The evolution mechanism of inclusions in Al-killed alloyed steel during secondary refining process was studied by industrial experiments and thermodynamic calculations. It is found that during the ...tapping process, Al–O deoxidization reaction is very close to equilibrium with the formation of many Al2O3 clusters. With the slag/steel reaction, inclusions vary with the route as Al2O3 inclusions→MgO–Al2O3 system inclusions→CaO–MgO–Al2O3 system inclusions, and finally change into globular inclusions surrounded by CaO–Al2O3 outer layer, of which the melting point is lower than liquid steel temperature. Since MgO is less stable than CaO and it is easier to be reduced by Al, dissolved Mg is generated earlier and faster than Ca before LF refining, thus the MgO–Al2O3 system inclusions form at first. The mapping photos of inclusions show that the evolution mechanism of MgO–Al2O3 system inclusions into CaO–MgO–Al2O3 system inclusions is Ca element substitution for Mg element in MgO–Al2O3 inclusions. The line scanning shows that there is also the reaction of Ca element substitution for Al element in the outer CaO–Al2O3 layer without MgO.
Macrosegregation is one of the most frequently observed defects in continuous casting blooms, which causes nonconformity in ultrasonic flaw detection of rolled products. To investigate the influence ...of combined EMS modes (M-EMS + F-EMS) on macrosegregation, a 3D multiphase solidification model based on the volume-averaged Eulerian approach was established to simulate the electromagnetic field, fluid flow, microstructural evolution, and solute transport of heavy-rail steel blooms subjected to different EMS processes. In this model, a hybrid model of the mushy zone and a back-diffusion model were introduced into the momentum and solute conservation equations to realize the calculation of microstructural evolution and solute transport with electromagnetic stirring. The predicted magnetic induction intensity, macrostructure, and macrosegregation were verified with Tesla meter measurements, etched macrostructure analysis, and infrared carbon-sulfur analysis. The calculation results showed that M-EMS had little effect on the improvement of the positive centerline segregation, whereas F-EMS effectively reduced the positive centerline segregation. Moreover, a combination of these EMS modes could further reduce the positive centerline segregation in continuous casting blooms. The change in solute concentration caused by M-EMS could be inherited by the position of F-EMS, which could enhance the metallurgical effects of F-EMS. These results were also verified through an industrial application.
Internal cracks are one of the main defects limiting the quality of continuous casting blooms. With bearing steel GCr15 chosen as the specific research steel, the temperature of internal crack ...initiation was determined by the solid-phase fraction (
f
s
) of the fracture, and by a simple and efficient high-temperature tensile method, the critical strain of internal crack initiation was determined. The thermal and mechanical behavior of continuous casting bloom was simulated and analyzed by a three-dimensional (3D) thermomechanical finite element model, and the model was verified by the online measured relationship between reduction force and amount of withdrawal and straightening units. Combining the critical criterion and strain distribution, an internal crack risk prediction model for continuous casting blooms was presented, and the predicted crack risk contour plot was verified by the morphology of actual bloom internal cracks. The maximum allowable single-roll reduction amount with different solidification phase fractions was analyzed for 320 mm × 425 mm GCr15 blooms, which concluded that the reasonable reduction strategy of continuous casting blooms could meet the requirement of relief center segregation and avoid internal crack risk simultaneously.
To further clarify the evolution of inclusions in Si-Mn-killed steel grades, industrial samples were taken during the LF (ladle furnace) refining process, and laboratory experiments were carried out ...to investigate the effects of alloy additions, refining slags, and refractory materials on the evolution of inclusions. It is found that the inclusions transform along the route of “MnO-SiO
2
-based inclusions → CaO-MnO-SiO
2
-based inclusions → CaO-SiO
2
-based inclusions”, and a small amount of MgO and Al
2
O
3
are contained in these inclusions. The increase of CaO content in inclusions is generally caused by the reduction of MnO and SiO
2
(especially MnO) by dissolved Ca in liquid steel, while the dissolved Al and Mg in liquid steel could also increase Al
2
O
3
and MgO in the inclusions. Both alloys and refining slag supply very limited Ca to liquid steel, which make it difficult to transform the MnO-SiO
2
-based inclusions into CaO-SiO
2
-based inclusions. The use of CaO-containing refractory is the main reason to cause the generation of CaO-MnO-SiO
2
-based inclusions in this study, and this kind of refractory is not recommended for tire cord and saw wire steel grades.
