Houfa Shen

Numerical simulation of macrosegregation in steel ingots using a two-phase model

A two-phase model for the prediction of macrosegregation formed during solidification is presented. This model incorporates the descriptions of heat transfer, melt convection, solute transport, and solid movement on the system scale with microscopic relations for grain nucleation and growth. Then the model is used to simulate the solidification of a benchmark industrial 3.3-t steel ingot. Simulations are performed to investigate the effects of grain motion and pipe shrinkage formation on the final macrosegregation pattern. The model predictions are compared with experimental data and numerical results from literatures. It is demonstrated that the model is able to express the overall macrosegregation patterns in the ingot. Furthermore, the results show that it is essential to consider the motion of equiaxed grains and the formation of pipe shrinkage in modelling. Several issues for future model improvements are identified.

Modeling of Species Transport and Macrosegregation in Heavy Steel Ingots

In the current study, two significant phenomena involved in heavy steel ingot casting, i.e., species transport and macrosegregation, were numerically simulated. First, a ladle–tundish–mold species transport model describing the entire multiple pouring process of heavy steel ingots was proposed. Carbon distribution and variation in both the tundish and the mold of a 292-ton steel ingot were predicted. Results indicate high carbon concentration in the bottom of the mold while low concentration carbon at the top of mold after the pouring process. Such concentration distribution helps in reducing both negative segregation in the bottom of the solidified ingot and positive segregation at the top. Second, a two-phase multiscale macrosegregation model was used to simulate the solidification process of industrial steel ingots. This model takes into account heat transfer, fluid flow, solute transport, and equiaxed grain motion on a system scale, as well as grain nucleation and growth on a microscopic scale. The model was first used to analyze a three-dimensional industry-scale steel ingot as a benchmark. Then, it was applied to study macrosegregation formation in a 53-ton steel ingot. Macrosegregation predicted by the numerical model was presented and compared with experimental measurements. Typical macrosegregation patterns in heavy steel ingots are found to be well reproduced with the two-phase model.

Water modelling of level fluctuation in thin slab continuous casting mould

Abstract A water modelling experiment was conducted to study the meniscus instability in a continuous thin slab casting mould using particle image visualisation. The results show that the level fluctuation, circulation centre position and jet impinging depth are unsteady and periodic with a similar period. The probability distributions of the fluctuating meniscus and wave height have been obtained with the highest frequency near the average position. The flow pattern and meniscus profile may be momentarily asymmetrical, and the phase difference of level fluctuation in the two sides of mould centreline is a half period. The average meniscus profile, the highest and lowest meniscus positions are generally symmetrical about the mould centreline. The wave height mainly depends on the jet impinging depth and circulation centre position. The wave height increases as the jet impinging position rises and the circulation centre approaches to the submerged entry nozzle.

Temperature distribution and dynamic control of secondary cooling in slab continuous casting

Abstract To predict and optimize the temperature distribution of slab continuous casting in steady operational state, a three-dimensional model (named “offline model”) based on the heat transfer and solidification theories was developed. Both heat transfer and flux distribution characteristics of the nozzle sprays on the slab were considered, and the complicated boundary conditions, such as spray cooling, natural convection, thermal radiation as well as contact cooling of individual rolls were involved in the model. By using the calibrated caster dependent model factors, the calculated temperature and shell thickness accorded well with the measured. Furthermore, a dynamic secondary water cooling control system was also developed on the basis of a two-dimensional transient heat transfer model (named “online model”) and incremental PID control algorithm to reduce slab surface temperature fluctuation in unsteady state. Compared with the traditional spray table control method, the present online model and dynamic PID control demonstrate a higher capability and flexibility to adjust cooling water flowrate and reduce slab surface temperature fluctuation when the casting speed is changed.

Inverse finite element modelling and identification of constitutive parameters of UHS steel based on Gleeble tensile tests at high temperature

The rheological behaviour of an ultra high strength (UHS) steel is investigated by Gleeble tensile tests at low-deformation rates and high temperature, from 1200°C to solidus temperature. Results show that large thermal gradients exist in specimens, resulting in heterogeneous deformation, which makes the identification of constitutive parameters difficult from the directly deduced nominal stress–strain curves. The advantages of an inverse identification method – associating a direct finite element model of Gleeble tests and an optimization module – are demonstrated in such conditions. The constitutive parameters identified by this technique have been successfully applied to additional tests, more complex in nature than those used for the identification of parameters. However, such tests combining successive loading and relaxation stages have revealed some limitations of the considered constitutive model.

