Intelligent casting:Empowering the future foundry industry

Emerging technological advances are reshaping the casting sector in latest decades.Casting technology is evolving towards intelligent casting paradigm that involves automation,greenization and intelligentization,which attracts more and more attention from the academic and industry communities.In this paper,the main features of casting technology were briefly summarized and forecasted,and the recent developments of key technologies and the innovative efforts made in promoting intelligent casting process were discussed.Moreover,the technical visions of intelligent casting process were also put forward.The key technologies for intelligent casting process comprise 3D printing technologies,intelligent mold technologies and intelligent process control technologies.In future,the intelligent mold that derived from mold with sensors,control devices and actuators will probably incorporate the Internet of Things,online inspection,embedded simulation,decision-making and control system,and other technologies to form intelligent cyber-physical casting system,which may pave the way to realize intelligent casting.It is promising that the intelligent casting process will eventually achieve the goal of real-time process optimization and full-scale control,with the defects,microstructure,performance,and service life of the fabricated castings can be accurately predicted and tailored.

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 per- formed 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.

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

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.

Numerical simulation of central shrinkage crack formation in a 234-t steel ingot

Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.

Analysis of internal crack in a six-ton P91 ingot

P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are f ive main portions of defects larger than 200 mm^3, four of which are interconnected. These initiated from continuous liquid f ilm, and then were torn apart by excessive tensile stress within the brittle temperature range(BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies' criterion and Katgerman's criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.

Simulation of macrosegregation in a 36-t steel ingot using a multiphase model

Macrosegregation is the major defect in large steel ingots caused by solute partitioning and melt convection during casting.In this study,a three-phase(liquid,columnar dendrites,and equiaxed grains)model is proposed to simulate macrosegregation in a 36-t steel ingot.A supplementary set of conservation equations are employed in the model such that two types of equiaxed grains,either settling or adhering to the solid shell,are well simulated.The predicted concentration agrees quantitatively with the experimental value.A negative segregation cone was located at the bottom owing to the grain settlement and solute-enriched melt leaving from the mushy zone.The interdendritic liquid flow was carefully analyzed,and the formation of A-type segregations in the mid-height of the ingot is discussed.Negative segregation was observed near the riser neck due to the specific relationship between flow direction and temperature gradient.Additionally,the as-cast macrostructure of the ingot is presented,including the grain size distribution and columnar–equiaxed transition.

Finite element modeling of macrosegregation coupled with shrinkage cavity in steel ingots using arbitrary Lagrangian-Eulerian model

Shrinkage cavity has significant influence on macrosegregation in steel ingots. An arbitrary Lagrangian-Eulerian (ALE) model based on volume averaging method is developed to predict the coupled formation progress of macrosegregation and shrinkage cavity during solidification of steel ingots. The combined effect of thermal-solutal convection and solidification shrinkage on macrosegregation is considered in the model. A specially designed mesh update algorithm is proposed to consider the formation of shrinkage cavity. The streamline-upwind/Petrov–Galerkin (SUPG) stabilized finite element algorithm is adopted to solve the conservation equations. Two solution methods for the energy conservation equation are proposed, i.e. the temperature-based solver and enthalpy-based solver. A Pb-48wt.%Sn solidification benchmark is used for validation. Then, the ALE model is applied to a Fe-3.6wt.%C industrial steel ingot. The formation progress of macrosegregation coupled with shrinkage cavity is predicted. By comparison with the predictions of the finite element model and finite volume model, the effect of shrinkage cavity formation on macrosegregation is investigated. Results show that the formation of shrinkage cavity can significantly change the segregation region and segregation degree at the hot top. It is demonstrated that the ALE model can predict the coupled formation of macrosegregation and shrinkage cavity in steel ingots.

Internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio

Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice.Water cooling is a usual way to enhance production efficiency.However,the combination of the two factors will generate internal defects,such as shrinkage porosity and hot crack.The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test.A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted.The billets were examined by X-ray high energy industrial CT,and the compactness was reconstructed in three dimensions.Microstructure near the crack was observed using scanning electron microscopy,and the solidification process and grain size were studied by high temperature confocal microscopy.Moreover,thermo-mechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack.A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot.The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio,and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities.Then,hot cracks begin to propagate along grain boundaries.The grain size in the upper and center of the ingot is large,which leads to an inverted cone shape defects zone in the ingot center.

A Numerical Study of the Effect of Multiple Pouring on Macrosegregation in a 438-Ton Steel Ingot

In present paper, a ladle-tundish-mold CFD model and a macrosegregation model were utilized to investigate the effects of the multiple pouring （MP） process on the macrosegregation in a 438-ton steel ingot. Firstly, the model was partially proved as compared to the measured carbon distributions along the transverse sections in the riser of ingot. Then, the comparison between the single pouring （SP） and MP process has been carried out to study their influences on the macrosegregation in ingot. Besides, the predicted macrosegregation results in MP process which introduced the improved riser fixed with an insulating sleeve were compared with that in traditional MP process. The traditional MP process leads to certain favorable initial carbon distribution in the mold, which has some favorable influence on suppressing the positive segregation in ingot. The holding time of the low carbon in the riser is the main factor to suppress the positive segregation in ingot. Improved insulating sleeve can prolong the holding time of the low carbon in the riser and release the positive segregation in the riser of ingot. Improvement of the insulating effect of the riser is an efficient method to control macrosegregation in large steel ingot.