Based on the Euler–Euler approach, a mathematical model is established to describe gas and liquid two-phase flow in the gas-stirred system for steelmaking, and the influences of the interphase force ...including turbulent dispersion force, drag force, and lift force are investigated. The modified
k–ε
model with extra source terms to account for the bubble-induced turbulence is adopted to model the turbulence in the system, and the simulation results of gas volume fraction, liquid velocity, and turbulent kinetic energy are compared with the measured data. The results show that the turbulent dispersion force dominates the bubbly plume shape and is responsible for successful prediction of the gas volume fraction. The bubble-induced turbulence has a significant influence on the liquid turbulence, and the conversion coefficient
C
b
, which denotes the fraction of bubble-induced energy converted into liquid turbulence, should be in the range of 0.8 and 0.9. The drag force also strongly influences the bubbly plume dynamics, and the coefficient model proposed by Kolev performs the best for determining the drag force; however, the lift force and bubble diameter do not have much effect on the current bubbly plume system. For different gas flow rates, the current Euler–Euler approach predictions are more consistent with the measured data than the Euler–Lagrange approach and the early Euler–Euler model.
A computation fluid dynamics–simultaneous reaction model (CFD–SRM) coupled model has been proposed to describe the desulfurization behavior in a gas-stirred ladle. For the desulfurization ...thermodynamics, different models were investigated to determine sulfide capacity and oxygen activity. For the desulfurization kinetic, the effect of bubbly plume flow, as well as oxygen absorption and oxidation reactions in slag eyes are considered. The thermodynamic and kinetic modification coefficients are proposed to fit the measured data, respectively. Finally, the effects of slag basicity and gas flow rate on the desulfurization efficiency are investigated. The results show that as the interfacial reactions (Al
2
O
3
)-(FeO)-(SiO
2
)-(MnO)-S-O simultaneous kinetic equilibrium is adopted to determine the oxygen activity, and the Young’s model with the modification coefficient
R
th
of 1.5 is adopted to determine slag sulfide capacity, the predicted sulfur distribution ratio LS agrees well with the measured data. With an increase of the gas blowing time, the predicted desulfurization rate gradually decreased, and when the modification parameter
R
k
is 0.8, the predicted sulfur content changing with time in ladle agrees well with the measured data. If the oxygen absorption and oxidation reactions in slag eyes are not considered in this model, then the sulfur removal rate in the ladle would be overestimated, and this trend would become more obvious with an increase of the gas flow rate and decrease of the slag layer height. With the slag basicity increasing, the total desulfurization ratio increases; however, the total desulfurization ratio changes weakly as the slag basicity exceeds 7. With the increase of the gas flow rate, the desulfurization ratio first increases and then decreases. When the gas flow rate is 200 NL/min, the desulfurization ratio reaches a maximum value in an 80-ton gas-stirred ladle.
