe2O3–Al2O3–P2O5 slags and liquid iron, i.e. the ion and molecule coexistence theory (IMCT)–LS model, has been developed for slags in a large variation range of slag oxidisation ability based on the IMCT. The developed IMCT–LS model has been verified with measured data of oxygen, phosphorus and sulphur distribution equilibria between the slags and liquid iron from the literature. The results indicate that the desulphurisation reaction is mainly controlled by reaction (CaO)+[S] = (CaS)+[O] in the reducing zone with the optimised standard molar Gibbs free energy change of ; the desulphurisation reaction is dominated by reaction (FeO)+[S] = (FeS)+[O] in the oxidising zone with the reported standard molar Gibbs free energy change of . The influence of CaO on the desulphurisation ability of the slags can be counteracted by that of FetO because higher CaO content corresponds to lower FetO content in the slags. An asymmetric V type relationship between sulphur distribution ratio LS and the mass action concentration ratio or or or , or the mass percentage ratio (%FeO)/(%CaO) or (%Fe2O3)/(%CaO) or (%FetO)/(%CaO) or the simplified complex basicity or optical basicity can be established for the slags equilibrated with liquid iron in a temperature range from 1811 to 1927 K (1538 to 1654°C). The abovementioned mass action concentration ratios or mass percentage ratios of various iron oxides to basic oxide CaO can be recommended to represent the comprehensive effect of iron oxides FetO and basic oxide CaO on desulphurisation ability of the slags.

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.

Effect of heating rate on microstructural evolution and mechanical properties of cold-rolled quenching and partitioning steel

Quenching and partitioning (Q&P) processes were performed using initial heating rates ranging from 0.5 to 300 K s−1 with either one step or two steps in the intercritical annealing stage. Increasing the heating rate strongly affected the recrystallisation of ferrite and spheroidisation of pearlite. The nucleation and growth mechanisms of austenite under various heating rates were discussed, and the characteristics of retained austenite in each annealed sample were evaluated. Tensile tests demonstrated that the mechanical properties of Q&P steel increased with increasing heating rate. The poor mechanical properties of the slowly heated samples are suggested to result from the linable distribution of martensite.

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.