Real-time observation of bainite formation at heterogeneous phases in a high-strength weathering steel

Due to the excellent comprehensive mechanical properties and toughness of bainite steels,bainite is regarded as a most desirable microstructure for the new generation of high-strength weathering steels.The formation of bainite was observed in real time in a high-strength weathering steel,and the results showed that bainite laths show impingement during phase transformation.The preferred regions of nucleation sites were identified,and the growth rate of bainite was measured.The growth mechanism of bainite was demonstrated to exhibit growth rate contributions from both the diffusion mechanism and the shear mechanism.Subsequently,the heterogeneous phases that form preferred sites for bainite nucleation were quantitatively identified by scanning electron microscopy(SEM),energy-dispersive X-ray spectrometry(EDS),and calculation of phase diagram(CALPHAD).The austenite grain sizes in crease with increasing austenite temperature,which leads to longer bainite laths.The influence of a small lattice disregistry between the heterogeneous phases and bainite on the bainite nucleation was studied.The disregistries between the favorable heterogeneous phases of VN,VC,TiN,or TiC and the ot-Fe in bainite are 2.9,3.1,3.9,and 4.6%,respectively.Therefore,VN,VC,TiN,and TiC can act as highly effective nuclei for bainite during the bainite transformation.

Numerical simulation of macrosegregation during solidification of binary alloys

Macrosegregation as a consequence of solidifi- cation of a binary Pb-48 wt% Sn alloy in two-dimensional rectangle cavity was investigated by two-phase model instead of the common used continuum model. The macroscopic transport of mass, momentum, species and energy was cou- pled with the microscopic descriptions of grain nucleation and growth. The mathematic equations of the two-phase model were discretized on staggered grids and solved by the finite volume method. The final tin concentrations along different heights over the bottom face were compared with the ex- periment measurements. Simulations with different mi- crostructure parameters （second dendrite arm spacing （SDAS）） were conducted to ascertain the influence of microstructure on the final macrosegregation. The results show that with the decrease in SDAS value, the drag force as the resistance to relative motion between solid and liquid phase increases, and the macrosegregation during solidification is alleviated.

Numerical Simulation of Macrosegregation Caused by Thermal–Solutal Convection and Solidification Shrinkage Using ALE Model

Solidifi cation shrinkage has been recognized as an important factor for macrosegregation formation. An arbitrary Lagrangian–Eulerian(ALE) model is constructed to predict the macrosegregation caused by thermal–solutal convection and solidi-fi cation shrinkage. A novel mesh update algorithm is developed to account for the domain change induced by solidifi cation shrinkage. The velocity–pressure coupling between the non-homogenous mass conservation equation and momentum equation is addressed by a modifi ed pressure correction method. The governing equations are solved by the streamline-upwind/Petrov–Galerkin-stabilized fi nite element algorithm. The application of the model to the Pb-19.2 wt%Sn alloy solidifi cation problem is considered. The inverse segregation is successfully predicted, and reasonable agreement with the literature results is obtained. Thus, the ALE model is established to be a highly effective tool for predicting the macrosegregation caused by solidifi cation shrinkage and thermal–solutal convection. Finally, the effect of solidifi cation shrinkage is analyzed. The results demonstrate that solidifi cation shrinkage delays the advance of the solidifi cation front and intensifi es the segregation.

Influence of N/C ratio on microstructure and properties of new highstrength weathering steels

Influence of the N/C ratio on the microstructure and properties of new-generation high-strength weathering steels was investigated using calculation of phase diagram(CALPHAD)and experiments.The microstructures of weathering steels containing different N/C ratios were predicted by a CALPHAD approach,and only three phases were predicted in these steels within the rolling temperature range of 850-1050℃.The precipitation fraction of VN/V(C,N)in creases with increasing N/C ratio.Microstructures of the four tested steels were all experimentally determined to contain bainite,ferrite,and VN precipitates after air cooling to room temperature.The bainite fraction increases with in creasing N/C ratio,and it is 85%in the steel containing 0.038% N and 0.032% C,The results of the tensile tests and impact tests demonstrated that the yield strength and tensile strength of the steel containing 0.038%N and 0.032%C are greater than 550 and 650 MPa,respectively,and the elongation is greater than 24%,which satisfies the design objectives for mechanical properties.The impact toughness values of the four steels at 0,-20,and-40℃ are all greater than 24 J.With increasing N/C ratio,the bainite fraction and the precipitation fraction of VN/V(C,N)increase,resulting in increasing yield strength and tensile strength.