Effect of electromagnetic brake on fluid flow in continuous slab casting mould

Abstract The present study describes a numerical model for fluid flow in a continuous slab casting mould with electromagnetic brake (EMBR). The model was solved using the finite difference method and the SIMPLER algorithm. A staggered grid system for fluid flow and electromagnetic field calculation was proposed for the discretisation of governing equations. The simulated fluid flow was validated with water-based experiments and a classic benchmark of driven cavity flow. Results show that the EMBR abates the flow velocities, alleviates the shearing stress at the mould narrow face and restrains the free surface fluctuation. It is also found that an intense electromagnetic field may change the flow pattern in the continuous casting mould.

Modelling of macrosegregation in steel ingots: benchmark validation and industrial application

The paper presents the recent progress made by the authors on modelling of macrosegregation in steel ingots. A two-phase macrosegregation model was developed that incorporates descriptions of heat transfer, melt convection, solute transport, and solid movement on the process scale with microscopic relations for grain nucleation and growth. The formation of pipe shrinkage at the ingot top is also taken into account in the model. Firstly, a recently proposed numerical benchmark test of macrosegregation was used to verify the model. Then, the model was applied to predict the macrosegregation in a benchmark industrial-scale steel ingot. The predictions were validated against experimental data from the literature. Furthermore, macrosegregation experiment of an industrial 53-t steel ingot was performed. The simulation results were compared with the measurements. It is indicated that the typical macrosegregation patterns encountered in steel ingots, including a positively segregated zone in the hot top and a negative segregation in the bottom part of the ingot, are well reproduced with the model.

Numerical simulation of fluid flow and solidification in continuous slab casting mould based on inverse heat transfer calculation

Abstract A mathematical model based on an inverse heat transfer calculation was built to determine the heat flux between the mould and slab based on the measured mould temperatures. With K–ϵ turbulence model, a mathematical model of three-dimensional heat transfer and solidification of molten steel in continuous slab casting mould is developed. Solidification has been taken into consideration, and flow in the mushy zone is modelled according to Darcy’s law as is the case of flow in the porous media. The heat flux prescribed on the boundaries is obtained in the inverse heat conduction calculation; thus, the effect of heat transfer in the mould has been taken into consideration. Results show that the calculated values of mould temperature coincide with the measured ones. Results also reveal that the temperature distribution and shell thickness are affected by the fluid flow and heat transfer of slab which is governed by the heat flux on the mould/slab interface.

Thermomechanical behavior in continuous bloom casting with different mold tapers

Abstract A two-dimensional finite element model was used to analyze the thermal and mechanical behavior during solidification of the strand in a continuous bloom casting mold. The coupled heat transfer and deformation were analyzed to simulate the formation of the air gap between the mold and the strand. The model was used to investigate the influence of mold taper on the temperature and stress distributions in the strand. The results show that the air gap mainly forms around the strand corner, causing a hotter and thinner solidifying shell in this region. The mold taper partially compensates for the strand shell shrinkage and reduces the influence of the air gap on the heat transfer. The mold taper compresses the shell and changes the stress state around the strand corner region. As the strand moves down into the mold, the mold constraint causes compressive stress beneath the corner surface, which reduces the hot tear that forms on the strand.

Finite Element Thermal-Mechanical Coupled Analysis of Strand Bulging Deformation in Continuous Casting

In order to reduce the internal cracks and control the quality of the steel strand in continuous casting, a thermal-mechanical model considering the movement of the strand, real roll arrangement and caster structure, has been established with the elasto-viscoplastic constitution to predict the three dimensional temperature distribution and bulging deformation of the strand. Using the sequentially coupled finite element method the model is solved numerically, and is validated by the comparisons of the simulation results and measured data. The model is suitable for future research on the whole strand bulging deformation to help improve the strand quality in continuous casting.