Industrial and laboratory experiments were carried out to investigate the formation and evolution of the inclusions in Ti-bearing Al-killed steel, and thermodynamic calculations were also conducted ...to check the stability of inclusions in steel. It is found that the addition of Ti influences the inclusions evidently. Before Ti addition, the inclusions transform along the route of “Al
2
O
3
→ MgO·Al
2
O
3
spinel → CaO-Al
2
O
3
(-MgO) system” with the generation of dissolved Mg and Ca in liquid steel. When there is no sufficient Ca in liquid steel, the addition of Ti during refining process would lead to the transformation of MgO·Al
2
O
3
spinel inclusions into MgO-Al
2
O
3
-TiO
x
inclusions. When a trace of dissolved Ca is formed, CaO-Al
2
O
3
-TiO
x
system inclusions can be generated from MgO-Al
2
O
3
-TiO
x
inclusions or CaO-Al
2
O
3
inclusions. Higher calcium content originated from calcium treatment would result in the formation of CaO·TiO
2
inclusions, and then weaken the castability of Ti-bearing Al-killed steel grades. Calcium treatment could also increase the frequency of macro-inclusions in steel, and evidently decline the quality of the steel. Cancellation of calcium treatment is suggested to improve the cleanliness of Ti-bearing Al-killed steel grades.
A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate solute diffusion controlled solidification in continuous steel casting. Constitutional and curvature ...undercooling were both solved to determine the equilibrium temperature and growth velocity of the solid/liquid interface. Simulations were firstly performed for both the free dendritic growth from an undercooled melt and the columnar dendritic growth in unidirectional solidification. Finally, competitive dendritic growth and columnar to equiaxed transition (CET) occurring in solidification of continuous casting process were reproduced by the present CA-FD model. The effect of the fragmentation of dendrites due to fluid flow induced by EMS in mould on nuclei was taken into consideration by increasing the grain density. The comparison between the simulated and experimentally observed results shows that the present model can be used to simulate solidification structure formation during the continuous casting process of steel. The influence of superheat on solidification structure was also analyzed, and it was found that increasing superheat increases the columnar dendritic growth and reduces the equiaxed ratio, as it is empirical well known.
The mechanism and kinetics of large TiC precipitates in Ti-Mo steel were investigated in situ with a high-temperature confocal laser scanning microscope and analytically evaluated through ...thermodynamic calculations in Thermo-Calc. The size of the austenite grains and TiC gradually decreased with increasing undercooling. During the solidification process, strong solute diffusion fields were formed via undercooling, which inhibited the nucleation of TiC; hence, the second phase precipitation temperature was lower than the equilibrium precipitation temperature. Moreover, there was a critical cooling rate between 0.2 and 1.5 K/s that rendered the capacity of γ formation equal to that of TiC formation. The nucleation location of TiC can be changed from grain boundary to grain interior by increasing undercooling. The growth pattern of TiC conformed to the Avrami equation; at higher undercooling, the time exponent was small, and the relationship between the growth length and time was approximately linear. During actual cooling, the driving force exceeded a critical value under a specific degree of undercooling, and the γ/TiC interface became morphologically unstable, leading to a cellular/dendritic pattern.
•During the solidification process, strong solute diffusion fields were formed.•The capacity of γ formation equal to that of largeTiC formation in Specific undercooling.•The growth pattern of largeTiC conformed to the Avrami equation.•The γ/TiC interface is a cellular/dendritic pattern.
The behavior of the meniscus region is crucial to the surface quality of continuous cast steel. A two-dimensional model was developed to study the evolution of the meniscus region during the initial ...casting stage of continuous casting process. The flow of multiphase fluids, transient heat transfer, melting and re-solidification of slag, and the solidification process of the steel shell were also considered in the model. The calculated results show a good agreement with experimental data of previous literature, which demonstrates the reliability of the model in predicting the behavior of the meniscus region. After the start of withdrawal, liquid slag infiltrates into the gaps, forming a slag layer structure composed of liquid slag films and solid slag films between the shell and the mold. As the distribution of each phase approaches stability, the initial shape of the meniscus was established. During the initial casting stage, as the casting speed increases, the curvature of the meniscus increases, accompanied by an elevated level of heat transfer within the meniscus region. Furthermore, from the perspective of the associated heat transfer and fluid flow variations during the increase in casting speed, the reasons for the decrease of the depth of the oscillation mark and the slag consumption are explained. This study provides new insights into the continuous and complex flow and heat transfer behavior within the meniscus region during the initial casting stage of continuous casting process.
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